The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying and terminating sessions with one or more participants.
SIP, the Session Initiation Protocol, is a signaling protocol for Internet conferencing, telephony, presence, events notification and instant messaging.
SIP can also be used for signaling Internet real-time fax delivery. This requires no major changes. Fax might be carried via RTP, TCP (e.g., the protocols discussed in the Internet fax WG) or other mechanisms.
SIP is independent of the packet layer and only requires an unreliable datagram service(UDP), as it provides its own reliability mechanism.
SIP provides the necessary protocol mechanisms so that end systems and proxy servers can provide services:
call forwarding, including
-the equivalent of 700-, 800- and 900- type calls
-call-forwarding no answer
-call-forwarding busy
-call-forwarding unconditional
-other address-translation services
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