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Network Working Group                                   Ken Morneault
INTERNET-DRAFT                                              Ram Dantu
                                                        Cisco Systems
                                                        Mallesh Kalla
                                                            Telcordia
                                                      Greg Sidebottom
                                                      Nortel Networks
                                                           Tom George
                                                              Alcatel


Expires in six months                                        Nov 2000



                  SS7 MTP2-User Adaptation Layer
                  <draft-ietf-sigtran-m2ua-05.txt>


Status of This Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups.  Note that other groups MAY also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and MAY be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as 'work in progress'.

The list of current Internet-Drafts can be accessed at
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To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).




Abstract

This Internet Draft defines a protocol for backhauling of SS7 MTP2
User signaling messages over IP using the Stream Control
Transmission Protocol (SCTP).  This protocol would be used between a
Signaling Gateway (SG) and Media Gateway Controller (MGC).  It is
assumed that the SG receives SS7 signaling over a standard SS7
interface using the SS7 Message Transfer Part (MTP) to provide
transport.  The Signaling Gateway would act as a Signaling Link
Terminal.




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                        TABLE OF CONTENTS

1.  Introduction..............................................2
  1.1  Scope..................................................2
  1.2  Terminology............................................3
  1.3  Signaling Transport Architecture.......................3
  1.4  Services Provide by the M2UA Adaptation Layer..........6
  1.5  Function Provided by the M2UA Layer....................8
  1.6  Definition of the M2UA Boundaries......................9
2.  Conventions...............................................9
3.  Protocol Elements.........................................9
  3.1  Common Message Header.................................10
  3.2  M2UA Message Header...................................11
  3.3  M2UA Messages.........................................11
4.  Procedures...............................................20
  4.1  Procedures to Support Service in Section 1.4.1........20
  4.2  Procedures to Support Service in Section 1.4.2........21
  4.3  Procedures to Support Service in Section 1.4.3........21
5.  Examples of MTP2 User Adaptation (M2UA) Procedures.......26
  5.1  Establishment of associations between SG and MGC......26
       examples
  5.2  MTP Level 2 / MTP Level 3 Boundary Examples...........28
  5.3  Layer Management Communication Examples...............29
6.  Security.................................................30
7.  IANA Considerations......................................31
8.  Acknowledgements.........................................31
9.  References...............................................32
10. Author's Addresses.......................................33
































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1.  Introduction

1.1 Scope

There is a need for Switched Circuit Network SCN signaling protocol
delivery from an Signaling Gateway (SG) to a Media Gateway
Controller (MGC) or IP Signaling Point (IPSP).  The delivery
mechanism SHOULD meet the following criteria:

*  Support for MTP Level 2 / MTP Level 3 interface boundary
*  Support for communication between Layer Management modules on SG
   and MGC
*  Support for management of active associations between the SG and MGC

In other words, the Signaling Gateway will transport MTP Level 3
messages to a Media Gateway Controller (MGC) or IP Signaling Point
(IPSP).  In the case of delivery from an SG to an IPSP, both the SG and
IPSP function as traditional SS7 nodes using the IP network as a new
type of SS7 link.  This allows for full MTP Level 3 message handling
and network management capabilities.

1.2 Terminology

MTP2-User - A protocol that normally uses the services of MTP Level 2
(i.e. MTP3).

Interface - For the purposes of this document, an interface is a SS7
signaling link.

Backhaul - Refers to the transport of signaling from the point of
interface for the associated data stream (i.e., SG function in the MGU)
back to the point of call processing (i.e., the MGCU), if this is not
local [4].

Association - An association refers to a SCTP association.  The
association will provide the transport for the delivery of protocol
data units for one or more interfaces.

Stream - A stream refers to an SCTP stream; a uni-directional logical
channel established from one SCTP endpoint to another associated SCTP
endpoint, within which all user messages are delivered in-sequence
except for those submitted to the un-ordered delivery service.

Interface Identifier - The Interface Identifier identifies the physical
interface at the SG for which the signaling messages are sent/received.
The format of the Interface Identifier parameter can be text or integer,
the values of which are assigned according to network operator policy.
The values used are of local significance only, coordinated between the
SG and ASP.

Application Server (AS) - A logical entity serving a specific application
instance.  An example of an Application Server is a MGC handling the
MTP Level 3 and call processing for SS7 links terminated by the
Signaling Gateways.  Practically speaking, an AS is modeled at the SG
as an ordered list of one or more related Application Server Processes
(e.g., primary, secondary, tertiary, ...).

Application Server Process (ASP) - A process instance of an Application
Server.  Examples of Application Server Processes are primary or backup
MGC instances.

Fail-over - The capability to re-route signaling traffic as required
to an alternate Application Server Process, or group of ASPs, within
an Application Server in the event of failure or unavailability of a
currently used Application Server Process.  Fail-back MAY apply upon
the return to service of a previously unavailable Application Server
Process.

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Signaling Link Terminal (SLT) - Refers to the means of performing all
of the functions defined at MTP level 2 regardless of their
implementation [2].

Layer Management - Layer Management is a nodal function in an SG or
ASP that handles the inputs and outputs between the M2UA layer and a
local management entity.

Network Byte Order: Most significant byte first, a.k.a Big Endian.

Host - The computing platform that the ASP process is running on.

1.3  M2UA Overview

The framework architecture that has been defined for SCN signaling
transport over IP [6] uses multiple components, including a signaling
common transport protocol and an adaptation module to support the
services expected by a particular SCN signaling protocol from its
underlying protocol layer.

Within this framework architecture, this document defines a SCN
adaptation module that is suitable for the transport of SS7 MTP2 User
messages.  The only SS7 MTP2 User is MTP3.  The M2UA uses the services
of the Stream Control Transmission Protocol [5] as the underlying
reliable signaling common transport protocol.

In a Signaling Gateway, it is expected that the SS7 MTP2-User signaling
is transmitted and received from the PSTN over a standard SS7 network
interface, using the SS7 Message Transfer Part Level 1 and Level 2 [3,4]
to provide reliable transport of the MTP3-User signaling messages to and
from an SS7 Signaling End Point (SEP) or Signaling Transfer Point (STP).
The SG then provides a functional inter-working of transport functions
with the IP transport, in order to transfer the MTP2-User signaling
messages to and from an Application Server Process where the peer MTP2-
User protocol layer exists.

1.3.1  Example - SG to MGC

In a Signaling Gateway, it is expected that the SS7 signaling is
received over a standard SS7 network termination, using the SS7 Message
Transfer Part (MTP) to provide transport of SS7 signaling messages to
and from an SS7 Signaling End Point (SEP) or SS7 Signaling Transfer
Point (STP).  In other words, the SG acts as a Signaling Link Terminal
(SLT) [2].  The SG then provides interworking of transport functions
with IP Signaling Transport, in order to transport the MTP3 signaling
messages to the MGC where the peer MTP3 protocol layer exists, as shown
below:








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    ******    SS7    ******      IP     *******
    *SEP *-----------* SG *-------------* MGC *
    ******           ******             *******

    +----+                              +----+
    |S7UP|                              |S7UP|
    +----+                              +----+
    |MTP +                              |MTP |
    | L3 |            (NIF)             |L3  |
    +----+         +----+----+          +----+
    |MTP |         |MTP |M2UA|          |M2UA|
    |    |         |    +----+          +----+
    |L2  |         |L2  |SCTP|          |SCTP|
    |L1  |         |L1  +----+          +----+
    |    |         |    |IP  |          |IP  |
    +----+         +---------+          +----+

    NIF  - Nodal Interworking Function
    SEP  - SS7 Signaling Endpoint
    IP   - Internet Protocol
    SCTP - Stream Control Transmission Protocol
           (Refer to Reference [5])

            Figure 1  M2UA in the SG to MGC Application

Note: STPs MAY be present in the SS7 path between the SEP and the SG.

It is recommended that the M2UA use the services of the Stream
Control Transmission Protocol (SCTP) as the underlying reliable
common signaling transport protocol.  The use of SCTP provides
the following features:

  - explicit packet-oriented delivery (not stream-oriented)
  - sequenced delivery of user messages within multiple streams,
    with an option for order-of-arrival delivery of individual
    user messages,
  - optional multiplexing of user messages into SCTP datagrams,
  - network-level fault tolerance through support of multi-homing
    at either or both ends of an association,
  - resistance to flooding and masquerade attacks, and
  - data segmentation to conform to discovered path MTU size

There are scenarios without redundancy requirements and
scenarios in which redundancy is supported below the transport
layer.  In these cases, the SCTP functions above MAY NOT be a
requirement and TCP can be used as the underlying common
transport protocol.

1.3.2  Support for the management of SCTP associations between the SG
       and ASPs

The M2UA layer at the SG maintains the availability state of all
dynamically registered remote ASPs, in order to manage the SCTP
Associations and the traffic between the SG and ASPs.  As well, the
active/inactive state of remote ASP(s) are also maintained.  Active
ASPs are those currently receiving traffic from the SG.

The M2UA layer MAY be instructed by local management to establish an
SCTP association to a peer M2UA node.  This can be achieved using the M-
SCTP ESTABLISH primitive to request, indicate and confirm the
establishment of an SCTP association with a peer M2UA node.

The M2UA layer MAY also need to inform local management of the status of
the underlying SCTP associations using the M-SCTP STATUS request and
indication primitive. For example, the M2UA MAY inform local management
of the reason for the release of an SCTP association, determined either
locally within the M2UA layer or by a primitive from the SCTP.

1.3.3 Signaling Network Architecture

A Signaling Gateway will support the transport of MTP2-User signaling
traffic received from the SS7 network to one or more distributed ASPs
(e.g., MGCs).  Clearly, the M2UA protocol description cannot in itself
meet any performance and reliability requirements for such transport.
A physical network architecture is required, with data on the
availability and transfer performance of the physical nodes involved in
any particular exchange of information.  However, the M2UA protocol MUST
be flexible enough allow its operation and management in a variety of
physical configurations that will enable Network Operators to meet
their performance and reliability requirements.

To meet the stringent SS7 signaling reliability and performance
requirements for carrier grade networks, these Network Operators SHOULD
ensure that there is no single point of failure provisioned in the end-
to-end network architecture between an SS7 node and an IP ASP.

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

An example physical network architecture relevant to carrier-grade
operation in the IP network domain is shown in Figure 2 below:




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

                    Figure 2 - Physical Model Example


For carrier grade networks, Operators SHOULD ensure that under failure
or isolation of a particular ASP, stable calls or transactions are not
lost.  This implies that ASPs need, in some cases, to share the call/-
transaction state or be able to pass the call/transaction state between
each other.  Also, in the case of ASPs performing call processing,
coordination MAY be required with the related Media Gateway to transfer
the MGC control for a particular trunk termination.  However, this
sharing or communication is outside the scope of this document.

1.3.4  ASP Fail-over Model and Terminology

The M2UA layer supports ASP fail-over functions in order to support a
high availability of call and transaction processing capability.  All
MTP2-User messages incoming to an SG from the SS7 network are assigned
to a unique Application Server, based on the Interface Identifier of
the message.

The Application Server is in practical terms a list of all ASPs currently
registered to process MTP2-User messages from certain Interface
Identifiers.  One or more ASPs in the list are normally active (i.e.,
handling traffic) while any others MAY be unavailable or inactive, to be
possibly used in the event of failure or unavailability of the active
ASP(s).

The fail-over model supports an n+k redundancy model, where n ASPs
is the minimum number of redundant ASPs required to handle traffic and
k ASPs are available to take over for a failed or unavailable ASP.
Note that 1+1 active/standby redundancy is a subset of this model.
A simplex 1+0 model is also supported as a subset, with no ASP
redundancy.



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Internet Draft         SS7 MTP2 User Adaptation Layer           Nov 2000


To avoid a single point of failure, it is recommended that a minimum of
two ASPs be in the list, resident in separate hosts  and therefore
available over different SCTP Associations.  For example, in the
network shown in Figure 2, all messages to DPC x could be sent to ASP1
in Host1 or ASP1 in Host2.  The AS list at SG1 might look like this:

    Interface Identiers - Application Server #1
        ASP1/Host1  - State = Active
        ASP1/Host2  - State = Inactive

In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
message for the Interface Identifiers registered.  ASP1 in Host2 would
normally be brought to the active state upon failure of, or loss of
connectivity to, ASP1/Host1.  In this example, both ASPs are Inactive
or Active, meaning that the related SCTP association and far-end M2UA
peer is ready.

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

1.3.5 Client/Server Model

It is recommended that the SG and ASP be able to support both client
and server operation.  The peer endpoints using M2UA SHOULD be
configured so that one always takes on the role of client and the
other the role of server for initiating SCTP associations.  The
default orientation would be for the SG to take on the role of server
while the ASP is the client.  In this case, ASPs SHOULD initiate the
SCTP association to the SG.

The SCTP (and UDP/TCP) Registered User Port Number Assignment for M2UA
is 2904.

1.4  Services Provided by the M2UA Adaptation Layer

The SS7 MTP3/MTP2(MTP2-User) interface is retained at the termination
point in the IP network, so that the M2UA protocol layer is required to
provide the equivalent set of services to its users as provided by the
MTP Level 2 to MTP Level 3.

This includes the following services:

1.4.1  Support for MTP Level 2 / MTP Level 3 interface boundary

Also provision is made for protocol elements that enable a seamless, or
as seamless as possible, operation of the MTP2-User peers in the SS7 and
IP domains.  This includes

Data

Provides the ability to transport MTP2 User information (in this case,
MTP Level 3 PDUs).

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Link Establish

Provides the ability to request MTP Level 2 to bring SS7 links
in-service.

Link Release

Provides the ability to request MTP Level 2 to take SS7 links out-of-
service.  Also, provides mechanism for MTP2 to autonomously indicate
that SS7 link(s) have gone out-of-service.

Link State

Provides the ability to request state change or information on a
per link basis.  Some examples would be the forcing of Local Processor
Outage or flushing buffers.

Link Status

Provides a means for asynchronous notification of link state changes to
be reported to the upper layer (MTP Level 3).  An example would be the
reporting of remote processor outage event.

Data Retrieval

Provides a mechanism to perform SS7 link changeover procedure in
the case of a SS7 link failure.

1.4.2  Support for communication between Layer Management modules
       on SG and MGC

It is envisioned that the M2UA layer needs to provide some messages that
will facilitate communication between Layer Management modules on the SG
and MGC. These primitives are shown below:

To facilitate reporting of errors that arise because of backhauling MTP
Level 3 scenario, the following primitive is defined:

M-ERROR

The M-ERROR message is used to indicate an error with a received
M2UA message (e.g., interface identifier value is not known to the SG).

1.4.3  Support for management of active associations between SG and MGC

The M2UA layer on the SG keeps the state of various ASPs it is associated
with.  A set of primitives between M2UA layer and the Layer Management
are defined below to help the Layer Management manage the association(s)
between the SG and the MGC. The M2UA layer can be instructed
by the Layer Management to establish a SCTP association to a peer M2UA
node.  This procedure can be achieved using the M-SCTP ESTABLISH
primitive.

M-SCTP ESTABLISH

The M-SCTP ESTABLISH primitive is used to request, indicate and confirm
the establishment of a SCTP association to a peer M2UA node.

M-SCTP RELEASE

The M-SCTP RELEASE primitives are used to request, indicate, and
confirm the release of a SCTP association to a peer M2UA node.

The M2UA layer MAY also need to inform the status of the SCTP
associations to the Layer Management.  This can be achieved using
the M-SCTP STATUS primitive.

The M2UA layer MAY also need to inform the status of the SCTP
association(s) to the Layer Management.  This can be achieved using
the following primitive.

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M-SCTP STATUS

The M-SCTP STATUS primitive is used to request and indicate the status
of underlying SCTP association(s).

The Layer Management MAY need to inform the M2UA layer of an AS/ASP
status (i.e., failure, active, etc.), so that messages can be exchanged
between M2UA layer peers to stop traffic to the local M2UA user.  This
can be achieved using the following primitive.

M-ASP STATUS

The ASP status is stored inside M2UA layer on both the SG and MGC
sides.  The M-ASP STATUS primitive can be used by Layer Management to
request the status of the Application Server Process from the M2UA
layer.  This primitive can also be used to indicate the status of the
Application Server Process.

M-ASP MODIFY

The M-ASP MODIFY primitive can be used by Layer Management to modify
the status of the Application Server Process.  In other words, the
Layer Management on the ASP side uses this primitive to initiate
the ASPM procedures.

M-AS STATUS

The M-AS STATUS primitive can be used by Layer Management to request
the status of the Application Server.  This primitive can also be
used to indicate the status of the Application Server.

1.5  Functions Provided by the M2UA Layer

1.5.1  Mapping

The M2UA layer MUST maintain a map of a Interface ID to a physical
interface on the Signaling Gateway.  A physical interface would be a
V.35 line, T1 line/timeslot, E1 line/timeslot, etc.   The M2UA layer
MUST also maintain a map of Interface Identifier to SCTP association
and to the related stream within the association.

The SG maps an Interface Identifier to an SCTP association/stream
only when an ASP sends an ASP Active message for a particular Interface
Identifier.  It MUST be noted, however, that this mapping is dynamic
and could change at any time due to a change of ASP state.  This mapping
could even temporarily be invalid, for example during failover of one
ASP to another.  Therefore, the SG MUST maintain the states of AS/ASP
and reference them during the routing of an messages to an AS/ASP.

An example of the logical view of relationship between SS7 link,
Interface Identifier, AS and ASP in the SG is shown below:

            /---------------------------------------------------+
           /   /------------------------------------------------|--+
          /   /                                                 v  |
         /   /    +----+             act+-----+    +-------+ -+--+-|+--+-
SS7 link1-------->|IID |-+          +-->| ASP |--->| Assoc |       v
           /      +----+ |  +----+  |   +-----+    +-------+ -+--+--+--+-
          /              +->| AS |--+                        Streams
         /        +----+ |  +----+   stb+-----+
SS7 link2-------->|IID |-+              | ASP |
                  +----+                +-----+

where IID = Interface Identifier

1.5.2 Status of ASPs

The M2UA layer on the SG MUST maintain the state of the ASPs it is
supporting.  The state of an ASP changes because of reception of
peer-to-peer messages (ASPM messages as described in Section 3.3.2)
or reception of indications from the local SCTP association.  ASP
state transition procedures are described in Section 4.3.1.

At a SG, an Application Server list MAY contain active and inactive
ASPs to support ASP fail-over procedures.  When, for example, both
a primary and a back-up ASP are available, M2UA peer protocol is
required to control which ASP is currently active.  The ordered
list of ASPs within a logical Application Server is kept updated in
the SG to reflect the active Application Server Process(es).

Also the M2UA layer MAY need to inform the local management of the
change in status of an ASP or AS.  This can be achieved using the M-ASP
STATUS or M-AS STATUS primitives.

1.5.3  SCTP Stream Management

SCTP allows user specified number of streams to be opened during the
initialization.  It is the responsibility of the M2UA layer to ensure
proper management of these streams.  Because of the unidirectional
nature of streams, M2UA layers are not aware of the stream information
from the peer M2UA layers.  Instead, the Interface Identifier is
in the M2UA message header.

The use of SCTP streams within M2UA is recommended in order to minimize
transmission and buffering delay, therefore improving the overall
performance and reliability of the signaling elements.  It is
recommended that a separate SCTP stream is used for each SS7 link.

1.5.4  Seamless SS7 Network Management Interworking

The M2UA layer on the SG SHOULD pass an indication of unavailability of
the M2UA-User (MTP3) to the local Layer Management, if the currently
active ASP moves from the ACTIVE state.  If the AS moves to the DOWN
state while SS7 links are in-service, the SG SHOULD follow the MTP 2
processor outage procedures [2].

1.5.5  Flow Control / Congestion

It is possible for the M2UA layer to be informed of IP network congestion
by means of an implementation-dependent function  (i.e. an indication
from the SCTP).  If the M2UA layer receives this indication, the action(s)
taken are implementation dependent.




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1.6  Definition of the M2UA Boundaries

1.6.1  Definition of the M2UA / MTP Level 3 boundary

DATA
ESTABLISH
RELEASE
STATE
STATUS
RETRIEVAL
DATA RETRIEVAL
DATA RETRIEVAL COMPLETE

1.6.2  Definition of the M2UA / MTP Level 2 boundary

DATA
ESTABLISH
RELEASE
STATE
STATUS
RETRIEVAL
DATA RETRIEVAL
DATA RETRIEVAL COMPLETE

1.6.3  Definition of the Lower Layer Boundary between M2UA and SCTP

The upper layer and layer management primitives provided by SCTP are
provided in Reference [5] Section 9.

1.6.4  Definition of Layer Management / M2UA Boundary

   M-SCTP ESTABLISH request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to establish an SCTP association with an SG.

   M-STCP ESTABLISH confirm
   Direction: M2UA -> LM
   Purpose: ASP confirms to LM that it has established an SCTP
            association with an SG.

   M-SCTP ESTABLISH indication
   Direction: M2UA -> LM
   Purpose: SG informs LM that an ASP has established an SCTP
            association.

   M-SCTP RELEASE request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to release an SCTP association with SG.

   M-SCTP RELEASE confirm
   Direction: M2UA -> LM
   Purpose: ASP confirms to LM that it has released SCTP association
            with SG.

   M-SCTP RELEASE indication
   Direction: M2UA -> LM
   Purpose: SG or IPSP informs LM that ASP has released an SCTP
            association.

   M-SCTP STATUS request
   Direction: LM -> M2UA
   Purpose: LM requests M2UA to report status of SCTP association.

   M-SCTP STATUS indication
   Direction: M2UA -> LM
   Purpose: M2UA reports status of SCTP association.

   M-ASP STATUS request
   Direction: LM -> M2UA
   Purpose: LM requests SG to report status of remote ASP.

   M-ASP STATUS indication
   Direction: M2UA -> LM
   Purpose: SG reports status of remote ASP.

   M-AS-STATUS request
   Direction: LM -> M2UA
   Purpose: LM requests SG to report status of AS.

   M-AS-STATUS indication
   Direction: M2UA -> LM
   Purpose: SG reports status of AS.

   M-NOTIFY indication
   Direction: M2UA -> LM
   Purpose: ASP reports that it has received a NOTIFY message
            from its peer.

   M-ERROR indication
   Direction: M2UA -> LM
   Purpose: ASP or SG reports that it has received an ERROR
            message from its peer.

   M-ASP-UP request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to start its operation and send an ASP UP
            message to the SG.

   M-ASP-UP confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that is has received an ASP UP Acknowledgement
            message from the SG.

   M-ASP-DOWN request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to stop its operation and send an ASP DOWN
            message to the SG.

   M-ASP-DOWN confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that is has received an ASP DOWN Acknowledgement
            message from the SG.

   M-ASP-ACTIVE request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to send an ASP ACTIVE message to the SG.

   M-ASP-ACTIVE confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that is has received an ASP ACTIVE Acknowledgement
            message from the SG.

   M-ASP-INACTIVE request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to send an ASP INACTIVE message to the SG.

   M-ASP-INACTIVE confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that is has received an ASP INACTIVE Acknowledgement
            message from the SG.

2.0 Conventions

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD
NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear
in this document, are to be interpreted as described in [RFC2119].

3.0  Protocol Elements

This section describes the format of various messages used in this
protocol.

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3.1  Common Message Header

The protocol messages for MTP2-User Adaptation require a message
structure which contains a version, message class, message type, message
length, and message contents.   This message header is common among all
signaling protocol adaptation layers:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Version    |     Spare     | Message Class | Message Type  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Message Length                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3  Common Message Header

All fields in an M2UA message MUST be transmitted in the network byte
order, unless otherwise stated.

3.1.1  Version

The version field (vers) contains the version of the M2UA adapation
layer.  The supported versions are:

       Value    Version
       -----    -------
         1      Release 1.0

3.1.2  Message Type

The following List contains the valid Message Classes:

   Message Class: 8 bits (unsigned integer)

         0        Management (MGMT) Message
         1        Reserved
         2        Reserved
         3        ASP State Maintenance (ASPSM) Messages
         4        ASP Traffic Maintenance (ASPTM) Messages
         5        Reserved
         6        MTP2 User Adaptation (MAUP) Messages
                  Messages
      7 to 255    Reserved

The following list contains the message types for the defined messages.

     MTP2 User Adaptatation (MAUP) Messages

         0        Reserved
         1        Data
         2        Establish Request
         3        Establish Confirm
         4        Release Request
         5        Release Confirm
         6        Release Indication
         7        State Request
         8        State Confirm
         9        State Indication
        10        Data Retrieval Request
        11        Data Retrieval Confirm
        12        Data Retrieval Indication
        13        Data Retrieval Complete Indication
     14 to 255    Reserved for MAUP Messages


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     Application Server Process State Maintenance (ASPSM) messages

         0        Reserved
         1        ASP Up (UP)
         2        ASP Down (DOWN)
         3        Heartbeat (BEAT)
         4        ASP Up Ack (UP ACK)
         5        ASP Down Ack (DOWN ACK)
         6        Heatbeat Ack (BEAT ACK)
       7 to 255   Reserved for ASPSM Messages

     Application Server Process Traffic Maintenance (ASTM) messages

         0        Reserved
         1        ASP Active (ACTIVE)
         2        ASP Inactive (INACTIVE)
         3        ASP Active Ack (ACTIVE ACK)
         4        ASP Inactive Ack (INACTIVE ACK)
       5 to 255   Reserved for ASPTM Messages

     Management (MGMT) Messages

         0        Error (ERR)
         1        Notify (NTFY)
       2 to 255   Reserved for Management Messages

3.1.3  Reserved

The Reserved field is 8-bits.  It SHOULD be set to all '0's and
ignored by the receiver.

3.1.4  Message Length

The Message length defines the length of the message in octets,
including the header.

3.1.5 Variable-Length Parameter Format

M2UA messages consist of a Common Header followed by zero or more
variable-length parameters, as defined by the message type.  The
variable-length parameters contained in a message are defined in a
Tag-Length-Value format as shown below.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Parameter Tag        |       Parameter Length        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  \                                                               \
  /                       Parameter Value                         /
  \                                                               \
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Parameter Tag: 16 bits (unsigned integer)

The Type field is a 16 bit identifier of the type of parameter.  It takes
a value of 0 to 65534.

The value of 65535 is reserved for IETF-defined extensions.  Values
other than those defined in specific parameter description are reserved
for use by the IETF.

Parameter Length: 16 bits (unsigned integer)

The Parameter Length field contains the size of the parameter in bytes,
including the Parameter Tag, Parameter Length, and Parameter Value
fields.  The Parameter Length does not include any padding bytes.

Parameter Value: variable-length.

The Parameter Value field contains the actual information to be
transferred in the parameter.

The total length of a parameter (including Tag, Parameter Length and Value
fields) MUST be a multiple of 4 bytes.  If the length of the parameter is
not a multiple of 4 bytes, the sender pads the Parameter at the end (i.e.,
after the Parameter Value field) with all zero bytes.  The length of the
padding is NOT included in the parameter length field.  A sender SHOULD
NOT pad with more than 3 bytes. The receiver MUST ignore the padding
bytes.

3.2  M2UA Message Header

In addition to the common message header, there will be a M2UA specific
message header.  The M2UA specific message header will immediately
follow the common message header, but will only be used with MAUP
messages.

This message header will contain the Interface Identifier.  The
Interface Identifier identifies the physical interface at the SG for
which the signaling messages are sent/received. The format of the
Interface Identifier parameter can be text or integer, the values of which
are assigned according to network operator policy.  The values used are of
local significance only, coordinated between the SG and ASP.

The integer formatted Interface Identifier MUST be supported.  The
text formatted Interface Identifier MAY optionally be supported.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x1)           |           Length=8            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier (integer)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4  M2UA Message Header (Integer-based Interface Identifier)


The Tag value for Integer-based Interface Identifier is 0x1.  The length is
always set to a value of 8.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x3)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                   Interface Identifier (text)                 |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 5  M2UA Message Header (Text-based Interface Identifier)

The Tag value for the Text-based Interface Identifier is 0x3.  The
length is variable.


3.3 M2UA Messages

The following section defines the messages and parameter contents.  The
M2UA messages will use the common message header (Figure 3) and the
M2UA message header (Figure 4).

3.3.1 MTP2 User Adaptation Messages

3.3.1.1 Data

The Data message contains an SS7 MTP2-User Protocol Data Unit (PDU).  The
Data message contains the protocol data.









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The format for the Data Message parameters is as follows:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                       Protocol Data                           |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The Protocol Data field contains the MTP2-User application message in
network byte order starting with the Signaling Information Octet (SIO).

3.3.1.2  Establish (Request, Confirmation)

The Establish Request message is used to establish the link or to
indicate that the channel has been established.  The MGC controls the state of the
SS7 link.  When the MGC desires the SS7 link to be in-service,
it will send the Establish Request message.  Note that the gateway
MAY already have the SS7 link established at its layer.  If so, upon
receipt of an Establish Request, the gateway takes no action except to
send an Establish Confirm.

When the MGC sends an M2UA Establish Request message, the MGC MAY
start a timer.  This timer would be stopped upon receipt of an M2UA
Establish Confirm.  If the timer expires, the MGC would re-send the
M2UA Establish Request message and restart the timer.  In other words,
the MGC MAY continue to request the establishment of the data link
on periodic basis until the desired state is achieved or take some
other action (notify the Management Layer).

The mode (Normal of Emergency) for bringing the link in service is
defaulted to Normal.  The State Request (described in Section 3.3.1.4
below) can be used to change the mode to Emergency.

3.3.1.3  Release (Request, Indication, Confirmation)

This Release Request message is used to release the channel.  The
Release Confirm and Indication messages are used to indicate that the
channel has been released.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Reason                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The valid values for Reason are shown in the following table.

      Define     Value           Description
   RELEASE_MGMT   0x0     Management layer generated release
   RELEASE_PHYS   0x1     Physical layer alarm generated release
   RELEASE_SIOS   0x2     Receipt of SIOS
   RELEASE_T6     0x3     Release due to expiration of Timer T6
   RELEASE_T7     0x4     Release due to expiration of Timer T7
   RELEASE_BSN    0x5     Release due to invalid BSN (2 of 3)
   RELEASE_FIB    0x6     Release due to invalid FIB (2 of 3)
   RELEASE_SUERM  0x7     Release due to failure reported by SUERM
   RELEASE_IAC    0x8     Release due to initial alignment failed
   RELEASE_OTHER  0x9     Other reason SS7 link out-of-service

For the Release Request, the only valid Reason is RELEASE_MGMT.  For
Release Confirm, the Reason field would also be RELEASE_MGMT (a
reflection of Release Request).  The other valid values for Reason
would be used for Release Indication.

3.3.1.4  State (Request, Confirm)

The State Request message can be sent from a MGC to cause an action
on a particular SS7 link supported by the Signaling Gateway.  The
gateway sends a State Confirm to the MGC if the action has been success-
fully completed.  The State Confirm reflects that state value received
in the State Request message.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             State                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The valid values for State are shown in the following table.

         Define           Value        Description
   STATUS_LPO_SET          0x0      Request local processor outage
   STATUS_LPO_CLEAR        0x1      Request local processor outage
                                    recovered
   STATUS_EMER_SET         0x2      Request emergency alignment
                                    procedure
   STATUS_EMER_CLEAR       0x3      Request normal alignment (cancel
                                    emergency) procedure
   STATUS_FLUSH_BUFFERS    0x4      Flush transmit and retransmit
                                    buffers
   STATUS_CONTINUE         0x5      Continue

3.3.1.5  State Indication

The MTP2 State Indication message can be sent from a gateway to an
ASP to indicate a condition on a link.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             State                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The valid values for State are shown in the following table.

       Define            Value          Description
   EVENT_ENTER_LPO        0x0      Entered local processor outage
   EVENT_EXIT_LPO         0x1      Exited local processor outage
   EVENT_ENTER_CONG       0x2      Entered a congested state
   EVENT_EXIT_CONG        0x3      Exited a congested state
   EVENT_PHYS_UP          0x4      Physical interface up
   EVENT_PHYS_DOWN        0x5      Physical interface down
   EVENT_PROTOCOL_ERR     0x6      Protocol error occurred
   EVENT_REM_ENTER_CONG   0xc      Remote entered congestion
   EVENT_REM_EXIT_CONG    0xd      Remote exited congestion
   EVENT_REM_ENTER_PO     0xe      Remote entered processor outage
   EVENT_REM_EXIT_PO      0xf      Remote exited processor outage

3.3.1.6  Retrieval (Request, Confirm)

The MTP2 Retrieval Request message is used during the MTP Level 3
changeover procedure to request the BSN, to retrieve PDUs from the
retransmit queue or to flush PDUs from the retransmit queue.




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Action                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            fsn_bsn                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The valid values for Action are shown in the following table.

        Define         Value       Description
   ACTION_RTRV_BSN      0x1     Retrieve the backward sequence number
   ACTION_RTRV_MSGS     0x2     Retrieve the PDUs from the retransmit
                                queue
   ACTION_DROP_MSGS     0x3     Drop the PDUs in the retransmit queue

In the Retrieval Request message, the fsn_bsn field SHOULD be ignored if
the Action field is ACTION_RTRV_BSN or ACTION_DROP_MSGS.  The fsn_bsn
field contains the FSN of the far end if the Action is ACTION_RTRV_MSGS.

When the Signaling Gateway sends a Retrieval Confirm to this request,
it echos the action and puts the BSN in the fsn_bsn field if the action
was ACTION_RTRV_BSN.  If there is a failure in retrieving the BSN, the
fsn_bsn SHOULD contain a -1 (0xffffffff).  For a Retrieval Confirm with
Action of ACTION_RTRV_MSGS or ACTION_DROP_MSGS, the value received in
the fsn_bsn field in the Request message will be sent.

3.3.1.7  Retrieval Indication

The Retrieval Indication message is sent by the Signaling Gateway
with a PDU from the retransmit queue.  The Retrieval Indication
message does not contain the Action or fsn_bsn fields, just a MTP3
Protocol Data Unit (PDU) from the retransmit queue.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     PDU from retransmit queue                 |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


3.3.1.8  Retrieval Complete Indication

The MTP2 Retrieval Complete Indication message is exactly the same as
the MTP2 Retrieval Indication message except that it also indicates that
it contains the last PDU from the retransmit queue.

3.3.2  Application Server Process Maintenance (ASPM) Messages

The ASPM messages will only use the common message header.

3.3.2.1  ASP UP (ASPUP)

The ASP UP (ASPUP) message is used to indicate to a remote M2UA peer
that the Adaptation layer is ready to receive traffic or maintenance
messages.

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The ASPUP message contains the following parameters

     Info String (optional)

The format for ASPUP Message parameters is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The optional INFO String parameter can carry any meaningful 8-bit ASCII
character string along with the message.  Length of the INFO String
parameter is from 0 to 255 characters.  No procedures are presently
identified for its use but the INFO String MAY be used for debugging
purposes.


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3.3.2.2 ASP Up Ack

The ASP UP Ack message is used to acknowledge an ASP Up message received
from a remote M2UA peer.

The ASPUP Ack message contains the following parameters:

     INFO String (optional)

The format for ASPUP Ack Message parameters is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x2)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                 Adaptation Layer Identifier*                  |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional Info String parameter is the
same as for the ASP UP message (See Section 3.3.2.1.)

3.3.2.3  ASP Down (ASPDN)

The ASP Down (ASPDN) message is used to indicate to a remote M2UA peer
that the adaptation layer is not ready to receive traffic or
maintenance messages.

The ASPDN message contains the following parameters

     Reason
     INFO String (Optional)

The format for the ASPDN message parameters is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Reason                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                         INFO String*                          |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional Info String parameter is the
same as for the ASP Up message (See Section 3.3.2.1.)

The Reason parameter indicates the reason that the remote M2UA
adaptation layer is unavailable.  The valid values for Reason are shown
in the following table.

     Value         Description
     0x1           Management

3.3.2.4 ASP Down Ack

The ASP Down Ack message is used to acknowledge an ASP Down message
received from a remote M2UA peer.

The ASP Down Ack message contains the following parameters:

     Reason
     INFO String (Optional)

The format for the ASPDN Ack message parameters is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Reason                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                         INFO String*                          |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional Info String parameter is the
same as for the ASP UP message (See Section 3.3.2.1.)

The format of the Reason parameter is the same as for the ASP Down message
(See Section 3.3.2.3).

3.3.2.5  ASP Active (ASPAC)

The ASPAC message is sent by an ASP to indicate to an SG that it is
Active and ready to be used.

The ASPAC message contains the following parameters

     Traffic Mode Type (Mandatory)
     Interface Identifier (Optional)
        - Combination of integer and integer ranges, OR
        - string (text formatted)
     INFO String (Optional)

The format for the ASPAC message using integer formatted Interface
Identifiers is as follows:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Tag (0x1=integer)         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifiers*                    |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Tag (0x8=integer range)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                            .
     .                                                            .
     .                                                            .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier StartN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier StopN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                    of Tag Type 0x1 or 0x8                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The format for the ASPAC message using text formatted (string)
Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifier*                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                        of Tag Type 0x3                        |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The Traffic Mode Type parameter identifies the traffic mode of
operation of the ASP within an AS. The valid values for Type are
shown in the following table:

    Value          Description
     0x1            Over-ride

Within a particular Interface Identifier, only one Type can be used.
The Over-ride value indicates that the ASP is operating in Over-ride
mode, where the ASP takes over all traffic in an Application Server
(i.e., primary/back-up operation), over-riding any currently active
ASPs in the AS.


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The optional Interface Identifiers parameter contains a list of
Interface Identifier integers (Type 0x1 or Type 0x8) or text strings
(Type 0x3)indexing the Application Server traffic that the sending
ASP is configured/registered to receive.  If integer formatted
Interface Identifiers are being used, the ASP can also send ranges of
Interface Identifiers (Type 0x8).  Interface Identifier types Integer
(0x1) and Integer Range (0x8) are allowed in the same message.  Text
formatted Interface Identifiers (0x3) cannot be used with either
Integer (0x1) or Integer Range (0x8) types.

If no Interface Identifiers are included, the message is for all
provisioned Interface Identifiers within the AS.  If only a subset
of Interface Identifiers are includes, the ASP is noted as Active
for all the Interface Identifiers provisioned for the AS.

Note:  If the optional Interface Identifier parameter is present, the
integer formatted Interface Identifier MUST be supported, while the
text formatted Interface Identifier MAY be supported.

An SG that receives an ASPAC with an incorrect Traffic Mode Type for
a particular Interface Identifier will respond with an Error Message
(Cause: Unsupported Traffic Handling Mode).

The format and description of the optional Info String parameter is the
same as for the ASP UP message (See Section 3.3.2.1.).

3.3.2.6 ASP Active Ack

The ASPAC Ack message is used to acknowledge an ASP-Active message
received from a remote M2UA peer.

The ASPAC Ack message contains the following parameters:

     Traffic Mode Type (Mandatory)
     Interface Identifier (Optional)
        - Combination of integer and integer ranges, OR
        - string (text formatted)
     INFO String (Optional)

The format for the ASPAC Ack message with Integer-formatted Interface
Identifiers is as follows:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Traffic Mode Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Tag (0x1=integer)         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifiers*                    |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Tag (0x8=integer range)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                            .
    .                                                            .
    .                                                            .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier StartN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier StopN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                    of Tag Type 0x1 or 0x8                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format for the ASP Active Ack message using text formatted (string)
Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifier*                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                        of Tag Type 0x3                        |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The format and description of the optional Info String parameter is the
same as for the ASP UP message (See Section 3.3.2.1.)

The format of the Type and Interface Identifier parameters is the same
as for the ASP Active message (See Section 3.3.2.5).

3.3.2.7  ASP Inactive (ASPIA)

The ASPIA message is sent by an ASP to indicate to an SG that it is no
longer an active ASP to be used from within a list of ASPs.  The SG will
respond with an ASPIA Ack message and either discard incoming messages
or buffer for a timed period and then discard.

The ASPIA message contains the following parameters

     Traffic Mode Type (Mandatory)
     Interface Identifiers (Optional)
        - Combination of integer and integer ranges, OR
        - string (text formatted)
     INFO String (Optional)

The format for the ASP Inactive message parameters using Integer
formatted Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Tag (0x1=integer)         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifiers*                    |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Tag (0x8=integer range)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                            .
    .                                                            .
    .                                                            .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier StartN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier StopN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                    of Tag Type 0x1 or 0x8                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The format for the ASP Inactive message using text formatted (string)
Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifier*                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                        of Tag Type 0x3                        |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The Traffic Mode Type parameter identifies the traffic mode of
operation of the ASP within an AS.  The valid values for Traffic Mode
Type are shown in the following table:

    Value          Description
     0x1            Over-ride

The format and description of the optional Interface Identifiers and
Info String parameters is the same as for the ASP Active message (See
Section 3.3.2.3.)

The optional Interface Identifiers parameter contains a list of
Interface Identifier integers indexing the Application Server traffic
that the sending ASP is configured/registered to receive, but does not
want to receive at this time.

3.3.2.8 ASP Inactive Ack

The ASPIA Ack message is used to acknowledge an ASP-Inactive message
received from a remote M2UA peer.

The ASPIA Ack message contains the following parameters:

     Traffic Mode Type (Mandatory)
     Interface Identifiers (Optional)
        - Combination of integer and integer ranges, OR
        - string (text formatted)
     INFO String (Optional)

The format for the ASPIA Ack message is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Tag (0x1=integer)         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifiers*                    |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Tag (0x8=integer range)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                             .
    .                                                             .
    .                                                             .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier StartN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier StopN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                    of Tag Type 0x1 or 0x8                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format for the ASP Inactive Ack message using text formatted
(string) Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifier*                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                        of Tag Type 0x3                        |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The format of the Traffic Mode Type and Interface Identifier
parameters is the same as for the ASP Inactive message
(See Section 3.3.2.7).

The format and description of the optional Info String parameter is
the same as for the ASP Up message (See Section 3.3.2.1).

3.3.2.9 Heartbeat (BEAT)

The Heartbeat message is optionally used to ensure that the M2UA peers
are still available to each other.  It is recommended for use when
the M2AU runs over a transport layer other than the SCTP, which has its
own heartbeat.

The BEAT message contains the following parameters:

     Heartbeat Data         Optional



The format for the BEAT message is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 9            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   |                       Heartbeat Data *                        |
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Heartbeat Data parameter contents are defined by the sending node.
The Heartbeat Data could include, for example, a Heartbeat Sequence
Number and, or Timestamp.  The receiver of a Heartbeat message does
not process this field as it is only of significance to the sender.
The receiver MUST respond with a Heartbeat Ack message.

3.3.2.10  Heartbeat Ack (BEAT-Ack)

The Heartbeat Ack message is sent in response to a received Heartbeat
message.  It includes all the parameters of the received Heartbeat
message, without any change.


3.3.3  Layer Management (MGMT) Messages

3.3.3.1  Error (ERR)

The Error message is used to notify a peer of an error event
associated with an incoming message.  For example, the message type
might be unexpected given the current state, or a parameter value might
be invalid.

The ERR message contains the following parameters:

     Error Code
     Diagnostic Information (optional)

The format for the ERR message is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Error Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x7)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Diagnostic Information*                   |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code parameter indicates the reason for the Error Message.
The Error parameter value can be one of the following values:

     Invalid Version                        0x1
     Invalid Interface Identifier           0x2
     Invalid Adaptation Layer Identifier    0x3
     Invalid Message Type                   0x4
     Unsupported Traffic Handling Mode      0x5
     Unexpected Message                     0x6
     Protocol Error                         0x7
     Invalid Stream Identifier              0x8

The "Invalid Version" error would be sent if a message was
received with an invalid or unsupported version.  The Error message
would contain the supported version in the Common header.  The
Error message could optionally provide the supported version in
the Diagnostic Information area.

The "Invalid Interface Identifier" error would be sent by a SG if
an ASP sends a message with an invalid (unconfigured) Interface
Identifier value.

The "Unsupported Traffic Handling Mode" error would be sent by a SG
if an ASP sends an ASP Active with an unsupported Traffic Handling
Mode.  An example would be a case in which the SG did not support
load-sharing.

The "Unexpected Message" error would be sent by an ASP if it received
a MAUP message from an SG while it was in the Inactive state.

The "Protocol Error" error would be sent for any protocol anomaly
(i.e. a bogus message).

The "Invalid Stream Identifier" error would be sent if a message
was received on an unexpected SCTP stream (i.e.  a stream that did
not have an Interface Identifier associated with it).

The "Unsupported Interface Identifier Type" error would be sent by
a SG if an ASP sends a Text formatted Interface Identifier and the
SG only supports Integer formatted Interface Identifiers.  When
the ASP receives this error, it will need to resend its message with
an Integer formatted Interface Identifier.

The optional Diagnostic information can be any information germain to
the error condition, to assist in identification of the error condition.
In the case of an Invalid Version Error Code the Diagnostic information
includes the supported Version parameter.  In the other cases, the
Diagnostic information MAY be the first 40 bytes of the offending message.

Morneault, et al                                               [Page 19]


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3.3.3.2  Notify (NTFY)

The Notify message used to provide an autonomous indication of M2UA
events to an M2UA peer.

The NTFY message contains the following parameters:

     Status Type
     Status Identification
     Interface Identifiers (Optional)
     INFO String (Optional)

The format for the Notify message with Integer-formatted Interface
Identifiers is as follows:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |    Status Identification      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Tag (0x1=integer)         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifiers*                    |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Tag (0x8=integer range)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                             .
    .                                                             .
    .                                                             .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier StartN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier StopN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                    of Tag Type 0x1 or 0x8                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The format for the Notify message with Text-formatted Interface
Identifiers is as follows:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |    Status Identification      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                     Interface Identifier*                     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |              Additional Interface Identifiers                 |
   |                        of Tag Type 0x3                        |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          INFO String*                         |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The Status Type parameter identifies the type of the Notify message.
The following are the valid Status Type values:

      Value          Description
       0x1   Application Server state change (AS_State_Change)
       0x2   Other

The Status Information parameter contains more detailed information for
the notification, based on the value of the Status Type.  If the Status
Type is AS_State_Change the following Status Information values are used:

      Value          Description
       1      Application Server Down (AS_Down)
       2      Application Server Inactive (AS_Inactive)
       3      Application Server Active (AS_Active)
       4      Application Server Pending (AS_Pending)

These notifications are sent from an SG to an ASP upon a change in status
of a particular Application Server.  The value reflects the new state of
the Application Server.

If the Status Type is Other, then the following Status Information values
are defined:

      Value          Description
       1      Insufficient ASP resources active in AS
       2      Alternate ASP Active

This notification is not based on the SG reporting the state change of an
ASP or AS.  In the Insufficient ASP Resources case, the SG is
indicating to an "Inactive" ASP(s) in the AS that another ASP is
required in order to handle the load of the AS (Load-sharing mode).
For the Alternate ASP Active case, an ASP is informed when an alternate
ASP transitions to the ASP-Active state in Over-ride mode.

The format and description of the optional Interface Identifiers and
Info String parameters is the same as for the ASP Active message
(See Section 3.3.2.3.)


Morneault, et al                                               [Page 20]


Internet Draft         SS7 MTP2 User Adaptation Layer           Nov 2000


4.0  Procedures

The M2UA layers needs to respond to various primitives it receives from
other layers as well as messages it receives from the peer-to-peer
messages.  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 M2UA layer's "Transport of MTP Level 2 /
MTP Level 3 boundary" service.

4.1.1  MTP Level 2 / MTP Level 3 Boundary Procedures

On receiving a primitive from the local upper layer, the M2UA layer will
send the corresponding MAUP message (see Section 2) to its peer.  The
M2UA layer MUST fill in 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 SS7 link.

4.1.2  MAUP Message Procedures

On receiving MAUP messages from a peer M2UA layer, the M2UA layer on an
SG or MGC needs to invoke the corresponding layer primitives to the
local MTP Level 2 or MTP Level 3 layer.

Morneault, et al                                               [Page 21]


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4.2  Procedures to Support Service in Section 1.4.2

These procedures achieve the M2UA layer's "Support for Communication
between Layer Managements" service.

4.2.1  Layer Management Primitives Procedure

On receiving these primitives from the local layer, the M2UA layer will
send the corresponding MGMT message (Error) to its peer.  The M2UA layer
MUST fill in the various fields of the common and specific headers
correctly.

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 M2UA 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 M2UA.

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 M2UA Notify or Error message respectively.  These indications
can also be generated based on local M2UA 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 M2UA 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 M2UA 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.

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

4.2.2 MGMT message procedures

Upon receipt of MGMT messages the M2UA layer MUST invoke the corresponding
Layer Management primitives indications (e.g., M-AS Status ind., M-ASP
Status ind., M-ERROR ind...) 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 M2UA
Notify or Error message.  These indications can also be generated
based on local M2UA 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 M2UA layer's "Support for management of
active associations between SG and MGC" service.

4.3.1 State Maintenance

The M2UA layer on the SG maintains the state of each AS, in each
Appliction Server that it is configured to receive traffic.

4.3.1.1  ASP States

The state of the each ASP, in each AS that it is configured, is
maintained in the M2UA layer on the SG. The state of an ASP changes
due to events. The events include

    * Reception of messages from peer M2UA layer at that ASP
    * Reception of some messages from the peer M2UA layer at other
      ASPs in the AS
    * Reception of indications from SCTP layer

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

ASP Down: Application Server Process is unavailable and/or the related
SCTP association is down. Initially all ASPs will be in this state.
An ASP in this state SHOULD NOT not be sent any M2UA messages.

ASP-INACTIVE: The remote M2UA 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-MAUP M2UA messages.

ASP-ACTIVE The remote M2UA peer at the ASP is available and
application traffic is active.


                 Figure 6  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 (M2UA) 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.


Morneault, et al                                               [Page 22]


Internet Draft         SS7 MTP2 User Adaptation Layer           Nov 2000


4.3.1.2  AS States

The state of the AS is maintained in the M2UA layer on the SG.

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

   * 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).

AS-ACTIVE: The Application Server is available and application traffic
is active. This state implies that 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.



                 Figure 7 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


Morneault, et al                                               [Page 23]


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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 M2UA layer at an ASP receives an M-SCTP
ESTABLISH request primitive from the Layer Management, the M2UA layer
will try to establish an SCTP association with the remote M2UA peer at
an SG.  Upon reception of an eventual SCTP-Communication Up confirm
primitive from the SCTP, the M2UA layer will invoke the primitive
M-SCTP ESTABLISH confirm to the Layer Management.

At the SG, the M2UA layer will receive an SCTP Communication Up
indication primitive from the SCTP. The M2UA layer will then invoke
the primitive M-SCTP ESTABLISH indication to the Layer Management.

Once the SCTP association is establishedand assuming that the local
M2UA-User is ready, the local ASP M2UA 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 M2UA layer on SG can communicate the
status of the application server using the M-AS STATUS primitives.
The Layer Managements and the M2UA layers 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 M2UA layer. The M2UA 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 M2UA 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 is used.

4.3.3.1 ASP-Inactive

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 M2UA 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 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-Up Ack message in response to a
received ASP Up message even if the ASP is already marked as "Inactive"
at the SG.

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

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 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 Data
messages before an ASP Up is received, the SG SHOULD discard.


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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 M2UA 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:

    - an ASP Down message is received from the ASP,
    - another ASPM message is received from the ASP and the SG has
      locked out the ASP for management reasons.

The SG sends anASP 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  M2UA 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.

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 MAUP messages before an ASP Active is received, the SG SHOULD
discard these messages.


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There is one mode of Application Server traffic handling in the SG
M2UA - Over-ride.  The Type parameter in the ASPAC messge 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.

For Over-ride mode AS, the 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.

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 Ids to indicate for which Application Servers
the ASPIA applies.

There is one mode of Application Server traffic handling in the SG
M2UA when withdrawing an ASP from service - Over-ride.  The Type
parameter in the ASPIA messge 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" (i.e., in the "Inactive" state).  An ASPIA Ack  message is
sent to the ASP, after ensuring that all traffic is stopped to the ASP.

If no other ASPs are Active in the Application Server, the SG 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 "Down" 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, 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.

4.3.3.6 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 M2UA, upon receiving
a BEAT message from the ASP, responds with a BEAT ACK message.  If no
BEAT message (or any other M2UA message) is received from the ASP
within the timer 2*T(beat), the SG will consider the remote M2UA 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 M2UA 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 M2UA
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".

5.0  Examples of MTP2 User Adaptation (M2UA) Procedures

5.1  Establishment of associations between SG and MGC examples

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

This scenario shows the example M2UA 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 M2UA message flows for the establishment
of traffic between an SG and two ASPs in the same Application Server,
where ASP1 is configured to be ┬čactive├Ě 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/transaction state or can communicate call state under
failure/withdrawal events.


       SG                        ASP1                        ASP2
        |                         |                          |
        |<--------ASP Up----------|                          |
        |-------ASP Up Ack------->|                          |
        |                         |                          |
        |<-----------------------------ASP Up----------------|
        |----------------------------ASP Up Ack------------->|
        |                         |                          |
        |                         |                          |
        |<-------ASP Active-------|                          |
        |-----ASP-Active Ack----->|                          |
        |                         |                          |




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5.2 ASP Traffic Fail-over Examples

5.2.1 (1+1 Sparing, withdrawal of ASP, Back-up Over-ride)

Following on from the example in Section 5.1.2, and ASP withdraws from
service:

       SG                       ASP1                       ASP2
        |                         |                          |
        |<-----ASP Inactive-------|                          |
        |----ASP Inactive Ack---->|                          |
        |--------------------NTFY(AS-Down) (Optional)------->|
        |                         |                          |
        |<------------------------------ 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 M2UA peer (M2UA 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)

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

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


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


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5.3  SG to MGC, MTP Level 2 to MTP Level 3 Boundary Procedures

When the M2UA layer on the ASP has a MAUP 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 SS7 link

  - Fill in the MAUP message, fill in M2UA Message Header, fill in
    Common Header

  - Send the MAUP message to the remote M2UA peer in the SG, over the
    SCTP association

When the M2UA layer on the SG has a MAUP 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 SS7 link

  - Fill in the MAUP message, fill in M2UA Message Header, fill in
    Common Header

  - Send the MAUP message to the remote M2UA peer in the ASP, over the
    SCTP association

5.3.1  SS7 Link Alignment

The MGC can request that a SS7 link be brought into alignment using the
normal or emergency procedure.  An example of the message flow to bring
a SS7 link in-service using the normal alignment procedure is shown
below.

          SG                                    ASP
           |                                     |
           |<---------Establish Req--------------|
           |                                     |
           |----------Establish Cfm------------->|
           |                                     |


An example of the message flow to bring a SS7 link in-service using the
emergency alignment procedure.

          SG                                    ASP
           |                                     |
           |<----State Req (STATUS_EMER_SET)-----|
           |                                     |
           |-----State Cfm (STATUS_EMER_SET)---->|
           |                                     |
           |<---------Establish Req--------------|
           |                                     |
           |----------Establish Cfm------------->|
           |                                     |



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5.3.2  SS7 Link Release

The MGC can request that a SS7 link be taken out-of-service.  It uses
the Release Request message as shown below.

          SG                                    ASP
           |                                     |
           |<-----Release Req (RELEASE_MGMT)-----|
           |                                     |
           |------------Release Cfm------------->|
           |                                     |

The SG can autonomously indicate that a SS7 link has gone out-of-service
as shown below.

          SG                                    ASP
           |                                     |
           |------Release Ind (RELEASE_PHYS)---->|
           |                                     |


5.3.3  Set and Clear Local Processor Outage

The MGC can set a Local Processor Outage condition.  It uses the
State Request message as shown below.

          SG                                    ASP
           |                                     |
           |<-----State Req (STATUS_LPO_SET)-----|
           |                                     |
           |------State Cfm (STATUS_LPO_SET)---->|
           |                                     |

The MGC can clear a Local Processor Outage condition.  It uses the
State Request message as shown below.

          SG                                    ASP
           |                                     |
           |<----State Req (STATUS_LPO_CLEAR)----|
           |                                     |
           |-----State Cfm (STATUS_LPO_CLEAR)--->|
           |                                     |

5.3.4  Notification of Processor Outage (local or remote)

The SG can indicate a Local or Remote Processor Outage condition.  It
uses the State Indication message as shown below.

          SG                                    ASP
           |                                     |
           |-----State Ind (EVENT_ENTER_LPO)---->|
           |                                     |
           |-----State Ind (EVENT_EXIT_LPO)----->|
           |                                     |

          SG                                    ASP
           |                                     |
           |-----State Ind (EVENT_ENTER_RPO)---->|
           |                                     |
           |-----State Ind (EVENT_EXIT_RPO)----->|
           |                                     |


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5.3.5  SS7 Link Changeover

An example of the message flow for a changeover is shown below.  In this
example, there were three messages in the retransmission queue that
needed to be retrieved.

           SG                                    ASP
           |                                     |
           |<---Retrieval Req (MTP2_RTRV_BSN)----|
           |                                     |
           |------Retrieval Cfm (with BSN)------>|
           |                                     |
           |<--Retrieval Req (MTP2_RTRV_MSGS)----|
           |                      with FSN       |
           |                                     |
           |-----------Retrieval Cfm------------>|
           |                                     |
           |-----------Retrieval Ind------------>|
           |-----------Retrieval Ind------------>|
           |-------Retrieval Complete Ind------->|
           |                                     |

Note:  The number of Retrieval Indication is dependent on the number of
messages in the retransmit queue that have been requested.  Only one
Retrieval Complete Indication SHOULD be sent.

6.0 Security

M2UA is designed to carry signaling messages for telephony services. As such,
M2UA 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, M2UA has the following security objectives:

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

M2UA runs on top of SCTP.  SCTP [5] provides certain transport related
security features, such as:

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

When M2UA 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" [8]
SHOULD be consulted for guidance.

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When the network in which M2UA 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 [9] 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

A request will be made to IANA to assign an M2UA value for the Payload
Protocol Identifier in SCTP Payload Data chunk.  The following SCTP Payload
Protocol Identifier will be registered:

        M2UA    0x10

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.

8.0  Acknowledgements

The authors would like to thank John Loughney, Neil Olson, Michael
Tuexen, Nikhil Jain, Steve Lorusso, Dan Brendes, Joe Keller, Heinz
Prantner, Barry Nagelberg, Naoto Makinae for their valuable comments
and suggestions.






















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9.0  References

[1] ITU-T Recommendation Q.700, 'Introduction To ITU-T Signalling
    System No. 7 (SS7)'

[2] ITU-T Recommendation Q.701-Q.705, 'Signalling System No. 7 (SS7) -
    Message Transfer Part (MTP)'

[3] ANSI T1.111 'Signalling System Number 7 - Message Transfer Part'

[4] Bellcore GR-246-CORE 'Bell Communications Research Specification
    of Signaling System Number 7', Volume 1, December 1995

[5] Stream Control Transmission Protocol, RFC 2960, October 2000

[6] Architectural Framework for Signaling Transport, RFC 2719 ,
    October 1999

[7] ITU-T Recommendation Q.2210, 'Message transfer part level 3
    functions and messages using the services of ITU-T
    Recommendation Q.2140'

[8] Site Security Handbook, RFC 2196, September 1997

[9] Security Architecture for the Internet Protocol, RFC 2401





























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10.0  Author's Addresses

Ken Morneault                                     Tel: +1-703-484-3323
Cisco Systems Inc.                           EMail: kmorneau@cisco.com
13615 Dulles Technology Drive
Herndon, VA. 20171
USA

Ram Dantu, Ph.D.                                   Tel +1-469-255-0716
Cisco Systems                                   EMail rdantu@cisco.com
17919 Waterview
Dallas, TX 75252
USA

Malleswar Kalla                                   Tel: +1-973-829-5212
Telcordia Technologies             EMail: kalla@research.telcordia.com
MCC 1J211R
445 South Street
Morristown, NJ 07960
USA

Greg Sidebottom                                   Tel: +1-613-763-7305
Nortel Networks                     EMail: gregside@nortelnetworks.com
3685 Richmond Rd,
Nepean, Ontario
Canada  K2H5B7

Tom George                                        Tel: +1-972-519-3168
Alcatel USA                          EMail: tom.george@usa.alcatel.com
1000 Coit Road
Plano, TX 74075
USA

This Internet Draft expires April 2001.


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