Appendix A SIP Messages

This appendix describes the SIP requests and responses.

SIP Requests

The following table lists the SIP requests.

Table A–1 SIP Request Methods


SIP Request	Description
INVITE	A client is being invited to participate in a call.
ACK	The client has confirmed the INVITE request.
BYE	The call has been terminated by either the caller or callee.
CANCEL	Cancel any pending requests.
OPTIONS	Queries the server for its capabilities.
REGISTER	Registers the client with the server according to the address in the To header.
PRACK	Similar to ACK, but a provisional confirmation.
SUBSCRIBE	Subscribes the device for an event notification.
NOTIFY	Notifies all subscribers of an event.
PUBLISH	Publishes an event to a server.
INFO	Sends information in the middle of a session that doesn't modify the session's state.
REFER	Asks the client to issue a SIP request, typically a call transfer.
MESSAGE	Sends an instant message using SIP.
UPDATE	Modifies a session's state without altering the dialog state.

For a list of all SIP requests and links to their definitions in their respective RFCs, see the SIP requests Wikipedia entry.

Table A–2 SIP Response Codes

For a list of all SIP responses, see the SIP responses Wikipedia entry.