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Re: [W3C] BPWG CTG Guidelines / HTTP field user-agent issues

From: Francois Daoust <fd@w3.org>
Date: Tue, 12 Aug 2008 15:06:43 +0200
Message-ID: <48A18AE3.3030202@w3.org>
To: casays <casays@yahoo.com>
CC: public-bpwg-comments@w3.org

Hi Eduardo,

Thanks for these additional detailed and constructive comments!

The Working Group will review them precisely and get back to you. Please 
note that the group is not having any regular call in the next two 
weeks, so it may take a bit of time.

Francois Daoust,
W3C Staff Contact,
Mobile Web Best Practices Working Group.



casays wrote:
> I posted the following message in the WMLProgramming mailing list.
> People have suggested that I publish it as a formal comment to the CTG
> draft, so here it is, under the heading "Allowing modifications of the
> HTTP header field user-agent: rationale missing".
> 
> Eduardo Casais
> areppim AG
> Bern Switzerland
> ------------------
> 
> I would like to review (a last time) an issue that reoccurs in all
> discussions about transcoders.
> 
>> Changing User Agent or other headers is not prohibited by HTTP
> 
> The first thing to stress is that the user-agent is essential to drive
> content selection and generation processes, both in the mobile Web and
> in the desktop Web. 
> 
> a) In the mobile Web, the user-agent is directly associated to the
> actual device, and hence serves as a key to characteristics such as
> screen dimensions, preferred content types, etc. The advent of
> uaprof/ccpp was supposed to make this mapping unncessary, but it is
> not the case: uaprof descriptions are often missing, point to invalid
> URL, omit important information, or are just plain unreliable. Device
> databases like WURFL, based on user-agent mappings, thus remain
> indispensible.
> b) In the desktop (non-mobile) Web, developers have long relied upon
> the user-agent to identify the browsers issuing requests in order to
> tailor content to their "quirks". This has been going on at least
> since the times of the Netscape / IE wars.
> 
> Let us now examine the use cases of a mobile Web browser accessing the
> Internet, and evaluate the relevance of the user-agent -- assuming
> that transcoders systematically substitue the original value with a
> new one.
> 
> 1.a User-agent-switcher.
> 	The Web server is able, based on the user-agent, to
> 	provide a mobile-optimized or a full-web service.
> 	It therefore needs the original user-agent; modifying
> 	it is unhelpful.
> 2.a Mobile Web only.
> 	2.a.1 Generic content.
> 	The server returns generic mobile content, without
> 	customizing it for any specific user-agent.
> 	This kind of applications is rare, and often
> 	corresponds to surviving examples of text-only
> 	services developed for older PDA and WAP 1 devices.
> 	Since the server does not use the original user
> 	agent, modifying it is useless.
> 	2.a.2 Mobile with default.
> 	The server returns mobile-optimized content. When
> 	not recognizing the user-agent, it returns a default,
> 	best-effort representation, perhaps with a message
> 	suggesting that the content is tailored for mobile
> 	devices.
> 	Since the server relies upon the user-agent, and is
> 	able to return a default representation, modifying it
> 	is unhelpful.
> 	2.a.3 No default.
> 	The server returns mobile-optimized content, but will
> 	return an error (page with "unsupported browser" warning
> 	or return code "request not acceptable") whenever it
> 	does not recognize the user-agent. In this case, modifying
> 	the original user-agent is most unhelpful, as it guarantees 
> 	that the server will not recognize it as a valid
> 	mobile one. If the server does not, for whatever reason
> 	(e.g. incomplete device database), recognize a mobile
> 	user-agent, then there might be a case for modifying it
> 	towards an acceptable mobile one -- but transcoders 
> 	precisely do the reverse: they change a mobile user-agent
> 	to an exotic full-Web one. Hence, a modification of the 
> 	original user-agent is unhelpful all situations.
> 3.a Full Web only.
> 	3.a.1 Generic content.
> 	The Web server serves generic full-Web content, without
> 	looking at the user-agent. In this case, modification of
> 	the original user-agent is useless.
> 	3.a.2 Tailored full-web with default.
> 	The server returns full-web content customized for specific
> 	full-web user-agents (e.g. IE 6.0, IE 7.0 and Firefox 2.0),
> 	and serves a default representation, perhaps with a warning
> 	("This site is best viewed with the following browsers:...")
> 	for other user-agents. In this case, modification of the
> 	original user-agent is either useless (in any case a default
> 	representation will be returned), or unhelpful (the default
> 	representation is probably better downgradable than one
> 	specifically customized for a very specific full-Web browser).
> 	3.a.3 Tailored with no default.
> 	The server returns content tailored for specific full-Web
> 	browsers, and an error for other unrecognized or unsupported
> 	user-agents.
> 	Here there is a case to substitute the original user-agent to
> 	force retrieval of content. However, this works only if the
> 	"fake" user-agent precisely corresponds to one of those
> 	accepted by the server -- but transcoders do not tailor their
> 	substitute user-agents with respect to the application server:
> 	the include only general hints (like Mozilla/x.y) in the hope
> 	this is enough to determine content generation.
> 	Hence, the generic substitution of user-agents performed by
> 	transcoders is not appropriate here.
> 
> Conclusion: in two cases, modification of the user-agent is useless,
> in three it is detrimental, in one it is either useless or
> detrimental, and in one it could be helpful, but it is currently done
> inappropriately.
> 
> Let us consider the interesting symetric use-cases: a full-Web mobile
> device accessing the Internet.
> 
> 1.b User-Agent switcher
> 	Following the same reasoning as in 1.a, we find that
>         modification of the original user-agent is unhelpful.
> 2.b Mobile Web only.
> 	2.b.1 Generic content.
> 	Whatever user-agent, the server returns generic mobile-optimized
> 	content. A modification of the original user-agent is useless.
> 	2.b.2 Tailored with default.
> 	The server returns mobile-optimized content, and a default
> 	representation for unrecognized user-agent. Modifying it is
> 	therefore useless -- the default representation will be returned
> 	whether the original (full-Web) or the substitute (pseudo-full
> 	Web) agent appears in the request.
> 	2.b.3 Tailored, no default.
> 	Following the same reasoning as in 2.a.3, the substitution of 
> 	original (full-Web) user-agent by a fake (full-Web) one is 
> 	useless, as it will anyway return an error.
> 3.b Full Web only
> 	3.b.1 Generic content.
> 	If the server returns generic full-Web content whatever the user
> 	agent, then modifications of the user-agent are useless.
> 	3.b.2 Tailored with default.
> 	Following the reasoning in 3.a.2, modifying the original
>         full-Web user agent is either unhelpful (because the server
>         could have recognized the mobile device's agent), or useless
>         (the same default representation would be returned).
> 	3.b.3 Tailored, no default.
> 	Here the server may recognize the full-Web user-agent of the
>         mobile device; it is therefore unhelpful to modify it. Or it
>         might not support that specific user-agent, in which case it
>         would be sensible to substitute one that is effectively
>         supported by the server; however, this is not what transcoders
>         do: they provide a generic, not a real user-agent instead --
>         this is inappropriate.
> 
> So one situation where it is detrimental, four where it is useless,
> one  where it is either useless or detrimental, and one where it is
> either useless or inappropriately done. It is also an acid test: do
> transcoders modify requests from full-Web capable mobile browsers? If
> so, something seriously weird is going on, as the excuse has generally
> been to make full-Web content available to non-full-Web capable devices.
> 
>>From this examination, one can only conclude that proponents of the
> preservation of the original user-agent do not have to justify their
> position and established practice. Rather, the onus is on the
> proponents of the substitution of the user-agent to argue in favour of
> their approach, which disrupts established practice. There is
> basically only one use case where changing the mobile user agent to a
> desktop user agent might help, but it remains to demonstrate:
> 
> a) The relevance of the scenario. Perhaps people at Google could let
> one of their crawlers roam over a few tens of thousands of WWW sites
> to gather statistics on the relative frequency of each aforementioned
> scenario.
> b) The benefits resulting from handling that specific scenario.
> c) That (a) and (b), taken together, are so overwhelming that they
> more than compensate the disruptions caused in all other use-cases.
> 
> If another use case outside the framework I have presented here pops
> up, this does not reduce the need for an assessment based on (a), (b),
> (c). 
> 
> As a final remark, I would like to note that transcoders have been
> operating in the mobile Web for a long time. It started with
> adaptation of HTML for PDA (Web clipping) and HTML to HDML conversion,
> continued with HTML to WML, before arriving at the current crop of
> content adaptation. In the old times, developers of content adaptation
> software were wary of modifying the user-agent: turning generic WWW
> content into a form suitable for mobile devices is so fraught with
> difficulties that one would take every chance to let a server return
> mobile optimized content (based on the user-agent) if it could. It is
> only fairly recently that, without much justification, transcoders
> have started in a
> systematic way to overwrite the user-agent field.
> 
> I think I have said everything I wanted regarding the CTG. The
> document  requires quite some rework -- nothing exceptional, since it
> is a draft. I will lean back and wait for the results of this round of
> revisions. Till then, readers of the WMLprogramming and W3C BPWG lists
> can rejoice in the knowledge that my long-winded posts are abating at
> last.
> 
> 
> E. Casais
> 
> 
> 
Received on Tuesday, 12 August 2008 13:07:19 UTC

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