A back-to-back user agent (B2BUA) is a logical network element in
Session Initiation Protocol (SIP) applications.
[, ''SIP: Session Initiation Protocol'', IETF, The Internet Society (2002)] SIP is a signaling protocol for managing multimedia
Voice over Internet Protocol
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
(VoIP) telephone calls. A back-to-back user agent operates between both end points of a communications session and divides the communication channel into two call legs, and mediates all SIP signaling between the endpoints of the session, from establishment to termination. As all control messages for each call flow through the B2BUA, a service provider may implement value-added features available during the call.
In the originating call leg, the B2BUA acts as a user agent server (UAS) and processes the request as a user agent client (UAC) to the destination end, handling the signaling between end points back-to-back. A B2BUA maintains complete state for the calls it handles. Each side of a B2BUA operates as a standard SIP
user agent network element as specified in .
In addition to call management, a B2BUA may provide billing services, internetworking for protocol conversions, and hiding of network-internal topology and information.
B2BUAs are often implemented in media gateways to bridge the media streams, in addition to the signaling path, for full control over the session.
A
signaling gateway, part of a
session border controller
A session border controller (SBC) is a network element deployed to protect SIP based voice over Internet Protocol (VoIP) networks.
Early deployments of SBCs were focused on the borders between two service provider networks in a peering environme ...
, is an example of a B2BUA.
Call flow diagram
B2BUA
Alice Server Bob
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See also
*
Real-time Transport Protocol
The Real-time Transport Protocol (RTP) is a network protocol for delivering audio and video over IP networks. RTP is used in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applicati ...
References
{{reflist
Voice over IP
Telephony
cs:IMS
fr:IP Multimedia Subsystem
ru:B2BUA