Stands for "Session Initiation Protocol." SIP is a protocol defined by the Internet Engineering Task Force (IETF). It is used for establishing sessions between two or more telecommunications devices over the Internet.
SIP has many applications, such as initiating video conferences, file transfers, instant messaging sessions, and multiplayer games. However, it is most well known for establishing voice and video calls over the Internet. VoIP companies, such as Vonage, Phone Power, and others, use SIP to provide Internet-based telephone services. This system, called "SIP trunking" allows clients to communicate over standard phone lines using IP phones or computers with VoIP software installed. A SIP server provides the translation from the VoIP connection to the public switched telephone network (PSTN).
Similar to HTTP, SIP uses simple request and response messages to initiate sessions. For example, the INVITE request message is used to invite a user to begin a session and ACK confirms the user has received the request. The response code 180 (Ringing) means the user is being alerted of the call and 200 (OK) indicates the request was successful. Once a session has been established, BYE is used to end the communication. While SIP codes are not always seen by users, they can be useful when troubleshooting unreliable connections.
NOTE: SIP may also stand for "Standard Interchange Protocol," which is a library system communication standard developed by 3M. SIP and SIP2 were designed to handle basic inventory operations, such as checking in and checking out library books. Both versions of the Standard Interchange Protocol have been largely replaced by the National Information Standards Organization Circulation Interchange Protocol (NCIP).