The Session Initiation Protocol (SIP) is a widely-used signaling protocol for controlling multimedia communication sessions such as voice and video calls over Internet Protocol (IP). The latest version of the specification is in RFC 3261 from the IETF Network Working Group.

SIP can be used for setting up and tearing down two-party (unicast) or multiparty (multicast) sessions consisting of one or several media streams. It can also modify such sessions by changing addresses or ports, inviting more participants, adding or deleting media streams, etc. Applications that can use SIP include video conferencing, streaming multimedia distribution, instant messaging, presence information, file transfer and online games.

SIP is now a 3GPP signaling protocol and permanent element of the IP Multimedia Subsystem (IMS) architecture for IP-based streaming multimedia services in cellular systems.