Some Tips For Washing Your Dog
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How Dog Food Brands Differ - A Brief Overview
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The Wonderful World of Dog Breeds
There are well over 150 different dog breeds in the world. This
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world and are in all shapes, colr before accepting the video invitation.
>
> b) No Required options, as the peer might not be able to support it, or
> because the re-INVITE might not neccesarily involve QoS establishment or
> user intervention.
>
> One option is that he can do try sending request with with scenario a)
> and then recieving 421 response (bad extension) on which he knows he can
> only send re-INVITE request without any extensions.
>
> The other option is to use only Supported extensions in re-INVITE
> request depending again the capability of peer, and then leave it upto
> the UAS of the re-INVITE request to chose the extensions it wants to use
> for the re-INVITE.
Well, it seems that if you require preconditions then you also must
require 100rel. So lets concentrate on the preconditions option.
3312 says:
An offerer MUST include this tag in the
Require header field if the offer contains one or more "mandatory"
strength-tags. If all the strength-tags in the description are
"optional" or "none", the offerer MUST include this tag in either a
Supported header field or in a Require header field.
So, on the reinvite the first decision is whether to use mandatory
strength-tags. If there is a need to do so, then just Require the
options and deal with the consequences if the request is rejected.
If not, I think you just put the options in Supported, include your
preconditions, and see what happens.
> I guess this would help out SIP implementations interop with UAs having
> different interpretations of scope of Required extensions in use-cases
> where the scope is not self evident (because even though the
> transaction-wise scope of extensions is agreed to, in many cases its
> scope is being derived on the basis of the use-case).
I just looked to refresh my memory, and I find nothing in 3262 or 3312
that implies the scope of these options extends beyond the INVITE
transaction in which they are used.
This does present an obscure problem: suppose you choose to revise your
session by sending an offer in an UPDATE rather than a reinvite, and you
also use preconditions, and perhaps even Require support for them in the
UPDATE. Even though it will take another UPDATE to complete the
precondition negotiation there is no guarantee that preconditions will
be supported in a subsequent UPDATE. But this seems to fall into the
category of "silly" behavior. If you allow an o/a with preconditions to
begin, there must be an implied contract to continue to allow
preconditions to be used to reach resolution. I suppose it would be good
if some future revision to 3312 mentioned this explicitly.
> Regards,
> Vineet.
>
>
>
> On Thu, Sep 11, 2008 at 12:36 AM, Robert Sparks <[EMAIL PROTECTED]
> <mailto:[EMAIL PROTECTED]>> wrote:
>
>
> On Sep 10, 2008, at 1:45 PM, Paul Kyzivat wrote:
>
> Robert,
>
> Thank you for this clarification. It is, AFAIK, consistent with
> observed practice. But I have not seen this level of detail
> written down anywhere before.
>
> Paraphrasing you, I guess the primary guarantee is that if you
> put Supported:X in a request, then you must be prepared to have
> X used in the response and/or to have a Require:X in the
> response. And if a Require:X is: in a request and not rejected
> by the response, or in a response following a Supported:X in the
> request, then that establishes an option-specific contract whose
> scope is defined by the option.
>
> Conversely a Supported:X in a response, or in a request and then
> not sealed with a Require:X in a response, establishes no
> contract and so no guarantees of any sort.
>
> But what about OPTIONS? 3261 says:
>
> The SIP method OPTIONS allows a UA to query another UA or a proxy
> server as to its capabilities. This allows a client to discover
> information about the supported methods, content types, extensions,
> codecs, etc. without "ringing" the other party. For example,
> before
> a client inserts a Require header field into an INVITE listing an
> option that it is not certain the destination UAS supports, the
> client can query the destination UAS with an OPTIONS to see if this
> option is returned in a Supported header field. All UAs MUST
> support
> the OPTIONS method.
>
> ...
>
> Allow, Accept, Accept-Encoding, Accept-Language, and Supported
> header
> fields SHOULD be present in a 200 (OK) response to an OPTIONS
> request.
>
> It would seem that this is just a "best effort" thing, and that
> there are no guarantees. Or perhaps an option should only be
> listed in Supported of an OPTIONS response if it is *always*
> supported.
>
>
> So this use of OPTIONS is something that isn't well implemented (at
> least not completely - there are several implementation that look
> for REFER to decide if they'll offer transfer).
>
> My read of the specs here is that you put stuff in OPTIONS that you
> would be able to use/support if it made sense in the context at the
> time. It's not a promise that you'll agree to use/support it at any
> particular point in time, but an indication that its possible you
> will. The utility is in providing a negative : "Don't bother trying
> to do this-strange-thing with me, because I can never do that"
>
>
>
>
> Thanks,
> Paul
>
> Robert Sparks wrote:
>
> 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
> <[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
> <mailto:[EMAIL PROTECTED]
> <mailto:[EMAIL PROTECTED]>>> 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
>
>
>
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