Re: [Sip] Doubt related to use of Supported and Require header.

[Robert Sparks <rjsparks at nostrum.com>]

lots inline.

On Sep 10, 2008, at 12:39 PM, Paul Kyzivat wrote:

> [Copying Robert in hopes there is some SIPit experience on this]
>
> Comments inline.
>
> 	Thanks,
> 	Paul
>
> Vineet Gupta wrote:
>> Hi Paul/All,
>> I totally agree with the points mentioned in your email. However, I  
>> still have doubts whether most implementations follow this approach  
>> or not.  *Firstly,* I share the same feeling with you with respect  
>> to presence of the Supported header in the request. The Require and  
>> Supported should not have common options in a request. Also,  
>> Supported in request is required if UAC wants to be generous enough  
>> to allow UAS to enforce an option, it itself does not require.
>> However, I was slightly confused by the following statement in  
>> RFC3261 with respect to the Supported header (Section 20.37 , [Page  
>> 178]):
>> /"The Supported header field enumerates all the extensions  
>> supported by the UAC or UAS"/.
>
> Yeah, it says that. Its also hopelessly naive. Sometimes options are  
> supported in one context, and not in another. There is no way to  
> express that. Some examples:
>
> - An option may be supported for one method but not another.
>
> - A B2BUA may only support an option if the device on the other
>  side also supports it. The device on the other side can change
>  over time.
>
>> *So, for inter-operatablity reasons, should all the application  
>> have same options in Require and Supported header or not?*
>
> I think, *when you can*, you should list all the options you support  
> all the time.

I think we can make this clear. I'm not sure I agree that the specs  
are that far away from clear on this - I have not seen any real issues  
with Supported/Require support at SIPits. Here's how I read the text  
and what I see implemented:

* The mechanics of Supported and Require operate at a transaction  
level. The semantics of any given extension can _use_ those mechanics  
differently. It's perfectly reasonable for one extension to have a  
semantic that is scoped entirely to one transaction, and for another  
extension to have a semantic that spans several transactions (maybe  
grouped together in a dialog, maybe not). The requirements of each  
particular extension will constrain the behavior of the endpoints to  
do the right thing with placing the tokens in the right fields in the  
right messages.
*What you place in a Supported header are extensions you are willing  
to use in the context of this transaction. The contract you are  
entering into is that you are willing to start doing the things the  
token you put in this header implies if the other side decides to  
require it. There is nothing that prevents you, in general, from  
deciding to not offer support for something that you've offered  
support for before. (But an extension may constrain you here once you  
or your peer has required it)
* What you place in a Required header are extensions that you are  
actively _using_ in the context of this transaction and that things  
will go wrong if the other side doesn't know it needs to do what the  
extension requires it to do.

>
>
>> *Secondly, *I had discussed the scope of options mentioned in  
>> Require/Supported header, in an earlier debate regarding this  
>> (whether it applies to a transaction/dialog) in this mailing list  
>> long back. As per my understanding, from RFC3261 the scope of  
>> Require/Supported header seems to be a transaction only. If we go  
>> by this assumption, once session in established using 100rel and  
>> precondition (as per 3gpp call-flow), a re-INVITE request can be  
>> sent without both these options (if for example the UAC realizes  
>> that no QoS establishment would be required).
>
>> However, I got a feeling that most of the implementions go by the  
>> assumption that Require/Supported extension apply through out the  
>> dialog.
>>
>
> This is an area that needs some clarification. Some people think the  
> scope of Supported/Require is a transaction, some a dialog, some for  
> as long as not changed, ... There is no great clarity on this in the  
> specs. Its more a matter of observation based on what behavior is  
> called for in various specs defining options.
See above.


>
>
>> Can you let me know in your opinion, what is the general consensus  
>> on this. *Are most SIP implementations prepared to handle change in  
>> Require/Supported options in re-INVITE request (They ofcourse can  
>> still reject such a re-INVITE by sending 420 Bad Extension/421  
>> Extension Required)?*
>
> I don't have data on this. Maybe there is some from SIPit.

I have seen changes like this successfully exercised at SIPits. The  
whole answer is wrapped into the endpoints behaving _per extension_ in  
the way the extension tells them to behave. If there's really a  
problem here, its in the individual extensions if that behavior is not  
clear. Can you point to a particular extension where that's the case?

100rel was brought up in this thread before. Support 100rel extension  
itself can come or go as the requestor sees fit for any given  
transaction. 3262 is pretty clear on that. Now some other extension  
_using_ 100rel might place more restrictions on endpoint behavior, and  
the documentation of those requirements has to live with that other  
extension.

>
>
> Note: this isn't really just about Require/Supported. There are  
> other headers with similar issues:
>
> Accept, Accept_Encoding, Accept-Language, Allow

Agreed, but again, I've not seen anything break at a SIPit yet because  
these things change between transactions in a given dialog (though  
that's not particularly common).

>
>
> 	Thanks,
> 	Paul
>
>> Regards,
>> Vineet.
>> On Tue, Sep 9, 2008 at 9:38 PM, Paul Kyzivat <pkyzivat at cisco.com <mailto:pkyzivat at cisco.com 
>> >> wrote:
>>    Vineet Gupta wrote:
>>        Hi,
>>         I have a small doubt related to use of Supported and Require
>>        header. While from RFC 3261 it is clear the Require header is
>>        used by the UAC and UAS to enforce the extension implemention
>>        for a particular session, while the Supported header is mostly
>>        used by UAC to express the extensions which can be enforced by
>>        UAS through the Require header.
>>         My doubt is that, is it *mandatary that every extension  
>> that is
>>        present in Require header must be present in Supported header
>>        *in the request? According to me, presence/absence of Required
>>        extensions in Supported header does not make any impact on UAS
>>        handling.  *Even if it is not mandatory, what is the preferred
>>        approach?*
>>    This is pretty loosely defined.
>>    For one thing, Require doesn't necessarily apply to a "particular
>>    session". The most that can be said with certainty is that is
>>    applies to a transaction. Often it applies longer, but how long
>>    isn't well defined.
>>    Second, there is some asymmetry in Supported/Require, but this
>>    varies based on the option. Require from a UAC states what it
>>    requires of the UAS, but doesn't necessarily imply that a Require
>>    for the same option, coming the other way, would be supported.
>>    Also, Supported and Require are often used together to do a  
>> negotiation:
>>    Supported in a request is paired with Require of the same option  
>> in
>>    the response to negotiate its use.
>>    In most cases there is no requirement to include Supported in
>>    requests. The main place it is essential is in a 420 response.
>>           Thanks,
>>           Paul

_______________________________________________
Sip mailing list  https://www.ietf.org/mailman/listinfo/sip
This list is for NEW development of the core SIP Protocol
Use sip-implementors at cs.columbia.edu for questions on current sip
Use sipping at ietf.org for new developments on the application of sip