Switching to VoIP

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Appendix A

APPENDIX A

SIP Methods and Responses

Table A-1 shows SIP methods.

Table A-2 shows SIP responses.

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Table A-1. Methods

INVITE A SIP device is being invited to participate in a call.

ACK Confirms that the client has received a final response to an INVITE request.

BYE Terminates a SIP call. Can be sent by any party involved.

CANCEL Cancels any pending call but does not terminate a call that has already been connected.

OPTIONS Queries the capabilities of servers without requesting to establish a call.

REGISTER Registers an IP with a SIP registrar.

PRACK Insures reliability of provisional 1xx responses if a UAS offers them.

UPDATE Updates a previously made offer for a not-yet-established session.

REFER Initiates a call transfer by telling the recipient (specified by URI) to contact a third party using the con-

tact information provided in the request.

SUBSCRIBE Subscribes to be notified of an event occurrence; for example a user presence update.

NOTIFY Used to notify that an event has occurred.

MESSAGE A method signifying the payload is an instant message.

Table A-2. Responses

100 Trying

180 Ringing

181 Call Is Being Forwarded

182 Queued

183 Session Progress

200 OK

202 Accepted

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