Session Initiation Protocol

Session Initiation Protocol (SIP) is a signaling protocol used to establish, modify, and terminate sessions for multimedia communication over IP networks. A session may be a voice call, video call, or multi-party conference. SIP does not carry the voice or video itself; it only handles the signaling portion, which concerns routing information about who is calling, who is being called, and what type of media is being used.

SIP is defined in documents issued by the IETF (Internet Engineering Task Force). It is a text-based protocol whose structure resembles HTTP: it consists of requests with a method and responses with status codes. For this reason, SIP messages are human-readable and can be examined directly.

How it works

A user has an address in the form of a SIP URI similar to an email address, for example sip:name@domain. When initiating a call, the calling device sends an INVITE message. Responses include 100 Trying, 180 Ringing, and 200 OK. When accepted, an ACK is sent, and call termination uses BYE. Other methods include REGISTER (for registering a user's location), CANCEL, OPTIONS, and SUBSCRIBE/NOTIFY.

Network components

A SIP system consists of user agents (the user's device, which can be a caller or a callee) and various servers: a proxy server that forwards requests, a registrar server that records a user's location, a redirect server that provides an alternative address, and a location service that maintains registration data.

Companion protocols

SIP alone does not complete a call. It is commonly used alongside SDP (Session Description Protocol), which describes codec types and media addresses, and RTP (Real-time Transport Protocol), which carries voice and video. SIP messages can be transmitted over UDP, TCP, or TLS; for encrypted media transmission, SRTP is used.

Uses and challenges

SIP is the foundation of VoIP (voice over internet) services, corporate PBX systems, and mobile telephone networks based on IMS (IP Multimedia Subsystem). Common challenges include problems with NAT and firewalls, addressed through solutions such as STUN, TURN, and ICE, as well as call spoofing attacks and service disruptions.