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Versions: 00 01 02 03 RFC 1819

ST Working Group                              L. Delgrossi and L. Berger
Internet-Draft                                                March 1995
File: draft-ietf-st2-spec-03.txt                    Expires:   July 1995



                Internet Stream Protocol Version 2 (ST2)


                 Protocol Specification - Version ST2+


Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months. Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time. It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as "work in
   progress".

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).


   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 its 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+.











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   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                                       14
           1.5.1  Streams                                                14
           1.5.2  Data Transmission                                      15
           1.5.3  Flow Specification                                     16
           1.6  Outline of This Document                                 17

   2  ST2 User Service Description                                       18
           2.1  Stream Operations and Primitive Functions                18
           2.2  State Diagrams                                           20
           2.3  State Transition Tables                                  23

   3  The ST2 Data Transfer Protocol                                     23
           3.1  Data Transfer with ST                                    24
           3.2  ST Protocol Functions                                    25
           3.2.1  Stream Identification                                  25
           3.2.2  Packet Discarding based on Data Priority               25

   4  SCMP Functional Description                                        25
           4.1  Types of Streams                                         27
           4.1.1  Stream Building                                        27
           4.1.2  Knowledge of Receivers                                 27
           4.2  Control PDUs                                             28
           4.3  SCMP Reliability                                         29
           4.4  Stream Options                                           30
           4.4.1  No Recovery                                            30
           4.4.2  Join Authorization Level                               31
           4.4.3  Record Route                                           31
           4.4.4  User Data                                              31
           4.5  Stream Setup                                             32
           4.5.1  Information from the Application                       32
           4.5.2  Initial Setup at the Origin                            32
           4.5.2.1  Invoking the Routing Function                        33
           4.5.2.2  Reserving Resources                                  33
           4.5.3  Sending CONNECT Messages                               34
           4.5.3.1  Empty Target List                                    34
           4.5.4  CONNECT Processing by an Intermediate ST agent         34
           4.5.5  CONNECT Processing at the Targets                      34



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           4.5.6  ACCEPT Processing by an Intermediate ST agent          35
           4.5.7  ACCEPT Processing by the Origin                        36
           4.5.8  REFUSE Processing by the Intermediate ST agent         36
           4.5.9  REFUSE Processing by the Origin                        36
           4.5.10  Other Functions during Stream Setup                   36
           4.6  Modifying an Existing Stream                             37
           4.6.1  The Origin Adding New Targets                          37
           4.6.2  The Origin Removing a Target                           38
           4.6.3  A Target Joining a Stream                              39
           4.6.3.1  Router as Origin                                     40
           4.6.4  A Target Deleting Itself                               40
           4.6.5  Changing a Stream's FlowSpec                           41
           4.7  Stream Tear Down                                         41

   5  Exceptional Cases                                                  42
           5.1  Long ST Messages                                         42
           5.1.1  Handling of Long Data Packets                          42
           5.1.2  Handling of Long Control Packets                       42
           5.2  Timeout Failures                                         43
           5.2.1  Failure due to ACCEPT Acknowledgment Timeout           44
           5.2.2  Failure due to CHANGE Acknowledgment Timeout           44
           5.2.3  Failure due to CHANGE Response Timeout                 44
           5.2.4  Failure due to CONNECT Acknowledgment Timeout          44
           5.2.5  Failure due to CONNECT Response Timeout                45
           5.2.6  Failure due to DISCONNECT Acknowledgment Timeout       45
           5.2.7  Failure due to JOIN Acknowledgment Timeout             45
           5.2.8  Failure due to JOIN Response Timeout                   45
           5.2.9  Failure due to JOIN-REJECT Acknowledgment Timeout      45
           5.2.10  Failure due to NOTIFY Acknowledgment Timeout          46
           5.2.11  Failure due to REFUSE Acknowledgment Timeout          46
           5.2.12  Failure due to STATUS Response Timeout                46
           5.3  Setup Failures due to Routing Failures                   46
           5.3.1  Path Convergence                                       47
           5.3.2  Other Cases                                            47
           5.4  Problems due to Routing Inconsistency                    48
           5.5  Problems in Reserving Resources                          49
           5.5.1  Mismatched FlowSpecs                                   49
           5.5.2  Unknown FlowSpec Version                               49
           5.5.3  LRM Unable to Process FlowSpec                         50
           5.5.4  Insufficient Resources                                 50
           5.6  Problems Caused by CHANGE Messages                       50
           5.7  Unknown Targets in DISCONNECT and CHANGE                 51

   6  Failure Detection and Recovery                                     52
           6.1  Failure Detection                                        52
           6.1.1  Network Failures                                       52
           6.1.2  Detecting ST Agents Failures                           53
           6.2  Failure Recovery                                         54



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           6.2.1  Problems in Stream Recovery                            57
           6.3  Stream Preemption                                        58

   7  A Group of Streams                                                 59
           7.1  Basic Group Relationships                                59
           7.1.1  Bandwidth Sharing                                      59
           7.1.2  Fate Sharing                                           60
           7.1.3  Route Sharing                                          61
           7.1.4  Subnet Resources Sharing                               61
           7.2  Relationships Orthogonality                              61

   8  Ancillary Functions                                                62
           8.1  Stream ID Generation                                     62
           8.2  Group Name Generator                                     62
           8.3  Checksum Computation                                     63
           8.4  Neighbour ST Agent Identification and                    63
                Information Collection
           8.5  Round Trip Time Estimation                               64
           8.6  Network MTU Discovery                                    64
           8.7  IP Encapsulation of ST                                   65
           8.8  IP Multicasting                                          66

   9  The ST2+ Flow Specification                                        67
           9.1  FlowSpec Version #0 - (Null FlowSpec)                    68
           9.2  FlowSpec Version #7 - ST2+ FlowSpec                      68
           9.2.1  QoS Classes                                            69
           9.2.2  Precedence                                             69
           9.2.3  Maximum Data Size                                      70
           9.2.4  Message Rate                                           70
           9.2.5  Delay and Delay Jitter                                 70
           9.2.6  ST2+ FlowSpec Format                                   70

   10  ST2 Protocol Data Units Specification                             72
           10.1  Data PDU                                                72
           10.1.1  ST Data Packets                                       74
           10.2  Control PDUs                                            74
           10.3  Common SCMP Elements                                    75
           10.3.1  FlowSpec                                              76
           10.3.2  Group                                                 76
           10.3.3  MulticastAddress                                      77
           10.3.4  Origin                                                78
           10.3.5  RecordRoute                                           78
           10.3.6  Target and TargetList                                 79
           10.3.7  UserData                                              80
           10.3.8  Handling of Undefined Parameters                      81
           10.4  ST Control Message PDUs                                 81
           10.4.1  ACCEPT                                                81
           10.4.2  ACK                                                   83



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           10.4.3  CHANGE                                                84
           10.4.4  CONNECT                                               84
           10.4.5  DISCONNECT                                            86
           10.4.6  ERROR                                                 87
           10.4.7  HELLO                                                 88
           10.4.8  JOIN                                                  89
           10.4.9  JOIN-REJECT                                           90
           10.4.10  NOTIFY                                               91
           10.4.11  REFUSE                                               92
           10.4.12  STATUS                                               94
           10.4.13  STATUS-RESPONSE                                      94
           10.5  Suggested Protocol Constants                            95
           10.5.1  SCMP Messages                                         95
           10.5.2  SCMP Parameters                                       96
           10.5.3  ReasonCode                                            96
           10.5.4  Timeouts and Other Constants                          98
           10.6  Data Notations                                          99

   11  Security Considerations                                           100

   12  Acknowledgments and Author's Addresses                            100

   13  References                                                        101




























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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 the provision of
   multimedia teleservices such as conferencing and mailing. In addition,



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   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
       work done by IBM Heidelberg.




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   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.

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



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   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 existing ST2 implementations.
   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 throughout the stream, i.e. it is not possible to use
   different flow specification formats for different parts of the same
   stream.





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   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 neighbours. 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

   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



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   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].

   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 1       |    and Router            :     |                   |
|  +-----------+     |  +-----------+           :     V                   V
|  |Application|<-+  |  |Application|<-+        :   +----+              +----+
|  |-----------|  |  |  |-----------|  |        :   |    |     Target 2 |    |
+->| ST Agent  |--+  +->| ST Agent  |--+        :   +----+     & Router +----+
   +-----------+        +-----------+           :  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

   1.5.1  Streams

   Streams form the core concepts of ST2. They are established between a



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   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 that can sit in the middle of the tree is call 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 a router. 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 neighbour 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
   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 an 12-byte header.




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   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 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,



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   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 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

   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.




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   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],

   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



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   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

   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 |



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           |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 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 the stream
   operations that can be executed at the 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



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       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.

   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 this 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

   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



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   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
   accordingly 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
   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



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   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.

   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



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   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.

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



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   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.

   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 joins
       then the stream autonomously.

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



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   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 routers. 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 downsteam targets.

   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 streams.

   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 into an incoming
                                   message
   7.      HELLO           to detect failures of neighbour 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



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   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.

   The ACCEPT, CHANGE, CONNECT, DISCONNECT, JOIN, JOIN-REJECT, NOTIFY and
   REFUSE 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 replied 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 response
   is not received in a timely manner. The ACCEPT, CHANGE, CONNECT,
   DISCONNECT, JOIN, JOIN-REJECT, NOTIFY, and REFUSE messages all must be



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   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 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.

   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 behaviour in case 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



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   modified by intermediate and target ST agents.

   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.

   4.4.4  User Data



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   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),

   o   creates local database entries to store information on the new



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       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.

   The membership of a stream in a group may affect the amount of



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   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 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.

   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



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   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 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



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   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 ACCEPT (or REFUSE) from all targets have been received at the
   origin, the application is notified that stream setup is complete.

   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 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



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   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 that have to be traversed to reach
       the targets, as part of the IP encapsulation mechanism described in
       Section 8.7. This is done by updating the IPHops field of the
       CONNECT message, see Section 10.4.4. This 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.

   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



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   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 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.

   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



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   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.
       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.



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   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  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 router ST 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 router must disconnect the
       target.

   o   if the stream FlowSpec is changed, the router 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 router 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 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



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   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.

   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




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   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
   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.



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   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.

   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 ACK has been received. Also, the STATUS message must be
   replied to with a STATUS-RESPONSE message.




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   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).

   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).



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   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 neighbour 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).

   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



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   an ACK from a neighbour 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 neighbour ST agent
   expects an ACK from that neighbour 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 neighbour 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
   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:




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   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 path, 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 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



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   affected targets. Such a CONNECT may reach an ST agent downstream of
   the failure before that ST agent has received a DISCONNECT from the
   neighbourhood 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 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



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   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 behaviour, 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.

   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



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   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.

   o   refuse to establish the stream along this path: the origin ST agent
       informs the application of the stream setup failure; an ST agent at
       a router or target issues 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



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   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
   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



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

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

   The purpose of the robustness protocol defined here is for ST agents
   to determine that the streams through a neighbour have been broken by
   the failure of the neighbour 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



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   the neighbouring ST agent remains intact. On the other hand, an ST
   agent may discover that communication with a neighbouring ST agent has
   ceased because it has not received any traffic from that neighbour 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 neighbour
   remains available, or the neighbour has failed while the network
   remains intact.

   6.1.2  Detecting ST Agents Failures

   Each ST agent periodically sends each neighbour 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 neighbour
   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 neighbour ST agent with no streams in common, e.g. to
   check whether it is active. STATUS messages can be used to poll status
   of neighbour ST agents, see Section 8.4.

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




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   Each ST stream has a RecoveryTimeout value associated with it. This
   value is assigned by the origin and carried into 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 neighbour 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
   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 neighbours
   at the rate necessary to support the smallest RecoveryTimeout of any
   active stream. Alternately, it may send HELLO messages to different
   neighbours independently at different rates corresponding to
   RecoveryTimeouts of individual streams.

   The ST agent that receives a HELLO message expects to receive at least
   one new HELLO message from a neighbour during the RecoveryTimeout of
   every active stream through that neighbour. It can detect duplicate or
   delayed HELLO messages by saving the HelloTimer field of the most
   recent valid HELLO message from that neighbour and comparing it with
   the HelloTimer field of incoming HELLO messages. It will only accept
   an incoming HELLO message from that neighbour if it has a HelloTimer
   field that is greater than the most recent valid HELLO message by the
   time elapsed since that message was received plus twice the maximum
   likely delay variance from that neighbour.

   If the ST agent does not receive a valid HELLO message within the
   RecoveryTimeout of a stream, it must assume that the neighbouring 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



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   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.

   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 of a failure should first verify that
       there was a failure. It can do this using STATUS messages to query
       its upstream neighbour. If it cannot communicate with that
       neighbour, then for each active stream from that neighbour it should
       first send a REFUSE message upstream with the appropriate ReasonCode
       (STAgentFailure). This is done to the neighbour to speed up the
       failure recovery in case the hop is unidirectional, i.e., the
       neighbour can hear the ST agent but the ST agent cannot hear the
       neighbour. The ST agent detecting the failure must then, for each
       active stream from that neighbour, 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
       neighbour using STATUS messages. If the neighbour 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



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       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.

   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.



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   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
   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



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   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 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.



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   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.

   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,



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   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
   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



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   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 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



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   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 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.



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   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
   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  Neighbour ST Agent Identification and Information Collection

   The STATUS message can be used to collect information about neighbour
   ST agents, streams the neighbour supports, and specific targets of
   streams the neighbour 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
   neighbour. It can be used to:

   o   identifying ST capable neighbours. If an ST agent wishes to check if
       a neighbour 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 neighbour'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



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       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
       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 neighbour's information related to on 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 algorithm is implementation dependent and it is outside
   the scope of this document. However, it must be reasonable enough not
   to cause excessive retransmission of SCMP message 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 neighbour, 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 neighbour.

   8.6  Network MTU Discovery




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   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
   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 appropriate
   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 network 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. The application or
   application interface will need to take into account ST data header
   size in order to the application to make it's evaluation.

   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



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       uniformly in the Internet, so its use can't be precisely defined
       here.

   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 ST agents
   make encapsulation decision based on information provided by routing
   function to indicate whether the routers in the path support ST.

   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
   way to the targets. The IPHops field value of the CONNECT message, see
   Section 10.4.4, is incremented at this purpose in case an ST agent
   uses IP encapsulation to reach its next-hop. The value is then
   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
   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



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   using unicast. The next-hop ST agents must be able to receive on the
   specified multicast address in order to accept the connection.

   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 loose
   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 (intermediated 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.

   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
   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:



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   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 average
       case, e.g. the "desired" message rate is the average rate for the
       transmission. Reservations are done for the average 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
       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



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   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 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 to the delay jitter.

   9.2.6  ST2+ FlowSpec Format

   The ST2+ FlowSpec has 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    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
       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.




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   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.

   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.



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   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.

   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) in the current ST version and are
   reserved for future use, e.g., as described in [WoHD95].



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   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

        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



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       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).

   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



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   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

   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



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   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

   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.






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   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

   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.






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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 = 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.
       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.




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   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.

        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.



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

   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 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 may 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 CONNECT to



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   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

   o   JN (bits 8 and 9) indicate the join authorization level for the



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       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.

   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.


















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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  OpCode = 5   |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.

   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.









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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 = 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.

   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



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       restarted, modulo the precision of the field. It is used to detect
       duplicate or delayed HELLO messages.

   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 of 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).

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



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   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 neighbour 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.

   10.5.1  SCMP Messages

   1)      ACCEPT
   2)      ACK
   3)      CHANGE



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   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.
   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



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                           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.
   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.



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   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
       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



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       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.

        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




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   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  Security Considerations

   This memo does not address security issues.

   12  Acknowledgments and Author's 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. We also thank Lynne Kendall Beltran
   for translating our drawings into ASCII. 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.

   Louis Berger
   BBN Systems and Technologies
   1300 North 17th Street, Suite 1200
   Arlington, VA 22209
   Phone: 703-284-4651
   EMail: lberger@bbn.com



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   Luca Delgrossi
   IBM ENC
   Multimedia Technology Center
   Vangerowstr. 18
   D69020 Heidelberg, Germany
   Phone: +49-6221-594330
   EMail: luca@heidelbg.ibm.com

   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

   13  References

   [RFC1071]       Braden, Borman, 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, RFC
                   1112, Stanford University, August 1989.

   [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, RFC 1122, USC/Information Sciences
                   Institute, October 1989.




L. Delgrossi and L. Berger (Ed.)                              [Page 101]


INTERNET-DRAFT        ST2+ Protocol Specification             March 1995


   [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]       C. Partridge: A Proposal Flow Specification, RFC 1363.

   [RFC791]        Postel: Internet Protocol, RFC 791, DARPA, September
                   1981.

   [RFC1700]       Reynolds, Postel: Assigned Numbers, RFC 1700, ISI,
                   October 1994.

   [RFC1190]       Topolcic C.: Internet Stream Protocol Version 2 (ST-II),
                   RFC1190, October 1990.

   [RFC1633]       R. Braden, D. Clark, S. Shenker: Integrated Services in
                   the Internet Architecture: an Overview, RFC1633, 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.

   [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. Internet Draft, work in progress, 1994.





L. Delgrossi and L. Berger (Ed.)                              [Page 102]


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