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<title>Back-to-back user agent</title>
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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Back-to-back user agent</span></span>
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<div id="mw-content-text" class="mw-body-content mw-content-ltr" lang="en" dir="ltr"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><p>A <b>back-to-back user agent</b> (<b>B2BUA</b>) is a logical network element in <a href="Session_Initiation_Protocol" title="Session Initiation Protocol">Session Initiation Protocol</a> (SIP) applications.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> SIP is a signaling protocol for managing multimedia <a href="Voice_over_Internet_Protocol" class="mw-redirect" title="Voice over Internet Protocol">Voice over Internet Protocol</a> (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.
</p><p>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 <a href="User_agent" title="User agent">user agent</a> network element as specified in <style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc3261">3261</a>.
</p><p>In addition to call management, a B2BUA may provide billing services, internetworking for protocol conversions, and hiding of network-internal topology and information.
</p><p>B2BUAs are often implemented in media gateways to bridge the media streams, in addition to the signaling path, for full control over the session.
</p><p>A <a href="Signaling_gateway" title="Signaling gateway">signaling gateway</a>, part of a <a href="Session_border_controller" title="Session border controller">session border controller</a>, is an example of a B2BUA.
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<div class="mw-heading mw-heading2"><h2 id="Call_flow_diagram">Call flow diagram</h2></div>
<pre> B2BUA
Alice Server Bob
| | | |
| INVITE F1 | | |
|------------------->| | |
| 100 Trying F2 | | |
|<-------------------| | INVITE F3 |
| | |------------------->|
| | | 100 Trying F4 |
| | |<-------------------|
| | | 180 Ringing F5 |
| 180 Ringing F6 | |<-------------------|
|<-------------------| | |
| | | 200 OK F7 |
| 200 OK F8 | |<-------------------|
|<-------------------| | ACK F9 |
| ACK F10 | |------------------->|
|------------------->| | |
| RTP Media | | RTP Media |
|<==================>| |<==================>|
| BYE F11 | | |
|------------------->| | BYE F12 |
| 200 OK F13 | |------------------->|
|<-------------------| | 200 OK F14 |
| | |<-------------------|
| | | |
</pre>
<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Real-time_Transport_Protocol" title="Real-time Transport Protocol">Real-time Transport Protocol</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc3261">3261</a>, <i>SIP: Session Initiation Protocol</i>, IETF, The Internet Society (2002)</span>
</li>
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This article is issued from <a class="external text" title="Last edited on 2023-07-19" href="https://en.wikipedia.org/wiki/?title=Back-to-back_user_agent&oldid=1166068690">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.
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