Back-to-Back User Agent (B2BUA)

The Back-to-Back User Agent (B2BUA) is a critical SIP element that acts as an intermediary between two or more SIP dialogs, functioning as an endpoint for each dialog and forwarding requests and responses between them. This unique capability makes B2BUAs one of the most frequently used application types in SIP-based communication services.

Despite their versatility, B2BUAs can sometimes be viewed as problematic due to their potential to disrupt end-to-end services. By mediating signaling between endpoints, a B2BUA may unintentionally interfere with services it is not explicitly designed to support. However, when properly implemented, B2BUAs offer a powerful toolset for developers to enable advanced SIP application functionality.

The SIP Servlet API fully supports the B2BUA pattern, providing new helper classes and methods that simplify its implementation. B2BUAs play a key role in advanced use cases such as call recording, bridging multiple call legs, and exerting fine-grained control over SIP sessions.