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rfc:rfc4166

Network Working Group L. Coene Request for Comments: 4166 Siemens Category: Informational J. Pastor-Balbas

                                                              Ericsson
                                                         February 2006
                Telephony Signalling Transport over
Stream Control Transmission Protocol (SCTP) Applicability Statement

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.

Coene & Pastor-Balbas Informational [Page 1] RFC 4166 Telephony Signalling over SCTP AS February 2006

Table of Contents

 1. Introduction ....................................................2
    1.1. Scope ......................................................2
    1.2. Terminology ................................................3
    1.3. Contributors ...............................................3
 2. SIGTRAN Architecture ............................................3
 3. Issues for Transporting Telephony Signalling over SCTP ..........5
    3.1. Congestion Control .........................................5
    3.2. Detection of Failures ......................................6
         3.2.1. Retransmission TimeOut (RTO) Calculation ............6
         3.2.2. Heartbeat ...........................................7
         3.2.3. Maximum Number of Retransmissions ...................7
    3.3. Shorten End-to-End Message Delay ...........................7
    3.4. Bundling Considerations ....................................7
    3.5. Stream Usage ...............................................7
 4. User Adaptation Layers ..........................................7
    4.1. Access Signalling .........................................10
         4.1.1. IUA (ISDN Q.921 User Adaptation) ...................10
         4.1.2. V5UA (V5.2-User Adaptation) Layer ..................12
         4.1.3. DUA (DPNSS/DASS User adaptation) Layer .............13
    4.2. Network Signalling ........................................13
         4.2.1. MTP lvl3 over IP ...................................14
         4.2.2. M3UA (SS7 MTP3 User Adaptation) Layer ..............17
         4.2.3. SUA (SS7 SCCP User Adaptation) Layer ...............18
 5. Security Considerations ........................................20
 6. Informative References .........................................20

1. Introduction

 This document is intended to describe how to transport telephony
 signalling protocols, used in classic telephony systems, over IP
 networks.  As described in [RFC2719], 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 [RFC3257].  The set of
 UALs has also been introduced to make it possible for different
 signalling protocols to use the SCTP layer.

1.1. Scope

 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.

Coene & Pastor-Balbas Informational [Page 2] RFC 4166 Telephony Signalling over SCTP AS February 2006

1.2. Terminology

 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.

1.3. Contributors

 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.

2. SIGTRAN Architecture

 The SIGTRAN architecture describes the transport of signalling
 information over IP infrastructure.

Coene & Pastor-Balbas Informational [Page 3] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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  [RFC3332] SS7 MTP3 users: SCCP, ISUP, TUP...
 o  [RFC3331] SS7 MTP2 users: MTP3
 o  [RFC3868] SS7 SCCP users: RANAP, MAP(+TCAP), INAP(+TCAP)...
 o  [RFC3057] ISDN Q.921 users: Q.931
 o  [RFC3807] 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.

Coene & Pastor-Balbas Informational [Page 4] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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.

3. Issues for Transporting Telephony Signalling over SCTP

 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.

3.1. Congestion Control

 The basic mechanism of congestion control in SCTP has been described
 in [RFC2960].  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.

Coene & Pastor-Balbas Informational [Page 5] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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.

3.2. Detection of Failures

 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.

3.2.1. Retransmission TimeOut (RTO) Calculation

 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 [ALLMAN99].
 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.

Coene & Pastor-Balbas Informational [Page 6] RFC 4166 Telephony Signalling over SCTP AS February 2006

3.2.2. Heartbeat

 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.

3.2.3. Maximum Number of Retransmissions

 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.

3.3. Shorten End-to-End Message Delay

 Telephony applications often require short end-to-end message delays.
 The method described in Section 3.2.1 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.

3.4. Bundling Considerations

 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.

3.5. Stream Usage

 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.

4. User Adaptation Layers

 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.

Coene & Pastor-Balbas Informational [Page 7] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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

Coene & Pastor-Balbas Informational [Page 8] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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 [RFC4129]
 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.

Coene & Pastor-Balbas Informational [Page 9] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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.

4.1. Access Signalling

 The SIGTRAN WG has developed UALs to transport the following Access
 Signalling protocols:
 o  ISDN Q.931
 o  V5.2
 o  DPNSS/DASS2

4.1.1. IUA (ISDN Q.921 User Adaptation)

 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.

Coene & Pastor-Balbas Informational [Page 10] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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 [RFC2719].
  • * 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".

Coene & Pastor-Balbas Informational [Page 11] RFC 4166 Telephony Signalling over SCTP AS February 2006

4.1.2. V5UA (V5.2-User Adaptation) Layer

 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".

Coene & Pastor-Balbas Informational [Page 12] RFC 4166 Telephony Signalling over SCTP AS February 2006

4.1.3. DUA (DPNSS/DASS User adaptation) Layer

 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".  .

4.2. Network Signalling

 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, ...

Coene & Pastor-Balbas Informational [Page 13] RFC 4166 Telephony Signalling over SCTP AS February 2006

4.2.1. MTP lvl3 over IP

 UALs:
 o  M2UA (SS7 MTP2 User Adaptation [RFC3331])
 o  M2PA (SS7 MTP2-User Peer-to-Peer Adaptation [RFC4165])

4.2.1.1. M2UA (SS7 MTP2-User Adaptation) Layer

 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

Coene & Pastor-Balbas Informational [Page 14] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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".

4.2.1.2. M2PA (SS7 MTP2-User Peer-to-Peer Adaptation)

 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

Coene & Pastor-Balbas Informational [Page 15] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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". Coene & Pastor-Balbas Informational [Page 16] RFC 4166 Telephony Signalling over SCTP AS February 2006 4.2.1.3. Main Differences between M2PA and M2UA 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. 4.2.2. M3UA (SS7 MTP3 User Adaptation) Layer 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 Coene & Pastor-Balbas Informational [Page 17] RFC 4166 Telephony Signalling over SCTP AS February 2006 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". 4.2.3. SUA (SS7 SCCP User Adaptation) Layer 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. Coene & Pastor-Balbas Informational [Page 18] RFC 4166 Telephony Signalling over SCTP AS February 2006 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
  1. IP Node to IP Node communication:
  • * * * IP * * * IPSP *——–* IPSP * * * * * **
                           +------+        +------+
                           | SUAP |        | SUAP |
                           +------+        +------+
                           | SUA  |        | SUA  |
                           +------+        +------+
                           | SCTP |        | SCTP |
                           +------+        +------+
                           |  IP  |        |  IP  |
                           +------+        +------+
              Figure 14: Intra-IP Communication using SUA

Coene & Pastor-Balbas Informational [Page 19] RFC 4166 Telephony Signalling over SCTP AS February 2006

 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".

5. Security Considerations

 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
 [RFC3788]).
 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 Section 11 of [RFC2960].
 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" [RFC3788]).  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 [RFC3436]
 for more information on using TLS with SCTP).  Recommendations of
 usage are also included.

6. Informative References

 [ALLMAN99]  Allman, M. and V. Paxson, "On Estimating End-to-End
             Network Path Properties", Proc. SIGCOMM'99, 1999.
 [RFC2960]   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", RFC 2960, October 2000.
 [RFC3257]   Coene, L., "Stream Control Transmission Protocol
             Applicability Statement", RFC 3257, April 2002.

Coene & Pastor-Balbas Informational [Page 20] RFC 4166 Telephony Signalling over SCTP AS February 2006

 [RFC2719]   Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene,
             L., Lin, H., Juhasz, I., Holdrege, M., and C. Sharp,
             "Framework Architecture for Signaling Transport", RFC
             2719, October 1999.
 [RFC3057]   Morneault, K., Rengasami, S., Kalla, M., and G.
             Sidebottom, "ISDN Q.921-User Adaptation Layer", RFC 3057,
             February 2001.
 [RFC3331]   Morneault, K., Dantu, R., Sidebottom, G., Bidulock, B.,
             and J. Heitz, "Signaling System 7 (SS7) Message Transfer
             Part 2 (MTP2) - User Adaptation Layer", RFC 3331,
             September 2002.
 [RFC3332]   Sidebottom, G., Morneault, K., and J. Pastor-Balbas,
             "Signaling System 7 (SS7) Message Transfer Part 3 (MTP3)
             - User Adaptation Layer (M3UA)", RFC 3332, September
             2002.
 [RFC3436]   Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
             Layer Security over Stream Control Transmission
             Protocol", RFC 3436, December 2002.
 [RFC3868]   Loughney, J., Sidebottom, G., Coene, L., Verwimp, G.,
             Keller, J., and B. Bidulock, "Signalling Connection
             Control Part User Adaptation Layer (SUA)", RFC 3868,
             October 2004.
 [RFC4165]   George, T., Dantu, R., Kalla, M., Schwarzbauer, H.J.,
             Sidebottom, G., Morneault, K.,"SS7 MTP2-User Peer-to-Peer
             Adaptation Layer", RFC 4165, September 2005.
 [RFC3807]   Weilandt, E., Khanchandani, N., and S. Rao, "V5.2-User
             Adaptation Layer (V5UA)", RFC 3807, June 2004.
 [RFC4129]   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", RFC 4129, September 2005.
 [RFC3788]   Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security
             Considerations for Signaling Transport (SIGTRAN)
             Protocols", RFC 3788, June 2004.

Coene & Pastor-Balbas Informational [Page 21] RFC 4166 Telephony Signalling over SCTP AS February 2006

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

Coene & Pastor-Balbas Informational [Page 22] RFC 4166 Telephony Signalling over SCTP AS February 2006

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Coene & Pastor-Balbas Informational [Page 23]

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