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RFC 4666

 
 
 

Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) - User Adaptation Layer (M3UA)

Part 5 of 5, p. 96 to 124
Prev RFC Part

 


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4.6.  MTP3 Restart

   In the case where the MTP3 in the SG undergoes an MTP restart, event
   communication SHOULD be handled as follows:

   When the SG discovers SS7 network isolation, the SGPs send an
   indication to all concerned available ASPs (i.e., ASPs in the ASP-
   ACTIVE state), using DUNA messages for the concerned destinations.

   When the SG has completed the MTP Restart procedure, the M3UA layers
   at the SGPs inform all concerned ASPs in the ASP-ACTIVE state of any
   available/restricted SS7 destinations, using the DAVA/DRST messages.
   No message is necessary for those destinations still unavailable
   after the restart procedure.

   When the M3UA layer at an ASP receives a DUNA message indicating SS7
   destination unavailability at an SG, MTP Users will receive an MTP-
   PAUSE indication and will stop any affected traffic to this
   destination.  When the M3UA receives a DAVA/DRST message, MTP Users
   will receive an MTP-RESUME indication and can resume traffic to the
   newly available SS7 destination, provided that the ASP is in the
   ASP-ACTIVE state towards this SGP.

   The ASP MAY choose to audit the availability of unavailable
   destinations by sending DAUD messages.  This would be the case when,
   for example, an AS becomes active at an ASP and does not have current
   destination statuses.  If MTP restart is in progress at the SG, the
   SGP returns a DUNA message for that destination, even if it received
   an indication that the destination became available or restricted.

   When an ASP becomes active for an AS and the SG is experiencing SS7
   network isolation or is performing the MTP Restart procedure for the
   AS, the SG MAY send a DUNA message for the concerned destinations to
   the newly active ASP to prevent the ASP from sending traffic.  These
   messages can be sent after receiving the ASP Active, and before
   sending the ASP Active Ack, to ensure that traffic is not initiated
   by the ASP to these destinations before the SSNM are received.  In
   addition to DUNA messages, SCON, DRST, and DAVA can also be sent.

   In the IPSP case, MTP restart could be considered if the IPSP also
   has connection to an SS7 network.  In that case, the same behavior as
   described above for the SGP would apply to the restarting IPSP.  This
   would also be the case if the IPSPs were perceived as exchanging MTP
   Peer PDUs, instead of MTP primitives between MTP User and MTP
   Provider.  In other words, M3UA does not provide the equivalent to
   Traffic Restart Allowed messages indicating the end of the restart
   procedure between peer IPSPs that would also be connected to an SS7
   network.

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4.7.  NIF Not Available

   Implementation Note: Although the NIF is decided to be an
   implementation dependent function, here are some guidelines that may
   be useful to follow:

   - If an SGP is isolated entirely from the NIF, the SGP should send
     ASP Down Ack to all its connected ASPs.  Upon receiving an ASP Up
     message while isolated from the NIF, the SGP should respond with an
     Error ("Refused - Management Blocking").

   - If an SGP suffers a partial failure (where an SGP can continue to
     service one or more active AS but due to a partial failure it is
     unable to service one or more other active AS), the SGP should send
     ASP Inactive Ack to all its connected ASPs for the affected AS.
     Upon receiving an ASP Active message for an affected AS while still
     partially isolated from the NIF, the SGP should respond with an
     Error ("Refused - Management Blocking").

   - If SG is isolated from NIF, it means that each SGP within an SG
     should follow the procedure mentioned above.

4.8.  M3UA Version Control

   If a 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 likely that the message
   having an unsupported version is an ASP Up message and therefore that
   the Error message would normally come from the SGP.

4.9.  M3UA Termination

   Whenever a M3UA node wants to stop the communication with the peer
   node, it MAY use one of the following procedures:

     a) Send the sequence of ASP-INACTIVE, DEREG (optionally whenever
        dynamic registration is used), and ASP-DOWN messages and perform
        the SCTP Shutdown procedure after that.

     b) Just do the SCTP Shutdown procedure.

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5.  Examples of M3UA Procedures

5.1.  Establishment of Association and Traffic between SGPs and ASPs

   These scenarios show examples of M3UA message flows for the
   establishment of traffic between an SGP and an ASP or between two
   IPSPs.  In all cases it is assumed that the SCTP association is
   already set up.

5.1.1.  Single ASP in an Application Server ("1+0" sparing),
        No Registration

   These scenarios show examples of M3UA message flows for the
   establishment of traffic between an SGP and an ASP where only one ASP
   is configured within an AS (no backup).

5.1.1.1.  Single ASP in an Application Server ("1+0" Sparing),
          No Registration

                 SGP                             ASP1
                  |                               |
                  |<-------------ASP Up-----------|
                  |-----------ASP Up Ack--------->|
                  |                               |
                  |-----NTFY(AS-INACTIVE)(RCn)--->|
                  |                               |
                  |<------- ASP Active(RCn)-------|  RC: Routing Context
                  |-----ASP Active Ack (RCn)----->|      (optional)
                  |                               |
                  |-----NTFY(AS-ACTIVE)(RCn)----->|
                  |                               |

   Note: If the ASP Active message contains an optional Routing Context
   parameter, the ASP Active message only applies for the specified RC
   value(s).  For an unknown RC value, the SGP responds with an Error
   message.

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5.1.1.2.  Single ASP in Application Server ("1+0" Sparing),
          Dynamic Registration

   This scenario is the same as for 5.1.1.1 but with the optional
   exchange of registration information.  In this case, the Registration
   is accepted by the SGP.

                SGP                             ASP1
                 |                               |
                 |<------------ASP Up------------|
                 |----------ASP Up Ack---------->|
                 |                               |
                 |                               |
                 |<----REGISTER REQ(LRCn,RKn)----|  LRC: Local Routing
                 |                               |       Key Id
                 |----REGISTER RESP(LRCn,RCn)--->|   RK: Routing Key
                 |                               |   RC: Routing Context
                 |----NTFY(AS-INACTIVE)(RCn)---->|
                 |                               |
                 |                               |
                 |<------- ASP Active(RCn)-------|
                 |-----ASP Active Ack (RCn)----->|
                 |                               |
                 |-----NTFY(AS-ACTIVE)(RCn)----->|
                 |                               |

   Note: In the case of an unsuccessful registration attempt (e.g.,
   invalid RKn), the Register Response message will contain an
   unsuccessful indication, and the ASP will not subsequently send an
   ASP Active message.

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5.1.1.3.  Single ASP in Multiple Application Servers (Each
          with "1+0" Sparing), Dynamic Registration (Case 1 - Multiple
          Registration Requests)

                SGP                             ASP1
                 |                               |
                 |<------------ASP Up------------|
                 |----------ASP Up Ack---------->|
                 |                               |
                 |<----REGISTER REQ(LRC1,RK1)----|  LRC: Local Routing
                 |                               |       Key Id
                 |----REGISTER RESP(LRC1,RC1)--->|   RK: Routing Key
                 |                               |   RC: Routing Context
                 |---NOTIFY(AS-INACTIVE)(RC1)--->|
                 |                               |
                 |                               |
                 |<------- ASP Active(RC1)-------|
                 |-----ASP Active Ack (RC1)----->|
                 |                               |
                 |----NOTIFY(AS-ACTIVE)(RC1)---->|
                 |                               |
                 ~                               ~
                 |                               |
                 |<----REGISTER REQ(LRCn,RKn)----|
                 |                               |
                 |----REGISTER RESP(LRCn,RCn)--->|
                 |                               |
                 |---NOTIFY(AS-INACTIVE)(RCn)--->|
                 |                               |
                 |<------- ASP Active(RCn)-------|
                 |-----ASP Active Ack (RCn)----->|
                 |                               |
                 |----NOTIFY(AS-ACTIVE)(RCn)---->|
                 |                               |

   Note: In the case of an unsuccessful registration attempt (e.g.,
   invalid RKn), the Register Response message will contain an
   unsuccessful indication, and the ASP will not subsequently send an
   ASP Active message.  Each LRC/RK pair registration is considered
   independently.

   It is not necessary to follow a Registration Request/Response message
   pair with an ASP Active message before sending the next Registration
   Request.  The ASP Active message can be sent at any time after the
   related successful registration.

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5.1.1.4.  Single ASP in Multiple Application Servers (each
          with "1+0" sparing), Dynamic Registration (Case 2 - Single
          Registration Request)

                  SGP                             ASP1
                   |                               |
                   |<------------ASP Up------------|
                   |----------ASP Up Ack---------->|
                   |                               |
                   |                               |
                   |<---REGISTER REQ({LRC1,RK1},   |
                   |                   ...,        |
                   |                 {LRCn,RKn}),--|
                   |                               |
                   |---REGISTER RESP({LRC1,RC1},-->|
                   |                  ...,         |
                   |                 (LRCn,RCn})   |
                   |                               |
                   |--NTFY(AS-INACTIVE)(RC1..RCn)->|
                   |                               |
                   |                               |
                   |<------- ASP Active(RC1)-------|
                   |-----ASP Active Ack (RC1)----->|
                   |                               |
                   |----NOTIFY(AS-ACTIVE)(RC1)---->|
                   |                               |
                   :                               :
                   :                               :
                   |                               |
                   |<------- ASP Active(RCn)-------|
                   |-----ASP Active Ack (RCn)----->|
                   |                               |
                   |----NOTIFY(AS-ACTIVE)(RCn)---->|
                   |                               |

   Note: In the case of an unsuccessful registration attempt (e.g.,
   Invalid RKn), the Register Response message will contain an
   unsuccessful indication, and the ASP will not subsequently send an
   ASP Active message.  Each LRC/RK pair registration is considered
   independently.

   The ASP Active message can be sent at any time after the related
   successful registration and may have more than one RC.

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5.1.2.  Two ASPs in Application Server ("1+1" Sparing)

   This scenario shows example M3UA message flows for the establishment
   of traffic between an SGP and two ASPs in the same Application
   Server, where ASP1 is configured to be in the ASP-ACTIVE state and
   ASP2 is to be a "backup" in the event of communication failure or the
   withdrawal from service of ASP1.  ASP2 may act as a hot, warm, or
   cold backup, depending on the extent to which ASP1 and ASP2 share
   call/transaction state or can communicate call state under
   failure/withdrawal events.  The example message flow is the same
   whether the ASP Active messages indicate "Override", "Loadshare", or
   "Broadcast" mode, although typically this example would use an
   Override mode.

         SGP                      ASP1                       ASP2
          |                        |                          |
          |<--------ASP Up---------|                          |
          |-------ASP Up Ack------>|                          |
          |                        |                          |
          |--NOTIFY(AS-INACTIVE)-->|                          |
          |                        |                          |
          |<----------------------------ASP Up----------------|
          |----------------------------ASP Up Ack------------>|
          |                        |                          |
          |--------------------------NOTIFY(AS-INACTIVE)----->|
          |                        |                          |
          |                        |                          |
          |<-------ASP Active------|                          |
          |------ASP Active Ack--->|                          |
          |                        |                          |
          |---NOTIFY(AS-ACTIVE)--->|                          |
          |--------------------------NOTIFY(AS-ACTIVE)------->|
          |                        |                          |

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5.1.3.  Two ASPs in an Application Server ("1+1" Sparing,
        Loadsharing Case)

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

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

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5.1.4.  Three ASPs in an Application Server ("n+k" Sparing,
        Loadsharing Case)

   This scenario shows example M3UA message flows for the establishment
   of traffic between an SGP and three ASPs in the same Application
   Server, where two of the ASPs are brought to the state ASP-ACTIVE and
   subsequently share the load.  In this case, a minimum of two ASPs are
   required to handle the total traffic load (2+1 sparing).

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

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5.2.  ASP Traffic Failover Examples

5.2.1.  1+1 Sparing, Withdrawal of ASP, Backup Override

   Following from the example in Section 5.1.2, ASP1 withdraws from
   service:

               SGP                      ASP1                       ASP2
                |                        |                          |
                |<-----ASP Inactive------|                          |
                |----ASP Inactive Ack--->|                          |
                |                        |                          |
                |----NTFY(AS-PENDING)--->|                          |
                |-----------------------NTFY(AS-PENDING)----------->|
                |                        |                          |
                |<----------------------------- ASP Active----------|
                |-----------------------------ASP Active Ack------->|
                |                        |                          |
                |----NTFY(AS-ACTIVE)---->|                          |
                |-----------------------NTFY(AS-ACTIVE)------------>|

   Note: If the SGP M3UA layer detects the loss of the M3UA peer (e.g.,
   M3UA heartbeat loss or detection of SCTP failure), the initial ASP
   Inactive message exchange (i.e., SGP to ASP1) would not occur.

5.2.2.  1+1 Sparing, Backup Override

   Following on from the example in Section 5.1.2, ASP2 wishes to
   Override ASP1 and take over the traffic:

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

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5.2.3.  n+k Sparing, Loadsharing Case, Withdrawal of ASP

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

        SGP                 ASP1                ASP2                ASP3
          |                   |                   |                   |
          |<----ASP Inact.----|                   |                   |
          |---ASP Inact Ack-->|                   |                   |
          |                   |                   |                   |
          |--NTFY(Ins. ASPs)->|                   |                   |
          |---------------------------------------NOTIFY(Ins. ASPs)-->|
          |                   |                   |                   |
          |                   |                   |                   |
          |<----------------------------------------ASP Act (Ldshr)---|
          |------------------------------------------ASP Act (Ack)--->|
          |                   |                   |                   |
          |-NTFY(AS-ACTIVE)-->|                   |                   |
          |-------------------NOTIFY(AS-ACTIVE)-->|                   |
          |---------------------------------------NOTIFY(AS-ACTIVE)-->|
          |                   |                   |                   |
          |                   |                   |                   |

   For the Notify message to be sent, the SG maintains knowledge of the
   minimum ASP resources required (e.g., if the SG knows that "n+k" =
   "2+1" for a Loadshare AS and "n" currently equals "1").

   Note: If the SGP detects loss of the ASP1 M3UA peer (e.g., M3UA
   heartbeat loss or detection of SCTP failure), the initial ASP
   Inactive message exchange (i.e., SGP-ASP1) would not occur.

5.3.  Normal Withdrawal of an ASP from an Application Server
      and Teardown of an Association

   An ASP that is now confirmed in the state ASP-INACTIVE (i.e., the ASP
   has received an ASP Inactive Ack message) may now proceed to the
   ASP-DOWN state, if it is to be removed from service.  Following from
   Section 5.2.1 or 5.2.3, where ASP1 has moved to the "Inactive" state:

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               SGP                            ASP1
                |                              |
                |<-----ASP Inactive (RCn)------|    RC: Routing Context
                |----ASP Inactive Ack (RCn)--->|
                |                              |
                |<-----DEREGISTER REQ(RCn)-----|    See Notes
                |                              |
                |---DEREGISTER RESP(LRCn,RCn)->|
                |                              |
                :                              :
                |                              |
                |<-----------ASP Down----------|
                |---------ASP Down Ack-------->|
                |                              |

   Note: The Deregistration procedure will typically be used if the ASP
   previously used the Registration procedures for configuration within
   the Application Server.  ASP Inactive and Deregister messages
   exchanges may contain multiple Routing Contexts.

   The ASP should be in the ASP-INACTIVE state and should have
   deregistered in all its Routing Contexts before attempting to move to
   the ASP-DOWN state.

5.4.  Auditing Examples

5.4.1.  SG State: Uncongested/Available

          ASP                          SGP
          ---                          ---
           |  -------- DAUD --------->  |
           |  <------ SCON(0) --------  |
           |  <------- DAVA ----------  |

5.4.2.  SG State: Congested (Congestion Level=2) / Available

          ASP                          SGP
          ---                          ---
           |  -------- DAUD --------->  |
           |  <------ SCON(2) --------  |
           |  <------- DAVA ----------  |

5.4.3.  SG State: Unknown/Available

          ASP                          SGP
          ---                          ---
           |  -------- DAUD --------->  |
           |  <------- DAVA ----------  |

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5.4.4.  SG State: Unavailable

          ASP                          SGP
          ---                          ---
           |  -------- DAUD --------->  |
           |  <------- DUNA ----------  |

5.5.  M3UA/MTP3-User Boundary Examples

5.5.1.  At an ASP

   This section describes the primitive mapping between the MTP3 User
   and the M3UA layer at an ASP.

5.5.1.1.  Support for MTP-TRANSFER Primitives at the ASP

5.5.1.1.1.  Support for MTP-TRANSFER Request Primitive

   When the MTP3-User on the ASP has data to send to a remote MTP3-User,
   it uses the MTP-TRANSFER request primitive.  The M3UA layer at the
   ASP will do the following when it receives an MTP-TRANSFER request
   primitive from the M3UA user:

      - Determine the correct SGP.

      - Determine the correct association to the chosen SGP.

      - Determine the correct stream in the association (e.g.,
        based on SLS).

      - Determine whether to complete the optional fields of the DATA
        message.

      - Map the MTP-TRANSFER request primitive into the Protocol Data
        field of a DATA message.

      - Send the DATA message to the remote M3UA peer at the SGP,
        over the SCTP association.

            SGP                       ASP
             |                         |
             |<-----DATA Message-------|<--MTP-TRANSFER req.
             |                         |

5.5.1.1.2.  Support for the MTP-TRANSFER Indication Primitive

   When the M3UA layer on the ASP receives a DATA message from the M3UA
   peer at the remote SGP, it will do the following:

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      - Evaluate the optional fields of the DATA message, if present.

      - Map the Protocol Data field of a DATA message into the
        MTP-TRANSFER indication primitive.

      - Pass the MTP-TRANSFER indication primitive to the user part.  In
        case of multiple user parts, the optional fields of the Data
        message are used to determine the concerned user part.

            SGP                       ASP
             |                         |
             |------Data Message------>|-->MTP-Transfer ind.
             |                         |

5.5.1.1.3.  Support for ASP Querying of SS7 Destination States

   There are situations such as temporary loss of connectivity to the
   SGP that may cause the M3UA layer at the ASP to audit SS7 destination
   availability/congestion states.  Note: there is no primitive for the
   MTP3-User to request this audit from the M3UA layer, as this is
   initiated by an internal M3UA management function.

            SGP                        ASP
             |                          |
             |<----------DAUD-----------|
             |<----------DAUD-----------|
             |<----------DAUD-----------|
             |                          |
             |                          |

5.5.2.  At an SGP

   This section describes the primitive mapping between the MTP3-User
   and the M3UA layer at an SGP.

5.5.2.1.  Support for MTP-TRANSFER Request Primitive at the SGP

   When the M3UA layer at the SGP has received DATA messages from its
   peer destined to the SS7 network, it will do the following:

      - Evaluate the optional fields of the DATA message, if present, to
        determine the Network Appearance.

      - Map the Protocol data field of the DATA message into an
        MTP-TRANSFER request primitive.

      - Pass the MTP-TRANSFER request primitive to the MTP3 of the
        concerned Network Appearance.

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                               SGP                        ASP
                                |                          |
           <---MTP-TRANSFER req.|<---------DATA -----------|
                                |                          |

5.5.2.2.  Support for MTP-TRANSFER Indication Primitive at the SGP

   When the MTP3 layer at the SGP has data to pass its user parts, it
   will use the MTP-TRANSFER indication primitive.  The M3UA layer at
   the SGP will do the following when it receives an MTP-TRANSFER
   indication primitive:

      - Determine the correct AS, using the distribution function;

      - Select an ASP in the ASP-ACTIVE state.

      - Determine the correct association to the chosen ASP.

      - Determine the correct stream in the SCTP association (e.g.,
        based on SLS).

      - Determine whether to complete the optional fields of the DATA
        message.

      - Map the MTP-TRANSFER indication primitive into the Protocol Data
        field of a DATA message.

      - Send the DATA message to the remote M3UA peer in the ASP, over
        the SCTP association.

                              SGP                        ASP
                               |                          |
          --MTP-TRANSFER ind.->|-----------DATA --------->|
                               |                          |

5.5.2.3.  Support for MTP-PAUSE, MTP-RESUME, MTP-STATUS Indication
          Primitives

   The MTP-PAUSE, MTP-RESUME, and MTP-STATUS indication primitives from
   the MTP3 upper layer interface at the SGP need to be made available
   to the remote MTP3 User Part lower-layer interface at the concerned
   ASP(s).

5.5.2.3.1.  Destination Unavailable

   The MTP3 layer at the SGP will generate an MTP-PAUSE indication
   primitive when it determines locally that an SS7 destination is
   unreachable.  The M3UA layer will map this primitive to a DUNA

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   message.  The SGP M3UA layer determines the set of concerned ASPs to
   be informed based on internal SS7 network information associated with
   the MTP-PAUSE indication primitive indication.

                      SGP                       ASP
                       |                         |
    --MTP-PAUSE ind.-->|---------DUNA----------->|--MTP-PAUSE ind.-->
                       |                         |
5.5.2.3.2.  Destination Available

   The MTP3 at the SGP will generate an MTP-RESUME indication primitive
   when it determines locally that an SS7 destination that was
   previously unreachable is now reachable.  The M3UA layer will map
   this primitive to a DAVA message.  The SGP M3UA determines the set of
   concerned ASPs to be informed based on internal SS7 network
   information associated with the MTP-RESUME indication primitive.

                        SGP                       ASP
                         |                         |
     --MTP-RESUME ind.-->|-----------DAVA--------->|--MTP-RESUME ind.-->
                         |                         |

5.5.2.3.3.  SS7 Network Congestion

   The MTP3 layer at the SGP will generate an MTP-STATUS indication
   primitive when it determines locally that the route to an SS7
   destination is congested.  The M3UA layer will map this primitive to
   a SCON message.  It will determine which ASP(s) to send the SCON
   message to, based on the intended Application Server.

                        SGP                       ASP
                         |                         |
     --MTP-STATUS ind.-->|-----------SCON--------->|--MTP-STATUS ind.-->
                         |                         |

5.5.2.3.4.  Destination User Part Unavailable

   The MTP3 layer at the SGP will generate an MTP-STATUS indication
   primitive when it receives an UPU message from the SS7 network.  The
   M3UA layer will map this primitive to a DUPU message.  It will
   determine which ASP(s) to send the DUPU to based on the intended
   Application Server.

                      SGP                       ASP
                       |                         |
   --MTP-STATUS ind.-->|----------DUPU---------->|--MTP-STATUS ind.-->
                       |                         |

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5.6.  Examples for IPSP Communication

   These scenarios show a basic example for IPSP communication for the
   three phases of the connection (establishment, data exchange,
   disconnection).  It is assumed that the SCTP association is already
   set up.  Both single exchange and double exchange behavior are
   included for illustrative purposes.

5.6.1.  Single Exchange

               IPSP-A                           IPSP-B
                 |                                |
                 |-------------ASP Up------------>|
                 |<----------ASP Up Ack-----------|
                 |                                |
                 |<------- ASP Active(RCb)--------|  RC: Routing Context
                 |-----ASP Active Ack (RCb)------>|      (optional)
                 |                                |
                 |                                |
                 |<=========  DATA (RCb) ========>|
                 |                                |
                 |<-----ASP Inactive (RCb)--------|  RC: Routing Context
                 |----ASP Inactive Ack (RCb)----->|      (optional)
                 |                                |
                 |<-----------ASP Down------------|
                 |---------ASP Down Ack---------->|
                 |                                |

   Routing Context is previously agreed to be the same in both
   directions.

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5.6.2.  Double Exchange

               IPSP-A                           IPSP-B
                 |                                |
                 |<-------------ASP Up------------|
                 |-----------ASP Up Ack---------->|
                 |                                |
                 |-------------ASP Up------------>|  (optional)
                 |<----------ASP Up Ack-----------|  (optional)
                 |                                |
                 |<------- ASP Active(RCb)--------|  RC: Routing Context
                 |-----ASP Active Ack (RCb)------>|      (optional)
                 |                                |
                 |------- ASP Active(RCa)-------->|  RC: Routing Context
                 |<-----ASP Active Ack (RCa)------|      (optional)
                 |                                |
                 |<=========  DATA (RCa) =========|
                 |==========  DATA (RCb) ========>|
                 |                                |
                 |<-----ASP Inactive (RCb)--------|  RC: Routing Context
                 |----ASP Inactive Ack (RCb)----->|
                 |                                |
                 |------ASP Inactive (RCa)------->|  RC: Routing Context
                 |<----ASP Inactive Ack (RCa)-----|
                 |                                |
                 |<-----------ASP Down------------|
                 |---------ASP Down Ack---------->|
                 |                                |
                 |------------ASP Down----------->|  (optional)
                 |<--------ASP Down Ack-----------|  (optional)
                 |                                |

   In this approach, only one single exchange of ASP Up message can be
   considered sufficient since the response by the other peer can be
   considered a notice that it is in ASP_UP state.

   For the same reason, only one ASP Down message is needed, since once
   an IPSP receives ASP_Down ack message it is itself considered to be
   in the ASP_Down state and not allowed to receive ASPSM messages.

6.  Security Considerations

   Implementations MUST follow the normative guidance of RFC3788 [11] on
   the integration and usage of security mechanisms in SIGTRAN
   protocols.

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7.  IANA Considerations

   This document contains no new actions for IANA.  The subsections
   below are retained for historical purposes.

7.1.  SCTP Payload Protocol Identifier

   IANA has assigned an M3UA value for the Payload Protocol Identifier
   in the SCTP DATA chunk.  The following SCTP Payload Protocol
   Identifier has been registered:

         M3UA    "3"

   The SCTP Payload Protocol Identifier value "3" SHOULD be included in
   each SCTP DATA chunk, to indicate that the SCTP is carrying the M3UA
   protocol.  The value "0" (unspecified) is also allowed but any other
   values MUST not be used.  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.  M3UA Port Number

   IANA has registered SCTP (and UDP/TCP) Port Number 2905 for M3UA.  It
   is recommended that SGPs use this SCTP port number for listening for
   new connections.  SGPs MAY also use statically configured SCTP port
   numbers instead.

7.3.  M3UA 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.
      - Through definition of additional message parameters.

   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 Guidelines for Writing an IANA Considerations Section in
   RFCs [23].

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

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7.3.1.  IETF-Defined Message Classes

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

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

7.3.2.  IETF Defined Message Types

   The documentation for a new message type MUST include 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 for 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 ("Unsupported
   Message Type").

7.3.3.  IETF-Defined 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 3.2.
      (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.

8.  Acknowledgements

   The authors would like to thank Antonio Roque Alvarez, Joyce
   Archibald, Tolga Asveren, Maria-Cruz Bartolome-Rodrigo, Dan Brendes,
   Antonio Canete, Nikhil Jain, Roland Jesske, Joe Keller, Kurt Kite,
   Ming Lin, Steve Lorusso, Naoto Makinae, Howard May, Francois
   Mouillaud, Barry Nagelberg, Neil Olson, Heinz Prantner, Shyamal

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   Prasad, Mukesh Punhani, Selvam Rengasami, John Schantz, Ray Singh,
   Michael Tuexen, Nitin Tomar, Gery Verwimp, Tim Vetter, Kazuo
   Watanabe, Ben Wilson, and many others for their valuable comments and
   suggestions.

9.  Document Contributors

   Ian Rytina - Ericsson
   Guy Mousseau - Nortel Networks
   Lyndon Ong - Ciena
   Hanns Juergen Schwarzbauer - Siemens
   Klaus Gradischnig - Detecon Inc.
   Mallesh Kalla - Telcordia
   Normand Glaude - Performance Technologies
   Brian Bidulock - OpenSS7
   John Loughney - Nokia
   Greg Sidebottom - Signatus Technologies

10.  References

10.1.  Normative References

   [1]  ITU-T Recommendations Q.761 to Q.767, "Signalling System No.7
        (SS7) - ISDN User Part (ISUP)"

   [2]  ANSI T1.113 - "Signaling System Number 7 - ISDN User Part"

   [3]  ETSI ETS 300 356-1 "Integrated Services Digital Network (ISDN);
        Signalling System No.7; ISDN User Part (ISUP) version 2 for the
        international interface; Part 1: Basic services"

   [4]  ITU-T Recommendations Q.711 to Q.715, "Signalling System No.  7
        (SS7) - Signalling Connection Control Part (SCCP)"

   [5]  ANSI T1.112 "Signaling System Number 7 - Signaling Connection
        Control Part"

   [6]  ETSI ETS 300 009-1, "Integrated Services Digital Network (ISDN);
        Signalling System No.7; Signalling Connection Control Part
        (SCCP) (connectionless and connection-oriented class 2) to
        support international interconnection; Part 1: Protocol
        specification"

   [7]  ITU-T Recommendations Q.700 to Q.705, "Signalling System No.  7
        (SS7) - Message Transfer Part (MTP)"

   [8]  ANSI T1.111 "Signaling System Number 7 - Message Transfer Part"

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   [9]  ETSI ETS 300 008-1, "Integrated Services Digital Network (ISDN);
        Signalling System No.7; Message Transfer Part (MTP) to support
        international interconnection; Part 1: Protocol specification"

   [10] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
        63, RFC 3629, November 2003.

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

10.2.  Informative References

   [12] 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.

   [13] ITU-T Recommendation Q.720, "Telephone User Part"

   [14] ITU-T Recommendations Q.771 to Q.775 "Signalling System No.  7
        (SS7) - Transaction Capabilities (TCAP)"

   [15] ANSI T1.114 "Signaling System Number 7 - Transaction
        Capabilities Application Part"

   [16] ETSI ETS 300 287-1, "Integrated Services Digital Network (ISDN);
        Signalling System No.7; Transaction Capabilities (TC) version 2;
        Part 1: Protocol specification"

   [17] 3G TS 25.410 V4.0.0 (2001-04) "Technical Specification - 3rd
        Generation partnership Project; Technical Specification Group
        Radio Access Network; UTRAN Iu Interface: General Aspects and
        Principles"

   [18] 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.

   [19] ITU-T Recommendation Q.2140 "B-ISDN ATM Adaptation Layer -
        Service Specific Coordination Function for signalling at the
        Network Node Interface (SSCF at NNI)"

   [20] ITU-T Recommendation Q.2110 "B-ISDN ATM Adaptation Layer -
        Service Specific Connection Oriented Protocol (SSCOP)"

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

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   [22] ITU-T Recommendation Q.2210 "Message Transfer Part Level 3
        functions and messages using the services of ITU Recommendation
        Q.2140"

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

   [24] Morneault, K., Dantu, R., Sidebottom, G., Bidulock, B., and J.
        Heitz, "Signaling System 7 (SS7) Message Transfer Part 2 (MTP2)
        - User Adaptation Layer", RFC 3331, September 2002.

   [25] George, T., Bidulock, B., Dantu, R., Schwarzbauer, H., and K.
        Morneault, "Signaling System 7 (SS7) Message Transfer Part 2
        (MTP2) - User Peer-to-Peer Adaptation Layer (M2PA)", RFC 4165,
        September 2005.

   [26] Telecommunication Technology Committee (TTC) Standard JT-Q704,
        "Message Transfer Part Signaling Network Functions", April 28,
        1992.

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Appendix A

A.1.  Signalling Network Architecture

   A Signalling Gateway is used to support the transport of MTP3-User
   signalling traffic received from the SS7 network to multiple
   distributed ASPs (e.g., MGCs and IP Databases).  Clearly, the M3UA
   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 provides support
   for reliable transport of signalling traffic over IP.  The M3UA
   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 stringent SS7 signalling reliability and performance
   requirements for carrier grade networks, Network Operators might
   require that no single point of failure is present in the end-to-end
   network architecture between an SS7 node and an IP-based application.
   This can typically be achieved through the use of redundant SGPs or
   SGs, redundant hosts, and the provision of redundant QOS-bounded IP
   network paths for SCTP Associations between SCTP End Points.
   Obviously, the reliability of the SG, the MGC, and other IP-based
   functional elements also needs to be taken into account.  The
   distribution of ASPs and SGPs within the available Hosts MAY also be
   considered.  As an example, for a particular Application Server, the
   related ASPs could be distributed over at least two Hosts.

   One example of a physical network architecture relevant to SS7
   carrier grade operation in the IP network domain is shown in Figure
   A-1, below:

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          SGs                                     MGCs

   Host#1 **************                          ************** Host#3
          *  ********__*__________________________*__********  *   =
          *  *SGP1.1*__*_____      _______________*__* ASP1 *  *  MGC1
          *  ********  *     \    /               *  ********  *
          *  ********__*______\__/________________*__********  *
          *  *SGP2.1*__*_______\/______      _____*__* ASP2 *  *
          *  ********  *       /\      |    |     *  ********  *
          *      :     *      /  \     |    |     *      :     *
          *  ********  *     /    \    |    |     *  ********  *
          *  * SGPn *  *     |    |    |    |     *  * ASPn *  *
          *  ********  *     |    |    |    |     *  ********  *
          **************     |    |    |    |     **************
                             |    |    \    /
   Host#2 **************     |    |     \  /      ************** Host#4
          *  ********__*_____|    |______\/_______*__********  *   =
          *  *SGP1.2*__*_________________/\_______*__* ASP1 *  *  MGC2
          *  ********  *                /  \      *  ********  *
          *  ********__*_______________/    \_____*__********  *
          *  *SGP2.2*__*__________________________*__* ASP2 *  *
          *  ********  *                          *  ********  *
          *      :     *     SCTP Associations    *      :     *
          *  ********  *                          *  ********  *
          *  * SGPn *  *                          *  * ASPn *  *
          *  ********  *                          *  ********  *
          **************                          **************

   SGP1.1 and SGP1.2 are part of SG1
   SGP2.1 and SGP2.2 are part of SG2

                         Figure A-1 - Physical Model

   In this model, each host may have many application processes.  In the
   case of the MGC, an ASP may provide service to one or more
   Application Servers, and is identified as an SCTP end point.  One or
   more Signalling Gateway Processes make up a single Signalling
   Gateway.

   This example model can also be applied to IPSP-IPSP signalling.  In
   this case, each IPSP may have its services distributed across 2 or
   more hosts, and may have multiple server processes on each host.

   In the example above, each signalling process (SGP, ASP, or IPSP) is
   the end point to more than one SCTP association, leading to more than
   one other signalling processes.  To support this, a signalling
   process must be able to support distribution of M3UA messages to many
   simultaneous active associations.  This message distribution function

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   is based on the status of provisioned Routing Keys, the status of the
   signalling routes to signalling points in the SS7 network, and the
   redundancy model (active-standby, load sharing, broadcast, n+k) of
   the remote signalling processes.

   For carrier grade networks, the failure or isolation of a particular
   signalling process should not cause stable calls or transactions to
   be lost.  This implies that signalling processes need, in some cases,
   to share the call/transaction state or be able to pass the call state
   information between each other.  In the case of ASPs performing call
   processing, coordination may also be required with the related Media
   Gateway to transfer the MGC control for a particular trunk
   termination.  However, this sharing or communication of
   call/transaction state information is outside the scope of this
   document.

   This model serves as an example.  M3UA imposes no restrictions as to
   the exact layout of the network elements, the message distribution
   algorithms, and the distribution of the signalling processes.
   Instead, it provides a framework and a set of messages that allow for
   a flexible and scalable signalling network architecture, aiming to
   provide reliability and performance.

A.2.  Redundancy Models

A.2.1.  Application Server Redundancy

   At the SGP, an Application Server list contains active and inactive
   ASPs to support ASP broadcast, loadsharing, and failover procedures.
   The list of ASPs within a logical Application Server is kept updated
   in the SGP to reflect the active Application Server Process(es).

   For example, in the network shown in Figure 1, all messages to DPC x
   could be sent to ASP1 in Host3 or ASP1 in Host4.  The AS list at SGP1
   in Host 1 might look like the following:

      Routing Key {DPC=x) - "Application Server #1"
         ASP1/Host3  - State = Active
         ASP1/Host4  - State = Inactive

   In this "1+1" redundancy case, ASP1 in Host3 would be sent any
   incoming message with DPC=x.  ASP1 in Host4 would normally be brought
   to the "active" state upon failure of, or loss of connectivity to,
   ASP1/Host1.

   The AS List at SGP1 in Host1 might also be set up in loadshare mode:

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      Routing Key {DPC=x) - "Application Server #1"
         ASP1/Host3 - State = Active
         ASP1/Host4 - State = Active

   In this case, both the ASPs would be sent a portion of the traffic.
   For example, the two ASPs could together form a database, where
   incoming queries may be sent to any active ASP.

   Care might need to be exercised by a Network Operator in the
   selection of the routing information to be used as the Routing Key
   for a particular AS.

   In the process of failover, it is recommended that, in the case of
   ASPs supporting call processing, stable calls do not fail.  It is
   possible that calls in "transition" may fail, although measures of
   communication between the ASPs involved can be used to mitigate this.

   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.  Any
   ASP-to-ASP protocol to support this function is outside the scope of
   this document.

A.2.2.  Signalling Gateway Redundancy

   Signalling Gateways may also be distributed over multiple hosts.
   Much like the AS model, SGs may comprise one or more SG Processes
   (SGPs), distributed over one or more hosts, using an active/backup or
   a loadsharing model.  Should an SGP lose all or partial SS7
   connectivity and other SGPs exist, the SGP may terminate the SCTP
   associations to the concerned ASPs.

   It is therefore possible for an ASP to route signalling messages
   destined to the SS7 network using more than one SGP.  In this model,
   a Signalling Gateway is deployed as a cluster of hosts acting as a
   single SG.  A primary/backup redundancy model is possible, where the
   unavailability of the SCTP association to a primary SGP could be used
   to reroute affected traffic to an alternate SGP.  A loadsharing model
   is possible, where the signalling messages are loadshared between
   multiple SGPs.  A broadcast model is also possible, where signalling
   messages are sent to each active SGP in the SG.  The distribution of
   the MTP3-user messages over the SGPs should be done in such a way to
   minimize message missequencing, as required by the SS7 User Parts.

   It may also be possible for an ASP to use more than one SG to access
   a specific SS7 end point, in a model that resembles an SS7 STP mated
   pair.  Typically, SS7 STPs are deployed in mated pairs, with traffic
   loadshared between them.  Other models are also possible, subject to
   the limitations of the local SS7 network provisioning guidelines.

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   From the perspective of the M3UA layer at an ASP, a particular SG is
   capable of transferring traffic to a provisioned SS7 destination X if
   an SCTP association with at least one SGP of the SG is established,
   the SGP has returned an acknowledgement to the ASP to indicate that
   the ASP is actively handling traffic for that destination X, the SGP
   has not indicated that the destination X is inaccessible, and the SGP
   has not indicated MTP Restart.  When an ASP is configured to use
   multiple SGPs for transferring traffic to the SS7 network, the ASP
   must maintain knowledge of the current capability of the SGPs to
   handle traffic to destinations of interest.  This information is
   crucial to the overall reliability of the service, for active/backup,
   loadsharing, and broadcast models, in the event of failures and
   recovery and maintenance activities.  The ASP M3UA may also use this
   information for congestion avoidance purposes.  The distribution of
   the MTP3-user messages over the SGPs should be done in such a way as
   to minimize message missequencing, as required by the SS7 User Parts.

Editors' Addresses

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

   EMail: kmorneau@cisco.com


   Javier Pastor-Balbas
   Ericsson Espana S.A.
   C/ Retama 1
   28045 Madrid - Spain

   EMail: j.javier.pastor@ericsson.com

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