Session Initiation Protocol

by Kris ZypMarch 26th, 2009

The Session Initiation Protocol (SIP) is another duplex communication protocol that is worth advising in Comet architectural design. SIP provides a mechanism for establishing a bi-directional communication channel with negotiation of acceptable media type and semantics for message routing and is used for voice over IP (VOIP). Of course browsers don’t directly support SIP, so it is not possible to implement true SIP messaging as a Comet technique, but many of concepts and semantics from this protocol are certainly reusable. SIP is an IETF standardized protocol and shares numerous interoperable techniques and syntax with other foundational IETF protocols for the Internet like HTTP.

SIP messages follow the MIME/HTTP style syntax, and SIP has also been extended to provide a powerful integration with HTTP for monitoring and subscribing to resource changes (along with an extension for efficient subscription to multiple resources at once). This technique bears a strong correspondence with the REST Channels mechanism, providing a means with integrating with a REST architecture using the semantics from HTTP (REST Channels is specifically designed for Comet though), allowing a client to subscribe to resources and be notified of changes.

Since SIP is transport agnostic, it would also be possible to layer on top of Comet transport layer like or WebSockets.

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