other network is preventing the request from being processed. 5.5.4 503 Service Unavailable This response indicates that the requested service is temporarily unavailable. The request can be retried after a few seconds, or after the expiration of the Retry-After header field. Instead of generating this response, a loaded server may just refuse the connection. This response code is important in that its receipt triggers a new DNS lookup to locate a backup server to obtain the desired service. The set of SIP DNS procedures for locating SIP servers is detailed in [9]. 5.5.5 504 Gateway Timeout This response indicates that the request failed due to a timeout encountered in the other network to which that the gateway connects. It is a server error class response because the call is failing due to a failure of the server in accessing resources outside the SIP network. 5.5.6 505 Version Not Supported This response indicates that the request has been refused by the server because of the SIP version number of the request. The detailed semantics of this response have not yet been defined because there is only one version of SIP (version 2.0) currently implemented. When additional version numbers are implemented in the future, the mechanisms for dealing with multiple protocol versions will need to be detailed. SIP Response Messages 5.5.7 513 Message Too Large The 513 Message Too Large response is used by a UAS to indicate that the request size was too large for it to process. 5.6 Global Error This response class indicates that the server knows that the request will fail wher- ever it is tried. As a result, the request should not be sent to other locations. Only a server that has definitive knowledge of the user identified by the Request-URI in every possible instance should send a global error class response. Otherwise, a client error class response should be sent. A Retry-After header field can be used to indicate when the request might be successful. 5.6.1 600 Busy Everywhere This response is the definitive version of the 