SIP a protocol
One of the mistakes that many people make when they look at SIP is comparing SIP to standards such as 802.11n for wireless LANs. IEEE and ITU specifications are rigid and clearly define how vendors must implement certain mechanisms. SIP is a protocol similar to HTTP that merely defines how two end-points find one another and exchange the information necessary to establish a session. But SIP, like HTTP, leaves the specifics of the session definition to the developers. SIP session requests leverage the “session description protocol (SDP)” to define session characteristics such as media type, codec, and so on. While the IETF and SIPforum have standardized many of the SDP elements, developers still have flexibility to implement SIP-based communications applications in their own way. Vendors argue this flexibility allows them to use better approaches for session management than their competitors, while end-users express frustration that one vendor’s SIP product won’t easily interoperate with anothers.
