    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.

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    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.

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    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.

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       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.

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       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.

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       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.

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    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.

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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 [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.

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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.

    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.

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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 [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".

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    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".

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    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, ...

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    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

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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".

    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

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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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    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

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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".

    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.

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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".

    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.

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    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.

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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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    Full Copyright Statement

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