[SIPForum-techwg] Question re Application Server

[Klaus Darilion]

Paul Kyzivat wrote:
> 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.

What about a "real direct routing" - Acme to Bozo without routing via a 
service provider? IMO that should happen.

regards
klaus

> 
> 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
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