Hi,

The B2BUA is not described in the RFC because B2BUA can do whatever 
it wants with the "SIP legs" but from the outside word each leg should 
be SIP compliant.

When B2BUA receives an INVITE it can modify the parameters and can 
send out the new INVITE which is independent from the received one. 
The task of the B2BUA to control both the legs and transfer the needed 
data from incoming to ougoing side.


When your server receives an INVITE, it may create an outgoing one by 
coping the necessary info: From,To,Reuqest URI if needed. You dont 
have to hide the From but the the COntact,via, record route. This will 
prevent that they can contact each other. 

Some advice:
Outgoing invite can contain the following headers:
- Request URI: Registered Contact of B
- From: original From
- To: original To
- Contact: address of the server
- via: only one line: address of the server,don't copy the via but store it 
on the incoming side to be able generate the responses
- don't copy record route but store it on the incoming side to be able 
generate the responses

When the reposnse should be "back to backed" create a response with 
the same fields but:
- Contact: address of the server
- via: which was strored from INViTE
- record route: which was stroed from INViTE (same order)

Basically you should implement an UAS and a UAC and they can transfer 
the needed info between each other the whatever way you want.

I know that my answer is not complete but I hope it helps.

br
Andras




 



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