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Internet Engineering Task Force                           Hemant Agrawal
INTERNET DRAFT                                             GlobeSpan Inc
                                                         Radhika R. Roy
Category: Informational                                             AT&T
Expires: October 10, 2000                                  Vipin Palawat
                                                      Wipro Technologies


                      SIP-H.323 Interworking Requirements
        <draft-agrawal-roy-palawat-sip-h323-interworking-00.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 docu-
ments of the Internet Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also distribute working docu-
ments as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsolete by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

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.

This document is a product of the SIP-H.323 Interworking Working
Group of the Internet Engineering Task Force (IETF).  Comments should be
submitted to the mailing list sip-h323@egroups.com.


Abstract

This document describes the requirements for the logical entity known
as the interworking function (IWF) that will allow for interworking
between the SIP and H.323 system.












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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

Table of Contents


      1.  Introduction ..............................................  3
      2.  Terminology ...............................................  3
      3.  Definitions ...............................................  3
      4.  Functionality within the IWF ..............................  3
      5.  Pre-Call Requirements .....................................  4
         5.1.  Registration with H.323 Gatekeeper ...................  4
         5.2.  Registration with SIP Server .........................  5
      6.  Resource Management .......................................  5
         6.1.  Resource Allocation and Reservation ..................  5
         6.2.  Resource Control .....................................  5
      7.  General Interworking Requirements .........................  6
         7.1. Performance Requirements ..............................  6
         7.2. Basic call Requirements ...............................  6
            7.2.1. Address Resolution ...............................  6
            7.2.2. Call with H.323 GK ...............................  7
            7.2.3. Call with SIP Servers ............................  7
            7.2.4. Call with both H.323 GK and SIP Server ...........  8
            7.2.5. Capability negotiation ...........................  8
            7.2.6. Opening of logical channels ......................  8
            7.2.7. Handling Media transmission and reception ........  9
         7.3. Requirements for support of fast connect procedures ...  9
         7.4. Requirements for support of H.245 tunnelling ..........  9
         7.5. Requirements for support of pre granted ARQ ...........  9
         7.6. Requirements for support of overlapped sending ........  9
         7.7. Requirements for support of early H.245 ...............  9
      8.  Transport ................................................. 10
         8.1.  Assumptions made for underlying network .............. 10
         8.2.  Transport Requirements ............................... 10
      9. Mapping between SIP and H.323 .............................. 10
         9.1  General Procedures .................................... 10
         9.2. H.323 Call Signalling (Q.931) and SIP Call Signalling . 11
         9.3. H.323 Call Control (H.245) and SIP Call Control(SDP) .. 11
         9.4. H.323 audio/video codec to SIP media formats .......... 11
         9.5. Call sequence ......................................... 11
     10.  State Machine Requirements ................................ 12
     11.  Security Requirements ..................................... 12
     12.  Current Issues ............................................ 12
     13.  Activities planned for next phase ......................... 13
     14.  Examples and Scenarios .................................... 13
     15.  Full Copyright Statement .................................. 16
     16.  References ................................................ 16
     17.  Acknowledgements .......................................... 17
     18.  Authors' addresses ........................................ 17




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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

1.  Introduction

This document describes requirements to be placed on the SIP-H.323 Interwoking function (IWF). This will become the guidelines for any Gateway converging the ITU-T H.323 and IETF SIP.

2.  Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" are to be interpreted as described in RFC 2119 [1] and indi-
cate requirement levels for the protocol.

3.  Definitions

3.1. H.323 GK
This is an optional component in H.323 network. If it is present, it must perform the address translation, bandwidth control, admission control and zone management.

3.2. IWF
Interworking function which allows interworking between the H.323 and SIP system.

3.3. SIP Server
This can be either SIP proxy or SIP redirect server.

3.4. Endpoint
This is an entity from which the media originates or finally terminates. This can either be H.323 terminal or SIP user agent.

3.5. Media switching fabric (MSF)
This will be a logical entity present in the IWF, which will perform
the task of switching RTP from one logical port to other.

4. Functionality within the IWF

This section provides the functional requirements of the SIP-H.323
interworking function.

IWF can be architectured in various ways. This may include the
coexistence of H.323 Gatekeeper or SIP servers with IWF. The location
of the H.323 GK and/or SIP server in conjunction with the IWF is a
matter of implementation and not a protocol issue. There will be no
assumptions made for the optional elements and components present in
either H.323 or SIP networks. The solution provided here will work for
a minimum configuration required for both the protocols. There will be
recommendations for other configurations, which includes optional
components.


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Internet draft      SIP-H.323 Interworking Requirements    10 April
2000

For instance, H.323 Gatekeeper is not a mandatory component of H.323
network. So, there will be no assumptions made for the basic
interworking which involves H.323 Gatekeeper either co-located with IWF
or exists separately on H.323 zone.

The introduction of IWF redundancy in the network is left for further
discussion.

Therefore, an IWF is assumed to contain the following functions:  a)
Call sequence mapping.  b) Address resolution.  c) Terminal Capability
transactions.  d) Opening and closing of media channels.  e) Mapping
media codecs for H.323 and SIP network.  f) Resource reservation and
release.  g
) Ability to provide the state of resources.
h) Call state machine.
i) Out of band signal processing.

There will be no processing on the media data at IWF. It is assumed
that both H.323 and SIP network uses RTP as a transport for carrying
media. In most of the cases RTP will be directly between the endpoints.
Even if the media from one endpoint terminated at IWF in special
scenario, the assumption is made that this will be simply switched to
other endpoint by a media switching fabric present in the IWF.

The inclusion of Network Management for IWF is left for further
discussion.

5.  Pre-Call Requirements

The IWF function shall have a table of reference for look up to resolve
the corresponding H.323 and SIP addresses to IP addresses. This can
either be accomplished by using the capabilities of H.323 Gatekeeper
and SIP servers. Since H.323 Gatekeeper and SIP Server are not
mandatory components of H.323 and SIP systems respectively, the IWF
function may keep the information for address resolution within itself,
which can be updated by using the H.323 Gatekeeper, SIP Server or any
other database.

5.1.  Registration with H.323 Gatekeeper

This is done to give the information about SIP side extensions of IWF
to H.323 Gatekeeper if it is present in the network. This information
will be used by H.323 Gatekeeper to direct the call whose destination
is in the SIP network. The registration information may be updated at
any time to the H.323 Gatekeeper. The way in which IWF gets the
information of SIP side extension is for further study.

IWF can register with one H.323 Gatekeeper only. The registration with
multiple GKs by the IWF is for further study.


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Internet draft      SIP-H.323 Interworking Requirements    10 April
2000

5.2.  Registration with SIP Server

This is done to give the information about H.323 side extensions of the
IWF to the SIP Server if it is present in the SIP network. This
information will be used by the SIP server to direct the call whose
destination is in H.323 network. The way in which IWF gets the
information of H.323 side extensions is for further study. IWF may
register with one SIP server only.

IWF may register with many SIP servers. This is for further
discussion.

6.  Resource Management

Resources for a call is the memory, processing time slot, logical ports
and other call related data within the IWF.  Resources in the IWF are
to be managed with respect to resource reservation and resource
control. This resource reservation is done for both signalling and
media switching fabric on a per call basis.

6.1. Resource Allocation and Reservation

The IWF shall:

6.1.1 Support reservation of logical ports for signalling and media
switching fabric for use by a particular call and support their
subsequent release (which may be implicit or explicit).

6.1.2 Allow release in a single exchange of message, of all resources
associated with a particular call.

6.1.3 Support release of resources if IWF detects that the call is no
longer active. The detection of call inactivity is for further
discussion.

6.1.4 Support the reservation and release of resources for opening,
reopening, changing and closing of media channels during the call. The
procedures for opening, reopening, closing and changing the existing
media sessions during a call is for further discussion.

The IWF may:

6.1.5 Support the reservation by priority based on the order of
capability
descriptors.

6.1.6 Support the reporting of resource reservation and connection completion.

6.2. Resource Control

The IWF shall:



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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

6.2.1 Support the prereservations for a particular call. These
reservations can be made before a call comes at IWF.

6.2.2 Support the restrictions that can be imposed on a particular
endpoint for the use of resources.

6.2.3 Support the prereservation of resources for a particular
endpoint.

6.2.4 Support the reporting for out of resource.

6.2.5 Support the denial of additional resource required during a call
for opening, reopening, closing and changing of sessions.

6.2.6 Support for force release of the resources associated with a
call.

7. General Interworking Requirements

The gateway shall use the H.323 Version 2.0 [5] and SIP Ver 2.0 [2].
The gateway should handle all mandatory features of H.323 Version 2 as
well as SIP Version 2.0. It should also provide backward compatibility
for earlier versions.

The IWF will provide the seamless interworking of the two protocols.
The functioning of IWF should not involve any modification to the H.323
and SIP protocols, but may involve specific profiles of these
protocols.  7.1. Performance Requirements

7.1.1. Minimizing the message exchange.

7.1.2. Recommended maximum processing delay at IWF. The figure for
maximum delay is left for further discussion.

7.1.3. Guidelines for the peak calling time. This is left for further
discussion.

7.1.4. There should be default settings, so that the transactions will
only be for a change or non-default parameters.

7.2. Basic call Requirements

7.2.1. Address Resolution

The IWF shall:

7.2.1.1. Support all the addressing schemes of both H.323 and SIP
protocol.





Agrawal, Roy, and Palawat                                      [Page 6]


Internet draft      SIP-H.323 Interworking Requirements    10 April
2000

7.2.1.2. Register itself to H.323 Gatekeeper and SIP Server if they are
present in the Network.

7.2.1.3. have a look up table for resolving the addresses. This may be
updated from the H.323 GK and SIP server.

The IWF may:

7.2.1.4. Use LDAP or X.500 for keeping the address resolution
information.

7.2.1.5. Use DNS for address resolution.

7.2.2. Call with H.323 GK

The IWF shall:

7.2.2.1. Resolve addresses with the help of H.323 GK when it is present
in the network.

7.2.2.2. Register itself to forward the SIP extensions supported on SIP
side of IWF.

7.2.2.3. not register with two different H.323 Gatekeepers. This is
left for further discussion.

The IWF may:

7.2.2.4. Update the newly added SIP extentions to H.323 Gatekeeper.
This is left for further discussion.

7.2.3. Call with SIP Server

The IWF shall:

7.2.3.1. Resolve addresses with the help of SIP Server if it is present
in the network.

7.2.3.2. Register itself with SIP Server to forward the H.323
extentions supported on H.323 side of IWF.

The IWF may:

7.2.3.3. Register with many SIP servers. This is left for further
discussion.

7.2.3.4. Update the newly added H.323 extensions to SIP Server. This is
left for f
urther discussion.




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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

7.2.4. Call with both H.323 GK and SIP Server

All the requirements of Section 7.2.2 and Section 7.2.3 will be met for this case.

7.2.5. Capability negotiation

The IWF shall:

7.2.5.1. not make any assumptions for the capabilities of either SIP
user agent or H.323 terminal. However, it may indicate a default
capability of H.323 terminal or SIP user agent even before doing
capability exchange with H.323(using H.245) and SIP (using SDP).  This
default capability is the mandatory capability requirements as defined
by the respective protocols.  For example, G.711 is mandatory for
higher bandwidth networks of H.323.

7.2.5.2. pass on all the capability descriptors of H.323 and SDP from
SIP in the maximum possible way to each other. The algorithm for
finding out the maximum mapping of capability descriptors with the
corresponding SDP is left for further discussion.

7.2.5.3. Provide mapping for common audio/video formats supported in
H.323 with the RTP/AVP formats.

The IWF may:

7.2.5.4. use OPTIONS message on the SIP side to do capability
negotiations.

7.2.5.5. supports extensions of H.245 and SDP for ATM and other
transport.

7.2.5.6. support re-negotiation of codec

7.2.6. Opening of logical channels

The IWF shall:

7.2.6.1. open the channels between the endpoints only wherever
possible. If it is not possible, then it can be opened at the media
switching fabric of IWF.

7.2.6.2. support unidirectional, symmetric bi-directional, and
asymmetric bi-directional opening of channels.

The IWF may:  7.2.6.3. respond to the mode request and/or to the
request for reopening and changing an existing logical channel .




Agrawal, Roy, and Palawat                                      [Page 8]


Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

7.2.6.4. support the flow control of H.323.

7.2.7. Handling Media transmission and reception

The IWF shall:

7.2.7.1. Not process RTP data going in and out from media switching fabric.

The IWF may:

7.2.7.3. Collect the statistics of media flow for the particular call. The parameters for collection of statistics is left for further discussion.

7.2.7.4. Use the media flow statistics to maintain the QOS.


7.3. Requirements for support of fast connect procedures

The IWF shall support the fastStart element.


7.4. Requirements for support of H.245 tunneling

The IWF shall support the H.245 tunneling in Setup message.

7.5. Requirements for support of pregranted ARQ

The IWF shall support the pregranted ARQ. In this case, the IWF may do the address resolution from H.323 GK using LRQ/LCF exchange.

7.6. Requirements for support of overlapped sending

The IWF shall support the overlapped sending of dialed digits.

7.7. Requirements for support of early H.245

This is left for further discussion.














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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

8.  Transport

8.1. Assumptions made for underlying network

8.1.1 It should support both the TCP and UDP i.e. both reliable and
non-reliable delivery of messages is supported.

8.1.2 The network of H.323 and SIP system can be anywhere. There are no
assumptions for the closeness of these networks.

8.1.3 The network is not assuring QOS.

8.1.4 There is no priority of signalling messages over other messages.

8.2. Transport Requirements

8.2.1 It is assumed that both H.323 and SIP network uses RTP for
carrying media. If this is not the case then a media gateway is
required.

8.2.2 Support for large fan-out.

9. Mapping between SIP and H.323

9.1. General Procedures

9.1.1. A clearer mapping between SIP and H.323 messages shall be
provided which reflects similar meaning in call sequence.

9.1.2. The call message sequence shall be maintained in both the
directions.

9.1.3. The IWF shall not take any decision on its own related to basic
functionality of a call like call setup and call teardown etc.

9.1.4. The messages, which do not have a match on the other side,
should be terminated on the IWF, and IWF should take the necessary
action on them.

9.1.5. In case the IWF is required to generate a message on its own in
any of the side, IWF should use the pre-configured default values for
the parameters.

9.1.6. The information elements of the respective messages are to be
converted as follows:

a) The contents of connection specific information elements (such as
Call Reference Value on H.323) shall be converted to respective






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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

information as required by SIP or SDP such as session ID, call leg and
Call-ID.

b) Information elements that are not in use on the H.323 side shall be
generated by the IWF as required by the SIP protocol and vice versa.

c) The SIP data fields are converted into the corresponding ASN.1
user-user information element structure. The user-user information
element structure shall be generated according to the specification in
Recommendation H.225.0 and H.245.

9.2 Call Signalling (H.225.0) and SIP Call Signalling

9.2.1. The IWF shall conform to the call signalling procedures
recommended for the SIP side independent from the H.323 side.

9.2.2. The IWF shall conform to the call signalling procedures
recommended for the H.323 side independent from the SIP side.

9.2.3. The IWF shall terminate the Q.931 Call Signalling Channel
between an H.323 endpoint or H.323 Gatekeeper (in case of GK routed
signalling) and the IWF on one hand and the call signalling (if any)
between the IWF and the SIP endpoint on the other side.

9.2.4. The IWF shall terminate the RAS Channel between H.323 Gatekeeper
(if any) and IWF.

9.2.5. Messages for supplementary services (FACILITY, NOTIFY, and the
INFORMATION messages) in H.323 side are to be processed by the IWF,
only if the service is supported.

9.3 H.323 Call Control (H.245) and SIP Call Control (SDP)

IWF should try to map the H.245 and SDP to the maximum extent.

9.4 H.323 audio/video codec to SIP media formats

The IWF should:

9.4.1. provide invisible support for all audio/video algorithms supported by either ITU or IANA.

9.4.2. Handling of dynamic payload types is for further discussion.

9.5. Call sequence

The call sequence should be maintained in such a way on both sides of IWF so that neither H.323 terminal nor SIP UA is aware of the IWF presence. The IWF should provide seamless interworking between the call

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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

flows of the two protocols. The IWF will not do any modifications to
the normal call flows of either protocols .The messages and parameters
which do not have direct mapping on the other side are to be generated
by the IWF with default parameters in most of the cases. In brief, the
H.323 endpoint should not be aware of the fact it is calling a SIP
endpoint and vice versa.

10. State Machine Requirements

The state machine for IWF will follow the following general
guidelines:

10.1. Unexpected messages in a particular state are treated as "Error"
messages.

10.2. All messages which do not change the state are treated as "Non
triggering or Informational" messages.

10.3. All messages which expect a change in state are treated as
"Triggering" m
essages.

For each state, there should be guidelines that classify all possible
messages into above three categories. Apart from this, it is required
to specify the processing i.e. action to be taken in the state machine
on the contents of the message.

This will result into a table given below as an illustration.

                                 State : Idle

Possible Messages         Message Category   Action         Next state

All RAS Msg.              Triggering         Add Reg.Info.  WaitForSetup
All Q.931 Msg.            Non Triggering
All H.245 Msg.            Error
All Msg. From SIP side    Triggering

11. Security Requirements

All the security requirements are for further discussion.

Assumptions for the endpoints:

11.1. All endpoints trying to use IWF are authorized with respective
H.323 Gatekeeper and SIP server if it is present in the network.

11.2. All endpoints trying to make a call using IWF are respectively
admitted to do so from H.323 Gatekeeper and SIP server if it is present
in the network.





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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

Required for IWF:

11.3 Procedures for preventing denial of service security attacks.

11.4 Maintaining a persistence data for authorized endpoints for future verifications.

12.  Current Issues

12.1. There will be no network management functionality. This is left
for further discussion.

12.2 There will be no redundancy for IWF in the network. This is left
for further discussion.

12.3 There will be no tone detection from RTP streams. This is left for
further discussion.

12.4 The supplementary services are planned for next phase of the
draft.

12.5 The conferencing is planned for next phase of the draft.

13. Activities planned for next phase

13.1 Simple call supplementary services like call forwarding, call hold
and call transfer.

13.2 Conferencing.

13.3 Session change (re-invite, mode request).

13.4 Security: Authentication, Authorization and privacy

13.5 QOS signalling.

13.6 Network Management.

13.7 Redundancy.

14. Examples and Scenarios

We are providing some examples of call scenarios that will show
primarily the input and output signaling messages of the IWF for
interworking between SIP and H.323. The important point is that the IWF
will perform the translation between the signaling messages of SIP and
H.323. However, we have not addressed how the mapping will be done in
this contribution although we have shown what should be the output
signaling message of the IWF for a given input signaling message in the
IWF.


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Internet draft      SIP-H.323 Interworking Requirements    10 April
2000

In performing the mapping, the IWF may have to face the following
situations:

a. It may so appear that there can be one-to-one mapping between the
signaling messages and the IWF will perform the translation
accordingly.

b. All parameters used in each signaling message one side may not match
exactly to the corresponding signaling message of the other side. In
this situation, some manipulations need to be done by the IWF so that
an agreed upon standard can be created based on common understanding
although all parameters do not exactly match.

c. For a given signaling message of a given protocol, there may not be
a corresponding signaling message of the other protocol that may appear
to be equivalent. The IWF needs to create a mapping between the
signaling messages or generate error messages based on common
understanding of an agreed upon standard.

Items a, b, and c as stated above are very critical to create the
interoperability standard between H.323 and SIP and we like to address
these in separate contributions. It may be mentioned that many problems
in those areas have already been addressed in the interworking between
SIP/SDP and H.323 document [1]. However, we have addressed the
configurations for call scenarios and the input-output messages of the
IWF that are required to provide interoperability between SIP and
H.323.

Following are the different configurations for the call scenarios:

14.1. Basic Configuration

H.323 EP  ---- IWF ---- SIP EP

14.2. Advanced Configurations

14.2.1. Calls using H.323 GK

H.323 EP ---- H.323 GK ---- IWF ---- SIP EP

14.2.2. Calls using SIP Server

H.323 EP ---- IWF ---- SIP Server ---- SIP EP

14.2.3. Calls using both H.323 GK and SIP Server

H.323 EP ---- H.323 GK ---- IWF ---- SIP Server ---- SIP EP

The different call scenarios for above configurations are:

a) Simple Call from H.323 terminal to SIP terminal.


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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

b) Call from H.323 terminal to SIP terminal using H.245 tunneling.

c) Call from H.323 terminal to SIP terminal using early H.245.

d) Call from H.323 terminal to SIP terminal using fast connect procedure.

e) Call from H.323 terminal to SIP terminal using overlapped sending.

f) Call from H.323 terminal to SIP terminal using pre granted ARQ (for configurations having H.323 GK).

g) Simple call from SIP terminal to H.323 terminal.

h) Call from SIP terminal to H.323 terminal using H.245 tunneling.

i) Call from SIP terminal to H.323 terminal using early H.245.

j) Call from SIP terminal to H.323 terminal using fast connect procedure.

k) Call from SIP terminal to H.323 terminal using overlapped sending.

l) Call from SIP terminal to H.323 terminal using pre granted ARQ (for configuration having H.323 GK).

Some call flow examples for the different configurations and call
scenarios are given below:

i) Simple call from H.323 terminal to SIP terminal using configuration
of 14.1

     H.323                        SIP
      EP    Setup   IWF           EP
       |------------>|    INVITE   |
       |             |------------>|
       |             | 180 RINGING |
       |   Alerting  |<------------|
       |<------------|   200 OK    |
       |  Connect    |<------------|
       |<------------|             |
       |   H.245     |             |
       |<----------->|    ACK      |
       |             |------------>|
       |            RTP            |
       |<------------------------->|

ii) Simple call from SIP terminal to H.323 terminal using configuration of 14.1





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Internet draft      SIP-H.323 Interworking Requirements    10 April 2000

   SIP                        H.323
    EP           IWF            EP
    |             |             |
    |   INVITE    |             |
    |------------>|   Setup     |
    |             |------------>|
    |             |  Alerting   |
    | 180 RINGING |<------------|
    |<------------|   Connect   |
    |             |<------------|
    |             |    H.245    |
    |     200 OK  |<----------->|
    |<------------|             |
    |     ACK     |             |
    |------------>|             |
    |            RTP            |
    |<------------------------->|


15.  Full Copyright Statement

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The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FIT-
NESS FOR A PARTICULAR PURPOSE."

16.  References

[1] Singh/Schulzrinne, "Interworking Between SIP/SDP and H.323", draft-

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singh-sip-h323-00.txt,IETF, January 2000.

[2] M. Handley, H.Schulzrinne, E.Schooler, and J.Rosenberg, "SIP:Session Initiation Prtocol", RFC 2543,IETF,March 1999.

[3] M. Handley and V. Jacobson, "SDP: Session Description Prtocol", RFC 2327, IETF, April 1998.

[4] S. Bradner,"Key words for use in RFCs to indicate requirement levels", RFC 2119,IETF, March 1997.

[5] "Packet based multimedia communication systems", Recommendation H.323,ITU-T,Geneva,Switzerland,Feb. 1998.


17.  Acknowledgements

The authors would like to acknowledge the many contributors who debated
the SIP-H.323 interworking architecture and requirements on the IETF ,
SIP and SG16 mailing lists. Contributions to this document have also
been made through internet-drafts and discussion with members of SIP,
H.323 and SG16 forums.

18.  Authors' addresses

        Hemant Agrawal
        GlobeSpan Inc.
        A-8,Sector 9,
        Noida (U.P.) - 201 301
        INDIA
        Tel: +91-1191-4544028 Extn: 121
        Fax: +91-1191-4544014
        Email: hemantag@globespan.net

        Radhika R. Roy
        AT&T
        Room C1-2B03
        200 Laurel Avenue S.
        Middletown,
        NJ 07748, USA
        Tel: +1-732-4201580
        Fax: +1-732-3681195
        Email: rrroy@att.com

        Vipin Palawat
        Wipro Technologies
        271, Sri Ganesha Complex,
          Hosur Main Road, Madiwala,
        Bangalore - 560 068 (Karnataka)
        INDIA

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          Tel: +91-80-5539134 Extn: 407
        Fax: +91-80-5539701/702
        Email: vipin.palawat@wipro.com














































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