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RFC 1190


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

Part 2 of 6, p. 25 to 49
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prevText      Top       Page 25 
         the ACCEPT message to the previous-hop along the same path
         traced by the CONNECT but in the reverse direction toward the
         origin.  The ACCEPT should not be propagated until all HID
         negotiations with the next-hop agent(s) have been successfully

         The FlowSpec is included in the ACCEPT message so that the
         origin and intermediate ST agents can gain access to the
         information that was accumulated as the CONNECT traversed the
         internet.  Note that the resources, as specified in the
         FlowSpec in the ACCEPT message, may differ from the resources
         that were reserved by the agent when the CONNECT was

      Agent A                     Agent 1                    Agent B

                                     +<-+<- ACCEPT B <-------<< [3.5]
                                     V  |   <RVLId=15><SVLId=44>
4.1.                 (wait for ACCEPTS) V   <Ref=410><LnkRef=110>
4.2.                                 V  +-> ACK --------------->+
4.3.    (wait until HID negotiated)<-+      <RVLId=44><SVLId=15>
                                  V         <Ref=410>
4.4.  <<--+<-- ACCEPT B <---------+

       Agent A                    Agent 2                    Agent C

                                     +<-+<- ACCEPT C <------<< [3.10]
                                     |  |   <RVLId=25><SVLId=54>
                                     |  V   <Ref=510><LnkRef=210>
4.5.                                 |  +-> ACK --------------->+
                                     |      <Ref=510>
                                     |      <RVLId=54><SVLId=25>
                                     |                       Agent D
                                     +<-+<- ACCEPT D <------<< [3.15]
                                     V  |   <RVLId=26><SVLId=64>
4.6.                 (wait for ACCEPTS) V   <Ref=610><LnkRef=215>
4.7.                                 V  +-> ACK --------------->+
4.8.    (wait until HID negotiated)<-+      <RVLId=64><SVLId=26>
                                  V         <Ref=610>
4.9.  <<--+<- ACCEPT C <----------+
              <RVLId=5><SVLId=23> |
4.10. <<--+<- ACCEPT D <----------+

         Figure 8.  ACCEPT Processing by an Intermediate Agent

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         originally processed.  However, the agent does not adjust the
         reservation in response to the ACCEPT.  It is expected that any
         excess resource allocation will be released for use by other
         stream or datagram traffic through an explicit CHANGE message
         initiated by the application at the origin if it does not wish
         to be charged for any excess resource allocations.

      3.1.8.        ACCEPT Processing by the Origin

         The origin will eventually receive an ACCEPT (or REFUSE or
         ERROR-IN-REQUEST) message from each of the targets.  As each
         ACCEPT is received, the application should be notified of the
         target and the resources that were successfully allocated along
         the path to it, as specified in the FlowSpec contained in the
         ACCEPT message.  The application may then use the information
         to either adopt or terminate the portion of the stream to each
         target.  When ACCEPTs (or failures) from all targets have been
         received at the origin, the application is notified that stream
         setup is complete, and that data may be sent.

         Application A   Agent A                  Agent 1   Agent 2

                            +<-- ACCEPT B <--------<< [4.4]
                            |    <RVLId=4><SVLId=14>
                            V    <Ref=115><LnkRef=10>
   5.1.                     +--> ACK ----------------->+
                            |    <RVLId=14><SVLId=4>
                            V    <Ref=115>
   5.2.        +<-- (inform A of B's FlowSpec)
               |            +<-- ACCEPT C <----------------<< [4.9]
               |            |    <RVLId=5><SVLId=23>
               |            V    <Ref=220><LnkRef=15>
   5.3.        |            +--> ACK ------------------------->+
               |            |    <RVLId=23><SVLId=5>
               |            V    <Ref=220>
   5.4.        +<-- (inform A of C's FlowSpec)
               |            +<-- ACCEPT D <----------------<< [4.10]
               |            |    <RVLId=5><SVLId=23>
               |            V    <Ref=225><LnkRef=15>
   5.5.        |            +--> ACK ------------------------->+
               |            |    <RVLId=23><SVLId=5>
               |            V    <Ref=225>
   5.6.        +<-- (inform A of D's FlowSpec)
   5.7.    (wait until HIDs negotiated)
   5.8.    (inform A open to B,C,D)

               Figure 9.  ACCEPT Processing by the Origin

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         There are several pieces of information contained in the
         FlowSpec that the application must combine before sending data
         through the stream.  The PDU size should be computed from the
         minimum value of the DesPDUBytes field from all ACCEPTs and the
         protocol layers above ST should be informed of the limit.  It
         is expected that the next higher protocol layer above ST will
         segment its PDUs accordingly.  Note, however, that the MTU may
         decrease over the life of the stream if new targets are
         subsequently added.  Whether the MTU should be increased as
         targets are dropped from a stream is left for further study.

         The available bandwidth and packet rate limits must also be
         combined.  In this case, however, it may not be possible to
         select a pair of values that may be used for all paths, e.g.,
         one path may have selected a low rate of large packets while
         another selected a high rate of small packets.  The application
         may remedy the situation by either tearing down the stream,
         dropping some participants, or creating a second stream.

         After any differences have been resolved (or some targets have
         been deleted by the application to permit resolution), the
         application at the origin should send a CHANGE message to
         release any excess resources along paths to those targets that
         exceed the resolved parameters for the stream, thereby reducing
         the costs that will be incurred by the stream.

      3.1.9.        Processing a REFUSE Message

         REFUSE messages are used to indicate a failure to reach an
         application at a target;  they are propagated toward the origin
         of a stream.  They are used in three situations:

          1  during stream setup or expansion to indicate that there
             is no satisfactory path from an ST agent to a target,

          2  when the application at the target either does not
             exist does not wish to be a participant, or wants to
             cease being a participant, and

          3  when a failure has been detected and the agents are
             trying to find a suitable path around the failure.

         The cases are distinguished by the ReasonCode field and an
         agent receiving a REFUSE message must examine that field in
         order to determine the proper action to be taken.  In
         particular, if the ReasonCode indicates that the CONNECT
         message reached the target then the REFUSE should be propagated
         back to the origin, releasing resources as appropriate along
         the way.  If the ReasonCode indicates that

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         the CONNECT message did not reach the target then the
         intermediate (origin) ST agent(s) should check for alternate
         routes to the target before propagating the REFUSE back another
         hop toward the origin.  This implies that an agent must keep
         track of the next-hops that it has tried, on a target by target
         basis, in order not to get caught in a loop.

         An ST agent that receives a REFUSE message must acknowledge it
         by sending an ACK to the next-hop.  The REFUSE must also be
         propagated back to the previous-hop ST agent.  Note that the ST
         agent may not have any information about the target in

   Appl.  Agent A                   Agent 2                 Agent E
                                               (proc E NOT listening)
1. (add E)
2.    +----->+-> CONNECT E ---------->+->+
                 <RVLId=23><SVLId=5>  |  |
                 <Ref=65>             V  |
3.           +<-- ACK <---------------+  |
                  <RVLId=5><SVLId=23>    V
4.                <Ref=65>         (routing to E)
5.                           (reserve resources 2 to E)
6.                                       +--> CONNECT E --------->+
                                              <RVLId=0><SVLId=27> |
                                              <Ref=115><HID=4600> |
7.                                    +<-+<- REFUSE B <-----------+
                                      |  |   <RVLId=27><SVLId=74>
                                      |  |   <Ref=705><LnkRef=115>
                                      |  V   <RC=SAPUnknown>
8.                                    |  +-> ACK ---------------->+
                                      |  |   <RVLId=74><SVLId=27> |
                                      |  V   <Ref=705>            |
9.                                    |  (free link 27)           V
10.                                   V              (free link 74)
11.          +<- REFUSE B <-----------+
             |   <RVLId=5><SVLId=23>  |
             |   <Ref=550><LnkRef=65> V
12.          |   <RC=SAPUnknown>  (free resources 2 to E)
13.          +-> ACK  --------------->+
             |   <RVLId=23><SVLId=5>  |
             |   <Ref=550>            V
14.          V             (keep link 23 for C,D)
15.  (keep link 5 for C,D)
16.  (inform application failed SAPUnknown)

                   Figure 10.  Sending REFUSE Message

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         the TargetList.  This may result from interacting DISCONNECT
         and REFUSE messages and should be logged and silently ignored.

         If, after deleting the specified target, the next-hop has no
         remaining targets, then those resources associated with that
         next-hop agent may be released.  Note that network resources
         may not actually be released if network multicasting is being

   Appl.   Agent A       Agent 2  Agent 1 Agent 3              Agent B

1.                                      (network from 1 to B fails)
2. (add B)
3.   +-> CONNECT B ----------------->+
         <RVLId=0><SVLId=6>          |
         <Ref=35><HID=100>           |
3.   +<- HID-APPROVE <---------------+
         <RVLId=6><SVLId=11>         |
         <Ref=35><HID=100>           V
4.                       (routing to B: no route)
5.   +<-+-- REFUSE B ----------------+
     |  |   <RVLId=6><SVLId=11>
     |  |   <Ref=155><LnkRef=35>
     |  V   <RC=NoRouteToDest>
6.   |  +-> ACK -------------------->+
     |  |   <RVLId=11><SVLId=6>      V
7.   |  V   <Ref=155>           (drop link 6)
8.   V  (drop link 11)
9.   (find alternative route: via agent 2)
10.  (resources from A to 2 already allocated:
     V   reuse control link & HID, no additional resources required)
11.  +-> CONNECT B -------->+->+
         <RVLId=23><SVLId=5>|  |
         <Ref=40>           V  |
12.  +<- ACK <--------------+  |
         <RVLId=5><SVLId=23>   V
13.      <Ref=40>    (routing to B: via agent 3)
14.                         +-> CONNECT B -->+
15.                      <RVLId=0><SVLId=24> +-> CONNECT B --------->+
                         <Ref=245><HID=4801> V   <RVLId=0><SVLId=32> |
16.                         +<- HID-APPROVE -+   <Ref=310><HID=6000> |
                                <RVLId=24><SVLId=33>                 |
                                <Ref=245><HID=4801>                  V
17.                                          +<- HID-APPROVE --------+
                                                 <Ref=310><HID=6000> V
18.        (ACCEPT handling follows normally to complete stream setup)

           Figure 11.  Routing Around a Failure

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         used since they may still be required for traffic to other
         next-hops in the multicast group.

         When the REFUSE reaches a origin, the origin sends an ACK and
         notifies the application via the next higher layer protocol
         that the target listed in the TargetList is no longer part of
         the stream and also if the stream has no remaining targets.  If
         there are no remaining targets, the application may wish to
         terminate the stream.

         Figure 10 illustrates the protocol exchanges for processing a
         REFUSE generated at the target, either because the target
         application is not running or that the target application
         rejects membership in the stream.  Figure 11 illustrates the
         case of rerouting around a failure by an intermediate agent
         that detects a failure or receives a refuse.  The protocol
         exchanges used by an application at the target to delete itself
         from the stream is discussed in Section 3.3.3 (page 35).

   3.2.       Data Transfer

      At the end of the connection setup phase, the origin, each target,
      and each intermediate ST agent has a database entry that allows it
      to forward the data packets from the origin to the targets and to
      recover from failures of the intermediate agents or networks.  The
      database should be optimized to make the packet forwarding task
      most efficient.  The time critical operation is an intermediate
      agent receiving a packet from the previous-hop agent and
      forwarding it to the next-hop agent(s).  The database entry must
      also contain the FlowSpec, utilization information, the address of
      the origin and previous-hop, and the addresses of the targets and
      next-hops, so it can perform enforcement and recover from

      An ST agent receives data packets encapsulated by an ST header.  A
      data packet received by an ST agent contains the non-zero HID
      assigned to the stream for the branch from the previous-hop to
      itself.  This HID was selected so that it is unique at the
      receiving ST agent and thus can be used, e.g., as an index into
      the database, to obtain quickly the necessary replication and
      forwarding information.

      The forwarding information will be network and implementation
      specific, but must identify the next-hop agent or agents and their
      respective HIDs.  It is suggested that the cached information for
      a next-hop agent include the local network address of the next-
      hop.  If the data packet must be forwarded to multiple next-hops
      across a single network that supports multicast, the database may
      specify a single HID and may identify the next-hops by a (local
      network) multicast address.

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      If the network does not support multicast, or the next-hops are on
      different networks, then the database must indicate multiple
      (next-hop, HID) tuples.  When multiple copies of the data packet
      must be sent, it may be necessary to invoke a packet replicator.

      Data packets should not require fragmentation as the next higher
      protocol layer at the origin was informed of the minimum MTU over
      all paths in the stream and is expected to segment its PDUs
      accordingly.  However, it may be the case that a data packet that
      is being rerouted around a failed network component may be too
      large for the MTU of an intervening network.  This should be a
      transient condition that will be corrected as soon as the new
      minimum MTU has been propagated back to the origin.  Disposition
      by a mechanism other than dropping of the too large PDUs is left
      for further study.

   3.3.       Modifying an Existing Stream

      Some applications may wish to change the parameters of a stream
      after it has been created.  Possible changes include adding or
      deleting targets and changing the FlowSpec.  These are described

      3.3.1.        Adding a Target

         It is possible for an application to add a new target to an
         existing stream any time after ST has incorporated information
         about the stream into its database.  At a high level, the
         application entities exchanges whatever information is
         necessary.  Although the mechanism or protocol used to
         accomplish this is not specified here, it is necessary for the
         higher layer protocol to inform the host ST agent at the origin
         of this event.  The host ST agent at the target must also be
         informed unless this had previously been done.  Generally, the
         transfer of a target list from an ST agent to another, or from
         a higher layer protocol to a host ST agent, will occur
         atomically when the CONNECT is received.  Any information
         concerning a new target received after this point can be viewed
         as a stream expansion by the receiving ST agent.  However, it
         may be possible that an ST agent can utilize such information
         if it is received before it makes the relevant routing
         decisions.  These implementation details are not specified
         here, but implementations must be prepared to receive CONNECT
         messages that represent expansions of streams that are still in
         the process of being setup.

         To expand an existing stream, the origin issues one or more
         CONNECT messages that contain the Name, the VLId, the FlowSpec,
         and the TargetList specifying the new target or targets.  The
         origin issues multiple CONNECT messages if

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

Application  Agent A               Agent 2                    Agent E

1.   (open E)
2.      V                                            (proc E listening)
3.      +->(routing to E)
4.         +-> (check resources from A to Agent 2: already allocated,
           V  reuse control link & HID, no additional resources needed)
5.         +-> CONNECT E --------->+->+
               <RVLId=23><SVLId=5> |  V
6.             <Ref=20>            V  (routing to E)
7.         +<- ACK <---------------+  V
               <RVLId=5><SVLId=23>    +->(reserve resources 2 to E)
               <Ref=20>                  V
8.                                       +-> CONNECT E --------->+
                                             <RVLId=0><SVLId=27> |
                                             <Ref=230><HID=4800> |
9.                                       +<- HID-APPROVE <-------+
                                             <Ref=230><HID=4800> V
10.                                               (proc E accepts)
11.                                    (wait until HID negotiated)
12.                                   +<-+<- ACCEPT E <----------+
                                      V  |   <RVLId=27><SVLId=74>
13.                  (wait for ACCEPTS)  V   <Ref=710><LnkRef=230>
14.                                   V  +-> ACK --------------->+
15.      (wait until HID negotiated)<-+      <RVLId=74><SVLId=27>
                                   V         <Ref=710>
16.           +<- ACCEPT E <-------+
              |   <RVLId=5><SVLId=23>
              V   <Ref=235><LnkRef=20>
17.           +-> ACK ------------>+
              |   <RVLId=23><SVLId=5>
              V   <Ref=235>
18.        +<-(inform A of E's FlowSpec)
19.     +<-(wait for ACCEPTS)
20.  +<-(wait until HID negotiated)
21.  (inform A open to E)

                 Figure 12.  Addition of Another Target

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         An ST agent that is already a node in the stream recognizes the
         RVLId and verifies that the Name of the stream is the same.  It
         then checks if the intersection of the TargetList and the
         targets of the established stream is empty.  If this is not the
         case, then the receiver responds with an ERROR-IN-REQUEST with
         the appropriate reason code (RouteLoop) that contains a
         TargetList of those targets that were duplicates;  see Section (page 106).

         For each new target in the TargetList, processing is much the
         same as for the original CONNECT;  see Sections 3.1.2-4 (pages
         19-20).  The CONNECT must be acknowledged, propagated, and
         network resources must be reserved.  However, it may be
         possible to route to the new targets using previously allocated
         paths or an existing multicast group.  In that case, additional
         resources do not need to be reserved but more next-hop(s) might
         have to be added to an existing multicast group.

         Nevertheless, the origin, or any intermediate ST agent that
         receives a CONNECT for an existing stream, can make a routing
         decision that is independent of any it may have made
         previously.  Depending on the routing algorithm that is used,
         the ST agent may decide to reach the new target by way of an
         established branch, or it may decide to create a new branch.
         The fact that a new target is being added to an existing stream
         may result in a suboptimal overall routing for certain routing
         algorithms.  We take this problem to be unavoidable since it is
         unlikely that the stream routing can be made optimal in
         general, and the only way to avoid this loss of optimality is
         to redefine the routing of potentially the entire stream, which
         would be too expensive and time consuming.

      3.3.2.        The Origin Removing a Target

         The application at the origin specifies a set of targets that
         are to be removed from the stream and an appropriate reason
         code (ApplDisconnect).  The targets are partitioned into
         multiple DISCONNECT messages based on the next-hop to the
         individual targets.  As with CONNECT messages, an ST agent that
         is sending a DISCONNECT must make sure that the message fits
         into the MTU for the intervening network.  If the message is
         too large, the TargetList must be further partitioned into
         multiple DISCONNECT messages.

         An ST agent that receives a DISCONNECT message must acknowledge
         it by sending an ACK back to the previous-hop.  The DISCONNECT
         must also be propagated to the relevant next-hop ST agents.
         Before propagating the message, however, the TargetList should
         be partitioned based on next-hop ST

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         agent and MTU, as described above.  Note that there may be
         targets in the TargetList for which the ST agent has no
         information.  This may result from interacting DISCONNECT and
         REFUSE messages and should be logged and silently ignored.

         If, after deleting the specified targets, any next-hop has no
         remaining targets, then those resources associated with that
         next-hop agent may be released.  Note that network resources
         may not actually be released if network multicasting is being
         used since they may still be required for traffic to other
         next-hops in the multicast group.

      Application                                         Application
            Agent A             Agent 1  Agent 2          Agent B    C

  1.  (close B,C ApplDisconnect)
  2.      +->+-+-> DISCONNECT B ----->+
  3.         | |   <RVLId=14><SVLId=4>+-+-> DISCONNECT B ------>+
             | |   <Ref=25>           | |   <RVLId=44><SVLId=15>|
             | V   <RC=ApplDisconnect>| |   <Ref=120>           |
  4.         | (free A to 1 resrc.)   | V   <RC=ApplDisconnect> |
  5.         |                        V (free 1 to B resrc.)    |
  6.         | +<- ACK <--------------+                         V
  7.         | |   <RVLId=4><SVLId=14>| +<- ACK <---------------+
             | V   <Ref=25>           | |   <RVLId=15><SVLId=44>|
  8.         | (free link 4)          V |   <Ref=120>           |
  9.         |           (free link 14) V                       |
  10.        |                          (free link 15)          V
  11.        |        (inform B that stream closed ApplDisconnect)
  12.        |                                     (free link 44)
  13.     +<-+-+-> DISCONNECT C ---------->+
  14.     |    |   <RVLId=23><SVLId=5>     +-+-> DISCONNECT C ------>+
          |    |   <Ref=30>                | |   <RVLId=54><SVLId=25>|
          |    V   <RC=ApplDisconnect>     | |   <Ref=240>           |
  15.     |    (keep A to 2 resrc for      | V   <RC=ApplDisconnect> |
  16.     |         data going to D,E)     | (free 2 to C resrc.)    |
          |                                V                         |
  17.     |    +<- ACK <-------------------+                         V
  18.     |    |   <RVLId=5><SVLId=23>     | +<- ACK <---------------+
          |    V   <Ref=30>                | |   <RVLId=25><SVLId=54>|
  19.     |    (keep link 5 for D,E)       V |   <Ref=240>           |
  20.     |           (keep link 23 for D,E) V                       |
  21.     |                           (free link 25)                 V
  22.     |              (inform C that stream closed ApplDisconnect>)
  23.     V                                             (free link 54)
  24.     (inform A closed to B,C ApplDisconnect)

                  Figure 13.  Origin Removing a Target

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         When the DISCONNECT reaches a target, the target sends an ACK
         and notifies the application that it is no longer part of the
         stream and the reason.  The application should then inform ST
         to terminate the stream, and ST should delete the stream from
         its database after performing any necessary management and
         accounting functions.

      3.3.3.        A Target Deleting Itself

         The application at the target may inform ST that it wants to be
         removed from the stream and the appropriate reason code
         (ApplDisconnect).  The agent then forms a REFUSE message with
         itself as the only entry in the TargetList.  The REFUSE is sent
         back to the origin via the previous-hop.  If a stream has
         multiple targets and one target leaves the stream using this
         REFUSE mechanism, the stream to the other targets is not
         affected;  the stream continues to exist.

         An ST agent that receives such a REFUSE message must
         acknowledge it by sending an ACK to the next-hop.  The target
         is deleted and, if the next-hop has no remaining targets, then
         the those resources associated with that next-hop agent may be
         released.  Note that network resources may not actually be
         released if network multicasting is being used since they may
         still be required for traffic to other next-hops in the
         multicast group.  The REFUSE must also be propagated back to
         the previous-hop ST agent.

                 Agent A          Agent 2          Agent E

            1.                             (close E ApplDisconnect)
            2.                         +<- REFUSE E --+
                                       |   <RVLId=27><SVLId=74>
                                       |   <Ref=720>
                                       V   <RC=ApplDisconnect>
            3.                      +<-+-> ACK ------>+
                                    |  |   <RVLId=74><SVLId=27>
            4.                      V  V   <Ref=720>
            5.    +<-+<- REFUSE E --+  (prune allocations)
                  |  |   <RVLId=5><SVLId=23>
                  |  |   <Ref=245>
                  |  V   <RC=ApplDisconnect>
            6.    |  +-> ACK ------>+
                  |  |   <RVLId=23><SVLId=5>
                  |  V   <Ref=245>
            7.    V  (prune allocations)
            8.    (inform application closed E ApplDisconnect)

                   Figure 14.  Target Deleting Itself

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         When the REFUSE reaches the origin, the origin sends an ACK and
         notifies the application that the target listed in the
         TargetList is no longer part of the stream.  If the stream has
         no remaining targets, the application may choose to terminate
         the stream.

      3.3.4.        Changing the FlowSpec

         An application may wish to change the FlowSpec of an
         established stream.  To do so, it informs ST of the new
         FlowSpec and the list of targets that are to be changed.  The
         origin ST agent then issues one or more CHANGE messages with
         the new FlowSpec and sends them to the relevant next-hop
         agents.  CHANGE messages are structured and processed similarly
         to CONNECT messages.  A next-hop agent that is an intermediate
         agent and receives a CHANGE message similarly determines if it
         can implement the new FlowSpec along the hop to each of its
         next-hop agents, and if so, it propagates the CHANGE messages
         along the established paths.  If this process succeeds, the
         CHANGE messages will eventually reach the targets, which will
         each respond with an ACCEPT message that is propagated back to
         the origin.

         Note that since a CHANGE may be sent containing a FlowSpec with
         a range of permissible values for bandwidth, delay, and/or
         error rate, and the actual values returned in the ACCEPTs may
         differ, then another CHANGE may be required to release excess
         resources along some of the paths.

   3.4.       Stream Tear Down

      A stream is usually terminated by the origin when it has no
      further data to send, but may also be partially torn down by the
      individual targets.  These cases will not be further discussed
      since they have already been described in Sections 3.3.2-3 (pages

      A stream is also torn down if the application should terminate
      abnormally.  Processing in this case is identical to the previous
      descriptions except that the appropriate reason code is different

      When all targets have left a stream, the origin notifies the
      application of that fact, and the application then is responsible
      for terminating the stream.  Note, however, that the application
      may decide to add a target(s) to the stream instead of terminating

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   3.5.       Exceptional Cases

      The previous descriptions covered the simple cases where
      everything worked.  We now discuss what happens when things do not
      succeed.  Included are situations where messages are lost, the
      requested resources are not available, the routing fails or is

      In order for the ST Control Message Protocol to be reliable over
      an unreliable internetwork, the problems of corruption,
      duplication, loss, and ordering must be addressed.  Corruption is
      handled through use of checksumming, as described in Section 4
      (page 76).  Duplication of control messages is detected by
      assigning a transaction number (Reference) to each control
      message;  duplicates are discarded.  Loss is detected using a
      timeout at the sender;  messages that are not acknowledged before
      the timeout expires are retransmitted;  see Section 3.7.6 (page
      66).  If a message is not acknowledged after a few retransmissions
      a fault is reported.  The protocol does not have significant
      ordering constraints.  However, minor sequencing of control
      messages for a stream is facilitated by the requirement that the
      Reference numbers be monotonically increasing;  see Section 4.2
      (page 78).

      3.5.1.        Setup Failure due to CONNECT Timeout

         If a response (an ERROR-IN-REQUEST, an ACK, a HID-REJECT, or a
         HID-APPROVE) has not been received within time ToConnect, the
         ST agent should retransmit the CONNECT message.  If no response
         has been received within NConnect retransmissions, then a fault
         occurs and a REFUSE message with the appropriate reason code
         (RetransTimeout) is sent back in the direction of the origin,
         and, in place of the CONNECT, a DISCONNECT is sent to the
         next-hop (in case the response to the CONNECT is the message
         that was lost).  The agent will expect an ACK for both the
         REFUSE and the DISCONNECT messages.  If it does not receive an
         ACK after retransmission time ToRefuse and ToDisconnect
         respectively, it will resend the REFUSE/DISCONNECT message.  If
         it does not receive ACKs after sending NRefuse/ NDisconnect
         consecutive REFUSE/DISCONNECT messages, then it simply gives up

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          Sending Agent              Receiving Agent

    1.   ->+----> CONNECT X ------>//// (message lost or garbled)
           |      <RVLId=0><SVLId=99>
           V      <Ref=1278><HID=1234>
    2. (timeout)
    3.     +----> CONNECT X ------------>+
    4.     |      <RVLId=0><SVLId=99>    +----> CONNECT X ----------->+
           |      <Ref=1278><HID=1234>   V      <RVLId=0><SVLId=1010> |
    5.     | //<- HID-APPROVE <----------+      <Ref=6666><HID=6666>  V
    6.     |      <RVLId=99><SVLId=88>      +<- HID-APPROVE <---------+
           V      <Ref=1278><HID=1234>          <RVLId=1010><SVLId=1111>
    7. (timeout)                                <Ref=6666><HID=6666>
    8.     +----> CONNECT X ------------>+
                  <RVLId=0><SVLId=99>    |
                  <Ref=1278><HID=1234>   V
    9.     +<-+<- HID-APPROVE <----------+
           |      <RVLId=99><SVLId=88>
           V      <Ref=1278><HID=1234>
     (cancel timer)

           Figure 15.  CONNECT Retransmission after a Timeout

      3.5.2.        Problems due to Routing Inconsistency

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

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

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

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         the previous-hop agent containing an appropriate reason code
         (RouteInconsist), pertinent TargetList, and in the
         NextHopIPAddress element the address of the previous-hop.  When
         the previous-hop receives the REFUSE, it will recompute the
         next-hop for the affected targets.  If there is a difference in
         the routing databases in the two agents, they may exchange
         CONNECT and REFUSE messages again.  Since such routing errors
         in the internet are assumed to be temporary, the situation
         should eventually stabilize.

      3.5.3.        Setup Failure due to a Routing Failure

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

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

          2  a deliberate source routing loop,

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

          4  an erroneous routing loop.

         The TargetList is used to distinguish the cases 1 and 2 (see
         also Section (page 107)) by comparing each newly
         received target with those of the previously existing stream:

          o  if the IP address of the targets differ, it is case 1;

          o  if the IP address of the targets match but the source
             route(s) are different, it is case 2;

          o  if the target (including any source route) matches a
             target (including any source route) in the existing
             stream, it may be case 3 or 4.

         It is expected that the joining of branches will become more
         common as routing decisions are based on policy issues and not
         just simple connectivity.  Unfortunately, there is no good way
         to merge the two parts of the stream back into a single stream.
         They must be treated independently with respect to processing
         in the agent.  In particular, a separate state machine is
         required, the Virtual Link Identifiers and HIDs from the
         previous-hops and to the next-hops must be different, and
         duplicate resources must be reserved in both the agent and in
         any next-hop networks.  Processing is the same for a deliberate
         source routing loop.

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         The remaining cases requiring recovery, a partially failed
         stream and an erroneous routing loop, are not easily
         distinguishable.  In attempting recovery of a failed stream, an
         agent may issue new CONNECT messages to the affected targets;
         for a full explanation see also Section 3.7.2 (page 51),
         Failure Recovery.  Such a CONNECT may reach an agent downstream
         of the failure before that agent has received a DISCONNECT from
         the neighborhood of the failure.  Until that agent receives the
         DISCONNECT, it cannot distinguish between a failure recovery
         and an erroneous routing loop.  That agent must therefore
         respond to the CONNECT with a REFUSE message with the affected
         targets specified in the TargetList and an appropriate reason
         code (StreamExists).

         The agent immediately preceding that point, i.e., the latest
         agent to send the CONNECT message, will receive the REFUSE
         message.  It must release any resources reserved exclusively
         for traffic to the listed targets.  If this agent was not the
         one attempting the stream recovery, then it cannot distinguish
         between a failure recovery and an erroneous routing loop.  It
         should repeat the CONNECT after a ToConnect timeout.  If after
         NConnect retransmissions it continues to receive REFUSE
         messages, it should propagate the REFUSE message toward the
         origin, with the TargetList that specifies the affected
         targets, but with a different error code (RouteLoop).

         The REFUSE message with this error code (RouteLoop) is
         propagated by each ST agent without retransmitting any CONNECT
         messages.  At each agent, it causes any resources reserved
         exclusively for the listed targets to be released.  The REFUSE
         will be propagated to the origin in the case of an erroneous
         routing loop.  In the case of stream recovery, it will be
         propagated to the ST agent that is attempting the recovery,
         which may be an intermediate agent or the origin itself.  In
         the case of a stream recovery, the agent attempting the
         recovery may issue new CONNECT messages to the same or to
         different next-hops.

         If an agent receives both a REFUSE message and a DISCONNECT
         message with a target in common then it can release the
         relevant resources and propagate neither the REFUSE nor the
         DISCONNECT (however, we feel that it is unlikely that most
         implementations will be able to detect this situation).

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

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         informed so that it may take whatever action it deems

      3.5.4.        Problems in Reserving Resources

         If the network or ST agent resources are not available, an ST
         agent may preempt one or more streams that have lower
         precedence than the one being created.  When it breaks a lower
         precedence stream, it must issue REFUSE and DISCONNECT messages
         as described in Sections (page 122) and (page
         110).  If there are no streams of lower precedence, or if
         preempting them would not provide sufficient resources, then
         the stream cannot be accepted by the ST agent.

         If an intermediate agent detects that it cannot allocate the
         necessary resources, then it sends a REFUSE that contains an
         appropriate reason code (CantGetResrc) and the pertinent
         TargetList to the previous-hop ST agent.  For further study are
         issues of reporting what resources are available, whether the
         resource shortage is permanent or transitory, and in the latter
         case, an estimate of how long before the requested resources
         might be available.

      3.5.5.        Setup Failure due to ACCEPT Timeout

         An ST agent that propagates an ACCEPT message backward toward
         the origin expects an ACK from the previous-hop.  If it does
         not receive an ACK within a timeout, called ToAccept, it will
         retransmit the ACCEPT.  If it does not receive an ACK after
         sending a number, called NAccept, of ACCEPT messages, then it
         will replace the ACCEPT with a REFUSE, and will send a
         DISCONNECT in the direction toward the target.  Both the REFUSE
         and DISCONNECT will identify the affected target(s) and specify
         an appropriate reason code (AcceptTimeout).  Both are also
         retransmitted until ACKed with timeout ToRefuse/ ToDisconnect
         and retransmit count NRefuse/NDisconnect.  If they are not
         ACKed, the agent simply gives up, letting the failure detection
         mechanism described in Section 3.7.1 (page 48) take care of any

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      3.5.6.        Problems Caused by CHANGE Messages

         An application must exercise care when changing a FlowSpec to
         prevent a failure.  A CHANGE might fail for two reasons.  The
         request may be for a larger amount of network resources when
         those resources are not available;  this failure may be
         prevented by requiring that the current level of service be
         contained within the ranges of the FlowSpec in the CHANGE.

         Alternatively, the local network might require all the former
         resources to be released before the new ones are requested and,
         due to unlucky timing, an unrelated request for network
         resources might be processed between the time the resources are
         released and the time the new resources are requested, so that
         the former resources are no longer available.  There is not
         much that an application or ST can do to prevent such failures.

         If the attempt to change the FlowSpec fails then the ST agent
         where the failure occurs must intentionally break the stream
         and invoke the stream recovery mechanism using REFUSE and
         DISCONNECT messages;  see Section 3.7.2 (page 51).  Note that
         the reserved resources after the failure of a CHANGE may not be
         the same as before, i.e., the CHANGE may have been partially
         completed.  The application is responsible for any cleanup
         (another CHANGE).

      3.5.7.        Notification of Changes Forced by Failures

         NOTIFY is issued by a an ST Agent to inform upsteam agents and
         the origin that resource allocation changes have occurred after
         a stream was established.  These changes occur when network
         components fail and when competing streams preempt resources
         previously reserved by a lower precedence stream.  We also
         anticipate that NOTIFY can be used in the future when
         additional resources become available, as is the case when
         network components recover or when higher precedence streams
         are deleted.

         NOTIFY is also used to inform upstream agents that a routing
         anomaly has occurred.  Such an example was cited in Section
         3.5.2 (page 38), where an agent notices that the next-hop agent
         is on the same network as the previous-hop agent;  the anomaly
         is that the previous-hop should have connected directly to the
         next-hop without using an intermediate agent.  Delays in
         propagating host status and routing information can cause such
         anomalies to occur.  NOTIFY allows ST to correct automatically
         such mistakes.

         NOTIFY reports a FlowSpec that reflects that revised guarantee
         that can be promised to the stream.  NOTIFY also

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         identifies those targets affected by the change.  In this way,
         NOTIFY is similar to ACCEPT.  NOTIFY includes a ReasonCode to
         identify the event that triggered the notification.  It also
         includes a TargetList, rather than a single Target, since a
         single event can affect a branch leading to several targets.

         NOTIFY is relayed by the ST agents back toward the origin,
         along the path established by the CONNECT but in the reverse
         direction.  NOTIFY must be acknowledged with an ACK at each
         hop.  If intermediate agent corrects the situation without
         causing any disruption to the data flow or guarantees, it can
         choose to drop the notification message before it reaches the
         origin.  If the originating agent receives a NOTIFY, it is then
         expected to adjust its own processing and data rates, and to
         submit any required CHANGE requests.  As with ACCEPT, the
         FlowSpec is not modified on this trip from the target back to
         the origin.  It is up to the origin to decide whether a CHANGE
         should be submitted.  (However, even though the FlowSpec has
         not been modified, the situation reported in the

   Application  Agent A            Agent 1                    Agent B

 1.                      (high precedence request preempts 10K of
                             the stream's original 30Kb bandwidth
                              allocated to the hop from 1 to B)
 2.   +<------+-- NOTIFY -------------+
      |       |   <RVLId=4><SVLId=14>
      |       |   <Ref=150>
      |       V   <FlowSpec=20Kb,...><TargList=B>
 3.   |       +-> ACK --------------->+
      |           <RVLId=14><SVLId=4>
      V           <Ref=150>
 4. (inform application)
 5. change(FlowSpec=20Kb,...)
 6.   +---------> CHANGE B ---------->+
 7.               <RVLId=14><SVLId=4> +--> CHANGE B ------------>+->+
                  <Ref=60>            |    <RVLId=44><SVLId=15>  |  |
                  <FlowSpec=20Kb,...> V    <Ref=160>             |  |
 8.           +<- ACK ----------------+    <FlowSpec=20Kb,...>   |  |
                  <RVLId=4><SVLId=14>                            V  |
 9.               <Ref=60>            +--- ACK ------------------+  |
                                             <RVLId=15><SVLId=44>   |
                                             <Ref=160>              V
              ... perform normal ACCEPT processing ...        <-----+

                 Figure 16.  Processing NOTIFY Messages

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         notify may have prevented the ST agents from meeting the
         original guarantees.)

   3.6.       Options

      Several options are defined in the CONNECT message.  The special
      processing required to support each will be described in the
      following sections.  The options are independent, i.e., can be set
      to one (1, TRUE) or zero (0, FALSE) in any combination.  However,
      the effect and implementation of the options is NOT necessarily
      independent, and not all combinations are supported.

      3.6.1.        HID Field Option

         The sender of a CONNECT message may or not specify an HID in
         the HID field.  If the HID Field option of the CONNECT message
         is not set (the H bit is 0), then the HID field does not
         contain relevant information and should be ignored.

         If this option is set (the H bit is 1), then the HID field
         contains a relevant value.  If this option is set and the HID
         field of the CONNECT contains a non-zero value, that value
         represents a proposed HID that initiates the HID negotiation.

         If the HID Field option is set but the HID field of the CONNECT
         message contains a zero, this means that the sender of that
         CONNECT message has chosen to defer selection of the HID to the
         next-hop agent (the receiver of a CONNECT message).  This
         choice can allow a more efficient mechanism for selecting HIDs
         and possibly a more efficient mechanism for forwarding data
         packets in the case when the previous-hop does not need to
         select the HID;  see also Section (page 105).

         Upon receipt of a CONNECT message with the HID Field option set
         and the HID field set to zero, a next-hop agent selects the HID
         for the hop, enters it into its appropriate data structure, and
         returns it in the HID field of the HID-APPROVE message.  The
         previous-hop takes the HID from the HID-APPROVE message and
         enters it into its appropriate data structure.

      3.6.2.        PTP Option

         The PTP option (Point-to-Point) is used to indicate that the
         stream will never have more than a single target.  It
         consequently implies that the stream will never need to support
         any form of multicasting.  Use of the PTP option may thus allow
         efficiencies in the way the stream is built or is

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         managed.  Specifically, the ST agents do not need to request
         that the intervening networks allocate multicast groups to
         support this stream.

         The PTP option can only be set to one (1) by the origin, and
         must be the same for the entire stream (i.e., propagated by ST
         agents).  The details of what this option does are
         implementation specific, and do not affect the protocol very

         If the application attempts to add a new target to an existing
         stream that was created with the PTP option set to one (1), the
         application should be informed of the error with an ERROR-IN-
         REQUEST message with the appropriate reason code.  If a CONNECT
         is received whose TargetList contains more than a single entry,
         an ERROR-IN-REQUEST message with the appropriate reason code
         (PTPError) should be returned to the previous-hop agent (note
         that such a CONNECT should never be received if the origin both
         implements the PTP option and is functioning properly).

         As implied in the last paragraph, a subsetted implementation
         might choose not to implement the PTP option.

      3.6.3.        FDx Option

         The FDx option is used to indicate that a second stream in the
         reverse direction, from the target to the origin, should
         automatically be created.  This option is most likely to be
         used when the TargetList has only a single entry.  If used when
         the TargetList has multiple entries, the resulting streams
         would allow bi-directional communication between the origin and
         the various targets, but not among the targets.  The FDx option
         can only be invoked by the origin, and must be propagated by
         intermediate agents.

         This option is specified by inclusion of both an RFlowSpec and
         an RHID parameter in the CONNECT message (possibly with an
         optional RGroup parameter).

         Any ST agent that receives a CONNECT message with both an
         RFlowSpec and an RHID parameter will create database entries
         for streams in both directions and will allocate resources in
         both directions for them.  By this we mean that an ST agent
         will reserve resources to the next-hop agent for the normal
         stream and resources back to the previous-hop agent for the
         reverse stream.  This is necessary since it is expected that
         network reservation interfaces will require the destination
         address(es) in order to make reservations, and because all ST
         agents must use the same reservation model.

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         The target agent will select a Name for the reverse stream and
         return it (in the RName parameter) and the resulting FlowSpec
         (in the RFlowSpec parameter) of the ACCEPT message.  Each agent
         that processes the ACCEPT will update its partial stream
         database entry for the reverse stream with the Name contained
         in the RName parameter.  We assume that the next higher
         protocol layer will use the same SAP for both streams.

      3.6.4.        NoRecovery Option

         The NoRecovery option is used to indicate that ST agents should
         not attempt recovery in case of network or component failure.
         If a failure occurs, the origin will be notified via a REFUSE
         message and the target(s) via a DISCONNECT, with an appropriate
         reason code of "failure" (i.e., one of DropFailAgt,
         DropFailHst, DropFailIfc, DropFailNet, IntfcFailure,
         NetworkFailure, STAgentFailure, FailureRecovery).  They can
         then decide whether to wait for the failed component to be
         fixed, or drop the target via DISCONNECT/REFUSE messages.  The
         NoRecovery option can only be set to one (1) by the origin, and
         must be the same for the entire stream.

      3.6.5.        RevChrg Option

         The RevChrg option bit in the FlowSpec is set to one (1) by the
         origin to request that the target(s) pay any charges associated
         with the stream (to the target(s));  see Section (page
         83).  If the target is not willing to accept charges, the bit
         should be set to zero (0) by the target before returning the
         FlowSpec to the origin in an ACCEPT message.

         If the FDx option is also specified, the target pays charges
         for both streams.

      3.6.6.        Source Route Option

         The Source Route Option may be used both for diagnostic
         purposes, and, in those hopefully infrequent cases where the
         standard routing mechanisms do not produce paths that satisfy
         some policy constraint, to allow the origin to prespecify the
         ST agents along the path to the target(s).  The idea is that
         the origin can explicitly specify the path to a target, either
         strictly hop-by-hop or more loosely by specification of one or
         more agents through which the path must pass.

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         The option is specified by including source routing information
         in the Target structure.  A target may contain zero or more
         SrcRoute options;  when multiple options are present, they are
         processed in the order in which they occur.  The parameter code
         indicates whether the portion of the path contained in the
         parameter is of the strict or loose variety.

         Since portions of a path may pass through portions of an
         internet that does not support ST agents, there are also forms
         of the SrcRoute option that are converted into the

Application  Agent A        Agent 2        Agent 3              Agent B

1.  (open B<SR=2,3>)
2.    V                                              (proc B listening)
3.   (source routed to 2)
4.   (check resources from A to Agent 2: already allocated,
      V   reuse control link & HID, no additional resources needed)
5.    +-> CONNECT B<SR=2,3>->-+-+
          <RVLId=23><SVLId=5> | |
6.        <Ref=50>            V |
7.    +<- ACK ----------------+ |
          <RVLId=5><SVLId=23>   |
          <Ref=50>              V
8.                 (source routed to 3)
9.            (reserve resources 2 to 3)
10.                       +-> CONNECT B<SR=3> ---->+
                              <RVLId=0><SVLId=24>  |
                              <Ref=280><HID=4801>  V
11.                       +<- HID-APPROVE <--------+
                              <RVLId=24><SVLId=33> |
                              <Ref=280><HID=4801>  |
                                           (routing to B)
                                 (reserve resources from 3 to B)
12.                                          +-> CONNECT B ---------->+
                                                 <RVLId=0><SVLId=32>  |
                                                 <Ref=330><HID=6000>  V
13.                                          +<- HID-APPROVE <--------+
                                                 <RVLId=32><SVLId=45> |
                                                 <Ref=330><HID=6000>  V
14.                                                    (proc B accepts)
                ... perform normal ACCEPT processing ...        <-----+

                    Figure 17.  Source Routing Option

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         corresponding IP Source Routing options by the ST agent that
         performs the encapsulation.

         The SrcRoute option is usually selected by the origin, but may
         be used by intermediate agents if specified as a result of the
         routing function.

         For example, in the topology of Figure 2, if A wants to add B
         back into the stream, its routing function might decide that
         the best path is via Agent 3.  Since the data is already being
         multicast across the network connected to C, D, and E, the
         route via Agent 3 might cost less than having A replicate the
         data packets and send them across A's network a second time.

   3.7.       Ancillary Functions

      There are several functions and procedures that are required by
      the ST Protocol.  They are described in subsequent sections.

      3.7.1.        Failure Detection

         The ST failure detection mechanism is based on two assumptions:

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

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

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

         Network Failures

            In this memo, a network is defined to be the protocol
            layer(s) below ST.  This function can be implemented in a
            hardware module separate from the ST agent, or as software
            modules within the ST agent itself, or as a combination of

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            both.  This specification and the robustness protocol do not
            differentiate between these alternatives.

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

         Detecting ST Stream Failures

            Each ST agent periodically sends each neighbor with which it
            shares a stream a HELLO message.  A HELLO message is ACKed
            if the Reference field is non-zero.  This message exchange
            is between ST agents, not entities representing streams or
            applications (there is no Name field in a HELLO message).
            That is, an ST agent need only send a single HELLO message
            to a neighbor regardless of the number of streams that flow
            between them.  All ST agents (host as well as intermediate)
            must participate in this exchange.  However, only agents
            that share active streams need to participate in this

            To facilitate processing of HELLO messages, an
            implementation may either create a separate Virtual Link
            Identifier for each neighbor having an active stream, or may
            use the reserved identifier of one (1) for the SVLId field
            in all its HELLO messages.

            An implementation that wishes to send its HELLO messages via
            a data path instead of the control path may setup a separate
            stream to its neighbor agent for that purpose.  The HELLO
            message would contain a HID of zero, indicating a control
            message, but would be identified to the next lower protocol
            layer as being part of the separate stream.

            As well as identifying the sender, the HELLO message has two
            fields;  a HelloTimer field that is in units of milliseconds
            modulo the maximum for the field size, and a

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