The Session Initiation Protocol (SIP) is a signaling protocol for initiating, managing and terminating voice and video sessions across packet networks. SIP sessions involve one or more participants and can use unicast or multicast communication. Borrowing from ubiquitous Internet protocols, such as HTTP and SMTP, SIP is text-encoded and highly extensible. SIP may be extended to accommodate features and services such as call control services, mobility, interoperability with existing telephony systems, and more. SIP is being developed by the SIP Working Group, within the Internet Engineering Task Force (IETF). The protocol is published as IETF RFC 2543 and currently has the status of a proposed standard. This paper describes the key constituents of SIP.
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