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RFC 1819

 
 
 

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

Part 2 of 4, p. 26 to 45
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3.  The ST2 Data Transfer Protocol

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

3.1  Data Transfer with ST

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

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

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

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

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

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

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

3.2  ST Protocol Functions

   The ST protocol provides two functions:

   o   stream identification

   o   data priority

3.2.1  Stream Identification

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

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

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

3.2.2  Packet Discarding based on Data Priority

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

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4.  SCMP Functional Description

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

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

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

   o   Add a peer at E,

   o   Drop peers B and C, and

   o   Let F join the stream

   o   Delete the stream.

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

                Figure 8:  Sample Topology for an ST Stream

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

4.1  Types of Streams

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

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4.1.1  Stream Building

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

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

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

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

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

4.1.2  Knowledge of Receivers

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

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

   This leads to two types of streams:

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

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

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

4.2  Control PDUs

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

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

   SCMP follows a request-response model with all requests expecting
   responses. Retransmission after timeout is used to allow for lost or
   ignored messages. Control messages do not extend across packet
   boundaries; if a control message is too large for the MTU of a hop,
   its information is partitioned and a control message per partition is
   sent, as described in Section 5.1.2.

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

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

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

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

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

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

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

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

4.3  SCMP Reliability

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

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

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

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

4.4  Stream Options

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

4.4.1  No Recovery

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

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4.4.2  Join Authorization Level

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

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

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

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

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

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

4.4.3  Record Route

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

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

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4.4.4  User Data

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

4.5  Stream Setup

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

4.5.1  Information from the Application

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

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

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

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

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

4.5.2  Initial Setup at the Origin

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

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

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

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

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

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

4.5.2.1  Invoking the Routing Function

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

   The result of the routing function is a set of next-hop ST agents.
   The set of next-hops selected by the routing function is not
   necessarily the same as the set of next-hops that IP would select
   given a number of independent IP datagrams to the same destinations.
   The routing algorithm may attempt to optimize parameters other than
   the number of hops that the packets will take, such as delay, local
   network bandwidth consumption, or total internet bandwidth
   consumption.  Alternatively, the routing algorithm may use a simple
   route lookup for each target.

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

4.5.2.2  Reserving Resources

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

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

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

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

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

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

4.5.3  Sending CONNECT Messages

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

   The TargetList may be empty, see Section 4.5.3.1; if the TargetList
   causes a too long CONNECT message to be generated, the CONNECT
   message is partitioned as explained in Section 5.1.2. If multiple
   next-hops are to be reached through a network that supports network
   level multicast, a different CONNECT message must nevertheless be
   sent to each next-hop since each will have a different TargetList.

4.5.3.1  Empty Target List

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

4.5.4  CONNECT Processing by an Intermediate ST agent

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

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4.5.5  CONNECT Processing at the Targets

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

   The application is presented with parameters from the CONNECT message
   including the SID, the selected stream options, Origin, FlowSpec,
   TargetList, and Group, if any, to be used as a basis for its
   decision.  The application is identified by a combination of the
   NextPcol field, from the Origin parameter, and the service access
   point, or SAP, field included in the correspondent (usually single
   remaining) Target of the TargetList. The contents of the SAP field
   may specify the port or other local identifier for use by the
   protocol layer above the host ST layer. Subsequently received data
   packets will carry the SID, that can be mapped into this information
   and be used for their delivery.

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

4.5.6  ACCEPT Processing by an Intermediate ST agent

   When an intermediate ST agent receives an ACCEPT, it first verifies
   that the message is a response to an earlier CONNECT. If not, it
   responds to the next-hop ST agent with an ERROR message, with
   ReasonCode (LnkRefUnknown). Otherwise, it responds to the next-hop ST
   agent with an ACK, and propagates the individual ACCEPT message to
   the previous-hop along the same path traced by the CONNECT but in the
   reverse direction toward the origin.

   The FlowSpec is included in the ACCEPT message so that the origin and
   intermediate ST agents can gain access to the information that was
   accumulated as the CONNECT traversed the internet. Note that the
   resources, as specified in the FlowSpec in the ACCEPT message, may
   differ from the resources that were reserved when the CONNECT was
   originally processed. Therefore, the ST agent presents the LRM with
   the FlowSpec included in the ACCEPT message. It is expected that each
   LRM adjusts local reservations releasing any excess resources. The

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

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

4.5.7  ACCEPT Processing by the Origin

   The origin will eventually receive an ACCEPT (or REFUSE) message from
   each of the targets. As each ACCEPT is received, the application is
   notified of the target and the resources that were successfully
   allocated along the path to it, as specified in the FlowSpec
   contained in the ACCEPT message. The application may then use the
   information to either adopt or terminate the portion of the stream to
   each target.

   When an ACCEPT is received by the origin, the path to the target is
   considered to be established and the ST agent is allowed to forward
   the data along this path as explained in Section 2 and in Section
   3.1.

4.5.8  REFUSE Processing by the Intermediate ST agent

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

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

4.5.9  REFUSE Processing by the Origin

   When the REFUSE message reaches the origin, the ST agent at the
   origin sends an ACK and notifies the application that the target is
   no longer part of the stream and also if the stream has no remaining
   targets. If there are no remaining targets, the application may wish
   to terminate the stream, or keep the stream active to allow addition

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   of targets or stream joining as described in Section 4.6.3.

4.5.10  Other Functions during Stream Setup

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

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

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

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

4.6  Modifying an Existing Stream

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

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

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4.6.1  The Origin Adding New Targets

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

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

   An intermediate ST agent that is already a participant in the stream
   looks at the SID and StreamCreationTime, and verifies that the stream
   is the same. It then checks if the intersection of the TargetList and
   the targets of the established stream is empty. If this is not the
   case, it responds with a REFUSE message with ReasonCode
   (TargetExists) that contains a TargetList of those targets that were
   duplicates. To indicate that the stream exists, and includes the
   listed targets, the ST agent sets to one (1) the E-bit of the REFUSE
   message, see Section 10.4.11.  The agent then proceeds processing
   each new target in the TargetList.

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

4.6.2  The Origin Removing a Target

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

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

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

4.6.3  A Target Joining a Stream

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

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

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

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

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

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

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

4.6.3.1  Intermediate Agent (Router) as Origin

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

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

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

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

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

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

4.6.4  A Target Deleting Itself

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

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   and with ReasonCode (ApplDisconnect). The REFUSE message is sent back
   to the origin via the previous-hop. If a stream has multiple targets
   and one target leaves the stream using this REFUSE mechanism, the
   stream to the other targets is not affected; the stream continues to
   exist.

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

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

4.6.5  Changing a Stream's FlowSpec

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

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

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

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

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

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

4.7  Stream Tear Down

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

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



(page 45 continued on part 3)

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