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> Chapter 34 - VoIP Solutions: SIP > SIP messages and media protocols > SIP response messages

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SIP response messages

SIP response messages (often just called SIP responses) provide status information in response to SIP request messages. All SIP response messages include a response code and a reason phrase. There are five SIP response message classes. They are described below.

There are also two types of SIP response messages, provisional and final. Final response messages convey the result of the request processing, and are sent reliably. Provisional responses provide information on the progress of the request processing, but may not be sent reliably. Provisional response messages start with 1xx and are also called informational response messages.

Informational (or provisional)

Informational or provisional responses indicate that a request message was received and imply that the endpoint is going to process the request. Information messages may not be sent reliably and may not require an acknowledgement.

If the SIP implementation uses Provisional Response Acknowledgment (PRACK) (RFC 3262) then informational or provisional messages are sent reliably and require a PRACK message to acknowledge that they have been received.

Informational responses can contain the following reason codes and reason phrases:

100 Trying

180 Ringing

181 Call is being forwarded

182 Queued

183 Session progress

Success

Success responses indicate that a request message was received, understood, and accepted. Success responses can contain the following reason codes and reason phrases:

200 OK

202 Accepted

Redirection

Redirection responses indicate that more information is required for the endpoint to respond to a request message. Redirection responses can contain the following reason codes and reason phrases:

300 Multiple choices

301 Moved permanently

302 Moved temporarily

305 Use proxy

380 Alternative service

Client error

Client error responses indicate that a request message was received by a server that contains syntax that the server cannot understand (i.e. contains a syntax error) or cannot comply with. Client error responses include the following reason codes and reason phrases:

400 Bad request               401 Unauthorized

402 Payment required          403 Forbidden

404 Not found                 405 Method not allowed

406 Not acceptable            407 Proxy authentication required

408 Request time-out          409 Conflict

410 Gone                      411 Length required

413 Request entity too large  414 Request-URL too large

415 Unsupported media type    420 Bad extension

480 Temporarily not available

481 Call leg/transaction does not exist

482 Loop detected

484 Address incomplete        483 Too many hops

486 Busy here                 485 Ambiguous

488 Not acceptable here       487 Request canceled

Server error

Server error responses indicate that a server was unable to respond to a valid request message. Server error responses include the following reason codes and reason phrases:

500 Server internal error

501 Not implemented

502 Bad gateway

502 Service unavailable

504 Gateway time-out

505 SIP version not supported

Global failure

Global failure responses indicate that there are no servers available that can respond to a request message. Global failure responses include the following reason codes and reason phrases:

600 Busy everywhere

603 Decline

604 Does not exist anywhere

606 Not acceptable