SIP Description/Call Flow
SIP, session initiation protocol, is the protocol used to facilitate the transfer of Non-Audio VoIP call data. It is used to initiate and maintain a call from start to end.
Common SIP Methods:
- ACK - Used as an acknowledgement of receipt and/or acceptance of a SIP message
- BYE - Used to end a session in progress (i.e. after a 200 OK)
- CANCEL - Used to cancel a session before it begins (i.e. before a 200 OK)
- INVITE - Used to initiate a new session
- RE-INVITE - This is a second invite sent with the same Call-ID. This is done to change aspects of the dialog.
Uncommon SIP Methods:
- OPTIONS - Used to ask a SIP server what methods are supported
- MESSAGE - Single message transmission used in instant messaging (RFC3428)
- UPDATE - Essentially like a re-invite, but it can be sent at any time during the dialog (i.e. both before and after the 200 OK or any other non-provisional response code)
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