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rfc:rfc1190

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

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

Status of this Memo

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

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] RFC 1190 Internet Stream Protocol October 1990

 1.1.       Table of Contents
               Status of this Memo .  .  .  .  .  .  .  .  .  .  .  .   1
       1.      Abstract   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .   1
       1.1.       Table of Contents   .  .  .  .  .  .  .  .  .  .  .   2
       1.2.       List of Figures  .  .  .  .  .  .  .  .  .  .  .  .   4
       2.      Introduction  .  .  .  .  .  .  .  .  .  .  .  .  .  .   7
       2.1.       Major Differences Between ST and ST-II   .  .  .  .   8
       2.2.       Concepts and Terminology  .  .  .  .  .  .  .  .  .   9
       2.3.       Relationship Between Applications and ST .  .  .  .  11
       2.4.       ST Control Message Protocol  .  .  .  .  .  .  .  .  12
       2.5.       Flow Specifications .  .  .  .  .  .  .  .  .  .  .  14
       3.      ST Control Message Protocol Functional Description   .  17
       3.1.       Stream Setup  .  .  .  .  .  .  .  .  .  .  .  .  .  18
       3.1.1.        Initial Setup at the Origin  .  .  .  .  .  .  .  18
       3.1.2.        Invoking the Routing Function   .  .  .  .  .  .  19
       3.1.3.        Reserving Resources .  .  .  .  .  .  .  .  .  .  19
       3.1.4.        Sending CONNECT Messages  .  .  .  .  .  .  .  .  20
       3.1.5.        CONNECT Processing by an Intermediate Agent .  .  22
       3.1.6.        Setup at the Targets   .  .  .  .  .  .  .  .  .  23
       3.1.7.        ACCEPT Processing by an Intermediate Agent  .  .  24
       3.1.8.        ACCEPT Processing by the Origin .  .  .  .  .  .  26
       3.1.9.        Processing a REFUSE Message  .  .  .  .  .  .  .  27
       3.2.       Data Transfer .  .  .  .  .  .  .  .  .  .  .  .  .  30
       3.3.       Modifying an Existing Stream .  .  .  .  .  .  .  .  31
       3.3.1.        Adding a Target  .  .  .  .  .  .  .  .  .  .  .  31
       3.3.2.        The Origin Removing a Target .  .  .  .  .  .  .  33
       3.3.3.        A Target Deleting Itself  .  .  .  .  .  .  .  .  35
       3.3.4.        Changing the FlowSpec  .  .  .  .  .  .  .  .  .  36
       3.4.       Stream Tear Down .  .  .  .  .  .  .  .  .  .  .  .  36
       3.5.       Exceptional Cases   .  .  .  .  .  .  .  .  .  .  .  37
       3.5.1.        Setup Failure due to CONNECT Timeout  .  .  .  .  37
       3.5.2.        Problems due to Routing Inconsistency .  .  .  .  38
       3.5.3.        Setup Failure due to a Routing Failure   .  .  .  39
       3.5.4.        Problems in Reserving Resources .  .  .  .  .  .  41
       3.5.5.        Setup Failure due to ACCEPT Timeout   .  .  .  .  41
       3.5.6.        Problems Caused by CHANGE Messages .  .  .  .  .  42
       3.5.7.        Notification of Changes Forced by Failures  .  .  42
       3.6.       Options .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  44
       3.6.1.        HID Field Option .  .  .  .  .  .  .  .  .  .  .  44
       3.6.2.        PTP Option .  .  .  .  .  .  .  .  .  .  .  .  .  44
       3.6.3.        FDx Option .  .  .  .  .  .  .  .  .  .  .  .  .  45
       3.6.4.        NoRecovery Option   .  .  .  .  .  .  .  .  .  .  46
       3.6.5.        RevChrg Option   .  .  .  .  .  .  .  .  .  .  .  46
       3.6.6.        Source Route Option .  .  .  .  .  .  .  .  .  .  46
       3.7.       Ancillary Functions .  .  .  .  .  .  .  .  .  .  .  48
       3.7.1.        Failure Detection   .  .  .  .  .  .  .  .  .  .  48
       3.7.1.1.         Network Failures .  .  .  .  .  .  .  .  .  .  48
       3.7.1.2.         Detecting ST Stream Failures .  .  .  .  .  .  49
       3.7.1.3.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  51

CIP Working Group [Page 2] RFC 1190 Internet Stream Protocol October 1990

       3.7.2.        Failure Recovery .  .  .  .  .  .  .  .  .  .  .  51
       3.7.2.1.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  55
       3.7.3.        A Group of Streams  .  .  .  .  .  .  .  .  .  .  56
       3.7.3.1.         Group Name Generator   .  .  .  .  .  .  .  .  57
       3.7.3.2.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  57
       3.7.4.        HID Negotiation  .  .  .  .  .  .  .  .  .  .  .  58
       3.7.4.1.         Subset  .  .  .  .  .  .  .  .  .  .  .  .  .  64
       3.7.5.        IP Encapsulation of ST .  .  .  .  .  .  .  .  .  64
       3.7.5.1.         IP Multicasting  .  .  .  .  .  .  .  .  .  .  65
       3.7.6.        Retransmission   .  .  .  .  .  .  .  .  .  .  .  66
       3.7.7.        Routing .  .  .  .  .  .  .  .  .  .  .  .  .  .  67
       3.7.8.        Security   .  .  .  .  .  .  .  .  .  .  .  .  .  67
       3.8.       ST Service Interfaces  .  .  .  .  .  .  .  .  .  .  68
       3.8.1.        Access to Routing Information   .  .  .  .  .  .  69
       3.8.2.        Access to Network Layer Resource Reservation   .  70
       3.8.3.        Network Layer Services Utilized .  .  .  .  .  .  71
       3.8.4.        IP Services Utilized   .  .  .  .  .  .  .  .  .  71
       3.8.5.        ST Layer Services Provided   .  .  .  .  .  .  .  72
       4.      ST Protocol Data Unit Descriptions .  .  .  .  .  .  .  75
       4.1.       Data Packets  .  .  .  .  .  .  .  .  .  .  .  .  .  76
       4.2.       ST Control Message Protocol Descriptions .  .  .  .  77
       4.2.1.        ST Control Messages .  .  .  .  .  .  .  .  .  .  79
       4.2.2.        Common SCMP Elements   .  .  .  .  .  .  .  .  .  80
       4.2.2.1.         DetectorIPAddress   .  .  .  .  .  .  .  .  .  80
       4.2.2.2.         ErroredPDU .  .  .  .  .  .  .  .  .  .  .  .  80
       4.2.2.3.         FlowSpec & RFlowSpec   .  .  .  .  .  .  .  .  81
       4.2.2.4.         FreeHIDs   .  .  .  .  .  .  .  .  .  .  .  .  84
       4.2.2.5.         Group & RGroup   .  .  .  .  .  .  .  .  .  .  85
       4.2.2.6.         HID & RHID .  .  .  .  .  .  .  .  .  .  .  .  86
       4.2.2.7.         MulticastAddress .  .  .  .  .  .  .  .  .  .  86
       4.2.2.8.         Name & RName  .  .  .  .  .  .  .  .  .  .  .  87
       4.2.2.9.         NextHopIPAddress .  .  .  .  .  .  .  .  .  .  88
       4.2.2.10.        Origin  .  .  .  .  .  .  .  .  .  .  .  .  .  88
       4.2.2.11.        OriginTimestamp  .  .  .  .  .  .  .  .  .  .  89
       4.2.2.12.        ReasonCode .  .  .  .  .  .  .  .  .  .  .  .  89
       4.2.2.13.        RecordRoute   .  .  .  .  .  .  .  .  .  .  .  94
       4.2.2.14.        SrcRoute   .  .  .  .  .  .  .  .  .  .  .  .  95
       4.2.2.15.        Target and TargetList  .  .  .  .  .  .  .  .  96
       4.2.2.16.        UserData   .  .  .  .  .  .  .  .  .  .  .  .  98
       4.2.3.        ST Control Message PDUs   .  .  .  .  .  .  .  .  99
       4.2.3.1.         ACCEPT  .  .  .  .  .  .  .  .  .  .  .  .  . 100
       4.2.3.2.         ACK  .  .  .  .  .  .  .  .  .  .  .  .  .  . 102
       4.2.3.3.         CHANGE-REQUEST   .  .  .  .  .  .  .  .  .  . 103
       4.2.3.4.         CHANGE  .  .  .  .  .  .  .  .  .  .  .  .  . 104
       4.2.3.5.         CONNECT .  .  .  .  .  .  .  .  .  .  .  .  . 105
       4.2.3.6.         DISCONNECT .  .  .  .  .  .  .  .  .  .  .  . 110
       4.2.3.7.         ERROR-IN-REQUEST .  .  .  .  .  .  .  .  .  . 111
       4.2.3.8.         ERROR-IN-RESPONSE   .  .  .  .  .  .  .  .  . 112
       4.2.3.9.         HELLO   .  .  .  .  .  .  .  .  .  .  .  .  . 113
       4.2.3.10.        HID-APPROVE   .  .  .  .  .  .  .  .  .  .  . 114
       4.2.3.11.        HID-CHANGE-REQUEST  .  .  .  .  .  .  .  .  . 115

CIP Working Group [Page 3] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.12.        HID-CHANGE .  .  .  .  .  .  .  .  .  .  .  . 116
       4.2.3.13.        HID-REJECT .  .  .  .  .  .  .  .  .  .  .  . 118
       4.2.3.14.        NOTIFY  .  .  .  .  .  .  .  .  .  .  .  .  . 120
       4.2.3.15.        REFUSE  .  .  .  .  .  .  .  .  .  .  .  .  . 122
       4.2.3.16.        STATUS  .  .  .  .  .  .  .  .  .  .  .  .  . 124
       4.2.3.17.        STATUS-RESPONSE  .  .  .  .  .  .  .  .  .  . 126
       4.3.       Suggested Protocol Constants .  .  .  .  .  .  .  . 127
       5.      Areas Not Addressed .  .  .  .  .  .  .  .  .  .  .  . 131
       6.      Glossary   .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 135
       7.      References .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 143
       8.      Security Considerations.  .  .  .  .  .  .  .  .  .  . 144
       9.      Authors' Addresses  .  .  .  .  .  .  .  .  .  .  .  . 145
       Appendix 1.      Data Notations   .  .  .  .  .  .  .  .  .  . 147
 1.2.       List of Figures
       Figure 1.    Protocol Relationships  .  .  .  .  .  .  .  .  .   6
       Figure 2.    Topology Used in Protocol Exchange Diagrams  .  .  16
       Figure 3.    Virtual Link Identifiers for SCMP Messages   .  .  16
       Figure 4.    HIDs Assigned for ST User Packets   .  .  .  .  .  18
       Figure 5.    Origin Sending CONNECT Message   .  .  .  .  .  .  21
       Figure 6.    CONNECT Processing by an Intermediate Agent  .  .  22
       Figure 7.    CONNECT Processing by the Target .  .  .  .  .  .  24
       Figure 8.    ACCEPT Processing by an Intermediate Agent   .  .  25
       Figure 9.    ACCEPT Processing by the Origin  .  .  .  .  .  .  26
       Figure 10.   Sending REFUSE Message  .  .  .  .  .  .  .  .  .  28
       Figure 11.   Routing Around a Failure   .  .  .  .  .  .  .  .  29
       Figure 12.   Addition of Another Target .  .  .  .  .  .  .  .  32
       Figure 13.   Origin Removing a Target   .  .  .  .  .  .  .  .  34
       Figure 14.   Target Deleting Itself  .  .  .  .  .  .  .  .  .  35
       Figure 15.   CONNECT Retransmission after a Timeout .  .  .  .  38
       Figure 16.   Processing NOTIFY Messages .  .  .  .  .  .  .  .  43
       Figure 17.   Source Routing Option   .  .  .  .  .  .  .  .  .  47
       Figure 18.   Typical HID Negotiation (No Multicasting) .  .  .  60
       Figure 19.   Multicast HID Negotiation  .  .  .  .  .  .  .  .  61
       Figure 20.   Multicast HID Re-Negotiation           .  .  .  .  62
       Figure 21.   ST Header   .  .  .  .  .  .  .  .  .  .  .  .  .  75
       Figure 22.   ST Control Message Format  .  .  .  .  .  .  .  .  77
       Figure 23.   ErroredPDU  .  .  .  .  .  .  .  .  .  .  .  .  .  80
       Figure 24.   FlowSpec & RFlowSpec .  .  .  .  .  .  .  .  .  .  81
       Figure 25.   FreeHIDs .  .  .  .  .  .  .  .  .  .  .  .  .  .  85
       Figure 26.   Group & RGroup .  .  .  .  .  .  .  .  .  .  .  .  85
       Figure 27.   HID & RHID  .  .  .  .  .  .  .  .  .  .  .  .  .  86
       Figure 28.   MulticastAddress  .  .  .  .  .  .  .  .  .  .  .  86
       Figure 29.   Name & RName   .  .  .  .  .  .  .  .  .  .  .  .  87
       Figure 30.   NextHopIPAddress  .  .  .  .  .  .  .  .  .  .  .  88

CIP Working Group [Page 4] RFC 1190 Internet Stream Protocol October 1990

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

CIP Working Group [Page 5] RFC 1190 Internet Stream Protocol October 1990

+——————–+ | Conference Control | +——————–+

                  |

+——-+ +——-+ |

Video Voice
Appl Appl SNMP Telnet FTP

+——-+ +——-+ | +—–+ +——+ +—–+ +—–+

  |        |      |     |        |     |            |
  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
Protocol Protocol

+—————-+ +—————-+

                  Figure 1.  Protocol Relationships

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

2. Introduction

 ST has been developed to support efficient delivery of streams of
 packets to either single or multiple destinations in applications
 requiring guaranteed data rates and controlled delay characteristics.
 The motivation for the original protocol was that IP [2] [15] did not
 provide the delay and data rate characteristics necessary to support
 voice applications.
 ST is an internet protocol at the same layer as IP, see Figure 1.  ST
 differs from IP in that IP, as originally envisioned, did not require
 routers (or intermediate systems) to maintain state information
 describing the streams of packets flowing through them.  ST
 incorporates the concept of streams across an internet.  Every
 intervening ST entity maintains state information for each stream
 that passes through it.  The stream state includes forwarding
 information, including multicast support for efficiency, and resource
 information, which allows network or link bandwidth and queues to be
 assigned to a specific stream.  This pre-allocation of resources
 allows data packets to be forwarded with low delay, low overhead, and
 a low probability of loss due to congestion.  The characteristics of
 a stream, such as the number and location of the endpoints, and the
 bandwidth required, may be modified during the lifetime of the
 stream.  This allows ST to give a real time application the
 guaranteed and predictable communication characteristics it requires,
 and is a good vehicle to support an application whose communications
 requirements are relatively predictable.
 ST proved quite useful in several early experiments that involved
 voice conferences in the Internet.  Since that time, ST has also been
 used to support point-to-point streams that include both video and
 voice.  Recently, multimedia conferencing applications have been
 developed that need to exchange real-time voice, video, and pointer
 data in a multi-site conferencing environment.  Multimedia
 conferencing across an internet is an application for which ST
 provides ideal support.  Simulation and wargaming applications [14]
 also place similar requirements on the communication system.  Other
 applications may include scientific visualization between a number of
 workstations and one or more remote supercomputers, and the
 collection and distribution of real-time sensor data from remote
 sensor platforms.  ST may also be useful to support activities that
 are currently supported by IP, such as bulk file transfer using TCP.
 Transport protocols above ST include the Packet Video Protocol (PVP)
 [5] and the Network Voice Protocol (NVP) [4], which are end-to-end
 protocols used directly by applications.  Other transport layer
 protocols that may be used over ST include TCP [16], VMTP [3], etc.
 They provide the user interface, flow control, and packet ordering.
 This specification does not describe these higher layer protocols.

CIP Working Group [Page 7] RFC 1190 Internet Stream Protocol October 1990

 2.1.       Major Differences Between ST and ST-II
    ST-II supports a wider variety of applications than did the
    original ST.  The differences between ST and ST-II are fairly
    straight forward yet provide great improvements.  Four of the more
    notable differences are:
       1  ST-II is decoupled from the Access Controller (AC).  The
          AC, as well as providing a rudimentary access control
          function, also served as a centralized repository and
          distributor of the conference information.  If an AC is
          necessary, it should be an entity in a higher layer
          protocol.  A large variety of applications such as
          conferencing, distributed simulations, and wargaming can
          be run without an explicit AC.
       2  The basic stream construct of ST-II is a directed tree
          carrying traffic away from a source to all the
          destinations, rather than the original ST's omniplex
          structure.  For example, a conference is composed of a
          number of such trees, one for traffic from each
          participant.  Although there are more (simplex) streams in
          ST-II, each is much simpler to manage, so the aggregate is
          much simpler.  This change has a minimal impact on the
          application.
       3  ST-II defines a number of the robustness and recovery
          mechanisms that were left undefined in the original ST
          specification.  In case of a network or ST Agent failure,
          a stream may optionally be repaired automatically (i.e.,
          without intervention from the user or the application)
          using a pruned depth first search starting at the ST Agent
          immediately preceding the failure.
       4  ST-II does not make an inherent distinction between
          streams connecting only two communicants and streams among
          an arbitrary number of communicants.
    This memo is the specification for the ST-II Protocol.  Since
    there should be no ambiguity between the original ST specification
    and the specification herein, the protocol is simply called ST
    hereafter.
    ST is the protocol used by ST entities to exchange information.
    The same protocol is used for communication among all ST entities,
    whether they communicate with a higher layer protocol or forward
    ST packets between attached networks.
    The remainder of this section gives a brief overview of the ST
    Protocol.  Section 3 (page 17) provides a detailed description of
    the operations required by the protocol.  Section 4 (page 75)
    provides descriptions of the ST Protocol Data Units exchanged

CIP Working Group [Page 8] RFC 1190 Internet Stream Protocol October 1990

    between ST entities.  Issues that have not yet been fully
    addressed are presented in Section 5 (page 131).  A glossary and
    list of references are in Sections 6 (page 135) and 7 (page 143),
    respectively.
    This memo also defines "subsets" of ST that can be implemented.  A
    subsetted implementation does not have full ST functionality, but
    it can interoperate with other similarly subsetted
    implementations, or with a full implementation, in a predictable
    and consistent manner.  This approach allows an implementation to
    be built and provide service with minimum effort, and gives it an
    immediate and well defined growth path.
 2.2.       Concepts and Terminology
    The ST packet header is not constrained to be compatible with the
    IP packet header, except for the IP Version Number (the first four
    bits) that is used to distinguish ST packets (IP Version 5) from
    IP packets (IP Version 4).  The ST packets, or protocol data units
    (PDUs), can be encapsulated in IP either to provide connectivity
    (possibly with degraded service) across portions of an internet
    that do not provide support for ST, or to allow access to services
    such as security that are not provided directly by ST.
    An internet entity that implements the ST Protocol is called an
    "ST Agent".  We refer to two kinds of ST agents:  "host ST
    agents", also called "host agents" and "intermediate ST agents",
    also called "intermediate agents".  The ST agents functioning as
    hosts are sourcing or sinking data to a higher layer protocol or
    application, while ST agents functioning as intermediate agents
    are forwarding data between directly attached networks.  This
    distinction is not part of the protocol, but is used for
    conceptual purposes only.  Indeed, a given ST agent may be
    simultaneously performing both host and intermediate roles.  Every
    ST agent should be capable of delivering packets to a higher layer
    protocol.  Every ST agent can replicate ST data packets as
    necessary for multi-destination delivery, and is able to send
    packets whether received from a network interface or a higher
    layer protocol.  There are no other kinds of ST agents.
    ST provides applications with an end-to-end flow oriented service
    across an internet.  This service is implemented using objects
    called "streams".  ST data packets are not considered to be
    totally independent as are IP data packets.  They are transmitted
    only as part of a point-to-point or point-to-multi- point stream.
    ST creates a stream during a setup phase before data is
    transmitted.  During the setup phase, routes are selected and
    internetwork resources are reserved.  Except for explicit changes
    to the stream, the routes remain in effect until the stream is
    explicitly torn down.

CIP Working Group [Page 9] RFC 1190 Internet Stream Protocol October 1990

    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.

CIP Working Group [Page 10] RFC 1190 Internet Stream Protocol October 1990

    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

CIP Working Group [Page 11] RFC 1190 Internet Stream Protocol October 1990

    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,

CIP Working Group [Page 12] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 13] RFC 1190 Internet Stream Protocol October 1990

    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.

CIP Working Group [Page 14] RFC 1190 Internet Stream Protocol October 1990

    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.

CIP Working Group [Page 15] RFC 1190 Internet Stream Protocol October 1990

  • * * * 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

CIP Working Group [Page 16] RFC 1190 Internet Stream Protocol October 1990

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.

CIP Working Group [Page 17] RFC 1190 Internet Stream Protocol October 1990

 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

CIP Working Group [Page 18] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 19] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 20] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 21] 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

CIP Working Group [Page 22] 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.

CIP Working Group [Page 23] 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

CIP Working Group [Page 24] 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

CIP Working Group [Page 25] 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

CIP Working Group [Page 26] 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

CIP Working Group [Page 27] 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

CIP Working Group [Page 28] 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

CIP Working Group [Page 29] 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.

CIP Working Group [Page 30] RFC 1190 Internet Stream Protocol October 1990

    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

CIP Working Group [Page 31] RFC 1190 Internet Stream Protocol October 1990

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

Application Agent A Agent 2 Agent E

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

         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

CIP Working Group [Page 32] RFC 1190 Internet Stream Protocol October 1990

       An ST agent that is already a node in the stream recognizes the
       RVLId and verifies that the Name of the stream is the same.  It
       then checks if the intersection of the TargetList and the
       targets of the established stream is empty.  If this is not the
       case, then the receiver responds with an ERROR-IN-REQUEST with
       the appropriate reason code (RouteLoop) that contains a
       TargetList of those targets that were duplicates;  see Section
       4.2.3.5 (page 106).
       For each new target in the TargetList, processing is much the
       same as for the original CONNECT;  see Sections 3.1.2-4 (pages
       19-20).  The CONNECT must be acknowledged, propagated, and
       network resources must be reserved.  However, it may be
       possible to route to the new targets using previously allocated
       paths or an existing multicast group.  In that case, additional
       resources do not need to be reserved but more next-hop(s) might
       have to be added to an existing multicast group.
       Nevertheless, the origin, or any intermediate ST agent that
       receives a CONNECT for an existing stream, can make a routing
       decision that is independent of any it may have made
       previously.  Depending on the routing algorithm that is used,
       the ST agent may decide to reach the new target by way of an
       established branch, or it may decide to create a new branch.
       The fact that a new target is being added to an existing stream
       may result in a suboptimal overall routing for certain routing
       algorithms.  We take this problem to be unavoidable since it is
       unlikely that the stream routing can be made optimal in
       general, and the only way to avoid this loss of optimality is
       to redefine the routing of potentially the entire stream, which
       would be too expensive and time consuming.
    3.3.2.        The Origin Removing a Target
       The application at the origin specifies a set of targets that
       are to be removed from the stream and an appropriate reason
       code (ApplDisconnect).  The targets are partitioned into
       multiple DISCONNECT messages based on the next-hop to the
       individual targets.  As with CONNECT messages, an ST agent that
       is sending a DISCONNECT must make sure that the message fits
       into the MTU for the intervening network.  If the message is
       too large, the TargetList must be further partitioned into
       multiple DISCONNECT messages.
       An ST agent that receives a DISCONNECT message must acknowledge
       it by sending an ACK back to the previous-hop.  The DISCONNECT
       must also be propagated to the relevant next-hop ST agents.
       Before propagating the message, however, the TargetList should
       be partitioned based on next-hop ST

CIP Working Group [Page 33] RFC 1190 Internet Stream Protocol October 1990

       agent and MTU, as described above.  Note that there may be
       targets in the TargetList for which the ST agent has no
       information.  This may result from interacting DISCONNECT and
       REFUSE messages and should be logged and silently ignored.
       If, after deleting the specified targets, any next-hop has no
       remaining targets, then those resources associated with that
       next-hop agent may be released.  Note that network resources
       may not actually be released if network multicasting is being
       used since they may still be required for traffic to other
       next-hops in the multicast group.
    Application                                         Application
          Agent A             Agent 1  Agent 2          Agent B    C
1.  (close B,C ApplDisconnect)
        V
2.      +->+-+-> DISCONNECT B ----->+
3.         | |   <RVLId=14><SVLId=4>+-+-> DISCONNECT B ------>+
           | |   <Ref=25>           | |   <RVLId=44><SVLId=15>|
           | V   <RC=ApplDisconnect>| |   <Ref=120>           |
4.         | (free A to 1 resrc.)   | V   <RC=ApplDisconnect> |
5.         |                        V (free 1 to B resrc.)    |
6.         | +<- ACK <--------------+                         V
7.         | |   <RVLId=4><SVLId=14>| +<- ACK <---------------+
           | V   <Ref=25>           | |   <RVLId=15><SVLId=44>|
8.         | (free link 4)          V |   <Ref=120>           |
9.         |           (free link 14) V                       |
10.        |                          (free link 15)          V
11.        |        (inform B that stream closed ApplDisconnect)
12.        |                                     (free link 44)
           V
13.     +<-+-+-> DISCONNECT C ---------->+
14.     |    |   <RVLId=23><SVLId=5>     +-+-> DISCONNECT C ------>+
        |    |   <Ref=30>                | |   <RVLId=54><SVLId=25>|
        |    V   <RC=ApplDisconnect>     | |   <Ref=240>           |
15.     |    (keep A to 2 resrc for      | V   <RC=ApplDisconnect> |
16.     |         data going to D,E)     | (free 2 to C resrc.)    |
        |                                V                         |
17.     |    +<- ACK <-------------------+                         V
18.     |    |   <RVLId=5><SVLId=23>     | +<- ACK <---------------+
        |    V   <Ref=30>                | |   <RVLId=25><SVLId=54>|
19.     |    (keep link 5 for D,E)       V |   <Ref=240>           |
20.     |           (keep link 23 for D,E) V                       |
21.     |                           (free link 25)                 V
22.     |              (inform C that stream closed ApplDisconnect>)
23.     V                                             (free link 54)
24.     (inform A closed to B,C ApplDisconnect)
                Figure 13.  Origin Removing a Target

CIP Working Group [Page 34] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 35] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 36] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 37] RFC 1190 Internet Stream Protocol October 1990

        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

CIP Working Group [Page 38] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 39] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 40] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 41] RFC 1190 Internet Stream Protocol October 1990

    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

CIP Working Group [Page 42] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 43] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 44] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 45] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 46] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 47] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 48] RFC 1190 Internet Stream Protocol October 1990

          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

CIP Working Group [Page 49] RFC 1190 Internet Stream Protocol October 1990

          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

CIP Working Group [Page 50] RFC 1190 Internet Stream Protocol October 1990

          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

CIP Working Group [Page 51] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 52] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 53] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 54] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 55] RFC 1190 Internet Stream Protocol October 1990

    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

CIP Working Group [Page 56] RFC 1190 Internet Stream Protocol October 1990

           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.

CIP Working Group [Page 57] RFC 1190 Internet Stream Protocol October 1990

    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

CIP Working Group [Page 58] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 59] RFC 1190 Internet Stream Protocol October 1990

       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)

CIP Working Group [Page 60] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 61] RFC 1190 Internet Stream Protocol October 1990

    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)

CIP Working Group [Page 62] RFC 1190 Internet Stream Protocol October 1990

    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)

CIP Working Group [Page 63] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 64] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 65] RFC 1190 Internet Stream Protocol October 1990

          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

CIP Working Group [Page 66] RFC 1190 Internet Stream Protocol October 1990

       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.

CIP Working Group [Page 67] RFC 1190 Internet Stream Protocol October 1990

       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).

CIP Working Group [Page 68] RFC 1190 Internet Stream Protocol October 1990

    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 >* >*

CIP Working Group [Page 69] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 70] RFC 1190 Internet Stream Protocol October 1990

    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

CIP Working Group [Page 71] RFC 1190 Internet Stream Protocol October 1990

          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

CIP Working Group [Page 72] RFC 1190 Internet Stream Protocol October 1990

       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

CIP Working Group [Page 73] RFC 1190 Internet Stream Protocol October 1990

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CIP Working Group [Page 74] RFC 1190 Internet Stream Protocol October 1990

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

CIP Working Group [Page 75] RFC 1190 Internet Stream Protocol October 1990

    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.

CIP Working Group [Page 76] RFC 1190 Internet Stream Protocol October 1990

 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

CIP Working Group [Page 77] RFC 1190 Internet Stream Protocol October 1990

       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).

CIP Working Group [Page 78] RFC 1190 Internet Stream Protocol October 1990

    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.

CIP Working Group [Page 79] RFC 1190 Internet Stream Protocol October 1990

    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

CIP Working Group [Page 80] RFC 1190 Internet Stream Protocol October 1990

             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

CIP Working Group [Page 81] RFC 1190 Internet Stream Protocol October 1990

          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.

CIP Working Group [Page 82] RFC 1190 Internet Stream Protocol October 1990

                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.

CIP Working Group [Page 83] RFC 1190 Internet Stream Protocol October 1990

             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

CIP Working Group [Page 84] RFC 1190 Internet Stream Protocol October 1990

             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

CIP Working Group [Page 85] RFC 1190 Internet Stream Protocol October 1990

          A GroupName has the same format as a Name;  see Figure 29.
       4.2.2.6.         HID & RHID
          The HID parameter (PCode = 5) is used in the NOTIFY message
          when the notification is related to a HID, and possibly in
          the STATUS-RESPONSE message to convey additional HIDs that
          are valid for a stream when there are more than one.  It
          consists of the PCode and PBytes bytes prepended to a HID;
          HIDs were described in Section 4 (page 76).
          The RHID parameter (PCode = 14) is used in conjunction with
          the FDx option to convey the HID that is to be used in the
          reverse direction.  It consists of the PCode and PBytes
          bytes prepended to a HID.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     PCode     |       4       |              HID              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 27.  HID & RHID
       4.2.2.7.         MulticastAddress
          The MulticastAddress parameter (PCode = 6) is an optional
          parameter that is used, when setting up a network level
          multicast group, to communicate an IP and/or local network
          multicast address to the next-hop agents that should become
          members of the group.
             LocalNetBytes is the length of the Local Net Multicast
             Address.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   PCode = 6   |    PBytes     | LocalNetBytes |       0       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     IP Multicast Address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                  Local Net Multicast Address  :    Padding    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 28.  MulticastAddress

CIP Working Group [Page 86] RFC 1190 Internet Stream Protocol October 1990

             IP Multicast Address is described in [6].  This field is
             zero (0) if no IP multicast address is known or is
             applicable.  The block of addresses 224.1.0.0 -
             224.1.255.255 has been allocated for use by ST.
             Local Net Multicast Address is the multicast address to
             be used on the local network.  It corresponds to the IP
             Multicast Address when the latter is non-zero.
       4.2.2.8.         Name & RName
          Each stream is uniquely (i.e., globally) identified by a
          Name.  A Name is created by the origin host ST agent and is
          composed of 1) a 16-bit number chosen to make the Name
          unique within the agent, 2) the IP address of the origin ST
          agent, and 3) a 32-bit timestamp.  If the origin has
          multiple IP addresses, then any that can be used to reach
          target may be used in the Name.  The intent is that the
          <Unique ID, IP Address> tuple be unique for the lifetime of
          the stream.  It is suggested that to increase robustness a
          Unique ID value not be reused for a period of time on the
          order of 5 minutes.
          The Timestamp is included both to make the Name unique over
          long intervals (e.g., forever) for purposes of network
          management and accounting/billing, and to protect against
          failure of an ST agent that causes knowledge of active
          Unique IDs to be lost.  The assumption is that all ST agents
          have access to some "clock".  If this is not the case, the
          agent should have access to some form of non-volatile memory
          in which it can store some number that at least gets
          incremented per restart.
          The Name parameter (PCode = 7) is used in most control
          messages to identify a stream.
          The RName parameter (PCode = 15) is used in conjunction with
          the FDx option to convey the Name of the reverse stream in
          an ACCEPT message.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     PCode     |       12      |            Unique ID          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          IP Address                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           Timestamp                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      Figure 29.  Name & RName

CIP Working Group [Page 87] RFC 1190 Internet Stream Protocol October 1990

       4.2.2.9.         NextHopIPAddress
          The NextHopIPAddress parameter (PCode = 8) is an optional
          parameter of NOTIFY (RouteBack) or REFUSE (RouteInconsist or
          RouteLoop) and contains the IP address of a suggested next-
          hop ST agent.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   PCode = 8   |       8       |               0               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       next-hop IP address                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 30.  NextHopIPAddress
       4.2.2.10.        Origin
          The Origin parameter (PCode = 9) is used to identify the
          origin of the stream, the next higher protocol, and the SAP
          being used in conjunction with that protocol.
             NextPcol is an 8-bit field used in demultiplexing
             operations to identify the protocol to be used above ST.
             The values of NextPcol are in the same number space as
             the IP Header's Protocol field and are consequently
             defined in the Assigned Numbers RFC [18].
             OriginSAPBytes specifies the length of the OriginSAP,
             exclusive of any padding required to maintain 32-bit
             alignment.
             OriginIPAddress is (one of) the IP address of the origin.
             OriginSAP identifies the origin's SAP associated with the
             NextPcol protocol.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   PCode = 9   |    PBytes     |    NextPcol   |OriginSAPBytes |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         OriginIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                           OriginSAP           :    Padding    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         Figure 31.  Origin

CIP Working Group [Page 88] RFC 1190 Internet Stream Protocol October 1990

       4.2.2.11.        OriginTimestamp
          The OriginTimestamp parameter (PCode = 10) is used to
          indicate the time at which the control message was sent.
          The units and format of the timestamp is that defined in the
          NTP protocol specification [13].  Note that discontinuities
          over leap seconds are expected.
          Note that the time synchronization implied by the use of
          such a parameter is the subject of systems management
          functions not described in this memo, e.g., NTP.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   PCode = 10  |      12       |               0               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-                          Timestamp                          -+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         Figure 32.  OriginTimestamp
       4.2.2.12.        ReasonCode
          Several errors may occur during protocol processing.  All ST
          error codes are taken from a single number space.  The
          currently defined values and their meaning is presented in
          the list below.  Note that new error codes may be defined
          from time to time.  All implementations are expected to
          handle new codes in a graceful manner.  If an unknown
          ReasonCode is encountered, it should be assumed to be fatal.
                  0                   1
                  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 |          ReasonCode           |
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 33.  ReasonCode

CIP Working Group [Page 89] RFC 1190 Internet Stream Protocol October 1990

                Name       Value                 Meaning
          ---------------- ----- ---------------------------------------
          AcceptTimeout      2   An Accept has not been
                                 acknowledged.
          AccessDenied       3   Access denied.
          AckUnexpected      4   An unexpected ACK was received.
          ApplAbort          5   The application aborted the stream
                                 abnormally.
          ApplDisconnect     6   The application closed the stream
                                 normally.
          AuthentFailed      7   The authentication function
                                 failed.
          CantGetResrc       8   Unable to acquire (additional)
                                 resources.
          CantRelResrc       9   Unable to release excess
                                 resources.
          CksumBadCtl       10   A received control PDU has a bad
                                 message checksum.
          CksumBadST        11   A received PDU has a bad ST Header
                                 checksum.
          DropExcdDly       12   A received PDU was dropped because
                                 it could not be processed within
                                 the delay specification.
          DropExcdMTU       13   A received PDU was dropped because
                                 its size exceeds the MTU.
          DropFailAgt       14   A received PDU was dropped because
                                 of a failed ST agent.
          DropFailHst       15   A received PDU was dropped because
                                 of a host failure.
          DropFailIfc       16   A received PDU was dropped because
                                 of a broken interface.
          DropFailNet       17   A received PDU was dropped because
                                 of a network failure.

CIP Working Group [Page 90] RFC 1190 Internet Stream Protocol October 1990

                Name       Value                 Meaning
          ---------------- ----- ---------------------------------------
          DropLimits        18   A received PDU was dropped because
                                 it exceeds the resource limits for
                                 its stream.
          DropNoResrc       19   A received PDU was dropped due to
                                 no available resources (including
                                 precedence).
          DropNoRoute       20   A received PDU was dropped because
                                 of no available route.
          DropPriLow        21   A received PDU was dropped because
                                 it has a priority too low to be
                                 processed.
          DuplicateIgn      22   A received control PDU is a
                                 duplicate and is being
                                 acknowledged.
          DuplicateTarget   23   A received control PDU contains a
                                 duplicate target, or an attempt to
                                 add an existing target.
          ErrorUnknown       1   An error not contained in this
                                 list has been detected.
          failure          N/A   An abbreviation used in the text
                                 for any of the more specific
                                 errors:  DropFailAgt, DropFailHst,
                                 DropFailIfc, DropFailNet,
                                 IntfcFailure, NetworkFailure,
                                 STAgentFailure, FailureRecovery.
          FailureRecovery   24   A notification that recovery is
                                 being attempted.
          FlowVerBad        25   A received control PDU has a
                                 FlowSpec Version Number that is
                                 not supported.
          GroupUnknown      26   A received control PDU contains an
                                 unknown Group Name.
          HIDNegFails       28   HID negotiation failed.
          HIDUnknown        29   A received control PDU contains an
                                 unknown HID.

CIP Working Group [Page 91] RFC 1190 Internet Stream Protocol October 1990

                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.

CIP Working Group [Page 92] RFC 1190 Internet Stream Protocol October 1990

                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.

CIP Working Group [Page 93] RFC 1190 Internet Stream Protocol October 1990

                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

CIP Working Group [Page 94] RFC 1190 Internet Stream Protocol October 1990

       4.2.2.14.        SrcRoute
          The SrcRoute parameter is used, in the Target structure
          shown in Figure 36, to specify the IP addresses of the ST
          agents through which the stream to the target should pass.
          There are two forms of the option, distinguished by the
          PCode.
          With loose source route (PCode = 18) each ST agent first
          examines the first next-hop IP address in the option.  If
          the address is (one of) the address of the current ST agent,
          that entry is removed, and the PBytes field reduced by four
          (4).  If the resulting PBytes field contains 4 (i.e., there
          are no more next-hop IP addresses) the parameter is removed
          from the Target.  In either case, the Target's TargetBytes
          field and the TargetList's PBytes field must be reduced
          accordingly.  The ST agent then routes toward the first
          next-hop IP address in the option, if one exists, or toward
          the target otherwise.  Note that the target's IP address is
          not included as the last entry in the list.
          With a strict source route (PCode = 19) each ST agent first
          examines the first next-hop IP address in the option.  If
          the address is not (one of) the address of the current ST
          agent, a routing error has occurred and should be reported
          with the appropriate reason code.  Otherwise that entry is
          removed, and the PBytes field reduced by four (4).  If the
          resulting PBytes field contains 4 (i.e., there are no more
          next-hop IP addresses) the parameter is removed from the
          Target.  In either case, the Target's TargetBytes field and
          the TargetList's PBytes field must be reduced accordingly.
          The ST agent then routes toward the first next-hop IP
          address in the option, if one exists, or toward the target
          otherwise.  Note that the target's IP address is not
          included as the last entry in the list.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      PCode    |     4+4*N     |               0               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      next-hop IP address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                              ...                              :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      next-hop IP address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Figure 35.  SrcRoute

CIP Working Group [Page 95] RFC 1190 Internet Stream Protocol October 1990

          Since it is possible that a single hop between ST agents is
          actually composed of multiple IP hops using IP
          encapsulation, it might be necessary to also specify an IP
          source routing option.  Two additional PCodes are used in
          this case.  See [15] for a description of IP routing
          options.
          An IP Loose Source Route (PCode = 16) indicates that PDUs
          for the next-hop ST agent should be encapsulated in IP and
          that the IP datagram should contain an IP Loose Source Route
          constructed from the list of IP router addresses contained
          in this option.
          An IP Strict Source Route (PCode = 17) is similarly used
          when the corresponding IP Strict Source Route option should
          be constructed.
          Consequently, the "routing parameter" may consist of a
          sequence of one or more separate parameters with PCodes 16,
          17, 18, or 19.
       4.2.2.15.        Target and TargetList
          Several control messages use a parameter called TargetList
          (PCode = 20), which contains information about the targets
          to which the message pertains.  For each Target in the
          TargetList, the information includes the IP addresses of the
          target, the SAP applicable to the next higher layer
          protocol, the length of the SAP (SAPBytes), and zero or more
          optional SrcRoute parameters;  see Section 4.2.2.14 (page
          95).  Consequently, a Target structure can be of variable
          length.  Each entry has the format shown in Figure 36.
          The optional SrcRoute parameter is only meaningful in a
          CONNECT messages;  if present in other messages, they are
          ignored.  Note that the presence of SrcRoute parameter(s)
          reduces the number of Targets that can be contained in a
          TargetList since the maximum size of a TargetList is 256
          bytes.  Consequently an implementation should be prepared to
          accept multiple TargetLists in a single message.
             TargetIPAddress is the IP Address of the Target.
             TargetBytes is the length of the Target structure,
             beginning with the TargetIPAddress and including any
             SrcRoute Parameter(s).
             SAPBytes is the length of the SAP, excluding any padding
             required to maintain 32-bit alignment.  I.e.,

CIP Working Group [Page 96] RFC 1190 Internet Stream Protocol October 1990

             there would be no padding required for SAPs with lengths
             of 2, 6, etc., bytes.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        TargetIPAddress                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  TargetBytes  |   SAPBytes    |                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-             -+-+-+-+-+-+-+-+-+
 :                              SAP              :    Padding    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     SrcRoute Parameter(s)                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         Figure 36.  Target
          We assume that the ST agents must know the maximum packet
          size of the networks to which they are connected (the MTU),
          and those maximum sizes will restrict the number of targets
          that can be specified in control messages.  We feel that
          this is not a serious drawback.  High bandwidth networks
          such as the Ethernet or the Terrestrial Wideband network
          support packet sizes large enough to allow well over one
          hundred targets to be specified, and we feel that
          conferences with a larger number of participants will not
          occur for quite some time.  Furthermore, we expect that
          future higher bandwidth networks will allow even larger
          packet sizes.  It may be desirable to send ST voice data
          packets in individual B-ISDN ATM cells, which are small, but
          network services on ATM will provide "adaptation layers" to
          implement network-level fragmentation that may be used to
          carry larger ST control messages.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   PCode = 20  |    PBytes     |        TargetCount = N        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                            Target 1                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                              ...                              :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                            Target N                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 37.  TargetList

CIP Working Group [Page 97] RFC 1190 Internet Stream Protocol October 1990

          If a message must pass across a network whose maximum packet
          size is too small, the message must be broken up into
          multiple messages, each of which carries part of the
          TargetList.  The function of the message can still be
          performed even if the message is so partitioned.  The effect
          in this partitioning is to compromise the performance, but
          still allows proper operation.  For example, if a CONNECT
          message were partitioned, the first CONNECT would establish
          the stream, and the rest of the CONNECTs would be processed
          as additions to the first.  The routing decisions might
          suffer, however, since they would be made on partial
          information.  Nevertheless, the stream would be created.
       4.2.2.16.        UserData
          The UserData parameter (PCode = 21) is an optional parameter
          that may be used by the next higher protocol or an
          application to convey arbitrary information to its peers.
          Note that since the size of control messages is limited by
          the smallest MTU in the path to the target(s), the maximum
          size of this parameter cannot be specified a priori.  If the
          parameter is too large for some network's MTU, a
          UserDataSize error will occur.  The parameter must be padded
          to a multiple of 32 bits.
             UserBytes specifies the number of valid UserInformation
             bytes.
             UserInformation is arbitrary data meaningful to the next
             higher protocol layer or application.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   PCode = 21  |    PBytes     |           UserBytes           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                        UserInformation        :    Padding    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 38.  UserData

CIP Working Group [Page 98] RFC 1190 Internet Stream Protocol October 1990

4.2.3. ST Control Message PDUs

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

CIP Working Group [Page 99] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.1.         ACCEPT
          ACCEPT (OpCode = 1) is issued by a target as a positive
          response to a CONNECT message.  It implies that the target
          is prepared to accept data from the origin along the stream
          that was established by the CONNECT.  The ACCEPT includes
          the FlowSpec that contains the cumulative information that
          was calculated by the intervening ST agents as the CONNECT
          made its way from the origin to the target, as well as any
          modifications made by the application at the target.  The
          ACCEPT is relayed by the ST agents from the target to the
          origin along the path established by the CONNECT but in the
          reverse direction.  The ACCEPT must be acknowledged with an
          ACK at each hop.
          The FlowSpec is not modified on this trip from the target
          back to the origin.  Since the cumulative FlowSpec
          information can be different for different targets, no
          attempt is made to combine the ACCEPTs from the various
          targets.  The TargetList included in each ACCEPT contains
          the IP address of only the target that issued the ACCEPT.
          Any SrcRoute parameters in the TargetList are ignored.
          Since an ACCEPT might be the first response from a next-hop
          on a control link (due to network reordering), the SVLId
          field may be the first source of the Virtual Link Identifier
          to be used in the RVLId field of subsequent control messages
          sent to that next-hop.
          When the FDx option has been selected to setup a second
          stream in the reverse direction, the ACCEPT will contain
          both RFlowSpec and RName parameters.  Each agent should
          update the state tables for the reverse stream with this
          information.
             TSR (bits 14 and 15) specifies the target's response for
             the use of data packet timestamps; see Section 4 (page
             76).  Its values and semantics are:
                00  Not implemented.
                01  No timestamps are permitted.
                10  Timestamps must always be present.
                11  Timestamps may optionally be present.
             Reference contains a number assigned by the agent sending
             the ACCEPT for use in the acknowledging ACK.
             LnkReference is the Reference number from the
             corresponding CONNECT or CHANGE.

CIP Working Group [Page 100] RFC 1190 Internet Stream Protocol October 1990

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 1   |     0     |TSR|           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |               0               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      FlowSpec Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     TargetList Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     RecordRoute Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      RFlowSpec Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         RName Parameter                       !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      UserData Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 39.  ACCEPT Control Message

CIP Working Group [Page 101] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.2.         ACK
          ACK (OpCode = 2) is used to acknowledge a request.  The
          Reference in the header is the Reference number of the
          control message being acknowledged.
          Since a ACK might be the first response from a next-hop on a
          control link, the SVLId field may be the first source of the
          Virtual Link Identifier to be used in the RVLId field of
          subsequent control messages sent to that next-hop.
             ReasonCode is usually NoError, but other possibilities
             exist, e.g., DuplicateIgn.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 2   |       0       |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |          ReasonCode           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               0                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 40.  ACK Control Message

CIP Working Group [Page 102] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.3.         CHANGE-REQUEST
          CHANGE-REQUEST (OpCode = 4) is used by an intermediate or
          target agent to request that the origin change the FlowSpec
          of an established stream.  The CHANGE-REQUEST message is
          propagated hop-by-hop to the origin, with an ACK at each
          hop.
          Any SrcRoute parameters in the targets of the TargetList are
          ignored.
             G (bit 8) is used to request a global, stream-wide
             change;  the TargetList parameter may be omitted when the
             G bit is specified.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 4   |G|      0      |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |               0               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                       FlowSpec Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     TargetList Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      UserData Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 41.  CHANGE-REQUEST Control Message

CIP Working Group [Page 103] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.4.         CHANGE
          CHANGE (OpCode = 3) is used to change the FlowSpec of an
          established stream.  Parameters are the same as for CONNECT
          but the TargetList is not required.  The CHANGE message is
          processed similarly to the CONNECT message, except that it
          travels along the path of an established stream.
          If the change to the FlowSpec is in a direction that makes
          fewer demands of the involved networks, then the change has
          a high probability of success along the path of the
          established stream.  Each ST agent receiving the CHANGE
          message makes the necessary requested changes to the network
          resource allocations, and if successful, propagates the
          CHANGE message along the established paths.  If the change
          cannot be made then the ST agent must recover using
          DISCONNECT and REFUSE messages as in the case of a network
          failure.  Note that a failure to change the resources
          requested for a specific target(s) should not cause other
          targets in the stream to be deleted.  The CHANGE must be
          ACKed.
          If the CHANGE is a result of a CHANGE-REQUEST the
          LnkReference field of the CHANGE will contain the value from
          the Reference field of the CHANGE-REQUEST.
          It is recommended that the origin only have one outstanding
          CHANGE per target;  if the application requests more that
          one to be outstanding at a time, it is the application's
          responsibility to deal with any sequencing problems that may
          arise.
          Any SrcRoute parameters in the targets of the
          TargetListParameter are ignored.
             G (bit 8) is used to request a global, stream-wide
             change;  the TargetList parameter may be omitted when the
             G bit is specified.

CIP Working Group [Page 104] RFC 1190 Internet Stream Protocol October 1990

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 3   |G|      0      |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |               0               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                       FlowSpec Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     TargetList Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      UserData Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 42.  CHANGE Control Message
       4.2.3.5.         CONNECT
          CONNECT (OpCode = 5) requests the setup of a new stream or
          an addition to or recovery of an existing stream.  Only the
          origin can issue the initial set of CONNECTs to setup a
          stream, and the first CONNECT to each next-hop is used to
          convey the initial suggestion for a HID.  If the stream's
          data packets will be sent to some set of next-hop ST agents
          by multicast then the CONNECTs to that set must suggest the
          same HID.  Otherwise, the HIDs in the various CONNECTs can
          be different.
          The CONNECT message must fit within the maximum allowable
          packet size (MTU) for the intervening network.  If a CONNECT
          message is too large, it must be fragmented into multiple
          CONNECT messages by partitioning the TargetList; see Section
          4.2 (page 77).  Any UserData parameter will be replicated in
          each fragment for delivery to all targets.

CIP Working Group [Page 105] RFC 1190 Internet Stream Protocol October 1990

          The next-hop can initially respond with any of the following
          five responses:
           1  ERROR-IN-REQUEST, which implies that the CONNECT was
              not valid and has been ignored,
           2  ACK, which implies that the CONNECT with the H bit not
              set was valid and is being processed,
           3  HID-APPROVE, which implies that the CONNECT with the
              H bit set was valid, and the suggested HID can be
              used or was deferred,
           4  HID-REJECT, which implies that the CONNECT with the H
              bit set was valid but the suggested HID cannot be
              used and another must be suggested in a subsequent
              HID-CHANGE message, or
           5  REFUSE, which implies that the CONNECT was valid but
              the included list of targets in the REFUSE cannot be
              processed for the stated reason.
          The next-hop will later relay back either an ACCEPT or
          REFUSE from each target not already specified in the REFUSE
          of case 5 above (note multiple targets may be included in a
          single REFUSE message).
          An intermediate ST agent that receives a CONNECT selects the
          next-hop ST agents, partitions the TargetList accordingly,
          reserves network resources in the direction toward the
          next-hop, updating the FlowSpec accordingly (see Section
          4.2.2.3 (page 81)), selects a proposed HID for each next-
          hop, and sends the resulting CONNECTs.
          If the intermediate ST agent that is processing a CONNECT
          fails to find a route to a target, then it responds with a
          REFUSE with the appropriate reason code.  If the next-hop to
          a target is by way of the network from which it received the
          CONNECT, then it sends a NOTIFY with the appropriate reason
          code (RouteBack).  In either case, the TargetList specifies
          the affected targets.  The intermediate ST agent will only
          route to and propagate a CONNECT to the targets for which it
          does not issue either an ERROR-IN-REQUEST or a REFUSE.

CIP Working Group [Page 106] RFC 1190 Internet Stream Protocol October 1990

          The processing of a received CONNECT message requires care
          to avoid routing loops that could result from delays in
          propagating routing information among ST agents.  If a
          received CONNECT contains a new Name, a new stream should be
          created (unless the Virtual Link Identifier matches a known
          link in which case an ERROR-IN-REQUEST should be sent).  If
          the Name is known, there are four cases:
           1  the Virtual Link Identifier matches and the Target
              matches a current Target -- the duplicate target
              should be ignored.
           2  the Virtual Link Identifier matches but the Target is
              new -- the stream should be expanded to include the
              new target.
           3  the Virtual Link Identifier differs and the Target
              matches a current Target -- an ERROR-IN-REQUEST
              message should be sent specifying that the target is
              involved in a routing loop.  If a reroute, the old
              path will eventually timeout and send a DISCONNECT;
              a subsequent retransmission of the rerouted CONNECT
              will then be processed under case 2 above.
           4  the Virtual Link Identifier differs but the Target is
              new -- a new (instance of the) stream should be
              created for the target that is deliberately part of
              a loop using a SrcRoute parameter.
          Note that the test for a known or matching Target includes
          comparing any SrcRoute parameter that might be present.
          Option bits are specified by either the origin's service
          user or by an intermediate agent, depending on the specific
          option.  Bits not specified below are currently unspecified,
          and should be set to zero (0) by the origin agent and not
          changed by other agents unless those agents know their
          meaning.
             H (bit 8) is used for the HID Field option; see Section
             3.6.1 (page 44).  It is set to one (1) only if the HID
             field contains either zero (when the HID selection is
             being deferred), or the proposed HID.  This bit is zero
             (0) if the HID field does not contain valid data and
             should be ignored.
             P (bit 9) is used for the PTP option; see Section 3.6.2
             (page 44).
             S (bit 10) is used for the NoRecovery option; see Section
             3.6.4 (page 46).

CIP Working Group [Page 107] RFC 1190 Internet Stream Protocol October 1990

             TSP (bits 14 and 15) specifies the origin's proposal for
             the use of data packet timestamps; see Section 4 (page
             76).  Its values and semantics are:
                00  No proposal.
                01  Cannot insert timestamps.
                10  Must always insert timestamps.
                11  Can insert timestamps if requested.
             RVLId, the receiver's Virtual Link Identifier, is set to
             zero in all CONNECT messages until its value arrives in
             the SVLId field of an acknowledgment to the CONNECT.
             SVLId, the sender's Virtual Link Identifier, is set to a
             value chosen by each hop to facilitate efficient
             dispatching of subsequent control messages.
             HID is the identifier that will be used with data packets
             moving through the stream in the direction from the
             origin to the targets.  It is a hop-by-hop shorthand
             identifier for the stream's Name, and is chosen by each
             agent for the branch to the next-hop agents.  The
             contents of the HID field are only valid, and a HID-
             REJECT or HID-APPROVE reply may only be sent, when the
             HID Field option (H bit) is set (1).  If the HID Field
             option is specified and the proposed HID is zero, the
             selection of the HID is deferred to the receiving next-
             hop agent.  If the HID Field option is not set (H bit is
             0), then the HID field does not contain valid data and
             should be ignored;  see Section 3.6.1 (page 44).
             TargetList is the list of IP addresses of the target
             processes.  It is of arbitrary size up to the maximum
             allowed for packets traveling across the specific
             network.

CIP Working Group [Page 108] RFC 1190 Internet Stream Protocol October 1990

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 5   |H|P|S|  0  |TSP|           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            RVLId/0            |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |             HID/0             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                       Origin Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      FlowSpec Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      TargetList Parameter(s)                  :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                        Group Parameter                        :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                   MulticastAddress Parameter                  :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     RecordRoute Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      RFlowSpec Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                        RGroup Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                        RHID Parameter                         !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      UserData Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 43.  CONNECT Control Message

CIP Working Group [Page 109] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.6.         DISCONNECT
          DISCONNECT (OpCode = 6) is used by an origin to tear down an
          established stream or part of a stream, or by an
          intermediate agent that detects a failure between itself and
          its previous-hop, as distinguished by the ReasonCode.  The
          DISCONNECT message specifies the list of targets that are to
          be disconnected.  An ACK is required in response to a
          DISCONNECT message.  The DISCONNECT message is propagated
          all the way to the specified targets.  The targets are
          expected to terminate their participation in the stream.
          Note that in the case of a failure it may be advantageous to
          retain state information as the stream should be repaired
          shortly;  see Section 3.7.2 (page 52).
             G (bit 8) is used to request a DISCONNECT of all the
             stream's targets; the TargetList parameter may be omitted
             when the G bit is set (1).
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 6   |G|      0      |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |          ReasonCode           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     TargetList Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      UserData Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 44.  DISCONNECT Control Message

CIP Working Group [Page 110] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.7.         ERROR-IN-REQUEST
          ERROR-IN-REQUEST (OpCode = 7) is sent in acknowledgment to a
          request in which an error is detected.  No action is taken
          on the erroneous request and no state information for the
          stream is retained.  Consequently it is appropriate for the
          SVLId to be zero (0).  No ACK is expected.
          An ERROR-IN-REQUEST is never sent in response to either an
          ERROR-IN-REQUEST or an ERROR-IN-RESPONSE;  however, the
          event should be logged for diagnostic purposes.  The
          receiver of an ERROR-IN-REQUEST is encouraged to try again
          without waiting for a retransmission timeout.
             Reference is the Reference number of the erroneous
             request.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 7   |       0       |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |            SVLId/0            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |          ReasonCode           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                          ErroredPDU                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      TargetList Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 45.  ERROR-IN-REQUEST Control Message

CIP Working Group [Page 111] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.8.         ERROR-IN-RESPONSE
          ERROR-IN-RESPONSE (OpCode = 8) is sent in acknowledgment to
          a response in which an error is detected.  No ACK is
          expected.  Action taken by the requester and responder will
          vary with the nature of the request.
          An ERROR-IN-REQUEST is never sent in response to either an
          ERROR-IN-REQUEST or an ERROR-IN-RESPONSE;  however, the
          event should be logged for diagnostic purposes.  The
          receiver of an ERROR-IN-RESPONSE is encouraged to try again
          without waiting for a retransmission timeout.
          Reference identifies the erroneous response.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 8   |       0       |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |          ReasonCode           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                          ErroredPDU                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      TargetList Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 46.  ERROR-IN-RESPONSE Control Message

CIP Working Group [Page 112] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.9.         HELLO
          HELLO (OpCode = 9) is used as part of the ST failure
          detection mechanism; see Section 3.7.1.2 (page 49).
             R (bit 8) is used for the Restarted bit.
             Reference is non-zero to inform the receiver that an ACK
             should be promptly sent so that the sender can update its
             round-trip time estimates.  If the Reference is zero, no
             ACK should be sent.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 9   |R|      0      |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            RVLId/0            |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Reference/0          |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |               0               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          HelloTimer                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                        OriginTimestamp                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 47.  HELLO Control Message

CIP Working Group [Page 113] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.10.        HID-APPROVE
          HID-APPROVE (OpCode = 10) is used by the agent that is
          responding to either a CONNECT or HID-CHANGE to agree to
          either use the proposed HID or to the addition or deletion
          of the specified HID.  In all cases but deletion, the newly
          approved HID is returned in the HID field;  for deletion,
          the HID field must be set to zero.  The HID-APPROVE is the
          acknowledgment of a CONNECT or HID-CHANGE.
          The optional FreeHIDs parameter provides the previous-hop
          agent with hints about what other HIDs are acceptable in
          case a multicast HID is being negotiated;  see Section
          4.2.2.4 (page 84).
          Since a HID-APPROVE might be the first response from a
          next-hop on a control link, the SVLId field may be the first
          source of the Virtual Link Identifier to be used in the
          RVLId field of subsequent control messages sent to that
          next-hop.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 10  |       0       |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |              HID              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               0                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      FreeHIDs Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 48.  HID-APPROVE Control Message

CIP Working Group [Page 114] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.11.        HID-CHANGE-REQUEST
          HID-CHANGE-REQUEST (OpCode = 12) is used by a next-hop agent
          that would like, for administrative reasons, to change the
          HID that is in use.  The receiving previous-hop agent
          acknowledges the request by either an ERROR-IN-REQUEST if it
          is unwilling to make the requested change, or with a HID-
          CHANGE if it can accommodate the request.
             A (bit 8) is used to indicate that the specified HID
             should be included in the set of HIDs for the specified
             Name.  When a HID is added, the acknowledging HID-APPROVE
             should contain a HID field whose contents is the HID just
             added.
             D (bit 9) is used to indicate that the specified HID
             should be removed in the set of HIDs for the specified
             Name.  When a HID is deleted, the acknowledging HID-
             APPROVE should contain a HID field whose contents is
             zero.  Note that the Reference field may be used to
             determine the HID that has been deleted.
             If neither bit is set, the specified HID should replace
             that currently in use with the specified Name.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 12  |A|D|     0     |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |              HID              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               0                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 49.  HID-CHANGE-REQUEST Control Message

CIP Working Group [Page 115] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.12.        HID-CHANGE
          HID-CHANGE (OpCode = 11) is used by the agent that issued a
          CONNECT and received a HID-REJECT to attempt to negotiate a
          suitable HID.  The HID in the HID-CHANGE message must be
          different from that in the CONNECT, or any previous HID-
          CHANGE messages for the given Name.  The agent receiving the
          HID-CHANGE must respond with a HID-APPROVE if the new HID is
          suitable, or a HID-REJECT if it is not.  In case of an
          error, either an ERROR-IN-REQUEST or a REFUSE may be
          returned as an acknowledgment.
          Since an agent may send CONNECT messages with the same HID
          to several next-hops in order to use multicast data
          transfer, any HID-CHANGE must also be sent to the same set
          of next-hops.  Therefore, a next-hop agent must be prepared
          to receive a HID-CHANGE before or after it has sent a HID-
          APPROVE response to the CONNECT or a previous HID-CHANGE.
          Only the last HID-CHANGE is relevant.  The previous-hop
          agent will ignore HID-APPROVE or HID-REJECT messages to
          previous CONNECT or HID-CHANGE messages.
          A DISCONNECT can be sent instead of a HID-CHANGE, or a
          REFUSE can be sent instead of a HID-APPROVE or HID-REJECT,
          to terminate fatally the HID negotiation and the agent's
          knowledge of the stream.
          The A and D bits are used to change a HID, e.g., when adding
          a new next-hop to a multicast group, in such a way that data
          packets that are flowing through the network will not be
          mishandled due to a race condition in processing the HID-
          CHANGE messages between the previous-hop and its next-hops.
          An implementation may choose to limit the number of
          simultaneous HIDs associated with a stream, but must allow
          at least two.
             A (bit 8) is used to indicate that the specified HID
             should be included in the set of HIDs for the specified
             Name.  When a HID is added, the acknowledging HID-APPROVE
             should contain a HID field whose contents is the HID just
             added.
             D (bit 9) is used to indicate that the specified HID
             should be removed from the set of HIDs for the specified
             Name.  When a HID is deleted, the acknowledging HID-
             APPROVE should contain a HID field whose contents is
             zero.  Note that the Reference field may be used to
             determine the HID that has been deleted.
             If neither bit is set, the specified HID should replace
             that currently in use for the specified Name.

CIP Working Group [Page 116] RFC 1190 Internet Stream Protocol October 1990

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 11  |A|D|     0     |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |              HID              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               0                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 50.  HID-CHANGE Control Message

CIP Working Group [Page 117] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.13.        HID-REJECT
          HID-REJECT (OpCode = 13) is used as an acknowledgment that a
          CONNECT or HID-CHANGE was received and is being processed,
          but means that the HID contained in the CONNECT or HID-
          CHANGE is not acceptable.  Upon receipt of this message the
          agent that issued the CONNECT or HID-CHANGE must now issue a
          HID-CHANGE to attempt to find a suitable HID.  The HID-
          CHANGE can cause another HID-REJECT but eventually the HID-
          CHANGE must be acknowledged with a HID-APPROVE to end
          successfully the HID negotiation.  The agent that issued the
          HID-REJECT may not issue an ACCEPT before it has found an
          acceptable HID.
          Since a HID-REJECT might be the first response from a next-
          hop on a control link, the SVLId field may be the first
          source of the Virtual Link Identifier to be used in the
          RVLId field of subsequent control messages sent to that
          next-hop.
          Either agent may terminate the negotiation by issuing either
          a DISCONNECT or a REROUTE.  The agent that issued the HID-
          REJECT may issue a REFUSE, or REROUTE at any time after the
          HID-REJECT.  In this case, the stream cannot be created, the
          HID negotiation need not proceed, and the previous-hop need
          not transmit any further messages;  any further messages
          that are received should be ignored.
          The optional FreeHIDs parameter provides the previous-hop
          agent with hints about what HIDs would have been acceptable;
          see Section 4.2.2.4 (page 84).

CIP Working Group [Page 118] RFC 1190 Internet Stream Protocol October 1990

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 13  |       0       |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |          RejectedHID          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               0                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      FreeHIDs Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 51.  HID-REJECT Control Message

CIP Working Group [Page 119] 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.

CIP Working Group [Page 120] RFC 1190 Internet Stream Protocol October 1990

             NextHopIPAddress is an optional parameter and contains
             the IP address of a suggested next-hop ST agent.
             TargetList is present when the notification is related to
             one or more targets.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 14  |       0       |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |          ReasonCode           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                          ErroredPDU                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      FlowSpec Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         HID Parameter                         !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                  NextHopIPAddress Parameter                   !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     RecordRoute Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      TargetList Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 52.  NOTIFY Control Message

CIP Working Group [Page 121] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.15.        REFUSE
          REFUSE (OpCode = 15) is issued by a target that either does
          not wish to accept a CONNECT message or wishes to remove
          itself from an established stream.  It might also be issued
          by an intermediate agent in response to a CONNECT or CHANGE
          either to terminate fatally a failing HID negotiation, to
          terminate a routing loop, or when a satisfactory next-hop to
          a target cannot be found.  It may also be a separate command
          when an existing stream has been preempted by a higher
          precedence stream or an agent detects the failure of a
          previous-hop, next-hop, or the network between them.  In all
          cases, the TargetList specifies the targets that are
          affected by the condition.  Each REFUSE must be acknowledged
          by an ACK.
          The REFUSE is relayed by the agents from the originating
          agent to the origin (or intermediate agent that created the
          CONNECT or CHANGE) along the path traced by the CONNECT.
          The agent receiving the REFUSE will process it differently
          depending on the condition that caused it, as specified in
          the ReasonCode field.  In some cases, such as if a next-hop
          cannot obtain resources, the agent can release any resources
          reserved exclusively for transmissions in the stream in
          question to the target specified in the TargetList, and the
          previous-hop can attempt to find an alternate route.  In
          some cases, such as a routing failure, the previous-hop
          cannot determine where the failure occurred, and must
          propagate the REFUSE back to the origin, which can attempt
          recovery of the stream by issuing a new CONNECT.
          No special effort is made to combine multiple REFUSE
          messages since it is considered most unlikely that separate
          REFUSEs will happen to both pass through an agent at the
          same time and be easily combined, e.g., have identical
          ReasonCodes and parameters.
          Since a REFUSE might be the first response from a next-hop
          on a control link, the SVLId field may be the first source
          of the Virtual Link Identifier to be used in the RVLId field
          of subsequent control messages sent to that next-hop.
             Reference contains a number assigned by the agent sending
             the REFUSE for use in the acknowledging ACK.
             LnkReference is either the Reference number from the
             corresponding CONNECT or CHANGE, if it is the result of
             such a message, or zero when the REFUSE was originated as
             a separate command.

CIP Working Group [Page 122] RFC 1190 Internet Stream Protocol October 1990

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 15  |       0       |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             RVLId             |             SVLId             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |          ReasonCode           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       DetectorIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                        Name Parameter                         !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     TargetList Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                          ErroredPDU                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                     RecordRoute Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      UserData Parameter                       :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 53.  REFUSE Control Message

CIP Working Group [Page 123] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.16.        STATUS
          STATUS (OpCode = 16) is used to inquire about the existence
          of a particular stream identified by either a HID (H bit
          set) or Name (Name Parameter present).
          When a stream has been identified, a STATUS-RESPONSE is
          returned that will contain the specified HID and/or Name but
          no other parameters if the specified stream is unknown, or
          will otherwise contain the current HID(s), Name, FlowSpec,
          TargetList, and possibly Group(s) of the stream.  Note that
          if a stream has no current HID, the HID field in the
          STATUS-RESPONSE will contain zero;  it will contain the
          first, or only, HID if a valid HID exists; additional valid
          HIDs will be returned in HID parameters.
          Use of STATUS is intended for diagnostic purposes and to
          assist in stream cleanup operations.  Note that if both a
          HID and Name are specified, but they do not correspond to
          the same stream, an ERROR-IN-REQUEST with the appropriate
          reason code (InconsistHID) would be returned.
          It is possible in cases of multiple failures or network
          partitioning for an ST agent to have information about a
          stream after the stream has either ceased to exist or has
          been rerouted around the agent.  When an agent concludes
          that a stream has not been used for a period of time and
          might no longer be valid, it can probe the stream's
          previous-hop or next-hop(s) to see if they believe that the
          stream still exists through the interrogating agent.  If
          not, those hops would reply with a STATUS-RESPONSE that
          contains the HID and/or Name but no other parameters;
          otherwise, if the stream is still valid, the hops would
          reply with the parameters of the stream.
             H (bit 8) is used to indicate whether (when 1) or not
             (when 0) a HID is present in the HID field.
             Q (bit 9) is set to one (1) for remote diagnostic
             purposes when the receiving agent should return a
             stream's parameters, whether or not the source of the
             message is believed to be a previous-hop or next-hop in
             the specified stream.  Note that this use has potential
             for disclosure of sensitive information.
             RVLId and SVLId may either or both be zero when STATUS is
             used for diagnostic purposes.

CIP Working Group [Page 124] RFC 1190 Internet Stream Protocol October 1990

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 16  |H|Q|     0     |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            RVLId/0            |            SVLId/0            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |             HID/0             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               0                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 54.  STATUS Control Message

CIP Working Group [Page 125] RFC 1190 Internet Stream Protocol October 1990

       4.2.3.17.        STATUS-RESPONSE
          STATUS-RESPONSE (OpCode = 17) is the reply to a STATUS
          message.  If the stream specified in the STATUS message is
          not known, the STATUS-RESPONSE will contain the specified
          HID and/or Name but no other parameters.  It will otherwise
          contain the current HID(s), Name, FlowSpec, TargetList, and
          possibly Group of the stream.  Note that if a stream has no
          current HID, the H bit in the STATUS-RESPONSE will be zero.
          The HID field will contain the first, or only, HID if a
          valid HID exists; additional valid HIDs will be returned in
          HID parameters.
             H (bit 8) is used to indicate whether (when 1) or not
             (when 0) a HID is present in the HID field.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  OpCode = 17  |H|Q|     0     |           TotalBytes          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            RVLId/0            |            SVLId/0            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reference           |         LnkReference          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SenderIPAddress                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Checksum           |             HID/0             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               0                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         Name Parameter                        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                       FlowSpec Parameter                      :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                        Group Parameter                        :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                         HID Parameter                         !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                      TargetList Parameter                     :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 55.  STATUS-RESPONSE Control Message

CIP Working Group [Page 126] RFC 1190 Internet Stream Protocol October 1990

 4.3.       Suggested Protocol Constants
    The ST Protocol uses several fields that must have specific values
    for the protocol to work, and also several values that an
    implementation must select.  This section specifies the required
    values and suggests initial values for others.  It is recommended
    that the latter be implemented as variables so that they may be
    easily changed when experience indicates better values.
    Eventually, they should be managed via the normal network
    management facilities.
    ST uses IP Version Number 5.
    When encapsulated in IP, ST uses IP Protocol Number 5.
     Value  ST Command Message Name       Value     ST Element Name
    ------- -----------------------      ------- ---------------------
       1    ACCEPT                          1    ErroredPDU
       2    ACK                             2    FlowSpec
       3    CHANGE                          3    FreeHIDs
       4    CHANGE-REQUEST                  4    Group
       5    CONNECT                         5    HID
       6    DISCONNECT                      6    MulticastAddress
       7    ERROR-IN-REQUEST                7    Name
       8    ERROR-IN-RESPONSE               8    NextHopIPAddress
       9    HELLO                           9    Origin
      10    HID-APPROVE                    10    OriginTimestamp
      11    HID-CHANGE                     11    RecordRoute
      12    HID-CHANGE-REQUEST             12    RFlowSpec
      13    HID-REJECT                     13    RGroup
      14    NOTIFY                         14    RHID
      15    REFUSE                         15    RName
      16    STATUS                         16    SrcRoute, IP Loose
      17    STATUS-RESPONSE                17    SrcRoute, IP Strict
                                           18    SrcRoute, ST Loose
                                           19    SrcRoute, ST Strict
                                           20    TargetList
                                           21    UserData
    A good choice for the minimum number of bits in the FreeHIDBitMask
    element of the FreeHIDs parameter is not yet known.  We suggest a
    minimum of 64 bits, i.e., N in Figure 25 has a value of two (2).
    HID value zero (0) is reserved for ST Control Messages.  HID
    values 1-3 are reserved for future use.

CIP Working Group [Page 127] RFC 1190 Internet Stream Protocol October 1990

    VLId value zero (0) may only be used in the RVLId field of an ST
    Control Message when the appropriate value has not yet been
    received from the other end of the virtual link;' except for an
    ERROR-IN-REQUEST or diagnostic message, the SVLId field may never
    contain a value of zero except in a diagnostic message.  VLId
    value 1 is reserved for use with HELLO messages by those agents
    whose implementation wishes to have all HELLOs so identified.
    VLId values 2-3 are reserved for future use.
    The following permanent IP multicast addresses have been assigned
    to ST:
       224.0.0.7    All ST routers
       224.0.0.8    All ST hosts
    In addition, a block of transient IP multicast addresses,
    224.1.0.0 - 224.1.255.255, has been allocated for ST multicast
    groups.  Note that in the case of Ethernet, an ST Multicast
    address of 224.1.cc.dd maps to an Ethernet Multicast address of
    01:00:5E:01:cc:dd (see [6]).
    SCMP uses retransmission to effect reliability and thus has
    several "retransmission timers".  Each "timer" is modeled by an
    initial time interval (ToXxx), which gets updated dynamically
    through measurement of control traffic, and a number of times
    (NXxx) to retransmit a message before declaring a failure.  All
    time intervals are in units of milliseconds.
     Value   Timeout  Name                      Meaning
    ------- ---------------------- ----------------------------------
      1000  ToAccept               Initial hop-by-hop timeout for
                                   acknowledgment of ACCEPT
         3  NAccept                ACCEPT retries before failure
      1000  ToConnect              Initial hop-by-hop timeout for
                                   acknowledgment of CONNECT
         5  NConnect               CONNECT retries before failure
      1000  ToDisconnect           Initial hop-by-hop timeout for
                                   acknowledgment of DISCONNECT
        3   NDisconnect            DISCONNECT retries before
                                   failure

CIP Working Group [Page 128] RFC 1190 Internet Stream Protocol October 1990

     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

CIP Working Group [Page 129] RFC 1190 Internet Stream Protocol October 1990

     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

CIP Working Group [Page 130] RFC 1190 Internet Stream Protocol October 1990

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

CIP Working Group [Page 131] RFC 1190 Internet Stream Protocol October 1990

 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

CIP Working Group [Page 132] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 133] RFC 1190 Internet Stream Protocol October 1990

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CIP Working Group [Page 134] RFC 1190 Internet Stream Protocol October 1990

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.)

CIP Working Group [Page 135] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 136] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 137] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 138] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 139] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 140] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 141] RFC 1190 Internet Stream Protocol October 1990

 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.

CIP Working Group [Page 142] RFC 1190 Internet Stream Protocol October 1990

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.

CIP Working Group [Page 143] RFC 1190 Internet Stream Protocol October 1990

 [12] Mallory, T., and A. Kullberg, "Incremental Updating of the
      Internet Checksum", RFC 1141, BBN Communications
      Corporation, January 1990.
 [13] Mills, D., "Network Time Protocol (Version 2) Specification
      and Implementation", RFC 1119, University of Delaware,
      September 1989 (Revised February 1990).
 [14] Pope, A., "The SIMNET Network and Protocols", BBN
      Report No. 7102, BBN Systems and Technologies, July 1989.
 [15] Postel, J., ed., "Internet Protocol - DARPA Internet Program
      Protocol Specification", RFC 791, DARPA, September 1981.
 [16] Postel, J., ed., "Transmission Control Protocol - DARPA
      Internet Program Protocol Specification", RFC 793, DARPA,
      September 1981.
 [17] Postel, J., "User Datagram Protocol", RFC 768,
      USC/Information Sciences Institute, August 1980.
 [18] Reynolds, J., Postel, J., "Assigned Numbers", RFC 1060,
      USC/Information Sciences Institute, March 1990.
 [19] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
      SDNS Secure Data Network System, Security Protocol 3 (SP3),
      SDN.301, Rev. 1.5, 1989-05-15.
 [20] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
      SDNS Secure Data Network System, Security Protocol 3 (SP3)
      Addendum 1, Cooperating Families, SDN.301.1, Rev. 1.2,
      1988-07-12.

8. Security Considerations

 See section 3.7.8.

CIP Working Group [Page 144] RFC 1190 Internet Stream Protocol October 1990

9. Authors' Addresses

    Stephen Casner
    USC/Information Sciences Institute
    4676 Admiralty Way
    Marina del Rey, CA 90292-6695
    Phone: (213) 822-1511 x153
    EMail: 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

CIP Working Group [Page 145] RFC 1190 Internet Stream Protocol October 1990

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CIP Working Group [Page 146] RFC 1190 Internet Stream Protocol October 1990

Appendix 1. Data Notations

 The convention in the documentation of Internet Protocols is to
 express numbers in decimal and to picture data with the most
 significant octet on the left and the least significant octet on the
 right.
 The order of transmission of the header and data described in this
 document is resolved to the octet level.  Whenever a diagram shows a
 group of octets, the order of transmission of those octets is the
 normal order in which they are read in English.  For example, in the
 following diagram the octets are transmitted in the order they are
 numbered.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       1       |       2       |       3       |       4       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       5       |       6       |       7       |       8       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       9       |      10       |      11       |      12       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 56.  Transmission Order of Bytes
 Whenever an octet represents a numeric quantity the left most bit in
 the diagram is the high order or most significant bit.  That is, the
 bit labeled 0 is the most significant bit.  For example, the
 following diagram represents the value 170 (decimal).
                          0 1 2 3 4 5 6 7
                         +-+-+-+-+-+-+-+-+
                         |1 0 1 0 1 0 1 0|
                         +-+-+-+-+-+-+-+-+
                  Figure 57.  Significance of Bits
 Similarly, whenever a multi-octet field represents a numeric quantity
 the left most bit of the whole field is the most significant bit.
 When a multi-octet quantity is transmitted the most significant octet
 is transmitted first.
 Fields whose length is fixed and fully illustrated are shown with a
 vertical bar (|) at the end;  fixed fields whose contents are
 abbreviated are shown with an exclamation point (!);  variable fields
 are shown with colons (:).

CIP Working Group [Page 147] RFC 1190 Internet Stream Protocol October 1990

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

CIP Working Group [Page 148]

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