B2BUA - (back-to-back user agent) is middleware in both SIP and RTP data streams. With SIP clients, a B2BUA behaves like a user agent server on one side of the connection and like a user agent client on the other. The point is to be able to manipulate the data streams.
B2BUA is specified in RFC 3261.
Examples for the application: Call management (including billing, call forwarding, automatic disconnection)Pairing of different networks (especially to adapt the different protocol dialects, depending on the manufacturer)Hiding the network structure (including private addresses, network topology)Basically, a B2BUA can be expanded to a proxy with integrated media gateway.