Network Working Group H. Schulzrinne
Request for Comments: 4123 Columbia University
Category: Informational C. Agboh
July 2005
Session Initiation Protocol (SIP)-H.323 Interworking Requirements
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 (2005).
IESG Note
This RFC is not a candidate for any level of Internet Standard. The
IETF disclaims any knowledge of the fitness of this RFC for any
purpose, and in particular notes that the decision to publish is not
based on IETF review for such things as security, congestion control,
or inappropriate interaction with deployed protocols. The RFC Editor
has chosen to publish this document at its discretion. Readers of
this document should exercise caution in evaluating its value for
implementation and deployment. See [RFC3932] for more information.
Abstract
This document describes the requirements for the logical entity known
as the Session Initiation Protocol (SIP)-H.323 Interworking Function
(SIP-H.323 IWF) that will allow the interworking between SIP and
H.323.
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Table of Contents
1. Introduction ....................................................3
2. Definitions .....................................................3
3. Functionality within the SIP-H.323 IWF ..........................4
4. Pre-Call Requirements ...........................................4
4.1. Registration with H.323 Gatekeeper .........................5
4.2. Registration with SIP Server ...............................5
5. General Interworking Requirements ...............................5
5.1. Basic Call Requirements ....................................5
5.1.1. General Requirements ................................5
5.1.2. Address Resolution ..................................6
5.1.3. Call with H.323 Gatekeeper ..........................6
5.1.4. Call with SIP Registrar .............................6
5.1.5. Capability Negotiation ..............................6
5.1.6. Opening of Logical Channels .........................7
5.2. IWF H.323 Features .........................................7
5.3. Overlapped Sending .........................................7
5.3.1. DTMF Support ........................................7
6. Transport .......................................................8
7. Mapping between SIP and H.323 ...................................8
7.1. General Requirements .......................................8
7.2. H.225.0 and SIP Call Signaling .............................8
7.3. Call Sequence ..............................................9
7.4. State Machine Requirements .................................9
8. Security Considerations ........................................10
9. Examples and Scenarios .........................................10
9.1. Introduction ..............................................10
9.2. IWF Configurations ........................................11
9.3. Call Flows ................................................11
9.3.1. Call from H.323 Terminal to SIP UA .................11
9.3.2. Call from SIP UA to H.323 Terminal .................12
10. Acknowledgments ...............................................12
11. Contributors ..................................................13
12. References ....................................................14
12.1. Normative References ....................................14
12.2. Informative References ..................................15
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1. Introduction
The SIP-H.323 Interworking function (IWF) converts between SIP
(Session Initiation Protocol) [RFC3261] and the ITU Recommendation
H.323 protocol [H.323]. This document describes requirements for
this protocol conversion.
2. Definitions
H.323 gatekeeper (GK): An H.323 gatekeeper is an optional component
in an H.323 network. If it is present, it performs address
translation, bandwidth control, admission control, and zone
management.
H.323 network: In this document, we refer to the collection of all
H.323-speaking components as the H.323 network.
SIP network: In this document, we refer to the collection of all SIP
servers and user agents as the SIP network.
Interworking Function (IWF): This function performs interworking
between H.323 and SIP. It belongs to both the H.323 and SIP
networks.
SIP server: A SIP server can be a SIP proxy, redirect server, or
registrar server.
Endpoint: An endpoint can call and be called. An endpoint is an
entity from which the media such as voice, video, or fax
originates or terminates. An endpoint can be H.323 terminal,
H.323 Gateway, H.323 MCU [H.323], or SIP user agent (UA)
[RFC3261].
Media-Switching Fabric (MSF): This is an optional logical entity
within the IWF. The MSF switches media such as voice, video, or
fax from one network association to another.
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3. Functionality within the SIP-H.323 IWF
This section summarizes the functional requirements of the SIP-H.323
interworking function (IWF).
A SIP-H.323 IWF may be integrated into an H.323 gatekeeper or SIP
server. Interworking should not require any optional components in
either the SIP or H.323 network, such as H.323 gatekeepers. IWF
redundancy in the network is beyond the scope of this document.
An IWF contains functions from the following list, inter alia:
o Mapping of the call setup and teardown sequences;
o Registering H.323 and SIP endpoints with SIP registrars and H.323
gatekeepers;
o Resolving H.323 and SIP addresses;
o Maintaining the H.323 and SIP state machines;
o Negotiating terminal capabilities;
o Opening and closing media channels;
o Mapping media-coding algorithms for H.323 and SIP networks;
o Reserving and releasing call-related resources;
o Processing of mid-call signaling messages;
o Handling of services and features.
The IWF should not process media. We assume that the same media
transport protocols, such as RTP, are used in both the SIP and H.323
networks. Thus, media packets are exchanged directly between the
endpoints. If a particular service requires the IWF to handle media,
we assume that the IWF simply forwards media packets without
modification from one network to the other, using a media-switching
fabric (MSF). The conversion of media from one encoding or format to
another is out of scope for SIP-H.323 protocol translation.
4. Pre-Call Requirements
The IWF function may use a translation table to resolve the H.323 and
SIP addresses to IP addresses. This translation table can be updated
by using an H.323 gatekeeper, a SIP proxy server, or a locally-
maintained database.
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4.1. Registration with H.323 Gatekeeper
An IWF may provide and update the H.323 gatekeeper with the addresses
of SIP UAs. A SIP user agent can make itself known to the H.323
network by registering with an IWF serving as a registrar. The IWF
creates an H.323 alias address and registers this alias, together
with its own network address, with the appropriate GK.
The gatekeeper can then use this information to route calls to SIP
UAs via the IWF, without being aware that the endpoint is not a
"native" H.323 endpoint.
The IWF can register SIP UAs with one or more H.323 gatekeepers.
4.2. Registration with SIP Server
The IWF can provide information about H.323 endpoints to a SIP
registrar. This allows the SIP proxy using this SIP registrar to
direct calls to the H.323 endpoints via the IWF.
The IWF can easily obtain information about H.323 endpoints if it
also serves as a gatekeeper. Other architectures require further
study.
If the H.323 endpoints are known through E.164 (telephone number)
addresses, the IWF can use IGREP [TGREP] or SLP [GWLOC] to inform the
SIP proxy server of these endpoints.
The IWF only needs to register with multiple SIP registrars if the
H.323 terminal is to appear under multiple, different addresses-of-
record.
5. General Interworking Requirements
The IWF should use H.323 Version 2 or later and SIP according to RFC
3261 [RFC3261]. The protocol translation function must not require
modifications or additions to either H.323 or SIP. However, it may
not be possible to support certain features of each protocol across
the IWF.
5.1. Basic Call Requirements
5.1.1. General Requirements
The IWF should provide default settings for translation parameters.
The IWF specification must identify these defaults.
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The IWF must release any call-related resource at the end of a call.
SIP session timers [RFC4028] may be used on the SIP side.
5.1.2. Address Resolution
The IWF should support all the addressing schemes in H.323, including
the H.323 URI [RFC3508], and the "sip", "sips", and "tel" URI schemes
in SIP. It should support the DNS-based SIP server location
mechanisms described in [RFC3263] and H.323 Annex O, which details
how H.323 uses DNS and, in particular, DNS SRV records.
The IWF should register with the H.323 Gatekeeper and the SIP
registrar when available.
The IWF may use any means to translate between SIP and H.323
addresses. Examples include translation tables populated by the
gatekeeper, SIP registrar or other database, LDAP, DNS or TRIP.
5.1.3. Call with H.323 Gatekeeper
When an H.323 GK is present in the network, the IWF should resolve
addresses with the help of the GK.
5.1.4. Call with SIP Registrar
The IWF applies normal SIP call routing and does not need to be aware
whether there is a proxy server.
5.1.5. Capability Negotiation
The IWF should not make any assumptions about the capabilities of
either the SIP user agent or the H.323 terminal. However, it may
indicate a guaranteed-to-be-supported list of codecs of the H.323
terminal or SIP user agent before exchanging capabilities with H.323
(using H.245) and SIP (using SDP [RFC2327]). H.323 defines mandatory
capabilities, whereas SIP currently does not. For example, the G.711
audio codec is mandatory for higher bandwidth H.323 networks.
The IWF should attempt to map the capability descriptors of H.323 and
SDP in the best possible fashion. The algorithm for finding the best
mapping between H.245 capability descriptors and the corresponding
SDP is left for further study.
The IWF should be able to map the common audio, video, and
application format names supported in H.323 to and from the
equivalent RTP/AVP [RFC3550] names.
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The IWF may use the SIP OPTIONS message to derive SIP UA
capabilities. It may support mid-call renegotiation of media
capabilities.
5.1.6. Opening of Logical Channels
The IWF should support the seamless exchange of messages for opening,
reopening, changing, and closing of media channels during a call.
The procedures for opening, reopening, closing, and changing the
existing media sessions during a call are for further study.
The IWF should open media channels between the endpoints whenever
possible. If this is not possible, then the channel can be opened at
the MSF of the IWF.
The IWF should support unidirectional, symmetric bi-directional, and
asymmetric bi-directional opening of channels.
The IWF may respond to the mode request and to the request for
reopening and changing an existing logical channel and may support
the flow control mechanism in H.323.
5.2. IWF H.323 Features
The IWF should support Fast Connect; i.e., H.245 tunneling in H.323
Setup messages. If IWF and GK are the same device, pre-granted ARQ
should be supported. If pre-granted ARQ is supported, the IWF may
perform the address resolution from H.323 GK using the LRQ/LCF
exchange.
5.3. Overlapped Sending
An IWF should follow the recommendations outlined in [RFC3578] when
receiving overlapped digits from the H.323 side. If the IWF receives
overlapped dialed digits from the SIP network, it may use the Q.931
Setup, Setup Ack, and Information Message in H.323.
The IWF may support the transfer of digits during a call by using the
appropriate SIP mechanism and UserInputIndication in H.245 (H.323).
5.3.1. DTMF Support
An IWF should support the mapping between DTMF and possibly other
telephony tones carried in signaling messages.
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6. Transport
The H.323 and SIP systems do not have to be in close proximity. The
IP networks hosting the H.323 and SIP systems do not need to assure
quality of service (QoS). In particular, the IWF should not assume
that signaling messages have priority over packets from other
applications. H.323 signaling over UDP (H.323 Annex E) is optional.
7. Mapping between SIP and H.323
7.1. General Requirements
o The call message sequence of both protocols must be maintained.
o The IWF must not set up or tear down calls on its own.
o Signaling messages that do not have a match for the destination
protocol should be terminated on the IWF, with the IWF taking the
appropriate action for them. For example, SIP allows a SIP UA to
discard an ACK request silently for a non-existent call leg.
o If the IWF is required to generate a message on its own, IWF
should use pre-configured default values for the message
parameters.
o The information elements and header fields of the respective
messages are to be converted as follows:
* The contents of connection-specific information elements, such
as Call Reference Value for H.323, are converted to similar
information required by SIP or SDP, such as the SDP session ID
and the SIP 'Call-ID'.
* The IWF generates protocol elements that are not available from
the other side.
7.2. H.225.0 and SIP Call Signaling
o The IWF must conform to the call signaling procedures recommended
for the SIP side regardless of the behavior of the H.323 elements.
o The IWF must conform to the call signaling procedures recommended
for the H.323 side regardless of the behavior of the SIP elements.
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o The IWF serves as the endpoint for the Q.931 Call Signaling
Channel to either an H.323 endpoint or H.323 Gatekeeper (in case
of GK routed signaling). The IWF also acts as a SIP user agent
client and server.
o The IWF also establishes a Registration, Admission, Status (RAS)
Channel to the H.323 GK, if available.
o The IWF should process messages for H.323 supplementary services
(FACILITY, NOTIFY, and the INFORMATION messages) only if the
service itself is supported.
7.3. Call Sequence
The call sequence on both sides should be maintained in such a way
that neither the H.323 terminal nor the SIP UA is aware of presence
of the IWF.
7.4. State Machine Requirements
The state machine for IWF will follow the following general
guidelines:
o Unexpected messages in a particular state shall be treated as
"error" messages.
o All messages that do not change the state shall be treated as
"non-triggering" or informational messages.
o All messages that expect a change in state shall be treated as
"triggering" messages.
For each state, an IWF specification must classify all possible
protocol messages into the above three categories. It must specify
the actions taken on the content of the message and the resulting
state. Below is an example of such a table:
State: Idle
Possible Messages Message Category Action Next state
-------------------------------------------------------------------
All RAS msg. Triggering Add Reg.Info. WaitForSetup
All H.245 msg. Error Send 4xx Idle
SIP OPTIONS Non Triggering Return cap. Idle
SIP INVITE Triggering Send SETUP WaitForConnect
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8. Security Considerations
Because the IWF whose requirements have been described in this
document combines both SIP and H.323 functionality, security
considerations for both of these protocols apply.
The eventual security solution for interworking must rely on the
standard mechanisms in RFC3261 [RFC3261] and H.323, without extending
them for the interworking function. Signaling security for H.323 is
described in H.235 [H.235].
Because all data elements in SIP or H.323 have to terminate at the
IWF, the resulting security cannot be expected to be end-to-end.
Thus, the IWF terminates not only the signalling protocols but also
the security in each domain. Therefore, users at the SIP or H.323
endpoint have to trust the IWF, like they would any other gateway, to
authenticate the other side correctly. Similarly, they have to trust
the gateway to respect the integrity of data elements and to apply
appropriate security mechanisms on the other side of the IWF.
The IWF must not indicate the identity of a user on one side without
first performing authentication. For example, if the SIP user was
not authenticated, it would be inappropriate to use mechanisms on the
H.323 side, such as H.323 Annex D, to indicate that the user identity
had been authenticated.
An IWF must not accept 'sips' requests unless it can guarantee that
the H.323 side uses equivalent H.235 [H.235] security mechanisms.
Similarly, the IWF must not accept H.235 sessions unless it succeeds
in using SIP-over-TLS (sips) on the SIP side of the IWF.
9. Examples and Scenarios
9.1. Introduction
We present some examples of call scenarios that will show the
signaling messages received and transmitted. The following
situations can occur:
o Some signaling messages can be translated one-to-one.
o In some cases, parameters on one side do not match those on the
other side.
o Some signaling messages do not have an equivalent message on the
other side. In some cases, the IWF can gather further information
and the signal on the other side. In some cases, only an error
indication can be provided.
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9.2. IWF Configurations
Below are some common architectures involving an IWF:
Basic Configuration: H.323 EP -- IWF -- SIP UA
Calls using H.323 GK: H.323 EP -- H.323 GK -- IWF -- SIP UA
Calls using SIP proxies: H.323 EP -- IWF -- SIP proxies -- SIP UA
Calls using both H.323 GK and SIP proxy: H.323 EP -- H.323 GK -- IWF
-- SIP proxies -- SIP UA
SIP trunking between H.323 networks: H.323 EP -- IWF -- SIP network
-- IWF -- H.323 EP
H.323 trunking between SIP networks: SIP EP -- IWF -- H.323 network
-- IWF -- SIP UA
9.3. Call Flows
Some call flow examples for two different configurations and call
scenarios are given below.
9.3.1. Call from H.323 Terminal to SIP UA
H.323 SIP
EP Setup IWF UA
|------------>| INVITE |
| |------------>|
| | 180 RINGING |
| Alerting |<------------|
|<------------| 200 OK |
| Connect |<------------|
|<------------| |
| H.245 | |
|<----------->| ACK |
| |------------>|
| RTP |
|<.........................>|
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9.3.2. Call from SIP UA to H.323 Terminal
SIP H.323
UA IWF EP
| | |
| INVITE | |
|------------>| Setup |
| |------------>|
| | Alerting |
| 180 RINGING |<------------|
|<------------| Connect |
| |<------------|
| | H.245 |
| 200 OK |<----------->|
|<------------| |
| ACK | |
|------------>| |
| RTP |
|<.........................>|
10. Acknowledgments
The authors would like to acknowledge the many contributors who
discussed the SIP-H.323 interworking architecture and requirements on
the IETF, SIP, and SG16 mailing lists. In particular, we would like
to thank Joon Maeng, Dave Walker, and Jean-Francois Mule.
Contributions to this document have also been made by members of the
H.323, aHIT!, TIPHON, and SG16 forums.
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11. Contributors
In addition to the editors, the following people provided substantial
technical and written contributions to this document. They are
listed alphabetically.
Hemant Agrawal
Telverse Communications
1010 Stewart Drive
Sunnyale, CA 94085
USA
EMail:
[email protected]
Alan Johnston
MCI WorldCom
100 South Fourth Street
St. Louis, MO 63102
USA
EMail:
[email protected]
Vipin Palawat
Cisco Systems Inc.
900 Chelmsford Street
Lowell, MA 01851
USA
EMail:
[email protected]
Radhika R. Roy
AT&T
Room C1-2B03
200 Laurel Avenue S.
Middletown, NJ 07748
USA
EMail:
[email protected]
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Kundan Singh
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
EMail:
[email protected]
David Wang
Nuera Communications Inc.
10445 Pacific Center Court
San Diego, CA 92121
USA
EMail:
[email protected]
12. References
12.1. Normative References
[H.235] International Telecommunication Union, "Security and
encryption for H-Series (H.323 and other H.245-based)
multimedia terminals", Recommendation H.235,
February 1998.
[H.323] International Telecommunication Union, "Packet based
multimedia communication systems", Recommendation H.323,
July 2003.
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC3508] Levin, O., "H.323 Uniform Resource Locator (URL) Scheme
Registration", RFC 3508, April 2003.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
12.2. Informative References
[GWLOC] Zhao, W. and H. Schulzrinne, "Locating IP-to-Public
Switched Telephone Network (PSTN) Telephony Gateways via
SLP", work in progress, February 2004.
[RFC3578] Camarillo, G., Roach, A., Peterson, J., and L. Ong,
"Mapping of Integrated Services Digital Network (ISDN)
User Part (ISUP) Overlap Signalling to the Session
Initiation Protocol (SIP)", RFC 3578, August 2003.
[RFC3932] Alvestrand, H., "The IESG and RFC Editor Documents:
Procedures", BCP 92, RFC 3932, October 2004.
[RFC4028] Donovan, S. and J. Rosenberg, "Session Timers in the
Session Initiation Protocol (SIP)", RFC 4028, April 2005.
[TGREP] Bangalore, M., "A Telephony Gateway REgistration Protocol
(TGREP)", work in progress, March 2004.
Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7042
EMail:
[email protected]
URI:
http://www.cs.columbia.edu
Charles Agboh
61 Bos Straat
3540 Herk-de-Stad
Belgium
Phone: +32479736250
EMail:
[email protected]
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