The Session Initiation Protocol (SIP) is an application-layer signaling protocol defined by the Internet Engineering Task Force (IETF) and Third Generation Partnership Project (3GPP) for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.

Functions:

SIP invitations used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different IP transport protocols including UDP, SCTP, and TCP.