Network Working Group               G. Sidebottom, L. Ong, Guy Mousseau
INTERNET-DRAFT                                          Nortel Networks
                                                             Ian Rytina
                                                               Ericsson
                          Hanns-Juergen Schwarzbauer, Klaus Gradischnig
                                                                Siemens
                                                          Ken Morneault
                                                                  Cisco
                                                          Mallesh Kalla
                                                              Telcordia
                                                         Normand Glaude
                                               Performance Technologies

Expires in six months                                         Sept                                         Nov 2000

                SS7 MTP3-User Adaptation Layer (M3UA)
                  <draft-ietf-sigtran-m3ua-04.txt>
                  <draft-ietf-sigtran-m3ua-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
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Abstract

This Internet Draft defines a protocol for supporting the transport of
any SS7 MTP3-User signalling (e.g., ISUP and SCCP messages) over IP
using the services of the Stream Control Transmission Protocol.  Also,
provision is made for protocol elements that enable a seamless
operation of the MTP3-User peers in the SS7 and IP domains. This
protocol would be used between a Signalling Gateway (SG) and a Media
Gateway Controller (MGC) or IP-resident Database.  It is assumed that
the SG receives SS7 signalling over a standard SS7 interface using the
SS7 Message Transfer Part (MTP) to provide transport.

                        TABLE OF CONTENTS

1. Introduction.......................................................3
    1.1 Scope.........................................................3
    1.2 Terminology...................................................3
    1.3 M3UA Overview.................................................5
    1.4 Functional Areas.............................................10
    1.5 Sample Configurations........................................18
    1.6 Definition of M3UA Boundaries................................21
2. Conventions.......................................................22
3. M3UA Protocol Elements............................................22
    3.1 Common Message Header........................................22
    3.2 Variable-Length Parameter Format
    3.3 Transfer Messages............................................24
    3.4 SS7 Signalling Network management (SSNM) Messages............26
    3.5 Application Server Process Maintenance Messages..............32
    3.6 Management Messages..........................................40
4. Procedures........................................................44
    4.1 Procedures to Support the Services of the M3UA Layer.........44
    4.2 Procedures to Support the M3UA Services in Section 1.4.2.....44
    4.3 Procedures to Support the M3UA Services in Section 1.4.4.....45
    4.4 Procedures to Support the M3UA Services in Section 1.4.3.....52
5. Examples of M3UA Procedures.......................................54
    5.1 Establishment of Association and Traffic
        Between SGs and ASPs.........................................54
    5.2 ASP traffic Fail-over Examples...............................56
    5.3 M3UA/MTP3-User Boundary Examples.............................57
6. Security..........................................................61
    6.1 Introduction.................................................61
    6.2 Threats......................................................61
    6.3 Protecting Confidentiality...................................62
7. IANA Considerations...............................................62
    7.1 SCTP Payload Protocol Identifier.............................62
    7.2 M3UA Protocol Extensions.....................................62
8. Acknowledgements..................................................62
9. References........................................................62
10. Author's Addresses...............................................65

1.  Introduction

1.1 Scope

There is a need for SCN signalling protocol delivery from an SS7
Signalling Gateway (SG) to a Media Gateway Controller (MGC) or IP-
resident Database as described in the Framework Architecture for
Signalling Transport [1].  The delivery mechanism should meet the
following criteria:

*  Support for the transfer of all SS7 MTP3-User Part messages (e.g.,
   ISUP, SCCP, TUP, etc.)
*  Support for the seamless operation of MTP3-User protocol peers
*  Support for the management of SCTP transport associations and
   traffic between an SG and one or more MGCs or IP-resident Databases
*  Support for MGC or IP-resident Database process fail-over and load-
   sharing
*  Support for the asynchronous reporting of status changes to
   management

In simplistic transport terms, the SG will terminate SS7 MTP2 and MTP3
protocol layers and deliver ISUP, SCCP and/or any other MTP3-User
protocol messages over SCTP transport associations to MTP3-User peers
in MGCs or IP-resident Databases.

1.2 Terminology

Application Server (AS) - A logical entity serving a specific Routing
Key. An example of an Application Server is a virtual switch element
handling all call processing for a unique range of PSTN trunks,
identified by an SS7 DPC/OPC/CIC_range.  Another example is a virtual
database element, handling all HLR transactions for a particular SS7
DPC/OPC/SCCP_SSN combination.  The AS contains a set of one or more
unique Application Server Processes, of which one or more is normally
actively processing traffic.

Application Server Process (ASP) - A process instance of an Application
Server. An Application Server Process serves as an active or standby
process of an Application Server (e.g., part of a distributed virtual
switch or database). Examples of ASPs are processes (or process
instances) of MGCs, IP SCPs or IP HLRs.  An ASP contains an SCTP end-
point and may be configured to process signalling traffic within more
than one Application Server.

Association - An association refers to an SCTP association.  The
association provides the transport for MTP3-User protocol data units
and M3UA adaptation layer peer messages.

IP Server Process (IPSP) - A process instance of an IP-based
application.  An IPSP is essentially the same as an ASP, except that it

uses MU3A in a peer-to-peer fashion.  Conceptually, an IPSP does not
use the services of a signalling gateway.

Signalling Gateway Process (SGP) - A process instance of a Signalling
Gateway.  It serves as an active, standby or load-sharing process of a
Signalling Gateway.

Signalling Process - A process instance that uses M3UA to communicate
with other signalling process.  An ASP, a signalling gateway process
and an IPSP are all signalling processes.

Routing Key: A Routing Key describes a set of SS7 parameter and
parameter values that uniquely define the range of signalling traffic
to be handled by a particular Application Server. For example, where
all traffic directed to an SS7 DPC, OPC and ISUP CIC_range(s) or SCCP
SSN is to be sent to a particular Application Server, that SS7 data
defines the associated Routing Key.  Routing Keys are unique in the
sense that a received SS7 signalling message cannot be directed to more
than one Routing Key.   Also, a Routing Key cannot extend across more
than a single SS7 DPC, in order to more easily support SS7 Management
procedures.  It is not necessary for the parameter range values within
a particular Routing Key to be contiguous.  For example, an ASP could
be configured to support call processing for multiple ranges of PSTN
trunks that are not represented by contiguous CIC values.

Routing Context - An Application Server Process may be configured to
process signalling traffic related to more than one Application Server,
over a single SCTP Association.  At an ASP, the Routing Context
parameter uniquely identifies the range of signalling traffic
associated with each Application Server that the ASP is configured to
receive.  There is a 1:1 relationship between a received Routing
Context value and a Routing Key entry at the sending node.  Therefore
the Routing Context can be viewed as an index into a sending node's
Message Distribution Table containing the Routing Key entries.

Fail-over - The capability to re-route signalling 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.

Signalling Point Management Cluster (SPMC) - The complete set of
Application Servers represented to the SS7 network under one specific
SS7 Point Code of one specific Network Appearance.  SPMCs are used to
sum the availability / congestion / User_Part status of an SS7
destination point code that is distributed in the IP domain, for the
purpose of supporting MTP3 management procedures at an SG.  In some
cases, the SG itself may also be a member of the SPMC.  In this case,
the SG availability / congestion / User_Part status must also be taken
into account when considering any supporting MTP3 management actions.

MTP - The Message Transfer Part of the SS7 protocol.

MTP3 - MTP Level 3, the signalling network layer of SS7
MTP3-User - Any protocol normally using the services of the SS7 MTP3
(e.g., ISUP, SCCP, TUP, etc.).

Network Appearance - The Network Appearance identifies an SS7 network
context for the purposes of logically separating the signalling traffic
between the SG and the Application Server Processes over a common SCTP
Association.  An example is where an SG is logically partitioned to
appear as an element in four separate national SS7 networks.  A Network
Appearance implicitly defines the SS7 Point Code(s), Network Indicator
and MTP3 protocol type/variant/version used within a specific SS7
network partition.  A physical SS7 route-set or link-set at an SG can
appear in only one network appearance. The Network Appearance is not
globally significant and requires coordination only between the SG and
the ASP. Therefore, in the case where an ASP is connected to more than
one SG, the same SS7 network context may be identified by different
Network Appearances depending over which SG a message is being
transmitted/received.

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

Layer Management - Layer Management is a nodal function that handles
the inputs and outputs between the M3UA layer and a local management
entity.

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

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.

1.3 M3UA Overview

1.3.1 Protocol Architecture.

The framework architecture that has been defined for SCN signalling
transport over IP [1] uses multiple components, including a common
signalling transport protocol and an adaptation module to support the
services expected by a particular SCN signalling protocol from its
underlying protocol layer.

Within the framework architecture, this document defines an MTP3-User
adaptation module suitable for supporting the transfer of messages of
any protocol layer that is identified to the MTP Level 3 layer, in SS7
terms, as a user part.  The list of these protocol layers include, but
is not limited to, ISDN User Part (ISUP) [2,3,4], Signalling Connection
Control Part (SCCP) [5,6,7] and Telephone User Part (TUP) [8].  TCAP

[9,10,11] or RANAP [12] messages are transferred transparently by the
M3UA as SCCP payload, as they are SCCP-User protocols.

It is recommended that the M3UA use the services of the Stream Control
Transmission Protocol (SCTP) [13] as the underlying reliable common
signalling transport protocol. This is to take advantage of various
SCTP features such as:

   - 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.

Under certain scenarios, such as back-to-back connections without
redundancy requirements, the SCTP functions above may not be necessary.
In these cases, it is acceptable to use TCP as the underlying common
transport protocol.

1.3.2 Services Provided by the M3UA Layer

The M3UA Layer at an ASP provides the equivalent set of primitives at
its upper layer to the MTP3-Users as provided by the MTP Level 3 to its
local users at an SS7 SEP.  In this way, the ISUP and/or SCCP layer at
an ASP is unaware that the expected MTP3 services are offered remotely
from an MTP3 Layer at an SG, and not by a local MTP3 layer.  The MTP3
layer at an SG may also be unaware that its local users are actually
remote user parts over M3UA.  In effect, the M3UA extends access to the
MTP3 layer services to a remote IP-based application.  The M3UA does
not itself provide the MTP3 services.  In the case where an ASP is
connected to more than one SG, however, the M3UA must maintain the
status of configured SS7 destinations and route messages according to
availability/congestion status of the routes to these destinations.

The M3UA Layer may also be used for point-to-point signalling between
two IP Server Processes (IPSPs).  In this case, the M3UA provides the
same set of primitives and services at its upper layer as the MTP3.
However, in this case the expected MTP3 services are not offered
remotely from an SG.  The MTP3 services are provided but the procedures
to support these services are a subset of the MTP3 procedures due to
the simplified point-to-point nature of the IPSP to IPSP relationship.

1.3.2.1 Support for the transport of MTP3-User Messages

The M3UA provides the transport of MTP-TRANSFER primitives across an
established SCTP association between an SG and an ASP and or between
IPSPs.

The MTP-TRANSFER primitives are primitive information is encoded as MTP3-User messages with
attached MTP3 Routing Labels as described in the message format
sections of the SCCP and ISUP recommendations. MTP3-User
messages.  In this way, the SCCP and ISUP messages received from the
SS7 network by the SG are not re-encoded into a different format for
transport to/from the server processes.  As
well, all between the required M3UA peers.  The MTP3 Service Information Octet
(SIO) and Routing Label information (OPC, DPC, SIO)
is available at the ASP and the IPSP SLS) are included, encoded as is
expected by the MTP3-User protocol layer.

At an ASP, in the case where a destination is reachable via multiple
SGs, the M3UA must also choose via which SG the message is to be routed
or support load balancing across the SGs, ensuring that no mis-
sequencing occurs.

The M3UA does not impose a 272-octet user information block limit as
specified by the SS7 MTP Level 3 protocol.  Larger information blocks
can be accommodated directly by M3UA/SCTP, without the need for an
upper layer segmentation/re-assembly procedure as specified in recent
SCCP or ISUP versions.  However, in the context of an SG, the maximum
272-octet block size must be followed when inter-working to a SS7
network that does not support the transfer of larger information blocks
to the final destination.  This avoids potential ISUP or SCCP
fragmentation requirements at the SG.  However, if the SS7 network is
provisioned to support the Broadband MTP [20] to the final SS7
destination, the information block size limit may be increased past 272
octets.

1.3.2.2 Native Management Functions

The M3UA provides management of the underlying SCTP transport protocol
to ensure that SG-ASP and IPSP-IPSP transport is available to the
degree called for by the MTP3-User signalling applications.

The M3UA provides the capability to indicate errors associated with
received M3UA messages and to notify, as appropriate, local management
and/or the peer M3UA.

1.3.2.3 Inter-working with MTP3 Network Management Functions

At the SG, the M3UA must also provide inter-working with MTP3
management functions to support seamless operation of the user SCN
signalling applications in the SS7 and IP domains.  This includes:

  - Providing an indication to MTP3-Users at an ASP that a remote
destination in the SS7 network is not reachable.

  - Providing an indication to MTP3-Users at an ASP that a remote
destination in the SS7 network is now reachable.

  - Providing an indication to MTP3-Users at an ASP that messages to a
remote MTP3-User peer in the SS7 network are experiencing SS7
congestion.

  - Providing an indication to MTP3-Users at an ASP that a remote MTP3-
User peer is unavailable.

The M3UA layer at an ASP may initiate an audit of the availability or
the congested state of remote SS7 destinations.  This information is
requested from the M3UA at the SG.

The M3UA layer at an ASP may also indicate to the SG that the M3UA
itself or the ASP or the ASP's host is congested.

1.3.2.4 Support for the management of SCTP associations between the SG
and ASPs.

The M3UA layer at the SG maintains the availability state of all
configured local and remote ASPs, in order to manage the SCTP
Associations and the traffic between the SG and ASPs. M3UA peers.  As well, the
active/inactive state of local and remote ASPs is also maintained - Active ASPs are those currently
receiving traffic from the SG. maintained.

The M3UA layer may be instructed by local management to establish an
SCTP association to a peer M3UA 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 M3UA node.  In order
to avoid multiple redundant SCTP associations between two IPSPs, M3UA peers, one side
must be designated to establish the SCTP association or the mutual SCTP
endpoint addresses must be pre-configured.

The M3UA 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 M3UA may inform local management
of the reason for the release of an SCTP association, determined either
locally within the M3UA layer or by a primitive from the SCTP.

Also the M3UA layer may need to inform the local management of the
change in status of an ASP or AS.  This can may be achieved using the M-ASP
STATUS request or M-AS STATUS request primitives.

1.3.2.5 Support for the management of connections to multiple SGs

As shown in Figure 1 an ASP may be connected to multiple SGs. In such a
case a particular SS7 destination may be reachable via more than SG,
i.e., via more than one route. As MTP3 users only maintain status on a
destination and not on a route basis M3UA must maintain the status
(availability and/or congestion of route to destination) of the
individual routes, derive the overall status of the destination from
the status of the individual routes, and inform the MTP3 users of this
derived status whenever it changes.

1.3.3 Signalling Network Architecture

A Signalling Gateway is used to support the transport of MTP3-User
signalling traffic received from the SS7 network to multiple

distributed ASPs (e.g., MGCs and IP Databases).  Clearly, the M3UA
protocol is not designed to meet the performance and reliability
requirements for such transport by itself.  However, the conjunction of
distributed architecture and redundant networks does allow for a
sufficiently reliable transport of signalling traffic over IP.  The
M3UA protocol is flexible enough to allow its operation and management
in a variety of physical configurations, enabling Network Operators to
meet their performance and reliability requirements.

To meet the stringent SS7 signalling reliability and performance
requirements for carrier grade networks, Network Operators should
ensure that no single point of failure is present in the end-to-end
network architecture between an SS7 node and an IP-based application.
This can typically be achieved through the use of redundant SGs,
redundant hosts, and the provision of redundant QOS-bounded IP network
paths for SCTP Associations between SCTP End Points. Obviously, the
reliability of the SG, the MGC and other IP-based functional elements
also needs to be taken into account.  The distribution of ASPs within
the available Hosts must also be considered.  As an example, for a
particular Application Server, the related ASPs should be distributed
over at least two Hosts.

Here is one

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

          SG                                     MGC

  Host#1 **************                          ************** Host#1
     =   *  ********__*__________________________*__********  *   =
    SG1  *  * SGP1 *__*_____      _______________*__* ASP1 *  *  MGC1
         *  ********  *     \    /               *  ********  *
         *  ********__*______\__/________________*__********  *
         *  * SGP2 *__*_______\/______      _____*__* ASP2 *  *
         *  ********  *       /\      |    |     *  ********  *
         *      :     *      /  \     |    |     *      :     *
         *  ********  *     /    \    |    |     *  ********  *
         *  * SGPn *  *     |    |    |    |     *  * ASPn *  *
         *  ********  *     |    |    |    |     *  ********  *
         **************     |    |    |    |     **************
                            |    |    \    /
  Host#2 **************     |    |     \  /      ************** Host#2
     =   *  ********__*_____|    |______\/_______*__********  *   =
    SG2  *  * SGP1 *__*_________________/\_______*__* ASP1 *  *  MGC2
         *  ********  *                /  \      *  ********  *
         *  ********__*_______________/    \_____*__********  *
         *  * SGP2 *__*__________________________*__* ASP2 *  *
         *  ********  *                          *  ********  *
         *      :     *     SCTP Associations    *      :     *
         *  ********  *                          *  ********  *
         *  * SGPn *  *                          *  * ASPn *  *
         *  ********  *                          *  ********  *
         **************                          **************

                      Figure 1 - Physical Model

In this model, each host has many application processes.  In the case
of the MGC, an ASP may provide service to one or more application
server, and is identified as an SCTP end point.  In the case of the SG,
a pair of signalling gateway processes may represent, as an example, a
single network appearance, serving a signalling point management
cluster.

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

In the example above, each signalling process (SGP, ASP or IPSP) is the
end point to more than one SCTP association, leading to many other
signalling processes.  To support this, a signalling process must be
able to support distribution of M3UA messages to many simultaneous
active associations.  This message distribution function is based on
the status of provisioned routing keys, the availability of signalling
points in the SS7 network, and the redundancy model (active-standby,
load-sharing, n+k) of the remote signalling processes.

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

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

1.4 Functional Areas

1.4.1 Signalling Point Code Representation

Within an SS7 network, a Signalling Gateway is charged with
representing a set of nodes in the IP domain into the SS7 network for
routing purposes.  The SG itself, as a physical node in the SS7
network, must be addressable with an SS7 Point Code for MTP3 Management
purposes. The SG Point Code may also be used for addressing any local
MTP3-Users at the SG such as an SG-resident SCCP function.

An SG may be logically partitioned to operate in multiple SS7 network
Appearances.  In such a case, the SG must be addressable with a Point
Code in each network appearance, and represents a set of nodes in the
IP domain into each SS7 network.  Alias PCs may also be used within an
SG network appearance.

The M3UA places no restrictions on the SS7 Point Code representation of
an AS.  Application Servers can be represented under the same PC of the
SG, their own individual Point Codes or grouped with other applications
for Point Code preservation purposes.  A single Point Code may be used
to represent the SG and all the ASPs together, if desired.

Where Application Servers are grouped under a Point Code address, an
SPMC will include more than one AS. If full advantage of SS7 management
procedures is to be taken (as is advisable in carrier grade networks)
care must be taken that, if (the connection to) one AS of an SPMC
fails, all AS of the SPMC fail or become unreachable from the SG. If
this is not the case, usage of SS7 transfer prohibited procedures by
the SG becomes problematic as either traffic to the failed AS cannot be
stopped/diverted or traffic to a still available AS will unnecessarily
be stopped/diverted. (Depending on the network configuration it may
even be necessary to assign an individual SS7 point code to each AS.)

Observing these principles is of particular importance if alternative
routing possibilities exist on the SS7 level (e.g. via mated SGs) or
application level (e.g. via another MGC/MG).

If an ASP or group of ASPs is available to the SS7 network via more
than one SG, each with its own Point Code, the ASP(s) should be
represented by a Point Code that is separate from any SG Point Code.
This allows these SGs to be viewed from the SS7 network as "STPs", each
having an ongoing  "route" to the same ASP(s).  Under failure
conditions where the  ASP(s) become(s) unavailable from one of the SGs,
this approach enables MTP3 route management messaging between the SG
and SS7 network, allowing simple SS7 re-routing through an alternate SG
without changing the Destination Point Code Address of SS7 traffic to
the ASP(s).

Where an AS can be reached via more than one SG it is equally important
that the corresponding Routing Keys in the involved SGs are identical.
(Note: It is possible for the Routing Key configuration data to be
temporarily out-of-synch during configuration updates).

                           +--------+
                           |        |
              +------------+  SG 1  +--------------+
  +-------+   |  SS7 links | "STP"  |  IP network  |     ----
  |  SEP  +---+            +--------+              +---/      \
  |   or  |                                           |  ASPs  |
  |  STP  +---+            +--------+              +---\      /
  +-------+   |            |        |              |     ----
              +------------+  SG 2  +--------------+
                           | "STP"  |
                           +--------+

Note: there is no SG-to-SG communication shown, so each SG can be
reached only via the direct linkset from the SS7 network.

The following example shows a signalling gateway partitioned into two
network appearances.

                               SG
  +-------+              +---------------+
  |  SEP  +--------------| SS7 Ntwk |M3UA|              ----
  +-------+   SS7 links  |   "A"    |    |            /      \
                         |__________|    +-----------+  ASPs  |
                         |          |    |            \      /
  +-------+              | SS7 Ntwk |    |              ----
  |  SEP  +--------------+   "B"    |    |
  +-------+              +---------------+

1.4.2 Message Distribution

1.4.2.1 Address Translation and Mapping at the SG

In order to direct messages received from the SS7 MTP3 network to the
appropriate IP destination, the SG must perform address translation and
mapping functions using information from the received MTP3-User
message.

To support this message distribution, the SG must maintain the
equivalent of a network address translation table, mapping incoming SS7
message information to an Application Server for a particular
application and range of traffic.  This is accomplished by comparing
elements of the incoming SS7 message to provisioned Routing Keys in the
SG.  These Routing Keys in turn make reference to an Application Server
that is enabled by one or more ASP.  These ASPs provide dynamic status
information to the SG using various management messages defined in the
M3UA protocol.  Possible SS7 address/routing information that comprise
a Routing Key entry includes, for example, the OPC, DPC, SIO found in
the MTP3 routing label, or other MTP3-User specific fields such as the
ISUP CIC, SCCP subsystem number, or TCAP transaction ID. Some example
routing keys are: the DPC alone, the DPC/OPC combination, the
DPC/OPC/CIC combination, or the DPC/SSN combination.  The particular
information used to define an M3UA Routing Key is application and
network dependent, and none of the above examples are mandated.

An Application Server contains a list of one or more ASPs that are
capable of processing the traffic.  This list is assumed to be dynamic,
taking into account the availability status of the individual ASPs in
the list, configuration changes, and possible fail-over mechanisms. The
M3UA protocol includes messages to convey the availability status of
the individual ASPs as input to a fail-over mechanism.

Normally, one or more ASPs is active in the ASP (i.e., currently
processing traffic) but in certain failure and transition cases it is
possible that there may not be an active ASP available.  Both load-
sharing and backup scenarios are supported.

When there is no Routing Key match for an incoming SS7 message, a
default treatment must be specified.  Possible solutions are to provide
a default Application Server at the SG that directs all unallocated
traffic to a (set of) default ASP(s), or to drop the messages and
provide a notification to management.  The treatment of unallocated
traffic is implementation dependent.

1.4.2.2 Address Translation and Mapping at the ASP

In order to direct messages to the SS7 network, the ASP must also
perform an address translation and mapping function in order to choose
the proper SG or SGP for a given message.  This is accomplished by
observing the Destination Point Code and other elements of the outgoing

message, SS7 network status, SG and SGP availability, and network
appearance configuration tables.

A remote Signalling Gateway may be composed of one or more SGPs that
are capable of routing SS7 traffic.  As is the case with ASPs, a
dynamic list of SGPs in an SG can be maintained, taking into account
the availability status of the individual SGPs, configuration changes
and fail-over mechanisms. There is, however, no M3UA messaging to
manage the status of an SGP. Whenever an SCTP association to an SGP
exists, it is assumed to be available.  Also, every SGP of one SG
communicating with one ASP regarding one AS provides identical SS7
connectivity to this ASP.

1.4.3 SS7 and M3UA Interworking

In the case of SS7 and M3UA inter-working, the M3UA adaptation layer is
designed to provide an extension of the MTP3 defined user primitives.

1.4.3.1 Signalling Gateway SS7 Layers

The SG is responsible for terminating MTP Level 3 of the SS7 protocol,
and offering an IP-based extension to its users.

>From an SS7 perspective, it is expected that the Signalling Gateway
(SG) transmits and receives SS7 Message Signalling Units (MSUs) to and
from
the PSTN over a standard SS7 network interface, using the SS7 Message
Transfer Part (MTP) [14,15,16] to provide reliable transport of the
messages.

As a standard SS7 network interface, the use of MTP Level 2 signalling
links is not the only possibility.  ATM-based High Speed Links can also
be used with the services of the Signalling ATM Adaptation Layer (SAAL)
[17,18].  It is possible for IP-based links to be present, using the
services of the MTP2-User Adaptation Layer (M2UA) [19].  These SS7
datalinks may be terminated at a Signalling Transfer Point (STP) or at
a Signalling End Point (SEP).  Using the services of MTP3, the SG may
be capable of communicating with remote SS7 SEPs in a quasi-associated
fashion, where STPs may be present in the SS7 path between the SEP and
the SG.

Where ATM-based High Speed Links are used in the SS7 network, it is
possible for the SG to use the services of the MTP-3b [20] for reliable
transport to and from an SS7 SEP or STP. The maximum Service Data Unit
(SDU) supported by the MTP-3b is 4096 octets compared to the 272-octet
maximum of the MTP3.  However, for MTP3-Users to take advantage of the
larger SDU between MTP3-User peers, network architects should ensure
that MTP3-b is used end-to-end between the SG and the SS7-resident
peer.

1.4.3.2 SS7 and M3UA Inter-Working at the SG

The SG provides a functional inter-working of transport functions
between the SS7 network and the IP network by also supporting the M3UA
adaptation layer.  It allows the transfer of MTP3-User signalling
messages to and from an IP-based Application Server Process where the
peer MTP3-User protocol layer exists.

The Signalling Gateway must maintain knowledge of SS7 node and
Signalling Point Management Cluster (SPMC) status in their respective
domains in order to perform a seamless inter-working of the IP-based
signalling and the SS7 domains.  For example, SG knowledge of the
availability and/or congestion status of the SPMC and SS7 nodes must be
maintained and disseminated in the respective networks, in order to
ensure that end-to-end operation is transparent to the communicating
SCN protocol peers at the SS7 node and ASP.

When the SG determines that the transport of SS7 messages to an SPMC
(or possibly to parts of an SPMC) is encountering congestion, the SG
should inform the MTP3 route management function (by an implementation-
dependent mechanism).  This information is used by the MTP3 to mark the
"route" to the affected destination as congested and to trigger MTP
Transfer Controlled (TFC) messages to any SS7 SEPs generating traffic
to the congested DPC, as per current MTP3 procedures.

When the SG determines that the transport of SS7 messages to all ASPs
in a particular SPMC is interrupted, then it should similarly inform
the MTP3 route management function.  This information is used by the
MTP3 to mark the "route" to the affected destination as unavailable,
and in the case of the SG acting as a signalling transfer point (i.e.,
the Point Code of the SG is different from that of the SPMC), to send
MTP Transfer Prohibited (TFP) messages to the relevant adjacent SS7
nodes, according to the local SS7 network procedures.

When the SG determines that the transport of SS7 messages to an ASP in
a particular SPMC can be resumed, the SG should similarly inform the
MTP3 route management function.  This information is used by the MTP3
to mark the route to the affected destination as available, and in the
case of a signalling transfer point, to send MTP Transfer Allowed (TFA)
messages to the relevant adjacent SS7 nodes, according to the local SS7
network procedures.

For SS7 user part management, it is required that the MTP3-User
protocols at ASPs receive indications of SS7 signalling point
availability, SS7 network congestion, and remote User Part
unavailability as would be expected in an SS7 SEP node.  To accomplish
this, the MTP-PAUSE, MTP-RESUME and MTP-STATUS indication primitives
received at the MTP3 upper layer interface at the SG need to be
propagated to the remote MTP3-User lower layer interface at the ASP.
(These indication primitives are, of course, also made available to any
existing local MTP3-Users at the SG, if present.)

It is important to clarify that MTP3 management messages such as TFPs
or TFAs received from the SS7 network are not "encapsulated" and sent
blindly to the ASPs.  Rather, the existing MTP3 management procedures
are followed within the MTP3 function of the SG to re-calculate the
MTP3 route set status and initiate any required signalling-route-set-
test procedures into the SS7 network.  Only when an SS7 destination
status changes are MTP-PAUSE or MTP-RESUME primitives invoked.  These
primitives can also be invoked due to local SS7 link set conditions as
per existing MTP3 procedures.

In case where the MTP in the SG undergoes an MTP restart, event
communication to the concerned ASPs should be handled as follows:

When the SG discovers SS7 network isolation, the SG sends an indication
to all concerned available ASPs (i.e., ASPs in the "active" or
"inactive" state), using a DUNA message.  For the purposes of MTP
Restart, all SPMCs with point codes different from that of the SG with
at least one ASP that is active or has sent an ASPAC message to the SG
during the first part of the restart procedure should be considered as
available.  If the M3UA at the SG receives any ASPAC messages during
the restart procedure, it delays the ASPAC-ACK messages until the end
of the restart procedure.  During the second part of the restart
procedure the M3UA at the SG informs all concerned ASPs in the "active"
or "inactive" state of any unavailable SS7 destinations.  At the end of
the restart procedure the M3UA sends an ASPAC-ACK message to all ASPs
in the "active" state.

1.4.3.2 Application Server

A cluster of application servers is responsible for providing the
overall support for one or more SS7 upper layers.  From an SS7
standpoint, a Signalling Point Management Cluster (SPMC) provides
complete support for the upper layer service for a given point code.
As an example, an SPMC providing MGC capabilities must provide complete
support for ISUP for a given point code, according to the local SS7
network specifications.

This measure is necessary to allow the SG to accurately represent the
signalling point on the local SS7 network.

In the case where an ASP is connected to more than one SG, the M3UA
must maintain the status of configured SS7 destinations and route
messages according to availability/congestion status of the routes to
these destinations.

When an ASP enters the "Inactive" state towards an SG the M3UA must
mark all SS7 destinations configured to be reachable via this SG as
available.

When the M3UA at an ASP receives a DUNA message indicating SS7 network
isolation at an SG, it will stop any affected traffic via this SG and

clear any unavailability state of SS7 destinations via this SG. When
the M3UA subsequently receives any DUNA messages from an SG it will
mark the effected SS7 destinations as unavailable via that SG.  When
the M3UA receives an ASPAC-ACK message it can resume traffic to
available SS7 destinations via this SG, provided the ASP is in the
active state towards this SG.

1.4.3.3 IPSP

Since IPSPs use M3UA in a point-to-point fashion, there is no concept
of routing of messages beyond the remote end.  Therefore, SS7 and M3UA
inter-working is not necessary for this model.

1.4.4 Redundancy Models

The network address translation and mapping function of the M3UA layer
supports signalling process fail-over functions in order to support a
high availability of call and transaction processing capability.

1.4.4.1 Application Server Redundancy

All MTP3-User messages (e.g., ISUP, SCCP) incoming to an SG from the
SS7 network are assigned to a unique Application Server, based on the
information in the message and the provisioned Routing Keys.

The Application Server is, in practical terms, a list of all ASPs
configured to process a range of MTP3-User traffic defined by one
Routing Key.  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.  A
"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.

At the SG, an Application Server list contains active and inactive ASPs
to support ASP load-sharing and fail-over procedures.  The list of ASPs
within a logical Application Server is kept updated in the SG to
reflect the active Application Server Process(es).

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 1, 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:

    Routing Key {DPC=x) - "Application Server #1"
        ASP1/Host1  - State=Up, Active
        ASP1/Host2  - State=Up, Inactive

In this "1+1" redundancy case, ASP1 in Host1 would be sent any incoming
message with DPC=x.  ASP1 in Host2 would normally be brought to the
active state upon failure of, or loss of connectivity to, ASP1/Host1.
In this example, both ASPs are Up, meaning that the related SCTP
association and far-end M3UA peer is ready.

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

    Routing Key {DPC=x) - "Application Server #1"
        ASP1/Host1 - State = Up, Active
        ASP1/Host2 - State = Up, Active

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

Care must be exercised by a Network Operator in the selection of the
routing information to be used as the Routing Key for a particular AS.
For example, where Application Servers are defined using ranges of ISUP
CIC values, the Operator is implicitly splitting up control of the
related circuit groups.  Some CIC value range assignments may interfere
with ISUP circuit group management procedures.

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 fail.  It
is possible that calls in "transition" may fail, although measures of
communication between the ASPs involved can be used to mitigate this.
For example, the two ASPs may share call state via shared memory, or
may use an ASP to ASP protocol to pass call state information.

1.4.4.2 Signalling Gateway Redundancy

Signalling Gateways may also be distributed over multiple hosts.  Much
like the AS model, SGs may be comprised of one or more SG Processes
(SGPs), distributed over one or more hosts, using an active/standby or
a load-sharing model.  An SGP is viewed as a remote SCTP end-point from
an ASP perspective.  There is, however, no M3UA protocol to manage the
status of an SGP. Whenever an SCTP association to an SGP exists, the
SGP is assumed to be available.  Also, every SGP within an SG
communicating with an ASP provides identical SS7 connectivity to this
ASP. Should an SGP lose all or partial SS7 connectivity and other SGPs
exist, the SGP must terminate the SCTP associations to the concerned
ASPs.

It is therefore possible for an ASP to route signalling messages
destined to the SS7 network using more than one SGP.  In this model, a

Signalling Gateway is deployed as a cluster of hosts acting as a single
SG.  A primary/back-up redundancy model is possible, where the
unavailability of the SCTP association to a primary SGP could be used
to reroute affected traffic to an alternate SGP.  A load-sharing model
is possible, where the signalling messages are load-shared between
multiple SGPs.

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

>From the perspective of the M3UA at an ASP, a particular SG is capable
of transferring traffic to an SS7 destination if an SCTP association
with at least one SGP of the SG is established, the SGP has received an
indication from the ASP M3UA that the ASP is actively handling traffic
for that destination, and the SG has not indicated that the destination
is inaccessible.  When an ASP is configured to use multiple SGs for
transferring traffic to the SS7 network, the ASP must maintain
knowledge of the current capability of the SGs to handle traffic to
destinations of interest.  This information is crucial to the overall
reliability of the service, for both active/standby and load-sharing
model, in the event of failures, recovery and maintenance activities.
The ASP M3UA may also use this information for congestion avoidance
purposes.

1.4.5 Flow Control
Local Management at an ASP may wish to stop traffic across an SCTP
association in order to temporarily remove the association from service
or to perform testing and maintenance activity.  The function could
optionally be used to control the start of traffic on to a newly
available SCTP association.

1.4.6 Congestion Management

The M3UA Layer is informed of local and IP network congestion by means
of an implementation-dependent function (e.g., an implementation-
dependent indication from the SCTP of IP network congestion). When an
SG determines that the transport of SS7 messages to a Signalling Point
Management Cluster (SPMC) is encountering congestion, the SG should
trigger SS7 MTP3 Transfer Controlled management messages to originating
SS7 nodes. The triggering of SS7 MTP3 Management messages from an SG is
an implementation-dependent function.

At an ASP, congestion is indicated to local MTP3-Users by means of an
MTP-Status primitive indicating congestion, to invoke appropriate upper
layer responses, as per current MTP3 procedures.

The M3UA should indicate local ASP congestion to the SG with an SCON
message.  When an SG receives an SCON message from an ASP it should

trigger SS7 MTP3 Transfer Controlled management messages to concerned
SS7 destinations according to established MTP procedures.

1.4.7 SCTP Stream Mapping.

The M3UA at both the SG and ASP also supports the assignment of
signalling traffic into streams within an SCTP association.  Traffic
that requires sequencing must be assigned to the same stream.  To
accomplish this, MTP3-User traffic may be assigned to individual
streams based on, for example, the SLS value in the MTP3 Routing Label
or the ISUP CIC assignment, subject of course to the maximum number of
streams supported by the underlying SCTP association.

The use of SCTP streams within M3UA is recommended in order to minimize
transmission and buffering delays, therefore improving the overall
performance and reliability of the signalling elements.  The
distribution of the MTP3 user messages over the various streams should
be done in such a way to minimize message mis-sequencing, as required
by the SS7 User Parts.

1.4.8 Client/Server Model

The SG takes on the role of server while the ASP is the client. ASPs
must initiate the SCTP association to the SG.

In the case of IPSP to IPSP communication, one side can be designated
as the initiator of the SCTP association and M3UA messaging.

The SCTP (and UDP/TCP) Registered User Port Number Assignment for M3UA
is 2905.

1.5 Sample Configurations

1.5.1 Example 1: ISUP message transport

  ********   SS7   *****************   IP   ********
  * SEP  *---------*      SG       *--------* ASP  *
  ********         *****************        ********

  +------+                                  +------+
  | ISUP |               (NIF)              | ISUP |
  +------+         +------+-+------+        +------+
  | MTP3 |         | MTP3 | | M3UA |        | M3UA |
  +------|         +------+ +------+        +------+
  | MTP2 |         | MTP2 | | SCTP |        | SCTP |
  +------+         +------+ +------+        +------+
  |  L1  |         |  L1  | |  IP  |        |  IP  |
  +------+         +------+ +------+        +------+
      |_______________|         |______________|

    SEP - SS7 Signalling End Point
    SCTP - Stream Control Transmission Protocol
    NIF - Nodal Inter-working Function

In this example, the SG provides an implementation-dependent nodal
inter-working function (NIF) that allows the MGC to exchange SS7
signalling messages with the SS7-based SEP.  The NIF within the SG
serves as the interface within the SG between the MTP3 and M3UA.  This
nodal inter-working function has no visible peer protocol with either
the MGC or SEP.  It also provides network status information to one or
both sides of the network.

For internal SG modeling purposes, at the NIF level, SS7 signalling
messages that are destined to the MGC are received as MTP-TRANSFER
indication primitives from the MTP Level 3 upper layer interface and
are sent to the local M3UA-resident message distribution function for
ongoing routing to the final IP destination.  MTP-TRANSFER primitives
received from the local M3UA network address translation and mapping
function are sent to the MTP Level 3 upper layer interface as MTP-
TRANSFER request primitives for on-going MTP Level 3 routing to an SS7
SEP.  For the purposes of providing SS7 network status information the
NIF also delivers MTP-PAUSE, MTP-RESUME and MTP-STATUS indication
primitives received from the MTP Level 3 upper layer interface to the
local M3UA-resident management function.

1.5.2  Example 2: SCCP Transport between IPSPs

   ********    IP    ********
   * IPSP *          * IPSP *
   ********          ********

   +------+          +------+
   |SCCP- |          |SCCP- |
   | User |          | User |
   +------+          +------+
   | SCCP |          | SCCP |
   +------+          +------+
   | M3UA |          | M3UA |
   +------+          +------+
   | SCTP |          | SCTP |
   +------+          +------+
   |  IP  |          |  IP  |
   +------+          +------+
       |________________|

This example shows an architecture where no Signalling Gateway is used.
In this example, SCCP messages are exchanged directly between two IP-
resident IPSPs with resident SCCP-User protocol instances, such as
RANAP or TCAP.  SS7 network inter-working is not required, therefore
there is no MTP3 network management status information for the SCCP and
SCCP-User protocols to consider.  Any MTP-PAUSE, -RESUME or -STATUS
indications from the M3UA to the SCCP should consider only the status
of the SCTP Association and underlying IP network.

1.5.3 Example 3: SG resident SCCP layer, with remote ASP

  ********   SS7   *****************   IP   ********
  * SEP  *---------*               *--------*      *
  *  or  *         *      SG       *        * ASP  *
  * STP  *         *               *        *      *
  ********         *****************        ********

  +------+         +---------------+        +------+
  | SCCP-|         |     SCCP      |        | SCCP-|
  | User |         +---------------+        | User |
  +------+           |   _____   |          +------+
  | SCCP |           |  |     |  |          | SCCP |
  +------+         +------+-+------+        +------+
  | MTP3 |         | MTP3 | | M3UA |        | M3UA |
  +------|         +------+ +------+        +------+
  | MTP2 |         | MTP2 | | SCTP |        | SCTP |
  +------+         +------+ +------+        +------+
  |  L1  |         |  L1  | |  IP  |        |  IP  |
  +------+         +------+ +------+        +------+
      |_______________|         |______________|

    STP - SS7 Signalling Transfer Point

In this example, the SG contains an instance of the SS7 SCCP protocol
layer that may, for example, perform the SCCP Global Title Translation
(GTT) function for messages logically addressed to the SG SCCP.  If the
result of a GTT for an SCCP message yields an SS7 DPC or DPC/SSN
address result of an SCCP peer located in the IP domain, the resulting
MTP-TRANSFER request primitive is sent to the local M3UA-resident
network address translation and mapping function for ongoing routing to
the final IP destination.

Similarly, the SCCP instance in an SG can perform the SCCP GTT service
for messages logically addressed to it from SCCP peers in the IP
domain.  In this case, MTP-TRANSFER messages are sent from the local
M3UA-resident network address translation and mapping function to the
SCCP for GTT.  If the result of the GTT yields the address of an SCCP
peer in the SS7 network then the resulting MTP-TRANSFER request is
given to the MTP3 for delivery to an SS7-resident node.

It is possible that the above SCCP GTT at the SG could yield the
address of an SCCP peer in the IP domain and the resulting MTP-TRANSFER
primitive would be sent back to the M3UA for delivery to an IP
destination.

For internal SG modeling purposes, this may be accomplished with the
use of an implementation-dependent nodal inter-working function within
the SG that effectively sits below the SCCP and routes MTP-TRANSFER
messages to/from both the MTP3 and the M3UA, based on the SS7 DPC or
DPC/SSN

address information.  This nodal inter-working function has no visible
peer protocol with either the ASP or SEP.

Note that the services and interface provided by the M3UA are the same
as in Example 1 and the functions taking place in the SCCP entity are
transparent to M3UA.  The SCCP protocol functions are not reproduced in
the M3UA protocol.

1.6 Definition of M3UA Boundaries

1.6.1 Definition of the boundary between M3UA and an MTP3-User.

>From ITU Q.701 [14]:

   MTP-TRANSFER request
   MTP-TRANSFER indication
   MTP-PAUSE indication
   MTP-RESUME indication
   MTP-STATUS indication

1.6.2 Definition of the boundary between M3UA and SCTP

The upper layer primitives provided by the SCTP are provided in [13]

1.6.3 Definition of the Boundary between M3UA and Layer Management

   M-SCTP ESTABLISH request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to establish an SCTP association with an SG
            or IPSP.

   M-STCP ESTABLISH confirm
   Direction: M3UA -> LM
   Purpose: ASP confirms to LM that it has established an SCTP
            association with an SG or IPSP.

   M-SCTP ESTABLISH indication
   Direction: M3UA -> LM
   Purpose: SG or IPSP M3UA informs LM that an a remote ASP has established an SCTP
            association.

   M-SCTP RELEASE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to release an SCTP association with SG or
            IPSP.

   M-SCTP RELEASE confirm
   Direction: M3UA -> LM
   Purpose: ASP confirms to LM that it has released SCTP association
            with SG.

   M-SCTP RELEASE indication
   Direction: M3UA -> LM
   Purpose: SG or IPSP M3UA informs LM that a remote ASP has released an SCTP
            association.
            Association or the SCTP association has failed.

   M-SCTP STATUS request
   Direction: LM -> M3UA
   Purpose: LM requests M3UA to report the status of an SCTP
            association.

   M-SCTP STATUS indication confirm
   Direction: M3UA -> LM
   Purpose: M3UA reports the status of an SCTP association.

   M-ASP STATUS request
   Direction: LM -> M3UA
   Purpose: LM requests SG or IPSP M3UA to report the status of a local or remote
            ASP.

   M-ASP STATUS indication confirm
   Direction: M3UA -> LM
   Purpose: SG or IPSP M3UA reports status of local or remote ASP.

   M-AS-STATUS

   M-AS STATUS request
   Direction: LM -> M3UA
   Purpose: LM requests SG or IPSP M3UA to report the status of an AS.

   M-AS-STATUS indication

   M-AS STATUS confirm
   Direction: M3UA -> LM
   Purpose: SG or IPSP M3UA reports the status of an AS.

   M-NOTIFY indication
   Direction: M3UA -> LM
   Purpose: ASP M3UA reports that it has received a NOTIFY message
            from its peer.

   M-ERROR indication
   Direction: M3UA -> LM
   Purpose: ASP, SG or IPSP M3UA reports that it has received an ERROR message from
            its peer.

   M-ASP-DOWN peer or that a local operation has been unsuccessful.

   M-ASP UP request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to stop start its operation and send an ASP-DOWN
            message ASP-UP
            Message to its peer.

   M-ASP UP confirm
   Direction: M3UA -> LM
   Purpose: M3UA confirms requested ASP-UP change has been successfully
            acknowledged by the SG.

   M-ASP-UP M3UA peer.

   M-ASP UP indication
   Direction: M3UA -> LM
   Purpose: M3UA reports it has successfully processed an incoming ASP-
            UP request from its peer.

   M-ASP DOWN request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to start stop its operation and send an ASP-UP
            message ASP-DOWN
            Message to the SG.

   M-ASP-INACTIVE request
   Direction: its peer.

   M-ASP DOWN confirm
   Direction: M3UA -> LM
   Purpose: M3UA confirms requested ASP-DOWN change has been
            successfully acknowledged by the M3UA peer.

   M-ASP DOWN indication
   Direction: M3UA -> LM
   Purpose: M3UA reports it has successfully processed an incoming ASP-
            DOWN request from its peer.

   M-ASP-ACTIVE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to stop data transfer and send an ASP-
            Inactive ASP-ACTIVE message to its peer
            and to start data transfer.

   M-ASP ACTIVE confirm
   Direction: M3UA -> LM
   Purpose: LM confirms requested ASP-ACTIVE change has been
            successfully acknowledged by the SG.

   M-ASP-ACTIVE M3UA peer.

   M-ASP ACTIVE indication
   Direction: M3UA -> LM
   Purpose: LM reports it has successfully processed an incoming ASP-
            ACTIVE request from its peer.

   M-ASP-INACTIVE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to start stop data transfer and send an ASP-
            Active
            Inactive message to the SG.

   M-ASP INACTIVE confirm
   Direction: LM -> M3UA
   Purpose: LM confirms requested ASP-INACTIVE change has been
            successfully acknowledged by the M3UA peer.

   M-ASP INACTIVE indication
   Direction: M3UA -> LM
   Purpose: LM reports it has successfully processed an incoming ASP-
            INACTIVE request from its peer.

   M-AS ACTIVE indication
   Direction: M3UA -> LM
   Purpose: LM reports that an AS has moved to the ACTIVE state.

   M-AS INACTIVE indication
   Direction: M3UA -> LM
   Purpose: LM reports that an AS has moved to the INACTIVE state.

   M-AS DOWN indication
   Direction: M3UA -> LM
   Purpose: LM reports that an AS has moved to the DOWN state.

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 M3UA Protocol Elements

The general M3UA message format includes a Common Message Header
followed by zero or more parameters as defined by the Message Type.
For forward compatibility, all Message Types may have attached
parameters even if none are specified in this version.

3.1 Common Message Header

The protocol messages for MTP3-User Adaptation require a message
structure that contains a version, message type, message length, and
message contents.  This message header is common among all signalling
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    |   Reserved    | Message Class | Message Type  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Message Length                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                                                               /

All fields in an M3UA message MUST be transmitted in the network byte
order, unless otherwise stated.

M3UA Protocol Version: 8 bits (unsigned integer)

   The version field contains the version of the M3UA adaptation layer.
   The supported versions are:

         1      Release 1.0

Message Class: 8 bits (unsigned integer)

   The following list contains the Message Type Classes for the defined
   messages.

         0        Management (MGMT) Message
         1        Transfer Messages
         2        SS7 Signalling Network Management (SSNM) Messages
         3        ASP State Maintenance (ASPSM) Messages
         4        ASP Traffic Maintenance (ASPTM) Messages
      5 to 255    Reserved

Message Type: 8 bits (unsigned integer)

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

     Management (MGMT) Message

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

     Transfer Messages

         0        Reserved
         1        Payload Data (DATA)
       2 to 255   Reserved for Transfer Messages

     SS7 Signalling Network Management (SSNM) Messages

         0        Reserved
         1        Destination Unavailable (DUNA)
         2        Destination Available (DAVA)
         3        Destination State Audit (DAUD)
         4        SS7 Network Congestion State (SCON)
         5        Destination User Part Unavailable (DUPU)
       6 to 255   Reserved for SSNM Messages
  ASP 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

  ASP Traffic Maintenance (ASPTM) 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

Reserved: 8 bits

   Should be set to all '0's and ignored by the receiver.

Message Length: 32-bits (unsigned integer)

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

3.2 Variable-Length Parameter Format

M3UA messages consist of a Common Header followed by zero or more
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)

   Tag 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 NEVER pad with more than 3
   bytes. The receiver MUST ignore the padding bytes.

3.3 Transfer Messages

The following section describes the Transfer messages and parameter
contents.

3.3.1 Payload Data Message (DATA)

The Data message contains the SS7 MTP3-User protocol data, which is an
MTP-TRANSFER primitive, including the complete MTP3 Routing Label. The
Data message contains the following variable length parameters:

     Network Appearance    Optional
     Protocol Data         Mandatory

The following format MUST be used for the Data Message:

    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 = 1            |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Network Appearance*                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 3            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                        Protocol Data                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Appearance: 32-bits (unsigned integer)

   The optional Network Appearance parameter identifies the SS7 network
   context for the message, for the purposes of logically separating
   the signalling traffic between the SG and the Application Server
   Process over a common SCTP Association.  An example is where an SG
   is logically partitioned to appear as an element in four different
   national SS7 networks.

   In a Data message, the Network Appearance implicitly defines the SS7
   Point Code format used, the SS7 Network Indicator value, and the
   MTP3/MTP3-User protocol type/variant/version used within the SS7
   network partition.  Where an SG operates in the context of a single
   SS7 network, or individual SCTP associations are dedicated to each
   SS7 network context, or the Network Indicator in the SIO of the MTP-
   Transfer primitive is sufficient, the Network Appearance parameter is not
   required.

   The Network Appearance parameter value is of local significance
   only, coordinated between the SG and ASP.

   Where the optional Network Appearance parameter is present, it must
   be the first parameter in the message as it defines the format of
   the Protocol Data field.

Protocol Data: variable length

   The Protocol Data field contains the SS7 MTP3-User application
   message, including the complete Service Information Octet and Routing Label.
   The Protocol Data parameter contains the following fields:

       Service Information Octet. Includes:
            Service Indicator,
            Network Indicator,
            and Spare/Priority codes

       MTP

       Routing Label. Includes:
            Destination Point Code,
            Originating Point Code,
            And Signalling Link Selection Code (SLS)

       User Protocol Data.  Includes MTP3-User protocol elements:
            ISUP, SCCP, or TUP parameters

   The format is as defined in the relevant MTP standards for the SS7
   protocol being transported.  The format is either implicitly known
   or identified by the Network Appearance parameter.

   For the ANSI protocol example, the Protocol Data field format is
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      SIO      |  DPC Network  |  DPC Cluster  |  DPC Member   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  OPC Network  |  OPC Cluster  |  OPC Member   |      SLS      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                        Protocol Data                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |MSB---------------------------------------------------------LSB|

   For the ITU international protocol example, the Protocol Data field
   is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      SIO      | DPC |      DPC      | DPC | OPC |     OPC     |
   |               |Zone |     Region    | SP  |Zone |    Region   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |*| OPC |  SLS  |                                               |
   |*| SP  |       |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   \                                                               \
   /                        Protocol Data                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |MSB---------------------------------------------------------LSB|

       * LSB of OPC Region

3.4 SS7 Signalling Network Management (SSNM) Messages

3.4.1 Destination Unavailable (DUNA)

The DUNA message is sent from the SG to all concerned ASPs to indicate
that the SG has determined that one or more SS7 destinations are
unreachable.  It is also sent in response to a message from the ASP to
an unreachable SS7 destination.  The MTP3-User at the ASP is expected
to stop traffic to the affected destination through the SG initiating
the DUNA as per the defined MTP3-User procedures.

The DUNA message contains the following parameters:

     Network Appearance      Optional
     Affected Destinations   Mandatory
     Info String             Optional

The format for DUNA 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 = 1            |           Length =8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Network Appearance*                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 5            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask      |                 Affected DPC 1                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask      |                 Affected DPC n                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Tag = 4           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Appearance: 32-bit unsigned integer

   The optional Network Appearance parameter identifies the SS7 network
   context for the message, for the purposes of logically separating
   the signalling traffic between the SG and the Application Server
   Process over a common SCTP Association.  An example is where an SG
   is logically partitioned to appear as an element in four different
   national SS7 networks.

   In an SSNM message, the Network Appearance parameter defines the
   format of the Affected DPC(s) in the Affected Destination parameter.
   The DPC point code length (e.g., 14-, 16-, or 24-bit) and sub-field
   definitions (e.g., ANSI 24-bit network/cluster/member, ITU-
   international 14-bit zone/region/signal_point, many national field
   variants, ...) are fixed within a particular Network Appearance.
   Where an SG operates in the context of a single SS7 network, or
   individual SCTP associations are dedicated to each SS7 network
   context, the Network Appearance parameter is not required and the
   format of the Affected DPC(s) is understood implicitly.

   The format of the Network Appearance parameter is an integer, the
   values used are of local significance only, coordinated between the
   SG and ASP.

   Where the optional Network Appearance parameter is present, it must
   be the first parameter in the message as it defines the format of
   the Affected DPCs in the Affected Destination parameter.

Affected Destinations: n x 32-bits

   The Affected Destinations parameter contains up to sixteen Affected
   Destination Point Code fields, each a three-octet parameter to allow
   for 14-, 16- and 24-bit binary formatted SS7 Point Codes.  Affected
   Point Codes that are less than 24-bits, are padded on the left to
   the 24-bit boundary.  The encoding is shown below for ANSI and ITU
   Point Code examples.

   ANSI 24-bit Point Code:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Mask      |    Network    |    Cluster    |     Member    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      |MSB-----------------------------------------LSB|

   ITU 14-bit Point Code:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Mask      |0 0 0 0 0 0 0 0 0 0|Zone |     Region    | SP  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                           |MSB--------------------LSB|

   It is optional to send an Affected Destinations parameter with more
   than one Affected DPC but it is mandatory to receive and process it.
   All the Affected DPCs included must be within the same Network
   Appearance.  Including multiple Affected DPCs may be useful when
   reception of an MTP3 management message or a linkset event
   simultaneously affects the availability status of a list of
   destinations at an SG.

Mask: 8-bit unsigned integer 8-bits (unsigned integer)

   The Mask field associated with each Affected DPC in the Affected
   Destinations parameter, used to identify a contiguous range of
   Affected Destination Point Codes, independent of the point code
   format.  Identifying a contiguous range of Affected DPCs may be
   useful when reception of an MTP3 management message or a linkset
   event simultaneously affects the availability status of a series of
   destinations at an SG.  For example, if all DPCs in an ANSI cluster
   are determined to be unavailable due to local linkset
   unavailability, the DUNA could identify potentially 256 Affected
   DPCs in a single Affected DPC field.

   The Mask parameter represents a bit mask that can be applied to the
   related Affected DPC field.  The bit mask identifies how many bits
   of the Affected DPC field are significant and which are effectively
   "wildcarded".  For example, a mask of "8" indicates that the last
   eight bits of the DPC is "wildcarded".  For an ANSI 24-bit Affected
   DPC, this is equivalent to signalling that all DPCs in an ANSI
   Cluster are unavailable.  A mask of "3" indicates that the last
   three bits of the DPC is "wildcarded".  For a 14-bit ITU Affected
   DPC, this is equivalent to signaling that an ITU Region is
   unavailable. A mask value equal to the number of bits in the DPC
   indicates that the entire network appearance is affected - this is
   used to indicate network isolation to the ASP.

Info String: variable length

   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 by
   Operators to identify in text form the location reflected by the
   Affected DPC for debugging purposes.

3.4.2 Destination Available (DAVA)

The DAVA message is sent from the SG to all concerned ASPs to indicate
that the SG has determined that one or more SS7 destinations are now
reachable, or in response to a DAUD message if appropriate. The ASP
MTP3-User protocol is expected to resume traffic to the affected
destination through the SG initiating the DUNA.

The DAVA message contains the following parameters:

     Network Appearance       Optional
     Affected Destinations    Mandatory
     Info String              Optional

The format and description of the Network Appearance, Affected
Destinations and Info String parameters is the same as for the DUNA
message (See Section 3.4.1.)

3.4.3 Destination State Audit (DAUD)

The DAUD message can be sent from the ASP to the SG to audit the
availability/congestion state of SS7 routes to one or more affected
destinations.

The DAUD message contains the following parameters:

     Network Appearance      Optional
     Affected Destinations   Mandatory
     Info String             Optional

The format and description of DAUD Message parameters is the same as
for the DUNA message (See Section 3.4.1.)

3.4.4 SS7 Network Congestion (SCON)

The SCON message can be sent from the SG to all concerned ASPs to
indicate the congestion level in the SS7 network to one or more
destinations destinations, or
to an ASP in response to a DATA or DAUD message, if message as appropriate.  For
some MTP protocol variants (e.g., ANSI MTP) the SCON may be sent when
the SS7 congestion level changes.  The SCON message is MAY also be sent
from the M3UA of an ASP to the SG or IPSP an M3UA peer indicating that the M3UA or the
ASP is congested.

The SCON message contains the following parameters:

     Network Appearance       Optional
     Affected Destinations    Mandatory
     Congestion Indications   Optional
     Info String              Optional

The format for SCON 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 = 1            |           Length =8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Network Appearance*                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 5            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Cong. Level 1      Mask     |                 Affected DPC 1                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Cong. Level n      Mask     |                 Affected DPC n                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4 14           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Reserved                    |  Cong. Level* |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String*                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Affected Destinations parameter differs from the Affected
Destinations parameter in the DUNA, DAVA, and DAUD in that a Congestion
Level field is included instead of a Mask field.  Therefore ranges of
congested Affected DPCs cannot be signaled, but this is consistent with
operation in the SS7 network.

The format and description of the Network Appearance Appearance, Affected
Destinations, and Info String parameters is the same as for the DUNA
message (See Section 3.3.2.1.) 3.4.1.)

The Affected Destinations parameter can be used to indicate congestion
of multiple destinations or ranges of destinations.  However, an SCON
MUST not be delayed in order to "collect" individual congested
destinations into a single SCON as any delay might affect the timing of
congestion indications to the M3UA Users.  One use for including a
range of Congested DPCs is when the SG supports an ANSI cluster route
set to the SS7 network that becomes congested due to outgoing link set
congestion.

Congested Indications: 32-bits

   The optional Congestion Indications parameter contains a Congestion
   Level field.  This optional parameter is used to communicate
   congestion levels in national MTP networks with multiple congestion
   thresholds, such as in ANSI MTP3.  For MTP congestion methods
   without multiple congestion levels (e.g., the ITU international
   method) the parameter is not included.

Congestion Level field: 8-bits (unsigned integer)

   The Congestion Level field, associated with each all of the Affected DPC
   DPC(s) in the Affected Destinations parameter, contains one of the following
   Following values:

         0     No Congestion or Undefined
         1     Congestion Level 1
         2     Congestion Level 2
         3     Congestion Level 3

   The congestion levels are as defined in the national congestion method in the
   appropriate national MTP recommendation recommendations [14,15].  For MTP
   congestion methods that do not employ congestion levels (e.g., the
   ITU international method, the parameter is always "Undefined".

When an SCON is received at the SG, a TFC message is generated into the
SS7 network.

Editors Note: May need a different message type (ASPCON) and specify
more detailed procedures at the SG or IPSP upon reception.

3.4.5 Destination User Part Unavailable (DUPU)

The DUPU message is used by an SG to inform an ASP that a remote peer
MTP3-User Part (e.g., ISUP or SCCP) at an SS7 node is unavailable.

The DUPU message contains the following parameters:

     Network Appearance       Optional
     Affected Destinations    Mandatory
     User/Cause               Mandatory
     Info String              Optional

The format for DUPU 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 = 1            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Network Appearance*                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 5            |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Reserved   Mask = 0    |                  Affected DPC                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 9            |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Cause             |            User               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

User/Cause: 32-bits

   The Unavailability Cause and MTP3-User Identity fields, associated
   with the Affected DPC in the Affected Destinations parameter, are
   encoded as follows:

Unavailability Cause field: 16-bits (unsigned integer)

   The Unavailability Cause parameter provides the reason for the
   unavailability of the MTP3-User.  The valid values for the
   Unavailability Cause parameter are shown in the following table.
   The values agree with those provided in the SS7 MTP3 User Part
   Unavailable message.  Depending on the MTP3 protocol used in the
   network appearance, additional values may be used - the
   specification of the relevant MTP3 protocol variant/version
   recommendation is definitive.

         0         Unknown
         1         Unequipped Remote User
         2         Inaccessible Remote User

MTP3-User Identity field: 16-bits (unsigned integer)

   The MTP3-User Identity describes the specific MTP3-User that is
   unavailable (e.g., ISUP, SCCP, ...).  Some of the valid values for
   the MTP3-User Identity are shown below.  The values agree with those
   provided in the SS7 MTP3 User Part Unavailable message and Service
   Indicator.  Depending on the MTP3 protocol variant/version used in
   the network appearance, additional values may be used - the specification of the used.  The relevant
   MTP3 protocol variant/version recommendation is definitive.

       0 to 2       Reserved
          3         SCCP
          4         TUP
          5         ISUP
       6 to 8       Reserved
          9         Broadband ISUP
          10        Satellite ISUP

The Affected Destinations parameter in a DUPU message differs from the
Affected Destinations parameter in the DUNA, DAVA, and DAUD in that a Reserved the
Mask field is included instead of not used and only a Mask field. Therefore, ranges single Affected DPC is attached.
Ranges and lists of
congested Affected DPCs cannot be signaled, but this is
consistent with operation in the SS7 network. The Affected Destinations
parameter in
the DUPU an MTP3 User Part Unavailable message can (UPU) received by an
SG from the SS7 network contains only contain one Affected DPC. destination.

The format and description of the Network Appearance and Info String
parameters is the same as for the DUNA message (See Section 3.4.1.).

3.5 Application Server Process Maintenance (ASPM) Messages

3.5.1 ASP Up (ASPUP)

The ASP UP (ASPUP) message is used to indicate to a remote M3UA peer
that the Adaptation layer is ready to receive  SSNM or ASPM management
messages for all Routing Keys that the ASP is configured to serve.

The ASPUP message contains the following parameters:

     Adaptation Layer Identifier   Optional

     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 = 2            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Adaptation Layer Identifier*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

Adaptation Layer Identifier: 32-bits

The optional Adaptation Layer Identifier (ALI) is a string that
identifies the adaptation layer.  This string must be set to "M3UA"
which results in a length of 8.  The ALI would normally only be used in
the initial ASP Up message across a new SCTP association to ensure both
peers are assuming the same adaptation layer protocol.

Editors Note: Info in SCTP (Payload Identifier) could be used - is
there any need for ALI anymore? 3.4.1.)

3.5.2 ASP Up Ack

The ASP UP Ack message is used to acknowledge an ASP-Up message
received from a  remote M3UA peer.

The ASPUP Ack message contains the following parameters:

     Adaptation Layer Identifier (optional)

     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 =2             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                 Adaptation Layer Identifier*                  /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag =4            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

The format and description of the optional Adaptation Layer Identifier
(ALI) parameter is the same as for the ASP-UP message. (See Section
3.4.1) 3.4.1.)

3.5.3 ASP Down (ASPDN)

The ASP Down (ASPDN) message is used to indicate to a remote M3UA peer
that the adaptation layer is not ready to receive traffic or
maintenance management
messages.

The ASPDN message contains the following parameters:

     Reason         Mandatory
     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag = 10            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Reason                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag =4             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String*                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional Info String parameter is the
same as for the DUNA message (See Section 3.3.2.1.) 3.4.1.)

Reason: 32-bit (unsigned integer)

   The Reason parameter indicates the reason that the remote M3UA
   adaptation layer is unavailable.  The valid values for Reason are
   shown in the following table.

         0       Unspecified
         1       User Unavailable
         2       Management Blocking

3.5.4 ASP Down Ack

The ASP Down Ack message is used to acknowledge an ASP-Down message
received from a remote M3UA peer.

The ASP Down Ack message contains the following parameters:

     Reason          Mandatory
     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag = 10            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Reason                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String*                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional Info String parameter is the
same as for the DUNA message (See Section 3.3.2.1.) 3.4.1.)

The format of the Reason parameter is the same as for the ASP-Down
message. (See Section 3.4.3)

3.5.5 ASP Active (ASPAC)

The ASPAC message is sent by an ASP to indicate to a remote M3UA peer
that it is Active and ready to process signalling traffic for a
particular Application Server.  The ASPAC affects only the ASP state
for the routing keys identified by the Routing Contexts, if present.

The ASPAC message contains the following parameters:

     Traffic Mode Type     Mandatory
     Routing Context       Optional
     INFO String           Optional

The format for the ASPAC 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 = 11            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Traffic Mode Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag =6             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context*                        /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 4              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Traffic Mode Type: 32-bit (unsigned integer)

   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.

         1         Over-ride
         2         Load-share
         3         Over-ride (Standby)
         4         Loadshare         Load-share (Standby)

   Within a particular Routing Context, Over-ride and Loadshare Load-share Types
   cannot be mixed.  The Over-ride value indicates that the ASP is
   operating in Over-ride mode, and the ASP wishes to take over all
   traffic in an Application Server (i.e., primary/back-up operation),
   over-riding any currently active ASP in the AS.  In Load-share mode,
   the ASP wishes to share in the traffic distribution with any other
   currently active ASPs.  The Standby versions of the Over-ride and
   Loadshare
   Load-share Types indicate that the ASP is declaring itself ready to
   accept traffic but leaves it up to the sender as to when the traffic
   is started.  Over-ride (Standby) indicates that the traffic sender
   continues to use the currently active ASP until it can no longer
   send/receive traffic (i.e., the currently active ASP transitions to
   Down or Inactive).  At this point the sender may immediately move
   the ASP to Active and commence traffic.  Loadshare  Load-share (Standby) is
   similar - the sender continues to loadshare load-share to the current ASPs
   until there it is determined that there is insufficient resources in
   the Loadshare Load-share group.  When there is insufficient ASPs, the sender
   may immediately move the ASP to Active.

Routing Context:

   The optional Routing Context parameter contains (a list of) 4-byte
   unsigned integers indexing the Application Server traffic that the
   sending ASP is configured/registered to receive.

   There is one-to-one relationship between an index entry and an SG
   Routing Key or AS Name.  Because an AS can only appear in one
   Network Appearance, the Network Appearance parameter is not required
   in the ASPAC message.

   An Application Server Process may be configured to process traffic
   for more than one logical Application Server.  From the perspective
   of an ASP, a Routing Context defines a range of signalling traffic
   that the ASP is currently configured to receive from the SG.  For
   example, an ASP could be configured to support call processing for
   multiple ranges of PSTN trunks and therefore receive related
   signalling traffic, identified by separate SS7 DPC/OPC/CIC_ranges.

The format and description of the optional Info String parameter is the
same as for the DUNA message (See Section 3.3.2.1.) 3.4.1.)

3.5.6 ASP Active Ack

The ASPAC Ack message is used to acknowledge an ASP-Active message
received from a remote M3UA peer.

The ASPAC Ack message contains the following parameters:

     Traffic Mode Type     Mandatory
     Routing Context       Optional
     INFO String           Optional

The format for the ASPAC 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag = 11            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Traffic Mode Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 6            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context*                        /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

The format of the Traffic Mode Type and Routing Context parameters is
the same as for the ASP-Active message. (See Section 3.4.5).

3.5.7  ASP Inactive (ASPIA)

The ASPIA message is sent by an ASP to indicate to a remote M3UA peer
that it is no longer processing signalling traffic within a particular
Application Server.  The ASPIA affects only the ASP state in the
Routing Keys identified by the Routing Contexts, if present.

The ASPIA message contains the following parameters:

     Traffic Mode Type       Mandatory
     Routing Context         Optional
     INFO String             Optional

The format for the ASPIA 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Type                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 6 11           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /
   |                      Traffic Mode Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 6            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context*                        /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Traffic Mode Type: 32-bit (unsigned integer)

   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.

      1            Over-ride
      2            Load-share

   Within a particular Routing Context, only one Type can be used.  The
   Over-ride value indicates that the ASP is operating in Over-ride
   mode, and will no longer handle traffic within an Application Server
   (i.e., it is now a backup in a primary/back-up arrangement).  The
   Load-share value indicates that the ASP is operating in Load-share
   mode and will no longer share in the traffic distribution with any
   other currently active ASPs.

   A node that receives an ASPIA with an incorrect Type for a
   particular routing Context will respond with an Error Message
   (Cause: Invalid Traffic Handling Mode. Mode).

The format and description of the optional Routing Context and Info
String parameters is the same as for the ASPAC message (See Section
2.3.3.3.)
3.5.5.)

3.5.8 ASP Inactive Ack

The ASPIA Ack message is used to acknowledge an ASP-Inactive message
received from a remote M3UA peer.

The ASPIA Ack message contains the following parameters:

     Traffic Mode Type     Mandatory
     Routing Context       Optional
     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag = 11            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Traffic Mode Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 6            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context*                        /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 4            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional Info String parameter is the
same as for the DUNA message (See Section 3.3.2.1.) 3.4.1.)

The format of the Traffic Mode Type and Routing Context parameters is
the same as for the ASP-Inactive message. (See Section 3.4.7). 3.5.7).

3.5.9 Heartbeat (BEAT)

The Heartbeat message is optionally used to ensure that the M3UA peers
are still available to each other.  It is recommended for use when the
M3UA runs over a transport layer other than the SCTP, which has its own
heartbeat.

The BEAT message contains the following parameters:

     Heatbeat 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 = 8            |            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 BEAT-Ack message.

3.5.10 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.

3.6  Management Messages

3.6.1  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                 Mandatory
     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 12           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Error Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 7            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                     Diagnostic Information*                   /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Error Code: 32-bits (unsigned integer)

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

     1      Invalid Version
     2      Invalid Network Appearance
     3      Invalid Adaptation Layer Identifier      Unsupported Message Type
     4      Invalid Message Type
     5      Invalid Traffic Handling Mode
     6      Unexpected Message Type
     7      Protocol Error
     8      Invalid Routing Context

Diagnostic Information: variable length

   When included, the optional Diagnostic information can be any
   information germane to the error condition, to assist in
   identification of the error condition.  In the case of an Invalid
   Network Appearance, Adaptation Layer Identifier, Traffic Handling Mode or Invalid Routing Context, the
   Diagnostic information includes the received parameter.  In the
   other cases, the Diagnostic information may be the first 40 bytes of
   the offending message.

   In the case of an Invalid Version Error Code, the Common Header
   contains the supported Version.

Error messages are not generated in response to other Error messages.

3.6.2 Notify (NTFY)

The Notify message used to provide an autonomous indication of M3UA
events to an M3UA peer.

The NTFY message contains the following parameters:

     Status Type                Mandatory
     Status Identification Type/ID              Mandatory
     Routing Context            Optional
     INFO String                Optional

The format for the NTFY 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 = 13             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |    Status Identification      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 6              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context*                        /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 4              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String*                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Status Type: 16-bits (unsigned integer)

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

         1     Application Server State Change (AS-StateChange)
         2     Other

Status Information: 16-bits (unsigned integer)

   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:

         1    reserved
         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:

         1    Insufficient ASP resources active in AS
         2    Alternate ASP Active

These notifications are not based on the SG reporting the state change
of an ASP or AS.  In the Insufficent 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 Routing Context and Info
String parameters is the same as for the ASPAC message (See Section
3.4.6.)

4.0 Procedures

The M3UA layer needs to respond to various local primitives it receives
from the SCTP and M3UA-User layers and Layer Management as well as the
messages that it receives from the peer M3UA layers.  This section
describes the M3UA procedures in response to these events.

4.1 Procedures to support the services of the M3UA layer

4.1.1 Receipt of primitives from the M3UA-User

On receiving an MTP-Transfer request primitive from an upper layer, or
the nodal inter-working function at an SG, the M3UA layer will send sends a
corresponding DATA message (see Section 3) to its M3UA peer.  The M3UA
layer must fill in the fields of
peer receiving the common and specific headers
correctly.

At Data message sends an SG, MTP-Transfer indication
primitive to the upper layer.

The M3UA address translation and mapping function determines the
Application Server (AS) based on the information in the incoming
message.  From the list of ASPs within the AS table, an Active ASP is
selected and a DATA message is constructed and issued on the
corresponding SCTP Association.  If more than one ASP is active (i.e.,
traffic is to be load-shared across all the active ASPs), one of the
active ASPs from the list is selected.  The selection algorithm is
implementation dependent but could, for example, be round-robin or
based on, for example, the SLS or ISUP CIC.  The appropriate selection
algorithm must be chosen carefully as it is dependent on application
assumptions and understanding of the degree of state coordination
between the active ASPs in the AS.

In addition, the message needs to be sent on the appropriate SCTP
stream, again taking care to meet the message sequencing needs of the
signalling application.

4.1.2 Receipt of primitives from the Layer Management

On receiving these primitives from the local Layer Management, the M3UA
layer will provide take the appropriate requested action and provide a response primitive across the
internal local to Layer Management interface.
Management.

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 M3UA node

An M-SCTP RELEASE request from Layer Management will initate initates the tear-
down of an SCTP association.  An M-SCTP RELEASE confirm will be is sent by to
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 M3UA

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

M-NOTIFY indication and M-ERROR indication indicate to  The M3UA responds with the
association status in an M-SCTP STATUS confirm.  No peer protocol is
invoked.

An M-ASP STATUS request supports a Layer Management query of the notification or error information contained in status
of a received M3UA
Notify or Error message.  These indications can also be generated based
on particular local or remote ASP.  The M3UA events. responds with the status
in an M-ASP STATUS request/indication and M-AS-STATUS request/indication
support confirm.  No M3UA peer protocol is invoked.

An M-AS STATUS request supports a Layer Management query of the local status
of a particular
ASP or AS.  The M3UA responds with an M-AS STATUS confirm.  No
M3UA peer protocol is invoked.

M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE M-ASP-ACTIVE request and M-ASP-
ACTIVE
INACTIVE request primitives allow Layer Management at an ASP to
initiate state
changes . changes.  Upon successful completion, a corresponding
confirm is provided by the M3UA to Layer Management.  If the invocation
is unsuccessful, an Error indication is provided.

These requests result in outgoing M3UA ASP-UP, ASP-DOWN,
ASP-INACTIVE and ASP-ACTIVE messages. and
ASP-INACTIVE messages to the remote M3UA peer at an SG or IPSP.

4.2 Receipt of M3UA Peer Management messages

Upon receipt of M3UA Management messages, successful state changes resulting from reception of M3UA ASP-UP,
ASP-DOWN, ASP-ACTIVE and ASP-INACTIVE messages from a peer M3UA, the
M3UA layer must invoke
the corresponding Layer Management primitive indications (e.g., M-AS
Status ind., M-ASP Status ind., M-ERROR ind., ...) UP, M-ASP DOWN, M-ASP ACTIVE and
M-ASP INACTIVE, M-AS ACTIVE, M-AS INACTIVE, and M-AS DOWN indications
as appropriate to the local layer
management. Layer Management.

M-NOTIFY indication and M-ERROR indication indicate to Layer Management
the notification or error information contained in a received M3UA
Notify or Error message.  These indications can also be generated based
on local M3UA events.

4.3 Procedures to support the M3UA Management services

These procedures support the M3UA management of SCTP Associations
between SGs and ASPs.

4.3.1 AS and ASP State Maintenance

The M3UA layer on the SG maintains the state of each remote ASP, in
each Application Server that the ASP is configured to receive traffic,
as input to the M3UA message distribution function.  Similarly, where
IPSPs use M3UA in a point-to-point fashion, the M3UA layer in an IPSP
maintains the state of remote ASPs in IPSPs. For the purposes of the
following procedures, only the SG/ASP case is described but the SG side
of the procedures also apply to an IPSP sending traffic to an AS
consisting of remote ASPs in IPSPs.

4.3.1.1  ASP States

The state of each remote ASP, in each AS that it is configured to
operate, is maintained in the M3UA layer in the SG or IPSP. The state
of a particular ASP in a particular AS changes due to events. The
events include:

   * Reception of messages from the peer M3UA layer at the ASP
   * Reception of some messages from the peer M3UA layer at other ASPs
     in the AS (e.g., ASPAC Take-over)
   * Reception of indications from the SCTP layer

The ASP state transition diagram is shown in Figure 4.  The possible
states of an ASP are:

ASP-DOWN: The remote M3UA peer at the ASP is unavailable and/or the
related SCTP association is down.  Initially all ASPs will be in this
state.  An ASP in this state should not be sent any M3UA messages.

ASP-INACTIVE: The remote M3UA 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-Data M3UA messages.

ASP-ACTIVE: The remote M3UA peer at the ASP is available and
application traffic is active (for a particular Routing Context or set
of Routing Contexts).

ASP-STANDBY: The remote M3UA peer at the ASP is available and ready to
receive application traffic at any time (for a particular Routing
Context or set of Routing Contexts).  In this state the ASP can be sent
any non-Data M3UA messages.

                 Figure 4: ASP State Transition Diagram

                                   +-------------+
                                   | ASP-ACTIVE  |
            +----------------------|      or     |
            |    Alternate +-------| ASP-STNDBY* |
            |       ASP    |       +-------------+
            |     Takeover |           ^     |
            |              |    ASP    |     | ASP
            |              |    Active |     | Inact
            |              |           |     v
            |              |       +-------------+
            |              |       |             |
            |              +------>|  ASP-INACT  |
            |                      +-------------+
            |                          ^    |
  ASP Down/ |                     ASP  |    | ASP Down /
  SCTP CDI  |                     Up   |    | SCTP CDI
            |                          |    v
            |                      +-------------+
            |                      |             |
            +--------------------->|             |
                                   |  ASP-DOWN   |
                                   +-------------+

*Note: ASP-ACTIVE and ASP-STNDBY differ only in whether the ASP is
currently receiving Data traffic.

SCTP CDI: The local SCTP layer's Communication Down Indication to the
Upper Layer Protocol (M3UA) 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 or COMMUNICATION LOST notification from the SCTP.

4.3.1.2  AS States

The state of the AS is maintained in the M3UA layer on the SG.

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

   * ASP state transitions
   * Recovery timer triggers

The possible states of an AS are:

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 at least one ASP is in the ASP-
ACTIVE state.

AS-PENDING: An active ASP has transitioned to inactive and it was the
last remaining active ASP in the AS (and no STANDBY ASPs are available.
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 5: AS State Transition Diagram

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

    Tr = Recovery Timer

4.3.2 M3UA Management 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 M3UA layer at an ASP receives an M-SCTP
ESTABLISH request primitive from the Layer Management, Management. the M3UA layer
will try to establish an SCTP association with the remote M3UA peer at
an SG.  Upon reception of an eventual SCTP-Communication Up confirm
primitive from the SCTP, the M3UA layer will invoke the primitive M-
SCTP ESTABLISH confirm to the Layer Management.

The M3UA layers at the SG will receive an SCTP-CommunicationUp
indication primitive from the SCTP when the association is successfully
set up.  The M3UA layer will then invoke the primitive M-SCTP ESTABLISH
indication to the Layer Management.

Once the SCTP association is established and assuming that the local
M3UA-User is ready, the local ASP M3UA 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.

If the M3UA layer subsequently receives an SCTP-Communication Down
indication from the underlying SCTP layer, it will inform the Layer
Management by invoking the M-SCTP STATUS indication primitive. The
state of the remote ASP will be moved to "Down".  At an ASP, the MTP3-
User at an ASP will be informed of the unavailability of any affected
SS7 destinations through the use of MTP-PAUSE primitives.  In the case
of SS7 network isolation, the local MTP3-Users may be informed by
implementation-dependent means as there is currently no primitive
defined for conveying this information.

At an ASP, the Layer Management may try to re-establish the SCTP
association using M-SCTP ESTABLISH request primitive.

4.3.3 M3UA Management procedures for peer-to-peer messages

All M3UA MGMT and ASP Maintenance messages are sent on a sequenced
stream to ensure ordering.  SCTP stream '0' is used.

4.3.3.1 ASP-Up

After an ASP has successfully established an SCTP association to an SG
or IPSP, the SG or IPSP waits for the ASP to send an ASP-Up message,
indicating that the ASP M3UA peer is available.  The ASP is always the
initiator of the ASP-Up exchange.  This action MAY be initiated at the
ASP by an M-ASP UP request primitive from Layer Management or may be
initiated automatically by an M3UA management function.

When an ASP-Up message is received at an SG or IPSP and internally the
remote ASP is not considered locked-out for local management reasons,
the SG marks the remote ASP as 'Inactive'. 'Inactive' and informs Layer Management
with an M-ASP-Up indication primitive.  If the SG knows via
configuration data which Application Servers that the ASP is configured
to operate in, it can update the ASP status to "Inactive" in each AS
pool that it is a member.  Alternatively, the SG may move the ASP into
a pool of Inactive ASPs available for future activation in AS pool(s)
denoted in the subsequent ASP-Active Routing Contexts.  The SG responds
with an ASP-Up Ack message in acknowledgement.  The SG sends an ASP-Up
Ack message in response to a received ASP-Up message even if the ASP is
already marked as "Inactive" at the SG.

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

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

When the Asp sends an ASP-Up it starts timer T(ack).  If the ASP does
not receive a response , or to an ASP-Down Ack is received, ASP-Up within T(ack), the ASP may MAY restart
T(ack) and resend ASP-Up messages every 2 seconds until it receives an ASP-Up Ack
message.  The ASP may decide to reduce the frequency (say to
every 5 seconds) if an ASP-Up Ack  T(ack) is not received after provisionable, with a default of 2 seconds.
Alternatively, retransmission of ASP-Up messages may be put under
control of Layer Management.  In this method, expiry of T(ack) results
in a M-ASP-Up confirmation carrying a few tries. negative indication.

The ASP must wait for the ASP-Up Ack message before sending any ASP
traffic control
messages (i.e., (e.g., ASPAC).  If the SG or IPSP remote peer receives any other M3UA
messages before an ASP Up is received, the SG or IPSP remote peer should
discard them.

4.3.3.2 ASP-Down

The ASP will send an ASP-Down to an SG or IPSP when the ASP wishes to
be removed from service in all Application Servers that it is a member
And no longer receive any M3UA traffic or management messages.  This
action MAY be initiated at the ASP by an M-ASP DOWN request primitive
from Layer Management or may be initiated automatically by an M3UA
management function.

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

The SG marks the ASP as "Down" "Down", informs Layer Management with an M-ASP-
Up indication primitive, 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 an ASP-Down Ack message in response to a received ASP-Down
message from the ASP even if the ASP is already marked as "Down" at the
SG.

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

When the Asp sends an ASP-Down it starts timer T(ack).  If the ASP does
not receive a response from the SG, to an ASP-Down within T(ack), the ASP may send MAY
restart T(ack) and resend 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 message.  T(ack) is not received after provisionable, with a default of 2
seconds.  Alternatively, retransmission of ASP-Down messages may be put
under control of Layer Management.  In this method, expiry of T(ack)
results in a M-ASP-Down confirmation carrying a few tries. negative indication.

4.3.3.3 M3UA Version Control

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

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

4.3.3.4 ASP-Active

Anytime after the ASP has received an ASP-Up Ack from the SG or IPSP,
the ASP sends an ASP-Active (ASPAC) to the SG indicating that the ASP
is ready to start processing traffic.  This action MAY be initiated at
the ASP by an M-ASP Active request primitive from Layer Management or
may be initiated automatically by an M3UA management function.   In the
case where an ASP wishes to process the traffic for more than one
Application Server across a common SCTP association, the ASPAC contains
a list of one or more Routing Contexts to indicate for which
Application Servers the ASPAC applies. In the case where an ASP-Active
message does not contain a Routing Context, the receiver must know, via
configuration data, which AS pools the ASP will be a member.

When an ASP Active (ASPAC) message is received, the SG or IPSP responds
to the ASP with an ASPAC Ack message, acknowledging that the ASPAC was
received and, depending on the ASPAC Type value received, moves the ASP
to the "Active" or "Standby" state within the associated Application
Server(s). Layer Management is informed with an ASP-Active indication
primitive.  The ASP MUST not send Data messages before receiving an
ASPAC Ack.  If the SG or IPSP receives any Data messages before an
ASPAC is received, the SG or IPSP should discard them.

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

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

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

There are two modes of Application Server traffic handling in the SG
M3UA - Over-ride and Load-share.  The Type parameter in the ASPAC
message indicates the traffic handling mode used in a particular
Application Server. If the SG determines that the mode indicated in an
ASPAC is incompatible with the mode currently used in the AS, the SG
responds with an Error message indicating "Invalid Traffic Handling
Mode".

In the case of an Over-ride mode AS, reception of an ASPAC message at
an SG causes the redirection of all traffic for the AS to the ASP that
sent the ASPAC.  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 or IPSP sends a Notify (Alternate ASP-Active) to the previously
active ASP in the AS, after stopping all traffic to that ASP.  In the
case of Over-ride (Standby) mode the actions are the same with the
exception that the traffic is not started to the ASP until the
previously active ASP transitions to "Inactive or "Down" state.  At
this point the ASP that sent the Over-Ride (Standby) ASPAC is moved to
the Active state and the traffic is redirected.  A Notify message is
not sent in this case.

In the case of a Load-share mode AS, reception of an ASPAC message at
an SG or IPSP causes the direction of traffic to the ASP sending the
ASPAC, in addition to all the other ASPs that are currently active in
the AS.  The algorithm at the SG for load-sharing traffic within an AS
to all the active ASPs is implementation dependent.  The algorithm
could, for example be round-robin or based on information in the Data
message (e.g., such as the SLS, SCCP SSN, ISUP CIC value).

Depending on the requirements of the application and the
call/transaction state handling assumptions of the collection of ASPs
in the AS.  The SG or IPSP responds to the ASPAC with an ASP-Active Ack
message to the ASP.  N

In the case of Loadshare Load-share (Standby) mode, the actions are the same with
the exception that the traffic is not started to the ASP until the SG
or IPSP determines that there are insufficient resources available in
the AS.  This is likely due to one of the active
loadsharing load-sharing ASPs
transitions to the "Inactive" or "Down" state.  At this point the ASP
that sent the Loadshare Load-share (Standby) ASPAC s moved to the Active state
and traffic is started.  A Notify message is not sent in this case.

All ASPs within a loadsharing load-sharing mode AS must be able to handle any
traffic within the AS, in order to accommodate any potential fail-over
or rebalancing of the offered load.

A node that receives an ASPAC with an incorrect Type for a particular
Routing Context will respond with an Error Message (Cause: Invalid
Traffic Handling Mode).  A node that receives an unknown Routing
Context value responds with an Error message (Cause: Invalid Routing
Context).

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 or IPSP.  In  This action MAY
be initiated at the case where
an ASP is processing the traffic for more than one Application by an M-ASP INACTIVE request primitive from
Layer Management or may be initiated automatically by an M3UA
management function.   In the case where an ASP is processing the
traffic for more than one Application Server across a common SCTP
association, the ASPIA contains one or more Routing Contexts to
indicate for which Application Servers the ASPIA applies.  In the case
where an ASP-Inactive message does not contain a Routing Context, the
receiver must know via configuration data which AS pools the ASP is a
member and move the ASP to the "Inactive" state in each AS.

There are two modes of Application Server traffic handling in the SG or
IPSP M3UA when withdrawing an ASP from service - Over-ride and Load-
share.  The Type parameter in the ASPIA message indicates the mode used
in a particular Application Server.  If the SG or IPSP determines that
the mode indicates in an ASPIA is inconsistent with the traffic
handling mode currently used in the AS, a this is reported to local
management indicating("Invalid Traffic Handling Mode").  The ASPIA is
still processed.

In the case of an Over-ride mode AS, where another ASP has already
taken over the traffic within the AS with an Over-ride ASPAC, the ASP
that sends the ASPIA is already considered by the SG to be "Inactive".
An ASPIA Ack message is sent to the ASP, after ensuring that all
traffic is stopped to the ASP.

In the case where another ASP has
already sent an Over-ride (Standby) ASPAC, the ASP is moved to the
"Inactive state" and traffic s immediately started to the standby ASP.

In the case of a Load-share mode AS, the SG moves the ASP to the
"Inactive" state and the AS traffic is re-allocated across the
remaining "active" ASPs per the load-sharing algorithm currently used

within the AS.  A NTFY(Insufficient ASPs) may be sent to all inactive
ASPs, if required.  However, if a Loadshare (Standby) ASP is available,
it may be now immediately included in the loadshare group and a Notify
message is not sent.  An ASPIA Ack message is sent to the ASP after all
traffic is halted. halted and Layer Management is informed with an ASP-INACTIVE
indication primitive.

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

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

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.  At the ASP, Layer Management is informed with
an M-NOTIFY indication primitive.

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

4.3.3.7 Heartbeat

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

After receiving an ASP-Up Ack message from an M3UA peer in response to
an ASP-Up message, an ASP may optionally send Beat messages

periodically, subject to a provisionable timer T(beat).  Upon receiving
a BEAT message, the M3UA peer MUST respond with a BEAT ACK message.  If
no BEAT ACK message (or any other M3UA message), is received by the ASP
within the timer 2*T(beat), the ASP will consider the remote M3UA peer
as "Down".

At the ASP, if no BEAT ACK message (or any other M3UA message) is
received from the M3UA peer within 2*T(beat), the remote M3UA peer is
considered unavailable.  Transmission of BEAT messages is stopped and
ASP-Up procedures are used to re-establish communication with the SG
M3UA 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".

4.4 Procedures to support the M3UA services

4.4.1 At an SG

On receiving an MTP-PAUSE, MTP-RESUME, or MTP-STATUS indication
primitive from the nodal inter-working function at an SG, the SG M3UA
layer will send a corresponding SSNM DUNA, DAVA, SCON, or DUPU message
(see Section 2) to the M3UA peers at concerned ASPs.  The M3UA layer
must fill in various fields of the SSNM messages consistently with the
information received in the primitives.

The SG M3UA determines the set of concerned ASPs to be informed based
on the SS7 network partition for which the primitive indication is
relevant. In this way, all ASPs configured to send/receive traffic
within a particular network appearance are informed.  If the SG
operates within a single SS7 network appearance, then all ASPs are
informed.

Optionally, the SG M3UA may filter further based on the Affected Point
Code in the MTP-PAUSE, MTP-Resume, or MTP-Status indication primitives.
In this way ASPs can be informed only of affected destinations to which
they actually communicate.  The SG M3UA may also suppress DUPU messages
to ASPs that do not implement an MTP3-User protocol peer for the
affected MTP3-User.

DUNA, DAVA, SCON messages must be sent on a sequenced stream as these
primitives should arrive in order.  Stream 0 is used.  Sequencing is
not required for the DUPU or DAUD message, which may optionally be sent
un-sequenced.  The same applies for the SCON message if the
international congestion method (see Q.704) is used.

4.4.2 At an ASP

4.4.2.1 Single SG configurations

At an ASP, upon receiving an SSNM message from the remote M3UA Peer,
the M3UA layer invokes the appropriate primitive indications to the
resident M3UA-Users.  Local management is informed.

In the case where a local event has caused the unavailability or
congestion status of SS7 destinations, the M3UA at the ASP should pass
up appropriate indications n the primitives to the M3UA User, as though
equivalent SSNM messages were received.  For example, the loss of an
SCTP association to an SG may cause the unavailability of a set of SS7
destinations.  MTP-Pause indications to the M3UA User is appropriate.
To accomplish this, the M3UA layer at an ASP maintains the status of
routes via the SG, much like an MTP3 layer maintains route-set status.

4.4.2.2 Multiple SG configurations

At an ASP, upon receiving an SSNM message from the remote M3UA Peer,
the
M3UA layer updates the status of the affected route(s) via the
originating SG and determines, whether or not the overall availability
or congestion status of the effected destination(s) has changed. In
this case the M3UA layer invokes the appropriate primitive indications
to the resident M3UA-Users.  Local management is informed.

4.4.3 ASP Auditing

An ASP may optionally initiate an audit procedure in order to enquire
of an SG the availability and, if the congestion method with multiple
congestion levels and message priorities is used, congestion status of
an SS7 destination or set of destinations.  A Destination Audit (DAUD)
message is sent from the ASP to the SG requesting the current
availability and congestion status of one or more SS7 Destination Point
Codes.

The DAUD may be sent un-sequenced. The DAUD may be sent by the ASP in
the following cases:

   - Periodic.  A Timer originally set upon reception of DUNA or SCON
     message has expired without a subsequent DAVA, DUNA or SCON
     updating the availability/congestion status of the affected
     Destination Point Codes.  The Timer is reset upon issuing a DAUD.
     In this case the DAUD is sent to the SG that originally sent the
     SSNM message.

   - the ASP is newly "Inactive" or "Active" or has been isolated from
     an SG for an extended period.  The ASP can request the
     availability/congestion status of one or more SS7 destinations to
     which it expects to communicate.

In the first case, the DAUD procedure must not be invoked for the case
of  received SCON containing a congestion level value of "no
congestion" or undefined" (i.e., congestion Level = "0").  This is
because the value indicates either congestion abatement or that the ITU
MTP3 international congestion method is being used.  In the
international congestion method, the MTP3 at the SG MTP3 does not
maintain the congestion status of any destinations and therefore the SG
cannot provide any congestion information in response to the DAUD.  For
the same reason, in the second case a DAUD cannot reveal any congested
destination(s).

The SG MUST respond to a DAUD with the MTP3 status of the routeset
associated with each Destination Point Code(s) in the DAUD.  The status
of each SS7 destination requested is indicated in a DUNA (if
unavailable), DAVA (if available/uncongested) or an SCON (if
available/congested).  Optionally, any DUNA or DAVA message in response
to a DAUD may contain a list of up to sixteen Affected Point Codes.

Note that from the point of view of an ASP sending an DAUD, the
subsequent reception of an SCON implies that the Affected Destination
is available.  The reception of a DAVA implies that the routeset to the
Affected Destination is not congested.  Obviously with the reception of
an DUNA, the routeset to the Affected Destination can not also be
congested.

5.0 Examples of M3UA Procedures

5.1 Establishment of Association and Traffic between SGs and ASPs

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

This scenario shows the example M3UA 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. The sending of DUNA/SCON messages by the
SG is not shown but would be similar to 5.1.2.

             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 M3UA message flows for the
establishment of traffic between an SG and two ASPs in the same
Application Server, where ASP1 is configured to be "active" and ASP2 a
"standby" in the event of communication failure or the withdrawal from
service of ASP1.  ASP2 may act as a hot, warm, or cold standby
depending on the extent to which ASP1 and ASP2 share call/transaction
state or can communicate call state under failure/withdrawal events.
The example message flow is the same whether the ASP-Active messages
are Over-ride or Load-share mode although typically this example would
use an Over-ride mode. In the case of MTP Restart, the SG starts
sending any relevant DUNA and SCON messages to the ASPs as soon as they
enter the ASP-INACTIVE state. The ASP-Active Ack message is only sent
after all relevant DUNA/SCON messages have been transmitted to the
concerned ASP.

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

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

This scenario shows a similar case to Section 4.1.2 but where the two
ASPs are brought to "active" and load-share the traffic load.  In this
case, one ASP is sufficient to handle the total traffic load. The
sending of DUNA/SCON messages by the SG is not shown but would be
similar to 5.1.2.

       SG                       ASP1                       ASP2
        |                         |                          |
        |<---------ASP Up---------|                          |
        |--------ASP-Up Ack------>|                          |
        |                         |                          |
        |<------------------------------ASP Up---------------|
        |-----------------------------ASP Up Ack------------>|
        |                         |                          |
        |                         |                          |
        |<--ASP Active (Ldshr)----|                          |
        |-----ASP-Active Ack----->|                          |
        |                         |                          |
        |<----------------------------ASP Active (Ldshr)-----|
        |-------------------------------ASP-Active Ack------>|
        |                         |                          |

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

This scenario shows the example M3UA message flows for the
establishment of traffic between an SG and three ASPs in the same
Application Server, where two of the ASPs are brought to "active" and
share the load. In this case, a minimum of two ASPs are required to
handle the total traffic load (2+1 sparing). The sending of DUNA/SCON
messages by the SG is not shown but would be similar to 5.1.2.

   SG                  ASP1                 ASP2                 ASP3
    |                    |                   |                   |
    |<------ASP Up-------|                   |                   |
    |-----ASP-Up Ack---->|                   |                   |
    |                    |                   |                   |
    |<--------------------------ASP Up-------|                   |
    |-------------------------ASP-Up Ack)--->|                   |
    |                    |                   |                   |
    |<---------------------------------------------ASP Up--------|
    |---------------------------------------------ASP-Up Ack---->|
    |                    |                   |                   |
    |                    |                   |                   |
    |<--ASP Act (Ldshr)--|                   |                   |
    |----ASP-Act Ack---->|                   |                   |
    |                    |                   |                   |
    |<--------------------ASP Act. (Ldshr)---|                   |
    |-----------------------ASP-Act Ack----->|                   |
    |                    |                   |                   |

5.2 ASP Traffic Fail-over Examples

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

Following on from the example in Section 4.1.2, and ASP withdraws from
service:

       SG                       ASP1                       ASP2
        |                         |                          |
        |<-----ASP Inactive-------|                          |
        |----ASP Inactive Ack---->|                          |
        |------------------------NTFY(AS-Inact.)(Optional)-->|
        |                         |                          |
        |<------------------------------ ASP Active----------|
        |------------------------------ASP-Active Ack)------>|
        |                                                    |

Note: If the SG detects loss of the M3UA peer (M3UA 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 4.1.2, and ASP2 wishes to
over-ride ASP1 and take over the traffic:

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

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

Following on from the example in Section 4.1.4, and ASP1 withdraws from
service:

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

The Notify message to ASP3 is optional, as well as the ASP-Active from
ASP3.  The optional Notify can only occur if the SG maintains knowledge
of the minimum ASP resources required - for example if the SG knows
that "n+k" = "2+1" for a load-share AS and "n" currently equals "1".

Note: If the SG detects loss of the ASP1 M3UA peer (M3UA heartbeat loss
or detection of SCTP failure), the first SG-ASP1 ASP Inactive message
exchange would not occur.

5.3  M3UA/MTP3-User Boundary Examples

5.3.1 At an ASP

This section describes the primitive mapping from the MTP3 User to M3UA
at an ASP.

5.3.1.1 Support for MTP-Transfer on the ASP

5.3.1.1.1 Support for MTP-Transfer Request
When the MTP3-User on the ASP has data to send into the SS7 network, it
will use the MTP-Transfer Request primitive.  The M3UA on the ASP will
do the following when it receives an MTP-Transfer Request primitive
from the M3UA user:

  - Determine the correct SG

  - Determine the correct association to the chosen SG

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

  - Determine whether to complete the optional fields of the Data
    message

  - Map the MTP-Transfer Request primitive into the Protocol Data
    field of an m3ua Data message

  - Send the Data message to the remote M3UA peer in the SG, over the
    SCTP association

        SG                       ASP
        |                         |
        |<-----Data Message-------|<--MTP-Transfer req.
        |                         |

5.3.1.1.2 Support for MTP Transfer Indication

When the M3UA on the ASP has received Data messages from the remote
M3UA peer in the SG it will do the following:

  - Evaluate the optional fields of the Data message if present

  - Map the Payload of a Data message into the MTP-Transfer Indication
    primitive

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

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

5.3.1.1.3 Support for ASP Querying of SS7 Destination States

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

The M3UA on the ASP normally sends Destination State Audit (DAUD)
messages for each of the destinations that the ASP supports.

       SG                        ASP
        |                         |
        |<-----DAUD Message ------|
        |<-----DAUD Message ------|
        |<-----DAUD Message ------|
        |                         |
        |                         |

5.3.2 At an SG

This section describes the MTP3 upper layer primitive mapping to the
M3UA at the SG.

5.3.2.1 Support for MTP-Transfer Request at the SG

When the M3UA on the SG has received Data messages from its peer
destined to the SS7 network it will do the following:

  - Evaluate the optional fields of the Data message if present to
    determine the network appearance

  - Map the Protocol data of the Data message into an MTP-Transfer
    Request primitive

  - Pass the MTP-Transfer Request primitive to the MTP3 of the
    concerned network appearance.

                            SG                        ASP
                             |                         |
        <---MTP-Transfer req.|<------Data Message------|
                             |                         |

5.3.2.2 Support for MTP-Transfer Indication at the SG

When the MTP3 on the SG has data to pass its user parts, it will use
the MTP-Transfer Indication primitive.  The M3UA on the SG will do the
following when it receives an MTP-Transfer Indication:

  - Determine the correct ASP

  - Determine the correct association to the chosen ASP

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

  - Determine whether to complete the optional fields of the Data
    message

  - Map the MTP-Transfer Indication primitive into the Protocol Data
    field of an M3UA Data message

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

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

5.3.2.3 Support for MTP-PAUSE, MTP-RESUME, MTP-STATUS

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

5.3.2.3.1 Destination Unavailable

The MTP3 on the SG will generate an MTP-PAUSE primitive when it
determines locally that an SS7 destination is unreachable.  The M3UA
will map this primitive to a Destination Unavailable (DUNA) message.
The SG M3UA determines the set of concerned ASPs to be informed based
on internal SS7 network information associated with the MTP-PAUSE
primitive indication.

                   SG                       ASP
                    |                         |
 --MTP-PAUSE ind.-->|------DUNA Message ----->|--MTP-PAUSE ind.-->
                    |                         |

5.3.2.3.2 Destination Available

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

                   SG                       ASP
                    |                         |
--MTP-RESUME ind.-->|------DAVA Message ----->|--MTP-RESUME ind.-->
                    |                         |

5.3.2.3.3 SS7 Network Congestion

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

                     SG                       ASP
                       |                         |
   --MTP-STATUS ind.-->|------SCON Message ----->|--MTP-STATUS ind.-->
                       |                         |

5.3.2.3.4 Destination User Part Unavailable

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

                      SG                       ASP
                       |                         |
   --MTP-STATUS ind.-->|------DUPU Message ----->|--MTP-STATUS ind.-->
                       |                         |

6.0 Security

6.1 Introduction

M3UA is designed to carry signalling messages for telephony services.
As such, M3UA 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.

6.2 Threats

There is no quick fix, one-size-fits-all solution for security.  As a
transport protocol, M3UA has the following security objectives:

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

M3UA runs on top of SCTP.  SCTP [6] provides certain transport related
security features, such as some protection against:

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

When M3UA 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"
[21] should be consulted for guidance.

When the network in which M3UA 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 [22]
for more information on configuring IPSEC services.

6.3 Protecting Confidentiality

Particularly for mobile users, user payload.  Consult [22]
for more information on configuring IPSEC services.

6.3 Protecting Confidentiality

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

7.0 IANA Considerations

7.1 SCTP Payload Protocol Identifier

A request will be made to IANA to assign an M3UA value for the Payload
Protocol Identifier in SCTP Payload Data chunk.  The following SCTP
Payload Protocol Identifier will be registered:

        M3UA    "3"

The SCTP Payload Protocol Identifier is included in each SCTP Data
chunk, to indicate which protocol the SCTP is carrying. This Payload

Protocol Identifier is not directly used by SCTP but MAY be used by
certain network entities to identify the type of information being
carried in a Data chunk.

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

7.2 M3UA Protocol Extensions

This protocol may also be extended through IANA in three ways:
 -- through definition of additional message classes,
 -- through definition of additional message types, and
 -- through definition of additional message parameters

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

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

7.2.1 IETF Defined Message Classes

The documentation for a new message class MUST include the following
information:
(a) A long and short name for the new message class;
(b) A detailed description of the purpose of the message class.

7.2.2 IETF Defined Message Types

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

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

7.2.3 IETF-defined TLV Parameter extension

Documentation of the requirement for confidentiality may
include message parameter MUST contain the masking following
information:

(a) Name of IP addresses and ports.  In this case
application level encryption is not sufficient; IPSEC ESP should be
used instead.  Regardless the parameter type.
(b) Detailed description of which level performs the encryption, structure of the
IPSEC ISAKMP service should be used for key management.

7.0 IANA Considerations

A request will be made to IANA parameter field.  This
    structure MUST conform to assign an M3UA value for the Payload
Protocol Identifier in SCTP Payload Data chunk.  The following SCTP
Payload Protocol Identifier will be registered:

        M3UA    "3"

The SCTP Payload Protocol Identifier is included general type-length-value format
    described in Section 3.1.5.
(c) Detailed definition of 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 component of the type parameter value.
(d) Detailed description of information being
carried in a Data chunk.

The User Adaptation peer may use the Payload Protocol Identifier as a
way intended use of determining additional information about this parameter type,
    and an indication of whether and under what circumstances multiple
    instances of this parameter type may be found within the data being
presented to it by SCTP. same
    message.

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.

9.0  References

[1] RFC 2719, "Framework Architecture for Signaling Transport"

[2] ITU-T Recommendations Q.761 to Q.767, 'Signalling System No.7 (SS7)
    - ISDN User Part (ISUP)'

[3] ANSI T1.113 - 'Signaling System Number 7 - ISDN User Part

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

[5] ITU-T Recommendations Q.711-715, 'Signalling System No. 7 (SS7) -
    Signalling Connection Control Part (SCCP)'

[6] ANSI T1.112 'Signaling System Number 7 - Signaling Connection
    Control Part'

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

[8] ITU-T Recommendations Q.720, 'Telephone User Part'

[9] ITU-T Recommendation Q.771-775 'Signalling System No. 7 SS7) -
    Transaction Capabilities (TCAP)

[10] ANSI T1.114 'Signaling System Number 7 - Transaction Capabilities
     Application Part'

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

[12] 3G TS 25.410 V3.1.0 (2000-01) Technical Specification - 3rd
     Generation partnership Project; Technical Specification Group
     Radio Access Network; UTRAN Iu Interface: General Aspects and
     Principles (3G TS 25.410 Version 3.1.0 Release 1999)

[13] Stream RFC 2719, "Stream Control Transport Protocol <draft-ietf-sigtran-sctp-
     13.txt>, July 2000, Work in Progress Protocol", R. Stewart et al,
     October 2000.

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

[15] ANSI T1.111 'Signaling System Number 7 - Message Transfer Part'

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

[17] ITU-T Recommendation Q.2140 'B-ISDN ATM Adaptation Layer - Service
     Specific Coordination Function for signalling at the Network Node
     Interface (SSCF at NNI)

[18] ITU-T Recommendation Q.2110 'B-ISDN ATM Adaptation Layer - Service
     Specific Connection Oriented Protocol (SSCOP)

[19] MTP2-User Adaptation Layer <draft-ietf-sigtran-m2ua-01.txt>, Nov.
     1999, Work in Progress

[20] ITU-T Recommendation Q.2210 'B-ISDN MTP'

[21] RFC 2196, "Site Security Handbook", B. Fraser Ed., September 1997

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

10.0  Author's Addresses

Lyndon Ong
Nortel Networks
4401 Great America Pkwy
Santa Clara, CA, USA  95054
long@nortelnetworks.com

Greg Sidebottom
Nortel Networks
3685 Richmond Rd,
Nepean, Ontario, Canada  K2H 5B7
gregside@nortelnetworks.com

Guy Mousseau
Nortel Networks
3685 Richmond Rd
Nepean, Ontario, Canada  K2H 5B7

Lyndon Ong
Nortel Networks
4401 Great America Pkwy
Santa Clara, CA, USA  95054
long@nortelnetworks.com

Ian Rytina
Ericsson Australia
37/360 Elizabeth Street
Melbourne, Victoria 3000, Australia
ian.rytina@ericsson.com

Hanns Juergen Schwarzbauer
SIEMENS AG
Hofmannstr. 51
81359 Munich, Germany
HannsJuergen.Schwarzbauer@icn.siemens.de

Klaus D. Gradischnig
SIEMENS AG
Hofmannstr. 51
81359 Munich, Germany
klaus.gradischnig@icn.siemens.de

Ken Morneault
Cisco Systems Inc.
13615 Dulles Technology Drive
Herndon, VA, USA  20171
EMail: kmorneau@cisco.com

Malleswar Kalla
Telcordia Technologies
MCC 1J211R
445 South Street
Morristown, NJ, USA  07960
EMail:
Email: kalla@research.telcordia.com

Normand Glaude
Performance Technologies
150 Metcalf Sreet, Suite 1300
Ottawa, Ontario, Canada  K2P 1P1
EMail: nglaude@microlegend.com

This draft expires March May 2000.