Framework Architecture for Signaling Transport
draft-ietf-sigtran-framework-arch-03

Internet Engineering Task Force                                        
INTERNET-DRAFT                                                  Authors
Transport Working Group                     Lyndon Ong, Nortel Networks
Category: Informational             Ian Rytina, Miguel Garcia, Ericsson
June   1999              HannsJuergen Schwarzbauer, Lode Coene, Siemens
Expires: January 2000                       Huai-an Paul Lin, Telcordia
                                                     Imre Juhasz, Telia
                                                  Matt Holdrege, Ascend
                                              Chip Sharp, Cisco Systems

         Framework Architecture for Signaling Transport
           < draft-ietf-sigtran-framework-arch-03.txt > 

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.

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

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. 

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Table of Contents

1. Introduction..............................................2
1.1 Overview.................................................2
1.2 Terminology..............................................2
1.3  Scope...................................................4
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...............................................13
4. Functional Requirements..................................15
5. Management...............................................18
6. Security.................................................18
7. Abbreviations............................................20
8. Acknowledgements.........................................20
9. References...............................................20
Authors' Contact Information................................21

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:

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Backhaul: 

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:

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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:

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- 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 
    function.
  - 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.

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       +---------------+                      +--------------+
       |               |                      |              |
 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.

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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.

         SGU                   
        +--------+            
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)
Notes: 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.

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               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.

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

     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.

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                         +--------------+
                         | 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.
   

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         +-- +--------------------------------+
         |   |      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

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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. 

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******   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.

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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

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4. Functional Requirements

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 supported.
- 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.

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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.

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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. 

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  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.
    RELEASE/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

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.

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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 draft-ietf-nat-terminology-02.txt.
   

    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 (RFC[2409]) can be applied.

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

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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] IP Network Address Translator (NAT) Terminology and Considerations
<draft-ietf-nat-terminology-02.txt>, P. Srisuresh and M. Holdrege, April
1999, work in progress.

[PSS1/QSIG] ECMA Standard ECMA-143 -Inter-Exchange Signalling Procedures 
and Protocol (QSIG-BC)

[Q.931/DSS1] ITU-T Recommendation Q.931, ISDN user-network interface layer 3 
specification (5/98)

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[SS7] ITU-T Recommendations Q.700-775, Signalling System No. 7

[SS7 MTP] ITU-T Recommendations Q.701-6, Message Transfer Part of SS7

[T1.607]  ANSI T1.607-1998, Digital Subscriber Signaling System Number 1 (DSS1)
 - Layer 3 Signaling Specification for Circuit-Switched Bearer Services

[Lin] Performance Requirements for Signaling in Internet Telephony, <draft-seth-sigtran-req-00.txt>, H. Lin, T. Seth, et al, work in progress.

[Lin2] Performance Requirements for TCAP Signaling in Internet Telephony, <draft-ietf-sigtran-tcap-perf-req-00.txt>, H. Lin, et al, work in progress.

Authors' Contact Information

Lyndon Ong                        Ian Rytina
Nortel Networks                   Ericsson Australia
4401 Great America Parkway        37/360 Elizabeth Street
Santa Clara, CA 95054, USA        Melbourne, Victoria 3000, Australia
long@nortelnetworks.com           ian.rytina@ericsson.com

Matt Holdrege                     Lode Coene
Ascend Communications             Siemens Atea
1701 Harbor Bay Parkway           Atealaan 34
Alameda, CA 94502  USA            Herentals, Belgium
matt@ascend.com                   lode.coene@ntnet.atea.be

Miguel-Angel Garcia               Chip Sharp
Ericsson Espana                   Cisco Systems
Retama 7                          7025 Kit Creek Road
28005 Madrid, Spain               Res Triangle Pk, NC 27709, USA
Miguel.A.Garcia@ericsson.com      chsharp@cisco.com

Imre Juhasz                       Haui-an Paul Lin
Telia                             Telcordia Technologies
Sweden                            Piscataway, NJ, USA
imre.i.juhasz@telia.se            hlin@research.telcordia.com

HannsJuergen Schwarzbauer 
SIEMENS AG
Hofmannstr. 51
81359 Munich,  Germany
HannsJuergen.Schwarzbauer@icn.siemens.de

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