Back-to-back user agent

The Back-to-Back User Agent (B2BUA) acts as a user agent to both ends of a Session Initiation Protocol (SIP) call. The B2BUA is responsible for handling all SIP signalling between both ends of the call, from call establishment to termination. Each call is tracked from beginning to end, allowing the operators of the B2BUA to offer value-added features to the call.

To SIP clients, the B2BUA acts as a User Agent server on one side and as a User Agent client on the other (back-to-back) side. The basic implementation of a B2BUA is defined in RFC 3261. The B2BUA may provide the following functionalities:

* call management (billing, automatic call disconnection, call transfer, etc.)
* network interworking (perhaps with protocol adaptation)
* hiding of network internals (private addresses, network topology, etc.)
* codec translation between two call legs

Because it maintains call state for all SIP calls it handles, failure of a B2BUA affects all these calls. Often, B2BUAs also terminate and bridge the media streams to have full control over the whole session.

A Signaling gateway, part of a Session Border Controller, or Asterisk PBX are good examples of a B2BUA.

ee also

*Session Border Controller
*VoIP
*User agent

External links

* [http://www.b2bua.org Open Source Sippy SIP B2BUA]