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RFC 3057

 
 
 

ISDN Q.921-User Adaptation Layer

Part 3 of 3, p. 45 to 66
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4.0  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" 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.

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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 set-
   up is complete.  An M-SCTP ESTABLISH indication is sent to Layer
   Management upon successful completion of an incoming SCTP association
   set-up from a peer IUA node

   An M-SCTP RELEASE request from Layer Management will initiate the
   tear-down 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 tear-down 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.

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   Upon receipt of a 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.

   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

   The ASP state transition diagram is shown in Figure 7.  The possible
   states of an ASP are the following:

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   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.

                   Figure 7  ASP State Transition Diagram

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

   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.

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:

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      *  ASP state transitions
      *  Recovery timer triggers

   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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                 Figure 8  AS State Transition Diagram

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

      Tr = Recovery Timer

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 "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, they 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.

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

   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 exchange.

   When an ASP Up message is received at an SG and internally the remote
   ASP is not considered locked-out for local management reasons, the SG
   marks the remote ASP as "Inactive".  The SG responds with an ASP Up
   Ack message in acknowledgement.  The SG sends an ASP-Up Ack message
   in response to a received ASP Up message even if the ASP is already
   marked as "Inactive" at the SG.

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   If for any local reason the SG cannot respond with an ASP Up, the SG
   responds to a ASP Up with a with an ASP-Down Ack message with Reason
   "Management Blocking".

   At the ASP, the ASP Up Ack message received from the SG is not
   acknowledged by the ASP.  If the ASP does not receive a response from
   the SG, or an ASP Down Ack is received, the ASP MAY resend ASP Up
   messages every 2 seconds until it receives a ASP Up Ack message from
   the SG.  The ASP MAY decide to reduce the frequency (say to every 5
   seconds) if an ASP Up Ack is not received after a few tries.

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

4.3.3.2 ASP Down

   The ASP will send an ASP Down to an SG when the ASP is to be removed
   from the list of ASPs in all Application Servers that it is a member
   and no longer receive any IUA traffic or management messages.

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

   The SG marks the ASP as "Down" and returns an ASP Down Ack message to
   the ASP if one of the following events occur:

      -  to acknowledge an ASP Down message from an ASP,
      -  to reply to ASPM messages from an ASP which is locked out for
         management reasons.

   The SG sends an ASP Down Ack message in response to a received ASP
   Down message from the ASP even if the ASP is already marked as "Down"
   at the SG.

   If the ASP does not receive a response from the SG, the ASP MAY send
   ASP Down messages every 2 seconds until it receives an ASP Down Ack
   message from the SG or the SCTP association goes down.  The ASP MAY
   decide to reduce the frequency (say to every 5 seconds) if an ASP
   Down Ack is not received after a few tries.

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.

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   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.

4.3.3.4 ASP Active

   Any time after the ASP has received a ASP Up Ack from the SG, the ASP
   sends an ASP-Active (ASPAC) to the SG indicating that the ASP is
   ready to start processing traffic.  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.

   When an ASP Active (ASPAC) message is received, the SG responds to
   the ASP with a ASPAC Ack message acknowledging that the ASPAC was
   received and starts sending traffic for the associated Application
   Server(s) to that ASP.

   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.

   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

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   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 a
   ASP-Active Ack message to the ASP.

4.3.3.5 ASP Inactive

   When an ASP wishes to withdraw from receiving traffic within an AS,
   the ASP sends an ASP Inactive (ASPIA) to the SG.  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 ASPIA 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. The Type parameter in the ASPIA 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, where normally another ASP has
   already taken over the traffic within the AS with an Over-ride ASPAC,
   the ASP which sends the ASPIA is already considered by the SG to be
   "Inactive".  An ASPIA Ack message is sent to the ASP, after ensuring
   that all traffic is stopped to the ASP.

   In the case of a Load-share mode AS, the SG moves the ASP to the
   "Inactive" state and the AS traffic is re-allocated across the
   remaining "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 NTFY (Insufficient ASPs) MAY be sent
   to all inactive ASPs, if required.

   If no other ASPs are Active in the Application Server, the SG sends a
   NTFY (AS-Pending) to all inactive ASPs of the AS and either discards
   all incoming messages for the AS or starts buffering the incoming
   messages for T(r)seconds, after which messages will be discarded.
   T(r) is configurable by the network operator.  If the SG receives an
   ASPAC from an ASP in the AS before expiry of T(r), the buffered
   traffic is directed to the ASP and the timer is cancelled.  If T(r)
   expires, the AS is moved to the "Inactive" state.

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4.3.3.6  Notify

   A Notify message reflecting a change in the AS state is sent to all
   ASPs in the AS, except those in the "Down" state, with appropriate
   Status Identification.

   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 force 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).

   After receiving an ASP Up Ack message from the SG in response to an
   ASP Up message, the ASP MAY optionally send Beat messages
   periodically, subject to a provisionable timer T(beat).  The SG IUA,
   upon receiving a BEAT message from the ASP, responds with a BEAT ACK
   message.  If no BEAT message (or any other IUA message) is received
   from the SG within the timer 2*T(beat), the SG will consider the
   remote IUA as "Down".  The SG will also send an ASP Down Ack message
   to the ASP.

   At the ASP, if no BEAT ACK message (or any other IUA message) is
   received from the SG within 2*T(beat), the SG is considered
   unavailable.  Transmission of BEAT messages is stopped and ASP Up
   procedures are used to re-establish communication with the SG 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 4 "ASP state
   transition diagram".

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5.0 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 set-up.

                SG                       ASP1
                 |
                 |<---------ASP Up----------|
                 |--------ASP Up Ack------->|
                 |                          |
                 |<-------ASP Active--------|
                 |------ASP Active Ack----->|
                 |                          |

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------->|                          |
           |                         |                          |
           |<-----------------------------ASP Up----------------|
           |----------------------------ASP Up Ack------------->|
           |                         |                          |
           |                         |                          |
           |<-------ASP Active-------|                          |
           |-----ASP Active Ack----->|                          |
           |                         |                          |

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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------>|                          |
           |                         |                          |
           |<------------------------------ASP Up---------------|
           |-----------------------------ASP Up Ack------------>|
           |                         |                          |
           |                         |                          |
           |<--ASP Active (Ldshr)----|                          |
           |----ASP Active Ack------>|                          |
           |                         |                          |
           |<----------------------------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---->|                   |                   |
       |                    |                   |                   |
       |<--------------------------ASP Up-------|                   |
       |------------------------ASPUp Ack)----->|                   |
       |                    |                   |                   |
       |<---------------------------------------------ASP Up--------|
       |--------------------------------------------ASP Up Ack----->|
       |                    |                   |                   |
       |                    |                   |                   |
       |<-ASP Act (Ldshr)---|                   |                   |
       |----ASP Act Ack---->|                   |                   |
       |                    |                   |                   |
       |<---------------------ASP Act (Ldshr)---|                   |
       |----------------------ASP Act Ack------>|                   |
       |                    |                   |                   |

5.2 ASP Traffic Fail-over Examples

5.2.1 (1+1 Sparing, withdrawal of ASP, Back-up 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) --------------->|
           |                         |                          |
           |<------------------------------ ASP Active----------|
           |-----------------------------ASP Active Ack)------->|
           |                                                    |

   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, Back-up Over-ride)

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

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          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.

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

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      -  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

      -  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.

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            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.

                  SG                       ASP

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

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

6.0 Security

   IUA is designed to carry signaling messages for telephony services.
   As such, IUA MUST involve the security needs of several parties the
   end users of the services; the network providers and the applications
   involved.  Additional requirements MAY come from local regulation.
   While having some overlapping security needs, any security solution
   SHOULD fulfill all of the different parties' needs.

6.1 Threats

   There is no quick fix, one-size-fits-all solution for security.  As a
   transport protocol, IUA has the following security objectives:

      *  Availability of reliable and timely user data transport.
      *  Integrity of user data transport.
      *  Confidentiality of user data.

   IUA runs on top of SCTP.  SCTP [3] provides certain transport related
   security features, such as

      *  Blind Denial of Service Attacks
      *  Flooding
      *  Masquerade
      *  Improper Monopolization of Services

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   When IUA is running in professionally managed corporate or service
   provider network, it is reasonable to expect that this network
   includes an appropriate security policy framework.  The "Site
   Security Handbook" [5] SHOULD be consulted for guidance.

   When the network in which IUA runs in involves more than one party,
   it MAY NOT be reasonable to expect that all parties have implemented
   security in a sufficient manner.  In such a case, it is recommended
   that IPSEC is used to ensure confidentiality of user payload.
   Consult [6] for more information on configuring IPSEC services.

6.2 Protecting Confidentiality

   Particularly for mobile users, the requirement for confidentiality
   MAY include the masking of IP addresses and ports.  In this case
   application level encryption is not sufficient; IPSEC ESP SHOULD be
   used instead.  Regardless of which level performs the encryption, the
   IPSEC ISAKMP service SHOULD be used for key management.

7.0 IANA Considerations

7.1 SCTP Payload Protocol Identifier

   A request will be made to IANA to assign an IUA value for the Payload
   Protocol Identifier in SCTP Payload Data chunk.  The following SCTP
   Payload Protocol Identifier will be 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.

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   The definition and use of new message classes, types and parameters
   is an integral part of SIGTRAN adaptation layers.  Thus, these
   extensions are assigned by IANA through an IETF Consensus action as
   defined in [RFC2434].

   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.
       ti3 (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 which 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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8.0 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.

9.0  References

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

   [2] T1S1.7/99-220 Contribution, 'Back-hauling of DSS1 protocol in a
       Voice over Packet Network'

   [3] 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.

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

   [5] Fraser, B., "Site Security Handbook", FYI 8, RFC 2196, September
       1997.

   [6] Kent, S. and R. Atkinson, "Security Architecture for the Internet
       Protocol", RFC 2401, November 1998.

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

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

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10.0 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
   Telcordia Technologies
   NVC-2Z439
   331 Newman Springs Road
   Red Bank, NJ 07701
   USA

   Phone: +1-732-758-5260
   EMail: srengasa@telcordia.com


   Greg Sidebottom
   Nortel Networks
   3685 Richmond Road
   Nepean, Ontario
   Canada  K2H5B7

   Phone: +1-613-763-7305
   EMail: gregside@nortelnetworks.com

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11. Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

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   the copyright notice or references to the Internet Society or other
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   The limited permissions granted above are perpetual and will not be
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.