path: root/doc/rfc/rfc1190.txt

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


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


Status of this Memo

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

1.         Abstract

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























CIP Working Group                                               [Page 1]
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RFC 1190                Internet Stream Protocol            October 1990


   1.1.       Table of Contents

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

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

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


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RFC 1190                Internet Stream Protocol            October 1990


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

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


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RFC 1190                Internet Stream Protocol            October 1990


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

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

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

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

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

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

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

   1.2.       List of Figures

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


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RFC 1190                Internet Stream Protocol            October 1990


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



























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RFC 1190                Internet Stream Protocol            October 1990


 +--------------------+
 | 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









CIP Working Group                                               [Page 6]
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RFC 1190                Internet Stream Protocol            October 1990


2.      Introduction

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

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

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

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





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RFC 1190                Internet Stream Protocol            October 1990


   2.1.       Major Differences Between ST and ST-II

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

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

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

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

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

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

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

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


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

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


   2.2.       Concepts and Terminology

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

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

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



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      An ST stream is:

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

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

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

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

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

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

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




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


   2.3.       Relationship Between Applications and ST

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

         o  identifying the participants,

         o  determining which are targets for which origins,

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

         o  specifying the protocol that resides above ST,

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

         o  ensuring security, if necessary.

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

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

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



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


   2.4.       ST Control Message Protocol

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

         o  create streams,

         o  refuse creation of a stream,

         o  delete a stream in whole or in part,

         o  negotiate or change a stream's parameters,

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

         o  reroute around network or component failures.

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

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

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

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

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

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




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         4  update the FlowSpec, and

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

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

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

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

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








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

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

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

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


   2.5.       Flow Specifications

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

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







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

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

























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

         Figure 2.  Topology Used in Protocol Exchange Diagrams






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

         Figure 3.  Virtual Link Identifiers for SCMP Messages


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3.      ST Control Message Protocol Functional Description

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

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

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

    o  Add a peer at E,

    o  Disconnect peers B and C, and

    o  D drops out of the stream.

   Other figures illustrate exchanges related to failure recovery.

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




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

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

             Figure 4.  HIDs Assigned for ST User Packets


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


   3.1.       Stream Setup


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


      3.1.1.        Initial Setup at the Origin

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




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


      3.1.2.        Invoking the Routing Function

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

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

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


      3.1.3.        Reserving Resources

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

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



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

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

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

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

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


      3.1.4.        Sending CONNECT Messages

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

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


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

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

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


         Application  Agent A                    Agent 1    Agent 2

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

               Figure 5.  Origin Sending CONNECT Message







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RFC 1190                Internet Stream Protocol            October 1990


      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




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RFC 1190                Internet Stream Protocol            October 1990


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


      3.1.6.        Setup at the Targets

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

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

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

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


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RFC 1190                Internet Stream Protocol            October 1990


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

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

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

              Figure 7.  CONNECT Processing by the Target


      3.1.7.        ACCEPT Processing by an Intermediate Agent

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


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RFC 1190                Internet Stream Protocol            October 1990


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

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


      Agent A                     Agent 1                    Agent B

                                     +<-+<- ACCEPT B <-------<< [3.5]
                                     V  |   <RVLId=15><SVLId=44>
4.1.                 (wait for ACCEPTS) V   <Ref=410><LnkRef=110>
4.2.                                 V  +-> ACK --------------->+
4.3.    (wait until HID negotiated)<-+      <RVLId=44><SVLId=15>
                                  V         <Ref=410>
4.4.  <<--+<-- ACCEPT B <---------+
               <RVLId=4><SVLId=14>
               <Ref=115><LnkRef=10>

       Agent A                    Agent 2                    Agent C

                                     +<-+<- ACCEPT C <------<< [3.10]
                                     |  |   <RVLId=25><SVLId=54>
                                     |  V   <Ref=510><LnkRef=210>
4.5.                                 |  +-> ACK --------------->+
                                     |      <Ref=510>
                                     |      <RVLId=54><SVLId=25>
                                     |
                                     |                       Agent D
                                     V
                                     +<-+<- ACCEPT D <------<< [3.15]
                                     V  |   <RVLId=26><SVLId=64>
4.6.                 (wait for ACCEPTS) V   <Ref=610><LnkRef=215>
4.7.                                 V  +-> ACK --------------->+
4.8.    (wait until HID negotiated)<-+      <RVLId=64><SVLId=26>
                                  V         <Ref=610>
4.9.  <<--+<- ACCEPT C <----------+
              <RVLId=5><SVLId=23> |
              <Ref=220><LnkRef=15>|
                                  V
4.10. <<--+<- ACCEPT D <----------+
              <RVLId=5><SVLId=23>
              <Ref=225><LnkRef=15>

         Figure 8.  ACCEPT Processing by an Intermediate Agent


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RFC 1190                Internet Stream Protocol            October 1990


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


      3.1.8.        ACCEPT Processing by the Origin

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


         Application A   Agent A                  Agent 1   Agent 2

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

               Figure 9.  ACCEPT Processing by the Origin



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RFC 1190                Internet Stream Protocol            October 1990


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

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

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


      3.1.9.        Processing a REFUSE Message

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

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

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

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

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




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RFC 1190                Internet Stream Protocol            October 1990


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

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


   Appl.  Agent A                   Agent 2                 Agent E
                                               (proc E NOT listening)
1. (add E)
2.    +----->+-> CONNECT E ---------->+->+
                 <RVLId=23><SVLId=5>  |  |
                 <Ref=65>             V  |
3.           +<-- ACK <---------------+  |
                  <RVLId=5><SVLId=23>    V
4.                <Ref=65>         (routing to E)
                                         V
5.                           (reserve resources 2 to E)
                                         V
6.                                       +--> CONNECT E --------->+
                                              <RVLId=0><SVLId=27> |
                                              <Ref=115><HID=4600> |
                                                                  V
7.                                    +<-+<- REFUSE B <-----------+
                                      |  |   <RVLId=27><SVLId=74>
                                      |  |   <Ref=705><LnkRef=115>
                                      |  V   <RC=SAPUnknown>
8.                                    |  +-> ACK ---------------->+
                                      |  |   <RVLId=74><SVLId=27> |
                                      |  V   <Ref=705>            |
9.                                    |  (free link 27)           V
10.                                   V              (free link 74)
11.          +<- REFUSE B <-----------+
             |   <RVLId=5><SVLId=23>  |
             |   <Ref=550><LnkRef=65> V
12.          |   <RC=SAPUnknown>  (free resources 2 to E)
             V
13.          +-> ACK  --------------->+
             |   <RVLId=23><SVLId=5>  |
             |   <Ref=550>            V
14.          V             (keep link 23 for C,D)
15.  (keep link 5 for C,D)
      V
16.  (inform application failed SAPUnknown)

                   Figure 10.  Sending REFUSE Message


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RFC 1190                Internet Stream Protocol            October 1990


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

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


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

1.                                      (network from 1 to B fails)
2. (add B)
3.   +-> CONNECT B ----------------->+
         <RVLId=0><SVLId=6>          |
         <Ref=35><HID=100>           |
3.   +<- HID-APPROVE <---------------+
         <RVLId=6><SVLId=11>         |
         <Ref=35><HID=100>           V
4.                       (routing to B: no route)
                                     V
5.   +<-+-- REFUSE B ----------------+
     |  |   <RVLId=6><SVLId=11>
     |  |   <Ref=155><LnkRef=35>
     |  V   <RC=NoRouteToDest>
6.   |  +-> ACK -------------------->+
     |  |   <RVLId=11><SVLId=6>      V
7.   |  V   <Ref=155>           (drop link 6)
8.   V  (drop link 11)
9.   (find alternative route: via agent 2)
10.  (resources from A to 2 already allocated:
     V   reuse control link & HID, no additional resources required)
11.  +-> CONNECT B -------->+->+
         <RVLId=23><SVLId=5>|  |
         <Ref=40>           V  |
12.  +<- ACK <--------------+  |
         <RVLId=5><SVLId=23>   V
13.      <Ref=40>    (routing to B: via agent 3)
                            V
14.                         +-> CONNECT B -->+
15.                      <RVLId=0><SVLId=24> +-> CONNECT B --------->+
                         <Ref=245><HID=4801> V   <RVLId=0><SVLId=32> |
16.                         +<- HID-APPROVE -+   <Ref=310><HID=6000> |
                                <RVLId=24><SVLId=33>                 |
                                <Ref=245><HID=4801>                  V
17.                                          +<- HID-APPROVE --------+
                                                 <RVLId=32><SVLId=45>|
                                                 <Ref=310><HID=6000> V
18.        (ACCEPT handling follows normally to complete stream setup)

           Figure 11.  Routing Around a Failure



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RFC 1190                Internet Stream Protocol            October 1990


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

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

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


   3.2.       Data Transfer

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

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

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



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

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


   3.3.       Modifying an Existing Stream

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


      3.3.1.        Adding a Target

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

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


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


Application  Agent A               Agent 2                    Agent E

1.   (open E)
2.      V                                            (proc E listening)
3.      +->(routing to E)
           V
4.         +-> (check resources from A to Agent 2: already allocated,
           V  reuse control link & HID, no additional resources needed)
5.         +-> CONNECT E --------->+->+
               <RVLId=23><SVLId=5> |  V
6.             <Ref=20>            V  (routing to E)
7.         +<- ACK <---------------+  V
               <RVLId=5><SVLId=23>    +->(reserve resources 2 to E)
               <Ref=20>                  V
8.                                       +-> CONNECT E --------->+
                                             <RVLId=0><SVLId=27> |
                                             <Ref=230><HID=4800> |
9.                                       +<- HID-APPROVE <-------+
                                             <RVLId=27><SVLId=74>|
                                             <Ref=230><HID=4800> V
10.                                               (proc E accepts)
11.                                    (wait until HID negotiated)
                                                                 V
12.                                   +<-+<- ACCEPT E <----------+
                                      V  |   <RVLId=27><SVLId=74>
13.                  (wait for ACCEPTS)  V   <Ref=710><LnkRef=230>
14.                                   V  +-> ACK --------------->+
15.      (wait until HID negotiated)<-+      <RVLId=74><SVLId=27>
                                   V         <Ref=710>
16.           +<- ACCEPT E <-------+
              |   <RVLId=5><SVLId=23>
              V   <Ref=235><LnkRef=20>
17.           +-> ACK ------------>+
              |   <RVLId=23><SVLId=5>
              V   <Ref=235>
18.        +<-(inform A of E's FlowSpec)
           V
19.     +<-(wait for ACCEPTS)
        V
20.  +<-(wait until HID negotiated)
     V
21.  (inform A open to E)

                 Figure 12.  Addition of Another Target


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

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

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


      3.3.2.        The Origin Removing a Target

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

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




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

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


      Application                                         Application
            Agent A             Agent 1  Agent 2          Agent B    C

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

                  Figure 13.  Origin Removing a Target



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


      3.3.3.        A Target Deleting Itself

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

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


                 Agent A          Agent 2          Agent E

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

                   Figure 14.  Target Deleting Itself


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


      3.3.4.        Changing the FlowSpec

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

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


   3.4.       Stream Tear Down

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

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

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





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   3.5.       Exceptional Cases

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

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


      3.5.1.        Setup Failure due to CONNECT Timeout

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












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          Sending Agent              Receiving Agent

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

           Figure 15.  CONNECT Retransmission after a Timeout


      3.5.2.        Problems due to Routing Inconsistency

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

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

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



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


      3.5.3.        Setup Failure due to a Routing Failure

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

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

          2  a deliberate source routing loop,

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

          4  an erroneous routing loop.

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

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

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

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

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



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

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

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

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

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




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


      3.5.4.        Problems in Reserving Resources

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

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


      3.5.5.        Setup Failure due to ACCEPT Timeout

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













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      3.5.6.        Problems Caused by CHANGE Messages

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

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

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


      3.5.7.        Notification of Changes Forced by Failures

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

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

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



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

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


   Application  Agent A            Agent 1                    Agent B

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

                 Figure 16.  Processing NOTIFY Messages



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


   3.6.       Options

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


      3.6.1.        HID Field Option

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

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

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

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


      3.6.2.        PTP Option

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




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

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

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

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


      3.6.3.        FDx Option

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

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

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




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


      3.6.4.        NoRecovery Option

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


      3.6.5.        RevChrg Option

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

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


      3.6.6.        Source Route Option

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







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

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


Application  Agent A        Agent 2        Agent 3              Agent B

1.  (open B<SR=2,3>)
2.    V                                              (proc B listening)
3.   (source routed to 2)
      V
4.   (check resources from A to Agent 2: already allocated,
      V   reuse control link & HID, no additional resources needed)
5.    +-> CONNECT B<SR=2,3>->-+-+
          <RVLId=23><SVLId=5> | |
6.        <Ref=50>            V |
7.    +<- ACK ----------------+ |
          <RVLId=5><SVLId=23>   |
          <Ref=50>              V
8.                 (source routed to 3)
                             V
9.            (reserve resources 2 to 3)
                          V
10.                       +-> CONNECT B<SR=3> ---->+
                              <RVLId=0><SVLId=24>  |
                              <Ref=280><HID=4801>  V
11.                       +<- HID-APPROVE <--------+
                              <RVLId=24><SVLId=33> |
                              <Ref=280><HID=4801>  |
                                                   V
                                           (routing to B)
                                                V
                                 (reserve resources from 3 to B)
                                             V
12.                                          +-> CONNECT B ---------->+
                                                 <RVLId=0><SVLId=32>  |
                                                 <Ref=330><HID=6000>  V
13.                                          +<- HID-APPROVE <--------+
                                                 <RVLId=32><SVLId=45> |
                                                 <Ref=330><HID=6000>  V
14.                                                    (proc B accepts)
                                                                      V
                ... perform normal ACCEPT processing ...        <-----+

                    Figure 17.  Source Routing Option


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

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

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


   3.7.       Ancillary Functions

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


      3.7.1.        Failure Detection

         The ST failure detection mechanism is based on two assumptions:

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

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

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


         3.7.1.1.         Network Failures

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




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

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


         3.7.1.2.         Detecting ST Stream Failures

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

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

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

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



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

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

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

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

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




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


         3.7.1.3.         Subset

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


      3.7.2.        Failure Recovery

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

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

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


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

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

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

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

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




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

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

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

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




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

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

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




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

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


         3.7.2.1.         Subset

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

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

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





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      3.7.3.        A Group of Streams

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

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

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

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

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


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

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


         3.7.3.1.         Group Name Generator

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

               AllocateGroupName()
                  -> result, GroupName

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

               ReleaseGroupName( GroupName )
                  -> result


         3.7.3.2.         Subset

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







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      3.7.4.        HID Negotiation

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

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

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



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

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

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

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


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

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

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


          Agent 3                      Agent B

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

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

         Figure 18.  Typical HID Negotiation (No Multicasting)


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


       Agent 2                           Agent C        Agent D

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

                 Figure 19.  Multicast HID Negotiation


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      Agent 2                  Agent C        Agent D     Agent 3

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

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


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      Agent 2                  Agent C        Agent D     Agent 3

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

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









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         3.7.4.1.         Subset

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


      3.7.5.        IP Encapsulation of ST

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

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

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

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

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



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

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

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

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

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

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


         3.7.5.1.         IP Multicasting

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



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

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

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

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

               224.0.0.7    All ST routers
               224.0.0.8    All ST hosts

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


      3.7.6.        Retransmission

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



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

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

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


      3.7.7.        Routing

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

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


      3.7.8.        Security

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





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

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

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


   3.8.       ST Service Interfaces

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

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





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      3.8.1.        Access to Routing Information

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

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

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

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

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

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




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


      3.8.2.        Access to Network Layer Resource Reservation

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

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

            ResourceProbe( requirements )
               -> likelihood

         Another call reserves the resources:

            ResourceReserve( requirements )
               -> result, reservation_id

         The third call adjusts the resource guarantees:

            ResourceAdjust( reservation_id, new requirements )
               -> result

         The final call allows the resources to be released:

            ResourceRelease( reservation_id )
               -> result






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      3.8.3.        Network Layer Services Utilized

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

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

            LeaveLocalGroup( network level group-address )
               -> result

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

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

            GetNotification( SAP )
               -> result, infop


      3.8.4.        IP Services Utilized

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

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

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

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

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

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

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




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

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

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


      3.8.5.        ST Layer Services Provided

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

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

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

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

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

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

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

         recvsignal( id )
            -> result, signal, info



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

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

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

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

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

         creategroup( members* )
            -> result, group name

         deletegroup( group name, members* )
            -> result






















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                      [This page intentionally left blank.]





















































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4.      ST Protocol Data Unit Descriptions

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

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

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

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

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

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

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


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

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

                         Figure 21.  ST Header





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

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

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

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


   4.1.       Data Packets

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

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












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   4.2.       ST Control Message Protocol Descriptions

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

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

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

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

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


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

                 Figure 22.  ST Control Message Format





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

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

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

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

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

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















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      4.2.1.        ST Control Messages

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

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

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

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

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










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      4.2.2.        Common SCMP Elements

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


         4.2.2.1.         DetectorIPAddress

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


         4.2.2.2.         ErroredPDU

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

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

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


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

                          Figure 23.  ErroredPDU




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

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


         4.2.2.3.         FlowSpec & RFlowSpec

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

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


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

                    Figure 24.  FlowSpec & RFlowSpec







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

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

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

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

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

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

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

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

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





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

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

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

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

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

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

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

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

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

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





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

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

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


         4.2.2.4.         FreeHIDs

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

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

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



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


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

                          Figure 25.  FreeHIDs


         4.2.2.5.         Group & RGroup

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

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


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

                       Figure 26.  Group & RGroup


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


         4.2.2.6.         HID & RHID

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

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


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

                         Figure 27.  HID & RHID


         4.2.2.7.         MulticastAddress

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

               LocalNetBytes is the length of the Local Net Multicast
               Address.


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

                      Figure 28.  MulticastAddress




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

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


         4.2.2.8.         Name & RName

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

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

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

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

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

                        Figure 29.  Name & RName


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         4.2.2.9.         NextHopIPAddress

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


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

                      Figure 30.  NextHopIPAddress


         4.2.2.10.        Origin

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

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

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

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

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

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

                           Figure 31.  Origin



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         4.2.2.11.        OriginTimestamp

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

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

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


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

                           Figure 32.  OriginTimestamp


         4.2.2.12.        ReasonCode

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


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

                         Figure 33.  ReasonCode









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

            AcceptTimeout      2   An Accept has not been
                                   acknowledged.

            AccessDenied       3   Access denied.

            AckUnexpected      4   An unexpected ACK was received.

            ApplAbort          5   The application aborted the stream
                                   abnormally.

            ApplDisconnect     6   The application closed the stream
                                   normally.

            AuthentFailed      7   The authentication function
                                   failed.

            CantGetResrc       8   Unable to acquire (additional)
                                   resources.

            CantRelResrc       9   Unable to release excess
                                   resources.

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

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

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

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

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

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

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

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





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

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

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

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

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

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

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

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

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

            FailureRecovery   24   A notification that recovery is
                                   being attempted.

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

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

            HIDNegFails       28   HID negotiation failed.

            HIDUnknown        29   A received control PDU contains an
                                   unknown HID.




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

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

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

            IntfcFailure      32   A network interface failure has
                                   been detected.

            InvalidHID        33   A received ST PDU contains an
                                   invalid HID.

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

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

            LnkRefUnknown     36   A received control PDU contains an
                                   unknown LnkReference.

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

            NetworkFailure    38   A network failure has been
                                   detected.

            NoError            0   No error has occurred.

            NoRouteToAgent    39   Cannot find a route to an ST
                                   agent.

            NoRouteToDest     40   Cannot find a route to the
                                   destination.

            NoRouteToHost     41   Cannot find a route to a host.

            NoRouteToNet      42   Cannot find a route to a network.

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

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

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



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

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

            ProtocolError     47   A protocol error was detected.

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

            RefUnknown        49   A received control PDU contains an
                                   unknown Reference.

            RestartLocal      50   The local ST agent has recently
                                   restarted.

            RemoteRestart     51   The remote ST agent has recently
                                   restarted.

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

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

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

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

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

            STAgentFailure    57   An ST agent failure has been
                                   detected.

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

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




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

            STVerBad          60   A received PDU is not ST Version
                                   2.

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

            TruncatedCtl      62   A received control PDU is shorter
                                   than expected.

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

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


         4.2.2.13.        RecordRoute

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

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


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

                          Figure 34.  RecordRoute







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         4.2.2.14.        SrcRoute

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

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

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


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

                          Figure 35.  SrcRoute




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

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

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

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


         4.2.2.15.        Target and TargetList

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

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

               TargetIPAddress is the IP Address of the Target.

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

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




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


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

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

                           Figure 36.  Target


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


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

                         Figure 37.  TargetList


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


         4.2.2.16.        UserData

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

               UserBytes specifies the number of valid UserInformation
               bytes.

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


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

                         Figure 38.  UserData












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4.2.3.        ST Control Message PDUs

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


















































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         4.2.3.1.         ACCEPT

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

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

            Any SrcRoute parameters in the TargetList are ignored.

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

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

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

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

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

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




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

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

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

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

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

                   Figure 39.  ACCEPT Control Message















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         4.2.3.2.         ACK

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

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

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


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

                    Figure 40.  ACK Control Message




















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         4.2.3.3.         CHANGE-REQUEST

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

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

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


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

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

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

               Figure 41.  CHANGE-REQUEST Control Message









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         4.2.3.4.         CHANGE

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

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

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

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

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

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
















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

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

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

                   Figure 42.  CHANGE Control Message




         4.2.3.5.         CONNECT

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

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



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

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

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

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

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

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

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

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

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











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

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

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

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

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


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

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

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

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

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


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

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

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

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

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

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



















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

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

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

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

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

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

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

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

                  Figure 43.  CONNECT Control Message

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         4.2.3.6.         DISCONNECT

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

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

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


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

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

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

                 Figure 44.  DISCONNECT Control Message






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RFC 1190                Internet Stream Protocol            October 1990


         4.2.3.7.         ERROR-IN-REQUEST

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

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

               Reference is the Reference number of the erroneous
               request.


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

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

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

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

              Figure 45.  ERROR-IN-REQUEST Control Message







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         4.2.3.8.         ERROR-IN-RESPONSE

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

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

            Reference identifies the erroneous response.


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

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

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

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

             Figure 46.  ERROR-IN-RESPONSE Control Message









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         4.2.3.9.         HELLO

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

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

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


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

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

                   Figure 47.  HELLO Control Message




















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         4.2.3.10.        HID-APPROVE

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

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

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


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

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

                Figure 48.  HID-APPROVE Control Message









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         4.2.3.11.        HID-CHANGE-REQUEST

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

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

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

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


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

             Figure 49.  HID-CHANGE-REQUEST Control Message









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         4.2.3.12.        HID-CHANGE

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

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

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

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

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

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

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



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RFC 1190                Internet Stream Protocol            October 1990


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

                 Figure 50.  HID-CHANGE Control Message



































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         4.2.3.13.        HID-REJECT

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

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

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

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























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

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

                 Figure 51.  HID-REJECT Control Message































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RFC 1190                Internet Stream Protocol            October 1990


         4.2.3.14.        NOTIFY

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

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

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

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

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

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

               ReasonCode identifies the reason for the notification.

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

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

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







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

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


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

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

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

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

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

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

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

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

                Figure 52.  NOTIFY Control Message


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         4.2.3.15.        REFUSE

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

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

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

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

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

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





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

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

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

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

                   Figure 53.  REFUSE Control Message





















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         4.2.3.16.        STATUS

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

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

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

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

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

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

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







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

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

                   Figure 54.  STATUS Control Message

































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         4.2.3.17.        STATUS-RESPONSE

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

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


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

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

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

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

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

                   Figure 55.  STATUS-RESPONSE Control Message


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   4.3.       Suggested Protocol Constants

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

      ST uses IP Version Number 5.

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


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

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


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


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





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


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

         224.0.0.7    All ST routers
         224.0.0.8    All ST hosts

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


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


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

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

           3  NAccept                ACCEPT retries before failure

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

           5  NConnect               CONNECT retries before failure

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

          3   NDisconnect            DISCONNECT retries before
                                     failure





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

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

           3  NHIDAck                HID-CHANGE-REQUEST retries
                                     before failure

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

           3  NHIDChange             HID-CHANGE retries before
                                     failure

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

           3  NNotify                NOTIFY retries before failure

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

           3  NRefuse                REFUSE retries before failure

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

           5  NReroute               CONNECT retries before failure

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

           0  NEnd2End               CONNECT retries before failure

















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

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

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

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

        2000  DefaultRecoveryTimeout Interval between successive
                                     HELLOs to/from active neighbors

           2  DefaultHelloFactor     HELLO filtering function factor




































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5.      Areas Not Addressed

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

   In other cases we feel that appropriate solutions are not clear at
   this time.  The following are examples of such issues:

   This document does not include a routing mechanism.  We do not feel
   that a routing strategy based on minimizing the number of hops from
   the source to the destination is necessarily appropriate.  An
   alternative strategy is to minimize the consumption of internet
   resources within some delay constraints.  Furthermore, it would be
   preferable if the routing function were to provide routes that
   incorporated bandwidth, delay, reliability, and perhaps other
   characteristics, not just connectivity.  This would increase the
   likelihood that a selected route would succeed.  This requirement
   would probably cause the ST agents to exchange more routing
   information than currently implemented.  We feel that further
   research and experimentation will be required before an appropriate
   routing strategy is well enough defined to be incorporated into the
   ST specification.

   Once the bandwidth for a stream has been agreed upon, it is not
   sufficient to rely on the origin to transmit traffic at that rate.
   The internet should not rely on the origin to operate properly.
   Furthermore, even if the origin sources traffic at the agreed rate,
   the packets may become aggregated unintentionally and cause local
   congestion.  There are several approaches to addressing this problem,
   such as metering the traffic in each stream as it passes through each
   agent.  Experimentation is necessary before such a mechanism is
   selected.

   The interface between the agent and the network is very limited.  A
   mechanism is provided by which the ST layer can query the network to
   determine the likelihood that a stream can be supported.  However,
   this facility will require practical experience before its
   appropriate use is defined.

   The simplex tree model of a stream does not easily allow for using
   multiple paths to support a greater bandwidth.  That is, at any given
   point in a stream, the entire incoming bandwidth must be transmitted
   to the same next-hop in order to get to some target.  If the
   bandwidth isn't available along any single path, the stream cannot be
   built to that target.  It may be the case that the bandwidth is not
   available along a single path, but if the data


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   flow is split along multiple paths, and so multiple next-hops,
   sufficient bandwidth would be available.  As currently specified, the
   ST agent at the point where the multiple flows converge will refuse
   the second connection because it can only be interpreted as a routing
   failure.  A mechanism that allows multiple paths in a stream and can
   protect against routing failures has not been defined.

   If sufficient bandwidth is not available, both preemption and
   rerouting are possible.  However, it is not clear when to use one or
   the other.  As currently specified, an ST agent that cannot obtain
   sufficient bandwidth will attempt to preempt lower precedence streams
   before attempting to reroute around the bottleneck.  This may lead to
   an undesirably high number of preemptions.  It may be that a higher
   precedence stream can be rerouted around lower precedence streams and
   still meet its performance requirements, whereas the preempted lower
   precedence streams cannot be reconstructed and still meet their
   performance requirements.  A simple and effective algorithm to allow
   a better decision has not been identified.

   In case a stream cannot be completed, ST does not report to the
   application the nature of the trouble in any great detail.
   Specifically, the application cannot determine where the bottleneck
   is, whether the problem is permanent or transitory, or the likely
   time before the trouble may be resolved.  The application can only
   attempt to build the stream at some later time hoping that the
   trouble has been resolved.  Schemes can be envisioned by which
   information is relayed back to the application.  However, only
   practical experience can evaluate the kind of trouble that is most
   likely encountered and the nature of information that would be most
   useful to the application.

   A mechanism is also not defined for cases where a stream cannot be
   completed not because of lack of resources but because of an
   unexpected failure that results in an ERROR-IN-REQUEST message.  An
   ERROR-IN-REQUEST message is returned in cases when an ST agent issues
   a malformed control message to a neighbor.  Such an occurrence is
   unexpected and may be caused by a bad or incomplete ST
   implementation.  In some cases a message, such as a NOTIFY should be
   sent to the origin.  Such a mechanism is not defined because it is
   not clear what information can be extracted and what the origin
   should do.

   No special action is taken when a target is removed from a stream.
   Removing a target may also remove a bottleneck either in bandwidth,
   packet rate or packet size, but advantage of this opportunity is not
   taken automatically.  The application may initiate a change to the
   stream's characteristics, but it is not in the best position to do
   this because the application may not know the nature of the
   bottleneck.  The ST layer may have the best information, but a





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   mechanism to do this may be very complex.  As a result, this concept
   requires further thought.

   An agent simply discards a stream's data packets if it cannot forward
   them.  The reason may be that the packets are too large or are
   arriving at too high a rate.  Alternative actions may include an
   attempt to do something with the packets, such as fragmenting them,
   or to notify the origin of the trouble.  Corrective measures may be
   too complex, so it may be preferable simply to notify the origin with
   a NOTIFY message.  However, if the incoming packet rate is causing
   congestion, then the NOTIFY messages themselves may cause more
   trouble.  The nature of the communication has yet to be defined.

   The FlowSpec includes a cost field, but its implementation has not
   been identified.  The units of cost can probably be defined
   relatively easily.  Cost of bandwidth can probably also be assigned.
   It is not clear how cost is assigned to other functions, such as high
   precedence or low delay, or how cost of the components of the stream
   are combined together.  It is clear that the cost to provide services
   will become more important in the near future, but it is not clear at
   this time how that cost is determined.

   A number of parameters of the FlowSpec are intended to be used as
   ranges, but some may be useful as discrete values.  For example, the
   FlowSpec may specify that bandwidth for a stream carrying voice
   should be reserved in a range from 16Kbps to 64Kbps because the voice
   codec has a variable coding rate.  However, the voice codec may be
   varied only among certain discrete values, such as 16Kbps, 32Kbps and
   64Kbps.  A stream that has 48Kbps of bandwidth is no better than one
   with 32Kbps.  The parameters of the FlowSpec where this may be
   relevant should optionally specify discrete values.  This is being
   considered.

   Groups are defined as a way to associate different streams, but the
   nature of the association is left for further study.  An example of
   such an association is to allow streams whose traffic is inherently
   not simultaneous to share the same allocated resources.  This may
   happen for example in a conference that has an explicit floor, such
   that only one site can generate video or audio traffic at any given
   time.  The grouping facility can be implemented based on this
   specification, but the implementation of the possible uses of groups
   will require new functionality to be added to the ST agents.  The
   uses for groups and the implementation to support them will be
   carried out as experience is gained and the need arises.

   We hope that the ST we here propose will act as a vehicle to study
   the use and performance of stream oriented services across packet
   switched networks.






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6.      Glossary

   appropriate reason code
      This phrase refers to one or perhaps a set of reason codes that
      indicate why a particular action is being taken.  Typically,
      these result from detection of errors or anomalous conditions.
      It can also indicate that an application component or agent has
      presented invalid parameters.

   DefaultRecoveryTimeout
      The DefaultRecoveryTimeout is maintained by each ST agent.  It
      indicates the default time interval to use for sending HELLO
      messages.

   downstream
      The direction in a stream from an origin toward its targets.

   element
      The fields and parameters of the ST control messages are
      collectively called elements.

   FlowSpec
      The Flow Specification, abbreviated "FlowSpec" is used by an
      application to specify required and desired characteristics of
      the stream.  The FlowSpec specifies bandwidth, delay, and
      reliability parameters.  Both minimal requirements and desired
      characteristics are included.  This information is then used to
      guide route selection and resource allocation decisions.  The
      desired vs. required characteristics are used to guide tradeoff
      decisions among competing stream requests.

   group
      A set of related streams can be associated as a group.  This is
      done by generating a Group Name and assigning it to each of the
      related streams.  The grouping information can then be used by
      the ST agents in making resource management and other control
      decisions.  For example, when preemption is necessary to
      establish a high precedence stream, we can exploit the group
      information to minimize the number of stream groups that are
      preempted.

   Group Name
      The Group Name is used to indicate that a collection of streams
      are related.  A Group Name is structured to ensure that it is
      unique across all hosts:  it includes the address of the host
      where it was generated combined with a unique number generated
      by that host.  A timestamp is added to ensure that the overall
      name is unique over all time.  (A Group Name has the same format
      as a stream Name.)





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   HelloLossFactor
      The HelloLossFactor is a parameter maintained by each ST agent.
      It identifies the expected number of consecutive HELLO messages
      typically lost due to transient factors.  Thus, an agent will be
      assumed to be down after we miss more than HelloLossFactor
      messages.

   HelloTimer
      The HelloTimer is a millisecond timer maintained by each ST
      agent.  It is included in each HELLO message.  It represents the
      time since the agent was restarted, modulo the precision of the
      field.  It is used to detect variations in the delay between the
      two agents, by comparing the arrival interval of two HELLO
      messages to the difference between their HelloTimer fields.

   HelloTimerHoldDown
      The HelloTimerHoldDown value is maintained by each ST agent.
      When an ST agent is restarted, it will set the "Restarted" bit
      in all HELLO messages it sends for HelloTimerHoldDown seconds.

   HID
      The Hop IDentifier, abbreviated as HID, is a numeric key stored
      in the header of each ST packet.  It is used by an ST agent to
      associate the packet with one of the incoming hops managed by
      the agent.  It can be used by receiving agent to map to
      the set of outgoing next-hops to which the message should be
      forwarded.  The HID field of an ST packet will generally need to
      be changed as it passes through each ST agent since there may be
      many HIDs associated with a single stream.

   hop
      A "hop" refers to the portion of a stream's path between two
      neighbor ST agents.  It is usually represented by a physical
      network.  However, a multicast hop can connect a single ST agent
      to several next-hop ST agents.

   host agents
      Synonym for host ST agents.

   host ST agents
      Host ST agents are ST agents that provide services to higher
      layer protocols and applications.  The services include methods
      for sourcing data from and sinking data to the higher layer or
      application, and methods for requesting and modifying streams.

   intermediate agents
      Synonym for intermediate ST agents.

   intermediate ST agents
      Intermediate ST agents are ST agents that can forward ST
      packets between the networks to which they are attached.



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   MTU
      The abbreviation for Maximum Transmission Unit, which is the
      maximum packet size in bytes that can be accepted by a given
      network for transmission.  ST agents determine the maximum
      packet size for a stream so that data written to the stream can
      be forwarded through the networks without fragmentation.

   multi-destination simplex
      The topology and data flow of ST streams are described as being
      multi-destination simplex:  all data flowing on the stream
      originates from a single origin and is passed to one or more
      destination targets.  Only control information, invisible to the
      application program, ever passes in the upstream direction.

   NAccept
      NAccept is an integer parameter maintained by each ST agent.  It
      is used to control retransmission of an ACCEPT message.  Since
      an ACCEPT request is relayed by agents back toward the origin,
      it must be acknowledged by each previous-hop agent.  If this ACK
      is not received within the appropriate timeout interval, the
      request will be resent up to NAccept times before giving up.

   Name
      Generally refers to the name of a stream.  A stream Name is
      structured to ensure that it is unique across all hosts: it
      includes the address of the host where it was generated combined
      with a unique number generated at that host.  A timestamp is
      added to ensure that the overall Name is unique over all time.
      (A stream Name has the same format as a Group Name.)

   NConnect
      NConnect is an integer parameter maintained by each ST agent.
      It is used to control retransmission of a CONNECT message.  A
      CONNECT request must be acknowledged by each next-hop agent as
      it is propagated toward the targets.  If a HID-ACCEPT,
      HID-REJECT, or ACK is not received for the CONNECT between any
      two agents within the appropriate timeout interval, the request
      will be resent up to NConnect times before giving up.

   NDisconnect
      NDisconnect is an integer parameter maintained by each ST
      agent.  It is used to control retransmission of a DISCONNECT
      message.  A DISCONNECT request must be acknowledged by each
      next-hop agent as it is propagated toward the targets.  If this
      ACK is not received for the DISCONNECT between any two agents
      within the appropriate timeout interval, the request will be
      resent up to NDisconnect times before giving up.







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   next protocol identifier
      The next protocol identifier is used by a target ST agent to
      identify to which of several higher layer protocols it should
      pass data packets it receives the network.  Examples of higher
      layer protocols include the Network Voice Protocol and the
      Packet Video Protocol.  These higher layer protocols will
      typically perform further demultiplexing among multiple
      application processes as part of their protocol processing
      activities.

   next-hop
      Synonym for next-hop ST agent.

   next-hop ST agent
      For each origin or intermediate ST agent managing a stream
      there are a set of next-hop ST agents.  The intermediate agent
      forwards each data packet it receives to all the next-hop ST
      agents, which in turn forward the data toward the target host
      agent (if the particular next-hop agent is another intermediate
      agent) or to the next higher protocol layer at the target (if
      the particular next-hop agent is a host agent).

   NextPcol
      NextPcol is a field in each Target of the CONNECT message used
      to convey the next protocol identifier.  See definition of next
      protocol identifier above for more details.

   NHIDAbort
      NHIDAbort is an integer parameter maintained by each ST agent.
      It is the number of unacceptable HID proposals before an ST
      agent aborts the HID negotiation process.

   NHIDAck
      NHIDAck is an integer parameter maintained by each ST agent.
      It is used to control retransmission of HID-CHANGE-REQUEST
      messages.  HID-CHANGE-REQUEST is sent by an ST agent to the
      previous-hop ST agent to request that the HID in use between
      those agents be changed.  The previous-hop acknowledges the
      HID-CHANGE-REQUEST message by sending a HID-CHANGE message.  If
      the HID-CHANGE is not received within the appropriate timeout
      interval, the request will be resent up to NHIDAck times before
      giving up.

   NHIDChange
      NHIDChange is an integer parameter maintained by each ST agent.
      It is used to control retransmission of the HID-CHANGE message.
      A HID-CHANGE message must be acknowledged by the next-hop agent.
      If this ACK is not received within the appropriate timeout
      interval, the request will be resent up to NHIDChange times
      before giving up.




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   NRefuse
      NRefuse is an integer parameter maintained by each ST agent.
      It is used to control retransmission of a REFUSE message.  As a
      REFUSE request is relayed by agents back toward the origin, it
      must be acknowledged by each previous-hop agent.  If this ACK is
      not received within the appropriate timeout interval, the
      request will be resent up to NRefuse times before giving up.

   NRetryRoute
      NRetryRoute is an integer parameter maintained by each ST
      agent.  It is used to control route exploration.  When an agent
      receives a REFUSE message whose ReasonCode indicates that the
      originally selected route is not acceptable, the agent should
      attempt to find an alternate route to the target.  If the agent
      has not found a viable route after a maximum of NRetryRoute
      choices, it should give up and notify the previous-hop or
      application that it cannot find an acceptable path to the
      target.

   origin
      The origin of a stream is the host agent where an application
      or higher level protocol originally requested that the stream be
      created.  The origin specifies the data to be sent through the
      stream.

   parameter
      Parameters are additional values that may be included in
      control messages.  Parameters are often optional.  They are
      distinguished from fields, which are always present.

   participants
      Participants are the end-users of a stream.

   PDU
      Abbreviation for Protocol Data Unit, defined below.

   peer
      The term peer is used to refer to entities at the same protocol
      layer.  It is used here to identify instances of an application
      or protocol layer above ST.  For example, data is passed through
      a stream from an originating peer process to its target peers.

   previous-hop
      Synonym for previous-hop ST agent.

   previous-hop ST agent
      The origin or intermediate agent from which an ST agent receives
      its data.






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   protocol data unit
      A protocol data unit (PDU) is the unit of data passed to a
      protocol layer by the next higher layer protocol or user.  It
      consists of control information and possibly user data.

   RecoveryTimeout
      RecoveryTimeout is specified in the FlowSpec of each stream.
      The minimum of these values over all streams between a pair of
      adjacent agents determines how often those agents must send
      HELLO messages to each other in order to ensure that failure of
      one of the agents will be detected quickly enough to meet the
      guarantee implied by the FlowSpec.

   Restarted bit
      The Restarted bit is part of the HELLO message.  When set, it
      indicates that the sending agent was restarted recently (within
      the last HelloTimerHoldDown seconds).

   round-trip time
      The round-trip-time is the time it takes a message to be sent,
      delivered, processed, and the acknowledgment received.  It
      includes both network and processing delays.

   RTT
      Abbreviation for round-trip-time.

   RVLId
      Abbreviation for Receiver's Virtual Link Identifier.  It
      uniquely identifies to the receiver the virtual link, and this
      stream, used to send it a message.  See definition for Virtual
      Link Identifier below.

   SAP
      Abbreviation for Service Access Point.

   SCMP
      Abbreviation for ST Control Message Protocol, defined below.

   Service Access Point
      A point where a protocol service provider makes available the
      services it offers to a next higher layer protocol or user.

   setup phase
      Before data can be transmitted through a stream, the ST agents
      must distribute state information about the stream to all agents
      along the path(s) to the target(s).  This is the setup phase.
      The setup phase ends when all the ACCEPT and REFUSE messages
      sent by the targets have been delivered to the origin.  At this
      point, the data transfer phase begins and data can be sent.
      Requests to modify the stream can be issued after the setup
      phase has ended, i.e., during the data transfer phase without
      disrupting the flow of data.


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   ST agent
      An ST agent is an entity that implements the ST Protocol.

   ST Control Message Protocol
      The ST Control Message Protocol is the subset of the overall ST
      Protocol responsible for creation, modification, maintenance,
      and tear down of a stream.  It also includes support for event
      notification and status monitoring.

   stream
      A stream is the basic object managed by the ST Protocol for
      transmission of data.  A stream has one origin where data are
      generated and one or more targets where the data are received
      for processing.  A flow specification, provided by the origin
      and negotiated among the origin, intermediate, and target ST
      agents, identifies the requirements of the application and the
      guarantees that can be assured by the ST agents.

   subsets
      Subsets of the ST Protocol are permitted, as defined in various
      sections of this specification.  Subsets are defined to allow
      simplified implementations that can still effectively
      interoperate with more complete implementations without causing
      disruption.

   SVLId
      Abbreviation for Sender's Virtual Link Identifier.  It uniquely
      identifies to the receiver the virtual link identifier that
      should be placed into the RVLId field of all replies sent over
      the virtual link for a given stream.  See definition for Virtual
      Link Identifier below.

   target
      An ST target is the destination where data supplied by the
      origin will be delivered for higher layer protocol or
      application processing.

   tear down
      The tear down phase of a stream begins when the origin indicates
      that it has no further data to send and the ST agents through
      which the stream passes should dismantle the stream and release
      its resources.

   ToAccept
      ToAccept is a timeout in seconds maintained by each ST agent.
      It sets the retransmission interval for ACCEPT messages.

   ToConnect
      ToConnect is a timeout in seconds maintained by each ST agent.
      It sets the retransmission interval a CONNECT messages.




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   ToDisconnect
      ToDisconnect is a timeout in seconds maintained by each ST
      agent.  It sets the retransmission interval for DISCONNECT
      messages.

   ToHIDAck
      ToHIDAck is a timeout in seconds maintained by each ST agent.
      It sets the retransmission interval for HID-CHANGE-REQUEST
      messages.

   ToHIDChange
      ToHIDChange is a timeout in seconds maintained by each ST agent.
      It sets the retransmission interval for HID-CHANGE messages.

   ToRefuse
      ToRefuse is a timeout in seconds maintained by each ST agent.
      It sets the retransmission interval for REFUSE messages.

   upstream
      The direction in a stream from a target toward the origin.

   Virtual Link
      A virtual link is one edge of the tree describing the path of
      data flow through a stream.  A separate virtual link is assigned
      to each pair of neighbor ST agents, even when multiple next-hops
      are be reached through a single network level multicast group.
      The virtual link allows efficient demultiplexing of ST Control
      Message PDUs received from a single physical link or network.

   Virtual Link Identifier
      For each ST Control Message sent, the sender provides its own
      virtual link identifier and that of the receiver (if known).
      Either of these identifiers, combined with the address of the
      corresponding host, can be used to identify uniquely the virtual
      control link to the agent.  However, virtual link identifiers
      are chosen by the associated agent so that the agent may
      precisely identify the stream, state machine, and other protocol
      processing data elements managed by that agent, without regard
      to the source of the control message.  Virtual link identifiers
      are not negotiated, and do not change during the lifetime of a
      stream.  They are discarded when the stream is torn down.













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7.      References

   [1] Braden, B., Borman, D., and C. Partridge, "Computing the
       Internet Checksum", RFC 1071, USC/Information Sciences
       Institute, Cray Research, BBN Laboratories, September
       1988.


   [2] Braden, R. (ed.), "Requirements for Internet Hosts --
       Communication Layers", RFC 1122, USC/Information Sciences
       Institute, October 1989.


   [3] Cheriton, D., "VMTP: Versatile Message Transaction Protocol
       Specification", RFC 1045, Stanford University, February 1988.


   [4] Cohen, D., "A Network Voice Protocol NVP-II", USC/Information
       Sciences Institute, April 1981.


   [5] Cole, E., "PVP - A Packet Video Protocol", W-Note 28,
       USC/Information Sciences Institute, August 1981.


   [6] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
       Stanford University, August 1989.


   [7] Edmond W., Seo K., Leib M., and C. Topolcic, "The DARPA
       Wideband Network Dual Bus Protocol", accepted for presentation
       at ACM SIGCOMM '90, September 24-27, 1990.


   [8] Forgie, J., "ST - A Proposed Internet Stream Protocol",
       IEN 119, M. I. T. Lincoln Laboratory, 7 September 1979.


   [9] Jacobs I., Binder R., and E. Hoversten E., "General Purpose
       Packet Satellite Network", Proc. IEEE, vol 66, pp 1448-1467,
       November 1978.


   [10] Jacobson, V., "Congestion Avoidance and Control", ACM
        SIGCOMM-88, August 1988.


   [11] Karn, P. and C. Partridge, "Round Trip Time Estimation",
        ACM SIGCOMM-87, August 1987.





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   [12] Mallory, T., and A. Kullberg, "Incremental Updating of the
        Internet Checksum", RFC 1141, BBN Communications
        Corporation, January 1990.


   [13] Mills, D., "Network Time Protocol (Version 2) Specification
        and Implementation", RFC 1119, University of Delaware,
        September 1989 (Revised February 1990).


   [14] Pope, A., "The SIMNET Network and Protocols", BBN
        Report No. 7102, BBN Systems and Technologies, July 1989.


   [15] Postel, J., ed., "Internet Protocol - DARPA Internet Program
        Protocol Specification", RFC 791, DARPA, September 1981.


   [16] Postel, J., ed., "Transmission Control Protocol - DARPA
        Internet Program Protocol Specification", RFC 793, DARPA,
        September 1981.


   [17] Postel, J., "User Datagram Protocol", RFC 768,
        USC/Information Sciences Institute, August 1980.


   [18] Reynolds, J., Postel, J., "Assigned Numbers", RFC 1060,
        USC/Information Sciences Institute, March 1990.


   [19] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
        SDNS Secure Data Network System, Security Protocol 3 (SP3),
        SDN.301, Rev. 1.5, 1989-05-15.


   [20] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
        SDNS Secure Data Network System, Security Protocol 3 (SP3)
        Addendum 1, Cooperating Families, SDN.301.1, Rev. 1.2,
        1988-07-12.

8.      Security Considerations

   See section 3.7.8.










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9.      Authors' Addresses

      Stephen Casner
      USC/Information Sciences Institute
      4676 Admiralty Way
      Marina del Rey, CA 90292-6695

      Phone: (213) 822-1511 x153
      EMail: Casner@ISI.Edu


      Charles Lynn, Jr.
      BBN Systems and Technologies,
      a division of Bolt Beranek and Newman Inc.
      10 Moulton Street
      Cambridge, MA  02138

      Phone: (617) 873-3367
      EMail: CLynn@BBN.Com


      Philippe Park
      BBN Systems and Technologies,
      a division of Bolt Beranek and Newman Inc.
      10 Moulton Street
      Cambridge, MA  02138

      Phone: (617) 873-2892
      EMail: ppark@BBN.COM


      Kenneth Schroder
      BBN Systems and Technologies,
      a division of Bolt Beranek and Newman Inc.
      10 Moulton Street
      Cambridge, MA  02138

      Phone: (617) 873-3167
      EMail: Schroder@BBN.Com


      Claudio Topolcic
      BBN Systems and Technologies,
      a division of Bolt Beranek and Newman Inc.
      10 Moulton Street
      Cambridge, MA  02138

      Phone: (617) 873-3874
      EMail: Topolcic@BBN.Com





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Appendix 1.      Data Notations

   The convention in the documentation of Internet Protocols is to
   express numbers in decimal and to picture data with the most
   significant octet on the left and the least significant octet on the
   right.

   The order of transmission of the header and data described in this
   document is resolved to the octet level.  Whenever a diagram shows a
   group of octets, the order of transmission of those octets is the
   normal order in which they are read in English.  For example, in the
   following diagram the octets are transmitted in the order they are
   numbered.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |       2       |       3       |       4       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |       6       |       7       |       8       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       9       |      10       |      11       |      12       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 56.  Transmission Order of Bytes


   Whenever an octet represents a numeric quantity the left most bit in
   the diagram is the high order or most significant bit.  That is, the
   bit labeled 0 is the most significant bit.  For example, the
   following diagram represents the value 170 (decimal).


                            0 1 2 3 4 5 6 7
                           +-+-+-+-+-+-+-+-+
                           |1 0 1 0 1 0 1 0|
                           +-+-+-+-+-+-+-+-+

                    Figure 57.  Significance of Bits


   Similarly, whenever a multi-octet field represents a numeric quantity
   the left most bit of the whole field is the most significant bit.
   When a multi-octet quantity is transmitted the most significant octet
   is transmitted first.

   Fields whose length is fixed and fully illustrated are shown with a
   vertical bar (|) at the end;  fixed fields whose contents are
   abbreviated are shown with an exclamation point (!);  variable fields
   are shown with colons (:).



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




















































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