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<pre>Network Working Group L. Coene
Request for Comments: 4166 Siemens
Category: Informational J. Pastor-Balbas
Ericsson
February 2006
<span class="h1">Telephony Signalling Transport over</span>
<span class="h1">Stream Control Transmission Protocol (SCTP) Applicability Statement</span>
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes the applicability of the several protocols
developed under the signalling transport framework. A description of
the main issues regarding the use of the Stream Control Transmission
Protocol (SCTP) and an explanation of each adaptation layer for
transport of telephony signalling information over IP infrastructure
are given.
<span class="grey">Coene & Pastor-Balbas Informational [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc4166">RFC 4166</a> Telephony Signalling over SCTP AS February 2006</span>
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
<a href="#section-1.1">1.1</a>. Scope ......................................................<a href="#page-2">2</a>
<a href="#section-1.2">1.2</a>. Terminology ................................................<a href="#page-3">3</a>
<a href="#section-1.3">1.3</a>. Contributors ...............................................<a href="#page-3">3</a>
<a href="#section-2">2</a>. SIGTRAN Architecture ............................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. Issues for Transporting Telephony Signalling over SCTP ..........<a href="#page-5">5</a>
<a href="#section-3.1">3.1</a>. Congestion Control .........................................<a href="#page-5">5</a>
<a href="#section-3.2">3.2</a>. Detection of Failures ......................................<a href="#page-6">6</a>
<a href="#section-3.2.1">3.2.1</a>. Retransmission TimeOut (RTO) Calculation ............<a href="#page-6">6</a>
<a href="#section-3.2.2">3.2.2</a>. Heartbeat ...........................................<a href="#page-7">7</a>
<a href="#section-3.2.3">3.2.3</a>. Maximum Number of Retransmissions ...................<a href="#page-7">7</a>
<a href="#section-3.3">3.3</a>. Shorten End-to-End Message Delay ...........................<a href="#page-7">7</a>
<a href="#section-3.4">3.4</a>. Bundling Considerations ....................................<a href="#page-7">7</a>
<a href="#section-3.5">3.5</a>. Stream Usage ...............................................<a href="#page-7">7</a>
<a href="#section-4">4</a>. User Adaptation Layers ..........................................<a href="#page-7">7</a>
<a href="#section-4.1">4.1</a>. Access Signalling .........................................<a href="#page-10">10</a>
<a href="#section-4.1.1">4.1.1</a>. IUA (ISDN Q.921 User Adaptation) ...................<a href="#page-10">10</a>
<a href="#section-4.1.2">4.1.2</a>. V5UA (V5.2-User Adaptation) Layer ..................<a href="#page-12">12</a>
<a href="#section-4.1.3">4.1.3</a>. DUA (DPNSS/DASS User adaptation) Layer .............<a href="#page-13">13</a>
<a href="#section-4.2">4.2</a>. Network Signalling ........................................<a href="#page-13">13</a>
<a href="#section-4.2.1">4.2.1</a>. MTP lvl3 over IP ...................................<a href="#page-14">14</a>
<a href="#section-4.2.2">4.2.2</a>. M3UA (SS7 MTP3 User Adaptation) Layer ..............<a href="#page-17">17</a>
<a href="#section-4.2.3">4.2.3</a>. SUA (SS7 SCCP User Adaptation) Layer ...............<a href="#page-18">18</a>
<a href="#section-5">5</a>. Security Considerations ........................................<a href="#page-20">20</a>
<a href="#section-6">6</a>. Informative References .........................................<a href="#page-20">20</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
This document is intended to describe how to transport telephony
signalling protocols, used in classic telephony systems, over IP
networks. As described in [<a href="./rfc2719" title=""Framework Architecture for Signaling Transport"">RFC2719</a>], the whole architecture is
called SIGTRAN (Signalling Transport) and is composed of a transport
protocol (SCTP) and several User Adaptation Layers (UALs). The
transport protocol SCTP has been developed to fulfill the stringent
requirements of telephony signalling networks [<a href="./rfc3257" title=""Stream Control Transmission Protocol Applicability Statement"">RFC3257</a>]. The set of
UALs has also been introduced to make it possible for different
signalling protocols to use the SCTP layer.
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Scope</span>
The scope of this document is the SIGTRAN user adaptation layers and
SCTP protocols and how they are used to transport telephony
signalling information over IP networks.
<span class="grey">Coene & Pastor-Balbas Informational [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc4166">RFC 4166</a> Telephony Signalling over SCTP AS February 2006</span>
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Terminology</span>
The following terms are commonly identified in related work:
Association: SCTP connection between two endpoints.
Stream: A uni-directional logical channel established within an
association, within which all user messages are
delivered in sequence except for those submitted to the
unordered delivery service.
SPU: Signalling protocol user, the application on top of the
User adaptation layer.
CTSP: Classical Telephony Signalling Protocol (examples
include: MTP level 2, MTP level 3, and SCCP).
UAL: User Adaptation Layer, the protocol that encapsulates
the upper layer telephony signalling protocols that are
to be transported over SCTP/IP.
ISEP: IP Signalling Endpoint, an IP node that implements SCTP
and a User adaptation layer.
SP: Signalling Point.
<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>. Contributors</span>
The following people contributed to the document: L. Coene (Editor),
M. Tuexen, G. Verwimp, J. Loughney, R.R. Stewart, Qiaobing Xie, M.
Holdrege, M.C. Belinchon, A. Jungmaier, J. Pastor, and L. Ong.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. SIGTRAN Architecture</span>
The SIGTRAN architecture describes the transport of signalling
information over IP infrastructure.
<span class="grey">Coene & Pastor-Balbas Informational [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc4166">RFC 4166</a> Telephony Signalling over SCTP AS February 2006</span>
Telephony signalling transport over IP normally uses the following
architecture:
Telephony Signalling Protocol
|
+------------------------------------+
| User Adaptation Layers |
+------------------------------------+
|
+------------------------------------+
|Stream Control Transmission Protocol|
| (SCTP) |
+------------------------------------+
|
Internet Protocol (IPv4/IPv6)
Figure 1: Telephony SIGnalling TRANsport Protocol Stack
The components of the protocol stack are:
1. Adaptation layers used when the telephony application needs to
preserve an existing primitive interface (e.g., management
indications or data operation primitives for a particular
user/application protocol).
2. SCTP, specially configured to meet the telephony application
performance requirements.
3. The standard Internet Protocol.
The telephony signalling protocols to be transported can be:
o [<a href="./rfc3332" title=""Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) - User Adaptation Layer (M3UA)"">RFC3332</a>] SS7 MTP3 users: SCCP, ISUP, TUP...
o [<a href="./rfc3331" title=""Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) - User Adaptation Layer"">RFC3331</a>] SS7 MTP2 users: MTP3
o [<a href="./rfc3868" title=""Signalling Connection Control Part User Adaptation Layer (SUA)"">RFC3868</a>] SS7 SCCP users: RANAP, MAP(+TCAP), INAP(+TCAP)...
o [<a href="./rfc3057" title=""ISDN Q.921-User Adaptation Layer"">RFC3057</a>] ISDN Q.921 users: Q.931
o [<a href="./rfc3807" title=""V5.2-User Adaptation Layer (V5UA)"">RFC3807</a>] V5.2 / DSS1
o ....
The user adaptation layers (UALs) are a set of protocols that
encapsulate a specific signalling protocol to be transported over
SCTP. The adaption is done in a way that the upper signalling
protocols, which are relayed, remain unaware that the lower layers
are different from the original lower telephony signalling layers.
In that sense, the upper interface of the user adaptation layers
needs to be the same as the upper layer interface is to its original
lower layer. If a MTP user is being relayed over the IP network, the
related UAL used to transport the MTP user will have the same upper
interface as MTP has.
<span class="grey">Coene & Pastor-Balbas Informational [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc4166">RFC 4166</a> Telephony Signalling over SCTP AS February 2006</span>
The Stream Control Transmission Protocol was designed to fulfill the
stringent transport requirements that classical signalling protocols
have and is therefore the recommended transport protocol to use for
this purpose.
SCTP provides the following functions:
o Reliable Data Transfer
o Multiple streams to help avoid head-of-line blocking
o Ordered and unordered data delivery on a per-stream basis
o Bundling and fragmentation of user data
o Congestion and flow control
o Support for continuous monitoring of reachability
o Graceful termination of association
o Support of multi-homing for added reliability
o Protection against blind denial-of-service attacks
o Protection against blind masquerade attacks
SCTP is used as the transport protocol for telephony signalling
applications. Message boundaries are preserved during data transport
by SCTP, so each UAL can specify its own message structure within the
SCTP user data. The SCTP user data can be delivered by the order of
transmission within a stream (in sequence delivery) or unordered.
SCTP can be used to provide redundancy at the transport layer and
below. Telephony applications needing this level of redundancy can
make use of SCTP's multi-homing support.
SCTP can be used for telephony applications where head-of-line
blocking is a concern. Such an application should use multiple
streams to provide independent ordering of telephony signalling
messages.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Issues for Transporting Telephony Signalling over SCTP</span>
Transport of telephony signalling requires special considerations.
In order to use SCTP, an implementation must take special care to
meet the performance, timing, and failure management requirements.
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Congestion Control</span>
The basic mechanism of congestion control in SCTP has been described
in [<a href="./rfc2960" title=""Stream Control Transmission Protocol"">RFC2960</a>]. SCTP congestion control sometimes conflicts with the
timing requirements of telephony signalling application messages
which are transported by SCTP. During congestion, messages may be
delayed by SCTP, thus sometimes violating the timing requirements of
those telephony applications.
<span class="grey">Coene & Pastor-Balbas Informational [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc4166">RFC 4166</a> Telephony Signalling over SCTP AS February 2006</span>
In an engineered network (e.g., a private intranet), in which network
capacity and maximum traffic are very well controlled, some telephony
signalling applications may choose to relax the congestion control
rules of SCTP in order to satisfy the timing requirements. In order
to do this, they should employ their own congestion control
mechanisms. This must be done without destabilizing the network;
otherwise, it would lead to potential congestion collapse of the
network.
Some telephony signalling applications may have their own congestion
control and flow control techniques. These techniques may interact
with the congestion control procedures in SCTP.
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Detection of Failures</span>
Often, telephony systems must have no single point of failure in
operation.
The UAL must meet certain service availability and performance
requirements according to the classical signalling layers they are
replacing. Those requirements may be specific for each UAL.
For example, telephony systems are often required to be able to
preserve stable calls during a component failure. Therefore, error
situations at the transport layer and below must be detected quickly
so that the UAL can take appropriate steps to recover and preserve
the calls. This poses special requirements on SCTP to discover
unreachability of a destination address or a peer.
<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>. Retransmission TimeOut (RTO) Calculation</span>
The SCTP protocol parameter RTO.Min value has a direct impact on the
calculation of the RTO itself. Some telephony applications want to
lower the value of the RTO.Min to less than 1 second. This would
allow the message sender to reach the maximum
number-of-retransmission threshold faster in the case of network
failures. However, lowering RTO.Min may have a negative impact on
network behaviour [<a href="#ref-ALLMAN99" title=""On Estimating End-to-End Network Path Properties"">ALLMAN99</a>].
In some rare cases, telephony applications might not want to use the
exponential timer back-off concept in RTO calculation in order to
speed up failure detection. The danger of doing this is that, when
network congestion occurs, not backing off the timer may worsen the
congestion situation. Therefore, this strategy should never be used
on the public Internet.
It should be noted that not using delayed SACK will also increase the
speed of failure detection.
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<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>. Heartbeat</span>
For faster detection of (un)availability of idle paths, the telephony
application may consider lowering the SCTP parameter HB.interval. It
should be noted this might result in a higher traffic load.
<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>. Maximum Number of Retransmissions</span>
Setting Path.Max.Retrans and Association.Max.Retrans SCTP parameters
to lower values will speed up both destination address and peer
failure detection. However, if these values are set too low, the
probability of false fault detections might increase.
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. Shorten End-to-End Message Delay</span>
Telephony applications often require short end-to-end message delays.
The method described in <a href="#section-3.2.1">Section 3.2.1</a> for lowering RTO may be
considered. The different paths within a single association will
have a different RTO, so using the path with the lowest RTO will lead
to a shorter end-to-end message delay for the application running on
top of the UALs.
<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>. Bundling Considerations</span>
Bundling small telephony signalling messages at transmission helps
improve the bandwidth usage efficiency of the network. On the
downside, bundling may introduce additional delay to some of the
messages. This should be taken into consideration when end-to-end
delay is a concern.
<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>. Stream Usage</span>
Telephony signalling traffic is often composed of multiple,
independent message sequences. It is highly desirable to transfer
those independent message sequences in separate SCTP streams. This
reduces the probability of head-of-line blocking in which the
retransmission of a lost message affects the delivery of other
messages not belonging to the same message sequence.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. User Adaptation Layers</span>
Users Adaptation Layers (UALs) are defined to encapsulate different
signalling protocols for transport over SCTP/IP.
There are UALs for both access signalling (DSS1) and trunk signalling
(SS7). A brief description of the standardized UALs follows in the
next sub-sections.
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The delivery mechanism in several UALs supports:
o Seamless operation of UALs user peers over an IP network
connection.
o The interface boundary that the UAL user had with the traditional
lower layer.
o Management of SCTP transport associations and traffic between SGs
and ISEPs or two ISEPs
o Asynchronous reporting of status changes to management.
Signalling User Adaptation Layers have been developed for both Access
and Trunk Telephony Signalling. They are defined as follows.
Access Signalling: This is the signalling that is needed between an
access device and an exchange in the core network in order to
establish, manage, or release the voice or data call paths. Several
protocols have been developed for this purpose.
Trunk Signalling: This is the signalling that is used between the
exchanges inside the core network in order to establish, manage, or
release the voice or data call paths. The most common protocols used
for this purpose are known as the SS7 system, which belongs to the
Common Channel Signalling (CCS) philosophy. The SS7 protocol stack
is depicted below:
+------+-----+-------+- -+-------+------+-----+------+
| | | | | | MAP | CAP | INAP |
+ | + RANAP |...| BSSAP +-------------------+
| ISUP | TUP | | | | TCAP |
+ | +---------------------------------------+
| | | SCCP |
+----------------------------------------------------+
| MTP3 |
+----------------------------------------------------+
| MTP2 |
+----------------------------------------------------+
| MTP1 |
+----------------------------------------------------+
Figure 2: SS7 Protocol Stack
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The Telephony Signalling Protocols to be transported with the already
designed UALS are:
o ISDN Q.921 Users: Q.931
o V5.2/DSS1
o DPNSS/DASS2 [<a href="./rfc4129" title=""Digital Private Network Signaling System (DPNSS)/Digital Access Signaling System 2 (DASS 2) Extensions to the IUA Protocol"">RFC4129</a>]
o SS7 MTP3 Users: SCCP, ISUP, TUP
o SS7 MTP2 Users: MTP3
o SS7 SCCP Users: TCAP, RANAP, BSSAP, ...
Two main scenarios have been developed to use the different UALS for
IP Signalling Transport:
1. Intercommunication of traditional Signalling transport nodes and
IP based nodes.
Traditional Telephony
Telephony Signalling
********* Signalling ********** over IP ********
* SEP *----------------* SG *--------------* ISEP *
********* ********** ********
+-------+ +-------+
|SigProt| |SigProt|
+-------+ +----+----+ +-------+
| | | |UAL | | UAL |
| | | +----+ +-------+
| TTST | |TTST|SCTP| | SCTP |
| | | +----+ +-------+
| | | | IP | | IP |
+-------+ +---------+ +-------+
SEP - Signalling Endpoint
SG - Signalling Gateway
ISEP - IP Signalling Endpoint
SigProt - Signalling Protocol
TTSP - Traditional Telephony Signalling Protocol
UAL - User Adaptation Layer
SCTP - Stream Control Transport Protocol
Figure 3: General Architecture of SS7-IP Interworking
This is also referred to as SG-to-AS communication. AS is the name
that UAL usually gives to the ISEP nodes. It stands for Application
Server.
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2. Communication inside the IP network.
Telephony
Signalling
********* over IP *********
* ISEP *------------------* ISEP *
********* *********
+-------+ +-------+
|SigProt| |SigProt|
+-------+ +-------+
| UAL | | UAL |
+-------+ +-------+
| SCTP | | SCTP |
+-------+ +-------+
| IP | | IP |
+-------+ +-------+
Figure 4: General Architecture of Intra-IP Communication
This is also referred to as IPSP communication. IPSP stands for IP
Signalling Point and describes the role that the UAL plays on an
IP-based node.
The first scenario is applied for both types of signalling (access
and trunk signalling). On the other hand, the peer-to-peer basis can
only be used for trunk signalling.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Access Signalling</span>
The SIGTRAN WG has developed UALs to transport the following Access
Signalling protocols:
o ISDN Q.931
o V5.2
o DPNSS/DASS2
<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>. IUA (ISDN Q.921 User Adaptation)</span>
UAL: IUA (ISDN Q.921 User Adaptation)
This document supports both ISDN Primary Rate Access (PRA) as well as
Basic Rate Access (BRA) including the support for both point-to-point
and point-to-multipoint modes of communication. This support
includes Facility Associated Signalling (FAS), Non-Facility
Associated Signalling (NFAS), and NFAS with backup D channel.
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It implements the client/server architecture. The default
orientation is for the SG to take on the role of server while the
ISEP is the client. The SCTP (and UDP/TCP) Registered User Port
Number Assignment for IUA is 9900.
Examples of the upper layers to be transported are Q.931 and QSIG.
The main scenario supported by this UAL is the SG-to-ISP
communication where the ISEP role is typically played by a node
called an MGC, as defined in [<a href="./rfc2719" title=""Framework Architecture for Signaling Transport"">RFC2719</a>].
****** ISDN ****** IP *******
*PBX *---------------* SG *--------------* MGC *
****** ****** *******
+-----+ +-----+
|Q.931| (NIF) |Q.931|
+-----+ +----------+ +-----+
| | | | IUA| | IUA |
| | | +----+ +-----+
|Q.921| |Q.921|SCTP| |SCTP |
| | | +----+ +-----+
| | | | IP | | IP |
+-----+ +-----+----+ +-----+
NIF - Nodal Interworking Function
PBX - Private Branch Exchange
SCTP - Stream Control Transmission Protocol
IUA - ISDN User Adaptation Layer Protocol
Figure 5: ISDN-IP Interworking using IUA
The SCTP (and UDP/TCP) Registered User Port Number Assignment for IUA
is 9900.
The value assigned by IANA for the Payload Protocol Identifier in the
SCTP Payload Data chunk is "1".
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<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>. V5UA (V5.2-User Adaptation) Layer</span>
UAL: V5UA (V5.2-User Adaptation)
V5UA is an extension from the IUA layer with the modifications needed
to support the differences between Q.921/Q.931, and V5.2 layer
2/layer 3. It supports analog telephone access, ISDN basic rate
access and ISDN primary rate access over a V5.2 interface. It is
typically implemented in an interworking scenario with SG.
****** V5.2 ****** IP *******
* AN *---------------* SG *--------------* MGC *
****** ****** *******
+-----+ +-----+
|V5.2 | (NIF) |V5.2 |
+-----+ +----------+ +-----+
| | | |V5UA| |V5UA |
| | | +----+ +-----+
|LAPV5| |LAPV5|SCTP| |SCTP |
| | | +----+ +-----+
| | | | IP + | IP |
+-----+ +-----+----+ +-----+
AN - Access Network
NIF - Nodal Interworking Function
LAPV5 - Link Access Protocol for the V5 channel
SCTP - Stream Control Transmission Protocol
Figure 6: V5.2-IP Interworking using V5UA
The SCTP (and UDP/TCP) Registered User Port Number Assignment for
V5UA is 5675.
The value assigned by IANA for the Payload Protocol Identifier in the
SCTP Payload Data chunk is "6".
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<span class="h4"><a class="selflink" id="section-4.1.3" href="#section-4.1.3">4.1.3</a>. DUA (DPNSS/DASS User adaptation) Layer</span>
UAL: DUA (DPNSS/DASS2 User Adaptation)
The DUA is built on top of IUA and defines the necessary extensions
to IUA for a DPNSS/DASS2 transport. DPNSS stands for Digital Private
Network Signalling System and DASS2 for Digital Access Signalling
System 2.
****** DPNSS ****** IP *******
*PBX *---------------* SG *--------------* MGC *
****** ****** *******
+-----+ +-----+
|DPNSS| (NIF) |DPNSS|
| L3 | | L3 |
+-----+ +-----+----+ +-----+
| | | | DUA| | DUA |
|DPNSS| |DPNSS+----+ +-----+
| L2 | | L2 |SCTP| |SCTP |
| | | +----+ +-----+
| | | | IP + | IP |
+-----+ +-----+----+ +-----+
PBX - Private Branch eXchange
NIF - Nodal Interworking Function
SCTP - Stream Control Transmission Protocol
DUA - DPNSS User Adaptation Layer Protocol
Figure 7: DPNSS-IP Interworking using DUA
The value assigned by IANA for the Payload Protocol Identifier in the
SCTP Payload Data chunk is "10". .
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Network Signalling</span>
The SIGTRAN WG has developed UALs to transport the following SS7
protocols:
o MTP2 Users: MTP3
o MTP3 Users: ISUP, TUP, SCCP
o SCCP Users: TCAP, RNSAP, RANAP, BSSAP, ...
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<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>. MTP lvl3 over IP</span>
UALs:
o M2UA (SS7 MTP2 User Adaptation [<a href="./rfc3331" title=""Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) - User Adaptation Layer"">RFC3331</a>])
o M2PA (SS7 MTP2-User Peer-to-Peer Adaptation [<a href="./rfc4165" title=""SS7 MTP2-User Peer-to-Peer Adaptation Layer"">RFC4165</a>])
<span class="h5"><a class="selflink" id="section-4.2.1.1" href="#section-4.2.1.1">4.2.1.1</a>. M2UA (SS7 MTP2-User Adaptation) Layer</span>
M2UA protocol is typically used between a Signalling Gateway (SG) and
Media Gateway Controller (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 an MGC.
MTP3 and MTP3b are the only SS7 MTP2 User protocols that are
transported by this UAL.
The SG provides an interworking of transport functions with the IP
transport to transfer MTP2-User signalling messages with an
Application Server (e.g., MGC) where the peer MTP2-User exists.
****** SS7 ****** IP *******
*SEP *-----------* SG *-------------* MGC *
****** ****** *******
+----+ +----+
|S7UP| |S7UP|
+----+ +----+
|MTP3| |MTP3|
| | (NIF) | |
+----+ +----+----+ +----+
| | | |M2UA| |M2UA|
| | | +----+ +----+
|MTP2| |MTP2|SCTP| |SCTP|
| | | +----+ +----+
| | | |IP | |IP |
+----+ +---------+ +----+
MGC - Media Gateway Controller
SG - Signalling Gateway
SEP - SS7 Signalling Endpoint
NIF - Nodal Interworking Function
IP - Internet Protocol
SCTP - Stream Control Transmission Protocol
Figure 8: SS7-IP Interworking using M2UA
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The SCTP (and UDP/TCP) Registered User Port Number Assignment for
M2UA is 2904.
The value assigned by IANA for the Payload Protocol Identifier in the
SCTP Payload Data chunk is "2".
<span class="h5"><a class="selflink" id="section-4.2.1.2" href="#section-4.2.1.2">4.2.1.2</a>. M2PA (SS7 MTP2-User Peer-to-Peer Adaptation)</span>
M2PA protocol is used between SS7 Signalling Points employing the MTP
Level 3 protocol. The SS7 Signalling Points may also use standard
SS7 links using the SS7 MTP Level 2 to provide transport of MTP Level
3 signalling messages.
Both configurations: communication of SS7 and IP with SG and
communication between ISEPs are possible.
Connection of SS7 and IP nodes:
******** SS7 *************** IP ********
* SEP *--------* SG *--------* IPSP *
******** *************** ********
+------+ +------+
| TCAP | | TCAP |
+------+ +------+
| SCCP | | SCCP |
+------+ +-------------+ +------+
| MTP3 | | MTP3 | | MTP3 |
+------+ +------+------+ +------+
| | | | M2PA | | M2PA |
| | | +------+ +------+
| MTP2 | | MTP2 | SCTP | | SCTP |
| | | +------+ +------+
| | | | IP | | IP |
+------+ +------+------+ +------+
SEP - SS7 Signalling Endpoint
Figure 9: SS7-IP Interworking with M2PA
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Communication between two IP nodes:
******** IP ********
* IPSP *--------* IPSP *
******** ********
+------+ +------+
| TCAP | | TCAP |
+------+ +------+
| SCCP | | SCCP |
+------+ +------+
| MTP3 | | MTP3 |
+------+ +------+
| M2PA | | M2PA |
+------+ +------+
| SCTP | | SCTP |
+------+ +------+
| IP | | IP |
+------+ +------+
IP - Internet Protocol
IPSP - IP Signalling Point
SCTP - Stream Control Transmission Protocol
Figure 10: Intra-IP Communication using M2PA
These figures are only an example. Other configurations are
possible. For example, IPSPs without traditional SS7 links could use
the protocol layers MTP3/M2PA/SCTP/IP to route SS7 messages in a
network with all IP links.
Another example is that two SGs could be connected over an IP network
to form an SG mated pair, similar to the way STPs are provisioned in
traditional SS7 networks.
The SCTP (and UDP/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 "5".
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<span class="h5"><a class="selflink" id="section-4.2.1.3" href="#section-4.2.1.3">4.2.1.3</a>. Main Differences between M2PA and M2UA</span>
o M2PA: IPSP processes MTP3/MTP2 primitives.
o M2UA: MGC transports MTP3/MTP2 primitives between the SG's MTP2
and the MGC's MTP3 (via the NIF) for processing.
o M2PA: SG-IPSP connection is an SS7 link.
o M2UA: SG-MGC connection is not an SS7 link. It is an extension of
MTP to a remote entity.
<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>. M3UA (SS7 MTP3 User Adaptation) Layer</span>
UAL: M3UA (SS7 MTP3 User Adaptation)
M3UA protocol supports the transport of any SS7 MTP3-User signalling
such as TUP, ISUP, and SCCP over IP using the services of SCTP.
Interconnection of SS7 and IP nodes:
******** SS7 ***************** IP ********
* SEP *---------* SGP *--------* ASP *
******** ***************** ********
+------+ +---------------+ +------+
| ISUP | | (NIF) | | ISUP |
+------+ +------+ +------+ +------+
| MTP3 | | MTP3 | | M3UA | | M3UA |
+------| +------+-+------+ +------+
| MTP2 | | MTP2 | | SCTP | | SCTP |
+------+ +------+ +------+ +------+
| L1 | | L1 | | IP | | IP |
+------+ +------+ +------+ +------+
SEP - SS7 Signalling End Point
SCTP - Stream Control Transmission Protocol
NIF - Nodal Interworking Function
Figure 11: SS7-IP Interworking using M3UA
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Communication between two IP nodes:
******** IP ********
* IPSP *----------* IPSP *
******** ********
+------+ +------+
|SCCP- | |SCCP- |
| User | | User |
+------+ +------+
| SCCP | | SCCP |
+------+ +------+
| M3UA | | M3UA |
+------+ +------+
| SCTP | | SCTP |
+------+ +------+
| IP | | IP |
+------+ +------+
Figure 12: Intra-IP Communication using M3UA
M3UA uses a client-server architecture. It is recommended that the
ISEP acts as the client and initiate the SCTP associations with the
SG. The port reserved by IANA is 2905. This is the port upon which
the SG should listen for possible client connections.
The assigned payload protocol identifier for the SCTP DATA chunks is
"3".
<span class="h4"><a class="selflink" id="section-4.2.3" href="#section-4.2.3">4.2.3</a>. SUA (SS7 SCCP User Adaptation) Layer</span>
UAL: SUA (SS7 SCCP User Adaptation)
SUA protocol supports the transport of any SS7 SCCP-User signalling
such as MAP, INAP, SMS, BSSAP, or RANAP over IP using the services of
SCTP. Each of the applications using SUA has its own set of timing
requirements that can be found in its respective standards documents.
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Possible configurations are showed in the pictures below.
- Interconnection of SS7 and IP:
******** *************** ********
* SEP * SS7 * * IP * *
* or *---------* SG *--------* ASP *
* STP * * * * *
******** *************** ********
+------ +------+
| SUAP | | SUAP |
+------+ +------+------+ +------+
| SCCP | | SCCP | SUA | | SUA |
+------+ +------+------+ +------+
| | | | | | |
| MTP3 | | MTP3 | SCTP | | SCTP |
| | | | | | |
+------+ +------+------+ +------+
| MTP2 | | MTP2 | IP | | IP |
+------+ +------+------+ +------+
SUAP - SCCP/SUA User Protocol (TCAP, for example)
STP - SS7 Signalling Transfer Point
Figure 13: SS7-IP Interworking using SUA
- IP Node to IP Node communication:
******** ********
* * IP * *
* IPSP *--------* IPSP *
* * * *
******** ********
+------+ +------+
| SUAP | | SUAP |
+------+ +------+
| SUA | | SUA |
+------+ +------+
| SCTP | | SCTP |
+------+ +------+
| IP | | IP |
+------+ +------+
Figure 14: Intra-IP Communication using SUA
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IANA has registered SCTP Port Number 14001 for SUA. It is
recommended that SGs use this SCTP port number for listening for new
connections. The payload protocol identifier for the SCTP DATA
chunks is "4".
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Security Considerations</span>
UALs are designated to carry signalling messages for telephony
services. As such, UALs 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. Although some security needs overlap, any security
solution should fulfill all the different parties' needs. See
specific Security Considerations in each UAL Technical specification
for details (for general security principles of SIGTRAN, see
[<a href="./rfc3788" title=""Security Considerations for Signaling Transport (SIGTRAN) Protocols"">RFC3788</a>]).
SCTP only tries to increase the availability of a network. SCTP does
not contain any protocol mechanisms directly related to communication
security, i.e., user message authentication, integrity, or
confidentiality functions. For such features, SCTP depends on
security protocols. In the field of system security, SCTP includes
mechanisms for reducing the risk of blind denial-of-service attacks
as described in <a href="./rfc2960#section-11">Section 11 of [RFC2960]</a>.
This document does not add any new components to the protocols
included in the discussion. For secure use of the SIGTRAN protocols,
readers should go through the "Security Considerations for SIGTRAN
Protocols" [<a href="./rfc3788" title=""Security Considerations for Signaling Transport (SIGTRAN) Protocols"">RFC3788</a>]). According to that document, the use of the
IPsec is the main requirement to secure SIGTRAN protocols in the
Internet, but Transport Layer Security (TLS) is also considered a
perfectly valid option for use in certain scenarios (see [<a href="./rfc3436" title=""Transport Layer Security over Stream Control Transmission Protocol"">RFC3436</a>]
for more information on using TLS with SCTP). Recommendations of
usage are also included.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Informative References</span>
[<a id="ref-ALLMAN99">ALLMAN99</a>] Allman, M. and V. Paxson, "On Estimating End-to-End
Network Path Properties", Proc. SIGCOMM'99, 1999.
[<a id="ref-RFC2960">RFC2960</a>] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", <a href="./rfc2960">RFC 2960</a>, October 2000.
[<a id="ref-RFC3257">RFC3257</a>] Coene, L., "Stream Control Transmission Protocol
Applicability Statement", <a href="./rfc3257">RFC 3257</a>, April 2002.
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<span class="grey"><a href="./rfc4166">RFC 4166</a> Telephony Signalling over SCTP AS February 2006</span>
[<a id="ref-RFC2719">RFC2719</a>] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene,
L., Lin, H., Juhasz, I., Holdrege, M., and C. Sharp,
"Framework Architecture for Signaling Transport", <a href="./rfc2719">RFC</a>
<a href="./rfc2719">2719</a>, October 1999.
[<a id="ref-RFC3057">RFC3057</a>] Morneault, K., Rengasami, S., Kalla, M., and G.
Sidebottom, "ISDN Q.921-User Adaptation Layer", <a href="./rfc3057">RFC 3057</a>,
February 2001.
[<a id="ref-RFC3331">RFC3331</a>] Morneault, K., Dantu, R., Sidebottom, G., Bidulock, B.,
and J. Heitz, "Signaling System 7 (SS7) Message Transfer
Part 2 (MTP2) - User Adaptation Layer", <a href="./rfc3331">RFC 3331</a>,
September 2002.
[<a id="ref-RFC3332">RFC3332</a>] Sidebottom, G., Morneault, K., and J. Pastor-Balbas,
"Signaling System 7 (SS7) Message Transfer Part 3 (MTP3)
- User Adaptation Layer (M3UA)", <a href="./rfc3332">RFC 3332</a>, September
2002.
[<a id="ref-RFC3436">RFC3436</a>] Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
Layer Security over Stream Control Transmission
Protocol", <a href="./rfc3436">RFC 3436</a>, December 2002.
[<a id="ref-RFC3868">RFC3868</a>] Loughney, J., Sidebottom, G., Coene, L., Verwimp, G.,
Keller, J., and B. Bidulock, "Signalling Connection
Control Part User Adaptation Layer (SUA)", <a href="./rfc3868">RFC 3868</a>,
October 2004.
[<a id="ref-RFC4165">RFC4165</a>] George, T., Dantu, R., Kalla, M., Schwarzbauer, H.J.,
Sidebottom, G., Morneault, K.,"SS7 MTP2-User Peer-to-Peer
Adaptation Layer", <a href="./rfc4165">RFC 4165</a>, September 2005.
[<a id="ref-RFC3807">RFC3807</a>] Weilandt, E., Khanchandani, N., and S. Rao, "V5.2-User
Adaptation Layer (V5UA)", <a href="./rfc3807">RFC 3807</a>, June 2004.
[<a id="ref-RFC4129">RFC4129</a>] Mukundan, R., Morneault, K., and N. Mangalpally, "Digital
Private Network Signaling System (DPNSS)/Digital Access
Signaling System 2 (DASS 2) Extensions to the IUA
Protocol", <a href="./rfc4129">RFC 4129</a>, September 2005.
[<a id="ref-RFC3788">RFC3788</a>] Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security
Considerations for Signaling Transport (SIGTRAN)
Protocols", <a href="./rfc3788">RFC 3788</a>, June 2004.
<span class="grey">Coene & Pastor-Balbas Informational [Page 21]</span></pre>
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Authors' Addresses
Lode Coene
Siemens
Atealaan 34
Herentals B-2200
Belgium
Phone: +32-14-252081
EMail: lode.coene@siemens.com
Javier Pastor-Balbas
Ericsson
Via de los Poblados 13
Madrid 28033
Spain
Phone: +34 91 339 1397
EMail: J.Javier.Pastor@ericsson.com
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<span class="grey"><a href="./rfc4166">RFC 4166</a> Telephony Signalling over SCTP AS February 2006</span>
Full Copyright Statement
Copyright (C) The Internet Society (2006).
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contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
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The IETF invites any interested party to bring to its attention any
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Acknowledgement
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Coene & Pastor-Balbas Informational [Page 23]
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