RFC1190

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.

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.

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

     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

     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

     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

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

+-------+ +-------+ | | Video | | Voice | | +-----+ +------+ +-----+ +-----+ Application | Appl | | Appl | | | SNMP| |Telnet| | FTP | ... | | Layer +-------+ +-------+ | +-----+ +------+ +-----+ +-----+

|        |      |     |        |     |            |
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

+----------------+ +----------------+ | (Sub-) Network |...| (Sub-) Network | (Sub-)Network | Protocol | | Protocol | Layer +----------------+ +----------------+

                Figure 1.  Protocol Relationships

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.

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

  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.

  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.

  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

  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,

     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.

  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.

  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.

                  ****                      ****
                 *    *     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

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.

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

     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

     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

     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

  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

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

                              |  V

<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

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

                              | V

<RVLId=5><SVLId=23> | +-> CONNECT C ---------->>

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

+->(reserve resources from 2 to D)

                                    V

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

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

     Figure 6.  CONNECT Processing by an Intermediate Agent

     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.

       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

     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>

(wait until HID negotiated)<-+ <RVLId=44><SVLId=15>

                              V         <Ref=410>

<<--+<-- 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>

| +-> ACK --------------->+

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

(wait until HID negotiated)<-+ <RVLId=64><SVLId=26>

                              V         <Ref=610>

<<--+<- 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

     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

     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

     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)

+----->+-> CONNECT E ---------->+->+

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

+<-- ACK <---------------+ |

              <RVLId=5><SVLId=23>    V

<Ref=65> (routing to E)

                                     V

(reserve resources 2 to E)

                                     V

+--> CONNECT E --------->+

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

+<-+<- REFUSE B <-----------+

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

| +-> ACK ---------------->+

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

| (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)

         V

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

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

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

  V

16. (inform application failed SAPUnknown)

               Figure 10.  Sending REFUSE Message

     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

+-> CONNECT B ----------------->+

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

+<- HID-APPROVE <---------------+

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

(routing to B: no route)

                                 V

+<-+-- REFUSE B ----------------+

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

| +-> ACK -------------------->+

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

(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)

                        V

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

     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.

  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

     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

+->(routing to E)

       V

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

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

+-> CONNECT E --------->+->+

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

+<- ACK <---------------+ V

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

+-> CONNECT E --------->+

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

+<- HID-APPROVE <-------+

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

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

                                                             V

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)

       V

19. +<-(wait for ACCEPTS)

    V

20. +<-(wait until HID negotiated)

 V

21. (inform A open to E)

             Figure 12.  Addition of Another Target

     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

     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

     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

     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.

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.

      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

     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.

     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

     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.

  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

     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

     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

     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.

     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.

     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

(source routed to 2)

  V

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

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

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

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

+<- ACK ----------------+ |

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

(source routed to 3)

                         V

(reserve resources 2 to 3)

                      V

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

     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

        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

        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

        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

     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.

     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.

     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.

     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.

  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

         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.

  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

     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.

     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)

     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

  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)

  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)

     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

     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.

        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

     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.

     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).

  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 >* >*

     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

  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

        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

     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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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

  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.

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

     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).

  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.

  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

           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

        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.

              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.

           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

           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

        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

           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

     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

     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

              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.

              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.

              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.

              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.

              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

     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

        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.,

           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

        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

ST Control Message PDUs

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

     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.

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

     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

     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

     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.

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.

        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.

        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).

           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.

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

     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

     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

     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

     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

     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

     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

     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.

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

     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).

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

     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.

           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

     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.

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

     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.

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

     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

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.

  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

   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

   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

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

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

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.

               [This page intentionally left blank.]

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.)

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.

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.

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.

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.

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.

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.

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.

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.

[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.

Security Considerations

See section 3.7.8.

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]

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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 (:).

Optional parameters are separated from control messages with a blank line. The order of any optional parameters is not meaningful.