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SIP messages
SIP messages, including SIP request messages and SIP response messages, are encoded in text
mode.
SIP request messages include INVITE, ACK, OPTIONS, BYE, CANCEL, and REGISTER. RFC
3261 defines the following six request messages:
• INVITE—Used to invite a user to join a call.
• ACK—Used to acknowledge the response to a request.
• OPTIONS—Used to query for the capabilities.
• BYE—Used to release an established call.
• CANCEL—Used to give up a call attempt.
• REGISTER—Used to register with the SIP registrar.
SIP response messages, used to respond to SIP requests, indicate the status of a call or registration,
succeeded or failed. Response messages are distinguished by status codes. Each status code is a
3-digit integer, where the first digit defines the class of a response, and the last two digits describe
the response message in more detail.
Table 237 Status codes of response messages
Code Description Class
100–199 Request is received and is being processed. Provisional
200–299 Request is successfully received, understood, and accepted. Success
300–399 Further action needs to be taken to process the request. Redirection
400–499 Request contains bad syntax, and therefore cannot be processed. Client error
500–599 Request cannot be processed due to UAS or server error. Server error
600–699 Request cannot be processed by any UAS or server. Global error
SIP fundamentals
Registration
In a complete SIP system, all SIP endpoints working as UAs should register with SIP registrars,
providing information such as location, session capabilities, and call policy.
Typically, a SIP UA sends its registrar a REGISTER request at startup or in response to an
administratively registration operation, carrying all the information that must be recorded. Upon
receipt of the request, the registrar sends back a response notifying receipt of the request, and a 200
OK (SUCCESS) message if the registration is accepted. The following figure shows the message
exchange.
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