CT: mandating respect of some heuristics

Hi,

Here is a summary of what I think the current discussion on ISSUE-286 is 
at. I say "we" below, but that should be read as my biased view of it :-)

The idea is to forbid content transformation for some of the "mobile 
heuristics" we currently have, when an explicit "mobile" flag is set in 
an HTTP response. Pending the resolution of the questions mentioned 
below, I haven't heard any objection to the idea.

The list of explicit heuristics is:
  * mobile doctypes (XHTML MP and Basic, WML, iMode)
  * <link rel="alternate" media="handheld" href=""/> and possibly <link 
rel="alternate" media="all" href=""/> (where the value of "href" is a 
same-document reference)
  * Content-Type in current appendix E [1], save "application/xhtml+xml"
  * possibly a mobileOK claim if that gets defined in time for this document
Slight adjustments may have to be done in that list. There are no other 
identified explicit mobile flags.

We identified a few cases where transformations might still be useful 
when mobile content is served, for instance to remove comments that may 
appear before an XML declaration, and that would prevent content 
rendering on the device. There is a general agreement that we are at the 
"SHOULD" level here and not at the "MUST" level.
We do not intend to provide a list of possible exceptions. The addition 
of header/footer and more generally any transformation done on a regular 
basis (i.e. regardless of the response) are not valid exceptions.


There are two open questions to which I add a minor third one:

1. Whether content transformation proxies that operate in "link-mode" 
(i.e. all URIs are re-written to go through the proxy) are allowed to 
rewrite links in mobile content, so that they may still provide 
transformation services for pages that are linked from the mobile page.

I note that such proxies also lose the possibility to propose their 
services when they receive a response with a "Cache-Control: 
no-transform" directive, since they cannot rewrite links in such cases 
either. I agree that this situation is not exactly the same. I wonder if 
there are many proxies around that operate in "link-mode" and need to be 
compliant with the guidelines?

Since this is a transformation that would be done on a regular basis, I 
do not think it fits as a possible exception to the "SHOULD" clause. 
Besides, if we add that as a normal exception, then the SHOULD 
significantly loses its meaning.

The proposal could be to complete the guideline with an explicit 
exception for links rewriting for proxies that operate in that mode.


2. Whether users may express their preference to have mobile pages still 
transformed.

This is actually already covered by the text of current section 4.2.2 
[2] that says:
  "Proxies must provide a means for users to express preferences for 
inhibiting content transformation. Those preferences must be maintained 
on a user by user and Web site by Web site basis."

Slight digression:
  [Argh! Don't do that! Focus! Well, I know, but...]
  I note we're talking about "inhibiting content transformation" here, 
not "allowing"... is it implied? If not, the sentence looks a bit 
strange as I thought we were against the expression of blanket user 
preferences to allow content transformation, but not against the 
expression of blanket user preferences to inhibit content 
transformation, so requiring that such a preference be maintained on a 
Web site by Web site basis seems weird.

I suggest that we leave the text as it stands (and address the above 
digression).


3. Whether optimizing operations are still allowed, in other words 
restructuring and recoding would be forbidden but not optimizing (we may 
already have agreed on that at some point, I haven't had a close look). 
Alternatively, we may use the notion of "proxies SHOULD behave 
transparently when...".


In short, a rough draft of the resulting guideline, that obviously would 
need to go through the hands of an editor with delicate fingers, could 
look like:
  Proxies SHOULD NOT restructure or recode the response if at least one 
of the following affirmations is true:
   - DOCTYPE is [foo]
   - Content-Type is [bar]
   - There's a <link rel="alternate" media="handheld" 
href="[same-document-reference]" /> directive
... possibly completed with a "Proxies that operate in link-mode 
[definition needed!] may still rewrite links in that case".

HTH,
Francois.

[1] 
http://www.w3.org/2005/MWI/BPWG/Group/TaskForces/CT/editors-drafts/Guidelines/081107#sec-Example-Content-Types
[2] 
http://www.w3.org/2005/MWI/BPWG/Group/TaskForces/CT/editors-drafts/Guidelines/081107#sec-administrative-arrangements

Received on Wednesday, 28 January 2009 14:01:29 UTC