Response Codes Regarding URIs that Cannot Be Routed to a Destination in the Session Initiation Protocol (SIP)
Pingtel
The Session Initiation Protocol (SIP) provides a number of response codes which a SIP agent can use to indicate that it cannot find a suitable destination to which to send a request. Unfortunately, this set of response codes was not carefully designed, making it difficult in many cases for a proxy to select a response code that accurately reflects the proxy's knowledge about the request URI. This document describes these response codes and their current usage.