Network Working Group L. Ong
Request for Comments: 2719 Nortel Networks
Category: Informational I. Rytina
M. Garcia
Ericsson
H. Schwarzbauer
L. Coene
Siemens
H. Lin
Telcordia
I. Juhasz
Telia
M. Holdrege
Lucent
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 (C) The Internet Society (1999). All Rights Reserved.
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.
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:
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
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.
+---------------+ +--------------+
| | | |
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.
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.
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.
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.
+--------------+
| 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
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.
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
SS7
[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
EMail: long@nortelnetworks.com
Ian Rytina
Ericsson Australia
37/360 Elizabeth Street
Melbourne, Victoria 3000, Australia
EMail: ian.rytina@ericsson.com
Matt Holdrege
Lucent Technologies
1701 Harbor Bay Parkway
Alameda, CA 94502 USA
EMail: holdrege@lucent.com
Lode Coene
Siemens Atea
Atealaan 34
Herentals, Belgium
EMail: lode.coene@siemens.atea.be
Miguel-Angel Garcia
Ericsson Espana
Retama 7
28005 Madrid, Spain
EMail: Miguel.A.Garcia@ericsson.com
Chip Sharp
Cisco Systems
7025 Kit Creek Road
Res Triangle Pk, NC 27709, USA
EMail: chsharp@cisco.com
Imre Juhasz
Telia
Sweden
EMail: imre.i.juhasz@telia.se
Haui-an Paul Lin
Telcordia Technologies
Piscataway, NJ, USA
EMail: hlin@research.telcordia.com
HannsJuergen Schwarzbauer
SIEMENS AG
Hofmannstr. 51
81359 Munich, Germany
EMail: HannsJuergen.Schwarzbauer@icn.siemens.de
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