A back-to-back user agent (B2BUA) is a logical network element in Session Initiation Protocol (SIP) applications.
A back-to-back user agent operates between both end points of a communications session and divides the communication channel into two call legs, and mediates all SIP signaling between the endpoints of the session, from establishment to termination.
In the originating call leg, the B2BUA acts as a user agent server (UAS) and processes the request as a user agent client (UAC) to the destination end, handling the signaling between end points back-to-back.
Each side of a B2BUA operates as a standard SIP user agent network element as specified in RFC 3261.
In addition to call management, a B2BUA may provide billing services, internetworking for protocol conversions, and hiding of network-internal topology and information.