RFC1045

Network Working Group David Cheriton Request for Comments: 1045 Stanford University

                                                       February 1988

          VMTP: VERSATILE MESSAGE TRANSACTION PROTOCOL
                     Protocol Specification

STATUS OF THIS MEMO

This RFC describes a protocol proposed as a standard for the Internet community. Comments are encouraged. Distribution of this document is unlimited.

OVERVIEW

This memo specifies the Versatile Message Transaction Protocol (VMTP) [Version 0.7 of 19-Feb-88], a transport protocol specifically designed to support the transaction model of communication, as exemplified by remote procedure call (RPC). The full function of VMTP, including support for security, real-time, asynchronous message exchanges, streaming, multicast and idempotency, provides a rich selection to the VMTP user level. Subsettability allows the VMTP module for particular clients and servers to be specialized and simplified to the services actually required. Examples of such simple clients and servers include PROM network bootload programs, network boot servers, data sensors and simple controllers, to mention but a few examples.

                       Table of Contents

Introduction 1

1.1. Motivation 2

   1.1.1. Poor RPC Performance                                     2
   1.1.2. Weak Naming                                              3
   1.1.3. Function Poor                                            3

1.2. Relation to Other Protocols 4 1.3. Document Overview 5

Protocol Overview 6

2.1. Entities, Processes and Principals 7 2.2. Entity Domains 9 2.3. Message Transactions 10 2.4. Request and Response Messages 11 2.5. Reliability 12

   2.5.1. Transaction Identifiers                                 13
   2.5.2. Checksum                                                14
   2.5.3. Request and Response Acknowledgment                     14
   2.5.4. Retransmissions                                         15
   2.5.5. Timeouts                                                15
   2.5.6. Rate Control                                            18

2.6. Security 19 2.7. Multicast 21 2.8. Real-time Communication 22 2.9. Forwarded Message Transactions 24 2.10. VMTP Management 25 2.11. Streamed Message Transactions 25 2.12. Fault-Tolerant Applications 28 2.13. Packet Groups 29 2.14. Runs of Packet Groups 31 2.15. Byte Order 32 2.16. Minimal VMTP Implementation 33 2.17. Message vs. Procedural Request Handling 33 2.18. Bibliography 34

VMTP Packet Formats 37

3.1. Entity Identifier Format 37 3.2. Packet Fields 38

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3.3. Request Packet 45 3.4. Response Packet 47

Client Protocol Operation 49

4.1. Client State Record Fields 49 4.2. Client Protocol States 51 4.3. State Transition Diagrams 51 4.4. User Interface 52 4.5. Event Processing 53 4.6. Client User-invoked Events 54

   4.6.1. Send                                                    54
   4.6.2. GetResponse                                             56

4.7. Packet Arrival 56

   4.7.1. Response                                                58

4.8. Management Operations 61

   4.8.1. HandleNoCSR                                             62

4.9. Timeouts 64

Server Protocol Operation 66

5.1. Remote Client State Record Fields 66 5.2. Remote Client Protocol States 66 5.3. State Transition Diagrams 67 5.4. User Interface 69 5.5. Event Processing 70 5.6. Server User-invoked Events 71

   5.6.1. Receive                                                 71
   5.6.2. Respond                                                 72
   5.6.3. Forward                                                 73
   5.6.4. Other Functions                                         74

5.7. Request Packet Arrival 74 5.8. Management Operations 78

   5.8.1. HandleRequestNoCSR                                      79

5.9. Timeouts 82

Concluding Remarks 84

I. Standard VMTP Response Codes 85

II. VMTP RPC Presentation Protocol 87

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II.1. Request Code Management 87

III. VMTP Management Procedures 89

III.1. Entity Group Management 100 III.2. VMTP Management Digital Signatures 101

IV. VMTP Entity Identifier Domains 102

IV.1. Domain 1 102 IV.2. Domain 3 104 IV.3. Other Domains 105 IV.4. Decentralized Entity Identifier Allocation 105

V. Authentication Domains 107

V.1. Authentication Domain 1 107 V.2. Other Authentication Domains 107

VI. IP Implementation 108

VII. Implementation Notes 109

VII.1. Mapping Data Structures 109 VII.2. Client Data Structures 111 VII.3. Server Data Structures 111 VII.4. Packet Group transmission 112 VII.5. VMTP Management Module 113 VII.6. Timeout Handling 114 VII.7. Timeout Values 114 VII.8. Packet Reception 115 VII.9. Streaming 116 VII.10. Implementation Experience 117

VIII. UNIX 4.3 BSD Kernel Interface for VMTP 118

Index 120

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                        List of Figures

Figure 1-1: Relation to Other Protocols 4 Figure 3-1: Request Packet Format 45 Figure 3-2: Response Packet Format 47 Figure 4-1: Client State Transitions 52 Figure 5-1: Remote Client State Transitions 68 Figure III-1: Authenticator Format 92 Figure VII-1: Mapping Client Identifier to CSR 109 Figure VII-2: Mapping Server Identifiers 110 Figure VII-3: Mapping Group Identifiers 111

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Introduction

The Versatile Message Transaction Protocol (VMTP) is a transport protocol designed to support remote procedure call (RPC) and general transaction-oriented communication. By transaction-oriented communication, we mean that:

- Communication is request-response: A client sends a request

 for a service to a server, the request is processed, and the
 server responds.  For example, a client may ask for the next
 page of a file as the service.  The transaction is terminated
 by the server responding with the next page.

- A transaction is initiated as part of sending a request to a

 server and terminated by the server responding.  There are no
 separate operations for setting up or terminating associations
 between clients and servers at the transport level.

- The server is free to discard communication state about a

 client between transactions without causing incorrect behavior
 or failures.

The term message transaction (or transaction) is used in the reminder of this document for a request-response exchange in the sense described above.

VMTP handles the error detection, retransmission, duplicate suppression and, optionally, security required for transport-level end-to-end reliability.

The protocol is designed to provide a range of behaviors within the transaction model, including:

- Minimal two packet exchanges for short, simple transactions.

- Streaming of multi-packet requests and responses for efficient

 data transfer.

- Datagram and multicast communication as an extension of the

 transaction model.

Example Uses:

- Page-level file access - VMTP is intended as the transport

 level for file access, allowing simple, efficient operation on
 a local network.  In particular, VMTP is appropriate for use
 by diskless workstations accessing shared network file

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 servers.

- Distributed programming - VMTP is intended to provide an

 efficient transport level protocol for remote procedure call
 implementations, distributed object-oriented systems plus
 message-based systems that conform to the request-response
 model.

- Multicast communication with groups of servers to: locate a

 specific object within the group, update a replicated object,
 synchronize the commitment of a distributed transaction, etc.

- Distributed real-time control with prioritized message

 handling, including datagrams, multicast and asynchronous
 calls.

The protocol is designed to operate on top of a simple unreliable datagram service, such as is provided by IP.

Motivation

VMTP was designed to address three categories of deficiencies with existing transport protocols in the Internet architecture. We use TCP as the key current transport protocol for comparison.

Poor RPC Performance

First, current protocols provide poor performance for remote procedure call (RPC) and network file access. This is attributable to three key causes:

- TCP requires excessive packets for RPC, especially for

 isolated calls.  In particular, connection setup and clear
 generates extra packets over that needed for VMTP to support
 RPC.

- TCP is difficult to implement, speaking purely from the

 empirical experience over the last 10 years.  VMTP was
 designed concurrently with its implementation, with focus on
 making it easy to implement and providing sensible subsets of
 its functionality.

- TCP handles packet loss due to overruns poorly. We claim that

 overruns are the key source of packet loss in a
 high-performance RPC environment and, with the increasing

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 performance of networks, will continue to be the key source.
 (Older machines and network interfaces cannot keep up with new
 machines and network interfaces.  Also, low-end network
 interfaces for high-speed networks have limited receive
 buffering.)

VMTP is designed for ease of implementation and efficient RPC. In addition, it provides selective retransmission with rate-based flow control, thus addressing all of the above issues.

Weak Naming

Second, current protocols provide inadequate naming of transport-level endpoints because the names are based on IP addresses. For example, a TCP endpoint is named by an Internet address and port identifier. Unfortunately, this makes the endpoint tied to a particular host interface, not specifically the process-level state associated with the transport-level endpoint. In particular, this form of naming causes problems for process migration, mobile hosts and multi-homed hosts. VMTP provides host-address independent names, thereby solving the above mentioned problems.

In addition, TCP provides no security and reliability guarantees on the dynamically allocated names. In particular, other than well-known ports, (host-addr, port-id)-tuples can change meaning on reboot following a crash. VMTP provides large identifiers with guarantee of stability, meaning that either the identifier never changes in meaning or else remains invalid for a significant time before becoming valid again.

Function Poor

TCP does not support multicast, real-time datagrams or security. In fact, it only supports pair-wise, long-term, streamed reliable interchanges. Yet, multicast is of growing importance and is being developed for the Internet (see RFC 966 and 988). Also, a datagram facility with the same naming, transmission and reception facilities as the normal transport level is a powerful asset for real-time and parallel applications. Finally, security is a basic requirement in an increasing number of environments. We note that security is natural to implement at the transport level to provide end-to-end security (as opposed to (inter)network level security). Without security at the transport level, a transport level protocol cannot guarantee the standard transport level service definition in the presence of an intruder. In particular, the intruder can interject packets or modify

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packets while updating the checksum, making mockery out of the transport-level claim of "reliable delivery".

In contrast, VMTP provides multicast, real-time datagrams and security, addressing precisely these weaknesses.

In general, VMTP is designed with the next generation of communication systems in mind. These communication systems are characterized as follows. RPC, page-level file access and other request-response behavior dominates. In addition, the communication substrate, both local and wide-area, provides high data rates, low error rates and relatively low delay. Finally, intelligent, high-performance network interfaces are common and in fact required to achieve performance that approximates the network capability. However, VMTP is also designed to function acceptably with existing networks and network interfaces.

Relation to Other Protocols

VMTP is a transport protocol that fits into the layered Internet protocol environment. Figure 1-1 illustrates the place of VMTP in the protocol hierarchy.

+-----------+ +----+ +-----------------+ +------+
|File Access| |Time| |Program Execution| |Naming|... Application
+-----------+ +----+ +-----------------+ +------+      Layer
   |           |           |             |      |
   +-----------+-----------+-------------+------+
                           |
                    +------------------+
                    | RPC Presentation |          Presentation
                    +------------------+          Layer
                              |
        +------+          +--------+
        |  TCP |          | VMTP   |              Transport
        +------+          +--------+              Layer
            |                  |
       +-----------------------------------+
       |       Internet Protocol & ICMP    |      Internetwork
       +-----------------------------------+      Layer

           Figure 1-1:   Relation to Other Protocols

The RPC presentation level is not currently defined in the Internet suite of protocols. Appendix II defines a proposed RPC presentation level for use with VMTP and assumed for the definition of the VMTP management procedures. There is also a need for the definition of the

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Application layer protocols listed above.

If internetwork services are not required, VMTP can be used without the IP layer, layered directly on top of the network or data link layers.

Document Overview

The next chapter gives an overview of the protocol, covering naming, message structure, reliability, flow control, streaming, real-time, security, byte-ordering and management. Chapter 3 describes the VMTP packet formats. Chapter 4 describes the client VMTP protocol operation in terms of pseudo-code for event handling. Chapter 5 describes the server VMTP protocol operation in terms of pseudo-code for event handling. Chapter 6 summarizes the state of the protocol, some remaining issues and expected directions for the future. Appendix I lists some standard Response codes. Appendix II describes the RPC presentation protocol proposed for VMTP and used with the VMTP management procedures. Appendix III lists the VMTP management procedures. Appendix IV proposes initial approaches for handling entity identification for VMTP. Appendix V proposes initial authentication domains for VMTP. Appendix VI provides some details for implementing VMTP on top of IP. Appendix VII provides some suggestions on host implementation of VMTP, focusing on data structures and support functions. Appendix VIII describes a proposed program interface for UNIX 4.3 BSD and its descendants and related systems.

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Protocol Overview

VMTP provides an efficient, reliable, optionally secure transport service in the message transaction or request-response model with the following features:

- Host address-independent naming with provision for multiple

 forms of names for endpoints as well as associated (security)
 principals.  (See Sections 2.1, 2.2, 3.1 and Appendix IV.)

- Multi-packet request and response messages, with a maximum

 size of 4 megaoctets per message.  (Sections 2.3 and 2.14.)

- Selective retransmission. (Section 2.13.) and rate-based flow

 control to reduce overrun and the cost of overruns.  (Section
 2.5.6.)

- Secure message transactions with provision for a variety of

 encryption schemes.  (Section 2.6.)

- Multicast message transactions with multiple response messages

 per request message.  (Section 2.7.)

- Support for real-time communication with idempotent message

 transactions with minimal server overhead and state (Section
 2.5.3), datagram request message transactions with no
 response, optional header-only checksum, priority processing
 of transactions, conditional delivery and preemptive handling
 of requests (Section 2.8)

- Forwarded message transactions as an optimization for certain

 forms of nested remote procedure calls or message
 transactions.  (Section 2.9.)

- Multiple outstanding (asynchronous) message transactions per

 client.  (Section 2.11.)

- An integrated management module, defined with a remote

 procedure call interface on top of VMTP providing a variety of
 communication services (Section 2.10.)

- Simple subset implementation for simple clients and simple

 servers.  (Section 2.16.)

This chapter provides an overview of the protocol as introduction to the basic ideas and as preparation for the subsequent chapters that describe the packet formats and event processing procedures in detail.

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In overview, VMTP provides transport communication between network- visible entities via message transactions. A message transaction consists of a request message sent by the client, or requestor, to a group of server entities followed by zero or more response messages to the client, at most one from each server entity. A message is structured as a message control portion and a segment data portion. A message is transmitted as one or more packet groups. A packet group is one or more packets (up to a maximum of 32 packets) grouped by the protocol for acknowledgment, sequencing, selective retransmission and rate control.

Entities and VMTP operations are managed using a VMTP management mechanism that is accessed through a procedural interface (RPC) implemented on top of VMTP. In particular, information about a remote entity is obtained and maintained using the Probe VMTP management operation. Also, acknowledgment information and requests for retransmission are sent as notify requests to the management module. (In the following description, reference to an "acknowledgment" of a request or a response refers to a management-level notify operation that is acknowledging the request or response.)

Entities, Processes and Principals

VMTP defines and uses three main types of identifiers: entity identifiers, process identifiers and principal identifiers, each 64-bits in length. Communication takes place between network-visible entities, typically mapping to, or representing, a message port or procedure invocation. Thus, entities are the VMTP communication endpoints. The process associated with each entity designates the agent behind the communication activity for purposes of resource allocation and management. For example, when a lock is requested on a file, the lock is associated with the process, not the requesting entity, allowing a process to use multiple entity identifiers to perform operations without lock conflict between these entities. The principal associated with an entity specifies the permissions, security and accounting designation associated with the entity. The process and principal identifiers are included in VMTP solely to make these values available to VMTP users with the security and efficiency provided by VMTP. Only the entity identifiers are actively used by the protocol.

Entity identifiers are required to have three properties;

Uniqueness Each entity identifier is uniquely defined at any given

            time.  (An entity identifier may be reused over time.)

Stability An entity identifier does not change between valid

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            meanings without suitable provision for removing
            references to the entity identifier.  Certain entity
            identifiers are strictly stable, (i.e. never changing
            meaning), typically being administratively assigned
            (although they need not be bound to a valid entity at
            all times), often called well-known identifiers.  All
            other entity identifiers are required to be T-stable,
            not change meaning without having remained invalid for
            at least a time interval T.

Host address independent

            An entity identifier is unique independent of the host
            address of its current host.  Moreover, an entity
            identifier is not tied to a single Internet host
            address.  An entity can migrate between hosts, reside on
            a mobile host that changes Internet addresses or reside
            on a multi-homed host.  It is up to the VMTP
            implementation to determine and maintain up to date the
            host addresses of entities with which it is
            communicating.

The stability of entity identifiers guarantees that an entity identifier represents the same logical communication entity and principal (in the security sense) over the time that it is valid. For example, if an entity identifier is authenticated as having the privileges of a given user account, it continues to have those privileges as long as it is continuously valid (unless some explicit notice is provided otherwise). Thus, a file server need not fully authenticate the entity on every file access request. With T-stable identifiers, periodically checking the validity of an entity identifier with period less than T seconds detects a change in entity identifier validity.

A group of entities can form an entity group, which is a set of zero or more entities identified by a single entity identifier. For example, one can have a single entity identifier that identifies the group of name servers. An entity identifier representing an entity group is drawn from the same name space as entity identifiers. However, single entity identifiers are flagged as such by a bit in the entity identifier, indicating that the identifier is known to identify at most one entity. In addition to the group bit, each entity identifier includes other standard type flags. One flag indicates whether the identifier is an alias for an entity in another domain (See Section 2.2 below.). Another flag indicates, for an entity group identifier, whether the identifier is a restricted group or not. A restricted group is one in which an entity can be added only by another entity with group management authorization. With an unrestricted group, an entity is allowed to add itself. If an entity identifier does not represent a

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group, a type bit indicates whether the entity uses big-endian or little-endian data representation (corresponding to Motorola 680X0 and VAX byte orders, respectively). Further specification of the format of entity identifiers is contained in Section 3.1 and Appendix IV.

An entity identifier identifies a Client, a Server or a group of Servers <1>. A Client is always identified by a T-stable identifier. A server or group of servers may be identified by a a T-stable identifier (group or single entity) or by strictly stable (statically assigned) entity group identifier. The same T-stable identifier can be used to identify a Client and Server simultaneously as long as both are logically associated with the same entity. The state required for reliable, secure communication between entities is maintained in client state records (CSRs), which include the entity identifier of the Client, its principal, its current or next transaction identifier and so on.

Entity Domains

An entity domain is an administration or an administration mechanism that guarantees the three required entity identifier properties of uniqueness, stability and host address independence for the entities it administers. That is, entity identifiers are only guaranteed to be unique and stable within one entity domain. For example, the set of all Internet hosts may function as one domain. Independently, the set of hosts local to one autonomous network may function as a separate domain. Each entity domain is identified by an entity domain identifier, Domain. Only entities within the same domain may communicate directly via VMTP. However, hosts and entities may participate in multiple entity domains simultaneously, possibly with different entity identifiers. For example, a file server may participate in multiple entity domains in order to provide file service to each domain. Each entity domain specifies the algorithms for allocation, interpretation and mapping of entity identifiers.

Domains are necessary because it does not appear feasible to specify one universal VMTP entity identification administration that covers all entities for all time. Domains limit the number of entities that need to be managed to maintain the uniqueness and stability of the entity

_______________

<1> Terms such as Client, Server, Request, Response, etc. are capitalized in this document when they refer to their specific meaning in VMTP.

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name space. Domains can also serve to separate entities of different security levels. For instance, allocation of a unclassified entity identifier cannot conflict with secret level entity identifiers because the former is interpreted only in the unclassified domain, which is disjoint from the secret domain.

It is intended that there be a small number of domains. In particular, there should be one (or a few) domains per installation "type", rather than per installation. For example, the Internet is expected to use one domain per security level, resulting in at most 8 different domains. Cluster-based internetwork architectures, those with a local cluster protocol distinct from the wide-area protocol, may use one domain for local use and one for wide-area use.

Additional details on the specification of specific domains is provided in Appendix IV.

Message Transactions

The message transaction is the unit of interaction between a Client that initiates the transaction and one or more Servers. A message transaction starts with a request message generated by a client. At the service interface, a server becomes involved with a transaction by receiving and accepting the request. A server terminates its involvement with a transaction by sending a response message. In a group message transaction, the server entity designated by the client corresponds to a group of entities. In this case, each server in the group receives a copy of the request. In the client's view, the transaction is terminated when it receives the response message or, in the case of a group message transaction, when it receives the last response message. Because it is normally impractical to determine when the last response message has been received. the current transaction is terminated by VMTP when the next transaction is initiated.

Within an entity domain, a transaction is uniquely identified by the tuple (Client, Transaction, ForwardCount). where Transaction is a 32-bit number and ForwardCount is a 4-bit value. A Client uses monotonically increasing Transaction identifiers for new message transactions. Normally, the next higher transaction number, modulo 2**32, is used for the next message transaction, although there are cases in which it skips a small range of Transaction identifiers. (See the description of the STI control flag.) The ForwardCount is used when a message transaction is forwarded and is zero otherwise.

A Client generates a stream of message transactions with increasing transaction identifiers, directed at a diversity of Servers. We say a

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Client has a transaction outstanding if it has invoked a message transaction, but has not received the last Response (or possibly any Response). Normally, a Client has only one transaction outstanding at a time. However, VMTP allows a Client to have multiple message transactions outstanding simultaneously, supporting streamed, asynchronous remote procedure call invocations. In addition, VMTP supports nested calls where, for example, procedure A calls procedure B which calls procedure C, each on a separate host with different client entity identifiers for each call but identified with the same process and principal.

Request and Response Messages

A message transaction consists of a request message and one or more Response messages. A message is structured as message control block (MCB) and segment data, passed as parameters, as suggested below.

 +-----------------------+
 | Message Control Block |
 +-----------------------+
 +-----------------------------------+
 |       segment data                |
 +-----------------------------------+

In the request message, the MCB specifies control information about the request plus an optional data segment. The MCB has the following format:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+                         ServerEntityId  (8 octets)            +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Flags       |         RequestCode                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+                         CoresidentEntity (8 octets)           +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>                         User Data (12 octets)                 <
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         MsgDelivery                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         SegmentSize                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The ServerEntityId is the entity to which the Request MCB is to be sent (or was sent, in the case of reception). The Flags indicate various options in the request and response handling as well as whether the

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CoresidentEntity, MsgDelivery and SegmentSize fields are in use. The RequestCode field specifies the type of Request. It is analogous to a packet type field of the Ethernet, acting as a switch for higher-level protocols. The CoresidentEntity field, if used, designates a subgroup of the ServerEntityId group to which the Request should be routed, namely those members that are co-resident with the specified entity (or entity group). The primary intended use is to specify the manager for a particular service that is co-resident with a particular entity, using the well-known entity group identifier for the service manager in the ServerEntityId field and the identifier for the entity in the CoresidentEntity field. The next 12 octets are user- or application-specified.

The MsgDelivery field is optionally used by the RPC or user level to specify the portions of the segment data to transmit and on reception, the portions received. It provides the client and server with (optional) access to, and responsibility for, a simple selective transmission and reception facility. For example, a client may request retransmission of just those portions of the segment that it failed to receive as part of the original Response. The primary intended use is to support highly efficient multi-packet reading from a file server. Exploiting user-level selective retransmission using the MsgDelivery field, the file server VMTP module need not save multi-packet Responses for retransmission. Retransmissions, when needed, are instead handled directly from the file server buffers.

The SegmentSize field indicates the size of the data segment, if present. The CoresidentEntity, MsgDelivery and SegmentSize fields are usable as additional user data if they are not otherwise used.

The Flags field provides a simple mechanism for the user level to communicate its use of VMTP options with the VMTP module as well as for VMTP modules to communicate this use among themselves. The use of these options is generally fixed for each remote procedure so that an RPC mechanism using VMTP can treat the Flags as an integral part of the RequestCode field for the purpose of demultiplexing to the correct stub.

A Response message control block follows the same format except the Response is sent from the Server to the Client and there is no Coresident Entity field (and thus 20 octets of user data).

Reliability

VMTP provides reliable, sequenced transfer of request and response messages as well as several variants, such as unreliable datagram requests. The reliability mechanisms include: transaction identifiers,

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checksums, positive acknowledgment of messages and timeout and retransmission of lost packets.

Transaction Identifiers

Each message transaction is uniquely identified by the pair (Client, Transaction). (We defer discussion of the ForwardCount field to Section 2.9.) The 32-bit transaction identifier is initialized to a random value when the Client entity is created or allocated its entity identifier. The transaction identifier is incremented at the end of each message transaction. All Responses with the same specified (Client, Transaction) pair are associated with this Request.

The transaction identifier is used for duplicate suppression at the Server. A Server maintains a state record for each Client for which it is processing a Request, identified by (Client, Transaction). A Request with the same (Client, Transaction) pair is discarded as a duplicate. (The ForwardCount field must also be equal.) Normally, this record is retained for some period after the Response is sent, allowing the Server to filter out subsequent duplicates of this Request. When a Request arrives and the Server does not have a state record for the sending Client, the Server takes one of three actions:

1. The Server may send a Probe request, a simple query

  operation, to the VMTP management module associated with the
  requesting Client to determine the Client's current
  Transaction identifier (and other information), initialize a
  new state record from this information, and then process the
  Request as above.

2. The Server may reason that the Request must be a new request

  because it does not have a state record for this Client if it
  keeps these state records for the maximum packet lifetime of
  packets in the network (plus the maximum VMTP retransmission
  time) and it has not been rebooted within this time period.
  That is, if the Request is not new either the Request would
  have exceeded the maximum packet lifetime or else the Server
  would have a state record for the Client.

3. The Server may know that the Request is idempotent or can be

  safely redone so it need not care whether the Request is a
  duplicate or not.  For example, a request for the current
  time can be responded to with the current time without being
  concerned whether the Request is a duplicate.  The Response
  is discarded at the Client if it is no longer of interest.

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Checksum

Each VMTP packet contains a checksum to allow the receiver to detect corrupted packets independent of lower level checks. The checksum field is 32 bits, providing greater protection than the standard 16-bit IP checksum (in combination with an improved checksum algorithm). The large packets, high packet rates and general network characteristics expected in the future warrant a stronger checksum mechanism.

The checksum normally covers both the VMTP header and the segment data. Optionally (for real-time applications), the checksum may apply only to the packet header, as indicated by the HCO control bit being set in the header. The checksum field is placed at the end of the packet to allow it to be calculated as part of a software copy or as part of a hardware transmission or reception packet processing pipeline, as expected in the next generation of network interfaces. Note that the number of header and data octets is an integral multiple of 8 because VMTP requires that the segment data be padded to be a multiple of 64 bits. The checksum field is appended after the padding, if any. The actual algorithm is described in Section 3.2.

A zero checksum field indicates that no checksum was transmitted with the packet. VMTP may be used without a checksum only when there is a host-to-host error detection mechanism and the VMTP security facility is not being used. For example, one could rely on the Ethernet CRC if communication is restricted to hosts on the same Ethernet and the network interfaces are considered sufficiently reliable.

Request and Response Acknowledgment

VMTP assumes an unreliable datagram network and internetwork interface. To guarantee delivery of Requests and Response, VMTP uses positive acknowledgments, retransmissions and timeouts.

A Request is normally acknowledged by receipt of a Response associated with the Request, i.e. with the same (Client, Transaction). With streamed message transactions, it may also be acknowledged by a subsequent Response that acknowledges previous Requests in addition to the transaction it explicitly identifies. A Response may be explicitly acknowledged by a NotifyVmtpServer operation requested of the manager for the Server. In the case of streaming, this is a cumulative acknowledgment, acknowledging all Responses with a lower transaction identifier as well.) In addition, with non-streamed communication, a subsequent Request from the same Client acknowledges Responses to all previous message transactions (at least in the sense that either the client received a Response or is no longer interested in Responses to

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those earlier message transactions). Finally, a client response timeout (at the server) acknowledges a Response at least in the sense that the server need not be prepared to retransmit the Response subsequently. Note that there is no end-to-end guarantee of the Response being received by the client at the application level.

Retransmissions

In general, a Request or Response is retransmitted periodically until acknowledged as above, up to some maximum number of retransmissions. VMTP uses parameters RequestRetries(Server) and ResponseRetries(Client) that indicate the number of retransmissions for the server and client respectively before giving up. We suggest the value 5 be used for both parameters based on our experience with VMTP and Internet packet loss. Smaller values (such as 3) could be used in low loss environments in which fast detection of failed hosts or communication channels is required. Larger values should be used in high loss environments where transport-level persistence is important.

In a low loss environment, a retransmission only includes the MCB and not the segment data of the Request or Response, resulting in a single (short) packet on retransmission. The intended recipient of the retransmission can request selective retransmission of all or part of the segment data as necessary. The selective retransmission mechanism is described in Section 2.13.

If a Response is specified as idempotent, the Response is neither retransmitted nor stored for retransmission. Instead, the Client must retransmit the Request to effectively get the Response retransmitted. The server VMTP module responds to retransmissions of the Request by passing the Request on to the server again to have it regenerate the Response (by redoing the operation), rather than saving a copy of the Response. Only Request packets for the last transaction from this client are passed on in this fashion; older Request packets from this client are discarded as delayed duplicates. If a Response is not idempotent, the VMTP module must ensure it has a copy of the Response for retransmission either by making a copy of the Response (either physically or copy-on-write) or by preventing the Server from continuing until the Response is acknowledged.

Timeouts

There is one client timer for each Client with an outstanding transaction. Similarly, there is one server timer for each Client transaction that is "active" at the server, i.e. there is a transaction

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record for a Request from the Client.

When the client transmits a new Request (without streaming), the client timer is set to roughly the time expected for the Response to be returned. On timeout, the Request is retransmitted with the APG (Acknowledge Packet Group) bit set. The timeout is reset to the expected roundtrip time to the Server because an acknowledgment should be returned immediately unless a Response has been sent. The Request may also be retransmitted in response to receipt of a VMTP management operation indicating that selected portions of the Request message segment need to be retransmitted. With streaming, the timeout applies to the oldest outstanding message transaction in the run of outstanding message transactions. Without streaming, there is one message transaction in the run, reducing to the previous situation. After the first packet of a Response is received, the Client resets the timeout to be the time expected before the next packet in the Response packet group is received, assuming it is a multi-packet Response. If not, the timer is stopped. Finally, the client timer is used to timeout waiting for second and subsequent Responses to a multicast Request.

The client timer is set at different times to four different values:

TC1(Server) The expected time required to receive a Response from

            the Server.  Set on initial Request transmission plus
            after its management module receives a NotifyVmtpClient
            operation, acknowledging the Request.

TC2(Server) The estimated round trip delay between the client and

            the server.  Set when retransmitting after receiving no
            Response for TC1(Server) time and retransmitting the
            Request with the APG bit set.

TC3(Server) The estimated maximum expected interpacket time for

            multi-packet Responses from the Server.  Set when
            waiting for subsequent Response packets within a packet
            group before timing out.

TC4 The time to wait for additional Responses to a group

            Request after the first Response is received.  This is
            specified by the user level.

These values are selected as follows. TC1 can be set to TC2 plus a constant, reflecting the time within which most servers respond to most requests. For example, various measurements of VMTP usage at Stanford indicate that 90 percent of the servers respond in less than 200 milliseconds. Setting TC1 to TC2 + 200 means that most Requests receive a Response before timing out and also that overhead for retransmission

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for long running transactions is insignificant. A sophisticated implementation may make the estimation of TC1 further specific to the Server.

TC2 may be estimated by measuring the time from when a Probe request is sent to the Server to when a response is received. TC2 can also be measured as the time between the transmission of a Request with the APG bit set to receipt of a management operation acknowledging receipt of the Request.

When the Server is an entity group, TC1 and TC2 should be the largest of the values for the members of the group that are expected to respond. This information may be determined by probing the group on first use (and using the values for the last responses to arrive). Alternatively, one can resort to default values.

TC3 is set initially to 10 times the transmission time for the maximum transmission unit (MTU) to be used for the Response. A sophisticated implementation may record TC3 per Server and refine the estimate based on measurements of actual interpacket gaps. However, a tighter estimate of TC3 only improves the reaction time when a packet is lost in a packet group, at some cost in unnecessary retransmissions when the estimate becomes overly tight.

The server timer, one per active Client, takes on the following values:

TS1(Client) The estimated maximum expected interpacket time. Set

            when waiting for subsequent Request packets within a
            packet group before timing out.

TS2(Client) The time to wait to hear from a client before

            terminating the server processing of a Request.  This
            limits the time spent processing orphan calls, as well
            as limiting how out of date the server's record of the
            Client state can be.  In particular, TS2 should be
            significantly less than the minimum time within which it
            is reasonable to reuse a transaction identifier.

TS3(Client) Estimated roundtrip time to the Client,

TS4(Client) The time to wait after sending a Response (or last

            hearing from a client) before discarding the state
            associated with the Request which allows it to filter
            duplicate Request packets and regenerate the Response.

TS5(Client) The time to wait for an acknowledgment after sending a

            Response before retransmitting the Response, or giving

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            up (after some number of retransmissions).

TS1 is set the same as TC3.

The suggested value for TS2 is TC1 + 3*TC2 for this server, giving the Client time to timeout waiting for a Response and retransmit 3 Request packets, asking for acknowledgments.

TS3 is estimated the same as TC1 except that refinements to the estimate use measurements of the Response-to-acknowledgment times.

In the general case, TS4 is set large enough so that a Client issuing a series of closely-spaced Requests to the same Server reuses the same state record at the Server end and thus does not incur the overhead of recreating this state. (The Server can recreate the state for a Client by performing a Probe on the Client to get the needed information.) It should also be set low enough so that the transaction identifier cannot wrap around and so that the Server does not run out of CSR's. We suggest a value in the range of 500 milliseconds. However, if the Server accepts non-idempotent Requests from this Client without doing a Probe on the Client, the TS4 value for this CSR is set to at least 4 times the maximum packet lifetime.

TS5 is TS3 plus the expected time for transmission and reception of the Response. We suggest that the latter be calculated as 3 times the transmission time for the Response data, allowing time for reception, processing and transmission of an acknowledgment at the Client end. A sophisticated implementation may refine this estimate further over time by timing acknowledgments to Responses.

Rate Control

VMTP is designed to deal with the present and future problem of packet overruns. We expect overruns to be the major cause of dropped packets in the future. A client is expected to estimate and adjust the interpacket gap times so as to not overrun a server or intermediate nodes. The selective retransmission mechanism allows the server to indicate that it is being overrun (or some intermediate point is being overrun). For example, if the server requests retransmission of every Kth block, the client should assume overrun is taking place and increase the interpacket gap times. The client passes the server an indication of the interpacket gap desired for a response. The client may have to increase the interval because packets are being dropped by an intermediate gateway or bridge, even though it can handle a higher rate. A conservative policy is to increase the interpacket gap whenever a packet is lost as part of a multi-packet packet group.

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The provision of selective retransmission allows the rate of the client and the server to "push up" against the maximum rate (and thus lose packets) without significant penalty. That is, every time that packet transmission exceeds the rate of the channel or receiver, the recovery cost to retransmit the dropped packets is generally far less than retransmitting from the first dropped packet.

The interpacket gap is expressed in 1/32nd's of the MTU packet transmission time. The minimum interpacket gap is 0 and the maximum gap that can be described in the protocol is 8 packet times. This places a limit on the slowest receivers that can be efficiently used on a network, at least those handling multi-packet Requests and Responses. This scheme also limits the granularity of adjustment. However, the granularity is relative to the speed of the network, as opposed to an absolute time. For entities on different networks of significantly different speed, we assume the interconnecting gateways can buffer packets to compensate<2>. With different network speeds and intermediary nodes subject to packet loss, a node must adjust the interpacket gap based on packet loss. The interpacket gap parameter may be of limited use.

Security

VMTP provides an (optional) secure mode that protects against the usual security threats of peeking, impostoring, message tampering and replays. Secure VMTP must be used to guarantee any of the transport-level reliability properties unless it is guaranteed that there are no intruders or agents that can modify packets and update the packet checksums. That is, non-secure VMTP provides no guarantees in the presence of an intelligent intruder.

The design closely follows that described by Birrell [1]. Authenticated information about a remote entity, including an encryption/decryption key, is obtained and maintained using a VMTP management operation, the authenticated Probe operation, which is executed as a non-secure VMTP message transaction. If a server receives a secure Request for which the server has no entity state, it sends a Probe request to the VMTP

_______________

<2> Gateways must also employ techniques to preserve or intelligently modify (if appropriate) the interpacket gaps. In particular, they must be sure not to arbitrarily remove interpacket gaps as a result of their forwarding of packets.

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management module of the client, "challenging" it to provide an authenticator that both authenticates the client as being associated with a particular principal as well as providing a key for encryption/decryption. The principal can include a real and effective principal, as used in UNIX <3>. Namely, the real principal is the principal on whose behalf the Request is being performed whereas the effective principal is the principal of the module invoking the request or remote procedure call.

Peeking is prevented by encrypting every Request and Response packet with a working Key that is shared between Client and Server. Impostoring and replays are detected by comparing the Transaction identifier with that stored in the corresponding entity state record (which is created and updated by VMTP as needed). Message tampering is detected by encryption of the packet including the Checksum field. An intruder cannot update the checksum after modifying the packet without knowing the Key. The cost of fully encrypting a packet is close to the cost of generating a cryptographic checksum (and of course, encryption is needed in the general case), so there is no explicit provision for cryptographic checksum without packet encryption.

A Client determines the Principal of the Server and acquires an authenticator for this Server and Principal using a higher level protocol. The Server cannot decrypt the authenticator or the Request packets unless it is in fact the Principal expected by the Client.

An encrypted VMTP packet is flagged by the EPG bit in the VMTP packet header. Thus, encrypted packets are easily detected and demultiplexed from unencrypted packets. An encrypted VMTP packet is entirely encrypted except for the Client, Version, Domain, Length and Packet Flags fields at the beginning of the packet. Client identifiers can be assigned, changed and used to have no real meaning to an intruder or to only communicate public information (such as the host Internet address). They are otherwise just a random means of identification and demultiplexing and do not therefore divulge any sensitive information. Further secure measures must be taken at the network or data link levels if this information or traffic behavior is considered sensitive.

VMTP provides multiple authentication domains as well as an encryption qualifier to accommodate different encryption algorithms and their

_______________

<3> Principal group membership must be obtained, if needed, by a higher level protocol.

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corresponding security/performance trade-offs. (See Appendix V.) A separate key distribution and authentication protocol is required to handle generation and distribution of authenticators and keys. This protocol can be implemented on top of VMTP and can closely follow the Birrell design as well.

Security is optional in the sense that messages may be secure or non-secure, even between consecutive message transactions from the same client. It is also optional in that VMTP clients and servers are not required to implement secure VMTP (although they are required to respond intelligently to attempts to use secure VMTP). At worst, a Client may fail to communicate with a Server if the Server insists on secure communication and the Client does not implement security or vice versa. However, a failure to communicate in this case is necessary from a security standpoint.

Multicast

The Server entity identifier in a message transaction can identify an entity group, in which case the Request is multicast to every Entity in this group (on a best-efforts basis). The Request is retransmitted until at least one Response is received (or an error timeout occurs) unless it is a datagram Request. The Client can receive multiple Responses to the Request.

The VMTP service interface does not directly provide reliable multicast because it is expensive to provide, rarely needed by applications, and can be implemented by applications using the multiple Response feature. However, the protocol itself is adequate for reliable multicast using positive acknowledgments. In particular, a sophisticated Client implementation could maintain a list of members for each entity group of interest and retransmit the Request until acknowledged by all members. No modifications are required to the Server implementations.

VMTP supports a simple form of subgroup addressing. If the CRE bit is set in a Request, the Request is delivered to the subgroup of entities in the Server group that are co-resident with one or more entities in the group (or individual entity) identified by the CoresidentEntity field of the Request. This is commonly used to send to the manager entity for a particular entity, where Server specifies the group of such managers. Co-resident means "using the same VMTP module", and logically on the same network host. In particular, a Probe request can be sent to the particular VMTP management module for an entity by specifying the VMTP management group as the Server and the entity in question as the CoResidentEntity.

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As an experimental aspect of the protocol, VMTP supports the Server sending a group Response which is sent to the Client as well as members of the destination group of Servers to which the original Request was sent. The MDG bit indicates whether the Client is a member of this group, allowing the Server module to determine whether separately addressed packet groups are required to send the Response to both the Client and the Server group. Normally, a Server accepts a group Response only if it has received the Request and not yet responded to the Client. Also, the Server must explicitly indicate it wants to accept group Responses. Logically, this facility is analogous to responding to a mail message sent to a distribution list by sending a copy of the Response to the distribution list.

Real-time Communication

VMTP provides three forms of support for real-time communication, in addition to its standard facilities, which make it applicable to a wide range of real-time applications. First, a priority is transmitted in each Request and Response which governs the priority of its handling. The priority levels are intended to correspond roughly to:

- urgent/emergency.

- important

- normal

- background.

with additional gradations for each level. The interpretation and implementation of these priority levels is otherwise host-specific, e.g. the assignment to host processing priorities.

Second, datagram Requests allow the Client to send a datagram to another entity or entity group using the VMTP naming, transmission and delivery mechanism, but without blocking, retransmissions or acknowledgment. (The client can still request acknowledgment using the APG bit although the Server does not expect missing portions of a multi-packet datagram Request to be retransmitted even if some are not received.) A datagram Request in non-streamed mode supersedes all previous Requests from the same Client. A datagram Request in stream mode is queued (if necessary) after previous datagram Requests on the same stream. (See Section 2.11.)

Finally, VMTP provides several control bit flags to modify the handling of Requests and Responses for real-time requirements. First, the

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conditional message delivery (CMD) flag causes a Request to be discarded if the recipient is not waiting for it when it arrives, similarly for the Response. This option allows a client to send a Request that is contingent on the server being able to process it immediately. The header checksum only (HCO) flag indicates that the checksum has been calculated only on the VMTP header and not on the data segment. Applications such as voice and video can avoid the overhead of calculating the checksum on data whose utility is insensitive to typical bit errors without losing protection on the header information. Finally, the No Retransmission (NRT) flag indicates that the recipient of a message should not ask for retransmission if part of the message is missing but rather either use what was received or discard it.

None of these facilities introduce new protocol states. In fact, the total processing overhead in the normal case is a bit flag test for CMD, HCO or NRT plus assignment of priority on packet transmission and reception. (In fact, CMD and NRT are not tested in the normal case.) The additional code complexity is minimal. We feel that the overhead for providing these real-time facilities is minimal and that these facilities are both important and adequate for a wide class of real-time applications.

Several of the normal facilities of VMTP appear useful for real-time applications. First, multicast is useful for distributed, replicated (fault-tolerant) real-time applications, allowing efficient state query and update for (for example) sensors and control state. Second, the DGM or idempotent flag for Responses has some real-time benefits, namely: a Request is redone to get the latest values when the Response is lost, rather than just returning the old values. The desirability of this behavior is illustrated by considering a request for the current time of day. An idempotent handling of this request gives better accuracy in returning the current time in the case that a retransmission is necessary. Finally, the request-response semantics (in the absence of streaming) of each new Request from a Client terminating the previous message transactions from that Client, if any, provides the "most recent is most important" handling of processing that most real-time applications require.

In general, a key design goal of VMTP was provide an efficient general-purpose transport protocol with the features required for real-time communication. Further experience is required to determine whether this goal has been achieved.

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Forwarded Message Transactions

A Server may invoke another Server to handle a Request. It is fairly common for the invocation of the second Server to be the last action performed by the first Server as part of handling the Request. For example, the original Server may function primarily to select a process to handle the Request. Also, the Server may simply check the authorization on the Request. Describing this situation in the context of RPC, a nested remote procedure call may be the last action in the remote procedure and the return parameters are exactly those of the nested call. (This situation is analogous to tail recursion.)

As an optimization to support this case, VMTP provides a Forward operation that allows the server to send the nested Request to the other server and have this other server respond directly to the Client.

If the message transaction being forwarded was not multicast, not secure or the two Servers are the same principal and the ForwardCount of the Request is less than the maximum forward count of 15, the Forward operation is implemented by the Server sending a Request onto the next Server with the forwarded Request identified by the same Client and Transaction as the original Request and a ForwardCount one greater than the Request received from the Client. In this case, the new Server responds directly to the Client. A forwarded Request is illustrated in the following figure.

+---------+   Request       +----------+
| Client  +---------------->| Server 1 |
+---------+                 +----------+
  ^                        |
  |                        | forwarded Request
  |                        V
  |   Response           +----------+
  +----------------------| Server 2 |
                         +----------+

If the message transaction does not meet the above requirements, the Server's VMTP module issues a nested call and simply maps the returned Response to a Response to original Request without further Server-level processing. In this case, the only optimization over a user-level nested call is one fewer VMTP service operation; the VMTP module handles the return to the invoking call directly. The Server may also use this form of forwarding when the Request is part of a stream of message transactions. Otherwise, it must wait until the forwarded message transaction completes before proceeding with the subsequent message transactions in the stream.

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Implementation of the user-level Forward operation is optional, depending on whether the server modules require this facility. Handling an incoming forwarded Request is a minor modification of handling a normal incoming Request. In particular, it is only necessary to examine the ForwardCount field when the Transaction of the Request matches that of the last message transaction received from the Client. Thus, the additional complexity in the VMTP module for the required forwarding support is minimal; the complexity is concentrated in providing a highly optimized user-level Forward primitive, and that is optional.

2.10. VMTP Management

VMTP management includes operations for creating, deleting, modifying and querying VMTP entities and entity groups. VMTP management is logically implemented by a VMTP management server module that is invoked using a message transaction addressed to the Server, VMTP_MANAGER_GROUP, a well-known group entity identifier, in conjunction with Coresident Entity mechanism introduced in Section 2.7. A particular Request may address the local module, the module managing a particular entity, the set of modules managing those entities contained in a specific group or all management modules, as appropriate.

The VMTP management procedures are specified in Appendix III.

2.11. Streamed Message Transactions

Streamed message transactions refer to two or more message transactions initiated by a Client before it receives the response to the first message transaction, with each transaction being processed and responded to in order but asynchronous relative to the initiation of the transactions. A Client streams messages transactions, and thereby has multiple message transactions outstanding, by sending them as part of a single run of message transactions. A run of message transactions is a sequence of message transactions with the same Client and Server and consecutive Transaction identifiers, with all but the first and last Requests and Responses flagged with the NSR (Not Start Run) and NER (Not End Run) control bits. (Conversely, the first Request and Response does not have the NSR set and the last Request and Response does not have the NER bit set.) The message transactions in a run use

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consecutive transaction identifiers (except if the STI bit <4> is used in one, in which case the transaction identifier for the next message transaction is 256 greater, rather than 1).

The Client retains a record for each outstanding transaction until it gets a Response or is timed out in error. The record provides the information required to retransmit the Request. On retransmission timeout, the client retransmits the last Request for which it has not received a Response the same as is done with non-streamed communication. (I.e. there need be only one timeout for all the outstanding message transactions associated with a single client.)

The consecutive transaction identifiers within a run of message transactions are used as sequence numbers for error control. The Server handles each message transaction in the sequence specified by its transaction identifier. When it receives a message transaction that is not marked as the beginning of a run, it checks that it previously received a message transaction with the predecessor transaction identifier, either 1 less than the current one or 256 less if the previous one had the STI bit set. If not, the Server sends a NotifyVmtpClient operation to the Client's manager indicating either: (1) the first message transaction was not fully received, or else (2) it has no record of the last one received. If the NRT control flag is set, it does not await nor expect retransmission but proceeds with handling this Request. This flag is used primarily when datagram Requests are used as part of a stream of message transactions. If NRT was not specified, the Client must retransmit from the first message transaction not fully received (either at all or in part) before the Server can proceed with handling this run of Requests or else restart the run of message transactions.

The Client expects to receive the Responses in a consecutive sequence, using the Transaction identifier to detect missing Responses. Thus, the Server must return Responses in sequence except possibly for some gaps, as follows. The Server can specify in the PGcount field in a Response, the number of consecutively previous Responses that this Response

_______________

<4> The STI bit is used by the Client to effectively allocate 255 transaction identifiers for use by the Server in returning a large Response or stream of Responses.

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corresponds to, up to a maximum of 255 previous Responses <5>. Thus, for example, a Response with Transaction identifier 46 and PGcount 3 represents Responses 43, 44, 45 and 46. This facility allows the Server to eliminate sending Responses to Requests that require no Response, effectively batching the Responses into one. It also allows the Server to effectively maintain strictly consecutive sequencing when the Client has skipped 256 Transaction identifiers using the STI bit and the Server does not have that many Responses to return.

If the Client receives a Response that is not consecutive, it retransmits the Request(s) for which the Response(s) is/are missing (unless, of course, the corresponding Requests were sent as datagrams). The Client should wait at the end of a run of message transactions for the last one to complete.

When a Server receives a Request with the NSR bit clear and a higher transaction identifier than it currently has for the Client, it terminates all processing and discards Responses associated with the previous Requests. Thus, a stream of message transactions is effectively aborted by starting a new run, even if the Server was in the middle of handling the previous run.

Using a mixture of datagram and normal Requests as part of a stream of message transactions, particularly with the use of the NRT bit, can lead to complex behavior under packet loss. It is recommended that a run of message transactions be all of one type to avoid problems, i.e. all normal or all datagrams. Finally, when a Server forwards a Request that is part of a run, it must suspend further processing of the subsequent Requests until the forwarded Request has been handled, to preserve order of processing. The simplest handling of this situation is to use a real nested call when forwarding with streamed message transactions.

Flow control of streamed message transactions relies on rate control at the Client plus receipt (or non-receipt) of management notify operations indicating the presence of overrunning. A Client must reduce the number of outstanding message transactions at the Server when it receives a NotifyVmtpServer operation with the MSGTRANS_OVERFLOW ResponseCode. The transact parameter indicates the last packet group that was accepted.

_______________

<5> PGcount actually corresponds to packet groups which are described in Section 2.13. This (simplified) description is accurate when there is one Request or Response per packet group.

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The implementation of multiple outstanding message transactions requires the ability to record, timeout and buffer multiple outstanding message transactions at the Client end as well as the Server end. However, this facility is optional for both the Client and the Server. Client systems with heavy-weight processes and high network access cost are most likely to benefit from this facility. Servers that serve a wide variety of client machines should implement streaming to accommodate these types of clients.

2.12. Fault-Tolerant Applications

One approach to fault-tolerant systems is to maintain a log of all messages sent at each node and replay the messages at a node when the node fails, after restarting it from the last checkpoint <6>. As an experimental facility, VMTP provides a Receive Sequence Number field in the NotifyVmtpClient and NotifyVmtpServer operations as well as the Next Receive Sequence (NRS) flag in the Response packet to allow a sender to log a receive sequence number with each message sent, allowing the packets to be replayed at a recovering node in the same sequence as they were originally received, thereby recovering to the same state as before.

Basically, each sending node maintains a receive sequence number for each receiving node. On sending a Request to a node, it presume that the receive sequence number is one greater than the one it has recorded for that node. If not, the receiving node sends a notify operation indicating the receive sequence number assigned the Request. The NRS in the Response confirms that the Request message was the next receive sequence number, so the sender can detect if it failed to receive the notify operation in the previous case. With Responses, the packets are ordered by the Transaction identifier except for multicast message transactions, in which there may be multiple Responses with the same identification. In this case, NotifyVmtpServer operations are used to provide receive sequence numbers.

This experimental extension of the protocol is focused on support for fault-tolerant real-time distributed systems required in various critical applications. It may be removed or extended, depending on further investigations.

_______________

<6> The sender-based logging is being investigated by Willy Zwaenepoel of Rice University.

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2.13. Packet Groups

A message (whether Request or Response) is sent as one or more packet groups. A packet group is one or more packets, each containing the same transaction identification and message control block. Each packet is formatted as below with the message control block logically embedded in the VMTP header.

+------------------------------------++---------------------+
|            VMTP Header             ||                     |
+------------+-----------------------||   segment data      |
|VMTP Control| Message Control Block ||                     |
+------------+-----------------------++---------------------+

The some fields of the VMTP control portion of the packet and data segment portion can differ between packets within the same packet group.

The segment data portion of a packet group represents up to 16 kilooctets of the segment specified in the message control block. The portion contained in each packet is indicated by the PacketDelivery field contained in the VMTP header. The PacketDelivery field as a bit mask has a similar interpretation to the MsgDelivery field in that each bit corresponds to a segment data block of 512 octets. The PacketDelivery field limits a packet group to 16 kilooctets and a maximum of 32 VMTP packets (with a minimum of 1 packet). Data can be sent in fewer packets by sending multiple data blocks per packet. We require that the underlying datagram service support delivery of (at minimum) the basic 580 octet VMTP packet <7>. To illustrate the use of the PacketDelivery field, consider for example the Ethernet which has a MTU of 1536 octets. so one would send 2 512-octet segment data blocks per packet. (In fact, if a third block is last in the segment and less than 512 octets and fits in the packet without making it too big, an Ethernet packet could contain three data blocks. Thus, an Ethernet packet group for a segment of size 0x1D00 octets (14.5 blocks) and MsgDelivery 0x000074FF consists of 6 packets indicated as follows <8>.

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<7> Note that with a 20 octet IP header, a VMTP packet is 600 octets. We propose the convention that any host implementing VMTP implicitly agrees to accept IP/VMTP packets of at least 600 octets.

<8> We use the C notation 0xHHHH to represent a hexadecimal number.

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Packet
Delivery  1 1  1 1  1 1  1 1  0 0  1 0  1 0  1 0  0 0 0 0 0 . . .
       0000 0400 0800 0C00 1000 1400 1800 1C00
      +----+----+----+----+----+----+----+-+
Segment  |....|....|....|....|....|....|....|.|
      +----+----+----+----+----+----+----+-+
      :    :    :    :    :    :  : /  /   :
      v    v    v    v    v    v  v   /|   v
      +----+----+----+----+    +----+  +---+
Packets  |  1 |  2 |  3 |  4 |    |  5 |  | 6 |
      +----+----+----+----+    +----+  +---+

Each '.' is 256 octets of data. The PacketDelivery masks for the 6 packets are: 0x00000003, 0x0000000C, 0x00000030, 0x000000C0, 0x00001400 and 0x00006000, indicating the segment blocks contained in each of the packets. (Note that the delivery bits are in little endian order.)

A packet group is sent as a single "blast" of packets with no explicit flow control. However, the sender should estimate and transmit at a rate of packet transmission to avoid congesting the network or overwhelming the receiver, as described in Section 2.5.6. Packets in a packet group can be sent in any order with no change in semantics.

When the first packet of a packet group is received (assuming the Server does not decide to discard the packet group), the Server saves a copy of the VMTP packet header, indicates it is currently receiving a packet group, initializes a "current delivery mask" (indicating the data in the segment received so far) to 0, accepts this packet (updating the current delivery mask) and sets the timer for the packet group. Subsequent packets in the packet group update the current delivery mask.

Reception of a packet group is terminated when either the current delivery mask indicates that all the packets in the packet group have been received or the packet group reception timer expires (set to TC3 or TS1). If the packet group reception timer expires, if the NRT bit is set in the Control flags then the packet group is discarded if not complete unless MDM is set. In this case, the MsgDelivery field in the message control block is set to indicate the segment data blocks actually received and the message control block and segment data received is delivered to application level.

If NRT is not set and not all data blocks have been received, a NotifyVmtpClient (if a Request) or NotifyVmtpServer (if a Response) is sent back with a PacketDelivery field indicating the blocks received. The source of the packet group is then expected to retransmit the missing blocks. If not all blocks of a Request are received after RequestAckRetries(Client) retransmissions, the Request is discarded and

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a NotifyVmtpClient operation with an error response code is sent to the client's manager unless MDM is set. With a Response, there are ResponseAckRetries(Server) retransmissions and then, if MDM is not set, the requesting entity is returned the message control block with an indication of the amount of segment data received extending contiguously from the start of the segment. E.g. if the sender sent 6 512-octet blocks and only the first two and the last two arrived, the receiver would be told that 1024 octets were received. The ResponseCode field is set to BAD_REPLY_SEGMENT. (Note that VMTP is only able to indicate the specific segment blocks received if MDM is set.)

The parameters RequestAckRetries(Client) and ResponseAckRetries(Server) could be set on a per-client and per-server basis in a sophisticated implementation based on knowledge of packet loss.

If the APG flag is set, a NotifyVmtpClient or NotifyVmtpServer operation is sent back at the end of the packet group reception, depending on whether it is a Request or a Response.

At minimum, a Server should check that each packet in the packet group contains the same Client, Server, Transaction identifier and SegmentSize fields. It is a protocol error for any field other than the Checksum, packet group control flags, Length and PacketDelivery in the VMTP header to differ between any two packets in one packet group. A packet group containing a protocol error of this nature should be discarded.

Notify operations should be sent (or invoked) in the manager whenever there is a problem with a unicast packet. i.e. negative acknowledgments are always sent in this case. In the case of problems with multicast packets, the default is to send nothing in response to an error condition unless there is some clear reason why no other node can respond positively. For example, the packet might be a Probe for an entity that is known to have been recently existing on the receiving host but now invalid and could not have migrated. In this case, the receiving host responds to the Probe indicating the entity is nonexistent, knowing that no other host can respond to the Probe. For packets and packet groups that are received and processed without problems, a Notify operation is invoked only if the APG bit is set.

2.14. Runs of Packet Groups

A run of packet groups is a sequence of packet groups, all Request packets or all Response packets, with the same Client and consecutive transaction identifiers, all but the first and last packets flagged with the NSR (Not Start Run) and NER (Not End Run) control bits. When each packet group in the run corresponds to a single Request or Response, it

Cheriton [page 31]

is identical to a run of message transactions. (See Section 2.11) However, a Request message or a Response message may consists of up to 256 packet groups within a run, for a maximum of 4 megaoctets of segment data. A message that is continued in the next packet group in the run is flagged in the current packet group by the CMG flag. Otherwise, the next packet group in the run (if any) is treated as a separate Request or Response.

Normally, each Request and Response message is sent as a single packet group and each run consists of a single packet group. In this case neither NSR or NER are set. For multi-packet group messages, the PacketDelivery mask in the i-th packet group of a message corresponds to the portion of the segment offset by i-1 times 16 kilooctets, designating the the first packet group to have i = 1.

2.15. Byte Order

For purposes of transmission and reception, the MCB is treated as consisting of 8 32-bit fields and the segment is a sequence of bytes. VMTP transmits the MCB in big-endian order, performing byte-swapping, if necessary, before transmission. A little-endian host must byte-swap the MCB on reception. (The data segment is transmitted as a sequence of bytes with no reordering.) The byte order of the sender of a message is indicated by the LEE bit in the entity identifier for the sender, the Client field if a Request and the Server field if a Response. The sender and receiver of a message are required to agree in some higher level protocol (such as an RPC presentation protocol) on who does further swapping of the MCB and data segment if required by the types of the data actually being transmitted. For example, the segment data may contain a record with 8-bit, 16-bit and 32-bit fields, so additional transformation is required to move the segment from a host of one byte order to another.

VMTP to date has used a higher-level presentation protocol in which segment data is sent in the native order of the sending host and byte-swapped as necessary by the receiving host. This approach minimizes the byte-swapping overhead between machines of common byte order (including when the communication is transparently local to one host), avoids a strong bias in the protocol to one byte-order, and allows for the sending entity to be sending to a group of hosts with different byte orders. (Note that the byte-swap overhead for the MCB is minimal.) The presentation-level overhead is minimal because most common operations, such as file access operations, have parameters that fit the MCB and data segment data types exactly.

Cheriton [page 32]

2.16. Minimal VMTP Implementation

A minimal VMTP client needs to be able to send a Request packet group and receive a Response packet group as well as accept and respond to Requests sent to its management module, including Probe and NotifyClient operations. It may also require the ability to invoke Probe and Notify operations to locate a Server and acknowledge responses. (the latter only if it is involved in transactions that are not idempotent or datagram message transactions. However, a simple sensor, for example, can transmit VMTP datagram Requests indicating its current state with even less mechanism.) The minimal client thus requires very little code and is suitable as a basis for (e.g.) a network boot loader.

A minimal VMTP server implements idempotent, non-encrypted message transactions, possibly with no segment data support. It should use an entity state record for each Request but need only retain it while processing the Request. Without segment data larger than a packet, there is no need for any timers, buffering (outside of immediate request processing) or queuing. In particular, it needs only as many records as message transactions it handles simultaneously (e.g. 1). The entity state record is required to recognize and respond to Request retransmissions during request processing.

The minimal server need only receive Requests and and be able to send Response packets. It need have only a minimal management module supporting Probe operations. (Support for the NotifyVmtpClient operation is only required if it does not respond immediately to a Request.) Thus the VMTP support for say a time server, sensor, or actuator can be extremely simple. Note that the server need never issue a Probe operation if it uses the host address of the Request for the Response and does not require the Client information returned by the Probe operation. The minimal server should also support reception of forwarded Requests.

2.17. Message vs. Procedural Request Handling

A request-response protocol can be used to implement two forms of semantics on reception. With procedural handling of a Request, a Request is handled by a process associated with the Server that effectively takes on the identity of the calling process, treating the Request message as invoking a procedure, and relinquishing its association to the calling process on return. VMTP supports multiple nested calls spanning multiple machines. In this case, the distributed call stack that results is associated with a single process from the standpoint of authentication and resource management, using the ProcessId field supported by VMTP. The entity identifiers effectively

Cheriton [page 33]

link these call frames together. That is, the Client field in a Request is effectively the return link to the previous call frame.

With message handling of a Request, a Request message is queued for a server process. The server process dequeues, reads, processes and responds to the Request message, executing as a separate process. Subsequent Requests to the same server are queued until the server asks to receive the next Request.

Procedural semantics have the advantage of allowing each Request (up to the resource limits of the Server) to execute concurrently at the Server, with Request-specific synchronization. Message semantics have the advantage that Requests are serialized at the Server and that the request processing logically executes with the priority, protection and independent execution of a separate process. Note that procedural and message handling of a request appear no differently to the client invoking the message transaction, except possibly for differences in performance.

We view the two Request handling approaches as appropriate under different circumstances. VMTP supports both models.

2.18. Bibliography

The basic protocol is similar to that used in the original form of the V kernel [3, 4] as well as the transport protocol of Birrell and Nelson's [2] remote procedure call mechanism. An earlier version of the protocol was described in SIGCOMM'86 [6]. The rate-based flow control is similar to the techniques of Netblt [9]. The support for idempotency draws, in part, on the favorable experience with idempotency in the V distributed system. Its use was originally inspired by the Woodstock File Server [11]. The multicast support draws on the multicast facilities in V [5] and is designed to work with, and is now implemented using, the multicast extensions to the Internet [8] described in RFC 966 and 988. The secure version of the protocol is similar to that described by Birrell [1] for secure RPC. The use of runs of packet groups is similar to Fletcher and Watson's delta-T protocol [10]. The use of "management" operations implemented using VMTP in place of specialized packet types is viewed as part of a general strategy of using recursion to simplify protocol architectures [7].

Finally, this protocol was designed, in part, to respond to the requirements identified by Braden in RFC 955. We believe that VMTP satisfies the requirements stated in RFC 955.

Cheriton [page 34]

[1] A.D. Birrell, "Secure Communication using Remote Procedure

  Calls", ACM. Trans. on Computer Systems 3(1), February, 1985.

[2] A. Birrell and B. Nelson, "Implementing Remote Procedure Calls",

  ACM Trans. on Computer Systems 2(1), February, 1984.

[3] D.R. Cheriton and W. Zwaenepoel, "The Distributed V Kernel and its

  Performance for Diskless Workstations", In Proceedings of the 9th
  Symposium on Operating System Principles,  ACM, 1983.

[4] D.R. Cheriton, "The V Kernel: A Software Base for Distributed

  Systems", IEEE Software 1(2), April, 1984.

[5] D.R. Cheriton and W. Zwaenepoel, "Distributed Process Groups in

  the V Kernel", ACM Trans. on Computer Systems 3(2), May, 1985.

[6] D.R. Cheriton, "VMTP: A Transport Protocol for the Next

  Generation of Communication Systems", In Proceedings of
  SIGCOMM'86, ACM, Aug 5-7, 1986.

[7] D.R. Cheriton, "Exploiting Recursion to Simplify an RPC

  Communication Architecture", in preparation, 1988.

[8] D.R. Cheriton and S.E. Deering, "Host Groups: A Multicast

  Extension for Datagram Internetworks", In 9th Data Communication 
  Symposium, IEEE Computer Society and ACM SIGCOMM, September, 1985.

[9] D.D. Clark and M. Lambert and L. Zhang, "NETBLT: A Bulk Data

  Transfer Protocol", Technical Report RFC 969, Defense Advanced 
  Research Projects Agency, 1985.

[10] J.G. Fletcher and R.W. Watson, "Mechanism for a Reliable Timer-

  based Protocol", Computer Networks 2:271-290, 1978.

Cheriton [page 35]

[11] D. Swinehart and G. McDaniel and D. Boggs, "WFS: A Simple File

  System for a Distributed Environment", In Proc. 7th Symp. 
  Operating Systems Principles, 1979.

Cheriton [page 36]

VMTP Packet Formats

VMTP uses 2 basic packet formats corresponding to Request packets and Response packets. These packet formats are identical in most of the fields to simplify the implementation.

We first describe the entity identifier format and the packet fields that are used in general, followed by a detailed description of each of the packet formats. These fields are described below in detail. The individual packet formats are described in the following subsections. The reader and VMTP implementor may wish to refer to Chapters 4 and 5 for a description of VMTP event handling and only refer to this detailed description as needed.

Entity Identifier Format

The 64-bit non-group entity identifiers have the following substructure.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| |L|R|
|A|0|E|E|      Domain-specific structure
|E| |E|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Domain-specific structure                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The field meanings are as follows:

RAE Remote Alias Entity - the entity identifier identifies

            an entity that is acting as an alias for some entity
            outside this entity domain.  This bit is used by
            higher-level protocols.  For instance, servers may take
            extra security and protection measures with aliases.

GRP Group - 0, for non-group entity identifiers.

LEE Little-Endian Entity - the entity transmits data in

            little-endian (VAX) order.

RES Reserved - must be 0.

The 64-bit entity group identifiers have the following substructure.

Cheriton [page 37]

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| |U|R|
|A|1|G|E|      Domain-specific structure
|E| |P|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Domain-specific structure                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The field meanings are as follows:

RAE Remote Alias Entity - same as for non-group entity

            identifier.

GRP Group - 1, for entity group identifiers.

UGP Unrestricted Group - no restrictions are placed on

            joining this group.  I.e. any entity can join limited
            only by implementation resources.

RES Reserved - must be 0.

The all-zero entity identifier is reserved and guaranteed to be unallocated in all domains. In addition, a domain may reserve part of the entity identifier space for statically allocated identifiers. However, this is domain-specific.

Description of currently defined entity identifier domains is provided in Appendix IV.

Packet Fields

Client 64-bit identifier for the client entity associated with

            this packet.  The structure, allocation and binding of
            this identifier is specific to the specified Domain.  An
            entity identifier always includes 4 types bits as
            specified in Section 3.1.

Version The 3-bit identifier specifying the version of the

            protocol.  Current version is version 0.

Domain The 13-bit identifier specifying the naming and

            administration domain for the client and server named in
            the packet.

Cheriton [page 38]

Packet Flags: 3 bits. (The normal case has none of the flags set.)

 HCO           Header checksum only - checksum has only been calculated
            on the header.  This is used in some real-time
            applications where the strict correctness of the data is
            not needed.

 EPG           Encrypted packet group - part of a secure message
            transaction.

 MPG           Multicast packet group - packet was multicast on
            transmission.

Length A 13-bit field that specifies the number of 32-bit words

            in the segment data portion of the packet (if any),
            excluding the checksum field.  (Every VMTP packet is
            required to be a multiple of 64 bits, possibly by
            padding out the segment data.)  The minimum legal Length
            is 0, the maximum length is 4096 and it must be an even
            number.

Control Flags: 9 bits. (The normal case has none of the flags set.)

 NRS           Next Receive Sequence - the associated Request message
            (in a Response) or previous Response (if a Request) was
            received consecutive with the last Request from this
            entity.  That is, there was no interfering messages
            received.

 APG           Acknowledge Packet Group - Acknowledge packet group on
            receipt.  If a Request, send back a Request to the
            client's manager providing an update on the state of the
            transaction as soon as the request packet group is
            received, independent of the response being available.
            If a Response, send an update to the server's manager as
            soon as possible after response packet group is received
            providing an update on the state of the transaction at
            the client

 NSR           Not Start Run - 1 if this packet is not part of the
            first packet group of a run of packet groups.

 NER           Not End Run - 1 if this packet is not part of the last
            packet group of a run of packet groups.

 NRT           No Retransmission - do not ask for retransmissions of
            this packet group if not all received within timeout

Cheriton [page 39]

            period, just deliver or discard.

 MDG           Member of Destination Group - this packet is sent to a
            group and the client is a member of this group.

 CMG           Continued Message - the message (Request or Response) is
            continued in the next packet group.  The next packet
            group has to be part of the same run of packet groups.

 STI           Skip Transaction Identifiers - the next transaction
            identifier that the Client plans to use is the current
            transaction plus 256, if part of the same run and at
            least this big if not.  In a Request, this authorizes
            the Server to send back up to 256 packet groups
            containing the Response.

 DRT           Delay Response Transmission - set by request sender if
            multiple responses are expected (as indicated by the MRD
            flag in the RequestCode) and it may be overrun by
            multiple responses.  The responder(s) should then
            introduce a short random delay in sending the Response
            to minimize the danger of overrunning the Client.  This
            is normally only used for responding to multicast
            Requests where the Client may be receiving a large
            number of Responses, as indicated by the MRD flag in the
            Request flags.  Otherwise, the Response is sent
            immediately.

RetransmitCount:

            3 bits - the ordinal number of transmissions of this
            packet group prior to this one, modulo 8.  This field is
            used in estimation of roundtrip times.  This count may
            wrap around during a message transaction.  However, it
            should be sufficient to match acknowledgments and
            responses with a particular transmission.

ForwardCount: 4 bits indicating the number of times this Request has

            been forwarded.  The original Request is always sent
            with a ForwardCount of 0.

Interpacket Gap: 8 bits.

            Indicates the recommended time to use between subsequent
            packet transmissions within a multi-packet packet group
            transmission.  The Interpacket Gap time is in 1/32nd of
            a network packet transmission time for a packet of size
            MTU for the node.  (Thus, the maximum gap time is 8
            packet times.)

Cheriton [page 40]

PGcount: 8 bits

            The number of packet groups that this packet group
            represents in addition to that specified by the
            Transaction field.  This is used in acknowledging
            multiple packet groups in streamed communication.

Priority 4-bit identifier for priority for the processing of this

            request both on transmission and reception.  The
            interpretation is:

            1100            urgent/emergency

            1000            important

            0000            normal

            0100            background

            Viewing the higher-order bit as a sign bit (with 1
            meaning negative), low values are high priority and high
            values are low priority.  The low-order 2 bits indicate
            additional (lower) gradations for each level.

Function Code: 1 bit - types of VMTP packets. If the low-order bit of

            the function code is 0, the packet is sent to the
            Server, else it is sent to the Client.

            0               Request

            1               Response

Transaction: 32 bits:

            Identifier for this message transaction.

PacketDelivery: 32 bits:

            Delivery indicates the segment blocks contained in this
            packet.  Each bit corresponds to one 512-octet block of
            segment data.  A 1 bit in the i-th bit position
            (counting the LSB as 0) indicates the presence of the
            i-th segment block.

Server: 64 bits

            Entity identifier for the server or server group
            associated with this transaction.  This is the receiver
            when a Request packet and the sender when a Response
            packet.

Cheriton [page 41]

Code: 32 bits The Request Code and Response Code, set either at the

            user level or VMTP level depending on use and packet
            type.  Both the Request and Response codes include 8
            high-order bits from the following set of control bits:

 CMD           Conditional Message Delivery -  only deliver the request
            or response if the receiving entity is waiting for it at
            the time of delivery, otherwise drop the message.

 DGM           DataGram Message - indicates that the message is being
            sent as a datagram.  If a Request message, do not wait
            for reply, or retransmit.  If a Response message, treat
            this message transaction as idempotent.

 MDM           Message Delivery Mask - indicates that the MsgDelivery
            field is being used.  Otherwise, the MsgDelivery field
            is available for general use.

 SDA           Segment Data Appended - segment data is appended to the
            message control block, with the total size of the
            segment specified by the SegmentSize field.  Otherwise,
            the segment data is null and the SegmentSize field is
            not used by VMTP and available for user- or RPC-level
            uses.

 CRE           CoResident Entity - indicates that the CoResidentEntity
            field in the message should be interpreted by VMTP.
            Otherwise, this field is available for additional user
            data.

 MRD           Multiple Responses Desired - multiple Responses are
            desired to to this Request if it is multicast.
            Otherwise, the VMTP module can discard subsequent
            Responses after the first Response.

 PIC           Public Interface Code - Values for Code with this bit
            set are reserved for definition by the VMTP
            specification and other standard protocols defined on
            top of VMTP.

 RES           Reserved for future use. Must be 0.

CoResidentEntity

            64-bit Identifier for an entity or group of entities
            with which the Server entity or entities must be
            co-resident, i.e. route only to entities (identified by
            Server) on the same host(s) as that specified by

Cheriton [page 42]

            CoResidentEntity, Only meaningful if CRE is set in the
            Code field.

User Data 12 octets Space in the header for the VMTP user to

            specify user-specific control and data.

MsgDelivery: 32 bits

            The segment blocks being transmitted (in total) in this
            packet group following the conventions for the
            PacketDelivery field.  This field is ignored by the
            protocol and treated as an additional user data field if
            MDM is 0.  On transmission, the user level sets the
            MsgDelivery to indicate those portions of the segment to
            be transmitted.  On receipt, the MsgDelivery field is
            modified by the VMTP module to indicate the segment data
            blocks that were actually received before the message
            control block is passed to the user or RPC level.  In
            particular, the kernel does not discard the packet group
            if segment data blocks are missing.  A Server or Client
            entity receiving a message with a MsgDelivery in use
            must check the field to ensure adequate delivery and
            retry the operation if necessary.

SegmentSize: 32 bits

            Size of segment in octets, up to a maximum of 16
            kilooctets without streaming and 4 megaoctets with
            streaming, if SDA is set.  Otherwise, this field is
            ignored by the protocol and treated as an additional
            user data field.

Segment Data: 0-16 kilooctets

            0 octets if SDA is 0, else the portion of the segment
            corresponding to the Delivery Mask, limited by the
            SegmentSize and the MTU, padded out to a multiple of 64
            bits.

Checksum: 32 bits.

            The 32-bit checksum for the header and segment data.

The VMTP checksum algorithm <9> develops a 32-bit checksum by computing

_______________

<9> This algorithm and description are largely due to Steve Deering of Stanford University.

Cheriton [page 43]

two 16-bit, ones-complement sums (like IP), each covering different parts of the packet. The packet is divided into clusters of 16 16-bit words. The first, third, fifth,... clusters are added to the first sum, and the second, fourth, sixth,... clusters are added to the second sum. Addition stops at the end of the packet; there is no need to pad out to a cluster boundary (although it is necessary that the packet be an integral multiple of 64 bits; padding octets may have any value and are included in the checksum and in the transmitted packet). If either of the resulting sums is zero, it is changed to 0xFFFF. The two sums are appended to the transmitted packet, with the first sum being transmitted first. Four bytes of zero in place of the checksum may be used to indicate that no checksum was computed.

The 16-bit, ones-complement addition in this algorithm is the same as used in IP and, therefore, subject to the same optimizations. In particular, the words may be added up 32-bits at a time as long as the carry-out of each addition is added to the sum on the following addition, using an "add-with-carry" type of instruction. (64-bit or 128-bit additions would also work on machines that have registers that big.)

A particular weakness of this algorithm (shared by IP) is that it does not detect the erroneous swapping of 16-bit words, which may easily occur due to software errors. A future version of VMTP is expected to include a more secure algorithm, but such an algorithm appears to require hardware support for efficient execution.

Not all of these fields are used in every packet. The specific packet formats are described below. If a field is not mentioned in the description of a packet type, its use is assumed to be clear from the above description.

Cheriton [page 44]

Request Packet

The Request packet (or packet group) is sent from the client to the server or group of servers to solicit processing plus the return of zero or more responses. A Request packet is identified by a 0 in the LSB of the fourth 32-bit word in the packet.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+                       Client (8 octets)                       +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver  |                         |H|E|M|                         |
|sion |          Domain         |C|P|P|      Length             |
|     |                         |O|G|G|                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|N|A|N|N|N|M|C|S|D|Retra|Forward|    Inter-     |       |R|R|R| |
|R|P|S|E|R|D|M|T|R|nsmit| Count |    Packet     | Prior |E|E|E|0|
|S|G|R|R|T|G|G|I|T|Count|       |     Gap       | -ity  |S|S|S| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Transaction                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     PacketDelivery                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+                    Server (8 octets)                          +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|D|M|S|R|C|M|P|                                               |
|M|G|D|D|E|R|R|I|        RequestCode                            |
|D|M|M|A|S|E|D|C|                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+                 CoResidentEntity (8 octets)                   +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>                   User Data (12 octets)                       <
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      MsgDelivery                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       SegmentSize                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>                  segment data, if any                         <
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Checksum                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 3-1:   Request Packet Format

The fields of the Request packet are set according to the semantics described in Section 3.2 with the following qualifications.

Cheriton [page 45]

InterPacketGap The estimated interpacket gap time the client would like

            for the Response packet group to be sent by the Server
            in responding to this Request.

Transaction Identifier for transaction, at least one greater than

            the previously issued Request from this Client.

Server Server to which this Request is destined.

RequestCode Request code for this request, indicating the operation

            to perform.

Cheriton [page 46]

Response Packet

The Response packet is sent from the Server to the Client in response to a Request, identified by a 1 in the LSB of the fourth 32-bit word in the packet.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+                       Client (8 octets)                       +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver  |                         |H|E|M|                         |
|sion |          Domain         |C|P|P|      Length             |
|     |                         |O|G|G|                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|N|A|N|N|N|R|C|S|R|Retra|Forward|               |       |R|R|R| |
|R|P|S|E|R|E|M|T|E|nsmit| Count |    PGcount    | Prior |E|E|E|1|
|S|G|R|R|T|S|G|I|S|Count|       |               | -ity  |S|S|S| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Transaction                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      PacketDelivery                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+                        Server (8 octets)                      +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|D|M|S|R|R|R|R|                                               |
|M|G|D|D|E|E|E|E|        ResponseCode                           |
|D|M|M|A|S|S|S|S|                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>                   UserData (20 octets)                        <
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     MsgDelivery                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Segment Size                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>                  segment data, if any                         <
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Checksum                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 3-2:   Response Packet Format

The fields of the Response packet are set according to the semantics described in Section 3.2 with the following qualifications.

Client, Version, Domain, Transaction

            Match those in the Request packet group to which this is

Cheriton [page 47]

            a response.

STI 1 if this Response is using one or more of the

            transaction identifiers skipped by the Client after the
            Request to which this is a Response.  STI in the Request
            essentially allocates up to 256 transaction identifiers
            for the Server to use in a run of Response packet
            groups.

RetransmitCount The retransmit count from the last Request packet

            received to which this is a response.

ForwardCount The number of times the corresponding Request was

            forwarded before this Response was generated.

PGcount The number of consecutively previous packet groups that

            this response is acknowledging in addition to the one
            identified by the Transaction identifier.

Server Server sending this response. This may differ from that

            originally specified in the Request packet if the
            original Server was a server group, or the request was
            forwarded.

The next two chapters describes the protocol operation using these packet formats, with the the Client and the Server portions described separately.

Cheriton [page 48]

Client Protocol Operation

This chapter describes the operation of the client portion of VMTP in terms of the procedures for handling VMTP user events, packet reception events, management operations and timeout events. Note that the client portion of VMTP is separable from the server portion. It is feasible to have a node that only implements the client end of VMTP.

To simplify the description, we define a client state record (CSR) plus some standard utility routines.

Client State Record Fields

In the following protocol description, there is one client state record (CSR) per (client,transaction) outstanding message transaction. Here is a suggested set of fields.

Link Link to next CSR when queued in one of the transmission,

            timeout or message queues.

QueuePtr Pointer to queue head in which this CSR is contained or

            NULL if none.  Queue could be one of transmission queue,
            timeout queue, server queue or response queue.

ProcessIdentification

            The process identification and address space.

Priority Priority for processing, network service, etc.

State One of the client states described below.

FinishupFunc Procedure to be executed on the CSR when it is completes

            its processing in transmission or timeout queues.

TimeoutCount Time to remain in timeout queue.

TimeoutLimit User-specified time after which the message transaction

            is aborted. The timeout is infinite if set to zero.

RetransCount Number of retransmissions since last hearing from the

            Server.

LastTransmitTime

            The time at which the last packet was sent.  This field
            is used to calculate roundtrip times, using the
            RetransmitCount to match the responding packet to a

Cheriton [page 49]

            particular transmission.  I.e. Response or management
            NotifyVmtpClient operation to Request and a management
            NotifyVmtpServer operation to a Response.

TimetoLive Time to live to be used on transmission of IP packets.

TransmissionMask

            Bit mask indicating the portions of the segment to
            transmit.  Set before entering the transmission queue
            and cleared incrementally as the 512-byte segment blocks
            of the segment are transmitted.

LocalClientLink Link to next CSR hashing to same hash index in the

            ClientMap.

LocalClient Entity identifier for client when this CSR is used to

            send a Request packet.

LocalTransaction

            Transaction identifier for current message transaction
            the local client has outstanding.

LocalPrincipal Account identification, possibly including key and key

            timeout.

LocalDelivery Bit mask of segment blocks that have not been

            acknowledged in the Request or have been received in the
            Response, depending on the state.

ResponseQueue Queue of CSR's representing the queued Responses for

            this entity.

VMTP Header Prototype VMTP header, used to generate and store the

            header portion of a Request for transmission and
            retransmission on timeout.

SegmentDesc Description of the segment data associated with the CSR,

            either the area storing the original Request data, the
            area for receiving Request data, or the area storing the
            Response data that is returned.

HostAddr The network or internetwork host address to which the

            Client last transmitted.  This field also indicates the
            type of the address, e.g. IP, Ethernet, etc.

Note: the CSR can be combined with a light-weight process descriptor with considerable benefit if the process is designed to block when it

Cheriton [page 50]

issues a message transaction. In particular, by combining the two descriptors, the implementation saves time because it only needs to locate and queue one descriptor with various operations (rather than having to locate two descriptors). It also saves space, given that the VMTP header prototype provides space such as the user data field which may serve to store processor state for when the process is preempted. Non-preemptive blocking can use the process stack to store the processor state so only a program counter and stack pointer may be required in the process descriptor beyond what we have described. (This is the approach used in the V kernel.)

Client Protocol States

A Client State Record records the state of message transaction generated by this host, identified by the (Client, Transaction) values in the CSR. As a client originating a transaction, it is in one of the following states.

AwaitingResponse

            Waiting for a Response packet group to arrive with the
            same (Client,Transaction) identification.

ReceivingResponse

            Waiting for additional packets in the Response packet
            group it is currently receiving.

"Other" Not waiting for a response, which can be Processing or

            some other operating system state, or one of the Server
            states if it also acts as a server.

This covers all the states for a client.

State Transition Diagrams

The client state transitions are illustrated in Figure 4-1. The client goes into the state AwaitingResponse on sending a request unless it is a datagram request. In the AwaitingResponse state, it can timeout and retry and eventually give up and return to the processing state unless it receives a Response. (A NotifyVmtpClient operation resets the timeout but does not change the state.) On receipt of a single packet response, it returns to the processing state. Otherwise, it goes to ReceivingResponse state. After timeout or final response packet is received, the client returns to the processing state. The processing state also includes any other state besides those associated with issuing a message transaction.

Cheriton [page 51]

+------------+ | Processing |<--------------------| | |<-------------| | | |<---| | | +|------^--^-+ Single Last |

Transmit  |  |    Packet    Response |
|      |  |    Response  Packet   |
|      |  |      |         |      |
+-DGM->+ Timeout |         |   Final timeout
|         |      |         |      |

+V-----------+ | +-----------+ | Awaiting |----+ | Receiving |->Response-+ | Response |->Response->| Response | | | | (multi- | |<----------+ +-|--------^-+ packet) +----------^+

 V        |                |        |
 +-Timeout+                +>Timeout+

             Figure 4-1:   Client State Transitions

User Interface

The RPC or user interface to VMTP is implementation-dependent and may use systems calls, functions or some other mechanism. The list of requests that follow is intended to suggest the basic functionality that should be available.

Send( mcb, timeout, segptr, segsize )

            Initiate a message transaction to the server and request
            message specified by mcb and return a response in mcb,
            if it is received within the specified timeout period
            (or else return USER_TIMEOUT in the Code field).  The
            segptr parameter specifies the location from which the
            segment data is sent and the location into which the
            response data is to be delivered.  The segsize field
            indicates the maximum length of this area.

GetResponse( responsemcb, timeout, segptr, segsize )

            Get the next response sent to this client as part of the
            current message transaction, returning the segment data,
            if any, into the memory specified by segptr and segsize.

This interface assumes that there is a client entity associated with the invoking process that is to be used with these operations. Otherwise, the client entity must be specified as an additional parameter.

Cheriton [page 52]

Event Processing

The following events may occur in the VMTP client:

- User Requests

    * Send

    * GetResponse

- Packet Arrival

    * Response Packet

    * Request

 The minimal Client implementation handles Request packets for
 its VMTP management (server) module and sends NotifyVmtpClient
 requests in response to others, indicating the specified
 server does not exist.

- Management Operation - NotifyVmtpClient

- Timeouts

    * Client Retransmission Timeout

The handling of these events is described in detail in the following subsections.

We first describe some conventions and procedures used in the description. A field of the received packet is indicated as (for example) p.Transaction, for the Transaction field. Optional portions of the code, such as the streaming handling code are prefixed with a "|" in the first column.

MapClient( client )

            Return pointer to CSR for client with the specified
            clientId, else NULL.

SendPacketGroup( csr )

            Send the packet group (Request, Response) according to
            that specified by the CSR.

NotifyClient( csr, p, code )

            Invoke the NotifyVmtpClient operation with the
            parameters csr.RemoteClient, p.control,

Cheriton [page 53]

            csr.ReceiveSeqNumber, csr.RemoteTransaction and
            csr.RemoteDelivery, and code.  If csr is NULL, use
            p.Client, p.Transaction and p.PacketDelivery instead and
            the global ReceiveSequenceNumber, if supported.  This
            function simplifies the description over calling
            NotifyVmtpClient directly in the procedural
            specification below.  (See Appendix III.)

NotifyServer( csr, p, code )

            Invoke the NotifyVmtpServer operation with the
            parameters p.Server, csr.LocalClient,
            csr.LocalTransaction, csr.LocalDelivery and code.  Use
            p.Client, P.Transaction and 0 for the clientId, transact
            and delivery parameters if csr is NULL.  This function
            simplifies the description over calling NotifyVmtpServer
            directly in the procedural specification below.  (See
            Appendix III.)

DGMset(p) True if DGM bit set in packet (or csr) else False.

            (Similar functions are used for other bits.)

Timeout( csr, timeperiod, func )

            Set or reset timer on csr record for timeperiod later
            and invoke func if the timeout expires.

Client User-invoked Events

A user event occurs when a VMTP user application invokes one of the VMTP interface procedures.

Send

Send( mcb, timeout, segptr, segsize )

map to main CSR for this client.
increment csr.LocalTransaction
Init csr and check parameters and segment if any.
Set SDA if sending appended data.
Flush queued replies from previous transaction, if any.
if local non-group server then
    deliver locally
    await response
    return
if GroupId(server) then
    Check for and deliver to local members.
    if CRE request and non-group local CR entity then

Cheriton [page 54]

       await response
       return
    endif
    set MDG if member of this group.
endif
clear csr.RetransCount
set csr.TransmissionMask
set csr.TimeLimit to timeout
set csr.HostAddr for csr.Server
SendPacketGroup( csr )
if DGMset(csr) then
   return
endif
set csr.State to AwaitingResponse
Timeout( rootcsr, TC1(csr.Server), LocalClientTimeout )
return

end Send

Notes:

1. Normally, the HostAddr is extracted from the ServerHost

  cache, which maps server entity identifiers to host
  addresses.  However, on cache miss, the client first queries
  the network using the ProbeEntity operation, as specified in
  Appendix III, determining the host address from the Response.
  The ProbeEntity operation is handled as a separate message
  transaction by the Client.

The stream interface incorporates a parameter to pass a responseHandler procedure that is invoked when the message transaction completes.

StreamSend( mcb, timeout, segptr, segsize, responseHandler )

map to main CSR for this client.

| Allocate a new csr if root in use. | lastcsr := First csr for last request. | if STIset(lastcsr) | csr.LocalTransaction := lastcsr.LocalTransaction + 256 | else | csr.LocalTransaction := lastcsr.LocalTransaction + 1

Init csr and check parameters and segment if any.
. . . ( rest is the same as for the normal Send)

Notes:

1. Each outstanding message transaction is represented by a CSR

  queued on the root CSR for this client entity.  The root CSR
  is used to handle timeouts, etc.  On timeout, the last packet

Cheriton [page 55]

  from the last packet group is retransmitted (with or without
  the segment data).

GetResponse

GetResponse( req, timeout, segptr, segsize )

csr := CurrentCSR;
if responses queued then return next response
  (in req, segptr to max of segsize )
if timeout is zero then return KERNEL_TIMEOUT error
set state to AWAITING_RESPONSE
Timeout( csr, timeout, ReturnKernelTimeout );

end GetResponse

Notes:

1. GetResponse is only used with multicast Requests, which is

  the only case in which multiple (different) Responses should
  be received.

2. A response must remain queued until the next message

  transaction is invoked to filter out duplicates of this
  response.

3. If the response is incomplete (only relevant if a

  multi-packet response), then the client may wait for the
  response to be fully received, including issuing requests for
  retransmission (using NotifyVmtpServer operations) before
  returning the response.

4. As an optimization, a response may be stored in the CSR of

  the client.  In this case, the response must be transferred
  to a separate buffer (for duplicate suppression) before
  waiting for another response.  Using this optimization, a
  response buffer is not allocated in the common case of the
  client receiving only one response.

Packet Arrival

In general, on packet reception, a packet is mapped to the client state record, decrypted if necessary using the key in the CSR. It then has its checksum verified and then is transformed to the right byte order. The packet is then processed fully relative to its packet function code. It is discarded immediately if it is addressed to a different domain than the domain(s) in which the receiving host participates.

Cheriton [page 56]

For each of the 2 packet types, we assume a procedure called with a pointer p to the VMTP packet and psize, the size of the packet in octets. Thus, generic packet reception is:

if not LocalDomain(p.Domain) then return;

csr := MapClient( p.Client )

if csr is NULL then

HandleNoCsr( p, psize )
return

if Secure(p) then

if SecureVMTP not supported then
    { Assume a Request. }
    if not Multicast(p) then
        NotifyClient(NULL, p, SECURITY_NOT_SUPPORTED )
    return
endif

| Decrypt( csr.Key, p, psize )

if p.Checksum not null then

if not VerifyChecksum(p, psize) then return;

if OppositeByteOrder(p) then ByteSwap( p, psize ) if psize not equal sizeof(VmtpHeader) + 4*p.Length then

NotifyClient(NULL, p, VMTP_ERROR )
return

Invoke Procedure[p.FuncCode]( csr, p, psize ) Discard packet and return

Notes:

1. The Procedure[p.FuncCode] refers to one of the 2 procedures

  corresponding to the two different packet types of VMTP,
  Requests and Responses.

2. In all the following descriptions, a packet is discarded on

  "return" unless otherwise stated.

3. The procedure HandleNoCSR is a management routine that

  allocates and initializes a CSR and processes the packet or
  else sends an error indication to the sender of the packet.
  This procedure is described in greater detail in Section
  4.8.1.

Cheriton [page 57]

Response

This procedure handles incoming Response packets.

HandleResponse( csr, p, psize )

if not LocalClient( csr ) then
    if Multicast then return

| if Migrated( p.Client ) then | NotifyServer(csr, p ENTITY_MIGRATED ) | else

        NotifyServer(csr, p, ENTITY_NOT_HERE )
    return
endif

if NSRset(p) then
    if Streaming not supported then
        NotifyServer(csr, p, STREAMING_NOT_SUPPORTED )
        return STREAMED_RESPONSE

| Find csr corresponding to p.Transaction | if none found then | NotifyServer(csr, p, BAD_TRANSACTION_ID ) | return

 else
  if csr.LocalTransaction not equal p.Transaction then
    NotifyServer(csr, p, BAD_TRANSACTION_ID )
    return
endif
Locate reply buffer rb for this p.Server
if found then
    if rb.State is not ReceivingResponse then
      { Duplicate }
        if APGset(p) or NERset(p) then
            { Send Response to stop response packets. }
            NotifyServer(csr, p, RESPONSE_DISCARDED )
        endif
        return
     endif
     { rb.State is ReceivingRequest}
     if new segment data then retain in CSR segment area.
     if packetgroup not complete then
         Timeout( rb, TC3(p.Server), LocalClientTimeout )
         return;
      endif
      goto EndPacketGroup
endif
{ Otherwise, a new response message. }

Cheriton [page 58]

if (NSRset(p) or NERset(p)) and NoStreaming then
    NotifyServer(csr, p, VMTP_ERROR )
    return

| if NSRset(p) then | { Check consecutive with previous packet group } | Find last packet group CSR from p.Server. | if p.Transaction not | lastcsr.RemoteTransaction+1 mod 2**32 then | { Out of order packet group } | NotifyServer(csr, p, BAD_TRANSACTION_ID) | return | endif | if lastcsr not completed then | NotifyServer(lastcsr, p, RETRY ) | endif | if CMG(lastcsr) then | Add segment data to lastcsr Response | Notify lastcsr with new packet group. | Clear lastcsr.VerifyInterval | else | if lastcsr available then | use it for this packet group | else allocate and initialize new CSR | Save message and segment data in new CSR area. | endif | else { First packet group }

    Allocate and init reply buffer rb for this response.
    if allocation fails then
        NotifyServer(csr, p, BUSY )
        return
    Set rb.State to ReceivingResponse
    Copy message and segment data to rb's segment area
     and set rb.PacketDelivery to that delivered.
    Save p.Server host address in ServerHost cache.
endif
if packetgroup not complete then
    Timeout( rb, TS1(p.Client), LocalClientTimeout )
    return;
endif

endPacketGroup:

{ We have received last packet in packet group. }
if APGset(p) then NotifyServer(csr, p, OK )

| if NERset(p) and CMGset(p) then | Queue waiting for continuation packet group. | Timeout( rb, TC2(rb.Server), LocalClientTimeout ) | return | endif

Cheriton [page 59]

{ Deliver response message. }
Deliver response to Client, or queue as appropriate.

end HandleResponse

Notes:

1. The mechanism for handling streaming is optional and can be

  replaced with the tests for use of streaming.  Note that the
  server should never stream at the Client unless the Client
  has streamed at the Server or has used the STI control bit.
  Otherwise, streamed Responses are a protocol error.

2. As an optimization, a Response can be stored into the CSR for

  the Client rather than allocating a separate CSR for a
  response buffer.  However, if multiple responses are handled,
  the code must be careful to perform duplicate detection on
  the Response stored there as well as those queued.  In
  addition, GetResponse must create a queued version of this
  Response before allowing it to be overwritten.

3. The handling of Group Responses has been omitted for brevity.

  Basically, a Response is accepted if there has been a Request
  received locally from the same Client and same Transaction
  that has not been responded to.  In this case, the Response
  is delivered to the Server or queued.

Cheriton [page 60]

Management Operations

VMTP uses management operations (invoked as remote procedure calls) to effectively acknowledge packet groups and request retransmissions. The following routine is invoked by the Client's management module on request from the Server.

NotifyVmtpClient( clientId,ctrl,receiveSeqNumber,transact,delivery,code)

Get csr for clientId
if none then return
if RemoteClient( csr ) and not NotifyVmtpRemoteClient then
   return

| else (for streaming) | Find csr with same LocalTransaction as transact | if csr is NULL then return

if csr.State not AwaitingResponse then return
if ctrl.PGcount then ack previous packet groups.
select on code
  case OK:
    Notify ack'ed segment blocks from delivery
    Clear csr.RetransCount;
    Timeout( csr, TC1(csr.Server), LocalClientTimeout )
    return
  case RETRY:
    Set csr.TransmissionMask to missing segment blocks,
        as specified by delivery
    SendPacketGroup( csr )
    Timeout( csr, TC1(csr.Server), LocalClientTimeout )
  case RETRY_ALL
    Set csr.TransmissionMask to retransmit all blocks.
    SendPacketGroup( csr )
    Timeout( csr, TC1(csr.Server), LocalClientTimeout )

| if streaming then | Restart transmission of packet groups, | starting from transact+1

     return
  case BUSY:
     if csr.TimeLimit exceeded then
         Set csr.Code to USER_TIMEOUT
         return Response to application
         return;
    Set csr.TransmissionMask for full retransmission
    Clear csr.RetransCount
    Timeout( csr, TC1(csr.Server), LocalClientTimeout )
    return
  case ENTITY_MIGRATED:
    Get new host address for entity

Cheriton [page 61]

    Set csr.TransmissionMask for full retransmission
    Clear csr.RetransCount
    SendPacketGroup( csr )
    Timeout( csr, TC1(csr.Server), LocalClientTimeout )
    return

  case STREAMING_NOT_SUPPORTED:
    Record that server does not support streaming
    if CMG(csr) then forget this packet group
    else resend Request as separate packet group.
    return
  default:
     Set csr.Code to code
     return Response to application
     return;
endselect

end NotifyVmtpClient

Notes:

1. The delivery parameter indicates the segment blocks received

  by the Server.  That is, a 1 bit in the i-th position
  indicates that the i-th segment block in the segment data of
  the Request was received.  All subsequent NotifyVmtpClient
  operations for this transaction should be set to acknowledge
  a superset of the segment blocks in this packet.  In
  particular, the Client need not be prepared to retransmit the
  segment data once it has been acknowledged by a Notify
  operation.

HandleNoCSR

HandleNoCSR is called when a packet arrives for which there is no CSR matching the client field of the packet.

HandleNoCSR( p, psize )

if Secure(p) then
    if SecureVMTP not supported then
        { Assume a Request }
        if not Multicast(p) then
            NotifyClient(NULL,p,SECURITY_NOT_SUPPORTED)
        return
    endif
    HandleRequestNoCSR( p, psize )
    return
endif

Cheriton [page 62]

if p.Checksum not null then
    if not VerifyChecksum(p, psize) then return;
if OppositeByteOrder(p) then ByteSwap( p, psize )
if psize not equal sizeof(VmtpHeader) + 4*p.Length then
    NotifyClient(NULL, p, VMTP_ERROR )
    return

if p.FuncCode is Response then

| if Migrated( p.Client ) then | NotifyServer(csr, p ENTITY_MIGRATED ) | else

        NotifyServer(csr, p, NONEXISTENT_ENTITY )
    return
endif

if p.FuncCode is Request then
   HandleRequestNoCSR( p, psize )
return

end HandleNoCSR

Notes:

1. The node need only check to see if the client entity has

  migrated if in fact it supports migration of entities.

2. The procedure HandleRequestNoCSR is specified in Section

  5.8.1.  In the minimal client version, it need only handle
  Probe requests and can do so directly without allocating a
  new CSR.

Cheriton [page 63]

Timeouts

A client with a message transaction in progress has a single timer corresponding to the first unacknowledged request message. (In the absence of streaming, this request is also the last request sent.) This timeout is handled as follows:

LocalClientTimeout( csr )

 select on csr.State
case AwaitingResponse:
  if csr.RetransCount > MaxRetrans(csr.Server) then
         terminate Client's message transactions up to
         and including the current message transaction.
         set return code to KERNEL_TIMEOUT
      return
  increment csr.RetransCount
  Resend current packet group with APG set.
  Timeout( csr, TC2(csr.Server), LocalClientTimeout )
  return
case ReceivingResponse:
  if DGMset(csr) or csr.RetransCount > Max then
     if MDMset(csr) then
        Set MCB.MsgDeliveryMask to blocks received.
     else
        Set csr.Code to BAD_REPLY_SEGMENT
     return to user Client
  endif
  increment csr.RetransCount
  NotifyServer with RETRY
  Timeout( csr, TC3(csr.Server), LocalClientTimeout )
  return
 end select

end LocalClientTimeout

Notes:

1. A Client can only request retransmission of a Response if the

  Response is not idempotent.  If idempotent, it must
  retransmit the Request.  The Server should generally support
  the MsgDeliveryMask for Requests that it treats as idempotent
  and that require multi-packet Responses.  Otherwise, there is
  no selective retransmission for idempotent message
  transactions.

2. The current packet group is the last one transmitted. Thus,

  with streaming, there may be several packet groups
  outstanding that precede the current packet group.

Cheriton [page 64]

3. The Request packet group should be retransmitted without the

  segment data, resulting in a single short packet in the
  retransmission.  The Server must then send a
  NotifyVmtpClient with a RETRY or RETRY_ALL code to get the
  segment data transmitted as needed.  This strategy minimizes
  the overhead on the network and the server(s) for
  retransmissions.

Cheriton [page 65]

Server Protocol Operation

This section describes the operation of the server portion of the protocol in terms of the procedures for handling VMTP user events, packet reception events and timeout events. Each server is assumed to implement the client procedures described in the previous chapter. (This is not strictly necessary but it simplifies the exposition.)

Remote Client State Record Fields

The CSR for a server is extended with the following fields, in addition to the ones listed for the client version.

RemoteClient Identifier for remote client that sent the Request that

            this CSR is handling.

RemoteClientLink

            Link to next CSR hashing to same hash index in the
            ClientMap.

RemoteTransaction

            Transaction identifier for Request from remote client.

RemoteDelivery The segment blocks received so far as part of a Request

            or yet to be acknowledged as part of a Response.

VerifyInterval Time interval since there was confirmation that the

            remote Client was still valid.

RemotePrincipal Account identification, possibly including key and key

            timeout for secure communication.

Remote Client Protocol States

A CSR in the server end is in one of the following states.

AwaitingRequest Waiting for a Request packet group. It may be marked as

            waiting on a specific Client, or on any Client.

ReceivingRequest

            Waiting to receive additional Request packets in a
            multi-packet group Request.

Responded The Response has been sent and the CSR is timing out,

            providing duplicate suppression and retransmission (if

Cheriton [page 66]

            the Response was not idempotent).

ResponseDiscarded

            Response has been acknowledged or has timed out so
            cannot be retransmitted.  However, duplicates are still
            filtered and CSR can be reused for new message
            transaction.

Processing Executing on behalf of the Client.

Forwarded The message transaction has been forwarded to another

            Server that is to respond directly to the Client.

State Transition Diagrams

The CSR state transitions in the server are illustrated in Figure 5-1. The CSR generally starts in the AwaitingRequest state. On receipt of a Request, the Server either has an up-to-date CSR for the Client or else it sends a Probe request (as a separate VMTP message transaction) to the VMTP management module associated with the Client. In the latter case, the processing of the Request is delayed until a Response to the Probe request is received. At that time, the CSR information is brought up to date and the Request is processed. If the Request is a single-packet request, the CSR is then set in the Processing state to handle the request. Otherwise (a multi-packet Request), the CSR is put into the ReceivingResponse state, waiting to receive subsequent Request packets that constitute the Request message. It exits the ReceivingRequest state on timeout or on receiving the last Request packet. In the former case, the request is delivered with an indication of the portion received, using the MsgDelivery field if MDM is set. After request processing is complete, either the Response is sent and the CSR enters the Responded state or the message transaction is forwarded and the CSR enters the Forwarded state.

In the Responded state, if the Response is not marked as idempotent, the Response is retransmitted on receipt of a retransmission of the corresponding Request, on receipt of a NotifyVmtpServer operation requesting retransmission or on timeout at which time APG is set, requesting an acknowledgment from the Client. The Response is retransmitted some maximum number of times at which time the Response is discarded and the CSR is marked accordingly. If a Request or a NotifyVmtpServer operation is received expecting retransmission of the Response after the CSR has entered the ResponseDiscarded state, a NotifyVmtpClient operation is sent back (or invoked in the Client management module) indicating that the response was discarded unless the Request was multicast, in which case no action is taken. After a

Cheriton [page 67]

 (Retransmit Forwarded Request and NotifyVmtpClient)
                Request/
                Ack/
               +Timeout+
               V       |
             +-|-------^-+
             |           |
      +-Time-| Forwarded |<-------------+
      |  out +-----------+              |
      |                                 |
      |          (Retransmit Response)  |
      |                      Request    |
      V                      Ack        |
      |                    +-Timeout-+  |
      |                    V         |  |
    +---------+ Ack/ +|---------^+ |
+-Time-|Response |<-Timeout--| Responded | |
|  out |Discarded|           +----^------+ |
|      +---------+                |        |
|  +------------+                 |        |
|  |            |->-Send Response-+        |
|  |            |->-forward Request--------+
+->| Processing |<----------------------+
|  |            |<----------------+     |
|  |            |<---|            |     |
|  +-|--------^-+    |          Last    |
| Receive     |      |          Request |
|    |   Timeout   Single       Packet  |
|    |        |    Packet         |   Timeout
|    |        |    Request        ^     ^
|    |        |      ^           +|-----|--+
|  +-V--------|-+    |           |Receiving|<-+Time
+->|  Awaiting  |->--+->Request->| Request |--+ out
|  Request   |    |  (multi-  +---------+
+------|-----+    ^  packet)
    Request       |
       |        Response
  Send Probe     to
       |        Probe
   +---V----+     |
   |Awaiting|     ^
   |Response|-->--+
   |to Probe|
   +--------+

         Figure 5-1:   Remote Client State Transitions

timeout corresponding to the time required to filter out duplicates, the

Cheriton [page 68]

CSR returns either to the AwaitingRequest state or to the Processing state. Note that "Ack" refers to acknowledgment by a Notify operation.

A Request that is forwarded leaves the CSR in the Forwarded state. In the Forwarded state, the forwarded Request is retransmitted periodically, expecting NotifyRemoteClient operations back from the Server to which the Request was forwarded, analogous to the Client behavior in the AwaitingResponse state. In this state, a NotifyRemoteClient from this Server acknowledges the Request or asks that it be retransmitted or reports an error. A retransmission of the Request from the Client causes a NotifyVmtpClient to be returned to the Client if APG is set. The CSR leaves the Forwarded state after timing out in the absence of NotifyRemoteClient operations from the forward Server or on receipt of a NotifyRemoteClient operation indicating the forward Server has sent a Response and received an acknowledgement. It then enters the ResponseDiscarded state.

Receipt of a new Request from the same Client aborts the current transaction, independent of its state, and initiates a new transaction unless the new Request is part of a run of message transactions. If it is part of a run of message transactions, the handling follows the state diagram except the new Request is not Processed until there has been a response sent to the previous transaction.

User Interface

The RPC or user interface to VMTP is implementation-dependent and may use systems calls, functions or some other mechanism. The list of requests that follow is intended to suggest the basic functionality that should be available.

AcceptMessage( reqmcb, segptr, segsize, client, transid, timeout )

            Accept a new Request message in the specified reqmcb
            area, placing the segment data, if any, in the area
            described by segptr and segsize.  This returns the
            Server in the entityId field of the reqmcb and actual
            segment size in the segsize parameters.  It also returns
            the Client and Transaction for this message transaction
            in the corresponding parameters.  This procedure
            supports message semantics for request processing.  When
            a server process executes this call, it blocks until a
            Request message has been queued for the server.
            AcceptMessage returns after the specified timeout period
            if a message has not been received by that time.

RespondMessage( responsemcb, client, transid, segptr )

Cheriton [page 69]

            Respond to the client with the specified response
            message and segment, again with message semantics.

RespondCall( responsemcb, segptr )

            Respond to the client with the specified response
            message and segment, with remote procedure call
            semantics.  This procedure does not return.  The
            lightweight process that executes this procedure is
            matched to a stack, program counter, segment area and
            priority from the information provided in a
            ModifyService call, as specified in Appendix III.

ForwardMessage( requestmcb, transid, segptr, segsize, forwardserver )

            Forward the client to the specified forwardserver with
            the request specified in mcb.

ForwardCall( requestmcb, segptr, segsize, forwardserver )

            Forward the client transaction to the specified
            forwardserver with the request specified by requestmcb.
            This procedure does not return.

GetRemoteClientId()

            Return the entityId for the remote client on whose
            behave the process is executing.  This is only
            applicable in the procedure call model of request
            handling.

GetForwarder( client )

            Return the entity that forwarded this Request, if any.

GetProcess( client )

            Return an identifier for the process associated with
            this client entity-id.

GetPrincipal( client )

            Return the principal associated with this client
            entity-id.

Event Processing

The following events may occur in VMTP servers.

- User Requests

    * Receive

Cheriton [page 70]

    * Respond

    * Forward

    * GetForwarder

    * GetProcess

    * GetPrincipal

- Packet Arrival

    * Request Packet

- Management Operations

    * NotifyVmtpServer

- Timeouts

    * Client State Record Timeout

The handling of these events is described in detail in the following subsections. The conventions of the previous chapter are followed, including the use of the various subroutines in the description.

Server User-invoked Events

A user event occurs when a VMTP server invokes one of the VMTP interface procedures.

Receive

AcceptMessage(reqmcb, segptr, segsize, client, transid, timeout)

Locate server's request queue.
if request is queued then
    Remember CSR associated with this Request.
    return Request in reqmcb, segptr and segsize
           and client and transaction id.
Wait on server's request queue for next request
up time timeout seconds.

end ReceiveCall

Notes:

Cheriton [page 71]

1. If a multi-packet Request is partially received at the time

  of the AcceptMessage, the process waits until it completes.

2. The behavior of a process accepting a Request as a

  lightweight thread is similar except that the process
  executes using the Request data logically as part of the
  requesting Client process.

Respond

RespondCall is described as one case of the Respond transmission procedure; RespondMessage is similar.

RespondCall( responsemcb, responsesegptr )

Locate csr for this client.
Check segment data accessible, if any
if local client then
    Handle locally
    return
endif
if responsemcb.Code is RESPONSE_DISCARDED then
    Mark as RESPONSE_DISCARDED
    return
SendPacketGroup( csr )
set csr.State to Responded.
if DGM reply then { Idempotent }
    release segment data
    Timeout( csr, TS4(csr.Client), FreeCsr );
else { Await acknowledgement or new Request else ask for ack. }
    Timeout( csr, TS5(csr.Client), RemoteClientTimeout )

end RespondCall

Notes:

1. RespondMessage is similar except the Server process must be

  synchronized with the release of the segment data (if any).

2. The non-idempotent Response with segment data is sent first

  without a request for an acknowledgement.  The Response is
  retransmitted after time TS5(client) if no acknowledgment or
  new Request is received from the client in the meantime.  At
  this point, the APG bit is sent.

3. The MCB of the Response is buffered in the client CSR, which

  remains for TS4 seconds, sufficient to filter old duplicates.
  The segment data (if any) must be retained intact until:  (1)

Cheriton [page 72]

  after transmission if idempotent or (2) after acknowledged or
  timeout has occurred if not idempotent.  Techniques such as
  copy-on-write might be used to keep a copy of the Response
  segment data without incurring the cost of a copy.

Forward

Forwarding is logically initiating a new message transaction between the Server (now acting as a Client) and the server to which the Request is forwarded. When the second server returns a Response, the same Response is immediately returned to the Client. The forwarding support in VMTP preserves these semantics while providing some performance optimizations in some cases.

ForwardCall( req, segptr, segsize, forwardserver )

Locate csr for this client.
Check segment data accessible, if any

if local client or Request was multicast or secure
   or csr.ForwardCount == 15 then
    Handle as a new Send operation
    return
if forwardserver is local then
    Handle locally
    return
Set csr.funccode to Request
Increment csr.ForwardCount
Set csr.State to Responded
SendPacketGroup( csr ) { To ForwardServer }
Timeout( csr, TS4(csr.Client), FreeAlien )

end ForwardCall

Notes:

1. A Forward is logically a new call or message transaction. It

  must be really implemented as a new message transaction if
  the original Request was multicast or secure (with the
  optional further refinement that it can be used with a secure
  message transaction when the Server and ForwardServer are the
  same principal and the Request was not multicast).

2. A Forward operation is never handled as an idempotent

  operation because it requires knowledge that the
  ForwardServer will treat the forwarded operation as
  idempotent as well.  Thus, a Forward operation that includes
  a segment should set APG on the first transmission of the

Cheriton [page 73]

  forwarded Request to get an acknowledgement for this data.
  Once the acknowledgement is received, the forwarding Server
  can discard the segment data, leaving only the basic CSR to
  handle retransmissions from the Client.

Other Functions

GetRemoteClient is a simple local query of the CSR. GetProcess and GetPrincipal also extract this information from the CSR. A server module may defer the Probe callback to the Client to get that information until it is requested by the Server (assuming it is not using secure communication and duplicate suppression is adequate without callback.) GetForwarder is implemented as a callback to the Client, using a GetRequestForwarder VMTP management operation. Additional management procedures for VMTP are described in Appendix III.

Request Packet Arrival

The basic packet reception follows that described for the Client routines. A Request packet is handled by the procedure HandleRequest.

HandleRequest( csr, p, psize )

if LocalClient(csr) then
    { Forwarded Request on local Client }
    if csr.LocalTransaction != p.Transaction then return
    if csr.State != AwaitingResponse then return
    if p.ForwardCount < csr.ForwardCount then
       Discard Request and return.
    Find a CSR for Client as a remote Client.
    if not found then
        if packet group complete then
            handle as a local message transaction
            return
        Allocate and init CSR
        goto newTransaction
    { Otherwise part of current transaction }
    { Handle directly below. }n
if csr.RemoteTransaction = p.Transaction then
  { Matches current transaction }
    if OldForward(p.ForwardCount,csr.ForwardCount) then
        return
    if p.ForwardCount > csr.ForwardCount then
      { New forwarded transaction }
        goto newTransaction

Cheriton [page 74]

    { Otherwise part of current transaction }
    if csr.State = ReceivingRequest then
        if new segment data then retain in CSR segment area.
        if Request not complete then
           Timeout( csr, TS1(p.Client), RemoteClientTimeout )
           return;
        endif
        goto endPacketGroup
    endif
    if csr.State is Responded then
      { Duplicate }
        if csr.Code is RESPONSE_DISCARDED
           and Multicast(p) then
            return
        endif
        if not DGM(csr) then { Not idempotent }
            if SegmentData(csr) then set APG
            { Resend Response or Request, if Forwarded }
            SendPacketGroup( csr )
            timeout=if SegmentData(csr) then TS5(csr.Client)
                      else TS4(csr.Client)
            Timeout( csr, timeout, RemoteClientTimeout )
            return
        { Else idempotent - fall thru to newTransaction }
    else { Presume it is a retransmission }
        NotifyClient( csr, p, OK )
        return

else if OldTransaction(csr.RemoteTransact,p.Transaction) then

    return
{ Otherwise, a new message transaction. }

newTransaction:

Abort handling of previous transactions for this Client.

if (NSRset(p) or NERset(p)) and NoStreaming then
    NotifyClient( csr, p, STREAMING_NOT_SUPPORTED )
    return

| if NSRset(p) then { Streaming } | { Check that consecutive with previous packet group } | Find last packet group CSR from this client. | if p.Transaction not lastcsr.RemoteTransaction+1 mod 2**32 | and not STIset(lastcsr) or | p.Transaction not lastcsr.RemoteTransaction+256 mod **32 | then | { Out of order packet group } | NotifyClient(csr, p, BAD_TRANSACTION_ID ) | return | endif

Cheriton [page 75]

| if lastcsr not completed then | NotifyClient( lastcsr, p, RETRY ) | endif | if lastcsr available then use it for this packet group | else allocate and initialize new CSR | if CMG(lastcsr) then | Add segment data to lastcsr Request | Keep csr as record of this packet group. | Clear lastcsr.VerifyInterval | endif | else { First packet group }

    if MultipleRemoteClients(csr) then ScavengeCsrs(p.Client)
    Set csr.RemoteTransaction, csr.Priority
    Copy message and segment data to csr's segment area
     and set csr.PacketDelivery to that delivered.
    Clear csr.PacketDelivery
    Clear csr.VerifyInterval
    SaveNetworkAddress( csr, p )
endif
if packetgroup not complete then
    Timeout( csr, TS3(p.Client), RemoteClientTimeout )
    return;
endif

endPacketGroup:

{ We have received complete packet group. }
if APG(p) then NotifyClient( csr, p, OK )
endif

| if NERset(p) and CMG(p) then | Queue waiting for continuation packet group. | Timeout( csr, TS3(csr.Client), RemoteClientTimeout ) | return | endif

{ Deliver request message. }
if GroupId(csr.Server) then
    For each server identified by csr.Server
        Replicate csr and associated data segment.
        if CMDset(csr) and Server busy then
           Discard csr and data
        else
           Deliver or invoke csr for each Server.
        if not DGMset(csr) then queue for Response
        else Timeout( csr, TS4(csr.Client), FreeCsr )
    endfor
 else
   if CMDset(csr) and Server busy then
       Discard csr and data
    else

Cheriton [page 76]

       Deliver or invoke csr for this server.
    if not DGMset(csr) then queue for Response
    else Timeout( csr, TS4(csr.Client), FreeCsr )
 endif

end HandleRequest

Notes:

1. A Request received that specifies a Client that is a local

  entity should be a Request forwarded by a remote server to a
  local Server.

2. An alternative structure for handling a Request sent to a

  group when there are multiple local group members is to
  create a remote CSR for each group member on reception of the
  first packet and deliver a copy of each packet to each such
  remote CSR as each packet arrives.

Cheriton [page 77]

Management Operations

VMTP uses management operations (invoked as remote procedure calls) to effectively acknowledge packet groups and request retransmissions. The following routine is invoked by the Server's management module on request from the Client.

NotifyVmtpServer(server,clientId,transact,delivery,code)

Find csr with same RemoteTransaction and RemoteClient
as clientId and transact.
if not found or csr.State not Responded then return
if DGMset(csr) then
    if transmission of Response in progress then
        Abort transmission
        if code is migrated then
           restart transmission with new host addr.
    if Retry then Report protocol error
    return
endif
select on code
  case RETRY:
    if csr.RetransCount > MaxRetrans(clientId) then
         if response data segment then
             Discard data and mark as RESPONSE_DISCARDED

| if NERset(csr) and subsequent csr then | Deallocate csr and use later csr for | future duplicate suppression | endif

         return
    endif
    increment csr.RetransCount
    Set csr.TransmissionMask to missing segment blocks,
        as specified by delivery
    SendPacketGroup( csr )
    Timeout( csr, TS3(csr.Client), RemoteClientTimeout )
  case BUSY:
    if csr.TimeLimit exceeded then
        if response data segment then
            Discard data and mark as RESPONSE_DISCARDED

| if NERset(csr) and subsequent csr then | Deallocate csr and use later csr for | future duplicate suppression | endif

         endif
    endif
    Set csr.TransmissionMask for full retransmission
    Clear csr.RetransCount

Cheriton [page 78]

    Timeout( csr, TS3(csr.Server), RemoteClientTimeout )
    return

  case ENTITY_MIGRATED:
    Get new host address for entity
    Set csr.TransmissionMask for full retransmission
    Clear csr.RetransCount
    SendPacketGroup( csr )
    Timeout( csr, TS3(csr.Server), RemoteClientTimeout )
    return

  case default:
    Abort transmission of Response if in progress.
    if response data segment then
       Discard data and mark as RESPONSE_DISCARDED
       if NERset(csr) and subsequent csr then
           Deallocate csr and use later csr for
           future duplicate suppression
       endif
    return
endselect

end NotifyVmtpServer

Notes:

1. A NotifyVmtpServer operation requesting retransmission of

  the Response is acceptable only if the Response was not
  idempotent.  When the Response is idempotent, the Client must
  be prepared to retransmit the Request to effectively request
  retransmission of the Response.

2. A NotifyVmtpServer operation may be received while the

  Response is being transmitted.  If an error return, as an
  efficiency, the transmission should be aborted, as suggested
  when the Response is a datagram.

3. A NotifyVmtpServer operation indicating OK or an error

  allows the Server to discard segment data and not provide for
  subsequent retransmission of the Response.

HandleRequestNoCSR

When a Request is received from a Client for which the node has no CSR, the node allocates and initializes a CSR for this Client and does a callback to the Client's VMTP management module to get the Principal, Process and other information associated with this Client. It also

Cheriton [page 79]

checks that the TransactionId is correct in order to filter out duplicates.

HandleRequestNoCSR( p, psize ) | if Secure(p) then | Allocate and init CSR | SaveSourceHostAddr( csr, p ) | ProbeRemoteClient( csr, p, AUTH_PROBE ) | if no response or error then | delete CSR | return | Decrypt( csr.Key, p, psize ) | if p.Checksum not null then | if not VerifyChecksum(p, psize) then return; | if OppositeByteOrder(p) then ByteSwap( p, psize ) | if psize not equal sizeof(VmtpHeader) + 4*p.Length then | NotifyClient(NULL, p, VMTP_ERROR ) | return | HandleRequest( csr, p, psize ) | return

if Server does not exist then
    NotifyClient( csr, p, NONEXISTENT_ENTITY )
    return
endif
if security required by server then
    NotifyClient(csr, p, SECURITY_REQUIRED )
    return
endif
Allocate and init CSR
SaveSourceHostAddr( csr, p );
if server requires Authentication then
    ProbeRemoteClient( csr, p, AUTH_PROBE )
    if no response or error then
       delete CSR
       return
endif
{ Setup immediately as a new message transaction }
set csr.Server to p.Server
set csr.RemoteTransaction to p.Transaction-1

HandleRequest( csr, p, psize )
endif

Notes:

1. A Probe request is always handled as a Request not requiring

  authentication so it never generates a callback Probe to the

Cheriton [page 80]

  Client.

2. If the Server host retains remote client CSR's for longer

  than the maximum packet lifetime and the Request
  retransmission time, and the host has been running for at
  least that long, then it is not necessary to do a Probe
  callback unless the Request is secure.  A Probe callback can
  take place when the Server asks for the Process or
  PrincipalId associated with the Client.

Cheriton [page 81]

Timeouts

The server must implement a timeout for remote client CSRs. There is a timeout for each CSR in the server.

RemoteClientTimeout( csr )

 select on csr.State
case Responded:
    if RESPONSE_DISCARDED then
        mark as timed out
        Make a candidate for reuse.
        return
    if csr.RetransCount > MaxRetrans(Client) then
        discard Response
        mark CSR as RESPONSE_DISCARDED
        Timeout(csr, TS4(Client), RemoteClientTimeout)
        return
    increment csr.RetransCount
    { Retransmit Response or forwarded Request }
    Set APG to get acknowledgement.
    SendPacketGroup( csr )
    Timeout( csr, TS3(Client), RemoteClientTimeout )
    return
case ReceivingRequest:
  if csr.RetransCount > MaxRetrans(csr.Client)
     or DGMset(csr) or NRTset(csr) then
      Modify csr.segmentSize and csr.MsgDelivery
      to indicate packets received.
      if MDMset(csr) then
          Invoke processing on Request
          return
      else
          discard Request and reuse CSR
          (Note: Need not remember Request discarded.)
          return
  increment csr.RetransCount
  NotifyClient( csr, p, RETRY )
  Timeout( csr, TS3(Client), RemoteClientTimeout )
  return
default:
    Report error - invalid state for RemoteClientTimeout
endselect

end RemoteClientTimeout

Notes:

1. When a CSR in the Responded state times out after discarding

Cheriton [page 82]

  the Response, it can be made available for reuse, either by
  the same Client or a different one.  The CSR should be kept
  available for reuse by the Client for as long as possible to
  avoid unnecessary callback Probes.

Cheriton [page 83]

Concluding Remarks

This document represents a description of the current state of the VMTP design. We are currently engaged in several experimental implementations to explore and refine all aspects of the protocol. Preliminary implementations are running in the UNIX 4.3BSD kernel and in the V kernel.

Several issues are still being discussed and explored with this protocol. First, the size of the checksum field and the algorithm to use for its calculation are undergoing some discussion. The author believes that the conventional 16-bit checksum used with TCP and IP is too weak for future high-speed networks, arguing for at least a 32-bit checksum. Unfortunately, there appears to be limited theory covering checksum algorithms that are suitable for calculation in software.

Implementation of the streaming facilities of VMTP is still in progress. This facility is expected to be important for wide-area, long delay communication.

Cheriton [page 84]

I. Standard VMTP Response Codes

The following are the numeric values of the response codes used in VMTP.

0 OK

1 RETRY

2 RETRY_ALL

3 BUSY

4 NONEXISTENT_ENTITY

5 ENTITY_MIGRATED

6 NO_PERMISSION

7 NOT_AWAITING_MSG

8 VMTP_ERROR

9 MSGTRANS_OVERFLOW

10 BAD_TRANSACTION_ID

11 STREAMING_NOT_SUPPORTED

12 NO_RUN_RECORD

13 RETRANS_TIMEOUT

14 USER_TIMEOUT

15 RESPONSE_DISCARDED

16 SECURITY_NOT_SUPPORTED

17 BAD_REPLY_SEGMENT

18 SECURITY_REQUIRED

19 STREAMED_RESPONSE

20 TOO_MANY_RETRIES

21 NO_PRINCIPAL

Cheriton [page 85]

22 NO_KEY

23 ENCRYPTION_NOT_SUPPORTED

24 NO_AUTHENTICATOR

25-63 Reserved for future VMTP assignment.

Other values of the codes are available for use by higher level protocols. Separate protocol documents will specify further standard values.

Applications are free to use values starting at 0x00800000 (hex) for application-specific return values.

Cheriton [page 86]

II. VMTP RPC Presentation Protocol

For complete generality, the mapping of the procedures and the parameters onto VMTP messages should be defined by a RPC presentation protocol. In the absence of an accepted standard protocol, we define an RPC presentation protocol for VMTP as follows.

Each procedure is assigned an identifying Request Code. The Request code serves effectively the same as a tag field of variant record, identifying the format of the Request and associated Response as a variant of the possible message formats.

The format of the Request for a procedure is its Request Code followed by its parameters sequentially in the message control block until it is full.

The remaining parameters are sent as part of the message segment data formatted according to the XDR protocol (RFC ??). In this case, the size of the segment is specified in the SegmentSize field.

The Response for a procedure consists of a ResponseCode field followed by the return parameters sequentially in the message control block, except if there is a parameter returned that must be transmitted as segment data, its size is specified in the SegmentSize field and the parameter is stored in the SegmentData field.

Attributes associated with procedure definitions should indicate the Flags to be used in the RequestCode. Request Codes are assigned as described below.

II.1. Request Code Management

Request codes are divided into Public Interface Codes and application-specific, according to whether the PIC value is set. An interface is a set of request codes representing one service or module function. A public interface is one that is to be used in multiple independently developed modules. In VMTP, public interface codes are allocated in units of 256 structured as

+-------------+----------------+-------------------+
| ControlFlags|  Interface     | Version/Procedure |
+-------------+----------------+-------------------+
8 bits          16 bits              8 bits

An interface is free to allocate the 8 bits for version and procedure as desired. For example, all 8 bits can be used for procedures. A module requiring more than 256 Version/Procedure values can be allocated

Cheriton [page 87]

multiple Interface values. They need not be consecutive Interface values.

Cheriton [page 88]

III. VMTP Management Procedures

Standard procedures are defined for VMTP management, including creation, deletion and query of entities and entity groups, probing to get information about entities, and updating message transaction information at the client or the server.

The procedures are implemented by the VMTP manager that constitutes a portion of every complete VMTP module. Each procedure is invoked by sending a Request to the VMTP manager that handles the entity specified in the operation or the local manager. The Request sent using the normal Send operation with the Server specified as the well-known entity group VMTP_MANGER_GROUP, using the CoResident Entity mechanism to direct the request to the specific manager that should handle the Request. (The ProbeEntity operation is multicast to the VMTP_MANAGER_GROUP if the host address for the entity is not known locally and the host address is determined as the host address of the responder. For all other operations, a ProbeEntity operation is used to determine the host address if it is not known.) Specifying co-resident entity 0 is interpreted as the co-resident with the invoking process. The co-resident entity identifier may also specify a group in which case, the Request is sent to all managers with members in this group.

The standard procedures with their RequestCode and parameters are listed below with their semantics. (The RequestCode range 0xVV000100 to 0xVV0001FF is reserved for use by the VMTP management routines, where VV is any choice of control flags with the PIC bit set. The flags are set below as required for each procedure.)

0x05000101 - ProbeEntity(CREntity, entityId, authDomain) -> (code,

            <staterec>) 
            Request and return information on the specified entity
            in the specified authDomain, sending the Request to the
            VMTP management module coresident with CREntity.  An
            error return is given if the requested information
            cannot be provided in the specified authDomain.  The
            <staterec> returned is structured as the following
            fields.

            Transaction identifier
                            The current or next transaction
                            identifier being used by the probed
                            entity.

            ProcessId: 64 bits 
                            Identifier for client process.  The
                            meaning of this is specified as part of

Cheriton [page 89]

                            the Domain definition.

            PrincipalId     The identifier for the principal or
                            account associated with the process
                            specified by ProcessId.  The meaning of
                            this field is specified as part of the
                            Domain definition.

            EffectivePrincipalId
                            The identifier for the principal or
                            account associated with the Client port,
                            which may be different from the
                            PrincipalId especially if this is an
                            nested call.  The meaning of this field
                            is specified as part of the Domain
                            definition.

            The code field indicates whether this is an error
            response or not.  The codes and their interpretation
            are:

              OK
            No error. Probe was completed OK.

              NONEXISTENT_ENTITY
            Specified entity does not exist.

              ENTITY_MIGRATED
            The entity has migrated and is no longer at the host to
            which the request was sent.

              NO_PERMISSION
            Entity has refused to provide ProbeResponse.

              VMTP_ERROR
            The Request packet group was in error relative to the
            VMTP protocol specification.

              "default"
            Some type of error - discard ProbeResponse.

0x0D000102 - AuthProbeEntity(CREntity,entityId,authDomain,randomId) ->

            (code,ProbeAuthenticator,EncryptType,EntityAuthenticator)
            
            Request authentication of the entity specified by
            entityId from the VMTP manager coresident with CREntity
            in authDomain authentication domain, returning the

Cheriton [page 90]

            information contained in the return parameters.  The
            fields are set the same as that specified for the basic
            ProbeResponse except as noted below.

            ProbeAuthenticator
                            20 bytes consisting of the EntityId, the
                            randomId and the probed Entity's current
                            Transaction value plus a 32-bit checksum
                            for these two fields (checksummed using
                            the standard packet Checksum algorithm),
                            all encrypted with the Key supplied in
                            the Authenticator.

            EncryptType     An identifier that identifies the
                            variant of encryption method being used
                            by the probed Entity for packets it
                            transmits and packets it is able to
                            receive.  (See Appendix V.)  The
                            high-order 8 bits of the EncryptType
                            contain the XOR of the 8 octets of the
                            PrincipalId associated with private key
                            used to encrypt the EntityAuthenticator.
                            This value is used by the requestor or
                            Client as an aid in locating the key to
                            decrypt the authenticator.

            EntityAuthenticator
                            (returned as segment data) The
                            ProcessId, PrincipalId,
                            EffectivePrincipal associated with the
                            ProbedEntity plus the private
                            encryption/decryption key and its
                            lifetime limit to be used for
                            communication with the Entity.  The
                            authenticator is encrypted with a
                            private key associated with the Client
                            entity such that it can be neither read
                            nor forged by a party not trusted by the
                            Client Entity.  The format of the
                            Authenticator in the message segment is
                            shown in detail in Figure III-1.

            Key: 64 bits    Encryption key to be used for encrypting
                            and decrypting packets sent to and
                            received from the probed Entity.  This
                            is the "working" key for packet
                            transmissions.  VMTP only uses private

Cheriton [page 91]

            +-----------------------------------------------+
            |            ProcessId   (8 octets)             |
            +-----------------------------------------------+
            |           PrincipalId  (8 octets)             |
            +-----------------------------------------------+
            |           EffectivePrincipalId  (8 octets)    |
            +-----------------------------------------------+
            |            Key  (8 octets)                    |
            +-----------------------------------------------+
            |              KeyTimeLimit                     |
            +-----------------------------------------------+
            |              AuthDomain                       |
            +-----------------------------------------------+
            |               AuthChecksum                    |
            +-----------------------------------------------+

              Figure III-1:   Authenticator Format

                            key encryption for data transmission.

            KeyTimeLimit: 32 bits 
                            The time in seconds since Dec. 31st,
                            1969 GMT at which one should cease to
                            use the Key.

            AuthDomain: 32 bits 
                            The authentication domain in which to
                            interpret the principal identifiers.
                            This may be different from the
                            authDomain specified in the call if the
                            Server cannot provide the authentication
                            information in the request domain.

            AuthChecksum: 32 bits 
                            Contains the checksum (using the same
                            Checksum algorithm as for packet) of
                            KeyTimeLimit, Key, PrincipalId and
                            EffectivePrincipalId.

            Notes:

               1. A authentication Probe Request and Response
                  are sent unencrypted in general because it is
                  used prior to there being a secure channel.
                  Therefore, specific fields or groups of
                  fields checksummed and encrypted to prevent
                  unauthorized modification or forgery.  In

Cheriton [page 92]

                  particular, the ProbeAuthenticator is
                  checksummed and encrypted with the Key.

               2. The ProbeAuthenticator authenticates the
                  Response as responding to the Request when
                  its EntityId, randomId and Transaction values
                  match those in the Probe request.  The
                  ProbeAutenticator is bound to the
                  EntityAutenticator by being encrypted by the
                  private Key contained in that authenticator.

               3. The authenticator is encrypted such that it
                  can be decrypted by a private key, known to
                  the Client.  This authenticator is presumably
                  obtained from a key distribution center that
                  the Client trusts.  The AuthChecksum prevents
                  undetected modifications to the
                  authenticator.

0x05000103 - ProbeEntityBlock( entityId ) -> ( code, entityId )

            Check whether the block of 256 entity identifiers
            associated with this entityId are in use.  The entityId
            returned should match that being queried or else the
            return value should be ignored and the operation redone.

0x05000104 - QueryVMTPNode( entityId ) -> (code, MTU, flags, authdomain,

            domains, authdomains, domainlist) 
            Query the VMTP management module for entityId to get
            various module- or node-wide parameters, including:  (1)
            MTU - Maximum transmission unit or packet size handled
            by this node.  (2) flags- zero or more of the following
            bit fields:

            1               Handles streamed Requests.

            2               Can issue streamed message transactions
                            for clients.

            4               Handles secure Requests.

            8               Can issue secure message transactions.

            The authdomain indicates the primary authentication
            domain supported.  The domains and authdomains
            parameters indicate the number of entity domains and
            authentication domains supported by this node, which are
            listed in the data segment parameter domainlist if

Cheriton [page 93]

            either parameter is non-zero. (All the entity domains
            precede the authentication domains in the data segment.)

0x05000105 - GetRequestForwarder( CREntity, entityId1 ) -> (code,

            entityId2, principal, authDomain) 
            Return the forwarding server's entity identifer and
            principal for the forwarder of entityId1.  CREntity
            should be zero to get the local VMTP management module.

0x05000106 - CreateEntity( entityId1 ) -> ( code, entityId2 )

            Create a new entity and return its entity identifier in
            entityId2.  The entity is created local to the entity
            specified in entityId1 and local to the requestor if
            entityId1 is 0.

0x05000107 - DeleteEntity( entityId ) -> ( code )

            Delete the entity specified by entityId, which may be a
            group.  If a group, the deletion is only on a best
            efforts basis.  The client must take additional measures
            to ensure complete deletion if required.

0x0D000108 -QueryEntity( entityId ) -> ( code, descriptor )

            Return a descriptor of entityId in arg of a maximum of
            segmentSize bytes.

0x05000109 - SignalEntity( entityId, arg )->( code )

            Send the signal specified by arg to the entity specified
            by entityId.  (arg is 32 bits.)

0x0500010A - CreateGroup(CREntity,entityGroupId,entityId,perms)->(code)

            Request that the VMTP manager local to CREntity create
            an new entity group, using the specified entityGroupId
            with entityId as the first member and permissions
            "perms", a 32-bit field described later.  The invoker is
            registered as a manager of the new group, giving it the
            permissions to add or remove members.  (Normally
            CREntity is 0, indicating the VMTP manager local to the
            requestor.)

0x0500010B - AddToGroup(CREntity, entityGroupId, entityId,

            perms)->(code) 
            Request that the VMTP manager local to CREntity add the
            specified entityId to the entityGroupId with the
            specified permissions.  If entityGroupId specifies a
            restricted group, the invoker must have permission to
            add members to the group, either because the invoker is

Cheriton [page 94]

            a manager of the group or because it was added to the
            group with the required permissions.  If CREntity is 0,
            then the local VMTP manager checks permissions and
            forwards the request with CREntity set to entityId and
            the entityId field set to a digital signature (see
            below) of the Request by the VMTP manager, certifying
            that the Client has the permissions required by the
            Request.  (If entityGroupId specifies an unrestricted
            group, the Request can be sent directly to the handling
            VMTP manager by setting CREntity to entityId.)

0x0500010C - RemoveFromGroup(CREntity, entityGroupId, entityId)->(code)

            Request that the VMTP manager local to CREntity remove
            the specified entityId from the group specified by
            entityGroupId.  Normally CREntity is 0, indicating the
            VMTP manager local to the requestor.  If CREntity is 0,
            then the local VMTP manager checks permissions and
            forwards the request with CREntity set to entityId and
            the entityId field a digital signature of the Request by
            the VMTP manager, certifying that the Client has the
            permissions required by the Request.

0x0500010D - QueryGroup( entityId )->( code, record )...

            Return information on the specified entity.  The
            Response from each responding VMTP manager is (code,
            record).  The format of the record is (memberCount,
            member1, member2, ...).  The Responses are returned on a
            best efforts basis; there is no guarantee that responses
            from all managers with members in the specified group
            will be received.

0x0500010E - ModifyService(entityId,flags,count,pc,threadlist)->(code,

            count) 
            Modify the service associated with the entity specified
            by entityId.  The flags may indicate a message service
            model, in which case the call "count" parameter
            indicates the maximum number of queued messages desired;
            the return "count" parameter indicates the number of
            queued message allowed.  Alternatively, the "flags"
            parameters indicates the RPC thread service model, in
            which case "count" threads are requested, each with an
            inital program counter as specified and stack, priority
            and message receive area indicated by the threadlist.
            In particular, "threadlist" consists of "count" records
            of the form
            (priority,stack,stacksize,segment,segmentsize), each one
            assigned to one of the threads.  Flags defined for the

Cheriton [page 95]

            "flags" parameter are:

            1               THREAD_SERVICE - otherwise the message
                            model.

            2               AUTHENTICATION_REQUIRED - Sent a Probe
                            request to determine principal
                            associated with the Client, if not
                            known.

            4               SECURITY_REQUIRED - Request must be
                            encrypted or else reject.

            8               INCREMENTAL - treat the count value as
                            an increment (or decrement) relative to
                            the current value rather than an
                            absolute value for the maximum number of
                            queued messages or threads.

            In the thread model, the count must be a positive
            increment or else 0, which disables the service.  Only a
            count of 0 terminates currently queued requests or
            in-progress request handling.

0x4500010F -

            NotifyVmtpClient(client,cntrl,recSeq,transact,delivery,code)->()
            
            Update the state associated with the transaction
            specified by client and transact, an entity identifier
            and transaction identifier, respectively.  This
            operation is normally used only by another VMTP
            management module.  (Note that it is a datagram
            operation.)  The other parameters are as follows:

            ctrl            A 32-bit value corresponding to 4th
                            32-bit word of the VMTP header of a
                            Response packet that would be sent in
                            response to the Request that this is
                            responding to.  That is, the control
                            flags, ForwardCount, RetransmitCount and
                            Priority fields match those of the
                            Request.  (The NRS flag is set if the
                            receiveSeqNumber field is used.)  The
                            PGCount subfield indicates the number of
                            previous Request packet groups being
                            acknowledged by this Notify operation.
                            (The bit fields that are reserved in

Cheriton [page 96]

                            this word in the header are also
                            reserved here and must be zero.)

            recSeq          Sequence number of reception at the
                            Server if the NRS flag is set in the
                            ctrl parameter, otherwise reserved and
                            zero.  (This is used for sender-based
                            logging of message activity for replay
                            in case of failure - an optional
                            facility.)

            delivery        Indicates the segment blocks of the
                            packet group have been received at the
                            Server.

            code            indicates the action the client should
                            take, as described below.

            The VMTP management module should take action on this
            operation according to the code, as specified below.

            OK              Do nothing at this time, continue
                            waiting for the response with a reset
                            timer.

            RETRY           Retransmit the request packet group
                            immediately with at least the segment
                            blocks that the Server failed to
                            receive, the complement of those
                            indicated by the delivery parameter.

            RETRY_ALL       Retransmit the request packet group
                            immediately with at least the segment
                            blocks that the Server failed to
                            receive, as indicated by the delivery
                            field plus all subsequently transmitted
                            packets that are part of this packet
                            run.  (The latter is applicable only for
                            streamed message transactions.)

            BUSY            The server was unable to accept the
                            Request at this time.  Retry later if
                            desired to continue with the message
                            transaction.

            NONEXISTENT_ENTITY
                            Specified Server entity does not exist.

Cheriton [page 97]

            ENTITY_MIGRATED The server entity has migrated and is no
                            longer at the host to which the request
                            was sent.  The Server should attempt to
                            determine the new host address of the
                            Client using the VMTP management
                            ProbeEntity operation (described
                            earlier).

            NO_PERMISSION   Server has not authorized reception of
                            messages from this client.

            NOT_AWAITING_MSG
                            The conditional message delivery bit was
                            set for the Request packet group and the
                            Server was not waiting for it so the
                            Request packet group was discarded.

            VMTP_ERROR      The Request packet group was in error
                            relative to the VMTP protocol
                            specification.

            BAD_TRANSACTION_ID
                            Transaction identifier is old relative
                            to the transaction identifier held for
                            the Client by the Server.

            STREAMING_NOT_SUPPORTED
                            Server does not support multiple
                            outstanding message transactions from
                            the same Client, i.e. streamed message
                            transactions.

            SECURITY_NOT_SUPPORTED
                            The Request was secure and this Server
                            does not support security.

            SECURITY_REQUIRED
                            The Server is refusing the Request
                            because it was not encrypted.

            NO_RUN_RECORD   Server has no record of previous packets
                            in this run of packet groups.  This can
                            occur if the first packet group is lost
                            or if the current packet group is sent
                            significantly later than the last one
                            and the Server has discarded its client
                            state record.

Cheriton [page 98]

0x45000110 - NotifyVmtpServer(server,client,transact,delivery,code)->()

            Update the server state associated with the transaction
            specified by client and transact, an entity identifier
            and transaction identifier, respectively.  This
            operation is normally used only by another VMTP
            management module.  (Note that it is a datagram
            operation.)  The other parameters are as follows:

            delivery        Indicates the segment blocks of the
                            Response packet group that have been
                            received at the Client.

            code            indicates the action the Server should
                            take, as listed below.

            The VMTP management module should take action on this
            operation according to the code, as specified below.

            OK              Client is satisfied with Response data.
                            The Server can discard the response
                            data, if any.

            RETRY           Retransmit the Response packet group
                            immediately with at least the segment
                            blocks that the Client failed to
                            receive, as indicated by the delivery
                            parameter.  (The delivery parameter
                            indicates those segment blocks received
                            by the Client).

            RETRY_ALL       Retransmit the Response packet group
                            immediately with at least the segment
                            blocks that the Client failed to
                            receive, as indicated by the (complement
                            of) the delivery parameter.  Also,
                            retransmit all Response packet groups
                            send subsequent to the specified packet
                            group.

            NONEXISTENT_ENTITY
                            Specified Client entity does not exist.

            ENTITY_MIGRATED The Client entity has migrated and is no
                            longer at the host to which the response
                            was sent.

            RESPONSE_DISCARDED

Cheriton [page 99]

                            The Response was discarded and no longer
                            of interest to the Client.  This may
                            occur if the conditional message
                            delivery bit was set for the Response
                            packet group and the Client was not
                            waiting for it so the Response packet
                            group was discarded.

            VMTP_ERROR      The Response packet group was in error
                            relative to the VMTP protocol
                            specification.

0x41000111 -

            NotifyRemoteVmtpClient(client,ctrl,recSeq,transact,delivery,code->()
            
            The same as NotifyVmtpClient except the co-resident
            addressing is not used.  This operation is used to
            update client state that is remote when a Request is
            forwarded.

Note the use of the CRE bit in the RequestCodes to route the request to the correct VMTP management module(s) to handle the request.

III.1. Entity Group Management

An entity in a group has a set of permissions associated with its membership, controling whether it can add or remove others, whether it can remove itself, and whether others can remove it from the group. The permissions for entity groups are as follows: VMTP_GRP_MANAGER 0x00000001 { Manager of group. } VMTP_REM_BY_SELF 0x00000002 { Can be removed self. } VMTP_REM_BY_PRIN 0x00000004 { Can be rem'ed by same principal} VMTP_REM_BY_OTHE 0x00000008 { Can be removed any others. } VMTP_ADD_PRIN 0x00000010 { Can add by same principal. } VMTP_ADD_OTHE 0x00000020 { Can add any others. } VMTP_REM_PRIN 0x00000040 { Can remove same principal. } VMTP_REM_OTHE 0x00000080 { Can remove any others. }

To remove an entity from a restricted group, the invoker must have permission to remove that entity and the entity must have permissions that allow it to be removed by that entity. With an unrestricted group, only the latter condition applies.

With a restricted group, a member can only be added by another entity with the permissions to add other entities. The creator of a group is given full permissions on a group. A entity adding another entity to a

Cheriton [page 100]

group can only give the entity it adds a subset of its permissions. With unrestricted groups, any entity can add itself to the group. It can also add other entities to the group providing the entity is not marked as immune to such requests. (This is an implementation restriction that individual entities can impose.)

III.2. VMTP Management Digital Signatures

As mentioned above, the entityId field of the AddToGroup and RemoveFromGroup is used to transmit a digital signature indicating the permission for the operation has been checked by the sending kernel. The digital signature procedures have not yet been defined. This field should be set to 0 for now to indicate no signature after the CREntity parameter is set to the entity on which the operation is to be performed.

Cheriton [page 101]

IV. VMTP Entity Identifier Domains

VMTP allows for several disjoint naming domains for its endpoints. The 64-bit entity identifier is only unique and meaningful within its domain. Each domain can define its own algorithm or mechanism for assignment of entity identifiers, although each domain mechanism must ensure uniqueness, stability of identifiers and host independence.

IV.1. Domain 1

For initial use of VMTP, we define the domain with Domain identifier 1 as follows:

+-----------+----------------+------------------------+
| TypeFlags | Discriminator  |    Internet Address    |
+-----------+----------------+------------------------+
4 bits          28 bits                32 bits

The Internet address is the Internet address of the host on which this entity-id is originally allocated. The Discriminator is an arbitrary value that is unique relative to this Internet host address. In addition, the host must guarantee that this identifier does not get reused for a long period of time after it becomes invalid. ("Invalid" means that no VMTP module considers in bound to an entity.) One technique is to use the lower order bits of a 1 second clock. The clock need not represent real-time but must never be set back after a crash. In a simple implementation, using the low order bits of a clock as the time stamp, the generation of unique identifiers is overall limited to no more than 1 per second on average. The type flags were described in Section 3.1.

An entity may migrate between hosts. Thus, an implementation can heuristically use the embedded Internet address to locate an entity but should be prepared to maintain a cache of redirects for migrated entities, plus accept Notify operations indicating that migration has occurred.

Entity group identifiers in Domain 1 are structured in one of two forms, depending on whether they are well-known or dynamically allocated identifiers. A well-known entity identifier is structured as:

+-----------+----------------+------------------------+
| TypeFlags |  Discriminator |Internet Host Group Addr|
+-----------+----------------+------------------------+
4 bits          28 bits                32 bits

Cheriton [page 102]

with the second high-order bit (GRP) set to 1. This form of entity identifier is mapped to the Internet host group address specified in the low-order 32 bits. The Discriminator distinguishes group identifiers using the same Internet host group. Well-known entity group identifiers should be allocated to correspond to the basic services provided by hosts that are members of the group, not specifically because that service is provided by VMTP. For example, the well-known entity group identifier for the domain name service should contain as its embedded Internet host group address the host group for Domain Name servers.

A dynamically allocated entity identifier is structured as:

+-----------+----------------+------------------------+
| TypeFlags |  Discriminator |   Internet Host Addr   |
+-----------+----------------+------------------------+
4 bits          28 bits             32 bits

with the second high-order bit (GRP) set to 1. The Internet address in the low-order 32 bits is a Internet address assigned to the host that dynamically allocates this entity group identifier. A dynamically allocated entity group identifier is mapped to Internet host group address 232.X.X.X where X.X.X are the low-order 24 bits of the Discriminator subfield of the entity group identifier.

We use the following notation for Domain 1 entity identifiers <10> and propose it use as a standard convention.

    <flags>-<discriminator>-<Internet address>

where <flags> are [X]{BE,LE,RG,UG}[A]

X = reserved
BE = big-endian entity
LE = little-endian entity
RG = restricted group
UG = unrestricted group
A  = alias

and <discriminator> is a decimal integer and <Internet address> is in standard dotted decimal IP address notation.

Examples:

_______________

<10> This notation was developed by Steve Deering.

Cheriton [page 103]

BE-25593-36.8.0.49 is big-endian entity #25593 created on host

            36.8.0.49.

RG-1-224.0.1.0 is the well-known restricted VMTP managers group.

UG-565338-36.8.0.77 is unrestricted entity group #565338 created on host

            36.8.0.77.

LEA-7823-36.8.0.77 is a little-endian alias entity #7823 created on host

            36.8.0.77.

This notation makes it easy to communicate and understand entity identifiers for Domain 1.

The well-known entity identifiers specified to date are:

VMTP_MANAGER_GROUP RG-1-224.0.1.0

            Managers for VMTP operations.

VMTP_DEFAULT_BECLIENT BE-1-224.0.1.0

            Client entity identifier to use when a (big-endian) host
            has not determined or been allocated any client entity
            identifiers.

VMTP_DEFAULT_LECLIENT LE-1-224.0.1.0

            Client entity identifier to use when a (little-endian)
            host has not determined or been allocated any client
            entity identifiers.

Note that 224.0.1.0 is the host group address assigned to VMTP and to which all VMTP hosts belong.

Other well-known entity group identifiers will be specified in subsequent extensions to VMTP and in higher-level protocols that use VMTP.

IV.2. Domain 3

Domain 3 is reserved for embedded systems that are restricted to a single network and are independent of IP. Entity identifiers are allocated using the decentralized approach described below. The mapping of entity group identifiers is specific to the type of network being used and not defined here. In general, there should be a simple algorithmic mapping from entity group identifier to multicast address, similar to that described for Domain 1. Similarly, the values for default client identifier are specific to the type of network and not

Cheriton [page 104]

defined here.

IV.3. Other Domains

Definition of additional VMTP domains is planned for the future. Requests for allocation of VMTP Domains should be addressed to the Internet protocol administrator.

IV.4. Decentralized Entity Identifier Allocation

The ProbeEntityBlock operation may be used to determine whether a block of entity identifiers is in use. ("In use" means valid or reserved by a host for allocation.) This mechanism is used to detect collisions in allocation of blocks of entity identifiers as part of the implementation of decentralized allocation of entity identifiers. (Decentralized allocation is used in local domain use of VMTP such as in embedded systems- see Domain 3.)

Basically, a group of hosts can form a Domain or sub-Domain, a group of hosts managing their own entity identifier space or subspace, respectively. As an example of a sub-Domain, a group of hosts in Domain 1 all identified with a particular host group address can manage the sub-Domain corresponding to all entity identifiers that contain that host group address. The ProbeEntityBlock operation is used to allocate the random bits of these identifiers as follows.

When a host requires a new block of entity identifiers, it selects a new block (randomly or by some choice algorithm) and then multicasts a ProbeEntityBlock request to the members of the (sub-)Domain some R times. If no response is received after R (re)transmissions, the host concludes that it is free to use this block of identifiers. Otherwise, it picks another block and tries again.

Notes:

1. A block of 256 identifiers is specified by an entity

  identifier with the low-order 8 bits all zero.

2. When a host allocates an initial block of entity identifiers

  (and therefore does not yet have a specified entity
  identifier to use) it uses VMTP_DEFAULT_BECLIENT (if
  big-endian, else VMTP_DEFAULT_LECLIENT if little-endian) as
  its client identifier in the ProbeEntityBlock Request and a
  transaction identifier of 0.  As soon as it has allocated a
  block of entity identifiers, it should use these identifiers

Cheriton [page 105]

RFC 1045                       VMTP                        February 1988 

  for all subsequent communication.  The default client
  identifier values are defined for each Domain.

3. The set of hosts using this decentralized allocation must not

  be subject to network partitioning.  That is, the R
  transmissions must be sufficient to ensure that every host
  sees the ProbeEntityBlock request and (reliably) sends a
  response.  (A host that detects a collision can retransmit
  the response multiple times until it sees a new
  ProbeEntityBlock operation from the same host/Client up to a
  maximum number of times.)  For instance, a set of machines
  connected by a single local network may able to use this type
  of allocation.

4. To guarantee T-stability, a host must prevent reuse of a

  block of identifiers if any of the identifiers in the block
  are currently valid or have been valid less than T seconds
  previously.  To this end, a host must remember recently used
  identifiers and object to their reuse in response to a
  ProbeEntityBlock operation.

5. Care is required in a VMTP implementation to ensure that

  Probe operations cannot be discarded due to lack of buffer
  space or queued or delayed so that a response is not
  generated quickly.  This is required not only to detect
  collisions but also to provide accurate roundtrip estimates
  as part of ProbeEntity operations.

Cheriton [page 106]

V. Authentication Domains

A VMTP authentication domain defines the format and interpretation for principal identifiers and encryption keys. In particular, an authentication domain must specify a means by which principal identifiers are allocated and guaranteed unique and stable. The currently defined authentication domains are as follows (0 is reserved).

Ideally, all entities within one entity domain are also associated with one authentication domain. However, authentication domains are orthogonal to entity domains. Entities within one domain may have different authentication domains. (In this case, it is generally necessary to have some correspondence between principals in the different domains.) Also, one entity identifier may be associated with multiple authentication domains. Finally, one authentication domain may be used across multiple entity domains.

V.1. Authentication Domain 1

A principal identifier is structured as follows.

+---------------------------+------------------------+
|     Internet Address      | Local User Identifier  |
+---------------------------+------------------------+
         32 bits                    32 bits

The Internet Address may specify an individual host (such as a UNIX machine) or may specify a host group address corresponding to a cluster of machines operating under a single adminstration. In both cases, there is assumed to be an adminstration associated with the embedded Internet address that guarantees the uniqueness and stability of the User Identifier relative to the Internet address. In particular, that administration is the only one authorized to allocate principal identifiers with that Internet address prefix, and it may allocate any of these identifiers.

In authentication domain 1, the standard EncryptionQualifiers are:

0 Clear text - no encryption.

1 use 64-bit CBC DES for encryption and decryption.

V.2. Other Authentication Domains

Other authentication domains will be defined in the future as needed.

Cheriton [page 107]

VI. IP Implementation

VMTP is designed to be implemented on the DoD IP Internet Datagram Protocol (although it may also be implemented as a local network protocol directly in "raw" network packets.)

VMTP is assigned the protocol number 81.

With a 20 octet IP header and one segment block, a VMTP packet is 600 octets. By convention, any host implementing VMTP implicitly agrees to accept VMTP/IP packets of at least 600 octets.

VMTP multicast facilities are designed to work with, and have been implemented using, the multicast extensions to the Internet [8] described in RFC 966 and 988. The wide-scale use of full VMTP/IP depends on the availability of IP multicast in this form.

Cheriton [page 108]

VII. Implementation Notes

The performance and reliability of a protocol in operation is highly dependent on the quality of its implementation, in addition to the "intrinsic" quality of the protocol design. One of the design goals of the VMTP effort was to produce an efficiently implementable protocol. The following notes and suggestions are based on experience with implementing VMTP in the V distributed system and the UNIX 4.3 BSD kernel. The following is described for a client and server handling only one domain. A multi-domain client or server would replicate these structures for each domain, although buffer space may be shared.

VII.1. Mapping Data Structures

The ClientMap procedure is implemented using a hash table that maps to the Client State Record whether this entity is local or remote, as shown in Figure VII-1.

         +---+---+--------------------------+
ClientMap   |   | x |                          |
         +---+-|-+--------------------------+
               |   +--------------+    +--------------+
               +-->| LocalClient  |--->| LocalClient  |
                   +--------------+    +--------------+
                   | RemoteClient |    | RemoteClient |-> ...
                   +--------------+    +--------------+
                   |              |    |              |
                   |              |    |              |
                   +--------------+    +--------------+

        Figure VII-1:   Mapping Client Identifier to CSR

Local clients are linked through the LocalClientLink, similarly for the RemoteClientLink. Once a CSR with the specified Entity Id is found, some field or flag indicates whether it is identifying a local or remote Entity. Hash collisions are handled with the overflow pointers LocalClientLink and RemoteClientLink (not shown) in the CSR for the LocalClient and RemoteClient fields, respectively. Note that a CSR representing an RPC request has both a local and remote entity identifier mapping to the same CSR.

The Server specified in a Request is mapped to a server descriptor using the ServerMap (with collisions handled by the overflow pointer.). The server descriptor is the root of a queue of CSR's for handling requests plus flags that modify the handling of the Request. Flags include:

Cheriton [page 109]

             +-------+---+-------------------------+
 ServerMap      |       | x |                         |
             +-------+-|-+-------------------------+
                       |   +--------------+
                       |   | OverflowLink |
                       |   +--------------+
                       +-->|   Server     |
                           +--------------+
                           | Flags | Lock |
                           +--------------+
                           | Head Pointer |
                           +--------------+
                           | Tail Pointer |
                           +--------------+

           Figure VII-2:   Mapping Server Identifiers

THREAD_QUEUE Request is to be invoked directly as a remote procedure

            invocation, rather than by a server process in the
            message model.

AUTHENTICATION_REQUIRED

            Sent a Probe request to determine principal associated
            with the Client, if not known.

SECURITY_REQUIRED

            Request must be encrypted or else reject.

REQUESTS_QUEUED Queue contains waiting requests, rather than free CSR's.

            Queue this request as well.

SERVER_WAITING The server is waiting and available to handle incoming

            Request immediately, as required by CMD.

Alternatively, the Server identifiers can be mapped to a CSR using the MapToClient mechanism with a pointer in the CSR refering to the server descriptor, if any. This scheme is attractive if there are client CSR's associated with a service to allow it to communicate as a client using VMTP with other services.

Finally, a similar structure is used to expand entity group identifiers to the local membership, as shown in Figure VII-3. A group identifier is hashed to an index in the GroupMap. The list of group descriptors rooted at that index in the GroupMap contains a group descriptor for each local member of the group. The flags are the group permissions defined in Appendix III.

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             +-------+---+----------------------------------+
 GroupMap       |       | x |                                  |
             +-------+-|-+----------------------------------+
                       |   +--------------+
                       |   | OverflowLink |
                       |   +--------------+
                       +-->|EntityGroupId |
                           +--------------+
                           | Flags        |
                           +--------------+
                           | Member Entity|
                           +--------------+

           Figure VII-3:   Mapping Group Identifiers

Note that the same pool of descriptors could be used for the server and group descriptors given that they are similar in size.

VII.2. Client Data Structures

Each client entity is represented as a client state record. The CSR contains a VMTP header as well as other bookkeeping fields, including timeout count, retransmission count, as described in Section 4.1. In addition, there is a timeout queue, transmission queue and reception queue. Finally, there is a ServerHost cache that maps from server entity-id records to host address, estimated round trip time, interpacket gap, MTU size and (optimally) estimated processing time for this server entity.

VII.3. Server Data Structures

The server maintains a heap of client state records (CSR), one for each (Client, Transaction). (If streams are not supported, there is, at worst, a CSR per Client with which the server has communicated with recently.) The CSR contains a VMTP header as well as various bookkeeping fields including timeout count, retransmission count. The server maintains a hash table mapping of Client to CSR as well as the transmission, timeout and reception queues. In a VMTP module implementing both the client and server functions, the same timeout queue and transmission queue are used for both.

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VII.4. Packet Group transmission

The procedure SendPacketGroup( csr ) transmits the packet group specified by the record CSR. It performs:

1. Fragmentation of the segment data, if any, into packets.

  (Note, segment data flagged by SDA bit.)

2. Modifies the VMTP header for each packet as required e.g.

  changing the delivery mask as appropriate.

3. Computes the VMTP checksum.

4. Encrypts the appropriate portion of the packet, if required.

5. Prepends and appends network-level header and trailer using

  network address from ServerHost cache, or from the responding
  CSR.

6. Transmits the packet with the interpacket gap specified in

  the cache.  This may involve round-robin scheduling between
  hosts as well as delaying transmissions slightly.

7. Invokes the finish-up procedure specified by the CSR record,

  completing the processing.  Generally, this finish-up
  procedure adds the record to the timeout queue with the
  appropriate timeout queue.

The CSR includes a 32-bit transmission mask that indicates the portions of the segment to transmit. The SendPacketGroup procedure is assumed to handle queuing at the network transmission queue, queuing in priority order according to the priority field specified in the CSR record. (This priority may be reflected in network transmission behavior for networks that support priority.)

The SendPacketGroup procedure only looks at the following fields of a CSR

- Transmission mask

- FuncCode

- SDA

- Client

- Server

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- CoResidentEntity

- Key

It modifies the following fields

- Length

- Delivery

- Checksum

In the case of encrypted transmission, it encrypts the entire packet, not including the Client field and the following 32-bits.

If the packet group is a Response, (i.e. lower-order bit of function code is 1) the destination network address is determined from the Client, otherwise the Server. The HostAddr field is set either from the ServerHost cache (if a Request) or from the original Request if a Response, before SendPacketGroup is called.

The CSR includes a timeout and TTL fields indicating the maximum time to complete the processing and the time-to-live for the packets to be transmitted.

SendPacketGroup is viewed as the right functionality to implement for transmission in an "intelligent" network interface.

Finally, it appears preferable to be able to assume that all portions of the segment remain memory-resident (no page faults) during transmission. In a demand-paged systems, some form of locking is required to keep the segment data in memory.

VII.5. VMTP Management Module

The implementation should implement the management operations as a separate module that is invoked from within the VMTP module. When a Request is received, either from the local user level or the network, for the VMTP management module, the management module is invoked as a remote or local procedure call to handle this request and return a response (if not a datagram request). By registering as a local server, the management module should minimize the special-case code required for its invocation. The management module is basically a case statement that selects the operation based on the RequestCode and then invokes the specified management operation. The procedure implementing the management operation, especially operations like NotifyVmtpClient and

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NotifyVmtpServer, are logically part of the VMTP module because they require full access to the basic data structures of the VMTP implementation.

The management module should be implemented so that it can respond quickly to all requests, particularly since the timing of management interactions is used to estimate round trip time. To date, all implementations of the management module have been done at the kernel level, along with VMTP proper.

VII.6. Timeout Handling

The timeout queue is a queue of CSR records, ordered by timeout count, as specified in the CSR record. On entry into the timeout queue, the CSR record has the timeout field set to the time (preferable in milliseconds or similar unit) to remain in the queue plus the finishup field set to the procedure to execute on removal on timeout from the queue. The timeout field for a CSR in the queue is the time relative to the record preceding it in the queue (if any) at which it is to be removed. Some system-specific mechanism decrements the time for the record at the front of the queue, invoking the finishup procedure when the count goes to zero.

Using this scheme, a special CSR is used to timeout and scan CSR's for non-recently pinged CSR's. That is, this CSR times out and invokes a finishup procedure that scans for non-recently pinged CSR that are "AwaitingResponse" and signals the request processing entity and deletes the CSR. It then returns to the timeout queue.

The timeout mechanism tends to be specific to an operating system. The scheme described may have to be adapted to the operating system in which VMTP is to be implemented.

This mechanism handles client request timeout and client response timeout. It is not intended to handle interpacket gaps given that these times are expected to be under 1 millisecond in general and possibly only a few microseconds.

VII.7. Timeout Values

Roundtrip timeout values are estimated by matching Responses or NotifyVmtpClient Requests to Request transmission, relying on the retransmitCount to identify the particular transmission of the Request that generated the response. A similar technique can be used with Responses and NotifyVmtpServer Requests. The retransmitCount is

Cheriton [page 114]

incremented each time the Response is sent, whether the retransmission was caused by timeout or retransmission of the Request.

The ProbeEntity request is recommended as a basic way of getting up-to-date information about a Client as well as predictable host machine turnaround in processing a request. (VMTP assumes and requires an efficient, bounded response time implementation of the ProbeEntity operation.)

Using this mechanism for measuring RTT, it is recommended that the various estimation and smoothing techniques developed for TCP RTT estimation be adapted and used.

VII.8. Packet Reception

Logically a network packet containing a VMTP packet is 5 portions:

- network header, possibly including lower-level headers

- VMTP header

- data segment

- VMTP checksum

- network trailer, etc.

It may be advantageous to receive a packet fragmented into these portions, if supported by the network module. In this case, ideally the VMTP header may be received directly into a CSR, the data segment into a page that can be mapped, rather than copied, to its final destination, with VMTP checksum and network header in a separate area (used to extract the network address corresponding to the sender).

Packet reception is described in detail by the pseudo-code in Section 4.7.

With a response, normally the CSR has an associated segment area immediately available so delivery of segment data is immediate. Similarly, server entities should be "armed" with CSR's with segment areas that provide for immediate delivery of requests. It is reasonable to discard segment data that cannot be immediately delivered in this way, providing that clients and servers are able to preallocate CSR's with segment areas for requests and responses. In particular, a client should be able to provide some number of additional CSR's for receiving multiple responses to a multicast request.

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The CSR data structure is intended to be the interface data structure for an intelligent network interface. For reception, the interface is "armed" with CSR's that may point to segment areas in main memory, into which it can deliver a packet group. Ideally, the interface handles all the processing of all packets, interacting with the host after receiving a complete Request or Response packet group. An implementation should use an interface based on SendPacketGroup(CSR) and ReceivePacketGroup(CSR) to facilitate the introduction of an intelligent network interface.

ReceivePacketGroup(csr) provides the interface with a CSR descriptor and zero or more bytes of main memory to receive segment data. The CSR describes whether it is to receive responses (and if so, for which client) or requests (and if so for which server).

The procedure ReclaimCSR(CSR) reclaims the specified record from the interface before it has been returned after receiving the specified packet group.

A finishup procedure is set in the CSR to be invoked when the CSR is returned to the host by the normal processing sequence in the interface. Similarly, the timeout parameter is set to indicate the maximum time the host is providing for the routine to perform the specified function. The CSR and associated segment memory is returned to the host after the timeout period with an indication of progress after the timeout period. It is not returned earlier.

VII.9. Streaming

The implementation of streaming is optional in both VMTP clients and servers. Ideally, all performance-critical servers should implement streaming. In addition, clients that have high context switch overhead, network access overhead or expect to be communicating over long delay links should also implement streaming.

A client stream is implemented by allocating a CSR for each outstanding message transaction. A stream of transactions is handled similarly to multiple outstanding transactions from separate clients except for the interaction between consecutive numbered transactions in a stream.

For the server VMTP module, streamed message transactions to a server are queued (if accepted) subordinate to the first unprocessed CSR corresponding to this Client. Thus, streamed transactions from a given Client are always performed in the order specified by the transaction identifiers.

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If a server does not implement streaming, it must refuse streamed message transactions using the NotifyVmtpClient operation. Also, all client VMTP's that support streaming must support the streamed interface to a server that does not support streaming. That is, it must perform the message transactions one at a time. Consequently, a program that uses the streaming interface to a non-streaming server experiences degraded performance, but not failure.

VII.10. Implementation Experience

The implementation experience to date includes a partial implementation (minus the streaming and full security) in the V kernel plus a similar preliminary implementation in the 4.3 BSD Unix kernel. In the V kernel implementation, the CSR's are part of the (lightweight) process descriptor.

The V kernel implementation is able to perform a VMTP message transaction with no data segment between two Sun-3/75's connected by 10 Mb Ethernet in 2.25 milliseconds. It is also able to transfer data at

megabits per second using 16 kilobyte Requests (but null checksums.)

The UNIX kernel implementation running on Microvax II's achieves a basic message transaction time of 9 milliseconds and data rate of 1.9 megabits per second using 16 kilobyte Responses. This implementation is using the standard VMTP checksum.

We hope to report more extensive implementation experience in future revisions of this document.

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VIII. UNIX 4.3 BSD Kernel Interface for VMTP

UNIX 4.3 BSD includes a socket-based design for program interfaces to a variety of protocol families and types of protocols (streams, datagrams). In this appendix, we sketch an extension to this design to support a transaction-style protocol. (Some familiarity with UNIX 4.2/3 IPC is assumed.) Several extensions are required to the system interface, rather than just adding a protocol, because no provision was made for supporting transaction protocols in the original design. These extensions include a new "transaction" type of socket plus new system calls invoke, getreply, probeentity, recreq, sendreply and forward.

A socket of type transaction bound to the VMTP protocol type IPPROTO_VMTP is created by the call

s = socket(AF_INET, SOCK_TRANSACT, VMTP);

This socket is bound to an entity identifier by

bind(s, &entityid, sizeof(entityid));

The first address/port bound to a socket is considered its primary name and is the one used on packet transmission. A message transaction is invoked between the socket named by s and the Server specified by mcb by

invoke(s, mcb, segptr, seglen, timeout );

The mcb is a message control block whose format was described in Section

The message control block specifies the request to send plus the

destination Server. The response message control block returned by the server is stored in mcb when invoke returns. The invoking process is blocked until a response is received or the message transaction times out unless the request is a datagram request. (Non-blocking versions with signals on completion could also be provided, especially with a streaming implementation.)

For multicast message transactions (sent to an entity group), the next response to the current message transaction (if it arrives in less than timeout milliseconds) is returned by

getreply( s, mcb, segptr, maxseglen, timeout );

The invoke operation sent to an entity group completes as soon as the first response is received. A request is retransmitted until the first reply is received (assuming the request is not a datagram). Thus, the system does not retransmit while getreply is timing out even if no replies are available.

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The state of an entity associated with entityId is probed using

probeentity( entityId, state );

A UNIX process acting as a VMTP server accepts a Request by the operation

recvreq(s, mcb, segptr, maxseglen );

The request message for the next queued transaction request is returned in mcb, plus the segment data of maximum length maxseglen, starting at segptr in the address space. On return, the message control block contains the values as set in invoke except: (1) the Client field indicates the Client that sent the received Request message. (2) the Code field indicates the type of request. (3) the MsgDelivery field indicates the portions of the segment actually received within the specified segment size, if MDM is 1 in the Code field. A segment block is marked as missing (i.e. the corresponding bit in the MsgDelivery field is 0) unless it is received in its entirety or it is all of the data in last segment contained in the segment.

To complete a transaction, the reply specified by mcb is sent to the client specified by the MCB using

sendreply(s, mcb, segptr );

The Client field of the MCB indicates the client to respond to.

Finally, a message transaction specified by mcb is forwarded to newserver as though it were sent there by its original invoker using

forward(s, mcb, segptr, timeout );

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Index

      Acknowledgment   14
      APG   16, 31, 39
      Authentication domain   20

      Big-endian   9

      Checksum   14, 43
      Checksum, not set   44
      Client   7, 10, 38
      Client timer   16
      CMD   42, 110
      CMG   32, 40
      Co-resident entity   25
      Code   42
      CoResidentEntity   42, 43
      CRE   21, 42

      DGM   42
      Digital signature, VMTP management   95, 101
      Diskless workstations   2
      Domain   9, 38
      Domain 1   102
      Domain 3   104

      Entity   7
      Entity domain   9
      Entity group   8
      Entity identifier   37
      Entity identifier allocation   105
      Entity identifier, all-zero   38
      EPG   20, 39

      Features   6
      ForwardCount   24
      Forwarding   24
      FunctionCode   41

      Group   8
      Group message transaction   10
      Group timeouts   16
      GRP   37

      HandleNoCSR   62
      HandleRequestNoCSR   79
      HCO   14, 23, 39

Cheriton [page 120]

      Host independence   8

      Idempotent   15
      Interpacket gap   18, 40
      IP   108

      Key   91

      LEE   32, 37
      Little-endian   9

      MCB   118
      MDG   22, 40
      MDM   30, 42
      Message control block   118
      Message size   6
      Message transaction   7, 10
      MPG   39
      MsgDelivery   43
      MSGTRANS_OVERFLOW   27
      Multicast   4, 21, 120
      Multicast, reliable   21

      Naming   6
      Negative acknowledgment   31
      NER   25, 31, 39
      NRT   26, 30, 39
      NSR   25, 27, 31, 39

      Object-oriented   2
      Overrun   18

      Packet group   7, 29, 39
      Packet group run   31
      PacketDelivery   29, 31, 41
      PGcount   26, 41
      PIC   42
      Principal   11
      Priority   41
      Process   11
      ProcessId   89
      Protocol number,IP   108

      RAE   37
      Rate control   18
      Real-time   2, 4
      Realtime   22

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      Reliability   12
      Request message   10
      RequestAckRetries   30
      RequestRetries   15
      Response message   10
      ResponseAckRetries   31
      ResponseRetries   15
      Restricted group   8
      Retransmission   15
      RetransmitCount   17
      Roundtrip time   17
      RPC   2
      Run   31, 39
      Run, message transactions   25

      SDA   42
      Security   4, 19
      Segment block   41
      Segment data   43
      SegmentSize   42, 43
      Selective retransmission   18
      Server   7, 10, 41
      Server group   8
      Sockets, VMTP   118
      STI   26, 40
      Streaming   25, 55
      Strictly stable   8
      Subgroups   21

      T-stable   8
      TC1(Server)   16
      TC2(Server)   16
      TC3(Server)   16
      TC4   16
      TCP   2
      Timeouts   15
      Transaction   10, 41
      Transaction identification   10
      TS1(Client)   17
      TS2(Client)   17
      TS3(Client)   17
      TS4(Client)   17
      TS5(Client)   17
      Type flags   8

      UNIX interface   118
      Unrestricted group   8, 38

Cheriton [page 122]

      NotifyVmtpClient   7, 26, 27, 30
      NotifyVmtpServer   7, 14, 30
      User Data   43

      Version   38
      VMTP Management digital signature   95, 101

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