B2BUA (Back‑to‑Back User Agent)

B2BUA (Back‑to‑Back User Agent) is a SIP element that relays signaling mechanisms while performing some alteration or modification of the messages that would otherwise not be permitted by a proxy server.

A logical entity that receives a request and processes it as a UAS (user agent server). In order to determine how the request should be answered, it acts as a UAC (user agent client) and generates requests. Unlike a proxy server it maintains dialog state and must participate in all requests sent on the dialogs it established.