Network Working Group Requirements for Management of Overload in the Session Initiation Protocol

Overload occurs in Session Initiation Protocol (SIP) networks when proxies and user agents have insuffient resources to complete the processing of a request. SIP provides limited support for overload handling through its 503 response code, which tells an upstream element that it is overloaded. However, numerous problems have been identified with this mechanism. This draft summarizes the problems with the existing 503 mechanism, and provides some requirements for a solution.