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draft-ietf-sigtran-m3ua-07

Description: Request For Comments

You can download source copies of the file as follows:

draft-ietf-sigtran-m3ua-07.txt in text format.

Listed below is the contents of file draft-ietf-sigtran-m3ua-07.txt.


Network Working Group                                   Greg Sidebottom
INTERNET-DRAFT                                             Guy Mousseau
                                                        Nortel Networks
                                                             Lyndon Ong
                                                                  Ciena
                                                             Ian Rytina
                                                               Ericsson
                                             Hanns Juergen Schwarzbauer
                                                                Siemens
                                                      Klaus Gradischnig
                                                                NeuStar
                                                          Ken Morneault
                                                                  Cisco
                                                          Mallesh Kalla
                                                              Telcordia
                                                         Normand Glaude
                                               Performance Technologies

Expires in six months                                         Jul 2001

                SS7 MTP3-User Adaptation Layer (M3UA)
                  <draft-ietf-sigtran-m3ua-07.txt>

Status of This Memo

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

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

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/1id-abstracts.html

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.

To learn the current status of any Internet-Draft, please check the 
'1id-abstracts.txt' listing contained in the Internet- Drafts Shadow 
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), 
munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or 
ftp.isi.edu (US West Coast).

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 

Sidebottom et al                                              [Page 1]

Internet Draft         SS7 MTP3-User Adaptation Layer         Jul 2001

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. 

Sidebottom et al                                              [Page 2]

Internet Draft         SS7 MTP3-User Adaptation Layer         Jul 2001

                        TABLE OF CONTENTS

1. Introduction.......................................................4
    1.1 Scope.........................................................4
    1.2 Terminology...................................................4
    1.3 M3UA Overview.................................................6
    1.4 Functional Areas.............................................12
    1.5 Sample Configurations........................................21
    1.6 Definition of M3UA Boundaries................................24
2. Conventions.......................................................28
3. M3UA Protocol Elements............................................28
    3.1 Common Message Header........................................28
    3.2 Variable-Length Parameter....................................31
    3.3 Transfer Messages............................................32
    3.4 SS7 Signalling Network management (SSNM) Messages............35
    3.5 ASP State Maintenance (ASPM) Messages........................43
    3.6 Routing Key Management (RKM) Messages........................46
    3.7 ASP Traffic Maintenance (ASPTM) Messages.....................55
    3.8 Management(MGMT) Messages....................................60
4. Procedures........................................................64
    4.1 Procedures to Support the M3UA-User and Layer Management
        Layers.......................................................64
    4.2 Procedures to Support the Management of SCTP Associations
        with M3UA Peers..............................................67
    4.3 Procedures to support the Unavailability or Congestion 
        Status of SS7 Destinations...................................81
    4.4 MTP3 Restart.................................................83
5. Examples of M3UA Procedures.......................................84
    5.1 Establishment of Association and Traffic 
        Between SGs and ASPs.........................................84
    5.2 ASP traffic Fail-over Examples...............................89
    5.3 Normal Withdrawal of an ASP from an Application Server 
        and Tear-down of an Association..............................90
    5.4.M3UA/MTP3-User Boundary Examples.............................91
6. Security..........................................................95
    6.1 Introduction.................................................95
    6.2 Threats......................................................95
    6.3 Protecting Confidentiality...................................95
7. IANA Considerations...............................................96
    7.1 SCTP Payload Protocol Identifier.............................96
    7.2 M3UA Protocol Extensions.....................................96
8. Acknowledgements..................................................97
9. References........................................................97
10.  Bibliography....................................................99
11.  Author's Addresses..............................................99

Sidebottom et al                                              [Page 3]

Internet Draft         SS7 MTP3-User Adaptation Layer         Jul 2001

1.  Introduction

1.1 Scope

There is a need for Switched Circuit Network (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, as well as certain MTP network management events, 
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 SIO/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.  An AS is contained within a 
single Network Appearance.  Note that there is a 1:1 relationship 
between an AS and a Routing Key.

Application Server Process (ASP) - A process instance of an Application 
Server. An Application Server Process serves as an active or back-up 
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. 

Sidebottom et al                                              [Page 4]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

Association - An association refers to an SCTP association.  The 
association provides the transport for the delivery of 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 M3UA in a point-to-point 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, back-up or load-sharing process of a 
Signalling Gateway.

Signalling Gateway - An SG is a signaling agent that receives/sends SCN 
native signaling at the edge of the IP network [1].  An SG appears to 
the SS7 network as an SS7 Signalling Point.  An SG contains a set of one 
or more unique Signalling Gateway Processes, of which one or more is 
normally actively processing traffic.  Where an SG contains more than 
one SGP, the SG is a logical entity and the contained SGPs must be 
coordinated into a single management view to the SS7 network and to the 
supported Application Servers.

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

Routing Key: A Routing Key describes a set of SS7 parameters and 
parameter values that uniquely define the range of signalling traffic 
to be handled by a particular Application Server. Parameters within the 
Routing Key cannot extend across more than a single SS7 Destination 
Point Code.

Routing Context - A value that uniquely identifies a Routing Key.
Routing Context values are either configured using a configuration 
management interface, or by using the routing key management procedures 
defined in this document.

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-over also applies 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

Sidebottom et al                                              [Page 5]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

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 uniquely identifies an SS7 
entity (Point Code) into an SS7 network, as presented by the SG.  It is 
used for the purposes of logically separating the signalling traffic 
between the SG and the Application Server Processes over a common SCTP 
association.  This partitioning is necessary where an SG is logically 
partitioned to appear as end node elements in multiple separate SS7 
networks, in which case there is a separate network appearance for each 
point code in the SS7 networks.  It is also necessary when an SG is 
configured as an STP hosting multiple point codes, or when configured 
as multiple end nodes within the same network, in which case each point 
code is a separate network appearance.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.  

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 

Sidebottom et al                                              [Page 6]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

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 protocol as SCCP payload, as they are SCCP-User protocols.  

It is recommended that 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 a 
requirement and TCP can be used as the underlying common transport 
protocol.  

1.3.2 Services Provided by the M3UA Layer

The M3UA Layer at an ASP or IPSP provides the equivalent set of 
primitives at its upper layer to the MTP3-Users as provided by the MTP 
Level 3 to its local MTP3-Users at an SS7 SEP.  In this way, the ISUP 
and/or SCCP layer at an ASP or IPSP is unaware that the expected MTP3 
services are offered remotely from an MTP3 Layer at an SGP, and not by 
a local MTP3 layer.  The MTP3 layer at an SGP 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 layer does not itself provide the MTP3 services.  
However, in the case where an ASP is connected to more than one SGP, 
the 
M3UA layer at an ASP must maintain the status of configured SS7 
destinations and route messages according to the availability and 
congestion status of the routes to these destinations via each SGP.

Sidebottom et al                                             [Page 7]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

The M3UA layer may also be used for point-to-point signalling between 
two IP Server Processes (IPSPs).  In this case, the M3UA layer 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 SGP.  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 layer provides the transport of MTP-TRANSFER primitives across 
an established SCTP association between an SGP and an ASP or between 
IPSPs.

The MTP-TRANSFER primitive information is encoded as in MTP3-User 
messages.  In this way, the SCCP and ISUP messages received from the 
SS7 network by the SGP are not re-encoded into a different format for 
transport between the M3UA peers.  The MTP3 Service Information Octet 
(SIO) and Routing Label (OPC, DPC, and SLS) are included, encoded as 
expected by the MTP3 and MTP3-User protocol layer. 

At an ASP, in the case where a destination is reachable via multiple 
SGPs, the M3UA layer must also choose via which SGP the message is to 
be routed or support load balancing across the SGPs, ensuring that no 
missequencing occurs.

The M3UA layer does not impose a 272-octet signalling information field 
(SIF) length limit as specified by the SS7 MTP Level 2 protocol [14] 
[15] [16].  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 SGPs.  
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 layer provides management of the underlying SCTP transport 
protocol to ensure that SGP-ASP and IPSP-IPSP transport is available to 
the degree called for by the MTP3-User signalling applications.

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

Sidebottom et al                                             [Page 8]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

1.3.2.3 Inter-working with MTP3 Network Management Functions

At the SGP, the M3UA layer 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 destination in the SS7 network are experiencing SS7 
    congestion.

  - Providing an indication to the M3UA layer at an ASP that the routes 
    to a remote destination in the SS7 network are restricted.

  - 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, the 
restricted or the congested state of remote SS7 destinations.  This 
information is requested from the M3UA layer at the SGP. 

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

1.3.2.4 Support for the Management of SCTP Associations between the SGP 
and ASPs.

The M3UA layer at the SGP maintains the availability state of all 
configured remote ASPs, in order to manage the SCTP Associations and 
the traffic between the M3UA peers.  As well, the active/inactive and 
congestion state of remote ASPs is 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 primitives to request, indicate and confirm the 
establishment of an SCTP association with a peer M3UA node.  In order 
to avoid redundant SCTP associations between two M3UA peers, one side 
(client) SHOULD be designated to establish the SCTP association, or 
M3UA configuration knowledge maintained to detect redundant 
associations (e.g., via knowledge of the expected local and remote SCTP 
endpoint addresses).

Local management MAY request from the M3UA layer the status of the 
underlying SCTP associations using the M-SCTP_STATUS request and 

Sidebottom et al                                              [Page 9]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

confirm primitives.  Also, the M3UA MAY autonomously 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 inform the local management of the change in 
status of an ASP or AS.  This may be achieved using the M-ASP_request 
or M-AS_STATUS request primitives.

1.3.2.5 Support for the Management of Connections to Multiple SGPs

As shown in Figure 1 an ASP may be connected to multiple SGPs. In such 
a case a particular SS7 destination may be reachable via more than one 
SGP, i.e., via more than one route. As MTP3 users only maintain status 
on a destination and not on a route basis, the M3UA layer must maintain 
the status (availability, restriction, and/or congestion of route to 
destination) of the individual routes, derive the overall availability 
or congestion 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 SGPs or 
SGs, redundant hosts, and the provision of redundant QOS-bounded IP 
network paths for SCTP Associations between SCTP End Points. Obviously, 
the reliability of the SG, the MGC and other IP-based functional 
elements also needs to be taken into account.  The distribution of ASPs 
and SGPs within the available Hosts SHOULD also be considered.  As an 
example, for a particular Application Server, the related ASPs SHOULD 
be distributed over at least two Hosts.

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

Sidebottom et al                                              [Page 10]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

          SG                                     MGC

  Host#1 **************                          ************** Host#3
     =   *  ********__*__________________________*__********  *   =
         *  * SGP1 *__*_____      _______________*__* ASP1 *  *  MGC1
         *  ********  *     \    /               *  ********  *
         *  ********__*______\__/________________*__********  *
         *  * SGP2 *__*_______\/______      _____*__* ASP2 *  *
         *  ********  *       /\      |    |     *  ********  *
         *      :     *      /  \     |    |     *      :     *
         *  ********  *     /    \    |    |     *  ********  *
         *  * SGPn *  *     |    |    |    |     *  * ASPn *  *
         *  ********  *     |    |    |    |     *  ********  *
         **************     |    |    |    |     **************
                            |    |    \    /
  Host#2 **************     |    |     \  /      ************** Host#4
     =   *  ********__*_____|    |______\/_______*__********  *   =
         *  * 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 
Servers, and is identified as an SCTP end point.  A pair of signalling 
gateway processes may represent, as an example, a single Signalling 
Gateway, 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.

Sidebottom et al                                             [Page 11]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

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

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

1.4 Functional Areas

1.4.1 Signalling Point Code Representation

For example, within an SS7 network, a Signalling Gateway might be 
charged with representing a set of nodes in the IP domain into the SS7 
network for routing purposes.  The SG itself, as a signalling point in 
the SS7 network, might also be addressable with an SS7 Point Code for 
MTP3 Management purposes. The SG Point Code might also 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 Point Codes [15] may also be 
used within an SG network appearance. 

Where an SG contains more than one SGP, the MTP3 routeset, SPMC and 
remote AS/ASP states of each SGP SHOULD be coordinated across all the 
SGPs.  Re-routing of traffic between the SGPs SHOULD also be supported

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

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 

Sidebottom et al                                             [Page 12]

Internet Draft       SS7 MTP3-User Adaptation Layer         Jul 2001

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 SGP it is equally 
important that the corresponding Routing Keys in the involved SGPs are 
identical.  (Note: It is possible for the SGP 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:.  SG-to SG communication is recommended for carrier grade 
networks, using an MTP3 linkset or an equivalent, to allow re-routing 
between the SGs in the event of route failures.  Where SGPs are used, 
inter-SGP communication is recommended.  Inter-SGP protocol is outside 
of the scope of this document.

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 Routing Contexts and Routing Keys

1.4.2.1 Overview

The distribution of SS7 messages between the SGP and the Application 
Servers is determined by the Routing Keys and their associated Routing
Contexts. A Routing Key is essentially a set of SS7 parameters used to 

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filter SS7 messages, whereas the Routing Context parameter is a 4-byte 
value (integer) that is associated to that Routing Key in a 1:1 
relationship. The Routing Context therefore can be viewed as an index 
into a sending node's Message Distribution Table containing the Routing 
Key entries.

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 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 Process may be configured to process signalling 
traffic related to more than one Application Server, over a single SCTP 
Association.  In ASP Active and ASP Inactive management messages, the 
signalling traffic to be started or stopped is discriminated by the 
Routing Context parameter.  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.

1.4.2.2 Routing Key Limitations

Routing Keys SHOULD be unique in the sense that each received SS7 
signalling message SHOULD have a single routing result to an 
Application Server. It is not necessary for the parameter range values 
within a particular Routing Key to be contiguous.  For example, an AS 
could be configured to support call processing for multiple ranges of 
PSTN trunks that are not represented by contiguous CIC values. 

1.4.2.3 Managing Routing Contexts and Routing Keys

There are two ways to provision a Routing Key at an SGP. A 
Routing Key may be configured statically using an implementation 
dependent management interface, or dynamically using the M3UA Routing 
Key registration procedure. A Routing Key may also be configured using 
the M3UA dynamic registration/deregistration procedures defined in this 
document.  An M3UA element must implement at least one method of 
Routing Key provisioning.

When using a management interface to configure Routing Keys, the 
message distribution function within the SGP is not limited to the set 
of parameters defined in this document.  Other implementation dependent 
distribution algorithms may be used.

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1.4.2.4 Message Distribution at the SGP

In order to direct messages received from the SS7 MTP3 network to the 
appropriate IP destination, the SGP must perform a message distribution 
function using information from the received MTP3-User message.

To support this message distribution, the SGP 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 currently defined  Routing Keys 
in the SGP.  These Routing Keys in turn make reference to an 
Application Server that is enabled by one or more ASPs.  These ASPs 
provide dynamic status information on their availability, traffic 
handling capability and congestion to the SGP using various management 
messages defined in the M3UA protocol.  

The list of ASPs in an AS is assumed to be dynamic, taking into account 
the availability, traffic handling capability and congestion status of 
the individual ASPs in the list, as well as configuration changes and 
possible fail-over mechanisms. 

Normally, one or more ASPs are active in the AS (i.e., currently 
processing traffic) but in certain failure and transition cases it is 
possible that there may be no active ASP available.  Both load-sharing 
and backup scenarios are supported.

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

1.4.2.5 Message Distribution at the ASP

In order to direct messages to the SS7 network, the ASP must also 
perform a message distribution function in order to choose the proper 
SGP for a given message.  This is accomplished by observing the 
Destination Point Code (and possibly other elements of the outgoing
message such as the SLS value).Where more than one route (or SGP) is 
possible for routing to the SS7 network, the ASP SHOULD maintain a 
dynamic table of available SGP routes for the SS7 destinations, taking 
into account the SS7 destination availability/restricted/congestion 
status received from the SGP(s), the availability status of the 
individual SGPs and configuration changes and fail-over mechanisms. 
There is, however, no M3UA messaging to manage the status of an SGP 
(e.g., SGP-Up/Down/Active/Inactive messaging).  Whenever an SCTP 

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association to an SGP exists, the SGP is assumed to be ready for the 
purposes of responding to M3UA ASPSM messages.  

Every SGP of one SG 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 
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].  

Note: It is also possible for IP-based interfaces to be present, using 
the services of the MTP2-User Adaptation Layer (M2UA) [23] or M2PA [].  
These may be terminated at a Signalling Transfer Point (STP) or 
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.

1.4.3.2 SS7 and M3UA Inter-Working at the SG

The SGP 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 relevant 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.  Where more than one SGP 

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constitutes an SG, the knowledge of the SGPs must be coordinated into 
an overall SG view.

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 also made available to any existing 
local MTP3-Users at the SG, if present.)

MTP3 management messages (such as TFPs or TFAs received from the SS7 
network) MUST NOT be encapsulated as Data message Payload Data and sent 
either from SG to ASP or from ASP to SG.  The SG MUST terminate these 
messages and generate M3UA messages as appropriate. 

1.4.3.3 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 (and any other MTP3 user located at the point code of 
the SPMC) 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 SGP, the M3UA 
layer must maintain the status of configured SS7 destinations and route 
messages according to availability/congestion/restricted status of the 
routes to these SS7 destinations.

1.4.3.4 IPSP Considerations

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.

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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/back-up redundancy is a subset of this model. A simplex 
"1+0" model is also supported as a subset, with no ASP redundancy.

At the SGP, 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 SGP 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 Host3 or ASP1 in Host4.  The AS list at SGP1 in Host 1 might look 
like the following:

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

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

The AS List at SGP1 in Host1 might also be set up in load-share mode:

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

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

Care 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.  

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

1.4.4.2 Signalling Gateway Redundancy

Signalling Gateways MAY also be distributed over multiple hosts.  Much 
like the AS model, SGs may comprise one or more SG Processes (SGPs), 
distributed over one or more hosts, using an active/back-up or a load-
sharing model.  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 SHOULD 
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.  The distribution of the MTP3-user messages over the 
SGPs should be done in such a way to minimize message mis-sequencing, 
as required by the SS7 User Parts.

It may also be possible for an ASP to use more than one SG to access a 
specific SS7 end point, in a model that resembles an SS7 STP mated 
pair.  Typically, SS7 STPs are deployed in mated pairs, with traffic 
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 layer 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 
returned an acknowledgement to the ASP to indicate that the ASP is 
actively handling traffic for that destination, and the SGP has not 
indicated that the destination is inaccessible.  When an ASP is 
configured to use multiple SGPs for transferring traffic to the SS7 
network, the ASP must maintain knowledge of the current capability of 
the SGPs to handle traffic to destinations of interest.  This 
information is crucial to the overall reliability of the service, for

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both active/back-up 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.  The distribution of the 
MTP3-user messages over the SGPs should be done in such a way to 
minimize message mis-sequencing, as required by the SS7 User Parts.

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). 

At an ASP or IPSP, the M3UA layer indicates congestion to local MTP3-
Users by means of an MTP-STATUS primitive, as per current MTP3 
procedures, to invoke appropriate upper layer responses.  

When an SG determines that the transport of SS7 messages to a 
Signalling Point Management Cluster (SPMC) is encountering congestion, 
the SG MAY trigger SS7 MTP3 Transfer Controlled management messages 
to originating SS7 nodes, per the congestion procedures of the relevant 
MTP3 standard. The triggering of SS7 MTP3 Management messages from an 
SG is an implementation-dependent function.

The M3UA layer at an ASP or IPSP should indicate local congestion to an 
M3UA peer with an SCON message.  When an SG receives a congestion 
message (SCON) from an ASP, and the SG determines that an SPMC is now 
encountering congestion, it MAY trigger SS7 MTP3 Transfer Controlled 
management messages to concerned SS7 destinations according to 
congestion procedures of the relevant MTP3 standard.

1.4.7 SCTP Stream Mapping.  

The M3UA layer at both the SGP 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 

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be done in such a way to minimize message mis-sequencing, as required 
by the SS7 User Parts.

1.4.8 Client/Server Model

It is recommended that the SGP and ASP be able to support both client and server 
operation. The peer endpoints using M3UA SHOULD be configured so that one always 
takes on the role of client and the other the role of server for initiating SCTP 
associations. The default orientation would be for the SGP to take on the role 
of server while the ASP is the client. In this case, ASPs SHOULD initiate the
SCTP association to the SGP

In the case of IPSP to IPSP communication, the peer endpoints using 
M3UA SHOULD be configured so that one always takes on the role of 
client and the other the role of server for initiating SCTP 
associations.

The SCTP Registered User Port Number Assignment for M3UA is 2905.

1.5 Sample Configurations

1.5.1 Example 1: ISUP Message Transport

  ********   SS7   *****************   IP   ********
  * SEP  *---------*      SGP      *--------* 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 SGP 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 SGP 
serves as the interface within the SGP 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.

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For internal SGP 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, 
translated to MTP-TRANSFER request primitives, and sent to the local 
M3UA-resident message distribution function for ongoing routing to the 
final IP destination.  Messages received from the local M3UA network 
address translation and mapping function as MTP-TRANSFER indication 
primitives 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. In addition, as an 
implementation and network option, restricted destinations are 
communicated from MTP network management 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, MTP-RESUME or MTP-
STATUS indications from the M3UA layer to the SCCP layer should 
consider the status of the SCTP Association and underlying IP network 
and any congestion information received from the remote site.    

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1.5.3 Example 3: SGP Resident SCCP Layer, with Remote ASP

  ********   SS7   *****************   IP   ********
  * SEP  *---------*               *--------*      *
  *  or  *         *      SGP      *        * 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 SGP 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 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 SGP can perform the SCCP GTT service 
for messages logically addressed to it from SCCP peers in the IP 
domain.  In this case, MTP-TRANSFER indication primitives 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 primitive is given to the MTP3 for delivery to an SS7-
resident node.

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

For internal SGP modeling purposes, this may be accomplished with the 
use of an implementation-dependent nodal inter-working function within 
the SGP that effectively sits below the SCCP and routes MTP-TRANSFER

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request/indication messages to/from both the MTP3 and the M3UA layer, 
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 layer are the 
same as in Example 1 and the functions taking place in the SCCP entity 
are transparent to the M3UA layer.  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

An example of the upper layer primitives provided by the SCTP are 
provided in Reference [13] Section 10.

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 its 
            peer.

   M-STCP_ESTABLISH confirm 
   Direction: M3UA -> LM
   Purpose: ASP confirms to LM that it has established an SCTP 
            association with its peer.

   M-SCTP_ESTABLISH indication 
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that 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 its 
            peer.

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

   M-SCTP_RELEASE indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that a remote ASP has released an SCTP 
            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 confirm 
   Direction: M3UA -> LM
   Purpose: M3UA responds with the status of an SCTP association.

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

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

   M-ASP_STATUS confirm 
   Direction: M3UA -> LM
   Purpose: M3UA reports status of local or remote ASP.

   M-AS_STATUS request 
   Direction: LM -> M3UA
   Purpose: LM requests M3UA to report the status of an AS.

   M-AS_STATUS confirm 
   Direction: M3UA -> LM
   Purpose: M3UA reports the status of an AS.

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

   M-ERROR indication 
   Direction: M3UA -> LM
   Purpose: M3UA reports that it has received an Error message from 
            its peer or that a local operation has been unsuccessful.

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   M-ASP_UP request 
   Direction: LM -> M3UA
   Purpose: LM requests ASP to start its operation and send an ASP Up
            message to its peer.
   M-ASP_UP confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that is has received an ASP UP Ack message from 
            its peer.

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

   M-ASP_DOWN request 
   Direction: LM -> M3UA
   Purpose: LM requests ASP to stop its operation and send an ASP-Down 
            message to its peer.

   M-ASP_DOWN confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that is has received an ASP Down
            Ack message from its peer.

   M-ASP_DOWN indication
   Direction: M3UA -> LM
   Purpose: M3UA reports it has successfully processed an incoming ASP 
            Down message from its peer, or the SCTP association has 
            been lost/reset.

   M-ASP_ACTIVE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to send an ASP Active message to its peer.

   M-ASP_ACTIVE confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that is has received an ASP Active
            Ack message from its peer.

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

   M-ASP_INACTIVE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to send an ASP Inactive message to its 
            peer.

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   M-ASP_INACTIVE confirm
   Direction: LM -> M3UA
   Purpose: ASP reports that is has received an ASP Inactive
            Ack message from its peer.

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

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

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

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

If dynamic registration of RK is supported by the M3UA layer, the layer 
MAY support the following additional primitives:

   M-RK_REG request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to register RK(s) with its peer by sending 
            REG REQ message

   M-RK_REG confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that it has received REG RSP message with 
            registration status as successful from its peer.

   M-RK_REG indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that it has successfully processed an 
            incoming REG REQ message.

   M-RK_DEREG request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to de-register RK(s) with its peer by 
            sending DEREG REQ message.

   M-RK_DEREG confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that it has received DEREG REQ message with de-
            registration status as successful from its peer.

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   M-RK_DEREG indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that it has successfully processed an 
            incoming DEREG REQ from its peer.

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. 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 header 
which contains the adaptation layer version, the message type, and 
message length.

    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.

3.1.1 M3UA Protocol Version: 8 bits (unsigned integer)

   The version field contains the version of the M3UA adaptation layer.  

   The supported versions are the following:

         1      Release 1.0

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3.1.2  Message Classes and Types

The following list contains the valid Message Classes:

Message Class: 8 bits (unsigned integer)

   The following list contains the valid Message Type Classes:

     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     Reserved for Other Sigtran Adaptation Layers
     6      Reserved for Other Sigtran Adaptation Layers
     7     Reserved for Other Sigtran Adaptation Layers
     8     Reserved for Other Sigtran Adaptation Layers
     9     Routing Key Management (RKM) Messages
  10 to 127 Reserved by the IETF
 128 to 255 Reserved for IETF-Defined Message Class extensions

Message Type: 8 bits (unsigned integer)

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

     Management (MGMT) Messages (See Section 3.6)

         0        Error (ERR)
         1        Notify (NTFY)
      2 to 127    Reserved by the IETF
    128 to 255    Reserved for IETF-Defined MGMT extensions

     Transfer Messages (See Section 3.3)

         0        Reserved
         1        Payload Data (DATA)      
      2 to 127    Reserved by the IETF
    128 to 255    Reserved for IETF-Defined Transfer extensions

     SS7 Signalling Network Management (SSNM) Messages (See Section 
3.4)

         0        Reserved
         1        Destination Unavailable (DUNA)
         2        Destination Available (DAVA)
         3        Destination State Audit (DAUD)
         4        SS7 Network Congestion (SCON)
         5        Destination User Part Unavailable (DUPU)
         6        Destination Restricted (DRST)      
      7 to 127    Reserved by the IETF
    128 to 255    Reserved for IETF-Defined SSNM extensions

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  ASP State Maintenance (ASPSM) Messages (See Section 3.5)

         0        Reserved
         1        ASP Up (ASPUP)
         2        ASP Down (ASPDN)         
         3        Heartbeat (BEAT)
         4        ASP Up Acknowledgement (ASPUP ACK)
         5        ASP Down Acknowledgement (ASPDN ACK)
         6        Heatbeat Acknowledgement (BEAT ACK)
      7 to 127    Reserved by the IETF
    128 to 255    Reserved for IETF-Defined ASPSM extensions

  ASP Traffic Maintenance (ASPTM) Messages (See Section 3.5)

         0        Reserved
         1        ASP Active (ASPAC)
         2        ASP Inactive (ASPIA)
         3        ASP Active Acknowledgement (ASPAC ACK) 
         4        ASP Inactive Acknowledgement (ASPIA ACK)
      5 to 127    Reserved by the IETF
    128 to 255    Reserved for IETF-Defined ASPTM extensions

  Routing Key Management (RKM) Messages (See Section 3.7)

         0        Reserved
         1        Registration Request (REG REQ)
         2        Registration Response (REG RSP)
         3        Deregistration Request (DEREG REQ) 
         4        Deregistration Response (DEREG RSP)
      5 to 127    Reserved by the IETF
    128 to 255    Reserved for IETF-Defined RKM extensions

3.1.3  Reserved: 8 bits

   The Reserved field SHOULD be set to all '0's and ignored by the 
   receiver.

3.1.4  Message Length: 32-bits (unsigned integer)

   The Message Length defines the length of the message in octets, 
   including the Common Header.  For messages with a final parameter 
   containing padding, the parameter padding MUST be included in the 
   Message Length. 

   Note: A receiver SHOULD accept the message whether or not the final 
   parameter padding is included in the message length.  

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3.2 Variable-Length Parameter Format

M3UA messages consist of a Common Header followed by zero or more 
variable length parameters, as defined by the message type.  All the 
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                         /
  \                                                               \
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Where more than one parameter is included in a message, the parameters 
may be in any order, except where explicitly mandated.  A receiver 
SHOULD accept the parameters in any order. 

Parameter Tag: 16 bits (unsigned integer)

   The Tag field is a 16-bit identifier of the type of parameter. It 
   takes a value of 0 to 65534.  Common parameters used by adaptation 
   layers are in the range of 0x00 to 0xff.   M3UA-specific parameters 
   have Tags in the range 0x80 to 0xbf.  The parameter Tags defined are 
   as follows:

         0x00        Reserved
         0x80        Network Appearance
         0x81        Protocol Data 1
         0x82        Protocol Data 2
         0x04        INFO String
         0x83        Affected Destinations
         0x06        Routing Context
         0x07        Diagnostic Information
         0x09        Heartbeat Data
         0x84        User/Cause
         0x0a        Reason
         0x0b        Traffic Mode Type
         0x0c        Error Code
         0x0d        Status 
         0x85        Congestion Indications
         0x86        Concerned Destination
         0x87        Routing Key
         0x88        Registration Result
         0x89        De-registration Result
         0x8a        Local_Routing Key Identifier
         0x8b        Destination Point Code
         0x8c        Service Indicators
  

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         0x8d        Subsystem Numbers
         0x8e        Originating Point Code List
         0x8f        Circuit Range
         0x90        Registration Results
         0x91        De-Registration Results
      0x92 to ffff...Reserved by the IETF
 
   The value of 65535 is reserved for IETF-defined extensions.  Values 
   other than those defined in specific parameter description are 
   reserved for use by the IETF. 

Parameter Length: 16 bits (unsigned integer)

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

Parameter Value: variable-length.

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

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

3.3 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 1 or 2     Mandatory

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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 = 0x80            |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Network Appearance                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Tag = 0x81 or 0x82       |             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 SGP and the ASP 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 and possibly the 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, the Network Appearance 
   parameter is not required.  In other cases the parameter MUST be 
   included.

   The Network Appearance parameter value is of local significance 
   only, coordinated between the SGP and ASP. Therefore, in the case 
   where an ASP is connected to more than one SGP, the same SS7 network 
   context may be identified by different Network Appearance values 
   depending over which SGP a message is being transmitted/received.

   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

   One of two possible Protocol Data parameters are included in a DATA 
   message: Protocol Data 1 or Protocol Data 2.  

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Protocol Data 1 or 2: variable length
 
   The Protocol Data 1 parameter contains the original SS7 MTP3 
   message, including the Service Information Octet and Routing Label. 

   The Protocol Data 1 parameter contains the following fields:

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

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

       User Protocol Data.  Includes:
            MTP3-User protocol elements (e.g., ISUP, SCCP, or TUP 
               parameters).
       
   The Protocol Data 2 parameter contains all the information in 
   Protocol Data 1 as described above, plus the MTP2 Length Indicator 
   octet.  The MTP2 Length Indicator (LI) octet appears before the SIO 
   and Routing Label information.  The MTP2 Length Indicator octet is 
   required for some national MTP variants that use the spare bits in 
   the LI to carry additional information of interest to the MTP3 and 
   MTP3-User (e.g., the Japan TTC standard use of LI spare bits to 
   indicate message priority) 

   The Payload Data 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.  
   Note: In the SS7 Recommendations, the format of the messages and 
   fields within the messages are based on bit transmission order.  In 
   these recommendations the Least Significant Bit (LSB) of each field 
   is positioned to the right.  The received SS7 fields are populated 
   octet by octet as received into the 4-octet word as shown in the two 
   examples below.

   For the ANSI protocol example, the Protocol Data 1 field format is 
   shown below:

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      SIO      |  DPC Member   |  DPC Cluster  |  DPC Network  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  OPC Member   |  OPC Cluster  |  OPC Network  |      SLS      | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                        Protocol Data                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Within each octet the Least Significant Bit (LSB) per the SS7 
   Recommendations is to the right (e.g., bit 7 of SIO is the LSB).

   For the ITU international protocol example (with the 3/8/3 Point 
   Code format), the Protocol Data 1 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 |OPC| DPC | DPC |   OPC       |@|
    |               | Region *| SP *|SP*|Zone*| reg.|  Region    *| |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  SLS  | OPC |$|      Protocol                                 |
    |      *|Zone*| |        Data                                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    * marks LSB of each field;  @ = OPC SP MSB;  $ = OPC region MSB

3.4 SS7 Signalling Network Management (SSNM) Messages

3.4.1 Destination Unavailable (DUNA)

The DUNA message is sent from all SGPs in an SG to all concerned ASPs 
to indicate that the SG has determined that one or more SS7 
destinations are unreachable.  It is also sent by an SGP in response to 
a message from the ASP to an unreachable SS7 destination.  As an 
implementation option the SG may suppress the sending of subsequent 
"response" DUNA messages regarding a certain unreachable SS7 
destination for a certain period in order to give the remote side time 
to react. The MTP3-User at the ASP is expected to stop traffic to the 
affected destination through the SGPs initiating the DUNA message as 
per the defined MTP3-User procedures. 

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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 = 0x80            |           Length =8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Network Appearance                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x83            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Mask      |                 Affected DPC 1                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Mask      |                 Affected DPC n                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 0x04           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Appearance: 32-bit unsigned integer

   See Section 3.3.1

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.

   

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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-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 least 
   significant 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 
   least significant three bits of the DPC is "wildcarded".  For a 14-
   bit ITU Affected DPC, this is equivalent to signaling that an ITU 

   
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   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 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 SGP to all concerned ASPs to indicate 
that the SG has determined that one or more SS7 destinations are now 
reachable (and not restricted), or in response to a DAUD message if 
appropriate. The ASP MTP3-User protocol is informed and may now resume 
traffic to the affected destination.  The ASP M3UA layer routes the 
MTP3_user traffic through the SGP(s) initiating the DAVA message. 

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 MAY be sent from the ASP to the SGP 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 SGP to all concerned ASPs to 
indicate congestion in the SS7 network to one or more destinations, or 

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to an ASP in response to a DATA or DAUD message as appropriate.  For 
some MTP protocol variants (e.g., ANSI MTP) the SCON message may be 
sent when the SS7 congestion level changes.  The SCON message MAY also 
be sent from the M3UA layer of an ASP to an M3UA peer indicating that 
the M3UA layer or the ASP is congested.

The SCON message contains the following parameters:

     Network Appearance       Optional
     Affected Destinations    Mandatory
     Concerned Destination    Optional
     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 = 0x80            |           Length =8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                       Network Appearance                      | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x83            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Mask     |                 Affected DPC 1                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Mask     |                 Affected DPC n                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x86            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    reserved   |                 Concerned DPC                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x85            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Reserved                    |  Cong. Level  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 0x04            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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).

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The Affected Destinations parameter can be used to indicate congestion 
of multiple destinations or ranges of destinations.  However, an SCON 
message MUST not be delayed in order to "collect" individual congested 
destinations into a single SCON message 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. 

Concerned Destination: 32-bits
   
   The optional Concerned Destination parameter is only used if the 
   SCON message is sent from an ASP to the SGP. It contains the point 
   code of the originator of the message that triggered the SCON 
   message. The Concerned Destination parameter contains one Concerned 
   Destination Point Code field, a three-octet parameter to allow for 
   14-, 16- and 24-bit binary formatted SS7 Point Codes.  A Concerned 
   Point Code that is less than 24-bits is padded on the left to the 
   24-bit boundary. Any resulting Transfer Controlled (TFC) message 
   from the SG is sent to the Concerned Point Code  using the single 
   Affected DPC contained in the SCON message to populate the 
  (affected) Destination field of the TFC message

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 all of the Affected 
   DPC(s) in the Affected Destinations parameter, contains one of the 
   Following values:

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

   The congestion levels are defined in the congestion method in the 
   appropriate national MTP recommendations [14,15]. 

3.4.5 Destination User Part Unavailable (DUPU)

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

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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 = 0x80            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                      Network Appearance                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x83            |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Mask = 0    |                  Affected DPC                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x84            |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Cause             |            User               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x04            |             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 

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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 align 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 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
         11        Reserved
         12        AAL type 2 Signalling
         13        Bearer Independent Call Control (BICC)
         14        Gateway Control Protocol 
         15        Reserved
                    
 
The format and description of the Affected Destinations parameter is 
the same as for the DUNA message (See Section 3.4.1.) except that the 
Mask field is not used and only a single Affected DPC is included.  
Ranges and lists of Affected DPCs cannot be signalled in a DUPU 
message, but this is consistent with UPU operation in the SS7 network. 
The Affected Destinations parameter in an MTP3 User Part Unavailable 
message (UPU) received by an SGP from the SS7 network contains only one 
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).

3.4.6 Destination Restricted (DRST)   

The DRST message is optionally sent from the SGP to all concerned ASPs 
to indicate that the SG has determined that one or more SS7 
destinations are now restricted from the point of view of the SGP, or 
in response to a DAUD message if appropriate. The M3UA layer at the ASP 
is expected to send traffic to the affected destination via an 
alternate SGP of equal priority, but only if such an alternate route 
exists and is available. If the affected destination is currently 
considered unavailable by the ASP, The MTP3-User should be informed 
that traffic to the affected destination can be resumed.  In this case, 
the M3UA layer should route the traffic through the SGP initiating the 
DRST message.

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This message is optional for the SGP to send and it is optional for the 
ASP to act on any information received in the message. It is for use in 
the "STP" case described in Section 1.4.1.

The DRST 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).

3.5 ASP State Maintenance (ASPSM) Messages

3.5.1 ASP Up

The ASP Up message is used to indicate to a remote M3UA peer that the 
adaptation layer is ready to receive any SSNM or ASPSM/ASPTM messages 
for all Routing Keys that the ASP is configured to serve.

The ASP Up message contains the following parameters:

     INFO String                   Optional

The format for ASP Up 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 = 0x04            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

3.5.2 ASP Up Acknowledgement (ASP Up Ack)

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

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The ASP Up Ack message contains the following parameters:

     INFO String (optional)

The format for ASP Up 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 =0x04             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional INFO String parameter is the 
same as for the DUNA message (See Section 3.4.1).  The INFO String in 
an ASP Up Ack message is independent from the INFO String in the ASP Up 
message (i.e., it does not have to echo back the INFO String received).

3.5.3 ASP Down

The ASP Down message is used to indicate to a remote M3UA peer that the 
adaptation layer is NOT ready to receive DATA, SSNM or ASPTM messages.

The ASP Down message contains the following parameters:

     Reason         Mandatory
     INFO String    Optional

The format for the ASP Down 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 = 0x0a            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                              Reason                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag =0x04             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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The format and description of the optional INFO String parameter is the 
same as for the DUNA message (See Section 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 Acknowledgement (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 ASP Down 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 = 0x0a            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Reason                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x04            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

The INFO String in an ASP Down Ack message is independent from the INFO 
String in the ASP Down message (i.e., it does not have to echo back the 
INFO String received).

The format of the Reason parameter is the same as for the ASP-Down 
message. (See Section 3.5.3).

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3.5.5 Heartbeat (BEAT)

The BEAT 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 = 0x09            |            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 BEAT message does not 
process this field as it is only of significance to the sender.  The 
receiver MUST respond with a BEAT Ack message.
  

3.5.6 Heartbeat Acknowledgement (BEAT Ack)

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

3.6 Routing Key Management (RKM) Messages

3.6.1 Registration Request (REG REQ)

The REG REQ message is sent by an ASP to indicate to a remote M3UA peer
that it wishes to register one or more given Routing Keys with the 
remote peer.  Typically, an ASP would send this message to an SGP, and 
expects to receive a REG RSP message in return with an associated 
Routing Context value.

The REG REQ message contains the following parameters:

     Routing Key           Mandatory

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The format for the REG REQ 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 = 0x87           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         Routing Key 1                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 0x87           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         Routing Key n                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Routing Key: variable length

   The Routing Key parameter is mandatory. The sender of this message
   expects that the receiver of this message will create a Routing
   Key entry and assign a unique Routing Context value to it, if the 
   Routing Key entry does not already exist.

   The Routing Key parameter may be present multiple times in the same
   message. This is used to allow the registration of multiple Routing 
   Keys in a single message.

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The format of the Routing Key parameter 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Local-RK-Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Traffic Mode Type                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Point Code                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Network Appearance (optional)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       SI (optional)                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         SSN (optional)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Origination Point Code List (optional)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Circuit Range List (optional)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Local-RK-Identifier: 32-bit integer

   The mandatory Local-RK-Identifier field is used to uniquely identify 
   the registration request. The Identifier value is assigned by the 
   ASP, and is used to correlate the response in an REG RSP message 
   with the original registration request. The Identifier value must 
   remain unique until the REG RSP message is received.

   The format of the Local-RK-Identifier field 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          ag = 0x8a            |         Length = 8            |   
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Local-RK-Identifier value                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Traffic Mode Type: 32-bit (unsigned integer)

   The Traffic Mode Type parameter is mandatory and identifies the 
   traffic mode of operation of the ASP(s) within an Application 
   Server.  The valid values for Traffic Mode Type are shown in the 
   following table:

      1     Over-ride
      2     Load-share

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If the receiver of the REG REQ creates a new Routing Key entry, then 
the Traffic Mode Type sets the traffic mode for the new Application 
Server.  If the receiver of the REG REQ determines that a matching 
Routing Key already exists, the Traffic Mode Type MUST match the 
existing traffic mode for the AS.

Destination Point Code:

   The Destination Point Code parameter is mandatory, and identifies 
   the Destination Point Code of incoming SS7 traffic for which the ASP 
   is registering.  The format is the same as described for the 
   Affected Destination parameter in the DUNA message (See Section 
   3.4.1). Its format is:

    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 = 0x8b            |         Length = 8            |   
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask = 0   |            Destination Point Code             |   
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Appearance:

   The optional Network Appearance parameter field identifies the SS7 
   network context for the Routing Key, and has the same format as in 
   the DATA message (See Section 3.3.1).  The absence of the Network 
   Appearance parameter in the Routing Key indicates the use 
   of any Network Appearance value, Its format is:

    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 = 0x80            |         Length = 8            |   
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   
   |                     Network Appearance                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Service Indicators (SI): n X 8-bit integers

   The optional SI field contains one or more Service Indicators from 
   the values as described in the MTP3-User Identity field of the DUPU 
   message.  The absence of the SI parameter in the Routing Key 
   indicates the use of any SI value, excluding of course MTP 
   management.  Where an SI parameter does not contain a multiple of 
   four SIs, the parameter is    padded out to 32-byte alignment.  An 
   SI value of zero is not valid    in M3UA.  The SI format is:

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x8c            |             Length            |   
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   
   |      SI #1    |     SI #2     |    SI #3      |    SI #4      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   
   /                              ...                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   
   |      SI #n    |             0 Padding, if necessary           |   
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Subsystem Numbers (SSN): n X 8-bit integers

   The optional SSN field contains one or more SCCP subsystem numbers, 
   and is used in conjunction with an SI values of 3 (i.e., SCCP) only. 
   The absence of the SSN parameter in the Routing Key indicates the 
   use of any SSN value, in the case of SCCP traffic.  Where an SSN 
   parameter does not contain a multiple of four SSNs, the parameter is 
   padded out to 32-byte alignment. The subsystem number values 
   associated are defined by the local network operator, and typically 
   follow ITU-T Recommendation Q.713 [5].  An SSN value of zero is not 
   valid in M3UA.  The format of this field 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 = 0x8d            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     SSN #1    |    SSN #2     |   SSN #3      |   SSN #4      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                              ...                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     SSN #n    |             0 Padding, if necessary           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

OPC List:

   The Originating Point Code List parameter contains one or more SS7 
   OPC entries, and its format is the same as the Destination Point 
   Code parameter.  The absence of the OPC List parameter in the 
   Routing Key indicates the use of any OPC value,

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x8e            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask = 0   |          Origination Point Code #1            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask = 0   |          Origination Point Code #2            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                              ...                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask = 0   |          Origination Point Code #n            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Circuit Range:

   An ISUP controlled circuit is uniquely identified by the SS7 OPC, 
   DPC and CIC value.  For the purposes of identifying Circuit Ranges 
   in an M3UA Routing Key, the optional Circuit Range parameter 
   includes one or more circuit ranges, each identified by an OPC and 
   Upper/Lower CIC value.  The DPC is implicit as it is mandatory and 
   already included in the DPC parameter of the Routing Key.  The 
   absence of the Circuit Range parameter in the Routing Key indicates 
   the use of any Circuit Range values, in the case of ISUP/TUP 
   traffic.  The Origination Point Code is encoded the same as the 
   Destination Point Code parameter, while the CIC values are 16-bit 
   integers.

   The Circuit Range format 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 = 0x8f            |              Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask = 0   |          Origination Point Code #1            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Lower CIC Value #1      |      Upper CIC Value #1       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask = 0   |          Origination Point Code #2            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Lower CIC Value #2      |      Upper CIC Value #2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                              ...                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Mask = 0   |          Origination Point Code #n            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Lower CIC Value #n      |      Upper CIC Value #n       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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3.6 2 Registration Response (REG RSP)

The REG RSP message is used as a response to the REG REQ message from a
remote M3UA peer.  It contains indications of success/failure for 
registration requests and returns a unique Routing Context value for 
successful registration requests, to be used in subsequent M3UA Traffic 
Management protocol.

The REG RSP message contains the following parameters:

     Registration Results   Mandatory

The format for the REG RSP 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 = 0x90           |              Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Registration Result 1                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                              ...                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Registration Result n                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Registration Results:

   The Registration Results parameter contains one or more results, 
   each containing the registration status for a single Routing Key in 
   an REG REQ message.  The number of results in a single REG RSP 
   message MAY match the number of Routing Key parameters found in the 
   corresponding REG REQ message.  The format of each result 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Local-RK-Identifier value                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Registration Status                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Routing Context                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Local-RK-Identifier: 32-bit integer

   The Local-RK-Identifier contains the same value as found in the
   matching Routing Key parameter found in the REG REQ message (See 
Section 3.5.5.1).

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Registration Status: 32-bit integer

   The Registration Result Status field indicates the success or the 
   reason for failure of a registration request.

   Its values may be:

        0           Successfully Registered
        1           Error - Unknown
        2           Error - Invalid DPC
        3           Error - Invalid Network Appearance
        4           Error - Invalid Routing Key
        5           Error - Permission Denied
        6           Error - Cannot Support Unique Routing
        7           Error - Routing Key not Currently Provisioned
        8           Error - Insufficient Resources
        9           Error - Unsupported RK parameter Field
       10           Error  Unsupported/Invalid Traffic Handling Mode

Routing Context: 32-bit integer

   The Routing Context field contains the Routing Context value for the
   associated Routing Key if the registration was successful. It is set
   to "0" if the registration was not successful.

3.6.3 De-Registration Request (DEREG REQ)

The DEREG REQ message is sent by an ASP to indicate to a remote M3UA
peer that it wishes to de-register a given Routing Key. Typically, an 
ASP would send this message to an SGP, and expects to receive a DEREG 
RSP message in return with the associated Routing Context value.

The DEREG REQ message contains the following parameters:

     Routing Context       Mandatory

The format for the DEREG REQ 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 = 0x06            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context                         / 
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Routing Context: n X 32-bit integers

   The Routing Context parameter contains (a list of) integers indexing 
   the Application Server traffic that the sending ASP is currently 
   registered to receive from the SGP but now wishes to deregister. 

3.6.4 De-Registration Response (DEREG RSP)

The DEREG RSP message is used as a response to the DEREG REQ message
from a remote M3UA peer.

The DEREG RSP message contains the following parameters:

     De-registration Results    Mandatory

The format for the DEREG RSP 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 = 0x89           |               Length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  De-Registration Result 1                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  De-Registration Result n                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

De-Registration Results:

   The De-Registration Results parameter contains one or more results, 
   each containing the de-registration status for a single Routing 
   Context in a DEREG REQ message.  The number of results in a single 
   DEREG RSP message MAY match the number of Routing Contexts found in 
   the corresponding DEREG REQ message.  The format of each result 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Routing Context                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    De-Registration Status                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Routing Context: 32-bit integer

   The Routing Context field contains the Routing Context value of the
   matching Routing Key to deregister, as found in the DEREG REQ 
   message.

De-Registration Status: 32-bit integer

   The De-Registration Result Status field indicates the success or the 
   reason for failure of the de-registration.

   Its values may be:
        0           Successfully De-registered
        1           Error - Unknown
        2           Error - Invalid Routing Context
        3           Error - Permission Denied
        4           Error - Not Registered
        5           Error  ASP Currently Active for Routing Context

3.7 ASP Traffic Maintenance (ASPTM) Messages

3.7.1 ASP Active

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

The ASP Active message contains the following parameters:

     Traffic Mode Type     Mandatory
     Routing Context       Optional
     INFO String           Optional

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The format for the ASP Active 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 = 0x0b            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Traffic Mode Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x06            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context                         / 
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |         Tag = 0x04            |             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 Traffic Mode 
   Type are shown in the following table:

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

   Within a particular Routing Context, Over-ride and Load-share, 
   either active or standby, MUST NOT be mixed.  The Over-ride value 
   indicates that the ASP is operating in Over-ride mode, and the ASP 
   takes over all traffic in an Application Server (i.e., primary/back-
   up operation), over-riding any currently active ASPs in the AS.  In 
   Load-share mode, the ASP will share in the traffic distribution with 
   any other currently active ASPs.  The Standby versions of the Over-
   ride and 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 state ASP-DOWN or ASP-ACTIVE).  At this point the 
   sender MUST move the standby ASP to the ASP-ACTIVE state and 
   commence traffic.  Load-share (Standby) is similar - the sender 
   continues to load-share to the current ASPs until it is determined 

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   that there is insufficient resources in the Load-share group.  When 
   there are insufficient ASPs, the sender MUST move the ASP to state 
   ASP-ACTIVE.

Routing Context: n X 32-bit integers

   The optional Routing Context parameter contains (a list of) 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 SGP 
   Routing Key or AS Name.  Because an AS can only appear in one 
   Network Appearance, the Network Appearance parameter is not required 
   in the ASP Active 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 SGP.  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 3.4.1).

3.7.2 ASP Active Acknowledgement (ASP Active Ack)

The ASP Active Ack message is used to acknowledge an ASP Active message 
received from a remote M3UA peer.  In the case where an ASP Active 
(Over-ride (standby)) or ASP Active (Load-share (standby)) message is 
received, a second ASP Active Ack message is sent when the ASP is moved 
from the ASP-STANDBY to the ASP-ACTIVE state.

The ASP Active Ack message contains the following parameters:

     Traffic Mode Type     Mandatory
     Routing Context       Optional
     INFO String           Optional

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The format for the ASP Active 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 = 0x0b          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Traffic Mode Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Tag = 0x06            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context                         / 
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |            Tag = 0x04         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

The INFO String in an ASP Active Ack message is independent from the 
INFO String in the ASP Active message (i.e., it does not have to echo 
back the INFO String received).

The format of the Traffic Mode Type and Routing Context parameters is 
the same as for the ASP Active message. (See Section 3.5.5).

3.7.3  ASP Inactive

The ASP Inactive message is sent by an ASP to indicate to a remote M3UA 
peer 
that it is no longer an active ASP to be used from within a list of 
ASPs.  The ASP Inactive message affects only the ASP state in the 
Routing Keys identified by the Routing Contexts, if present. 

The ASP Inactive message contains the following parameters:

     Routing Context         Optional
     INFO String             Optional

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The format for the ASP Inactive 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 = 0x06            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context                         / 
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |         Tag = 0x04            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format and description of the optional Routing Context and INFO 
String parameters is the same as for the ASP Active message (See 
Section 3.5.5.)

3.7.4 ASP Inactive Acknowledgement (ASP Inactive Ack)

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

The ASP Inactive Ack message contains the following parameters:

     Routing Context       Optional
     INFO String           Optional

The format for the ASP Inactive 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 = 0x06            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context*                        / 
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |         Tag = 0x04            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

The INFO String in an ASP Inactive Ack message is independent from the 
INFO String in the ASP Inactive message (i.e., it does not have to echo 
back the INFO String received).

The format of the Routing Context parameter is the same as for the ASP
Inactive message. (See Section 3.5.7).

3.8  Management (MGMT) Messages

3.8.1  Error

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 Error message contains the following parameters:

     Error Code                 Mandatory
     Diagnostic Information     Optional

The format for the Error 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 = 0x0c           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Error Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 0x07           |            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      Unsupported Message Class

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     4      Unsupported Message Type 
     5      Unsupported/Invalid Traffic Handling Mode 
     6      Unexpected Message
     7      Protocol Error
     8      Invalid Routing Context
     9      Invalid Stream Identifier
    10      Invalid Parameter Value 
    11      Refused - Management Blocking
    12      Unknown Routing Context

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

The "Invalid Network Appearance" error is sent by a SGP if an ASP sends 
a message with an invalid (unconfigured) Network Appearance value.

The "Unsupported Message Class" error is sent if a message with an 
unexpected or unsupported Message Class is received.

The "Unsupported Message Type" error is sent if a message with an 
unexpected or unsupported Message Type is received.

The "Unsupported/Invalid Traffic Handling Mode" error is sent by a SGP 
if an ASP sends an ASP Active message with an unsupported Traffic Mode 
Type  or a Traffic Mode Type that is inconsistent with the presently 
configured mode for the Application Server.  An example would be a case 
in which the SGP did not support load-sharing.

The "Unexpected Message" error MAY be sent if a defined and recognized 
message is received that is not expected in the current state (in some 
cases the ASP may optionally silently discard the message and not send 
an Error message).  For example, silent discard is used by an ASP if it 
received a DATA message from an SGP while it was in the ASP-INACTIVE 
state. 

The "Protocol Error" error is sent for any protocol anomaly(i.e., 
reception of a parameter that is syntactically correct but unexpected 
in the current situation.

The "Invalid Routing Context" error is sent if a message is received 
from a peer with an invalid (unconfigured) Routing Context value.

The "Invalid Stream Identifier" error is sent if a message is received 
on an unexpected SCTP stream (e.g., a Management message was received 
on a stream other than "0").

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The " Invalid Parameter Value " error is sent if a message is received 
with an invalid parameter value (e.g., a DUPU message was received with 
a Mask value other than "0").

The "Refused - Management Blocking" error is sent when an ASP-Up or 
ASP-Active message is received and the request is refused for 
management reasons (e.g., management lock-out").

The "Unknown Routing Context" Error is sent if a message is received 
from a peer without a Routing Context parameter and it is not known by 
configuration data which Application Servers are referenced.

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, Traffic Handling Mode, Routing Context or 
   Parameter Value, the Diagnostic information parameter MUST be added 
   and include the offending parameter.  In the other cases, the 
   Diagnostic information MAY be the first 40 bytes of the offending 
   message.

Error messages MUST NOT be generated in response to other Error 
messages.

3.8.2 Notify

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

The Notify message contains the following parameters:

     Status                     Mandatory
     Routing Context            Optional
     INFO String                Optional

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The format for the Notify 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 = 0x0d             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |    Status Information      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Tag = 0x06             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                       Routing Context                         / 
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |         Tag = 0x04            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                          INFO String                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Status Type: 16-bits (unsigned integer)

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

         1     Application Server State Change (AS-State_Change)
         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 SGP to an ASP upon a change in 
   status of a particular Application Server. The value reflects the 
   new state of the Application Server.

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   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 SGP reporting the state change 
of an ASP or AS.  In the Insufficent ASP Resources case, the SGP is 
indicating to an ASP_INACTIVE ASP 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 ASP Active message (See 
Section 
3.5.5.)

4. Procedures

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

4.1 Procedures to Support the M3UA-User and Layer Management Layers

4.1.1 Receipt of Primitives from the M3UA-User

On receiving an MTP-TRANSFER request primitive from an upper layer at 
an ASP/IPSP, or the nodal inter-working function at an SGP, the M3UA 
layer sends a corresponding DATA message (see Section 3) to its M3UA 
peer.  The M3UA peer receiving the DATA message sends an MTP-TRANSFER 
indication primitive to the upper layer.

The M3UA message distribution function (see Section 1.4.2.1) determines 
the Application Server (AS) based on comparing the information in the 
MTP-TRANSFER request primitive with a provisioned Routing Key. 

>From the list of ASPs within the AS table, an ASP in the ASP-ACTIVE 
state is selected and a DATA message is constructed and issued on the 
corresponding SCTP association.  If more than one ASP is in the ASP-
ACTIVE state (i.e., traffic is to be load-shared across more than one 
ASP), one of the ASPs in the ASP_ACTIVE state is selected from the 
list.  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 ASP_ACTIVE ASPs in the AS. 

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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. 

When there is no Routing Key match, or only a partial match, for an 
incoming SS7 message, a default treatment MUST be specified.  Possible 
solutions are to provide a default Application Server at the SGP that 
directs all unallocated traffic to a (set of) default ASP(s), or to 
drop the message and provide a notification to Layer Management in an 
M-ERROR indication primitive.  The treatment of unallocated traffic is 
implementation dependent.

4.1.2 Receipt of Primitives from the Layer Management

On receiving primitives from the local Layer Management, the M3UA layer 
will take the requested action and provide an appropriate response 
primitive to Layer Management.

An M-SCTP_ESTABLISH request primitive from Layer Management at an ASP 
or IPSP will initiate the establishment of an SCTP association.  The 
M3UA layer will attempt to establish an SCTP association with the 
remote M3UA peer by sending an SCTP-ASSOCIATE primitive to the local 
SCTP layer.  

When an SCTP association has been successfully established, the SCTP 
will send an SCTP-COMMUNICATION_UP notification primitive to the local 
M3UA layer.  At the SGP or IPSP that initiated the request, the M3UA 
layer will send an M-SCTP_ESTABLISH confirm primitive to Layer 
Management when the association set-up is complete.  At the peer M3UA 
layer, an M-SCTP_ESTABLISH indication primitive is sent to Layer 
Management upon successful completion of an incoming SCTP association 
set-up.

An M-SCTP_RELEASE request primitive from Layer Management initates the 
tear-down of an SCTP association.  The M3UA layer accomplishes a 
graceful shutdown of the SCTP association by sending an SCTP-SHUTDOWN 
primitive to the SCTP layer.

When the graceful shutdown of the SCTP association has been 
accomplished, the SCTP layer returns an SCTP-SHUTDOWN_COMPLETE 
notification primitive to the local M3UA layer.  At the M3UA Layer that 
initiated the request, the M3UA layer will send an M-SCTP_RELEASE 
confirm primitive to Layer Management when the association teardown is 
complete.   At the peer M3UA Layer, an M-SCTP_RELEASE indication 
primitive is sent to Layer Management upon successful tear-down of an 
SCTP association.

An M-SCTP_STATUS request primitive supports a Layer Management query of 
the local status of a particular SCTP association.  The M3UA layer 
simply maps the M-SCTP_STATUS request primitive to an SCTP-STATUS 

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primitive to the SCTP layer.  When the SCTP responds, the M3UA layer 
maps the association status information to an M-SCTP_STATUS confirm 
primitive.  No peer protocol is invoked.

Similar LM-to-M3UA-to-SCTP and/or SCTP-to-M3UA-to-LM primitive mappings 
can be described for the various other SCTP Upper Layer primitives in 
RFC2960 [13]  such as INITIALIZE, SET PRIMARY, CHANGE HEARTBEAT, 
REQUEST HEARTBEAT, GET SRTT REPORT, SET FAILURE THRESHOLD, SET PROTOCOL 
PARAMETERS, DESTROY SCTP INSTANCE, SEND FAILURE, AND NETWORK STATUS 
CHANGE.  Alternatively, these SCTP Upper Layer primitives (and Status 
as well) can be considered for modeling purposes as a Layer Management 
interaction directly with the SCTP Layer.  

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

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

M-ASP_UP request, M-ASP_DOWN request, M-ASP_ACTIVE request and M-ASP_
INACTIVE request primitives allow Layer Management at an ASP to 
initiate state changes.  Upon successful completion, a corresponding 
confirm primitive is provided by the M3UA layer to Layer Management.  
If an invocation is unsuccessful, an Error indication primitive is 
provided in the primitive.

These requests result in outgoing ASP Up, ASP Down, ASP Active and 
ASP Inactive messages to the remote M3UA peer at an SGP or IPSP. 

4.1.3 Receipt of M3UA Peer Management Messages

Upon successful state changes resulting from reception of ASP Up, 
ASP Down, ASP Active and ASP Inactive messages from a peer M3UA, the 
M3UA layer SHOULD invoke corresponding M-ASP_UP, M-ASP_DOWN, M-
ASP_ACTIVE and M-ASP_INACTIVE, M-AS_ACTIVE, M-AS_INACTIVE, and M-
AS_DOWN indication primitives to the local Layer Management.

M-NOTIFY indication and M-ERROR indication primitives 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.

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4.2 Procedures to Support the Management of SCTP Associations with M3UA 
Peers

These procedures support the M3UA management of SCTP Associations 
between SGPs and ASPs or between IPSPs.

4.2.1 AS and ASP State Maintenance

The M3UA layer on the SGP 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 IPSPs. For the purposes of the following 
procedures, only the SGP/ASP case is described but the SGP side of the 
procedures also apply to an IPSP sending traffic to an AS consisting of 
a set of remote IPSPs.

4.2.1.1 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 SGP. 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., ASP Active message indicating "Over-ride");
   * Reception of indications from the SCTP layer; or
   * Local Management intervention.

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 MAY 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 MAY be sent 
any non-Data M3UA messages.

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                 Figure 4: ASP State Transition Diagram

                                   +--------------+ 
                                   |  ASP-ACTIVE  |
            +----------------------|      or      |
            |      Other   +-------| ASP-STANDBY* |
            |   ASP in AS  |       +--------------+
            |   Overrides  |           ^     |
            |              |    ASP    |     | ASP
            |              |    Active |     | Inactive
            |              |           |     v
            |              |       +--------------+
            |              |       |              |
            |              +------>| ASP-INACTIVE |
            |                      +--------------+
            |                          ^     |
  ASP Down/ |                     ASP  |     | ASP Down /
  SCTP CDI  |                     Up   |     | SCTP CDI
            |                          |     v
            |                      +--------------+
            |                      |              |
            +--------------------->|   ASP-DOWN   |
                                   |              |
                                   +--------------+

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

SCTP CDI: The SCTP CDI denotes the local SCTP layer's Communication 
Down Indication to the Upper Layer Protocol (M3UA) on an SGP. The local 
SCTP layer 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 layer.

4.2.1.2  AS States

The state of the AS is maintained in the M3UA layer on the SGP.  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. An Application Server MUST be in the AS-
DOWN state before it can be removed from a configuration.

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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 or ASP-STANDBY states).  The 
recovery timer 
T(r) is not running or has expired. 

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

AS-PENDING: An active ASP has transitioned to ASP-INACTIVE or ASP-DOWN  
and it was the last remaining active ASP in the AS (and no ASPs in the 
ASP-STANDBY state are available.  A recovery timer T(r) SHOULD be 
started and all incoming signalling messages SHOULD be queued by the 
SGP. If an ASP becomes ASP-ACTIVE before T(r) expires, the AS is moved 
to the AS-ACTIVE state and all the queued messages will be 
sent to the ASP. 

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

Figure 5 shows an example AS state machine for the case where the 
AS/ASP data is pre-configured.  For other cases where the AS/ASP 
configuration data is created dynamically, there would be differences 
in the state machine, especially at creation of the AS.  

For example, where the AS/ASP configuration data is not created until 
Registration of the first ASP, the AS-INACTIVE state is entered 
directly upon the first successful REG REQ from an ASP.  Another 
example is where the AS/ASP configuration data is not created until the 
first ASP successfully enters the ASP-ACTIVE state.  In this case the 
AS-ACTIVE state is entered directly.

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                 Figure 5: AS State Transition Diagram

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

    Tr = Recovery Timer

4.2.2 M3UA Management Procedures for Primitives

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

Once the SCTP association is established (see Section 4.1.2) and 
assuming that the local M3UA-User is ready, the local M3UA ASP 
Maintenance (ASPM) function will initiate the relevant procedures, 
using the ASP Up/ASP Down/ASP Active/ASP Inactive messages to convey 
the ASP state to the SGP (see Section 4.3.3).

If the M3UA layer subsequently receives an SCTP-COMMUNICATION_DOWN 
or SCTP-RESTART indication primitive from the underlying SCTP layer, it 
will inform the Layer Management by invoking the M-SCTP_STATUS 
indication primitive. The state of the ASP will be moved to ASP-DOWN  
At an ASP, the MTP3-User will be informed of the unavailability of any 
affected SS7 destinations through the use of MTP-PAUSE indication 
primitives.  In the case 

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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.

In the case of SCTP-COMMUNICATION_DOWN, the SCTP client MAY try to re-
establish the SCTP Association.  This MAY be done by the M3UA layer 
automatically, or Layer Management MAY re-establish using the M-
SCTP_ESTABLISH request primitive. 

In the case of an SCTP-RESTART indication at an ASP, the ASP is now 
considered by its M3UA peer to be in the ASP-DOWN state.  The ASP, if 
it is to recover, must begin any recovery with the ASP-Up procedure.

4.2.3 M3UA Management Procedures for Peer-to-Peer Messages

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

4.2.3.1 ASP Up Procedures

After an ASP has successfully established an SCTP association to an 
SGP, the SGP 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 message.  This action MAY be initiated at the ASP by an 
M-ASP_UP request primitive from Layer Management or MAY be initiated 
automatically by an M3UA management function.

When an ASP Up message is received at an SGP and internally the remote 
ASP is in the ASP-DOWN state and not considered locked-out for local 
management reasons, the SGP marks the remote ASP in the state ASP-
INACTIVE and informs Layer Management with an M-ASP_Up indication 
primitive.  If the SGP is aware, via current configuration data, which 
Application Servers the ASP is configured to operate in, the SGP 
updates the ASP state to ASP-INACTIVE in each AS that it is a member.  
Alternatively, the SGP may move the ASP into a pool of Inactive ASPs 
available for future configuration within Application Server(s), 
determined in a subsequent Registration Request or ASP Active 
procedure.  The SGP responds with an ASP Up Ack message in 
acknowledgement.  The SGP sends an ASP Up Ack message in response to a 
received ASP Up message even if the ASP is already marked as ASP-
INACTIVE at the SGP.  

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

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

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enters the ASP-Inactive state from the ASP_Down state towards an SGP 
the M3UA MUST mark all SS7 destinations configured to be reachable via 
this SGP as available.

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

The ASP must wait for the ASP Up Ack message before sending any other 
M3UA messages (e.g., ASP Active or REG REQ).  If the SGP receives any 
other M3UA messages before an ASP Up message is received, the SGP 
SHOULD discard them.

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

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

4.2.3.1.1 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 SGP.

4.2.3.1.2 IPSP Considerations

In the case of peer-to-peer IPSPs, either of the IPSPs (IPSP_A) may 
start operations by sending an ASP Up message to the remote peer 
(IPSP_B).  When the ASP Up message is received at IPSP_B and internally 
the remote IPSP_A is in the ASP-DOWN state and not considered locked-
out for local management reasons, IPSP_B marks the remote IPSP_A in the 
state ASP-INACTIVE and informs Layer Management with an M-ASP_Up 
indication primitive.  IPSP_B returns an ASP-Up Ack message to IPSP_A.  
IPSP_A moves IPSP_B to the ASP-INACTIVE state upon reception of an ASP 
Up Ack message, if is not already in the ASP_INACTIVE state, and 
informs Layer Management with an M-ASP_UP confirmation primitive.

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If for any local reason (e.g., management lock-out) the IPSP_B cannot 
respond with an ASP Up Ack message, it responds to an ASP Up message 
with an Error message with Reason "Refused - Management Blocking" and 
leaves IPSP_A in the ASP-DOWN state.  

4.2.3.2 ASP-Down Procedures

The ASP will send an ASP Down message to an SGP when the ASP wishes to 
be 
removed from service in all Application Servers that it is a member and 
no longer receive any DATA, SSNM or ASPTM messages.  This action MAY be 
initiated at the ASP by an M-ASP_DOWN request primitive from Layer 
Management or MAY be initiated automatically by an M3UA management 
function.   

Whether the ASP is permanently removed from any AS is a function of 
configuration management.  In the case where the ASP previously used 
the Registration procedures (see Section 3.5.5) to register within 
Application Servers but has not deregistered from all of them prior to 
sending the ASP Down message, the SGP SHOULD consider the ASP as 
Deregistered in all Application Servers that it is still a member.

The SGP marks the ASP as ASP-DOWN, informs Layer Management with an M-
ASP_Down indication primitive, and returns an ASP Down Ack message to 
the ASP. has locked out the ASP for management reasons.

The SGP 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 ASP-DOWN 
at the SGP.  The SGP sends an ASP Down Ack message even if the reason 
in the received ASP Down message is considered invalid.

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

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

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4.2.3.4 ASP-Active Procedures

Anytime after the ASP has received an ASP Up Ack message from the SGP 
or IPSP, the ASP sends an ASP Active message to the SGP 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 
ASP Active message(s) SHOULD contain a list of one or more Routing 
Contexts to indicate for which Application Servers the ASP Active 
message applies. It is not necessary for the ASP to include all Routing 
Contexts of interest in a single ASP Active message, thus requesting to 
become active in all Routing Contexts at the same time.  Multiple ASP 
Active messages MAY be used to activate within the Application Servers 
independently, or in sets.  In the case where an ASP Active message 
does not contain a Routing Context parameter, the receiver must know, 
via configuration data, which Application Server(s) the ASP is a 
member.

For the Application Servers that the ASP can be successfully activated, 
he SGP or IPSP responds 
with one or more ASP Active Ack messages, including the associated 
Routing Context and Traffic Mode Type values.  The Routing Context 
parameter MUST be included in the Asp Active Ack message if the 
received ASP Active message contained any Routing Contexts.  Depending 
on the ASP Active Message Traffic Mode Type request, the SGP moves the 
ASP to the correct ASP traffic state within the associated Application 
Server(s). Layer Management is informed with an M-ASP_Active 
indication. If the SGP or IPSP receives any Data messages before an ASP 
Active message is received, the SGP or IPSP MAY discard them.  By 
sending an ASP Active Ack message, the SGP or IPSP is now ready to 
receive and send traffic for the related Routing Context(s).  The ASP 
SHOULD NOT send Data messages for the related Routing Context(s) before 
receiving an ASP Active Ack message, or it will risk message loss.

Multiple ASP Active Ack messages MAY be used in response to an ASP 
Active message containing multiple Routing Contexts, allowing the SGP 
or IPSP to independently acknowledge the ASP Active message for 
different (sets of) Routing Contexts.  The SGP or IPSP sends an Error 
message ("Invalid Routing Context") for each Routing Context value that 
the ASP cannot be successfully activated . 

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

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

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

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

There are four modes of Application Server traffic handling in the SGP 
M3UA layer - Over-ride, Over-ride (Standby), Load-share and Load-share 
(Standby).  The Traffic Mode Type parameter in the ASP Active message 
indicates the traffic handling mode used in a particular Application 
Server. If the SGP determines that the mode indicated in an ASP Active 
message is unsupported or incompatible with the mode currently 
configured for the AS, the SGP responds with an Error message 
("Unsupported / Invalid Traffic Handling Mode").  If the Traffic 
Handling mode of the Application Server is not already known via 
configuration data, then the Traffic Handling mode indicated in the 
first ASP Active message causing the transition of the Application 
Server state to AS-ACTIVE MAY be used to set the mode.

In the case of an Over-ride mode AS, reception of an ASP Active message 
at an SGP causes the (re)direction of all traffic for the AS to the ASP 
that sent the ASP Active message.  Any previously active ASP in the AS 
is now considered to be in state ASP-INACTIVE and SHOULD no longer 
receive traffic from the SGP within the AS.  The SGP or IPSP then MUST 
send a Notify message ("Alternate ASP-Active") to the previously active 
ASP in the AS, and SHOULD stop traffic to/from that ASP.  The ASP 
receiving this Notify MUST consider itself now in the ASP-INACTIVE 
state, if it is not already aware of this via inter-ASP communication 
with the Over-riding ASP.

In the case of Over-ride (Standby) mode the traffic is not started to 
the ASP until the currently active ASP transitions to the ASP-INACTIVE 
or ASP-DOWN state.  At this point the ASP that sent the ASP Active 
message ("Over-Ride (Standby)") is moved to the ASP-ACTIVE state and 
the traffic is redirected.  A second ASP Active Ack message with a new 
Traffic Mode Type ("Over-ride", previously "Over-ride(Standby)") is 
sent to the ASP. A Notify message ("Alternate ASP-Active") is not sent 
in this case.

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If there is no currently active ASP, an ASP Active Ack message ("Over-
ride") is returned right away and the traffic is directed to the ASP.

In the case of a Load-share mode AS, reception of an ASP Active message 
at an SGP or IPSP causes the direction of traffic to the ASP sending 
the ASP Active message, in addition to all the other ASPs that are 
currently active in the AS.  The algorithm at the SGP 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., the SLS, SCCP SSN, ISUP CIC 
value).  

An SGP or IPSP, upon reception of an ASP Active message for the first 
ASP in a Loadshare AS, MAY choose not to direct traffic to a newly 
active ASP until it determines that there are sufficient resources to 
handle the expected load (e.g., until there are "n" ASPs in state ASP-
ACTIVE in the AS).

In the case of a Load-share (Standby) mode AS, the traffic is not 
started to the ASP until the SGP or IPSP determines that there are 
insufficient resources available in the AS.  This is likely when one of 
the active load-sharing ASPs transitions to either the ASP-INACTIVE or 
ASP-DOWN state.  At this point the ASP that sent the ASP Active message 
("Load-share (Standby)") is moved to the ASP_ACTIVE state and traffic 
is started.  A second ASP Active Ack message with a new Traffic Mode 
Type ("Load-share" - previously "Loadshare(Standby)") is sent to the 
ASP. A Notify message ("Insufficient ASP resources active in AS ") is 
not sent in this case.

If there is no currently active ASP, an ASP Active Ack message 
("Loadshare") is returned right away and the traffic is directed to the 
ASP.

All ASPs within a load-sharing mode AS must be able to process any 
Data message received for the AS, in order to accommodate any potential 
fail-over or rebalancing of the offered load.

4.2.3.5 ASP Inactive Procedures

When an ASP wishes to withdraw from receiving traffic within an AS, the 
ASP sends an ASP Inactive message to the SGP or IPSP.  This action MAY 
be initiated at the ASP by an M-ASP_INACTIVE request primitive from 
Layer Management or MAY be initiated automatically by an 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 ASP Inactive message contains one or more Routing 
Contexts to indicate for which Application Servers the ASP Inactive 
message applies.  In the case where an ASP Inactive message does not 
contain a Routing Context parameter, the receiver must know, via

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configuration data, which Application Servers the ASP is a member and 
move the ASP to the ASP-INACTIVE state in each all Application Servers.  
In the case of an Over-ride mode AS, where another ASP has already 
taken over the traffic within the AS with an ASP Active ("Over-ride") 
message, the ASP that sends the ASP Inactive message is already 
considered by the SGP to be in state ASP-INACTIVE. .An ASP Inactive Ack 
message is sent to the ASP, after ensuring that all traffic is stopped 
to the ASP. 

In the case of a Load-share mode AS, the SGP moves the ASP to the ASP-
INACTIVE state and the AS traffic is re-allocated across the remaining 
ASPs in the state ASP-ACTIVE, as per the load-sharing algorithm 
currently used within the AS.  A Notify message("Insufficient ASP 
resources active in AS") 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 ASP Inactive Ack message is sent to the ASP after all traffic 
is halted and Layer Management is informed with an M-ASP_INACTIVE 
indication primitive.

Multiple ASP Inactive Ack messages MAY be used in response to an ASP 
Inactive message containing multiple Routing Contexts, allowing the SGP 
or IPSP to independently acknowledge for different (sets of) Routing 
Contexts.  The SGP or IPSP sends an Error message ("Invalid Routing 
Context") message for each invalid or un-configured Routing Context 
value in a received ASP Inactive message message.

The SGP MUST send an ASP Inactive Ack message in response to a received 
ASP Inactive message from the ASP and the ASP is already marked as ASP-
INACTIVE at the SGP.  

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

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

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4.2.3.6 Notify Procedures

A Notify message reflecting a change in the AS state SHOULD be sent to 
all ASPs in the AS, except those in the ASP-DOWN state, with 
appropriate Status Information.  The Notify message MUST be sent after 
any ASP State or Traffic Management acknowledgement messages (e.g., ASP 
Up Ack, ASP Down Ack, ASP Active Ack, or ASP Inactive Ack).  At the 
ASP, Layer Management is informed with an M-NOTIFY indication 
primitive.

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

In the case where a Notify message does not contain a Routing Context
parameter, the receiver must know, via configuration data, of which
Application Servers the ASP is a member and take the appropriate action for
the ASP in each AS.

4.2.3.7 Heartbeat Procedures

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 SCTP).  

After receiving an ASP Up Ack message from an M3UA peer in response to 
an ASP Up message, an ASP may optionally send Heartbeat messages 
periodically, subject to a provisionable timer T(beat).  Upon receiving 
a Heartbeat message, the M3UA peer MUST respond with a Heartbeat ACK 
message.  

At the ASP, if no Heartbeat 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 Heartbeat messages is stopped 
and the ASP SHOULD attempt to re-establish communication with the SGP 
M3UA peer.

The Heartbeat message may optionally contain an opaque Heartbeat Data 
parameter that MUST be echoed back unchanged in the related Heartbeat 
Ack message.  The ASP, upon examining the contents of the returned 
Heartbeat Ack message, MAY choose to consider the remote M3UA peer 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).

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Note: Heartbeat related events are not shown in Figure 4 "ASP state 
transition diagram".  

4.2.4 Routing Key Management Procedures

4.2.4.1 Registration

An ASP MAY dynamically register with an SGP as an ASP within an 
Application Server using the REG REQ message. A Routing Key parameter 
in the REG REQ message specifies the parameters associated with the 
Routing Key.  

The SGP examines the contents of the received Routing Key parameter and 
compares it with the currently provisioned Routing Keys.  If the 
received Routing Key matches an existing SGP Routing Key entry, and the 
ASP is not currently included in the list of ASPs for the related 
Application Server, the SGP MAY authorize the ASP to be added to the 
AS.  Or, if the Routing Key does not currently exist and the received 
Routing Key data is valid and unique, an SGP supporting dynamic 
configuration MAY authorize the creation of a new Routing Key and 
related Application Server and add the ASP to the new AS.  In either 
case, the SGP returns a Registration Response message to the ASP, 
containing the same Local-RK-Identifier as provided in the initial 
request, and a Registration Result "Successfully Registered".  A unique 
Routing Context value assigned to the SGP Routing Key is included. The 
method of Routing Context value assignment at the SG/SGP is 
implementation dependent but must be guaranteed to be unique across all 
SGPs in an SG.

If the SGP determines that the received Routing Key data is invalid, or 
contains invalid parameter values, the SGP returns a Registration 
Response message to the ASP, containing a Registration Result "Error - 
Invalid Routing Key", "Error - Invalid DPC", "Error - Invalid Network 
Appearance" as appropriate.

If the SGP determines that a unique Routing Key cannot be created, the 
SGP returns a Registration Response message to the ASP, with a 
Registration Status of "Error - "Cannot Support Unique Routing"  An 
incoming signalling message received at an SGP should not match against 
more than one Routing Key.

If the SGP does not authorize the registration request, the SGP returns 
a REG RSP message to the ASP containing the Registration Result "Error 
 Permission Denied".

If an SGP determines that a received Routing Key does not currently 
exist and the SGP does not support dynamic configuration, the SGP 
returns a Registration Response message to the ASP, containing a 
Registration Result "Error - Routing Key not Currently Provisioned".

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If an SGP determines that a received Routing Key does not currently 
exist and the SGP supports dynamic configuration but does not have the 
capacity to add new Routing Key and Application Server entries, the SGP 
returns a Registration Response message to the ASP, containing a 
Registration Result "Error - Insufficient Resources".

If an SGP determines that one or more of the Routing Key parameters are 
not supported for the purpose of creating new Routing Key entries, the 
SGP returns a Registration Response message to the ASP, containing a 
Registration Result "Error  Unsupported RK parameter field".  This 
result MAY be used if, for example, the SGP does not support RK Circuit 
Range Lists in a Routing Key because the SGP does not support ISUP 
traffic, or does not provide CIC range granularity.

A Registration Response "Error  Unsupported Traffic Handling Mode" is 
returned if the Routing Key in the REG REQ contains an Traffic Handling 
Mode that is inconsistent with the presently configured mode for the 
matching Application Server.
An ASP MAY register multiple Routing Keys at once by including a number 
of Routing Key parameters in a single REG REQ message.  The SGP MAY 
respond to each registration request in a single REG RSP message, 
indicating the success or failure result for each Routing Key in a 
separate Registration Result parameter.  Alternatively the SGP MAY 
respond with multiple REG RSP messages, each with one or more 
Registration Result parameters.  The ASP uses the Local-RK-Identifier 
parameter to correlate the requests with the responses.

Upon successful registration of an ASP in an AS, the SGP can now send 
related SS7 Signalling Network Management messaging, if this did not 
previously start upon the ASP transitioning to state ASP-INACTIVE

4.2.4.2 Deregistration

An ASP MAY dynamically deregister with an SGP as an ASP within an 
Application Server using the DEREG REQ message.  A Routing Context 
parameter in the DEREG REQ message specifies which Routing Keys to de-
register.  An ASP SHOULD move to the ASP-INACTIVE state for an 
Application Server before attempting to deregister the Routing Key 
(i.e., deregister after receiving an ASP Inactive Ack).  Also, an ASP 
SHOULD deregister from all Application Servers that it is a member 
before attempting to move to the ASP-Down state.  

The SGP examines the contents of the received Routing Context parameter 
and validates that the ASP is currently registered in the Application 
Server(s) related to the included Routing Context(s).  If validated, 
the ASP is de-registered as an ASP in the related Application Server.  

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The deregistration procedure does not necessarily imply the deletion of
Routing Key and Application Server configuration data at the SGP. Other 
ASPs may continue to be associated with the Application Server, in 
which case the Routing Key data MUST NOT be deleted.  If a 
Deregistration results in no more ASPs in an Application Server, an SGP 
MAY delete the Routing Key data.

The SGP acknowledges the deregistration request by returning a DEREG 
RSP message to the requesting ASP.  The result of the deregistration is 
found in the Deregistration Result parameter, indicating success or 
failure with cause.

An ASP MAY deregister multiple Routing Contexts at once by including a 
number of Routing Contexts in a single DEREG REQ message.  The SGP MAY 
respond to each deregistration request in a single DEREG RSP message, 
indicating the success or failure result for each Routing Context in a 
separate Deregistration Result parameter.  

4.3 Procedures to Support the Availability or Congestion Status of SS7 
Destination

4.3.1 At an SGP

On receiving an MTP-PAUSE, MTP-RESUME or MTP-STATUS indication 
primitive from the nodal inter-working function at an SGP, the SGP M3UA 
layer will send a corresponding SS7 Signalling Network Management 
(SSNM) DUNA, DAVA, SCON, or DUPU message (see Section 3.4) 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 SGP M3UA layer 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 SGP 
operates within a single SS7 network appearance, then all ASPs are 
informed.  

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 SGP M3UA layer MAY also suppress DUPU 
messages to ASPs that do not implement an MTP3-User protocol peer for 
the affected MTP3-User.

DUNA, DAVA, SCON, and DRST messages MUST be sent sequentially and processed at 
the receiver in the order sent.  SCTP stream "0" is used to provide the 
sequencing.  .  The only exception to this is if the international congestion 
method (see Q.704) is used.  If so, the Unordered bit in the SCTP DATA chunk MAY 
be used for the SCON message.

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Sequencing is not required for the DUPU or DAUD messages, which MAY 
be sent un-sequenced.  Again, SCTP stream 0 is used, with optional use 
of the Unordered bit in the SCTP DATA chunk.

4.3.2 At an ASP

4.3.2.1 Single SGP Configurations

At an ASP, upon receiving an SS7 Signalling Network Management (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 layer at the ASP MUST 
pass up appropriate indications in the primitives to the M3UA User, as 
though equivalent SSNM messages were received.  For example, the loss 
of an SCTP association to an SGP may cause the unavailability of a set 
of SS7 destinations.  MTP-PAUSE indication primitives to the M3UA User 
are appropriate.  To accomplish this, the M3UA layer at an ASP 
maintains the status of routes via the SG(P), much like an MTP3 layer 
maintains route-set status.

4.3.2.2 Multiple SGP Configurations

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

An M3UA layer at the ASP MAY choose to maintain knowledge of which SGPs 
are included in Signalling Gateways for the purpose of interpreting 
SSNM messaging from one SGP so as to apply to all the SGPs in the SG.  

4.3.3 ASP Auditing

An ASP may optionally initiate an audit procedure in order to enquire 
of an SGP the availability and, if the national 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 SGP requesting the 
current availability and congestion status of one or more SS7 
Destination Point Codes.

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

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   - Periodic.  A Timer originally set upon reception of a DUNA, SCON 
                or DRST message has expired without a subsequent DAVA, 
                DUNA, SCON or DRST message 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 
                SGP that originally sent the SSNM message.
 
   - Isolation. The ASP is newly ASP-INACTIVE or ASP-ACTIVE or has been 
                isolated from an SGP for an extended period.  The ASP 
                MAY request the availability/congestion status of one 
                or more SS7 destinations to which it expects to 
                communicate.

In the first of the cases above, the auditing  procedure must not be 
invoked for the case of a received SCON message 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 layer at the SGP does not 
maintain the congestion status of any destinations and therefore the 
SGP cannot provide any congestion information in response to the DAUD.  
For the same reason, in the second of the cases above a DAUD message 
cannot reveal any congested destination(s).

The SGP MUST respond to a DAUD message with the MTP3 
availability/congested status of the routeset associated with each 
Destination Point Code(s) in the DAUD message.  The status of each SS7 
destination requested is indicated in a DUNA message (if unavailable), 
a DAVA message (if available), or a DRST (if restricted and the SGP 
supports this feature).  If the SS7 destination is available and 
congested, the SGP responds with an SCON message in addition to the 
DAVA message.  If the SS7 destination is restricted and congested, the 
SGP responds with an SCON message in addition to the DRST.  If the SGP 
has no information on the availability/congestion status of the SS7 
destination, the SGP responds with a DUNA message, as it has no routing 
information to allow it to route traffic to this destination

Any DUNA or DAVA message in response to a DAUD message MAY contain a 
list of up to sixteen Affected Point Codes. 

4.4 MTP3 Restart

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

When the SG discovers SS7 network isolation, the SGPs send an indication 
to all concerned available ASPs (i.e., ASPs in the ASP-ACTIVE, ASP-
STANDBY or ASP-INACTIVE state) using a DUNA message.  For the purpose of 
MTP restart, all Signalling Point Management Clusters with point codes 

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different from that of the SG with at least one ASP in the ASP-ACTIVE 
state or that has sent an ASP ACTIVE message to the SG during the first 
part of the restart procedure should be considered as available.  If the 
M3UA layer at the SGP receives any ASP ACTIVE messages during the 
restart procedure, it delays the ASP ACTIVE ACK messages until the end 
of the restart procedure.  During the second part of the restart 
procedure the SGP M3UA layers at the SGPs inform all concerned ASPs in 
the ASP-ACTIVE, ASP-STANDBY or ASP-INACTIVE states of any unavailable 
SS7 destinations using the DUNA message.  At the end of the restart 
procedure the SGP M3UA layers send an ASP ACTIVE ACK message to all ASPs 
in the ASP-ACTIVE state.

When the M3UA layer at an ASP receives a DUNA message indicating SS7 
network isolation at an SG, it will stop any affected traffic via this 
route. When the M3UA subsequently receives any DUNA messages from an SGP 
it will mark the affected SS7 destinations as unavailable via that SG.  
When the M3UA receives an ASP ACTIVE ACK message it can resume traffic 
to available SS7 destinations via this SGP, provided the ASP is in the 
ASP-ACTIVE state towards this SGP.  The ASP MAY choose to audit the 
availability of any unavailable destinations

5. Examples of M3UA Procedures

5.1 Establishment of Association and Traffic between SGPs and ASPs

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

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

This scenario shows the example M3UA message flows for the 
establishment of traffic between an SGP and an ASP, where only one ASP 
is configured within an AS (no backup).  It is assumed that the SCTP 
association is already set-up. The sending of any DUNA/SCON messages by 
the SGP is not shown but is similar to the case described in Section 
5.1.2.

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

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

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

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

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

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

5.1.1.3 Single ASP in Multiple Application Servers (each with "1+0" 
sparing), Dynamic Registration (Case 1  Multiple Registration 
Requests)

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

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

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

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

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

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

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

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

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

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

Note: It is also possible for ASP2 to send an ASP-Active ("Over-ride-
Standby") message after ASP1 goes ASP-ACTIVE  A similar sparing 
arrangement is created, except that the SGP may re-direct traffic to 
ASP2 more quickly in certain fail-over cases.

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

This scenario shows a similar case to Section 5.1.2 but where the two 
ASPs are brought to the state ASP-ACTIVE and subsequently load-share 
the traffic.  In this case, one ASP is sufficient to handle the total 
traffic load. The sending of DUNA, DRST and SCON messages by the SGP is 
not shown but is similar to the case described in Section 5.1.2.

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       SGP                       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 SGP and three ASPs in the same 
Application Server, where two of the ASPs are brought to the state ASP-
ACTIVE and subsequently share the load. In this case, a minimum of two 
ASPs are required to handle the total traffic load (2+1 sparing). The 
sending of DUNA, DRST and SCON messages by the SGP is not shown but is 
similar to the case described in Section 5.1.2.

   SGP                  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----->|                   | 
    |                    |                   |                   |

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Note: It is also possible for ASP3 to send an ASP Active message 
("Loadshare-Standby") after ASP1 and ASP2 go to the ASP-ACTIVE state  A 
similar sparing arrangement is created, except that the SGP may 
redirect traffic to ASP3 more quickly in certain fail-over cases.

5.2 ASP Traffic Fail-over Examples

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

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

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

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

5.2.2 (1+1 Sparing, Back-up Over-ride)

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

       SGP                       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 5.1.4, and ASP1 withdraws from 
service:

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

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

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

5.3 Normal Withdrawal of an ASP from an Application Server and Tear-
down of an Association

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

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

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

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The ASP SHOULD be ASP-INACTIVE and de-registered in all its Routing 
Contexts before attempting to move to the ASP-DOWN state.  

5.4  M3UA/MTP3-User Boundary Examples

5.4.1 At an ASP

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

5.4.1.1 Support for MTP-TRANSFER Primitives at the ASP

5.4.1.1.1 Support for MTP-TRANSFER Request Primitive

When the MTP3-User on the ASP has data to send into the SS7 network, it 
uses the MTP-TRANSFER request primitive.  The M3UA layer at the ASP 
will do the following when it receives an MTP-TRANSFER request 
primitive from the M3UA user:

  - Determine the correct SGP;

  - Determine the correct association to the chosen SGP;

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

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

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

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

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

        
5.4.1.1.2 Support for the MTPTRANSFER Indication Primitive

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

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

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

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

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

5.4.1.1.3 Support for ASP Querying of SS7 Destination States

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

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

5.4.2 At an SGP 

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

5.4.2.1 Support for MTP-TRANSFER Request Primitive at the SGP

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

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

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

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

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

5.4.2.2 Support for MTP-TRANSFER Indication Primitive at the SGP

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

  - Determine the correct 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 a DATA message;

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

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

5.4.2.3 Support for MTP-PAUSE, MTP-RESUME, MTP-STATUS Indication 
Primitives

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

5.4.2.3.1 Destination Unavailable 

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

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                   SGP                       ASP   
                    |                         |
 --MTP-PAUSE ind.-->|---------DUNA----------->|--MTP-PAUSE ind.-->
                    |                         |

5.4.2.3.2 Destination Available

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

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

5.4.2.3.3 SS7 Network Congestion 

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

5.4.2.3.4 Destination User Part Unavailable

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

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

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6. 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 fulfil 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 is recommended to be transported on SCTP.  SCTP [13] 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, the requirement for confidentiality may 
include the masking of IP addresses and ports.  In this case 
application level encryption is not sufficient; IPSEC ESP [23] SHOULD 
be used instead.  Regardless of which level performs the encryption, 
the IPSEC ISAKMP [24] service SHOULD be used for key management.

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7. IANA Considerations

7.1 SCTP Payload Protocol Identifier

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

        M3UA    "3"

The SCTP Payload Protocol Identifier value "3" SHOULD be included in 
each SCTP DATA chunk, to indicate that the SCTP is carrying the M3UA 
protocol. The value "0" (unspecified) is also allowed but any other 
values MUST not be used.  This Payload Protocol Identifier is not 
directly used by SCTP but MAY be used by certain network entities to 
identify the type of information being carried in a DATA chunk.

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

7.2 M3UA Port Number

IANA has registered SCTP (and UDP/TCP) Port Number 2905 for M3UA.  

7.3 M3UA Protocol Extensions

This protocol may also be extended through IANA in three ways:
 -- through definition of additional message classes,
 -- through definition of additional message types, 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 
Guidelines for Writing an IANA Considerations Section in RFCs (25]

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

7.3.1 IETF Defined Message Classes

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

7.3.2 IETF Defined Message Types

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

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(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 ("Unsupported 
Message Type").

7.3.3 IETF Defined Parameter Extension

Documentation of the message parameter MUST contain the following 
information:

(a) Name of the parameter type;
(b) Detailed description of the structure of the parameter field.  This 
    structure MUST conform to the general type-length-value format 
    described in Section 3.2;
(c) Detailed definition of each component of the parameter value;
(d) Detailed description of the intended use of this parameter type, 
    and an indication of whether and under what circumstances multiple 
    instances of this parameter type may be found within the same 
    message.

8. Acknowledgements

The authors would like to thank Antonio Roque Alvarez, Joyce Archibald, 
Tolga Asveren, Brian Bidulock, Dan Brendes, Nikhil Jain, Joe Keller, 
Kurt Kite, Ming Lin, Steve Lorusso, John Loughney, Naoto Makinae, 
Howard May, Barry Nagelberg, Neil Olson, Heinz Prantner, Shyamal 
Prasad, Mukesh Punhani, Selvam Rengasami, Ray Singh, Michael Tuexen, 
Nitin Tomar, Gery Verwimp, Kazuo Watanabe, Ben Wilson and many others 
for their valuable comments and suggestions.

9.  References

[1] RFC 2719, "Framework Architecture for Signaling Transport", L. Ong 
et al, October 1999

[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"

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[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 to Q.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 Recommendation Q.720, "Telephone User Part"

[9] ITU-T Recommendations Q.771 to Q.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 V4.0.0 (2001-04) "Technical Specification - 3rd  
     Generation partnership Project; Technical Specification Group  
     Radio Access Network; UTRAN Iu Interface: General Aspects and  
     Principles" 

[13] RFC 2960, "Stream Control Transport Protocol", R. Stewart et al, 
     October 2000. 
 
[14] ITU-T Recommendations Q.701 to 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)"

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

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

[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.

[23] RFC 2406, "IP Encapsulating Security Payload (ESP)", S.Kent and 
     R. Atkinson, November 1998.

[24] RFC 2408, "Internet Security Association and Key Management
     Protocol", D. Maughan, M. Schertler, M. Schneider and J. Turner,
     November 1998.

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

10. Bibliography 

[26] <draft-ietf-sigtran-m2ua-07.txt>, "MTP2-User Adaptation Layer",
     K. Morneault et al, February 2001 (Work in Progress)

11. Author's Addresses

Greg Sidebottom
Kanata, Ontario, Canada
gregside@home.com

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

Lyndon Ong
Ciena
10480 Ridgeview Court
Cupertino, CA 95014
lyong@ciena.com

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

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

Klaus D. Gradischnig
NeuStar, Inc
1120 Vermont Ave. N.W.Suite 400
Washington D.C 20005
klaus.gradischnig@neustar.com

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: 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 December 2001.

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