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Session Initiation Protocol (SIP) - Call Setup

The Session Initiation Protocol (SIP) is an IETF proposed standard for multimedia conferencing over IP. SIP is an application-layer control protocol that can be used to establish, maintain, and terminate calls between two or more end points.

A SIP networks consits of clients and a server. SIP clients (phones or softphones) initiate SIP requests and respond to requests.

SIP servers include:

  • Proxy server: an intermediate device that receives SIP requests from a client and then forwards them to the next SIP server in the network.
  • Redirect server: provides information about the next hop or hops that a request should take.
The animation shows SIP operation in the different modes: a direct call between two clients, a call through a proxy server and through a redirect server.