PRACK signaling a Provisional Response ACKnowledgment allows reliable information to be signaled concerning the progress of a request ■SUBSCRIBE/NOTIFY Enables SIP UAs to be signaled with event notifications
◼ PRACK signaling a Provisional Response ACKnowledgment allows reliable information to be signaled concerning the progress of a request ◼ SUBSCRIBE/NOTIFY Enables SIP UAs to be signaled with event notifications
UPDATE Allows a session to be modified ■REFER Enables call transfer ■MESSAGE Used for instant messaging
◼ UPDATE Allows a session to be modified ◼ REFER Enables call transfer ◼ MESSAGE Used for instant messaging
SIP message format a text-based protocol similar in many respects to HTTP The following is an example of a request line: INVITE sip:charlie@example.com SIP/2.0 where: INVITE indicates the SIP method. sip:charlie@example.com is a SIP URI. SIP/2.0 indicates the protocol version. The following is an example of a status line: SIP/2.0 180 Ringing where: SIP/2.0 indicates the protocol version. ◆ 180 provides a numerical status code of the transaction. Ringing provides a human-readable format of the transaction status
SIP message format ◼ a text-based protocol similar in many respects to HTTP