path: root/doc/rfc/rfc1819.txt

Network Working Group                                  ST2 Working Group
Request for Comments: 1819           L. Delgrossi and L. Berger, Editors
Obsoletes: 1190, IEN 119                                     August 1995
Category: Experimental


                Internet Stream Protocol Version 2 (ST2)
                 Protocol Specification - Version ST2+

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

IESG NOTE

   This document is a revision of RFC1190. The charter of this effort
   was clarifying, simplifying and removing errors from RFC1190 to
   ensure interoperability of implementations.

   NOTE WELL: Neither the version of the protocol described in this
   document nor the previous version is an Internet Standard or under
   consideration for that status.

   Since the publication of the original version of the protocol, there
   have been significant developments in the state of the art.  Readers
   should note that standards and technology addressing alternative
   approaches to the resource reservation problem are currently under
   development within the IETF.

Abstract

   This memo contains a revised specification of the Internet STream
   Protocol Version 2 (ST2). ST2 is an experimental resource reservation
   protocol intended to provide end-to-end real-time guarantees over an
   internet. It allows applications to build multi-destination simplex
   data streams with a desired quality of service. The revised version
   of ST2 specified in this memo is called ST2+.

   This specification is a product of the STream Protocol Working Group
   of the Internet Engineering Task Force.








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Table of Contents

     1  Introduction                                                   6
             1.1  What is ST2?                                         6
             1.2  ST2 and IP                                           8
             1.3  Protocol History                                     8
             1.3.1  RFC1190 ST and ST2+ Major Differences              9
             1.4  Supporting Modules for ST2                          10
             1.4.1  Data Transfer Protocol                            11
             1.4.2  Setup Protocol                                    11
             1.4.3  Flow Specification                                11
             1.4.4  Routing Function                                  12
             1.4.5  Local Resource Manager                            12
             1.5  ST2 Basic Concepts                                  15
             1.5.1  Streams                                           16
             1.5.2  Data Transmission                                 16
             1.5.3  Flow Specification                                17
             1.6  Outline of This Document                            19

     2  ST2 User Service Description                                  19
             2.1  Stream Operations and Primitive Functions           19
             2.2  State Diagrams                                      21
             2.3  State Transition Tables                             25

     3  The ST2 Data Transfer Protocol                                26
             3.1  Data Transfer with ST                               26
             3.2  ST Protocol Functions                               27
             3.2.1  Stream Identification                             27
             3.2.2  Packet Discarding based on Data Priority          27

     4  SCMP Functional Description                                   28
             4.1  Types of Streams                                    29
             4.1.1  Stream Building                                   30
             4.1.2  Knowledge of Receivers                            30
             4.2  Control PDUs                                        31
             4.3  SCMP Reliability                                    32
             4.4  Stream Options                                      33
             4.4.1  No Recovery                                       33
             4.4.2  Join Authorization Level                          34
             4.4.3  Record Route                                      34
             4.4.4  User Data                                         35
             4.5  Stream Setup                                        35
             4.5.1  Information from the Application                  35
             4.5.2  Initial Setup at the Origin                       35
             4.5.2.1  Invoking the Routing Function                   36
             4.5.2.2  Reserving Resources                             36
             4.5.3  Sending CONNECT Messages                          37
             4.5.3.1  Empty Target List                               37



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             4.5.4  CONNECT Processing by an Intermediate ST agent    37
             4.5.5  CONNECT Processing at the Targets                 38
             4.5.6  ACCEPT Processing by an Intermediate ST agent     38
             4.5.7  ACCEPT Processing by the Origin                   39
             4.5.8  REFUSE Processing by the Intermediate ST agent    39
             4.5.9  REFUSE Processing by the Origin                   39
             4.5.10  Other Functions during Stream Setup              40
             4.6  Modifying an Existing Stream                        40
             4.6.1  The Origin Adding New Targets                     41
             4.6.2  The Origin Removing a Target                      41
             4.6.3  A Target Joining a Stream                         42
             4.6.3.1  Intermediate Agent (Router) as Origin           43
             4.6.4  A Target Deleting Itself                          43
             4.6.5  Changing a Stream's FlowSpec                      44
             4.7  Stream Tear Down                                    45

     5  Exceptional Cases                                             45
             5.1  Long ST Messages                                    45
             5.1.1  Handling of Long Data Packets                     45
             5.1.2  Handling of Long Control Packets                  46
             5.2  Timeout Failures                                    47
             5.2.1  Failure due to ACCEPT Acknowledgment Timeout      47
             5.2.2  Failure due to CHANGE Acknowledgment Timeout      47
             5.2.3  Failure due to CHANGE Response Timeout            48
             5.2.4  Failure due to CONNECT Acknowledgment Timeout     48
             5.2.5  Failure due to CONNECT Response Timeout           48
             5.2.6  Failure due to DISCONNECT Acknowledgment Timeout  48
             5.2.7  Failure due to JOIN Acknowledgment Timeout        48
             5.2.8  Failure due to JOIN Response Timeout              49
             5.2.9  Failure due to JOIN-REJECT Acknowledgment Timeout 49
             5.2.10  Failure due to NOTIFY Acknowledgment Timeout     49
             5.2.11  Failure due to REFUSE Acknowledgment Timeout     49
             5.2.12  Failure due to STATUS Response Timeout           49
             5.3  Setup Failures due to Routing Failures              50
             5.3.1  Path Convergence                                  50
             5.3.2  Other Cases                                       51
             5.4  Problems due to Routing Inconsistency               52
             5.5  Problems in Reserving Resources                     53
             5.5.1  Mismatched FlowSpecs                              53
             5.5.2  Unknown FlowSpec Version                          53
             5.5.3  LRM Unable to Process FlowSpec                    53
             5.5.4  Insufficient Resources                            53
             5.6  Problems Caused by CHANGE Messages                  54
             5.7  Unknown Targets in DISCONNECT and CHANGE            55







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     6  Failure Detection and Recovery                                55
             6.1  Failure Detection                                   55
             6.1.1  Network Failures                                  56
             6.1.2  Detecting ST Agents Failures                      56
             6.2  Failure Recovery                                    58
             6.2.1  Problems in Stream Recovery                       60
             6.3  Stream Preemption                                   62

     7  A Group of Streams                                            63
             7.1  Basic Group Relationships                           63
             7.1.1  Bandwidth Sharing                                 63
             7.1.2  Fate Sharing                                      64
             7.1.3  Route Sharing                                     65
             7.1.4  Subnet Resources Sharing                          65
             7.2  Relationships Orthogonality                         65

     8  Ancillary Functions                                           66
             8.1  Stream ID Generation                                66
             8.2  Group Name Generator                                66
             8.3  Checksum Computation                                67
             8.4  Neighbor ST Agent Identification and
                     Information Collection                           67
             8.5  Round Trip Time Estimation                          68
             8.6  Network MTU Discovery                               68
             8.7  IP Encapsulation of ST                              69
             8.8  IP Multicasting                                     70

     9  The ST2+ Flow Specification                                   71
             9.1  FlowSpec Version #0 - (Null FlowSpec)               72
             9.2  FlowSpec Version #7 - ST2+ FlowSpec                 72
             9.2.1  QoS Classes                                       73
             9.2.2  Precedence                                        74
             9.2.3  Maximum Data Size                                 74
             9.2.4  Message Rate                                      74
             9.2.5  Delay and Delay Jitter                            74
             9.2.6  ST2+ FlowSpec Format                              75

     10  ST2 Protocol Data Units Specification                        77
             10.1  Data PDU                                           77
             10.1.1  ST Data Packets                                  78
             10.2  Control PDUs                                       78
             10.3  Common SCMP Elements                               80
             10.3.1  FlowSpec                                         80
             10.3.2  Group                                            81
             10.3.3  MulticastAddress                                 82
             10.3.4  Origin                                           82
             10.3.5  RecordRoute                                      83
             10.3.6  Target and TargetList                            84



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             10.3.7  UserData                                         85
             10.3.8  Handling of Undefined Parameters                 86
             10.4  ST Control Message PDUs                            86
             10.4.1  ACCEPT                                           86
             10.4.2  ACK                                              88
             10.4.3  CHANGE                                           89
             10.4.4  CONNECT                                          89
             10.4.5  DISCONNECT                                       92
             10.4.6  ERROR                                            93
             10.4.7  HELLO                                            94
             10.4.8  JOIN                                             95
             10.4.9  JOIN-REJECT                                      96
             10.4.10  NOTIFY                                          97
             10.4.11  REFUSE                                          98
             10.4.12  STATUS                                         100
             10.4.13  STATUS-RESPONSE                                100
             10.5  Suggested Protocol Constants                      101
             10.5.1  SCMP Messages                                   102
             10.5.2  SCMP Parameters                                 102
             10.5.3  ReasonCode                                      102
             10.5.4  Timeouts and Other Constants                    104
             10.6  Data Notations                                    105
     11  References                                                  106
     12  Security Considerations                                     108
     13  Acknowledgments and Authors' Addresses                      108


























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1.  Introduction

1.1  What is ST2?

   The Internet Stream Protocol, Version 2 (ST2) is an experimental
   connection-oriented internetworking protocol that operates at the
   same layer as connectionless IP. It has been developed to support the
   efficient delivery of data streams to single or multiple destinations
   in applications that require guaranteed quality of service. ST2 is
   part of the IP protocol family and serves as an adjunct to, not a
   replacement for, IP. The main application areas of the protocol are
   the real-time transport of multimedia data, e.g., digital audio and
   video packet streams, and distributed simulation/gaming, across
   internets.

   ST2 can be used to reserve bandwidth for real-time streams across
   network routes. This reservation, together with appropriate network
   access and packet scheduling mechanisms in all nodes running the
   protocol, guarantees a well-defined Quality of Service (QoS) to ST2
   applications. It ensures that real-time packets are delivered within
   their deadlines, that is, at the time where they need to be
   presented.  This facilitates a smooth delivery of data that is
   essential for time- critical applications, but can typically not be
   provided by best- effort IP communication.

                      DATA PATH                         CONTROL PATH
                      =========                         ============
       Upper     +------------------+                     +---------+
       Layer     | Application data |                     | Control |
                 +------------------+                     +---------+
                          |                                    |
                          |                                    V
                          |                     +-------------------+
       SCMP               |                     |   SCMP  |         |
                          |                     +-------------------+
                          |                             |
                          V                             V
            +-----------------------+      +------------------------+
       ST   | ST |                  |      | ST |         |         |
            +-----------------------+      +------------------------+
            D-bit=1                       D-bit=0

                   Figure 1: ST2 Data and Control Path

   Just like IP, ST2 actually consists of two protocols: ST for the data
   transport and SCMP, the Stream Control Message Protocol, for all
   control functions. ST is simple and contains only a single PDU format
   that is designed for fast and efficient data forwarding in order to



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   achieve low communication delays. SCMP, however, is more complex than
   IP's ICMP. As with ICMP and IP, SCMP packets are transferred within
   ST packets as shown in Figure 1.

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

                   Figure 2.  Protocol Relationships



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1.2  ST2 and IP

   ST2 is designed to coexist with IP on each node. A typical
   distributed multimedia application would use both protocols: IP for
   the transfer of traditional data and control information, and ST2 for
   the transfer of real-time data. Whereas IP typically will be accessed
   from TCP or UDP, ST2 will be accessed via new end-to-end real-time
   protocols. The position of ST2 with respect to the other protocols of
   the Internet family is represented in Figure 2.

   Both ST2 and IP apply the same addressing schemes to identify
   different hosts. ST2 and IP packets differ in the first four bits,
   which contain the internetwork protocol version number: number 5 is
   reserved for ST2 (IP itself has version number 4). As a network layer
   protocol, like IP, ST2 operates independently of its underlying
   subnets. Existing implementations use ARP for address resolution, and
   use the same Layer 2 SAPs as IP.

   As a special function, ST2 messages can be encapsulated in IP
   packets.  This is represented in Figure 2 as a link between ST2 and
   IP. This link allows ST2 messages to pass through routers which do
   not run ST2.  Resource management is typically not available for
   these IP route segments. IP encapsulation is, therefore, suggested
   only for portions of the network which do not constitute a system
   bottleneck.

   In Figure 2, the RTP protocol is shown as an example of transport
   layer on top of ST2. Others include the Packet Video Protocol (PVP)
   [Cole81], the Network Voice Protocol (NVP) [Cohe81], and others such
   as the Heidelberg Transport Protocol (HeiTP) [DHHS92].

1.3  Protocol History

   The first version of ST was published in the late 1970's and was used
   throughout the 1980's for experimental transmission of voice, video,
   and distributed simulation. The experience gained in these
   applications led to the development of the revised protocol version
   ST2. The revision extends the original protocol to make it more
   complete and more applicable to emerging multimedia environments. The
   specification of this protocol version is contained in Internet RFC
   1190 which was published in October 1990 [RFC1190].

   With more and more developments of commercial distributed multimedia
   applications underway and with a growing dissatisfaction at the
   transmission quality for audio and video over IP in the MBONE,
   interest in ST2 has grown over the last years. Companies have
   products available incorporating the protocol. The BERKOM MMTS
   project of the German PTT [DeAl92] uses ST2 as its core protocol for



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   the provision of multimedia teleservices such as conferencing and
   mailing. In addition, implementations of ST2 for Digital Equipment,
   IBM, NeXT, Macintosh, PC, Silicon Graphics, and Sun platforms are
   available.

   In 1993, the IETF started a new working group on ST2 as part of
   ongoing efforts to develop protocols that address resource
   reservation issues.  The group's mission was to clean up the existing
   protocol specification to ensure better interoperability between the
   existing and emerging implementations. It was also the goal to
   produce an updated experimental protocol specification that reflected
   the experiences gained with the existing ST2 implementations and
   applications. Which led to the specification of the ST2+ protocol
   contained in this document.

1.3.1  RFC1190 ST and ST2+ Major Differences

   The protocol changes from RFC1190 were motivated by protocol
   simplification and clarification, and codification of extensions in
   existing implementations. This section provides a list of major
   differences, and is probably of interest only to those who have
   knowledge of RFC1190. The major differences between the versions are:

o   Elimination of "Hop IDentifiers" or HIDs. HIDs added much complexity
    to the protocol and was found to be a major impediment to
    interoperability. HIDs have been replaced by globally unique
    identifiers called "Stream IDentifiers" or SIDs.

o   Elimination of a number of stream options. A number of options were
    found to not be used by any implementation, or were thought to add
    more complexity than value. These options were removed. Removed
    options include: point-to-point, full-duplex, reverse charge, and
    source route.

o   Elimination of the concept of "subset" implementations. RFC1190
    permitted subset implementations, to allow for easy implementation
    and experimentation. This led to interoperability problems. Agents
    implementing the protocol specified in this document, MUST implement
    the full protocol. A number of the protocol functions are best-
    effort. It is expected that some implementations will make more
    effort than others in satisfying particular protocol requests.

o   Clarification of the capability of targets to request to join a
    steam. RFC1190 can be interpreted to support target requests, but
    most implementors did not understand this and did not add support
    for this capability. The lack of this capability was found to be a
    significant limitation in the ability to scale the number of
    participants in a single ST stream. This clarification is based on



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    work done by IBM Heidelberg.

o   Separation of functions between ST and supporting modules. An effort
    was made to improve the separation of functions provided by ST and
    those provided by other modules. This is reflected in reorganization
    of some text and some PDU formats. ST was also made FlowSpec
    independent, although it does define a FlowSpec for testing and
    interoperability purposes.

o   General reorganization and re-write of the specification. This
    document has been organized with the goal of improved readability
    and clarity. Some sections have been added, and an effort was made
    to improve the introduction of concepts.

1.4  Supporting Modules for ST2

   ST2 is one piece of a larger mosaic. This section presents the
   overall communication architecture and clarifies the role of ST2 with
   respect to its supporting modules.

   ST2 proposes a two-step communication model. In the first step, the
   real-time channels for the subsequent data transfer are built. This
   is called stream setup. It includes selecting the routes to the
   destinations and reserving the correspondent resources. In the second
   step, the data is transmitted over the previously established
   streams.  This is called data transfer. While stream setup does not
   have to be completed in real-time, data transfer has stringent real-
   time requirements. The architecture used to describe the ST2
   communication model includes:

o   a data transfer protocol for the transmission of real-time data
    over the established streams,

o   a setup protocol to establish real-time streams based on the flow
    specification,

o   a flow specification to express user real-time requirements,

o   a routing function to select routes in the Internet,

o   a local resource manager to appropriately handle resources involved
    in the communication.

   This document defines a data protocol (ST), a setup protocol (SCMP),
   and a flow specification (ST2+ FlowSpec). It does not define a
   routing function and a local resource manager. However, ST2 assumes
   their existence.




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   Alternative architectures are possible, see [RFC1633] for an example
   alternative architecture that could be used when implementing ST2.

1.4.1  Data Transfer Protocol

   The data transfer protocol defines the format of the data packets
   belonging to the stream. Data packets are delivered to the targets
   along the stream paths previously established by the setup protocol.
   Data packets are delivered with the quality of service associated
   with the stream.

   Data packets contain a globally unique stream identifier that
   indicates which stream they belong to. The stream identifier is also
   known by the setup protocol, which uses it during stream
   establishment. The data transfer protocol for ST2, known simply as
   ST, is completely defined by this document.

1.4.2  Setup Protocol

   The setup protocol is responsible for establishing, maintaining, and
   releasing real-time streams. It relies on the routing function to
   select the paths from the source to the destinations. At each
   host/router on these paths, it presents the flow specification
   associated with the stream to the local resource manager. This causes
   the resource managers to reserve appropriate resources for the
   stream.  The setup protocol for ST2 is called Stream Control Message
   Protocol, or SCMP, and is completely defined by this document.

1.4.3  Flow Specification

   The flow specification is a data structure including the ST2
   applications' QoS requirements. At each host/router, it is used by
   the local resource manager to appropriately handle resources so that
   such requirements are met. Distributing the flow specification to all
   resource managers along the communication paths is the task of the
   setup protocol. However, the contents of the flow specification are
   transparent to the setup protocol, which simply carries the flow
   specification. Any operations on the flow specification, including
   updating internal fields and comparing flow specifications are
   performed by the resource managers.

   This document defines a specific flow specification format that
   allows for interoperability among ST2 implementations. This flow
   specification is intended to support a flow with a single
   transmission rate for all destinations in the stream. Implementations
   may support more than one flow specification format and the means are
   provided to add new formats as they are defined in the future.
   However, the flow specification format has to be consistent



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   throughout the stream, i.e., it is not possible to use different flow
   specification formats for different parts of the same stream.

1.4.4  Routing Function

   The routing function is an external unicast route generation
   capability. It provides the setup protocol with the path to reach
   each of the desired destinations. The routing function is called on a
   hop-by-hop basis and provides next-hop information. Once a route is
   selected by the routing function, it persists for the whole stream
   lifetime. The routing function may try to optimize based on the
   number of targets, the requested resources, or use of local network
   multicast or bandwidth capabilities. Alternatively, the routing
   function may even be based on simple connectivity information.

   The setup protocol is not necessarily aware of the criteria used by
   the routing function to select routes. It works with any routing
   function algorithm. The algorithm adopted is a local matter at each
   host/router and different hosts/routers may use different algorithms.
   The interface between setup protocol and routing function is also a
   local matter and therefore it is not specified by this document.

   This version of ST does not support source routing. It does support
   route recording. It does include provisions that allow identification
   of ST capable neighbors. Identification of remote ST hosts/routers is
   not specifically addressed.

1.4.5  Local Resource Manager

   At each host/router traversed by a stream, the Local Resource Manager
   (LRM) is responsible for handling local resources. The LRM knows
   which resources are on the system and what capacity they can provide.
   Resources include:

o   CPUs on end systems and routers to execute the application and
    protocol software,

o   main memory space for this software (as in all real-time systems,
    code should be pinned in main memory, as swapping it out would have
    detrimental effects on system performance),

o   buffer space to store the data, e.g., communication packets, passing
    through the nodes,

o   network adapters, and






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o   transmission networks between the nodes. Networks may be as simple
    as point-to-point links or as complex as switched networks such as
    Frame Relay and ATM networks.

   During stream setup and modification, the LRM is presented by the
   setup protocol with the flow specification associated to the stream.
   For each resource it handles, the LRM is expected to perform the
   following functions:

o   Stream Admission Control: it checks whether, given the flow
    specification, there are sufficient resources left to handle the new
    data stream. If the available resources are insufficient, the new
    data stream must be rejected.

o   QoS Computation: it calculates the best possible performance the
    resource can provide for the new data stream under the current
    traffic conditions, e.g., throughput and delay values are computed.

o   Resource Reservation: it reserves the resource capacities required
    to meet the desired QoS.

   During data transfer, the LRM is responsible for:

o   QoS Enforcement: it enforces the QoS requirements by appropriate
    scheduling of resource access. For example, data packets from an
    application with a short guaranteed delay must be served prior to
    data from an application with a less strict delay bound.

   The LRM may also provide the following additional functions:

o   Data Regulation: to smooth a stream's data traffic, e.g., as with the
    leaky bucket algorithm.

o   Policing: to prevent applications exceed their negotiated QoS, e.g.,
    to send data at a higher rate than indicated in the flow
    specification.

o   Stream Preemption: to free up resources for other streams with
    higher priority or importance.

   The strategies adopted by the LRMs to handle resources are resource-
   dependent and may vary at every host/router. However, it is necessary
   that all LRMs have the same understanding of the flow specification.
   The interface between setup protocol and LRM is a local matter at
   every host and therefore it is not specified by this document. An
   example of LRM is the Heidelberg Resource Administration Technique
   (HeiRAT) [VoHN93].




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   It is also assumed that the LRM provides functions to compare flow
   specifications, i.e., to decide whether a flow specification requires
   a greater, equal, or smaller amount of resource capacities to be
   reserved.















































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1.5  ST2 Basic Concepts

   The following sections present at an introductory level some of the
   fundamental ST2 concepts including streams, data transfer, and flow
   specification.

            Hosts Connections...                :      ...and Streams
            ====================                :      ==============
        data       Origin                       :          Origin
       packets +-----------+                    :          +----+
          +----|Application|                    :          |    |
          |    |-----------|                    :          +----+
          +--->| ST Agent  |                    :           |  |
               +-----------+                    :           |  |
                     |                          :           |  |
                     V                          :           |  |
              +-------------+                   :           |  |
              |             |                   :           |  |
+-------------|  Network A  |                   :   +-------+  +--+
|             |             |                   :   |             |
|             +-------------+                   :   |     Target 2|
|                    |     Target 2             :   |     & Router|
|     Target 1       |    and Router            :   |             |
|  +-----------+     |  +-----------+           :   V             V
|  |Application|<-+  |  |Application|<-+        : +----+        +----+
|  |-----------|  |  |  |-----------|  |        : |    |        |    |
+->| ST Agent  |--+  +->| ST Agent  |--+        : +----+        +----+
   +-----------+        +-----------+           :Target 1         |  |
                              |                 :                 |  |
                              V                 :                 |  |
                    +-------------+             :                 |  |
                    |             |             :                 |  |
      +-------------|  Network B  |             :           +-----+  |
      |             |             |             :           |        |
      |             +-------------+             :           |        |
      |    Target 3        |    Target 4        :           |        |
      |  +-----------+     |  +-----------+     :           V        V
      |  |Application|<-+  |  |Application|<-+  :         +----+ +----+
      |  |-----------|  |  |  |-----------|  |  :         |    | |    |
      +->| ST Agent  |--+  +->| ST Agent  |--+  :         +----+ +----+
         +-----------+        +-----------+     :      Target 3 Target 4
                                                :

                         Figure 3: The Stream Concept







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1.5.1  Streams

   Streams form the core concepts of ST2. They are established between a
   sending origin and one or more receiving targets in the form of a
   routing tree. Streams are uni-directional from the origin to the
   targets. Nodes in the tree represent so-called ST agents, entities
   executing the ST2 protocol; links in the tree are called hops. Any
   node in the middle of the tree is called an intermediate agent, or
   router. An agent may have any combination of origin, target, or
   intermediate capabilities.

   Figure 3 illustrates a stream from an origin to four targets, where
   the ST agent on Target 2 also functions as an intermediate agent. Let
   us use this Target 2/Router node to explain some basic ST2
   terminology: the direction of the stream from this node to Target 3
   and 4 is called downstream, the direction towards the Origin node
   upstream. ST agents that are one hop away from a given node are
   called previous-hops in the upstream, and next-hops in the downstream
   direction.

   Streams are maintained using SCMP messages. Typical SCMP messages are
   CONNECT and ACCEPT to build a stream, DISCONNECT and REFUSE to close
   a stream, CHANGE to modify the quality of service associated with a
   stream, and JOIN to request to be added to a stream.

   Each ST agent maintains state information describing the streams
   flowing through it. It can actively gather and distribute such
   information. It can recognize failed neighbor ST agents through the
   use of periodic HELLO message exchanges. It can ask other ST agents
   about a particular stream via a STATUS message. These ST agents then
   send back a STATUS-RESPONSE message. NOTIFY messages can be used to
   inform other ST agents of significant events.

   ST2 offers a wealth of functionalities for stream management. Streams
   can be grouped together to minimize allocated resources or to process
   them in the same way in case of failures. During audio conferences,
   for example, only a limited set of participants may talk at once.
   Using the group mechanism, resources for only a portion of the audio
   streams of the group need to be reserved. Using the same concept, an
   entire group of related audio and video streams can be dropped if one
   of them is preempted.

1.5.2  Data Transmission

   Data transfer in ST2 is simplex in the downstream direction. Data
   transport through streams is very simple. ST2 puts only a small
   header in front of the user data. The header contains a protocol
   identification that distinguishes ST2 from IP packets, an ST2 version



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   number, a priority field (specifying a relative importance of streams
   in cases of conflict), a length counter, a stream identification, and
   a checksum. These elements form a 12-byte header.

   Efficiency is also achieved by avoiding fragmentation and reassembly
   on all agents. Stream establishment yields a maximum message size for
   data packets on a stream. This maximum message size is communicated
   to the upper layers, so that they provide data packets of suitable
   size to ST2.

   Communication with multiple next-hops can be made even more efficient
   using MAC Layer multicast when it is available. If a subnet supports
   multicast, a single multicast packet is sufficient to reach all
   next-hops connected to this subnet. This leads to a significant
   reduction of the bandwidth requirements of a stream. If multicast is
   not provided, separate packets need to be sent to each next-hop.

   As ST2 relies on reservation, it does not contain error correction
   mechanisms features for data exchange such as those found in TCP. It
   is assumed that real-time data, such as digital audio and video,
   require partially correct delivery only. In many cases, retransmitted
   packets would arrive too late to meet their real-time delivery
   requirements. Also, depending on the data encoding and the particular
   application, a small number of errors in stream data are acceptable.
   In any case, reliability can be provided by layers on top of ST2 when
   needed.

1.5.3  Flow Specification

   As part of establishing a connection, SCMP handles the negotiation of
   quality-of-service parameters for a stream. In ST2 terminology, these
   parameters form a flow specification (FlowSpec) which is associated
   with the stream. Different versions of FlowSpecs exist, see
   [RFC1190], [DHHS92] and [RFC1363], and can be distinguished by a
   version number.  Typically, they contain parameters such as average
   and maximum throughput, end-to-end delay, and delay variance of a
   stream. SCMP itself only provides the mechanism for relaying the
   quality-of-service parameters.

   Three kinds of entities participate in the quality-of-service
   negotiation: application entities on the origin and target sites as
   the service users, ST agents, and local resource managers (LRM). The
   origin application supplies the initial FlowSpec requesting a
   particular service quality. Each ST agent which obtains the FlowSpec
   as part of a connection establishment message, it presents the local
   resource manager with it. ST2 does not determine how resource
   managers make reservations and how resources are scheduled according
   to these reservations; ST2, however, assumes these mechanisms as its



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   basis.

   An example of the FlowSpec negotiation procedure is illustrated in
   Figure 4. Depending on the success of its local reservations, the LRM
   updates the FlowSpec fields and returns the FlowSpec to the ST agent,
   which passes it downstream as part of the connection message.
   Eventually, the FlowSpec is communicated to the application at the
   target which may base its accept/reject decision for establishing the
   connection on it and may finally also modify the FlowSpec. If a
   target accepts the connection, the (possibly modified) FlowSpec is
   propagated back to the origin which can then calculate an overall
   service quality for all targets. The application entity at the origin
   may later request a CHANGE to adjust reservations.

                 Origin                 Router               Target 1
                +------+      1a       +------+      1b      +------+
                |      |-------------->|      |------------->|      |
                +------+               +------+              +------+
                 ^  | ^                                          |
                 |  | |                    2                     |
                 |  | +------------------------------------------+
                 +  +
 +-------------+  \  \             +-------------+       +-------------+
 |Max Delay: 12|   \  \            |Max Delay: 12|       |Max Delay: 12|
 |-------------|    \  \           |-------------|       |-------------|
 |Min Delay:  2|     \  \          |Min Delay:  5|       |Min Delay:  9|
 |-------------|      \  \         |-------------|       |-------------|
 |Max Size:4096|       +  +        |Max Size:2048|       |Max Size:2048|
 +-------------+       |  |        +-------------+       +-------------+
    FlowSpec           |  | 1
                       |  +---------------+
                       |                  |
                       |                  V
                     2 |               +------+
                       +---------------|      |
                                       +------+
                                       Target 2
                                   +-------------+
                                   |Max Delay: 12|
                                   |-------------|
                                   |Min Delay:  4|
                                   |-------------|
                                   |Max Size:4096|
                                   +-------------+

        Figure 4:  Quality-of-Service Negotiation with FlowSpecs





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1.6  Outline of This Document

   This document contains the specification of the ST2+ version of the
   ST2 protocol. In the rest of the document, whenever the terms "ST" or
   "ST2" are used, they refer to the ST2+ version of ST2.

   The document is organized as follows:

o   Section 2 describes the ST2 user service from an application point
    of view.

o   Section 3 illustrates the ST2 data transfer protocol, ST.

o   Section 4 through Section 8 specify the ST2 setup protocol, SCMP.

o   the ST2 flow specification is presented in Section 9.

o   the formats of protocol elements and PDUs are defined in Section 10.

2.  ST2 User Service Description

   This section describes the ST user service from the high-level point
   of view of an application. It defines the ST stream operations and
   primitive functions. It specifies which operations on streams can be
   invoked by the applications built on top of ST and when the ST
   primitive functions can be legally executed. Note that the presented
   ST primitives do not specify an API. They are used here with the only
   purpose of illustrating the service model for ST.

2.1  Stream Operations and Primitive Functions

   An ST application at the origin may create, expand, reduce, change,
   send data to, and delete a stream. When a stream is expanded, new
   targets are added to the stream; when a stream is reduced, some of
   the current targets are dropped from it. When a stream is changed,
   the associated quality of service is modified.

   An ST application at the target may join, receive data from, and
   leave a stream. This translates into the following stream operations:

o   OPEN: create new stream [origin], CLOSE: delete stream [origin],

o   ADD: expand stream, i.e., add new targets to it [origin],

o   DROP: reduce stream, i.e., drop targets from it [origin],

o   JOIN: join a stream [target], LEAVE: leave a stream [target],




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o   DATA: send data through stream [origin],

o   CHG: change a stream's QoS [origin],

   Each stream operation may require the execution of several primitive
   functions to be completed. For instance, to open a new stream, a
   request is first issued by the sender and an indication is generated
   at one or more receivers; then, the receivers may each accept or
   refuse the request and the correspondent indications are generated at
   the sender. A single receiver case is shown in Figure 5 below.

                Sender             Network             Receiver
                  |                   |                   |
     OPEN.req     |                   |                   |
                  |-----------------> |                   |
                  |                   |-----------------> |
                  |                   |                   | OPEN.ind
                  |                   |                   | OPEN.accept
                  |                   |<----------------- |
                  |<----------------- |                   |
  OPEN.accept-ind |                   |                   |
                  |                   |                   |

           Figure 5: Primitives for the OPEN Stream Operation



























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   Table 1 defines the ST service primitive functions associated to each
   stream operation. The column labelled "O/T" indicates whether the
   primitive is executed at the origin or at the target.

           +===================================================+
           |Primitive      | Descriptive                   |O/T|
           |===================================================|
           |OPEN.req       | open a stream                 | O |
           |OPEN.ind       | connection request indication | T |
           |OPEN.accept    | accept stream                 | T |
           |OPEN.refuse    | refuse stream                 | T |
           |OPEN.accept-ind| connection accept indication  | O |
           |OPEN.refuse-ind| connection refuse indication  | O |
           |ADD.req        | add targets to stream         | O |
           |ADD.ind        | add request indication        | T |
           |ADD.accept     | accept stream                 | T |
           |ADD.refuse     | refuse stream                 | T |
           |ADD.accept-ind | add accept indication         | O |
           |ADD.refuse-ind | add refuse indication         | O |
           |JOIN.req       | join a stream                 | T |
           |JOIN.ind       | join request indication       | O |
           |JOIN.reject    | reject a join                 | O |
           |JOIN.reject-ind| join reject indication        | T |
           |DATA.req       | send data                     | O |
           |DATA.ind       | receive data indication       | T |
           |CHG.req        | change stream QoS             | O |
           |CHG.ind        | change request indication     | T |
           |CHG.accept     | accept change                 | T |
           |CHG.refuse     | refuse change                 | T |
           |CHG.accept-ind | change accept indication      | O |
           |CHG.refuse-ind | change refuse indication      | O |
           |DROP.req       | drop targets                  | O |
           |DROP.ind       | disconnect indication         | T |
           |LEAVE.req      | leave stream                  | T |
           |LEAVE.ind      | leave stream indication       | O |
           |CLOSE.req      | close stream                  | O |
           |CLOSE.ind      | close stream indication       | T |
           +---------------------------------------------------+

                              Table 1: ST Primitives

2.2  State Diagrams

   It is not sufficient to define the set of ST stream operations. It is
   also necessary to specify when the operations can be legally
   executed.  For this reason, a set of states is now introduced and the
   transitions from one state to the others are specified. States are
   defined with respect to a single stream. The previously defined



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   stream operations can be legally executed only from an appropriate
   state.

   An ST agent may, with respect to an ST stream, be in one of the
   following states:

o   IDLE: the stream has not been created yet.

o   PENDING: the stream is in the process of being established.

o   ACTIVE: the stream is established and active.

o   ADDING: the stream is established. A stream expansion is underway.

o   CHGING: the stream is established. A stream change is underway.

   Previous experience with ST has lead to limits on stream operations
   that can be executed simultaneously. These restrictions are:


   1.  A single ADD or CHG operation can be processed at one time. If
       an ADD or CHG is already underway, further requests are queued
       by the ST agent and handled only after the previous operation
       has been completed. This also applies to two subsequent
       requests of the same kind, e.g., two ADD or two CHG operations.
       The second operation is not executed until the first one has
       been completed.

   2.  Deleting a stream, leaving a stream, or dropping targets from a
       stream is possible only after stream establishment has been
       completed. A stream is considered to be established when all
       the next-hops of the origin have either accepted or refused the
       stream.  Note that stream refuse is automatically forced after
       timeout if no reply comes from a next-hop.

   3.  An ST agent forwards data only along already established paths
       to the targets, see also Section 3.1. A path is considered to
       be established when the next-hop on the path has explicitly
       accepted the stream. This implies that the target and all other
       intermediate ST agents are ready to handle the incoming data
       packets. In no cases an ST agent will forward data to a
       next-hop ST agent that has not explicitly accepted the stream.
       To be sure that all targets receive the data, an application
       should send the data only after all paths have been
       established, i.e., the stream is established.






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   4.  It is allowed to send data from the CHGING and ADDING states.
       While sending data from the CHGING state, the quality of
       service to the targets affected by the change should be assumed
       to be the more restrictive quality of service. When sending
       data from the ADDING state, the targets that receive the data
       include at least all the targets that were already part of the
       stream at the time the ADD operation was invoked.

   The rules introduced above require ST agents to queue incoming
   requests when the current state does not allow to process them
   immediately. In order to preserve the semantics, ST agents have to
   maintain the order of the requests, i.e., implement FIFO queuing.
   Exceptionally, the CLOSE request at the origin and the LEAVE request
   at the target may be immediately processed: in these cases, the queue
   is deleted and it is possible that requests in the queue are not
   processed.

   The following state diagrams define the ST service. Separate diagrams
   are presented for the origin and the targets.

   The symbol (a/r)* indicates that all targets in the target list have
   explicitly accepted or refused the stream, or refuse has been forced
   after timeout. If the target list is empty, i.e., it contains no
   targets, the (a/r)* condition is immediately satisfied, so the empty
   stream is created and state ESTBL is entered.

   The separate OPEN and ADD primitives at the target are for conceptual
   purposes only. The target is actually unable to distinguish between
   an OPEN and an ADD. This is reflected in Figure 7 and Table 3 through
   the notation OPEN/ADD.





















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                        +------------+
                        |            |<-------------------+
            +---------->|    IDLE    |-------------+      |
            |           |            |    OPEN.req |      |
            |           +------------+             |      |
 CLOSE.req  |      CLOSE.req ^   ^ CLOSE.req       V      | CLOSE.req
            |                |   |            +---------+ |
            |                |   |            | PENDING |-|-+ JOIN.reject
            |                |   -------------|         |<|-+
            |    JOIN.reject |                +---------+ |
            |    DROP.req +----------+             |      |
            |       +-----|          |             |      |
            |       |     |  ESTDL   | OPEN.(a/r)* |      |
            |       +---->|          |<------------+      |
            |             +----------+                    |
            |              |  ^  |  ^                     |
            |              |  |  |  |                     |
       +----------+ CHG.req|  |  |  | Add.(a/r)*    +----------+
       |          |<-------+  |  |  +-------------- |          |
       |  CHGING  |           |  |                  |  ADDING  |
       |          |-----------+  +----------------->|          |
       +----------+ CHG.(a/r)*         JOIN.ind     +----------+
           |   ^                         ADD.req        |   ^
           |   |                                        |   |
           +---+                                        +---+
           DROP.req                                    DROP.req
           JOIN.reject                                 JOIN.reject

                  Figure 6: ST Service at the Origin

                 +--------+
                 |        |-----------------------+
                 |  IDLE  |                       |
                 |        |<---+                  | OPEN/ADD.ind
                 +--------+    | CLOSE.ind        | JOIN.req
                     ^         | OPEN/ADD.refuse  |
                     |         | JOIN.refect-ind  |
         CLOSE.ind   |         |                  V
         DROP.ind    |         |             +---------+
         LEAVE.req   |         +-------------|         |
                     |                       | PENDING |
                 +-------+                   |         |
                 |       |                   +---------+
                 | ESTBL |    OPEN/ADD.accept     |
                 |       |<-----------------------+
                 +-------+

                     Figure 7: ST Service at the Target



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2.3  State Transition Tables

   Table 2 and Table 3 define which primitives can be processed from
   which states and the possible state transitions.

+======================================================================+
|Primitive      |IDLE|    PENDING    |  ESTBL |    CHGING  |    ADDING |
|======================================================================|
|OPEN.req       | ok | -             | -      | -          | -         |
|OPEN.accept-ind| -  |if(a,r)*->ESTBL| -      | -          | -         |
|OPEN.refuse-ind| -  |if(a,r)*->ESTBL| -      | -          | -         |
|ADD.req        | -  | queued        |->ADDING| queued     | queued    |
|ADD.accept-ind | -  | -             | -      | -          |if(a,r)*   |
|               | -  | -             | -      | -          |->ESTBL    |
|ADD.refuse-ind | -  | -             | -      | -          |if(a,r)*   |
|               | -  | -             | -      | -          |->ESTBL    |
|JOIN.ind       | -  | queued        |->ADDING| queued     |queued     |
|JOIN.reject    | -  | OK            | ok     | ok         | ok        |
|DATA.req       | -  | -             | ok     | ok         | ok        |
|CHG.req        | -  | queued        |->CHGING| queued     |queued     |
|CHG.accept-ind | -  | -             | -      |if(a,r)*    | -         |
|               | -  | -             | -      |->ESTBL     | -         |
|CHG.refuse.ind | -  | -             | -      |if(a,r)*    | -         |
|               | -  | -             | -      |->ESTBL     | -         |
|DROP.req       | -  | -             | ok     | ok         | ok        |
|LEAVE.ind      | -  | OK            | ok     | ok         | ok        |
|CLOSE.req      | -  | OK            | ok     | ok         | ok        |
+----------------------------------------------------------------------+
                Table 2: Primitives and States at the Origin

             +======================================================+
             | Primitive       |   IDLE    |  PENDING   |   ESTBL   |
             |======================================================|
             | OPEN/ADD.ind    | ->PENDING | -          | -         |
             | OPEN/ADD.accept | -         | ->ESTBL    | -         |
             | OPEN/ADD.refuse | -         | ->IDLE     | -         |
             | JOIN.req        | ->PENDING | -          | -         |
             | JOIN.reject-ind |-          | ->IDLE     | -         |
             | DATA.ind        | -         | -          | ok        |
             | CHG.ind         | -         | -          | ok        |
             | CHG.accept      | -         | -          | ok        |
             | DROP.ind        | -         | ok         | ok        |
             | LEAVE.req       | -         | ok         | ok        |
             | CLOSE.ind       | -         | ok         | ok        |
             | CHG.ind         | -         | -          | ok        |
             +------------------------------------------------------+
                Table 3: Primitives and States at the Target




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3.  The ST2 Data Transfer Protocol

   This section presents the ST2 data transfer protocol, ST. First, data
   transfer is described in Section 3.1, then, the data transfer
   protocol functions are illustrated in Section 3.2.

3.1  Data Transfer with ST

   Data transmission with ST is unreliable. An application is not
   guaranteed that the data reaches its destinations and ST makes no
   attempts to recover from packet loss, e.g., due to the underlying
   network. However, if the data reaches its destination, it should do
   so according to the quality of service associated with the stream.

   Additionally, ST may deliver data corrupted in transmission. Many
   types of real-time data, such as digital audio and video, require
   partially correct delivery only. In many cases, retransmitted packets
   would arrive too late to meet their real-time delivery requirements.
   On the other hand, depending on the data encoding and the particular
   application, a small number of errors in stream data are acceptable.
   In any case, reliability can be provided by layers on top of ST2 if
   needed.

   Also, no data fragmentation is supported during the data transfer
   phase. The application is expected to segment its data PDUs according
   to the minimum MTU over all paths in the stream. The application
   receives information on the MTUs relative to the paths to the targets
   as part of the ACCEPT message, see Section 8.6. The minimum MTU over
   all paths can be calculated from the MTUs relative to the single
   paths. ST agents silently discard too long data packets, see also
   Section 5.1.1.

   An ST agent forwards the data only along already established paths to
   targets. A path is considered to be established once the next-hop ST
   agent on the path sends an ACCEPT message, see Section 2.2. This
   implies that the target and all other intermediate ST agents on the
   path to the target are ready to handle the incoming data packets. In
   no cases will an ST agent forward data to a next-hop ST agent that
   has not explicitly accepted the stream.

   To be reasonably sure that all targets receive the data with the
   desired quality of service, an application should send the data only
   after the whole stream has been established. Depending on the local
   API, an application may not be prevented from sending data before the
   completion of stream setup, but it should be aware that the data
   could be lost or not reach all intended targets. This behavior may
   actually be desirable to applications, such as those application that
   have multiple targets which can each process data as soon as it is



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   available (e.g., a lecture or distributed gaming).

   It is desirable for implementations to take advantage of networks
   that support multicast. If a network does not support multicast, or
   for the case where the next-hops are on different networks, multiple
   copies of the data packet must be sent.

3.2  ST Protocol Functions

   The ST protocol provides two functions:

   o   stream identification

   o   data priority

3.2.1  Stream Identification

   ST data packets are encapsulated by an ST header containing the
   Stream IDentifier (SID). This SID is selected at the origin so that
   it is globally unique over the Internet. The SID must be known by the
   setup protocol as well. At stream establishment time, the setup
   protocol builds, at each agent traversed by the stream, an entry into
   its local database containing stream information. The SID can be used
   as a reference into this database, to obtain quickly the necessary
   replication and forwarding information.

   Stream IDentifiers are intended to be used to make the packet
   forwarding task most efficient. The time-critical operation is an
   intermediate ST agent receiving a packet from the previous-hop ST
   agent and forwarding it to the next-hop ST agents.

   The format of data PDUs including the SID is defined in Section 10.1.
   Stream IDentifier generation is discussed in Section 8.1.

3.2.2  Packet Discarding based on Data Priority

   ST provides a well defined quality of service to its applications.
   However, there may be cases where the network is temporarily
   congested and the ST agents have to discard certain packets to
   minimize the overall impact to other streams. The ST protocol
   provides a mechanism to discard data packets based on the Priority
   field in the data PDU, see Section 10.1. The application assigns each
   data packet with a discard-priority level, carried into the Priority
   field. ST agents will attempt to discard lower priority packets first
   during periods of network congestion. Applications may choose to send
   data at multiple priority levels so that less important data may be
   discarded first.




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4.  SCMP Functional Description

   ST agents create and manage streams using the ST Control Message
   Protocol (SCMP). Conceptually, SCMP resides immediately above ST (as
   does ICMP above IP). SCMP follows a request-response model. SCMP
   messages are made reliable through the use of retransmission after
   timeout.

   This section contains a functional description of stream management
   with SCMP. To help clarify the SCMP exchanges used to setup and
   maintain ST streams, we include an example of a simple network
   topology, represented in Figure 8. Using the SCMP messages described
   in this section it will be possible for an ST application to:

   o   Create a stream from A to the peers at B, C and D,

   o   Add a peer at E,

   o   Drop peers B and C, and

   o   Let F join the stream

   o   Delete the stream.




























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                                               +---------+    +---+
                                               |         |----| B |
               +---------+      +----------+   |         |    +---+
               |         |------| Router 1 |---| Subnet2 |
               |         |      +----------+   |         |
               |         |                     |         |
               |         |                     +---------+
               |         |                         |
               | Subnet1 |                         |
               |         |                     +----------+
               |         |                     | Router 3 |
       +---+   |         |                     +----------+
       | A |---|         |    +----------+           |
       +---+   |         |----| Router 2 |           |
               |         |    +----------+           |
               +---------+         |                 |
                                   |                 |
                                   |          +----------+    +---+
                                   +----------|          |----| C |
                                              |          |    +---+
                         +---------+          |  Subnet3 |
                 +---+   |         |   +---+  |          |    +---+
                 | F |---| Subnet4 |---| E |--|          |----| D |
                 +---+   |         |   +---+  +----------+    +---+
                         +---------+

                Figure 8:  Sample Topology for an ST Stream

   We first describe the possible types of stream in Section 4.1;
   Section 4.2 introduces SCMP control message types; SCMP reliability
   is discussed in Section 4.3; stream options are covered in Section
   4.4; stream setup is presented in Section 4.5; Section 4.6
   illustrates stream modification including stream expansion,
   reduction, changes of the quality of service associated to a stream.
   Finally, stream deletion is handled in Section 4.7.

4.1  Types of Streams

   SCMP allows for the setup and management of different types of
   streams. Streams differ in the way they are built and the information
   maintained on connected targets.










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4.1.1  Stream Building

   Streams may be built in a sender-oriented fashion, receiver-oriented
   fashion, or with a mixed approach:

o   in the sender-oriented fashion, the application at the origin
    provides the ST agent with the list of receivers for the stream. New
    targets, if any, are also added from the origin.

o   in the receiver-oriented approach, the application at the origin
    creates an empty stream that contains no targets. Each target then
    joins the stream autonomously.

o   in the mixed approach, the application at the origin creates a
    stream that contains some targets and other targets join the stream
    autonomously.

   ST2 provides stream options to support sender-oriented and mixed
   approach steams. Receiver-oriented streams can be emulated through
   the use of mixed streams. The fashion by which targets may be added
   to a particular stream is controlled via join authorization levels.
   Join authorization levels are described in Section 4.4.2.

4.1.2  Knowledge of Receivers

   When streams are built in the sender-oriented fashion, all ST agents
   will have full information on all targets down stream of a particular
   agent. In this case, target information is relayed down stream from
   agent-to-agent during stream set-up.

   When targets add themselves to mixed approach streams, upstream ST
   agents may or may not be informed. Propagation of information on
   targets that "join" a stream is also controlled via join
   authorization levels. As previously mentioned, join authorization
   levels are described in Section 4.4.2.

   This leads to two types of streams:

o   full target information is propagated in a full-state stream. For
    such streams, all agents are aware of all downstream targets
    connected to the stream. This results in target information being
    maintained at the origin and at intermediate agents. Operations on
    single targets are always possible, i.e., change a certain target,
    or, drop that target from the stream. It is also always possible for
    any ST agent to attempt recovery of all downstream targets.






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o   in light-weight streams, it is possible that the origin and other
    upstream agents have no knowledge about some targets. This results
    in less maintained state and easier stream management, but it limits
    operations on specific targets. Special actions may be required to
    support change and drop operations on unknown targets, see Section
    5.7. Also, stream recovery may not be possible. Of course, generic
    functions such as deleting the whole stream, are still possible. It
    is expected that applications that will have a large number of
    targets will use light-weight streams in order to limit state in
    agents and the number of targets per control message.

   Full-state streams serve well applications as video conferencing or
   distributed gaming, where it is important to have knowledge on the
   connected receivers, e.g., to limit who participates. Light-weight
   streams may be exploited by applications such as remote lecturing or
   playback applications of radio and TV broadcast where the receivers
   do not need to be known by the sender. Section 4.4.2 defines join
   authorization levels, which support two types of full-state streams
   and one type of light-weight stream.

4.2  Control PDUs

   SCMP defines the following PDUs (the main purpose of each PDU is also
   indicated):

1.      ACCEPT          to accept a new stream
2.      ACK             to acknowledge an incoming message
3.      CHANGE          to change the quality of service associated with
                                a stream
4.      CONNECT         to establish a new stream or add new targets to
                                an existing stream
5.      DISCONNECT      to remove some or all of the stream's targets
6.      ERROR           to indicate an error contained in an incoming
                                message
7.      HELLO           to detect failures of neighbor ST agents
8.      JOIN            to request stream joining from a target
9.      JOIN-REJECT     to reject a stream joining request from a target
10.     NOTIFY          to inform an ST agent of a significant event
11.     REFUSE          to refuse the establishment of a new stream
12.     STATUS          to query an ST agent on a specific stream
13.     STATUS-RESPONSE to reply queries on a specific stream

   SCMP follows a request-response model with all requests expecting
   responses. Retransmission after timeout 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 is partitioned and a control message per partition is
   sent, as described in Section 5.1.2.



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   CONNECT and CHANGE request messages are answered with ACCEPT messages
   which indicate success, and with REFUSE messages which indicate
   failure. JOIN messages are answered with either a CONNECT message
   indicating success, or with a JOIN-REJECT message indicating failure.
   Targets may be removed from a stream by either the origin or the
   target via the DISCONNECT and REFUSE messages.

   The ACCEPT, CHANGE, CONNECT, DISCONNECT, JOIN, JOIN-REJECT, NOTIFY
   and REFUSE messages must always be explicitly acknowledged:

o   with an ACK message, if the message was received correctly and it
    was possible to parse and correctly extract and interpret its
    header, fields and parameters,

o   with an ERROR message, if a syntax error was detected in the header,
    fields, or parameters included in the message. The errored PDU may
    be optionally returned as part of the ERROR message. An ERROR
    message indicates a syntax error only. If any other errors are
    detected, it is necessary to first acknowledge with ACK and then
    take appropriate actions. For instance, suppose a CHANGE message
    contains an unknown SID: first, an ACK message has to be sent, then
    a REFUSE message with ReasonCode (SIDUnknown) follows.

   If no ACK or ERROR message are received before the correspondent
   timer expires, a timeout failure occurs. The way an ST agent should
   handle timeout failures is described in Section 5.2.

   ACK, ERROR, and STATUS-RESPONSE messages are never acknowledged.

   HELLO messages are a special case. If they contain a syntax error, an
   ERROR message should be generated in response. Otherwise, no
   acknowledgment or response should be generated. Use of HELLO messages
   is discussed in Section 6.1.2.

   STATUS messages containing a syntax error should be answered with an
   ERROR message. Otherwise, a STATUS-RESPONSE message should be sent
   back in response. Use of STATUS and STATUS-RESPONSE are discussed in
   Section 8.4.

4.3  SCMP Reliability

   SCMP is made reliable through the use of retransmission when a
   response is not received in a timely manner. The ACCEPT, CHANGE,
   CONNECT, DISCONNECT, JOIN, JOIN-REJECT, NOTIFY, and REFUSE messages
   all must be answered with an ACK message, see Section 4.2. In
   general, when sending a SCMP message which requires an ACK response,
   the sending ST agent needs to set the Toxxxx timer (where xxxx is the
   SCMP message type, e.g., ToConnect). If it does not receive an ACK



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   before the Toxxxx timer expires, the ST agent should retransmit the
   SCMP message. If no ACK has been received within Nxxxx
   retransmissions, then a SCMP timeout condition occurs and the ST
   agent enters its SCMP timeout recovery state. The actions performed
   by the ST agent as the result of the SCMP timeout condition differ
   for different SCMP messages and are described in Section 5.2.

   For some SCMP messages (CONNECT, CHANGE, JOIN, and STATUS) the
   sending ST agent also expects a response back (ACCEPT/REFUSE,
   CONNECT/JOIN- REJECT) after ACK has been received. For these cases,
   the ST agent needs to set the ToxxxxResp timer after it receives the
   ACK. (As before, xxxx is the initiating SCMP message type, e.g.,
   ToConnectResp).  If it does not receive the appropriate response back
   when ToxxxxResp expires, the ST agent updates its state and performs
   appropriate recovery action as described in Section 5.2. Suggested
   constants are given in Section 10.5.4.

   The timeout and retransmission algorithm is implementation dependent
   and it is outside the scope of this document. Most existing
   algorithms are based on an estimation of the Round Trip Time (RTT)
   between two agents. Therefore, SCMP contains a mechanism, see Section
   8.5, to estimate this RTT. Note that the timeout related variable
   names described above are for reference purposes only, implementors
   may choose to combine certain variables.

4.4  Stream Options

   An application may select among some stream options. The desired
   options are indicated to the ST agent at the origin when a new stream
   is created. Options apply to single streams and are valid during the
   whole stream's lifetime. The options chosen by the application at the
   origin are included into the initial CONNECT message, see Section
   4.5.3. When a CONNECT message reaches a target, the application at
   the target is notified of the stream options that have been selected,
   see Section 4.5.5.

4.4.1  No Recovery

   When a stream failure is detected, an ST agent would normally attempt
   stream recovery, as described in Section 6.2. The NoRecovery option
   is used to indicate that ST agents should not attempt recovery for
   the stream. The protocol behavior in the case that the NoRecovery
   option has been selected is illustrated in Section 6.2. The
   NoRecovery option is specified by setting the S-bit in the CONNECT
   message, see Section 10.4.4. The S-bit can be set only by the origin
   and it is never modified by intermediate and target ST agents.





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4.4.2  Join Authorization Level

   When a new stream is created, it is necessary to define the join
   authorization level associated with the stream. This level determines
   the protocol behavior in case of stream joining, see Section 4.1 and
   Section 4.6.3. The join authorization level for a stream is defined
   by the J-bit and N-bit in the CONNECT message header, see Section
   10.4.4.  One of the following authorization levels has to be
   selected:

   o   Level 0 - Refuse Join (JN = 00): No targets are allowed to join this
       stream.

   o   Level 1 - OK, Notify Origin (JN = 01): Targets are allowed to join
       the stream. The origin is notified that the target has joined.

   o   Level 2 - OK (JN = 10): Targets are allowed to join the stream. No
       notification is sent to the stream origin.

   Some applications may choose to maintain tight control on their
   streams and will not permit any connections without the origin's
   permission. For such streams, target applications may request to be
   added by sending an out-of-band, i.e., via regular IP, request to the
   origin. The origin, if it so chooses, can then add the target
   following the process described in Section 4.6.1.

   The selected authorization level impacts stream handling and the
   state that is maintained for the stream, as described in Section 4.1.

4.4.3  Record Route

   The RecordRoute option can be used to request the route between the
   origin and a target be recorded and delivered to the application.
   This option may be used while connecting, accepting, changing, or
   refusing a stream. The results of a RecordRoute option requested by
   the origin, i.e., as part of the CONNECT or CHANGE messages, are
   delivered to the target. The results of a RecordRoute option
   requested by the target, i.e., as part of the ACCEPT or REFUSE
   messages, are delivered to the origin.

   The RecordRoute option is specified by adding the RecordRoute
   parameter to the mentioned SCMP messages. The format of the
   RecordRoute parameter is shown in Section 10.3.5. When adding this
   parameter, the ST agent at the origin must determine the number of
   entries that may be recorded as explained in Section 10.3.5.






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4.4.4  User Data

   The UserData option can be used by applications to transport
   application specific data along with some SCMP control messages. This
   option can be included with ACCEPT, CHANGE, CONNECT, DISCONNECT, and
   REFUSE messages. The format of the UserData parameter is shown in
   Section 10.3.7. This option may be included by the origin, or the
   target, by adding the UserData parameter to the mentioned SCMP
   messages. This option may only be included once per SCMP message.

4.5  Stream Setup

   This section presents a description of stream setup. For simplicity,
   we assume that everything succeeds, e.g., any required resources are
   available, messages are properly delivered, and the routing is
   correct. Possible failures in the setup phase are handled in Section
   5.2.

4.5.1  Information from the Application

   Before stream setup can be started, the application has to collect
   the necessary information to determine the characteristics for the
   connection. This includes identifying the participants and selecting
   the QoS parameters of the data flow. Information passed to the ST
   agent by the application includes:

o   the list of the stream's targets (Section 10.3.6). The list may be
    empty (Section 4.5.3.1),

o   the flow specification containing the desired quality of service for
    the stream (Section 9),

o   information on the groups in which the stream is a member, if any
    (Section 7),

o   information on the options selected for the stream (Section 4.4).

4.5.2  Initial Setup at the Origin

   The ST agent at the origin then performs the following operations:

o   allocates a stream ID (SID) for the stream (Section 8.1),

o   invokes the routing function to determine the set of next-hops for
    the stream (Section 4.5.2.1),

o   invokes the Local Resource Manager (LRM) to reserve resources
    (Section 4.5.2.2),



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o   creates local database entries to store information on the new
    stream,

o   propagates the stream creation request to the next-hops determined
    by the routing function (Section 4.5.3).

4.5.2.1  Invoking the Routing Function

   An ST agent that is setting up a stream invokes the routing function
   to find the next-hop to reach each of the targets specified by the
   target list provided by the application. This is similar to the
   routing decision in IP. However, in this case the route is to a
   multitude of targets with QoS requirements rather than to a single
   destination.

   The result of the routing function is a set of next-hop ST agents.
   The set of next-hops selected by the routing function 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.  Alternatively, the routing algorithm may use a simple
   route lookup for each target.

   Once a next-hop is selected by the routing function, it persists for
   the whole stream lifetime, unless a network failure occurs.

4.5.2.2  Reserving Resources

   The ST agent invokes the Local Resource Manager (LRM) to perform the
   appropriate reservations. The ST agent presents the LRM with
   information including:

o   the flow specification with the desired quality of service for the
    stream (Section 9),

o   the version number associated with the flow specification
    (Section 9).

o   information on the groups the stream is member in, if any
    (Section 7),

   The flow specification contains information needed by the LRM to
   allocate resources. The LRM updates the flow specification contents
   information before returning it to the ST agent. Section 9.2.3
   defines the fields of the flow specification to be updated by the
   LRM.



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   The membership of a stream in a group may affect the amount of
   resources that have to be allocated by the LRM, see Section 7.

4.5.3  Sending CONNECT Messages

   The ST agent sends a CONNECT message to each of the next-hop ST
   agents identified by the routing function. Each CONNECT message
   contains the SID, the selected stream options, the FlowSpec, and a
   TargetList. The format of the CONNECT message is defined by Section
   10.4.4. In general, the FlowSpec and TargetList depend on both the
   next-hop and the intervening network. Each TargetList is a subset of
   the original TargetList, identifying the targets that are to be
   reached through the next-hop to which the CONNECT message is being
   sent.

   The TargetList may be empty, see Section 4.5.3.1; if the TargetList
   causes a too long CONNECT message to be generated, the CONNECT
   message is partitioned as explained in Section 5.1.2. 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.

4.5.3.1  Empty Target List

   An application at the origin may request the local ST agent to create
   an empty stream. It does so by passing an empty TargetList to the
   local ST agent during the initial stream setup. When the local ST
   agent receives a request to create an empty stream, it allocates the
   stream ID (SID), updates its local database entries to store
   information on the new stream and notifies the application that
   stream setup is complete. The local ST agent does not generate any
   CONNECT message for streams with an empty TargetList. Targets may be
   later added by the origin, see Section 4.6.1, or they may
   autonomously join the stream, see Section 4.6.3.

4.5.4  CONNECT Processing by an Intermediate ST agent

   An ST agent receiving a CONNECT message, assuming no errors, responds
   to the previous-hop with an ACK. The ACK message must identify the
   CONNECT message to which it corresponds by including the reference
   number indicated by the Reference field of the CONNECT message. The
   intermediate ST agent calls the routing function, invokes the LRM to
   reserve resources, and then propagates the CONNECT messages to its
   next-hops, as described in the previous sections.







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4.5.5  CONNECT Processing at the Targets

   An ST agent that is the target of a CONNECT message, assuming no
   errors, responds to the previous-hop with an ACK. The ST agent
   invokes the LRM to reserve local resources and then queries the
   specified application process whether or not it is willing to accept
   the connection.

   The application is presented with parameters from the CONNECT message
   including the SID, the selected stream options, Origin, FlowSpec,
   TargetList, and Group, if any, to be used as a basis for its
   decision.  The application is identified by a combination of the
   NextPcol field, from the Origin parameter, and the service access
   point, or SAP, field included in the correspondent (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 the SID, that can be mapped into this information
   and be used for their delivery.

   Finally, based on the application's decision, the ST agent sends to
   the previous-hop from which the CONNECT message was received either
   an ACCEPT or REFUSE message. Since the ACCEPT (or REFUSE) message has
   to be acknowledged by the previous-hop, it is assigned a new
   Reference number that will be returned in the ACK. The CONNECT
   message to which ACCEPT (or REFUSE) is a reply is identified by
   placing the CONNECT's Reference number in the LnkReference field of
   ACCEPT (or REFUSE). The ACCEPT message contains the FlowSpec as
   accepted by the application at the target.

4.5.6  ACCEPT Processing by an Intermediate ST 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 message, with
   ReasonCode (LnkRefUnknown). Otherwise, it responds to the next-hop ST
   agent with an ACK, and propagates the individual ACCEPT message to
   the previous-hop along the same path traced by the CONNECT but in the
   reverse direction toward the origin.

   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 when the CONNECT was
   originally processed. Therefore, the ST agent presents the LRM with
   the FlowSpec included in the ACCEPT message. It is expected that each
   LRM adjusts local reservations releasing any excess resources. The



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   LRM may choose not to adjust local reservations when that adjustment
   may result in the loss of needed resources. It may also choose to
   wait to adjust allocated resources until all targets in transition
   have been accepted or refused.

   In the case where the intermediate ST agent is acting as the origin
   with respect to this target, see Section 4.6.3.1, the ACCEPT message
   is not propagated upstream.

4.5.7  ACCEPT Processing by the Origin

   The origin will eventually receive an ACCEPT (or REFUSE) message from
   each of the targets. As each ACCEPT is received, the application is
   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 an ACCEPT is received by the origin, the path to the target is
   considered to be established and the ST agent is allowed to forward
   the data along this path as explained in Section 2 and in Section
   3.1.

4.5.8  REFUSE Processing by the Intermediate ST agent

   If an application at a target does not wish to participate in the
   stream, it sends a REFUSE message back to the origin with ReasonCode
   (ApplDisconnect). An intermediate ST agent that receives a REFUSE
   message with ReasonCode (ApplDisconnect) acknowledges it by sending
   an ACK to the next-hop, invokes the LRM to adjusts reservations as
   appropriate, deletes the target entry from the internal database, and
   propagates the REFUSE message back to the previous-hop ST agent.

   In the case where the intermediate ST agent is acting as the origin
   with respect to this target, see Section 4.6.3.1, the REFUSE message
   is only propagated upstream when there are no more downstream agents
   participating in the stream. In this case, the agent indicates that
   the agent is to be removed from the stream propagating the REFUSE
   message with the G-bit set (1).

4.5.9  REFUSE Processing by the Origin

   When the REFUSE message reaches the origin, the ST agent at the
   origin sends an ACK and notifies the application that the target 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, or keep the stream active to allow addition



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   of targets or stream joining as described in Section 4.6.3.

4.5.10  Other Functions during Stream Setup

   Some other functions have to be accomplished by an ST agent as
   CONNECT messages travel downstream and ACCEPT (or REFUSE) messages
   travel upstream during the stream setup phase. They were not
   mentioned in the previous sections to keep the discussion as simple
   as possible. These functions include:

   o   computing the smallest Maximum Transmission Unit size over the path
       to the targets, as part of the MTU discovery mechanism presented in
       Section 8.6. This is done by updating the MaxMsgSize field of the
       CONNECT message, see Section 10.4.4. This value is carried back to
       origin in the MaxMsgSize field of the ACCEPT message, see Section
       10.4.1.

   o   counting the number of IP clouds to be traversed to reach the
       targets, if any. IP clouds are traversed when the IP encapsulation
       mechanism is used. This mechanism described in Section 8.7.
       Encapsulating agents update the IPHops field of the CONNECT message,
       see Section 10.4.4. The resulting value is carried back to origin in
       the IPHops field of the ACCEPT message, see Section 10.4.1.

   o   updating the RecoveryTimeout value for the stream based on what can
       the agent can support. This is part of the stream recovery
       mechanism, in Section 6.2. This is done by updating the
       RecoveryTimeout field of the CONNECT message, see Section 10.4.4.
       This value is carried back to origin in the RecoveryTimeout field of
       the ACCEPT message, see Section 10.4.1.

4.6  Modifying an Existing Stream

   Some applications may wish to modify a stream after it has been
   created. Possible changes include expanding a stream, reducing it,
   and changing its FlowSpec. The origin may add or remove targets as
   described in Section 4.6.1 and Section 4.6.2. Targets may request to
   join the stream as described in Section 4.6.3 or, they may decide to
   leave a stream as described in Section 4.6.4. Section 4.6.5 explains
   how to change a stream's FlowSpec.

   As defined by Section 2, an ST agent can handle only one stream
   modification at a time. If a stream modification operation is already
   underway, further requests are queued and handled when the previous
   operation has been completed. This also applies to two subsequent
   requests of the same kind, e.g., two subsequent changes to the
   FlowSpec.




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4.6.1  The Origin Adding New Targets

   It is possible for an application at the origin to add new targets to
   an existing stream any time after the stream has been established.
   Before new targets are added, the application has to collect the
   necessary information on the new targets. Such information is passed
   to the ST agent at the origin.

   The ST agent at the origin issues a CONNECT message that contains the
   SID, the FlowSpec, and the TargetList specifying the new targets.
   This is similar to sending a CONNECT message during stream
   establishment, with the following exceptions: the origin checks that
   a) the SID is valid, b) the targets are not already members of the
   stream, c) that the LRM evaluates the FlowSpec of the new target to
   be the same as the FlowSpec of the existing stream, i.e., it requires
   an equal or smaller amount of resources to be allocated. If the
   FlowSpec of the new target does not match the FlowSpec of the
   existing stream, an error is generated with ReasonCode
   (FlowSpecMismatch). Functions to compare flow specifications are
   provided by the LRM, see Section 1.4.5.

   An intermediate ST agent that is already a participant in the stream
   looks at the SID and StreamCreationTime, and verifies that 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, it responds with a REFUSE message with ReasonCode
   (TargetExists) that contains a TargetList of those targets that were
   duplicates. To indicate that the stream exists, and includes the
   listed targets, the ST agent sets to one (1) the E-bit of the REFUSE
   message, see Section 10.4.11.  The agent then proceeds processing
   each new target in the TargetList.

   For each new target in the TargetList, processing is much the same as
   for the original CONNECT. The CONNECT is acknowledged, propagated,
   and network resources are reserved. Intermediate or target ST agents
   that are not already participants in the stream behave as in the case
   of stream setup (see Section 4.5.4 and Section 4.5.5).

4.6.2  The Origin Removing a Target

   It is possible for an application at the origin to remove existing
   targets of a stream any time after the targets have accepted the
   stream. The application at the origin specifies the set of targets
   that are to be removed and informs the local ST agent. Based on this
   information, the ST agent sends DISCONNECT messages with the
   ReasonCode (ApplDisconnect) to the next-hops relative to the targets.





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   An ST agent that receives a DISCONNECT message must acknowledge it by
   sending an ACK to the previous-hop. The ST agent updates its state
   and notifies the LRM of the target deletion so that the LRM can
   modify reservations as appropriate. When the DISCONNECT message
   reaches the target, the ST agent also notifies the application that
   the target is no longer part of the stream. When there are no
   remaining targets that can be reached through a particular next-hop,
   the ST agent informs the LRM and it deletes the next-hop from its
   next-hops set.

   SCMP also provides a flooding mechanism to delete targets that joined
   the stream without notifying the origin. The special case of target
   deletion via flooding is described in Section 5.7.

4.6.3  A Target Joining a Stream

   An application may request to join an existing stream. It has to
   collect information on the stream including the stream ID (SID) and
   the IP address of the stream's origin. This can be done out-of-band,
   e.g., via regular IP. The information is then passed to the local ST
   agent. The ST agent generates a JOIN message containing the
   application's request to join the stream and sends it toward the
   stream origin.

   An ST agent receiving a JOIN message, assuming no errors, responds
   with an ACK. The ACK message must identify the JOIN message to which
   it corresponds by including the Reference number indicated by the
   Reference field of the JOIN message. If the ST agent is not traversed
   by the stream that has to be joined, it propagates the JOIN message
   toward the stream's origin. Once a JOIN message has been
   acknowledged, ST agents do not retain any state information related
   to the JOIN message.

   Eventually, an ST agent traversed by the stream or the stream's
   origin itself is reached. This agent must respond to a received JOIN
   first with an ACK to the ST agent from which the message was
   received, then, it issues either a CONNECT or a JOIN-REJECT message
   and sends it toward the target. The response to the join request is
   based on the join authorization level associated with the stream, see
   Section 4.4.2:

o   If the stream has authorization level #0 (refuse join):
    The ST agent sends a JOIN-REJECT message toward the target with
    ReasonCode (JoinAuthFailure).

o   If the stream has authorization level #1 (ok, notify origin):
    The ST agent sends a CONNECT message toward the target with a
    TargetList including the target that requested to join the stream.



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    This eventually results in adding the target to the stream. When
    the ST agent receives the ACCEPT message indicating that the new
    target has been added, it does not propagate the ACCEPT message
    backwards (Section 4.5.6). Instead, it issues a NOTIFY message
    with ReasonCode (TargetJoined) so that upstream agents, including
    the origin, may add the new target to maintained state
    information. The NOTIFY message includes all target specific
    information.

o   If the stream has authorization level #2 (ok):
    The ST agent sends a CONNECT message toward the target with a
    TargetList including the target that requested to join the stream.
    This eventually results in adding the target to the stream. When
    the ST agent receives the ACCEPT message indicating that the new
    target has been added, it does not propagate the ACCEPT message
    backwards (Section 4.5.6), nor does it notify the origin. A NOTIFY
    message is generated with ReasonCode (TargetJoined) if the target
    specific information needs to be propagated back to the origin. An
    example of such information is change in MTU, see Section 8.6.

4.6.3.1  Intermediate Agent (Router) as Origin

   When a stream has join authorization level #2, see Section 4.4.2, it
   is possible that the stream origin is unaware of some targets
   participating in the stream. In this case, the ST intermediate agent
   that first sent a CONNECT message to this target has to act as the
   stream origin for the given target. This includes:

o   if the whole stream is deleted, the intermediate agent must
    disconnect the target.

o   if the stream FlowSpec is changed, the intermediate agent must
    change the FlowSpec for the target as appropriate.

o   proper handling of ACCEPT and REFUSE messages, without propagation
    to upstream ST agents.

o   generation of NOTIFY messages when needed. (As described above.)

   The intermediate agent behaves normally for all other targets added
   to the stream as a consequence of a CONNECT message issued by the
   origin.

4.6.4  A Target Deleting Itself

   The application at the target may inform the local ST agent that it
   wants to be removed from the stream. The ST agent then forms a REFUSE
   message with the target itself as the only entry in the TargetList



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   and with ReasonCode (ApplDisconnect). The REFUSE message 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 a REFUSE message acknowledges it by sending
   an ACK to the next-hop. The target is deleted and the LRM is notified
   so that it adjusts reservations as appropriate. The REFUSE message is
   also propagated back to the previous-hop ST agent except in the case
   where the agent is acting as the origin. In this case a NOTIFY may be
   propagated instead, see Section 4.6.3.

   When the REFUSE reaches the origin, the origin sends an ACK and
   notifies the application that the target is no longer part of the
   stream.

4.6.5  Changing a Stream's FlowSpec

   The application at the origin may wish to change the FlowSpec of an
   established stream. Changing the FlowSpec is a critical operation and
   it may even lead in some cases to the deletion of the affected
   targets. Possible problems with FlowSpec changes are discussed in
   Section 5.6.

   To change the stream's FlowSpec, the application informs the ST agent
   at the origin of the new FlowSpec and of the list of targets relative
   to the change. The ST agent at the origin then issues one CHANGE
   message per next-hop including the new FlowSpec and sends it to the
   relevant next-hop ST agents. If the G-bit field of the CHANGE message
   is set (1), the change affects all targets in the stream.

   The CHANGE message contains a bit called I-bit, see Section 10.4.3.
   By default, the I-bit is set to zero (0) to indicate that the LRM is
   expected to try and perform the requested FlowSpec change without
   risking to tear down the stream. Applications that desire a higher
   probability of success and are willing to take the risk of breaking
   the stream can indicate this by setting the I-bit to one (1).
   Applications that require the requested modification in order to
   continue operating are expected to set this bit.

   An intermediate ST agent that receives a CHANGE message first sends
   an ACK to the previous-hop and then provides the FlowSpec to the LRM.
   If the LRM can perform the change, the ST agent propagates the CHANGE
   messages along the established paths.






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   If the whole process succeeds, the CHANGE messages will eventually
   reach the targets. Targets respond with an ACCEPT (or REFUSE) message
   that is propagated back to the origin. In processing the ACCEPT
   message on the way back to the origin, excess resources may be
   released by the LRM as described in Section 4.5.6. The REFUSE message
   must have the ReasonCode (ApplRefused).

   SCMP also provides a flooding mechanism to change targets that joined
   the stream without notifying the origin. The special case of target
   change via flooding is described in Section 5.7.

4.7  Stream Tear Down

   A stream is usually terminated by the origin when it has no further
   data to send. A stream is also torn down if the application should
   terminate abnormally or if certain network failures are encountered.
   Processing in this case is identical to the previous descriptions
   except that the ReasonCode (ApplAbort, NetworkFailure, etc.) is
   different.

   When all targets have left a stream, the origin notifies the
   application of that fact, and the application is then responsible for
   terminating the stream. Note, however, that the application may
   decide to add targets to the stream instead of terminating it, or may
   just leave the stream open with no targets in order to permit stream
   joins.

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 exceed a network MTU, are
   lost, the requested resources are not available, the routing fails or
   is inconsistent.

5.1  Long ST Messages

   It is possible that an ST agent, or an application, will need to send
   a message that exceeds a network's Maximum Transmission Unit (MTU).
   This case must be handled but not via generic fragmentation, since
   ST2 does not support generic fragmentation of either data or control
   messages.

5.1.1  Handling of Long Data Packets

   ST agents discard data packets that exceed the MTU of the next-hop
   network. No error message is generated. Applications should avoid
   sending data packets larger than the minimum MTU supported by a given



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   stream. The application, both at the origin and targets, can learn
   the stream minimum MTU through the MTU discovery mechanism described
   in Section 8.6.

5.1.2  Handling of Long Control Packets

   Each ST agent knows the MTU of the networks to which it is connected,
   and those MTUs restrict the size of the SCMP message it can send. An
   SCMP message size can exceed the MTU of a given network for a number
   of reasons:

o   the TargetList parameter (Section 10.3.6) may be too long;

o   the RecordRoute parameter (Section 10.3.5) may be too long.

o   the UserData parameter (Section 10.3.7) may be too long;

o   the PDUInError field of the ERROR message (Section 10.4.6) may be
    too long;

   An ST agent receiving or generating a too long SCMP message should:

o   break the message into multiple messages, each carrying part of the
    TargetList. Any RecordRoute and UserData parameters are replicated
    in each message for delivery to all targets. Applications that
    support a large number of targets may avoid using long TargetList
    parameters, and are expected to do so, by exploiting the stream
    joining functions, see Section 4.6.3. One exception to this rule
    exists. In the case of a long TargetList parameter to be included in
    a STATUS-RESPONSE message, the TargetList parameter is just
    truncated to the point where the list can fit in a single message,
    see Section 8.4.

o   for down stream agents: if the TargetList parameter contains a
    single Target element and the message size is still too long, the ST
    agent should issue a REFUSE message with ReasonCode
    (RecordRouteSize) if the size of the RecordRoute parameter causes
    the SCMP message size to exceed the network MTU, or with ReasonCode
    (UserDataSize) if the size of the UserData parameter causes the SCMP
    message size to exceed the network MTU. If both RecordRoute and
    UserData parameters are present the ReasonCode (UserDataSize) should
    be sent. For messages generated at the target: the target ST agent
    must check for SCMP messages that may exceed the MTU on the complete
    target-to-origin path, and inform the application that a too long
    SCMP messages has been generated. The format for the error reporting
    is a local implementation issue. The error codes are the same as
    previously stated.




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   ST agents generating too long ERROR messages, simply truncate the
   PDUInError field to the point where the message is smaller than the
   network MTU.

5.2  Timeout Failures

   As described in Section 4.3, SCMP message delivery is made reliable
   through the use of acknowledgments, timeouts, and retransmission. The
   ACCEPT, CHANGE, CONNECT, DISCONNECT, JOIN, JOIN-REJECT, NOTIFY, and
   REFUSE messages must always be acknowledged, see Section 4.2. In
   addition, for some SCMP messages (CHANGE, CONNECT, JOIN) the sending
   ST agent also expects a response back (ACCEPT/REFUSE, CONNECT/JOIN-
   REJECT) after an ACK has been received. Also, the STATUS message must
   be answered with a STATUS-RESPONSE message.

   The following sections describe the handling of each of the possible
   failure cases due to timeout situations while waiting for an
   acknowledgment or a response. The timeout related variables, and
   their names, used in the next sections are for reference purposes
   only. They may be implementation specific. Different implementations
   are not required to share variable names, or even the mechanism by
   which the timeout and retransmission behavior is implemented.

5.2.1  Failure due to ACCEPT Acknowledgment Timeout

   An ST agent that sends an ACCEPT message upstream expects an ACK from
   the previous-hop ST agent. If no ACK is received before the ToAccept
   timeout expires, the ST agent should retry and send the ACCEPT
   message again. After NAccept unsuccessful retries, the ST agent sends
   a REFUSE message toward the origin, and a DISCONNECT message toward
   the targets. Both REFUSE and DISCONNECT must identify the affected
   targets and specify the ReasonCode (RetransTimeout).

5.2.2  Failure due to CHANGE Acknowledgment Timeout

   An ST agent that sends a CHANGE message downstream expects an ACK
   from the next-hop ST agent. If no ACK is received before the ToChange
   timeout expires, the ST agent should retry and send the CHANGE
   message again. After NChange unsuccessful retries, the ST agent
   aborts the change attempt by sending a REFUSE message toward the
   origin, and a DISCONNECT message toward the targets. Both REFUSE and
   DISCONNECT must identify the affected targets and specify the
   ReasonCode (RetransTimeout).








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5.2.3  Failure due to CHANGE Response Timeout

   Only the origin ST agent implements this timeout. After correctly
   receiving the ACK to a CHANGE message, an ST agent expects to receive
   an ACCEPT, or REFUSE message in response. If one of these messages is
   not received before the ToChangeResp timer expires, the ST agent at
   the origin aborts the change attempt, and behaves as if a REFUSE
   message with the E-bit set and with ReasonCode (ResponseTimeout) is
   received.

5.2.4  Failure due to CONNECT Acknowledgment Timeout

   An ST agent that sends a CONNECT message downstream expects an ACK
   from the next-hop ST agent. If no ACK is received before the
   ToConnect timeout expires, the ST agent should retry and send the
   CONNECT message again. After NConnect unsuccessful retries, the ST
   agent sends a REFUSE message toward the origin, and a DISCONNECT
   message toward the targets. Both REFUSE and DISCONNECT must identify
   the affected targets and specify the ReasonCode (RetransTimeout).

5.2.5  Failure due to CONNECT Response Timeout

   Only the origin ST agent implements this timeout. After correctly
   receiving the ACK to a CONNECT message, an ST agent expects to
   receive an ACCEPT or REFUSE message in response. If one of these
   messages is not received before the ToConnectResp timer expires, the
   origin ST agent aborts the connection setup attempt, acts as if a
   REFUSE message is received, and it sends a DISCONNECT message toward
   the targets.  Both REFUSE and DISCONNECT must identify the affected
   targets and specify the ReasonCode (ResponseTimeout).

5.2.6  Failure due to DISCONNECT Acknowledgment Timeout

   An ST agent that sends a DISCONNECT message downstream expects an ACK
   from the next-hop ST agent. If no ACK is received before the
   ToDisconnect timeout expires, the ST agent should retry and send the
   DISCONNECT message again. After NDisconnect unsuccessful retries, the
   ST agent simply gives up and it assumes the next-hop ST agent is not
   part in the stream any more.

5.2.7  Failure due to JOIN Acknowledgment Timeout

   An ST agent that sends a JOIN message toward the origin expects an
   ACK from a neighbor ST agent. If no ACK is received before the ToJoin
   timeout expires, the ST agent should retry and send the JOIN message
   again. After NJoin unsuccessful retries, the ST agent sends a JOIN-
   REJECT message back in the direction of the target with ReasonCode
   (RetransTimeout).



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5.2.8  Failure due to JOIN Response Timeout

   Only the target agent implements this timeout. After correctly
   receiving the ACK to a JOIN message, the ST agent at the target
   expects to receive a CONNECT or JOIN-REJECT message in response. If
   one of these message is not received before the ToJoinResp timer
   expires, the ST agent aborts the stream join attempt and returns an
   error corresponding with ReasonCode (RetransTimeout) to the
   application.

   Note that, after correctly receiving the ACK to a JOIN message,
   intermediate ST agents do not maintain any state on the stream
   joining attempt. As a consequence, they do not set the ToJoinResp
   timer and do not wait for a CONNECT or JOIN-REJECT message. This is
   described in Section 4.6.3.

5.2.9  Failure due to JOIN-REJECT Acknowledgment Timeout

   An ST agent that sends a JOIN-REJECT message toward the target
   expects an ACK from a neighbor ST agent. If no ACK is received before
   the ToJoinReject timeout expires, the ST agent should retry and send
   the JOIN-REJECT message again. After NJoinReject unsuccessful
   retries, the ST agent simply gives up.

5.2.10  Failure due to NOTIFY Acknowledgment Timeout

   An ST agent that sends a NOTIFY message to a neighbor ST agent
   expects an ACK from that neighbor ST agent. If no ACK is received
   before the ToNotify timeout expires, the ST agent should retry and
   send the NOTIFY message again. After NNotify unsuccessful retries,
   the ST agent simply gives up and behaves as if the ACK message was
   received.

5.2.11  Failure due to REFUSE Acknowledgment Timeout

   An ST agent that sends a REFUSE message upstream expects an ACK from
   the previous-hop ST agent. If no ACK is received before the ToRefuse
   timeout expires, the ST agent should retry and send the REFUSE
   message again. After NRefuse unsuccessful retries, the ST agent gives
   up and it assumes it is not part in the stream any more.

5.2.12  Failure due to STATUS Response Timeout

   After sending a STATUS message to a neighbor ST agent, an ST agent
   expects to receive a STATUS-RESPONSE message in response. If this
   message is not received before the ToStatusResp timer expires, the ST
   agent sends the STATUS message again. After NStatus unsuccessful
   retries, the ST agent gives up and assumes that the neighbor ST agent



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   is not active.

5.3  Setup Failures due to Routing Failures

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

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

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

   3.  a routing loop.

   The TargetList contained in the CONNECT is used to distinguish the
   different cases by comparing each newly received target with those of
   the previously existing stream:

o   if the IP address of the target(s) differ, it is case #1;

o   if the target matches a target in the existing stream, it may be
    case #2 or #3.

   Case #1 is handled in Section 5.3.1, while the other cases are
   handled in Section 5.3.2.

5.3.1  Path Convergence

   It is possible for an ST agent to receive a CONNECT message that
   contains a known SID, but from an ST agent other than the previous-
   hop ST agent of the stream with that SID. This might be the result of
   two branches of the tree forming the stream have joined back
   together.  Detection of this case and other possible sources was
   discussed in Section 5.2.

   SCMP does not allow for streams which have converged paths, i.e.,
   streams are always tree-shaped and not graph-like. At the point of
   convergence, the ST agent which detects the condition generates a
   REFUSE message with ReasonCode (PathConvergence). Also, as a help to
   the upstream ST agent, the detecting agent places the IP address of
   one of the stream's connected targets in the ValidTargetIPAddress
   field of the REFUSE message. This IP address will be used by upstream
   ST agents to avoid splitting the stream.

   An upstream ST agent that receives the REFUSE with ReasonCode
   (PathConvergence) will check to see if the listed IP address is one



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   of the known stream targets. If it is not, the REFUSE is propagated
   to the previous-hop agent. If the listed IP address is known by the
   upstream ST agent, this ST agent is the ST agent that caused the
   split in the stream. (This agent may even be the origin.) This agent
   then avoids splitting the stream by using the next-hop of that known
   target as the next-hop for the refused targets. It sends a CONNECT
   with the affected targets to the existing valid next-hop.

   The above process will proceed, hop by hop, until the
   ValidTargetIPAddress matches the IP address of a known target. The
   only case where this process will fail is when the known target is
   deleted prior to the REFUSE propagating to the origin. In this case
   the origin can just reissue the CONNECT and start the whole process
   over again.

5.3.2  Other Cases

   The remaining cases including a partially failed stream and a routing
   loop, are not easily distinguishable. In attempting recovery of a
   failed stream, an ST agent may issue new CONNECT messages to the
   affected targets. Such a CONNECT may reach an ST agent downstream of
   the failure before that ST agent has received a DISCONNECT from the
   neighborhood of the failure. Until that ST agent receives the
   DISCONNECT, it cannot distinguish between a failure recovery and an
   erroneous routing loop. That ST agent must therefore respond to the
   CONNECT with a REFUSE message with the affected targets specified in
   the TargetList and an appropriate ReasonCode (StreamExists).

   The ST agent immediately preceding that point, i.e., the latest ST
   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 ST 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, see Section 5.2.4. 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 ReasonCode
   (RouteLoop).

   The REFUSE message with this ReasonCode (RouteLoop) is propagated by
   each ST agent without retransmitting any CONNECT messages. At each ST
   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 ST agent or the origin
   itself. In the case of a stream recovery, the ST agent attempting the



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   recovery may issue new CONNECT messages to the same or to different
   next-hops.

   If an ST agent receives both a REFUSE message and a DISCONNECT
   message with a target in common then it can, for the each target in
   common, release the relevant resources and propagate neither the
   REFUSE nor the DISCONNECT.

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

5.4  Problems due to Routing Inconsistency

   When an intermediate ST agent receives a CONNECT, it invokes the
   routing algorithm to select the next-hop ST 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 ST agent an informative NOTIFY message with the
   appropriate ReasonCode (RouteBack), pertinent TargetList, and in the
   NextHopIPAddress element the address of the next-hop ST agent
   returned by its routing algorithm.

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

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



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5.5  Problems in Reserving Resources

   As mentioned in Section 1.4.5, resource reservation is handled by the
   LRM. The LRM may not be able to satisfy a particular request during
   stream setup or modification for a number of reasons, including a
   mismatched FlowSpec, an unknown FlowSpec version, an error in
   processing a FlowSpec, and an inability to allocate the requested
   resource. This section discusses these cases and specifies the
   ReasonCodes that should be used when these error cases are
   encountered.

5.5.1  Mismatched FlowSpecs

   In some cases the LRM may require a requested FlowSpec to match an
   existing FlowSpec, e.g., when adding new targets to an existing
   stream, see Section 4.6.1. In case of FlowSpec mismatch the LRM
   notifies the processing ST agent which should respond with ReasonCode
   (FlowSpecMismatch).

5.5.2  Unknown FlowSpec Version

   When the LRM is invoked, it is passed information including the
   version of the FlowSpec, see Section 4.5.2.2. If this version is not
   known by the LRM, the LRM notifies the ST agent. The ST agent should
   respond with a REFUSE message with ReasonCode (FlowVerUnknown).

5.5.3  LRM Unable to Process FlowSpec

   The LRM may encounter an LRM or FlowSpec specific error while
   attempting to satisfy a request. An example of such an error is given
   in Section 9.2.1. These errors are implementation specific and will
   not be enumerated with ST ReasonCodes. They are covered by a single,
   generic ReasonCode. When an LRM encounters such an error, it should
   notify the ST agent which should respond with the generic ReasonCode
   (FlowSpecError).

5.5.4  Insufficient Resources

   If the LRM cannot make the necessary reservations because sufficient
   resources are not available, an ST agent may:

o   try alternative paths to the targets: the ST agent calls the routing
    function to find a different path to the targets. If an alternative
    path is found, stream connection setup continues in the usual way,
    as described in Section 4.5.






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o   refuse to establish the stream along this path: the origin ST agent
    informs the application of the stream setup failure; intermediate
    and target ST agents issue a REFUSE message (as described in Section
    4.5.8) with ReasonCode (CantGetResrc).

   It depends on the local implementations whether an ST agent tries
   alternative paths or refuses to establish the stream. In any case, if
   enough resources cannot be found over different paths, the ST agent
   has to explicitly refuse to establish the stream.

5.6  Problems Caused by CHANGE Messages

   A CHANGE might fail for several reasons, including:

o   insufficient resources: the request may be for a larger amount of
    network resources when those resources are not available, ReasonCode
    (CantGetResrc);

o   a target application not agreeing to the change, ReasonCode
    (ApplRefused);

   The affected stream can be left in one of two states as a result of
   change failures: a) the stream can revert back to the state it was in
   prior to the CHANGE message being processed, or b) the stream may be
   torn down.

   The expected common case of failure will be when the requested change
   cannot be satisfied, but the pre-change resources remain allocated
   and available for use by the stream. In this case, the ST agent at
   the point where the failure occurred must inform upstream ST agents
   of the failure. (In the case where this ST agent is the target, there
   may not actually be a failure, the application may merely have not
   agreed to the change). The ST agent informs upstream ST agents by
   sending a REFUSE message with ReasonCode (CantGetResrc or
   ApplRefused). To indicate that the pre-change FlowSpec is still
   available and that the stream still exists, the ST agent sets the E-
   bit of the REFUSE message to one (1), see Section 10.4.11. Upstream
   ST agents receiving the REFUSE message inform the LRM so that it can
   attempt to revert back to the pre-change FlowSpec. It is permissible,
   but not desirable, for excess resources to remain allocated.

   For the case when the attempt to change the stream results in the
   loss of previously reserved resources, the stream is torn down. This
   can happen, for instance, when the I-bit is set (Section 4.6.5) and
   the LRM releases pre-change stream resources before the new ones are
   reserved, and neither new nor former resources are available. In this
   case, the ST agent where the failure occurs must inform other ST
   agents of the break in the affected portion of the stream. This is



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   done by the ST agent by sending a REFUSE message upstream and a
   DISCONNECT message downstream, both with the ReasonCode
   (CantGetResrc). To indicate that pre-change stream resources have
   been lost, the E-bit of the REFUSE message is set to zero (0).

   Note that a failure to change the resources requested for specific
   targets should not cause other targets in the stream to be deleted.

5.7  Unknown Targets in DISCONNECT and CHANGE

   The handling of unknown targets listed in a DISCONNECT or CHANGE
   message is dependent on a stream's join authorization level, see
   Section 4.4.2. For streams with join authorization levels #0 and #1,
   see Section 4.4.2, all targets must be known. In this case, when
   processing a CHANGE message, the agent should generate a REFUSE
   message with ReasonCode (TargetUnknown). When processing a DISCONNECT
   message, it is possible that the DISCONNECT is a duplicate of an old
   request so the agent should respond as if it has successfully
   disconnected the target. That is, it should respond with an ACK
   message.

   For streams with join authorization level #2, it is possible that the
   origin is not aware of some targets that participate in the stream.
   The origin may delete or change these targets via the following
   flooding mechanism.

   If no next-hop ST agent can be associated with a target, the CHANGE/
   DISCONNECT message including the target is replicated to all known
   next-hop ST agents. This has the effect of propagating the CHANGE/
   DISCONNECT message to all downstream ST agents. Eventually, the ST
   agent that acts as the origin for the target (Section 4.6.3.1) is
   reached and the target is deleted.

   Target deletion/change via flooding is not expected to be the normal
   case. It is included to present the applications with uniform
   capabilities for all stream types. Flooding only applies to streams
   with join authorization level #2.

6.  Failure Detection and Recovery

6.1  Failure Detection

   The SCMP 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.



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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, the recovery procedures described in
   Section 6.2 are initiated by the ST agents.

6.1.1  Network Failures

   An ST agent can detect network failures by two mechanisms:

   o   the network can report a failure, or

   o   the ST agent can discover a failure by itself.

   They differ in the amount of information that an 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. 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.

6.1.2  Detecting ST Agents Failures

   Each ST agent periodically sends each neighbor with which it shares
   one or more streams a HELLO message. This message exchange is between
   ST agents, not entities representing streams or applications. 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 ST agents that share active streams can
   participate in this exchange and it is an error to send a HELLO
   message to a neighbor ST agent with no streams in common, e.g., to
   check whether it is active. STATUS messages can be used to poll the
   status of neighbor ST agents, see Section 8.4.

   For the purpose of HELLO message exchange, stream existence is
   bounded by ACCEPT and DISCONNECT/REFUSE processing and is defined for
   both the upstream and downstream case. A stream to a previous-hop is



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   defined to start once an ACCEPT message has been forwarded upstream.
   A stream to a next-hop is defined to start once the received ACCEPT
   message has been acknowledged. A stream is defined to terminate once
   an acknowledgment is sent for a received DISCONNECT or REFUSE
   message, and an acknowledgment for a sent DISCONNECT or REFUSE
   message has been received.

   The HELLO message has two fields:

   o   a HelloTimer field that is in units of milliseconds modulo the
       maximum for the field size, and

   o   a Restarted-bit specifying that the ST agent has been restarted
       recently.

   The HelloTimer must appear to be incremented every millisecond
   whether a HELLO message is sent or not. 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 in all HELLO messages sent in the following
   HelloTimerHoldDown seconds.

   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 state.
   If it shares streams with that neighbor, it must initiate stream
   recovery activity, see Section 6.2. If it does not share streams with
   that neighbor, it should not attempt to create one until that bit is
   no longer set. If an ST agent receives a CONNECT message from a
   neighbor whose Restarted-bit is still set, the agent must respond
   with an ERROR message with the appropriate ReasonCode
   (RestartRemote). If an agent receives a CONNECT message while the
   agent's own Restarted- bit is set, the agent must respond with an
   ERROR message with the appropriate ReasonCode (RestartLocal).

   Each ST stream has an associated RecoveryTimeout value. This value is
   assigned by the origin and carried in the CONNECT message, see
   Section 4.5.10. Each agent checks to see if it can support the
   requested value. If it can not, it updates the value to the smallest
   timeout interval it can support. The RecoveryTimeout used by a
   particular stream is obtained from the ACCEPT message, see Section
   4.5.10, and is the smallest value seen across all ACCEPT messages
   from participating targets.

   An ST agent must send HELLO messages to its neighbor with a period
   shorter than the smallest RecoveryTimeout of all the active streams
   that pass between the two ST agents, regardless of direction. This
   period must be smaller by a factor, called HelloLossFactor, which is



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   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.

   An ST agent must expect to receive at least one new HELLO message
   from each neighbor at least as frequently as the smallest
   RecoveryTimeout of any active stream in common with that neighbor.
   The agent can detect duplicate or delayed HELLO messages by comparing
   the HelloTimer field of the most recent valid HELLO message from that
   neighbor with the HelloTimer field of an incoming HELLO message.
   Valid incoming HELLO messages will have a HelloTimer field that is
   greater than the field contained in the previously received valid
   HELLO message by the time elapsed since the previous message was
   received. Actual evaluation of the elapsed time interval should take
   into account the maximum likely delay variance from that neighbor.

   If the ST agent does not receive a valid HELLO message within the
   RecoveryTimeout period 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, as described
   below in Section 6.2.

6.2  Failure Recovery

   If an intermediate ST agent fails or a network or part of a network
   fails, the previous-hop ST agent and the various next-hop ST agents
   will discover the fact by the failure detection mechanism described
   in Section 6.1.

   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 typically a homogeneous system,
   failure recovery in the network may be a relatively faster and
   simpler operation. Therefore an ST agent that detects a failure
   should attempt to fix the network failure before attempting recovery
   of the ST stream. If the stream that existed between two ST agents
   before the failure cannot be reconstructed by network recovery
   mechanisms alone, then the ST stream recovery mechanism must be
   invoked.



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

o   An ST agent that is a next-hop from a failure 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 for each active stream from that neighbor it should
    first send a REFUSE message upstream with the appropriate ReasonCode
    (STAgentFailure). This is done to the neighbor to speed up the
    failure recovery in case the hop is unidirectional, i.e., the
    neighbor can hear the ST agent but the ST agent cannot hear the
    neighbor. The ST agent detecting the failure must then, for each
    active stream from that neighbor, send DISCONNECT messages with the
    same ReasonCode toward the targets. All downstream ST agents process
    this DISCONNECT message just like the DISCONNECT that tears down the
    stream. If recovery is successful, targets will receive new CONNECT
    messages.

o   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 try and reconstruct
    (explained below) the affected streams, for those streams that do
    not have the NoRecovery option selected. If it cannot communicate
    with the next-hop, then the ST agent detecting the failure sends a
    DISCONNECT message, for each affected stream, with the appropriate
    ReasonCode (STAgentFailure) 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.

   Based on the NoRecovery option, the ST agent that is the previous-hop
   before the failed component takes the following actions:

o   If the NoRecovery option is selected, then the ST agent sends, per
    affected stream, a REFUSE message with the appropriate ReasonCode
    (STAgentFailure) to the previous-hop. The TargetList in these
    messages contains all the targets that were reached through the
    broken branch. As discussed in Section 5.1.2, 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. The application at the origin can attempt recovery of
    the stream by sending a new CONNECT to the affected targets. For
    established streams, the new CONNECT will be treated by intermediate
    ST agents as an addition of new targets into the established stream.






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o   If the NoRecovery option is not selected, the ST agent can attempt
    recovery of the affected streams. It does so one a stream by stream
    basis 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
    ReasonCode (CantRecover) 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 ST 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 are 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.

   Upon receiving an ACCEPT during the a stream recovery, the agent
   reconstructing the stream must ensure that the FlowSpec and other
   stream attributes (e.g., MaxMsgSize and RecoveryTimeout) of the re-
   established stream are equal to, or are less restrictive, than the
   pre-failure stream. If they are more restrictive, the recovery
   attempt must be aborted. If they are equal, or are less restrictive,
   then the recovery attempt is successful. When the attempt is a
   success, failure recovery related ACCEPTs are not forwarded upstream
   by the recovering agent.

   Any ST agent that decides that enough recovery attempts have been
   made, or that recovery attempts have no chance of succeeding, may
   indicate that no further attempts at recovery should be made. This is
   done by setting the N-bit in the REFUSE message, see Section 10.4.11.
   This bit must be set by agents, including the target, that know that
   there is no chance of recovery succeeding. An ST agent that receives
   a REFUSE message with the N-bit set (1) will not attempt recovery,
   regardless of the NoRecovery option, and it will set the N-bit when
   propagating the REFUSE message upstream.

6.2.1  Problems in Stream Recovery

   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 ERROR messages for the new CONNECTs that are
   caused by internet routing errors. The ST agent attempting the



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   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 ST 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 message with the
   appropriate ReasonCode (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. Therefore, the ST agent that receives the ERROR message with
   ReasonCode (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 ReasonCode
   (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 ST agent receives a second CONNECT message for a target
   that recently resulted in a REFUSE, that ST 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.

   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. FlowSpec comparison is done by the
   LRM. If the selections of the parameters are different, then the ST



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   agent that is attempting recovery will send the origin a NOTIFY
   message with the appropriate ReasonCode (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.

6.3  Stream Preemption

   As mentioned in Section 1.4.5, it is possible that the LRM decides to
   break a stream intentionally. This is called stream preemption.
   Streams are expected to be preempted in order to free resources for a
   new stream which has a higher priority.

   If the LRM decides that it is necessary to preempt one or more of the
   stream traversing it, the decision on which streams have to be
   preempted has to be made. There are two ways for an application to
   influence such decision:

   1.  based on FlowSpec information. For instance, with the ST2+
       FlowSpec, streams can be assigned a precedence value from 0
       (least important) to 256 (most important). This value is
       carried in the FlowSpec when the stream is setup, see Section
       9.2, so that the LRM is informed about it.

   2.  with the group mechanism. An application may specify that a set
       of streams are related to each other and that they are all
       candidate for preemption if one of them gets preempted. It can
       be done by using the fate-sharing relationship defined in
       Section 7.1.2. This helps the LRM making a good choice when
       more than one stream have to be preempted, because it leads to
       breaking a single application as opposed to as many
       applications as the number of preempted streams.

   If the LRM preempts a stream, it must notify the local ST agent. The
   following actions are performed by the ST agent:

o   The ST agent at the host where the stream was preempted sends
    DISCONNECT messages with the appropriate ReasonCode
    (StreamPreempted) toward the affected targets. It sends a REFUSE
    message with the appropriate ReasonCode (StreamPreempted) to the
    previous-hop.

o   A previous-hop ST agent of the preempted stream acts as in case of
    failure recovery, see Section 6.2.

o   A next-hop ST agent of the preempted stream acts as in case of
    failure recovery, see Section 6.2.



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   Note that, as opposite to failure recovery, there is no need to
   verify that the failure actually occurred, because this is explicitly
   indicated by the ReasonCode (StreamPreempted).

7.  A Group of Streams

   There may be 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.

   A group consists of a set of streams and a relationship. The set of
   streams may be empty. The relationship applies to all group members.
   Each group is identified by a group name. The group name must be
   globally unique.

   Streams belong to the same group if they have the same GroupName in
   the GroupName field of the Group parameter, see Section 10.3.2. The
   relationship is defined by the Relationship field. Group membership
   must be specified at stream creation time and persists for the whole
   stream lifetime. A single stream may belong to multiple groups.

   The ST agent that creates a new group is called group initiator. Any
   ST agent can be a group initiator. The initiator allocates the
   GroupName and the Relationship among group members. The initiator may
   or may not be the origin of a stream belonging to the group.
   GroupName generation is described in Section 8.2.

7.1  Basic Group Relationships

   This version of ST defines four basic group relationships. An ST2+
   implementation must support all four basic relationships. Adherence
   to specified relationships are usually best effort. The basic
   relationships are described in detail below in Section 7.1.1 -
   Section 7.1.4.

7.1.1  Bandwidth Sharing

   Streams associated with the same group share the same network
   bandwidth. The intent is to support applications such as audio
   conferences where, of all participants, only some are allowed to
   speak at one time. In such a scenario, global bandwidth utilization
   can be lowered by allocating only those resources that can be used at
   once, e.g., it is sufficient to reserve bandwidth for a small set of
   audio streams.

   The basic concept of a shared bandwidth group is that the LRM will
   allocate up to some specified multiplier of the most demanding stream
   that it knows about in the group. The LRM will allocate resources



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   incrementally, as stream setup requests are received, until the total
   group requirements are satisfied. Subsequent setup requests will
   share the group's resources and will not need any additional
   resources allocated. The procedure will result in standard allocation
   where only one stream in a group traverses an agent, and shared
   allocations where multiple streams traverse an agent.

   To illustrate, let's call the multiplier mentioned above "N", and the
   most demanding stream that an agent knows about in a group Bmax. For
   an application that intends to allow three participants to speak at
   the same time, N has a value of three and each LRM will allocate for
   the group an amount of bandwidth up to 3*Bmax even when there are
   many more steams in the group. The LRM will reserve resources
   incrementally, per stream request, until N*Bmax resources are
   allocated. Each agent may be traversed by a different set and number
   of streams all belonging to the same group.

   An ST agent receiving a stream request presents the LRM with all
   necessary group information, see Section 4.5.2.2. If maximum
   bandwidth, N*Bmax, for the group has already been allocated and a new
   stream with a bandwidth demand less than Bmax is being established,
   the LRM won't allocate any further bandwidth.

   If there is less than N*Bmax resources allocated, the LRM will expand
   the resources allocated to the group by the amount requested in the
   new FlowSpec, up to N*Bmax resources. The LRM will update the
   FlowSpec based on what resources are available to the stream, but not
   the total resources allocated for the group.

   It should be noted that ST agents and LRMs become aware of a group's
   requirements only when the streams belonging to the group are
   created.  In case of the bandwidth sharing relationship, an
   application should attempt to establish the most demanding streams
   first to minimize stream setup efforts. If on the contrary the less
   demanding streams are built first, it will be always necessary to
   allocate additional bandwidth in consecutive steps as the most
   demanding streams are built. It is also up to the applications to
   coordinate their different FlowSpecs and decide upon an appropriate
   value for N.

7.1.2  Fate Sharing

   Streams belonging to this group share the same fate. If a stream is
   deleted, the other members of the group are also deleted. This is
   intended to support stream preemption by indicating which streams are
   mutually related. If preemption of multiple streams is necessary,
   this information can be used by the LRM to delete a set of related
   streams, e.g., with impact on a single application, instead of making



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   a random choice with the possible effect of interrupting several
   different applications. This attribute does not apply to normal
   stream shut down, i.e., ReasonCode (ApplDisconnect). On normal
   disconnect, other streams belonging to such groups remain active.

   This relationship provides a hint on which streams should be
   preempted. Still, the LRM responsible for the preemption is not
   forced to behave accordingly, and other streams could be preempted
   first based on different criteria.

7.1.3  Route Sharing

   Streams belonging to this group share the same paths as much as is
   possible. This can be desirable for several reasons, e.g., to exploit
   the same allocated resources or in the attempt to maintain the
   transmission order. An ST agent attempts to select the same path
   although the way this is implemented depends heavily on the routing
   algorithm which is used.

   If the routing algorithm is sophisticated enough, an ST agent can
   suggest that a stream is routed over an already established path.
   Otherwise, it can ask the routing algorithm for a set of legal routes
   to the destination and check whether the desired path is included in
   those feasible.

   Route sharing is a hint to the routing algorithm used by ST. Failing
   to route a stream through a shared path should not prevent the
   creation of a new stream or result in the deletion of an existing
   stream.

7.1.4  Subnet Resources Sharing

   This relationship provides a hint to the data link layer functions.
   Streams belonging to this group may share the same MAC layer
   resources. As an example, the same MAC layer multicast address may be
   used for all the streams in a given group. This mechanism allows for
   a better utilization of MAC layer multicast addresses and it is
   especially useful when used with network adapters that offer a very
   small number of MAC layer multicast addresses.

7.2  Relationships Orthogonality

   The four basic relationships, as they have been defined, are
   orthogonal. This means, any combinations of the basic relationships
   are allowed. For instance, let's consider an application that
   requires full-duplex service for a stream with multiple targets.
   Also, let's suppose that only N targets are allowed to send data back
   to the origin at the same time. In this scenario, all the reverse



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   streams could belong to the same group. They could be sharing both
   the paths and the bandwidth attributes. The Path&Bandwidth sharing
   relationship is obtained from the basic set of relationships. This
   example is important because it shows how full-duplex service can be
   efficiently obtained in ST.

8.  Ancillary Functions

   Certain functions are required by ST host and intermediate agent
   implementations. Such functions are described in this section.

8.1  Stream ID Generation

   The stream ID, or SID, is composed of 16-bit unique identifier and
   the stream origin's 32-bit IP address. Stream IDs must be globally
   unique.  The specific definition and format of the 16 -bit field is
   left to the implementor. This field is expected to have only local
   significance.

   An ST implementation has to provide a stream ID generator facility,
   so that an application or higher layer protocol can obtain a unique
   IDs from the ST layer. This is a mechanism for the application to
   request the allocation of stream ID that is independent of the
   request to create a stream. The Stream ID is used by the application
   or higher layer protocol when creating the streams.

   For instance, the following two functions could be made available:

   o   AllocateStreamID() -> result, StreamID

   o   ReleaseStreamID(StreamID) -> result

   An implementation may also provide a StreamID deletion function.

8.2  Group Name Generator

   GroupName generation is similar to Stream ID generation. The
   GroupName includes a 16-bit unique identifier, a 32-bit creation
   timestamp, and a 32-bit IP address. Group names are globally unique.
   A GroupName includes the creator's IP address, so this reduces a
   global uniqueness problem to a simple local problem. The specific
   definitions and formats of the 16-bit field and the 32-bit creation
   timestamp are left to the implementor. These fields must be locally
   unique, and only have local significance.

   An ST implementation has to provide a group name generator facility,
   so that an application or higher layer protocol can obtain a unique
   GroupName from the ST layer. This is a mechanism for the application



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   to request the allocation of a GroupName that is independent of the
   request to create a stream. The GroupName is used by the application
   or higher layer protocol when creating the streams that are to be
   part of the group.

   For instance, the following two functions could be made available:

   o   AllocateGroupName() -> result, GroupName

   o   ReleaseGroupName(GroupName) -> result

   An implementation may also provide a GroupName deletion function.

8.3  Checksum Computation

   The standard Internet checksum algorithm is used for ST: "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 (0)." See
   [RFC1071], [RFC1141], and [RFC791] for suggestions for efficient
   checksum algorithms.

8.4  Neighbor ST Agent Identification and Information Collection

   The STATUS message can be used to collect information about neighbor
   ST agents, streams the neighbor supports, and specific targets of
   streams the neighbor supports. An agent receiving a STATUS message
   provides the requested information via a STATUS-RESPONSE message.

   The STATUS message can be used to collect different information from
   a neighbor. It can be used to:

o   identify ST capable neighbors. If an ST agent wishes to check if
    a neighbor is ST capable, it should generate a STATUS message with
    an SID which has all its fields set to zero. An agent receiving a
    STATUS message with such SID should answer with a STATUS-RESPONSE
    containing the same SID, and no other stream information. The
    receiving ST agent must answer as soon as possible to aid in Round
    Trip Time estimation, see Section 8.5;

o   obtain information on a particular stream. If an ST agent wishes to
    check a neighbor's general information related to a specific
    stream, it should generate a STATUS message containing the stream's
    SID. An ST agent receiving such a message, will first check to see
    if the stream is known. If not known, the receiving ST agent sends a
    STATUS-RESPONSE containing the same SID, and no other stream
    information. If the stream is known, the receiving ST agent sends a
    STATUS-RESPONSE containing the stream's SID, IPHops, FlowSpec, group



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    membership (if any), and as many targets as can be included in a
    single message as limited by MTU, see Section 5.1.2. Note that all
    targets may not be included in a response to a request for general
    stream information. If information on a specific target in a stream
    is desired, the mechanism described next should be used.

o   obtain information on particular targets in a stream. If an ST agent
    wishes to check a neighbor's information related to one or more
    specific targets of a specific stream, it should generate a STATUS
    message containing the stream's SID and a TargetList parameter
    listing the relevant targets. An ST agent receiving such a message,
    will first check to see if the stream and target are known. If the
    stream is not known, the agent follows the process described above.
    If both the stream and targets are known, the agent responds with
    STATUS-RESPONSE containing the stream's SID, IPHops, FlowSpec, group
    membership (if any), and the requested targets that are known. If
    the stream is known but the target is not, the agent responds with a
    STATUS-RESPONSE containing the stream's SID, IPHops, FlowSpec, group
    membership (if any), but no targets.

   The specific formats for STATUS and STATUS-RESPONSE messages are
   defined in Section 10.4.12 and Section 10.4.13.

8.5  Round Trip Time Estimation

   SCMP is made reliable through use of retransmission when an expected
   acknowledgment is not received in a timely manner. Timeout and
   retransmission algorithms are implementation dependent and are
   outside the scope of this document. However, it must be reasonable
   enough not to cause excessive retransmission of SCMP messages while
   maintaining the robustness of the protocol. Algorithms on this
   subject are described in [WoHD95], [Jaco88], [KaPa87].

   Most existing algorithms are based on an estimation of the Round Trip
   Time (RTT) between two hosts. With SCMP, if an ST agent wishes to
   have an estimate of the RTT to and from a neighbor, it should
   generate a STATUS message with an SID which has all its fields set to
   zero. An ST agent receiving a STATUS message with such SID should
   answer as rapidly as possible with a STATUS-RESPONSE message
   containing the same SID, and no other stream information. The time
   interval between the send and receive operations can be used as an
   estimate of the RTT to and from the neighbor.

8.6  Network MTU Discovery

   At connection setup, the application at the origin asks the local ST
   agent to create streams with certain QoS requirements. The local ST
   agent fills out its network MTU value in the MaxMsgSize parameter in



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   the CONNECT message and forwards it to the next-hop ST agents. Each
   ST agent in the path checks to see if it's network MTU is smaller
   than the one specified in the CONNECT message and, if it is, the ST
   agent updates the MaxMsgSize in the CONNECT message to it's network
   MTU. If the target application decides to accept the stream, the ST
   agent at the target copies the MTU value in the CONNECT message to
   the MaxMsgSize field in the ACCEPT message and sends it back to the
   application at the origin. The MaxMsgSize field in the ACCEPT message
   is the minimum MTU of the intervening networks to that target. If the
   application has multiple targets then the minimum MTU of the stream
   is the smallest MaxMsgSize received from all the ACCEPT messages. It
   is the responsibility of the application to segment its PDUs
   according to the minimum MaxMsgSize of the stream since no data
   fragmentation is supported during the data transfer phase. If a
   particular target's MaxMsgSize is unacceptable to an application, it
   may disconnect the target from the stream and assume that the target
   cannot be supported.  When evaluating a particular target's
   MaxMsgSize, the application or the application interface will need to
   take into account the size of the ST data header.

8.7  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-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, see [RFC1700], to indicate an ST packet is enclosed,
    as opposed to TCP or UDP, for example.

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, see [RFC1700]. This feature is not implemented
    uniformly in the Internet, so its use can't be precisely defined
    here.



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   IP encapsulation adds little difficulty for the ST agent that
   receives the packet. However, when IP encapsulation is performed it
   must be done in both directions. To process the encapsulated IP
   message, the ST agents simply remove the IP header and proceed with
   ST header 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. The
   routing function provides ST agents with the route and capability
   information needed to support encapsulation.

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

   Applications are informed about the number of IP hops traversed on
   the path to each target. The IPHops field of the CONNECT message, see
   Section 10.4.4, carries the number of traversed IP hops to the target
   application. The field is incremented by each ST agent when IP
   encapsulation will be used to reach the next-hop ST agent. The number
   of IP hops traversed is returned to the origin in the IPHops field of
   the ACCEPT message, Section 10.4.1.

   When using IP Encapsulation, the MaxMsgSize field will not reflect
   the MTU of the IP encapsulated segments. This means that IP
   fragmentation and reassembly may be needed in the IP cloud to support
   a message of MaxMsgSize. IP fragmentation can only occur when the MTU
   of the IP cloud, less IP header length, is the smallest MTU in a
   stream's network path.

8.8  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 may be used if
   it is implemented in the next-hop ST agents and in the intervening IP
   routers.

   When the stream is established, the collection of next-hop ST agents
   must be set up as an IP multicast group. The ST agent must allocate
   an appropriate IP multicast address (see Section 10.3.3) and fill
   that address in the IPMulticastAddress field of the CONNECT message.
   The IP multicast address in the CONNECT message is used to inform the
   next-hop ST agents that they should join the multicast group to
   receive subsequent PDUs. Obviously, the CONNECT message itself must
   be sent using unicast. The next-hop ST agents must be able to receive
   on the specified multicast address in order to accept the connection.



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   If the next-hop ST agent can not receive on the specified multicast
   address, it sends a REFUSE message with ReasonCode (BadMcastAddress).
   Upon receiving the REFUSE, the upstream agent can choose to retry
   with a different multicast address. Alternatively, it can choose to
   lose the efficiency of multicast and use unicast delivery.

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

           224.0.0.7 All ST routers (intermediate agents)
           224.0.0.8 All ST hosts (agents)

   In addition, a block of transient IP multicast addresses, 224.1.0.0 -
   224.1.255.255, has been allocated for ST multicast groups. For
   instance, the following two functions could be made available:

   o   AllocateMcastAddr() -> result, McastAddr

   o   ListenMcastAddr(McastAddr) -> result

   o   ReleaseMcastAddr(McastAddr) -> result

9.  The ST2+ Flow Specification

   This section defines the ST2+ flow specification. The flow
   specification contains the user application requirements in terms of
   quality of service. Its contents are LRM dependent and are
   transparent to the ST2 setup protocol. ST2 carries the flow
   specification as part of the FlowSpec parameter, which is described
   in Section 10.3.1. The required ST2+ flow specification is included
   in the protocol only to support interoperability. ST2+ also defines a
   "null" flow specification to be used only to support testing.

   ST2 is not dependent on a particular flow specification format and it
   is expected that other versions of the flow specification will be
   needed in the future. Different flow specification formats are
   distinguished by the value of the Version field of the FlowSpec
   parameter, see Section 10.3.1. A single stream is always associated
   with a single flow specification format, i.e., the Version field is
   consistent throughout the whole stream. The following Version field
   values are defined:










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   0 - Null FlowSpec       /* must be supported */
   1 - ST Version 1
   2 - ST Version 1.5
   3 - RFC 1190 FlowSpec
   4 - HeiTS FlowSpec
   5 - BerKom FlowSpec
   6 - RFC 1363 FlowSpec
   7 - ST2+ FlowSpec       /* must be supported */

   FlowSpecs version #0 and #7 must be supported by ST2+
   implementations.  Version numbers in the range 1-6 indicate flow
   specifications are currently used in existing ST2 implementations.
   Values in the 128-255 range are reserved for private and experimental
   use.

   In general, a flow specification may support sophisticated flow
   descriptions. For example, a flow specification could represent sub-
   flows of a particular stream. This could then be used to by a
   cooperating application and LRM to forward designated packets to
   specific targets based on the different sub-flows. The reserved bits
   in the ST2 Data PDU, see Section 10.1, may be used with such a flow
   specification to designate packets associated with different sub-
   flows. The ST2+ FlowSpec is not so sophisticated, and is intended for
   use with applications that generate traffic at a single rate for
   uniform delivery to all targets.

9.1  FlowSpec Version #0 - (Null FlowSpec)

   The flow specification identified by a #0 value of the Version field
   is called the Null FlowSpec. This flow specification causes no
   resources to be allocated. It is ignored by the LRMs. Its contents
   are never updated. Stream setup takes place in the usual way leading
   to successful stream establishment, but no resources are actually
   reserved.

   The purpose of the Null FlowSpec is that of facilitating
   interoperability tests by allowing streams to be built without
   actually allocating the correspondent amount of resources. The Null
   FlowSpec may also be used for testing and debugging purposes.

   The Null FlowSpec comprises the 4-byte FlowSpec parameter only, see
   Section 10.3.1. The third byte (Version field) must be set to 0.

9.2  FlowSpec Version #7 - ST2+ FlowSpec

   The flow specification identified by a #7 value of the Version field
   is the ST2+ FlowSpec, to be used by all ST2+ implementations. It
   allows the user applications to express their real-time requirements



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   in the form of a QoS class, precedence, and three basic QoS
   parameters:

   o   message size,

   o   message rate,

   o   end-to-end delay.

   The QoS class indicates what kind of QoS guarantees are expected by
   the application, e.g., strict guarantees or predictive, see Section
   9.2.1. QoS parameters are expressed via a set of values:

o   the "desired" values indicate the QoS desired by the application.
    These values are assigned by the application and never modified by
    the LRM.

o   the "limit" values indicate the lowest QoS the application is
    willing to accept. These values are also assigned by the application
    and never modified by the LRM.

o   the "actual" values indicate the QoS that the system is able to
    provide. They are updated by the LRM at each node. The "actual"
    values are always bounded by the "limit" and "desired" values.

9.2.1  QoS Classes

   Two QoS classes are defined:

   1 - QOS_PREDICTIVE      /* QoSClass field value = 0x01, must be
                              supported*/
   2 - QOS_GUARANTEED      /* QoSClass field value = 0x10, optional */

o   The QOS_PREDICTIVE class implies that the negotiated QoS may be
    violated for short time intervals during the data transfer. An
    application has to provide values that take into account the
    "normal" case, e.g., the "desired" message rate is the allocated rate
    for the transmission. Reservations are done for the "normal" case as
    opposite to the peak case required by the QOS_GUARANTEED service
    class. This QoS class must be supported by all implementations.

o   The QOS_GUARANTEED class implies that the negotiated QoS for the
    stream is never violated during the data transfer. An application
    has to provide values that take into account the worst possible
    case, e.g., the "desired" message rate is the peak rate for the
    transmission. As a result, sufficient resources to handle the peak
    rate are reserved. This strategy may lead to overbooking of
    resources, but it provides strict real-time guarantees. Support of



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    this QoS class is optional.

   If a LRM that doesn't support class QOS_GUARANTEED receives a
   FlowSpec containing QOS_GUARANTEED class, it informs the local ST
   agent. The ST agent may try different paths or delete the
   correspondent portion of the stream as described in Section 5.5.3,
   i.e., ReasonCode (FlowSpecError).

9.2.2  Precedence

   Precedence is the importance of the connection being established.
   Zero represents the lowest precedence. The lowest level is expected
   to be used by default. 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.

9.2.3  Maximum Data Size

   This parameter is expressed in bytes. It represents the maximum
   amount of data, excluding ST and other headers, allowed to be sent in
   a messages as part of the stream. The LRM first checks whether it is
   possible to get the value desired by the application (DesMaxSize). If
   not, it updates the actual value (ActMaxSize) with the available size
   unless this value is inferior to the minimum allowed by the
   application (LimitMaxSize), in which case it informs the local ST
   agent that it is not possible to build the stream along this path.

9.2.4  Message Rate

   This parameter is expressed in messages/second. It represents the
   transmission rate for the stream. The LRM first checks whether it is
   possible to get the value desired by the application (DesRate). If
   not, it updates the actual value (ActRate) with the available rate
   unless this value is inferior to the minimum allowed by the
   application (LimitRate), in which case it informs the local ST agent
   that it is not possible to build the stream along this path.

9.2.5  Delay and Delay Jitter

   The delay parameter is expressed in milliseconds. It represents the
   maximum end-to-end delay for the stream. The LRM first checks whether
   it is possible to get the value desired by the application
   (DesMaxDelay). If not, it updates the actual value (ActMaxDelay) with
   the available delay unless this value is greater than the maximum
   delay allowed by the application (LimitMaxDelay), in which case it
   informs the local ST agent that it is not possible to build the



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   stream along this path.

   The LRM also updates at each node the MinDelay field by incrementing
   it by the minimum possible delay to the next-hop. Information on the
   minimum possible delay allows to calculate the maximum end-to-end
   delay range, i.e., the time interval in which a data packet can be
   received. This interval should not exceed the DesMaxDelayRange value
   indicated by the application. The maximum end-to-end delay range is
   an upper bound of the delay jitter.

9.2.6  ST2+ FlowSpec Format

   The ST2+ FlowSpec has the following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    QosClass   |  Precedence   |            0(unused)          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             DesRate                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            LimitRate                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             ActRate                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            DesMaxSize         |           LimitMaxSize        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            ActMaxSize         |           DesMaxDelay         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LimitMaxDelay      |           ActMaxDelay         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            DesMaxDelayRange   |           ActMinDelay         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 9: The ST2+ FlowSpec.

   The LRM modifies only "actual" fields, i.e., those beginning with
   "Act". The user application assigns values to all other fields.

o   QoSClass indicates which of the two defined classes of service
    applies. The two classes are: QOS_PREDICTIVE (QoSClass = 1) and
    QOS_GUARANTEED (QoSClass = 2).

o   Precedence indicates the stream's precedence. Zero represents the
    lowest precedence, and should be the default value.

o   DesRate is the desired transmission rate for the stream in messages/
    second. This field is set by the origin and is not modified by



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    intermediate agents.

o   LimitRate is the minimum acceptable transmission rate in messages/
    second. This field is set by the origin and is not modified by
    intermediate agents.

o   ActRate is the actual transmission rate allocated for the stream in
    messages/second. Each agent updates this field with the available
    rate unless this value is less than LimitRate, in which case a
    REFUSE is generated.

o   DesMaxSize is the desired maximum data size in bytes that will be
    sent in a message in the stream. This field is set by the origin.

o   LimitMaxSize is the minimum acceptable data size in bytes. This
    field is set by the origin

o   ActMaxSize is the actual maximum data size that may be sent in a
    message in the stream. This field is updated by each agent based on
    MTU and available resources. If available maximum size is less than
    LimitMaxSize, the connection must be refused with ReasonCode
    (CantGetResrc).

o   DesMaxDelay is the desired maximum end-to-end delay for the stream
    in milliseconds. This field is set by the origin.

o   LimitMaxDelay is the upper-bound of acceptable end-to-end delay for
    the stream in milliseconds. This field is set by the origin.

o   ActMaxDelay is the maximum end-to-end delay that will be seen by
    data in the stream. Each ST agent adds to this field the maximum
    delay that will be introduced by the agent, including transmission
    time to the next-hop ST agent. If the actual maximum exceeds
    LimitMaxDelay, then the connection is refused with ReasonCode
    (CantGetResrc).

o   DesMaxDelayRange is the desired maximum delay range that may be
    encountered end-to-end by stream data in milliseconds. This value is
    set by the application at the origin.

o   ActMinDelay is the actual minimum end-to-end delay that will be
    encountered by stream data in milliseconds. Each ST agent adds to
    this field the minimum delay that will be introduced by the agent,
    including transmission time to the next-hop ST agent. Each agent
    must add at least 1 millisecond. The delay range for the stream can
    be calculated from the actual maximum and minimum delay fields. It
    is expected that the range will be important to some applications.




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10.  ST2 Protocol Data Units Specification

10.1  Data PDU

   IP and ST packets can be distinguished by the IP Version Number
   field, i.e., the first four (4) bits of the packet; ST has been
   assigned the value 5 (see [RFC1700]). There is no requirement for
   compatibility between IP and ST packet headers beyond the first four
   bits. (IP uses value 4.)

   The ST PDUs sent between ST agents consist of an ST Header
   encapsulating either a higher layer PDU or an ST Control Message.
   Data packets are distinguished from control messages via the D-bit
   (bit 8) in the ST header.

   The ST Header also includes an ST Version Number, a total length
   field, a header checksum, a unique id, and the stream origin 32-bit
   IP address. The unique id and the stream origin 32-bit IP address
   form the stream id (SID). This is shown in Figure 10. Please refer to
   Section 10.6 for an explanation of the notation.

        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=3 |D| Pri |   0   |            TotalBytes         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          HeaderChecksum       |            UniqueID           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         OriginIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 10: ST Header

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

o   Ver is the ST Version Number. The value for the current ST2+ version
    is 3.

o   D (bit 8) is set to 1 in all ST data packets and to 0 in all SCMP
    control messages.

o   Pri (bits 9-11) is the packet-drop priority field with zero (0)
    being lowest priority and seven the highest. The field is to be used
    as described in Section 3.2.2.






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

o   HeaderChecksum covers only the ST Header (12 bytes). The ST Protocol
    uses 16-bit checksums here in the ST Header and in each Control
    Message. For checksum computation, see Section 8.3.

o   UniqueID is the first element of the stream ID (SID). It is locally
    unique at the stream origin, see Section 8.1.

o   OriginIPAddress is the second element of the SID. It is the 32-bit
    IP address of the stream origin, see Section 8.1.

   Bits 12-15 must be set to zero (0) when using the flow specifications
   defined in this document, see Section 9. They may be set accordingly
   when other flow specifications are used, e.g., as described in
   [WoHD95].

10.1.1  ST Data Packets

   ST packets whose D-bit is non-zero are 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.

10.2  Control PDUs

   SCMP control messages are exchanged between neighbor ST agents using
   a D-bit of zero (0). The control protocol follows a request-response
   model with all requests expecting responses. Retransmission after
   timeout (see Section 4.3) 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
   is partitioned and a control message per partition is sent (see
   Section 5.1.2). All control messages have the following format












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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  OpCode       |     Options   |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Reference            |          LnkReference         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |            ReasonCode         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                      OpCodeSpecificData                       :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 11: ST Control Message Format

o   OpCode identifies the type of control message.

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

o   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 (4).

o   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 (0) 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.

o   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 from the same ST agent. Examples are CONNECT and CHANGE
    messages that are first acknowledged hop-by-hop and then lead to an
    ACCEPT or REFUSE response from each target.

o   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).





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o   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 to be error free before
    it is acted upon.

o   ReasonCode is set to zero (0 = NoError) in most SCMP messages.
    Otherwise, it can be set to an appropriate value to indicate an
    error situation as defined in Section 10.5.3.

o   OpCodeSpecificData 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 (0) are included to allow the
    format to match that of the corresponding request message. The
    OpCodeSpecificData may also contain optional parameters. The
    specifics of OpCodeSpecificData are defined in Section 10.3.

10.3  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 four
   (4) bytes, the parameter is padded with one to three zero (0) bytes.
   PBytes is thus always a multiple of four (4). Parameters can be
   present in any order.

10.3.1  FlowSpec

   The FlowSpec parameter (PCode = 1) is used in several SCMP messages
   to convey the ST2 flow specification. The FlowSpec parameter has the
   following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   PCode = 1   |    PBytes     |   Version     |       0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                        FlowSpec detail                        :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 12: FlowSpec Parameter




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o   the Version field contains the FlowSpec version.

o   the FlowSpec detail field contains the flow specification and is
    transparent to the ST agent. It is the data structure to be passed
    to the LRM. It must be 4-byte aligned.

   The Null FlowSpec, see Section 9.1, has no FlowSpec detail field.
   PBytes is set to four (4), and Version is set to zero (0). The ST2+
   FlowSpec, see Section 9.2, is a 32-byte data structure. PBytes is set
   to 36, and Version is set to seven (7).

10.3.2  Group

   The Group parameter (PCode = 2) is an optional argument used to
   indicate that the stream is a member in the specified group.

        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 = 2    |   PBytes = 16 |           GroupUniqueID       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        GroupCreationTime                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     GroupInitiatorIPAddress                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Relationship       |                 N             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 13: Group Parameter

o   GroupUniqueID, GroupInitiatorIPAddress, and GroupCreationTime
    together form the GroupName field. They are allocated by the group
    name generator function, see Section 8.2. GroupUniqueID and
    GroupCreationTime are implementation specific and have only local
    definitions.

o   Relationship has the following format:

                                            0
                        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       |    0 (unused)         |S|P|F|B|
                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 14: Relationship Field






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   The B, F, P, S bits correspond to Bandwidth, Fate, Path, and Subnet
   resources sharing, see Section 7. A value of 1 indicates that the
   relationship exists for this group. All combinations of the four bits
   are allowed. Bits 0-11 of the Relationship field are reserved for
   future use and must be set to 0.

o   N contains a legal value only if the B-bit is set. It is the value
    of the N parameter to be used as explained in Section 7.1.1.

10.3.3  MulticastAddress

   The MulticastAddress parameter (PCode = 3) is an optional parameter
   that is used when using IP encapsulation and setting up an IP
   multicast group. This parameter is used to communicate the desired IP
   multicast address to next-hop ST agents that should become members of
   the group, see Section 8.8.

        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    |   PBytes = 8  |                0              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        IPMulticastAddress                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 15:  MulticastAddress

o   IPMulticastAddress is the 32-bit IP multicast address to be used to
    receive data packets for the stream.

10.3.4  Origin

   The Origin parameter (PCode = 4) is used to identify the next higher
   protocol, and the SAP being used in conjunction with that 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 = 5    |   PBytes      | NextPcol      |OriginSAPBytes |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                OriginSAP                      :     Padding   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Figure 16: Origin







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o   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 [RFC1700].

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

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

   Note that the 32-bit IP address of the stream origin is not included
   in this parameter because it is always available as part of the ST
   header.

10.3.5  RecordRoute

   The RecordRoute parameter (PCode = 5) is used to request that the
   route between the origin and a target be recorded and delivered to
   the user application. The ST agent at the origin (or target)
   including this parameter, has to determine the parameter's length,
   indicated by the PBytes field. ST agents processing messages
   containing this parameter add their receiving IP address in the
   position indicated by the FreeOffset field, space permitting. If no
   space is available, the parameter is passed unchanged. When included
   by the origin, all agents between the origin and the target add their
   IP addresses and this information is made available to the
   application at the target. When included by the target, all agents
   between the target and the origin, inclusive, add their IP addresses
   and this information is made available to the application at the
   origin.

        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 = 5   |     PBytes    |       0       |  FreeOffset   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          IP Address 1                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                              ...                              :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          IP Address N                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 17: RecordRoute

o   PBytes is the length of the parameter in bytes. Length is determined
    by the agent (target or origin) that first introduces the parameter.



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    Once set, the length of the parameter remains unchanged.

o   FreeOffset indicates the offset, relative to the start of the
    parameter, for the next IP address to be recorded. When the
    FreeOffset is greater than, or equal to, PBytes the RecordRoute
    parameter is full.

o   IP Address is filled in, space permitting, by each ST agent
    processing this parameter.

10.3.6  Target and TargetList

   Several control messages use a parameter called TargetList (PCode =
   6), which contains information about the targets to which the message
   pertains. For each Target in the TargetList, the information includes
   the 32-bit IP address of the target, the SAP applicable to the next
   higher layer protocol, and the length of the SAP (SAPBytes).
   Consequently, a Target structure can be of variable length. Each
   entry has the format shown in Figure 18.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Target IP Address                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  TargetBytes  |  SAPBytes     |     SAP       :    Padding    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Figure 18: Target

o   TargetIPAddress is the 32-bit IP Address of the Target.

o   TargetBytes is the length of the Target structure, beginning with
    the TargetIPAddress.

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

o   SAP may be longer than 2 bytes and it includes a padding when
    required. There would be no padding required for SAPs with lengths
    of 2, 6, 10, etc., bytes.










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

                           Figure 19: TargetList
10.3.7  UserData

   The UserData parameter (PCode = 7) is an optional parameter that may
   be used by the next higher protocol or an application to convey
   arbitrary information to its peers. This parameter is propagated in
   some control messages and its contents have no significance to ST
   agents. Note that since the size of control messages is limited by
   the smallest MTU in the path to the targets, the maximum size of this
   parameter cannot be specified a priori. If the size of this parameter
   causes a message to exceed the network MTU, an ST agent behaves as
   described in Section 5.1.2. The parameter must be padded to a
   multiple of 32 bits.

        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 = 7    |   PBytes      |           UserBytes           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                      UserInfo                 :   Padding     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 20:  UserData

o   UserBytes specifies the number of valid UserInfo bytes.

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











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10.3.8  Handling of Undefined Parameters

   An ST agent must be able to handle all parameters listed above. To
   support possible future uses, parameters with unknown PCodes must
   also be supported. If an agent receives a message containing a
   parameter with an unknown Pcode value, the agent should handle the
   parameter as if it was a UserData parameter. That is, the contents of
   the parameter should be ignored, and the message should be
   propagated, as appropriate, along with the related control message.

10.4  ST Control Message PDUs

   ST Control messages are described in the following section. Please
   refer to Section 10.6 for an explanation of the notation.

10.4.1  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.  ACCEPT is also issued as a positive response to a CHANGE
   message. It implies that the target accepts the proposed stream
   modification.

   ACCEPT is relayed by the ST agents from the target to the origin
   along the path established by CONNECT (or CHANGE) but in the reverse
   direction. ACCEPT must be acknowledged with ACK at each hop.
























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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  OpCode = 1   |      0        |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |         LnkReference          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode = 0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          MaxMsgSize           |          RecoveryTimeout      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      StreamCreationTime                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   IPHops      |                        0                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           FlowSpec                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           TargetList                          :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           RecordRoute                         :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           UserData                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 21: ACCEPT Control Message

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

o   LnkReference is the Reference number from the corresponding CONNECT
    (or CHANGE)

o   MaxMsgSize indicates the smallest MTU along the path traversed by
    the stream. This field is only set when responding to a CONNECT
    request.

o   RecoveryTimeout reflects 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. This field
    represents what can actually be supported by each participating
    agent, and is only set when responding to a CONNECT request.

o   StreamCreationTime is the 32- bits system dependent timestamp copied
    from the corresponding CONNECT request.



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o   IPHops is the number of IP encapsulated hops traversed by the
    stream. This field is set to zero by the origin, and is incremented
    at each IP encapsulating agent.

10.4.2  ACK

   ACK (OpCode = 2) is used to acknowledge a request. The ACK message is
   not propagated beyond the previous-hop or 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  OpCode = 2   |     0         |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Reference               |           LnkReference = 0    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Checksum                |           ReasonCode          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 22: ACK Control Message

o   Reference is the Reference number of the control message being
    acknowledged.

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























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10.4.3  CHANGE

   CHANGE (OpCode = 3) is used to change the FlowSpec of an established
   stream. The CHANGE message is processed similarly to CONNECT, except
   that it travels along the path of an established stream. CHANGE must
   be propagated until it reaches the related stream's targets. CHANGE
   must be acknowledged with ACK at each 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 = 3   |G|I|     0     |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Reference           |          LnkReference = 0     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        SenderIPAddress                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode = 0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                            FlowSpec                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           TargetList                          :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           RecordRoute                         :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                            UserData                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 23: CHANGE Control Message

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

o   I (bit 7) is used to indicate that the LRM is permitted to interrupt
    and, if needed, break the stream in the process of trying to satisfy
    the requested change.

o   Reference contains a number assigned by the ST agent sending CHANGE
    for use in the acknowledging ACK.

10.4.4  CONNECT

   CONNECT (OpCode = 4) 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



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   CONNECT to each next-hop is used to convey the SID.

   The next-hop initially responds with an ACK, which implies that the
   CONNECT was valid and is being processed. The next-hop will later
   relay back either an ACCEPT or REFUSE from each target. An
   intermediate ST agent that receives a CONNECT behaves as explained in
   Section 4.5.

        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   |J N|S|    0    |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Reference           |          LnkReference = 0     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |          ReasonCode = 0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           MaxMsgSize          |          RecoveryTimeout      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        StreamCreationTime                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   IPHops      |                        0                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                             Origin                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           FlowSpec                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                          TargetList                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                          RecordRoute                          :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                             Group                             :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                        MulticastAddress                       :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                            UserData                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 24: CONNECT Control Message





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o   JN (bits 8 and 9) indicate the join authorization level for the
    stream, see Section 4.4.2.

o   S (bit 10) indicates the NoRecovery option (Section 4.4.1). When the
    S-bit is set (1), the NoRecovery option is specified for the stream.

o   Reference contains a number assigned by the ST agent sending CONNECT
    for use in the acknowledging ACK.

o   MaxMsgSize indicates the smallest MTU along the path traversed by
    the stream. This field is initially set to the network MTU of the
    agent issues the CONNECT.

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

o   StreamCreationTime is the 32- bits system dependent timestamp
    generated by the ST agent issuing the CONNECT.

o   IPHops is the number of IP encapsulated hops traversed by the
    stream. This field is set to zero by the origin, and is incremented
    at each IP encapsulating agent.




























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10.4.5  DISCONNECT

   DISCONNECT (OpCode = 5) is used by an origin to tear down an
   established stream or part of a stream, or by an intermediate ST
   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.

        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   |G|    0        |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |     LnkReference = 0          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      GeneratorIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           TargetList                          :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                            UserData                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 25: DISCONNECT Control Message

o   G (bit 8) is used to request a DISCONNECT of all the stream's
    targets. TargetList should be omitted when the G-bit is set (1). If
    TargetList is present, it is ignored.

o   Reference contains a number assigned by the ST agent sending
    DISCONNECT for use in the acknowledging ACK.

o   ReasonCode reflects the event that initiated the message.

o   GeneratorIPAddress is the 32-bit IP address of the host that first
    generated the DISCONNECT message.







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10.4.6  ERROR

   ERROR (OpCode = 6) is sent in acknowledgment to a request in which an
   error is detected. No action is taken on the erroneous request. No
   ACK is expected. The ERROR message is not propagated beyond the
   previous-hop or next-hop ST agent. An ERROR is never sent in response
   to another ERROR. The receiver of an ERROR is encouraged to try again
   without waiting for a retransmission timeout.

        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   |       0       |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |     LnkReference = 0          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |        ReasonCode             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           PDUInError                          :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 26: ERROR Control Message

o   Reference is the Reference number of the erroneous request.

o   ReasonCode indicates the error that triggered the message.

o   PDUInError is the PDU in error, beginning with the ST Header. This
    parameter is optional. Its length is limited by network MTU, and may
    be truncated when too long.



















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10.4.7  HELLO

   HELLO (OpCode = 7) is used as part of the ST failure detection
   mechanism, see Section 6.1.

        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 = 7   |R|    0        |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Reference = 0           |        LnkReference = 0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Checksum              |          ReasonCode = 0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          HelloTimer                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 27: HELLO Control Message

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

o   HelloTimer represents the time in millisecond since the agent was
    restarted, modulo the precision of the field. It is used to detect
    duplicate or delayed HELLO messages.

























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10.4.8  JOIN

   JOIN (OpCode = 8) is used as part of the ST steam joining mechanism,
   see Section 4.6.3.

        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          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |         LnkReference = 0      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode = 0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      GeneratorIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                          TargetList                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 28: JOIN Control Message

o   Reference contains a number assigned by the ST agent sending JOIN
    for use in the acknowledging ACK.

o   GeneratorIPAddress is the 32-bit IP address of the host that
    generated the JOIN message.

o   TargetList is the information associated with the target to be added
    to the stream.




















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10.4.9  JOIN-REJECT

   JOIN-REJECT (OpCode = 9) is used as part of the ST steam joining
   mechanism, see Section 4.6.3.

        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   |      0        |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |          LnkReference         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      GeneratorIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 29: JOIN-REJECT Control Message

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

o   LnkReference is the Reference number from the corresponding JOIN
    message.

o   ReasonCode reflects the reason why the JOIN request was rejected.

o   GeneratorIPAddress is the 32-bit IP address of the host that first
    generated the JOIN-REJECT message.




















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10.4.10  NOTIFY

   NOTIFY (OpCode = 10) is issued by an ST agent to inform other ST
   agents of events that may be significant. NOTIFY may be propagated
   beyond the previous-hop or next-hop ST agent depending on the
   ReasonCode, see Section 10.5.3; NOTIFY must be acknowledged with an
   ACK.

        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          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |         LnkReference = 0      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      DetectorIPAddress                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          MaxMsgSize           |          RecoveryTimeout      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           FlowSpec                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           TargetList                          :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           UserData                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 30: NOTIFY Control Message

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

o   ReasonCode identifies the reason for the notification.

o   DetectorIPAddress is the 32-bit IP address of the ST agent that
    detects the event.

o   MaxMsgSize is set when the MTU of the listed targets has changed
    (e.g., due to recovery), or when the notification is generated after
    a successful JOIN. Otherwise it is set to zero (0).





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o   RecoveryTimeout is set when the notification is generated after a
    successful JOIN. Otherwise it is set to zero (0).

o   FlowSpec is present when the notification is generated after a
    successful JOIN.

o   TargetList is present when the notification is related to one or
    more targets, or when MaxMsgSize is set

o   UserData is present if the notification is generated after a
    successful JOIN and the UserData parameter was set in the ACCEPT
    message.

10.4.11  REFUSE

   REFUSE (OpCode = 11) 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 ST
   agent in response to a CONNECT or CHANGE either 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 ST 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 back by the ST agents to the origin (or
   intermediate ST agent that created the CONNECT or CHANGE) along the
   path traced by the CONNECT. The ST agent receiving the REFUSE will
   process it differently depending on the condition that caused it, as
   specified in the ReasonCode field. 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 ST agent at
   the same time and be easily combined, e.g., have identical
   ReasonCodes and parameters.
















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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  OpCode = 11  |G|E|N|    0    |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |         LnkReference          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       DetectorIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       ValidTargetIPAddress                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                          TargetList                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                         RecordRoute                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                            UserData                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 31: REFUSE Control Message

o   G (bit 8) is used to indicate that all targets down stream from the
    sender are refusing. It is expected that this will be set most
    commonly due to network failures. The TargetList parameter is
    ignored or not present when this bit is set, and must be included
    when not set.

o   E (bit 9) is set by an ST agent to indicate that the request failed
    and that the pre-change stream attributes, including resources, and
    the stream itself still exist.

o   N (bit 10) is used to indicate that no further attempts to recover
    the stream should be made. This bit must be set when stream recovery
    should not be attempted, even in the case where the target
    application has shut down normally (ApplDisconnect).

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

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



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o   DetectorIPAddress is the 32-bit IP address of the host that first
    generated the REFUSE message.

o   ValidTargetIPAddress is the 32-bit IP address of a host that is
    properly connected as part of the stream. This parameter is only
    used when recovering from stream convergence, otherwise it is set to
    zero (0).

10.4.12  STATUS

   STATUS (OpCode = 12) is used to inquire about the existence of a
   particular stream identified by the SID. Use of STATUS is intended
   for collecting information from an neighbor ST agent, including
   general and specific stream information, and round trip time
   estimation. The use of this message type is described in Section 8.4.

        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   |       0       |           TotalBytes          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |       LnkReference = 0        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |          ReasonCode = 0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                          TargetList                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 32: STATUS Control Message

o   Reference contains a number assigned by the ST agent sending STATUS
    for use in the replying STATUS-RESPONSE.

o   TargetList is an optional parameter that when present indicates that
    only information related to the specific targets should be relayed
    in the STATUS-RESPONSE.

10.4.13  STATUS-RESPONSE

   STATUS-RESPONSE (OpCode = 13) 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 SID but no other parameters. It
   will otherwise contain the current SID, FlowSpec, TargetList, and
   possibly Groups of the stream. It the full target list can not fit in
   a single message, only those targets that can be included in one



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   message will be included. As mentioned in Section 10.4.12, it is
   possible to request information on a specific target.

        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          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reference                |       LnkReference = 0        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SenderIPAddress                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |       ReasonCode = 0          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           FlowSpec                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                           Groups                              :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :                          TargetList                           :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 33: STATUS-RESPONSE Control Message

o   Reference contains a number assigned by the ST agent sending the
    STATUS.

10.5  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.









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10.5.1  SCMP Messages

   1)      ACCEPT
   2)      ACK
   3)      CHANGE
   4)      CONNECT
   5)      DISCONNECT
   6)      ERROR
   7)      HELLO
   8)      JOIN
   9)      JOIN-REJECT
   10)     NOTIFY
   11)     REFUSE
   12)     STATUS
   13)     STATUS-RESPONSE

10.5.2  SCMP Parameters

   1)      FlowSpec
   2)      Group
   3)      MulticastAddress
   4)      Origin
   5)      RecordRoute
   6)      TargetList
   7)      UserData

10.5.3  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. The
   ReasonCode is an 8-bit field. Following values are defined:

1       NoError         No error has occurred.
2       ErrorUnknown    An error not contained in this list has been
                        detected.
3       AccessDenied    Access denied.
4       AckUnexpected   An unexpected ACK was received.
5       ApplAbort       The application aborted the stream abnormally.
6       ApplDisconnect  The application closed the stream normally.
7       ApplRefused     Applications refused requested connection or
                        change.
8       AuthentFailed   The authentication function failed.
9       BadMcastAddress IP Multicast address is unacceptable in CONNECT
10      CantGetResrc    Unable to acquire (additional) resources.



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11      CantRelResrc    Unable to release excess resources.
12      CantRecover     Unable to recover failed stream.
13      CksumBadCtl     Control PDU has a bad message checksum.
14      CksumBadST      PDU has a bad ST Header checksum.
15      DuplicateIgn    Control PDU is a duplicate and is being
                        acknowledged.
16      DuplicateTarget Control PDU contains a duplicate target, or an
                        attempt to add an existing target.
17      FlowSpecMismatch        FlowSpec in request does not match
                                existing FlowSpec.
18      FlowSpecError   An error occurred while processing the FlowSpec
19      FlowVerUnknown  Control PDU has a FlowSpec Version Number that
                        is not supported.
20      GroupUnknown    Control PDU contains an unknown Group Name.
21      InconsistGroup  An inconsistency has been detected with the
                        streams forming a group.
22      IntfcFailure    A network interface failure has been detected.
23      InvalidSender   Control PDU has an invalid SenderIPAddress
                        field.
24      InvalidTotByt   Control PDU has an invalid TotalBytes field.
25      JoinAuthFailure Join failed due to stream authorization level.
26      LnkRefUnknown   Control PDU contains an unknown LnkReference.
27      NetworkFailure  A network failure has been detected.
28      NoRouteToAgent  Cannot find a route to an ST agent.
29      NoRouteToHost   Cannot find a route to a host.
30      NoRouteToNet    Cannot find a route to a network.
31      OpCodeUnknown   Control PDU has an invalid OpCode field.
32      PCodeUnknown    Control PDU has a parameter with an invalid
                        PCode.
33      ParmValueBad    Control PDU contains an invalid parameter value.
34      PathConvergence Two branches of the stream join during the
                        CONNECT setup.
35      ProtocolUnknown Control PDU contains an unknown next-higher
                        layer protocol identifier.
36      RecordRouteSize RecordRoute parameter is too long to permit
                        message to fit a network's MTU.
37      RefUnknown      Control PDU contains an unknown Reference.
38      ResponseTimeout Control message has been acknowledged but not
                        answered by an appropriate control message.
39      RestartLocal    The local ST agent has recently restarted.
40      RestartRemote   The remote ST agent has recently restarted.
41      RetransTimeout  An acknowledgment has not been received after
                        several retransmissions.
42      RouteBack       Route to next-hop through same interface as
                        previous-hop and is not previous-hop.
43      RouteInconsist  A routing inconsistency has been detected.
44      RouteLoop       A routing loop has been detected.




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45      SAPUnknown      Control PDU contains an unknown next-higher
                        layer SAP (port).
46      SIDUnknown      Control PDU contains an unknown SID.
47      STAgentFailure  An ST agent failure has been detected.
48      STVer3Bad       A received PDU is not ST Version 3.
49      StreamExists    A stream with the given SID already exists.
50      StreamPreempted The stream has been preempted by one with a
                        higher precedence.
51      TargetExists    A CONNECT was received that specified an
                        existing target.
52      TargetUnknown   A target is not a member of the specified
                        stream.
53      TargetMissing   A target parameter was expected and is not
                        included, or is empty.
54      TruncatedCtl    Control PDU is shorter than expected.
55      TruncatedPDU    A received ST PDU is shorter than the ST Header
                        indicates.
56      UserDataSize    UserData parameter too large to permit a
                        message to fit into a network's MTU.

10.5.4  Timeouts and Other Constants

   SCMP uses retransmission to effect reliability and thus has several
   "retransmission timers". Each "timer" is modeled by an initial time
   interval (ToXxx), which may get 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. Note that the variables are described
   for reference purposes only, different implementations may not
   include the identical variables.

Value   Timeout Name    Meaning
------------------------------------------------------------------------
  500   ToAccept        Initial hop-by-hop timeout for acknowledgment of
                        ACCEPT
    3   NAccept         ACCEPT retries before failure
  500   ToChange        Initial hop-by-hop timeout for acknowledgment of
                        CHANGE
    3   NChange         CHANGE retries before failure
 5000   ToChangeResp    End-to-End CHANGE timeout for receipt of ACCEPT
                        or REFUSE
  500   ToConnect       Initial hop-by-hop timeout for acknowledgment of
                        CONNECT
    5   NConnect        CONNECT retries before failure
 5000   ToConnectResp   End-to-End CONNECT timeout for receipt of ACCEPT
                        or REFUSE from targets by origin
  500   ToDisconnect    Initial hop-by-hop timeout for acknowledgment of
                        DISCONNECT



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    3   NDisconnect     DISCONNECT retries before failure
  500   ToJoin          Initial hop-by-hop timeout for acknowledgment of
                        JOIN
    3   NJoin           JOIN retries before failure
  500   ToJoinReject    Initial hop-by-hop timeout for acknowledgment of
                        JOIN-REJECT
    3   NJoinReject     JOIN-REJECT retries before failure
 5000   ToJoinResp      Timeout for receipt of CONNECT or JOIN-REJECT
                        from origin or intermediate hop
  500   ToNotify        Initial hop-by-hop timeout for acknowledgment of
                        NOTIFY
    3   NNotify         NOTIFY retries before failure
  500   ToRefuse        Initial hop-by-hop timeout for acknowledgment of
                        REFUSE
    3   NRefuse         REFUSE retries before failure
  500   ToRetryRoute    Timeout for receipt of ACCEPT or REFUSE from
                        targets during failure recovery
    5   NRetryRoute     CONNECT retries before failure
 1000   ToStatusResp    Timeout for receipt of STATUS-RESPONSE
    3   NStatus         STATUS retries before failure
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

10.6  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.











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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       1       |       2       |       3       |       4       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       5       |       6       |       7       |       8       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       9       |      10       |      11       |      12       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 34:  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 35: Significance of Bits

   Similarly, whenever a multi-octet field represents a numeric quantity
   the left most bit of the whole field is the most significant bit.
   When a multi-octet quantity is transmitted the most significant octet
   is transmitted first.

   Fields whose length is fixed and fully illustrated are shown with a
   vertical bar (|) at the end; fixed fields whose contents are
   abbreviated are shown with an exclamation point (!); variable fields
   are shown with colons (:). Optional parameters are separated from
   control messages with a blank line. The order of parameters is not
   meaningful.

11.  References

[RFC1071]       Braden, R., Borman, D., and C. Partridge,
                "Computing the Internet Checksum", RFC 1071,
                USC/Information Sciences Institute,
                Cray Research, BBN Laboratories, September 1988.

[RFC1112]       Deering, S., "Host Extensions for IP Multicasting",
                STD 5, RFC 1112, Stanford University, August 1989.






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[WoHD95]        L. Wolf, R. G. Herrtwich, L. Delgrossi: Filtering
                Multimedia Data in Reservation-based Networks,
                Kommunikation in Verteilten Systemen 1995 (KiVS),
                Chemnitz-Zwickau, Germany, February 1995.

[RFC1122]       Braden, R., "Requirements for Internet Hosts --
                Communication Layers", STD 3, RFC 1122,
                USC/Information Sciences Institute, October 1989.

[Jaco88]        Jacobson, V.: Congestion Avoidance and Control, ACM
                SIGCOMM-88, August 1988.

[KaPa87]        Karn, P. and C. Partridge: Round Trip Time Estimation,
                ACM SIGCOMM-87, August 1987.

[RFC1141]       Mallory, T., and A. Kullberg, "Incremental Updating
                of the Internet Checksum", RFC 1141, BBN, January 1990.

[RFC1363]       Partridge, C., "A Proposal Flow Specification",
                RFC 1363, BBN, September 1992.

[RFC791]        Postel, J., "Internet Protocol", STD 5, RFC 791,
                DARPA, September 1981.

[RFC1700]       Reynolds, J., and J. Postel, "Assigned Numbers",
                STD 2, RFC 1700, USC/Information Sciences Institute,
                October 1994.

[RFC1190]       Topolcic C., "Internet Stream Protocol Version 2
                (ST-II)", RFC 1190, CIP Working Group, October 1990.

[RFC1633]       Braden, R., Clark, D., and S. Shenker, "Integrated
                Services in the Internet Architecture: an Overview",
                RFC 1633, USC/Information Sciences Institute,
                MIT, Xerox PARC, June 1994.

[VoHN93]        C. Vogt, R. G. Herrtwich, R. Nagarajan: HeiRAT: the
                Heidelberg Resource Administration Technique - Design
                Philosophy and Goals, Kommunikation In Verteilten
                Systemen, Munich, Informatik Aktuell, Springer-Verlag,
                Heidelberg, 1993.

[Cohe81]        D. Cohen: A Network Voice Protocol NVP-II, University of
                Southern California, Los Angeles, 1981.

[Cole81]        R. Cole: PVP - A Packet Video Protocol, University of
                Southern California, Los Angeles, 1981.




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[DeAl92]        L. Delgrossi (Ed.) The BERKOM-II Multimedia Transport
                System, Version 1, BERKOM Working Document, October,
                1992.

[DHHS92]        L. Delgrossi, C. Halstrick, R. G. Herrtwich, H.
                Stuettgen: HeiTP: a Transport Protocol for ST-II,
                GLOBECOM'92, Orlando (Florida), December 1992.

[Schu94]        H. Schulzrinne: RTP: A Transport Protocol for Real-Time
                Applications. Work in Progress, 1994.


12.  Security Considerations

   Security issues are not discussed in this memo.

13.  Acknowledgments and Authors' Addresses

   Many individuals have contributed to the work described in this memo.
   We thank the participants in the ST Working Group for their input,
   review, and constructive comments. George Mason University C3I Center
   for hosting an interim meeting. Murali Rajagopal for his efforts on
   ST2+ state machines. Special thanks are due to Steve DeJarnett, who
   served as working group co-chair until summer 1993.

   We would also like to acknowledge the authors of [RFC1190]. All
   authors of [RFC1190] should be considered authors of this document
   since this document contains much of their text and ideas.























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   Louis Berger
   BBN Systems and Technologies
   1300 North 17th Street, Suite 1200
   Arlington, VA 22209

   Phone: 703-284-4651
   EMail: lberger@bbn.com


   Luca Delgrossi
   Andersen Consulting Technology Park
   449, Route des Cretes
   06902 Sophia Antipolis, France

   Phone: +33.92.94.80.92
   EMail: luca@andersen.fr


   Dat Duong
   BBN Systems and Technologies
   1300 North 17th Street, Suite 1200
   Arlington, VA 22209

   Phone: 703-284-4760
   EMail: dat@bbn.com


   Steve Jackowski
   Syzygy Communications Incorporated
   269 Mt. Hermon Road
   Scotts Valley, CA 95066

   Phone: 408-439-6834
   EMail: stevej@syzygycomm.com


   Sibylle Schaller
   IBM ENC
   Broadband Multimedia Communications
   Vangerowstr. 18
   D69020 Heidelberg, Germany

   Phone: +49-6221-5944553
   EMail: schaller@heidelbg.ibm.com







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