Difference between revisions of "RFC1190"

Revision as of 23:45, 22 September 2020

Network Working Group CIP Working Group Request for Comments: 1190 C. Topolcic, Editor Obsoletes: IEN-119 October 1990

       Experimental Internet Stream Protocol, Version 2 (ST-II)

Status of this Memo

  This memo defines a revised version of the Internet Stream Protocol,
  originally defined in IEN-119 [8], based on results from experiments
  with the original version, and subsequent requests, discussion, and
  suggestions for improvements.  This is a Limited-Use Experimental
  Protocol.  Please refer to the current edition of the "IAB Official
  Protocol Standards" for the standardization state and status of this
  protocol.  Distribution of this memo is unlimited.

1. Abstract

  This memo defines the Internet Stream Protocol, Version 2 (ST-II), an
  IP-layer protocol that provides end-to-end guaranteed service across
  an internet.  This specification obsoletes IEN 119 "ST - A Proposed
  Internet Stream Protocol" written by Jim Forgie in 1979, the previous
  specification of ST.  ST-II is not compatible with Version 1 of the
  protocol, but maintains much of the architecture and philosophy of
  that version.  It is intended to fill in some of the areas left
  unaddressed, to make it easier to implement, and to support a wider
  range of applications.

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

  1.1.       Table of Contents

                Status of this Memo .  .  .  .  .  .  .  .  .  .  .  .   1
        1.      Abstract   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .   1
        1.1.       Table of Contents   .  .  .  .  .  .  .  .  .  .  .   2
        1.2.       List of Figures  .  .  .  .  .  .  .  .  .  .  .  .   4

        2.      Introduction  .  .  .  .  .  .  .  .  .  .  .  .  .  .   7
        2.1.       Major Differences Between ST and ST-II   .  .  .  .   8
        2.2.       Concepts and Terminology  .  .  .  .  .  .  .  .  .   9
        2.3.       Relationship Between Applications and ST .  .  .  .  11
        2.4.       ST Control Message Protocol  .  .  .  .  .  .  .  .  12
        2.5.       Flow Specifications .  .  .  .  .  .  .  .  .  .  .  14

        3.      ST Control Message Protocol Functional Description   .  17
        3.1.       Stream Setup  .  .  .  .  .  .  .  .  .  .  .  .  .  18
        3.1.1.        Initial Setup at the Origin  .  .  .  .  .  .  .  18
        3.1.2.        Invoking the Routing Function   .  .  .  .  .  .  19
        3.1.3.        Reserving Resources .  .  .  .  .  .  .  .  .  .  19
        3.1.4.        Sending CONNECT Messages  .  .  .  .  .  .  .  .  20
        3.1.5.        CONNECT Processing by an Intermediate Agent .  .  22
        3.1.6.        Setup at the Targets   .  .  .  .  .  .  .  .  .  23
        3.1.7.        ACCEPT Processing by an Intermediate Agent  .  .  24
        3.1.8.        ACCEPT Processing by the Origin .  .  .  .  .  .  26
        3.1.9.        Processing a REFUSE Message  .  .  .  .  .  .  .  27
        3.2.       Data Transfer .  .  .  .  .  .  .  .  .  .  .  .  .  30
        3.3.       Modifying an Existing Stream .  .  .  .  .  .  .  .  31
        3.3.1.        Adding a Target  .  .  .  .  .  .  .  .  .  .  .  31
        3.3.2.        The Origin Removing a Target .  .  .  .  .  .  .  33
        3.3.3.        A Target Deleting Itself  .  .  .  .  .  .  .  .  35
        3.3.4.        Changing the FlowSpec  .  .  .  .  .  .  .  .  .  36
        3.4.       Stream Tear Down .  .  .  .  .  .  .  .  .  .  .  .  36
        3.5.       Exceptional Cases   .  .  .  .  .  .  .  .  .  .  .  37
        3.5.1.        Setup Failure due to CONNECT Timeout  .  .  .  .  37
        3.5.2.        Problems due to Routing Inconsistency .  .  .  .  38
        3.5.3.        Setup Failure due to a Routing Failure   .  .  .  39
        3.5.4.        Problems in Reserving Resources .  .  .  .  .  .  41
        3.5.5.        Setup Failure due to ACCEPT Timeout   .  .  .  .  41
        3.5.6.        Problems Caused by CHANGE Messages .  .  .  .  .  42
        3.5.7.        Notification of Changes Forced by Failures  .  .  42
        3.6.       Options .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  44
        3.6.1.        HID Field Option .  .  .  .  .  .  .  .  .  .  .  44
        3.6.2.        PTP Option .  .  .  .  .  .  .  .  .  .  .  .  .  44
        3.6.3.        FDx Option .  .  .  .  .  .  .  .  .  .  .  .  .  45
        3.6.4.        NoRecovery Option   .  .  .  .  .  .  .  .  .  .  46
        3.6.5.        RevChrg Option   .  .  .  .  .  .  .  .  .  .  .  46
        3.6.6.        Source Route Option .  .  .  .  .  .  .  .  .  .  46
        3.7.       Ancillary Functions .  .  .  .  .  .  .  .  .  .  .  48
        3.7.1.        Failure Detection   .  .  .  .  .  .  .  .  .  .  48
        3.7.1.1.         Network Failures .  .  .  .  .  .  .  .  .  .  48
        3.7.1.2.         Detecting ST Stream Failures .  .  .  .  .  .  49
        3.7.1.3.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  51

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        3.7.2.        Failure Recovery .  .  .  .  .  .  .  .  .  .  .  51
        3.7.2.1.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  55
        3.7.3.        A Group of Streams  .  .  .  .  .  .  .  .  .  .  56
        3.7.3.1.         Group Name Generator   .  .  .  .  .  .  .  .  57
        3.7.3.2.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  57
        3.7.4.        HID Negotiation  .  .  .  .  .  .  .  .  .  .  .  58
        3.7.4.1.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  64
        3.7.5.        IP Encapsulation of ST .  .  .  .  .  .  .  .  .  64
        3.7.5.1.         IP Multicasting  .  .  .  .  .  .  .  .  .  .  65
        3.7.6.        Retransmission   .  .  .  .  .  .  .  .  .  .  .  66
        3.7.7.        Routing .  .  .  .  .  .  .  .  .  .  .  .  .  .  67
        3.7.8.        Security   .  .  .  .  .  .  .  .  .  .  .  .  .  67
        3.8.       ST Service Interfaces  .  .  .  .  .  .  .  .  .  .  68
        3.8.1.        Access to Routing Information   .  .  .  .  .  .  69
        3.8.2.        Access to Network Layer Resource Reservation   .  70
        3.8.3.        Network Layer Services Utilized .  .  .  .  .  .  71
        3.8.4.        IP Services Utilized   .  .  .  .  .  .  .  .  .  71
        3.8.5.        ST Layer Services Provided   .  .  .  .  .  .  .  72

        4.      ST Protocol Data Unit Descriptions .  .  .  .  .  .  .  75
        4.1.       Data Packets  .  .  .  .  .  .  .  .  .  .  .  .  .  76
        4.2.       ST Control Message Protocol Descriptions .  .  .  .  77
        4.2.1.        ST Control Messages .  .  .  .  .  .  .  .  .  .  79
        4.2.2.        Common SCMP Elements   .  .  .  .  .  .  .  .  .  80
        4.2.2.1.         DetectorIPAddress   .  .  .  .  .  .  .  .  .  80
        4.2.2.2.         ErroredPDU .  .  .  .  .  .  .  .  .  .  .  .  80
        4.2.2.3.         FlowSpec & RFlowSpec   .  .  .  .  .  .  .  .  81
        4.2.2.4.         FreeHIDs   .  .  .  .  .  .  .  .  .  .  .  .  84
        4.2.2.5.         Group & RGroup   .  .  .  .  .  .  .  .  .  .  85
        4.2.2.6.         HID & RHID .  .  .  .  .  .  .  .  .  .  .  .  86
        4.2.2.7.         MulticastAddress .  .  .  .  .  .  .  .  .  .  86
        4.2.2.8.         Name & RName  .  .  .  .  .  .  .  .  .  .  .  87
        4.2.2.9.         NextHopIPAddress .  .  .  .  .  .  .  .  .  .  88
        4.2.2.10.        Origin  .  .  .  .  .  .  .  .  .  .  .  .  .  88
        4.2.2.11.        OriginTimestamp  .  .  .  .  .  .  .  .  .  .  89
        4.2.2.12.        ReasonCode .  .  .  .  .  .  .  .  .  .  .  .  89
        4.2.2.13.        RecordRoute   .  .  .  .  .  .  .  .  .  .  .  94
        4.2.2.14.        SrcRoute   .  .  .  .  .  .  .  .  .  .  .  .  95
        4.2.2.15.        Target and TargetList  .  .  .  .  .  .  .  .  96
        4.2.2.16.        UserData   .  .  .  .  .  .  .  .  .  .  .  .  98
        4.2.3.        ST Control Message PDUs   .  .  .  .  .  .  .  .  99
        4.2.3.1.         ACCEPT  .  .  .  .  .  .  .  .  .  .  .  .  . 100
        4.2.3.2.         ACK  .  .  .  .  .  .  .  .  .  .  .  .  .  . 102
        4.2.3.3.         CHANGE-REQUEST   .  .  .  .  .  .  .  .  .  . 103
        4.2.3.4.         CHANGE  .  .  .  .  .  .  .  .  .  .  .  .  . 104
        4.2.3.5.         CONNECT .  .  .  .  .  .  .  .  .  .  .  .  . 105
        4.2.3.6.         DISCONNECT .  .  .  .  .  .  .  .  .  .  .  . 110
        4.2.3.7.         ERROR-IN-REQUEST .  .  .  .  .  .  .  .  .  . 111
        4.2.3.8.         ERROR-IN-RESPONSE   .  .  .  .  .  .  .  .  . 112
        4.2.3.9.         HELLO   .  .  .  .  .  .  .  .  .  .  .  .  . 113
        4.2.3.10.        HID-APPROVE   .  .  .  .  .  .  .  .  .  .  . 114
        4.2.3.11.        HID-CHANGE-REQUEST  .  .  .  .  .  .  .  .  . 115

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.12.        HID-CHANGE .  .  .  .  .  .  .  .  .  .  .  . 116
        4.2.3.13.        HID-REJECT .  .  .  .  .  .  .  .  .  .  .  . 118
        4.2.3.14.        NOTIFY  .  .  .  .  .  .  .  .  .  .  .  .  . 120
        4.2.3.15.        REFUSE  .  .  .  .  .  .  .  .  .  .  .  .  . 122
        4.2.3.16.        STATUS  .  .  .  .  .  .  .  .  .  .  .  .  . 124
        4.2.3.17.        STATUS-RESPONSE  .  .  .  .  .  .  .  .  .  . 126
        4.3.       Suggested Protocol Constants .  .  .  .  .  .  .  . 127

        5.      Areas Not Addressed .  .  .  .  .  .  .  .  .  .  .  . 131

        6.      Glossary   .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 135

        7.      References .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 143

        8.      Security Considerations.  .  .  .  .  .  .  .  .  .  . 144

        9.      Authors' Addresses  .  .  .  .  .  .  .  .  .  .  .  . 145

        Appendix 1.      Data Notations   .  .  .  .  .  .  .  .  .  . 147

  1.2.       List of Figures

        Figure 1.    Protocol Relationships  .  .  .  .  .  .  .  .  .   6
        Figure 2.    Topology Used in Protocol Exchange Diagrams  .  .  16
        Figure 3.    Virtual Link Identifiers for SCMP Messages   .  .  16
        Figure 4.    HIDs Assigned for ST User Packets   .  .  .  .  .  18
        Figure 5.    Origin Sending CONNECT Message   .  .  .  .  .  .  21
        Figure 6.    CONNECT Processing by an Intermediate Agent  .  .  22
        Figure 7.    CONNECT Processing by the Target .  .  .  .  .  .  24
        Figure 8.    ACCEPT Processing by an Intermediate Agent   .  .  25
        Figure 9.    ACCEPT Processing by the Origin  .  .  .  .  .  .  26
        Figure 10.   Sending REFUSE Message  .  .  .  .  .  .  .  .  .  28
        Figure 11.   Routing Around a Failure   .  .  .  .  .  .  .  .  29
        Figure 12.   Addition of Another Target .  .  .  .  .  .  .  .  32
        Figure 13.   Origin Removing a Target   .  .  .  .  .  .  .  .  34
        Figure 14.   Target Deleting Itself  .  .  .  .  .  .  .  .  .  35
        Figure 15.   CONNECT Retransmission after a Timeout .  .  .  .  38
        Figure 16.   Processing NOTIFY Messages .  .  .  .  .  .  .  .  43
        Figure 17.   Source Routing Option   .  .  .  .  .  .  .  .  .  47
        Figure 18.   Typical HID Negotiation (No Multicasting) .  .  .  60
        Figure 19.   Multicast HID Negotiation  .  .  .  .  .  .  .  .  61
        Figure 20.   Multicast HID Re-Negotiation           .  .  .  .  62
        Figure 21.   ST Header   .  .  .  .  .  .  .  .  .  .  .  .  .  75
        Figure 22.   ST Control Message Format  .  .  .  .  .  .  .  .  77
        Figure 23.   ErroredPDU  .  .  .  .  .  .  .  .  .  .  .  .  .  80
        Figure 24.   FlowSpec & RFlowSpec .  .  .  .  .  .  .  .  .  .  81
        Figure 25.   FreeHIDs .  .  .  .  .  .  .  .  .  .  .  .  .  .  85
        Figure 26.   Group & RGroup .  .  .  .  .  .  .  .  .  .  .  .  85
        Figure 27.   HID & RHID  .  .  .  .  .  .  .  .  .  .  .  .  .  86
        Figure 28.   MulticastAddress  .  .  .  .  .  .  .  .  .  .  .  86
        Figure 29.   Name & RName   .  .  .  .  .  .  .  .  .  .  .  .  87
        Figure 30.   NextHopIPAddress  .  .  .  .  .  .  .  .  .  .  .  88

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        Figure 31.   Origin   .  .  .  .  .  .  .  .  .  .  .  .  .  .  88
        Figure 32.   OriginTimestamp   .  .  .  .  .  .  .  .  .  .  .  89
        Figure 33.   ReasonCode  .  .  .  .  .  .  .  .  .  .  .  .  .  89
        Figure 34.   RecordRoute .  .  .  .  .  .  .  .  .  .  .  .  .  94
        Figure 35.   SrcRoute .  .  .  .  .  .  .  .  .  .  .  .  .  .  95
        Figure 36.   Target   .  .  .  .  .  .  .  .  .  .  .  .  .  .  97
        Figure 37.   TargetList  .  .  .  .  .  .  .  .  .  .  .  .  .  97
        Figure 38.   UserData .  .  .  .  .  .  .  .  .  .  .  .  .  .  98
        Figure 39.   ACCEPT Control Message  .  .  .  .  .  .  .  .  . 101
        Figure 40.   ACK Control Message  .  .  .  .  .  .  .  .  .  . 102
        Figure 41.   CHANGE-REQUEST Control Message   .  .  .  .  .  . 103
        Figure 42.   CHANGE Control Message  .  .  .  .  .  .  .  .  . 105
        Figure 43.   CONNECT Control Message .  .  .  .  .  .  .  .  . 109
        Figure 44.   DISCONNECT Control Message .  .  .  .  .  .  .  . 110
        Figure 45.   ERROR-IN-REQUEST Control Message .  .  .  .  .  . 111
        Figure 46.   ERROR-IN-RESPONSE Control Message   .  .  .  .  . 112
        Figure 47.   HELLO Control Message   .  .  .  .  .  .  .  .  . 113
        Figure 48.   HID-APPROVE Control Message   .  .  .  .  .  .  . 114
        Figure 49.   HID-CHANGE-REQUEST Control Message  .  .  .  .  . 115
        Figure 50.   HID-CHANGE Control Message .  .  .  .  .  .  .  . 117
        Figure 51.   HID-REJECT Control Message .  .  .  .  .  .  .  . 119
        Figure 52.   NOTIFY Control Message  .  .  .  .  .  .  .  .  . 121
        Figure 53.   REFUSE Control Message  .  .  .  .  .  .  .  .  . 123
        Figure 54.   STATUS Control Message  .  .  .  .  .  .  .  .  . 125
        Figure 55.   STATUS-RESPONSE Control Message  .  .  .  .  .  . 126
        Figure 56.   Transmission Order of Bytes   .  .  .  .  .  .  . 147
        Figure 57.   Significance of Bits .  .  .  .  .  .  .  .  .  . 147

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

+--------------------+
| Conference Control |
+--------------------+
                   |

   |        |      |     |        |     |            |
   V        V      |     |        |     |            |   ------------
+-----+  +-----+   |     |        |     |            |
| PVP |  | NVP |   |     |        |     |            |
+-----+  +-----+   +     |        |     |            |
 |   \      | \     \    |        |     |            |
 |    +-----|--+-----+   |        |     |            |
 |     Appl.|control  V  V        V     V            V
 | ST  data |         +-----+    +-------+        +-----+
 | & control|         | UDP |    |  TCP  |    ... |     | Transport
 |          |         +-----+    +-------+        +-----+   Layer
 |         /|          / | \       / / |          / /|
 |\       / |  +------+--|--\-----+-/--|--- ... -+ / |
 | \     /  |  |         |   \     /   |          /  |
 |  \   /   |  |         |    \   +----|--- ... -+   |   -----------
 |   \ /    |  |         |     \ /     |             |
 |    V     |  |         |      V      |             |
 | +------+ |  |         |   +------+  |   +------+  |
 | | SCMP | |  |         |   | ICMP |  |   | IGMP |  |    Internet
 | +------+ |  |         |   +------+  |   +------+  |     Layer
 |    |     |  |         |      |      |      |      |
 V    V     V  V         V      V      V      V      V

+-----------------+ +-----------------------------------+ | STream protocol |->| Internet Protocol | +-----------------+ +-----------------------------------+

              | \   / |
              |  \ /  |
              |   X   |                                  ------------
              |  / \  |
              | /   \ |
              VV     VV

                   Figure 1.  Protocol Relationships

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

2. Introduction

  ST has been developed to support efficient delivery of streams of
  packets to either single or multiple destinations in applications
  requiring guaranteed data rates and controlled delay characteristics.
  The motivation for the original protocol was that IP [2] [15] did not
  provide the delay and data rate characteristics necessary to support
  voice applications.

  ST is an internet protocol at the same layer as IP, see Figure 1.  ST
  differs from IP in that IP, as originally envisioned, did not require
  routers (or intermediate systems) to maintain state information
  describing the streams of packets flowing through them.  ST
  incorporates the concept of streams across an internet.  Every
  intervening ST entity maintains state information for each stream
  that passes through it.  The stream state includes forwarding
  information, including multicast support for efficiency, and resource
  information, which allows network or link bandwidth and queues to be
  assigned to a specific stream.  This pre-allocation of resources
  allows data packets to be forwarded with low delay, low overhead, and
  a low probability of loss due to congestion.  The characteristics of
  a stream, such as the number and location of the endpoints, and the
  bandwidth required, may be modified during the lifetime of the
  stream.  This allows ST to give a real time application the
  guaranteed and predictable communication characteristics it requires,
  and is a good vehicle to support an application whose communications
  requirements are relatively predictable.

  ST proved quite useful in several early experiments that involved
  voice conferences in the Internet.  Since that time, ST has also been
  used to support point-to-point streams that include both video and
  voice.  Recently, multimedia conferencing applications have been
  developed that need to exchange real-time voice, video, and pointer
  data in a multi-site conferencing environment.  Multimedia
  conferencing across an internet is an application for which ST
  provides ideal support.  Simulation and wargaming applications [14]
  also place similar requirements on the communication system.  Other
  applications may include scientific visualization between a number of
  workstations and one or more remote supercomputers, and the
  collection and distribution of real-time sensor data from remote
  sensor platforms.  ST may also be useful to support activities that
  are currently supported by IP, such as bulk file transfer using TCP.

  Transport protocols above ST include the Packet Video Protocol (PVP)
  [5] and the Network Voice Protocol (NVP) [4], which are end-to-end
  protocols used directly by applications.  Other transport layer
  protocols that may be used over ST include TCP [16], VMTP [3], etc.
  They provide the user interface, flow control, and packet ordering.
  This specification does not describe these higher layer protocols.

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

  2.1.       Major Differences Between ST and ST-II

     ST-II supports a wider variety of applications than did the
     original ST.  The differences between ST and ST-II are fairly
     straight forward yet provide great improvements.  Four of the more
     notable differences are:

        1  ST-II is decoupled from the Access Controller (AC).  The
           AC, as well as providing a rudimentary access control
           function, also served as a centralized repository and
           distributor of the conference information.  If an AC is
           necessary, it should be an entity in a higher layer
           protocol.  A large variety of applications such as
           conferencing, distributed simulations, and wargaming can
           be run without an explicit AC.

        2  The basic stream construct of ST-II is a directed tree
           carrying traffic away from a source to all the
           destinations, rather than the original ST's omniplex
           structure.  For example, a conference is composed of a
           number of such trees, one for traffic from each
           participant.  Although there are more (simplex) streams in
           ST-II, each is much simpler to manage, so the aggregate is
           much simpler.  This change has a minimal impact on the
           application.

        3  ST-II defines a number of the robustness and recovery
           mechanisms that were left undefined in the original ST
           specification.  In case of a network or ST Agent failure,
           a stream may optionally be repaired automatically (i.e.,
           without intervention from the user or the application)
           using a pruned depth first search starting at the ST Agent
           immediately preceding the failure.

        4  ST-II does not make an inherent distinction between
           streams connecting only two communicants and streams among
           an arbitrary number of communicants.

     This memo is the specification for the ST-II Protocol.  Since
     there should be no ambiguity between the original ST specification
     and the specification herein, the protocol is simply called ST
     hereafter.

     ST is the protocol used by ST entities to exchange information.
     The same protocol is used for communication among all ST entities,
     whether they communicate with a higher layer protocol or forward
     ST packets between attached networks.

     The remainder of this section gives a brief overview of the ST
     Protocol.  Section 3 (page 17) provides a detailed description of
     the operations required by the protocol.  Section 4 (page 75)
     provides descriptions of the ST Protocol Data Units exchanged

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

     between ST entities.  Issues that have not yet been fully
     addressed are presented in Section 5 (page 131).  A glossary and
     list of references are in Sections 6 (page 135) and 7 (page 143),
     respectively.

     This memo also defines "subsets" of ST that can be implemented.  A
     subsetted implementation does not have full ST functionality, but
     it can interoperate with other similarly subsetted
     implementations, or with a full implementation, in a predictable
     and consistent manner.  This approach allows an implementation to
     be built and provide service with minimum effort, and gives it an
     immediate and well defined growth path.

  2.2.       Concepts and Terminology

     The ST packet header is not constrained to be compatible with the
     IP packet header, except for the IP Version Number (the first four
     bits) that is used to distinguish ST packets (IP Version 5) from
     IP packets (IP Version 4).  The ST packets, or protocol data units
     (PDUs), can be encapsulated in IP either to provide connectivity
     (possibly with degraded service) across portions of an internet
     that do not provide support for ST, or to allow access to services
     such as security that are not provided directly by ST.

     An internet entity that implements the ST Protocol is called an
     "ST Agent".  We refer to two kinds of ST agents:  "host ST
     agents", also called "host agents" and "intermediate ST agents",
     also called "intermediate agents".  The ST agents functioning as
     hosts are sourcing or sinking data to a higher layer protocol or
     application, while ST agents functioning as intermediate agents
     are forwarding data between directly attached networks.  This
     distinction is not part of the protocol, but is used for
     conceptual purposes only.  Indeed, a given ST agent may be
     simultaneously performing both host and intermediate roles.  Every
     ST agent should be capable of delivering packets to a higher layer
     protocol.  Every ST agent can replicate ST data packets as
     necessary for multi-destination delivery, and is able to send
     packets whether received from a network interface or a higher
     layer protocol.  There are no other kinds of ST agents.

     ST provides applications with an end-to-end flow oriented service
     across an internet.  This service is implemented using objects
     called "streams".  ST data packets are not considered to be
     totally independent as are IP data packets.  They are transmitted
     only as part of a point-to-point or point-to-multi- point stream.
     ST creates a stream during a setup phase before data is
     transmitted.  During the setup phase, routes are selected and
     internetwork resources are reserved.  Except for explicit changes
     to the stream, the routes remain in effect until the stream is
     explicitly torn down.

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     An ST stream is:

        o  the set of paths that data generated by an application
           entity traverses on its way to its peer application
           entity(s) that receive it,

        o  the resources allocated to support that transmission of
           data, and

        o  the state information that is maintained describing that
           transmission of data.

     Each stream is identified by a globally unique "Name";  see
     Section 4.2.2.8 (page 87).  The Name is specified in ST control
     operations, but is not used in ST data packets.  A set of streams
     may be related as members of a larger aggregate called a "group".
     A group is identified by a "Group Name";  see Section 3.7.3 (page
     56).

     The end-users of a stream are called the "participants" in the
     stream.  Data travels in a single direction through any given
     stream.  The host agent that transmits the data into the stream is
     called the "origin", and the host agents that receive the data are
     called the "targets".  Thus, for any stream one participant is the
     origin and the others are the targets.

     A stream is "multi-destination simplex" since data travels across
     it in only one direction:  from the origin to the targets.  A
     stream can be viewed as a directed tree in which the origin is the
     root, all the branches are directed away from the root toward the
     targets, which are the leaves.  A "hop" is an edge of that tree.
     The ST agent that is on the end of an edge in the direction toward
     the origin is called the "previous-hop ST agent", or the
     "previous-hop".  The ST agents that are one hop away from a
     previous-hop ST agent in the direction toward the targets are
     called the "next-hop ST agents", or the "next-hops".  It is
     possible that multiple edges between a previous-hop and several
     next-hops are actually implemented by a network level multicast
     group.

     Packets travel across a hop for one of two purposes:  data or
     control.  For ST data packet handling, hops are marked by "Hop
     IDentifiers" (HIDs) used for efficient forwarding instead of the
     stream's Name.  A HID is negotiated among several agents so that
     data forwarding can be done efficiently on both a point-to-point
     and multicast basis.  All control message exchange is done on a
     point-to-point basis between a pair of agents.  For control
     message handling, Virtual Link Identifiers are used to quickly
     dispatch the control messages to the proper stream's state
     machine.

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     ST requires routing decisions to be made at several points in the
     stream setup and management process.  ST assumes that an
     appropriate routing algorithm exists to which ST has access; see
     Section 3.8.1 (page 69).  However, routing is considered to be a
     separate issue.  Thus neither the routing algorithm nor its
     implementation is specified here.  A routing algorithm may attempt
     to minimize the number of hops to the target(s), or it may be more
     intelligent and attempt to minimize the total internet resources
     consumed.  ST operates equally well with any reasonable routing
     algorithm.  The availability of a source routing option does not
     eliminate the need for an appropriate routing algorithm in ST
     agents.

  2.3.       Relationship Between Applications and ST

     It is the responsibility of an ST application entity to exchange
     information among its peers, usually via IP, as necessary to
     determine the structure of the communication before establishing
     the ST stream.  This includes:

        o  identifying the participants,

        o  determining which are targets for which origins,

        o  selecting the characteristics of the data flow between any
           origin and its target(s),

        o  specifying the protocol that resides above ST,

        o  identifying the Service Access Point (SAP), port, or
           socket relevant to that protocol at every participant, and

        o  ensuring security, if necessary.

     The protocol layer above ST must pass such information down to the
     ST protocol layer when creating a stream.

     ST uses a flow specification, abbreviated herein as "FlowSpec", to
     describe the required characteristics of a stream.  Included are
     bandwidth, delay, and reliability parameters.  Additional
     parameters may be included in the future in an extensible manner.
     The FlowSpec describes both the desired values and their minimal
     allowable values.  The ST agents thus have some freedom in
     allocating their resources.  The ST agents accumulate information
     that describes the characteristics of the chosen path and pass
     that information to the origin and the targets of the stream.

     ST stream setup control messages carry some information that is
     not specifically relevant to ST, but is passed through the
     interface to the protocol that resides above ST.  The "next

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     protocol identifier" ("NextPcol") allows ST to demultiplex streams
     to a number of possible higher layer protocols.  The SAP
     associated with each participant allows the higher layer protocol
     to further demultiplex to a specific application entity.  A
     UserData parameter is provided;  see Section 4.2.2.16 (page 98).

  2.4.       ST Control Message Protocol

     ST agents create and manage a stream using the ST Control Message
     Protocol (SCMP).  Conceptually, SCMP resides immediately above ST
     (as does ICMP above IP) but is an integral part of ST.  Control
     messages are used to:

        o  create streams,

        o  refuse creation of a stream,

        o  delete a stream in whole or in part,

        o  negotiate or change a stream's parameters,

        o  tear down parts of streams as a result of router or
           network failures, or transient routing inconsistencies,
           and

        o  reroute around network or component failures.

     SCMP follows a request-response model.  SCMP reliability is
     ensured through use of retransmission after timeout;  see Section
     3.7.6 (page 66).

     An ST application that will transmit data requests its local ST
     agent, the origin, to create a stream.  While only the origin
     requests creation of a stream, all the ST agents from the origin
     to the targets participate in its creation and management.  Since
     a stream is simplex, each participant that wishes to transmit data
     must request that a stream be created.

     An ST agent that receives an indication that a stream is being
     created must:

        1  negotiate a HID with the previous-hop identifying the
           stream,

        2  map the list of targets onto a set of next-hop ST agents
           through the routing function,

        3  reserve the local and network resources required to
           support the stream,

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        4  update the FlowSpec, and

        5  propagate the setup information and partitioned target
           list to the next-hop ST agents.

     When a target receives the setup message, it must inquire from the
     specified application process whether or not it is willing to
     accept the stream, and inform the origin accordingly.

     Once a stream is established, the origin can safely send data.  ST
     and its implementations are optimized to allow fast and efficient
     forwarding of data packets by the ST agents using the HIDs, even
     at the cost of adding overhead to stream creation and management.
     Specifically, the forwarding decisions, that is, determining the
     set of next-hop ST agents to which a data packet belonging to a
     particular stream will be sent, are made during the stream setup
     phase.  The shorthand HIDs are negotiated at that time, not only
     to reduce the data packet header size, but to access efficiently
     the stream's forwarding information.  When possible, network-layer
     multicast is used to forward a data packet to multiple next-hop ST
     agents across a network.  Note that when network-layer multicast
     is used, all members of the multicast group must participate in
     the negotiation of a common HID.

     An established stream can be modified by adding or deleting
     targets, or by changing the network resources allocated to it.  A
     stream may be torn down by either the origin or the targets.  A
     target can remove itself from a stream leaving the others
     unaffected.  The origin can similarly remove any subset of the
     targets from its stream leaving the remainder unaffected.  An
     origin can also remove all the targets from the stream and
     eliminate the stream in its entirety.

     A stream is monitored by the involved ST agents.  If they detect a
     failure, they can attempt recovery.  In general, this involves
     tearing down part of the stream and rebuilding it to bypass the
     failed component(s).  The rebuilding always occurs from the origin
     side of the failure.  The origin can optionally specify whether
     recovery is to be attempted automatically by intermediate ST
     agents or whether a failure should immediately be reported to the
     origin.  If automatic recovery is selected but an intermediate
     agent determines it cannot effect the repair, it propagates the
     failure information backward until it reaches an agent that can
     effect repair.  If the failure information propagates back to the
     origin, then the application can decide if it should abort or
     reattempt the recovery operation.

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     Although ST supports an arbitrary connection structure, we
     recognize that certain stream topologies will be common and
     justify special features, or options, which allow for optimized
     support.  These include:

        o  streams with only a single target (see Section 3.6.2 (page
           44)), and

        o  pairs of streams to support full duplex communication
           between two points (see Section 3.6.3 (page 45)).

     These features allow the most frequently occurring topologies to
     be supported with less setup delay, with fewer control messages,
     and with less overhead than the more general situations.

  2.5.       Flow Specifications

     Real time data, such as voice and video, have predictable
     characteristics and make specific demands of the networks that
     must transfer it.  Specifically, the data may be transmitted in
     packets of a constant size that are produced at a constant rate.
     Alternatively, the bandwidth may vary, due either to variable
     packet size or rate, with a predefined maximum, and perhaps a
     non-zero minimum.  The variation may also be predictable based on
     some model of how the data is generated.  Depending on the
     equipment used to generate the data, the packet size and rate may
     be negotiable.  Certain applications, such as voice, produce
     packets at the given rate only some of the time.  The networks
     that support real time data must add minimal delay and delay
     variance, but it is expected that they will be non-zero.

     The FlowSpec is used for three purposes.  First, it is used in the
     setup message to specify the desired and minimal packet size and
     rate required by the origin.  This information is used by ST
     agents when they attempt to reserve the resources in the
     intervening networks.  Second, when the setup message reaches the
     target, the FlowSpec contains the packet size and rate that was
     actually obtained along the path from the origin, and the accrued
     mean delay and delay variance expected for data packets along that
     path.  This information is used by the target to determine if it
     wishes to accept the connection.  The target may reduce reserved
     resources if it wishes to do so and if the possibility is still
     available.  Third, if the target accepts the connection, it
     returns the updated FlowSpec to the origin, so that the origin can
     decide if it still wishes to participate in the stream with the
     characteristics that were actually obtained.

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     When the data transmitted by stream users is generated at varying
     rates, including bursts of varying rate and duration, there is an
     opportunity to provide service to more subscribers by providing
     guaranteed service for the average data rate of each stream, and
     reserving additional network capacity, shared among all streams,
     to service the bursts.  This concept has been recognized by analog
     voice network providers leading to the principle of time assigned
     speech interpolation (TASI) in which only the talkspurts of a
     speech conversation are transmitted, and, during silence periods,
     the circuit can be used to send the talkspurts of other
     conversations.  The FlowSpec is intended to assist algorithms that
     perform similar kinds of functions.  We do not propose such
     algorithms here, but rather expect that this will be an area for
     experimentation.  To allow for experiments, and a range of ways
     that application traffic might be characterized, a "DutyFactor" is
     included in the FlowSpec and we expect that a "burst descriptor"
     will also be needed.

     The FlowSpec will need to be revised as experience is gained with
     connections involving numerous participants using multiple media
     across heterogeneous internetworks.  We feel a change of the
     FlowSpec does not necessarily require a new version of ST, it only
     requires the FlowSpec version number be updated and software to
     manage the new FlowSpec to be distributed.  We further suggest
     that if the change to the FlowSpec involves additional information
     for improved operation, such as a burst descriptor, that it be
     added to the end of the FlowSpec and that the current parameters
     be maintained so that obsolete software can be used to process the
     current parameters with minimum modifications.

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                     ****                      ****
                    *    *     ST Agent 1     *    *       +---+
                   *      *------- o ---------*    *-------+ B |
                   *      *                   *    *       +---+
                   *      *                    ****
     +---+         *      *                     |
     |   |         *      *                     |
     | A +---------*      *                     o ST Agent 3
     |   |         *      *                     |
     +---+         *      *                     |
                   *      *                    ***
                   *      *                   *   *        +---+
                   *      *    ST Agent 2    *     *-------+ C |
                   *      *------- o --------*     *       +---+
                    *    *                   *     *
                     ****                    *     *
                                             *     *
                                +---+        *     *       +---+
                                | E +--------*     *-------+ D |
                                +---+         *   *        +---+
                                               ***

        Figure 2.  Topology Used in Protocol Exchange Diagrams

                     ****     ST Agent 1       ****
                    * +--+---14--- o -----15--+----+--44---+---+
                   *  | +-+--11---   -----16--+-+  *       | B |
                   *  | | *                   * |+-+--45---+---+
                   *  | | *                    *++*
     +---+         *  | | *                  34 ||32
     |   +----4----+--+ | *                     ||
     | A +----6----+----+ *                     o ST Agent 3
     |   +----5----+---+  *                     |
     +---+         *   |  *                     | 33
                   *   |  *       ST           *+*
                   *   |  *      Agent        * | *
                   *   |  *        2 -----24-+--+  *       +---+
                   *   +--+--23--- o -----25-+-----+--54---+ C |
                    *    *           -----26-+---+ *       +---+
                     ****            -----27-+-+ | *
                                             * | | *
                                +---+        * | | *       +---+
                                | E +---74---+-+ +-+--64---+ D |
                                +---+         *   *        +---+
                                               ***

        Figure 3.  Virtual Link Identifiers for SCMP Messages

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3. ST Control Message Protocol Functional Description

  This section contains a functional description of the ST Control
  Message Protocol (SCMP); Section 4 (page 75) specifies the formats of
  the control message PDUs.  We begin with a description of stream
  setup.  Mechanisms used to deal with the exceptional cases are then
  presented.  Complications due to options that an application or a ST
  agent may select are then detailed.  Once a stream has been
  established, the data transfer phase is entered; it is described.
  Once the data transfer phase has been completed, the stream must be
  torn down and resources released; the control messages used to
  perform this function are presented.  The resources or participants
  of a stream may be changed during the lifetime of the stream; the
  procedures to make changes are described.  Finally, the section
  concludes with a description of some ancillary functions, such as
  failure detection and recovery, HID negotiation, routing, security,
  etc.

  To help clarify the SCMP exchanges used to setup and maintain ST
  streams, we have included a series of figures in this section.  The
  protocol interactions in the figures assume the topology shown in
  Figure 2.  The figures, taken together,

   o  Create a stream from an application at A to three peers at B,
      C and D,

   o  Add a peer at E,

   o  Disconnect peers B and C, and

   o  D drops out of the stream.

  Other figures illustrate exchanges related to failure recovery.

  In order to make the dispatch function within SCMP more uniform and
  efficient, each end of a hop is assigned, by the agent at that end, a
  Virtual Link Identifier that uniquely (within that agent) identifies
  the hop and associates it with a particular stream's state
  machine(s).  The identifier at the end of a link that is sending a
  message is called the Sender Virtual Link Identifier (SVLId);  that
  at the receiving end is called the Receiver Virtual Link Identifier
  (RVLId).  Whenever one agent sends a control message for the other to
  receive, the sender will place the receiver's identifier into the
  RVLId field of the message and its own identifier in the SVLId field.
  When a reply to the message is sent, the values in SVLId and RVLId
  fields will be reversed, reflecting the fact the sender and receiver
  roles are reversed.  VLIds with values zero through three are
  received and should not be assigned in response to CONNECT messages.
  Figure 3 shows the hops that will be used in the examples and
  summarizes the VLIds that will be assigned to them.

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  Similarly, Figure 4 summarizes the HIDs that will eventually be
  negotiated as the stream is created.

                     ****     ST Agent 1       ****
                    *  +>+--1200-> o -------->+--->+-3600->+---+
                   *   ^  *                   *    *       | B |
                   *   |  *                   * +->+-6000->+---+
                   *   |  *                    *+**
     +---+         *   |  *                     ^
     |   +-------->+-->+  *                     |
     | A |         *      *                     o St Agent 3
     |   +-------->+-->+  *                     ^
     +---+         *   |  *                     | 4801
                   *   |  *                    *+*
                   *   V  *   ST Agent 2      * ^ *        +---+
                    *  +>+--2400-> o ------->+->+->+-4800->+ C |
                     ****                    *  |  * 4801  +---+
                                             *  |  *
                                +---+        *  V  *       +---+
                                | E +<-4800--+<-+->+-4800->+ D |
                                +---+         *   *  4801  +---+
                                               ***

            Figure 4.  HIDs Assigned for ST User Packets

  Some of the diagrams that follow form a progression.  For example,
  the steps required initially to establish a connection are spread
  across five figures.  Within a progression, the actions on the first
  diagram are numbered 1.1, 1.2, etc.;  within the second diagram they
  are numbered 2.1, 2.2, etc.  Points where control leaves one diagram
  to enter another are identified with a continuation arrow "-->>", and
  are continued with "[a.b] >>-->" in the other diagram.  The number in
  brackets shows the label where control left the earlier diagram.  The
  reception of simple acknowledgments, e.g., ACKs, in one figure from
  another is omitted for clarity.

  3.1.       Stream Setup

     This section presents a description of stream setup assuming that
     everything succeeds -- HIDs are approved, any required resources
     are available, and the routing is correct.

     3.1.1.        Initial Setup at the Origin

        As described in Section 2.3 (page 11), the application has
        collected the information necessary to determine the

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        participants in the communication before passing it to the host
        ST agent at the origin.  The host ST agent will take this
        information, allocate a Name for the stream (see Section
        4.2.2.8 (page 87)), and create a stream.

     3.1.2.        Invoking the Routing Function

        An ST agent that is setting up a stream invokes a routing
        function to find a path to reach each of the targets specified
        in the TargetList.  This is similar to the routing decision in
        IP.  However, in this case the route is to a multitude of
        targets rather than to a single destination.

        The set of next-hops that an ST agent would select is not
        necessarily the same as the set of next hops that IP would
        select given a number of independent IP datagrams to the same
        destinations.  The routing algorithm may attempt to optimize
        parameters other than the number of hops that the packets will
        take, such as delay, local network bandwidth consumption, or
        total internet bandwidth consumption.

        The result of the routing function is a set of next-hop ST
        agents and the parameters of the intervening network(s).  The
        latter permit the ST agent to determine whether the selected
        network has the resources necessary to support the level of
        service requested in the FlowSpec.

     3.1.3.        Reserving Resources

        The intent of ST is to provide a guaranteed level of service by
        reserving internet resources for a stream during a setup phase
        rather than on a per packet basis.  The relevant resources are
        not only the forwarding information maintained by the ST
        agents, but also packet switch processor bandwidth and buffer
        space, and network bandwidth and multicast group identifiers.
        Reservation of these resources can help to increase the
        reliability and decrease the delay and delay variance with
        which data packets are delivered.  The FlowSpec contains all
        the information needed by the ST agent to allocate the
        necessary resources.  When and how these resources are
        allocated depends on the details of the networks involved, and
        is not specified here.

        If an ST agent must send data across a network to a single
        next-hop ST agent, then only the point-to-point bandwidth needs
        to be reserved.  If the agent must send data to multiple next-
        hop agents across one network and network layer multicasting is
        not available, then bandwidth must be reserved for all of them.
        This will allow the ST agent to

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        use replication to send a copy of the data packets to each
        next-hop agent.

        If multicast is supported, its use will decrease the effort
        that the ST agent must expend when forwarding packets and also
        reduces the bandwidth required since one copy can be received
        by all next-hop agents.  However, the setup phase is more
        complicated.  A network multicast address must be allocated
        that contains all those next-hop agents, the sender must have
        access to that address, the next-hop agents must be informed of
        the address so they can join the multicast group identified by
        it (see Section 4.2.2.7 (page 86)), and a common HID must be
        negotiated.

        The network should consider the bandwidth and multicast
        requirements to determine the amount of packet switch
        processing bandwidth and buffer space to reserve for the
        stream.  In addition, the membership of a stream in a Group may
        affect the resources that have to be allocated;  see Section
        3.7.3 (page 56).

        Few networks in the Internet currently offer resource
        reservation, and none that we know of offer reservation of all
        the resources specified here.  Only the Terrestrial Wideband
        Network (TWBNet) [7] and the Atlantic Satellite Network
        (SATNET) [9] offer(ed) bandwidth reservation.  Multicasting is
        more widely supported.  No network provides for the reservation
        of packet switch processing bandwidth or buffer space.  We hope
        that future networks will be designed to better support
        protocols like ST.

        Effects similar to reservation of the necessary resources may
        be obtained even when the network cannot provide direct support
        for the reservation.  Certainly if total reservations are a
        small fraction of the overall resources, such as packet switch
        processing bandwidth, buffer space, or network bandwidth, then
        the desired performance can be honored if the degree of
        confidence is consistent with the requirements as stated in the
        FlowSpec.  Other solutions can be designed for specific
        networks.

     3.1.4.        Sending CONNECT Messages

        A VLId and a proposed HID must be selected for each next-hop
        agent.  The control packets for the next-hop must carry the
        VLId in the SVLId field.  The data packets transmitted in the
        stream to the next-hop must carry the HID in the ST Header.

        The ST agent sends a CONNECT message to each of the ST agents
        identified by the routing function.  Each CONNECT message
        contains the VLId, the proposed HID (the HID Field option bit

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        must be set, see Section 3.6.1 (page 44)), an updated FlowSpec,
        and a TargetList.  In general, the HID, FlowSpec, and
        TargetList will depend on both the next-hop and the intervening
        network.  Each TargetList is a subset of the received (or
        original) TargetList, identifying the targets that are to be
        reached through the next-hop to which the CONNECT message is
        being sent.  Note that a CONNECT message to a single next-hop
        might have to be fragmented into multiple CONNECTs if the
        single CONNECT is too large for the intervening network's MTU;
        fragmentation is performed by further dividing the TargetList.

        If multiple next-hops are to be reached through a network that
        supports network level multicast, a different CONNECT message
        must nevertheless be sent to each next-hop since each will have
        a different TargetList;  see Section 4.2.3.5 (page 105).
        However, since an identical copy of each ensuing data packet
        will reach each member of the multicast group, all the CONNECT
        messages must propose the same HID.  See Section 3.7.4 (page
        58) for a detailed discussion on HID selection.

        In the example of Figure 2, the routing function might return
        that B is reachable via Agent 1 and C and D are reachable via
        Agent 2.  Thus A would create two CONNECT messages, one each
        for Agents 1 and 2, as illustrated in Figure 5.  Assuming that
        the proposed HIDs are available in the receiving agents, they
        would each send a responding HID-APPROVE back to Agent A.

        Application  Agent A                    Agent 1    Agent 2

   1.1. (open B,C,D)
              V
   1.2.       +-> (routing to B,C,D)
                        V
   1.3.                 +->(reserve resources from A to Agent 1)
                        |  V
   1.4.                 |  +-> CONNECT B --------->>
                        |      <RVLId=0><SVLId=4>
                        |      <Ref=10><HID=1200>
                        V
   1.5.                 +->(reserve resources from A to Agent 2)
                           V
   1.6.                    +-> CONNECT C,D ------------------>>
                               <RVLId=0><SVLId=5>
                               <Ref=15><HID=2400>

              Figure 5.  Origin Sending CONNECT Message

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     3.1.5.        CONNECT Processing by an Intermediate Agent

        An ST agent receiving a CONNECT message should, assuming no
        errors, quickly select a VLId and respond to the previous-hop
        with either an ACK, a HID-REJECT, or a HID-APPROVE message, as
        is appropriate.  This message must identify the CONNECT to
        which it corresponds by including the CONNECT's Reference
        number in its Reference field.  Note that the VLId that this
        agent selects is placed in the SVLId of the response, and the
        previous-hop's VLId (which is contained in the SVLId of the
        CONNECT) is copied into the RVLId of the response.  If the
        agent is not a target, it must then invoke the routing
        function, reserve resources, and send a CONNECT message(s) to
        its next-hop(s), as described in Sections 3.1.2-4 (pages 19-
        20).

      Agent A                   Agent 1                      Agent B

   [1.4] >>-> CONNECT B -------->+--+
              <RVLId=0><SVLId=4> |  V

2.1. <Ref=10><HID=1200> | (routing to B)

                                 |  V

2.2. V +->(reserve resources from 1 to B) 2.3. +<- HID-APPROVE <------+ V 2.4. <RVLId=4><SVLId=14> +-> CONNECT B ---------->>

              <Ref=10><HID=1200>           <RVLId=0><SVLId=15>
                                           <Ref=110><HID=3600>

      Agent A                   Agent 2                      Agent C

   [1.6] >>-> CONNECT C,D ------>+-+
              <RVLId=0><SVLId=5> | V

2.5. <Ref=15><HID=2400> | (routing to C,D)

| V

2.6. V +-->(reserve resources from 2 to C) 2.7. +<- HID-APPROVE <------+ | V 2.8. <RVLId=5><SVLId=23> | +-> CONNECT C ---------->>

              <Ref=15><HID=2400>   |       <RVLId=0><SVLId=25>
                                   |       <Ref=210><HID=4800>
                                   |
                                   |                         Agent D
                                   V

2.9. +->(reserve resources from 2 to D)

2.10. +-> CONNECT D ---------->>

                                           <RVLId=0><SVLId=26>
                                           <Ref=215><HID=4800>

        Figure 6.  CONNECT Processing by an Intermediate Agent

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RFC 1190 Internet Stream Protocol October 1990

        The resources listed as Desired in a received FlowSpec may not
        correspond to those actually reserved in either the ST agent
        itself or in the network(s) used to reach the next-hop
        agent(s).  As long as the reserved resources are sufficient to
        meet the specified Limits, the copy of the FlowSpec sent to a
        next-hop must have the Desired resources updated to reflect the
        resources that were actually obtained.  For example, the
        Desired bandwidth might be reduced because the network to the
        next-hop could not provide all of the desired bandwidth.  Also,
        the delay and delay variance are appropriately increased, and
        the link MTU may require that the DesPDUBytes field be reduced.
        (The minimum requirements that the origin had entered into the
        FlowSpec Limits fields cannot be altered by the intermediate or
        target agents.)

     3.1.6.        Setup at the Targets

        An ST agent that is the target of a CONNECT, whether from an
        intermediate ST agent, or directly from the origin host ST
        agent, must respond first (assuming no errors) with either a
        HID-REJECT or HID-APPROVE.  After inquiring from the specified
        application process whether or not it is willing to accept the
        connection, the agent must also respond with either an ACCEPT
        or a REFUSE.

        In particular, the application must be presented with
        parameters from the CONNECT, such as the Name, FlowSpec,
        Options, and Group, to be used as a basis for its decision.
        The application is identified by a combination of the NextPcol
        field and the SAP field in the (usually) single remaining
        Target of the TargetList.  The contents of the SAP field may
        specify the "port" or other local identifier for use by the
        protocol layer above the host ST layer.  Subsequently received
        data packets will carry a short hand identifier (the HID) that
        can be mapped into this information and be used for their
        delivery.

        The responses to the CONNECT message are sent to the previous-
        hop from which the CONNECT was received.  An ACCEPT contains
        the Name of the stream and the updated FlowSpec.  Note that the
        application might have reduced the desired level of service in
        the received FlowSpec before accepting it.  The target must not
        send the ACCEPT until HID negotiation has been successfully
        completed.

        Since the ACCEPT or REFUSE message must be acknowledged by the
        previous-hop, it is assigned a new Reference number that will
        be returned in the ACK.  The CONNECT to which the ACCEPT or
        REFUSE is a reply is identified by placing the CONNECT's
        Reference number in the LnkReference field of the ACCEPT or
        REFUSE.

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RFC 1190 Internet Stream Protocol October 1990

          Agent 1                    Agent B       Application B
3.1.                                             (proc B listening)
        [2.4] >>-> CONNECT B ---------->+------------------+
                   <RVLId=0><SVLId=15>  |                  |
3.2.               <Ref=110><HID=3600>  V          (proc B accepts)
3.3.           +<- HID-APPROVE <--------+                  |
                   <RVLId=15><SVLId=44>                    |
                   <Ref=110><HID=3600>                     V
3.4.                       (wait until HID negotiated) <---+
                                        V
3.5.       <<--+<- ACCEPT B <-----------+
                   <RVLId=15><SVLId=44>
                   <Ref=410><LnkRef=110>

          Agent 2                    Agent C       Application C
3.6.                                             (proc C listening)
        [2.8] >>-> CONNECT C ---------->+------------------+
                   <RVLId=0><SVLId=25>  |                  |
3.7.               <Ref=210><HID=4800>  V          (proc C accepts)
3.8.           +<- HID-APPROVE <--------+                  |
                   <RVLId=25><SVLId=54>                    |
                   <Ref=210><HID=4800>                     V
3.9.                       (wait until HID negotiated) <---+
                                        V
3.10.      <<--+<- ACCEPT C <-----------+
                   <RVLId=25><SVLId=54>
                   <Ref=510><LnkRef=210>

          Agent 2                    Agent D       Application D
3.11.                                            (proc D listening)
       [2.10] >>-> CONNECT D ---------->+------------------+
                   <RVLId=0><SVLId=26>  |                  |
3.12.              <Ref=215><HID=4800>  V          (proc D accepts)
3.13.          +<- HID-APPROVE <--------+                  |
                   <RVLId=26><SVLId=64>                    |
                   <Ref=215><HID=4800>                     V
3.14.                      (wait until HID negotiated) <---+
                                        V
3.15.      <<--+<- ACCEPT D <-----------+
                   <RVLId=26><SVLId=64>
                   <Ref=610><LnkRef=215>

             Figure 7.  CONNECT Processing by the Target

     3.1.7.        ACCEPT Processing by an Intermediate Agent

        When an intermediate ST agent receives an ACCEPT, it first
        verifies that the message is a response to an earlier CONNECT.
        If not, it responds to the next-hop ST agent with an ERROR-IN-
        REPLY (LnkRefUnknown) message.  Otherwise, it responds to the
        next-hop ST agent with an ACK, and propagates

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        the ACCEPT message to the previous-hop along the same path
        traced by the CONNECT but in the reverse direction toward the
        origin.  The ACCEPT should not be propagated until all HID
        negotiations with the next-hop agent(s) have been successfully
        completed.

        The FlowSpec is included in the ACCEPT message so that the
        origin and intermediate ST agents can gain access to the
        information that was accumulated as the CONNECT traversed the
        internet.  Note that the resources, as specified in the
        FlowSpec in the ACCEPT message, may differ from the resources
        that were reserved by the agent when the CONNECT was

     Agent A                     Agent 1                    Agent B

                                    +<-+<- ACCEPT B <-------<< [3.5]
                                    V  |   <RVLId=15><SVLId=44>

4.1. (wait for ACCEPTS) V <Ref=410><LnkRef=110> 4.2. V +-> ACK --------------->+ 4.3. (wait until HID negotiated)<-+ <RVLId=44><SVLId=15>

                                 V         <Ref=410>

4.4. <<--+<-- ACCEPT B <---------+

              <RVLId=4><SVLId=14>
              <Ref=115><LnkRef=10>

      Agent A                    Agent 2                    Agent C

                                    +<-+<- ACCEPT C <------<< [3.10]
                                    |  |   <RVLId=25><SVLId=54>
                                    |  V   <Ref=510><LnkRef=210>

4.5. | +-> ACK --------------->+

                                    |      <Ref=510>
                                    |      <RVLId=54><SVLId=25>
                                    |
                                    |                       Agent D
                                    V
                                    +<-+<- ACCEPT D <------<< [3.15]
                                    V  |   <RVLId=26><SVLId=64>

4.6. (wait for ACCEPTS) V <Ref=610><LnkRef=215> 4.7. V +-> ACK --------------->+ 4.8. (wait until HID negotiated)<-+ <RVLId=64><SVLId=26>

                                 V         <Ref=610>

4.9. <<--+<- ACCEPT C <----------+

             <RVLId=5><SVLId=23> |
             <Ref=220><LnkRef=15>|
                                 V

4.10. <<--+<- ACCEPT D <----------+

             <RVLId=5><SVLId=23>
             <Ref=225><LnkRef=15>

        Figure 8.  ACCEPT Processing by an Intermediate Agent

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RFC 1190 Internet Stream Protocol October 1990

        originally processed.  However, the agent does not adjust the
        reservation in response to the ACCEPT.  It is expected that any
        excess resource allocation will be released for use by other
        stream or datagram traffic through an explicit CHANGE message
        initiated by the application at the origin if it does not wish
        to be charged for any excess resource allocations.

     3.1.8.        ACCEPT Processing by the Origin

        The origin will eventually receive an ACCEPT (or REFUSE or
        ERROR-IN-REQUEST) message from each of the targets.  As each
        ACCEPT is received, the application should be notified of the
        target and the resources that were successfully allocated along
        the path to it, as specified in the FlowSpec contained in the
        ACCEPT message.  The application may then use the information
        to either adopt or terminate the portion of the stream to each
        target.  When ACCEPTs (or failures) from all targets have been
        received at the origin, the application is notified that stream
        setup is complete, and that data may be sent.

        Application A   Agent A                  Agent 1   Agent 2

                           +<-- ACCEPT B <--------<< [4.4]
                           |    <RVLId=4><SVLId=14>
                           V    <Ref=115><LnkRef=10>
  5.1.                     +--> ACK ----------------->+
                           |    <RVLId=14><SVLId=4>
                           V    <Ref=115>
  5.2.        +<-- (inform A of B's FlowSpec)
              |            +<-- ACCEPT C <----------------<< [4.9]
              |            |    <RVLId=5><SVLId=23>
              |            V    <Ref=220><LnkRef=15>
  5.3.        |            +--> ACK ------------------------->+
              |            |    <RVLId=23><SVLId=5>
              |            V    <Ref=220>
  5.4.        +<-- (inform A of C's FlowSpec)
              |            +<-- ACCEPT D <----------------<< [4.10]
              |            |    <RVLId=5><SVLId=23>
              |            V    <Ref=225><LnkRef=15>
  5.5.        |            +--> ACK ------------------------->+
              |            |    <RVLId=23><SVLId=5>
              |            V    <Ref=225>
  5.6.        +<-- (inform A of D's FlowSpec)
              V
  5.7.    (wait until HIDs negotiated)
              V
  5.8.    (inform A open to B,C,D)

              Figure 9.  ACCEPT Processing by the Origin

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        There are several pieces of information contained in the
        FlowSpec that the application must combine before sending data
        through the stream.  The PDU size should be computed from the
        minimum value of the DesPDUBytes field from all ACCEPTs and the
        protocol layers above ST should be informed of the limit.  It
        is expected that the next higher protocol layer above ST will
        segment its PDUs accordingly.  Note, however, that the MTU may
        decrease over the life of the stream if new targets are
        subsequently added.  Whether the MTU should be increased as
        targets are dropped from a stream is left for further study.

        The available bandwidth and packet rate limits must also be
        combined.  In this case, however, it may not be possible to
        select a pair of values that may be used for all paths, e.g.,
        one path may have selected a low rate of large packets while
        another selected a high rate of small packets.  The application
        may remedy the situation by either tearing down the stream,
        dropping some participants, or creating a second stream.

        After any differences have been resolved (or some targets have
        been deleted by the application to permit resolution), the
        application at the origin should send a CHANGE message to
        release any excess resources along paths to those targets that
        exceed the resolved parameters for the stream, thereby reducing
        the costs that will be incurred by the stream.

     3.1.9.        Processing a REFUSE Message

        REFUSE messages are used to indicate a failure to reach an
        application at a target;  they are propagated toward the origin
        of a stream.  They are used in three situations:

         1  during stream setup or expansion to indicate that there
            is no satisfactory path from an ST agent to a target,

         2  when the application at the target either does not
            exist does not wish to be a participant, or wants to
            cease being a participant, and

         3  when a failure has been detected and the agents are
            trying to find a suitable path around the failure.

        The cases are distinguished by the ReasonCode field and an
        agent receiving a REFUSE message must examine that field in
        order to determine the proper action to be taken.  In
        particular, if the ReasonCode indicates that the CONNECT
        message reached the target then the REFUSE should be propagated
        back to the origin, releasing resources as appropriate along
        the way.  If the ReasonCode indicates that

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        the CONNECT message did not reach the target then the
        intermediate (origin) ST agent(s) should check for alternate
        routes to the target before propagating the REFUSE back another
        hop toward the origin.  This implies that an agent must keep
        track of the next-hops that it has tried, on a target by target
        basis, in order not to get caught in a loop.

        An ST agent that receives a REFUSE message must acknowledge it
        by sending an ACK to the next-hop.  The REFUSE must also be
        propagated back to the previous-hop ST agent.  Note that the ST
        agent may not have any information about the target in

  Appl.  Agent A                   Agent 2                 Agent E
                                              (proc E NOT listening)

1. (add E) 2. +----->+-> CONNECT E ---------->+->+

                <RVLId=23><SVLId=5>  |  |
                <Ref=65>             V  |

3. +<-- ACK <---------------+ |

                 <RVLId=5><SVLId=23>    V

4. <Ref=65> (routing to E)

5. (reserve resources 2 to E)

6. +--> CONNECT E --------->+

                                             <RVLId=0><SVLId=27> |
                                             <Ref=115><HID=4600> |
                                                                 V

7. +<-+<- REFUSE B <-----------+

                                     |  |   <RVLId=27><SVLId=74>
                                     |  |   <Ref=705><LnkRef=115>
                                     |  V   <RC=SAPUnknown>

8. | +-> ACK ---------------->+

                                     |  |   <RVLId=74><SVLId=27> |
                                     |  V   <Ref=705>            |

9. | (free link 27) V 10. V (free link 74) 11. +<- REFUSE B <-----------+

            |   <RVLId=5><SVLId=23>  |
            |   <Ref=550><LnkRef=65> V

12. | <RC=SAPUnknown> (free resources 2 to E)

13. +-> ACK --------------->+

            |   <RVLId=23><SVLId=5>  |
            |   <Ref=550>            V

14. V (keep link 23 for C,D) 15. (keep link 5 for C,D)

16. (inform application failed SAPUnknown)

                  Figure 10.  Sending REFUSE Message

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RFC 1190 Internet Stream Protocol October 1990

        the TargetList.  This may result from interacting DISCONNECT
        and REFUSE messages and should be logged and silently ignored.

        If, after deleting the specified target, the next-hop has no
        remaining targets, then those resources associated with that
        next-hop agent may be released.  Note that network resources
        may not actually be released if network multicasting is being

  Appl.   Agent A       Agent 2  Agent 1 Agent 3              Agent B

1. (network from 1 to B fails) 2. (add B) 3. +-> CONNECT B ----------------->+

        <RVLId=0><SVLId=6>          |
        <Ref=35><HID=100>           |

3. +<- HID-APPROVE <---------------+

        <RVLId=6><SVLId=11>         |
        <Ref=35><HID=100>           V

4. (routing to B: no route)

5. +<-+-- REFUSE B ----------------+

    |  |   <RVLId=6><SVLId=11>
    |  |   <Ref=155><LnkRef=35>
    |  V   <RC=NoRouteToDest>

6. | +-> ACK -------------------->+

    |  |   <RVLId=11><SVLId=6>      V

7. | V <Ref=155> (drop link 6) 8. V (drop link 11) 9. (find alternative route: via agent 2) 10. (resources from A to 2 already allocated:

    V   reuse control link & HID, no additional resources required)

11. +-> CONNECT B -------->+->+

        <RVLId=23><SVLId=5>|  |
        <Ref=40>           V  |

12. +<- ACK <--------------+ |

        <RVLId=5><SVLId=23>   V

13. <Ref=40> (routing to B: via agent 3)

14. +-> CONNECT B -->+ 15. <RVLId=0><SVLId=24> +-> CONNECT B --------->+

                        <Ref=245><HID=4801> V   <RVLId=0><SVLId=32> |

16. +<- HID-APPROVE -+ <Ref=310><HID=6000> |

                               <RVLId=24><SVLId=33>                 |
                               <Ref=245><HID=4801>                  V

17. +<- HID-APPROVE --------+

                                                <RVLId=32><SVLId=45>|
                                                <Ref=310><HID=6000> V

18. (ACCEPT handling follows normally to complete stream setup)

          Figure 11.  Routing Around a Failure

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        used since they may still be required for traffic to other
        next-hops in the multicast group.

        When the REFUSE reaches a origin, the origin sends an ACK and
        notifies the application via the next higher layer protocol
        that the target listed in the TargetList is no longer part of
        the stream and also if the stream has no remaining targets.  If
        there are no remaining targets, the application may wish to
        terminate the stream.

        Figure 10 illustrates the protocol exchanges for processing a
        REFUSE generated at the target, either because the target
        application is not running or that the target application
        rejects membership in the stream.  Figure 11 illustrates the
        case of rerouting around a failure by an intermediate agent
        that detects a failure or receives a refuse.  The protocol
        exchanges used by an application at the target to delete itself
        from the stream is discussed in Section 3.3.3 (page 35).

  3.2.       Data Transfer

     At the end of the connection setup phase, the origin, each target,
     and each intermediate ST agent has a database entry that allows it
     to forward the data packets from the origin to the targets and to
     recover from failures of the intermediate agents or networks.  The
     database should be optimized to make the packet forwarding task
     most efficient.  The time critical operation is an intermediate
     agent receiving a packet from the previous-hop agent and
     forwarding it to the next-hop agent(s).  The database entry must
     also contain the FlowSpec, utilization information, the address of
     the origin and previous-hop, and the addresses of the targets and
     next-hops, so it can perform enforcement and recover from
     failures.

     An ST agent receives data packets encapsulated by an ST header.  A
     data packet received by an ST agent contains the non-zero HID
     assigned to the stream for the branch from the previous-hop to
     itself.  This HID was selected so that it is unique at the
     receiving ST agent and thus can be used, e.g., as an index into
     the database, to obtain quickly the necessary replication and
     forwarding information.

     The forwarding information will be network and implementation
     specific, but must identify the next-hop agent or agents and their
     respective HIDs.  It is suggested that the cached information for
     a next-hop agent include the local network address of the next-
     hop.  If the data packet must be forwarded to multiple next-hops
     across a single network that supports multicast, the database may
     specify a single HID and may identify the next-hops by a (local
     network) multicast address.

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     If the network does not support multicast, or the next-hops are on
     different networks, then the database must indicate multiple
     (next-hop, HID) tuples.  When multiple copies of the data packet
     must be sent, it may be necessary to invoke a packet replicator.

     Data packets should not require fragmentation as the next higher
     protocol layer at the origin was informed of the minimum MTU over
     all paths in the stream and is expected to segment its PDUs
     accordingly.  However, it may be the case that a data packet that
     is being rerouted around a failed network component may be too
     large for the MTU of an intervening network.  This should be a
     transient condition that will be corrected as soon as the new
     minimum MTU has been propagated back to the origin.  Disposition
     by a mechanism other than dropping of the too large PDUs is left
     for further study.

  3.3.       Modifying an Existing Stream

     Some applications may wish to change the parameters of a stream
     after it has been created.  Possible changes include adding or
     deleting targets and changing the FlowSpec.  These are described
     below.

     3.3.1.        Adding a Target

        It is possible for an application to add a new target to an
        existing stream any time after ST has incorporated information
        about the stream into its database.  At a high level, the
        application entities exchanges whatever information is
        necessary.  Although the mechanism or protocol used to
        accomplish this is not specified here, it is necessary for the
        higher layer protocol to inform the host ST agent at the origin
        of this event.  The host ST agent at the target must also be
        informed unless this had previously been done.  Generally, the
        transfer of a target list from an ST agent to another, or from
        a higher layer protocol to a host ST agent, will occur
        atomically when the CONNECT is received.  Any information
        concerning a new target received after this point can be viewed
        as a stream expansion by the receiving ST agent.  However, it
        may be possible that an ST agent can utilize such information
        if it is received before it makes the relevant routing
        decisions.  These implementation details are not specified
        here, but implementations must be prepared to receive CONNECT
        messages that represent expansions of streams that are still in
        the process of being setup.

        To expand an existing stream, the origin issues one or more
        CONNECT messages that contain the Name, the VLId, the FlowSpec,
        and the TargetList specifying the new target or targets.  The
        origin issues multiple CONNECT messages if

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RFC 1190 Internet Stream Protocol October 1990

        either the targets are to be reached through different next-hop
        agents, or a single CONNECT message is too large for the
        network MTU.  The HID Field option is not set since the HID has
        already been (or is being) negotiated for the hop;
        consequently, the CONNECT is acknowledged with an ACK instead
        of a HID-REJECT or HID-APPROVE.

Application Agent A Agent 2 Agent E

1. (open E) 2. V (proc E listening) 3. +->(routing to E)

4. +-> (check resources from A to Agent 2: already allocated,

          V  reuse control link & HID, no additional resources needed)

5. +-> CONNECT E --------->+->+

              <RVLId=23><SVLId=5> |  V

6. <Ref=20> V (routing to E) 7. +<- ACK <---------------+ V

              <RVLId=5><SVLId=23>    +->(reserve resources 2 to E)
              <Ref=20>                  V

8. +-> CONNECT E --------->+

                                            <RVLId=0><SVLId=27> |
                                            <Ref=230><HID=4800> |

9. +<- HID-APPROVE <-------+

                                            <RVLId=27><SVLId=74>|
                                            <Ref=230><HID=4800> V

10. (proc E accepts) 11. (wait until HID negotiated)

12. +<-+<- ACCEPT E <----------+

                                     V  |   <RVLId=27><SVLId=74>

13. (wait for ACCEPTS) V <Ref=710><LnkRef=230> 14. V +-> ACK --------------->+ 15. (wait until HID negotiated)<-+ <RVLId=74><SVLId=27>

                                  V         <Ref=710>

16. +<- ACCEPT E <-------+

             |   <RVLId=5><SVLId=23>
             V   <Ref=235><LnkRef=20>

17. +-> ACK ------------>+

             |   <RVLId=23><SVLId=5>
             V   <Ref=235>

18. +<-(inform A of E's FlowSpec)

19. +<-(wait for ACCEPTS)

20. +<-(wait until HID negotiated)

21. (inform A open to E)

                Figure 12.  Addition of Another Target

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RFC 1190 Internet Stream Protocol October 1990

        An ST agent that is already a node in the stream recognizes the
        RVLId and verifies that the Name of the stream is the same.  It
        then checks if the intersection of the TargetList and the
        targets of the established stream is empty.  If this is not the
        case, then the receiver responds with an ERROR-IN-REQUEST with
        the appropriate reason code (RouteLoop) that contains a
        TargetList of those targets that were duplicates;  see Section
        4.2.3.5 (page 106).

        For each new target in the TargetList, processing is much the
        same as for the original CONNECT;  see Sections 3.1.2-4 (pages
        19-20).  The CONNECT must be acknowledged, propagated, and
        network resources must be reserved.  However, it may be
        possible to route to the new targets using previously allocated
        paths or an existing multicast group.  In that case, additional
        resources do not need to be reserved but more next-hop(s) might
        have to be added to an existing multicast group.

        Nevertheless, the origin, or any intermediate ST agent that
        receives a CONNECT for an existing stream, can make a routing
        decision that is independent of any it may have made
        previously.  Depending on the routing algorithm that is used,
        the ST agent may decide to reach the new target by way of an
        established branch, or it may decide to create a new branch.
        The fact that a new target is being added to an existing stream
        may result in a suboptimal overall routing for certain routing
        algorithms.  We take this problem to be unavoidable since it is
        unlikely that the stream routing can be made optimal in
        general, and the only way to avoid this loss of optimality is
        to redefine the routing of potentially the entire stream, which
        would be too expensive and time consuming.

     3.3.2.        The Origin Removing a Target

        The application at the origin specifies a set of targets that
        are to be removed from the stream and an appropriate reason
        code (ApplDisconnect).  The targets are partitioned into
        multiple DISCONNECT messages based on the next-hop to the
        individual targets.  As with CONNECT messages, an ST agent that
        is sending a DISCONNECT must make sure that the message fits
        into the MTU for the intervening network.  If the message is
        too large, the TargetList must be further partitioned into
        multiple DISCONNECT messages.

        An ST agent that receives a DISCONNECT message must acknowledge
        it by sending an ACK back to the previous-hop.  The DISCONNECT
        must also be propagated to the relevant next-hop ST agents.
        Before propagating the message, however, the TargetList should
        be partitioned based on next-hop ST

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RFC 1190 Internet Stream Protocol October 1990

        agent and MTU, as described above.  Note that there may be
        targets in the TargetList for which the ST agent has no
        information.  This may result from interacting DISCONNECT and
        REFUSE messages and should be logged and silently ignored.

        If, after deleting the specified targets, any next-hop has no
        remaining targets, then those resources associated with that
        next-hop agent may be released.  Note that network resources
        may not actually be released if network multicasting is being
        used since they may still be required for traffic to other
        next-hops in the multicast group.

     Application                                         Application
           Agent A             Agent 1  Agent 2          Agent B    C

 1.  (close B,C ApplDisconnect)
         V
 2.      +->+-+-> DISCONNECT B ----->+
 3.         | |   <RVLId=14><SVLId=4>+-+-> DISCONNECT B ------>+
            | |   <Ref=25>           | |   <RVLId=44><SVLId=15>|
            | V   <RC=ApplDisconnect>| |   <Ref=120>           |
 4.         | (free A to 1 resrc.)   | V   <RC=ApplDisconnect> |
 5.         |                        V (free 1 to B resrc.)    |
 6.         | +<- ACK <--------------+                         V
 7.         | |   <RVLId=4><SVLId=14>| +<- ACK <---------------+
            | V   <Ref=25>           | |   <RVLId=15><SVLId=44>|
 8.         | (free link 4)          V |   <Ref=120>           |
 9.         |           (free link 14) V                       |
 10.        |                          (free link 15)          V
 11.        |        (inform B that stream closed ApplDisconnect)
 12.        |                                     (free link 44)
            V
 13.     +<-+-+-> DISCONNECT C ---------->+
 14.     |    |   <RVLId=23><SVLId=5>     +-+-> DISCONNECT C ------>+
         |    |   <Ref=30>                | |   <RVLId=54><SVLId=25>|
         |    V   <RC=ApplDisconnect>     | |   <Ref=240>           |
 15.     |    (keep A to 2 resrc for      | V   <RC=ApplDisconnect> |
 16.     |         data going to D,E)     | (free 2 to C resrc.)    |
         |                                V                         |
 17.     |    +<- ACK <-------------------+                         V
 18.     |    |   <RVLId=5><SVLId=23>     | +<- ACK <---------------+
         |    V   <Ref=30>                | |   <RVLId=25><SVLId=54>|
 19.     |    (keep link 5 for D,E)       V |   <Ref=240>           |
 20.     |           (keep link 23 for D,E) V                       |
 21.     |                           (free link 25)                 V
 22.     |              (inform C that stream closed ApplDisconnect>)
 23.     V                                             (free link 54)
 24.     (inform A closed to B,C ApplDisconnect)

                 Figure 13.  Origin Removing a Target

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        When the DISCONNECT reaches a target, the target sends an ACK
        and notifies the application that it is no longer part of the
        stream and the reason.  The application should then inform ST
        to terminate the stream, and ST should delete the stream from
        its database after performing any necessary management and
        accounting functions.

     3.3.3.        A Target Deleting Itself

        The application at the target may inform ST that it wants to be
        removed from the stream and the appropriate reason code
        (ApplDisconnect).  The agent then forms a REFUSE message with
        itself as the only entry in the TargetList.  The REFUSE is sent
        back to the origin via the previous-hop.  If a stream has
        multiple targets and one target leaves the stream using this
        REFUSE mechanism, the stream to the other targets is not
        affected;  the stream continues to exist.

        An ST agent that receives such a REFUSE message must
        acknowledge it by sending an ACK to the next-hop.  The target
        is deleted and, if the next-hop has no remaining targets, then
        the those resources associated with that next-hop agent may be
        released.  Note that network resources may not actually be
        released if network multicasting is being used since they may
        still be required for traffic to other next-hops in the
        multicast group.  The REFUSE must also be propagated back to
        the previous-hop ST agent.

                Agent A          Agent 2          Agent E

           1.                             (close E ApplDisconnect)
                                                     V
           2.                         +<- REFUSE E --+
                                      |   <RVLId=27><SVLId=74>
                                      |   <Ref=720>
                                      V   <RC=ApplDisconnect>
           3.                      +<-+-> ACK ------>+
                                   |  |   <RVLId=74><SVLId=27>
           4.                      V  V   <Ref=720>
           5.    +<-+<- REFUSE E --+  (prune allocations)
                 |  |   <RVLId=5><SVLId=23>
                 |  |   <Ref=245>
                 |  V   <RC=ApplDisconnect>
           6.    |  +-> ACK ------>+
                 |  |   <RVLId=23><SVLId=5>
                 |  V   <Ref=245>
           7.    V  (prune allocations)
           8.    (inform application closed E ApplDisconnect)

                  Figure 14.  Target Deleting Itself

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        When the REFUSE reaches the origin, the origin sends an ACK and
        notifies the application that the target listed in the
        TargetList is no longer part of the stream.  If the stream has
        no remaining targets, the application may choose to terminate
        the stream.

     3.3.4.        Changing the FlowSpec

        An application may wish to change the FlowSpec of an
        established stream.  To do so, it informs ST of the new
        FlowSpec and the list of targets that are to be changed.  The
        origin ST agent then issues one or more CHANGE messages with
        the new FlowSpec and sends them to the relevant next-hop
        agents.  CHANGE messages are structured and processed similarly
        to CONNECT messages.  A next-hop agent that is an intermediate
        agent and receives a CHANGE message similarly determines if it
        can implement the new FlowSpec along the hop to each of its
        next-hop agents, and if so, it propagates the CHANGE messages
        along the established paths.  If this process succeeds, the
        CHANGE messages will eventually reach the targets, which will
        each respond with an ACCEPT message that is propagated back to
        the origin.

        Note that since a CHANGE may be sent containing a FlowSpec with
        a range of permissible values for bandwidth, delay, and/or
        error rate, and the actual values returned in the ACCEPTs may
        differ, then another CHANGE may be required to release excess
        resources along some of the paths.

  3.4.       Stream Tear Down

     A stream is usually terminated by the origin when it has no
     further data to send, but may also be partially torn down by the
     individual targets.  These cases will not be further discussed
     since they have already been described in Sections 3.3.2-3 (pages
     33-35).

     A stream is also torn down if the application should terminate
     abnormally.  Processing in this case is identical to the previous
     descriptions except that the appropriate reason code is different
     (ApplAbort).

     When all targets have left a stream, the origin notifies the
     application of that fact, and the application then is responsible
     for terminating the stream.  Note, however, that the application
     may decide to add a target(s) to the stream instead of terminating
     it.

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  3.5.       Exceptional Cases

     The previous descriptions covered the simple cases where
     everything worked.  We now discuss what happens when things do not
     succeed.  Included are situations where messages are lost, the
     requested resources are not available, the routing fails or is
     inconsistent.

     In order for the ST Control Message Protocol to be reliable over
     an unreliable internetwork, the problems of corruption,
     duplication, loss, and ordering must be addressed.  Corruption is
     handled through use of checksumming, as described in Section 4
     (page 76).  Duplication of control messages is detected by
     assigning a transaction number (Reference) to each control
     message;  duplicates are discarded.  Loss is detected using a
     timeout at the sender;  messages that are not acknowledged before
     the timeout expires are retransmitted;  see Section 3.7.6 (page
     66).  If a message is not acknowledged after a few retransmissions
     a fault is reported.  The protocol does not have significant
     ordering constraints.  However, minor sequencing of control
     messages for a stream is facilitated by the requirement that the
     Reference numbers be monotonically increasing;  see Section 4.2
     (page 78).

     3.5.1.        Setup Failure due to CONNECT Timeout

        If a response (an ERROR-IN-REQUEST, an ACK, a HID-REJECT, or a
        HID-APPROVE) has not been received within time ToConnect, the
        ST agent should retransmit the CONNECT message.  If no response
        has been received within NConnect retransmissions, then a fault
        occurs and a REFUSE message with the appropriate reason code
        (RetransTimeout) is sent back in the direction of the origin,
        and, in place of the CONNECT, a DISCONNECT is sent to the
        next-hop (in case the response to the CONNECT is the message
        that was lost).  The agent will expect an ACK for both the
        REFUSE and the DISCONNECT messages.  If it does not receive an
        ACK after retransmission time ToRefuse and ToDisconnect
        respectively, it will resend the REFUSE/DISCONNECT message.  If
        it does not receive ACKs after sending NRefuse/ NDisconnect
        consecutive REFUSE/DISCONNECT messages, then it simply gives up
        trying.

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         Sending Agent              Receiving Agent

   1.   ->+----> CONNECT X ------>//// (message lost or garbled)
          |      <RVLId=0><SVLId=99>
          V      <Ref=1278><HID=1234>
   2. (timeout)
          V
   3.     +----> CONNECT X ------------>+
   4.     |      <RVLId=0><SVLId=99>    +----> CONNECT X ----------->+
          |      <Ref=1278><HID=1234>   V      <RVLId=0><SVLId=1010> |
   5.     | //<- HID-APPROVE <----------+      <Ref=6666><HID=6666>  V
   6.     |      <RVLId=99><SVLId=88>      +<- HID-APPROVE <---------+
          V      <Ref=1278><HID=1234>          <RVLId=1010><SVLId=1111>
   7. (timeout)                                <Ref=6666><HID=6666>
          V
   8.     +----> CONNECT X ------------>+
                 <RVLId=0><SVLId=99>    |
                 <Ref=1278><HID=1234>   V
   9.     +<-+<- HID-APPROVE <----------+
          |      <RVLId=99><SVLId=88>
          V      <Ref=1278><HID=1234>
    (cancel timer)

          Figure 15.  CONNECT Retransmission after a Timeout

     3.5.2.        Problems due to Routing Inconsistency

        When an intermediate agent receives a CONNECT, it selects the
        next-hop agents based on the TargetList and the networks to
        which it is connected.  If the resulting next-hop to any of the
        targets is across the same network from which it received the
        CONNECT (but not the previous-hop itself), there may be a
        routing problem.  However, the routing algorithm at the
        previous-hop may be optimizing differently than the local
        algorithm would in the same situation.  Since the local ST
        agent cannot distinguish the two cases, it should permit the
        setup but send back to the previous-hop agent an informative
        NOTIFY message with the appropriate reason code (RouteBack),
        pertinent TargetList, and in the NextHopIPAddress element the
        address of the next-hop ST agent returned by its routing
        algorithm.

        The agent that receives such a NOTIFY should ACK it.  If the
        agent is using an algorithm that would produce such behavior,
        no further action is taken;  if not, the agent should send a
        DISCONNECT to the next-hop agent to correct the problem.

        Alternatively, if the next-hop returned by the routing function
        is in fact the previous-hop, a routing inconsistency has been
        detected.  In this case, a REFUSE is sent back to

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        the previous-hop agent containing an appropriate reason code
        (RouteInconsist), pertinent TargetList, and in the
        NextHopIPAddress element the address of the previous-hop.  When
        the previous-hop receives the REFUSE, it will recompute the
        next-hop for the affected targets.  If there is a difference in
        the routing databases in the two agents, they may exchange
        CONNECT and REFUSE messages again.  Since such routing errors
        in the internet are assumed to be temporary, the situation
        should eventually stabilize.

     3.5.3.        Setup Failure due to a Routing Failure

        It is possible for an agent to receive a CONNECT message that
        contains a known Name, but from an agent other than the
        previous-hop agent of the stream with that Name.  This may be:

         1  that two branches of the tree forming the stream have
            joined back together,

         2  a deliberate source routing loop,

         3  the result of an attempted recovery of a partially
            failed stream, or

         4  an erroneous routing loop.

        The TargetList is used to distinguish the cases 1 and 2 (see
        also Section 4.2.3.5 (page 107)) by comparing each newly
        received target with those of the previously existing stream:

         o  if the IP address of the targets differ, it is case 1;

         o  if the IP address of the targets match but the source
            route(s) are different, it is case 2;

         o  if the target (including any source route) matches a
            target (including any source route) in the existing
            stream, it may be case 3 or 4.

        It is expected that the joining of branches will become more
        common as routing decisions are based on policy issues and not
        just simple connectivity.  Unfortunately, there is no good way
        to merge the two parts of the stream back into a single stream.
        They must be treated independently with respect to processing
        in the agent.  In particular, a separate state machine is
        required, the Virtual Link Identifiers and HIDs from the
        previous-hops and to the next-hops must be different, and
        duplicate resources must be reserved in both the agent and in
        any next-hop networks.  Processing is the same for a deliberate
        source routing loop.

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        The remaining cases requiring recovery, a partially failed
        stream and an erroneous routing loop, are not easily
        distinguishable.  In attempting recovery of a failed stream, an
        agent may issue new CONNECT messages to the affected targets;
        for a full explanation see also Section 3.7.2 (page 51),
        Failure Recovery.  Such a CONNECT may reach an agent downstream
        of the failure before that agent has received a DISCONNECT from
        the neighborhood of the failure.  Until that agent receives the
        DISCONNECT, it cannot distinguish between a failure recovery
        and an erroneous routing loop.  That agent must therefore
        respond to the CONNECT with a REFUSE message with the affected
        targets specified in the TargetList and an appropriate reason
        code (StreamExists).

        The agent immediately preceding that point, i.e., the latest
        agent to send the CONNECT message, will receive the REFUSE
        message.  It must release any resources reserved exclusively
        for traffic to the listed targets.  If this agent was not the
        one attempting the stream recovery, then it cannot distinguish
        between a failure recovery and an erroneous routing loop.  It
        should repeat the CONNECT after a ToConnect timeout.  If after
        NConnect retransmissions it continues to receive REFUSE
        messages, it should propagate the REFUSE message toward the
        origin, with the TargetList that specifies the affected
        targets, but with a different error code (RouteLoop).

        The REFUSE message with this error code (RouteLoop) is
        propagated by each ST agent without retransmitting any CONNECT
        messages.  At each agent, it causes any resources reserved
        exclusively for the listed targets to be released.  The REFUSE
        will be propagated to the origin in the case of an erroneous
        routing loop.  In the case of stream recovery, it will be
        propagated to the ST agent that is attempting the recovery,
        which may be an intermediate agent or the origin itself.  In
        the case of a stream recovery, the agent attempting the
        recovery may issue new CONNECT messages to the same or to
        different next-hops.

        If an agent receives both a REFUSE message and a DISCONNECT
        message with a target in common then it can release the
        relevant resources and propagate neither the REFUSE nor the
        DISCONNECT (however, we feel that it is unlikely that most
        implementations will be able to detect this situation).

        If the origin receives such a REFUSE message, it should attempt
        to send a new CONNECT to all the affected targets.  Since
        routing errors in an internet are assumed to be temporary, the
        new CONNECTs will eventually find acceptable routes to the
        targets, if one exists.  If no further routes exist after
        NRetryRoute tries, the application should be

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        informed so that it may take whatever action it deems
        necessary.

     3.5.4.        Problems in Reserving Resources

        If the network or ST agent resources are not available, an ST
        agent may preempt one or more streams that have lower
        precedence than the one being created.  When it breaks a lower
        precedence stream, it must issue REFUSE and DISCONNECT messages
        as described in Sections 4.2.3.15 (page 122) and 4.2.3.6 (page
        110).  If there are no streams of lower precedence, or if
        preempting them would not provide sufficient resources, then
        the stream cannot be accepted by the ST agent.

        If an intermediate agent detects that it cannot allocate the
        necessary resources, then it sends a REFUSE that contains an
        appropriate reason code (CantGetResrc) and the pertinent
        TargetList to the previous-hop ST agent.  For further study are
        issues of reporting what resources are available, whether the
        resource shortage is permanent or transitory, and in the latter
        case, an estimate of how long before the requested resources
        might be available.

     3.5.5.        Setup Failure due to ACCEPT Timeout

        An ST agent that propagates an ACCEPT message backward toward
        the origin expects an ACK from the previous-hop.  If it does
        not receive an ACK within a timeout, called ToAccept, it will
        retransmit the ACCEPT.  If it does not receive an ACK after
        sending a number, called NAccept, of ACCEPT messages, then it
        will replace the ACCEPT with a REFUSE, and will send a
        DISCONNECT in the direction toward the target.  Both the REFUSE
        and DISCONNECT will identify the affected target(s) and specify
        an appropriate reason code (AcceptTimeout).  Both are also
        retransmitted until ACKed with timeout ToRefuse/ ToDisconnect
        and retransmit count NRefuse/NDisconnect.  If they are not
        ACKed, the agent simply gives up, letting the failure detection
        mechanism described in Section 3.7.1 (page 48) take care of any
        cleanup.

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     3.5.6.        Problems Caused by CHANGE Messages

        An application must exercise care when changing a FlowSpec to
        prevent a failure.  A CHANGE might fail for two reasons.  The
        request may be for a larger amount of network resources when
        those resources are not available;  this failure may be
        prevented by requiring that the current level of service be
        contained within the ranges of the FlowSpec in the CHANGE.

        Alternatively, the local network might require all the former
        resources to be released before the new ones are requested and,
        due to unlucky timing, an unrelated request for network
        resources might be processed between the time the resources are
        released and the time the new resources are requested, so that
        the former resources are no longer available.  There is not
        much that an application or ST can do to prevent such failures.

        If the attempt to change the FlowSpec fails then the ST agent
        where the failure occurs must intentionally break the stream
        and invoke the stream recovery mechanism using REFUSE and
        DISCONNECT messages;  see Section 3.7.2 (page 51).  Note that
        the reserved resources after the failure of a CHANGE may not be
        the same as before, i.e., the CHANGE may have been partially
        completed.  The application is responsible for any cleanup
        (another CHANGE).

     3.5.7.        Notification of Changes Forced by Failures

        NOTIFY is issued by a an ST Agent to inform upsteam agents and
        the origin that resource allocation changes have occurred after
        a stream was established.  These changes occur when network
        components fail and when competing streams preempt resources
        previously reserved by a lower precedence stream.  We also
        anticipate that NOTIFY can be used in the future when
        additional resources become available, as is the case when
        network components recover or when higher precedence streams
        are deleted.

        NOTIFY is also used to inform upstream agents that a routing
        anomaly has occurred.  Such an example was cited in Section
        3.5.2 (page 38), where an agent notices that the next-hop agent
        is on the same network as the previous-hop agent;  the anomaly
        is that the previous-hop should have connected directly to the
        next-hop without using an intermediate agent.  Delays in
        propagating host status and routing information can cause such
        anomalies to occur.  NOTIFY allows ST to correct automatically
        such mistakes.

        NOTIFY reports a FlowSpec that reflects that revised guarantee
        that can be promised to the stream.  NOTIFY also

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        identifies those targets affected by the change.  In this way,
        NOTIFY is similar to ACCEPT.  NOTIFY includes a ReasonCode to
        identify the event that triggered the notification.  It also
        includes a TargetList, rather than a single Target, since a
        single event can affect a branch leading to several targets.

        NOTIFY is relayed by the ST agents back toward the origin,
        along the path established by the CONNECT but in the reverse
        direction.  NOTIFY must be acknowledged with an ACK at each
        hop.  If intermediate agent corrects the situation without
        causing any disruption to the data flow or guarantees, it can
        choose to drop the notification message before it reaches the
        origin.  If the originating agent receives a NOTIFY, it is then
        expected to adjust its own processing and data rates, and to
        submit any required CHANGE requests.  As with ACCEPT, the
        FlowSpec is not modified on this trip from the target back to
        the origin.  It is up to the origin to decide whether a CHANGE
        should be submitted.  (However, even though the FlowSpec has
        not been modified, the situation reported in the

  Application  Agent A            Agent 1                    Agent B

1.                      (high precedence request preempts 10K of
                            the stream's original 30Kb bandwidth
                             allocated to the hop from 1 to B)
                                     |
                                     V
2.   +<------+-- NOTIFY -------------+
     |       |   <RVLId=4><SVLId=14>
     |       |   <Ref=150>
     |       V   <FlowSpec=20Kb,...><TargList=B>
3.   |       +-> ACK --------------->+
     |           <RVLId=14><SVLId=4>
     V           <Ref=150>
4. (inform application)
     ....
5. change(FlowSpec=20Kb,...)
     V
6.   +---------> CHANGE B ---------->+
7.               <RVLId=14><SVLId=4> +--> CHANGE B ------------>+->+
                 <Ref=60>            |    <RVLId=44><SVLId=15>  |  |
                 <FlowSpec=20Kb,...> V    <Ref=160>             |  |
8.           +<- ACK ----------------+    <FlowSpec=20Kb,...>   |  |
                 <RVLId=4><SVLId=14>                            V  |
9.               <Ref=60>            +--- ACK ------------------+  |
                                            <RVLId=15><SVLId=44>   |
                                            <Ref=160>              V
             ... perform normal ACCEPT processing ...        <-----+

                Figure 16.  Processing NOTIFY Messages

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        notify may have prevented the ST agents from meeting the
        original guarantees.)

  3.6.       Options

     Several options are defined in the CONNECT message.  The special
     processing required to support each will be described in the
     following sections.  The options are independent, i.e., can be set
     to one (1, TRUE) or zero (0, FALSE) in any combination.  However,
     the effect and implementation of the options is NOT necessarily
     independent, and not all combinations are supported.

     3.6.1.        HID Field Option

        The sender of a CONNECT message may or not specify an HID in
        the HID field.  If the HID Field option of the CONNECT message
        is not set (the H bit is 0), then the HID field does not
        contain relevant information and should be ignored.

        If this option is set (the H bit is 1), then the HID field
        contains a relevant value.  If this option is set and the HID
        field of the CONNECT contains a non-zero value, that value
        represents a proposed HID that initiates the HID negotiation.

        If the HID Field option is set but the HID field of the CONNECT
        message contains a zero, this means that the sender of that
        CONNECT message has chosen to defer selection of the HID to the
        next-hop agent (the receiver of a CONNECT message).  This
        choice can allow a more efficient mechanism for selecting HIDs
        and possibly a more efficient mechanism for forwarding data
        packets in the case when the previous-hop does not need to
        select the HID;  see also Section 4.2.3.5 (page 105).

        Upon receipt of a CONNECT message with the HID Field option set
        and the HID field set to zero, a next-hop agent selects the HID
        for the hop, enters it into its appropriate data structure, and
        returns it in the HID field of the HID-APPROVE message.  The
        previous-hop takes the HID from the HID-APPROVE message and
        enters it into its appropriate data structure.

     3.6.2.        PTP Option

        The PTP option (Point-to-Point) is used to indicate that the
        stream will never have more than a single target.  It
        consequently implies that the stream will never need to support
        any form of multicasting.  Use of the PTP option may thus allow
        efficiencies in the way the stream is built or is

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        managed.  Specifically, the ST agents do not need to request
        that the intervening networks allocate multicast groups to
        support this stream.

        The PTP option can only be set to one (1) by the origin, and
        must be the same for the entire stream (i.e., propagated by ST
        agents).  The details of what this option does are
        implementation specific, and do not affect the protocol very
        much.

        If the application attempts to add a new target to an existing
        stream that was created with the PTP option set to one (1), the
        application should be informed of the error with an ERROR-IN-
        REQUEST message with the appropriate reason code.  If a CONNECT
        is received whose TargetList contains more than a single entry,
        an ERROR-IN-REQUEST message with the appropriate reason code
        (PTPError) should be returned to the previous-hop agent (note
        that such a CONNECT should never be received if the origin both
        implements the PTP option and is functioning properly).

        As implied in the last paragraph, a subsetted implementation
        might choose not to implement the PTP option.

     3.6.3.        FDx Option

        The FDx option is used to indicate that a second stream in the
        reverse direction, from the target to the origin, should
        automatically be created.  This option is most likely to be
        used when the TargetList has only a single entry.  If used when
        the TargetList has multiple entries, the resulting streams
        would allow bi-directional communication between the origin and
        the various targets, but not among the targets.  The FDx option
        can only be invoked by the origin, and must be propagated by
        intermediate agents.

        This option is specified by inclusion of both an RFlowSpec and
        an RHID parameter in the CONNECT message (possibly with an
        optional RGroup parameter).

        Any ST agent that receives a CONNECT message with both an
        RFlowSpec and an RHID parameter will create database entries
        for streams in both directions and will allocate resources in
        both directions for them.  By this we mean that an ST agent
        will reserve resources to the next-hop agent for the normal
        stream and resources back to the previous-hop agent for the
        reverse stream.  This is necessary since it is expected that
        network reservation interfaces will require the destination
        address(es) in order to make reservations, and because all ST
        agents must use the same reservation model.

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        The target agent will select a Name for the reverse stream and
        return it (in the RName parameter) and the resulting FlowSpec
        (in the RFlowSpec parameter) of the ACCEPT message.  Each agent
        that processes the ACCEPT will update its partial stream
        database entry for the reverse stream with the Name contained
        in the RName parameter.  We assume that the next higher
        protocol layer will use the same SAP for both streams.

     3.6.4.        NoRecovery Option

        The NoRecovery option is used to indicate that ST agents should
        not attempt recovery in case of network or component failure.
        If a failure occurs, the origin will be notified via a REFUSE
        message and the target(s) via a DISCONNECT, with an appropriate
        reason code of "failure" (i.e., one of DropFailAgt,
        DropFailHst, DropFailIfc, DropFailNet, IntfcFailure,
        NetworkFailure, STAgentFailure, FailureRecovery).  They can
        then decide whether to wait for the failed component to be
        fixed, or drop the target via DISCONNECT/REFUSE messages.  The
        NoRecovery option can only be set to one (1) by the origin, and
        must be the same for the entire stream.

     3.6.5.        RevChrg Option

        The RevChrg option bit in the FlowSpec is set to one (1) by the
        origin to request that the target(s) pay any charges associated
        with the stream (to the target(s));  see Section 4.2.2.3 (page
        83).  If the target is not willing to accept charges, the bit
        should be set to zero (0) by the target before returning the
        FlowSpec to the origin in an ACCEPT message.

        If the FDx option is also specified, the target pays charges
        for both streams.

     3.6.6.        Source Route Option

        The Source Route Option may be used both for diagnostic
        purposes, and, in those hopefully infrequent cases where the
        standard routing mechanisms do not produce paths that satisfy
        some policy constraint, to allow the origin to prespecify the
        ST agents along the path to the target(s).  The idea is that
        the origin can explicitly specify the path to a target, either
        strictly hop-by-hop or more loosely by specification of one or
        more agents through which the path must pass.

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        The option is specified by including source routing information
        in the Target structure.  A target may contain zero or more
        SrcRoute options;  when multiple options are present, they are
        processed in the order in which they occur.  The parameter code
        indicates whether the portion of the path contained in the
        parameter is of the strict or loose variety.

        Since portions of a path may pass through portions of an
        internet that does not support ST agents, there are also forms
        of the SrcRoute option that are converted into the

Application Agent A Agent 2 Agent 3 Agent B

1. (open B<SR=2,3>) 2. V (proc B listening) 3. (source routed to 2)

4. (check resources from A to Agent 2: already allocated,

     V   reuse control link & HID, no additional resources needed)

5. +-> CONNECT B<SR=2,3>->-+-+

         <RVLId=23><SVLId=5> | |

6. <Ref=50> V | 7. +<- ACK ----------------+ |

         <RVLId=5><SVLId=23>   |
         <Ref=50>              V

8. (source routed to 3)

9. (reserve resources 2 to 3)

10. +-> CONNECT B<SR=3> ---->+

                             <RVLId=0><SVLId=24>  |
                             <Ref=280><HID=4801>  V

11. +<- HID-APPROVE <--------+

                             <RVLId=24><SVLId=33> |
                             <Ref=280><HID=4801>  |
                                                  V
                                          (routing to B)
                                               V
                                (reserve resources from 3 to B)
                                            V

12. +-> CONNECT B ---------->+

                                                <RVLId=0><SVLId=32>  |
                                                <Ref=330><HID=6000>  V

13. +<- HID-APPROVE <--------+

                                                <RVLId=32><SVLId=45> |
                                                <Ref=330><HID=6000>  V

14. (proc B accepts)

                                                                     V
               ... perform normal ACCEPT processing ...        <-----+

                   Figure 17.  Source Routing Option

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        corresponding IP Source Routing options by the ST agent that
        performs the encapsulation.

        The SrcRoute option is usually selected by the origin, but may
        be used by intermediate agents if specified as a result of the
        routing function.

        For example, in the topology of Figure 2, if A wants to add B
        back into the stream, its routing function might decide that
        the best path is via Agent 3.  Since the data is already being
        multicast across the network connected to C, D, and E, the
        route via Agent 3 might cost less than having A replicate the
        data packets and send them across A's network a second time.

  3.7.       Ancillary Functions

     There are several functions and procedures that are required by
     the ST Protocol.  They are described in subsequent sections.

     3.7.1.        Failure Detection

        The ST failure detection mechanism is based on two assumptions:

         1  If a neighbor of an ST agent is up, and has been up
            without a disruption, and has not notified the ST agent
            of a problem with streams that pass through both, then
            the ST agent can assume that there has not been any
            problem with those streams.

         2  A network through which an ST agent has routed a stream
            will notify the ST agent if there is a problem that
            affects the stream data packets but does not affect the
            control packets.

        The purpose of the robustness protocol defined here is for ST
        agents to determine that the streams through a neighbor have
        been broken by the failure of the neighbor or the intervening
        network.  This protocol should detect the overwhelming majority
        of failures that can occur.  Once a failure is detected,
        recovery procedures are initiated.

        3.7.1.1.         Network Failures

           In this memo, a network is defined to be the protocol
           layer(s) below ST.  This function can be implemented in a
           hardware module separate from the ST agent, or as software
           modules within the ST agent itself, or as a combination of

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           both.  This specification and the robustness protocol do not
           differentiate between these alternatives.

           An ST agent can detect network failures by two mechanisms;
           the network can report a failure, or the ST agent can
           discover a failure by itself.  They differ in the amount of
           information that ST agent has available to it in order to
           make a recovery decision.  For example, a network may be
           able to report that reserved bandwidth has been lost and the
           reason for the loss and may also report that connectivity to
           the neighboring ST agent remains intact.  In this case, the
           ST agent may request the network to allocate bandwidth anew.
           On the other hand, an ST agent may discover that
           communication with a neighboring ST agent has ceased because
           it has not received any traffic from that neighbor in some
           time period.  If an ST agent detects a failure, it may not
           be able to determine if the failure was in the network while
           the neighbor remains available, or the neighbor has failed
           while the network remains intact.

        3.7.1.2.         Detecting ST Stream Failures

           Each ST agent periodically sends each neighbor with which it
           shares a stream a HELLO message.  A HELLO message is ACKed
           if the Reference field is non-zero.  This message exchange
           is between ST agents, not entities representing streams or
           applications (there is no Name field in a HELLO message).
           That is, an ST agent need only send a single HELLO message
           to a neighbor regardless of the number of streams that flow
           between them.  All ST agents (host as well as intermediate)
           must participate in this exchange.  However, only agents
           that share active streams need to participate in this
           exchange.

           To facilitate processing of HELLO messages, an
           implementation may either create a separate Virtual Link
           Identifier for each neighbor having an active stream, or may
           use the reserved identifier of one (1) for the SVLId field
           in all its HELLO messages.

           An implementation that wishes to send its HELLO messages via
           a data path instead of the control path may setup a separate
           stream to its neighbor agent for that purpose.  The HELLO
           message would contain a HID of zero, indicating a control
           message, but would be identified to the next lower protocol
           layer as being part of the separate stream.

           As well as identifying the sender, the HELLO message has two
           fields;  a HelloTimer field that is in units of milliseconds
           modulo the maximum for the field size, and a

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           Restarted bit specifying that the ST agent has been
           restarted recently.  The HelloTimer must appear to be
           incremented every millisecond whether a HELLO message is
           sent or not, but it is allowable for an ST agent to create a
           new HelloTimer only when it sends a HELLO message.  The
           HelloTimer wraps around to zero after reaching the maximum
           value.  Whenever an ST agent suffers a catastrophic event
           that may result in it losing ST state information, it must
           reset its HelloTimer to zero and must set the Restarted bit
           for the following HelloTimerHoldDown seconds.

           An ST agent must send HELLO messages to its neighbor with a
           period shorter than the smallest RecoveryTimeout parameter
           of the FlowSpecs of all the active streams that pass between
           the two agents, regardless of direction.  This period must
           be smaller by a factor, called HelloLossFactor, which is at
           least as large as the greatest number of consecutive HELLO
           messages that could credibly be lost while the communication
           between the two ST agents is still viable.

           An ST agent may send simultaneous HELLO messages to all its
           neighbors at the rate necessary to support the smallest
           RecoveryTimeout of any active stream.  Alternately, it may
           send HELLO messages to different neighbors independently at
           different rates corresponding to RecoveryTimeouts of
           individual streams.

           The agent that receives a HELLO message expects to receive
           at least one new HELLO message from a neighbor during the
           RecoveryTimeout of every active stream through that
           neighbor.  It can detect duplicate or delayed HELLO messages
           by saving the HelloTimer field of the most recent valid
           HELLO message from that neighbor and comparing it with the
           HelloTimer field of incoming HELLO messages.  It will only
           accept an incoming HELLO message from that neighbor if it
           has a HelloTimer field that is greater than the most recent
           valid HELLO message by the time elapsed since that message
           was received plus twice the maximum likely delay variance
           from that neighbor.  If the ST agent does not receive a
           valid HELLO message within the RecoveryTimeout of a stream,
           it must assume that the neighboring ST agent or the
           communication link between the two has failed and it must
           initiate stream recovery activity.

           Furthermore, if an ST agent receives a HELLO message that
           contains the Restarted bit set, it must assume that the
           sending ST agent has lost its ST state.  If it shares
           streams with that neighbor, it must initiate stream recovery
           activity.  If it does not share streams with that neighbor,
           it should not attempt to create one until that

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           bit is no longer set.  If an ST agent receives a CONNECT
           message from a neighbor whose Restarted bit is still set, it
           must respond with ERROR-IN-REQUEST with the appropriate
           reason code (RemoteRestart).  If it receives a CONNECT
           message while its own Restarted bit is set, it must respond
           with ERROR-IN-REQUEST with the appropriate reason code
           (RestartLocal).

        3.7.1.3.         Subset

           This failure detection mechanism subsets by reducing the
           complexity of the timing and decisions.  A subsetted ST
           agent sends HELLO messages to all its ST neighbors
           regardless of whether there is an active ST stream between
           them or not.  The RecoveryTimeout parameter of the FlowSpec
           is ignored and is assumed to be the DefaultRecoveryTimeout.
           Note that this implies that a REFUSE should be sent for all
           CONNECT or CHANGE messages whose RecoveryTimeout is less
           than DefaultRecoveryTimeout.  An ST agent will accept an
           incoming HELLO message if it has a HelloTimer field that is
           greater than the most recent valid HELLO message by
           DefaultHelloFactor times the time elapsed since that message
           was received.

     3.7.2.        Failure Recovery

        Streams can fail from various causes;  an ST agent can break, a
        network can break, or an ST agent can intentionally break a
        stream in order to give the stream's resources to a higher
        precedence stream.  We can envision several approaches to
        recovery of broken streams, and we consider the one described
        here the simplest and therefore the most likely to be
        implemented and work.

        If an intermediate agent fails or a network or part of a
        network fails, the previous-hop agent and the various next-hop
        agents will discover the fact by the failure detection
        mechanism described in Section 3.7.1 (page 48).  An ST agent
        that intentionally breaks a stream obviously knows of the
        event.

        The recovery of an ST stream is a relatively complex and time
        consuming effort because it is designed in a general manner to
        operate across a large number of networks with diverse
        characteristics.  Therefore, it may require information to be
        distributed widely, and may require relatively long timers.  On
        the other hand, since a network is a homogeneous system,
        failure recovery in the network may be a relatively faster and
        simpler operation.  Therefore an ST agent that detects a
        failure should attempt to fix the network failure before

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        attempting recovery of the ST stream.  If the stream that
        existed between two ST agents before the failure cannot be
        reconstructed by network recovery mechanisms alone, then the ST
        stream recovery mechanism must be invoked.

        If stream recovery is necessary, the different ST agents may
        need to perform different functions, depending on their
        relation to the failure.

        An intermediate agent that breaks the stream intentionally
        sends DISCONNECT messages with the appropriate reason code
        (StreamPreempted) toward the affected targets.  If the
        NoRecovery option is selected, it sends a REFUSE message with
        the appropriate reason code(StreamPreempted) toward the origin.
        If the NoRecovery option is not selected, then this agent
        attempts recovery of the stream, as described below.

        A host agent that is a target of the broken stream or is itself
        the next-hop of the failed component should release resources
        that are allocated to the stream, but should maintain the
        internal state information describing the stream.  It should
        inform any next higher protocol of the failure.  It is
        appropriate for that protocol to expect that the stream will be
        fixed shortly by some alternate path and so maintain, for some
        time period, whatever information in the ST layer, the next
        higher layer, and the application is necessary to reactivate
        quickly entries for the stream as the alternate path develops.
        The agent should use a timeout to delete all the stream
        information in case the stream cannot be fixed in a reasonable
        time.

        An intermediate agent that is a next-hop of a failure that was
        not due to a preemption should first verify that there was a
        failure.  It can do this using STATUS messages to query its
        upstream neighbor.  If it cannot communicate with that
        neighbor, then it should first send a REFUSE message with the
        appropriate reason code of "failure" to the neighbor to speed
        up the failure recovery in case the hop is unidirectional,
        i.e., the neighbor can hear the agent but the agent cannot hear
        the neighbor.  The ST agent detecting the failure must then
        send DISCONNECT messages with the same reason code toward the
        targets.  The intermediate agents process this DISCONNECT
        message just like the DISCONNECT that tears down the stream.
        However, a target ST agent that receives a DISCONNECT message
        with the appropriate reason code (StreamPreempted, or
        "failure") will maintain the stream state and notify the next
        higher protocol of the failure.  In effect, these DISCONNECT
        messages tear down the stream from the point of the failure to
        the targets, but inform the targets that the stream may be
        fixed shortly.

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        An ST agent that is the previous-hop before the failed
        component first verifies that there was a failure by querying
        the downstream neighbor using STATUS messages.  If the neighbor
        has lost its state but is available, then the ST agent may
        reconstruct the stream if the NoRecovery option is not
        selected, as described below.  If it cannot communicate with
        the next-hop, then the agent detecting the failure releases any
        resources that are dedicated exclusively to sending data on the
        broken branch and sends a DISCONNECT message with the
        appropriate reason code ("failure") toward the affected
        targets.  It does so to speed up failure recovery in case the
        communication may be unidirectional and this message might be
        delivered successfully.

        If the NoRecovery option is selected, then the ST agent that
        detects the failure sends a REFUSE message with the appropriate
        reason code ("failure") to the previous-hop.  If it is breaking
        the stream intentionally, it sends a REFUSE message with the
        appropriate reason code (StreamPreempted) to the previous-hop.
        The TargetList in these messages contains all the targets that
        were reached through the broken branch.  Multiple REFUSE
        messages may be required if the PDU is too long for the MTU of
        the intervening network.  The REFUSE message is propagated all
        the way to the origin, which can attempt recovery of the stream
        by sending a new CONNECT to the affected targets.  The new
        CONNECT will be treated by intermediate ST agents as an
        addition of new targets into the established stream.

        If the NoRecovery option is not selected, the ST agent that
        breaks the stream intentionally or is the previous-hop before
        the failed component can attempt recovery of the stream.  It
        does so by issuing a new CONNECT message to the affected
        targets.  If the ST agent cannot find new routes to some
        targets, or if the only route to some targets is through the
        previous-hop, then it sends one or more REFUSE messages to the
        previous-hop with the appropriate reason code ("failure" or
        StreamPreempted) specifying the affected targets in the
        TargetList.  The previous-hop can then attempt recovery of the
        stream by issuing a CONNECT to those targets.  If it cannot
        find an appropriate route, it will propagate the REFUSE message
        toward the origin.

        Regardless of which agent attempts recovery of a damaged
        stream, it will issue one or more CONNECT messages to the
        affected targets.  These CONNECT messages are treated by
        intermediate ST agents as additions of new targets into the
        established stream.  The FlowSpecs of the new CONNECT messages
        should be the same as the ones contained in the most recent
        CONNECT or CHANGE messages that the ST agent had sent toward
        the affected targets when the stream was operational.

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        The reconstruction of a broken stream may not proceed smoothly.
        Since there may be some delay while the information concerning
        the failure is propagated throughout an internet, routing
        errors may occur for some time after a failure.  As a result,
        the ST agent attempting the recovery may receive REFUSE or
        ERROR-IN-REQUEST messages for the new CONNECTs that are caused
        by internet routing errors.  The ST agent attempting the
        recovery should be prepared to resend CONNECTs before it
        succeeds in reconstructing the stream.  If the failure
        partitions the internet and a new set of routes cannot be found
        to the targets, the REFUSE messages will eventually be
        propagated to the origin, which can then inform the application
        so it can decide whether to terminate or to continue to attempt
        recovery of the stream.

        The new CONNECT may at some point reach an ST agent downstream
        of the failure before the DISCONNECT does.  In this case, the
        agent that receives the CONNECT is not yet aware that the
        stream has suffered a failure, and will interpret the new
        CONNECT as resulting from a routing failure.  It will respond
        with an ERROR-IN-REQUEST message with the appropriate reason
        code (StreamExists).  Since the timeout that the ST agents
        immediately preceding the failure and immediately following the
        failure are approximately the same, it is very likely that the
        remnants of the broken stream will soon be torn down by a
        DISCONNECT message with the appropriate reason code
        ("failure").  Therefore, the ST agent that receives the ERROR-
        IN-REQUEST message with reason code (StreamExists) should
        retransmit the CONNECT message after the ToConnect timeout
        expires.  If this fails again, the request will be retried for
        NConnect times.  Only if it still fails will the ST agent send
        a REFUSE message with the appropriate reason code (RouteLoop)
        to its previous-hop.  This message will be propagated back to
        the ST agent that is attempting recovery of the damaged stream.
        That ST agent can issue a new CONNECT message if it so chooses.
        The REFUSE is matched to a CONNECT message created by a
        recovery operation through the LnkReference field in the
        CONNECT.

        ST agents that have propagated a CONNECT message and have
        received a REFUSE message should maintain this information for
        some period of time.  If an agent receives a second CONNECT
        message for a target that recently resulted in a REFUSE, that
        agent may respond with a REFUSE immediately rather than
        attempting to propagate the CONNECT.  This has the effect of
        pruning the tree that is formed by the propagation of CONNECT
        messages to a target that is not reachable by the routes that
        are selected first.  The tree will pass through any given ST
        agent only once, and the stream setup phase will be completed
        faster.

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        The time period for which the failure information is maintained
        must be consistent with the expected lifetime of that
        information.  Failures due to lack of reachability will remain
        relevant for time periods large enough to allow for network
        reconfigurations or repairs.  Failures due to routing loops
        will be valid only until the relevant routing information has
        propagated, which can be a short time period.  Lack of
        bandwidth resulting from over-allocation will remain valid
        until streams are terminated, which is an unpredictable time,
        so the time that such information is maintained should also be
        short.

        If a CONNECT message reaches a target, the target should as
        efficiently as possible use the state that it has saved from
        before the stream failed during recovery of the stream.  It
        will then issue an ACCEPT message toward the origin.  The
        ACCEPT message will be intercepted by the ST agent that is
        attempting recovery of the damaged stream, if not the origin.
        If the FlowSpec contained in the ACCEPT specifies the same
        selection of parameters as were in effect before the failure,
        then the ST agent that is attempting recovery will not
        propagate the ACCEPT.  If the selections of the parameters are
        different, then the agent that is attempting recovery will send
        the origin a NOTIFY message with the appropriate reason code
        (FailureRecovery) that contains a FlowSpec that specifies the
        new parameter values.  The origin may then have to change its
        data generation characteristics and the stream's parameters
        with a CHANGE message to use the newly recovered subtree.

        3.7.2.1.         Subset

           Subsets of this mechanism may reduce the functionality in
           the following ways.  A host agent might not retain state
           describing a stream that fails with a DISCONNECT message
           with the appropriate reason code ("failure" or
           StreamPreempted).

           An agent might force the NoRecovery option always to be set.
           In this case, it will allow the option to be propagated in
           the CONNECT message, but will propagate the REFUSE message
           with the appropriate reason code ("failure" or
           StreamPreempted) without attempting recovery of the damaged
           stream.

           If an ST agent allows stream recovery and attempts recovery
           of a stream, it might choose a FlowSpec to specify exactly
           the current values of the parameters, with no ranges or
           options.

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     3.7.3.        A Group of Streams

        There may be a need to associate related streams.  The Group
        mechanism is simply an association technique that allows ST
        agents to identify the different streams that are to be
        associated.  Streams are in the same Group if they have the
        same Group Name in the GroupName field of the (R)Group
        parameter.  At this time there are no ST control messages that
        modify Groups.  Group Names have the same format as stream
        Names, and can share the same name space.  A stream that is a
        member of a Group can specify one or more (Subgroup Identifier,
        Relation) tuples.  The Relation specifies how the members of
        the Subgroup of the Group are related.  The Subgroups
        Identifiers need only be unique within the Group.

        Streams can be associated into Groups to support activities
        that deal with a number of streams simultaneously.  The
        operation of Groups of streams is a matter for further study,
        and this mechanism is provided to support that study.  This
        mechanism allows streams to be identified as belonging to a
        given Group and Subgroup, but in order to have any effect, the
        behavior that is expected of the Relation must be implemented
        in the ST agents.  Possible applications for this mechanism
        include the following:

         o  Associating streams that are part of a floor-controlled
            conference.  In this case, only one origin can send data
            through its stream at any given time.  Therefore, at any
            point where more than one stream passes through a branch
            or network, only enough bandwidth for one stream needs
            to be allocated.

         o  Associating streams that cannot exist independently.  An
            example of this may be the various streams that carry
            the audio, video, and data components of a conference,
            or the various streams that carry data from the
            different participants in a conference.  In this case,
            if some ST agent must preempt more than a single stream,
            and it has selected any one of the streams so
            associated, then it should also preempt the rest of the
            members of that Subgroup rather than preempting any
            other streams.

         o  Associating streams that must not be completed
            independently.  This example is similar to the preceding
            one, but relates to the stream setup phase.  In this
            example, any single member of a Subgroup of streams need
            not be completed unless the rest are also completed.
            Therefore, if one stream becomes blocked, all the others
            will also be blocked.  In this case, if there are not
            enough resources to support all the conferences that are
            attempted, some number of the conferences will complete

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            and other will be blocked, rather than all conferences
            be partially completed and partially blocked.

        This document assumes that the creation and membership of the
        Group will be managed by the next protocol above ST, with the
        assistance of ST.  For example, the next higher protocol
        would request ST to create a unique Group Name and a set of
        Subgroups with specified characteristics.  The next higher
        protocol would distribute this information to the other
        participants that were to be members of the Group.  Each
        would transfer the Group Name, Subgroups, and Relations to
        the ST layer, which would simply include them in the stream
        state.

        3.7.3.1.         Group Name Generator

           This facility is provided so that an application or higher
           layer protocol can obtain a unique Group Name from the ST
           layer.  This is a mechanism for the application to request
           the allocation of a Group Name that is independent of the
           request to create a stream.  The Group Name is used by the
           application or higher layer protocol when creating the
           streams that are to be part of a group.  All that is
           required is a function of the form:

              AllocateGroupName()
                 -> result, GroupName

           A corresponding function to release a Group Name is also
           desirable;  its form is:

              ReleaseGroupName( GroupName )
                 -> result

        3.7.3.2.         Subset

           Since Groups are currently intended to support
           experimentation, and it is not clear how best to use them,
           it is appropriate for an implementation not to support
           Groups.  At this time, a subsetted ST agent may ignore the
           Group parameter.  It is expected that in the future, when
           Groups transition from being an experimental concept to an
           operational one, it may be the case that such subsetting
           will no longer be acceptable.  At that time, a new
           subsetting option may be defined.

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     3.7.4.        HID Negotiation

        Each data packet must carry a value to identify the stream to
        which it belongs, so that forwarding can be performed.
        Conceptually, this value could be the Name of the stream.  A
        shorthand identifier is desirable for two reasons.  First,
        since each data packet must carry this identifier, network
        bandwidth efficiency suggests that it be as small as
        possible.  This is particularly important for applications
        that use small data packets, and that use low bandwidth
        networks, such as voice across packet radio networks.
        Second, the operation of mapping this identifier into a data
        object that contains the forwarding information must be
        performed at each intermediate ST agent in the stream.  To
        minimize delay and processing overhead, this operation should
        be as efficient as possible.  Most likely, this identifier
        will be used to index into an internal table.  To meet these
        goals, ST has chosen to use a 16-bit hop-by-hop identifier
        (HID).  It is large enough to handle the foreseen number of
        streams during the expected life of the protocol while small
        enough not to preclude its use as a forwarding table index.
        Note, however, that HID 0 is reserved for control messages,
        and that HIDs 1-3 are also reserved for future use.

        When ST makes use of multicast ability in networks that
        provide it, a data packet multicast by an ST agent will be
        received identically by several next-hop ST agents.  In a
        multicast environment, the HID must be selected either by
        some network-wide mechanism that selects unique identifiers,
        or it must be selected by the sender of the CONNECT message.
        Since we feel any network-wide mechanism is outside the scope
        of this protocol, we propose that the previous-hop agent
        select the HID and send it in the CONNECT message (with the
        HID Field option set, see Section 3.6.1 (page 44)) subject to
        the approval of the next-hop agents.  We call this "HID
        negotiation".

        As an origin ST agent is creating a stream or as an
        intermediate agent is propagating a CONNECT message, it must
        make a routing decision to determine which targets will be
        reached through which next-hop ST agents.  In some cases,
        several next-hops can be reached through a network that
        supports multicast delivery.  If so, those next-hops will be
        made members of a multicast group and data packets will be
        sent to the group.  Different CONNECT messages are sent to
        the several next-hops even if the data packets will be sent
        to the multicast group, because the CONNECT messages contain
        different TargetLists and are acknowledged and accepted
        separately.  However, the HID contained by the different
        CONNECT message must be identical.  The ST agent selects a
        16-bit quantity to be the HID and inserts it into each

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        CONNECT message that is then sent to the appropriate
        next-hop.

        The next-hop agents that receive the CONNECT messages must
        propagate the CONNECT messages toward the targets, but must
        also look at the HID and decide whether they can approve it.
        An ST agent can only receive data packets with a given HID if
        they belong to a single stream.  If the ST agent already has
        an established stream that uses the proposed HID, this is a
        HID collision, and the agent cannot approve the HID for the
        new stream.  Otherwise the agent can approve the HID.  If it
        can approve the HID, then it must make note of that HID and
        it must respond with a HID-APPROVE message (unless it can
        immediately respond with an ERROR-IN-REQUEST or a REFUSE).
        If it cannot approve the HID then it must respond with a
        HID-REJECT message.

        An agent that sends a CONNECT message with the H bit set
        awaits its acknowledgment message (which could be a
        HID-ACCEPT, HID-REJECT, or an ERROR-IN-REQUEST) from the
        next-hops independently of receiving ACCEPT messages.  If it
        does not receive an acknowledgment within timeout ToConnect,
        it will resend the CONNECT.  If each next-hop agent responds
        with a HID-ACCEPT, this implies that they have each approved
        of the HID, so it can be used for all subsequent data
        packets.  If one or more next-hops respond with an
        HID-REJECT, then the agent that selected the HID must select
        another HID and send it to each next-hop in a set of
        HID-CHANGE messages.  The next-hop agents must respond to
        (and thus acknowledge) these HID-CHANGE messages with either
        a HID-ACCEPT or a HID-REJECT (or, in the case of an error, an
        ERROR-IN-REQUEST, or a REFUSE if the next-hop agent wants to
        abort the HID negotiation process after rejecting NHIDAbort
        proposed HIDs).  If the agent does not receive such a
        response within timeout ToHIDChange, it will resend the
        HID-CHANGE up to NHIDChange times.  If any next-hop agents
        respond with a REFUSE message that specifies all the targets
        that were included in the corresponding CONNECT, then that
        next-hop is removed from the negotiation.  The overall
        negotiation is complete only when the agent receives a
        HID-ACCEPT to the same proposed HID from all the next-hops
        that do not respond with an ERROR-IN-REQUEST or a REFUSE.

        This negotiation may continue an indeterminate length of
        time.  In fact, the CONNECT messages could propagate to the
        targets and their ACCEPT messages may potentially propagate
        back to the origin before the negotiation is complete.  If
        this were permitted, the origin would not be aware of the
        incomplete negotiation and could begin to send data packets.
        Then the agent that is attempting to select a HID would have
        to discard any data rather than sending it to the next-hops
        since it might not have a valid HID to send with the data.

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        To prevent this situation, an ACCEPT should not be propagated
        back to the previous-hop until the HID negotiation with the
        next-hops has been completed.

        Although it is possible that the negotiation extends for an
        arbitrary length of time, we consider this to be very
        unlikely.  Since the HID is only relevant across a single
        hop, we can estimate the probability that a randomly selected
        HID will conflict with the HID of an established stream.
        Consider a stream in which the hop from an ST agent to ten
        next-hop agents is through the multicast facility of a given
        network.  Assume also that each of the next-hop agents
        participates in 1000 other streams, and that each has been
        created with a different HID.  A randomly selected 16-bit HID
        will have a probability of greater than 85.9% of succeeding
        on the first try, 98.1% of succeeding on the second, and
        99.8% of succeeding on the third.  We therefore suggest that
        a 16-bit HID space is sufficiently large to support ST until
        better multicast HID selection procedures, e.g., HID servers,
        can be deployed.

        An obvious way to select the HID is for the ST agents to use
        a random number generator as suggested above.  An alternate
        mechanism is for the intermediate agents to use the HID
        contained in the incoming CONNECT message for all the
        outgoing CONNECT messages, and generate a random number only
        as a second choice.  In this case, the origin ST agent would

         Agent 3                      Agent B

     1.     +-> CONNECT B -------------->+
                <RVLId=0><SVLId=32>      |
                <Ref=315><HID=5990>      V
     2.             (Check HID Table, 5990 busy, 6000-11 unused)
                                         V
     3.     +<- HID-REJECT --------------+
            |   <RVLId=32><SVLId=45>
            |   <Ref=315><HID=5990>
            V   <FreeHIDs=5990:0000FFF0>
     4.     +-> HID-CHANGE  ------------>+
                <RVLId=45><SVLId=32>     |
                <Ref=320><HID=6000>      V
     5.             (Check HID Table, 6000 (still) available)
                                         V
     6.     +<- HID-APPROVE -------------+
                <RVLId=32><SVLId=45>
                <Ref=320><HID=6000>

     7.     (Both parties have now agreed to use HID 6000)

        Figure 18.  Typical HID Negotiation (No Multicasting)

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        be responsible for generating the HID, and the same HID could
        be propagated for the entire stream.  This approach has the
        marginal advantage that the HID could be created by a higher
        layer protocol that might have global knowledge and could
        select small, globally unique HIDs for all the streams.  While
        this is possible, we leave it for further study.

      Agent 2                           Agent C        Agent D

  1.    +->+-> CONNECT ---------------------------------->+
           |   <RVLId=0><SVLId=26>                        |
           |   <Ref=250><HID=4824>                        |
           V   <Mcast=224.1.18.216,01:00:5E:01:12:d8>     |
  2.       +-> CONNECT --------------------+              |
               <RVLId=0><SVLId=25>         |              |
               <Ref=252><HID=4824>         |              V
  3.           <Mcast=224.1.18.216,        V      (Check HID Table)
  4.            01:00:5E:01:12:d8> (Check HID Table)  (4824 ok)
                                       (4824 busy)  (4800-4809 ok)
                                     (4800-4820 ok)       |
                                           V              |
  5.       +<- HID-REJECT -----------------+              |
           |   <RVLId=25><SVLId=54>                       |
           |   <Ref=252><HID=4824>                        |
           V   <FreeHIDs=4824:FFFFF800>                   V
  6.    +<-+<- HID-APPROVE -------------------------------+
        |      <RVLId=26><SVLId=64>
        |      <Ref=250><HID=4824>
        V      <FreeHIDs=4824:FFC00080>
        (find common HID 4800)
        V
  7.    +->+-> HID-CHANGE ------------------------------->+
           |   <RVLId=64><SVLId=26>                       |
           V   <Ref=253><HID=4800>                        |
  8.       +-> HID-CHANGE ---------------->+              |
               <RVLId=54><SVLId=25>        |              V
  9.           <Ref=254><HID=4800>         V      (Check HID Table)
  10.                              (Check HID Table)   (4800 ok)
                                     (4800-4820 ok) (4800-4809 ok)
                                           V              |
  11.      +<- HID-APPROVE ----------------+              |
           |   <RVLId=25><SVLId=54>                       |
           |   <Ref=254><HID=4800>                        |
           V   <FreeHIDs=4800:7FFFF800>                   V
  12.   +<-+<- HID-APPROVE -------------------------------+
        |      <RVLId=26><SVLId=64>
        |      <Ref=253><HID=4800>
        V      <FreeHIDs=4800:7FC00080>
  13.   (all parties have now agreed to use HID 4800)

                Figure 19.  Multicast HID Negotiation

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     Agent 2                  Agent C        Agent D     Agent 3

 1.   +----> CONNECT B ------------------------------------>+
             <RVLId=0><SVLId=24>                            V
 2.          <Ref=260><HID=4800>                    (Check HID Table)
             <Mcast=224.1.18.216,             (4800 busy, 4801-4810 ok)
              01:00:5E:01:12:d8>                            V
 3.   +<---- HID-REJECT <-----------------------------------+
      |      <RVLId=24><SVLId=33>
      |      <Ref=260><HID=4824>
      V      <FreeHIDs=4824:7FE00000>
 4.   (find common HID 4810)
      V
 5.   +->+-> HID-CHANGE ----------------------------------->+
         |   <RVLId=33><SVLId=24>                           |
         V   <Ref=262><HID=4810>                            |
 6.      +-> HID-CHANGE-ADD ------------------->+           |
         |   <RVLId=64><SVLId=26>               |           V
 7.      V   <Ref=263><HID=4810>                |   (Check HID Table)
 8.      +-> HID-CHANGE-ADD ---->+              |     (4801-4815 ok)
             <RVLId=54><SVLId=25>|              V           |
 9.          <Ref=265><HID=4810> V      (Check HID Table)   |
 10.                     (Check HID Table) (4810 busy)      |
                           (4801-4812 ok) (4801-4807 ok)    |
                                 V              |           |
 11.     +<- HID-APPROVE <-------+              |           |
         |   <RVLId=25><SVLId=54>               |           |
         |   <Ref=265><HID=4810>                |           |
         V   <FreeHIDs=4810:7FD8000>            V           |
 12.     +<- HID-REJECT <-----------------------+           |
         |   <RVLId=26><SVLId=64>                           |
         |   <Ref=263><HID=4810>                            |
         V   <FreeHIDs=4810:7F000000>                       V
 13.  +<-+<- HID-APPROVE <----------------------------------+
      |      <RVLId=24><SVLId=33>
      |      <Ref=262><HID=4810>
      V      <FreeHIDs=4810:7FDF0000>
 14.  +->+-> HID-CHANGE-DELETE ---------------------------->+
      |  |   <RVLId=33><SVLId=24>                           |
      |  V   <Ref=266><HID=4810>                            |
 15.  |  +-> HID-CHANGE-DELETE ->+                          |
      |      <RVLId=54><SVLId=25>|                          |
      |      <Ref=268><HID=4810> V                          |
 16.  |  +<- HID-APPROVE --------+                          |
      |      <RVLId=25><SVLId=54>                           |
      |      <Ref=268><HID=0>                               V
 17.  |  +<- HID-APPROVE -----------------------------------+
      |      <RVLId=24><SVLId=33>
      V      <Ref=266><HID=0>
 18.  (find common HID 4801)

               Figure 20.  Multicast HID Re-Negotiation (part 1)

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     Agent 2                  Agent C        Agent D     Agent 3

 18.  (find common HID 4801)
      V
 19.  +->+-> HID-CHANGE ----------------------------------->+
         |   <RVLId=33><SVLId=24>                           |
         V   <Ref=270><HID=4801>                            |
 20.     +-> HID-CHANGE-ADD ------------------->+           |
         |   <RVLId=64><SVLId=26>               |           V
 21.     V   <Ref=273><HID=4801>                |   (Check HID Table)
 22.     +-> HID-CHANGE-ADD ---->+              |     (4801-4815 ok)
             <RVLId=54><SVLId=25>|              V           |
 23.         <Ref=274><HID=4801> V      (Check HID Table)   |
 24.                     (Check HID Table)(4801-4807 ok)    |
                           (4801-4812 ok)       |           |
                                 V              |           |
 25.     +<- HID-APPROVE <-------+              |           |
         |   <RVLId=25><SVLId=54>               |           |
         |   <Ref=274><HID=4801>                |           |
         V   <FreeHIDs=4801:3FF80000>           V           |
 26.     +<- HID-APPROVE <----------------------+           |
         |   <RVLId=26><SVLId=64>                           |
         |   <Ref=273><HID=4801>                            |
         V   <FreeHIDs=4801:3F000000>                       V
 27.  +<-+<- HID-APPROVE <----------------------------------+
      |      <RVLId=24><SVLId=33>
      |      <Ref=270><HID=4801>
      V      <FreeHIDs=4801:3FFF0000>
 28.  (switch data stream to HID 4801, drop 4800)
      V
 29.  +->+-> HID-CHANGE-DELETE ---------------->+
         |   <RVLId=64><SVLId=26>               |
         V   <Ref=275><HID=4800>                |
 30.     +-> HID-CHANGE-DELETE ->+              |
             <RVLId=54><SVLId=25>|              |
             <Ref=277><HID=4800> V              |
 31.  +<-+<- HID-APPROVE --------+              |
      |      <RVLId=25><SVLId=54>               |
      V      <Ref=277><HID=0>                   V
 32.  +<-+<- HID-APPROVE -----------------------+
      |      <RVLId=26><SVLId=64>
      V      <Ref=275><HID=0>
      (all parties have now agreed to use HID 4801)

               Figure 20.  Multicast HID Re-Negotiation (part 2)

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        3.7.4.1.         Subset

           The above mechanism can operate exactly as described even if
           the ST agents do not all use the entire 16 bits of the HID.
           A low capacity ST agent that cannot support a large number
           of simultaneous streams may use only some of the bits in the
           HID, say for example the low order byte.  This may allow
           this disadvantaged agent to use smaller internal data
           structures at the expense of causing HID collisions to occur
           more often.  However, neither the disadvantaged agent's
           previous-hop nor its next-hops need be aware of its
           limitations.  In the HID negotiation, the negotiators still
           exchange a 16-bit quantity.

     3.7.5.        IP Encapsulation of ST

        ST packets may be encapsulated in IP to allow them to pass
        through routers that don't support the ST Protocol.  Of course,
        ST resource management is precluded over such a path, and
        packet overhead is increased by encapsulation, but if the
        performance is reasonably predictable this may be better than
        not communicating at all.  IP encapsulation may also be
        required either for enhanced security (see Section 3.7.8 (page
        67)) or for user-space implementations of ST in hosts that
        don't allow demultiplexing on the IP Version Number field (see
        Section 4 (page 75)), but do allow access to raw IP packets.

        IP-encapsulated ST packets begin with a normal IP header.  Most
        fields of the IP header should be filled in according to the
        same rules that apply to any other IP packet.  Three fields of
        special interest are:

         o  Protocol is 5 to indicate an ST packet is enclosed, as
            opposed to TCP or UDP, for example.  The assignment of
            protocol 5 to ST is an arranged coincidence with the
            assignment of IP Version 5 to ST [18].

         o  Destination Address is that of the next-hop ST agent.
            This may or may not be the target of the ST stream.
            There may be an intermediate ST agent to which the
            packet should be routed to take advantage of service
            guarantees on the path past that agent.  Such an
            intermediate agent would not be on a directly-connected
            network (or else IP encapsulation wouldn't be needed),
            so it would probably not be listed in the normal routing
            table.  Additional routing mechanisms, not defined here,
            will be required to learn about such agents.

         o  Type-of-Service may be set to an appropriate value for
            the service being requested (usually low delay, high

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        throughput, normal reliability).  This feature is not
        implemented uniformly in the Internet, so its use can't be
        precisely defined here.

        Since there can be no guarantees made about performance across
        a normal IP network, the ST agent that will encapsulate should
        modify the Desired FlowSpec parameters when the stream is being
        established to indicate that performance is not guaranteed.  In
        particular, Reliability should be set to the minimum value
        (1/256), and suitably large values should be added to the
        Accumulated Mean Delay and Accumulated Delay Variance to
        reflect the possibility that packets may be delayed up to the
        point of discard when there is network congestion.  A suitably
        large value is 255 seconds, the maximum packet lifetime as
        defined by the IP Time-to-Live field.

        IP encapsulation adds little difficulty for the ST agent that
        receives the packet.  The IP header is simply removed, then the
        ST header is processed as usual.

        The more difficult part is during setup, when the ST agent must
        decide whether or not to encapsulate.  If the next-hop ST agent
        is on a remote network and the route to that network is through
        a router that supports IP but not ST, then encapsulation is
        required.  As mentioned in Section 3.8.1 (page 69), routing
        table entries must be expanded to indicate whether the router
        supports ST.

        On forwarding, the (mostly constant) IP Header must be inserted
        and the IP checksum appropriately updated.

        On a directly connected network, though, one might want to
        encapsulate only when sending to a particular destination host
        that does not allow demultiplexing on the IP Version Number
        field.  This requires the routing table to include host-route
        as well as network-route entries.  Host-route entries might
        require static definition if the hosts do not participate in
        the routing protocols.  If packet size is not a critical
        performance factor, one solution is always to encapsulate on
        the directly connected network whenever some hosts require
        encapsulation.  Those that don't require the encapsulation
        should be able to remove it upon reception.

        3.7.5.1.         IP Multicasting

           If an ST agent must use IP encapsulation to reach multiple
           next-hops toward different targets, then either the packet
           must be replicated for transmission to each next-hop, or IP
           multicasting [6] may be used if it is implemented in the
           next-hop ST agents and in the intervening IP routers.

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           This is analogous to using network-level service to
           multicast to several next-hop agents on a directly connected
           network.

           When the stream is established, the collection of next-hop
           ST agents must be set up as an IP multicast group.  It may
           be necessary for the ST agent that wishes to send the IP
           multicast to allocate a transient multicast group address
           and then tell the next-hop agents to join the group.  Use of
           the MulticastAddress parameter (see Section 4.2.2.7 (page
           86)) provides one way that the information may be
           communicated, but other techniques are possible.  The
           multicast group address in inserted in the Destination
           Address field of the IP encapsulation when data packets are
           transmitted.

           A block of transient IP multicast addresses, 224.1.0.0 -
           224.1.255.255, has been allocated for this purpose.  There
           are 2^16 addresses in this block, allowing a direct mapping
           with 16-bit HIDs, if appropriate.  The mechanisms for
           allocating these addresses are not defined here.

           In addition, two permanent IP multicast addresses have been
           assigned to facilitate experimentation with exchange of
           routing or other information among ST agents.  Those
           addresses are:

              224.0.0.7    All ST routers
              224.0.0.8    All ST hosts

           An ST router is an ST agent that can pass traffic between
           attached networks;  an ST host is an ST agent that is
           connected to a single network or is not permitted to pass
           traffic between attached networks.  Note that the range of
           these multicasts is normally just the attached local
           network, limited by setting the IP time-to-live field to 1
           (see [6]).

     3.7.6.        Retransmission

        The ST Control Message Protocol is made reliable through use of
        retransmission when an expected acknowledgment is not received
        in a timely manner.  The problem of when to send a
        retransmission has been studied for protocols such as TCP [2]
        [10] [11].  The problem should be simpler for ST since control
        messages usually only have to travel a single hop and they do
        not contain very much data.  However, the algorithms developed
        for TCP are sufficiently simple that their use is recommended
        for ST as well;  see [2].  An implementor might, for example,
        choose to keep statistics separately for each

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        neighboring ST agent, or combined into a single statistic for
        an attached network.

        Estimating the packet round-trip time (RTT) is a key function
        in reliable transport protocols such as TCP.  Estimation must
        be dynamic, since congestion and resource contention result in
        varying delays.  If RTT estimates are too low, packets will be
        retransmitted too frequently, wasting network capacity.  If RTT
        estimates are too high, retransmissions will be delayed
        reducing network throughput when transmission errors occur.
        Article [11] identifies problems that arise when RTT estimates
        are poor, outlines how RTT is used and how retransmission
        timeouts (RTO) are estimated, and surveys several ways that RTT
        and RTO estimates can be improved.

        Note the HELLO/ACK mechanism described in Section 3.7.1.2 (page
        49) can give an estimate of the RTT and its variance.  These
        estimates are also important for use with the delay and delay
        variance entries in the FlowSpec.

     3.7.7.        Routing

        ST requires access to routing information in order to select a
        path from an origin to the destination(s).  However, routing is
        considered to be a separate issue and neither the routing
        algorithm nor its implementation is specified here.  ST should
        operate equally well with any reasonable routing algorithm.

        While ST may be capable of using several types of information
        that are not currently available, the minimal information
        required is that provided by IP, namely the ability to find an
        interface and next hop router for a specified IP destination
        address and Type of Service.  Methods to make more information
        available and to use it are left for further study.  For
        initial ST implementations, any routing information that is
        required but not automatically provided will be assumed to be
        manually configured into the ST agents.

     3.7.8.        Security

        The ST Protocol by itself does not provide security services.
        It is more vulnerable to misdelivery and denial of service than
        IP since the ST Header only carries a 16-bit HID for
        identification purposes.  Any information, such as source and
        destination addresses, which a higher-layer protocol might use
        to detect misdelivery are the responsibility of either the
        application or higher-layer protocol.

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        ST is less prone to traffic analysis than IP since the only
        identifying information contained in the ST Header is a hop-
        by-hop identifier (HID).  However, the use of a HID is also
        what makes ST more vulnerable to denial of service since an ST
        agent has no reliable way to detect when bogus traffic is
        injected into, and thus consumes bandwidth from, a user's
        stream.  Detection can be enhanced through use of per-interface
        forwarding tables and verification of local network source and
        destination addresses.

        We envision that applications that require security services
        will use facilities, such as the Secure Digital Networking
        System (SDNS) layer 3 Security Protocol (SP3/D) [19] [20].  In
        such an environment, ST PDUs would first be encapsulated in an
        IP Header, using IP Protocol 5 (ST) as described in Section
        3.7.5 (page 64).  These IP datagrams would then be secured
        using SP3/D, which results in another IP Protocol 5 PDU that
        can be passed between ST agents.

        This memo does not specify how an application invokes security
        services.

  3.8.       ST Service Interfaces

     ST has several interfaces to other modules in a communication
     system.  ST provides its services to applications or transport-
     level protocols through its "upper" interface (or SAP).  ST in
     turn uses the services provided by network layers, management
     functions (e.g., address translation and routing), and IP.  The
     interfaces to these modules are described in this section in the
     form of subroutine calls.  Note that this does not mean that an
     implementation must actually be implemented as subroutines, but is
     instead intended to identify the information to be passed between
     the modules.

     In this style of outlining the module interfaces, the information
     passed into a module is shown as arguments to the subroutine call.
     Return information and/or success/failure indications are listed
     after the arrow ("->") that follows the subroutine call.  In
     several cases, a list of values must either be passed to or
     returned from a module interface.  Examples include a set of
     target addresses, or the mappings from a target list to a set of
     next hop addresses that span the route to the originally listed
     targets.  When such a list is appropriate, the values repeated for
     each list element are bracketed and an asterisk is added to
     indicate that zero, one, or many list elements can be passed
     across the interface (e.g., "<target>*" means zero, one, or more
     targets).

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     3.8.1.        Access to Routing Information

        The design of routing functions that can support a variety of
        resource management algorithms is difficult.  In this section
        we suggest a set of preliminary interfaces suitable for use in
        initial experiments.  We expect that these interfaces will
        change as we gain more insight into how routing, resource
        allocation, and decision making elements are best divided.

        Routing functions are required to identify the set of potential
        routes to each destination site.  The routing functions should
        make some effort to identify routes that are currently
        available and that meet the resource requirements. However,
        these properties need not be confirmed until the actual
        resource allocation and connection setup propagation are
        performed.

        The minimum capability required of the interface to routing is
        to identify the network interface and next hop toward a given
        target.  We expect that the traditional routing table will need
        to be extended to include information that ST requires such as
        whether or not a next hop supports ST, and, if so, whether or
        not IP encapsulation (see Section 3.7.5 (page 64)) is required
        to communicate with it.  In particular, host entries will be
        required for hosts that can only support ST through
        encapsulation because the IP software either is not capable of
        demultiplexing datagrams based on the IP Version Number field,
        or the application interface only supports access to raw IP
        datagrams.  This interface is illustrated by the function:

           FindNextHop( destination, TOS )
              -> result, < interface, next hop, ST-capable,
                 MustEncapsulate >*

        However, the resource management functions can best tradeoff
        among alternative routes when presented with a matrix of all
        potential routes.  The matrix entry corresponding to a
        destination and a next hop would contain the estimated
        characteristics of the corresponding pathway.  Using this
        representation, the resource management functions can quickly
        determine the next hop sets that cover the entire destination
        list, and compare the various parameters of the tradeoff
        between the guarantees that can be promised by each set.  An
        interface that returns a compressed matrix, listing the
        suitable routes by next hop and the destinations reachable
        through each, is illustrated by the function:

           FindNextHops( < destination >*, TOS )
              -> result, < destination, < interface, next hop,
                 ST-capable, MustEncapsulate >* >*

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        We hope that routing protocols will be available that propagate
        additional metrics of bandwidth, delay, bit/burst error rate,
        and whether a router has ST capability.  However, propagating
        this information in a timely fashion is still a key research
        issue.

     3.8.2.        Access to Network Layer Resource Reservation

        The resources required to reach the next-hops associated with
        the chosen routes must be allocated.  These allocations will
        generally be requested and released incrementally.  As the
        next-hop elements for the routes are chosen, the network
        resources between the current node and the next-hops must be
        allocated.  Since the resources are not guaranteed to be
        available -- a network or node further down the path might have
        failed or needed resources might have been allocated since the
        routing decisions where made -- some of these allocations may
        have to be released, another route selected, and a new
        allocation requested.

        There are four basic interface functions needed for the network
        resource allocator.  The first checks to see if the required
        resources are available, returning the likelihood that an
        ensuing resource allocation will succeed.  A probability of 0%
        indicates the resources are not available or cannot promise to
        meet the required guarantees.  Low probabilities indicate that
        most of the resource has been allocated or that there is a lot
        of contention for using the resource.  This call does not
        actually reserve the resources:

           ResourceProbe( requirements )
              -> likelihood

        Another call reserves the resources:

           ResourceReserve( requirements )
              -> result, reservation_id

        The third call adjusts the resource guarantees:

           ResourceAdjust( reservation_id, new requirements )
              -> result

        The final call allows the resources to be released:

           ResourceRelease( reservation_id )
              -> result

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     3.8.3.        Network Layer Services Utilized

        ST requires access to the usual network layer functions to send
        and receive packets and to be informed of network status
        information.  In addition, it requires functions to enable and
        disable reception of multicast packets.  Such functions might
        be defined as:

           JoinLocalGroup( network level group-address )
              -> result, multicast_id

           LeaveLocalGroup( network level group-address )
              -> result

           RecvNet( SAP )
              -> result, src, dst, len, BufPTR )

           SendNet( src, dst, SAP, len, BufPTR )
              -> result

           GetNotification( SAP )
              -> result, infop

     3.8.4.        IP Services Utilized

        Since ST packets might be sent or received using IP
        encapsulation, IP level routines to join and leave multicast
        groups are required in addition to the usual services defined
        in the IP specification (see the IP specification [2] [15] and
        the IP multicast specification [6] for details).

           JoinHostGroup( IP level group-address, interface )
              -> result, multicast_id

           LeaveHostGroup( IP level group-address, interface )
              -> result

           GET_SRCADDR( remote IP addr, TOS )
              -> local IP address

           SEND( src, dst, prot, TOS, TTL, BufPTR, len, Id, DF,
                 opt )
              -> result

           RECV( BufPTR, prot )
              -> result, src, dst, SpecDest, TOS, len, opt

           GET_MAXSIZES( local, remote, TOS )
              -> MMS_R, MMS_S

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           ADVISE_DELIVPROB( problem, local, remote, TOS )
              -> result

           SEND_ICMP( src, dst, TOS, TTL, BufPTR, len, Id, DF, opt )
              -> result

           RECV_ICMP( BufPTR )
              -> result, src, dst, len, opt

     3.8.5.        ST Layer Services Provided

        Interface to the ST layer services may be modeled using a set
        of subroutine calls (but need not be implemented as such).
        When the protocol is implemented as part of an operating
        system, these subroutines may be used directly by a higher
        level protocol processing layer.

        These subroutines might also be provided through system service
        calls to provide a raw interface for use by an application.
        Often, this will require further adaptation to conform with the
        idiom of the particular operating system.  For example, 4.3 BSD
        UNIX (TM) provides sockets, ioctls and signals for network
        programming.

        open( connect/listen, SAPBytes, local SAP, local host,
              account, authentication info, < foreign host,
              SAPBytes, foreign SAP, options >*, flow spec,
              precedence, group name, optional parameters )
            -> result, id, stream name, < foreign host,
              foreign SAPBytes, foreign SAP, result, flow spec,
              rname, optional parameters >*

        Note that an open by a target in "listen mode" may cause ST to
        create a state block for the stream to facilitate rendezvous.

        add( id, SAPBytes, local SAP, local host, < foreign host,
             SAPBytes, foreign SAP, options >*, flow spec,
             precedence, group name, optional parameters )
           -> result, < foreign host, foreign SAPBytes,
              foreign SAP, result,
              flow spec, rname, optional parameters >*

        send( id, buffer address, byte count, priority )
           -> result, next send time, burst send time

        recv( id, buffer address, max byte count )
           -> result, byte count

        recvsignal( id )
           -> result, signal, info

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        receivecontrol( id )
           -> result, id, stream name, < foreign host,
              foreign SAPBytes, foreign SAP, result, flow spec,
              rname, optional parameters >*

        sendcontrol( id, flow spec, precedence, options,
              < foreign host, SAPBytes, foreign SAP, options >*)
           -> result, < foreign host, foreign SAPBytes,
              foreign SAP, result, flow spec, rname,
              optional parameters >*

        change( id, flow spec, precedence, options,
              < foreign host, SAPBytes, foreign SAP, options >*)
           -> result, < foreign host, foreign SAPBytes,
              foreign SAP, result, flow spec, rname,
              optional parameters >*

        close( id, < foreign host, SAPBytes, foreign SAP >*,
              optional parameters )
           -> result

        status( id/stream name/group name )
           -> result, account, group name, protocol,
              < stream name, < foreign host, SAPbytes,
              foreign SAP, state, options, flow spec,
              routing info, rname >*, precedence, options >*

        creategroup( members* )
           -> result, group name

        deletegroup( group name, members* )
           -> result

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4. ST Protocol Data Unit Descriptions

  The ST PDUs sent between ST agents consist of an ST Header
  ncapsulating either a higher layer PDU or an ST Control Message.
  Since ST operates as an extension of IP, the packet arrives at the
  same network service access point that IP uses to receive IP
  datagrams, e.g., ST would use the same ethertype (0x800) as does IP.
  The two types of packets are distinguished by the IP Version Number
  field (the first four bits of the packet);  IP currently uses a value
  of 4, while ST has been assigned the value 5 [18].  There is no
  requirement for compatibility between IP and ST packet headers beyond
  the first four bits.

  The ST Header also includes an ST Version Number, a total length
  field, a header checksum, and a HID, as shown in Figure 21.  See
  Appendix 1 (page 147) for an explanation of the notation.

     ST is the IP Version Number assigned to identify ST packets.  The
     value for ST is 5.

     Ver is the ST Version Number.  This document defines ST Version 2.

     Pri is the priority of the packet.  It is used in data packets to
     indicate those packets to drop if a stream is exceeding its
     allocation.  Zero is the lowest priority and 7 the highest.

     T (bit 11) is used to indicate that a Timestamp is present
     following the ST Header but before any next higher layer protocol
     data.  The Timestamp is not permitted on ST Control Messages
     (which may use the OriginTimestamp option).

     Bits 12 through 15 are spares and should be set to 0.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  ST=5 | Ver=2 | Pri |T| Bits  |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |              HID              |        HeaderChecksum         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-                          Timestamp                          -+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 21.  ST Header

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     TotalBytes is the length, in bytes, of the entire ST packet, it
     includes the ST Header and optional Timestamp but does not include
     any local network headers or trailers.  In general, all length
     fields in the ST Protocol are in units of bytes.

     HID is the 16-bit hop-by-hop stream identifier.  It is an
     abbreviation for the Name of the stream and is used both to reduce
     the packet header length and, by the receiver of the data packet,
     to make the forwarding function more efficient.  Control Messages
     have a HID value of zero.  HIDs are negotiated by the next-hop and
     previous-hop agents to make the abbreviation unique.  It is used
     here in the ST Header and in various Control Messages.  HID values
     1-3 are reserved for future use.

     HeaderChecksum covers only the ST Header and Timestamp, if
     present.  The ST Protocol uses 16-bit checksums here in the ST
     Header and in each Control Message.  The standard Internet
     checksum algorithm is used:  "The checksum field is the 16-bit
     one's complement of the one's complement sum of all 16-bit words
     in the header.  For purposes of computing the checksum, the value
     of the checksum field is zero."  See [1] [12] [15] for suggestions
     for efficient checksum algorithms.

     Timestamp is an optional timestamp inserted into data packets by
     the origin.  It is only present when the T bit, described above,
     is set (1).  Its use is negotiated at connection setup time;  see
     Sections 4.2.3.5 (page 108) and 4.2.3.1 (page 100).  The Timestamp
     has the NTP format;  see [13].

  4.1.       Data Packets

     ST packets whose HID is not zero to three are user data packets.
     Their interpretation is a matter for the higher layer protocols
     and consequently is not specified here.  The data packets are not
     protected by an ST checksum and will be delivered to the higher
     layer protocol even with errors.

     ST agents will not pass data packets over a new hop whose setup is
     not complete, i.e., a HID must have been negotiated and either an
     ACCEPT or REFUSE has been received for all targets specified in
     the CONNECT.

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  4.2.       ST Control Message Protocol Descriptions

     ST Control Messages are between a previous-hop agent and its
     next-hop agent(s) using a HID of zero.  The control protocol
     follows a request-response model with all requests expecting
     responses.  Retransmission after timeout (see Section 3.7.6 (page
     66)) is used to allow for lost or ignored messages.  Control
     messages do not extend across packet boundaries; if a control
     message is too large for the MTU of a hop, its information
     (usually a TargetList) is partitioned and a control message per
     partition is sent.  All control messages have the following
     format:

        OpCode identifies the type of control message.  Each is
        described in detail in following sections.

        Options is used to convey OpCode-specific variations for a
        control message.

        TotalBytes is the length of the control message, in bytes,
        including all OpCode specific fields and optional parameters.
        The value is always divisible by four.

        RVLId is used to convey the Virtual Link Identifier of the
        receiver of the control message, when known, or zero in the
        case of an initial CONNECT or diagnostic message.  The RVLId is
        intended to permit efficient dispatch to the portion of a
        stream's state machine containing information about a specific
        operation in progress over the link.  RVLId values 1-3 are
        reserved; see Sections 3 (page 17) and 3.7.1.2 (page 49).

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     OpCode    |    Options    |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |                               :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                             -+
  :                      OpCode Specific Data                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 22.  ST Control Message Format

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        SVLId is used to convey the Virtual Link Identifier of the
        sender of the control message.  Except for ERROR-IN-REQUEST and
        diagnostic messages, it must never be zero.  SVLId values 1-3
        are reserved; see Sections 3 (page 17) and 3.7.1.2 (page 49).

        Reference is a transaction number.  Each sender of a request
        control message assigns a Reference number to the message that
        is unique with respect to the stream.  The Reference number is
        used by the receiver to detect and discard duplicates.  Each
        acknowledgment carries the Reference number of the request
        being acknowledged.  Reference zero is never used, and
        Reference numbers are assumed to be monotonically increasing
        with wraparound so that the older-than and more-recent-than
        relations are well defined.

        LnkReference contains the Reference field of the request
        control message that caused this request control message to be
        created.  It is used in situations where a single request leads
        to multiple "responses".  Examples are CONNECT and CHANGE
        messages that must be acknowledged hop-by-hop and will also
        lead to an ACCEPT or REFUSE from each target in the TargetList.

        SenderIPAddress is the 32-bit IP address of the network
        interface that the ST agent used to send the control message.
        This value changes each time the packet is forwarded by an ST
        agent (hop-by-hop).

        Checksum is the checksum of the control message.  Because the
        control messages are sent in packets that may be delivered with
        bits in error, each control message must be checked before it
        is acted upon;  see Section 4 (page 76).

        OpCode Specific Data contains any additional information that
        is associated with the control message.  It depends on the
        specific control message and is explained further below.  In
        some response control messages, fields of zero are included to
        allow the format to match that of the corresponding request
        message.  The OpCode Specific Data may also contain any of the
        optional Parameters defined in Section 4.2.2 (page 80).

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     4.2.1.        ST Control Messages

        The CONNECT and CHANGE messages are used to establish or modify
        branches in the stream.  They propagate in the direction from
        the origin toward the targets.  They are end-to-end messages
        created by the origin.  They propagate all the way to the
        targets, and require ERROR-IN-REQUEST, ACK, HID-REJECT, HID-
        APPROVE, ACCEPT, or REFUSE messages in response.  The CONNECT
        message is the stream setup message.  The CHANGE message is
        used to change the characteristics of an established stream.
        The CONNECT message is also used to add one or more targets to
        an existing stream and during recovery of a broken stream.
        Both messages have a TargetList parameter and are processed
        similarly.

        The DISCONNECT message is used to tear down streams or parts of
        streams.  It propagates in the direction from the origin toward
        the targets.  It is either used as an end-to-end message
        generated by the origin that is used to completely tear down a
        stream, or is generated by an intermediate ST agent that
        preempts a stream or detects the failure of its previous-hop
        agent or network in the stream.  In the latter case, it is used
        to tear down the part of the stream from the failure to the
        targets, thus the message propagates all the way to the
        targets.

        The REFUSE message is sent by a target to refuse to join or
        remove itself from a stream;  in these cases, it is an end-to-
        end message.  An intermediate ST agent issues a REFUSE if it
        cannot find a route to a target, can only find a route to a
        target through the previous-hop, preempts a stream, or detects
        a failure in a next-hop ST agent or network.  In all cases a
        REFUSE propagates in the direction toward the origin.

        The ACCEPT message is an end-to-end message generated by a
        target and is used to signify the successful completion of the
        setup of a stream or part of a stream, or the change of the
        FlowSpec.  There are no other messages that are similar to it.

        The following sections contain descriptions of common fields
        and parameters, followed by descriptions of the individual
        control messages, both listed in alphabetical order.  A brief
        description of the use of the control message is given.  The
        packet format is shown graphically.

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     4.2.2.        Common SCMP Elements

        Several fields and parameters (referred to generically as
        "elements") are common to two or more PDUs.  They are described
        in detail here instead of repeating their description several
        times.  In many cases, the presence of a parameter is optional.
        To permit the parameters to be easily defined and parsed, each
        is identified with a PCode byte that is followed by a PBytes
        byte indicating the length of the parameter in bytes (including
        the PCode, PByte, and any padding bytes).  If the length of the
        information is not a multiple of 4 bytes, the parameter is
        padded with one to three zero (0) bytes.  PBytes is thus always
        a multiple of four.  Parameters can be present in any order.

        4.2.2.1.         DetectorIPAddress

           Several control messages contain the DetectorIPAddress
           field.  It is used to identify the agent that caused the
           first instance of the message to be generated, i.e., before
           it was propagated.  It is copied from the received message
           into the copy of the message that is to be propagated to a
           previous-hop or next-hop.  It use is primarily diagnostic.

        4.2.2.2.         ErroredPDU

           The ErroredPDU parameter (PCode = 1) is used for diagnostic
           purposes to encapsulate a received ST PDU that contained an
           error.  It may be included in the ERROR-IN-REQUEST, ERROR-
           IN-RESPONSE, or REFUSE messages.  It use is primarily
           diagnostic.

              PDUBytes indicates how many bytes of the PDUInError are
              actually present.

              ErrorOffset contains the number of bytes into the errored
              PDU to the field containing the error.  At least as much
              of the PDU in error must be included to

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 1   |     PBytes    |   PDUBytes    |  ErrorOffset  |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                          PDUInError           :    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 23.  ErroredPDU

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              include the field or parameter identified by ErrorOffset;
              an ErrorOffset of zero would imply a problem with the IP
              Version Number or ST Version Number fields.

              PDUInError is the PDU in error, beginning with the ST
              Header.

        4.2.2.3.         FlowSpec & RFlowSpec

           The FlowSpec is used to convey stream service requirements
           end-to-end.  We expect that other versions of FlowSpec will
           be needed in the future, which may or may not be subsets or
           supersets of the version described here.  PBytes will allow
           new constraints to be added to the end without having to
           simultaneously update all implementations in the field.
           Implementations are expected to be able to process in a
           graceful manner a Version 4 (or higher) structure that has
           more elements than shown here.

           The FlowSpec parameter (PCode = 2) is used in several
           messages to convey the FlowSpec.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     PCode     |     PBytes    |  Version = 3  |       0       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   DutyFactor  |   ErrorRate   |   Precedence  |  Reliability  |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Tradeoffs           |        RecoveryTimeout        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          LimitOnCost          |         LimitOnDelay          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        LimitOnPDUBytes        |        LimitOnPDURate         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         MinBytesXRate                         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         AccdMeanDelay                         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       AccdDelayVariance                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          DesPDUBytes          |          DesPDURate           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 24.  FlowSpec & RFlowSpec

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           The RFlowSpec parameter (PCode = 12) is used in conjunction
           with the FDx option to convey the FlowSpec that is to be
           used in the reverse direction.

              Version identifies the version of the FlowSpec.  Version
              3 is defined here.

              DutyFactor is the estimated proportion of the time that
              the requested bandwidth will actually be in use.  Zero is
              taken to represent 256 and signify a duty factor of 1.
              Other values are to be divided by 256 to yield the duty
              factor.

              ErrorRate expresses the error rate as the negative
              exponent of 10 in the error rate.  One (1) represents a
              bit error rate of 0.1 and 10 represents 0.0000000001.

              Precedence is the precedence of the connection being
              established.  Zero represents the lowest precedence.
              Note that non-zero values of this parameter should be
              subject to authentication and authorization checks, which
              are not specified here.  In general, the distinction
              between precedence and priority is that precedence
              specifies streams that are permitted to take previously
              committed resources from another stream, while priority
              identifies those PDUs that a stream is most willing to
              have dropped when the stream exceeds its guaranteed
              limits.

              Reliability is modified by each intervening ST agent as a
              measure of the probability that a given offered data
              packet will be forwarded and not dropped.  Zero is taken
              to represent 256 and signify a probability of 1.  Other
              values are to be divided by 256 to yield the probability.

              Tradeoffs is incompletely defined at this time.  Bits
              currently specified are as follows:

                 The most significant bit in the field, bit 0 in the
                 Figure 24, when one (1) means that each ST agent must
                 "implement" all constraints in the FlowSpec even if
                 they are not shown in the figure, e.g., when the
                 FlowSpec has been extended.  When zero (0), unknown
                 constraints may be ignored.

                 The second most significant bit in the field, bit 1,
                 when one (1) means that one or more constraints are
                 unknown and have been ignored.  When zero (0), all
                 constraints are known and have been processed.

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                 The third most significant bit in the field, bit 2, is
                 used for RevChrg;  see Section 3.6.5 (page 46).

                 Other bits are currently unspecified, and should be
                 set to zero (0) by the origin ST agent and not changed
                 by other agents unless those agents know their
                 meaning.

              RecoveryTimeout specifies the nominal number of
              milliseconds that the application is willing to wait for
              a failed system component to be detected and any
              corrective action to be taken.

              LimitOnCost specifies the maximum cost that the origin is
              willing to expend.  A value of zero indicates that the
              application is not willing to incur any direct charges
              for the resources used by the stream.  The meaning of
              non-zero values is left for further study.

              LimitOnDelay specifies the maximum end-to-end delay, in
              milliseconds, that can be tolerated by the origin.

              LimitOnPDUBytes is the smallest packet size, in terms of
              ST-user data bytes, that can be tolerated by the origin.

              LimitOnPDURate is the lowest packet rate that can be
              tolerated by the origin, expressed as tenths of a packet
              per second.

              MinBytesXRate is the minimum bandwidth that can be
              tolerated by the origin, expressed as a product of bytes
              and tenths of a packet per second.

              AccdMeanDelay is modified by each intervening ST agent.
              This provides a means of reporting the total expected
              delay, in milliseconds, for a data packet.  Note that it
              is implicitly assumed that the requested mean delay is
              zero and there is no limit on the mean delay, so there
              are no parameters to specify these explicitly.

              AccdDelayVariance is also modified by each intervening ST
              agent as a measure, in milliseconds squared, of the
              packet dispersion.  This quantity can be used by the
              target or origin in determining whether the resulting
              stream has an adequate quality of service to support the
              application.  Note that it is implicitly assumed that the
              requested delay variance is zero and there is no limit on
              the delay variance, so there are no parameters to specify
              these explicitly.

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              DesPDUBytes is the desired PDU size in bytes.  This is
              not necessarily the same as the minimum necessary PDU
              size.  This value may be made smaller by intervening ST
              agents so long as it is not made smaller than
              LimitOnPDUBytes.  The *PDUBytes limits measure the size
              of the PDUs of next-higher protocol layer, i.e., the user
              information contained in a data packet.  An ST agent must
              account for both the ST Header (including possible IP
              encapsulation) and any local network headers and trailers
              when comparing a network's MTU with *PDUBytes.  In an
              ACCEPT message, the value of this field will be no larger
              than the MTU of the path to the specified target.

              DesPDURate is the requested PDU rate, expressed as tenths
              of a packet per second.  This value may be made smaller
              by intervening ST agents so long as it is not made
              smaller than LimitOnPDURate.

              It is expected that the next parameter to be added to the
              FlowSpec will be a Burst Descriptor.  This parameter will
              describe the burstiness of the offered traffic.  For
              example, this may include the simple average rate, peak
              rate and variance values, or more complete descriptions
              that characterize the distribution of expected burst
              rates and their expected duration.  The nature of the
              algorithms that deal with the traffic's burstiness and
              the information that needs to be described by this
              parameter will be subjects of further experimentation.
              It is expected that a new FlowSpec with Version = 4 will
              be defined that looks like Version 3 but has a Burst
              Descriptor parameter appended to the end.

        4.2.2.4.         FreeHIDs

           The FreeHIDs parameter (PCode = 3) is used to communicate to
           the previous-hop suggestions for a HID.  It consists of
           BaseHID and FreeHIDBitMask fields.  Experiments will
           determine how long the mask should be for practical use of
           this parameter.  The parameter (if implemented) should be
           included in all HID-REJECTs, and in HID-APPROVEs that are
           linked to a multicast CONNECT, e.g., one containing the
           MulticastAddress parameter.

              BaseHID was the suggested value in a HID-CHANGE or
              CONNECT.  BaseHID is chosen to be the suggested HID value
              to insure that the masks from multiple FreeHIDs
              parameters will overlap.

              FreeHIDBitMask identifies available HID values as
              follows.  Bit 0 in the FreeHIDBitMask corresponds to a

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              HID with a value equal to BaseHID with the 5 least
              significant bits set to zero, bit 1 corresponds to that
              value + 1, etc.  This alignment of the mask on a 32-bit
              boundary is used so that masks from several FreeHIDs
              parameters might more easily be combined using a bit-wise
              AND function to find a free HID.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 3   |     4+4*N     |            BaseHID            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                        FreeHIDBitMask                         :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 25.  FreeHIDs

        4.2.2.5.         Group & RGroup

           The Group parameter (PCode = 4) is an optional argument
           used only for the creation of a stream.  This parameter
           contains a GroupName; the GroupName may be the same as the
           Name of one of the group's streams.  In addition, there
           may be some number of <SubGroupId, Relation> tuples that
           describe the meaning of the grouping and the relation
           between the members of the group.  The forms of grouping
           are for further study.

           The RGroup parameter (PCode = 13) is an optional argument
           used only for the creation of a stream in the reverse
           direction that is a member of a Group;  see the FDx
           option, Section 3.6.3 (page 45).  This parameter has the
           same format as the Group parameter.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     PCode     |    12+4*N     |                               !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                             -+
  !                           GroupName                           !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           SubGroupId          |            Relation           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :              ...              :              ...              :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           SubGroupId          |            Relation           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 26.  Group & RGroup

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RFC 1190 Internet Stream Protocol October 1990

           A GroupName has the same format as a Name;  see Figure 29.

        4.2.2.6.         HID & RHID

           The HID parameter (PCode = 5) is used in the NOTIFY message
           when the notification is related to a HID, and possibly in
           the STATUS-RESPONSE message to convey additional HIDs that
           are valid for a stream when there are more than one.  It
           consists of the PCode and PBytes bytes prepended to a HID;
           HIDs were described in Section 4 (page 76).

           The RHID parameter (PCode = 14) is used in conjunction with
           the FDx option to convey the HID that is to be used in the
           reverse direction.  It consists of the PCode and PBytes
           bytes prepended to a HID.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     PCode     |       4       |              HID              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 27.  HID & RHID

        4.2.2.7.         MulticastAddress

           The MulticastAddress parameter (PCode = 6) is an optional
           parameter that is used, when setting up a network level
           multicast group, to communicate an IP and/or local network
           multicast address to the next-hop agents that should become
           members of the group.

              LocalNetBytes is the length of the Local Net Multicast
              Address.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 6   |    PBytes     | LocalNetBytes |       0       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     IP Multicast Address                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                  Local Net Multicast Address  :    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 28.  MulticastAddress

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RFC 1190 Internet Stream Protocol October 1990

              IP Multicast Address is described in [6].  This field is
              zero (0) if no IP multicast address is known or is
              applicable.  The block of addresses 224.1.0.0 -
              224.1.255.255 has been allocated for use by ST.

              Local Net Multicast Address is the multicast address to
              be used on the local network.  It corresponds to the IP
              Multicast Address when the latter is non-zero.

        4.2.2.8.         Name & RName

           Each stream is uniquely (i.e., globally) identified by a
           Name.  A Name is created by the origin host ST agent and is
           composed of 1) a 16-bit number chosen to make the Name
           unique within the agent, 2) the IP address of the origin ST
           agent, and 3) a 32-bit timestamp.  If the origin has
           multiple IP addresses, then any that can be used to reach
           target may be used in the Name.  The intent is that the
           <Unique ID, IP Address> tuple be unique for the lifetime of
           the stream.  It is suggested that to increase robustness a
           Unique ID value not be reused for a period of time on the
           order of 5 minutes.

           The Timestamp is included both to make the Name unique over
           long intervals (e.g., forever) for purposes of network
           management and accounting/billing, and to protect against
           failure of an ST agent that causes knowledge of active
           Unique IDs to be lost.  The assumption is that all ST agents
           have access to some "clock".  If this is not the case, the
           agent should have access to some form of non-volatile memory
           in which it can store some number that at least gets
           incremented per restart.

           The Name parameter (PCode = 7) is used in most control
           messages to identify a stream.

           The RName parameter (PCode = 15) is used in conjunction with
           the FDx option to convey the Name of the reverse stream in
           an ACCEPT message.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     PCode     |       12      |            Unique ID          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          IP Address                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           Timestamp                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 29.  Name & RName

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RFC 1190 Internet Stream Protocol October 1990

        4.2.2.9.         NextHopIPAddress

           The NextHopIPAddress parameter (PCode = 8) is an optional
           parameter of NOTIFY (RouteBack) or REFUSE (RouteInconsist or
           RouteLoop) and contains the IP address of a suggested next-
           hop ST agent.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 8   |       8       |               0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       next-hop IP address                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 30.  NextHopIPAddress

        4.2.2.10.        Origin

           The Origin parameter (PCode = 9) is used to identify the
           origin of the stream, the next higher protocol, and the SAP
           being used in conjunction with that protocol.

              NextPcol is an 8-bit field used in demultiplexing
              operations to identify the protocol to be used above ST.
              The values of NextPcol are in the same number space as
              the IP Header's Protocol field and are consequently
              defined in the Assigned Numbers RFC [18].

              OriginSAPBytes specifies the length of the OriginSAP,
              exclusive of any padding required to maintain 32-bit
              alignment.

              OriginIPAddress is (one of) the IP address of the origin.

              OriginSAP identifies the origin's SAP associated with the
              NextPcol protocol.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 9   |    PBytes     |    NextPcol   |OriginSAPBytes |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         OriginIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                           OriginSAP           :    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 31.  Origin

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RFC 1190 Internet Stream Protocol October 1990

        4.2.2.11.        OriginTimestamp

           The OriginTimestamp parameter (PCode = 10) is used to
           indicate the time at which the control message was sent.

           The units and format of the timestamp is that defined in the
           NTP protocol specification [13].  Note that discontinuities
           over leap seconds are expected.

           Note that the time synchronization implied by the use of
           such a parameter is the subject of systems management
           functions not described in this memo, e.g., NTP.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 10  |      12       |               0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-                          Timestamp                          -+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 32.  OriginTimestamp

        4.2.2.12.        ReasonCode

           Several errors may occur during protocol processing.  All ST
           error codes are taken from a single number space.  The
           currently defined values and their meaning is presented in
           the list below.  Note that new error codes may be defined
           from time to time.  All implementations are expected to
           handle new codes in a graceful manner.  If an unknown
           ReasonCode is encountered, it should be assumed to be fatal.

                   0                   1
                   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  |          ReasonCode           |
                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 33.  ReasonCode

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RFC 1190 Internet Stream Protocol October 1990

                 Name       Value                 Meaning
           ---------------- ----- ---------------------------------------

           AcceptTimeout      2   An Accept has not been
                                  acknowledged.

           AccessDenied       3   Access denied.

           AckUnexpected      4   An unexpected ACK was received.

           ApplAbort          5   The application aborted the stream
                                  abnormally.

           ApplDisconnect     6   The application closed the stream
                                  normally.

           AuthentFailed      7   The authentication function
                                  failed.

           CantGetResrc       8   Unable to acquire (additional)
                                  resources.

           CantRelResrc       9   Unable to release excess
                                  resources.

           CksumBadCtl       10   A received control PDU has a bad
                                  message checksum.

           CksumBadST        11   A received PDU has a bad ST Header
                                  checksum.

           DropExcdDly       12   A received PDU was dropped because
                                  it could not be processed within
                                  the delay specification.

           DropExcdMTU       13   A received PDU was dropped because
                                  its size exceeds the MTU.

           DropFailAgt       14   A received PDU was dropped because
                                  of a failed ST agent.

           DropFailHst       15   A received PDU was dropped because
                                  of a host failure.

           DropFailIfc       16   A received PDU was dropped because
                                  of a broken interface.

           DropFailNet       17   A received PDU was dropped because
                                  of a network failure.

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RFC 1190 Internet Stream Protocol October 1990

                 Name       Value                 Meaning
           ---------------- ----- ---------------------------------------

           DropLimits        18   A received PDU was dropped because
                                  it exceeds the resource limits for
                                  its stream.

           DropNoResrc       19   A received PDU was dropped due to
                                  no available resources (including
                                  precedence).

           DropNoRoute       20   A received PDU was dropped because
                                  of no available route.

           DropPriLow        21   A received PDU was dropped because
                                  it has a priority too low to be
                                  processed.

           DuplicateIgn      22   A received control PDU is a
                                  duplicate and is being
                                  acknowledged.

           DuplicateTarget   23   A received control PDU contains a
                                  duplicate target, or an attempt to
                                  add an existing target.

           ErrorUnknown       1   An error not contained in this
                                  list has been detected.

           failure          N/A   An abbreviation used in the text
                                  for any of the more specific
                                  errors:  DropFailAgt, DropFailHst,
                                  DropFailIfc, DropFailNet,
                                  IntfcFailure, NetworkFailure,
                                  STAgentFailure, FailureRecovery.

           FailureRecovery   24   A notification that recovery is
                                  being attempted.

           FlowVerBad        25   A received control PDU has a
                                  FlowSpec Version Number that is
                                  not supported.

           GroupUnknown      26   A received control PDU contains an
                                  unknown Group Name.

           HIDNegFails       28   HID negotiation failed.

           HIDUnknown        29   A received control PDU contains an
                                  unknown HID.

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                 Name       Value                 Meaning
           ---------------- ----- ---------------------------------------

           InconsistHID      30   An inconsistency has been detected
                                  with a stream Name and
                                  corresponding HID.

           InconsistGroup    31   An inconsistency has been detected
                                  with the streams forming a group.

           IntfcFailure      32   A network interface failure has
                                  been detected.

           InvalidHID        33   A received ST PDU contains an
                                  invalid HID.

           InvalidSender     34   A received control PDU has an
                                  invalid SenderIPAddress field.

           InvalidTotByt     35   A received control PDU has an
                                  invalid TotalBytes field.

           LnkRefUnknown     36   A received control PDU contains an
                                  unknown LnkReference.

           NameUnknown       37   A received control PDU contains an
                                  unknown stream Name.

           NetworkFailure    38   A network failure has been
                                  detected.

           NoError            0   No error has occurred.

           NoRouteToAgent    39   Cannot find a route to an ST
                                  agent.

           NoRouteToDest     40   Cannot find a route to the
                                  destination.

           NoRouteToHost     41   Cannot find a route to a host.

           NoRouteToNet      42   Cannot find a route to a network.

           OpCodeUnknown     43   A received control PDU has an
                                  invalid OpCode field.

           PCodeUnknown      44   A received control PDU has a
                                  parameter with an invalid PCode.

           ParmValueBad      45   A received control PDU contains an
                                  invalid parameter value.

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                 Name       Value                 Meaning
           ---------------- ----- ---------------------------------------

           PcolIdUnknown     46   A received control PDU contains an
                                  unknown next-higher layer protocol
                                  identifier.

           ProtocolError     47   A protocol error was detected.

           PTPError          48   Multiple targets were specified
                                  for a stream created with the PTP
                                  option.

           RefUnknown        49   A received control PDU contains an
                                  unknown Reference.

           RestartLocal      50   The local ST agent has recently
                                  restarted.

           RemoteRestart     51   The remote ST agent has recently
                                  restarted.

           RetransTimeout    52   An acknowledgment to a control
                                  message has not been received
                                  after several retransmissions.

           RouteBack         53   The routing function indicates
                                  that the route to the next-hop is
                                  through the same interface as the
                                  previous-hop and is not the
                                  previous-hop.

           RouteInconsist    54   A routing inconsistency has been
                                  detected, e.g., a route loop.

           RouteLoop         55   A CONNECT was received that
                                  specified an existing target.

           SAPUnknown        56   A received control PDU contains an
                                  unknown next-higher layer SAP
                                  (port).

           STAgentFailure    57   An ST agent failure has been
                                  detected.

           StreamExists      58   A stream with the given Name or
                                  HID already exists.

           StreamPreempted   59   The stream has been preempted by
                                  one with a higher precedence.

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                 Name       Value                 Meaning
           ---------------- ----- ---------------------------------------

           STVerBad          60   A received PDU is not ST Version
                                  2.

           TooManyHIDs       61   Attempt to add more HIDs to a
                                  stream than the implementation
                                  supports.

           TruncatedCtl      62   A received control PDU is shorter
                                  than expected.

           TruncatedPDU      63   A received ST PDU is shorter than
                                  the ST Header indicates.

           UserDataSize      64   The UserData parameter is too
                                  large to permit a control message
                                  to fit into a network's MTU.

        4.2.2.13.        RecordRoute

           The RecordRoute parameter (PCode = 11) may be used to
           request that the route between the origin and a target be
           recorded and returned to the agent specified in the
           DetectorIPAddress field.

           FreeOffset is the offset to the position where the next
           next-hop IP address should be inserted.  It is initialized
           to four (4) and incremented by four each time an agent
           inserts its IP address.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 11  |     PBytes    |       0       |  FreeOffset   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       next-hop IP address                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                              ...                              :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       next-hop IP address                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 34.  RecordRoute

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RFC 1190 Internet Stream Protocol October 1990

        4.2.2.14.        SrcRoute

           The SrcRoute parameter is used, in the Target structure
           shown in Figure 36, to specify the IP addresses of the ST
           agents through which the stream to the target should pass.
           There are two forms of the option, distinguished by the
           PCode.

           With loose source route (PCode = 18) each ST agent first
           examines the first next-hop IP address in the option.  If
           the address is (one of) the address of the current ST agent,
           that entry is removed, and the PBytes field reduced by four
           (4).  If the resulting PBytes field contains 4 (i.e., there
           are no more next-hop IP addresses) the parameter is removed
           from the Target.  In either case, the Target's TargetBytes
           field and the TargetList's PBytes field must be reduced
           accordingly.  The ST agent then routes toward the first
           next-hop IP address in the option, if one exists, or toward
           the target otherwise.  Note that the target's IP address is
           not included as the last entry in the list.

           With a strict source route (PCode = 19) each ST agent first
           examines the first next-hop IP address in the option.  If
           the address is not (one of) the address of the current ST
           agent, a routing error has occurred and should be reported
           with the appropriate reason code.  Otherwise that entry is
           removed, and the PBytes field reduced by four (4).  If the
           resulting PBytes field contains 4 (i.e., there are no more
           next-hop IP addresses) the parameter is removed from the
           Target.  In either case, the Target's TargetBytes field and
           the TargetList's PBytes field must be reduced accordingly.
           The ST agent then routes toward the first next-hop IP
           address in the option, if one exists, or toward the target
           otherwise.  Note that the target's IP address is not
           included as the last entry in the list.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      PCode    |     4+4*N     |               0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      next-hop IP address                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                              ...                              :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      next-hop IP address                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 35.  SrcRoute

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RFC 1190 Internet Stream Protocol October 1990

           Since it is possible that a single hop between ST agents is
           actually composed of multiple IP hops using IP
           encapsulation, it might be necessary to also specify an IP
           source routing option.  Two additional PCodes are used in
           this case.  See [15] for a description of IP routing
           options.

           An IP Loose Source Route (PCode = 16) indicates that PDUs
           for the next-hop ST agent should be encapsulated in IP and
           that the IP datagram should contain an IP Loose Source Route
           constructed from the list of IP router addresses contained
           in this option.

           An IP Strict Source Route (PCode = 17) is similarly used
           when the corresponding IP Strict Source Route option should
           be constructed.

           Consequently, the "routing parameter" may consist of a
           sequence of one or more separate parameters with PCodes 16,
           17, 18, or 19.

        4.2.2.15.        Target and TargetList

           Several control messages use a parameter called TargetList
           (PCode = 20), which contains information about the targets
           to which the message pertains.  For each Target in the
           TargetList, the information includes the IP addresses of the
           target, the SAP applicable to the next higher layer
           protocol, the length of the SAP (SAPBytes), and zero or more
           optional SrcRoute parameters;  see Section 4.2.2.14 (page
           95).  Consequently, a Target structure can be of variable
           length.  Each entry has the format shown in Figure 36.

           The optional SrcRoute parameter is only meaningful in a
           CONNECT messages;  if present in other messages, they are
           ignored.  Note that the presence of SrcRoute parameter(s)
           reduces the number of Targets that can be contained in a
           TargetList since the maximum size of a TargetList is 256
           bytes.  Consequently an implementation should be prepared to
           accept multiple TargetLists in a single message.

              TargetIPAddress is the IP Address of the Target.

              TargetBytes is the length of the Target structure,
              beginning with the TargetIPAddress and including any
              SrcRoute Parameter(s).

              SAPBytes is the length of the SAP, excluding any padding
              required to maintain 32-bit alignment.  I.e.,

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RFC 1190 Internet Stream Protocol October 1990

              there would be no padding required for SAPs with lengths
              of 2, 6, etc., bytes.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        TargetIPAddress                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  TargetBytes  |   SAPBytes    |                               :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-             -+-+-+-+-+-+-+-+-+
  :                              SAP              :    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     SrcRoute Parameter(s)                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 36.  Target

           We assume that the ST agents must know the maximum packet
           size of the networks to which they are connected (the MTU),
           and those maximum sizes will restrict the number of targets
           that can be specified in control messages.  We feel that
           this is not a serious drawback.  High bandwidth networks
           such as the Ethernet or the Terrestrial Wideband network
           support packet sizes large enough to allow well over one
           hundred targets to be specified, and we feel that
           conferences with a larger number of participants will not
           occur for quite some time.  Furthermore, we expect that
           future higher bandwidth networks will allow even larger
           packet sizes.  It may be desirable to send ST voice data
           packets in individual B-ISDN ATM cells, which are small, but
           network services on ATM will provide "adaptation layers" to
           implement network-level fragmentation that may be used to
           carry larger ST control messages.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 20  |    PBytes     |        TargetCount = N        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                            Target 1                           :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                              ...                              :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                            Target N                           :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 37.  TargetList

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RFC 1190 Internet Stream Protocol October 1990

           If a message must pass across a network whose maximum packet
           size is too small, the message must be broken up into
           multiple messages, each of which carries part of the
           TargetList.  The function of the message can still be
           performed even if the message is so partitioned.  The effect
           in this partitioning is to compromise the performance, but
           still allows proper operation.  For example, if a CONNECT
           message were partitioned, the first CONNECT would establish
           the stream, and the rest of the CONNECTs would be processed
           as additions to the first.  The routing decisions might
           suffer, however, since they would be made on partial
           information.  Nevertheless, the stream would be created.

        4.2.2.16.        UserData

           The UserData parameter (PCode = 21) is an optional parameter
           that may be used by the next higher protocol or an
           application to convey arbitrary information to its peers.
           Note that since the size of control messages is limited by
           the smallest MTU in the path to the target(s), the maximum
           size of this parameter cannot be specified a priori.  If the
           parameter is too large for some network's MTU, a
           UserDataSize error will occur.  The parameter must be padded
           to a multiple of 32 bits.

              UserBytes specifies the number of valid UserInformation
              bytes.

              UserInformation is arbitrary data meaningful to the next
              higher protocol layer or application.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   PCode = 21  |    PBytes     |           UserBytes           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                        UserInformation        :    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 38.  UserData

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RFC 1190 Internet Stream Protocol October 1990

4.2.3. ST Control Message PDUs

        Each control message is described in a following section.  See
        Appendix 1 (page 147) for an explanation of the notation.

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RFC 1190 Internet Stream Protocol October 1990

        4.2.3.1.         ACCEPT

           ACCEPT (OpCode = 1) is issued by a target as a positive
           response to a CONNECT message.  It implies that the target
           is prepared to accept data from the origin along the stream
           that was established by the CONNECT.  The ACCEPT includes
           the FlowSpec that contains the cumulative information that
           was calculated by the intervening ST agents as the CONNECT
           made its way from the origin to the target, as well as any
           modifications made by the application at the target.  The
           ACCEPT is relayed by the ST agents from the target to the
           origin along the path established by the CONNECT but in the
           reverse direction.  The ACCEPT must be acknowledged with an
           ACK at each hop.

           The FlowSpec is not modified on this trip from the target
           back to the origin.  Since the cumulative FlowSpec
           information can be different for different targets, no
           attempt is made to combine the ACCEPTs from the various
           targets.  The TargetList included in each ACCEPT contains
           the IP address of only the target that issued the ACCEPT.

           Any SrcRoute parameters in the TargetList are ignored.

           Since an ACCEPT might be the first response from a next-hop
           on a control link (due to network reordering), the SVLId
           field may be the first source of the Virtual Link Identifier
           to be used in the RVLId field of subsequent control messages
           sent to that next-hop.

           When the FDx option has been selected to setup a second
           stream in the reverse direction, the ACCEPT will contain
           both RFlowSpec and RName parameters.  Each agent should
           update the state tables for the reverse stream with this
           information.

              TSR (bits 14 and 15) specifies the target's response for
              the use of data packet timestamps; see Section 4 (page
              76).  Its values and semantics are:

                 00  Not implemented.
                 01  No timestamps are permitted.
                 10  Timestamps must always be present.
                 11  Timestamps may optionally be present.

              Reference contains a number assigned by the agent sending
              the ACCEPT for use in the acknowledging ACK.

              LnkReference is the Reference number from the
              corresponding CONNECT or CHANGE.

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RFC 1190 Internet Stream Protocol October 1990

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 1   |     0     |TSR|           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |               0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      FlowSpec Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     TargetList Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     RecordRoute Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      RFlowSpec Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         RName Parameter                       !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      UserData Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 39.  ACCEPT Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.2.         ACK

           ACK (OpCode = 2) is used to acknowledge a request.  The
           Reference in the header is the Reference number of the
           control message being acknowledged.

           Since a ACK might be the first response from a next-hop on a
           control link, the SVLId field may be the first source of the
           Virtual Link Identifier to be used in the RVLId field of
           subsequent control messages sent to that next-hop.

              ReasonCode is usually NoError, but other possibilities
              exist, e.g., DuplicateIgn.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 2   |       0       |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |          ReasonCode           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               0                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 40.  ACK Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.3.         CHANGE-REQUEST

           CHANGE-REQUEST (OpCode = 4) is used by an intermediate or
           target agent to request that the origin change the FlowSpec
           of an established stream.  The CHANGE-REQUEST message is
           propagated hop-by-hop to the origin, with an ACK at each
           hop.

           Any SrcRoute parameters in the targets of the TargetList are
           ignored.

              G (bit 8) is used to request a global, stream-wide
              change;  the TargetList parameter may be omitted when the
              G bit is specified.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 4   |G|      0      |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |               0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                       FlowSpec Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     TargetList Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      UserData Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 41.  CHANGE-REQUEST Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.4.         CHANGE

           CHANGE (OpCode = 3) is used to change the FlowSpec of an
           established stream.  Parameters are the same as for CONNECT
           but the TargetList is not required.  The CHANGE message is
           processed similarly to the CONNECT message, except that it
           travels along the path of an established stream.

           If the change to the FlowSpec is in a direction that makes
           fewer demands of the involved networks, then the change has
           a high probability of success along the path of the
           established stream.  Each ST agent receiving the CHANGE
           message makes the necessary requested changes to the network
           resource allocations, and if successful, propagates the
           CHANGE message along the established paths.  If the change
           cannot be made then the ST agent must recover using
           DISCONNECT and REFUSE messages as in the case of a network
           failure.  Note that a failure to change the resources
           requested for a specific target(s) should not cause other
           targets in the stream to be deleted.  The CHANGE must be
           ACKed.

           If the CHANGE is a result of a CHANGE-REQUEST the
           LnkReference field of the CHANGE will contain the value from
           the Reference field of the CHANGE-REQUEST.

           It is recommended that the origin only have one outstanding
           CHANGE per target;  if the application requests more that
           one to be outstanding at a time, it is the application's
           responsibility to deal with any sequencing problems that may
           arise.

           Any SrcRoute parameters in the targets of the
           TargetListParameter are ignored.

              G (bit 8) is used to request a global, stream-wide
              change;  the TargetList parameter may be omitted when the
              G bit is specified.

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RFC 1190 Internet Stream Protocol October 1990

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 3   |G|      0      |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |               0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                       FlowSpec Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     TargetList Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      UserData Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 42.  CHANGE Control Message

        4.2.3.5.         CONNECT

           CONNECT (OpCode = 5) requests the setup of a new stream or
           an addition to or recovery of an existing stream.  Only the
           origin can issue the initial set of CONNECTs to setup a
           stream, and the first CONNECT to each next-hop is used to
           convey the initial suggestion for a HID.  If the stream's
           data packets will be sent to some set of next-hop ST agents
           by multicast then the CONNECTs to that set must suggest the
           same HID.  Otherwise, the HIDs in the various CONNECTs can
           be different.

           The CONNECT message must fit within the maximum allowable
           packet size (MTU) for the intervening network.  If a CONNECT
           message is too large, it must be fragmented into multiple
           CONNECT messages by partitioning the TargetList; see Section
           4.2 (page 77).  Any UserData parameter will be replicated in
           each fragment for delivery to all targets.

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RFC 1190 Internet Stream Protocol October 1990

           The next-hop can initially respond with any of the following
           five responses:

            1  ERROR-IN-REQUEST, which implies that the CONNECT was
               not valid and has been ignored,

            2  ACK, which implies that the CONNECT with the H bit not
               set was valid and is being processed,

            3  HID-APPROVE, which implies that the CONNECT with the
               H bit set was valid, and the suggested HID can be
               used or was deferred,

            4  HID-REJECT, which implies that the CONNECT with the H
               bit set was valid but the suggested HID cannot be
               used and another must be suggested in a subsequent
               HID-CHANGE message, or

            5  REFUSE, which implies that the CONNECT was valid but
               the included list of targets in the REFUSE cannot be
               processed for the stated reason.

           The next-hop will later relay back either an ACCEPT or
           REFUSE from each target not already specified in the REFUSE
           of case 5 above (note multiple targets may be included in a
           single REFUSE message).

           An intermediate ST agent that receives a CONNECT selects the
           next-hop ST agents, partitions the TargetList accordingly,
           reserves network resources in the direction toward the
           next-hop, updating the FlowSpec accordingly (see Section
           4.2.2.3 (page 81)), selects a proposed HID for each next-
           hop, and sends the resulting CONNECTs.

           If the intermediate ST agent that is processing a CONNECT
           fails to find a route to a target, then it responds with a
           REFUSE with the appropriate reason code.  If the next-hop to
           a target is by way of the network from which it received the
           CONNECT, then it sends a NOTIFY with the appropriate reason
           code (RouteBack).  In either case, the TargetList specifies
           the affected targets.  The intermediate ST agent will only
           route to and propagate a CONNECT to the targets for which it
           does not issue either an ERROR-IN-REQUEST or a REFUSE.

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RFC 1190 Internet Stream Protocol October 1990

           The processing of a received CONNECT message requires care
           to avoid routing loops that could result from delays in
           propagating routing information among ST agents.  If a
           received CONNECT contains a new Name, a new stream should be
           created (unless the Virtual Link Identifier matches a known
           link in which case an ERROR-IN-REQUEST should be sent).  If
           the Name is known, there are four cases:

            1  the Virtual Link Identifier matches and the Target
               matches a current Target -- the duplicate target
               should be ignored.

            2  the Virtual Link Identifier matches but the Target is
               new -- the stream should be expanded to include the
               new target.

            3  the Virtual Link Identifier differs and the Target
               matches a current Target -- an ERROR-IN-REQUEST
               message should be sent specifying that the target is
               involved in a routing loop.  If a reroute, the old
               path will eventually timeout and send a DISCONNECT;
               a subsequent retransmission of the rerouted CONNECT
               will then be processed under case 2 above.

            4  the Virtual Link Identifier differs but the Target is
               new -- a new (instance of the) stream should be
               created for the target that is deliberately part of
               a loop using a SrcRoute parameter.

           Note that the test for a known or matching Target includes
           comparing any SrcRoute parameter that might be present.

           Option bits are specified by either the origin's service
           user or by an intermediate agent, depending on the specific
           option.  Bits not specified below are currently unspecified,
           and should be set to zero (0) by the origin agent and not
           changed by other agents unless those agents know their
           meaning.

              H (bit 8) is used for the HID Field option; see Section
              3.6.1 (page 44).  It is set to one (1) only if the HID
              field contains either zero (when the HID selection is
              being deferred), or the proposed HID.  This bit is zero
              (0) if the HID field does not contain valid data and
              should be ignored.

              P (bit 9) is used for the PTP option; see Section 3.6.2
              (page 44).

              S (bit 10) is used for the NoRecovery option; see Section
              3.6.4 (page 46).

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RFC 1190 Internet Stream Protocol October 1990

              TSP (bits 14 and 15) specifies the origin's proposal for
              the use of data packet timestamps; see Section 4 (page
              76).  Its values and semantics are:

                 00  No proposal.
                 01  Cannot insert timestamps.
                 10  Must always insert timestamps.
                 11  Can insert timestamps if requested.

              RVLId, the receiver's Virtual Link Identifier, is set to
              zero in all CONNECT messages until its value arrives in
              the SVLId field of an acknowledgment to the CONNECT.

              SVLId, the sender's Virtual Link Identifier, is set to a
              value chosen by each hop to facilitate efficient
              dispatching of subsequent control messages.

              HID is the identifier that will be used with data packets
              moving through the stream in the direction from the
              origin to the targets.  It is a hop-by-hop shorthand
              identifier for the stream's Name, and is chosen by each
              agent for the branch to the next-hop agents.  The
              contents of the HID field are only valid, and a HID-
              REJECT or HID-APPROVE reply may only be sent, when the
              HID Field option (H bit) is set (1).  If the HID Field
              option is specified and the proposed HID is zero, the
              selection of the HID is deferred to the receiving next-
              hop agent.  If the HID Field option is not set (H bit is
              0), then the HID field does not contain valid data and
              should be ignored;  see Section 3.6.1 (page 44).

              TargetList is the list of IP addresses of the target
              processes.  It is of arbitrary size up to the maximum
              allowed for packets traveling across the specific
              network.

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 5   |H|P|S|  0  |TSP|           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            RVLId/0            |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |             HID/0             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                       Origin Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      FlowSpec Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      TargetList Parameter(s)                  :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                        Group Parameter                        :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                   MulticastAddress Parameter                  :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     RecordRoute Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      RFlowSpec Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                        RGroup Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                        RHID Parameter                         !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      UserData Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 43.  CONNECT Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.6.         DISCONNECT

           DISCONNECT (OpCode = 6) is used by an origin to tear down an
           established stream or part of a stream, or by an
           intermediate agent that detects a failure between itself and
           its previous-hop, as distinguished by the ReasonCode.  The
           DISCONNECT message specifies the list of targets that are to
           be disconnected.  An ACK is required in response to a
           DISCONNECT message.  The DISCONNECT message is propagated
           all the way to the specified targets.  The targets are
           expected to terminate their participation in the stream.

           Note that in the case of a failure it may be advantageous to
           retain state information as the stream should be repaired
           shortly;  see Section 3.7.2 (page 52).

              G (bit 8) is used to request a DISCONNECT of all the
              stream's targets; the TargetList parameter may be omitted
              when the G bit is set (1).

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 6   |G|      0      |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |          ReasonCode           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     TargetList Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      UserData Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 44.  DISCONNECT Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.7.         ERROR-IN-REQUEST

           ERROR-IN-REQUEST (OpCode = 7) is sent in acknowledgment to a
           request in which an error is detected.  No action is taken
           on the erroneous request and no state information for the
           stream is retained.  Consequently it is appropriate for the
           SVLId to be zero (0).  No ACK is expected.

           An ERROR-IN-REQUEST is never sent in response to either an
           ERROR-IN-REQUEST or an ERROR-IN-RESPONSE;  however, the
           event should be logged for diagnostic purposes.  The
           receiver of an ERROR-IN-REQUEST is encouraged to try again
           without waiting for a retransmission timeout.

              Reference is the Reference number of the erroneous
              request.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 7   |       0       |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |            SVLId/0            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |          ReasonCode           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                          ErroredPDU                           :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      TargetList Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 45.  ERROR-IN-REQUEST Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.8.         ERROR-IN-RESPONSE

           ERROR-IN-RESPONSE (OpCode = 8) is sent in acknowledgment to
           a response in which an error is detected.  No ACK is
           expected.  Action taken by the requester and responder will
           vary with the nature of the request.

           An ERROR-IN-REQUEST is never sent in response to either an
           ERROR-IN-REQUEST or an ERROR-IN-RESPONSE;  however, the
           event should be logged for diagnostic purposes.  The
           receiver of an ERROR-IN-RESPONSE is encouraged to try again
           without waiting for a retransmission timeout.

           Reference identifies the erroneous response.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 8   |       0       |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |          ReasonCode           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                          ErroredPDU                           :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      TargetList Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 46.  ERROR-IN-RESPONSE Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.9.         HELLO

           HELLO (OpCode = 9) is used as part of the ST failure
           detection mechanism; see Section 3.7.1.2 (page 49).

              R (bit 8) is used for the Restarted bit.

              Reference is non-zero to inform the receiver that an ACK
              should be promptly sent so that the sender can update its
              round-trip time estimates.  If the Reference is zero, no
              ACK should be sent.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 9   |R|      0      |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            RVLId/0            |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Reference/0          |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |               0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          HelloTimer                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                        OriginTimestamp                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 47.  HELLO Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.10.        HID-APPROVE

           HID-APPROVE (OpCode = 10) is used by the agent that is
           responding to either a CONNECT or HID-CHANGE to agree to
           either use the proposed HID or to the addition or deletion
           of the specified HID.  In all cases but deletion, the newly
           approved HID is returned in the HID field;  for deletion,
           the HID field must be set to zero.  The HID-APPROVE is the
           acknowledgment of a CONNECT or HID-CHANGE.

           The optional FreeHIDs parameter provides the previous-hop
           agent with hints about what other HIDs are acceptable in
           case a multicast HID is being negotiated;  see Section
           4.2.2.4 (page 84).

           Since a HID-APPROVE might be the first response from a
           next-hop on a control link, the SVLId field may be the first
           source of the Virtual Link Identifier to be used in the
           RVLId field of subsequent control messages sent to that
           next-hop.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 10  |       0       |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |              HID              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               0                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      FreeHIDs Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 48.  HID-APPROVE Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.11.        HID-CHANGE-REQUEST

           HID-CHANGE-REQUEST (OpCode = 12) is used by a next-hop agent
           that would like, for administrative reasons, to change the
           HID that is in use.  The receiving previous-hop agent
           acknowledges the request by either an ERROR-IN-REQUEST if it
           is unwilling to make the requested change, or with a HID-
           CHANGE if it can accommodate the request.

              A (bit 8) is used to indicate that the specified HID
              should be included in the set of HIDs for the specified
              Name.  When a HID is added, the acknowledging HID-APPROVE
              should contain a HID field whose contents is the HID just
              added.

              D (bit 9) is used to indicate that the specified HID
              should be removed in the set of HIDs for the specified
              Name.  When a HID is deleted, the acknowledging HID-
              APPROVE should contain a HID field whose contents is
              zero.  Note that the Reference field may be used to
              determine the HID that has been deleted.

              If neither bit is set, the specified HID should replace
              that currently in use with the specified Name.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 12  |A|D|     0     |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |              HID              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               0                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 49.  HID-CHANGE-REQUEST Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.12.        HID-CHANGE

           HID-CHANGE (OpCode = 11) is used by the agent that issued a
           CONNECT and received a HID-REJECT to attempt to negotiate a
           suitable HID.  The HID in the HID-CHANGE message must be
           different from that in the CONNECT, or any previous HID-
           CHANGE messages for the given Name.  The agent receiving the
           HID-CHANGE must respond with a HID-APPROVE if the new HID is
           suitable, or a HID-REJECT if it is not.  In case of an
           error, either an ERROR-IN-REQUEST or a REFUSE may be
           returned as an acknowledgment.

           Since an agent may send CONNECT messages with the same HID
           to several next-hops in order to use multicast data
           transfer, any HID-CHANGE must also be sent to the same set
           of next-hops.  Therefore, a next-hop agent must be prepared
           to receive a HID-CHANGE before or after it has sent a HID-
           APPROVE response to the CONNECT or a previous HID-CHANGE.
           Only the last HID-CHANGE is relevant.  The previous-hop
           agent will ignore HID-APPROVE or HID-REJECT messages to
           previous CONNECT or HID-CHANGE messages.

           A DISCONNECT can be sent instead of a HID-CHANGE, or a
           REFUSE can be sent instead of a HID-APPROVE or HID-REJECT,
           to terminate fatally the HID negotiation and the agent's
           knowledge of the stream.

           The A and D bits are used to change a HID, e.g., when adding
           a new next-hop to a multicast group, in such a way that data
           packets that are flowing through the network will not be
           mishandled due to a race condition in processing the HID-
           CHANGE messages between the previous-hop and its next-hops.
           An implementation may choose to limit the number of
           simultaneous HIDs associated with a stream, but must allow
           at least two.

              A (bit 8) is used to indicate that the specified HID
              should be included in the set of HIDs for the specified
              Name.  When a HID is added, the acknowledging HID-APPROVE
              should contain a HID field whose contents is the HID just
              added.

              D (bit 9) is used to indicate that the specified HID
              should be removed from the set of HIDs for the specified
              Name.  When a HID is deleted, the acknowledging HID-
              APPROVE should contain a HID field whose contents is
              zero.  Note that the Reference field may be used to
              determine the HID that has been deleted.

              If neither bit is set, the specified HID should replace
              that currently in use for the specified Name.

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 11  |A|D|     0     |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |              HID              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               0                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 50.  HID-CHANGE Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.13.        HID-REJECT

           HID-REJECT (OpCode = 13) is used as an acknowledgment that a
           CONNECT or HID-CHANGE was received and is being processed,
           but means that the HID contained in the CONNECT or HID-
           CHANGE is not acceptable.  Upon receipt of this message the
           agent that issued the CONNECT or HID-CHANGE must now issue a
           HID-CHANGE to attempt to find a suitable HID.  The HID-
           CHANGE can cause another HID-REJECT but eventually the HID-
           CHANGE must be acknowledged with a HID-APPROVE to end
           successfully the HID negotiation.  The agent that issued the
           HID-REJECT may not issue an ACCEPT before it has found an
           acceptable HID.

           Since a HID-REJECT might be the first response from a next-
           hop on a control link, the SVLId field may be the first
           source of the Virtual Link Identifier to be used in the
           RVLId field of subsequent control messages sent to that
           next-hop.

           Either agent may terminate the negotiation by issuing either
           a DISCONNECT or a REROUTE.  The agent that issued the HID-
           REJECT may issue a REFUSE, or REROUTE at any time after the
           HID-REJECT.  In this case, the stream cannot be created, the
           HID negotiation need not proceed, and the previous-hop need
           not transmit any further messages;  any further messages
           that are received should be ignored.

           The optional FreeHIDs parameter provides the previous-hop
           agent with hints about what HIDs would have been acceptable;
           see Section 4.2.2.4 (page 84).

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 13  |       0       |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |          RejectedHID          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               0                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      FreeHIDs Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 51.  HID-REJECT Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.14.        NOTIFY

           NOTIFY (OpCode = 14) is issued by a an agent to inform other
           agents, the origin, or target(s) of events that may be
           significant.  The action taken by the receiver of a NOTIFY
           depends on the ReasonCode.  Possible events are suspected
           routing problems or resource allocation changes that occur
           after a stream has been established.  These changes occur
           when network components fail and when competing streams
           preempt resources previously reserved by a lower precedence
           stream.  We also anticipate that NOTIFY can be used in the
           future when additional resources become available, as is the
           case when network components recover or when higher
           precedence streams are deleted.

           NOTIFY may contain a FlowSpec that reflects that revised
           guarantee that can be promised to the stream.  NOTIFY may
           also identify those targets that are affected by the change.
           In this way, NOTIFY is similar to ACCEPT.

           NOTIFY may be relayed by the ST agents back to the origin,
           along the path established by the CONNECT but in the reverse
           direction.  It is up to the origin to decide whether a
           CHANGE should be submitted.

           When NOTIFY is received at the origin, the application
           should be notified of the target and the change in resources
           allocated along the path to it, as specified in the FlowSpec
           contained in the NOTIFY message.  The application may then
           use the information to either adjust or terminate the
           portion of the stream to each affected target.

           The NOTIFY may be propagated beyond the previous-hop or
           next-hop agent; it must be acknowledged with an ACK.

              Reference contains a number assigned by the agent sending
              the NOTIFY for use in the acknowledging ACK.

              ReasonCode identifies the reason for the notification.

              LnkReference, when non-zero, is the Reference number from
              a command that is the subject of the notification.

              HID is present when the notification is related to a HID.

              Name is present when the notification is related to a
              stream.

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RFC 1190 Internet Stream Protocol October 1990

              NextHopIPAddress is an optional parameter and contains
              the IP address of a suggested next-hop ST agent.

              TargetList is present when the notification is related to
              one or more targets.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 14  |       0       |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |          ReasonCode           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                          ErroredPDU                           :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      FlowSpec Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         HID Parameter                         !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                  NextHopIPAddress Parameter                   !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     RecordRoute Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      TargetList Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 52.  NOTIFY Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.15.        REFUSE

           REFUSE (OpCode = 15) is issued by a target that either does
           not wish to accept a CONNECT message or wishes to remove
           itself from an established stream.  It might also be issued
           by an intermediate agent in response to a CONNECT or CHANGE
           either to terminate fatally a failing HID negotiation, to
           terminate a routing loop, or when a satisfactory next-hop to
           a target cannot be found.  It may also be a separate command
           when an existing stream has been preempted by a higher
           precedence stream or an agent detects the failure of a
           previous-hop, next-hop, or the network between them.  In all
           cases, the TargetList specifies the targets that are
           affected by the condition.  Each REFUSE must be acknowledged
           by an ACK.

           The REFUSE is relayed by the agents from the originating
           agent to the origin (or intermediate agent that created the
           CONNECT or CHANGE) along the path traced by the CONNECT.
           The agent receiving the REFUSE will process it differently
           depending on the condition that caused it, as specified in
           the ReasonCode field.  In some cases, such as if a next-hop
           cannot obtain resources, the agent can release any resources
           reserved exclusively for transmissions in the stream in
           question to the target specified in the TargetList, and the
           previous-hop can attempt to find an alternate route.  In
           some cases, such as a routing failure, the previous-hop
           cannot determine where the failure occurred, and must
           propagate the REFUSE back to the origin, which can attempt
           recovery of the stream by issuing a new CONNECT.

           No special effort is made to combine multiple REFUSE
           messages since it is considered most unlikely that separate
           REFUSEs will happen to both pass through an agent at the
           same time and be easily combined, e.g., have identical
           ReasonCodes and parameters.

           Since a REFUSE might be the first response from a next-hop
           on a control link, the SVLId field may be the first source
           of the Virtual Link Identifier to be used in the RVLId field
           of subsequent control messages sent to that next-hop.

              Reference contains a number assigned by the agent sending
              the REFUSE for use in the acknowledging ACK.

              LnkReference is either the Reference number from the
              corresponding CONNECT or CHANGE, if it is the result of
              such a message, or zero when the REFUSE was originated as
              a separate command.

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 15  |       0       |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             RVLId             |             SVLId             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |          ReasonCode           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       DetectorIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                        Name Parameter                         !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     TargetList Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                          ErroredPDU                           :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                     RecordRoute Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      UserData Parameter                       :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 53.  REFUSE Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.16.        STATUS

           STATUS (OpCode = 16) is used to inquire about the existence
           of a particular stream identified by either a HID (H bit
           set) or Name (Name Parameter present).

           When a stream has been identified, a STATUS-RESPONSE is
           returned that will contain the specified HID and/or Name but
           no other parameters if the specified stream is unknown, or
           will otherwise contain the current HID(s), Name, FlowSpec,
           TargetList, and possibly Group(s) of the stream.  Note that
           if a stream has no current HID, the HID field in the
           STATUS-RESPONSE will contain zero;  it will contain the
           first, or only, HID if a valid HID exists; additional valid
           HIDs will be returned in HID parameters.

           Use of STATUS is intended for diagnostic purposes and to
           assist in stream cleanup operations.  Note that if both a
           HID and Name are specified, but they do not correspond to
           the same stream, an ERROR-IN-REQUEST with the appropriate
           reason code (InconsistHID) would be returned.

           It is possible in cases of multiple failures or network
           partitioning for an ST agent to have information about a
           stream after the stream has either ceased to exist or has
           been rerouted around the agent.  When an agent concludes
           that a stream has not been used for a period of time and
           might no longer be valid, it can probe the stream's
           previous-hop or next-hop(s) to see if they believe that the
           stream still exists through the interrogating agent.  If
           not, those hops would reply with a STATUS-RESPONSE that
           contains the HID and/or Name but no other parameters;
           otherwise, if the stream is still valid, the hops would
           reply with the parameters of the stream.

              H (bit 8) is used to indicate whether (when 1) or not
              (when 0) a HID is present in the HID field.

              Q (bit 9) is set to one (1) for remote diagnostic
              purposes when the receiving agent should return a
              stream's parameters, whether or not the source of the
              message is believed to be a previous-hop or next-hop in
              the specified stream.  Note that this use has potential
              for disclosure of sensitive information.

              RVLId and SVLId may either or both be zero when STATUS is
              used for diagnostic purposes.

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RFC 1190 Internet Stream Protocol October 1990

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 16  |H|Q|     0     |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            RVLId/0            |            SVLId/0            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |             HID/0             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               0                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 54.  STATUS Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

        4.2.3.17.        STATUS-RESPONSE

           STATUS-RESPONSE (OpCode = 17) is the reply to a STATUS
           message.  If the stream specified in the STATUS message is
           not known, the STATUS-RESPONSE will contain the specified
           HID and/or Name but no other parameters.  It will otherwise
           contain the current HID(s), Name, FlowSpec, TargetList, and
           possibly Group of the stream.  Note that if a stream has no
           current HID, the H bit in the STATUS-RESPONSE will be zero.
           The HID field will contain the first, or only, HID if a
           valid HID exists; additional valid HIDs will be returned in
           HID parameters.

              H (bit 8) is used to indicate whether (when 1) or not
              (when 0) a HID is present in the HID field.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  OpCode = 17  |H|Q|     0     |           TotalBytes          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            RVLId/0            |            SVLId/0            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Reference           |         LnkReference          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         SenderIPAddress                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |             HID/0             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               0                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         Name Parameter                        !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                       FlowSpec Parameter                      :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                        Group Parameter                        :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                         HID Parameter                         !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  :                      TargetList Parameter                     :
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 55.  STATUS-RESPONSE Control Message

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

  4.3.       Suggested Protocol Constants

     The ST Protocol uses several fields that must have specific values
     for the protocol to work, and also several values that an
     implementation must select.  This section specifies the required
     values and suggests initial values for others.  It is recommended
     that the latter be implemented as variables so that they may be
     easily changed when experience indicates better values.
     Eventually, they should be managed via the normal network
     management facilities.

     ST uses IP Version Number 5.

     When encapsulated in IP, ST uses IP Protocol Number 5.

      Value  ST Command Message Name       Value     ST Element Name
     ------- -----------------------      ------- ---------------------

        1    ACCEPT                          1    ErroredPDU
        2    ACK                             2    FlowSpec
        3    CHANGE                          3    FreeHIDs
        4    CHANGE-REQUEST                  4    Group
        5    CONNECT                         5    HID
        6    DISCONNECT                      6    MulticastAddress
        7    ERROR-IN-REQUEST                7    Name
        8    ERROR-IN-RESPONSE               8    NextHopIPAddress
        9    HELLO                           9    Origin
       10    HID-APPROVE                    10    OriginTimestamp
       11    HID-CHANGE                     11    RecordRoute
       12    HID-CHANGE-REQUEST             12    RFlowSpec
       13    HID-REJECT                     13    RGroup
       14    NOTIFY                         14    RHID
       15    REFUSE                         15    RName
       16    STATUS                         16    SrcRoute, IP Loose
       17    STATUS-RESPONSE                17    SrcRoute, IP Strict
                                            18    SrcRoute, ST Loose
                                            19    SrcRoute, ST Strict
                                            20    TargetList
                                            21    UserData

     A good choice for the minimum number of bits in the FreeHIDBitMask
     element of the FreeHIDs parameter is not yet known.  We suggest a
     minimum of 64 bits, i.e., N in Figure 25 has a value of two (2).

     HID value zero (0) is reserved for ST Control Messages.  HID
     values 1-3 are reserved for future use.

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RFC 1190 Internet Stream Protocol October 1990

     VLId value zero (0) may only be used in the RVLId field of an ST
     Control Message when the appropriate value has not yet been
     received from the other end of the virtual link;' except for an
     ERROR-IN-REQUEST or diagnostic message, the SVLId field may never
     contain a value of zero except in a diagnostic message.  VLId
     value 1 is reserved for use with HELLO messages by those agents
     whose implementation wishes to have all HELLOs so identified.
     VLId values 2-3 are reserved for future use.

     The following permanent IP multicast addresses have been assigned
     to ST:

        224.0.0.7    All ST routers
        224.0.0.8    All ST hosts

     In addition, a block of transient IP multicast addresses,
     224.1.0.0 - 224.1.255.255, has been allocated for ST multicast
     groups.  Note that in the case of Ethernet, an ST Multicast
     address of 224.1.cc.dd maps to an Ethernet Multicast address of
     01:00:5E:01:cc:dd (see [6]).

     SCMP uses retransmission to effect reliability and thus has
     several "retransmission timers".  Each "timer" is modeled by an
     initial time interval (ToXxx), which gets updated dynamically
     through measurement of control traffic, and a number of times
     (NXxx) to retransmit a message before declaring a failure.  All
     time intervals are in units of milliseconds.

      Value   Timeout  Name                      Meaning
     ------- ---------------------- ----------------------------------

       1000  ToAccept               Initial hop-by-hop timeout for
                                    acknowledgment of ACCEPT

          3  NAccept                ACCEPT retries before failure

       1000  ToConnect              Initial hop-by-hop timeout for
                                    acknowledgment of CONNECT

          5  NConnect               CONNECT retries before failure

       1000  ToDisconnect           Initial hop-by-hop timeout for
                                    acknowledgment of DISCONNECT

         3   NDisconnect            DISCONNECT retries before
                                    failure

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      Value   Timeout  Name                      Meaning
     ------- ---------------------- ----------------------------------

       1000  ToHIDAck               Initial hop-by-hop timeout for
                                    acknowledgment of
                                    HID-CHANGE-REQUEST

          3  NHIDAck                HID-CHANGE-REQUEST retries
                                    before failure

       1000  ToHIDChange            Initial hop-by-hop timeout for
                                    acknowledgment of HID-CHANGE

          3  NHIDChange             HID-CHANGE retries before
                                    failure

       1000  ToNotify               Initial hop-by-hop timeout for
                                    acknowledgment of NOTIFY

          3  NNotify                NOTIFY retries before failure

       1000  ToRefuse               Initial hop-by-hop timeout for
                                    acknowledgment of REFUSE

          3  NRefuse                REFUSE retries before failure

       1000  ToReroute              Timeout for receipt of ACCEPT or
                                    REFUSE from targets during
                                    failure recovery

          5  NReroute               CONNECT retries before failure

       5000  ToEnd2End              End-to-End timeout for receipt
                                    of ACCEPT or REFUSE from targets
                                    by origin

          0  NEnd2End               CONNECT retries before failure

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      Value   Parameter  Name                    Meaning
     ------- ---------------------- ----------------------------------

         10  NHIDAbort              Number of rejected HID proposals
                                    before aborting the HID
                                    negotiation process

      10000  HelloTimerHoldDown     Interval that Restarted bit must
                                    be set after ST restart

          5  HelloLossFactor        Number of consecutively missed
                                    HELLO messages before declaring
                                    link failure

       2000  DefaultRecoveryTimeout Interval between successive
                                    HELLOs to/from active neighbors

          2  DefaultHelloFactor     HELLO filtering function factor

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5. Areas Not Addressed

  There are a number of issues that will need to be addressed in the
  long run but are not addressed here.  Some issues are network or
  implementation specific.  For example, the management of multicast
  groups depends on the interface that a network provides to the ST
  agent, and an UP/DOWN protocol based on ST HELLO messages depends on
  the details of the ST agents.  Both these examples may impact the ST
  implementations, but we feel it is inappropriate to specify them
  here.

  In other cases we feel that appropriate solutions are not clear at
  this time.  The following are examples of such issues:

  This document does not include a routing mechanism.  We do not feel
  that a routing strategy based on minimizing the number of hops from
  the source to the destination is necessarily appropriate.  An
  alternative strategy is to minimize the consumption of internet
  resources within some delay constraints.  Furthermore, it would be
  preferable if the routing function were to provide routes that
  incorporated bandwidth, delay, reliability, and perhaps other
  characteristics, not just connectivity.  This would increase the
  likelihood that a selected route would succeed.  This requirement
  would probably cause the ST agents to exchange more routing
  information than currently implemented.  We feel that further
  research and experimentation will be required before an appropriate
  routing strategy is well enough defined to be incorporated into the
  ST specification.

  Once the bandwidth for a stream has been agreed upon, it is not
  sufficient to rely on the origin to transmit traffic at that rate.
  The internet should not rely on the origin to operate properly.
  Furthermore, even if the origin sources traffic at the agreed rate,
  the packets may become aggregated unintentionally and cause local
  congestion.  There are several approaches to addressing this problem,
  such as metering the traffic in each stream as it passes through each
  agent.  Experimentation is necessary before such a mechanism is
  selected.

  The interface between the agent and the network is very limited.  A
  mechanism is provided by which the ST layer can query the network to
  determine the likelihood that a stream can be supported.  However,
  this facility will require practical experience before its
  appropriate use is defined.

  The simplex tree model of a stream does not easily allow for using
  multiple paths to support a greater bandwidth.  That is, at any given
  point in a stream, the entire incoming bandwidth must be transmitted
  to the same next-hop in order to get to some target.  If the
  bandwidth isn't available along any single path, the stream cannot be
  built to that target.  It may be the case that the bandwidth is not
  available along a single path, but if the data

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  flow is split along multiple paths, and so multiple next-hops,
  sufficient bandwidth would be available.  As currently specified, the
  ST agent at the point where the multiple flows converge will refuse
  the second connection because it can only be interpreted as a routing
  failure.  A mechanism that allows multiple paths in a stream and can
  protect against routing failures has not been defined.

  If sufficient bandwidth is not available, both preemption and
  rerouting are possible.  However, it is not clear when to use one or
  the other.  As currently specified, an ST agent that cannot obtain
  sufficient bandwidth will attempt to preempt lower precedence streams
  before attempting to reroute around the bottleneck.  This may lead to
  an undesirably high number of preemptions.  It may be that a higher
  precedence stream can be rerouted around lower precedence streams and
  still meet its performance requirements, whereas the preempted lower
  precedence streams cannot be reconstructed and still meet their
  performance requirements.  A simple and effective algorithm to allow
  a better decision has not been identified.

  In case a stream cannot be completed, ST does not report to the
  application the nature of the trouble in any great detail.
  Specifically, the application cannot determine where the bottleneck
  is, whether the problem is permanent or transitory, or the likely
  time before the trouble may be resolved.  The application can only
  attempt to build the stream at some later time hoping that the
  trouble has been resolved.  Schemes can be envisioned by which
  information is relayed back to the application.  However, only
  practical experience can evaluate the kind of trouble that is most
  likely encountered and the nature of information that would be most
  useful to the application.

  A mechanism is also not defined for cases where a stream cannot be
  completed not because of lack of resources but because of an
  unexpected failure that results in an ERROR-IN-REQUEST message.  An
  ERROR-IN-REQUEST message is returned in cases when an ST agent issues
  a malformed control message to a neighbor.  Such an occurrence is
  unexpected and may be caused by a bad or incomplete ST
  implementation.  In some cases a message, such as a NOTIFY should be
  sent to the origin.  Such a mechanism is not defined because it is
  not clear what information can be extracted and what the origin
  should do.

  No special action is taken when a target is removed from a stream.
  Removing a target may also remove a bottleneck either in bandwidth,
  packet rate or packet size, but advantage of this opportunity is not
  taken automatically.  The application may initiate a change to the
  stream's characteristics, but it is not in the best position to do
  this because the application may not know the nature of the
  bottleneck.  The ST layer may have the best information, but a

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  mechanism to do this may be very complex.  As a result, this concept
  requires further thought.

  An agent simply discards a stream's data packets if it cannot forward
  them.  The reason may be that the packets are too large or are
  arriving at too high a rate.  Alternative actions may include an
  attempt to do something with the packets, such as fragmenting them,
  or to notify the origin of the trouble.  Corrective measures may be
  too complex, so it may be preferable simply to notify the origin with
  a NOTIFY message.  However, if the incoming packet rate is causing
  congestion, then the NOTIFY messages themselves may cause more
  trouble.  The nature of the communication has yet to be defined.

  The FlowSpec includes a cost field, but its implementation has not
  been identified.  The units of cost can probably be defined
  relatively easily.  Cost of bandwidth can probably also be assigned.
  It is not clear how cost is assigned to other functions, such as high
  precedence or low delay, or how cost of the components of the stream
  are combined together.  It is clear that the cost to provide services
  will become more important in the near future, but it is not clear at
  this time how that cost is determined.

  A number of parameters of the FlowSpec are intended to be used as
  ranges, but some may be useful as discrete values.  For example, the
  FlowSpec may specify that bandwidth for a stream carrying voice
  should be reserved in a range from 16Kbps to 64Kbps because the voice
  codec has a variable coding rate.  However, the voice codec may be
  varied only among certain discrete values, such as 16Kbps, 32Kbps and
  64Kbps.  A stream that has 48Kbps of bandwidth is no better than one
  with 32Kbps.  The parameters of the FlowSpec where this may be
  relevant should optionally specify discrete values.  This is being
  considered.

  Groups are defined as a way to associate different streams, but the
  nature of the association is left for further study.  An example of
  such an association is to allow streams whose traffic is inherently
  not simultaneous to share the same allocated resources.  This may
  happen for example in a conference that has an explicit floor, such
  that only one site can generate video or audio traffic at any given
  time.  The grouping facility can be implemented based on this
  specification, but the implementation of the possible uses of groups
  will require new functionality to be added to the ST agents.  The
  uses for groups and the implementation to support them will be
  carried out as experience is gained and the need arises.

  We hope that the ST we here propose will act as a vehicle to study
  the use and performance of stream oriented services across packet
  switched networks.

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6. Glossary

  appropriate reason code
     This phrase refers to one or perhaps a set of reason codes that
     indicate why a particular action is being taken.  Typically,
     these result from detection of errors or anomalous conditions.
     It can also indicate that an application component or agent has
     presented invalid parameters.

  DefaultRecoveryTimeout
     The DefaultRecoveryTimeout is maintained by each ST agent.  It
     indicates the default time interval to use for sending HELLO
     messages.

  downstream
     The direction in a stream from an origin toward its targets.

  element
     The fields and parameters of the ST control messages are
     collectively called elements.

  FlowSpec
     The Flow Specification, abbreviated "FlowSpec" is used by an
     application to specify required and desired characteristics of
     the stream.  The FlowSpec specifies bandwidth, delay, and
     reliability parameters.  Both minimal requirements and desired
     characteristics are included.  This information is then used to
     guide route selection and resource allocation decisions.  The
     desired vs. required characteristics are used to guide tradeoff
     decisions among competing stream requests.

  group
     A set of related streams can be associated as a group.  This is
     done by generating a Group Name and assigning it to each of the
     related streams.  The grouping information can then be used by
     the ST agents in making resource management and other control
     decisions.  For example, when preemption is necessary to
     establish a high precedence stream, we can exploit the group
     information to minimize the number of stream groups that are
     preempted.

  Group Name
     The Group Name is used to indicate that a collection of streams
     are related.  A Group Name is structured to ensure that it is
     unique across all hosts:  it includes the address of the host
     where it was generated combined with a unique number generated
     by that host.  A timestamp is added to ensure that the overall
     name is unique over all time.  (A Group Name has the same format
     as a stream Name.)

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  HelloLossFactor
     The HelloLossFactor is a parameter maintained by each ST agent.
     It identifies the expected number of consecutive HELLO messages
     typically lost due to transient factors.  Thus, an agent will be
     assumed to be down after we miss more than HelloLossFactor
     messages.

  HelloTimer
     The HelloTimer is a millisecond timer maintained by each ST
     agent.  It is included in each HELLO message.  It represents the
     time since the agent was restarted, modulo the precision of the
     field.  It is used to detect variations in the delay between the
     two agents, by comparing the arrival interval of two HELLO
     messages to the difference between their HelloTimer fields.

  HelloTimerHoldDown
     The HelloTimerHoldDown value is maintained by each ST agent.
     When an ST agent is restarted, it will set the "Restarted" bit
     in all HELLO messages it sends for HelloTimerHoldDown seconds.

  HID
     The Hop IDentifier, abbreviated as HID, is a numeric key stored
     in the header of each ST packet.  It is used by an ST agent to
     associate the packet with one of the incoming hops managed by
     the agent.  It can be used by receiving agent to map to
     the set of outgoing next-hops to which the message should be
     forwarded.  The HID field of an ST packet will generally need to
     be changed as it passes through each ST agent since there may be
     many HIDs associated with a single stream.

  hop
     A "hop" refers to the portion of a stream's path between two
     neighbor ST agents.  It is usually represented by a physical
     network.  However, a multicast hop can connect a single ST agent
     to several next-hop ST agents.

  host agents
     Synonym for host ST agents.

  host ST agents
     Host ST agents are ST agents that provide services to higher
     layer protocols and applications.  The services include methods
     for sourcing data from and sinking data to the higher layer or
     application, and methods for requesting and modifying streams.

  intermediate agents
     Synonym for intermediate ST agents.

  intermediate ST agents
     Intermediate ST agents are ST agents that can forward ST
     packets between the networks to which they are attached.

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  MTU
     The abbreviation for Maximum Transmission Unit, which is the
     maximum packet size in bytes that can be accepted by a given
     network for transmission.  ST agents determine the maximum
     packet size for a stream so that data written to the stream can
     be forwarded through the networks without fragmentation.

  multi-destination simplex
     The topology and data flow of ST streams are described as being
     multi-destination simplex:  all data flowing on the stream
     originates from a single origin and is passed to one or more
     destination targets.  Only control information, invisible to the
     application program, ever passes in the upstream direction.

  NAccept
     NAccept is an integer parameter maintained by each ST agent.  It
     is used to control retransmission of an ACCEPT message.  Since
     an ACCEPT request is relayed by agents back toward the origin,
     it must be acknowledged by each previous-hop agent.  If this ACK
     is not received within the appropriate timeout interval, the
     request will be resent up to NAccept times before giving up.

  Name
     Generally refers to the name of a stream.  A stream Name is
     structured to ensure that it is unique across all hosts: it
     includes the address of the host where it was generated combined
     with a unique number generated at that host.  A timestamp is
     added to ensure that the overall Name is unique over all time.
     (A stream Name has the same format as a Group Name.)

  NConnect
     NConnect is an integer parameter maintained by each ST agent.
     It is used to control retransmission of a CONNECT message.  A
     CONNECT request must be acknowledged by each next-hop agent as
     it is propagated toward the targets.  If a HID-ACCEPT,
     HID-REJECT, or ACK is not received for the CONNECT between any
     two agents within the appropriate timeout interval, the request
     will be resent up to NConnect times before giving up.

  NDisconnect
     NDisconnect is an integer parameter maintained by each ST
     agent.  It is used to control retransmission of a DISCONNECT
     message.  A DISCONNECT request must be acknowledged by each
     next-hop agent as it is propagated toward the targets.  If this
     ACK is not received for the DISCONNECT between any two agents
     within the appropriate timeout interval, the request will be
     resent up to NDisconnect times before giving up.

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  next protocol identifier
     The next protocol identifier is used by a target ST agent to
     identify to which of several higher layer protocols it should
     pass data packets it receives the network.  Examples of higher
     layer protocols include the Network Voice Protocol and the
     Packet Video Protocol.  These higher layer protocols will
     typically perform further demultiplexing among multiple
     application processes as part of their protocol processing
     activities.

  next-hop
     Synonym for next-hop ST agent.

  next-hop ST agent
     For each origin or intermediate ST agent managing a stream
     there are a set of next-hop ST agents.  The intermediate agent
     forwards each data packet it receives to all the next-hop ST
     agents, which in turn forward the data toward the target host
     agent (if the particular next-hop agent is another intermediate
     agent) or to the next higher protocol layer at the target (if
     the particular next-hop agent is a host agent).

  NextPcol
     NextPcol is a field in each Target of the CONNECT message used
     to convey the next protocol identifier.  See definition of next
     protocol identifier above for more details.

  NHIDAbort
     NHIDAbort is an integer parameter maintained by each ST agent.
     It is the number of unacceptable HID proposals before an ST
     agent aborts the HID negotiation process.

  NHIDAck
     NHIDAck is an integer parameter maintained by each ST agent.
     It is used to control retransmission of HID-CHANGE-REQUEST
     messages.  HID-CHANGE-REQUEST is sent by an ST agent to the
     previous-hop ST agent to request that the HID in use between
     those agents be changed.  The previous-hop acknowledges the
     HID-CHANGE-REQUEST message by sending a HID-CHANGE message.  If
     the HID-CHANGE is not received within the appropriate timeout
     interval, the request will be resent up to NHIDAck times before
     giving up.

  NHIDChange
     NHIDChange is an integer parameter maintained by each ST agent.
     It is used to control retransmission of the HID-CHANGE message.
     A HID-CHANGE message must be acknowledged by the next-hop agent.
     If this ACK is not received within the appropriate timeout
     interval, the request will be resent up to NHIDChange times
     before giving up.

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  NRefuse
     NRefuse is an integer parameter maintained by each ST agent.
     It is used to control retransmission of a REFUSE message.  As a
     REFUSE request is relayed by agents back toward the origin, it
     must be acknowledged by each previous-hop agent.  If this ACK is
     not received within the appropriate timeout interval, the
     request will be resent up to NRefuse times before giving up.

  NRetryRoute
     NRetryRoute is an integer parameter maintained by each ST
     agent.  It is used to control route exploration.  When an agent
     receives a REFUSE message whose ReasonCode indicates that the
     originally selected route is not acceptable, the agent should
     attempt to find an alternate route to the target.  If the agent
     has not found a viable route after a maximum of NRetryRoute
     choices, it should give up and notify the previous-hop or
     application that it cannot find an acceptable path to the
     target.

  origin
     The origin of a stream is the host agent where an application
     or higher level protocol originally requested that the stream be
     created.  The origin specifies the data to be sent through the
     stream.

  parameter
     Parameters are additional values that may be included in
     control messages.  Parameters are often optional.  They are
     distinguished from fields, which are always present.

  participants
     Participants are the end-users of a stream.

  PDU
     Abbreviation for Protocol Data Unit, defined below.

  peer
     The term peer is used to refer to entities at the same protocol
     layer.  It is used here to identify instances of an application
     or protocol layer above ST.  For example, data is passed through
     a stream from an originating peer process to its target peers.

  previous-hop
     Synonym for previous-hop ST agent.

  previous-hop ST agent
     The origin or intermediate agent from which an ST agent receives
     its data.

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  protocol data unit
     A protocol data unit (PDU) is the unit of data passed to a
     protocol layer by the next higher layer protocol or user.  It
     consists of control information and possibly user data.

  RecoveryTimeout
     RecoveryTimeout is specified in the FlowSpec of each stream.
     The minimum of these values over all streams between a pair of
     adjacent agents determines how often those agents must send
     HELLO messages to each other in order to ensure that failure of
     one of the agents will be detected quickly enough to meet the
     guarantee implied by the FlowSpec.

  Restarted bit
     The Restarted bit is part of the HELLO message.  When set, it
     indicates that the sending agent was restarted recently (within
     the last HelloTimerHoldDown seconds).

  round-trip time
     The round-trip-time is the time it takes a message to be sent,
     delivered, processed, and the acknowledgment received.  It
     includes both network and processing delays.

  RTT
     Abbreviation for round-trip-time.

  RVLId
     Abbreviation for Receiver's Virtual Link Identifier.  It
     uniquely identifies to the receiver the virtual link, and this
     stream, used to send it a message.  See definition for Virtual
     Link Identifier below.

  SAP
     Abbreviation for Service Access Point.

  SCMP
     Abbreviation for ST Control Message Protocol, defined below.

  Service Access Point
     A point where a protocol service provider makes available the
     services it offers to a next higher layer protocol or user.

  setup phase
     Before data can be transmitted through a stream, the ST agents
     must distribute state information about the stream to all agents
     along the path(s) to the target(s).  This is the setup phase.
     The setup phase ends when all the ACCEPT and REFUSE messages
     sent by the targets have been delivered to the origin.  At this
     point, the data transfer phase begins and data can be sent.
     Requests to modify the stream can be issued after the setup
     phase has ended, i.e., during the data transfer phase without
     disrupting the flow of data.

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  ST agent
     An ST agent is an entity that implements the ST Protocol.

  ST Control Message Protocol
     The ST Control Message Protocol is the subset of the overall ST
     Protocol responsible for creation, modification, maintenance,
     and tear down of a stream.  It also includes support for event
     notification and status monitoring.

  stream
     A stream is the basic object managed by the ST Protocol for
     transmission of data.  A stream has one origin where data are
     generated and one or more targets where the data are received
     for processing.  A flow specification, provided by the origin
     and negotiated among the origin, intermediate, and target ST
     agents, identifies the requirements of the application and the
     guarantees that can be assured by the ST agents.

  subsets
     Subsets of the ST Protocol are permitted, as defined in various
     sections of this specification.  Subsets are defined to allow
     simplified implementations that can still effectively
     interoperate with more complete implementations without causing
     disruption.

  SVLId
     Abbreviation for Sender's Virtual Link Identifier.  It uniquely
     identifies to the receiver the virtual link identifier that
     should be placed into the RVLId field of all replies sent over
     the virtual link for a given stream.  See definition for Virtual
     Link Identifier below.

  target
     An ST target is the destination where data supplied by the
     origin will be delivered for higher layer protocol or
     application processing.

  tear down
     The tear down phase of a stream begins when the origin indicates
     that it has no further data to send and the ST agents through
     which the stream passes should dismantle the stream and release
     its resources.

  ToAccept
     ToAccept is a timeout in seconds maintained by each ST agent.
     It sets the retransmission interval for ACCEPT messages.

  ToConnect
     ToConnect is a timeout in seconds maintained by each ST agent.
     It sets the retransmission interval a CONNECT messages.

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  ToDisconnect
     ToDisconnect is a timeout in seconds maintained by each ST
     agent.  It sets the retransmission interval for DISCONNECT
     messages.

  ToHIDAck
     ToHIDAck is a timeout in seconds maintained by each ST agent.
     It sets the retransmission interval for HID-CHANGE-REQUEST
     messages.

  ToHIDChange
     ToHIDChange is a timeout in seconds maintained by each ST agent.
     It sets the retransmission interval for HID-CHANGE messages.

  ToRefuse
     ToRefuse is a timeout in seconds maintained by each ST agent.
     It sets the retransmission interval for REFUSE messages.

  upstream
     The direction in a stream from a target toward the origin.

  Virtual Link
     A virtual link is one edge of the tree describing the path of
     data flow through a stream.  A separate virtual link is assigned
     to each pair of neighbor ST agents, even when multiple next-hops
     are be reached through a single network level multicast group.
     The virtual link allows efficient demultiplexing of ST Control
     Message PDUs received from a single physical link or network.

  Virtual Link Identifier
     For each ST Control Message sent, the sender provides its own
     virtual link identifier and that of the receiver (if known).
     Either of these identifiers, combined with the address of the
     corresponding host, can be used to identify uniquely the virtual
     control link to the agent.  However, virtual link identifiers
     are chosen by the associated agent so that the agent may
     precisely identify the stream, state machine, and other protocol
     processing data elements managed by that agent, without regard
     to the source of the control message.  Virtual link identifiers
     are not negotiated, and do not change during the lifetime of a
     stream.  They are discarded when the stream is torn down.

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7. References

  [1] Braden, B., Borman, D., and C. Partridge, "Computing the
      Internet Checksum", RFC 1071, USC/Information Sciences
      Institute, Cray Research, BBN Laboratories, September
      1988.

  [2] Braden, R. (ed.), "Requirements for Internet Hosts --
      Communication Layers", RFC 1122, USC/Information Sciences
      Institute, October 1989.

  [3] Cheriton, D., "VMTP: Versatile Message Transaction Protocol
      Specification", RFC 1045, Stanford University, February 1988.

  [4] Cohen, D., "A Network Voice Protocol NVP-II", USC/Information
      Sciences Institute, April 1981.

  [5] Cole, E., "PVP - A Packet Video Protocol", W-Note 28,
      USC/Information Sciences Institute, August 1981.

  [6] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
      Stanford University, August 1989.

  [7] Edmond W., Seo K., Leib M., and C. Topolcic, "The DARPA
      Wideband Network Dual Bus Protocol", accepted for presentation
      at ACM SIGCOMM '90, September 24-27, 1990.

  [8] Forgie, J., "ST - A Proposed Internet Stream Protocol",
      IEN 119, M. I. T. Lincoln Laboratory, 7 September 1979.

  [9] Jacobs I., Binder R., and E. Hoversten E., "General Purpose
      Packet Satellite Network", Proc. IEEE, vol 66, pp 1448-1467,
      November 1978.

  [10] Jacobson, V., "Congestion Avoidance and Control", ACM
       SIGCOMM-88, August 1988.

  [11] Karn, P. and C. Partridge, "Round Trip Time Estimation",
       ACM SIGCOMM-87, August 1987.

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  [12] Mallory, T., and A. Kullberg, "Incremental Updating of the
       Internet Checksum", RFC 1141, BBN Communications
       Corporation, January 1990.

  [13] Mills, D., "Network Time Protocol (Version 2) Specification
       and Implementation", RFC 1119, University of Delaware,
       September 1989 (Revised February 1990).

  [14] Pope, A., "The SIMNET Network and Protocols", BBN
       Report No. 7102, BBN Systems and Technologies, July 1989.

  [15] Postel, J., ed., "Internet Protocol - DARPA Internet Program
       Protocol Specification", RFC 791, DARPA, September 1981.

  [16] Postel, J., ed., "Transmission Control Protocol - DARPA
       Internet Program Protocol Specification", RFC 793, DARPA,
       September 1981.

  [17] Postel, J., "User Datagram Protocol", RFC 768,
       USC/Information Sciences Institute, August 1980.

  [18] Reynolds, J., Postel, J., "Assigned Numbers", RFC 1060,
       USC/Information Sciences Institute, March 1990.

  [19] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
       SDNS Secure Data Network System, Security Protocol 3 (SP3),
       SDN.301, Rev. 1.5, 1989-05-15.

  [20] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
       SDNS Secure Data Network System, Security Protocol 3 (SP3)
       Addendum 1, Cooperating Families, SDN.301.1, Rev. 1.2,
       1988-07-12.

8. Security Considerations

  See section 3.7.8.

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

9. Authors' Addresses

     Stephen Casner
     USC/Information Sciences Institute
     4676 Admiralty Way
     Marina del Rey, CA 90292-6695

     Phone: (213) 822-1511 x153
     EMail: [email protected]

     Charles Lynn, Jr.
     BBN Systems and Technologies,
     a division of Bolt Beranek and Newman Inc.
     10 Moulton Street
     Cambridge, MA  02138

     Phone: (617) 873-3367
     EMail: [email protected]

     Philippe Park
     BBN Systems and Technologies,
     a division of Bolt Beranek and Newman Inc.
     10 Moulton Street
     Cambridge, MA  02138

     Phone: (617) 873-2892
     EMail: [email protected]

     Kenneth Schroder
     BBN Systems and Technologies,
     a division of Bolt Beranek and Newman Inc.
     10 Moulton Street
     Cambridge, MA  02138

     Phone: (617) 873-3167
     EMail: [email protected]

     Claudio Topolcic
     BBN Systems and Technologies,
     a division of Bolt Beranek and Newman Inc.
     10 Moulton Street
     Cambridge, MA  02138

     Phone: (617) 873-3874
     EMail: [email protected]

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

                  [This page intentionally left blank.]

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

Appendix 1. Data Notations

  The convention in the documentation of Internet Protocols is to
  express numbers in decimal and to picture data with the most
  significant octet on the left and the least significant octet on the
  right.

  The order of transmission of the header and data described in this
  document is resolved to the octet level.  Whenever a diagram shows a
  group of octets, the order of transmission of those octets is the
  normal order in which they are read in English.  For example, in the
  following diagram the octets are transmitted in the order they are
  numbered.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       1       |       2       |       3       |       4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       5       |       6       |       7       |       8       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       9       |      10       |      11       |      12       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 56.  Transmission Order of Bytes

  Whenever an octet represents a numeric quantity the left most bit in
  the diagram is the high order or most significant bit.  That is, the
  bit labeled 0 is the most significant bit.  For example, the
  following diagram represents the value 170 (decimal).

                           0 1 2 3 4 5 6 7
                          +-+-+-+-+-+-+-+-+
                          |1 0 1 0 1 0 1 0|
                          +-+-+-+-+-+-+-+-+

                   Figure 57.  Significance of Bits

  Similarly, whenever a multi-octet field represents a numeric quantity
  the left most bit of the whole field is the most significant bit.
  When a multi-octet quantity is transmitted the most significant octet
  is transmitted first.

  Fields whose length is fixed and fully illustrated are shown with a
  vertical bar (|) at the end;  fixed fields whose contents are
  abbreviated are shown with an exclamation point (!);  variable fields
  are shown with colons (:).

CIP Working Group

RFC 1190 Internet Stream Protocol October 1990

  Optional parameters are separated from control messages with a blank
  line.  The order of any optional parameters is not meaningful.

CIP Working Group

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