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RFC 4233

 
 
 

Integrated Services Digital Network (ISDN) Q.921-User Adaptation Layer

Part 3 of 3, p. 46 to 73
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4.  Procedures

   The IUA layer needs to respond to various primitives it receives from
   other layers as well as messages it receives from the peer IUA layer.
   This section describes various procedures involved in response to
   these events.

4.1.  Procedures to Support Service in Section 1.4.1

   These procedures achieve the IUA layer's "Transport of Q.921/Q.931
   boundary primitives" service.

4.1.1.  Q.921 or Q.931 Primitives Procedures

   On receiving these primitives from the local layer, the IUA layer
   will send the corresponding QPTM message (Data, Unit Data, Establish,
   Release) to its peer.  While doing so, the IUA layer needs to fill
   various fields of the common and specific headers correctly.  In
   addition, the message needs to be sent on the SCTP stream that
   corresponds to the D channel (Interface Identifier).

4.1.2.  QPTM Message Procedures

   On receiving QPTM messages from a peer IUA layer, the IUA layer on an
   SG or MGC needs to invoke the corresponding layer primitives
   (DL-ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921
   or Q.931 layer.

4.2.  Procedures to Support Service in Section 1.4.2

   These procedures achieve the IUA layer's "Support for Communication
   between Layer Managements" service.

4.2.1.  Layer Management Primitives Procedures

   On receiving these primitives from the local Layer Management, the
   IUA layer will provide the appropriate response primitive across the
   internal local Layer Management interface.

   An M-SCTP ESTABLISH request from Layer Management will initiate the
   establishment of an SCTP association.  An M-SCTP ESTABLISH confirm
   will be sent to Layer Management when the initiated association setup
   is complete.  An M-SCTP ESTABLISH indication is sent to Layer
   Management upon successful completion of an incoming SCTP association
   setup from a peer IUA node.

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   An M-SCTP RELEASE request from Layer Management will initiate the
   teardown of an SCTP association.  An M-SCTP RELEASE confirm will be
   sent by Layer Management when the association teardown is complete.
   An M-SCTP RELEASE indication is sent to Layer Management upon
   successful teardown of an SCTP association initiated by a peer IUA.

   M-SCTP STATUS request and indication support a Layer Management query
   of the local status of a particular SCTP association.

   M-NOTIFY indication and M-ERROR indication indicate to Layer
   Management the notification or error information contained in a
   received IUA Notify or Error message, respectively.  These
   indications can also be generated based on local IUA events.

   M-ASP STATUS request/indication and M-AS-STATUS request/indication
   support a Layer Management query of the local status of a particular
   ASP or AS.  No IUA peer protocol is invoked.

   M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE request, and
   M-ASP-ACTIVE request allow Layer Management at an ASP to initiate
   state changes.  These requests result in outgoing IUA ASP UP, ASP
   DOWN, ASP INACTIVE, and ASP ACTIVE messages.

   M-ASP-UP confirmation, M-ASP-DOWN confirmation, M-ASP-INACTIVE
   confirmation, and M-ASP-ACTIVE confirmation indicate to Layer
   Management that the previous request has been confirmed.

   Upon receipt of an M-TEI Status primitive from Layer Management, the
   IUA will send the corresponding MGMT message (TEI Status) to its
   peer.  While doing so, the IUA layer needs to fill various fields of
   the common and specific headers correctly.

   All MGMT messages are sent on a sequenced stream to ensure ordering.
   SCTP stream '0' SHOULD be used.

4.2.2.  Receipt of IUA Peer Management Messages

   Upon receipt of IUA Management messages, the IUA layer MUST invoke
   the corresponding Layer Management primitive indications (e.g., M-AS
   Status ind., M-ASP Status ind., M-ERROR ind., M-TEI STATUS) to the
   local layer management.

   M-NOTIFY indication and M-ERROR indication indicate to Layer
   Management the notification or error information contained in a
   received IUA Notify or Error message.  These indications can also be
   generated based on local IUA events.

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   All MGMT messages are sent on a sequenced stream to ensure ordering.
   SCTP stream '0' SHOULD be used.

4.3.  Procedures to Support Service in Section 1.4.3

   These procedures achieve the IUA layer's "Support for management of
   active associations between SG and MGC" service.

4.3.1.  AS and ASP State Maintenance

   The IUA layer on the SG needs to maintain the states of each ASP as
   well as the state of the AS.

4.3.1.1.  ASP States

   The state of the each ASP, in each AS that it is configured, is
   maintained in the IUA layer on the SG.  The state of an ASP changes
   due to the following type of events:

      *  Reception of messages from peer IUA layer at that ASP
      *  Reception of some messages from the peer IUA layer at other
         ASPs in the AS
      *  Reception of indications from SCTP layer
      *  Local Management intervention

   The ASP state transition diagram is shown in Figure 6.  The possible
   states of an ASP are the following:

   ASP-DOWN: Application Server Process is unavailable and/or the
   related SCTP association is down.  Initially, all ASPs will be in
   this state.  An ASP in this state SHOULD NOT be sent any IUA
   messages.

   ASP-INACTIVE: The remote IUA peer at the ASP is available (and the
   related SCTP association is up) but application traffic is stopped.
   In this state, the ASP can be sent any non-QPTM IUA messages (except
   for TEI Status messages).

   ASP-ACTIVE: The remote IUA peer at the ASP is available and
   application traffic is active.

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                                    +--------------+
             +----------------------|              |
             |   Alternate  +-------|  ASP-ACTIVE  |
             |       ASP    |       +--------------+
             |    Takeover  |           ^     |
             |              |    ASP    |     | ASP Inactive /
             |              |    Active |     | ASP Up
             |              |           |     v
             |              |       +--------------+
             |              |       |              |
             |              +------>| ASP-INACTIVE |
             |                      +--------------+
             |                          ^    |
   ASP Down/ |                     ASP  |    | ASP Down /
   SCTP CDI/ |                     Up   |    | SCTP CDI /
   SCTP RI   |                          |    v SCTP RI
             |                      +--------------+
             +--------------------->|              |
                                    |   ASP-DOWN   |
                                    +--------------+

                  Figure 6.  ASP State Transition Diagram

   SCTP CDI:  The local SCTP layer's Communication Down Indication to
   the Upper Layer Protocol (IUA) on an SG.  The local SCTP will send
   this indication when it detects the loss of connectivity to the ASP's
   peer SCTP layer.  SCTP CDI is understood as either a SHUTDOWN
   COMPLETE notification and COMMUNICATION LOST notification from the
   SCTP.

   SCTP RI: The local SCTP layer's Restart indication to the upper layer
   protocol (IUA) on an SG.  The local SCTP will send this indication
   when it detects a restart from the ASP's peer SCTP layer.

4.3.1.2.  AS States

   The state of the AS is maintained in the IUA layer on the SG.

   The state of an AS changes due to events.  These events include the
   following:

      *  ASP state transitions
      *  Recovery timer triggers

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   The possible states of an AS are the following:

   AS-DOWN: The Application Server is unavailable.  This state implies
   that all related ASPs are in the ASP-DOWN state for this AS.
   Initially, the AS will be in this state.

   AS-INACTIVE: The Application Server is available but no application
   traffic is active (i.e., one or more related ASPs are in the
   ASP-INACTIVE state, but none in the ASP-ACTIVE state).  The recovery
   timer T(r) is not running or has expired.

   AS-ACTIVE: The Application Server is available and application
   traffic is active.  This state implies that at least one ASP is in
   the ASP-ACTIVE state.

   AS-PENDING: An active ASP has transitioned from active to inactive or
   down and it was the last remaining active ASP in the AS.  A recovery
   timer T(r) will be started and all incoming SCN messages will be
   queued by the SG.  If an ASP becomes active before T(r) expires, the
   AS will move to AS-ACTIVE state and all the queued messages will be
   sent to the active ASP.

   If T(r) expires before an ASP becomes active, the SG stops queuing
   messages and discards all previously queued messages.  The AS will
   move to AS-INACTIVE if at least one ASP is in ASP-INACTIVE state,
   otherwise it will move to AS-DOWN state.

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      +----------+ one ASP trans to ASP-ACTIVE +-------------+
      |    AS-   |---------------------------->|     AS-     |
      | INACTIVE |                             |   ACTIVE    |
      |          |<---                         |             |
      +----------+    \                        +-------------+
         ^   |         \ Tr Expiry,                ^    |
         |   |          \ at least one             |    |
         |   |           \ ASP in ASP-INACTIVE     |    |
         |   |            \                        |    |
         |   |             \                       |    |
         |   |              \                      |    |
 one ASP |   | all ASP       \            one ASP  |    | Last ACTIVE
 trans   |   | trans to       \           trans to |    | ASP trans to
 to      |   | ASP-DOWN        -------\   ASP-     |    | ASP-INACTIVE
 ASP-    |   |                         \  ACTIVE   |    | or ASP-DOWN
 INACTIVE|   |                          \          |    |  (start Tr)
         |   |                           \         |    |
         |   |                            \        |    |
         |   v                             \       |    v
      +----------+                          \  +-------------+
      |          |                           --|             |
      | AS-DOWN  |                             | AS-PENDING  |
      |          |                             |  (queueing) |
      |          |<----------------------------|             |
      +----------+    Tr Expiry and no ASP     +-------------+
                     in ASP-INACTIVE state

     Tr = Recovery Timer

                 Figure 7: AS State Transition Diagram

4.3.2.  ASPM Procedures for Primitives

   Before the establishment of an SCTP association, the ASP state at
   both the SG and ASP is assumed to be in the state ASP-DOWN.

   As the ASP is responsible for initiating the setup of an SCTP
   association to an SG, the IUA layer at an ASP receives an M-SCTP
   ESTABLISH request primitive from the Layer Management, the IUA layer
   will try to establish an SCTP association with the remote IUA peer at
   an SG.  Upon reception of an eventual SCTP-Communication Up confirm
   primitive from the SCTP, the IUA layer will invoke the primitive
   M-SCTP ESTABLISH confirm to the Layer Management.

   At the SG, the IUA layer will receive an SCTP Communication Up
   indication primitive from the SCTP.  The IUA layer will then invoke
   the primitive M-SCTP ESTABLISH indication to the Layer Management.

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   Once the SCTP association is established and assuming that the local
   IUA-User is ready, the local ASP IUA Application Server Process
   Maintenance (ASPM) function will initiate the ASPM procedures, using
   the ASP Up/-Down/-Active/-Inactive messages to convey the ASP state
   to the SG (see Section 4.3.3).

   The Layer Management and the IUA layer on SG can communicate the
   status of the application server using the M-AS_STATUS primitives.
   The Layer Management and the IUA layer on both the SG and ASP can
   communicate the status of an SCTP association using the M-SCTP_STATUS
   primitives.

   If the Layer Management on SG or ASP wants to bring down an SCTP
   association for management reasons, it would send M-SCTP RELEASE
   request primitive to the local IUA layer.  The IUA layer would
   release the SCTP association and upon receiving the SCTP-
   COMMUNICATION_DOWN indication from the underlying SCTP layer, it
   would inform the local Layer Management using M-SCTP_RELEASE confirm
   primitive.

   If the IUA layer receives an SCTP-COMMUNICATION_DOWN indication from
   the underlying SCTP layer, it will inform the Layer Management by
   invoking the M-SCTP RELEASE indication primitive.  The state of the
   ASP will be moved to "Down" at both the SG and ASP.

   At an ASP, the Layer Management MAY try to reestablish the SCTP
   association using M-SCTP_ESTABLISH request primitive.

   In the case of an SCTP-RESTART indication at an ASP, the ASP is now
   considered by its IUA peer to be in the ASP-DOWN state.  The ASP, if
   it is to recover, must begin any recovery with the ASP Up procedure.

4.3.3.  ASPM Procedures for Peer-to-Peer Messages

   All ASPM messages are sent on a sequenced stream to ensure ordering.
   SCTP stream '0' SHOULD be used.

4.3.3.1.  ASP Up Procedures

   After an ASP has successfully established an SCTP association to an
   SG, the SG waits for the ASP to send an ASP Up message, indicating
   that the ASP IUA peer is available.  The ASP is always the initiator
   of the ASP Up message.  This action MAY be initiated at the ASP by an
   M-ASP_UP request primitive from Layer Management or MAY be initiated
   automatically by an IUA management function.

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   When an ASP Up message is received at an SG and internally the remote
   ASP is in the ASP-DOWN state and not considered locked out for local
   management reasons, the SG marks the remote ASP in the state
   ASP-INACTIVE and informs Layer Management with an M-ASP_Up indication
   primitive.  If the SG is aware, via current configuration data, which
   Application Servers the ASP is configured to operate in, the SG
   updates the ASP state to ASP-INACTIVE in each AS that it is a member.

   Alternatively, the SG may move the ASP into a pool of Inactive ASPs
   available for future configuration within Application Server(s),
   determined in a subsequent ASP Active procedure.  If the ASP Up
   message contains an ASP Identifier, the SG should save the ASP
   Identifier for that ASP.  The SG MUST send an ASP Up Ack message in
   response to a received ASP Up message even if the ASP is already
   marked as ASP-INACTIVE at the SG.

   If for any local reason (e.g., management lockout) the SG cannot
   respond with an ASP Up Ack message, the SG responds to an ASP Up
   message with an Error message with reason "Refused - Management
   Blocking".

   At the ASP, the ASP Up Ack message received is not acknowledged.
   Layer Management is informed with an M-ASP_UP confirm primitive.

   When the ASP sends an ASP Up message, it starts timer T(ack).  If the
   ASP does not receive a response to an ASP Up message within T(ack),
   the ASP MAY restart T(ack) and resend ASP Up messages until it
   receives an ASP Up Ack message.  T(ack) is provisionable, with a
   default of 2 seconds.  Alternatively, retransmission of ASP Up
   messages MAY be put under control of Layer Management.  In this
   method, expiry of T(ack) results in an M-ASP_UP confirm primitive
   carrying a negative indication.

   The ASP must wait for the ASP Up Ack message before sending any other
   IUA messages (e.g., ASP Active).  If the SG receives any other IUA
   messages before an ASP Up message is received (other than ASP Down;
   see Section 4.3.3.2), the SG MAY discard them.

   If an ASP Up message is received and internally the remote ASP is in
   the ASP-ACTIVE state, an ASP Up Ack message is returned, as well as
   an Error message ("Unexpected Message"), and the remote ASP state is
   changed to ASP-INACTIVE in all relevant Application Servers.

   If an ASP Up message is received and internally the remote ASP is
   already in the ASP-INACTIVE state, an ASP Up Ack message is returned
   and no further action is taken.

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4.3.3.2.  ASP Down Procedures

   The ASP will send an ASP Down message to an SG when the ASP wishes to
   be removed from the list of ASPs in all Application Servers that it
   is a member and no longer receive any IUA QPTM or ASPTM messages.
   This action MAY be initiated at the ASP by an M-ASP_DOWN request
   primitive from Layer Management or MAY be initiated automatically by
   an IUA management function.

   Whether the ASP is permanently removed from an AS is a function of
   configuration management.

   The SG marks the ASP as ASP-DOWN, informs Layer Management with an
   M-ASP_Down indication primitive, and returns an ASP Down Ack message
   to the ASP.

   The SG MUST send an ASP Down Ack message in response to a received
   ASP Down message from the ASP even if the ASP is already marked as
   ASP-DOWN at the SG.

   At the ASP, the ASP Down Ack message received is not acknowledged.
   Layer Management is informed with an M-ASP_DOWN confirm primitive.
   If the ASP receives an ASP Down Ack without having sent an ASP Down
   message, the ASP should now consider itself as in the ASP-DOWN state.
   If the ASP was previously in the ASP-ACTIVE or ASP-INACTIVE state,
   the ASP should then initiate procedures to return itself to its
   previous state.

   When the ASP sends an ASP Down message, it starts timer T(ack).  If
   the ASP does not receive a response to an ASP Down message within
   T(ack), the ASP MAY restart T(ack) and resend ASP Down messages until
   it receives an ASP Down Ack message.  T(ack) is provisionable, with a
   default of 2 seconds.  Alternatively, retransmission of ASP Down
   messages MAY be put under control of Layer Management.  In this
   method, expiry of T(ack) results in an M-ASP_DOWN confirm primitive
   carrying a negative indication.

4.3.3.3.  IUA Version Control

   If a ASP Up message with an unsupported version is received, the
   receiving end responds with an Error message, indicating the version
   the receiving node supports and notifies Layer Management.

   This is useful when protocol version upgrades are being performed in
   a network.  A node upgraded to a newer version SHOULD support the
   older versions used on other nodes it is communicating with.  Because
   ASPs initiate the ASP Up procedure it is assumed that the Error
   message would normally come from the SG.

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4.3.3.4.  ASP Active Procedures

   Any time after the ASP has received an ASP Up Ack from the SG, the
   ASP sends an ASP Active message to the SG indicating that the ASP is
   ready to start processing traffic.  This action MAY be initiated at
   the ASP by an M-ASP_ACTIVE request primitive from Layer Management or
   MAY be initiated automatically by an IUA management function.  In the
   case where an ASP is configured/registered to process the traffic for
   more than one Application Server across an SCTP association, the
   ASPAC contains one or more Interface Identifiers to indicate for
   which Application Servers the ASPAC applies.

   If the Application Server can be successfully activated, the SG
   responds to the ASP with an ASPAC Ack message acknowledging that the
   ASPAC message was received and starts sending traffic for the
   Application Server to that ASP.

   In the case where an "out-of-the-blue" ASP Active message is received
   (i.e., the ASP has not registered with the SG or the SG has no static
   configuration data for the ASP), the message MAY be silently
   discarded.

   The SG MUST send an ASP Active Ack message in response to a received
   ASP Active message from the ASP, if the ASP is already marked in the
   ASP-ACTIVE state at the SG.

   At the ASP, the ASP Active Ack message received is not acknowledged.
   Layer Management is informed with an M-ASP_ACTIVE confirm primitive.
   It is possible for the ASP to receive Data message(s) before the ASP
   Active Ack message as the ASP Active Ack and Data messages from an SG
   may be sent on different SCTP streams.  Message loss is possible as
   the ASP does not consider itself in the ASP-ACTIVE state until
   reception of the ASP Active Ack message.

   When the ASP sends an ASP Active message, it starts timer T(ack).  If
   the ASP does not receive a response to an ASP Active message within
   T(ack), the ASP MAY restart T(ack) and resend ASP Active messages
   until it receives an ASP Active Ack message.  T(ack) is
   provisionable, with a default of 2 seconds.  Alternatively,
   retransmission of ASP Active messages MAY be put under control of
   Layer Management.  In this method, expiry of T(ack) results in an M-
   ASP_ACTIVE confirm primitive carrying a negative indication.

   The ASP MUST wait for the ASP Active Ack message from the SG before
   sending any Data messages or it will risk message loss.  If the SG
   receives QPTM messages before an ASP Active is received, the SG
   SHOULD discard these messages.

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   There are two modes of Application Server traffic handling in the SG
   IUA: Over-ride and Load-sharing.  The Type parameter in the ASPAC
   message indicates the mode used in a particular Application Server.
   If the SG determines that the mode indicates in an ASPAC is
   incompatible with the traffic handling mode currently used in the AS,
   the SG responds with an Error message indicating Unsupported Traffic
   Handling Mode.

   In the case of an Over-ride mode AS, reception of an ASPAC message at
   an SG causes the redirection of all traffic for the AS to the ASP
   that sent the ASPAC.  The SG responds to the ASPAC with an ASP Active
   Ack message to the ASP.  Any previously active ASP in the AS is now
   considered Inactive and will no longer receive traffic from the SG
   within the AS.  The SG sends a Notify (Alternate ASP Active) to the
   previously active ASP in the AS, after stopping all traffic to that
   ASP.

   In the case of a load-share mode AS, reception of an ASPAC message at
   an SG causes the direction of traffic to the ASP sending the ASPAC,
   in addition to all the other ASPs that are currently active in the
   AS.  The algorithm at the SG for load-sharing traffic within an AS to
   all the active ASPs is implementation dependent.  The algorithm
   could, for example, be round-robin or based on information in the
   Data message, such as Interface Identifier, depending on the
   requirements of the application and the call state handling
   assumptions of the collection of ASPs in the AS.  The SG responds to
   the ASPAC with an ASP Active Ack message to the ASP.

4.3.3.5.  ASP Inactive Procedures

   When an ASP wishes to withdraw from receiving traffic within an AS,
   the ASP sends an ASP Inactive message to the SG.  This action MAY be
   initiated at the ASP by an M-ASP_INACTIVE request primitive from
   Layer Management or MAY be initiated automatically by an IUA
   management function.  In the case where an ASP is configured/
   registered to process the traffic for more than one Application
   Server across an SCTP association, the ASPIA contains one or more
   Interface Identifiers to indicate for which Application Servers the
   ASP Inactive message applies.

   There are two modes of Application Server traffic handling in the SG
   IUA when withdrawing an ASP from service: Over-ride and Load-sharing.
   In the case of an Over-ride mode AS, where normally another ASP has
   already taken over the traffic within the AS with an Over-ride ASPAC
   message, the ASP that sends the ASPIA message is already considered
   by the SG to be ASP-INACTIVE.  An ASPIA Ack message is sent to the
   ASP, after ensuring that all traffic is stopped to the ASP.

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   In the case of a Load-share mode AS, the SG moves the ASP to the
   ASP-INACTIVE state and the AS traffic is re-allocated across the
   remaining ASP-ACTIVE ASPs per the load-sharing algorithm currently
   used within the AS.  An ASPIA Ack message is sent to the ASP after
   all traffic is halted to the ASP.  A Notify (Insufficient ASPs)
   message MAY be sent to all inactive ASPs, if required.

   When the ASP sends an ASP Inactive message it starts timer T(ack).
   If the ASP does not receive a response to an ASP Inactive message
   within T(ack), the ASP MAY restart T(ack) and resend ASP Inactive
   messages until it receives an ASP Inactive Ack message.  T(ack) is
   provisionable, with a default of 2 seconds.  Alternatively,
   retransmission of ASP Inactive messages MAY be put under control of
   Layer Management.  In this method, expiry of T(ack) results in a M-
   ASP_Inactive confirm primitive carrying a negative indication.

   If no other ASPs in the Application Server are in the state
   ASP-ACTIVE, the SG MUST send a Notify ("AS-Pending") message to all
   of the ASPs in the AS that are in the state ASP-INACTIVE.  The SG
   SHOULD start buffering the incoming messages for T(r) seconds, after
   which messages MAY be discarded.  T(r) is configurable by the network
   operator.  If the SG receives an ASP Active message from an ASP in
   the AS before expiry of T(r), the buffered traffic is directed to
   that ASP and the timer is cancelled.  If T(r) expires, the AS is
   moved to the AS-INACTIVE state.

   At the ASP, the ASP Inactive Ack message received is not
   acknowledged.  Layer Management is informed with an M-ASP_INACTIVE
   confirm primitive.  If the ASP receives an ASP Inactive Ack without
   having sent an ASP Inactive message, the ASP should now consider
   itself as in the ASP-INACTIVE state.  If the ASP was previously in
   the ASP-ACTIVE state, the ASP should then initiate procedures to
   return itself to its previous state.

4.3.3.6.  Notify Procedures

   A Notify message reflecting a change in the AS state MUST be sent to
   all ASPs in the AS, except those in the ASP-DOWN state, with
   appropriate Status Information and any ASP Identifier of the failed
   ASP.  At the ASP, Layer Management is informed with an M-NOTIFY
   indication primitive.  The Notify message must be sent whether the AS
   state change was a result of an ASP failure or reception of an ASP
   State Management (ASPSM) / ASP Traffic Management (ASPTM) message.
   In the second case, the Notify message MUST be sent after any related
   acknowledgement messages  (e.g., ASP Up Ack, ASP Down Ack, ASP Active
   Ack, or ASP Inactive Ack).

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   In the case where a Notify ("AS-Pending") message is sent by an SG
   that now has no ASPs active to service the traffic, or a NTFY
   ("Insufficient ASPs") is sent in the Load-share mode, the Notify does
   not explicitly compel the ASP(s) receiving the message to become
   active.  The ASPs remain in control of what (and when) action is
   taken.

4.3.3.7.  Heartbeat

   The optional Heartbeat procedures MAY be used when operating over
   transport layers that do not have their own heartbeat mechanism for
   detecting loss of the transport association (i.e., other than the
   SCTP).

   Either IUA peer may optionally send Heartbeat messages periodically,
   subject to a provisionable timer T(beat).  Upon receiving a Heartbeat
   message, the IUA peer MUST respond with a Heartbeat Ack message.

   If no Heartbeat Ack message (or any other IUA message) is received
   from the IUA peer within 2*T(beat), the remote IUA peer is considered
   unavailable.  Transmission of Heartbeat messages is stopped and the
   signaling process SHOULD attempt to re-establish communication if it
   is configured as the client for the disconnected IUA peer.

   The BEAT message MAY optionally contain an opaque Heartbeat Data
   parameter that MUST be echoed back unchanged in the related Beat Ack
   message.  The ASP upon examining the contents of the returned BEAT
   Ack message MAY choose to consider the remote ASP as unavailable.
   The contents/format of the Heartbeat Data parameter is implementation
   dependent and only of local interest to the original sender.  The
   contents MAY be used, for example, to support a Heartbeat sequence
   algorithm (to detect missing Heartbeats), and/or a timestamp
   mechanism (to evaluate delays).

   Note:  Heartbeat-related events are not shown in Figure 6, "ASP State
   Transition Diagram".

5.  Examples

5.1.  Establishment of Association and Traffic between SGs and ASPs

5.1.1.  Single ASP in an Application Server (1+0 sparing)

   This scenario shows the example IUA message flows for the
   establishment of traffic between an SG and an ASP, where only one ASP
   is configured within an AS (no backup).  It is assumed that the SCTP
   association is already setup.

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                SG                       ASP1
                 |
                 |<---------ASP Up----------|
                 |--------ASP Up Ack------->|
                 |                          |
                 |-----NTFY(AS-INACTIVE)--->|
                 |                          |
                 |<-------ASP Active--------|
                 |------ASP Active Ack----->|
                 |                          |
                 |------NTFY(AS-ACTIVE)---->|
                 |                          |

5.1.2.  Two ASPs in Application Server (1+1 sparing)

   This scenario shows the example IUA message flows for the
   establishment of traffic between an SG and two ASPs in the same
   Application Server, where ASP1 is configured to be Active and ASP2 a
   standby in the event of communication failure or the withdrawal from
   service of ASP1.  ASP2 MAY act as a hot, warm, or cold standby
   depending on the extent to which ASP1 and ASP2 share call state or
   can communicate call state under failure/withdrawal events.  The
   example message flow is the same whether the ASP Active messages are
   Over-ride or Load-share mode although typically this example would
   use an Over-ride mode.

          SG                        ASP1                        ASP2
           |                         |                          |
           |<--------ASP Up----------|                          |
           |-------ASP Up Ack------->|                          |
           |                         |                          |
           |----NTFY(AS-INACTIVE)--->|                          |
           |                         |                          |
           |<-----------------------------ASP Up----------------|
           |----------------------------ASP Up Ack------------->|
           |                         |                          |
           |                         |                          |
           |<-------ASP Active-------|                          |
           |-----ASP Active Ack----->|                          |
           |                         |                          |
           |-----NTFY(AS-ACTIVE)---->|                          |
           |----------------------NTFY(AS-ACTIVE)-------------->|

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5.1.3.  Two ASPs in an Application Server (1+1 sparing, load-sharing
        case)

   This scenario shows a similar case to Section 5.1.2 but where the two
   ASPs are brought to active and load-share the traffic load.  In this
   case, one ASP is sufficient to handle the total traffic load.

          SG                       ASP1                       ASP2
           |                         |                          |
           |<---------ASP Up---------|                          |
           |--------ASP Up Ack------>|                          |
           |                         |                          |
           |----NTFY(AS-INACTIVE)--->|                          |
           |                         |                          |
           |<------------------------------ASP Up---------------|
           |-----------------------------ASP Up Ack------------>|
           |                         |                          |
           |                         |                          |
           |<--ASP Active (Ldshr)----|                          |
           |----ASP Active Ack------>|                          |
           |                         |                          |
           |-----NTFY(AS-ACTIVE)---->|                          |
           |----------------------NTFY(AS-ACTIVE)-------------->|
           |                         |                          |
           |<----------------------------ASP Active (Ldshr)-----|
           |-----------------------------ASP Active Ack-------->|
           |                         |                          |

5.1.4.  Three ASPs in an Application Server (n+k sparing, load-sharing
        case)

   This scenario shows the example IUA message flows for the
   establishment of traffic between an SG and three ASPs in the same
   Application Server, where two of the ASPs are brought to active and
   share the load.  In this case, a minimum of two ASPs are required to
   handle the total traffic load (2+1 sparing).

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      SG                  ASP1                ASP2                ASP3
       |                    |                   |                   |
       |<------ASP Up-------|                   |                   |
       |-----ASP Up Ack---->|                   |                   |
       |                    |                   |                   |
       |-NTFY(AS-INACTIVE)->|                   |                   |
       |                    |                   |                   |
       |<--------------------------ASP Up-------|                   |
       |-----------------------ASP Up Ack------>|                   |
       |                    |                   |                   |
       |<---------------------------------------------ASP Up--------|
       |--------------------------------------------ASP Up Ack----->|
       |                    |                   |                   |
       |                    |                   |                   |
       |<-ASP Act (Ldshr)---|                   |                   |
       |----ASP Act Ack---->|                   |                   |
       |                    |                   |                   |
       |<---------------------ASP Act (Ldshr)---|                   |
       |----------------------ASP Act Ack------>|                   |
       |                    |                   |                   |
       |--NTFY(AS-ACTIVE)-->|                   |                   |
       |---------------NTFY(AS-ACTIVE)--------->|                   |
       |------------------------NTFY(AS-ACTIVE)-------------------->|

5.1.5.  Interface Identifier Configuration Mismatch Example

   This scenario shows the example IUA message flows for the
   establishment of traffic between an SG and an ASP in which some of
   the Interface Identifiers have been misconfigured on the ASP side.
   The SG in this case has Interface Identifiers 1-5 configured for
   ASP1.

                SG                               ASP1
                 |                                |
                 |                                |
                 |<----ASP Active (IIDs 1-10)-----|
                 |---ASP Active Ack (IIDs 1-5)--->|
                 |-------Error (IIDs 6)---------->|
                 |-------Error (IIDs 7)---------->|
                 |-------Error (IIDs 8)---------->|
                 |-------Error (IIDs 9)---------->|
                 |-------Error (IIDs 10)--------->|
                 |                                |

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5.2.  ASP Traffic Fail-over Examples

5.2.1.  (1+1 Sparing, withdrawal of ASP, Backup Over-ride)

   The following example shows a case in which an ASP withdraws from
   service:

          SG                       ASP1                       ASP2
           |                         |                          |
           |<-----ASP Inactive-------|                          |
           |----ASP Inactive Ack---->|                          |
           |                         |                          |
           |----NTFY(AS-Pending)---->|                          |
           |-------------------NTFY(AS-Pending)---------------->|
           |                         |                          |
           |<------------------------------ ASP Active----------|
           |-----------------------------ASP Active Ack)------->|
           |                         |                          |
           |----NTFY(AS-ACTIVE)----->|                          |
           |-------------------NTFY(AS-ACTIVE)----------------->|

   In this case, the SG notifies ASP2 that the AS has moved to the Down
   state.  The SG could have also (optionally) sent a Notify message
   when the AS moved to the Pending state.

   Note:  If the SG detects loss of the IUA peer (IUA heartbeat loss or
   detection of SCTP failure), the initial SG-ASP1 ASP Inactive message
   exchange would not occur.

5.2.2.  (1+1 Sparing, Backup Over-ride)

   The following example shows a case in which ASP2 wishes to override
   ASP1 and take over the traffic:

          SG                       ASP1                       ASP2
           |                         |                          |
           |<-------------------------------ASP Active----------|
           |-----------------------------ASP Active Ack-------->|
           |----NTFY( Alt ASP-Act)-->|
           |                         |                          |

   In this case, the SG notifies ASP1 that an alternative ASP has
   overridden it.

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5.2.3.  (n+k Sparing, Load-sharing case, withdrawal of ASP)

   Following on from the example in Section 5.1.4, and ASP1 withdraws
   from service:

     SG                  ASP1                 ASP2                 ASP3
      |                    |                   |                   |
      |<----ASP Inact------|                   |                   |
      |---ASP Inact Ack--->|                   |                   |
      |                    |                   |                   |
      |---------------------------------NTFY(Ins. ASPs)----------->|
      |                    |                   |                   |
      |<-----------------------------------------ASP Act (Ldshr)---|
      |-------------------------------------------ASP Act (Ack)--->|
      |                    |                   |                   |

   In this case, the SG has knowledge of the minimum ASP resources
   required (implementation dependent), for example, if the SG knows
   that n+k = 2+1 for a load-share AS and n currently equals 1.

   Note:  If the SG detects loss of the ASP1 IUA peer (IUA heartbeat
   loss or detection of SCTP failure), the first SG-ASP1 ASP Inactive
   message exchange would not occur.

5.3.  Q.921/Q.931 Primitives Backhaul Examples

   When the IUA layer on the ASP has a QPTM message to send to the SG,
   it will do the following:

      -  Determine the correct SG

      -  Find the SCTP association to the chosen SG

      -  Determine the correct stream in the SCTP association based on
         the D channel

      -  Fill in the QPTM message, fill in IUA Message Header, fill in
         Common Header

      -  Send the QPTM message to the remote IUA peer in the SG, over
         the SCTP association

   When the IUA layer on the SG has a QPTM message to send to the ASP,
   it will do the following:

      -  Determine the AS for the Interface Identifier

      -  Determine the Active ASP (SCTP association) within the AS

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      -  Determine the correct stream in the SCTP association based on
         the D channel

      -  Fill in the QPTM message, fill in IUA Message Header, fill in
         Common Header

      -  Send the QPTM message to the remote IUA peer in the ASP, over
         the SCTP association

   An example of the message flows for establishing a data link on a
   signaling channel, passing PDUs and releasing a data link on a
   signaling channel is shown below.  An active association between MGC
   and SG is established (Section 5.1) prior to the following message
   flows.

            SG                             ASP

                        <----------- Establish Request
      Establish Confirm  ---------->

                        <----------- Data Request
         Data Indication ----------->
                        <----------- Data Request
         Data Indication ----------->
                        <----------- Data Request
                        <----------- Data Request
         Data Indication ----------->

                        <----------- Release Request (RELEASE_MGMT)
        Release Confirm  ---------->

   An example of the message flows for a failed attempt to establish a
   data link on the signaling channel is shown below.  In this case, the
   gateway has a problem with its physical connection (e.g., Red Alarm),
   so it cannot establish a data link on the signaling channel.

            SG                             ASP

                        <----------- Establish Request (ESTABLISH_START)
      Release Indication ---------->
      (RELEASE_PHYS)

5.4.  Layer Management Communication Examples

   An example of the message flows for communication between Layer
   Management modules between SG and ASP is shown below.  An active
   association between ASP and SG is established (Section 5.1) prior to
   the following message flows.

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                  SG                       ASP

                        <----------- Data Request
        Error Indication ---------->
         (INVALID_TEI)

                        <----------- TEI Status Request
      TEI Status Confirm ---------->
           (Unassigned)

6.  Security

   The security considerations discussed in "Security Considerations for
   SIGTRAN Protocols", RFC 3788 [3], apply to this document.

7.  IANA Considerations

7.1.  SCTP Payload Protocol Identifier

   The IANA has assigned an IUA value for the Payload Protocol
   Identifier in SCTP Payload Data chunk.  The following SCTP Payload
   Protocol Identifier has been registered:

         IUA    "1"

   The SCTP Payload Protocol Identifier is included in each SCTP Data
   chunk, to indicate which protocol the SCTP is carrying.  This Payload
   Protocol Identifier is not directly used by SCTP but MAY be used by
   certain network entities to identify the type of information being
   carried in a Data chunk.

   The User Adaptation peer MAY use the Payload Protocol Identifier as a
   way of determining additional information about the data being
   presented to it by SCTP.

7.2.  IUA Protocol Extensions

   This protocol may also be extended through IANA in three ways:

      -- through definition of additional message classes,
      -- through definition of additional message types, and
      -- through definition of additional message parameters.

   The definition and use of new message classes, types, and parameters
   are an integral part of SIGTRAN adaptation layers.  Thus, these
   extensions are assigned by IANA through an IETF Consensus action as
   defined in [7].

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   The proposed extension must in no way adversely affect the general
   working of the protocol.

7.2.1.  IETF-Defined Message Classes

   The documentation for a new message class MUST include the following
   information:

   (a) A long and short name for the message class.
   (b) A detailed description of the purpose of the message class.

7.2.2.  IETF-Defined Message Types

   Documentation of the message type MUST contain the following
   information:

   (a) A long and short name for the new message type.
   (b) A detailed description of the structure of the message.
   (c) A detailed definition and description of intended use of each
       field within the message.
   (d) A detailed procedural description of the use of the new
       message type within the operation of the protocol.
   (e) A detailed description of error conditions when receiving this
       message type.

   When an implementation receives a message type that it does not
   support, it MUST respond with an Error (ERR) message with an Error
   Code of Unsupported Message Type.

7.2.3.  IETF-Defined TLV Parameter Extension

   Documentation of the message parameter MUST contain the following
   information:

   (a) Name of the parameter type.
   (b) Detailed description of the structure of the parameter field.
       This structure MUST conform to the general type-length-value
       format described in Section 3.1.5.
   (c) Detailed definition of each component of the parameter value.
   (d) Detailed description of the intended use of this parameter type,
       and an indication of whether and under what circumstances
       multiple instances of this parameter type may be found within the
       same message type.

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8.  Timer Values

   The following are suggestions for default timer values.

   T(r)                                    3-5 seconds
   T(ack)                                  2-5 seconds
   T(beat)   Heartbeat Timer               30 seconds

9.  Acknowledgements

   The authors would like to thank Alex Audu, Maria Sonia Vazquez
   Arevalillo, Ming-te Chao, Keith Drage, Norm Glaude, Nikhil Jain,
   Bernard Kuc, Ming Lin, Stephen Lorusso, John Loughney, Barry
   Nagelberg, Neil Olson, Lyndon Ong, Heinz Prantner, Jose Luis Jimenez
   Ramirez, Ian Rytina, Michael Tuexen, and Hank Wang for their valuable
   comments and suggestions.

10.   References

10.1.  Normative References

   [1]  ITU-T Recommendation Q.920, 'Digital Subscriber signaling System
        No. 1 (DSS1) - ISDN User-Network Interface Data Link Layer -
        General Aspects'

   [2]  Coded Character Set--7-Bit American Standard Code for
        Information Interchange, ANSI X3.4-1986.

   [3]  Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security
        Considerations for Signaling Transport (SIGTRAN) Protocols", RFC
        3788, June 2004.

10.2.  Informative References

   [4]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
        H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson,
        "Stream Control Transmission Protocol", RFC 2960, October 2000.

   [5]  Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
        Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Framework
        Architecture for Signaling Transport", RFC 2719, October 1999.

   [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [7]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

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   [8]  Stone, J., Stewart, R., and D. Otis, "Stream Control
        Transmission Protocol (SCTP) Checksum Change", RFC 3309,
        September 2002.

11.  Change Log

   Below is a list of the major changes between this document and RFC
   3057.

   1.  The TEI Query message was added.

   2.  An explanation of the DLCI format (shown in Figure 6) is
       provided.

   3.  Aligned the ASP and AS procedures in Section 4 with RFC3331 and
       RFC3332.

   4.  Alinged the format of the ASPSM and ASPTM messages with RFC3331
       and RFC3332.  These changes include removing the Reason field
       from the ASP Down and ASP Down Ack messages and the Traffic Mode
       Type field from the ASP Inactive and ASP Inactive Ack messages.

   5.  Sections 1.3.3 and 1.3.4 were moved to Appendix A.  A new section
       was added in place of Section 1.3.3.

   6.  The references have been split between Normative and Informative.

   7.  The new Sigtran security document is referenced and Section 6 has
       been updated appropriately.

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Appendix A

A.1.  Signaling Network Architecture

   A Signaling Gateway is used to support the transport of Q.921-User
   signaling traffic to one or more distributed ASPs (e.g., MGCs).
   Clearly, the IUA protocol is not designed to meet the performance and
   reliability requirements for such transport by itself.  However, the
   conjunction of distributed architecture and redundant networks does
   allow for a sufficiently reliable transport of signaling traffic over
   IP.  The IUA protocol is flexible enough to allow its operation and
   management in a variety of physical configurations, enabling Network
   Operators to meet their performance and reliability requirements.

   To meet the ISDN signaling reliability and performance requirements
   for carrier grade networks, Network Operators SHOULD ensure that
   there is no single point of failure provisioned in the end-to-end
   network architecture between an ISDN node and an IP ASP.

   Depending of course on the reliability of the SG and ASP functional
   elements, this can typically be met by the provision of redundant
   Quality of Service (QoS)-bounded IP network paths for SCTP
   Associations between SCTP End Points, and redundant Hosts, and
   redundant SGs.  The distribution of ASPs within the available Hosts
   is also important.  For a particular Application Server, the related
   ASPs SHOULD be distributed over at least two Hosts.

   An example logical network architecture relevant to carrier-grade
   operation in the IP network domain is shown in Figure 8 below:

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                                                          Host1
     ********                                         **************
     *      *_________________________________________*  ********  *
     *      *                                _________*  * ASP1 *  *
     *  SG1 *   SCTP Associations           |         *  ********  *
     *      *_______________________        |         *            *
     ********                       |       |         **************
                                    |       |
     ********                       |       |
     *      *_______________________________|
     *      *                       |
     *  SG2 *    SCTP Associations  |
     *      *____________           |
     *      *            |          |                     Host2
     ********            |          |                 **************
                         |          |_________________*  ********  *
                         |____________________________*  * ASP1 *  *
                                                      *  ********  *
                                                      *            *
                                                      **************
                                                              .
                                                              .
                                                              .

                      Figure 8.  Logical Model Example

   For carrier-grade networks, the failure or isolation of a particular
   ASP SHOULD NOT cause stable calls to be dropped.  This implies that
   ASPs need, in some cases, to share the call state or be able to pass
   the call state between each other.  However, this sharing or
   communication of call state information is outside the scope of this
   document.

A.2.  Application Server Process Redundancy

   To avoid a single point of failure, it is recommended that a minimum
   of two ASPs be in the list, resident in separate hosts and therefore
   available over different SCTP Associations.  For example, in the
   network shown in Figure 8, all messages from a particular D Channel
   (Interface Identifier) could be sent to ASP1 in Host1 or ASP1 in
   Host2.  The AS list at SG1 might look like the following:

      Interface Identifier(s) - Application Server #1
          ASP1/Host1  - State=Up, Active
          ASP1/Host2  - State=Up, Inactive

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   In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
   message for the Interface Identifiers registered.  ASP1 in Host2
   would normally be brought to the active state upon failure of, or
   loss of connectivity to, ASP1/Host1.  In this example, both ASPs are
   Up, meaning that the related SCTP association and far-end IUA peer
   are ready.

   The AS List at SG1 might also be set up in load-share mode as shown
   below:

      Interface Identifier(s) - Application Server #1
          ASP1/Host1 - State=Up, Active
          ASP1/Host2 - State=Up, Active

   In this case, both the ASPs would be sent a portion of the traffic.

   In the process of fail-over, it is recommended that in the case of
   ASPs supporting call processing, stable calls do not get released.
   It is possible that calls in transition MAY fail, although measures
   of communication between the ASPs involved can be used to mitigate
   this problem.  For example, the two ASPs MAY share call state via
   shared memory, or MAY use an ASP-to-ASP protocol to pass call state
   information.  The ASP-to-ASP protocol is outside the scope of this
   document.

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Authors' Addresses

   Ken Morneault
   Cisco Systems Inc.
   13615 Dulles Technology Drive
   Herndon, VA. 20171
   USA

   Phone: +1-703-484-3323
   EMail: kmorneau@cisco.com


   Malleswar Kalla
   Telcordia Technologies
   PYA 2J-341
   3 Corporate Place
   Piscataway, NJ 08854
   USA

   Phone: +1-732-699-3728
   EMail: mkalla@telcordia.com


   Selvam Rengasami
   Tridea Works

   Phone: +1-732-512-0969
   EMail: selvam@trideaworks.com


   Greg Sidebottom
   Signatus Technologies
   Kanata, Ontario, Canada

   EMail: greg@signatustechnologies.com

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