Network Working Group
Request for Comments: 2719
Category: Informational
L. Ong
Nortel Networks
I. Rytina
M. Garcia
H. Schwarzbauer
L. Coene
H. Lin
I. Juhasz
M. Holdrege
C. Sharp
Cisco Systems
October 1999

Framework Architecture for Signaling Transport

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 © The Internet Society (1999). All Rights Reserved.


This document defines an architecture framework and functional requirements for transport of signaling information over IP. The framework describes relationships between functional and physical entities exchanging signaling information, such as Signaling Gateways and Media Gateway Controllers. It identifies interfaces where signaling transport may be used and the functional and performance requirements that apply from existing Switched Circuit Network (SCN) signaling protocols.

Table of Contents

   1. Introduction..................................................2
   1.1 Overview.....................................................2
   1.2 Terminology..................................................3
   1.3  Scope.......................................................5
   2.  Signaling Transport Architecture.............................5
   2.1  Gateway Component Functions.................................5
   2.2  SS7 Interworking for Connection Control.....................6
   2.3  ISDN Interworking for Connection Control....................8
   2.4  Architecture for Database Access............................9
   3. Protocol Architecture........................................10
   3.1 Signaling Transport Components..............................10
   3.2 SS7 access for Media Gateway Control........................11
   3.3 Q.931 Access to MGC.........................................12
   3.4 SS7 Access to IP/SCP........................................12
   3.5 SG to SG....................................................14
   4. Functional Requirements......................................15
   4.1 Transport of SCN Signaling Protocols........................15
   4.2 Performance of SCN Signaling Protocols......................17
   4.2.1 SS7 MTP Requirements......................................17
   4.2.2 SS7 MTP Level 3 Requirements..............................17
   4.2.3 SS7 User Part Requirements................................18
   4.2.4 ISDN Signaling Requirements...............................18
   5. Management...................................................19
   6. Security Considerations......................................19
   6.1 Security Requirements.......................................19
   6.2 Security Mechanisms Currently Available in IP Networks......20
   7. Abbreviations................................................21
   8. Acknowledgements.............................................21
   9. References...................................................21
   Authors' Addresses..............................................22
   Full Copyright Statement........................................24

1. Introduction

1.1 Overview

This document defines an architecture framework for transport of message-based signaling protocols over IP networks. The scope of this work includes definition of encapsulation methods, end-to-end protocol mechanisms and use of existing IP capabilities to support the functional and performance requirements for signaling transport.

The framework portion describes the relationships between functional and physical entities used in signaling transport, including the framework for control of Media Gateways, and other scenarios where signaling transport may be required.

The requirements portion describes functional and performance requirements for signaling transport such as flow control, in- sequence delivery and other functions that may be required for specific SCN signaling protocols.

1.2 Terminology

The following are general terms are used in this document:


Backhaul refers to the transport of signaling from the point of interface for the associated data stream (i.e., SG function in the MGU) back to the point of call processing (i.e., the MGCU), if this is not local.

Signaling Transport (SIG):

SIG refers to a protocol stack for transport of SCN signaling protocols over an IP network. It will support standard primitives to interface with an unmodified SCN signaling application being transported, and supplements a standard IP transport protocol underneath with functions designed to meet transport requirements for SCN signaling.

Switched Circuit Network (SCN):

The term SCN is used to refer to a network that carries traffic within channelized bearers of pre-defined sizes. Examples include Public Switched Telephone Networks (PSTNs) and Public Land Mobile Networks (PLMNs). Examples of signaling protocols used in SCN include Q.931, SS7 MTP Level 3 and SS7 Application/User parts.

The following are terms for functional entities relating to signaling transport in a distributed gateway model.

Media Gateway (MG):

   A MG terminates SCN media streams, packetizes the media data,, if it
   is not already packetized, and delivers packetized traffic  to the
   packet network.  It performs these functions in reverse order for
   media streams flowing from the packet network to the SCN.

Media Gateway Controller (MGC):

An MGC handles the registration and management of resources at the MG. The MGC may have the ability to authorize resource usage based on local policy. For signaling transport purposes, the MGC serves as a possible termination and origination point for SCN application protocols, such as SS7 ISDN User Part and Q.931/DSS1.

Signaling Gateway (SG):

An SG is a signaling agent that receives/sends SCN native signaling at the edge of the IP network. The SG function may relay, translate or terminate SS7 signaling in an SS7-Internet Gateway. The SG function may also be co-resident with the MG function to process SCN signaling associated with line or trunk terminations controlled by the MG (e.g., signaling backhaul).

The following are terms for physical entities relating to signaling transport in a distributed gateway model:

Media Gateway Unit (MGU)

An MG-Unit is a physical entity that contains the MG function. It may contain other functions, esp. an SG function for handling facility-associated signaling.

Media Gateway Control Unit (MGCU)

An MGC-Unit is a physical entity containing the MGC function.

Signaling Gateway Unit (SGU)

An SG-Unit is a physical entity containing the SG function.

Signaling End Point (SEP):

This is a node in an SS7 network that originates or terminates signaling messages. One example is a central office switch.

Signal Transfer Point (STP):

This is a node in an SS7 network that routes signaling messages based on their destination point code in the SS7 network.

1.3 Scope

   Signaling transport provides transparent transport of message-based
   signaling protocols over IP networks.   The scope of this work
   includes definition of encapsulation methods, end-to-end protocol
   mechanisms and use of IP capabilities to support the functional and
   performance requirements for signaling.

Signaling transport shall be used for transporting SCN signaling between a Signaling Gateway Unit and Media Gateway Controller Unit. Signaling transport may also be used for transport of message-based signaling between a Media Gateway Unit and Media Gateway Controller Unit, between dispersed Media Gateway Controller Units, and between two Signaling Gateway Units connecting signaling endpoints or signal transfer points in the SCN.

Signaling transport will be defined in such a way as to support encapsulation and carriage of a variety of SCN protocols. It is defined in such a way as to be independent of any SCN protocol translation functions taking place at the endpoints of the signaling transport, since its function is limited to the transport of the SCN protocol.

Since the function being provided is transparent transport, the following areas are considered outside the scope of the signaling transport work:

   -  definition of the SCN protocols themselves.
   -  signaling interworking such as conversion from Channel Associated
      Signaling (CAS) to message signaling protocols.
   -  specification of the functions taking place within the SGU or MGU
   -  in particular, this work does not address whether the SGU provides
      mediation/interworking, as this is transparent to the transport
   -  similarly, some management and addressing functions taking place
      within the SGU or MGU are also considered out of scope, such as
      determination of the destination IP address for signaling, or
      specific procedures for assessing the performance of the transport
      session (i.e., testing and proving functions).

2. Signaling Transport Architecture

2.1 Gateway Component Functions

Figure 1 defines a commonly defined functional model that separates out the functions of SG, MGC and MG. This model may be implemented in a number of ways, with functions implemented in separate devices or combined in single physical units.

Where physical separation exists between functional entities, Signaling Transport can be applied to ensure that SCN signaling information is transported between entities with the required functionality and performance.

        +---------------+                      +--------------+
        |               |                      |              |
  SCN<-------->[SG]  <--+---------O------------+--> [SG]  <------> SCN
 signal |       |       |                      |     |        |   signal
        +-------|-------+                      +-----|--------+
       Signaling|gateway                    Signaling|gateway (opt)
                O                                    O
                |                                    |
        +-------|-------+                      +-----|--------+
        |       |       |                      |     |        |
        |      [MGC] <--+--------O-------------+--> [MGC]     |
        |       |       |                      |     |        |
        |       |       |                      |     |        |
        +-------|-------+                      +-----|--------+
        Gateway | controller                 Gateway | controller (opt)
                O                                    O
                |                                    |
        +-------|-------+                      +-----|--------+
  Media |       |       |                      |     |        | Media
 <------+---->[MG]  <---+-----RTP stream-------+-> [MG]  <----+-------->
  stream|               |                      |              | stream
        +---------------+                      +--------------+
        Media gateway                           Media gateway

Figure 1: Sigtran Functional Model

As discussed above, the interfaces pertaining to signaling transport include SG to MGC, SG to SG. Signaling transport may potentially be applied to the MGC to MGC or MG to MGC interfaces as well, depending on requirements for transport of the associated signaling protocol.

2.2 SS7 Interworking for Connection Control

Figure 2 below shows some example implementations of these functions in physical entities as used for interworking of SS7 and IP networks for Voice over IP, Voice over ATM, Network Access Servers, etc. No recommendation is made as to functional distribution and many other examples are possible but are not shown to be concise. The use of signaling transport is independent of the implementation.

For interworking with SS7-controlled SCN networks, the SG terminates the SS7 link and transfers the signaling information to the MGC using signaling transport. The MG terminates the interswitch trunk and controls the trunk based on the control signaling it receives from the MGC. As shown below in case (a), the SG, MGC and MG may be implemented in separate physical units, or as in case (b), the MGC and MG may be implemented in a single physical unit.

In alternative case (c), a facility-associated SS7 link is terminated by the same device (i.e., the MGU) that terminates the interswitch trunk. In this case, the SG function is co-located with the MG function, as shown below, and signaling transport is used to "backhaul" control signaling to the MGCU.

Note: SS7 links may also be terminated directly on the MGCU by cross-connecting at the physical level before or at the MGU.

   SS7<------>[SG]  |
   (ISUP)  |   |    |
            ST |                SGU                       MGCU
           +---|----+           +--------+                +--------+
           | [MGC]  |      SS7---->[SG]  |                | [MGC]  |
           |   |    |           |   |    |                |  | |   |
           +---|----+           +---|----+                +--|-|---+
          MGCU |                 ST |                        | |
               |                    |                     ST | |
     Media +---|----+     Media +---|----+                +--|-|---+
      ------->[MG]  |      ----->[MG/MGC]|      SS7 link-->[SG]|   |
    stream |        |    stream |        |       Media------> [MG] |
           +--------+           +--------+       stream   +--------+
           MGU                  MGU                       MGU

(a) (b) (c)


ST = Signaling Transport used to carry SCN signaling

Figure 2: Example Implementations

In some implementations, the function of the SG may be divided into multiple physical entities to support scaling, signaling network management and addressing concerns. Thus, Signaling Transport can be used between SGs as well as from SG to MGC. This is shown in Figure 3 below.

               SGU                                 MGCU
             +---------+                         +---------+
             |         |          ST             |         |
             |  [SG2]------------------------------>[MGC]  |
             |   ^ ^   |                         |         |
             +---|-|---+                         +---------+
                 | |
                 | |             ST
               ST| +--------------------------------+
                 |                                  |
                 |                                  |
        SS7  +---|----------+             SS7  +----|---------+
   -----------> [SG1]       |        -----------> [SG1]       |
    media    |              |         media    |              |
   ------------------->[MG] |        ------------------->[MG] |
    stream   +--------------+         stream   +--------------+
              MGU                                MGU

Figure 3: Multiple SG Case

In this configuration, there may be more than one MGU handling facility associated signaling (i.e. more than one containing it's own SG function), and only a single SGU. It will therefore be possible to transport one SS7 layer between SG1 and SG2, and another SS7 layer between SG2 and MGC. For example, SG1 could transport MTP3 to SG2, and SG2 could transport ISUP to MGC.

2.3 ISDN Interworking for Connection Control

In ISDN access signaling, the signaling channel is carried along with data channels, so that the SG function for handling Q.931 signaling is co-located with the MG function for handling the data stream. Where Q.931 is then transported to the MGC for call processing, signaling transport would be used between the SG function and MGC. This is shown in Figure 3 below.

                             |    [MGC]    |
                             |     | |     |
                                   | |
                                   | O device control
                                   | |
                          Q.931/ST O |
                                   | |
                             |     | |     |
                       Q.931---->[SG]|     |
                      signals|       |     |
                             |       |     |
                    Media---->[MG]   |
                    stream   |             |

Figure 4: Q.931 transport model

2.4 Architecture for Database Access

Transaction Capabilities (TCAP) is the application part within SS7 that is used for non-circuit-related signaling.

TCAP signaling within IP networks may be used for cross-access between entities in the SS7 domain and the IP domain, such as, for example:

- access from an SS7 network to a Service Control Point (SCP) in IP. - access from an SS7 network to an MGC.
- access from an MGC to an SS7 network element.
- access from an IP SCP to an SS7 network element.

A basic functional model for TCAP over IP is shown in Figure 5.

                            | IP SCP       |
                               |    |
            SGU                |    |                SGU
           +--------------+    |    |    +--------------+
           |              |    |    |    |              |
   SS7<--------->[SG] ---------+    |    |     [SG]<---------> SS7
   (TCAP)  |      |       |         |    |      |       |
           +------|-------+         |    +------|-------+
                  |                 |           |
                  O    +------------+           O
          MGCU    |    |                        | MGCU
          +-------|----|--+               +-----|--------+
          |       |    |  |               |     |        |
          |      [MGC]    |               |    [MGC]     |
          |       |       |               |     |        |
          +-------|-------+               +-----|--------+
                  |                             |
          +-------|-------+               +-----|------+
    Media |       |       |               |     |      | Media
   <------+---->[MG]  <---+--RTP stream---+--> [MG]  <-+-------->
    stream|               |               |            | stream
          +---------------+               +------------+
          MGU                             MGU

Figure 5: TCAP Signaling over IP

3. Protocol Architecture

This section provides a series of examples of protocol architecture for the use of Signaling Transport (SIG).

3.1 Signaling Transport Components

Signaling Transport in the protocol architecture figures below is assumed to consist of three components (see Figure 6):

   1) an adaptation sub-layer that supports specific primitives, e.g.,
      management indications, required by a particular SCN signaling
      application protocol.
   2) a Common Signaling Transport Protocol that supports a common set
      of reliable transport functions for signaling transport.
   3) a standard, unmodified IP transport protocol.
                 +-- +--------------------------------+
                 |   |      SCN adaptation module     |
                 |   +--------------------------------+
                 |                  |
               S |   +--------------------------------+
               I |   | Common Signaling Transport     |
               G |   +--------------------------------+
                 |                  |
                 |   +--------------------------------+
                 |   |     standard IP transport      |
                 +-- +--------------------------------+

Figure 6: Signaling Transport Components

3.2. SS7 access for Media Gateway Control

This section provides a protocol architecture for signaling transport supporting SS7 access for Media Gateway Control.

****** SS7 ******* SS7 ****** IP *******

*SEP *--------* STP *------* SG *------------* MGC *

****** ******* ****** *******

          +----+                                       +-----+
          |ISUP|                                       | ISUP|
          +----+        +-----+      +---------+       +-----+
          |MTP |        |MTP  |      |MTP | SIG|       | SIG |
          |L1-3|        |L1-3 |      |L1-3+----+       +-----+
          |    |        |     |      |    | IP |       | IP  |
          +----+        +-----+      +---------+       +-----+

STP - Signal Transfer Point SEP - Signaling End Point

SG - Signaling Gateway SIG - Signaling Transport

MGC - Media Gateway Controller

Figure 7: SS7 Access to MGC

3.3. Q.931 Access to MGC

This section provides a protocol architecture for signaling transport supporting ISDN point-to-point access (Q.931) for Media Gateway Control.

****** ISDN ********* IP *******

* EP *--------------* SG/MG *------------* MGC *

****** ********* *******

            +----+                                   +-----+
            |Q931|                                   | Q931|
            +----+              +---------+          +-----+
            |Q921|              |Q921| SIG|          | SIG |
            +    +              +    +----+          +-----+
            |    |              |    | IP |          | IP  |
            +----+              +---------+          +-----+

MG/SG - Media Gateway with SG function for backhaul
EP - ISDN End Point

Figure 8: ISDN Access

3.4. SS7 Access to IP/SCP

This section provides a protocol architecture for database access, for example providing signaling between two IN nodes or two mobile network nodes. There are a number of scenarios for the protocol stacks and the functionality contained in the SIG, depending on the SS7 application.

In the diagrams, SS7 Application Part (S7AP) is used for generality to cover all Application Parts (e.g. MAP, IS-41, INAP, etc). Depending on the protocol being transported, S7AP may or may not include TCAP. The interface to the SS7 layer below S7AP can be either the TC-user interface or the SCCP-user interface.

Figure 9a shows the scenario where SCCP is the signaling protocol being transported between the SG and an IP Signaling Endpoint (ISEP), that is, an IP destination supporting some SS7 application protocols.

****** SS7 ******* SS7 ****** IP *******

*SEP *--------* STP *------* SG *-------------* ISEP*

****** ******* ****** *******

          +-----+                                       +-----+
          |S7AP |                                       |S7AP |
          +-----+                                       +-----+
          |SCCP |                                       |SCCP |
          +-----+        +-----+      +---------+       +-----+
          |MTP  |        |MTP  |      |MTP |SIG |       |SIG  |
          +     +        +     +      +    +----+       +-----+
          |     |        |     |      |    | IP |       |IP   |
          +-----+        +-----+      +---------+       +-----+

Figure 9a: SS7 Access to IP node - SCCP being transported

Figure 9b shows the scenario where S7AP is the signaling protocol being transported between SG and ISEP. Depending on the protocol being transported, S7AP may or may not include TCAP, which implies that SIG must be able to support both the TC-user and the SCCP-user interfaces.

****** SS7 ******* SS7 ****** IP *******

*SEP *--------* STP *------* SG *-------------* ISEP*

****** ******* ****** *******

          +-----+                                       +-----+
          |S7AP |                                       |S7AP |
          +-----+                     +----+----+       +-----+
          |SCCP |                     |SCCP|    |       |     |
          +-----+        +-----+      +----|SIG |       |SIG  |
          |MTP  |        |MTP  |      |MTP |    |       |     |
          +     +        +     +      +    +----+       +-----+
          |     |        |     |      |    |IP  |       |IP   |
          +-----+        +-----+      +---------+       +-----+

Figure 9b: SS7 Access to IP node - S7AP being transported

3.5. SG to SG

This section identifies a protocol architecture for support of signaling between two endpoints in an SCN signaling network, using signaling transport directly between two SGs.

The following figure describes protocol architecture for a scenario with two SGs providing different levels of function for interworking of SS7 and IP. This corresponds to the scenario given in Figure 3.

The SS7 User Part (S7UP) shown is an SS7 protocol using MTP directly for transport within the SS7 network, for example, ISUP.

In this scenario, there are two different usage cases of SIG, one which transports MTP3 signaling, the other which transports ISUP signaling.

****** SS7 ****** IP ****** IP ******

*SEP *-------* SG1*----------* SG2*-------*MGC *

****** ****** ****** ******

            +----+                                    +----+
            |S7UP|                                    |S7UP|
            +----+                     +----+----+    +----+
            |MTP3|                     |MTP3|    |    |    |
            +----+    +---------+      +----+ SIG|    |SIG |
            |MTP2|    |MTP2|SIG |      |SIG |    |    |    |
            +    +    +    +----+      +----+----+    +----+
            |    |    |    | IP |      |   IP    |    | IP |
            +----+    +----+----+      +----+----+    +----+

S7UP - SS7 User Part

Figure 10: SG to SG Case 1

The following figure describes a more generic use of SS7-IP interworking for transport of SS7 upper layer signaling across an IP network, where the endpoints are both SS7 SEPs.

****** SS7 ****** IP ****** SS7 ******

*SEP *--------* SG *-----------* SG *--------*SEP *

****** ****** ****** ******

            +----+                                       +-----+
            |S7UP|                                       | S7UP|
            +----+                                       +-----+
            |MTP3|                                       | MTP3|
            +----+        +---------+     +---------+    +-----+
            |MTP2|        |MTP2| SIG|     |SIG |MTP2|    | MTP2|
            +    +        +    +----+     +----+    +    +     +
            |    |        |    | IP |     | IP |    |    |     |
            +----+        +----+----+     +----+----+    +-----+

Figure 11: SG to SG Case 2

4. Functional Requirements

4.1 Transport of SCN Signaling Protocols

Signaling transport provides for the transport of native SCN protocol messages over a packet switched network.

Signaling transport shall:

1) Transport of a variety of SCN protocol types, such as the application and user parts of SS7 (including MTP Level 3, ISUP, SCCP, TCAP, MAP, INAP, IS-41, etc.) and layer 3 of the DSS1/PSS1 protocols (i.e. Q.931 and QSIG).

2) Provide a means to identify the particular SCN protocol being transported.

3) Provide a common base protocol defining header formats, security extensions and procedures for signaling transport, and support extensions as necessary to add individual SCN protocols if and when required.

4) In conjunction with the underlying network protocol (IP), provide the relevant functionality as defined by the appropriate SCN lower layer.

Relevant functionality may include (according to the protocol being transported):

- flow control
- in sequence delivery of signaling messages within a control stream

   -  logical identification of the entities on which the signaling
      messages originate or terminate
   -  logical identification of the physical interface controlled by the
      signaling message
   -  error detection
   -  recovery from failure of components in the transit path
   -  retransmission and other error correcting methods
   -  detection of unavailability of peer entities.

For example:

   -  if the native SCN protocol is ISUP or SCCP, the relevant
      functionality provided by MTP2/3 shall be provided.
   -  if the native SCN protocol is TCAP, the relevant functionality
      provided by SCCP connectionless classes and MTP 2/3 shall be
   -  if the native SCN protocol is Q.931, the relevant functionality
      provided by Q.921 shall be supported.
   -  if the native SCN protocol is MTP3, the relevant functionality of
      MTP2 shall be supported.

5) Support the ability to multiplex several higher layer SCN sessions on one underlying signaling transport session. This allows, for example, several DSS1 D-Channel sessions to be carried in one signaling transport session.

In general, in-sequence delivery is required for signaling messages within a single control stream, but is not necessarily required for messages that belong to different control streams. The protocol should if possible take advantage of this property to avoid blocking delivery of messages in one control stream due to sequence error within another control stream. The protocol should also allow the SG to send different control streams to different destination ports if desired.

6) Be able to transport complete messages of greater length than the underlying SCN segmentation/reassembly limitations. For example, signaling transport should not be constrained by the length limitations defined for SS7 lower layer protocol (e.g. 272 bytes in the case of narrowband SS7) but should be capable of carrying longer messages without requiring segmentation.

7) Allow for a range of suitably robust security schemes to protect signaling information being carried across networks. For example, signaling transport shall be able to operate over proxyable sessions, and be able to be transported through firewalls.

8) Provide for congestion avoidance on the Internet, by supporting appropriate controls on signaling traffic generation (including signaling generated in SCN) and reaction to network congestion.

4.2 Performance of SCN Signaling Protocols

This section provides basic values regarding performance requirements of key SCN protocols to be transported. Currently only message-based SCN protocols are considered. Failure to meet these requirements is likely to result in adverse and undesirable signaling and call behavior.

4.2.1 SS7 MTP requirements

The performance requirements below have been specified for transport of MTP Level 3 network management messages. The requirements given here are only applicable if all MTP Level 3 messages are to be transported over the IP network.

  • Message Delay - MTP Level 3 peer-to-peer procedures require response within 500 to 1200 ms. This value includes round trip time and processing at the remote end. Failure to meet this limitation will result in the initiation of error procedures for specific timers, e.g., timer T4 of ITU-T Recommendation Q.704.

4.2.2 SS7 MTP Level 3 requirements

The performance requirements below have been specified for transport of MTP Level 3 user part messages as part of ITU-T SS7 Recommendations [SS7].

  • Message Loss - no more than 1 in 10E+7 messages will be lost due to transport failure
  • Sequence Error - no more than 1 in 10E+10 messages will be delivered out-of- sequence (including duplicated messages) due to transport failure
  • Message Errors - no more than 1 in 10E+10 messages will contain an error that is undetected by the transport protocol (requirement is 10E+9 for ANSI specifications)
  • Availability - availability of any signaling route set is 99.9998% or better, i.e., downtime 10 min/year or less. A signaling route set is the complete set of allowed signaling paths from a given signaling point towards a specific destination.
  • Message length (payload accepted from SS7 user parts) - 272 bytes for narrowband SS7, 4091 bytes for broadband SS7

4.2.3 SS7 User Part Requirements

More detailed analysis of SS7 User Part Requirements can be found in [Lin].

ISUP Message Delay - Protocol Timer Requirements

  • one example of ISUP timer requirements is the Continuity Test procedure, which requires that a tone generated at the sending end be returned from the receiving end within 2 seconds of sending an IAM indicating continuity test. This implies that one way signaling message transport, plus accompanying nodal functions need to be accomplished within 2 seconds.
      ISUP Message Delay - End-to-End Requirements
  • the requirement for end-to-end call setup delay in ISUP is that an end-to-end response message be received within 20-30 seconds of the sending of the IAM. Note: while this is the protocol guard timer value, users will generally expect faster response time.

TCAP Requirements - Delay Requirements

  • TCAP does not itself define a set of delay requirements. Some work has been done [Lin2] to identify application-based delay requirements for TCAP applications.

4.2.4 ISDN Signaling Requirements

      Q.931 Message Delay
  • round-trip delay should not exceed 4 seconds. A Timer of this length is used for a number of procedures, esp. RELASE/RELEASE COMPLETE and CONNECT/CONNECT ACK where excessive delay may result in management action on the channel, or release of a call being set up. Note: while this value is indicated by protocol timer specifications, faster response time is normally expected by the user.

- 12 sec. timer (T309) is used to maintain an active call in case of loss of the data link, pending re-establishment. The related ETSI documents specify a maximum value of 4 seconds while ANSI specifications [T1.607] default to 90 seconds.

5. Management

Operations, Administration & Management (OA&M) of IP networks or SCN networks is outside the scope of SIGTRAN. Examples of OA&M include legacy telephony management systems or IETF SNMP managers. OA&M implementors and users should be aware of the functional interactions of the SG, MGC and MG and the physical units they occupy.

6. Security Considerations

6.1 Security Requirements

When SCN related signaling is transported over an IP network two possible network scenarios can be distinguished:

  • Signaling transported only within an Intranet; Security measures are applied at the discretion of the network owner.
  • Signaling transported, at least to some extent, in the public Internet; The public Internet should be regarded generally as an "insecure" network and usage of security measures is required.

Generally security comprises several aspects

  • Authentication: It is required to ensure that the information is sent to/from a known and trusted partner.
  • Integrity: It is required to ensure that the information hasn't been modified while in transit.
  • Confidentiality: It might be sometimes required to ensure that the transported information is encrypted to avoid illegal use.
  • Availability: It is required that the communicating endpoints remain in service for authorized use even if under attack.

6.2 Security Mechanisms Currently Available in IP Networks

Several security mechanisms are currently available for use in IP networks.

  • IPSEC ([RFC2401]): IPSEC provides security services at the IP layer that address the above mentioned requirements. It defines the two protocols AH and ESP respectively that essentially provide data integrity and data confidentiality services.

The ESP mechanism can be used in two different modes: - Transport mode;
- Tunnel mode.

In Transport mode IPSEC protects the higher layer protocol data portion of an IP packet, while in Tunnel mode a complete IP packet is encapsulated in a secure IP tunnel.

If the SIG embeds any IP addresses outside of the SA/DA in the IP header, passage through a NAT function will cause problems. The same is true for using IPsec in general, unless an IPsec ready RSIP function is used as described in RFC 2663 [NAT].

The use of IPSEC does not hamper the use of TCP or UDP as the underlying basis of SIG. If automated distribution of keys is required the IKE protocol ([RFC2409]) can be applied.

  • SSL, TLS ([RFC2246]): SSL and TLS also provide appropriate security services but operate on top of TCP/IP only.

It is not required to define new security mechanisms in SIG, as the use of currently available mechanisms is sufficient to provide the necessary security. It is recommended that IPSEC or some equivalent method be used, especially when transporting SCN signaling over public Internet.

7. Abbreviations

   CAS   Channel-Associated Signaling
   DSS1  Digital Subscriber Signaling
   INAP  Intelligent Network Application Part
   ISEP  IP Signaling End Point
   ISUP  Signaling System 7 ISDN User Part
   MAP   Mobile Application Part
   MG    Media Gateway
   MGU   Media Gateway Unit
   MGC   Media Gateway Controller
   MGCU  Media Gateway Controller Unit
   MTP   Signaling System 7 Message Transfer Part
   PLMN  Public Land Mobile Network
   PSTN  Public Switched Telephone Network
   S7AP  SS7 Application Part
   S7UP  SS7 User Part
   SCCP  SS7 Signaling Connection Control Part
   SCN   Switched Circuit Network
   SEP   Signaling End Point
   SG    Signaling Gateway
   SIG   Signaling Transport protocol stack
   SS7   Signaling System No. 7
   TCAP  Signaling System 7 Transaction Capabilities Part

8. Acknowledgements

The authors would like to thank K. Chong, I. Elliott, Ian Spiers, Al Varney, Goutam Shaw, C. Huitema, Mike McGrew and Greg Sidebottom for their valuable comments and suggestions.

9. References

   [NAT]        Srisuresh P. and M. Holdrege, "IP Network Address
                Translator (NAT) Terminology and Considerations", RFC
                2663, August 1999.
   [PSS1/QSIG]   ISO/IEC 11572 Ed. 2 (1997-06), "Information technology
                - Telecommunications and information exchange between
                systems - Private Integrated Services Network - Circuit
                mode bearer services - Inter-exchange signalling
                procedures and protocol"
   [Q.931/DSS1] ITU-T Recommendation Q.931, ISDN user-network interface
                layer 3 specification (5/98)
   [SS7]        ITU-T Recommendations Q.700-775, Signalling System No. 7
   [SS7 MTP]    ITU-T Recommendations Q.701-6, Message Transfer Part of
   [T1.607]     ANSI T1.607-1998, Digital Subscriber Signaling System
                Number 1 (DSS1) - Layer 3 Signaling Specification for
                Circuit-Switched Bearer Services
   [Lin]        Lin, H., Seth, T., et al., "Performance Requirements for
                Signaling in Internet Telephony", Work in Progress.
   [Lin2]       Lin, H., et al., "Performance Requirements for TCAP
                Signaling in Internet Telephony", Work in Progress.
   [RFC2246]    Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
                RFC 2246, January 1999.
   [RFC2409]    Harkins, D. and C. Carrel, "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.
   [RFC2401]    Kent, S. and R. Atkinson, "Security Architecture for the
                Internet Protocol", RFC 2401, November 1998.

Authors' Addresses

Lyndon Ong
Nortel Networks
4401 Great America Parkway
Santa Clara, CA 95054, USA


Ian Rytina
Ericsson Australia
37/360 Elizabeth Street
Melbourne, Victoria 3000, Australia

   Matt Holdrege
   Lucent Technologies
   1701 Harbor Bay Parkway
   Alameda, CA 94502  USA


Lode Coene
Siemens Atea
Atealaan 34
Herentals, Belgium


Miguel-Angel Garcia
Ericsson Espana
Retama 7
28005 Madrid, Spain


Chip Sharp
Cisco Systems
7025 Kit Creek Road
Res Triangle Pk, NC 27709, USA


Imre Juhasz


Haui-an Paul Lin
Telcordia Technologies
Piscataway, NJ, USA

   HannsJuergen Schwarzbauer
   Hofmannstr. 51
   81359 Munich,  Germany

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