SIP uses Methods / Requests and corresponding Responses to communicate and establish a call session.
SIP Requests:
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 Responses:
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.