Back-to-Back User Agent

A back-to-back user agent (B2BUA) is a logical entity that receives a request and processes it as a user agent server (UAS). To determine how the request should be answered, the B2BUA acts as a user agent client (UAC) and generates requests. Unlike a proxy server, the B2BUA maintains a dialog state and participates in all requests sent on the dialogs it has established. Since it is a concatenation of a UAC and UAS, no explicit definitions are needed for its behavior.