Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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

share|improve this question

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).

share|improve this answer

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.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.