What are SIP methods - requests and responses?
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)
3xx = Redirection responses
Further reading