<P> A back - to - back user agent (B2BUA) is a logical network element in Session Initiation Protocol (SIP) applications . SIP is a signaling protocol to manage multimedia Voice over Internet Protocol (VoIP) telephone calls . A back - to - back user agent operates between both end points of a phone call or communications session and divides the communication channel into two call legs and mediates all SIP signaling between both ends of the call, from call 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 network element as specified in RFC 3261 . </P>

What is sip back to back user agent
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