The Session Initiation Protocol (SIP) is a communication protocol that is used in internet telephony, as well as instant messaging. SIP is used for signaling and controlling multimedia communication session such as voice and video calls over Internet Protocol (IP) networks.
The SIP protocol administrates initiation and termination of messages between two end points as all we all other essential elements of the transmission. The protocol is used to create, modify and termination of sessions that can contain one or more different types of media streams. This text based application layer protocol, incorporating many elements of HTTP (Hypertext Transfer Protocol) was developed independently to the underlying transport layer.
SIP can work in parallel with other application layer protocols including Session Description Protocol (SDP), Real-time Transport Protocol (RTP) or Secure Real-time Transport Protocol (SRTP) for streaming media etc. Transport Layer Security (TLS) can be used to encrypt data for sending secure SIP messages.