Network Working Group R. Fielding Request for Comments: 2616 ...

If the response could be unacceptable, a user agent SHOULDtemporarily stop receipt of more data and query the user for adecision on further actions.10.4.8 407 Proxy Authentication RequiredThis code is similar to 401 (Unauthorized), but indicates that theclient must first authenticate itself with the proxy. The proxy MUSTreturn a Proxy-Authenticate header field (section 14.33) containing achallenge applicable to the proxy for the requested resource. Theclient MAY repeat the request with a suitable Proxy-Authorizationheader field (section 14.34). HTTP access authentication is explainedin "HTTP Authentication: Basic and Digest Access Authentication"[43].10.4.9 408 Request TimeoutThe client did not produce a request within the time that the serverwas prepared to wait. The client MAY repeat the request withoutmodifications at any later time.10.4.10 409 ConflictThe request could not be completed due to a conflict with the currentstate of the resource. This code is only allowed in situations whereit is expected that the user might be able to resolve the conflictand resubmit the request. The response body SHOULD include enoughFielding, et al. Standards Track [Page 67]RFC 2616 HTTP/1.1 June 1999information for the user to recognize the source of the conflict.Ideally, the response entity would include enough information for theuser or user agent to fix the problem; however, that might not bepossible and is not required.Conflicts are most likely to occur in response to a PUT request. Forexample, if versioning were being used and the entity being PUTincluded changes to a resource which conflict with those made by anearlier (third-party) request, the server might use the 409 responseto indicate that it can't complete the request. In this case, theresponse entity would likely contain a list of the differencesbetween the two versions in a format defined by the responseContent-Type.10.4.11 410 Gone