| 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[how?] for keepalive purposes.[citation needed] | 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.[how?] | 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 |