Request for Comments: 3331 Cisco Systems
Category: Standards Track R. Dantu
NetRake
G. Sidebottom
Signatus Technologies
B. Bidulock
OpenSS7
J. Heitz
Lucent
September 2002
Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) -
User Adaptation Layer
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a protocol for the backhauling of Signaling
System 7 Message Transfer Part 2 (SS7 MTP2) User signalling messages
over IP using the Stream Control Transmission Protocol (SCTP). This
protocol would be used between a Signalling Gateway (SG) and Media
Gateway Controller (MGC). It is assumed that the SG receives SS7
signalling over a standard SS7 interface using the SS7 Message
Transfer Part (MTP) to provide transport. The Signalling Gateway
would act as a Signalling Link Terminal.
Table of Contents
1. Introduction.............................................. 2
1.1 Scope.................................................. 3
1.2 Terminology............................................ 3
1.3 M2UA Overview.......................................... 5
1.4 Services Provided by the M2UA Adaptation Layer......... 7
1.5 Functions Provided by the M2UA Layer................... 9
1.6 Definition of the M2UA Boundaries..................... 12
2. Conventions.............................................. 16
3. Protocol Elements........................................ 16
3.1 Common Message Header................................. 16
3.2 M2UA Message Header................................... 22
3.3 M2UA Messages......................................... 23
4. Procedures............................................... 58
4.1 Procedures to Support the M2UA-User Layer............. 58
4.2 Receipt of Primitives from the Layer Management....... 59
4.3 AS and ASP State Maintenance.......................... 61
4.4 Link Key Management Procedures........................ 73
5. Examples of MTP2 User Adaptation (M2UA) Procedures....... 75
5.1 Establishment of associations between SGP and MGC..... 75
examples
5.2 ASP Traffic Fail-over Examples........................ 77
5.3 SGP to MGC, MTP Level 2 to MTP Level 3 Boundary
Procedures............................................ 78
6. Timer Values............................................. 85
7. Security Considerations.................................. 85
7.1 Threats................................................ 85
7.2 Protecting Confidentiality............................. 86
8. IANA Considerations...................................... 86
8.1 SCTP Payload Protocol Identifier....................... 86
8.2 M2UA Protocol Extensions............................... 86
9. Acknowledgements......................................... 87
10. References............................................... 88
Appendix A: Signalling Network Architecture.................. 90
Authors' Addresses........................................... 92
Full Copyright Statement..................................... 94
1. Introduction
This document defines a protocol for the backhauling of SS7 [1] MTP2
User [2] [3] [4] (i.e. MTP3) signalling messages over IP using the
Stream Control Transmission Protocol (SCTP) [8]. This protocol would
be used between a Signalling Gateway (SG) and Media Gateway
Controller (MGC).
1.1 Scope
There is a need for Switched Circuit Network (SCN) signalling
protocol delivery from a Signalling Gateway (SG) to a Media Gateway
Controller (MGC) [9]. The delivery mechanism addresses the following
objectives:
* Support for MTP Level 2 / MTP Level 3 interface boundary
* Support for communication between Layer Management modules on SG
and MGC
* Support for management of SCTP active associations between the SG
and MGC
The SG will terminate up to MTP Level 2 and the MGC will terminate
MTP Level 3 and above. In other words, the SG will transport MTP
Level 3 messages over an IP network to a MGC.
1.2 Terminology
Application Server (AS) - A logical entity serving a specific
application instance. An example of an Application Server is a MGC
handling the MTP Level 3 and call processing for SS7 links terminated
by the Signalling 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
active or standby MGC instances.
Association - An association refers to a SCTP association. The
association will provide the transport for the delivery of protocol
data units for one or more interfaces.
Backhaul - Refers to the transport of signalling from the point of
interface for the associated data stream (i.e., SG function in the
MGU) back to the point of call processing (i.e., the MGCU), if this
is not local [9].
Fail-over - The capability to reroute signalling traffic as required
to an alternate Application Server Process within an Application
Server in the event of failure or unavailability of a currently used
Application Server Process. Fail-back MAY apply upon the return to
service of a previously unavailable Application Server Process.
Host - The computing platform that the ASP process is running on.
Interface - For the purposes of this document, an interface is a SS7
signalling link.
Interface Identifier - The Interface Identifier identifies the
physical interface at the SG for which the signalling 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.
Layer Management - Layer Management is a nodal function in an SG or
ASP that handles the inputs and outputs between the M2UA layer and a
local management entity.
Link Key - The link key is a locally unique (between ASP and SG)
value that identifies a registration request for a particular
Signalling Data Link and Signalling Terminal pair.
MTP - The Message Transfer Part of the SS7 protocol
MTP2 - MTP Level 2, the signalling data link layer of SS7
MTP3 - MTP Level 3, the signalling network layer of SS7
MTP2-User - A protocol that uses the services of MTP Level 2 (i.e.
MTP3).
Network Byte Order: Most significant byte first, a.k.a Big Endian.
Signalling Data Link - An SDL refers to a specific communications
facility that connects two Signalling Link Terminals.
Signalling Gateway (SG) - An SG is a signalling agent at the edge of
the IP network. An SG appears to the SS7 as one or more Signalling
Link Terminals that are connected to one or more Signalling Data
Links in the SS7 network. An SG contains a set of one or more unique
Signalling Gateway Processes, on which one or more is normally
actively processing traffic. Where an SG contains more than one SGP,
the SG is a logical entity.
Signalling Gateway Process (SGP) - A process instance that uses M2UA
to communicate to and from a Signalling Link Terminal. It serves as
an active, backup or load-sharing process of a Signalling Gateway.
Signalling Link Terminal (SLT) - Refers to the means of performing
all of the functions defined at MTP level 2 regardless of their
implementation [2,3].
Stream - A stream refers to an SCTP stream; a unidirectional 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 unordered delivery service.
1.3 M2UA Overview
The framework architecture that has been defined for SCN signalling
transport over IP [9] uses two components: a signalling common
transport protocol and an adaptation module to support the services
expected by a particular SCN signalling protocol from its underlying
protocol layer.
Within this framework architecture, this document defines a SCN
adaptation module that is suitable for the transport of SS7 MTP2 User
messages. The only SS7 MTP2 User is MTP3. The M2UA uses the
services of the Stream Control Transmission Protocol [8] as the
underlying reliable signalling common transport protocol.
In a Signalling Gateway, it is expected that the SS7 MTP2-User
signalling is transmitted and received from the PSTN over a standard
SS7 network interface, using the SS7 Message Transfer Part Level 1
and Level 2 [2,3,4] to provide reliable transport of the MTP3-User
signalling messages to and from an SS7 Signalling End Point (SEP) or
Signalling Transfer Point (STP). The SG then provides an
interworking of transport functions with the IP transport, in order
to transfer the MTP2-User signalling messages to and from an
Application Server Process where the peer MTP2-User protocol layer
exists.
1.3.1 Example - SG to MGC
In a Signalling Gateway, it is expected that the SS7 signalling is
received over a standard SS7 network termination, using the SS7
Message Transfer Part (MTP) to provide transport of SS7 signalling
messages to and from an SS7 Signalling End Point (SEP) or SS7
Signalling Transfer Point (STP). In other words, the SG acts as a
Signalling Link Terminal (SLT) [2,3]. The SG then provides an
interworking of transport functions with IP Signalling Transport, in
order to transport the MTP3 signalling messages to the MGC where the
peer MTP3 protocol layer exists, as shown below:
****** SS7 ****** IP *******
*SEP *-----------* SG *-------------* MGC *
****** ****** *******
+----+ +----+
|S7UP| |S7UP|
+----+ +----+
|MTP + |MTP |
| L3 | (NIF) |L3 |
+----+ +----+----+ +----+
|MTP | |MTP |M2UA| |M2UA|
| | | +----+ +----+
|L2 | |L2 |SCTP| |SCTP|
|L1 | |L1 +----+ +----+
| | | |IP | |IP |
+----+ +---------+ +----+
NIF - Nodal Interworking Function
SEP - SS7 Signalling Endpoint
IP - Internet Protocol
SCTP - Stream Control Transmission Protocol (Reference [8])
Figure 1 M2UA in the SG to MGC Application
Note: STPs MAY be present in the SS7 path between the SEP and the SG.
It is recommended that the M2UA use the services of the Stream
Control Transmission Protocol (SCTP) [8] as the underlying reliable
common signalling 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 the support of multi-homing
at either or both ends of an association,
- resistance to flooding and masquerade attacks, and
- data segmentation to conform to discovered path MTU size
There are scenarios without redundancy requirements and scenarios in
which redundancy is supported below the transport layer. In these
cases, the SCTP functions above MAY NOT be a requirement and TCP can
be used as the underlying common transport protocol.
1.3.2 ASP Fail-over Model and Terminology
The M2UA layer supports ASP fail-over functions in order to support a
high availability of call and transaction processing capability. All
MTP2-User messages incoming to a SGP from the SS7 network are
assigned to the unique Application Server, based on the Interface
Identifier of the message.
The M2UA layer supports a n+k redundancy model (active-standby, load
sharing, broadcast) where n is the minimum number of redundant ASPs
required to handle traffic and k ASPs are available to take over for
a failed or unavailable ASP. Note that 1+1 active/standby redundancy
is a subset of this model. A simplex 1+0 model is also supported as
a subset, with no ASP redundancy.
1.3.3 Client/Server Model
It is recommended that the SGP and ASP be able to support both client
and server operation. The peer endpoints using M2UA SHOULD be
configured so that one always takes on the role of client and the
other the role of server for initiating SCTP associations. The
default orientation would be for the SGP to take on the role of
server while the ASP is the client. In this case, ASPs SHOULD
initiate the SCTP association to the SGP.
The SCTP and TCP Registered User Port Number Assignment for M2UA is
2904.
1.4 Services Provided by the M2UA Adaptation Layer
The SS7 MTP3/MTP2(MTP2-User) interface is retained at the termination
point in the IP network, so that the M2UA protocol layer is required
to provide the equivalent set of services to its users as provided by
the MTP Level 2 to MTP Level 3.
1.4.1 Support for MTP Level 2 / MTP Level 3 interface boundary
M2UA supports a MTP Level 2 / MTP Level 3 interface boundary that
enables a seamless, or as seamless as possible, operation of the
MTP2-User peers in the SS7 and IP domains. An example of the
primitives that need to be supported can be found in [10].
1.4.2 Support for communication between Layer Management modules on SG
and MGC
The M2UA layer needs to provide some messages that will facilitate
communication between Layer Management modules on the SG and MGC. To
facilitate reporting of errors that arise because of the backhauling
MTP Level 3 scenario, the following primitive is defined:
M-ERROR
The M-ERROR message is used to indicate an error with a received M2UA
message (e.g., an interface identifier value is not known to the SG).
1.4.3 Support for management of active associations between SG and MGC
The M2UA layer on the SG keeps the state of the configured ASPs. A
set of primitives between M2UA layer and the Layer Management are
defined below to help the Layer Management manage the association(s)
between the SG and the MGC. The M2UA layer can be instructed by the
Layer Management to establish a SCTP association to a peer M2UA node.
This procedure can be achieved using the M-SCTP ESTABLISH primitive.
M-SCTP_ESTABLISH
The M-SCTP_ESTABLISH primitive is used to request, indicate and
confirm the establishment of a SCTP association to a peer M2UA node.
M-SCTP_RELEASE
The M-SCTP_RELEASE primitives are used to request, indicate, and
confirm the release of a SCTP association to a peer M2UA node.
The M2UA layer MAY also need to inform the status of the SCTP
association(s) to the Layer Management. This can be achieved using
the following primitive.
M-SCTP_STATUS
The M-SCTP_STATUS primitive is used to request and indicate the
status of underlying SCTP association(s).
The Layer Management MAY need to inform the M2UA layer of an AS/ASP
status (i.e., failure, active, etc.), so that messages can be
exchanged between M2UA layer peers to stop traffic to the local M2UA
user. This can be achieved using the following primitive.
M-ASP_STATUS
The ASP status is stored inside the M2UA layer on both the SG and MGC
sides. The M-ASP_STATUS primitive can be used by Layer Management to
request the status of the Application Server Process from the M2UA
layer. This primitive can also be used to indicate the status of the
Application Server Process.
M-ASP_MODIFY
The M-ASP_MODIFY primitive can be used by Layer Management to modify
the status of the Application Server Process. In other words, the
Layer Management on the ASP side uses this primitive to initiate the
ASPM procedures.
M-AS_STATUS
The M-AS_STATUS primitive can be used by Layer Management to request
the status of the Application Server. This primitive can also be
used to indicate the status of the Application Server.
1.5 Functions Provided by the M2UA Layer
1.5.1 Mapping
The M2UA layer MUST maintain a map of an Interface ID to a physical
interface on the Signalling Gateway. A physical interface would be a
V.35 line, T1 line/time slot, E1 line/time slot, etc. The M2UA layer
MUST also maintain a map of the Interface Identifier to SCTP
association and to the related stream within the association.
The SGP 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
fail-over of one ASP to another. Therefore, the SGP MUST maintain
the states of AS/ASP and reference them during the routing of any
messages to an AS/ASP.
Note that only one SGP SHOULD provide Signalling Link Terminal
services to an SS7 link. Therefore, within an SG, an Application
Server SHOULD be active for only one SGP at any given point in time.
An example of the logical view of the relationship between an SS7
link, Interface Identifier, AS and ASP in an SGP is shown below:
/-------------------------------------------------+
/ /----------------------------------------------|--+
/ / v |
/ / +----+ act+-----+ +-------+ -+--+|-+-
SS7 link1-------->|IID |-+ +-->| ASP |-->| Assoc | v
/ +----+ | +----+ | +-----+ +-------+ -+--+--+-
/ +->| AS |--+ Streams
/ +----+ | +----+ stb+-----+
SS7 link2-------->|IID |-+ | ASP |
+----+ +-----+
where IID = Interface Identifier
A SGP MAY support more than one AS. An AS MAY support more than one
Interface Identifier.
1.5.2 Support for the management of SCTP associations between the SGPs
and ASPs
The M2UA layer at the SG maintains the availability state of all
configured 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. The Active ASP(s) are the
one(s) currently receiving traffic from the SG.
The M2UA layer MAY be instructed by local management to establish an
SCTP association to a peer M2UA node. This can be achieved using the
M-SCTP_ESTABLISH primitive to request, indicate and confirm the
establishment of an SCTP association with a peer M2UA node.
The M2UA layer MAY also need to inform local management of the status
of the underlying SCTP associations using the M-SCTP_STATUS request
and the indication primitive. For example, the M2UA MAY inform local
management of the reason for the release of an SCTP association,
determined either locally within the M2UA layer or by a primitive
from the SCTP.
Also the M2UA layer may need to inform the local management of the
change in status of an ASP or AS. This may be achieved using the M-
ASP STATUS request or M-AS_STATUS request primitives.
1.5.3 Status of ASPs
The M2UA layer on the SG MUST maintain the state of the ASPs it is
supporting. The state of an ASP changes because of the reception of
peer-to-peer messages (ASPM messages as described in Section 3.3.2)
or the reception of indications from the local SCTP association. The
ASP state transition procedures are described in Section 4.3.1.
At a SGP, an Application Server list MAY contain active and inactive
ASPs to support ASP fail-over procedures. When, for example, both a
primary and a backup ASP are available, the M2UA peer protocol is
required to control which ASP is currently active. The ordered list
of ASPs within a logical Application Server is kept updated in the
SGP to reflect the active Application Server Process.
Also the M2UA layer MAY need to inform the local management of the
change in status of an ASP or AS. This can be achieved using the M-
ASP_STATUS or M-AS_STATUS primitives.
1.5.4 SCTP Specifics
1.5.4.1 SCTP Stream Management
SCTP allows a user specified number of streams to be opened during
initialization of the association. It is the responsibility of the
M2UA layer to ensure proper management of these streams. Because of
the unidirectional nature of streams, a M2UA layer is not aware of
the stream information from its peer M2UA layer. For this reason,
the Interface Identifier is in the M2UA message header.
The use of SCTP streams within M2UA is recommended in order to
minimize transmission and buffering delay, thereby, improving the
overall performance and reliability of the signalling elements. A
separate SCTP stream can be used for each SS7 link. Or, an
implementation may choose to split the SS7 link across several
streams based on SLS. This method may be of particular interest for
high speed SS7 links (MTP3b) since high speed links have a 24-bit
sequence number and the stream sequence number is 16-bits.
SCTP Stream '0' SHOULD NOT be used for MTP2 User Adaptation (MAUP)
messages (see Section 3) since stream '0' SHOULD only be used for ASP
Management (ASPM) messages (see Section 4.3.3).
1.5.5 Seamless SS7 Network Management Interworking
The M2UA layer on the SGP SHOULD pass an indication of unavailability
of the M2UA-User (MTP3) to the local Layer Management, if the
currently active ASP moves from the ACTIVE state. The actions taken
by M2UA on the SGP with regards to MTP Level 2 should be in
accordance with the appropriate MTP specifications.
1.5.6 Flow Control / Congestion
It is possible for the M2UA layer to be informed of the IP network
congestion onset and abatement by means of an implementation
dependent function (i.e. an indication from the SCTP). The handling
of this congestion indication by M2UA is implementation dependent.
However, the actions taken by the SG should be in accordance with the
appropriate MTP specification and should enable SS7 functionality
(e.g. flow control) to be correctly maintained.
1.5.7 Audit of SS7 Link State
After a fail-over of one ASP to another ASP, it may be necessary for
the M2UA on the ASP to audit the current SS7 link state to ensure
consistency. The M2UA on the SGP would respond to the audit request
with information regarding the current state of the SS7 link (i.e.
in-service, out-of-service, congestion state, LPO/RPO state).
1.6 Definition of the M2UA Boundaries
1.6.1 Definition of the M2UA / MTP Level 3 boundary
DATA
ESTABLISH
RELEASE
STATE
DATA RETRIEVAL
DATA RETRIEVAL COMPLETE
1.6.2 Definition of the M2UA / MTP Level 2 boundary
DATA
ESTABLISH
RELEASE
STATE
DATA RETRIEVAL
DATA RETRIEVAL COMPLETE
1.6.3 Definition of the Lower Layer Boundary between M2UA and SCTP
The upper layer and layer management primitives provided by SCTP are
provided in Reference [8] Section 10.
1.6.4 Definition of Layer Management / M2UA Boundary
M-SCTP_ESTABLISH request
Direction: LM -> M2UA
Purpose: LM requests ASP to establish an SCTP association with an
SGP.
M-SCTP_ESTABLISH confirm
Direction: M2UA -> LM
Purpose: ASP confirms to LM that it has established an
SCTP association with an SGP.
M-SCTP_ESTABLISH indication
Direction: M2UA -> LM
Purpose: SGP informs LM that an ASP has established an SCTP
association.
M-SCTP_RELEASE request
Direction: LM -> M2UA
Purpose: LM requests ASP to release an SCTP association with SGP.
M-SCTP_RELEASE confirm
Direction: M2UA -> LM
Purpose: ASP confirms to LM that it has released SCTP association
with SGP.
M-SCTP_RELEASE indication
Direction: M2UA -> LM
Purpose: SGP informs LM that ASP has released an SCTP association.
M-SCTP_RESTART indication
Direction: M2UA -> LM
Purpose: M2UA informs LM that a SCTP Restart indication has
been received.
M-SCTP_STATUS request
Direction: LM -> M2UA
Purpose: LM requests M2UA to report status of SCTP association.
M-SCTP_STATUS indication
Direction: M2UA -> LM
Purpose: M2UA reports status of SCTP association.
M-ASP_STATUS request
Direction: LM -> M2UA
Purpose: LM requests SGP to report status of remote ASP.
M-ASP_STATUS indication
Direction: M2UA -> LM
Purpose: SGP reports status of remote ASP.
M-AS_STATUS request
Direction: LM -> M2UA
Purpose: LM requests SG to report status of AS.
M-AS_STATUS indication
Direction: M2UA -> LM
Purpose: SG reports status of AS.
M-NOTIFY indication
Direction: M2UA -> LM
Purpose: ASP reports that it has received a NOTIFY message
from its peer.
M-ERROR indication
Direction: M2UA -> LM
Purpose: ASP or SGP reports that it has received an ERROR
message from its peer.
M-ASP_UP request
Direction: LM -> M2UA
Purpose: LM requests ASP to start its operation and send an ASP UP
message to the SGP.
M-ASP_UP confirm
Direction: M2UA -> LM
Purpose: ASP reports that it has received an ASP UP Acknowledgment
message from the SGP.
M-ASP_DOWN request
Direction: LM -> M2UA
Purpose: LM requests ASP to stop its operation and send an ASP DOWN
message to the SGP.
M-ASP_DOWN confirm
Direction: M2UA -> LM
Purpose: ASP reports that is has received an ASP DOWN Acknowledgment
message from the SGP.
M-ASP_ACTIVE request
Direction: LM -> M2UA
Purpose: LM requests ASP to send an ASP ACTIVE message to the SGP.
M-ASP_ACTIVE confirm
Direction: M2UA -> LM
Purpose: ASP reports that is has received an ASP ACTIVE
Acknowledgment message from the SGP.
M-ASP_INACTIVE request
Direction: LM -> M2UA
Purpose: LM requests ASP to send an ASP INACTIVE message to the SGP.
M-ASP_INACTIVE confirm
Direction: M2UA -> LM
Purpose: ASP reports that is has received an ASP INACTIVE
Acknowledgment message from the SGP.
M-LINK_KEY_REG Request
Direction: LM -> M2UA
Purpose: LM requests ASP to register Link Key with SG by sending REG
REQ message.
M-LINK_KEY_REG Confirm
Direction: M2UA -> LM
Purpose: ASP reports to LM that it has successfully received a REG
RSP message from SG.
M-LINK_KEY_REG Indication
Direction: M2UA -> LM
Purpose: SG reports to LM that it has successfully processed an
incoming REG REQ message from ASP.
M-LINK_KEY_DEREG Request
Direction: LM -> M2UA
Purpose: LM requests ASP to de-register Link Key with SG by sending
DEREG REQ message.
M-LINK_KEY_DEREG Confirm
Direction: M2UA -> LM
Purpose: ASP reports to LM that it has successfully received a
DEREG RSP message from SG.
M-LINK_KEY_DEREG Indication
Direction: M2UA -> LM
Purpose: SG reports to LM that it has successfully processed an
incoming DEREG REQ message from ASP.
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 MTP2-User Adaptation require a message
structure that contains a version, message class, message type,
message length, and message contents. This message header is common
among all signalling protocol adaptation layers:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Spare | Message Class | Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 Common Message Header
All fields in an M2UA message MUST be transmitted in the network byte
order, unless otherwise stated.
3.1.1 Version
The version field contains the version of the M2UA adaptation layer.
The supported versions are:
Value Version
----- -------
1 Release 1.0
3.1.2 Spare
The Spare field is 8-bits. It SHOULD be set to all '0's by the
sender and ignored by the receiver.
3.1.3 Message Class
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 Transport (QPTM)
Messages [IUA]
6 MTP2 User Adaptation (MAUP) Messages [M2UA]
7 Connectionless Messages [SUA]
8 Connection-Oriented Messages [SUA]
9 Routing Key Management (RKM) Messages (M3UA)
10 Interface Identifier Management (IIM) Messages (M2UA)
11 to 127 Reserved by the IETF
128 to 255 Reserved for IETF-Defined Message Class extensions
3.1.4 Message Type
The following List contains the Message Types for the valid Message
Classes:
MTP2 User Adaptation (MAUP) Messages
0 Reserved
1 Data
2 Establish Request
3 Establish Confirm
4 Release Request
5 Release Confirm
6 Release Indication
7 State Request
8 State Confirm
9 State Indication
10 Data Retrieval Request
11 Data Retrieval Confirm
12 Data Retrieval Indication
13 Data Retrieval Complete Indication
14 Congestion Indication
15 Data Acknowledge
16 to 127 Reserved by the IETF
128 to 255 Reserved for IETF-Defined MAUP extensions
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 Heartbeat Ack (BEAT ACK)
7 to 127 Reserved by the IETF
128 to 255 Reserved for IETF-Defined ASPSM extensions
Application Server Process Traffic Maintenance (ASPTM) messages
0 Reserved
1 ASP Active (ACTIVE)
2 ASP Inactive (INACTIVE)
3 ASP Active Ack (ACTIVE ACK)
4 ASP Inactive Ack (INACTIVE ACK)
5 to 127 Reserved by the IETF
128 to 255 Reserved for IETF-Defined ASPTM extensions
Management (MGMT) Messages
0 Error (ERR)
1 Notify (NTFY)
2 to 127 Reserved by the IETF
128 to 255 Reserved for IETF-Defined MGMT extensions
Interface Identifier Management (IIM) Messages
0 Reserved
1 Registration Request (REG REQ)
2 Registration Response (REG RSP)
3 Deregistration Request (DEREG REQ)
4 Deregistration Response (DEREG RSP)
5 to 127 Reserved by the IETF
128 to 255 Reserved for IETF-Defined IIM extensions
3.1.5 Message Length
The Message Length defines the length of the message in octets,
including the header. The Message Length MUST include parameter
padding bytes, if any. The Message Length MUST NOT be longer than a
MTP3 message [2,3,4,5] plus the length of the common and M2UA message
headers.
3.1.6 Variable-Length Parameter Format
M2UA messages consist of a Common Header followed by zero or more
variable-length parameters, as defined by the message type. The
variable-length parameters contained in a message are defined in a
Tag-Length-Value format as shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Tag | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Parameter Value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Mandatory parameters MUST be placed before optional parameters in a
message.
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 common parameters used by the
adaptation layers are in the range of 0x00 to 0xff. The M2UA
specific parameters have Tags in the range 0x300 to 0x3ff.
The common parameter tags (used by all User Adaptation layers) that
M2UA uses are defined below:
Parameter Value Parameter Name
--------------- --------------
0 (0x00) Reserved
1 (0x01) Interface Identifier (Integer)
2 (0x02) Unused
3 (0x03) Interface Identifier (Text)
4 (0x04) Info String
5 (0x05) Unused
6 (0x06) Unused
7 (0x07) Diagnostic Information
8 (0x08) Interface Identifier (Integer Range)
9 (0x09) Heartbeat Data
10 (0x0a) Unused
11 (0x0b) Traffic Mode Type
12 (0x0c) Error Code
13 (0x0d) Status Type/Information
14 (0x0e) Unused
15 (0x0f) Unused
16 (0x10) Unused
17 (0x11) ASP Identifier
18 (0x12) Unused
19 (0x13) Correlation Id
18-255 Reserved
The M2UA specific parameter Tags defined are as follows:
Parameter Value Parameter Name
--------------- --------------
768 (0x0300) Protocol Data 1
769 (0x0301) Protocol Data 2 (TTC)
770 (0x0302) State Request
771 (0x0303) State Event
772 (0x0304) Congestion Status
773 (0x0305) Discard Status
774 (0x0306) Action
775 (0x0307) Sequence Number
776 (0x0308) Retrieval Result
777 (0x0309) Link Key
778 (0x030a) Local-LK-Identifier
779 (0x030b) Signalling Data Terminal (SDT) Identifier
780 (0x030c) Signalling Data Link (SDL) Identifier
781 (0x030d) Registration Result
782 (0x030e) Registration Status
783 (0x030f) De-Registration Result
784 (0x0310) De-Registration Status
Parameter Length: 16 bits (unsigned integer)
The Parameter Length field contains the size of the parameter in
bytes, including the Parameter Tag, Parameter Length, and Parameter
Value fields. Thus, a parameter with a zero-length Parameter Value
field would have a Length field of 4. The Parameter Length does not
include any padding bytes.
Parameter Value: variable-length.
The Parameter Value field contains the actual information to be
transferred in the parameter.
The total length of a parameter (including Tag, Parameter Length and
Value fields) MUST be a multiple of 4 bytes. If the length of the
parameter is not a multiple of 4 bytes, the sender pads the Parameter
at the end (i.e., after the Parameter Value field) with all zero
bytes. The length of the padding is NOT included in the parameter
length field. A sender MUST NOT pad with more than 3 bytes. The
receiver MUST ignore the padding bytes.
3.2 M2UA Message Header
In addition to the common message header, there will be a M2UA
specific message header. The M2UA specific message header will
immediately follow the common message header, but will only be used
with MAUP messages.
This message header will contain the Interface Identifier. The
Interface Identifier identifies the physical interface at the SG for
which the signalling messages are sent/received. The format of the
Interface Identifier parameter can be text or integer, the values of
which are assigned according to network operator policy. The values
used are of local significance only, coordinated between the SG and
ASP.
The integer formatted Interface Identifier MUST be supported. The
text formatted Interface Identifier MAY optionally be supported.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x1) | Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Identifier (integer) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 M2UA 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x3) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ \
\ Interface Identifier (text) /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 M2UA Message Header (Text-based Interface Identifier)
The Tag value for the Text-based Interface Identifier is 0x3. The
encoding of the Identifier is ANSI X3.4-1986 [7]. The maximum string
length of the text-based Interface Identifier is 255 octets. The tag
length is equal to the string length of the Interface Identifier name
plus four bytes for the Tag and Length fields.
3.3 M2UA Messages
The following section defines the messages and parameter contents.
The M2UA messages will use the common message header (Figure 2) and
the M2UA message header (Figure 3 and Figure 4).
3.3.1 MTP2 User Adaptation Messages
3.3.1.1 Data
The Data message contains an SS7 MTP2-User Protocol Data Unit (PDU).
The Data message contains the following parameter:
Protocol Data (mandatory)
Correlation Id (optional)
The format for the Data Message parameters is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x300) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ \
\ Protocol Data /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x13) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Correlation Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protocol Data field contains the MTP2-User application message in
network byte order starting with the Signalling Information Octet
(SIO). The Correlation Id parameter uniquely identifies the MSU
carried in the Protocol Data within an AS. This Correlation Id
parameter is assigned by the sending M2UA. The purpose of the
Correlation Id is to permit the newly active ASP to synchronize its
processing of the traffic in each ordered stream with other ASPs in
the broadcast group.
The format for a Data Message with TTC PDU 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 (0x301) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ \
\ TTC Protocol Data /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x13) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Correlation Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protocol Data field contains the MTP2-User application message in
network byte order starting with the Length Indicator (LI) octet.
The Japanese TTC variant uses the spare bits of the LI octet for
priority.
The length of the Protocol Data and TTC Protocol Data MUST NOT exceed
the length of a MTP2-User application message [2,3,5].
3.3.1.2 Data Acknowledge Message
The Data Acknowledge message contains the Correlation Id of the Data
message that the sending M2UA is acknowledging as successfully
processed to the peer M2UA.
The Data Acknowledge message contains the following parameter:
Correlation Id Mandatory
The following format MUST be used for the Data Ack Message:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x13) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Correlation Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Correlation Id parameter of the Data message and the Data Ack
message provide a mechanism, for those SG implementations capable of
taking advantage of them, to obtain an acknowledgment that the MSU
has been transferred to the M2UA peer before acknowledging the MSU to
the SS7 peer, removing the risk of losing messages due to association
failure or SCTP congestion.
The Data Ack message MUST be sent if a Correlation Id parameter is
received from the peer. Otherwise, the Data Ack message MUST NOT be
sent.
If the Data Acknowledge is not sent for Correlation Id(s) or is sent
with Invalid Correlation Id(s), the SS7 link will eventually fail due
to lack of MTP Level 2 acknowledgments of the SS7 peer's MSUs.
3.3.1.3 Establish (Request, Confirmation)
The Establish Request message is used to establish the SS7 link or to
indicate that the channel has been established. The MGC controls the
state of the SS7 link. When the MGC desires the SS7 link to be in-
service, it will send the Establish Request message. Note that the
SGP MAY already have the SS7 link established at its layer. If so,
upon receipt of an Establish Request, the SGP takes no action except
to send an Establish Confirm.
When the MGC sends an M2UA Establish Request message, the MGC MAY
start a timer. This timer would be stopped upon receipt of an M2UA
Establish Confirm. If the timer expires, the MGC would resend the
M2UA Establish Request message and restart the timer. In other
words, the MGC MAY continue to request the establishment of the data
link on a periodic basis until the desired state is achieved or some
other action is taken (notify the Management Layer).
The mode (Normal or Emergency) for bringing the SS7 link in service
is defaulted to Normal. The State Request (described in Section
3.3.1.5 below) can be used to change the mode to Emergency.
3.3.1.4 Release (Request, Indication, Confirmation)
This Release Request message is used to release the channel. The
Release Confirm and Indication messages are used to indicate that the
channel has been released.
3.3.1.5 State Request
The State Request message can be sent from a MGC to cause an action
on a particular SS7 link supported by the Signalling Gateway Process.
The SGP sends a State Confirm to the MGC if the action has been
successfully completed. The State Confirm reflects that state value
received in the State Request message.
The State Request message contains the following parameter:
State (mandatory)
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 (0x302) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for State are shown in the following table.
Define Value Description
STATUS_LPO_SET 0x0 Request local processor outage
STATUS_LPO_CLEAR 0x1 Request local processor outage
recovered
STATUS_EMER_SET 0x2 Request emergency alignment
STATUS_EMER_CLEAR 0x3 Request normal alignment (cancel
emergency)
STATUS_FLUSH_BUFFERS 0x4 Flush or clear receive, transmit
and retransmit queues
STATUS_CONTINUE 0x5 Continue or Resume
STATUS_CLEAR_RTB 0x6 Clear the retransmit queue
STATUS_AUDIT 0x7 Audit state of link
STATUS_CONG_CLEAR 0x8 Congestion cleared
STATUS_CONG_ACCEPT 0x9 Congestion accept
STATUS_CONG_DISCARD 0xa Congestion discard
3.3.1.6 State Confirm
The State Confirm message will be sent by the SGP in response to a
State Request from the MGC. The State Confirm reflects that state
value received in the State Request message.
The State Confirm message contains the following parameter:
State (mandatory)
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 (0x302) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for State are shown in the following table. The
value of the State field SHOULD reflect the value received in the
State Request message.
Define Value Description
STATUS_LPO_SET 0x0 Request local processor outage
STATUS_LPO_CLEAR 0x1 Request local processor outage
recovered
STATUS_EMER_SET 0x2 Request emergency alignment
STATUS_EMER_CLEAR 0x3 Request normal alignment (cancel
emergency)
STATUS_FLUSH_BUFFERS 0x4 Flush or clear receive, transmit
and retransmit queues
STATUS_CONTINUE 0x5 Continue or Resume
STATUS_CLEAR_RTB 0x6 Clear the retransmit queue
STATUS_AUDIT 0x7 Audit state of link
STATUS_CONG_CLEAR 0x8 Congestion cleared
STATUS_CONG_ACCEPT 0x9 Congestion accept
STATUS_CONG_DISCARD 0xa Congestion discard
3.3.1.7 State Indication
The MTP2 State Indication message can be sent from a SGP to an ASP to
indicate a condition on a SS7 link.
The State Indication message contains the following parameter:
Event (mandatory)
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 (0x303) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Event |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for Event are shown in the following table.
Define Value Description
EVENT_RPO_ENTER 0x1 Remote entered processor outage
EVENT_RPO_EXIT 0x2 Remote exited processor outage
EVENT_LPO_ENTER 0x3 Link entered processor outage
EVENT_LPO_EXIT 0x4 Link exited processor outage
3.3.1.8 Congestion Indication
The Congestion Indication message can be sent from a Signalling
Gateway Process to an ASP to indicate the congestion status and
discard status of a SS7 link. When the MSU buffer fill increases
above an Onset threshold or decreases below an Abatement threshold or
crosses a Discard threshold in either direction, the SGP SHALL send a
congestion indication message when it supports SS7 MTP2 variants that
support multiple congestion levels.
The SGP SHALL send the message only when there is actually a change
in either the discard level or the congestion level to report,
meaning it is different from the previously sent message. In
addition, the SGP SHALL use an implementation dependent algorithm to
limit the frequency of congestion indication messages.
An implementation may optionally send Congestion Indication messages
on a "high priority" stream in order to potentially reduce delay.
The Congestion Indication message contains the following parameters:
Congestion Status (mandatory)
Discard Status (optional)
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 (0x304) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Congestion Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x305) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discard Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for Congestion Status and Discard Status are shown
in the following table.
Define Value Description
LEVEL_NONE 0x0 No congestion
LEVEL_1 0x1 Congestion Level 1
LEVEL_2 0x2 Congestion Level 2
LEVEL_3 0x3 Congestion Level 3
For SS7 networks that do not support multiple levels of congestion,
only the LEVEL_NONE and LEVEL_3 values will be used. For SS7
networks that support multiple levels of congestion, it is possible
for all values to be used. Refer to [2], [3] and [12] for more
details on the Congestion and Discard Status of SS7 signalling links.
3.3.1.9 Retrieval Request
The MTP2 Retrieval Request message is used during the MTP Level 3
changeover procedure to request the BSN, to retrieve PDUs from the
transmit and retransmit queues or to flush PDUs from the retransmit
queue. Examples of the use of Retrieval Request for SS7 Link
Changeover are provided in Section 5.3.6.
The Retrieval Request message contains the following parameters:
Action (mandatory)
Sequence Number (optional)