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RFC 1819

 
 
 

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

Part 3 of 4, p. 45 to 76
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5.  Exceptional Cases

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

5.1  Long ST Messages

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

5.1.1  Handling of Long Data Packets

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

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

5.1.2  Handling of Long Control Packets

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

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

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

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

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

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

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

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

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

5.2  Timeout Failures

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

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

5.2.1  Failure due to ACCEPT Acknowledgment Timeout

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

5.2.2  Failure due to CHANGE Acknowledgment Timeout

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

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5.2.3  Failure due to CHANGE Response Timeout

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

5.2.4  Failure due to CONNECT Acknowledgment Timeout

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

5.2.5  Failure due to CONNECT Response Timeout

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

5.2.6  Failure due to DISCONNECT Acknowledgment Timeout

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

5.2.7  Failure due to JOIN Acknowledgment Timeout

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

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5.2.8  Failure due to JOIN Response Timeout

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

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

5.2.9  Failure due to JOIN-REJECT Acknowledgment Timeout

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

5.2.10  Failure due to NOTIFY Acknowledgment Timeout

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

5.2.11  Failure due to REFUSE Acknowledgment Timeout

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

5.2.12  Failure due to STATUS Response Timeout

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

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   is not active.

5.3  Setup Failures due to Routing Failures

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

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

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

   3.  a routing loop.

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

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

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

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

5.3.1  Path Convergence

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

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

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

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

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

5.3.2  Other Cases

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

   The ST agent immediately preceding that point, i.e., the latest ST
   agent to send the CONNECT message, will receive the REFUSE message.
   It must release any resources reserved exclusively for traffic to the
   listed targets. If this ST agent was not the one attempting the
   stream recovery, then it cannot distinguish between a failure
   recovery and an erroneous routing loop. It should repeat the CONNECT
   after a ToConnect timeout, see Section 5.2.4. If after NConnect
   retransmissions it continues to receive REFUSE messages, it should
   propagate the REFUSE message toward the origin, with the TargetList
   that specifies the affected targets, but with a different ReasonCode
   (RouteLoop).

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

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

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

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

5.4  Problems due to Routing Inconsistency

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

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

   Alternatively, if the next-hop returned by the routing function is in
   fact the previous-hop, a routing inconsistency has been detected. In
   this case, a REFUSE is sent back to the previous-hop ST agent
   containing an appropriate ReasonCode (RouteInconsist), pertinent
   TargetList, and in the NextHopIPAddress element the address of the
   previous-hop. When the previous-hop receives the REFUSE, it will
   recompute the next-hop for the affected targets. If there is a
   difference in the routing databases in the two ST agents, they may
   exchange CONNECT and REFUSE messages again. Since such routing errors
   in the internet are assumed to be temporary, the situation should
   eventually stabilize.

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5.5  Problems in Reserving Resources

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

5.5.1  Mismatched FlowSpecs

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

5.5.2  Unknown FlowSpec Version

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

5.5.3  LRM Unable to Process FlowSpec

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

5.5.4  Insufficient Resources

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

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

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

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

5.6  Problems Caused by CHANGE Messages

   A CHANGE might fail for several reasons, including:

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

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

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

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

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

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

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

5.7  Unknown Targets in DISCONNECT and CHANGE

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

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

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

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

6.  Failure Detection and Recovery

6.1  Failure Detection

   The SCMP failure detection mechanism is based on two assumptions:

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

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

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

6.1.1  Network Failures

   An ST agent can detect network failures by two mechanisms:

   o   the network can report a failure, or

   o   the ST agent can discover a failure by itself.

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

6.1.2  Detecting ST Agents Failures

   Each ST agent periodically sends each neighbor with which it shares
   one or more streams a HELLO message. This message exchange is between
   ST agents, not entities representing streams or applications. That
   is, an ST agent need only send a single HELLO message to a neighbor
   regardless of the number of streams that flow between them. All ST
   agents (host as well as intermediate) must participate in this
   exchange. However, only ST agents that share active streams can
   participate in this exchange and it is an error to send a HELLO
   message to a neighbor ST agent with no streams in common, e.g., to
   check whether it is active. STATUS messages can be used to poll the
   status of neighbor ST agents, see Section 8.4.

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

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

   The HELLO message has two fields:

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

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

   The HelloTimer must appear to be incremented every millisecond
   whether a HELLO message is sent or not. The HelloTimer wraps around
   to zero after reaching the maximum value. Whenever an ST agent
   suffers a catastrophic event that may result in it losing ST state
   information, it must reset its HelloTimer to zero and must set the
   Restarted-bit in all HELLO messages sent in the following
   HelloTimerHoldDown seconds.

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

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

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

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   at least as large as the greatest number of consecutive HELLO
   messages that could credibly be lost while the communication between
   the two ST agents is still viable.

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

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

   If the ST agent does not receive a valid HELLO message within the
   RecoveryTimeout period of a stream, it must assume that the
   neighboring ST agent or the communication link between the two has
   failed and it must initiate stream recovery activity, as described
   below in Section 6.2.

6.2  Failure Recovery

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

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

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

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

o   An ST agent that is the previous-hop before the failed component
    first verifies that there was a failure by querying the downstream
    neighbor using STATUS messages. If the neighbor has lost its state
    but is available, then the ST agent may try and reconstruct
    (explained below) the affected streams, for those streams that do
    not have the NoRecovery option selected. If it cannot communicate
    with the next-hop, then the ST agent detecting the failure sends a
    DISCONNECT message, for each affected stream, with the appropriate
    ReasonCode (STAgentFailure) toward the affected targets. It does so
    to speed up failure recovery in case the communication may be
    unidirectional and this message might be delivered successfully.

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

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

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o   If the NoRecovery option is not selected, the ST agent can attempt
    recovery of the affected streams. It does so one a stream by stream
    basis by issuing a new CONNECT message to the affected targets. If
    the ST agent cannot find new routes to some targets, or if the only
    route to some targets is through the previous-hop, then it sends one
    or more REFUSE messages to the previous-hop with the appropriate
    ReasonCode (CantRecover) specifying the affected targets in the
    TargetList. The previous-hop can then attempt recovery of the stream
    by issuing a CONNECT to those targets. If it cannot find an
    appropriate route, it will propagate the REFUSE message toward the
    origin.

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

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

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

6.2.1  Problems in Stream Recovery

   The reconstruction of a broken stream may not proceed smoothly. Since
   there may be some delay while the information concerning the failure
   is propagated throughout an internet, routing errors may occur for
   some time after a failure. As a result, the ST agent attempting the
   recovery may receive ERROR messages for the new CONNECTs that are
   caused by internet routing errors. The ST agent attempting the

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   recovery should be prepared to resend CONNECTs before it succeeds in
   reconstructing the stream. If the failure partitions the internet and
   a new set of routes cannot be found to the targets, the REFUSE
   messages will eventually be propagated to the origin, which can then
   inform the application so it can decide whether to terminate or to
   continue to attempt recovery of the stream.

   The new CONNECT may at some point reach an ST agent downstream of the
   failure before the DISCONNECT does. In this case, the ST agent that
   receives the CONNECT is not yet aware that the stream has suffered a
   failure, and will interpret the new CONNECT as resulting from a
   routing failure. It will respond with an ERROR message with the
   appropriate ReasonCode (StreamExists). Since the timeout that the ST
   agents immediately preceding the failure and immediately following
   the failure are approximately the same, it is very likely that the
   remnants of the broken stream will soon be torn down by a DISCONNECT
   message. Therefore, the ST agent that receives the ERROR message with
   ReasonCode (StreamExists) should retransmit the CONNECT message after
   the ToConnect timeout expires. If this fails again, the request will
   be retried for NConnect times. Only if it still fails will the ST
   agent send a REFUSE message with the appropriate ReasonCode
   (RouteLoop) to its previous-hop. This message will be propagated back
   to the ST agent that is attempting recovery of the damaged stream.
   That ST agent can issue a new CONNECT message if it so chooses. The
   REFUSE is matched to a CONNECT message created by a recovery
   operation through the LnkReference field in the CONNECT.

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

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

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   agent that is attempting recovery will send the origin a NOTIFY
   message with the appropriate ReasonCode (FailureRecovery) that
   contains a FlowSpec that specifies the new parameter values. The
   origin may then have to change its data generation characteristics
   and the stream's parameters with a CHANGE message to use the newly
   recovered subtree.

6.3  Stream Preemption

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

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

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

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

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

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

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

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

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

7.  A Group of Streams

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

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

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

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

7.1  Basic Group Relationships

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

7.1.1  Bandwidth Sharing

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

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

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

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

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

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

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

7.1.2  Fate Sharing

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

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

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

7.1.3  Route Sharing

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

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

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

7.1.4  Subnet Resources Sharing

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

7.2  Relationships Orthogonality

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

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

8.  Ancillary Functions

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

8.1  Stream ID Generation

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

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

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

   o   AllocateStreamID() -> result, StreamID

   o   ReleaseStreamID(StreamID) -> result

   An implementation may also provide a StreamID deletion function.

8.2  Group Name Generator

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

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

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

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

   o   AllocateGroupName() -> result, GroupName

   o   ReleaseGroupName(GroupName) -> result

   An implementation may also provide a GroupName deletion function.

8.3  Checksum Computation

   The standard Internet checksum algorithm is used for ST: "The
   checksum field is the 16-bit one's complement of the one's complement
   sum of all 16-bit words in the header. For purposes of computing the
   checksum, the value of the checksum field is zero (0)." See
   [RFC1071], [RFC1141], and [RFC791] for suggestions for efficient
   checksum algorithms.

8.4  Neighbor ST Agent Identification and Information Collection

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

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

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

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

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

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

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

8.5  Round Trip Time Estimation

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

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

8.6  Network MTU Discovery

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

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

8.7  IP Encapsulation of ST

   ST packets may be encapsulated in IP to allow them to pass through
   routers that don't support the ST Protocol. Of course, ST resource
   management is precluded over such a path, and packet overhead is
   increased by encapsulation, but if the performance is reasonably
   predictable this may be better than not communicating at all.

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

o   Protocol is 5, see [RFC1700], to indicate an ST packet is enclosed,
    as opposed to TCP or UDP, for example.

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

o   Type-of-Service may be set to an appropriate value for the service
    being requested, see [RFC1700]. This feature is not implemented
    uniformly in the Internet, so its use can't be precisely defined
    here.

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   IP encapsulation adds little difficulty for the ST agent that
   receives the packet. However, when IP encapsulation is performed it
   must be done in both directions. To process the encapsulated IP
   message, the ST agents simply remove the IP header and proceed with
   ST header as usual.

   The more difficult part is during setup, when the ST agent must
   decide whether or not to encapsulate. If the next-hop ST agent is on
   a remote network and the route to that network is through a router
   that supports IP but not ST, then encapsulation is required. The
   routing function provides ST agents with the route and capability
   information needed to support encapsulation.

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

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

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

8.8  IP Multicasting

   If an ST agent must use IP encapsulation to reach multiple next-hops
   toward different targets, then either the packet must be replicated
   for transmission to each next-hop, or IP multicasting may be used if
   it is implemented in the next-hop ST agents and in the intervening IP
   routers.

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

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

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

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

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

   o   AllocateMcastAddr() -> result, McastAddr

   o   ListenMcastAddr(McastAddr) -> result

   o   ReleaseMcastAddr(McastAddr) -> result

9.  The ST2+ Flow Specification

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

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

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

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

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

9.1  FlowSpec Version #0 - (Null FlowSpec)

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

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

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

9.2  FlowSpec Version #7 - ST2+ FlowSpec

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

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

   o   message size,

   o   message rate,

   o   end-to-end delay.

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

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

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

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

9.2.1  QoS Classes

   Two QoS classes are defined:

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

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

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

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    this QoS class is optional.

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

9.2.2  Precedence

   Precedence is the importance of the connection being established.
   Zero represents the lowest precedence. The lowest level is expected
   to be used by default. In general, the distinction between precedence
   and priority is that precedence specifies streams that are permitted
   to take previously committed resources from another stream, while
   priority identifies those PDUs that a stream is most willing to have
   dropped.

9.2.3  Maximum Data Size

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

9.2.4  Message Rate

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

9.2.5  Delay and Delay Jitter

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

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   stream along this path.

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

9.2.6  ST2+ FlowSpec Format

   The ST2+ FlowSpec has the following format:

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

                        Figure 9: The ST2+ FlowSpec.

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

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

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

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

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    intermediate agents.

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

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

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

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

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

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

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

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

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

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


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