20.03.12 We are not quite dead yet. :) There are many updates for the B2BUA in the pipeline and also there are some new projects to be anounced. Stay tuned.

28.05.10 Source code repository moved to Git at  SourceForge.

30.11.09 Bugfix version 1.0.3 of Sippy B2BUA has been released.

20.11.09 Bugfix version 1.0.2 of Sippy B2BUA has been released.

15.10.09 Follow us on  Twitter!

17.08.09  RFC3326 support in Sippy B2BUA.

16.08.09  IPv6 support in Sippy B2BUA.

10.08.09 New stable version 1.0.1 of Sippy B2BUA has been released.

01.08.09 Asterisk Codec Negotiation Patch has been updated for Asterisk 1.4.25.

Older Entries..

About Sippy B2BUA

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.

Logically, it consists of two independent signaling SIP UA connected by the call control logic.


  • 5,000-10,000 simultaneous sessions per server;
  • 150-200 call setups/tear-downs per second;
  • Realtime calls control and call data records (CDRs) generation;
  • Optional ability to use Sippy RTPproxy for media relaying;
  • Optional ability to perform Cisco-compatible RADIUS AAA (Authentication, Authorization and Accounting);
  •  RFC3261 compliance;
  •  RFC3326 support;
  • IPv6 support;
  • Seamless compatibility with majority of popular SIP software and hardware on the market today;
  • Robustness and Resilience;
  • Simple and clean, yet flexible, internal design making implementing new features and services easy;
  • B2BUA could be easily combined with other Open Source software, such as  SIP Express Router to build complete softswitch solution.


The development of the software started at the late 2003 after negative experience with using  Vovida B2BUA. It was designed with robustness and performance in mind and now being used by several hundreds ITSPs all around the globe. The software now is being actively developed and maintained by the  Sippy Software, Inc.

Getting B2BUA


Current Versions

Source code via read-only Git

The latest development B2BUA source code (unstable) can be downloaded from the Berlios CVS using the following command:

git clone git://

You can also  browse the source code.


The Sippy B2BUA is distributed under version 2 of GNU Public License (GPL). You can find the full text of the license  here. For a license to use the SIPPY software under conditions other than those described here, or to purchase support for this software, please contact  Sippy Software, Inc.


The B2BUA documentation is available here. Please note that it is still work in progress (last updated on 7 August 2009), please make sure to check for updates periodically.

Getting Help

You can obtain free best-effort help from the B2BUA community. To participate on the mailing list, subscribe at the following web address: and stick to problem reporting guidelines. Discussion of development, new features and B2BUA status as on CVS takes place at the following mailing list:

For a commercial support, including any custom development, please contact  Sippy Software, Inc..

Related Projects

Sippy RTPproxy software
Asterisk Codec Negotiation Patch
Python Module for SER, OpenSIPS, Kamailio