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RFC 1190


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

Part 1 of 6, p. 1 to 24
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Obsoleted by:    1819


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


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


Status of this Memo

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

1.         Abstract

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

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   1.1.       Table of Contents

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

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

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

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

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

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

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

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

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

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

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

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

   1.2.       List of Figures

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

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

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 +--------------------+
 | Conference Control |
 +--------------------+
                    |
+-------+ +-------+ |
| Video | | Voice | | +-----+ +------+ +-----+     +-----+ Application
| Appl  | | Appl  | | | SNMP| |Telnet| | FTP | ... |     |    Layer
+-------+ +-------+ | +-----+ +------+ +-----+     +-----+
    |        |      |     |        |     |            |
    V        V      |     |        |     |            |   ------------
 +-----+  +-----+   |     |        |     |            |
 | PVP |  | NVP |   |     |        |     |            |
 +-----+  +-----+   +     |        |     |            |
  |   \      | \     \    |        |     |            |
  |    +-----|--+-----+   |        |     |            |
  |     Appl.|control  V  V        V     V            V
  | ST  data |         +-----+    +-------+        +-----+
  | & control|         | UDP |    |  TCP  |    ... |     | Transport
  |          |         +-----+    +-------+        +-----+   Layer
  |         /|          / | \       / / |          / /|
  |\       / |  +------+--|--\-----+-/--|--- ... -+ / |
  | \     /  |  |         |   \     /   |          /  |
  |  \   /   |  |         |    \   +----|--- ... -+   |   -----------
  |   \ /    |  |         |     \ /     |             |
  |    V     |  |         |      V      |             |
  | +------+ |  |         |   +------+  |   +------+  |
  | | SCMP | |  |         |   | ICMP |  |   | IGMP |  |    Internet
  | +------+ |  |         |   +------+  |   +------+  |     Layer
  |    |     |  |         |      |      |      |      |
  V    V     V  V         V      V      V      V      V
+-----------------+  +-----------------------------------+
| STream protocol |->|      Internet     Protocol        |
+-----------------+  +-----------------------------------+
               | \   / |
               |  \ /  |
               |   X   |                                  ------------
               |  / \  |
               | /   \ |
               VV     VV
+----------------+   +----------------+
| (Sub-) Network |...| (Sub-) Network |                  (Sub-)Network
|    Protocol    |   |    Protocol    |                     Layer
+----------------+   +----------------+

                    Figure 1.  Protocol Relationships

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2.      Introduction

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

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

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

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

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   2.1.       Major Differences Between ST and ST-II

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

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

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

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

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

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

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

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

Top       Page 9 
      between ST entities.  Issues that have not yet been fully
      addressed are presented in Section 5 (page 131).  A glossary and
      list of references are in Sections 6 (page 135) and 7 (page 143),
      respectively.

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


   2.2.       Concepts and Terminology

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

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

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

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      An ST stream is:

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

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

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

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

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

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

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

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


   2.3.       Relationship Between Applications and ST

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

         o  identifying the participants,

         o  determining which are targets for which origins,

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

         o  specifying the protocol that resides above ST,

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

         o  ensuring security, if necessary.

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

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

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

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


   2.4.       ST Control Message Protocol

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

         o  create streams,

         o  refuse creation of a stream,

         o  delete a stream in whole or in part,

         o  negotiate or change a stream's parameters,

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

         o  reroute around network or component failures.

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

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

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

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

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

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

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         4  update the FlowSpec, and

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

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

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

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

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

Top       Page 14 
      Although ST supports an arbitrary connection structure, we
      recognize that certain stream topologies will be common and
      justify special features, or options, which allow for optimized
      support.  These include:

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

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

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


   2.5.       Flow Specifications

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

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

Top       Page 15 
      When the data transmitted by stream users is generated at varying
      rates, including bursts of varying rate and duration, there is an
      opportunity to provide service to more subscribers by providing
      guaranteed service for the average data rate of each stream, and
      reserving additional network capacity, shared among all streams,
      to service the bursts.  This concept has been recognized by analog
      voice network providers leading to the principle of time assigned
      speech interpolation (TASI) in which only the talkspurts of a
      speech conversation are transmitted, and, during silence periods,
      the circuit can be used to send the talkspurts of other
      conversations.  The FlowSpec is intended to assist algorithms that
      perform similar kinds of functions.  We do not propose such
      algorithms here, but rather expect that this will be an area for
      experimentation.  To allow for experiments, and a range of ways
      that application traffic might be characterized, a "DutyFactor" is
      included in the FlowSpec and we expect that a "burst descriptor"
      will also be needed.

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

Top       Page 16 
                      ****                      ****
                     *    *     ST Agent 1     *    *       +---+
                    *      *------- o ---------*    *-------+ B |
                    *      *                   *    *       +---+
                    *      *                    ****
      +---+         *      *                     |
      |   |         *      *                     |
      | A +---------*      *                     o ST Agent 3
      |   |         *      *                     |
      +---+         *      *                     |
                    *      *                    ***
                    *      *                   *   *        +---+
                    *      *    ST Agent 2    *     *-------+ C |
                    *      *------- o --------*     *       +---+
                     *    *                   *     *
                      ****                    *     *
                                              *     *
                                 +---+        *     *       +---+
                                 | E +--------*     *-------+ D |
                                 +---+         *   *        +---+
                                                ***

         Figure 2.  Topology Used in Protocol Exchange Diagrams






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

         Figure 3.  Virtual Link Identifiers for SCMP Messages

Top       Page 17 
3.      ST Control Message Protocol Functional Description

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

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

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

    o  Add a peer at E,

    o  Disconnect peers B and C, and

    o  D drops out of the stream.

   Other figures illustrate exchanges related to failure recovery.

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

Top       Page 18 
   Similarly, Figure 4 summarizes the HIDs that will eventually be
   negotiated as the stream is created.

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

             Figure 4.  HIDs Assigned for ST User Packets


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


   3.1.       Stream Setup


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


      3.1.1.        Initial Setup at the Origin

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

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


      3.1.2.        Invoking the Routing Function

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

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

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


      3.1.3.        Reserving Resources

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

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

Top       Page 20 
         use replication to send a copy of the data packets to each
         next-hop agent.

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

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

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

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


      3.1.4.        Sending CONNECT Messages

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

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

Top       Page 21 
         must be set, see Section 3.6.1 (page 44)), an updated FlowSpec,
         and a TargetList.  In general, the HID, FlowSpec, and
         TargetList will depend on both the next-hop and the intervening
         network.  Each TargetList is a subset of the received (or
         original) TargetList, identifying the targets that are to be
         reached through the next-hop to which the CONNECT message is
         being sent.  Note that a CONNECT message to a single next-hop
         might have to be fragmented into multiple CONNECTs if the
         single CONNECT is too large for the intervening network's MTU;
         fragmentation is performed by further dividing the TargetList.

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

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


         Application  Agent A                    Agent 1    Agent 2

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

               Figure 5.  Origin Sending CONNECT Message

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      3.1.5.        CONNECT Processing by an Intermediate Agent

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


       Agent A                   Agent 1                      Agent B

    [1.4] >>-> CONNECT B -------->+--+
               <RVLId=0><SVLId=4> |  V
2.1.           <Ref=10><HID=1200> |  (routing to B)
                                  |  V
2.2.                              V  +->(reserve resources from 1 to B)
2.3.       +<- HID-APPROVE <------+     V
2.4.           <RVLId=4><SVLId=14>      +-> CONNECT B ---------->>
               <Ref=10><HID=1200>           <RVLId=0><SVLId=15>
                                            <Ref=110><HID=3600>

       Agent A                   Agent 2                      Agent C

    [1.6] >>-> CONNECT C,D ------>+-+
               <RVLId=0><SVLId=5> | V
2.5.           <Ref=15><HID=2400> | (routing to C,D)
                                  | V
2.6.                              V +-->(reserve resources from 2 to C)
2.7.       +<- HID-APPROVE <------+ |   V
2.8.           <RVLId=5><SVLId=23>  |   +-> CONNECT C ---------->>
               <Ref=15><HID=2400>   |       <RVLId=0><SVLId=25>
                                    |       <Ref=210><HID=4800>
                                    |
                                    |                         Agent D
                                    V
2.9.                                +->(reserve resources from 2 to D)
                                        V
2.10.                                   +-> CONNECT D ---------->>
                                            <RVLId=0><SVLId=26>
                                            <Ref=215><HID=4800>

         Figure 6.  CONNECT Processing by an Intermediate Agent

Top       Page 23 
         The resources listed as Desired in a received FlowSpec may not
         correspond to those actually reserved in either the ST agent
         itself or in the network(s) used to reach the next-hop
         agent(s).  As long as the reserved resources are sufficient to
         meet the specified Limits, the copy of the FlowSpec sent to a
         next-hop must have the Desired resources updated to reflect the
         resources that were actually obtained.  For example, the
         Desired bandwidth might be reduced because the network to the
         next-hop could not provide all of the desired bandwidth.  Also,
         the delay and delay variance are appropriately increased, and
         the link MTU may require that the DesPDUBytes field be reduced.
         (The minimum requirements that the origin had entered into the
         FlowSpec Limits fields cannot be altered by the intermediate or
         target agents.)


      3.1.6.        Setup at the Targets

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

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

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

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

Top       Page 24 
           Agent 1                    Agent B       Application B
 3.1.                                             (proc B listening)
         [2.4] >>-> CONNECT B ---------->+------------------+
                    <RVLId=0><SVLId=15>  |                  |
 3.2.               <Ref=110><HID=3600>  V          (proc B accepts)
 3.3.           +<- HID-APPROVE <--------+                  |
                    <RVLId=15><SVLId=44>                    |
                    <Ref=110><HID=3600>                     V
 3.4.                       (wait until HID negotiated) <---+
                                         V
 3.5.       <<--+<- ACCEPT B <-----------+
                    <RVLId=15><SVLId=44>
                    <Ref=410><LnkRef=110>

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

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

              Figure 7.  CONNECT Processing by the Target


      3.1.7.        ACCEPT Processing by an Intermediate Agent

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


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