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Versions: 00 01 02 03 rfc1819                                           
ST Working Group                              L. Delgrossi and L. Berger
Internet-Draft                                             February 1995
File: draft-ietf-st2-spec-02.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+.


   Editor's Note:

   This memo is available both in ASCII format (file: draft-ietf-st2-
   spec-02.txt) and in PostScript (file: draft-ietf-st2-spec-02.ps).
   This draft is considered to be complete, and will be used as the
   basis for an RFC once comments are integrated.



L. Delgrossi and L. Berger (Ed.)                                [Page 1]


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   1 Introduction                                                         6
           1.1 What is ST2?                                               6
           1.2 ST2 and IP                                                 8
           1.3 Protocol History                                           8
           1.4 Supporting Modules for ST2                                 9
           1.4.1 Data Transfer Protocol                                   9
           1.4.2 Setup Protocol                                          10
           1.4.3 Flow Specification                                      10
           1.4.4 Routing Function                                        10
           1.4.5 Local Resource Manager                                  11
           1.5 ST2 Basic Concepts                                        12
           1.5.1 Streams                                                 12
           1.5.2 Data Transmission                                       14
           1.5.3 Flow Specification                                      15
           1.6 Outline of This Document                                  16

   2 ST2 User Service Description                                        17
           2.1 Stream Operations and Primitive Functions                 17
           2.2 State Diagrams                                            19
           2.3 State Transition Tables                                   22

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

   4  SCMP Functional Description                                        24
           4.1 Types of Streams                                          26
           4.1.1 Stream Building                                         26
           4.1.2 Knowledge of Receivers                                  26
           4.2 Control PDUs                                              27
           4.3 SCMP Reliability                                          28
           4.4 Stream Options                                            29
           4.4.1 No Recovery                                             29
           4.4.2 Join Authorization Level                                29
           4.4.3 Record Route                                            30
           4.4.4 User Data                                               30
           4.5 Stream Setup                                              30
           4.5.1 Information from the Application                        31
           4.5.2 Initial Setup at the Origin                             31
           4.5.2.1 Invoking the Routing Function                         31
           4.5.2.2 Reserving Resources                                   32
           4.5.3 Sending CONNECT Messages                                32
           4.5.3.1 Empty Target List                                     33
           4.5.4 CONNECT Processing by an Intermediate ST agent          33
           4.5.5 CONNECT Processing at the Targets                       33
           4.5.6 ACCEPT Processing by an Intermediate ST agent           34



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           4.5.7 ACCEPT Processing by the Origin                         34
           4.5.8 REFUSE Processing by the Intermediate ST agent          34
           4.5.9 REFUSE Processing by the Origin                         35
           4.5.10 Other Functions during Stream Setup                    35
           4.6 Modifying an Existing Stream                              36
           4.6.1 The Origin Adding New Targets                           36
           4.5.4 and Section 4.5.5).                                     36
           4.6.2 The Origin Removing a Target                            36
           4.6.3 A Target Joining a Stream                               37
           4.6.3.1 Router as Origin                                      38
           4.6.4 A Target Deleting Itself                                38
           4.6.5 Changing a Stream's FlowSpec                            39
           4.7 Stream Tear Down                                          39

   5 Exceptional Cases                                                   40
           5.1 Long ST Messages                                          40
           5.1.1 Handling of Long Data Packets                           40
           5.1.2 Handling of Long Control Packets                        40
           5.2 Timeout Failures                                          41
           5.2.1 Failure due to ACCEPT Acknowledgment Timeout            42
           5.2.2 Failure due to CHANGE Acknowledgment Timeout            42
           5.2.3 Failure due to CHANGE Response Timeout                  42
           5.2.4 Failure due to CONNECT Acknowledgment Timeout           42
           5.2.5 Failure due to CONNECT Response Timeout                 42
           5.2.6 Failure due to DISCONNECT Acknowledgment Timeout        43
           5.2.7 Failure due to JOIN Acknowledgment Timeout              43
           5.2.8 Failure due to JOIN Response Timeout                    43
           5.2.9 Failure due to JOIN-REJECT Acknowledgment Timeout       43
           5.2.10 Failure due to NOTIFY Acknowledgment Timeout           43
           5.2.11 Failure due to REFUSE Acknowledgment Timeout           43
           5.2.12 Failure due to STATUS Response Timeout                 44
           5.3 Setup Failures due to Routing Failures                    44
           5.3.1 Path Convergence                                        44
           5.3.2 Other Cases                                             45
           5.4 Problems due to Routing Inconsistency                     46
           5.5 Problems in Reserving Resources                           47
           5.5.1 Mismatched FlowSpecs                                    47
           5.5.2 Unknown FlowSpec Version                                47
           5.5.3 LRM Unable to Process FlowSpec                          47
           5.5.4 Insufficient Resources                                  47
           5.6 Problems Caused by CHANGE Messages                        48
           5.7 Unknown Targets in DISCONNECT and CHANGE                  49

   6 Failure Detection and Recovery                                      49
           6.1 Failure Detection                                         49
           6.1.1 Network Failures                                        50
           6.1.2 Detecting ST Agents Failures                            50
           6.2 Failure Recovery                                          51



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           6.2.1 Problems in Stream Recovery                             53
           6.3 Stream Preemption                                         55

   7 A Group of Streams                                                  56
           7.1 Basic Group Relationships                                 56
           7.1.1 Bandwidth Sharing                                       56
           7.1.2 Fate Sharing                                            57
           7.1.3 Route Sharing                                           58
           7.1.4 Subnet Resources Sharing                                58
           7.2 Relationships Orthogonality                               58

   8 Ancillary Functions                                                 58
           8.1 Stream ID Generation                                      58
           8.2 Group Name Generator                                      59
           8.3 Checksum Computation                                      59
           8.4 Collecting Information From Neighbour ST Agent            60
           8.5 Round Trip Time Estimation                                61
           8.6 Network MTU Discovery                                     61
           8.7 IP Encapsulation of ST                                    61
           8.8 IP Multicasting                                           62

   9 The ST2 Flow Specification                                          63
           9.1 FlowSpec Version #0 - (Null FlowSpec)                     64
           9.2 FlowSpec Version #7 - ST2+ FlowSpec                       64
           9.2.1 QoS Classes                                             65
           9.2.2 Precedence                                              65
           9.2.3 Maximum Data Size                                       66
           9.2.4 Message Rate                                            66
           9.2.5 Delay and Delay Jitter                                  66
           9.2.6 ST2+ FlowSpec Format                                    66

   10  ST2 Protocol Data Units Specification                             68
           10.1  Data PDU                                                68
           10.1.1 ST Data Packets                                        69
           10.2 Control PDUs                                             70
           10.3 Common SCMP Elements                                     71
           10.3.1 FlowSpec                                               71
           10.3.2 Group                                                  72
           10.3.3 MulticastAddress                                       73
           10.3.4 Origin                                                 73
           10.3.5 RecordRoute                                            74
           10.3.6 Target and TargetList                                  75
           10.3.7 UserData                                               76
           10.3.8 Handling of Undefined Parameters                       77
           10.4 ST Control Message PDUs                                  77
           10.4.1 ACCEPT                                                 77
           10.4.2 ACK                                                    79
           10.4.3 CHANGE                                                 80



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           10.4.4 CONNECT                                                80
           10.4.5 DISCONNECT                                             83
           10.4.6 ERROR                                                  83
           10.4.7 HELLO                                                  84
           10.4.8 JOIN                                                   85
           10.4.9 JOIN-REJECT                                            85
           10.4.10 NOTIFY                                                86
           10.5.3; NOTIFY must be acknowledged with an ACK.              86
           10.4.11 REFUSE                                                87
           10.4.12 STATUS                                                89
           10.4.13 STATUS-RESPONSE                                       89
           10.5 Suggested Protocol Constants                             90
           10.5.1 SCMP Messages                                          90
           10.5.2 SCMP Parameters                                        91
           10.5.3 ReasonCode                                             91
           10.5.4 Timeouts and Other Constants                           93
           10.6 Data Notations                                           94

   11 Acknowledgments and Author's Addresses                             95

   12  References                                                        96






























L. Delgrossi and L. Berger (Ed.)                                [Page 5]


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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.

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

                      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



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        +--------------------+
        | Conference Control |
        +--------------------+
                           |
       +-------+ +-------+ |
       | Video | | Voice | | +-----+ +------+ +-----+     +-----+ Application
       | Appl  | | Appl  | | | SNMP| |Telnet| | FTP | ... |     |    Layer
       +-------+ +-------+ | +-----+ +------+ +-----+     +-----+
           |        |      |     |        |     |            |
           V        V      |     |        |     |            |   ------------
        +-----+  +-----+   |     |        |     |            |
        | PVP |  | NVP |   |     |        |     |            |
        +-----+  +-----+   +     |        |     |            |
         |   \      | \     \    |        |     |            |
         |    +-----|--+-----+   |        |     |            |
         |     Appl.|control  V  V        V     V            V
         | ST  data |         +-----+    +-------+        +-----+
         | & control|         | UDP |    |  TCP  |    ... | 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






L. Delgrossi and L. Berger (Ed.)                                [Page 7]


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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, containing 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.  Alternatives 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, implementations of ST2 for Digital
   Equipment, IBM, NeXT, Macintosh, PC, Silicon Graphics, and Sun platforms are



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   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. This has led to the
   specification of the version of ST2, called ST2+, contained in this
   document.

   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.

   1.4.1 Data Transfer Protocol



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   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 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 node, 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 the same
   stream.

   1.4.4 Routing Function

   The routing function is an external unicast route generation capability. It
   provides the setup protocol with the path to reach each of the desired
   destinations. Once a route is selected by the routing function, it persists
   for the whole stream lifetime. The routing function may try to optimize



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   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 well with any reasonable routing
   function algorithm. The algorithm adopted is a local matter at each host and
   different hosts 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.

   1.4.5 Local Resource Manager

   At each host 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 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 is sufficient capacity left to handle the new
           data stream. If the available capacity is 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.



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   During data transfer, the LRM is responsible for:

   o       QoS Enforcement: it enforces the QoS requirements by appropriate
           scheduling of resource access. For instance, an application with a
           short guaranteed delay must be served prior to 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. 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.

   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.

   1.5.1 Streams

   Streams form the core concepts of ST2. They are established between a
   sending origin and one or more receiving targets in the form of a routing
   tree.  Streams are uni-directional from the origin to the targets. Nodes in
   the tree represent so-called ST agents, entities executing the ST2 protocol;
   links in the tree are called hops.

   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



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   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.

            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


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



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   Each ST agent maintains state information describing the streams flowing
   through it. It can actively gather and distribute such information. If, for
   example, an intermediate ST agent fails, the neighbouring ST agents can
   recognize this via HELLO messages that are periodically exchanged between ST
   agents that share streams. STATUS packets can be used to ask other ST agents
   about a particular stream.  These ST agents then send back a STATUS-RESPONSE
   message. NOTIFY messages serve to inform 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.

   Efficiency is also achieved by avoiding fragmentation and reassembly on
   router nodes. 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. 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 retransmission known from 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. 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.





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   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 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, 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 origin may later issue a
   CHANGE request to adjust reservations.




















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

   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.




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   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 ST primitives do not form 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 request is
   first issued by the sender and an indication is generated at the receiver;
   then, the receiver may accept or refuse the request and the correspondent
   indication is generated at the sender. This is shown in Figure 5 below.










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                   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 |
           |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 |



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           |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 suggested to impose limits on the stream
   operations that can be executed at the same time. These restrictions are:

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

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

   3.      An ST agent forwards data only along already established paths to
   the targets, see also Section 3.1. A path is considered to be established
   when the next-hop on the path has explicitly accepted the stream. This



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   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. To keep the figure simple, only
   the primitives that cause state transitions are represented.

   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.




















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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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   It should be noted that 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.

   2.3 State Transition Tables

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

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

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



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   3 The ST2 Data Transfer Protocol

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

   3.1 Data Transfer with ST

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

   Additionally, ST may deliver data corrupted in transmission. 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.  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 to send 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).

   Implementations must be able to handle networks that support multicast. If a



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   network does not support multicast, or for the case where the next-hops are
   on different networks, multiple copies of the data packet must be sent.

   3.2 ST Protocol Functions

   The ST protocol provides two functions:

   o       stream identification

   o       data priority

   3.2.1 Stream Identification

   ST data packets are encapsulated by an ST header containing the Stream
   IDentifier (SID). This SID is selected at the origin so that it is globally
   unique over the Internet. The SID must be known by the setup protocol as
   well.  At stream establishment time, the setup protocol builds, at each host
   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.

   4  SCMP Functional Description

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

   This section contains a functional description of stream management with



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   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 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



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   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 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 streams, up-stream ST agents may or may
   not be informed. Propagation of information on targets that "join" a stream
   is also controlled via join authorization levels. As previously mentioned,
   join authorization levels are described in Section 4.4.2.

   This leads to two types of streams:

   o       full target information is propagated in a controlled stream. For
           such streams, all agents are aware of all down-stream targets
           connected to the stream. This results in target information



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           being maintained at the origin and routers. Operations on single
           targets are always possible, i.e.  charge a certain target, or,
           drop that target from the stream. It is also always possible for
           any ST agent to attempt recovery of all down-steam targets.

   o       in light-weight streams, it is possible that the origin and other
           up-stream 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.

   Controlled 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 controlled 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
   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.



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   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 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



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   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 hosts. Therefore,
   SCMP contains a mechanism to estimate this RTT, see Section 8.5. 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 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



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   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 steam. 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

   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.

   4.5 Stream Setup




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   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
           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
   a path 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



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   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 route 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 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.




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   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 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, FlowSpec, Group, and
   UserData, if any, to be used as a basis for its decision. The application is
   identified by a combination of the NextPcol field and the 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



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   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 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 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



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   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 not
   propagated upstream.

   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 stream joining as described in Section 4.6.3.

   4.5.10 Other Functions during Stream Setup

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

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

   o       counting the number of IP clouds 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, as part of the
           stream recovery mechanism presented 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.



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   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 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) the FlowSpec of the new target, if
   present, matches 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 node in the stream looks at the
   SID 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 (RouteLoop) that contains a TargetList of those targets that were
   duplicates.

   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 nodes 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



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   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 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).




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   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 tar-
           get has been added, it does not propagate the ACCEPT message
           backwards (Section 4.5.6). Instead, it issues a NOTIFY message with
           ReasonCode (TargetJoined) and with TargetList including the target
           to inform the origin of the new target.

   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 tar-
           get has been added, it does not propagate the ACCEPT message
           backwards (Section 4.5.6), nor does it notify the origin.

   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.

   Of course, 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.




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   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, see Section 4.6.3.1.

   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 stream. 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 fragmentation, since ST2 does not support
   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.

   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;



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   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 with a STATUS-RESPONSE
   message.

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



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   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 and it
   sends a REFUSE message toward the origin with ReasonCode (RetransTimeout).

   5.2.3 Failure due to CHANGE Response 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
   aborts the change attempt and it sends a REFUSE message toward the origin
   with ReasonCode (ResponseTimeout).

   5.2.4 Failure due to CONNECT Acknowledgment Timeout

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

   5.2.5 Failure due to CONNECT Response Timeout

   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 message
   is not received before the ToConnectResp timer expires, the ST agent aborts
   the connection setup attempt and it 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 (ResponseTimeout).




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   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

   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 it returns an error
   correspondent to ReasonCode (RetransTimeout) to the application.

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

   5.2.9 Failure due to JOIN-REJECT Acknowledgment Timeout

   An ST agent that sends a JOIN-REJECT message toward the target expects an
   ACK from a 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



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   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.      an erroneous routing loop.

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

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

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

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

   5.3.1 Path Convergence

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

   SCMP does not allow for streams which have converged 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



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   (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 an erroneous
   routing loop, are not easily distinguishable. In attempting recovery of a
   failed stream, an ST agent may issue new CONNECT messages to the affected
   targets. Such a CONNECT may reach an ST agent downstream of the failure
   before that ST agent has received a DISCONNECT from the 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



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   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 release the relevant resources and propagate
   neither the REFUSE nor the DISCONNECT.

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

   5.4 Problems due to Routing Inconsistency

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

   The ST agent that receives such a NOTIFY should ACK it. If the ST agent is
   using an algorithm that would produce such 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.



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   5.5 Problems in Reserving Resources

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

   5.5.1 Mismatched FlowSpecs

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

   5.5.2 Unknown FlowSpec Version

   When the LRM is invoked, it is passed information including the version of
   the FlowSpec, see Section 4.5.2.2. If this version is not know 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 error 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).




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

   5.6 Problems Caused by CHANGE Messages

   A CHANGE might fail for several reasons, including:

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

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

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

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




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   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, the all
   targets must be known. In this case, when processing a CHANGE message, the
   agent should generate a refuse with ReasonCode (TargetUnknown).  When
   processing a DISCONNECT message, it is possible that the DISCONNECT is a
   duplicate of an old request so the agent should respond as if it has
   successfully disconnected the target. That is, it should respond with an ACK
   message.

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

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

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

   6 Failure Detection and Recovery

   6.1 Failure Detection

   The SCMP failure detection mechanism is based on two assumptions:

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

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

   The purpose of the robustness protocol defined here is for ST agents to
   determine that the streams through a neighbor have been broken by the
   failure of the neighbor or the intervening network. This protocol should
   detect the overwhelming majority of failures that can occur.  Once a failure
   is detected, the recovery procedures described in Section 6.2 are initiated



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   by the ST agents.

   6.1.1 Network Failures

   An ST agent can detect network failures by two mechanisms:

   o       the network can report a failure, or

   o       the ST agent can discover a failure by itself.

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

   6.1.2 Detecting ST Agents Failures

   Each ST agent periodically sends each 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.



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   If an ST agent receives a HELLO message that contains the Restarted-bit set,
   it must assume that the sending ST agent has lost its state. If it shares
   streams with that neighbor, it must initiate stream recovery activity, see
   Section 6.2. If it does not share streams with that neighbor, it should not
   attempt to create one until that bit is no longer set. If an ST agent
   receives a CONNECT message from a neighbor whose Restarted-bit is still set,
   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).

   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.

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

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

   If the ST agent does not receive a valid HELLO message within the
   RecoveryTimeout of a stream, it must assume that the neighboring ST agent or
   the communication link between the two has failed and it must initiate
   stream recovery activity, 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



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   effort because it is designed in a general manner to operate across a large
   number of networks with diverse characteristics.  Therefore, it may require
   information to be distributed widely, and may require relatively long
   timers. On the other hand, since a network is a homogeneous system, failure
   recovery in the network may be a relatively faster and simpler operation.
   Therefore an ST agent that detects a failure should attempt to fix the
   network failure before attempting recovery of the ST stream. If the stream
   that existed between two ST agents before the failure cannot be
   reconstructed by network recovery mechanisms alone, then the ST stream
   recovery mechanism must be invoked.

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

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

   o       An ST agent that is the previous-hop before the failed component
           first verifies that there was a failure by querying the
           downstream neighbor using STATUS messages. If the neighbor has
           lost its state but is available, then the ST agent may try and
           reconstruct the stream, unless the NoRecovery option was
           selected, as explained below. If it cannot communicate with the
           next-hop, then the ST agent detecting the failure sends a
           DISCONNECT message 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 a
           REFUSE message with the appropriate ReasonCode (STAgentFailure)
           to the previous-hop. The TargetList in these messages contains
           all the targets that were reached through the broken branch. As
           discussed in Section 5.1.2, multiple REFUSE messages may be



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           required if the PDU is too long for the MTU of the intervening
           network. The REFUSE message is propagated all the way to the ori-
           gin. 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 stream. It does so by issuing a new CONNECT
           message to the affected targets. If the ST agent cannot find new
           routes to some targets, or if the only route to some targets is
           through the previous-hop, then it sends one or more REFUSE
           messages to the previous-hop with the appropriate ReasonCode
           (CantRecover) specifying the affected targets in the TargetList.
           The previous-hop can then attempt recovery of the stream by
           issuing a CONNECT to those targets. If it cannot find an
           appropriate route, it will propagate the REFUSE message toward
           the origin.

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

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

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

   6.2.1 Problems in Stream Recovery




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   The reconstruction of a broken stream may not proceed smoothly. Since there
   may be some delay while the information concerning the failure is propagated
   throughout an internet, routing errors may occur for some time after a
   failure. As a result, the ST agent attempting the recovery may receive 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 with the appropriate ReasonCode
   ("failure"). Therefore, the ST agent that receives the ERROR message with
   ReasonCode (StreamExists) should retransmit the CONNECT message after the
   ToConnect timeout expires. If this fails again, the request will be retried
   for NConnect times. Only if it still fails will the ST agent send a REFUSE
   message with the appropriate ReasonCode (RouteLoop) to its previous-hop.
   This message will be propagated back to the ST agent that is attempting
   recovery of the damaged stream. That ST agent can issue a new CONNECT
   message if it so chooses. The REFUSE is matched to a CONNECT message created
   by a recovery operation through the LnkReference field in the CONNECT.

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

   If a CONNECT message reaches a target, the target should as efficiently as
   possible use the state that it has saved from before the stream failed
   during recovery of the stream. It will then issue an ACCEPT message toward
   the origin. The ACCEPT message will be intercepted by the ST agent that is
   attempting recovery of the damaged stream, if not the origin. If the
   FlowSpec contained in the ACCEPT specifies the same selection of parameters
   as were in effect before the failure, then the ST agent that is attempting



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   recovery will not propagate the ACCEPT. 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.

   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



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   ReasonCode (StreamPreempted).

   7 A Group of Streams

   There may be need to associate related streams. The group mechanism is
   simply an association technique that allows ST agents to identify the
   different streams that are to be associated.

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

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

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

   7.1 Basic Group Relationships

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

   7.1.1 Bandwidth Sharing

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

   The basic concept of a shared bandwidth group is that the LRM will allocate
   up to some specified multiplier of the most demanding stream that it knows
   about in the group. The LRM will allocate resources 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 a host,
   and shared allocations where multiple streams traverse a host.



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   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 host 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 also Section 4.5.2.2. If maximum bandwidth, N*Bmax,
   for the group has already been allocated and a new stream with a bandwidth
   demand less than Bmax is being established, the LRM won't allocate any
   further bandwidth.

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

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

   7.1.2 Fate Sharing

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




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   7.1.3 Route Sharing

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

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

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

   7.1.4 Subnet Resources Sharing

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

   7.2 Relationships Orthogonality

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



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   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

   8.2 Group Name Generator

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

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

   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.




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   8.4 Collecting Information From Neighbour ST Agent

   The STATUS message can be used to collect information about neighbor 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       identify a neighbour is ST capable. 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 8.4;

   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 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.




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   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 maintain the robustness of
   the protocol. Algorithms on this subject are described in [WoHD95],
   [Jaco88], [KaPa87].

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

   8.6 Network MTU Discovery

   At connection setup, the application at the origin asks the local ST agent
   to create streams with certain QoS requirements. The local ST agent fills
   out its network MTU value in the MaxMsgSize parameter in the CONNECT message
   and forwards it to the next-hop ST agents.  Each ST agent in the path checks
   to see if its 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.

   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.



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

   o       Protocol is 5 to indicate an ST packet is enclosed, as opposed to
           TCP or UDP, for example.  The assignment of protocol 5 to ST is
           an arranged coincidence with the assignment of IP Version 5 to ST
           [RFC1190].

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

   o       Type-of-Service may be set to an appropriate value for the service
           being requested (usually low delay, high throughput, normal
           reliability).  This feature is not implemented 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.

   8.8 IP Multicasting

   If an ST agent must use IP encapsulation to reach multiple next-hops toward



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   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 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
           224.0.0.8 All ST hosts

   In addition, a block of transient IP multicast addresses, 224.1.0.0 -
   224.1.255.255, has been allocated for ST multicast groups. 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 transparent to the setup protocol. The setup protocol
   carries the flow specification as part of the FlowSpec parameter, which is
   described in Section 10.3.1.

   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.



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   the Version field is consistent throughout the whole stream. The following
   Version field values are defined:

   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 have to 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 128-256 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 in the current version of ST2. It allows the user
   applications to express their real-time requirements in the form of a QoS
   class, precedence, and 3 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



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   parameters are expressed via a set of values:

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

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

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

   9.2.1 QoS Classes

   Two QoS classes are defined:

   1 - QOS_PREDICTIVE      /* QoSClass field value = 0x01, must be supported*/

   2 - QOS_GUARANTEED      /* QoSClass field value = 0x10, optional */

   o       The QOS_PREDICTIVE class implies that the negotiated QoS may be
           violated for short time intervals during the data transfer. An
           application has to provide values that take into account the
           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



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   represents the lowest precedence. In general, the distinction between
   precedence and priority is that precedence specifies streams that are
   permitted to take previously committed resources from another stream, while
   priority identifies those PDUs that a stream is most willing to have
   dropped.

   9.2.3 Maximum Data Size

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

   9.2.4 Message Rate

   This parameter is expressed in messages/seconds. 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 assignees 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.

   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

   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.

   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 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. 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; IP currently uses a value of 4, while
   ST has been assigned the value 5 (see [RFC791]). There is no requirement for
   compatibility between IP and ST packet headers beyond the first four bits.

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

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



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   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].

   10.1.1 ST Data Packets

   ST packets whose D-bit is non-zero are data packets. Their interpretation is



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   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
           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



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           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 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 any of the optional parameters
           defined in Section 10.3.

   10.3 Common SCMP Elements

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

   10.3.1 FlowSpec

   The FlowSpec parameter (PCode = 1) is used in several SCMP messages to



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   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 it is
           transparent to the ST agent.  It is the data structure to be
           passed to the LRM. It must be 4-byte aligned.

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

   10.3.2 Group

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

         0                   1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |  PCode = 2    |   PBytes = 16 |           GroupUniqueID       |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                        GroupCreationTime                      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                     GroupInitiatorIPAddress                   |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |            Relationship       |                 N             |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 13: Group Parameter

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



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           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.

   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.







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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 = 4    |   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 [RFC1060].

   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 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 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 is 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 is only
           set when responding to a CONNECT request.

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




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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 are permitted to risk and
           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 each next-hop is used



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   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
           stream, see Section 4.4.2.



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

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

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

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

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

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































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   10.4.5 DISCONNECT

   DISCONNECT (OpCode = 5) is used by an origin to tear down an established
   stream or part of a stream, or by an intermediate ST agent that detects a
   failure between itself and its previous-hop, as distinguished by the
   ReasonCode. The DISCONNECT message specifies the list of targets that are
   to be disconnected. An ACK is required in response to a DISCONNECT message.
   The DISCONNECT message is propagated all the way to the specified targets.
   The targets are expected to terminate their participation in the stream.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  OpCode = 5   |G|    0        |           TotalBytes          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Reference                |     LnkReference = 0          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         SenderIPAddress                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Checksum           |          ReasonCode           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      GeneratorIPAddress                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                           TargetList                          :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                            UserData                           :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 25: DISCONNECT Control Message

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

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

   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



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   retransmission timeout.

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

                      Figure 26: ERROR Control Message

   o       Reference is the Reference number of the erroneous request.

   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 first
           generated the JOIN message.

   o       TargetList IP address of a single target i.e. the origin of the
           stream.

   10.4.9 JOIN-REJECT

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












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

   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         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         SenderIPAddress                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Checksum           |          ReasonCode           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      DetectorIPAddress                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                          TargetList                           :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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

   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 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 a change failed and
           pre-change resources and the stream 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.

   o       DetectorIPAddress is the 32-bit IP address of the host that first



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           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.

   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.

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




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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  OpCode = 13  |    0          |           TotalBytes          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Reference                |     LnkReference              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         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-RESPONSE.

   o       LnkReference is the Reference number from the corresponding 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
   4)      CONNECT



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   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
                           acknowledged.
   16      DuplicateTarget Control PDU contains a duplicate target, or an



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                           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, e.g., a
                           route loop.
   44      RouteLoop       A CONNECT was received that specified an existing
                           target.
   45      SAPUnknown      Control PDU contains an unknown next-higher layer
                           SAP (port).
   46      SIDUnknown      Control PDU contains an unknown SID.
   47      STAgentFailure  An ST agent failure has been detected.
   48      STVer3Bad       A received PDU is not ST Version 3.
   49      StreamExists    A stream with the given SID already exists.
   50      StreamPreempted The stream has been preempted by one with a higher
                           precedence.



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   51      TargetUnknown   A target is not a member of the specified stream.
   52      TargetMissing   A target parameter was expected and is not
                           included, or is empty.
   53      TruncatedCtl    Control PDU is shorter than expected.
   54      TruncatedPDU    A received ST PDU is shorter than the ST Header
                           indicates.
   55      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
       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



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                           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

   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).









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                                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 any optional parameters is not meaningful.

   11 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. 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

   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



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   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

   12  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.

   [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.

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




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

   [RFC1060]       Reynolds, Postel: Assigned Numbers, RFC 1060, USC/ISI, March
                   1990.

   [RFC1190]       Topolcic C.: Internet Stream Protocol Version 2 (ST2),
                   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.



















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