[SIPForum-techwg] Question re Application Server

[Paul Kyzivat]

Inline comment after Francois' response.

	Paul

Francois Audet wrote:
>  
>> 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.

After I studied the wording of the document more carefully, I realize 
that its scope is carefully limited to Enterprise-PSTN interconnect. 
With that limited scope, there never would be any direct routing from 
Acme to Bozo.

Conceptually, calls from Acme to Bozo would go via a pair of application 
servers connected by a PSTN link. If you collapse that to IP routing 
following these rules, you then need the B2BUA to do the readdressing 
that the SGWs otherwise do.

But realistically doing this without going to the PSTN, and without a 
B2BUA, is going to be valuable. IMO the kind of usage I'm showing above, 
while out of scope for the current document, can be viewed as a backward 
compatible extension to the current document.

	Paul