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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 3057

Network Working Group                                    Ken Morneault
INTERNET-DRAFT                                           Cisco Systems
                                                       Malleswar Kalla
                                                      Selvam Rengasami
                                                Telcordia Technologies
                                                       Greg Sidebottom
                                                       Nortel Networks


Expires in six months                                         Nov 2000




                  ISDN Q.921-User Adaptation Layer
                  <draft-ietf-sigtran-iua-09.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
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The list of current Internet-Drafts can be accessed at
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To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).



Abstract

This Internet Draft defines a protocol for backhauling of ISDN Q.921
User messages over IP using the Stream Control Transmission Protocol
(SCTP). This protocol would be used between a Signaling Gateway (SG)
and Media Gateway Controller (MGC). It is assumed that the SG receives
ISDN signaling over a standard ISDN interface.






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                        TABLE OF CONTENTS

1.  Introduction.....................................................3
  1.1  Scope.........................................................3
  1.2  Terminology...................................................3
  1.3  IUA Overview..................................................4
  1.4  Services Provided by the IUA Layer............................8
  1.5  Functions Implemented by the IUA Layer.......................10
  1.6  Definition of IUA Boundaries.................................11
2.  Conventions.....................................................14
3.  Protocol Elements...............................................14
  3.1  Common Message Header........................................14
  3.2  IUA Message Header...........................................17
  3.3  Description of Messages......................................18
4.  Procedures......................................................35
  4.1  Procedures to Support Service in Section 1.4.1...............36
  4.2  Procedures to Support Service in Section 1.4.2...............36
  4.3  Procedures to Support Service in Section 1.4.3...............37
5. Examples.........................................................44
  5.1 Establishment of associations between SG and MGC examples.....44
  5.2 ASP Traffic Fail-over Examples................................46
  5.3 Q.921/Q.931 primitives backhaul Examples......................47
  5.4 Layer Management Communication Examples.......................48
6.  Security........................................................48
  6.1 Threats.......................................................49
  6.2 Protecting Confidentiality ...................................49
7.  IANA Considerations.............................................49
  7.1 SCTP Payload Protocol Identifier..............................49
  7.2 IUA Protocol Extensions.......................................49
8.  Acknowledgements................................................50
9.  References......................................................50
10. Author's Addresses..............................................50
























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1.  Introduction

In this document, the term Q.921 user refers to an upper layer which
uses the services of Q.921, not the user side of ISDN interface [1].
Examples of the upper layer would be Q.931 and QSIG.

This section describes the need for ISDN Q.921 User Adaptation (IUA)
layer protocol as well as how this protocol shall be implemented.

1.1 Scope

There is a need for Switched Circuit Network (SCN) signaling protocol
delivery from an ISDN Signaling Gateway (SG) to a Media Gateway
Controller (MGC) as described in the Framework Architecture for
signaling Transport [4].  The delivery mechanism SHOULD meet the
following criteria:

*  Support for transport of the Q.921 / Q.931 boundary primitives
*  Support for communication between Layer Management modules on SG
   and MGC
*  Support for management of active associations between SG and MGC

This draft supports both ISDN Primary Rate Access (PRA) as well as
Basic Rate Access (BRA) including the support for both point-to-point
mode and point-to-multipoint modes of communication.  This support
includes Facility Associated Signaling (FAS), Non-Facility Associated
Signaling (NFAS) and NFAS with backup D channel.  QSIG adaptation
layer requirements do not differ from Q.931 adaptation layer, hence
the procedures described in this draft are also applicable for a QSIG
adaptation layer.  For simplicity, only Q.931 will be mentioned in the
rest of this document.

1.2 Terminology

Interface - For the purposes of this document an interface supports the
relevant ISDN signaling channel.  This signaling channel MAY be a
16 kbps D channel for an ISDN BRA as well as 64 kbps primary or backup
D channel for an ISDN PRA.  For QSIG, the signaling channel is a Qc
channel.

Q.921-User - Any protocol normally using the services of the ISDN
Q.921 (e.g., Q.931, QSIG, etc.).

Backhaul - A SG terminates the lower layers of an SCN protocol and
backhauls the upper layer(s) to MGC for call processing.  For the
purposes of this draft the SG terminates Q.921 and backhauls Q.931 to
MGC.

Association - An association refers to a SCTP association.  The
association will provide the transport for the delivery of Q.921-User
protocol data units and IUA adaptation layer peer messages.


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

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

Application Server (AS) - A logical entity serving a specific
application instance.  An example of an Application Server is a MGC
handling the Q.931 and call processing for D channels terminated by
the Signaling Gateways.  Practically speaking, an AS is modeled at
the SG as an ordered list of one or more related Application Server
Processes (e.g., primary, secondary, tertiary).

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

Fail-over - The capability to re-route signaling traffic as required
between related ASPs in the event of failure or unavailability of the
currently used ASP (e.g. from primary MGC to back-up MGC).  Fail-over
also applies upon the return to service of a previously unavailable
process.

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

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

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

1.3 IUA Overview

The architecture that has been defined [4] for SCN signaling transport
over IP uses multiple components, including an IP transport
protocol, a signaling common transport protocol and an adaptation
module to support the functions expected by a particular SCN signaling
protocol from its underlying protocol layer.

This document defines an adaptation module that is suitable for the
transport of ISDN Q.921 User (Q.931).








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1.3.1  Example - SG to MGC

In a Signaling Gateway, it is expected that the ISDN signaling is
received over a standard ISDN network termination. The SG then
provides interworking of transport functions with IP Signaling
Transport, in order to transport the Q.931 signaling messages to the
MGC where the peer Q.931 protocol layer exists, as shown below:


******   ISDN        ******      IP      *******
* EP *---------------* SG *--------------* MGC *
******               ******              *******

+-----+                                  +-----+
|Q.931|              (NIF)               |Q.931|
+-----+           +----------+           +-----+
|     |           |     | IUA|           | IUA |
|     |           |     +----+           +-----+
|Q.921|           |Q.921|SCTP|           |SCTP |
|     |           |     +----+           +-----+
|     |           |     | IP |           | IP  |
+-----+           +-----+----+           +-----+

NIF  - Nodal Interworking Function
EP   - ISDN End Point
SCTP - Stream Control Transmission Protocol (Refer to [3])
IUA  - ISDN User Adaptation Layer Protocol

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

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

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







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

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

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

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

1.3.3  Signaling Network Architecture

A Signaling Gateway is used to support the transport of Q.921-User
signaling traffic to one or more distributed ASPs (e.g., MGCs).
Clearly, the IUA protocol is not designed to meet the performance
and reliability requirements for such transport by itself.  However,
the conjunction of distributed architecture and redundant networks
does allow for a sufficiently reliable transport of signaling
traffic over IP.  The IUA protocol is flexible enough to allow its
operation and management in a variety of physical configurations,
enabling Network Operators to meet their performance and reliability
requirements.

To meet the ISDN signaling reliability and performance requirements
for carrier grade networks, Network Operators SHOULD ensure that
there is no single point of failure provisioned in the end-to-end
network architecture between an ISDN 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 provision of redundant
QOS-bounded IP network paths for SCTP Associations between SCTP End
Points, and redundant Hosts, and redundant SGs. 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 logical network architecture relevant to carrier-grade
operation in the IP network domain is shown in Figure 1 below:


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

                    Figure 2 - Logical Model Example


For carrier grade networks, the failure or isolation of a particular
ASP SHOULD NOT cause stable calls to be dropped.  This implies that
ASPs need, in some cases, to share the call state or be able to pass
the call state between each other. However, this sharing or
communication of call state information is outside the scope of this
document.

1.3.4 ASP Fail-over Model and Terminology

The IUA layer supports ASP fail-over functions in order to support a
high availability of call processing capability.  All Q.921-User
messages incoming to an SG 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
configured to process Q.921-User messages from certain Interface
Identifiers.  One or more ASPs in the list are normally active
(i.e., handling traffic) while any others MAY be unavailable or
inactive, to be possibly used in the event of failure or
unavailability of the active ASP(s).

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


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

    Interface Identifier(s) - Application Server #1
        ASP1/Host1  - State=Up, Active
        ASP1/Host2  - State=Up, Inactive

In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
message 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 IUA peer
is ready.

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

    Interface Identifier(s) - 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
get released. It is possible that calls in transition MAY fail,
although measures of communication between the ASPs involved can
be used to mitigate this problem.  For example, the two ASPs MAY share
call state via shared memory, or MAY use an ASP to ASP protocol to
pass call state information.  The ASP to ASP protocol is outside the
scope of this document.

1.3.5  Client/Server Model

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

The SCTP (and UDP/TCP) Registered User Port Number Assignment for
IUA is 9900.






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1.4  Services Provided by the IUA Layer

1.4.1  Support for transport of Q.921/Q.931 boundary primitives

In the backhaul scenario, the Q.921/Q.931 boundary primitives are
exposed.  IUA layer needs to support all of the primitives of this
boundary to successfully backhaul Q.931.

This includes the following primitives [1]:

DL-ESTABLISH

The DL-ESTABLISH primitives are used to request, indicate and confirm
the outcome of the procedures for establishing multiple frame
operation.

DL-RELEASE

DL-RELEASE primitives are used to request, indicate, and confirm the
outcome of the procedures for terminating a previously established
multiple frame operation, or for reporting an unsuccessful
establishment attempt.

DL-DATA

The DL-DATA primitives are used to request and indicate layer 3
(Q.931) messages which are to be transmitted, or have been
received, by the Q.921 layer using the acknowledged information
transfer service.

DL-UNIT DATA

The DL-UNIT DATA primitives are used to request and indicate layer 3
(Q.931) messages which are to be transmitted, by the Q.921 layer
using the unacknowledged information transfer service.

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

It is envisioned that the IUA layer needs to provide some services
that will facilitate communication between Layer Management modules on
the SG and MGC. These primitives are pointed out in [2], which are
shown below:

M-TEI STATUS

The M-TEI STATUS primitives are used to request, confirm and
indicate the status (assigned/unassigned) of a TEI.

M-ERROR

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


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1.4.3 Support for management of active associations between SG and MGC

A set of primitives between the IUA layer and the Layer Management are
defined below to help the Layer Management manage the SCTP
association(s) between the SG and MGC.  The IUA layer can be instructed
by the Layer Management to establish an SCTP association to a peer IUA
node.  This procedure can be achieved using the M-SCTP ESTABLISH
primitive.

M-SCTP ESTABLISH

The M-SCTP ESTABLISH primitives are used to request, indicate, and
confirm the establishment of an SCTP association to a peer IUA node.

M-SCTP RELEASE

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

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

M-SCTP STATUS

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

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

M-ASP STATUS

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

M-ASP MODIFY

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

M-AS STATUS

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


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1.5 Functions Implemented by the IUA Layer

1.5.1 Mapping

The IUA layer MUST maintain a map of the Interface Identifier to a
physical interface on the Signaling Gateway.  A physical interface
would be a T1 line, E1 line, etc. and could include the TDM timeslot.
In addition, for a given interface the SG MUST be able to identify
the associated signaling channel.  IUA layers on both SG and MGC
need to maintain the status of TEIs and SAPIs.

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

One example of the logical view of relationship between D channel,
Interface Identifier, AS and ASP in the SG is shown below:

          /---------------------------------------------------+
         /   /------------------------------------------------|--+
        /   /                                                 v  |
       /   /    +----+             act+-----+    +-------+ -+--+-|+--+-
D chan1-------->|IID |-+          +-->| ASP |--->| Assoc |       v
         /      +----+ |  +----+  |   +-----+    +-------+ -+--+--+--+-
        /              +->| AS |--+                        Streams
       /        +----+ |  +----+   stb+-----+
D chan2-------->|IID |-+              | ASP |
                +----+                +-----+

where IID = Interface Identifier

Note that an ASP can be in more than one AS.

1.5.2 Status of ASPs

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

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

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


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1.5.3 SCTP Stream Management

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

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

1.5.4 Seamless Network Management Interworking

The IUA layer on the SG SHOULD pass an indication of unavailability of
the IUA-User (Q.931) to the local Layer Management, if the currently
active ASP moves from the ACTIVE state. The Layer Management could
instruct Q.921 to take some action, if it deems appropriate.

Likewise, if an SCTP association fails, the IUA layer on both the
SG and ASP sides MAY generate Release primitives to take the D
channels out-of-service.

1.5.5 Congestion Management

If the IUA layer becomes congested (implementation dependent), it
MAY stop reading from the SCTP association to flow control from
the peer IUA.

1.6 Definition of IUA Boundaries

1.6.1 Definition of IUA/Q.921 boundary

   DL-ESTABLISH
   DL-RELEASE
   DL-DATA
   DL-UNIT DATA

1.6.2 Definition of IUA/Q.931 boundary

   DL-ESTABLISH
   DL-RELEASE
   DL-DATA
   DL-UNIT DATA

1.6.3 Definition of SCTP/IUA Boundary

An example of the upper layer primitives provided by SCTP are
available in Reference [3] section 9.





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1.6.4 Definition of IUA/Layer-Management Boundary

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

   M-TEI STATUS request
   Direction: LM -> IUA
   Purpose: LM requests ASP to send a TEI status request to the SG.

   M-TEI STATUS indication
   Direction: IUA -> LM
   Purpose: ASP reports that is has received a TEI status indication
            from the SG.

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   M-TEI STATUS confirm
   Direction: IUA -> LM
   Purpose: ASP reports that is has received a TEI status confirm from the
            SG.

2.0 Conventions

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

3.0 Protocol Elements

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

3.1 Common Message Header

The protocol messages for Q.921 User Adaptation require a message
header which contains the adaptation layer version, the message type,
and message length.


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

                  Figure 3 Common Header Format

All fields in an IUA 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 IUA adaptation
layer.  The supported versions are the following:

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









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3.1.2  Message Classes and Types

The following List contains the valid Message Classes:

   Message Class: 8 bits (unsigned integer)

         0        Management (MGMT) Message [IUA/M2UA/M3UA/SUA]
         1        Transfer Messages [M3UA]
         2        SS7 Signalling Network Management (SSNM) Messages [M3UA/SUA]
         3        ASP State Maintenance (ASPSM) Messages [IUA/M2UA/M3UA/SUA]
         4        ASP Traffic Maintenance (ASPTM) Messages [IUA/M2UA/M3UA/SUA]
         5        Q.921/Q.931 Boundary Primitives Tranport (QPTM)
                  Messages [IUA]
         6        MTP2 User Adaptatation (MAUP) Messages [M2UA]
         7        Connectionless Messages [SUA]
         8        Connection-Oriented Messages [SUA]
      9 to 127    Reserved by the IETF
    128 to 255    Reserved for IETF-Defined Message Class Extentions

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

     Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages

         0        Reserved
         1        Data Request Message
         2        Data Indication Message
         3        Unit Data Request Message
         4        Unit Data Indication Message
         5        Establish Request
         6        Establish Confirm
         7        Establish Indication
         8        Release Request
         9        Release Confirm
        10        Release Indication
      11 to 127   Reserved by the IETF
     128 to 255   Reserved for IETF-Defined Message Class Extentions


     Application Server Process State Maintenance (ASPSM) messages

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

     Application Server Process Traffic Maintenance (ASTM) messages

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


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     Management (MGMT) Messages

         0        Error (ERR)
         1        Notify (NTFY)
         2        TEI Status Request
         3        TEI Status Confirm
         4        TEI Status Indication
       5 to 255   Reserved for Management Messages


3.1.3  Reserved

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

3.1.4  Message Length

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

3.1.5  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.  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.


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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 IUA Message Header

In addition to the common message header, there will be a specific
message header for QPTM and the TEI Status MGMT messages.  The IUA
message header will immediately follow the Common header in these
messages.

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

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


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x1)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier (integer)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x5)           |             Length=8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            DLCI               |              Spare            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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









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

   |                   Interface Identifier (text)                 |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x5)           |             Length=8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            DLCI               |             Spare             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

The DLCI format is shown below in Figure 6.

      0     1     2     3     4     5     6     7
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |  0  | SPR |      SAPI                         |
   +-----------------------------------------------+
   |  1  |            TEI                          |
   +-----------------------------------------------+

              Figure 6  DLCI Format

SPR:  Spare 2nd bit in octet 1, (1 bit)

SAPI: Service Access Point Identifier, 3rd through 8th bits in octet 1
      (6 bits)

TEI:  Terminal Endpoint Identifier, 2nd through 8th bits in octet 2
      (7 bits)

The DLCI field (including the SAPI and TEI) is coded in accordance
with Q.921.


3.3 IUA  Messages

The following section defines the messages and parameter contents.
The IUA messages will use the common message header (Figure 3) and
the IUA message header (Figure 4 and Figure 5).






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3.3.1 Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages

3.3.1.1  Establish Messages (Request, Confirm, Indication)

The Establish Messages are used to establish a data link on the
signaling channel or to confirm that a data link on the signaling
channel has been established.  The MGC controls the state of the
D channel.  When the MGC desires the D channel to be in-service,
it will send the Establish Request message.

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

When the SG receives an IUA Establish Request from the MGC, the SG
shall send the Q.921 Establish Request primitive to the its Q.921
entity.  In addition, the SG shall map any response received from the
Q.921 entity to the appropriate message to the MGC.  For example, if
the Q.921 entity responds with a Q.921 Establish Confirm primitive, the
IUA layer shall map this to an IUA Establish Confirm message.  As
another example, if the IUA Layer receives a Q.921 Release Confirm or
Release Indication as an apparent response to the Q.921 Establish
Request primitive, the IUA Layer shall map these to the corresponding
IUA Release Confirm or Release Indication messages.

The Establish messages contain the common message header followed by
IUA message header.  It does not contain any additional parameters.

3.3.1.2  Release Messages (Request, Indication, Confirmation)

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

If a response to the Release Request message is not received, the MGC
MAY resend the Release Request message.  If no response is received,
the MGC can consider the data link as being released.  In this case,
signaling traffic on that D channel is not expected from the SG and
signaling traffic will not be sent to the SG for that D channel.

The Release messages contain the common message header followed by
IUA message header. The Release confirm message is in response to
a Release Request message and it does not contain any additional
parameters. The Release Request and Indication messages contain the
following parameter:

     REASON



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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xf)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              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_DM     0x2     Specific to a request. Indicates Layer 2
                          SHOULD release and deny all requests from
                          far end to establish a data link on the
                          signaling channel (i.e. if SABME is
                          received send a DM)
   RELEASE_OTHER  0x3     Other reasons

Note:  Only RELEASE_MGMT, RELEASE_DM and RELEASE_OTHER are valid
reason codes for a Release Request message.

3.3.1.3 Data Messages (Request, Indication)

The Data message contains an ISDN Q.921-User Protocol Data Unit (PDU)
corresponding to acknowledged information transfer service.

The Data messages contain the common message header followed by IUA
message header. The Data message contains the following parameters:

     PROTOCOL DATA

    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 (0xe)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          Protocol Data                        |

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

The protocol data contains upper layer signaling message e.g.
Q.931, QSIG.  The Data message SHOULD NOT contain any padding
bytes.



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3.3.1.4 Unit Data Messages (Request, Indication)

The Unit Data message contains an ISDN Q.921-User Protocol Data Unit
(PDU) corresponding to unacknowledged information transfer service.

The Unit Data messages contain the common message header followed by
IUA message header. The Unit Data message contains the following
parameters

     PROTOCOL DATA

    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 (0xe)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                          Protocol Data                        |

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

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 sent by an ASP to indicate to an SG that
it is ready to receive traffic or maintenance messages.

The ASPUP message contains the following parameters:

     Info String (optional)

The format for ASPUP Message parameters is as follows:

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

   |                          INFO String*                         |

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

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

3.3.2.2 ASP Up Ack

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


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The ASPUP Ack message contains the following parameters:

     INFO String (optional)

The format for ASPUP Ack Message parameters is as follows:

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

3.3.2.3  ASP Down (ASPDN)

The ASP Down (ASPDN) message is sent by an ASP to indicate to an SG that
it 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xa)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              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.).




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

     Value         Description
     0x1          Management Inhibit

If a ASP is removed from Management Inhibit, the ASP will send
an ASP Up message.

3.3.2.4  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 ASP Down Ack message parameters is as follows:

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

   |                         INFO String*                          |

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

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

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

3.3.2.5  ASP Active (ASPAC)

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

The ASPAC message contains the following parameters

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

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


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

   |                     Interface Identifiers*                    |

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

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

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

   |                          INFO String*                         |

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














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

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

   |                     Interface Identifier*                     |

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

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

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

   |                          INFO String*                         |

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


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

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

Within a particular Interface Identifier, only one Traffic Mode 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.

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

If no Interface Identifiers are included, the message is for all
provisioned Interface Identifiers within the AS(s) in which the
ASP is provisioned.  If only a subset of Interface Identifiers are
included, the ASP is noted as Active for all the Interface Identifiers
provisioned for that AS.

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


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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.).

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

3.3.2.6 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:

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

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


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

   |                     Interface Identifiers*                    |

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

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

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

   |                          INFO String*                         |

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


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

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

   |                     Interface Identifier*                     |

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

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

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

   |                          INFO String*                         |

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


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

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

3.3.2.7  ASP Inactive (ASPIA)

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

The ASPIA message contains the following parameters

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

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





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

   |                     Interface Identifiers*                    |

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

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

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

   |                          INFO String*                         |

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

















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

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

   |                     Interface Identifier*                     |

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

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

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

   |                          INFO String*                         |

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


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

    Value          Description
     0x1            Over-ride
     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 3.3.2.3.).

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

3.3.2.8  ASP Inactive Ack

The ASP Inactive (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:

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


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The format for the ASPIA Ack message is as follows:

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

   |                     Interface Identifiers*                    |

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

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

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

   |                          INFO String*                         |

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













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

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

   |                     Interface Identifier*                     |

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

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

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

   |                          INFO String*                         |

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


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

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

3.3.2.9  Heartbeat (BEAT)

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

The BEAT message contains the following parameters:

     Heartbeat Data         Optional



The format for the BEAT message is as follows:

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

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

3.3.2.10  Heartbeat Ack (BEAT-Ack)

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

3.3.3  Layer Management (MGMT) Messages

3.3.3.1  Error (ERR)

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

The Error message will only have the common message header.  The Error
message contains the following parameters:

     Error Code
     Diagnostic Information (optional)


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The format for the ERR message is as follows:

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

   |                     Diagnostic Information*                   |

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

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

     Invalid Version                               0x01
     Invalid Interface Identifier                  0x02
     Invalid Adaptation Layer Identifier           0x03
     Invalid Message Type                          0x04
     Unsupported Traffic Handling Mode             0x05
     Unexpected Message                            0x06
     Protocol Error                                0x07
     Unsupported Interface Identifier Type         0x08
     Invalid Stream Identifier                     0x09
     Unassigned TEI                                0x0a
     Unrecognized SAPI                             0x0b
     Invalid TEI, SAPI combination                 0x0c
     Unexpected Message Class                      0x0d

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

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

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

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

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

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


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

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

The "Unrecognized SAPI" error would handle the case of using a SAPI
that is not recognized by the SG.  The "Invalid TEI, SAPI combination"
error identify errors where the TEI is assigned and the the SAPI is
recognized, but the combination is not valid for the interface
(e.g., on a BRI the MGC tries to send Q.921 Management messages via
IUA when Layer Management at the SG SHOULD be performing this function).

The optional Diagnostic information can be any information germane to
the error condition, to assist in identification of the error
condition.  To enhance debugging, the Diagnostic information could
contain the first 40 bytes of the offending message.

3.3.3.2  Notify (NTFY)

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

The Notify message will only use the common message header.  The Notify
message contains the following parameters:

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















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


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

   |                     Interface Identifiers*                    |

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

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

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

   |                          INFO String*                         |

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


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


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

   |                     Interface Identifier*                     |

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

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

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

   |                          INFO String*                         |

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

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

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

The Status Identification 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 Identification values
are used:

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

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

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

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

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


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

3.3.3.3 TEI Status Messages (Request, Confirm and Indication)

The TEI Status messages are exchanged between IUA layer peers to
request, confirm and indicate the status of a particular TEI.

The TEI Status messages contain the common message header followed by
IUA message header. The TEI Status Request message does not contain
any additional parameters.

In the integrated ISDN Layer 2/3 model (e.g., in traditional ISDN
switches), it is assumed that the Layer Management for the Q.921
Layer and the Q.931 layer are co-located.  When backhauling ISDN,
this assumption is not necessarily valid.  The TEI status messages
allow the two Layer Management entities to communicate the status of
the TEI.  In addition, knowing that a TEI is in service allows the ASP
to request the SG to establish the datalink to the terminal (via the
IUA Establish message) for signaling if the ASP wants to be in control
of data link establishment.  Another use of the TEI status procedure
is where the Layer Management at the ASP can prepare for send/-
receive signaling to/from a given TEI and confirm/verify the
establishment of a datalink to that TEI.  For example, if a datalink is
established for a TEI that the ASP did not know was assigned, the ASP
can check to see whether it was assigned or whether there was an error
in the signaling message.  Also, knowing that a TEI is out of service,
the ASP need not request the SG to establish a datalink to that TEI.

The TEI Status Indication, and Confirm messages contain the following
parameter:

     STATUS


    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 (0x10)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Status                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Define     Value           Description
   ASSIGNED       0x0        TEI is considered assigned by Q.921
   UNASSIGNED     0x1        TEI is considered unassigned by Q.921

4.0  Procedures

The IUA 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.


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4.1  Procedures to support service in section 1.4.1

These procedures achieve the IUA layer's "Transport of Q.921/Q.931
boundary" service.

4.1.1  Q.921 or Q.931 primitives procedures

On receiving these primitives from the local layer, the IUA layer will
send the corresponding QPTM message (Data, Unit Data, Establish,
Release) to its peer.  While doing so, the IUA layer needs to fill
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 D channel (Interface Identifier).

4.1.2  QPTM message procedures

On receiving QPTM messages from a peer IUA layer, the IUA layer on an
SG or MGC needs to invoke the corresponding layer primitives
(DL-ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921
or Q.931 layer.

4.2  Procedures to support service in section 1.4.2

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

4.2.1 Layer Management primitives procedures

On receiving these primitives from the local Layer Management, the
IUA layer will provide the appropriate response primitive across the
internal local Layer Management interface.

An M-SCTP ESTABLISH request from Layer Management will initiate the
establishment of an SCTP association.  An M-SCTP ESTABLISH confirm
will be sent to Layer Management when the initiated association set-up
is complete.  An M-SCTP ESTABLISH indication is sent to Layer
Management upon successful completion of an incoming SCTP association
set-up from a peer IUA node

An M-SCTP RELEASE request from Layer Management will initiate the
tear-down of an SCTP association.  An M-SCTP RELEASE confirm will
be sent by Layer Management when the association teardown is complete.
An M-SCTP RELEASE indication is sent to Layer Management upon
successful tear-down of an SCTP association initiated by a peer IUA.

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

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

M-ASP STATUS request/indication and M-AS-STATUS request/indication
support a Layer Management query of the local status of a particular
ASP or AS.  No IUA peer protocol is invoked.


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M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE request and
M-ASP-ACTIVE request allow Layer Management at an ASP to initiate
state changes.  These requests result in outgoing IUA ASP UP,
ASP DOWN, ASP INACTIVE and ASP ACTIVE messages.

M-ASP-UP confirmation, M-ASP-DOWN confirmation, M-ASP-INACTIVE
confirmation and M-ASP-ACTIVE confirmation indicate to Layer
Management that the previous request has been confirmed.

Upon receipt of a M-TEI Status primitive from Layer Management, the
IUA will send the corresponding MGMT message (TEI Status) to its peer.
While doing so, the IUA layer needs to fill various fields of the
common and specific headers correctly.

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

4.2.2  Receipt of IUA Peer Management messages

Upon receipt of IUA Management messages, the IUA layer MUST invoke
the corresponding Layer Management primitive indications (e.g., M-AS
Status ind., M-ASP Status ind., M-ERROR ind., M-TEI STATAUS...) to the
local layer management.

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

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

4.3 Procedures to support service in section 1.4.3

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

4.3.1 AS and ASP State Maintenance

The IUA layer on the SG needs to maintain the states of each ASP as
well as the state of the AS.

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 the following type of events:

    * 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


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The ASP state transition diagram is shown in Figure 7.  The possible
states of an ASP are the following:

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

ASP-INACTIVE: The remote IUA peer at the ASP is available (and the
related SCTP association is up) but application traffic is stopped.
In this state the ASP can be sent any non-QPTM IUA messages (except
for TEI Status messages).

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


                 Figure 7  ASP State Transition Diagram

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



SCTP CDI:  The local SCTP layer's Communication Down Indication to the
Upper Layer Protocol (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.  SCTP CDI is understood as either a SHUTDOWN COMPLETE
notification and COMMUNICATION LOST notification from the SCTP.









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

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

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

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



















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               Figure 8  AS State Transition Diagram

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

    Tr = Recovery Timer


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.


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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 Management and the IUA layer 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 '0' SHOULD be used.

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

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

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 Ack is received, the ASP MAY resend ASP Up
messages every 2 seconds until it receives a ASP Up Ack message from
the SG.  The ASP MAY decide to reduce the frequency (say to every 5
seconds) if an ASP Up Ack is not received after a few tries.

The ASP MUST wait for the ASP Up Ack message from the SG before
sending any ASP traffic control messages (ASPAC or ASPIA) or Data
messages or it will risk message loss.  If the SG receives QPTM,
ASP Active or ASP Inactive messages before an ASP Up is received,
the SG SHOULD discard these messages.


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4.3.3.2 ASP Down

The ASP will send an ASP Down to an SG when the ASP is to be removed
from the list of ASPs in all Application Servers that it is a member
and no longer receive any IUA traffic or management messages.

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

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

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

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

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 IUA 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 and notifies Layer Management.

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

4.3.3.4 ASP Active

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

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

The ASP MUST wait for the ASP-Active Ack message from the SG before
sending any Data messages or it will risk message loss.  If the SG
receives QPTM messages before an ASP Active is received, the SG SHOULD
discard these messages.


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

In the case of an Over-ride mode AS, 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 load-sharing traffic within an AS to all
the active ASPs is implementation dependent.  The algorithm could, for
example be round-robin or based on information in the Data message,
such as Interface Identifier, 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 Identifiers 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-sharing.
The Type parameter in the ASPIA message indicates the mode used in a
particular Application Server. If the SG determines that the mode
indicates in an ASPAC is incompatible with the traffic handling mode
currently used in the AS, the SG responds with an Error message
indicating Unsupported Traffic Handling Mode.

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

In the case of a Load-share mode AS, the SG moves the ASP to the
"Inactive" state and the AS traffic is re-allocated across the
remaining "active" ASPs per the load-sharing algorithm currently used
within the AS.  An ASPIA Ack message is sent to the ASP after all
traffic is halted to the ASP.  A NTFY (Insufficient ASPs) MAY be sent
to all inactive ASPs, if required.


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

4.3.3.6  Notify

A Notify message reflecting a change in the AS state is sent to all
ASPs in the AS, except those in the "Down" state, with appropriate
Status Identification.

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

4.3.3.7  Heartbeat

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

After receiving an ASP Up Ack message from the SG in response to an
ASP Up message, the ASP MAY optionally send Beat messages periodically,
subject to a provisionable timer T(beat).  The SG IUA, upon receiving
a BEAT message from the ASP, responds with a BEAT ACK message.  If no
BEAT message (or any other IUA message) is received from the SG
within the timer 2*T(beat), the SG will consider the remote IUA as
"Down".  The SG will also send an ASP Down Ack message to the ASP.

At the ASP, if no BEAT ACK message (or any other IUA 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 IUA
peer.

The BEAT message MAY optionally contain an opaque Heartbeat Data
parameter that MUST be echoed back unchanged in the related Beat
Ack message.  The ASP upon examining the contents of the returned
BEAT Ack message MAY choose to consider the remote ASP as
unavailable. The contents/format of the Heartbeat Data parameter is
implementation-dependent and only of local interest to the original
sender.  The contents MAY be used, for example, to support a
Heartbeat sequence algorithm (to detect missing Heartbeats), and/or
a timestamp mechanism (to evaluate delays).

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

5.0 Examples

5.1 Establishment of Association and Traffic between SGs and ASPs

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

This scenario shows the example IUA 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.




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

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

This scenario shows the example IUA message flows for the
establishment of traffic between an SG and two ASPs in the same
Application Server, where ASP1 is configured to be Active and ASP2 a
standby in the event of communication failure or the withdrawal from
service of ASP1.  ASP2 MAY act as a hot, warm, or cold standby
depending on the extent to which ASP1 and ASP2 share call 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----->|                          |
        |                         |                          |


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

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


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





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

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

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


5.2 ASP Traffic Fail-over Examples

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

The following example shows a case in which an ASP withdraws from
service:

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

In this case, the SG notifies ASP2 that the AS has moved to the
Down state.  The SG could have also (optionally) sent a Notify
message when the AS moved to the Pending state.

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


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

The following example shows a case in which ASP2 wishes to
over-ride ASP1 and take over the traffic:

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

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

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 n+k = 2+1 for a load-share AS and n currently equals 1.

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

5.3 Q.921/Q.931 primitives backhaul Examples

When the IUA layer on the ASP has a QPTM message to send to the SG, it
will do the following:

  - Determine the correct SG

  - Find the SCTP association to the chosen SG

  - Determine the correct stream in the SCTP association based on
    the D channel

  - Fill in the QPTM message, fill in IUA Message Header, fill in
    Common Header

  - Send the QPTM message to the remote IUA peer in the SG, over the
    SCTP association

When the IUA layer on the SG has a QPTM message to send to the ASP, it
will do the following:

  - Determine the AS for the Interface Identifier

  - Determine the Active ASP (SCTP association) within the AS

  - Determine the correct stream in the SCTP association based on
    the D channel

  - Fill in the QPTM message, fill in IUA Message Header, fill in
    Common Header

  - Send the QPTM message to the remote IUA peer in the ASP, over the
    SCTP association

An example of the message flows for establishing a data link on a
signaling channel, passing PDUs and releasing a data link on a
signaling channel is shown below. An active association between MGC
and SG is established (Section 5.1) prior to the following message
flows.








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            SG                             ASP

                        <----------- Establish Request
      Establish Confirm  ---------->

                        <----------- Data Request
         Data Indication ----------->
                        <----------- Data Request
         Data Indication ----------->
                        <----------- Data Request
                        <----------- Data Request
         Data Indication ----------->

                        <----------- Release Request (RELEASE_MGMT)
        Release Confirm  ---------->


An example of the message flows for a failed attempt to establish a
data link on the signaling channel is shown below.  In this case, the
gateway has a problem with its physical connection (e.g. Red Alarm),
so it cannot establish a data link on the signaling channel.


            SG                             ASP

                        <----------- Establish Request (ESTABLISH_START)
      Release Indication ---------->
      (RELEASE_PHYS)

5.4 Layer Management Communication Examples

An example of the message flows for communication between Layer
Management modules between SG and ASP is shown below. An active
association between ASP and SG is established (Section 5.1) prior to
the following message flows.

                  SG                       ASP

                        <----------- Data Request
        Error Indication ---------->
         (INVALID_TEI)

                        <----------- TEI Status Request
      TEI Status Confirm ---------->
           (Unassigned)

6.0 Security

IUA is designed to carry signaling messages for telephony services.
As such, IUA 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.


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6.1 Threats

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

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

IUA runs on top of SCTP.  SCTP [3] provides certain transport related
security features, such as

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

When IUA 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" [5] SHOULD be consulted for guidance.

When the network in which IUA 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 [6] for more information on configuring IPSEC services.

6.2 Protecting Confidentiality

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


7.0 IANA Considerations

7.1 SCTP Payload Protocol Identifier

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

        IUA    "1"

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

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

7.2  IUA Protocol Extensions

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

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

The proposed extention must in no way adversly affect the general working of
the protocol.

7.2.1 IETF Defined Message Classes

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

7.2.2 IETF Defined Message Types

Documentation of the message type MUST contain the following information:

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

When an implementation receives a message type which it does not support, it MUST
respond with an Error (ERR) message, with an Error Code = Unsupported Message Type.
[Editor's note: this Error Code should be added to all of the UAs]

7.2.3 IETF-defined TLV Parameter Extension

Documentation of the message parameter MUST contain the following information:

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

Morneault, Kalla, Rengasami, & Sidebottom                    [Page 50]

Internet Draft        ISDN Q.921 User Adaptation Layer        Nov 2000


8.0 Acknowledgements

The authors would like to thank Alex Audu, Maria Sonia Vazquez
Arevalillo, Ming-te Chao, Keith Drage, Norm Glaude, Nikhil Jain,
Bernard Kuc, Ming Lin, Stephen Lorusso, John Loughney, Barry Nagelberg,
Neil Olson, Lyndon Ong, Heinz Prantner, Jose Luis Jimenez Ramirez,
Ian Rytina, Michael Tuexen and Hank Wang for their valuable comments
and suggestions.

9.0  References

[1] ITU-T Recommendation Q.920, 'Digital Subscriber signaling System
    No. 1 (DSS1) - ISDN User-Network Interface Data Link Layer -
    General Aspects'

[2] T1S1.7/99-220 Contribution, 'Back-hauling of DSS1 protocol in a
    Voice over Packet Network'

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

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

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

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

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

Selvam Rengasami                                  Tel +1-732-758-5260
Telcordia Technologies                   EMail srengasa@telcordia.com
NVC-2Z439
331 Newman Springs Road
Red Bank, NJ 07701
USA

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

Morneault, Kalla, Rengasami, & Sidebottom                    [Page 51]


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