Editing Session Initiation Protocol (section)

===Responses===
{{Main|List of SIP response codes}}
Responses are sent by the user agent server indicating the result of a received request. Several classes of responses are recognized, determined by the numerical range of result codes:<ref>Stallings, pp.216-217<!--which William Stallings book are we referring to here?--></ref>
* 1xx: Provisional responses to requests indicate the request was valid and is being processed.
* 2xx: Successful completion of the request. As a response to an INVITE, it indicates a call is established. The most common code is 200, which is an unqualified success report.
* 3xx: Call redirection is needed for completion of the request. The request must be completed with a new destination.
* 4xx: The request cannot be completed at the server for a variety of reasons, including bad request syntax (code 400).
* 5xx: The server failed to fulfill an apparently valid request, including server internal errors (code 500).
* 6xx: The request cannot be fulfilled at any server. It indicates a global failure, including call rejection by the destination.