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BraCa
Asked: 2011-10-17 14:35:17 +0800 CST

What is a SIP B2BUA, and how is it different from a UA?

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I've been reading some stuff SIP related, and I'm confused, about what is a SIP back-to-back user agent(B2BUA) and a 'normal' user agent?

Can anyone explain the differences?

From what I read I can't differentiate a B2BUA from a UA... For me a UA acts as a UAS (User Agent Server) when needs to answer a request, and as a UAC (User Agent Client) when needs to send a request..

But I can't understand how different a B2BUA is..

Sorry if this doesnt make sense, but I'm a little bit confused

sip useragent

3 Answers

  1. The RFCs don't define what B2BUAs can or can't do, so my answer's a bit wooly, of necessity.

    A B2BUA is anything is not a proxy but that does have calls flowing through it. For instance, you could use a B2BUA as a gateway between two SIP networks.

    Hence the name: some connection running through the B2BUA results in a SIP call on one side (where the B2BUA acts as a UAS) and another SIP call on the other side (where the B2BUA acts as a UAC).

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  2. You may find an interesting article explaining B2BUAs here:

    https://andrewjprokop.wordpress.com/2013/12/27/the-back-to-back-user-agent-b2bua/

    In a few words, a B2BUA is a SIP entity which can "split" a call into two call legs: terminating a call leg on one side and creating a new one on the other side. Those two call legs could be unaware of one another, thus the B2BUA acts as a demarcation point. The term B2BUA also refers to a SIP entity's capability of manipulating SIP packets in terms of SIP headers and SDP body.

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  3. A B2BUA is a network element used in SIP applications. Its behavior depends on its role in a SIP signalling flow. It behaves as a UAS to the originator of the request, in the meantime, it behaves as a UAC to the destination of the request (from different legs) in a SIP signalling.

    Its role is not defined as a stateless proxy, it can manipulate, change, delete or add header fields to SIP messages.

    Alice ----------------> B2BUA -------------------> Bob

    Alice <---------------- B2BUA <------------------- Bob

    Considering a SIP signalling as above, Alice inviting Bob, all the dialog and transaction identifiers between Alice-B2BUA and B2BUA-Bob will be different. In the first INVITE request, B2BUA will appear as a UAS to Alice and as a UAC to Bob.

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