Sippy B2BUA is a RFC3261-compliant Session Initiation Protocol (SIP)
Back-to-back user agent (B2BUA) server software.

The B2BUA is a SIP call controlling component. Unlike a SIP proxy server,
which only maintains transaction state, the B2BUA maintains complete call
state and participates in all call requests. For this reason it can perform
number of functions that are not possible to implement using SIP proxy, such
as for example accurate call accounting, pre-paid rating and billing, fail
over call routing etc. Unlike PBX-type solutions such as Asterisk for example,
the B2BUA doesn't perform any media relaying or processing, therefore it
doesn't introduce any additional packet loss, delay or jitter into the
media path.

The Sippy B2BUA is distributed under version 2 of GNU Public License (GPL).