SIP - 4xx: Client Failure Responses

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Client error responses indicate that the request cannot be fulfilled as some errors are identified from the UAC side. The response codes are generally sent by UAS. Upon receiving an error message, the client should resend the request by modifying it based on the response. Discussed below are some of the important client error responses.

400 Bad Request

• This indicates that the server could not understood the request.

• Request might be missing required header fields such as To, From, Call-ID, or CSeq.

401 Unauthorized

• It indicates that the request need to perform authentication.

• 401 Unauthorized is normally sent by a registrar server for REGISTER request.

• The response contains WWW-Authenticate header field which requests for correct credentials from the calling user agent.

• A subsequent REGISTER will trigger from the User Agent with correct credentials.

402 Payment Required

• It indicates payment is required for further processing of request.

403 Forbidden

• 403 Forbidden is sent when the server has understood the request, found the request to be correctly formulated, but will not service the request.

• This response is not used when authorization is required.

404 Not Found

• It indicates that server has not found the indicated SIP URI by the User.

405 Method Not Allowed

• It indicates that the request has contains list of methods that are not allowed.

• Example: A REGISTER request might be sent to a user agent.

• It contains an Allow field which inform the UAC as to what methods are acceptable.

406 Not Acceptable

• This response indicates that the request cannot be processed due to a requirement in the request message.

• The Accept header field in the request did not contain any options supported by the UAS.

407 Proxy Authentication Required

• This request sent by a proxy indicates that the UAC first has to authenticate itself with the proxy before the request can be processed.

• The response should contain Proxy-Authenticate header which informs about the type of credentials required by the proxy.

• The request can be resent with the proper credentials in a Proxy-Authorization header field.

408 Request Timeout

• When the specified time period mentioned in the Expires header field of INVITE request has passed, this response comes.

• It could be sent by a forking proxy or a user agent.

• The request can be retried at any time by the UAC.

422 Session Timer Interval Too Small

• The response is used to reject a request containing a Session-Expires header field.

• The minimum allowed interval is indicated in the required Min-SE header field.

• The calling party may retry the request without the Session-Expires header field or with a value less than or equal to the specified minimum.

423 Interval Too Brief

• The response is returned by a registrar that is rejecting a registration request because the requested expiration time on one or more Contacts is too brief.

• The response must contain a Min-Expires header field listing the minimum expiration interval that the registrar will accept.

480 Temporarily Unavailable

• This response indicates that the request has reached the correct destination, but the called party is not available for some reason.

• The response should contain a Retry-After header indicating when the request may be able to be fulfilled.

481 Dialog/Transaction Does Not Exist

• This response indicates that a response referencing an existing call or transaction has been received for which the server has no records or state information.

483 Too Many Hops

• This response indicates that the request has been forwarded the maximum number of times as set by the Max-Forwards header that is 70 in the request.

• This is indicated by the receipt of a Max-Forward: 0 header in a request.

486 Busy Here

• This indicates the user agent is busy and cannot accept the call.

487 Request Terminated

• This response can be sent by a UA that has received a CANCEL request for a pending INVITE request.

• A 200 OK is sent to acknowledge the CANCEL, and a 487 is sent to cancel the INVITE transaction