Session Initiation Protocol (SIP, RFC 2543) – part of IETF‘s multimedia data and control protocol framework. SIP is a powerful client-server signaling protocol used in VoIP networks. SIP handles the setup and tear down of multimedia sessions between speakers; these sessions can include multimedia conferences, telephone calls, and multimedia distribution.
SIP is a text-based signaling protocol transported over either transmission control protocol (TCP) or user datagram protocol (UDP), and is designed to be lightweight. It inherited some design philosophy and architecture from the hypertext transfer protocol (HTTP) and simple mail transfer protocol (SMTP) to ensure its simplicity, efficiency and extensibility.
SIP uses invitations to create session description protocol (SDP) messages to carry out capability exchange and to setup call control channel use. These invitations allow participants to agree on a set of compatible media types.
SIP supports user mobility by proxying and redirecting requests to the user’s current location. Users can inform the server of their current location (IP address or URL) by sending a registration message to a registrar. This function is powerful and often needed for a highly mobile voice user base.