The Critical Role of
Sip&H.323
Internetworking in NextGeneration Telephony
Dr. Samir Chatterjee
Associate Professor
School of Information Science
909-607-4651; samir.chatterjee@cgu.edu
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Outline
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Definition of SIP and H.323
Comparison of SIP and H.323
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Complexity
Extensibility
Scalability
Services
Security Mechanisms used in SIP and H.323
Market Analysis
Conclusions
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Definition – H.323
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ITU H.323 series of recommendations (“Packet Based
Multimedia Communications Systems”) defines
protocols and procedures for multimedia
communications on the Internet.
It is an umbrella standard that provides a well-defined
system architecture and implementation guidelines.
It includes
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H.245 for control
H.225.0 for connection establishment
H.332 for large conferences
H.450(.1,.2,.3) for supplementary services
H.235 for security
H.246 for interoperability with circuit-switched services.
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Definition – SIP
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The Session Initiation Protocol (SIP), developed by
MMUSIC working group of the IETF, is a signaling
protocol for establishing real-time calls and
conferences over IP networks.
It resembles HTTP and SMTP.
It uses SDP for media description.
It is not as strictly defined as a complete system like
H.323. Therefore, it is flexible and can be adapted to
a number of implementations.
It allows for the use of established protocols from
other applications, such as HTTP and HTML.
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Definition - Functional Entities
Terminal
Terminal
Gatekeeper
Terminal
H.323
Zone
PSTN
MCU
Gateway
H.323/SIP
gateway
H.323
network
UA
(softphone)
SIP telephony gateway
Proxy/
Registrar
SIP
network
Enterprise
network
Redirect
server
UA (IP phone)
SIP Realm
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Comparison - Complexity
H.323
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Rather complex protocol
Defines hundreds of elements
Uses binary representation for
its messages
→ therefore it requires special
code generators to parse
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Uses several protocol
components
→therefore, many services require
interaction between many of
them
→ this also complicates firewall
traversal
Source: schulzrinne and Rosenberg
SIP
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Simpler protocol
Defines only 37 headers
Encodes its messages as text,
similar to HTTP
→ this allows simple parsing and
generation
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Uses a single request that
contains all necessary
information
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Comparison - Extensibility
H.323
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SIP
Provides extensibility generally
by use of nonstandardParam
fields
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→ this allows for different vendors to
develop their own extensions
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Extensions are limited only to
those places where a nonstandard parameter has been
added
It has no mechanisms for
allowing terminals to exchange
information about which
extensions each supports.
Source: schulzrinne and Rosenberg
Built in a rich set of
extensibility and compatibility
functions
Numerical error codes are
hierarchically organized
→ this allows for additional
features to be added by
defining semantics for the error
codes in a class, while
achieving compatibility
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Uses textual encoding which is
self describing
→ this enables developers to
determine usage from the
name
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Comparison - Scalability
H.323
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Large Number of Domains
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It provides no easy way to
perform loop detection in
complex multi-domain
searches.
Server Processing
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SIP
The complexity of signaling
makes it less scalable.
Large Number of Domains
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Server Processing
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Conference Sizes
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Three distinct mechanisms
exits to support different
conference sizes.
Source: schulzrinne and Rosenberg
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It uses a loop detection
algorithm which can be
performed in a stateless
manner.
Simple signaling
mechanism makes it more
scalable.
Conference Sizes
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It scales all different
conference sizes.
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Comparison - Services
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H.323 and SIP offer roughly equivalent call control
services.
H.323 provides a much richer set of functionality for
capabilities exchange services.
SIP provides rich support for personal mobility
services.
H.323 supports various conference control services.
Sip does not provide conference control, rather it
relies on other protocols for this service.
Source: schulzrinne and Rosenberg
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Security Mechanisms
H.323/H.235
SIP
Two mechanisms that provide
 End-to-end mechanisms
Authentication or/and Integrity are:
 Basic authentication
 Annex D - Baseline Security Profile
 Digest authentication
 Hop-by-hop processing
 S/MIME
 Password based security
 Hop-by-hop mechanisms
 Shared Secret-Key
 Transport Layer Security (TLS)
 Digest (Hashing) Algorithm
 IP Security (IPSec)
 Annex E - Signature Security Profile
 The SIPS URI schema
 Signature Profile – Public Key
(source: Ben Campbell presentation)
Infrastructure (PKI)
 Certificate Based Security
 Scalable - applicable for “Global” IP
Telephony
 Hop-by-Hop and End-to-End
security
 Digest Algorithms
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(Source: Radvision PPT)
SIP Authentication
SIP Server
SIP Client
REQUEST
Generate the
Nonce value
CHALLENGE
Nonce, realm
Compute response = F(nonce,
Username, password, realm)
F= MD5
REQUEST
Nonce, realm,
Username, response
Authenticate: compute
F(nonce, username, password, realm)
And compare with response
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Market Analysis
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Chart 1 summarizes the
technology supported
by the 77 products.
(source: Wind River
White Paper)
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Chart 2 summarizes the
technology supported
by VoIP Service
Providers. (source:
Wind River White
Paper)
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Interoperability
Source: Ho et al.
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Conclusion
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If SIP is better, why is H.323 important?
 Huge installed base and backward compatibility is important.
 However, newer products may not need H.323.
In videoconferencing world, H.323 is still a dominant player.
Most VoIP products support H.323 and SIP together. But this has
the potential to increase the cost, size and power requirements of
the products.
An all-SIP network is simple and cleaner to run/manage but we
will see H.323/SIP for a long time.
Security mechanisms (authentication, privacy, authorization,
integrity, non-repudiation) may well decide their fate.
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References
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www.ietf.org drafts and RFCs (3261, 2543) for SIP
ITU-T and H.323 specifications.
SIP Vs. H.323:A Business Analysis, white paper from WindRiver.
SIP versus H.323, iptel.org/info/trends/sip.html
H.323 versus SIP: A Comparison, packetizer analysis at
http://www.packetizer.com/iptel/h323_vs_sip/
A Comparison of SIP and H.323 for Internet Telephony
Henning Schulzrinne and Jonathan Rosenberg
Network and Operating System Support for Digital Audio and
Video (NOSSDAV), (Cambridge, England), July 1998.
For our work on SIP/H.323 security, see
http://middleware.internet2.edu/video/
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