The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. It can be used to create two-party, multiparty, or multicast sessions that include Internet telephone calls, multimedia distribution, and multimedia conferences. SIP is designed to be independent of the underlying transport layer; it can run on TCP, UDP, or SCTP. It is widely used as a signaling protocol for Voice over IP, along with H.323 and others.
SIP has the following characteristics:
a) Transport-independent, because SIP can be used with UDP, TCP, SCTP & so on.
b) Text-based, allowing for humans to read SIP messages.