reening proxy. Any INVITE request received directly would automatically receive this response without user intervention. 5.3.5 380 Alternative Service This response returns a URI that indicates the type of service that the called party would like. An example might be a redirect to a voicemail server. 5.4 Client Error This class of response is used by a server or UAS to indicate that the request can- not be fulfilled as it was submitted. The specific client error response or the pres- ence of certain header fields should indicate to the UAC the nature of the error and how the request can be reformulated. The UAC should not resubmit the SIP: Understanding the Session Initiation Protocol request without modifying the request based on the response. The same request, however, can be tried in other locations. A forking proxy receipt of a 4xx response does not terminate the search. Typically, client error responses will require user intervention before a new request can be generated. 5.4.1 400 Bad Request This response indicates that the request was not understood by the server. An example might be a request that is missing required header fields such as To, From, Call ID, or CSeq. This response is also used if a UAS receives mul- tiple INVITE requests (not retransmissions) for the same Call ID. 5.4.2 401 Unauthorized This response indicates that the request requires the user to perform authentica- tion. This response is generally sent by a user agent, since the 407 Proxy Authentication Required (Section 5.4.8) is sent by a proxy that requires authentication. The exception is a registrar server, which sends a 401 Unauthorized response to a REGISTER message that does not contain the proper credentials. An example of this response is: SIP/2.0 401 Unathorized Via: SIP/2.0/UDP proxy.globe.org:5060;branch=z9hG4bK2311ff5d.1 ;received=192.0.2. Via: SIP/2.0/UDP 173.23.43.1:5060;branch=z9hG4bK4545 From: <sip:explorer@geographic.org>;tag=341323 To: <sip:printer@maps-r-us.com>;tag=19424103 From: Copernicus<sip:copernicus@globe.org