INTERNET-DRAFT                                             L. Coene(Ed)
Internet Engineering Task Force                                 Siemens
Issued:  April  November 2002                                        J. Pastor
Expires: September 2002 May 2003                                              Ericsson

  Telephony Signalling Transport over SCTP applicability statement

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026. Internet-Drafts are working
    documents of the Internet Engineering Task Force (IETF), its areas,
    and its working groups.  Note that other groups may also distribute
    working documents as Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet-Drafts as
    reference material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at

    The list of Internet-Draft Shadow Directories can be accessed at


    This document describes the applicability of the new protocols
    developed under the signaling signalling transport framework[RFC2719]. A
    description of the main issues regarding the use of the Stream
    Control Transmission Protocol (SCTP)[RFC2960] and each adaptation
    layer for transport of telephony signalling information over IP
    infrastructure is explained.

Draft                Telephony Signalling AS                 April                October 2002

                            Table of contents

   Telephony signalling over SCTP Applicability statement .........   ii
   Chapter 1: Introduction ........................................    2
   Chapter 1.1: Scope ..... .......................................    3
   Chapter 1.2: Terminology .......................................    3
   Chapter 1.3: Contributors ......................................    3
   Chapter 2: SIGTRAN architecture ................................    4
   Chapter 2.1: Overview .........................................     4
   Chapter 3: Issues for transporting Telephony signalling
   information over SCTP ..........................................    6
   Chapter 3.1: Congestion control ................................    6
   Chapter 3.2: Detection of failures .............................    6
   Chapter 3.2.1: Retransmission TimeOut (RTO) calculation ........    7
   Chapter 3.2.2: Heartbeat .......................................    7
   Chapter 3.2.3: Maximum Number of retransmissions ...............    7
   Chapter 3.3:  Shorten end-to-end message delay .................    7
   Chapter 3.4: Bundling considerations ...........................    8
   Chapter 3.5: Stream Usage ......................................    8
   Chapter 4: User Adaptation Layers...............................    8
   Chapter 4.1: Access Signalling..................................   11
   Chapter 4.1.1: IUA (ISDN Q.921 User Adaptation) ..................   10 ................   11
   Chapter 4.2: 4.1.2: V5UA (V5.2-User Adaptation) Layer .................   11 ...............   12
   Chapter 4.3: 4.1.3: DUA (DPNSS/DASS User adaptation) Layer ............   12 ..........   13
   Chapter 4.2: Network Signalling ................................   13
   Chapter 4.2.1: MTP lvl3 over IP ................................   14
   Chapter 4.4: M2UA (SS7 MTP2 User MTP2-User Adaptation) Layer .............   12 .........   14
   Chapter 4.5: M2PA (SS7 MTP2-User Peer-to-Peer Adaptation) Layer.   13 ..   15
   Chapter Main difference between M2PA and M2UA .........   16
   Chapter 4.6: 4.2.2: M3UA (SS7 MTP3 User Adaptation) Layer .............   15 ...........   17
   Chapter 4.7: 4.2.3: SUA (SS7 SCCP User Adaptation) Layer ..............   16 ............   18
   Chapter 5: Security considerations .............................   18   20
   Chapter 6: References and related work .........................   18   20
   Chapter 7: Acknowledgments .....................................   19   21
   Chapter 8: Author's address ....................................   19   22


Draft                Telephony Signalling AS                October 2002

    This document intends to inform how to transport telephony
    signalling protocols, used in classic telephony systems, over IP
    networks. The whole architecture is called SIGTRAN (Signalling

Draft                 Telephony Signalling AS                 April 2002
    Transport) as described in RFC2719 and is composed of a transport
    protocol(SCTP) and several User Adaptation (UAL) layers. layers(UAL). The
    transport protocol SCTP has been been developed to fulfill the
    stringent requirements that telephony signalling networks have. The
    set of User Adaptation layers have also been introduced to make it
    possible that different signalling protocols can use the SCTP layer.

1.1 Scope

    The scope of this document is to explain the way that user
    adaptation layers and SCTP protocols have to be used to transport
    Telephony signalling information over IP.

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

    SPU: Signalling protocol user, the application on top of the User
    adaptation layer.

    CTSP: Classical Telephony Signalling protocol(examples: MTP level2,
    MTP level 3, SCCP....).

    UAL: User adaptation layer: the protocol that encapsulate the upper
    layer telephony signalling protocols that are to be transported over

    ISEP: IP signalling endpoint: a IP node that implements SCTP and a
    User adapatation layer.

    SP: signalling point

1.3 Contributors

Draft                Telephony Signalling AS                October 2002

    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.

Draft                 Telephony Signalling AS                 April 2002

2 SIGTRAN architecture

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

    Telephony Signalling transport over IP normally uses the following

                 Telephony Signalling Application
             |   Signalling Adaptation Layers     |
             |Stream Control Transmission Protocol|
             |             (SCTP)                 |
               Internet Protocol (IPv4/IPv6)

    Figure 1.1: Telephony signalling transport protocol stack

    The components of the protocol stack are :

    (1) Adaptation modules are used when the telephony application needs
    to preserve an existing primitive interface. (e.g. management
    indications, 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:

    - SS7 MTP3 users: SCCP, ISUP, TUP...

    - SS7 MTP2 users: MTP3

    - SS7 SCCP users: RANAP, MAP(+TCAP), INAP(+TCAP)...

Draft                Telephony Signalling AS                October 2002

    - ISDN Q.921 users: Q.931

    - V5.2/DSS1

    - ....

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    Every classic telephony protocol can have a corresponding UAL

    The user adaptation layers(UALs) are a set of protocols that
    encapsulate a specific signalling protocol to be transported over
    SCTP. The adapation is done in a way that the upper signalling
    protocols that are relayed remain unaware that the lower layers are
    different to the originail lower telephony signalling layers. In
    that sense, the upper interface of the user adapatation layers need
    to be the same as the upper layer interface 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.

    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.

    The following functions are provided by SCTP:

    - Reliable Data Transfer

    - Multiple streams to help avoid head-of-line blocking

    - Ordered and unordered data delivery on a per-stream basis

    - Bundling and fragmentation of user data

    - Congestion and flow control

    - Support continuous monitoring of reachability

    - Graceful termination of association

    - Support of multi-homing for added reliability

    - Protection against blind denial-of-service attacks

    - 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 and so each UA UAL can specify its own message
    structure withing the SCTP user data. The SCTP user data can be
    delivered by the order of transmission within a stream(in sequence
    delivery) or unordered.

Draft                Telephony Signalling AS                October 2002

    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.

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    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

3 Issues for transporting telephony signalling over SCTP

    Transport of telephony signalling requires special
    considerations. In order to use SCTP, special care must be taken to
    meet the performance, timing and failure management requirements.

3.1 Congestion Control

    The basic mechanism of congestion control in SCTP have 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.

    In an engineered network (e.g. a private intranet), in which network
    capacity and maximum traffic are very well understood, 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. But this should be done without
    destabilising the network, otherwise this 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

    Telephony systems often must have no single point of failure in

    The UA UAL must meet certain service availability and performance
    requirements according to the classical signalling layers they are

Draft                Telephony Signalling AS                October 2002

    replacing. Those requirements may be specific for each UA. 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

Draft                 Telephony Signalling AS                 April 2002
    so that the UA UAL can take approriate steps to recover and preserve the
    calls. This poses special requirements on SCTP to discover
    unreachablility 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
    in public Internet.

    It should be noted that not using delayed SACK will also help faster
    failure detection.

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

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.

Draft                Telephony Signalling AS                October 2002

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 on lowering RTO may

Draft                 Telephony Signalling AS                 April 2002
    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 UA's. UAL's.

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 have been (UALs) are defined to encapsulate different
    signalling protocols in order to transport them over SCTP/IP. 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.

    The delivery mechanism in the several UALs

       - Supports seamless operation of UALs user peers over an IP
          network connection.

       - Supports the interface boundary that the UAL user had with the
          traditional lower layer.

Draft                Telephony Signalling AS                October 2002

       - Supports management of SCTP transport associations and traffic
          between SGs and ISEPs or two ISEPs

       - Supports asynchronous reporting of status changes to management.

Draft                 Telephony

    Signalling AS                 April 2002

    Two main scenarios User Adaptation Layers have been developed for Signalling Transport:

    - Intercommunication of traditional Signalling transport nodes both:
    Access and IP
    based nodes.

                       Traditional               Telephony Trunk Telephony Signalling. They are defined as follows.

    Access Signalling: This is the signalling that is needed between and
      access device and an exchange in the core network in order to
      establish, manage or release the voice or data call paths. There
      are several protocols that 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 that belongs to
      the Common Channel Signalling
              *******   Signalling    **********   over IP    ********
              * SEP *----------------*   SG   *--------------* ISEP *
              *******                **********              ********

              +-----+                                        +------+
              | SPU |                                        |  SPU (CCS) philosophy. The SS7 protocol
      stack is depicted below:

           +------+-----+-------+- -+-------+------+-----+------+
              +-----+      |     |       |   |       |  MAP | CAP | INAP |
           +      |     + RANAP |...| BSSAP +-------------------+
           | ISUP | TUP |       |   |       |       TCAP        |
           +      |     +---------------------------------------+
           |      |     |                  SCCP                 |
           |                          MTP3                      |
           |                          MTP2                      |
           |                          MTP1                      |

    The Telephony Signalling Protocols to be transported with the already
    designed UALS are:

       - ISDN Q.921 Users: Q.931
       - V5.2/DSS1
       - DPNSS/DASS2
       - SS7 MTP3 Users: SCCP, ISUP, TUP
       - SS7 MTP2 Users: MTP3
       - SS7 SCCP Users: TCAP, RANAP, BSSAP, ...

    Two main scenarios have been developed to use the different UALS for
    IP Signalling Transport:

Draft                Telephony Signalling AS                October 2002

    (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  |
          |       |                |    +----+             +------+
              |CTSP             +-------+
          | TTST  |                |CTSP|SCTP|                |TTST|SCTP|             | SCTP  |
          |       |                |    +----+             +------+             +-------+
          |       |                |    | IP |             |  IP   |
          +-------+                +---------+             +------+

                  SEP:             +-------+

                SEP     -   Signalling Endpoint
                SG      -   Signalling Gateway
                ISEP    -   IP Signalling Endpoint
                SigProt -   Signalling Protocol User
                  CTSP: Classical
                TTST    -   Traditional Telephony Signalling Protocol
                  UAL: Transport
                UAL     -   User Adaptation Layer
                SCTP    -   Stream Control Transport Protocol

       It is also referred as SG to AS communication. AS is I the name that
       UAL usually gives to the ISEP nodes. It stands for Application

Draft                 Telephony Signalling AS                 April 2002


    (2) Communication inside the IP networks. network.

                      *********     over IP      ********      *********
                      * ISEP  *------------------*  ISEP *
                          ********                  ********

                          +------+                  +------+
                          | SPU  |                  | SPU  |
                          +------+                  +------+
                      *********                  *********

                      +-------+                  +-------+
                      |SigProt|                  |SigProt|
                      +-------+                  +-------+
                      |  UAL  |                  |  UAL  |
                          +------+                  +------+
                      +-------+                  +-------+
                      | SCTP  |                  | SCTP  |
                          +------+                  +------+
                      +-------+                  +-------+
                      |  IP   |                  |  IP   |
                          +------+                  +------+
                      +-------+                  +-------+

       This is also referred to as IPSP communication. IPSP is the name
    given to

Draft                Telephony Signalling AS                October 2002

       of the role that a an UAL plays on an IP-based node. It stands for
       IP Signalling Point.

    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 have developed UALs to transport the following Access
    Signalling protocols:

    - ISDN Q.931
    - V5.2

 4.1.1  ISDN Q.931 over IP

    UAL: IUA (ISDN Q.921 User Adaptation)

    This protocol 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.

    It implements the client/server architecture. The default orientation is
    would be 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 would be Q.931 and

    The main scenario supported by this UAL is the SG to ISEP
    communication where the ISEP role is typically played by a node
    called an MGC, as MGC defined in [RFC2719].

Draft                 Telephony Signalling AS                 April 2002

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

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

Draft                Telephony Signalling AS                October 2002

                NIF  - Nodal Interworking Function
                PBX  - ISDN End Point Private Branch Exchange
                SCTP - Stream Control Transmission Protocol
                IUA  - ISDN User Adaptation Layer Protocol

    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".

4.2 1

 4.1.2  V5UA over IP

    UAL: V5UA (V5.2-User Adaptation) Layer

    It 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 basically
    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

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            LAPV5 -  Link Access Protocol for the V5 channel
            SCTP  - Stream Control Transmission Protocol

    The SCTP (and UDP/TCP) Registered User Port Number Assignment for
    V5UA is 5675.

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    The value assigned by IANA for the Payload Protocol Identifier in the
    SCTP Payload Data chunk is "6".


 4.1.3  DPNSS/DASS2 over IP

    UAL: DUA (DPNSS/DASS 2 (DPNSS/DASS2 User Adaptation) Layer

    The DUA is built on top of IUA and defines defining 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 No 2. 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

    The value assigned by IANA for the Payload Protocol Identifier in the
    SCTP Payload Data chunk is "TBD".


 4.2  Network Signalling

    The SIGTRAN WG have developed UALs to transport the following SS7

    - MTP2 Users: MTP3
    - MTP3 Users: ISUP, TUP, SCCP
    - SCCP Users: TCAP, RNSAP, RANAP, BSSAP, ...

 4.2.1     MTP lvl3 over IP

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    - M2UA (SS7 MTP2 User Adaptation) Layer

    - M2PA (SS7 MTP2-User Peer-to-Peer Adaptation)   M2UA (SS7 MTP2 User Adaptation)

    M2UA protocol is typically mainly used between a Signalling Gateway (SG) and
    Media Gateway Controler 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 a

Draft                 Telephony Signalling AS                 April 2002

    MTP3 and MTP3b are the

    The only SS7 MTP2 Users User is MTP3 that are is the protocol transported by
    this UAL.

    The SG provides a interworking of transport functions with the IP
    transport, in order to transfer the MTP2-User signalling messages with to
    and from an Application Server (e.g. MGC) where the peer MTP2-
    User at an application server(e.g. MGC). protocol layer exists.

               ******    SS7    ******      IP     *******
               *SEP *-----------* SG *-------------* MGC *
               ******           ******             *******

               +----+                              +----+
               |S7UP|                              |S7UP|
               +----+                              +----+
        |MTP +                              |MTP
               |MTP3|                              |MTP3|
               |    | L3 |            (NIF)             |L3             |    |
               +----+         +----+----+          +----+
               |         |MTP    |         |    |M2UA|          |M2UA|
               |    |         |    +----+          +----+
        | L2 |         | L2 |SCTP|
               |MTP2|         |MTP2|SCTP|          |SCTP|
               | L1    |         | L1    +----+          +----+
               |    |         |    |IP  |          |IP  |
               +----+         +---------+          +----+

               MGC  - Media Gateway Controler
               SG   - Signalling Gateway
               SEP  - SS7 Signalling Endpoint
               NIF  - Nodal Interworking Function
               IP   - Internet Protocol
               SCTP - Stream Control Transmission Protocol

    The SCTP (and UDP/TCP) Registered User Port Number Assignment for
    M2UA is 2904.

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    The value assigned by IANA for the Payload Protocol Identifier in the
    SCTP Payload Data chunk is "2".

4.5 "2"  M2PA (SS7 MTP2-User Peer-to-Peer Adaptation) Layer


    M2PA protocol is used between SS7 Signalling Points using employing the MTP
    Level 3 protocol. The SS7 Signalling Points may also employ standard
    SS7 links using the SS7 MTP Level Layer 2 to provide transport of MTP
    Level Layer
    3 signalling messages.

    Both configurations: interworking intercommunication of SS7 and IP with SG and
    communication between ISEPs are possible.

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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

    Interconnection of SS7 and IP nodes:

Draft                Telephony Signalling AS                October 2002

            ********  SS7   ***************   IP   ********
            * SEP  *--------*     SG      *--------* IPSP *
            ********        ***************        ********

            +------+                               +------+
            | TCAP |                               | TCAP |
            +------+                               +------+
            | SCCP |                               | SCCP |
            +------+        +-------------+        +------+
            | MTP3 |        |    MTP3     |        | MTP3 |
            +------+        +------+------+        +------+
            | MTP2      |        | MTP2      | M2PA |        | M2PA |
            |      |        |      +------+        +------+------+        +------+
            | MTP1 MTP2 |        | MTP1 MTP2 | SCTP |        | SCTP |
            |      |        |      +------+        +------+
            |      |        |      | IP   |        | IP   |
            +------+        +------+------+        +------+

                    SEP   - SS7 Signalling Endpoint

    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 TBD.

    The value assigned by IANA for the Payload Protocol Identifier in the
    SCTP Payload Data chunk is "5".

    Differences TBD  Main differences between M2PA and M2UA include:

Draft                 Telephony Signalling AS                 April 2002 M2UA:

       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 necessarily an SS7 node and may not have a 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.

Draft                Telephony Signalling AS                October 2002

       e. M2PA: relies on MTP3 for management procedures.
          M2UA: uses M2UA management procedures.


 4.3  MTP lvl3-Users (ISUP, TUP, SCCP) over IP

    UAL: M3UA (SS7 MTP3 User Adaptation) Layer

    This adaptation layer

    M3UA protocol supports the transport of any SS7 MTP3-User signalling
    such as TUP, ISUP and SCCP over IP using the services of SCTP.

    This protocol allows both:

       - Interworking

    Interconnection of SS7 and IP nodes
       - Communication between two IP nodes 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

    Communication between two IP nodes:

Draft                Telephony Signalling AS                 April                October 2002

                        ********    IP    ********
                        * IPSP *----------* IPSP *
                        ********          ********

                        +------+          +------+
                        |SCCP- |          |SCCP- |
                        | User |          | User |
                        +------+          +------+
                        | SCCP |          | SCCP |
                        +------+          +------+
                        | M3UA |          | M3UA |
                        +------+          +------+
                        | SCTP |          | SCTP |
                        +------+          +------+
                        |  IP  |          |  IP  |
                        +------+          +------+

    It works using the client-server architecture. It philosophy. ISEP is recommended
    that the ISEP act as the to
    be client and initiate SCTP associations when talking with
    the a SG. The port reserved port by IANA is port number 2905. this is the
    port upon which the SG should 2905
    to listen for to possible client connections.

    The assigned payload protocol identifier for the SCTP DATA chunks is


 4.4  SCCP-Users over IP

    UAL: SUA (SS7 SCCP User Adaptation) Layer

    This adaptation layer

    SUA protocol supports the transport of any SS7 SCCP-User signalling
    such as MAP, INAP, SMS, BSSAP, RANAP over IP using the services of

    For message relaying, SUA should have the same can support only non-call related signalling.

    SUA does not pose stringent timing constraints as
    SCCP .  For on SCTP due to the end-to-end approach,
    fact that SUA applications may have
    broader broad timing requirements requirement (from 100 10 of milliseconds
    seconds to hours) which allows the applications to guard themselves. themselves and the
    timing supervision of the application is end-to-end, not hop-by-
    hop(as with ISUP).

    Possible configurations are showed in the pictures below: below.

    - Interworking Interconnection of SS7 and IP
       - IP Node to IP Node communication IP:

Draft                Telephony Signalling AS                 April                October 2002

            ********   SS7         ***************   IP        ********
            * SEP  *---------*             *--------*  *    IP   *  or             *   IP   *      SG      *
            *  or  *---------*     SG      *--------* ASP  *
            * STP  *         *             *        *      *
            ********         ***************        ********

            +------                                 +------+
            | SUAP |                                | SUAP |
            +------+         +------+------+        +------+
            | SCCP |         | SCCP | SUA  |        | SUA  |
            +------+         +------+------+        +------+
            | MTP3      |         | MTP3      |      |        |      |
            +------+         +------+
            | MTP3 |         | MTP3 | SCTP |        | SCTP |
            | MTP2      |         | MTP2      |      |        |      |
            +------+         +------+------+        +------+
            |  L1 MTP2 |         |  L1 MTP2 |  IP  |        |  IP  |
            +------+         +------+------+        +------+

              SUAP - SCCP/SUA User Protocol (TCAP, for example)
              STP  - SS7 Signalling Transfer Point


    - IP Node to IP Node communication:

                          ********        ********
                          *      *--------*      *   IP   * IPSP      *
                          * IPSP *--------* IPSP *
                          *      *        *      *
                          ********        ********

                          +------+        +------+
                          | SUAP |        | SUAP |
                          +------+        +------+
                          | SUA  |        | SUA  |
                          +------+        +------+
                          | SCTP |        | SCTP |
                          +------+        +------+
                          |  IP  |        |  IP  |
                          +------+        +------+

    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".

Draft                Telephony Signalling AS                 April                October 2002

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.  While having some overlapping security needs, any
    security solution should fulfill all of the different parties'
    needs.  See specific Security considerations in each UAL technical

    SCTP only tries to increase the availability of a network. SCTP does
    not contain any protocol mechanisms which are directly related to
    user message authentication, integrity and confidentiality
    functions. For such features, it depends on the IPSEC protocols and
    architecture and/or on security features of its user protocols.

    Mechanisms for reducing the risk of blind denial-of-service attacks
    and masquerade attacks are built into SCTP protocol. See RFC2960,
    section 11 for detailed information.

    Currently the IPSEC working group is investigating the support of
    multihoming by IPSEC protocols. At the present time to use IPSEC,
    one must use 2 * N * M security associations if one endpoint uses N
    addresses and the other M addresses.

6 References and related work

    [RFC2960] Stewart, R. R., Xie, Q., Morneault, K., Sharp, C. , ,
    Schwarzbauer, H. J., Taylor, T., Rytina, I., Kalla, M., Zhang,
    L. and Paxson, V, "Stream Control Transmission Protocol", RFC2960,
    October 2000.


    [RF3257] Coene, L., Tuexen, M., Verwimp, G., Loughney, J., Stewart,
    R.  R., Xie, Q., Holdrege, M., Belinchon, M.C., and Jungmayer, A.,
    "Stream Control Transmission Protocol Applicability statement",
    RFC3257, April 2002.

    [RFC2719] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene,
    L., Lin, H., Juhasz, I., Holdrege, M., Sharp, C., "Framework
    Architecture for Signalling Transport", RFC2719, October 1999.

    [RFC3057] Morneault, K., Rengasami, S., Kalla, M., Sidebottom, G.,

Draft                Telephony Signalling AS                October 2002

    "ISDN Q.921-User Adaptation Layer", RFC3057, February 2001.


    [RFC3331] Morneault, K., Dantu, R., Sidebottom, G., George, T.,

Draft                 Telephony Signalling AS                 April 2002
    Bidulock, B., Heitz , J., "Signaling System 7 (SS7) Message Transfer
    Part (MTP) 2 - User Adaptation Layer", RFCxxxx, May RFC3331, September 2002.


    [RFC3332] Sidebottom, G., Pastor-Balbas, J., Rytina, I., Mousseau,
    G., Ong, L., Schwarzbauer, H.J., Gradischnig, K., Morneault, K.,
    Kalla, M., Glaude, N., Bidulock, B., Loughney, J., "SS7 MTP3-User
    Adaptation Layer (M3UA)", RFCyyyy, May RFC3332, September 2002.

    [RFCzzzz] Loughney, J., Sidebottom, G., Mousseau, G., Lorusso, S.,
    Coene, L., Verwimp, G., Keller, J., Escobar, F., Sully, W., Furniss,
    S., Bidulock, B.,"SS7 SCCP-User Adaptation Layer (SUA)", RFCzzzz,
    May 2002.

    [RFCwwww] George, T., Dantu, R., Kalla, M., Schwarzbauer, H.J.,
    Sidebottom, G., Morneault, K.,"SS7 MTP2-User Peer-to-Peer Adaptation
    Layer", RFCwwww, June 2002.

    [RFCqqqq] Weilandt, E., Khanchandani, N., Rao, S.,"V5.2-User
    Adaptation Layer (V5UA)", RFCqqqq, June 2002

    [RFCtttt] Vydyam, A., Mukundan, R., Mangalpally, N., Morneault,
    K.,"DPNSS/DASS 2 extensions to the IUA protocol", RFCtttt, August

    [ALLMAN99] Allman, M. and Paxson, V., "On Estimating End-to-End
    Network Path Properties", Proc. SIGCOMM'99, 1999.

7 Acknowledgments

    This document was initially developed by a design team consisting of
    Lode Coene, John Loughney, Michel Tuexen, Randall R. Stewart,
    Qiaobing Xie, Matt Holdrege, Maria-Carmen Belinchon, Andreas
    Jungmaier, Gery Verwimp and Lyndon Ong.

    The authors wish to thank Renee Revis, H.J. Schwarzbauer, T. Taylor,
    G.  Sidebottom, K. Morneault, T. George, M. Stillman and many others
    for their invaluable comments.

8 Author's Address

Draft                Telephony Signalling AS                October 2002

Lode Coene                  Phone: +32-14-252081
Siemens Atea                EMail:

Draft                 Telephony Signalling AS                 April 2002
Atealaan 34
B-2200    Herentals

Javier Pastor-Balbas        Phone:
Ericsson Espana S.A.        Email:
C/ Ombu 3 Retama 1
28045 Madrid

Expires: August 2002

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Draft                Telephony Signalling AS                 April                October 2002