[SIPForum-techwg] Question re Application Server

[Francois Audet]

> Paul Kyzivat wrote:
> > I am trying to understand the nuances of draft 5, and there is 
> > something I am unsure of: Is the Application Server 
> required to act as 
> > a B2BUA, or is it permitted to act as a proxy? There is no 
> mention of 
> > B2BUAs in the document, but I think it may be implied.
> > 
> > In particular, suppose we have one service provider servicing two
> > enterprises: acme.com and bozo.com.
> > 
> > Acme has a phone number +12125551234, and Bozo has 
> +14085556789. Then 
> > Acme calls Bozo. According to the spec, the call from Acme 
> is addressed as:
> > 
> > 	INVITE sip:+14085556789 at serviceprovider.net;user=phone SIP/2.0
> > 	To: <sip:+14085556789 at serviceprovider.net;user=phone>
> > 	From: <sip:+12125551234 at acme.com;user=phone>
> > 
> > But then the instructions for service provider addressing say that 
> > when this is sent to Bozo that it should be addressed as:
> > 
> > 	INVITE sip:+14085556789 at bozo.com;user=phone SIP/2.0
> > 	To: <sip:+14085556789 at bozo.com;user=phone>
> > 	From: <sip:+12125551234 at serviceprovider.net;user=phone>
> > 
> > To change the To and From fields in this way requires that the 
> > Application Server be a B2BUA. If this is routed using 
> proxy routing, 
> > then the message going to Bozo would perhaps look like:
> > 
> > 	INVITE sip:+14085556789 at bozo.com;user=phone SIP/2.0
> > 	To: <sip:+14085556789 at serviceprovider.net;user=phone>
> > 	From: <sip:+12125551234 at acme.com;user=phone>
> > 
> > So, is it a requirement to have a B2BUA in the path, or is 
> proxy routing ok?


I hadn't notice this and agree with Paul.

We shouldn't mandate the use of a B2BUA and therefore the From and 
To field should be left intact. The current procedures actually would
fail with RFC 4474.