Re: [Sip] Alert-Info Idea (was Re: Comment on draft-willis-sip-answeralert-01)

[Dean Willis <[e-mail]>]

On Oct 28, 2005, at 8:08 AM, Paul Kyzivat wrote:

>
>
>
>> there's a chance that the request might be delivered to a node  
>> that  doesn't grok the extension, and the resulting 4xx response  
>> might be  HERFP-eaten by the proxy if it gets a "better" response  
>> from some  other branch.
>> So what?
>> Either a "better" response goes back to the UAC (perhaps from a   
>> branch that understands the require),  or it doesn't. Big loss.  
>> All  that happens is that we don't get an error response back from  
>> a node  we didn't want to talk to anyhow.
>>
>
> The problem is that there could be two UAs, one of which  
> understands the A-I and another that doesn't. The one that supports  
> the A-I isn't going to answer, because nobody is manning it. The  
> one that doesn't support the A-I is ready and willing to answer.
>
> The caller wants to use the A-I, but will fall back to not using it  
> if necessary.
>
> So the call is tried, and the UA that supports the A-I alerts for  
> awhile using the chosen alert, but never answers. Eventually the  
> caller gives up, and tries without the A-I. Now both UAs alert and  
> the call is answered.
>
> Of course the answer here is to not put the Required header if you  
> don't really require the feature. That is a reasonable in this  
> scenario. I wonder if there are any scenarios where the Require is  
> helpful?
>

Exactly. If you don't actually require that the end point support the  
extension, don't insert a Require header for it. You can still offer  
the feature, and if it gets used, that's nice.

If we use an Alert-Info header for the "don't bother the user because  
this is just an administrative loopback test call" function allowed  
for in draft-willis-answeralert, we get some mileage out of having a  
Require. Because we REALLY DON'T want the phone to ring and get  
picked up, resulting in a very confused user. We WANT the call to  
fail if the client doesn't support the extension. There are probably  
other cases, but as with most SIP extensions, sending a Require is  
generally the exception, not the rule.

>
>> We could certainly insert caveats about Require and HERFP in the   
>> draft. Oddly enough, the answer-mode/alert-mode draft I did for  
>> OMA  suffers from exactly the same quirk, but nobody complained  
>> there.
>>
>
> Probably because it applies in a narrow context. But I would have  
> to go back and look at it again.
>

Well, please go have another look at that draft, because they're  
about to start gestating cattle over in OMA for the lack of a  
solution to this problem.

--
Dean


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