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draft-ietf-sigtran-m2ua-04

Description: Request For Comments

You can download source copies of the file as follows:

draft-ietf-sigtran-m2ua-04.txt in text format.

Listed below is the contents of file draft-ietf-sigtran-m2ua-04.txt.


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

Expires in six months                                      March 2000

                  SS7 MTP2-User Adaptation Layer
                  <draft-ietf-sigtran-m2ua-04.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/ietf/1id-abstracts.txt

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

Morneault, et al                                                [Page 1]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

                        TABLE OF CONTENTS

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

Morneault, et al                                                [Page 2]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

1.  Introduction

1.1 Scope

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

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

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

1.2 Terminology

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

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

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

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

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

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

Application Server Process Path (ASP Path or just Path) - A Path to a 
remote Application Server Process instance.  A Path maps 1:1 to an SCTP 
association.

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

Morneault, et al                                                [Page 3]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

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

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

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

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

Stream - A stream refers to an SCTP stream.

1.3 Signaling Transport Architecture

The framework architecture that has been defined for SCN signaling 
transport over IP [5] uses multiple components, including a signaling 
common transport protocol and an adaptation module to support the 
services expected by a particular SCN signaling protocol from its 
underlying protocol layer.  

Within this framework architecture, this document defines a SCN 
adaptation module that is suitable for the transport of SS7 MTP2 User 
messages.  The only SS7 MTP2 User is MTP3.  The M2UA uses the services 
of the Simple Common Transport protocol [6] as the underlying reliable 
signaling common transport protocol. 

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

1.3.1  Example - SG to MGC

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

Morneault, et al                                                [Page 4]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

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

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

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

            Figure 1  M2UA in the SG to MGC Application

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

1.3.2 Signaling Network Architecture

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

To meet the stringent SS7 signaling reliability and performance 
requirements for carrier grade networks, these Network Operators should 
ensure that there is no single point of failure provisioned in the end-
to-end network architecture between an SS7 node and an IP ASP.  Depending 
of course on the reliability of the SG and ASP functional elements, this 
can typically be met by the spreading links in a linkset across SGs, the 
provision of redundant QOS-bounded IP network paths for SCTP Associations 
between SCTP End Points, and redundant Hosts.  The distribution of ASPs 
within the available Hosts is also important.  For a particular 
Application Server, the related ASPs should be distributed over at least 
two Hosts.

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

Morneault, et al                                                [Page 5]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

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

                    Figure 2 - Physical Model Example

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

1.3.4  ASP Fail-over Model and Terminology

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

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

The fail-over model supports an ôn+kö redundancy model, where ônö ASPs 
is the minimum number of redundant ASPs required to handle traffic and 
ôkö ASPs are available to take over for a failed or unavailable ASP.  
Note that ô1+1ö active/standby redundancy is a subset of this model.  
A simplex ô1+0ö model is also supported as a subset, with no ASP 
redundancy.

Morneault, et al                                                [Page 6]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

To avoid a single point of failure, it is recommended that a minimum of 
two ASPs be in the list, resident in separate hosts  and therefore 
available over different SCTP Associations.  For example, in the 
network shown in Figure 1, all messages to DPC x could be sent to ASP1 
in Host1 or ASP1 in Host2.  The AS list at SG1 might look like this:

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

In this ô1+1ö redundancy case, ASP1 in Host1 would be sent any incoming 
message for the Interface Identifiers registered.  ASP1 in Host2 would 
normally be brought to the active state upon failure of, or loss of 
connectivity to, ASP1/Host1.  In this example, both ASPs are Up, meaning 
that the related SCTP association and far-end M2UA peer is ready.

The AS List at SG1 might also be set up in loadshare mode:

    Interface Identifiers - Application Server #1
        ASP1/Host1 - State = Up, Active
        ASP1/Host2 - State = Up, Active

In this case, both the ASPs would be sent a portion of the traffic.  

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

1.3.5 Client/Server Model

It is recommended that the SG take on the role of server while the 
ASP is the client.  ASPs should initiate the SCTP association to the 
SG.

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

1.4  Services Provided by the M2UA Adaptation Layer

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

This includes the following services:
 
1.4.1  Support for MTP Level 2 / MTP Level 3 interface boundary

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

Data

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

Morneault, et al                                                [Page 7]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

Link Establish

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

Link Release

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

Link State

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

Link Status

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

Data Retrieval

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

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

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

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

M-ERROR

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

1.4.3  Support for management of active associations between SG and MGC

The M2UA layer on the SG keeps the state of various ASPs it is associated
with.  A set of primitives between M2UA layer and the Layer Management 
are defined below to help the Layer Management manage the association(s) 
between the SG and the MGC.

M-SCTP ESTABLISH

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

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

Morneault, et al                                                [Page 8]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

M-SCTP STATUS

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

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

M-ASP STATUS

The M-ASP STATUS primitive is used by the Layer Management to indicate
the status of the local M2UA user to the M2UA layer.

1.5  Functions Provided by the M2UA Layer 

1.5.1  Mapping

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

1.5.2  Flow Control / Congestion

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

1.5.3  SCTP Stream Management

SCTP allows user specified number of streams to be opened during the
initialization.  It is the responsibility of the M2UA layer to ensure 
proper management of these streams.  SCTP streams provide a means for
avoiding head of line blocking.  For that reason, a stream should be used 
per SS7 signaling link terminated by the Signaling Gateway.  The SS7 
signaling link can be identified by the optional Interface Identifier 
in the M2UA specific message header (refer to Section 2.2).

1.5.4  Seamless SS7 Network Management Interworking

If the SG loses the SCTP association to the MGC, it should follow
MTP 2 processor outage procedures [2].

Morneault, et al                                                [Page 9]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

1.5.5  Active Association Control

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

1.6  Definition of the M2UA Boundaries

1.6.1  Definition of the M2UA / MTP Level 3 boundary

DATA 
ESTABLISH 
RELEASE 
STATE 
STATUS
RETRIEVAL 
DATA RETRIEVAL 
DATA RETRIEVAL COMPLETE 

1.6.2  Definition of the M2UA / MTP Level 2 boundary

DATA 
ESTABLISH 
RELEASE 
STATE 
STATUS
RETRIEVAL 
DATA RETRIEVAL 
DATA RETRIEVAL COMPLETE 

1.6.3  Definition of the Lower Layer Boundary between M2UA and SCTP

The upper layer and layer management primitives provided by SCTP are 
provided in Reference [6] Section 9.

1.6.4  Definition of Layer Management / M2UA Boundary

M-ERROR
M-SCTP ESTABLISH
M-SCTP STATUS
M-ASP STATUS

2.0 Conventions

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

3.0  Protocol Elements

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

Morneault, et al                                               [Page 10]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

3.1  Common Message Header

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

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

                  Figure 3  Common Message Header

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

3.1.1  Version

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

      Release 1.0 	0x1

3.1.2  Message Type

The valid message types are defined in Section 3.2.2 and the message 
contents are described in Section 3.3.  Each message can contain 
parameters.

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

     MTP2 User Adaptatation (MAUP) Messages

        Data                                       0x0601
        Establish Request                          0x0602
        Establish Confirm                          0x0603
        Release Request                            0x0604
        Release Confirm                            0x0605
        Release Indication                         0x0606
        State Request                              0x0607
        State Confirm                              0x0608
        State Indication                           0x0609
        Data Retrieval Request                     0x060a
        Data Retrieval Confirm                     0x060b
        Data Retrieval Indication                  0x060c
        Data Retrieval Complete Indication         0x060d

     Application Server Process Maintenance (ASPM) Messages

        ASP Up                                     0x0301
        ASP Down                                   0x0302
        Heartbeat                                  0x0303
        ASP Up Ack                                 0x0304
        ASP Down Ack                               0x0305
        ASP Active                                 0x0401
        ASP Inactive                               0x0402
        ASP Active                                 0x0403
        ASP Inactive                               0x0404

Morneault, et al                                               [Page 11]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

     Management (MGMT) Messages

        Error                                      0x0000
        Notify                                     0x0001
       
3.1.3  Message Length

The Message length defines the length of the message in octets, not 
including the header.

3.1.4 Variable-Length Parameter Format

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

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

Parameter Tag: 16 bits (unsigned integer)

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

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

Parameter Length: 16 bits (unsigned integer)

The Parameter Length field contains the size of the parameter in bytes, 
including the Parameter Tag, Parameter Length, and Parameter Value 
fields.  Thus, a parameter with a zero-length Parameter Value field 
would have a Length field of 4.  The Parameter Length does not include 
any padding bytes.

Parameter Value: variable-length.

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

The total length of a parameter (including Tag, Parameter Length and Value 
fields) MUST be a multiple of 4 bytes.  If the length of the parameter is 
not a multiple of 4 bytes, the sender pads the Parameter at the end (i.e., 
after the Parameter Value field) with all zero bytes.  The length of the 
padding is NOT included in the parameter length field.  A sender should 
NEVER pad with more than 3 bytes. The receiver MUST ignore the padding 
bytes.

3.2  M2UA Message Header

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

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

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

                Figure 4  M2UA Message Header

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

3.3 M2UA Messages

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

3.3.1 MTP2 User Adaptation Messages

3.3.1.1 Data 

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

Morneault, et al                                               [Page 12]

Internet Draft         SS7 MTP2 User Adaptation Layer           Jul 2000

The format for the Data Message parameters is as follows:

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

   |                       Protocol Data                           |

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

The Protocol Data field contains the MTP2-User application message in
network byte order. 

3.3.1.2  Establish (Request, Confirmation)

The Establish Request message is used to establish the link or to 
indicate that the channel has been established.  Note that the gateway 
may already have the SS7 link established at its layer.  If so, upon 
receipt of an Establish Request, the gateway takes no action except to 
send an Establish Confirm.

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

3.3.1.3  Release (Request, Indication, Confirmation)

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

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

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

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

3.3.1.4  State (Request, Confirm)

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

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

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

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

3.3.1.5  State Indication

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

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

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

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

3.3.1.6  Retrieval (Request, Confirm)

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

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

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

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

In the Retrieval Request message, the fsn_bsn field contains the FSN of 
the far end if the action is ACTION_RTRV_MSGS. 

When the Signaling Gateway sends a Retrieval Confirm to this request, 
it echos the action and puts the BSN in the fsn_bsn field if the action 
was ACTION_RTRV_BSN.  If there is a failure in retrieving the BSN, the
fsn_bsn should contain a -1 (0xffffffff).

3.3.1.7  Retrieval Indication

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

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

   |                     PDU from retransmit queue                 |

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

3.3.1.8  Retrieval Complete Indication

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

3.3.2  Application Server Process Maintenance (ASPM) Messages

The ASPM messages will only use the common message header.

3.3.2.1  ASP UP (ASPUP)

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

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The ASPUP message contains the following parameters:

     Adaptation Layer Identifer (optional)
     Protocol Identifier (optional)
     INFO String (optional)
     
The format for ASPUP Message parameters is as follows:

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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    
   |                          INFO String*                         |

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

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

The optional Adaptation Layer Identifier (ALI) is a string that 
identifies the adaptation layer.  This string must be set to "M2UA" 
which results in a length of 8.  The ALI would normally only be used in 
the initial ASP Up message across a new SCTP association to ensure both 
peers are assuming the same adaptation layer protocol.

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3.3.3.2 ASP Up Ack

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

The ASPUP Ack message contains the following parameters:

     Adaptation Layer Identifer (optional)
     INFO String (optional)
     
The format for ASPUP Ack Message parameters is as follows:

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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    
   |                          INFO String*                         |

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

The format and description of the optional Info String parameter is the 
same as for the ASP UP message (See Section 3.3.3.1.)

The format and description of the optional Adaptation Layer Identifier (ALI) 
parameter is the same as for the ASP UP message (See Section 3.3.3.1).

3.3.3.3  ASP Down (ASPDN)

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

The ASPDN message contains the following parameters

     Reason
     INFO String (Optional)

The format for the ASPDN message parameters is as follows:

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

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

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

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

     Value         Description
     0x1        Management Inhibit

3.4.4 ASP Down Ack

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

The ASP Down Ack message contains the following parameters:

     Reason
     INFO String (Optional)

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

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

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

The format and description of the optional Info String parameter is the 
same as for the ASP UP message (See Section 3.3.3.1.)

The format of the Reason parameter is the same as for the ASP-Down message 
(See Section 3.3.3.3).

3.3.3.5  ASP Active (ASPAC)

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

The ASPAC message contains the following parameters

     Type
     Interface Identifier (Optional)
     INFO String (Optional)

The format for the ASPAC message is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Type                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xx)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
   |                     Interface Identifiers*                    |

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

   |                          INFO String*                         |

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

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

    Value          Description
     0x1            Over-ride
     0x2            Load-share

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

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The optional Interface Identifiers parameter contains a list of
Interface Identifier integers indexing the Application Server traffic
that the sending ASP is configured/registered to receive.  There is
one-to-one relationship between an Interface Identifier and an AS
Name. 

An SG that receives an ASPAC with an incorrect type for a particular
Interface Identifier will respond with an Error Message.

The format and description of the optional Info String parameter is the
same as for the ASP UP message (See Section 3.3.3.1.)

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

3.3.3.6 ASP Active Ack

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

The ASPAC Ack message contains the following parameters:

     Type
     Interface Identifier (Optional)
     INFO String (Optional)

The format for the ASPAC Ack message is as follows:

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

   |                     Interface Identifiers*                    |

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

   |                          INFO String*                         |

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

The format and description of the optional Info String parameter is the 
same as for the ASP UP message (See Section 3.3.3.1.)

The format of the Type and Interface Identifier parameters is the same 
as for the ASP Active message (See Section 3.3.3.5).

3.3.3.7  ASP Inactive (ASPIA)

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

The ASPIA message contains the following parameters

     Type
     Interface Identifiers (Optional)
     INFO String (Optional)

The format for the ASPIA message parameters is as follows:

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

   |                     Interface Identifiers*                    |

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

   |                          INFO String*                         |

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

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

    Value          Description
     0x1            Over-ride
     0x2            Load-share

The format and description of the optional Interface Identifiers and
Info String parameters is the same as for the ASP Active message (See
Section 3.3.3.3.)

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

3.3.3.8 ASP Inactive Ack

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

The ASPIA Ack message contains the following parameters:

     Type
     Routing Context (Optional)
     INFO String (Optional)

The format for the ASPIA Ack message is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                             Type                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x6)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
   |                       Routing Context*                        | 

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

   |                          INFO String*                         |

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

The format and description of the optional Info String parameter is 
the same as for the ASP UP message (See Section 3.3.3.1.)

The format of the Type and Routing Context parameters is the same as 
for the ASP-Inctive message (See Section 3.3.3.7).

3.4.9 Heartbeat (BEAT)

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

The BEAT message contains no parameters.

3.3.3  Layer Management (MGMT) Messages

3.3.3.1  Error (ERR)

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

The ERR message contains the following parameters:

     Error Code
     Diagnostic Information (optional)

The format for the ERR message is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Error Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xx)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
   |                     Diagnostic Information*                   |
 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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3.3.3.2  Notify (NTFY)

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

The NTFY message contains the following parameters:

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

The format for the NTFY message is as follows:

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

   |                      Interface Identifiers*                   |

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

   |                          INFO String*                         |

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

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

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

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

      Value          Description     
       0x1    Application Server Down (AS_Down)
       0x2    Application Server Up (AS_Up)
       0x3    Application Server Active (AS_Active)
       0x4    Application Server Pending (AS_Pending)
       0x5    Alternate ASP Active

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

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

      Value          Description     
       0x1    Insufficient ASP resources active in AS

This notification is not based on the SG reporting the state change of an 
ASP or AS.  For the value defined the SG is indicating to an ASP(s) in 
the AS that another ASP is required in order to handle the load of the AS. 

The format and description of the optional Interface Identifiers and
Info String parameters is the same as for the ASP Active message
(See Section 3.3.3.3.)

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4.0  Procedures

The M2UA layers needs to respond to various primitives it receives from 
other layers as well as messages it receives from the peer-to-peer 
messages.  This section describes various procedures involved in 
response to these events.

4.1  Procedures to Support Service in Section 1.4.1

These procedures achieve the M2UA layer's "Transport of MTP Level 2 / 
MTP Level 3 boundary" service.

4.1.1  MTP Level 2 / MTP Level 3 Boundary Procedures

On receiving a primitive from the local upper layer, the M2UA layer will 
send the corresponding MAUP message (see Section 2) to its peer.  The 
M2UA layer must fill in various fields of the common and specific headers
correctly.  In addition the message needs to be sent on the SCTP stream 
that corresponds to the SS7 link.

4.1.2  MAUP Message Procedures

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

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4.2  Procedures to Support Service in Section 1.4.2

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

4.2.1  Layer Management Primitives Procedure

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

4.2.2 MGMT message procedures

Upon receipt of MGMT messages the M2UA layer must invoke the corresponding
Layer Management primitives (M-ERROR) to the local layer management.

4.3 Procedures to Support Service in Section 1.4.3

These procedures achieve the M2UA layer's "Support for management of 
active associations between SG and MGC" service.

4.3.1 State Maintenance

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

4.3.1.1  ASP States

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

    * Reception of messages from peer IUA layer at that ASP
    * Reception of some messages from the peer IUA layer at other
      ASPs in the AS
    * Reception of indications from SCTP layer
    * Switch-over Time triggers

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

ASP-DOWN Application Server Process is unavailable and/or the SCTP
association is down. Initially all ASPs will be in this state.

ASP-UP The remote IUA peer at the ASP is available (and the SCTP
association is up) but application traffic is stopped.

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

                
                 Figure 4 ASP State Transition Diagram

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

SCTP CDI The local SCTP layer's Communication Down Indication to the  
Upper Layer Protocol (IUA) on an SG. The local SCTP will send this 
indication when it detects the loss of connectivity to the ASP's peer 
SCTP layer.

Ts Switch-over Time Triggers.  This timer is configurable by the 
Operator on a per AS basis.  The default value of this timer should
be three seconds.

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4.3.1.2  AS States

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

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

   * ASP state transitions
   * Recovery timer triggers

The possible states of an AS are the following:

AS-DOWN The Application Server is unavailable.  This state implies 
that all related ASPs are in the ASP-DOWN state for this AS.
Initially the AS will be in this state.

AS-UP The Application Server is available but no application traffic
is active (i.e., one or more related ASPs are in the ASP-UP state,
but none in the ASP-Active state). 

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

AS-PENDING An active ASP has transitioned from active to inactive or
down and it was the last remaining active ASP in the AS. A recovery 
timer T(r) will be started and all incoming SCN messages will be
queued by the SG. If an ASP becomes active before T(r) expires, the
AS will move to AS-ACTIVE state and all the queued messages will be
sent to the active ASP. 

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

                 Figure 5 AS State Transition Diagram

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

    Tr = Recovery Timer

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4.3.2 ASPM procedures for primitives

Before the establishment of an SCTP association the ASP state at both 
the SG and ASP is assumed to be "Down".  

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

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

Once the SCTP association is establishedand assuming that the local 
IUA-User is ready, the local ASP IUA Application Server Process 
Maintenance (ASPM) function will initiate the ASPM procedures, using 
the ASP-Up/-Down/-Active/-Inactive messages to convey the ASP-state to 
the SG - see Section 4.3.3. 

The Layer Management and the IUA layer on SG can communicate the
status of the application server using the M-AS STATUS primitives.
The Layer Managements and the IUA layers on both the SG and ASP
can communicate the status of an SCTP association using the 
M-SCTP STATUS primitives.

If the Layer Management on SG or ASP wants to bring down an SCTP
association for management reasons, they would send M-SCTP RELEASE
request primitive to the local IUA layer. The IUA layer would release
the SCTP association and upon receiving the SCTP Communication Down
indication from the underlying SCTP layer, it would inform the local
Layer Management using M-SCTP RELEASE confirm primitive.

If the IUA layer receives an SCTP-Communication Down indication
from the underlying SCTP layer, it will inform the Layer 
Management by invoking the M-SCTP RELEASE indication primitive. The 
state of the ASP will be moved to "Down" at both the SG and ASP.

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

4.3.3 ASPM procedures for peer-to-peer messages

All ASPM messages are sent on a sequenced stream to ensure ordering.  
SCTP stream æ0Æ is used.

4.3.3.2 ASP-Up

After an ASP has successfully established an SCTP association to an SG, 
the SG waits for the ASP to send an ASP-Up message, indicating that the 
ASP IUA peer is available.  The ASP is always the initiator of the 
ASP-Up exchange.  

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

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

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

The ASP must wait for the ASP-Up Ack message from the SG before 
sending any ASP traffic control messages (ASPAC or ASPIA) or Data 
messages or it will risk message loss.  If the SG receives Data 
messages before an ASP Up is received, the SG should discard.

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4.3.3.2 ASP-Down

The ASP will send an ASP-Down to an SG when the ASP is to be removed 
from the list of ASPs in all Application Servers that it is a member.

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

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

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

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

4.3.3.3  M2UA Version Control

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

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

4.3.3.4 ASP-Active

Any time after the ASP has received a ASP-Up Ack from the SG, the ASP 
sends an ASP-Active (ASPAC) to the SG indicating that the ASP is ready 
to start processing traffic.  In the case where an ASP is configured/-
registered to process the traffic for more than one Application Server 
across an SCTP association, the ASPAC contains one or more Interface 
Identifiers to indicate for which Application Servers the ASPAC applies. 

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

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

In the case of an Over-ride mode AS, reception of an ASPAC message at 
an SG causes the redirection of all traffic for the AS to the ASP that 
sent the ASPAC.  The SG responds to the ASPAC with an ASP-Active Ack 
message to the ASP.  Any previously active ASP in the AS is now 
considered Inactive and will no longer receive traffic from the SG 
within the AS.  The SG sends a Notify (Alternate ASP-Active) to the 
previously active ASP in the AS, after stopping all traffic to that 
ASP.

In the case of a load-share mode AS, reception of an ASPAC message at
an SG causes the direction of traffic to the ASP sending the ASPAC,
in addition to all the other ASPs that are currently active in the AS.  
The algorithm at the SG for loadsharing traffic within an AS to all
the active ASPs is application and network dependent.  The algorithm
could, for example be round-robin or based on information in the Data
message, such as Interface ID, depending on the requirements of the
application and the call state handling assumptions of the collection
of ASPs in the AS. The SG responds to the ASPAC with a ASP-Active Ack
message to the ASP.

4.3.3.5 ASP Inactive

When an ASP wishes to withdraw from receiving traffic within an AS,  
the ASP sends an ASP Inactive (ASPIA) to the SG.  In the case where 
an ASP is configured/registered to process the traffic for more than 
one Application Server across an SCTP association, the ASPIA contains 
one or more Interface Ids to indicate for which Application Servers 
the ASPIA applies.

There are two modes of Application Server traffic handling in the SG
IUA when withdrawing an ASP from service - Over-ride and Load-balancing.  
The Type parameter in the ASPIA messge indicates the mode used in a 
particular Application Server. If the SG determines that the mode 
indicates in an ASPAC is incompatible with the traffic handling mode 
currently used in the AS, the SG responds with an Error message 
indicating Invalid Traffic Handling Mode.

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

In the case of a Loadshare mode AS, the SG moves the ASP to the "Up" 
state and the AS traffic is re-allocated across the remaining 
"active" ASPs per the load-sharing algorithm currently used within 
the AS.  AnASPIA Ack message is sent to the ASP after all traffic 
is halted to the ASP.

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

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4.3.3.5 Notify

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

4.3.3.6 Heartbeat

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

Once the ASP sends an ASP-Up message to the SG, the ASP sends Beat 
messages periodically, subject to a provisionable timer T(beat).  
The SG M3UA, upon receiving a BEAT message from the ASP, responds 
with a BEAT message.  If no BEAT message (or any other M3UA message), 
is received from the ASP within the timer 2*T(beat), the ASP will 
consider the remote M3UA as 'Down".

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

Note:  Heartbeat related events are not shown in Figure 4 "ASP state 
transition diagram".  
                                 
5.0  Examples of MTP2 User Adaptation (M2UA) Procedures

5.1  Establishment of associations between SG and MGC examples

5.1.1 Single ASP in an Application Server (ô1+0ö sparing)

This scenario shows the example M2UA message flows for the establishment 
of traffic between an SG and an ASP, where only one ASP is configured 
within an AS (no backup).  It is assumed that the SCTP association is 
already set-up.

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

5.1.2 Two ASPs in Application Server (ô1+1ö sparing)

This scenario shows the example M2UA message flows for the establishment 
of traffic between an SG and two ASPs in the same Application Server, 
where ASP1 is configured to be ôactiveö and ASP2 a ôstandbyö in the event 
of communication failure or the withdrawal from service of ASP1.  ASP2 may 
act as a hot, warm, or cold standby depending on the extent to which ASP1 
and ASP2 share call/transaction state or can communicate call state under 
failure/withdrawal events.  The example message flow is the same whether 
the ASP-Active messages are Over-ride or Load-share mode although typically 
this example would use an Over-ride mode.

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

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5.1.3 Two ASPs in an Application Server (ô1+1ö sparing, load-sharing case)

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

       SG                       ASP1                       ASP2
        |                         |                          |
        |<---------ASP Up---------|                          | 
        |---------ASP-UpAck------>|                          |
        |                         |                          |
        |<------------------------------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 (ôn+kö sparing, load-sharing case) 

This scenario shows the example M2UA message flows for the establishment 
of traffic between an SG and three ASPs in the same Application Server, 
where two of the ASPs are brought to ôactiveö and share the load. In 
this case, a minimum of two ASPs are required to handle the total traffic 
load (2+1 sparing).

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

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5.2 ASP Traffic Fail-over Examples

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

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

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

Note: If the SG detects loss of the M2UA peer (M2UA heartbeat loss or 
detection of SCTP failure), the initial SG-ASP1 ASP Inactive message 
exchange would not occur.

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

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

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

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

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

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

The Notify message to ASP3 is optional, as well as the ASP-Active from 
ASP3.  The optional Notify can only occur if the SG maintains knowledge 
of the minimum ASP resources required - for example if the SG knows that 
ôn+kö = ô2+1ö for a loadshare AS and ônö currently equals ô1ö. 

Note: If the SG detects loss of the ASP1 M2UA peer (M2UA heartbeat loss 
or detection of SCTP failure), the first SG-ASP1 ASP Inactive message 
exchange would not occur.  

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

5.3.1  SS7 Link Alignment

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

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

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

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5.3.2  SS7 Link Release

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

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

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

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

5.3.3  Set and Clear Local Processor Outage

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

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

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

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

5.3.4  Notification of Processor Outage (local or remote)

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

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

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

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5.3.5  SS7 Link Changeover

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

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

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

6.0 Security

6.1 Introduction

M2UA is designed to carry signaling messages for telephony services. As such, 
M2UA must involve the security needs of several parties: the end users
of the services; the network providers and the applications involved.
Additional requirements may come from local regulation.  While having some
overlapping security needs, any security solution should fulfill all of the
different parties' needs. 

6.2 Threats

There is no quick fix, one-size-fits-all solution for security.  As a
transport protocol, M2UA has the following security objectives:

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

M2UA runs on top of SCTP.  SCTP [6] provides certain transport related
security features, such as:

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

When M2UA is running in professionally managed corporate or service 
provider network, it is reasonable to expect that this network includes 
an appropriate security policy framework. The "Site Security Handbook" [9]
should be consulted for guidance.

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When the network in which M2UA runs in involves more than one party, it 
may not be reasonable to expect that all parties have implemented security 
in a sufficient manner.  In such a case, it is recommended that IPSEC is 
used to ensure confidentiality of user payload.  Consult [10] for more 
information on configuring IPSEC services.

6.3 Protecting Confidentiality 

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

7.0 IANA Considerations

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

        M2UA    tbd

The SCTP Payload Protocol Identifier is included in each SCTP Data chunk, 
to indicate which protocol the SCTP is carrying.  This Payload Protocol 
Identifier is not directly used by SCTP but may be used by certain network 
entities to identify the type of information being carried in a Data chunk.

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

The authors would like to thank Ian Rytina, Hanns Juergen Schwarzbauer 
and ZhangYi for their valuable comments and suggestions.

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9.0  References

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

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

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

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

[5] Framework Architecture for Signaling Transport, draft-ietf-sigtran-
    framework-arch-03.txt, June 1999

[6] Simple Control Transmission Protocol, draft-ietf-sigtran-sctp-07.txt, 
    March 2000

[7] Media Gateway Control Protocol (MGCP), draft-huitema-megaco-mgcp-
    v1-03.txt, August 1999

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

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

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

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10.0  Author's Addresses

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

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

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

Ram Dantu, Ph.D.                     Tel +1-972-234-6070 extension 211
IPmobile                                     EMail rdantu@ipmobile.com
1651 North Glenville, Suite 216
Richardson, TX 75081
USA

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

This Internet Draft expires December 2000.


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