SIP request types
RFC 3261 (SIP) uses six types (methods) of requests:
- INVITE — Indicates a client is being invited to participate in a call session.
- ACK — Confirms that the client has received a final response to an INVITE request.
- BYE — Terminates a call and can be sent by either the caller or the callee.
- CANCEL — Cancels any pending searches but does not terminate a call that has already been accepted.
- OPTIONS — Queries the capabilities of servers.
- REGISTER — Registers the address listed in the To header field with a SIP server.
RFC 3262 Reliability of Provisional Responses in SIP
- PRACK – Provisional acknowledgement.
RFC 3265 extends the basic requests, to support notification:
- SUBSCRIBE — Subscribes for an Event of Notification from the Notifier.
- NOTIFY — Notify the subscriber of a new Event.
RFC 3903 extends the basic requests, to publish event changes to the server:
- PUBLISH – publishes an event to the Server.
Other requests introduced in SIP are:
- INFO – Sends mid-session information that does not modify the session state. (RFC 2976)
- REFER – Ask recipient to issue SIP request (call transfer.) (RFC 3515)
- MESSAGE – The MESSAGE is used to transport instant messages using SIP. (RFC 3428)
- UPDATE – The UPDATE method is used to modify the state of a session without changing the state of the dialog. (RFC 3311)