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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

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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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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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