List of SIP request methods

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SIP requests are the messages in the Session Initiation Protocol that initiate a functionality of the protocol. They are sent by a user agent client to the server, and are answered with one or more SIP responses, which return a result code of the transaction, and generally indicate the success, failure, or other state of the transaction.

SIP requests
Request name Description Notes RFC references
INVITE The invite message initiates a SIP dialog with the intent to establish a call. It is sent by a user agent client to a user agent server. When sent during an established dialog (reinvite) it modifies the sessions, for example placing a call on hold. RFC 3261
ACK Confirm that an entity has received a final response to an INVITE request. RFC 3261
BYE This method signals termination of a dialog and ends a call. This message may be sent by either endpoint of a dialog. RFC 3261
CANCEL Cancel any pending request. Usually means terminating a SIP call while it is still ringing, before answer. RFC 3261
OPTIONS Query the capabilities of an endpoint. It is often used for keepalive purposes. RFC 3261
REGISTER Register the SIP URI listed in the To header field with a location server and associates it with the network address given in a Contact header field. It implements a location service. RFC 3261
PRACK Provisional acknowledgement. PRACK is sent in response to provisional response (1xx). RFC 3262
SUBSCRIBE Initiates a subscription for notification of events from a notifier. RFC 6665
NOTIFY Inform a subscriber of notifications of a new event. RFC 6665
PUBLISH Publish an event to a notification server. RFC 3903
INFO Send mid-session information that does not modify the session state. This method is often used for DTMF relay. RFC 6086
REFER Ask recipient to issue SIP request for the purpose of call transfer. RFC 3515
MESSAGE Transport text messages. RFC 3428
UPDATE Modifies the state of a session without changing the state of the dialog. RFC 3311