Editing Session Initiation Protocol (section)

===Requests===
Requests 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.

{|class="wikitable"
|+ SIP requests
! Request name !! Description !! Notes !! RFC references
|-
| REGISTER || Register the URI listed in the To-header field with a location server and associates it with the network address given in a ''Contact'' header field. || The command implements a location service. || {{IETF RFC|3261}}
|-
| INVITE || Initiate a dialog for establishing a call. The request 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. || {{IETF RFC|3261}}
|-
| ACK || Confirm that an entity has received a final response to an INVITE request. || || {{IETF RFC|3261}}
|-
| BYE || Signal termination of a dialog and end a call. || This message may be sent by either endpoint of a dialog. || {{IETF RFC|3261}}
|-
| CANCEL || Cancel any pending request. || Usually means terminating a call while it is still ringing, before answer. || {{IETF RFC|3261}}
|-
| UPDATE || Modify the state of a session without changing the state of the dialog. || || {{IETF RFC|3311}}
|-
| REFER || Ask recipient to issue a request for the purpose of call transfer. || || {{IETF RFC|3515}}
|-
| PRACK || Provisional acknowledgement. || PRACK is sent in response to provisional response (1xx). || {{IETF RFC|3262}}
|-
| SUBSCRIBE || Initiates a subscription for notification of events from a notifier. || || {{IETF RFC|6665}}
|-
| NOTIFY || Inform a subscriber of notifications of a new event. || || {{IETF RFC|6665}}
|-
| PUBLISH || Publish an event to a notification server. || || {{IETF RFC|3903}}
|-
| MESSAGE || Deliver a text message. || Used in instant messaging applications. || {{IETF RFC|3428}}
|-
| INFO || Send mid-session information that does not modify the session state. || This method is often used for DTMF relay.  || {{IETF RFC|6086}}
|-
| OPTIONS || Query the capabilities of an endpoint. || It is often used for NAT [[keepalive]] purposes. || {{IETF RFC|3261}}
|-
|}