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Page 1 of 4 H.323 versus SIP: A Comparison Provided with permission from Packetizer. This is, frankly, the best comparison of H.323 and SIP available anywhere. Virtually all of the others are misleading, out-of-date, and just plain wrong. To compound the problem--to further propagate the error, as it were--we have also seen several papers written by naive students and rank-and-file engineers that blindly parrot what they have read in these comparisons. Furthermore, many, many people have formed their opinions of H.323 and SIP based not on each protocol's merits but solely on the misinformation provided by these comparisons and through other information provided by largely the same sources. To counter this misinformation, we decided to put together this thorough, up-to-date comparison. As with ours, please consider the financial interests of the source of any information on this subject, be it an author, speaker, institution, forum, company, web site, or conference. Are the people providing information on this issue involved in both of these--and other--protocols and have nothing besides perhaps an honest academic interest in one or the other protocol, or have they otherwise "hitched their wagon" to one?
Like everything else on the web, this is a living document which we will be updating as the standards evolve. In fact, there is much work in progress for both H.323 and SIP, but, in order to compare apples to apples and make this comparison meaningful, we have chosen to focus on what is currently defined rather than on what might be defined in the future. Also, note that commentary that is not vital to the main comparison text appears in a smaller font immediately below it. | | H.323 | SIP | | Philosophy | H.323 was designed with a good understanding of the requirements for multimedia communication over IP networks, including audio, video, and data conferencing. It defines an entire, unified system for performing these functions, leveraging the strengths of the IETF and ITU-T protocols. As a result, it might be reasonable for users to expect about the same level of robustness and interoperability as is found on the PSTN today, although this admittedly varies across the globe. H.323 was designed to scale to add new functionality. The most widely deployed use of H.323 is "Voice over IP" followed by "Videoconferencing", both of which are described in the H.323 specifications. | SIP was designed to setup a "session" between two points and to be a modular, flexible component of the Internet architecture. It has a loose concept of a call (that being a "session" with media streams), has no support for multimedia conferencing, and the integration of sometimes disparate standards is largely left up to each vendor. As a result, SIP is now a 10-year old protocol with a vast number of interoperability problems. While SIP has been successfully deployed in some environments, those are generally "closed" environments where the means of interoperability has been PSTN gateways. | | Reliability | H.323 has defined a number of features to handle failure of intermediate network entities, including "alternate gatekeepers", "alternate endpoints", and a means of recovering from connection failures. | SIP has not defined procedures for handling device failure. If a proxy fails, the user agent detects this through timer expiration. It is the responsibility of the user-agent to send a re-INVITE to another proxy, leading to long delays in call establishment. | | Message Definition | ASN.1, a standardized, extremely precise, easy-to-understand structural notation that is used by many other systems. | ABNF, or Augmented Backus-Naur Form, a syntactical notation. SIP uses the ABNF as defined in RFC 2234. | | Message Encoding | H.323 encodes messages in a compact binary format that is suitable for narrowband and broadband connections. Messages are efficiently encoded and decoded by machines, with decoders widely available (e.g., Ethereal). | SIP messages are encoded in ASCII text format, suitable for humans to read. As a consequence, the messages are large and less suitable for networks where bandwidth, delay, and/or processing are a concern. SIP messages get so large that they sometimes exceed the MTU size when going over WAN links, resulting in delays, packet loss, etc. As a result, effort has been made to binary encode SIP (e.g., RFC 3485 and RFC 3486).
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