Session Initiation Protocol (SIP) is used for controlling multimedia communication sessions over an IP network. Common applications include voice over IP (VoIP), videoconferencing, streaming multimedia, on-line gaming, and instant messaging. SIP is the protocol of choice for VoIP, and is used to create, modify, and terminate VoIP sessions, including functions such as call transfer, conference calls, and call hold.
This very high-level protocol operates primarily in the Application Layer (Layer 7) of the OSI model. Because SIP runs independently of the Transport Layer (Layer 4), it works with most transport protocols, including TCP and UDP.
Much like HTTP, SIP is a text-based protocol. SIP messages contain only as much information as is needed for each session, so it’s very efficient and can expand and contract to meet each application’s specific requirements. This extensibility makes SIP incredibly versatile, enabling it to cover functions ranging from simple VoIP calls to complex multi-user videoconferencing.
SIP uses proxy servers to route requests, authenticate users, and provide features such as voice mail. SIP performs five basic functions: