SIP Methods / Requests and Responses
SIP uses methods / requests and corresponding responses to communicate and establish a call session.
There are fourteen SIP request methods of which the first six are the most basic request / method types:
- INVITE = Establishes a session.
- ACK = Confirms an INVITE request.
- BYE = Ends a session.
- CANCEL = Cancels establishing of a session.
- REGISTER = Communicates user location (host name, IP).
- OPTIONS = Communicates information about the capabilities of the calling and receiving SIP phones.
- PRACK = Provisional acknowledgement.
- SUBSCRIBE = Subscribes for notification from the notifier.
- NOTIFY = Notifies the subscriber of a new event.
- PUBLISH = Publishes an event to the server.
- INFO = Sends mid session information.
- REFER = Asks the recipient to issue call transfer.
- MESSAGE = Transports instant messages.
- UPDATE = Modifies the state of a session.
SIP requests are answered with SIP responses, of which there are six classes:
1xx = Informational responses, such as 180 (ringing).
2xx = Success responses.
3xx = Redirection responses.
4XX = Request failures.
5xx = Server errors.
6xx = Global failures.