SIP stands for Session Initiation Protocol and is the most widely used protocol in IP telephony. SIP controls how a call is established, how voice data is transferred during the call, as well as the termination of the connection. It can be used for other communication services as well. The SIP protocol was developed by the Internet Engineering Task Force (IETF).
The Session Initiation Protocol is text based and only used for establishing a communication session. The actual data transmission is done by the Transmission Control Protocol (TCP) or the User Datagramm Protocol (UDP) on layer 5 of the OSI model. The Session Description Protocol SDP controls which of the protocols is used.
SIP is very similar to the Hypertext Transfer Protocol (HTTP), which makes it easy to integrate it into web browsers, applications, services and electronic devices. Since most electronic devices have access to the internet nowadays, it is easy to have them all use the SIP protocol for instant messaging, presence information, file transfer and voice calls.
In IP telephony SIP is used by all types of services. IP phones are using SIP to communicate with other IP phones, IP phone systems (PBXs), gateways and SIP telephony providers. SIP is used in local networks (LAN) and the internet.