This document summarizes the design and implementation of a conference gateway that supports interoperability between the H.323 and SIP signaling protocols for multiparty video conferencing. Key points: - The gateway allows H.323 and SIP clients to participate in the same conferencing session over an IP network. It acts as a gatekeeper, multipoint controller, and proxy server to translate between the protocols. - Design decisions addressed mapping conference IDs, capability descriptions, membership control, and ongoing conference information between H.323 and SIP. Media streams are forwarded via the gateway without transcoding. - An implementation was tested with up to 5 clients on each side. Performance evaluation showed comparable call setup times between the protocols ro

1. A Conference Gateway Supporting
Interoperability Between SIP and H.323
Jiann-Min Ho (Presenter) Jia-Cheng Hu
Information Networking Institute
Peter Steenkiste
School of Computer Science
Dept. of Electrical and Computer Engineering
Carnegie Mellon University

2. Agenda
Overview
Target network and goal
Signaling protocols and comparison
Design
Implementation
Performance evaluation
Discussion
Related work
Conclusion

3. Overview -
Multi-party Media Conference
Media conference application drivers
 Network bandwidth and endpoint capability
Signaling and media streaming protocols
 Signaling protocols:
 H.323 (ITU-T) vs. SIP (IETF)
 Media transport protocol: RTP (RFC1889)

4. Target Network and Goal
Target network –
 Packet switching network (e.g. IP), not PSTN
 Future network infrastructure
Goal – two folds
 Build a practical system allowing SIP and H.323
clients to participate in one video conferencing
session.
 A set of recommendations and for developers and
standard bodies that would improve
interoperability issues.

5. Signal Protocols -
Generic H.323 Call Flow
H.323 Endpoint 1
Setup (E2, CID = N, create)
Alerting/Call Proceeding
ARQ
ACF
Connect (E2, H.245 TA)
terminalCapabilitySet Exchange
Master/Slave Determination
H.323 Endpoint 2 (may contain MC)
H.225.0 RAS
Procedures
H.225.0
Call Signaling
Procedures
H.245
Control
Procedures
Gatekeeper

6. Signal Protocols -
Generic SIP Call Flow

7. Signal Protocols -
Comparison of H.323 and SIP
Modularity
 Synchronization of the mapping procedures during
operation
Message presentation
 Translation of syntax
Mapping of an H.323 conference and a SIP
session
 Map H.323 conference ID and SIP session ID
 Map the H.245 capabilityDescriptor structure to SDP
syntax

8. Signal Protocols -
Comparison of H.323 and SIP(cont.)
Advertising
 Conference information availability
Determination of conference media capability
Applicable solution:
 Central determination via an intermediate agent serving
as an H.323 mc
Control over membership
 Generic admission control mechanisms and strategies
handling conference membership

9. Design – GCCG Functionality

10. Design - Functionality of GCCG
in SIP and H.323 perspective
On the H.323 side
 Gatekeeper (GK), Multipoint Controller (MC) and
Multipoint Processor (MP) without transcoding
 Independent conferencing components
On the SIP side
 Proxy Server and Conferencing Server

11. Design – Key Design Decisions
Conference Call Messages Translation
H.323 ⇔ SIP
Conference Create -
 H.225 SETUP (? H.245 CapSet ) ⇒ SDP message
 N/A (no SAP supported) ⇐ SDP message
Conference Invite - depend on conferencing scenariosdepend on conferencing scenarios
 H.225 SETUP ⇒ INVITE (? SDP) message
(? FastStart) + OPTION (obtain/check SIP invitee Media Cap. - Invitable?)
 H.225 SETUP ⇐ INVITE (SDP) message
Conference Join -
 H.225 SETUP ⇒ N/A (trigger IGMP message)
 N/A ⇐ N/A (only IGMP message)

12. Design – Key Design Decisions
(Cont.)
Central Determination of Conference Media
Capability
Ongoing Conference Information
Conference Management: Membership
Control and Session Management

13. Design – Example: Conference
Invite and Join
H.323 Endpoint (E1) GCCG SIP Client C1
Setup (C1 {usrID@addr}, CID = N, invite,
FastStart {openLogicalChannel structures})
Alerting/Call Proceeding
ARQ
ACF
Connect (GCCG H.245 TA)
Ringing
Accept
Invite ( C1 {usrID}, Session Info)
Bye
Terminal Left Conference
No H.245 control channel…
Option ( C1 {usrID})
Option_Reply(Media type)
SDP messages to MBone cloud
• H.323 Client Invites SIP Client via Fast Connection Procedure
NOTE: Conference is created after the invitee’s media cap is received.

14. Design – Example: Conference
Invite and Join (Cont.)
H.323 Endpoint (E2) GCCG H.323 Participants (E1)
Setup (GCCG, CID = N or
session name, Join)
Alerting/Call Proceeding
ARQ
ACF
Connect (GCCG H.245 TA)
terminalCapabilitySet Exchange
Master/Slave Determination
CommunicationModeCommand
MultipointConference Indication MultipointConference Indication
CommunicationModeCommand
• A H.323 Endpoint joins an ongoing conference.

15. Implementation
RAS
Signaling
Channel
Handler
Call Signaling
Routing
Handler
H.323 connection
per endpoint
H.225
Call Signal
Channel
Handler
H.245
Call Control
Channel
Handler
Conference
Control
Logics/Functions
SIP/SDP
Message
HandlerTranslated Signal
FIFO Queues
H.323
Message
Handler
Internal
Conference
Control Data
Media Data
Switching
Internal Data Flow
Active Thread
Program Control Interaction
Internal Data Storage
Internal Message Queue
SDP
Receive
Handler
SDP/SIP
Send
Handler
SIP
Receive
Handler
GCCG Internal Architecture

16. Implementation –
Operation and Status
Only video is available.
Simple Conference Media Mode
Determination
H.323 clients are lack of conference
information, LDAP is not supported.
Media streams (RTP) are forwarded via
GCCG; no support from media
mixing/transcoding components/gateways.

17. Performance Evaluation
Correctness of conferencing signaling flow via
GCCG
Testbed configuration
 One GCCG Server (Linux PC) and Five PC Clients (NT)
 Video Conferencing Software
 MS NetMeeting version 3.01 and the MBone tools SDR v2.9 and VIC
v2.8.
 Common communication mode - H.261
 Network Configuration
 10 Mbs shared Ethernet

18. Performance Evaluation (cont.)
GCCG
Server
H.323
client
H.323
client
H.323
client
H.323
client
H.323
client
RTT
Measurement
Media Streams
RTP(H.261)
GCCG
Server
SIP
client
H.323
client
SIP
client
SIP
client
SIP
client
RTT
Measurement
Media Streams
RTP(H.261)
multicast

19. Performance Evaluation (cont.)
6
8
10
12
14
16
18
1 2 3 4 5
Number of Participants
AverageRTT(ms)
Scenario 1(H.323 clients)
Scenario 2(SIP clients)
Comparison of Average RTT

20. Discussion
Negotiation of Media Capabilities
 Common conference media type (no transcoding GW)
 H.323 – determined by MC via H.245 procedures
 SIP – use a proxy server to query media capability information.
OPTION message can be applied. Standard? Draft?
 Translation of message syntax between H.245 and SDP
 Change of media stream codec gracefully?

21. Discussion (cont.)
Session Advertising
 Conference information advertisement
 SIP – operate with SAP (push)
 H.323 – LDAP can be used (pull) if clients require this feature
 Conference information mapping
Adaptation of Call Signaling Semantics
 Reduce signaling overhead via H.323 FastConnect
Conference Control
 H.323 – ITU-T T series, e.g. T.124, GCCP…
 SIP – drafting, standards?

22. Discussion (cont.)
IP Multicast
 Characteristic of the architecture – open conference
 Authorization, authentication, encryption
 Application level support – conference server (GCCG) and
clients (H.323/SIP)
 IP multicast support – match application needs
 Access media streaming multicast address
 H.323 – determined by MC
 SIP – from SAP messages
 malloc

23. Related Work
Several active groups, e.g. aHIT! from IMTC,
ITU-T SG 16 and TIPHON, and …
H.323/SIP signaling gateway
 - Columbia University
Our prototype implementation focus
 Multi-party media conferencing signaling support
 IP multicast efficiency

24. Conclusion
Completion of interoperability for H323 and
SIP in multiparty media conferencing
sessions
Recommendation for raised interoperability
issues.

25. Q & A