6xx-Class Responses Considered Harmful in the Session Initiation Protocol (SIP)
Pingtel
The specification of the Session Initiation Protocol (SIP) permits a user agent (UA) that receives a call to generate a response in the "6xx class" that not only rejects the call but also requires the termination of all attempts to route the call to alternative destinations. As the call may have reached the UA through multiple forwardings whose meanings cannot be known by the UA, the global termination of alternative routing can only rarely be correctly requested by the UA. Because such responses are almost never appropriate, ability of a UA to generate such responses is harmful, and all 6xx class responses should be replaced with 4xx class responses with similar meanings. However, there is no 4xx class response similar to "603 Decline", and so one needs to be created.