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Collaboration, Voice & AV · Foundational

SIP INVITE

The SIP request method that initiates a session, normally carrying an SDP offer describing the caller's media.

Definition

An INVITE starts a three-way handshake: INVITE from caller, a final 2xx response from the callee, and an ACK confirming it. A re-INVITE within an established dialog modifies the session, for example to place a call on hold, change codecs or add video. Other core methods include ACK, BYE, CANCEL, REGISTER and OPTIONS, with extensions such as UPDATE, REFER, PRACK, SUBSCRIBE and NOTIFY.

Example

A re-INVITE with a=sendonly in the SDP places a call on hold.

Exam tip

Know the basic flow INVITE, 100 Trying, 180 Ringing, 200 OK, ACK, then BYE and 200 OK to end the call.

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