Re: [sipcore] Question on Require in responses

["Christer Holmberg" <[e-mail]>]

Hi, 

>>The option tag is a means of 1) discovering whether or not the far- 
>>end supports a specific extension and of 2) declaring the criticality 
>>of understanding that extension relative to satisfying a request and 
>>3) informing the UAC of what extensions were applied to the request 
>>in order to generate the response. This is purely a function of 
>>capability negotiation, not one of invocation.
>>
>>The extension itself (typically, an extension header field, although 
>>there are other types of extensions) is what causes the extension to 
>>be invoked.
>Depends a bit how you define extension, but I would argue that if I 
>require an extension to be supported on a transaction/dialog by the 
>UAS and the UAS accepts the request that this means that for this call 
>the UAS behaves as it should according to the extensions 
>specification.
>
>Saying that an extensions invokes itself is meaningless to me.
>
>A UAS may choose to only invoke the extension behaviour if it finds 
>that the extension is required. That comes close to saying that the 
>require:extension invokes the behaviour in the UAS, but it is not at 
>all the same.
>
>
>The Require doesn't control whether the UAS invokes the extension. The
UAS may invoke the extension on its own recognizance.  Most likely, it
chooses to do so based on 1) the presence of a Supported 
>indicator for the extension in a request as evidence that the UAC
supports the extension or the presence of an extension header in the
request that also indicates support in the UAC and 2) support for 
>that extension in the UAS. Other extensions may not require support in
the UAC, so may be enacted by this UAS purely on local policy.

100rel does not carry any extension headers in the request, nor does
RFC3262 say that the UAC must insert Supported in addition to Require.

But, RFC3262 DOES say:

"The UAS MUST send any non-100 provisional response reliably if the
initial request contained a Require header field with the option tag
100rel."

So, in my opinion Require:100rel means:

1. The UAC supports 100rel (no Suppored needed)
2. The UAS MUST use the extension (if sending non-100 provisional
responses).

I have nothing against saying that the definition of each option-tag
should specify how it is used, and what Require really means as far as
the UAS is concerned, but I don't think the generic definition of
Supported and Require that you are proposing fits all cases.

Regards,

Christer