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Network Working Group Tom George
INTERNET-DRAFT Alcatel
Ram Dantu
Netrake
Malleswar Kalla
Telcordia
Hanns Juergen Schwarzbauer
Siemens
Greg Sidebottom
Ken Morneault
Cisco Systems
Brian Bidulock
OpenSS7
Expires February 2003 August 28, 2002
SS7 MTP2-User Peer-to-Peer Adaptation Layer
<draft-ietf-sigtran-m2pa-06.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
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Internet Draft SS7 MTP2-User Peer-to-Peer Adaptation Layer Aug 2002
Abstract
This Internet Draft defines a protocol supporting the transport of
Signaling System Number 7 (SS7) Message Transfer Part (MTP) Level 3
signaling messages over Internet Protocol (IP) using the services of
the Stream Control Transmission Protocol (SCTP). This protocol would
be used between SS7 Signaling Points using the MTP Level 3
protocol. The SS7 Signaling Points may also use standard SS7 links
using the SS7 MTP Level 2 to provide transport of MTP Level 3
signaling messages.
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Internet Draft SS7 MTP2-User Peer-to-Peer Adaptation Layer Aug 2002
TABLE OF CONTENTS
1. Introduction............................................. 4
1.1 Scope................................................. 4
1.2 Terminology........................................... 4
1.3 Abbreviations......................................... 5
1.4 Conventions........................................... 6
1.5 Signaling Transport Architecture...................... 6
1.6 Services Provided by M2PA............................. 8
1.7 Functions Provided by M2PA............................ 9
1.8 Definition of the M2PA Boundaries.....................10
1.9 Differences Between M2PA and M2UA.....................12
2. Protocol Elements........................................14
2.1 Common Message Header.................................14
2.2 M2PA Header...........................................15
2.3 M2PA Messages.........................................16
3. M2PA Link State Control..................................19
4. Procedures...............................................23
4.1 Procedures to Support MTP2 Features...................23
4.2 Procedures to Support the MTP3/MTP2 Interface.........33
4.3 SCTP Considerations...................................37
5. Examples of M2PA Procedures..............................39
5.1 Link Initialization (Alignment).......................39
5.2 Message Transmission and Reception....................41
5.3 Link Status Indication................................41
5.4 Link Status Message (Processor Outage)................42
5.5 Level 2 Flow Control..................................43
5.6 MTP3 Signaling Link Congestion........................44
5.7 Link Deactivation.....................................44
5.8 Link Changeover.......................................45
6. Security.................................................46
6.1 Threats...............................................46
6.2 Protecting Confidentiality............................46
7. IANA Considerations......................................47
7.1 SCTP Payload Protocol Identifier......................47
7.2 M2PA Protocol Extensions..............................47
8. Acknowledgements.........................................48
9. References...............................................49
10. Author's Addresses.......................................50
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1. Introduction
1.1 Scope
There is a need for Switched Circuit Network (SCN) signaling protocol
delivery over an IP network. This includes message transfer between
the following:
- a Signaling Gateway (SG) and a Media Gateway Controller (MGC)
[1]
- a SG and an IP Signaling Point (IPSP)
- an IPSP and an IPSP
This could allow for convergence of some signaling and data
networks. SCN signaling nodes would have access to databases and other
devices in the IP network domain that do not use SS7 signaling
links. Likewise, IP telephony applications would have access to SS7
services. There may also be operational cost and performance
advantages when traditional signaling links are replaced by IP network
"connections".
The delivery mechanism described in this document allows for full MTP3
message handling and network management capabilities between any two
SS7 nodes, communicating over an IP network. An SS7 node equipped with
an IP network connection is called an IP Signaling Point (IPSP). The
IPSPs function as traditional SS7 nodes using the IP network instead
of SS7 links.
The delivery mechanism should
- Support seamless operation of MTP3 protocol peers over an IP
network connection.
- Support the MTP Level 2 / MTP Level 3 interface boundary.
- Support management of SCTP transport associations and traffic
instead of MTP2 Links.
- Support asynchronous reporting of status changes to management.
1.2 Terminology
MTP - The Message Transfer Part of the SS7 protocol [2] [3].
MTP2 - MTP Level 2, the MTP signaling link layer.
MTP3 - MTP Level 3, the MTP signaling network layer.
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MTP2-User - A protocol that normally uses the services of MTP
Level 2. The only MTP2 user is MTP3. The MTP2 user is equivalent to
the M2PA user.
Signaling End Point (SEP) - A node in an SS7 network that originates
or terminates signaling messages. One example is a central office
switch.
IP Signaling Point (IPSP) - An SS7 Signaling Point with an IP
network connection used for SS7 over IP.
Signaling Gateway (SG) - A signaling agent that receives/sends SCN
native signaling at the edge of the IP network [4]. In this context,
an SG is an SS7 Signaling Point that has both an IP network connection
used for SS7 over IP, and a traditional (non-IP) link to an SS7
network.
Signaling Transfer Point (STP) - A node in an SS7 network that routes
signaling messages based on their destination point code in the SS7
network.
Association - An association refers to a SCTP association [5]. The
association provides the transport for MTP3 protocol data units and
M2PA adaptation layer peer messages.
Network Byte Order - Most significant byte first, also known as "Big
Endian". See [14], Appendix B Data Transmission Order.
Stream - A stream refers to a SCTP stream [5].
1.3 Abbreviations
BSNT - Backward Sequence Number to be Transmitted
FSNC - Forward Sequence Number of last message accepted by
remote level 2
LI - Length Indicator
MSU - Message Signal Unit
SCCP - Signaling Connection Control Part
SCN - Switched Circuit Network
SCTP - Stream Control Transmission Protocol
SIF - Signaling Information Field
SIO - Service Information Octet
SLC - Signaling Link Code
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SS7 - Signaling System Number 7
SSN - Stream Sequence Number
STP - Signal Transfer Point
1.4 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
[8].
1.5 Signaling Transport Architecture
The architecture that has been defined [4] for Switched Circuit
Network (SCN) signaling transport over IP uses multiple components,
including an IP transport protocol, the Stream Control Transmission
Protocol (SCTP), and an adaptation module to support the services
expected by a particular SCN signaling protocol from its underlying
protocol layer.
Within this framework architecture, this document defines an SCN
adaptation module that is suitable for the transport of SS7 MTP3
messages.
Figure 1 shows the seamless interworking at the MTP3 layer. MTP3 is
adapted to the SCTP layer using the MTP2 User Peer-to-peer Adaptation
Layer (M2PA). All the primitives between MTP3 and MTP2 are supported
by M2PA. The SCTP association acts as one SS7 link between the IPSPs.
An IPSP may have the Signaling Connection Control Part (SCCP) and
other SS7 layers above MTP3.
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******** IP ********
* IPSP *--------* IPSP *
******** ********
+------+ +------+
| TCAP | | TCAP |
+------+ +------+
| SCCP | | SCCP |
+------+ +------+
| MTP3 | | MTP3 |
+------+ +------+
| M2PA | | M2PA |
+------+ +------+
| SCTP | | SCTP |
+------+ +------+
| IP | | IP |
+------+ +------+
IP - Internet Protocol
IPSP - IP Signaling Point
SCTP - Stream Control Transmission Protocol
(see Reference [5])
Figure 1: M2PA Symmetrical Peer-to-Peer Architecture
Figure 2 shows an example of M2PA used in a Signaling Gateway
(SG). The SG is an IPSP equipped with both traditional SS7 and IP
network connections. In effect, the Signaling Gateway acts as a
Signal Transfer Point (STP). Any of the nodes in the diagram could
have SCCP or other SS7 layers. STPs may or may not be present in the
SS7 path between the SEP and the SG.
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******** SS7 *************** IP ********
* SEP *--------* SG *--------* IPSP *
******** *************** ********
+------+ +------+
| TCAP | | TCAP |
+------+ +------+
| SCCP | | SCCP |
+------+ +-------------+ +------+
| MTP3 | | MTP3 | | MTP3 |
+------+ +------+------+ +------+
| MTP2 | | MTP2 | M2PA | | M2PA |
+------+ +------+------+ +------+
| MTP1 | | MTP1 | SCTP | | SCTP |
| | | +------+ +------+
| | | | IP | | IP |
+------+ +------+------+ +------+
SEP - SS7 Signaling Endpoint
Figure 2: M2PA in IP Signaling Gateway
Figure 2 is only an example. Other configurations are possible. In
short, M2PA uses the SCTP association as an SS7 link. The M2PA/SCTP/IP
stack can be used in place of an MTP2/MTP1 stack.
1.5.1 Point Code Representation
The MTP specification requires that each node with an MTP3 layer is
identified by an SS7 point code. In particular, each IPSP must have
its own SS7 point code.
1.6 Services Provided by M2PA
The SS7 MTP3/MTP2 (MTP2-User) interface is retained in the IPSP. The
M2PA protocol layer is required to provide the equivalent set of
services to its user as provided by MTP Level 2 to MTP Level 3.
These services are described in the following subsections.
1.6.1 Support for MTP Level 2 / MTP Level 3 interface boundary
This interface is the same as the MTP2/MTP3 interface described in
[2], [3] and [10], with the addition of support for larger sequence
numbers in [3] and [7].
Because M2PA uses larger sequence numbers than MTP2, the MTP3
Changeover procedure must use the Extended Changeover Order and
Extended Changeover Acknowledgment messages described in [7] and [3].
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Also, the following MTP3/MTP2 primitives must use the larger sequence
numbers:
- BSNT Confirmation
- Retrieval Request and FSNC
1.6.2 Support for peer-to-peer communication
In SS7, MTP Level 2 sends three types of messages, known as signal
units: Message Signal Units (MSUs), Link Status Signal Units (LSSUs),
and Fill-In Signal Units (FISUs).
MSUs originate at a higher level than MTP2, and are destined for a
peer at another node. Likewise, M2PA passes these messages from MTP3
to SCTP as data for transport across a link. These are called User
Data messages in M2PA.
LSSUs allow peer MTP2 layers to exchange status information. Analogous
messages are needed for M2PA. The Link Status message serves this
purpose.
FISUs are sent when no other signal units are waiting to be sent. This
purpose is served by the heartbeat messages in SCTP. FISUs also carry
acknowledgment of messages. This function is performed by the M2PA
User Data and Link Status messages. Therefore, it is unnecessary for
M2PA to provide a protocol data unit like the FISU. Furthermore, since
an IP network is a shared resource, it would be undesirable to have a
message type that is sent continuously as the FISUs are.
1.7 Functions Provided by M2PA
1.7.1 Support of MTP3/MTP2 Primitives
M2PA receives the primitives sent from MTP3 to its lower layer. M2PA
processes these primitives or maps them to appropriate primitives at
the M2PA/SCTP interface. Likewise, M2PA sends primitives to MTP3 like
those used in the MTP3/MTP2 interface.
1.7.2 MTP2 Functionality
M2PA provides MTP2 functionality that is not provided by SCTP. This
includes
- Data retrieval to support the MTP3 changeover procedure
- Reporting of link status changes to MTP3
- Processor outage procedure
- Link alignment procedure
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SCTP provides reliable, sequenced delivery of messages.
1.7.3 Mapping of SS7 and IP Entities
The M2PA layer must maintain a map of each of its SS7 links to the
corresponding SCTP association.
1.7.4 SCTP Stream Management
SCTP allows a user-specified number of streams to be opened during the
initialization. It is the responsibility of the M2PA layer to ensure
proper management of the streams allowed within each association.
M2PA uses two streams in each direction for each association. Stream 0
in each direction is designated for Link Status messages. Stream 1 is
designated for User Data messages. Separating the Link Status and User
Data messages onto separate streams allows M2PA to prioritize the
messages in a manner similar to MTP2.
1.7.5 Retention of MTP3 in the SS7 Network
M2PA allows MTP3 to perform all of its Message Handling and Network
Management functions with IPSPs as with other SS7 nodes.
1.8 Definition of the M2PA Boundaries
1.8.1 Definition of the M2PA / MTP Level 3 boundary
The upper layer primitives provided by M2PA are the same as those
provided by MTP2 to MTP3. These primitives are described in [2], [3],
and [10]. Following is a list of the primitives.
Primitives sent from MTP3 to M2PA:
Data Request - Used to send a Data Message for transmission.
Start Request - Used to activate a link.
Stop Request - Used to deactivate a link.
Retrieve BSNT Request - Request the BSNT for the changeover procedure.
Retrieval Request and FSNC - Request retrieval of unacknowledged and
unsent messages. This request includes the FSNC received from the
remote end.
Local Processor Outage Request - Informs M2PA of a local processor
outage condition.
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Local Processor Outage Recovered Request - Informs M2PA that a local
processor outage condition has ceased.
Flush Buffers Request - Requests that all transmit and receive
buffers be emptied.
Continue Request - Requests that processing resume after a processor
outage.
Emergency Request - Requests that M2PA use the emergency alignment
procedure.
Emergency Ceases Request - Requests that M2PA use the normal alignment
procedure.
Primitives sent from M2PA to MTP3:
Data Indication - Used to deliver received Data Message to MTP3.
Congestion Indication - Indicates change in congestion status. The
indication includes the congestion status, if the protocol is using the
optional congestion levels. The indication also includes the discard
status.
In Service Indication - Indicates that the link is in service.
Out of Service Indication - Indicates that the link is out of service.
Retrieved Messages Indication - Indicates delivery of unacknowledged
and unsent messages.
Retrieval Complete Indication - Indicates that delivery of
unacknowledged and unsent messages is complete.
BSNT Confirm - Replies to the BSNT Request. The confirmation includes
the BSNT.
BSNT Not Retrievable Confirm - Replies to the BSNT Request when the
BSNT cannot be determined.
Remote Processor Outage Indication - Indicates processor outage at
remote end.
Remote Processor Recovered Indication - Indicates recovery from
processor outage at remote end.
1.8.2 Definition of the Lower Layer Boundary between M2PA and SCTP
The upper layer primitives provided by SCTP are described in Reference
[5] Section 10 "Interface with Upper Layer".
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1.9 Differences Between M2PA and M2UA
The MTP2 User Adaptation Layer (M2UA) [6] also adapts the MTP3 layer
to the SCTP/IP stack. It does so through a backhauling architecture
[4]. This section intends to clarify some of the differences between
the M2PA and M2UA approaches.
A possible M2PA architecture is shown in Figure 3. Here the IPSP's
MTP3 uses its underlying M2PA as a replacement for MTP2. Communication
between the two layers MTP3/M2PA is defined by the same primitives as
in SS7 MTP3/MTP2. M2PA performs functions similar to MTP2.
A comparable architecture for M2UA is shown in Figure 4. In M2UA, the
MGC's MTP3 uses the SG's MTP2 as its lower SS7 layer. Likewise, the
SG's MTP2 uses the MGC's MTP3 as its upper SS7 layer. In SS7,
communication between the MTP3 and MTP2 layers is defined by
primitives. In M2UA, the MTP3/MTP2 communication is defined as M2UA
messages and sent over the IP connection.
******** SS7 *************** IP ********
* SEP *--------* SG *--------* IPSP *
******** *************** ********
+------+ +-------------+ +------+
| SCCP | | SCCP | | SCCP |
+------+ +-------------+ +------+
| MTP3 | | MTP3 | | MTP3 |
+------+ +------+------+ +------+
| MTP2 | | MTP2 | M2PA | | M2PA |
+------+ +------+------+ +------+
| MTP1 | | MTP1 | SCTP | | SCTP |
| | | +------+ +------+
| | | | IP | | IP |
+------+ +------+------+ +------+
Figure 3: M2PA in IP Signaling Gateway
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******** SS7 *************** IP ********
* SEP *--------* SG *--------* MGC *
******** *************** ********
+------+ +------+
| SCCP | | SCCP |
+------+ +------+
| MTP3 | (NIF) | MTP3 |
+------+ +------+------+ +------+
| MTP2 | | MTP2 | M2UA | | M2UA |
+------+ +------+------+ +------+
| MTP1 | | MTP1 | SCTP | | SCTP |
| | | +------+ +------+
| | | | IP | | IP |
+------+ +------+------+ +------+
NIF - Nodal Interworking Function
Figure 4: M2UA in IP Signaling Gateway
M2PA and M2UA are similar in that:
a. Both transport MTP3 data messages.
b. Both present an MTP2 upper interface to MTP3.
Differences between M2PA and M2UA include:
a. M2PA: IPSP processes MTP3/MTP2 primitives.
M2UA: MGC transports MTP3/MTP2 primitives between the SG's MTP2
and the MGC's MTP3 (via the NIF) for processing.
b. M2PA: SG-IPSP connection is an SS7 link.
M2UA: SG-MGC connection is not an SS7 link. It is an
extension of MTP to a remote entity.
c. M2PA: SG is an SS7 node with a point code.
M2UA: SG is not an SS7 node and has no point code.
d. M2PA: SG can have upper SS7 layers, e.g., SCCP.
M2UA: SG does not have upper SS7 layers since it has no MTP3.
e. M2PA: relies on MTP3 for management procedures.
M2UA: uses M2UA management procedures.
Potential users of M2PA and M2UA should be aware of these differences
when deciding how to use them for SS7 signaling transport over IP
networks.
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2. Protocol Elements
This section describes the format of various messages used in this
protocol.
All fields in an M2PA message must be transmitted in the network byte
order, i.e., most significant byte first, unless otherwise stated.
2.1 Common Message Header
The protocol messages for M2PA require a message header structure
which contains a version, message class, message type, and message
length. The header structure is shown in Figure 5.
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 5: Common Message Header
2.1.1 Version
The version field contains the version of M2PA. The supported versions
are:
Value Version
----- -------
1 Release 1.0 of M2PA protocol
2.1.2 Spare
The Spare field SHOULD be set to all zeroes (0's) by the sender and
ignored by the receiver. The Spare field SHOULD NOT be used for
proprietary information.
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2.1.3 Message Class
The following List contains the valid Message Classes:
Value
(decimal) Message Class
--------- -------------
11 M2PA Messages
Other values are invalid for M2PA.
2.1.4 Message Type
The following list contains the message types for the defined
messages.
Value Message Type
----- ------------
1 User Data
2 Link Status
Other values are invalid.
2.1.5 Message Length
The Message Length defines the length of the message in octets,
including the Common Header.
2.2 M2PA Header
All protocol messages for M2PA require an M2PA-specific header. The
header structure is shown in Figure 6.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused | BSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused | FSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: M2PA-specific Message Header
2.2.1 Backward Sequence Number
This is the FSN of the message last received from the peer.
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2.2.2 Forward Sequence Number
This is the M2PA sequence number of the User Data message being sent.
2.3 M2PA Messages
The following section defines the messages and parameter contents. An
M2PA message consists of a Common Message Header and M2PA Header
followed by the data appropriate to the message.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
| Common Message Header |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| M2PA-specific Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
| Message Data |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.1 User Data
The User Data is the data sent from MTP3. The format for the User Data
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
| Data |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Data field contains the following fields of the MTP Message Signal
Unit (MSU):
- Length Indicator (LI), including the two undefined bits between
the SIO and LI fields.
- Service Information Octet (SIO)
- Signaling Information Field (SIF)
The MTP MSU described in [2] Q.703, section 2.2 Signal Unit Format,
and [3] T1.111.3 section 2.2 Signal Unit Format.
M2PA does not add padding to the MTP3 message.
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Note that the Data field SHALL NOT contain other components of the MTP
MSU format:
- Flag
- Backward Sequence Number (BSN)
- Backward Indicator Bit (BIB)
- Forward Sequence Number (FSN)
- Forward Indicator Bit (FIB)
- Check bits (CK)
The Data field SHALL be transmitted in the byte order as defined by
MTP3.
It is not necessary to put the message length in the LI octet as in
MTP2. The LI octet is included because the two spare bits in the LI
octet are used by MTP3 in at least one national version of SS7 to
carry MTP3 information. For example, the Japanese TTC standard uses
these spare bits as an MTP3 Message Priority field. See [9], section
14 "Common Characteristics of message signal unit formats", section
14.2 (A) Priority Indicator (PRI). For versions of MTP that do not use
these two bits, the entire octet is spare.
Therefore in M2PA the format of the LI octet is:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|PRI| spare | (followed by SIO, SIF)
+-+-+-+-+-+-+-+-+
PRI - Priority used only in national MTP defined in [9].
Spare for other MTP versions.
Since the LI octet is not used for a message length, there is no need
to support the expanded LI field in [2], Q.703 Annex A. Therefore the
LI field in M2PA is always one octet.
Note: In the SS7 Recommendations, the format of the messages and
fields within the messages are based on bit transmission order. In
these recommendations the Least Significant Bit (LSB) of each field is
positioned to the right. The received SS7 fields are populated octet
by octet as received into the 4-octet word as shown below.
As an example, in the ANSI MTP protocol, the Data field format is
shown below:
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|MSB---------------------------------------------------------LSB|
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PRI| spare | SIO | SIF octet | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ : \
/ : /
\ : \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | ... | ... | SIF octet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Within each octet the Least Significant Bit (LSB) per the SS7
Recommendations is to the right (e.g., bit 15 of SIO is the LSB).
2.3.2 Link Status
The MTP2 Link Status message can be sent between M2PA peers to
indicate link status. This message performs a function similar to the
the Link Status Signal Unit in MTP2. The format for the Link Status
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for State are shown in the following table.
Value
(decimal) Description
--------- -----------
1 Alignment
2 Proving Normal
3 Proving Emergency
4 Ready
5 Processor Outage
6 Processor Outage Ended
7 Busy
8 Busy Ended
9 Out of Service
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2.3.2.1 Link Status Proving
The Link Status Proving message may optionally carry additional
bytes. If the optional bytes are used, the format for the 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| filler |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:
:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| filler |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It is recommended that the length of the Link Status Proving message
be similar to the size of the User Data messages that will be carried
on the link.
It is recommended that the filler field contain a number pattern which
varies among the Link Status Proving messages, and that will allow the
SCTP checksum to be used to verify the accuracy of transmission.
3. M2PA Link State Control
The M2PA link moves from one state to another in response to various
events. The events that may result in a change of state include:
- MTP3 primitive requests
- SCTP notifications
- Receipt of Status messages from the peer M2PA
- Expiration of certain timers
Figure 7 illustrates state changes together with the causing events.
Note that some of the error conditions are not shown in the state
diagram.
Following is a list of the M2PA Link States and a description of each.
IDLE - State of the link during power-up initialization.
OOS - Out Of Service. Power-up initialization is complete.
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AIP - Alignment In Progress. M2PA is attempting to exchange Alignment
messages with its peer.
PROVING - M2PA is sending Link Status Proving messages to its peer.
ALIGNED READY - Proving is complete. M2PA is waiting until peer
completes proving.
INS - In Service. Link is ready for traffic.
RETRIEVAL - Link no longer carries traffic. M2PA is waiting for
request for message retrieval from MTP3.
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+-----------+
| IDLE |
+-----------+
|
| Power On
|
| +--------------------------+
| | (Associate) |
V V |
+-----------+ |
+----------------------->| OOS |<--+ |
| +-----------+ | Link Configured |
| | | | (Associate) |
| | +-----+ |
| | |
| | MTP3 Start |
| V |
| +-----------+ |
+<-----------------------| AIP |----------------------->+
| MTP3 Stop +-----------+ SCTP Comm Error |
| OR T1 Expiry ^ | OR SCTP Comm Lost |
| | | |
| | | |
| +----------------------+ | Send and |
| | | Receive LS Alignment |
| | V |
| | +-----------+ |
+<-----------------------| PROVING |----------------------->+
| | MTP3 Stop +-----------+ SCTP Comm Error |
| | OR Receive LS OOS | OR SCTP Comm Lost |
| | | |
| | | T4 Expiry |
| | V |
| | +-----------+ |
| | | ALIGNED | |
+<-----------------------| READY |----------------------->+
| | MTP3 Stop +-----------+ SCTP Comm Error
| | OR T3 Expiry | OR SCTP Comm Lost
| | OR Receive LS OOS |
| | | Receive LS Ready
| | | OR Receive User Data
| | |
| | V
| | +-----------+
| | | INS |
| | +-----------+
| | |
| | | MTP3 Stop
| | | OR Receive LS OOS
| | | OR SCTP Comm Error
| | | OR SCTP Comm Lost
| | | OR T6 Expiry
| | | OR T7 Expiry
| | | OR M2PA Link Failure
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| | |
| | |
| | |
| | |
| | V
| | MTP3 Start +-----------+
| +<-------------------| RETRIEVAL |
+<-----------------------+-----------+
Retrieval Complete
Figure 7: M2PA Link State Transition Diagram
Figure 8 illustrates state changes in the M2PA management of the SCTP
association together with the causing events. Note that some of the
error conditions are not shown in the state diagram.
Following is a list of the M2PA Association States and a description
of each.
IDLE - State of the association during power-up initialization.
ASSOCIATE - M2PA is attempting to establish an SCTP association.
ESTABLISHED - SCTP association is established.
+-----------+
| IDLE |
+-----------+
|
| Associate
| (Issue SCTP associate)
|
| +----------------------+
| | (Issue SCTP |
V V associate) |
+-----------+ |
| ASSOCIATE |------------------->+
+-----------+ SCTP Comm Error |
| |
| |
| SCTP Comm Up |
| |
V |
+-------------+ |
| ESTABLISHED |----------------->+
+-------------+ SCTP Comm Error
OR SCTP Comm Lost
Figure 8: M2PA Association State Transition Diagram
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4. Procedures
4.1 Procedures to Support MTP2 Features
4.1.1 Signal Unit Format, Delimitation, Acceptance
Messages for transmission across the network must follow the format
described in section 2.
SCTP provides reliable, in-sequence delivery. Therefore the related
functionality of MTP2 is not needed. SCTP does not provide functions
related to Link State Control in MTP2. These functions must be
provided by M2PA.
4.1.2 MTP and SCTP Entities
This section describes how M2PA relates MTP and SCTP entities.
Each MTP link corresponds to an SCTP association. To prevent duplicate
associations from being established, it is recommended that each
endpoint know the IP address (or IP addresses, if multi-homing is
used) and port number of both endpoints. SCTP prevents two
associations with the same IP addresses and port numbers from being
established.
It is necessary for at least one of the endpoints to be listening on
the port on which the other endpoint is trying to establish the
association. Therefore, at least one of the port numbers should be the
M2PA registered port.
If only one association is to be established between these two IP
addresses, then the association should be established using the M2PA
registered port at each endpoint.
If it is desirable to create multiple associations (for multiple
links) between the two IP addresses, different port numbers can be
used for each association. Nevertheless, the M2PA registered port
number should be used at one end of each association.
Each combination of IP address/port for the two endpoints (i.e., each
association) must be mapped to the same Signaling Link Code (SLC) at
each endpoint, so that each endpoint knows which link is being created
at the time the SCTP association is established. However, M2PA does
not do any processing based on the SLC.
Following are examples of the relationships between associations and
links. Note that a link is an SCTP association identified by two
endpoints. Each endpoint is identified by an IP address and port
number. Each association is mapped to an SLC.
Figure 9 shows a case with two IPSPs, each with two IP addresses. Two
associations are the links that connect the two IPSPs. Since these
links are in the same link set, they must have different SLCs.
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Table 1 shows the relationships in tabular form. Table 1 is only
conceptual. The actual method for mapping the SCTP associations to the
SLCs is implementation dependent.
IPSP X IPSP Y
+-------------+ +-------------+
| | SCTP | |
| IPA | association 1 | IPB |
| port = PW +---------------+ port = PW |
| SLC = a | | SLC = a |
| | | |
| | | |
| | SCTP | |
| IPC | association 2 | IPD |
| port = PW +---------------+ port = PW |
| SLC = b | | SLC = b |
| | | |
| | | |
+-------------+ +-------------+
IPx = IP address
PW = Registered port number for M2PA
Figure 9: Associations and Links -
Two IPSPs with Two IP Addresses Each
+-------------+---------------------------------------+-----+
| Association | IPSP X | IPSP Y | SLC |
| +------------+------+------------+------+ |
| | IP address | Port | IP address | Port | |
+=============+============+======+============+======+=====+
| 1 | IPA | PW | IPB | PW | a |
+-------------+------------+------+------------+------+-----+
| 2 | IPC | PW | IPD | PW | b |
+-------------+------------+------+------------+------+-----+
Table 1: Associations and SLCs -
Two IPSPs with Two IP Addresses Each
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Figure 10 and Table 2 show an example with three IPSPs. Note that in
this example, the two links are in different link sets. Therefore, it
is possible that the values a and b may be equal.
IPSP X IPSP Y
+-------------+ +-------------+
| | SCTP | |
| IPA | association 1 | IPB |
| port = PW +---------------+ port = PW |
| SLC = a | | SLC = a |
| | | |
| | | |
| | SCTP | |
| IPC | association 2 | |
| port = PW +-------+ | |
| SLC = b | | | |
| | | | |
| | | | |
+-------------+ | +-------------+
|
|
| IPSP Z
|
| +-------------+
| | |
| | IPD |
+-------+ port = PW |
| SLC = b |
| |
| |
| |
| |
| |
| |
| |
| |
+-------------+
IPx = IP address
PW = Registered port number for M2PA
Figure 10: Associations and Links -
One IPSP Connected to Two IPSPs
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+-------------+---------------------------------------+-----+
| Association | IPSP X | IPSP Y | SLC |
| +------------+------+------------+------+ |
| | IP address | Port | IP address | Port | |
+=============+============+======+============+======+=====+
| 1 | IPA | PW | IPB | PW | a |
+-------------+------------+------+------------+------+-----+
| 2 | IPC | PW | IPD | PW | b |
+-------------+------------+------+------------+------+-----+
Table 2: Associations and SLCs -
One IPSP Connected to Two IPSPs
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Figure 11 and Table 3 show two associations between the same IP
addresses. This is accomplished by using different port numbers for
each association at one endpoint.
IPSP X IPSP Y
+-------------+ +-------------+
| | SCTP | |
| IPA | association 1 | IPB |
| port = P1 +---------------+ port = PW |
| SLC = a | | SLC = a |
| | | |
| | | |
| | SCTP | |
| IPA | association 2 | IPB |
| port = PW +---------------+ port = PW |
| SLC = b | | SLC = b |
| | | |
| | | |
+-------------+ +-------------+
IPx = IP address
P1 = Pre-selected port number
PW = Registered port number for M2PA
Figure 11: Associations and SLCs -
Multiple Associations Between Two IP Addresses
+-------------+---------------------------------------+-----+
| Association | IPSP X | IPSP Y | SLC |
| +------------+------+------------+------+ |
| | IP address | Port | IP address | Port | |
+=============+============+======+============+======+=====+
| 1 | IPA | P1 | IPB | PW | a |
+-------------+------------+------+------------+------+-----+
| 2 | IPA | PW | IPB | PW | b |
+-------------+------------+------+------------+------+-----+
Table 3: Associations and SLCs -
Multiple Associations Between Two IP Addresses
The association shall contain two streams in each direction. Stream 0
is designated for Link Status messages. Stream 1 is designated for
User Data messages.
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4.1.3 Link Alignment
The purposes of the alignment procedure are:
(1) To provide a handshaking procedure so that both endpoints are
prepared to send SS7 traffic, and to prevent traffic from being
sent before the other end is ready.
(2) To verify that the SCTP association is suitable for use as an
SS7 link.
Link alignment takes place after the association is established. If
SCTP fails to establish the association, and M2PA has received a Start
Request from its MTP3, then M2PA shall report to MTP3 that the link is
out of service.
After the association is established, M2PA shall send a Link Status
Out of Service message to its peer.
Once the association is established and M2PA has received a Start
Request from MTP3, M2PA sends the Link Status Alignment message to its
peer. If M2PA has not already received the Link Status Alignment
message from its peer, then M2PA starts timer T1.
(Note that if the remote M2PA has not received a Start Request from
its MTP3, it will not send the Link Status Alignment message to the
local M2PA. Eventually timer T1 in the local M2PA will expire. If the
remote M2PA receives a Start Request from its MTP3 and sends Link
Status Alignment before the local M2PA's timer T1 expires, the
alignment procedure can continue.)
M2PA stops timer T1 when it has received the Link Status Alignment
message from its peer.
If timer T1 expires, then M2PA reports to MTP3 that the link is out of
service. M2PA sends Link Status Out of Service to its peer. M2PA
should leave the association established. M2PA waits for MTP3 to
initiate the alignment procedure again.
Note: Between the time M2PA sends Link Status Alignment to its peer
and receives Link Status Alignment from its peer, M2PA may receive
Link Status Out of Service from its peer. This message is
ignored. After receiving Link Status Alignment from the peer, receipt
of a Link Status Out of Service message causes M2PA to send Out of
Service to MTP3 and return to the Out of Service state.
When M2PA has both sent and received the Link Status Alignment
message, it has completed alignment and moves to the proving state.
M2PA starts the proving period timer T4. During the proving period,
M2PA sends Link Status Proving messages to its peer at an interval
defined by the protocol parameter Proving_Rate. M2PA sends either the
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Proving Normal or Proving Emergency message, according to the
Emergency and Emergency Ceases commands from MTP3. M2PA uses the value
of T4 corresponding to the Normal or Emergency state. However, if M2PA
receives a Link Status Proving Emergency message from its peer, then
M2PA shall initiate the Emergency proving period value for T4, but it
shall continue to send the Proving message (Normal or Emergency)
determined by its own upper layer MTP3.
When the proving period timer T4 expires, M2PA shall start the timer
T3 and send Link Status Ready messages to its peer at interval
Status_Interval. These messages are used to verify that both ends have
completed proving.
M2PA shall stop timer T3 when it receives a Link Status Ready or User
Data message from its peer. If timer T3 expires, then M2PA reports to
MTP3 that the link is out of service. M2PA sends Link Status Out of
Service to its peer. M2PA should leave the association
established. M2PA waits for MTP3 to initiate the alignment procedure
again.
Note that if M2PA has already received a Link Status Ready message
from its peer when its timer T4 expires, there is no need to start
timer T3. M2PA can just send Link Status Ready to the peer and
continue along.
When all of the following are true:
(a) M2PA has received a Start Request from MTP3.
(b) M2PA's proving period T4 has expired.
(c) M2PA has sent a Link Status Ready to its peer.
(d) M2PA has received a Link Status Ready OR User Data
message from its peer.
(e) M2PA has not received Link Status Out of Service from its peer
since it received Link Status Alignment.
then M2PA shall send Link In Service to its MTP3.
If there is a local processor outage condition during the alignment
procedure, M2PA sends Link Status Processor Outage to its peer. When
the local processor outage condition ends, then M2PA shall send Link
Status Processor Outage Ended to its peer. M2PA shall attempt to
complete the alignment procedure during the local processor outage
condition.
If M2PA receives a Link Status Processor Outage during alignment, and
M2PA had received a Start Request from its MTP3, M2PA shall report
Remote Processor Outage to MTP3. M2PA shall attempt to complete the
alignment procedure during the remote processor outage condition.
If M2PA receives a Stop command from its MTP3 during alignment, M2PA
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shall send Link Status Out of Service to its peer and terminate the
alignment procedure.
Anomalous messages received during alignment should be
discarded. Examples include:
(a) User Data received before proving begins.
(b) Link Status Alignment received during proving.
Recommended values:
T1 Alignment - Range: 1-60 seconds Default: 10 seconds
T4 Proving -
Normal - Range: 1-60 seconds Default: 10 seconds
Emergency - Range: 400-600 milliseconds Default: 500 milliseconds
T3 Ready - Range: 1-60 seconds Default: 10 seconds
Status_Interval - implementation dependent.
Proving_Rate - implementation dependent.
4.1.4 Processor Outage
A processor outage occurs when M2PA cannot transfer messages because
of a condition at a higher layer than M2PA.
When M2PA detects a local processor outage, it sends a Link Status
message to its peer with status Processor Outage. M2PA shall also
cease sending User Data messages to SCTP for transmission. M2PA shall
stop receiving incoming messages from SCTP.
M2PA should periodically send a Link Status Processor Outage message
as long as there is a local processor outage and the link is in
service. If the link is out of service, M2PA should locally mark that
it is in local processor outage.
The peer M2PA, upon receiving the Link Status Processor Outage
message, shall report Remote Processor Outage to its MTP3. The peer
M2PA ceases sending User Data messages. M2PA stops the Remote
Congestion timer T6 (see section 4.1.5 Level 2 Flow Control) if it is
running.
MTP3 may send a Flush Buffers or Continue command to M2PA as part of
its processor outage procedure (See section 4.2.4 Flush Buffers,
Continue). Alternatively, MTP3 may perform data retrieval as part of a
changeover procedure.
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When the processor outage ceases, MTP3 sends a Local Processor
Recovered indication to M2PA. The local M2PA notifies its peer by
sending a Link Status message with status Processor Outage Ended. The
peer uses the Remote Processor Recovered Indication to notify its MTP3
that the remote processor outage condition has ceased.
4.1.5 Level 2 Flow Control
The determination of receive congestion in M2PA is implementation
dependent.
If M2PA determines that it is in receive congestion for an
association, M2PA shall send a Link Status Busy message to its peer on
that association. M2PA shall continue to acknowledge incoming
messages.
M2PA should periodically send a Link Status Busy message as long as
it is in receive congestion.
M2PA shall continue transmitting messages while it is in receive
congestion.
When the peer M2PA receives the Link Status Busy message, it shall
start the Remote Congestion timer T6. If timer T6 expires, M2PA shall
take the link out of service. M2PA sends a Link Status Out of Service
message to its peer, and goes to the Retrieval state.
The peer M2PA should continue transmitting messages to SCTP while its
T6 timer is running, i.e., while the other end is Busy.
The peer M2PA shall not fail the link due to expiration of timer T7
excessive delay of acknowledgement (see section 4.2.1 Sending and
receiving messages) while its T6 timer is running, i.e., while the
other end is Busy.
If M2PA is no longer in receive congestion for the association, M2PA
shall send a Link Status Busy Ended message to its peer on that
association.
When the peer M2PA receives the Link Status Busy Ended message, it
shall stop timer T6.
Recommended value of T6 is 1-6 seconds.
4.1.6 Error Monitoring
If M2PA loses the SCTP association for a link, M2PA shall report to
MTP3 that the link is out of service.
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4.1.7 Transmission and Reception Priorities
In MTP, Link Status messages have priority over User Data messages
([2] Q.703, section 11.2). To achieve this in M2PA, M2PA shall send
Link Status and User Data messages on separate streams in its SCTP
association. All messages are sent using the ordered delivery option.
M2PA SHOULD give higher priority to Link Status messages than to User
Data messages when sending messages to SCTP.
M2PA SHOULD give higher priority to reading the Link Status stream
than to reading the User Data stream.
M2PA SHOULD give higher priority to receiving notifications from SCTP
than to reading either the Link Status stream or the User Data stream.
4.1.8 M2PA Version Control
A node upgraded to a newer version of M2PA SHOULD support the older
versions used on other nodes with which it is communicating. If that
is the case, then alignment can proceed normally.
In particular, it is recommended that for future modifications to this
protocol:
- Any newer version should be able to process the messages from a
lower version.
- A newer version of M2PA should refrain from sending messages to an
older version of M2PA messages that the older version cannot
process.
- If an older version of M2PA receives a message that it cannot
process, it should discard the message.
- In cases where different processing is done in two versions for the
same format of a message, then the newer version should contain
procedures to recognize this and handle it appropriately.
In case a newer version of M2PA is incompatible with an older version,
the newer version should recognize this and prevent the alignment of
the link. If a Link Status Alignment message with an unsupported
version is received by the newer version, the receiving end's M2PA
shall not complete the alignment procedure.
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4.2 Procedures to Support the MTP3/MTP2 Interface
4.2.1 Sending and receiving messages
When MTP3 sends a message for transmission to M2PA, M2PA passes the
corresponding M2PA message to SCTP using the SEND primitive.
M2PA Link Status messages are passed to SCTP using the SEND primitive.
Link Status and User Data messages shall be sent via SCTP on separate
streams.
When M2PA receives a User Data message from SCTP, M2PA passes the
message to MTP3.
If M2PA receives a message from SCTP with an invalid Message Class or
unsupported Message Type in the Common Message Header, M2PA shall
discard the message.
The first User Data message sent after the link is placed in service
is given a Forward Sequence Number (FSN) of 1.
The Forward Sequence Number of the header is incremented by 1 for each
User Data message sent. When the FSN reaches the maximum value, the
next FSN is 0.
For message types other than User Data, the Forward Sequence Number is
set to the FSN of the last User Data message sent.
The Backward Sequence Number is set to the FSN of the last User Data
message M2PA received from its peer. This serves as an M2PA-level
acknowledgement of the message. After the link is placed in service
and before a User Data message has been received, the BSN is set to 0.
When M2PA receives a User Data message with Backward Sequence Number
equal to 'n', it may remove all messages with Forward Sequence Number
<= n from its queue.
If M2PA receives a User Data message with an FSN that is out of order,
M2PA shall fail the link.
M2PA should follow the criterion stated in [2] Q.703, section 5.3.1
for incorrect BSNs: If any two BSNs in three consecutively received
User Data messages are not the same as the previous one or any of the
FSNs of the User Data messages in the M2PA transmit buffer at the time
they are received, then MTP3 should be informed that the link is
faulty.
M2PA should ignore the FSN and BSN contained in a Link Status message.
Note: In all calculations involving FSN and BSN, the programmer should
be aware that the value wraps around to 0 after reaching its maximum
value.
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If there is no other User Data message to be sent when there is a
message to acknowledge, M2PA may send a User Data message with no data
payload. The FSN for this empty User Data message is incremented as
with any other User Data message.
Note that since SCTP provides reliable delivery and ordered delivery
within the stream, M2PA does not perform retransmissions.
Timer T7 provides an indication of excessive delay of
acknowledgement. If the following conditions are true:
(a) There is at least one message in the M2PA transmit buffer that
has been sent.
(b) The remote M2PA is not in a Busy condition.
(c) M2PA has not received an acknowledgement in the span of T7.
Then M2PA should fail the link.
The recommended range for timer T7 is 0.5-2 seconds.
4.2.2 Link activation and restoration
When MTP3 requests that M2PA activate or restore a link by a Start
Request, M2PA shall follow the alignment procedure in section 4.1.3.
4.2.3 Link deactivation
When MTP3 requests that M2PA deactivate a link by a Stop command, M2PA
shall send a Link Status Out of Service message to its peer.
The peer M2PA, upon receiving Link Status Out of Service, shall notify
its upper layer MTP3 that the link is out of service.
4.2.4 Flush Buffers, Continue
The Flush Buffers and Continue commands allow M2PA to resume normal
operations (i.e., transmission of messages to SCTP and receiving
messages from SCTP) after a processor outage (local and/or remote)
ceases.
If M2PA receives a Flush Buffers command from MTP3, M2PA:
(a) shall not transmit any messages to SCTP that are currently
waiting to be transmitted to SCTP. These messages shall be
discarded.
(b) shall discard all messages currently waiting to be passed
to MTP3.
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If M2PA receives either a Flush Buffers or Continue command from MTP3,
and the processor outage condition ceases, M2PA shall resume receiving
and transmitting messages.
4.2.5 MTP3 Signaling Link Congestion
M2PA shall detect transmit congestion in its buffers according to the
requirements for signaling link transmit congestion in [2] Q.704,
section 3.8.
M2PA shall use the Congestion Indication primitive to notify its upper
layer MTP3 of changes in the signaling link congestion status and the
signaling link discard status. For national networks with multiple
congestion threshold levels, M2PA shall notify MTP3 of the congestion
and discard status levels.
4.2.6 Changeover
The objective of the changeover is to ensure that signaling traffic
carried by the unavailable signaling link is diverted to the
alternative signaling link(s) as quickly as possible while avoiding
message loss, duplication, or mis-sequencing. For this purpose, the
changeover procedure includes data retrieval, which is performed
before opening the alternative signaling links to the diverted
traffic. Data retrieval consists of these steps:
(1) buffer updating, i.e., identifying all those User Data
messages in the retransmission buffer of the unavailable
signaling link which have not been received by the far end
M2PA, as well as untransmitted messages, and
(2) transferring those messages to the transmission buffers of the
alternate links.
Note that only User Data messages are retrieved and transmitted over
the alternate links. Link Status messages shall not be retrieved and
transmitted over the alternate links.
M2PA's Sequence Numbers are 24 bits long. MTP2's Forward and Backward
Sequence Numbers are only seven bits long. Hence it is necessary for
MTP3 to accommodate the larger sequence numbers. This is done through
the use of the Extended Changeover Order (XCO) and Extended Changeover
Acknowledgement (XCA) messages instead of the Changeover Order (COO)
and Changeover Acknowledgement (COA) messages. The XCO and XCA
messages are specified in Reference [7] section 9.8.1 and Reference
[3] T1.111.4, section 15.4. Only the XCO and XCA messages from [7] or
[3] are required. The BSN is placed in the XCO/XCA message as
explained in [7] and [3].
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Also, the following MTP3/MTP2 primitives must use the larger sequence
numbers:
- BSNT Confirmation
- Retrieval Request and FSNC
For data retrieval, MTP3 requests the Backward Sequence Number to be
Transmitted (BSNT) from M2PA through the Retrieve BSNT request. M2PA
determines the Forward Sequence Number of the last User Data message
received from the peer. This value is the BSNT. M2PA sends the BSNT
value to MTP3 in the BSNT confirmation. In the same way, the remote
end also detects its BSNT. The MTP3 layers exchange BSNT values
through the XCO and XCA messages. The BSNT received from the other end
is called the FSNC. When MTP3 receives the FSNC from the other end,
MTP3 retrieves all the unsent and unacknowledged messages starting
with sequence number (FSNC + 1). This is accomplished through a
Retrieval Request and FSNC request. After all the messages are sent
from M2PA to MTP3, M2PA sends a Retrieval Complete indication to MTP3.
If there are any messages on the M2PA or SCTP receive queues that have
not been acknowledged by M2PA, M2PA SHOULD discard these messages. The
peer will retransmit them on an alternate link. Any messages
acknowledged by M2PA MUST NOT be discarded. These messages must be
delivered to MTP3.
If M2PA receives a Retrieve BSNT request from MTP3, M2PA shall respond
with the BSNT confirmation. The BSNT value is the Forward Sequence
Number of the last User Data message received from the peer.
If M2PA receives a Retrieval Request and FSNC request from MTP3, M2PA
shall retrieve from its buffers in order and deliver to MTP3:
(a) any transmitted User Data messages beginning with the first
unacknowledged message with FSN greater than FSNC.
(b) any untransmitted User Data messages.
Then M2PA shall send the Retrieval Complete indication to MTP3.
For emergency changeover, MTP3 retrieves only the unsent messages for
transmission on the alternate link(s). If M2PA receives a Retrieval
Request and FSNC request with no FSNC value, or with an invalid FSNC,
then M2PA shall retrieve from its buffers in order and deliver to
MTP3:
(a) any untransmitted User Data messages.
Then M2PA shall send the Retrieval Complete indication to MTP3.
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Note: For the Japanese version of MTP defined in [9], MTP3 retrieves
both unsent and unacknowleged messages for transmission on the
alternate link(s). In this version of MTP, if M2PA receives a Retrieval
Request and FSNC request with no FSNC value, or with an invalid FSNC,
then M2PA shall retrieve from its buffers in order and deliver to
MTP3:
(a) any transmitted but unacknowledged User Data messages.
(b) any untransmitted User Data messages.
Then M2PA shall send the Retrieval Complete indication to MTP3.
4.2.6.1 Multiple User Data Streams and Changeover
The changeover procedure makes it problematic for M2PA to have
multiple User Data streams in one direction for a link. Buffer
updating would have to be done for each User Data stream separately to
avoid duplication or loss of messages. But MTP3 provides for only one
XCO/XCA message for sending the last-received sequence number.
Even with sequence numbering of User Data messages at the M2PA layer,
it is necessary to perform buffer updating on each stream. Since the
M2PA messages would be delivered over multiple streams, there could be
a gap in the M2PA sequence numbers at the receiving end when the
changeover procedure begins. If only the M2PA sequence number is used
in the XCO/XCA message, there would be a possibility of losing the
messages in the gap, or duplicating messages after the gap.
M2PA links with multiple User Data streams would be possible if a
multiple-BSNT XCO/XCA message is defined in MTP3, or MTP3 allows
multiple XCO/XCA messages (one for each User Data stream) to be sent
during a changeover. This is beyond the scope of this document.
4.3 SCTP Considerations
Some M2PA procedures may be affected by the use of SCTP as a transport
layer. These considerations are discussed in this section.
4.3.1 SCTP Slow Start
SCTP contains a slow start algorithm to control the amount of data
being injected into the network. The algorithm allows SCTP to probe
the network to determine the available capacity. The algorithm is
invoked when transmission begins on an association, after a
sufficiently long idle period, or after repairing loss detected by the
SCTP retransmission timer.
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It is possible that transmission of M2PA messages may be delayed by
SCTP slow start under certain conditions, including the following:
(a) Link Alignment. Link alignment takes place after an association
is established. SCTP invokes the slow start algorithm since
transmission is beginning on the association.
(b) Changeover. Messages are retrieved from one link (association)
and transferred to another for transmission. If the second link
had previously been idle, or is in the process of link
alignment, SCTP may invoke the slow start algorithm.
(c) Path failure (multi-homing). If SCTP switches from a failed
path to a new path, and the new path had previously been idle,
SCTP may invoke the slow start algorithm.
(d) Reduced traffic volume. Any time that M2PA sends a low volume
of traffic on a link and then the volume increases, SCTP may
invoke the slow start algorithm.
Programmers should be aware of this condition and how it may affect
M2PA performance. In some cases, it may be possible to avoid the
negative effects of slow start. For example, the Link Status Proving
messages sent during the proving period may be used to complete slow
start before the link is placed in service.
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5. Examples of M2PA Procedures
In general, messages passed between MTP3 and M2PA are the same as
those passed between MTP3 and MTP2. M2PA interprets messages from
MTP3 and sends the appropriate message to SCTP. Likewise, messages
from SCTP are used to generate a meaningful message to MTP3.
Note that throughout this section, the primitives between MTP3 and
M2PA are named using the MTP terminology [1][2]. Communications
between M2PA and SCTP are named using SCTP terminology.
5.1 Link Initialization (Alignment)
An example of the message flow to bring an SS7 link in service is
shown below. Alignment is done by both ends of the link. To simplify
the diagram, alignment is shown on one end only. It is assumed in this
example that SCTP has been initialized.
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Associate
------------>
(SCTP Association
procedure)
Communication Up Communication Up
<------------ ------------>
Link Status Out of Service
------------------------------------>
Emergency OR
Emergency Ceases
------------>
Start
------------>
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MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Link Status Alignment
------------------------------------>
Start timer T1
Link Status Alignment
<------------------------------------
Stop timer T1
Start timer T4
Proving period begins. (Messages from remote end not shown.)
Link Status Proving
------------------------------------>
------------------------------------>
------------------------------------>
------------------------------------>
------------------------------------>
------------------------------------>
Timer T4 expires
Send Link Status Ready until the remote end completes its proving
period.
Start timer T3
Link Status Ready
------------------------------------>
------------------------------------>
------------------------------------>
------------------------------------>
Link Status Ready
<------------------------------------
Stop timer T3
In Service In Service
<------------ ------------>
At this point, MTP3 may begin sending data messages.
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5.2 Message Transmission and Reception
Messages are transmitted using the Data Request primitive from MTP3 to
M2PA. The diagram shows the case where the Link is In Service. The
message is passed from MTP3 of the source to MTP3 of the destination.
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Message for
transmission
------------>
Send
(Data Message)
------------>
(SCTP sends message)
Receive
------------>
Received message
------------>
5.3 Link Status Indication
If SCTP sends a Communication Lost primitive to M2PA, M2PA notifies
MTP3 that the link is out of service. MTP3 responds in its usual way.
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Communication Lost
<------------
Out of Service
<------------
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5.4 Link Status Message (Processor Outage)
This example shows how M2PA responds to a local processor outage. M2PA
sends a Link Status message to its peer. The peer M2PA notifies MTP3
of the outage. MTP3 can then follow the processor outage procedures in
[2].
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
M2PA detects
Local Processor
Outage
Link Status
Processor Outage
------------>
(SCTP sends message)
Receive
------------>
Remote Processor
Outage
------------>
Link Status
Processor Outage
Ended
------------>
(SCTP sends message)
Receive
------------>
Remote Processor
Outage Ceases
------------>
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5.5 Level 2 Flow Control
This illustrates the Level 2 Flow Control procedure. In the first
diagram, congestion ceases before timer T6 expires. The second diagram
shows the case where T6 expires.
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Implementation dependent
determination of M2PA
receive congestion onset
Link Status Busy
------------------------------------>
Start
Timer T6
Implementation dependent
determination of M2PA
receive congestion abatement
Link Status Busy Ended
------------------------------------>
Stop
Timer T6
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Implementation dependent
determination of M2PA
receive congestion onset
Link Status Busy
------------------------------------>
Start
Timer T6
:
:
Timer T6
Expires
Link Status Out of Service
<------------------------------------
Out of Service
------------>
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5.6 MTP3 Signaling Link Congestion
In this example, M2PA notifies MTP3 of congestion onset and
abatement. The notification includes the congestion level, if there
are levels of congestion defined.
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Implementation dependent
determination of M2PA
transmit congestion
onset (level)
Congestion Indication
(level)
<------------
Implementation dependent
determination of M2PA
transmit congestion
abatement (level)
Congestion Indication
(level)
<------------
5.7 Link Deactivation
The MTP3 can request that a SS7-IP link be taken out of service.
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Stop
------------>
Link Status Out of Service
------------------------------------>
Out of Service
<------------
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5.8 Link Changeover
In this example, MTP3 performs a changeover because the link went out
of service. MTP3 selects a different link to retransmit the
unacknowledged and unsent messages.
Note that in this example, the sequence numbers and messages requested
by MTP3 are sent from SCTP to M2PA in the Communication Lost
primitive. In general, the retrieval of sequence numbers and messages
is implementation dependent.
MTP3 M2PA SCTP SCTP M2PA MTP3
---- ---- ---- ---- ---- ----
Communication Lost
<------------
Out of Service
<------------
Retrieve BSNT
------------>
BSNT Confirmation
<------------
XCO (BSNT) on another link
------------------------------------------------------------>
Retrieve BSNT
<------------
BSNT Confirmation
------------>
XCA (BSNT)
<------------------------------------------------------------
Retrieval Request
and FSNC
------------>
Retrieved Message
<------------
:
:
<------------
Retrieval Complete
<------------
Send messages on another link.
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6. Security
M2PA is designed to carry signaling messages for telephony
services. As such, M2PA MUST involve the security needs of several
parties: the end users of the services, the network providers, and the
applications involved. Additional requirements MAY come from local
regulation. While having some overlapping security needs, any
security solution SHOULD fulfill all of the different parties' needs.
6.1 Threats
There is no quick-fix, one-size-fits-all solution for security. As a
transport protocol, M2PA has the following security objectives:
- Availability of reliable and timely user data transport.
- Integrity of user data transport.
- Confidentiality of user data.
M2PA runs on top of SCTP. SCTP [5] provides certain transport related
security features, such as:
- Blind Denial of Service Attacks
- Flooding
- Masquerade
- Improper Monopolization of Services
When M2PA 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" [11] SHOULD be consulted for guidance.
When the network in which M2PA runs involves more than one party
(e.g., a non-private network), 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 be used to ensure confidentiality
of user payload. Consult [12] 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.
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7. IANA Considerations
7.1 SCTP Payload Protocol Identifier
The SCTP (and TCP) Registered User Port Number Assignment for M2PA
is 3565.
The value assigned by IANA for the Payload Protocol Identifier in the
SCTP Payload Data chunk is
M2PA 5
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 M2PA Protocol Extensions
This protocol may be extended through IANA in three ways:
- through definition of additional message classes,
- through definition of additional message types, and
- through definition of additional message parameters.
The definition and use of new message classes, types, and parameters
is an integral part of SIGTRAN adaptation layers. Thus, these
extensions are assigned by IANA through an IETF Consensus action as
defined in [13].
The proposed extension must in no way adversely affect the general
working of the protocol.
7.2.1 IETF Defined Message Classes
The documentation for a new message class MUST include the following
information:
(a) A long and short name for the message class.
(b) A detailed description of the purpose of the message class.
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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 the 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 discard the message.
7.2.3 IETF-defined Parameter Extension
Documentation of the message parameter MUST contain the following
information:
(a) Name of the parameter type.
(b) Detailed description of the structure of the parameter field.
(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.
8. Acknowledgements
The authors would like to thank the following for their valuable
comments and suggestions: Brian Tatum, Jeff Copley, Monique Bernard,
Wayne Davis, Cliff Thomas, Ian Rytina, Al Varney.
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9. References
[1] ITU-T Recommendation Q.700, 'Introduction To ITU-T Signalling
System No. 7 (SS7)'.
[2] ITU-T Recommendation Q.701-Q.705, 'Signalling System No. 7
(SS7) - Message Transfer Part (MTP)'.
[3] ANSI T1.111-2000, American National Standard for
Telecommunications - Signaling System Number 7 (SS7) -
Message Transfer Part (MTP), 2000.
[4] RFC 2719, Framework Architecture for Signaling Transport,
October 1999.
[5] RFC 2960, Stream Control Transmission Protocol,
October 2000.
[6] SS7 MTP2-User Adaptation Layer, draft-ietf-sigtran-m2ua-15.txt,
February 2002.
[7] ITU-T Recommendation Q.2210, 'Message transfer part level 3
functions and messages using the services of ITU-T
Recommendation Q.2140'.
[8] Bradner, S. "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[9] Telecommunication Technology Committee (TTC) Standard JT-Q704,
'Message Transfer Part Signaling Network Functions',
April 28, 1992.
[10] ITU-T Recommendation Q.2140, 'B-ISDN ATM Adaptation Layer',
February 1995.
[11] RFC 2196, Site Security Handbook, September 1997.
[12] RFC 2401, Security Architecture for the Internet Protocol,
November 1998.
[13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[14] RFC 791, Internet Protocol, September 1981.
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10. Author's Addresses
Tom George Tel: +1-972-519-3168
Alcatel USA, Inc. EMail: [email protected]
1000 Coit Road
Plano, TX 75075
USA
Ram Dantu, Ph.D. Tel: +1-214-291-1111
Netrake Corporation EMail: [email protected]
3000 Technology Drive, Suite 100
Plano 75074
USA
Malleswar Kalla Tel: +1-973-829-5212
Telcordia Technologies EMail: [email protected]
MCC 1J211R
445 South Street
Morristown, NJ 07960
USA
Hanns Juergen Schwarzbauer Tel: +49-89-722-24236
SIEMENS AG [email protected]
Hofmannstr. 51
81359 Munich
Germany
Greg Sidebottom
gregside consulting EMail: [email protected]
Kanata, Ontario
Canada
Ken Morneault Tel: +1-703-484-3323
Cisco Systems Inc. EMail: [email protected]
13615 Dulles Technology Drive
Herndon, VA 20171
USA
Brian Bidulock Tel +1-780-490-1141
OpenSS7 Corporation EMail: [email protected]
1469 Jeffreys Crescent
Edmonton, AB T6L 6T1
Canada
This Internet Draft expires February 2003.
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