What is Session Initiation Protocol SIP
SIP stands for "Session Initiation Protocol" It was designed in 1996 by the joined efforts of Henning Schulzrinne and Mark Handley. "SIP is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences." (Wikipedia.com, 2007)
Its design was decided in such a way so that complexity can be reduced and it can become lightweight. It just has six methods in all to reduce the complexity. The next goal was to implement a transport independent protocol, this need raised due to its heterogeneous nature. It can be executed over other protocols like "UDP, TCP, ATM, etc". SIP is text based to make it user-friendly; this feature differentiates SIP from its competitors.
"SIP clients use TCP or UDP typically using port 5060 to connect to SIP servers and other SIP endpoints. SIP is primarily used in setting up and tearing down voice or video calls" (Wikipedia.com, 2007). SIP provides necessary mechanisms for end systems and proxy servers to leverage the services like, call forwarding, callee & calling number delivery, location-independent addressing, terminal-type negotiation & selection, terminal capability negotiation, caller and callee authentication, blind & supervised call transfer and invitation to multicast conferences (Schulzrinne, 2007).
There is a lot of enhancement in SIP from its development till present. Various new features have been added into its functionality. Few examples are "allow click-to-dial services, fully meshed conferences and connections to multipoint control units (MCUs)". This protocol is very simple and that's why this is very popular. An address like email is used by the SIP for addressing and SMTP EXPN is used for expanding the address.
This feature is used by many latest applications like browsers, email programs. A direct call can be made using the default application based on telephony which is implemented on the desktops. This is done by providing a link like URL; user can click the link to make the call directly. SIP addresses are as simple as URLs that allows them to made part of the web pages (Admin, 2007). These can be used directly, without any modifications for "Internet Fax delivery". Other terms used for these addresses are "SIP, H.323 or telephone (E.164)". A reliable datagram service is not the mandatory requirement for it because it does not depend on the packet layer. Instead of running over UDP or TCP, it can also be run on "IPX, carrier pigeons, frame relay, ATM AAL5, X.25".