W3C home > Mailing lists > Public > public-media-fragment@w3.org > December 2009

Re: Processing requirements

From: Philip Jägenstedt <philipj@opera.com>
Date: Tue, 29 Dec 2009 14:23:02 +0100
To: Philip Jägenstedt <philipj@opera.com>, "Silvia Pfeiffer" <silviapfeiffer1@gmail.com>
Cc: "Jack Jansen" <Jack.Jansen@cwi.nl>, "Media Fragment" <public-media-fragment@w3.org>
Message-ID: <op.u5pbgothatwj1d@sisko.linkoping.osa>
On Wed, 23 Dec 2009 15:05:55 +0100, Philip Jägenstedt <philipj@opera.com>  

> On Wed, 23 Dec 2009 13:43:02 +0100, Silvia Pfeiffer
> <silviapfeiffer1@gmail.com> wrote:
>> Hi Philip,
>> Thanks for continuing to give implementer/browser developer feedback -
>> it's really awesome to have this input! And such a shame you cannot
>> join Davy, Conrad and I at FOMS to discuss this further.
>> On Wed, Dec 23, 2009 at 11:28 PM, Philip Jägenstedt <philipj@opera.com>  
>> wrote:
>>> On Sat, 12 Dec 2009 05:46:53 +0100, Silvia Pfeiffer
>>> <silviapfeiffer1@gmail.com> wrote:
>>>> On Thu, Dec 3, 2009 at 10:01 AM, Philip Jägenstedt <philipj@opera.com>
>>>> wrote:
>>>>> On Wed, 02 Dec 2009 21:51:47 +0100, Jack Jansen <Jack.Jansen@cwi.nl>
>>>>> wrote:
>>>>>> On 2 dec 2009, at 12:55, Philip Jägenstedt wrote:
>>>>>>> Following up on my previous email and todays IRC-conference (for  
>>>>>>> me).
>>>>>>> I won't get involved in the editors stylistic choices between ABNF,
>>>>>>> equivalent parsing algorithms (only the side effects of which are
>>>>>>> normative)
>>>>>>> or any other spec technique, but would request that at least the
>>>>>>> following
>>>>>>> are defined:
>>>>>>> 1. Splitting of name-value pairs
>>>>>>> The current ABNF only allows joining timesegment / spacesegment /
>>>>>>> tracksegment by "&", which means that e.g. #t=5& is not allowed  
>>>>>>> because
>>>>>>> it
>>>>>>> has a trailing &, which is very easy to get by accident if you  
>>>>>>> write a
>>>>>>> script like this:
>>>>>>> urifrag = '#':
>>>>>>> for d in dimensions:
>>>>>>>  urifrag += d + '&'
>>>>>> I'm not thrilled by this idea. The web has a long history of  
>>>>>> features
>>>>>> where an initial implementation was syntactically forgiving because  
>>>>>> it
>>>>>> was
>>>>>> deemed to be user-friendly at the time. Many of these have been  
>>>>>> causing
>>>>>> endless headaches until today. Think of the ability to use filenames
>>>>>> (especially Windows filenames) in the URL-bar, or in attributes in  
>>>>>> the
>>>>>> HTML
>>>>>> code. Think of global variables in JavaScript.
>>>>> Let's be clear that validity and processing requirements are separate
>>>>> things. That the processing for a certain input is well defined does  
>>>>> not
>>>>> mean that said input is valid. The validity definition is useful for
>>>>> authors
>>>>> to check their syntax against (using a validator) to find some  
>>>>> mistakes,
>>>>> etc. In my opinion, processing requirements should be as strict as
>>>>> possible
>>>>> (staying close to the valid syntax) while still being easy to  
>>>>> understand
>>>>> (for test suite writers, implementors and actual authors) and  
>>>>> degrading
>>>>> gracefully for forward-compatibility in the contexts where it is
>>>>> necessary.
>>>>> I am not suggesting relaxing e.g. any of the temporal syntaxes  
>>>>> because
>>>>> there
>>>>> is no benefit in doing so -- they are fixed and will not be changed  
>>>>> by
>>>>> future spec revisions.
>>>>> The Web platform is full of ugly and broken features, but that is not
>>>>> because specs had unambiguous but lax processing requirements, it is
>>>>> because
>>>>> they either did not exist or left processing ambiguous or undefined.  
>>>>> This
>>>>> results in poor interoperability and an inevitable race towards the  
>>>>> most
>>>>> forgiving parsing possible. We absolutely do not want this to happen  
>>>>> yet
>>>>> again with media fragments.
>>>> I have added two paragraphs to the the ABNF specification section, see
>>>> http://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-spec/#naming-syntax,
>>>> which specifies how we look at media fragment URIs. I think this is
>>>> necessary. I have kept it slightly more generic than just specifying
>>>> "&" as a separator and also allowed ";" as a separator, since that is
>>>> being used often by applications as a separator (see
>>>> http://en.wikipedia.org/wiki/Query_string). I think that's a good
>>>> compromise to take to address Philip's concern.
>>> I am a bit skeptical of allowing both & and ; as separators, as it  
>>> adds a
>>> little bit of complexity without any obvious benefit. Wikipedia links  
>>> back
>>> to
>>> <http://www.w3.org/TR/1999/REC-html401-19991224/appendix/notes.html#h-B.2.2>,
>>> but has that advice been followed? Personally I can't remember ever  
>>> reading
>>> or writing a query string using ; as a separator. I'd like to see some
>>> research on actual deployed software to see if not allowing ; as a  
>>> separator
>>> would cause problems. Also, if ; stays the ABNF needs to be updated.
>>> I appreciate this part: "A conformant server or user agent will need  
>>> to be
>>> able to parse a random URI query or fragment string for a media  
>>> resource and
>>> identify the relevant parts. E.g. the relevant field-value pair out of  
>>> a
>>> media fragment URI like this
>>> http://www.example.com/video.ogv#&&=&=tom;jerry=&t=34&t=meow:0# is  
>>> t=34."
>>> However, it's vague on what exactly the conformance requirements are.  
>>> I'd
>>> like the spec to be explicit about how to split the fragment into  
>>> segments,
>>> especially if & and ; are both allowed as separators. Having defined  
>>> that,
>>> simply refer to the ABNF and say that any string which is not "a valid
>>> production of the mediasegment syntax" should be discarded. I suppose  
>>> the
>>> error handling section is an appropriate place, although it is  
>>> currently
>>> defined in terms of MF concepts and not strings. Perhaps the error  
>>> handling
>>> section should be split into two parts, one which gets us from an  
>>> arbitrary
>>> string to a list of dimensions, and then the existing section that  
>>> defines
>>> which of those dimensions actually apply.
>> It is actually deliberately vague, because we are piggybacking onto a
>> mechanism that has been developed outside the media fragments working
>> group and is not part of what we should be specifying: how to compose
>> a query string. We are already leaning out of the window by also
>> applying it to URI fragments, but believe that is acceptable.
> Wikipedia points to
> <http://www.w3.org/TR/REC-html40/interact/forms.html#form-content-type>
> which states: "The control names/values are listed in the order they
> appear in the document. The name is separated from the value by `=' and
> name/value pairs are separated from each other by `&'." This is much too
> vague though.
> HTML5  
> <http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#url-encoded-form-data>  
> is a lot more specific and defines how to encode something which looks  
> like a query string. However, what we need is a spec for how to decode  
> it. If such a spec already exists we should reference it explicitly and  
> make it part of the conformance requirements, the same for query strings  
> and URI fragments.

I have searched high and low and consulted with #whatwg [1] and it seems  
there simply is no specification for how to decode query strings, not even  
a vague one. It looks as though the most relevant spec we could be  
referencing is IRI [2] or IRI bis [3], which would replace our reference  
to URI (RFC 3986).

The IRI spec (both versions) states: "Specifications that define the  
syntax of any of the above components MAY divide them further and define  
smaller parts to be components according to this document. [snip] Even  
where the components are not defined formally, it may be helpful to think  
about some syntax in terms of components and to apply the relevant  
restrictions.  For example, for the usual name/value syntax in query  
parts, it is convenient to treat each name and each value as a component."

Note that IRI DOES NOT define any kind of name/value pair syntax, the  
smallest components defined are iquery and ifragment. Any smaller  
subdivision must be defined by the MF spec, including the processing  
requirements with error handling. However, I believe it is inappropriate  
and unnecessary to have any normative requirements involving the query  
syntax, as explained below.

>> I do not think that the media fragment URI specification is the place
>> to define how a query string on a media resource has to be parsed.
>> There could be any number of other query parameters used in a query
>> string and they could be perfectly valid because the particular client
>> and the server both support them. So, we cannot actually write an
>> algorithm that expresses all possible query parameters in a media
>> fragment URI. We can only hint at it saying then where "?" or ";" are
>> being used as separators on a media resource and the particular
>> parameters that we specify are in use, we can prescribe what they
>> mean.
> If MF should not be concerned with composing query strings or URI  
> fragments, a position which I am inclined to support, then we are  
> already violating spec layers in e.g. the axissegment ABNF (defining how  
> to compose query string with "&") and in timesegment, spacesegment,  
> tracksegment and namesegment (defining how to join name-value pairs with  
> "=").
> I would support layering MF on top of an abstract list of name-value  
> pairs, ignoring the underlying string encoding. If we can find no  
> specification which defines how to map an arbitrary string to that  
> abstract list, then we could put it in an appendix or publish it as a  
> separate spec. Thoughts?

Please read the parts that I have highlighted in the IRC discussion [1].  
IMHO, it is best if this WG does not say anything about the query  
component at all. Defining smaller parts of iquery is similar in most  
regards to defining smaller parts of ipath, i.e. having the spec say that  
URIs on the form http://example.com/video/id=chapter-1/t=5/ have some  
special meaning. The format and parsing of both the path and query  
component have always been server-specific, it would make our job simpler  
if we didn't try to change that. Otherwise, what does MF say about all the  
URLs already out there that use t=0 in their query strings?

I realize that this WG has spent quite some effort on discussing and  
writing spec text for URI queries, so I am not suggesting just throwing  
that away. See suggestion at bottom.

>> BTW: for the same reasoning, we cannot exclude ";" as a separator - if
>> for years it has been proposed to be used as separator, then that's
>> what it should be. I believe, however, that ";" is not a separator
>> between parameters, but probably rather between parameter values and
>> we can totally make use of that.
> But are deployed servers handling it? I honestly don't know. If there is  
> no spec for how to handle ; perhaps we should reverse-engineer current  
> popular server software and do what they do. This is surely the most  
> real-world compatible solution.

I did some testing of the query string parsing of PHP, ASP (VBScript) and  

In their default configurations, all of these split on '&' and then split  
the name/value pair on the first occurence of '=', so that e.g. v==b  
results in a ('v','=b') pair. This is as expected. The only interesting  
differences are in how % encoded entities are decoded. For example, ASP  
silently discards some malformed entities such as '%%' while PHP preserves  
them. JSP is quite aggressive in throwing away any key/value pair with  
malformed % encoding. For duplicate names, PHP uses the last value, while  
ASP and JSP represents the values as a list (in the order they appeared).  
I haven't run it myself, but it looks like Perl CGI splits on either & or  
;. I also want to test what mod_perl does, but haven't yet.

There is no spec for parsing *query* components and I don't think there  
should be one. Instead, I propose that we define a parsing algorithm for  
splitting a *fragment* component into name/value pairs that is close to  
what PHP, ASP and JSP do for the query component. This is only so that  
authors can use techniques they are familiar with to produce media  
fragments, e.g. using urlencode to escape each part. We need to decide  
what to do about '+' as that is usually used to encode a space. (Consider  
what would happen if someone tried to use #t=clock:2009-12-29T13:58+01:00  
without escaping the + as %2B.)

Having defined how the fragment component is parsed into a list of  
name/value pairs, we can define media fragments in terms of that list.  
Then we should make all the text about query components non-normative and  
note that some server software may support evaluating the fragment  
server-side for some resources if the name/value pairs are given to it in  
a format it understands, be that  
http://example.com/video?t=4&xywh=160,120,320,240 or  

Is this a course of action the WG would be willing to pursue? The lack of  
processing requirements for the fragment component is a blocking issue for  
implementation. By discussing it up-front I hope we can find a solution  
everyone is happy with, instead of letting the first implementor make  
something up and then having to rely on that behavior forever.

I would very much appreciate if we can discuss this by email and not  
teleconference, as it is much easier to follow and to refer to afterwards.  
(I'll also be idling in #whatwg most of the time if anyone cares to  
discuss some details on IRC.)

[1] http://krijnhoetmer.nl/irc-logs/whatwg/20091229#l-5
[2] http://tools.ietf.org/html/rfc3987
[3] http://tools.ietf.org/html/draft-duerst-iri-bis-07

Philip Jägenstedt
Core Developer
Opera Software
Received on Tuesday, 29 December 2009 13:23:53 UTC

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