e processed due to a require- ment in the request message. The Accept header field in the request did not contain any options supported by the UAS. 5.4.8 407 Proxy Authentication Required This request sent by a proxy indicates that the UAC must first authenticate itself with the proxy before the request can be processed. The response should contain information about the type of credentials required by the proxy in a Proxy- Authenticate header field. The request can be resubmitted with the proper credentials in a Proxy-Authorization header field. Unlike in HTTP, this response may not be used by a proxy to authenticate another proxy. SIP/2.0 407 Proxy Authorization Required Via: SIP/2.0/UDP discrete.sampling.org: 5060 branch=z9hG4bK6563 ; received=65.64.140 From: Shannon <sip:shannon@sampling.org>it tag=59204 To: Schockley ip:shockley@transistor.com>;t tag=142334 Call-ID: adf8gasdd7fld@discrete.sampling.o SIP: Understanding the Session Initiation Protocol CSeq: 1 INVITE Proxy-Authenticate: Digest realm="sampling.org" qop="auth", e="9c8e88df84df1cec4341ae6cbe5a359" opaque=" ", stale=FALSE, algorithm=MD5 Content-Length: 0 5.4.9 408 Request Timeout This response is sent when an Expires header field is present in an INVITE request, and the specified time period has passed. This response could be sent by a forking proxy or a user agent. The request can be retried at any time by the UAC, perhaps with a longer time period in the Expires header field or no Expires header field at all. Alternatively, a stateful proxy can send this response after the request transaction times out without receiving a final response. 5.4.10 409 Conflict This response code has been removed from RFC 3261 but is defined in RFC 2543. It indicates that the request cannot be processed due to a conflict in the request. This response is used by registrars to reject a registration with a conflict- ing action parameter. 5.4.11 410 Gone This response is similar to the 404 Not Found response but contains the hint that the requested user will not be availa