What is SIP?
SIP (Session Initiation Protocol) is a protocol that uses to create, modify, and terminate sessions with one user or more. SIP was published as working standard by IETF (Internet Engineering Task Force) in 2002. RFC 3261 is IETF specification for SIP. Sessions of SIP can consist of voice communications, instant messaging, and multimedia applications.
SIP is often used to initiate and terminate VoIP (Voice over Internet Protocol) calls. A common SIP session involves a client and a SIP server. The client sends the request to the SIP server, after server received the request; it returns a response to the user to show that there is availability of session.
SIP is ASCII text-based. It shares some common characteristics with HTTP. SIP relies on a peer-to-peer architecture which uses the intelligent network elements for advanced functions of call processing and management. Both endpoints of SIP referred as user agent client and user agent server. SIP proxy server has advanced call processing functions includes authentication, security, and call routing. Beside, RTP (Real-Time Transport) protocol is used to provide the voice or video content at the application layer between the endpoints.
SIP is widely used by instant messaging applications, for example, Microsoft MSN Messenger and Apple iChat also use SIP client for transporting voice and video free of charge.
XMPP (Extensible Messaging and Presence Protocol) and SIMPLE (Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions) are two SIP based protocols which allow presence information to exchange between the endpoints. An example of presence indicator is the icon (in instant messaging client) shows that the user is online, so that other people can chat with.
However, SIP has limitation of emergency calling and law enforcement interception activities. Despite these, SIP is gaining popularity due to its ease of deployment and management, scalability, and low cost.