A back-to-back user agent (B2BUA) operates like a user agent (UA), but on both endpoints of a SIP connection, Session Initiation Protocol
(SIP). The B2BUA agents take care of SIP signaling between the two endpoints, as well as connection setup and termination. Each individual call over the SIP connection is tracked. B2BUA operators can make the call more attractive by providing additional value-added services. With respect to SIP clients, B2BUA behaves like a user agent server on one side of the connection and like a user agent client on the other. B2BUA is described in RFC 3261.