[SIPForum-techwg] Question re Application Server

[Paul Kyzivat]

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?

	Thanks,
	Paul