Understanding Session Initiation Protocol (SIP)

Session Initiation Protocol (SIP) is a signaling protocol used for a variety of purposes in IP networks.

SIP is principally a mechanism employed to seamlessly create, modify and terminate sessions involving multiple participants. Such sessions could be Internet telephone calls, multimedia conferences or multicast sessions. SIP can work with any type of media content. SIP is independent of the transport layer and can therefore be used with multiple transport protocols as TCP, UDP or SCTP.

The IMS components can be implemented using SIP agents and application servers. SIP was designed in 1996 and specified by International Engineering Task Force (IETF).

SIP is an application-layer protocol. It runs above the User Datagram Protocol (UDP). SIP is used for starting, modifying and ending communication, conference and collaborative sessions on the platform of Internet Protocol (IP) networks. SIP helps users to invite other participants to an existing session where the participants may be persons, automated service or a physical device such as a handset.

The SIP session has four key components:

  • SIP User Agents
  • SIP Register Servers
  • SIP Proxy Servers
  • SIP Redirect Servers

These components are of vital importance to IMS as they are commonly employed to instantiate IMS components. The SIP components work in tandem for the delivery of messages after defining their content and characteristics, thus completing a SIP session.

The SIP User Agents (UA) are the devices which are used by the end users. These devices could be PCs, PDAs etc. that are capable of supporting the SIP session. The User Agent is present in the form of a client in the UE and in the form of a server in the network infrastructure. The message is created by the User Agent Client and the response to this message is given by the User Agent Server.

SIP Registrar Servers are the databases that hold the data concerning the User Agents in a domain. These servers obtain and send the IP addresses of the participants and also other information to the SIP Proxy Server.

SIP Proxy Servers accept session requests made by SIP UAs. Proxy Servers obtain the UA’s addressing information from the SIP Registrar server. If the recipient UA resides in the same domain, then this information thus retrieved is forwarded to the recipient UA. If the recipient UA resides in another domain, then the information retrieved is passed on to a Proxy Server.

SIP Redirect Server passes on the SIP session invitation to the external domains. The SIP redirect servers are located in the SIP Registrar Servers and SIP Proxy Servers.

SIP is essential to IMS as it is used for signaling between various IMS network elements for control and orchestration.