[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Finishing the XML-tagging discussion

> > Second, the definition of a global parameter namespace is far from the worst
> > thing about this. The biggest problem this approach has is that now you have
> > what amounts to a mandatory parameter, one which has to appear with every
> > appearance of a given media type and must have a specific value in those
> > cases.

> actually, I don't view the parameter as mandatory; I view it as advisory.
> The right way to handle this is to treat it as an image/svg.  Depending
> on how you look at it, the information in the $superclass parameter is
> either a fallback way to handle the type if you don't know how to handle
> image/svg, or additional information that can be used by generic "index
> everything that is XML" processors.

An advisory parameter of this sort is a worthless parameter. If you cannot
depend on it appearing for every instance of a given content type you
cannot use it for anything. (Or worse, you will use it and fail when it
isn't present.)

> > This is completely contrary to the entire intent of the parameter
> > space: Parameters were intended to convey information that isn't
> > invariant with the media type.

> it's true that parameters were intended to convey type-specific information,
> but I don't see the harm in defining another set of parameters (disjoint
> from the type-specific parameters), which convey other information about
> the media type.

Well, all I can say is that I do see the potential for tremendous harm in doing
it, many times more harm than I see in the suffix that you seem to think is so

> > Third, this introduces the possibility of silly states in a major way.

> As far as I can tell, parameters already have tremendous potential for
> silly states - nothing stops you from adding any parameter to any media
> type (so you can for example have a charset parameter on an image).

While this may be a silly state in some sense, it is an entirely harmless one.
The silly states this superclass nonsense introduces are anything but.

> But in practice, and despite widespread misuse of certain parameter
> names with content-types that don't define those parameters, it doesn't
> seem to cause big problems.  Unrecognized parameters are generally
> ignored.

Exactly. We were careful to define parameters in this way so that bad
behavior as a result of silly states was minimized. But this isn't possible
with the sort of parameter you're proposing.

> > Fourth, the infrastructure upgrades to handle this are far nastier
> > than you seem to think, and have to happen at both the sending and receiving
> > end.

> I guess the question in my mind is whether those upgrades affect every
> MIME handling agent or whether (for the time being) they only affect
> things that care specifically about XML.

In order for this to be useful they have to affect every agent.

> The latter set seems a lot
> smaller than the former set, and I'm less worried about the impact of
> this on XML-aware agents (which after are in an emerging space anyway)
> than I am worried about the impact of -xml frobs on things that need
> to do content-negotiation (which includes every HTTP client and server).

This is a complete red herring. Nobody is proposing that the suffix be
used for negotiation purposes. Negotiation is a different problem than

> > Fifth, as I said before, this doesn't work with places where only media
> > types and not parameters are carried. Such places do exist and fixing
> > them all is going to be nearly impossible.

> Since I see this parameter as advisory, to me it doesn't matter much.
> Applications for which the advisory parameter is of sufficient utility
> will get fixed to interpret or generate that parameter; those that don't
> benefit sufficiently will not get fixed.

Again, an advisory parameter of this sort accomplishes nothing.

> To put it another way, if it's absolutely important that every instance
> of image/svg be externally labelled as XML then I'd agree with you.
> But I don't see this as absolutely important.

I do.

> > In summary, what this amounts to is nothing less that a complete redesign of
> > how  MIME works.

> "complete redesign" seems a bit overstated.  It is a slightly different
> way of using parameters than originally envisioned.  But it doesn't
> interfere with the existing parameter space.

Actually, I think I seriously understated it. It isn't just a complete
redesign, it is a revamping of the underlying principles and agreements MIME is
based on.

> And while I understand that many implementations don't have the ability
> to generate or the ability to interpret content-type parameters, I don't
> see how this breaks anything that works now.  by contrast, the -xml frob
> coupled with the desire for content-negotiation languages (http accept
> and conneg) to recognize anything that is XML does break things.
> I'm just trying to minimize breakage.

Please cite a single, solitary example of where it breaks things. I'd like
to hear of one. Thus far all you have cited are easily surmountable
problems, like the ordering of future additional suffixes (assuming there
ever are any).

> a concrete example of something that this breaks would be helpful
> in getting me to understand your concerns.

It breaks so many things in so many ways... Some exmaples:

(1) Silly state problems. Consider the possible effect of image/jpeg;
    $superclass=text/xml on a handler only prepared to accept XML text.
    (And compare it with the effect of image/jpeg; charset=us-ascii on
    any existing handler.)

(2) Problems with sending agents not including the tag. Suppose an application
    is deployed that depends on the superclass tag. (This is inevitable once
    the tag is defined, you can call it advisory until you are blue in the
    face but if it is used at all it won't be taken as such.) Now consider
    what happens when something that's incapable of generating the tag sends
    to the agent that requires it.

(3) Problems with places where parameters aren't expected/allowed. Once
    the tag is required there will be pressure to generate it. This in turn
    will lead to sending agents upgrading producing it and thereby cranking
    out parameters for the first time. Some of these agents are now used to
    generate values for fields that don't allow parameters. The upgrade will
    cause these fields to become synatically invalid.

I could go on to list many more, but hopefully this gives some flavor
of how bad I think this idea is.