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RE: ACTION-550: Draft some initial material for Section 2.3 of the Guidelines

From: Jo Rabin <jrabin@mtld.mobi>
Date: Mon, 24 Sep 2007 22:49:09 +0100
Message-ID: <C8FFD98530207F40BD8D2CAD608B50B47326D9@mtldsvr01.DotMobi.local>
To: "Sean Patterson" <SPatterson@Novarra.com>, <public-bpwg-ct@w3.org>
Hi Sean


Thanks for this, certainly food for thought. I have a number of detailed
comments about the wording. E.g. I am not sure that it is true that
completely accurate to say "Although web servers are technically
supposed to base the content they send to browsers on the Accept header
[6]," as my reading of the HTTP spec is that the server can vary its
response on any aspect of the request, and that is what the Vary header
indicates. Although I agree that if you read the section on the Accept
header in isolation it does strongly suggest that this is the sole basis
of decision making. However, it is pretty clear from other places in the
spec that this is not the case.


Also I have various other comments on the assumptions behind your
proposed 2.3.1 and 2.3.2 and the specifics of the text, especially
"Content transformation servers typically want the origin server to send
the desktop version of the site since the desktop version is usually
more functional." which seems to ignore the intentions of the content
owner and the preferences of the content consumer. It also appears to
make the assumption that a mobile version is a dumbed down version of a
desktop version, whereas what we advocate is really the opposite, i.e.
that a mobile version is a carefully crafted rendition that is
especially tailored for the mobile user. 


This is really not primarily to do with the technical limitations of
format/content-type but is about understanding the use of your content
in the mobile context. If the content provide has crafted their content
to be especially useful in that context, then the transformation
provider saying that 'there is a more functional desktop presentation'
is actually completely wrong. It would be more functional if the user
had a desktop, but the point precisely is they don't. The content owner
knows that, and is in a _much_ better position than some automatic
process to determine what is a "functional user experience" (q.v) of
_their_ content in the mobile context.


That said, my main observations relate to your proposed mechanisms in


This is of course just a sketch, not proposed text:


As a matter of priority I would prefer that we find a mechanism that
primarily, or exclusively, calls on HTTP headers to figure out what is
going on. I'd also, personally, prefer that we don't call on HTTP header
extensions if that is possible. Extending the values or interpretation
of the values of existing headers seems more in line with the spirit of
it all.


A Via header consists of a sequence of hosts or pseudonyms (pseudonyms
are syntactically HTTP tokens) and optional comments. I'm unsure of why
you think that it is unreliable. My understanding is that although the
comment field MAY be stripped the fact that a proxy has been involved
MUST be preserved (all MUSTards RFC2119 wise, of course). Hence a via
header consists of a sequence of hosts or pseudonyms. We could suggest a
practice in which transcoding proxies identify themselves by adding a
#transcoding suffix to either the pseudonym or the host. We could
suggest that a standard URI is added to the list to identify that the
previous proxy is a transforming proxy. Something along those lines.


(Incidentally, I think that we need to be clear, also, that where it is
the user agent itself that exhibits transcoding or adaptation behavior
it should also add a via header and should make it clear that it is the
user agent itself that is doing that - by being the first in the chain
and by some other mechanism ...).


I believe that the transcoding proxy MAY alter the User Agent Header in
addition to being obliged to add a Via field. This is only to avoid
blocking hosts. If it does modify the user agent header it MUST include
a standard notification that it has done so. (that would imo be a URI) .
I believe that a server that has alternative representations SHOULD
always add a Vary: User-Agent (or Vary: *) to indicate that it has
alternative representations. If a server receives a request with a Via
header that contains #transcoding (or whatever) it MUST respond with
Vary to indicate that it requires the correct user agent header. On
receipt of a Vary header, having presented a modified user agent header,
the Transcoding proxy MUST re-present the request with the User Agent
header that it originally received. The transcoding proxy SHOULD cache
the information that the server varies its response according to the
User Agent header and not modify it in the future. (Subject to cache
expiry etc.)


The server may in addition add cache-control: no-transform to any
response. And this MUST be respected. 


I realise that what I am proposing puts the onus on servers that have
more than one representation to do something that they don't today, and
I realize also that I am suggesting that transcoding proxies can modify
the user agent string. In light of recent intemperate discussion, on
this and other lists, let me repeat that what I mean is that they can do
this only in order to avoid a blocking response. If, in response to a
request, they get a Vary: User-Agent or * they MUST re-request the URI
with the User-Agent they received and remember that setting for that URI
(using a mechanism similar to authentication realms in which user agents
are expected to remember the URI path and re-present authentication
credentials without prompting the user for URIs in that path). Proxies
must pass on the Vary.


My justification for putting the onus on servers in this way is that if
the server is dumb you can't do anything other than spoof the UA string.
If on the other hand the server is smart and varies the response, then
the RFC says "An HTTP/1.1 server SHOULD include a Vary header field with
any cacheable response that is subject to server-driven negotiation."
Delete "cacheable" for the purposes of this discussion.


I think that in addition transforming proxies MAY take account of clues,
like the content-type header or DOCTYPE suggesting mobile content.
However, I think that this is likely to be a short term solution that
locks in a very unsatisfactory status quo on XMHTL-MP and Basic
mish-mash. Since XHTML Basic content is also XHTML content and since the
DOCTYPE and content-type markers work extremely imperfectly and not
really at all together I think it is something we'd like not to rely on.
We should be able to send HTML content to UAs that support it and be
able to inform transcoding proxies that we really do mean this for those
user agents.


I agree with your idea of adding Warning: headers. I suggest adding some
standard URIs to mean various things.


I think we should consider what, if anything, to say about 300 HTTP
responses. An important principle, imo, is that if there is a choice and
the user is interested in it, they should be given it. It seems that the
300 response is a vehicle for that. How should we make use of it?


I think we could consider what role HEAD has in tasting whether a server
can Vary.


I like your idea of using links rel="handheld" but a) don't think this
extends generally enough (what is a handheld) and b) would prefer to see
the Link header reinstated in HTTP for this purpose 


I wonder what we should say about increasing the overhead on requests
and reponses. We should at least introduce a statement of principle?


We haven't touched on how a transcoding proxy can advertise its
capabilities nor how a consenting server can take advantage of them.


Hope this makes some kind of sense.






From: public-bpwg-ct-request@w3.org
[mailto:public-bpwg-ct-request@w3.org] On Behalf Of Sean Patterson
Sent: 24 September 2007 20:14
To: public-bpwg-ct@w3.org
Subject: ACTION-550: Draft some initial material for Section 2.3 of the


I apologize for not getting this sent out to the group sooner.  Here is
my draft for section 2.3 of the "Guidelines for Using Content
Transformation".  (Hopefully the formatting will be OK.  Is there a
better/preferred way to submit material to the group?)

Sean Patterson

+1 630 773 0000 ext. 289


Powering the Mobile Generation(tm)

2       Guidance for Delivery Chain Component Developers

2.3     Guidance for Content Transformation Server Developers

Content transformation servers have the ability to transform content
into a form that is suitable for a requesting entity's delivery context.
However, a content transformation server that is invisible from browsers
and other servers on the network can cause problems.  These problems
include transforming content that should not be transformed, multiple
transformations, and sub-optimal transformation.  This section contains
guidelines for developers of content transformation servers to help
avoid these problems.

2.3.1   The Need for Content Transformation Servers Variation of device capabilities

While there are many mobile devices in existence today that give their
users the ability to browse the web, the majority of devices are not
capable of accessing web content.  Even for those devices that can
access the internet, there are large variations in their web browsing
capabilities.  Content transformation servers can transform web content
into a form that works well on any particular device. Most content is not designed for mobile devices

The majority of web sites are designed for users of desktop (or laptop)
computers.  These computers have large screens, a mouse, full-size
keyboards, fast CPUs, large amounts of memory, and are fully connected
to the Internet, typically at broadband speeds.  Mobile devices
(especially mobile phones) normally have none of these characteristics.
Regular web content frequently assumes that it will be displayed using
the hardware of a desktop computer.  Content transformation servers can
reduce the hardware requirements of the content so that it works better
on a mobile device. Most content is not designed for mobile browsers

Most web content is designed to be displayed on web browsers that run on
desktop computers.  These are full-featured browsers that can display
web sites that use complex HTML, CSS, and JavaScript as well as
multimedia content such as Flash and video.  In addition, most desktop
web sites assume that the user has a mouse or other pointing device.
Mobile devices frequently have much more limited web browsers.  Regular
web content may not display properly or at all on the web browser in a
mobile device.  Even if a desktop web site displays reasonably well, it
may be difficult to use on a mobile phone.  Content transformation can
transform the content into a simpler form that can be displayed and used
on a mobile browser. Variation of mobile content

There is a wide variation of what is considered "mobile content."
Mobile content that is designed for a high-end mobile device may not
display well or be useable on lower-end mobile devices.  In this case it
makes sense for a content transformation server to transform the content
developed for a higher-end mobile device into content that is suitable
for a lower-end device. Eliminates the need for a least common denominator solution

One approach to the problem of the variation of mobile devices is to
create a "least common denominator" page that works on all (or almost
all) mobile devices.  This approach is simpler than having multiple
versions of the page (see the next section), but limits the end user
experience.  An example of a least common denominator approach is
writing content that will work with the Default Delivery Context" (DDC)
defined in the "Mobile Web Best Practices 1.0" W3C Proposed
Recommendation [1]. The "Default Delivery Context" outlines the baseline
characteristics that a device must implement in order to be suitable for
browsing the web.  If a content transformation server exists on the
network, the least common denominator approach is not necessary.
Instead, a rich version of the site can be created with the knowledge
that it will be "reduced down" for any requesting entity that is less
capable. Reduces the need for multiple versions of a site

Another way to handle the variation of mobile devices is to create
multiple versions of a web site to deal with the multiple types of
mobile devices that can access the site.  This approach is costly to
establish and maintain across the increasingly diverse range of handsets
available.  When a content transformation server exists in the network,
the need to create multiple versions for different mobile devices is
reduced.  Again, a single, rich version of the site can be created and
easily maintained. A content transformation server can do a better job of following
mobile best practices

The "Mobile Web Best Practices 1.0" W3C Proposed Recommendation [1]
contains many recommendations for authoring content that is intended for
viewing on a mobile device.  A well-designed content transformation
server can do a better job of following the mobile best practices than a
human author, especially when taking into account the capabilities of
the many different mobile devices.  The result will be a more
consistent, uniform experience.

2.3.2   Guidelines of how content transformation servers should
communicate with the rest of the delivery chain Identifying the content transformation server

HTTP 1.1 requires that all proxy servers append a string to the Via
header [2] for any request or response they forward.  This string
consists of the name of the protocol of the received message, the
version number of the protocol, the hostname (or a pseudonym if the
hostname is sensitive information), and an optional comment.  (The name
of the protocol is assumed to be HTTP if not specified.)  Content
transformation servers should identify themselves in the comment of the
string they put in the Via header.  Here is an example where a content
transformation server at zzz.net adds itself to the Via header:

Via: 1.1 nowhere.com (Apache/1.1), 1.1 zzz.net (CT-Server-2000/1.0)

Unfortunately, the HTTP 1.1 protocol specification [3] allows subsequent
servers that receive the message to remove comments in the Via header.
So, while it is recommended that content transformation servers identify
themselves in the Via header, it is not always reliable.

A more reliable method for identifying a content transformation server
is to use the X-Mobile-Gateway header.  The syntax of the
X-Mobile-Gateway header is as follows (expressed in Augmented BNF form
as described in [4]):

X-Mobile-Gateway  = "X-Mobile-Gateway" ":" 1*( product | comment )

An example would be:

X-Mobile-Gateway: CT-Server-2000/1.0 (Server-Only; Linux i686; en-US),
Super-CT-Server/2.0 (Headers, Footers; MS Windows XP i686; en-US)

The syntax for each content transformation server in the
X-Mobile-Gateway header is the same as for the User-Agent and Server
headers.  It is recommended that value of this header contain the
product name and version of the content transformation server as well as
a comment in parentheses that contains useful characteristics of the
content transformation server separated by semicolons.  See [5] for the
syntax of "product".

Each subsequent content transformation server in the request/response
chain appends its information to the end of the X-Mobile-Gateway header.
In contrast to the Via header, content transformation servers are only
allowed to append to the end of the X-Mobile-Gateway header; no other
modifications are allowed. The User-Agent header

It is frequently necessary for content transformation servers to replace
the User-Agent header in requests with a value that is the same as used
by a desktop browser.  For example, the content transformation server
might use the following User-Agent header:

User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:
Gecko/20070725 Firefox/

Although web servers are technically supposed to base the content they
send to browsers on the Accept header [6], it is very common for web
servers to use the User-Agent header to make decisions about the content
to return to a particular browser.  For example, a web site that has
both a desktop and mobile version may examine the User-Agent header and
send the desktop version of the site if the User-Agent is recognized as
a desktop browser and return the mobile version of the site if the
User-Agent is recognized as a mobile browser on a mobile device.
Content transformation servers typically want the origin server to send
the desktop version of the site since the desktop version is usually
more functional.  This is the reason that content transformation servers
frequently send a User-Agent header from a desktop browser.

If the origin server needs to know what the actual User-Agent header is
from the original device that made the request, it can examine the
X-Device-User-Agent header (see section Identifying the mobile browser

Since content transformation servers typically replace the User-Agent
header in the original request from the mobile browser with a desktop
User-Agent string, there needs to be a way for the origin server to
identify the mobile browser that made the original request.  This is
done with the X-Device-User-Agent header.  The syntax for the
X-Device-User-Agent header is as follows:

X-Device-User-Agent  = "X-Device-User-Agent" ":" 1*( product | comment )

(The syntax is the same as for the User-Agent header.)

When a content transformation server replaces the User-Agent header with
a desktop User-Agent string, an X-Device-User-Agent header should be
added to the request and the original User-Agent value from the mobile
browser should be copied without modification to the X-Device-User-Agent
header.  This will allow the origin server to detect the type of mobile
browser and mobile device that made the request if it needs this

Content transformation servers should not modify the X-Device-User-Agent
header if it already exists. Determining whether or not a web page should be transformed

There are times when the origin server wants a web page to be sent to
the mobile web browser unchanged.  The origin server can signal that it
does not want a web page to be transformed by a content transformation
server (or any other proxy) by using the Cache-Control [7] header.  The
no-transform directive [8] is used to specify that the entity body of a
response from the origin server should not be modified.

Cache-Control: no-transform

The Cache-Control header must be honored for both requests and
responses.  A content transformation server must not modify the entity
body of any request or response that uses the Cache-Control:
no-transform header.  In addition there are a handful of headers that
should not be modified as well.  See [9] for a list of those headers.

The Cache-Control: no-transform header can be added by content
transformation servers but it should not be modified by content
transformation servers. Notification that transformation has been applied

If a content transformation server makes changes (i.e., transformations)
to the entity body in a response, the content transformation server must
set the Warning header [10] to "214":

Warning: 214 zzz.net "Transformation applied"

This lets the browser and any other content transformation servers in
the request/response Identification of mobile content

Content can be identified as intended for mobile browsers by one of the
following methods:

*       The Content-Type header of the response is one of the following

o       application/vnd.wap.xhtml+xml

o       text/vnd.wap.wml

*       The document type of the response document is

o       <!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.0//EN"

o       <!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.1//EN"

o       <!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.2//EN"

o       <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"

o       <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.1//EN"

*       There is a link element in the response document with a media
attribute that has a value of "handheld" that points to a mobile
document.  Here is an example:

<link rel="alternate" media="handheld" href="www.mobileversion.com/" />

Origin servers that want to present a choice to the user of whether to
view the desktop version of a web page or the mobile version may use
this technique.  (The mobile browser would need to have the capability
of presenting the choice to the user for this to work.)

Identifying mobile content is important when the content transformation
server is deciding which transformations to apply to the response
content received from the origin server.

*       if the response content is identified as mobile, the content
transformation server should be conservative and try to perform only
non-layout and non-format changing transformations.  For example, it
would be OK to accelerate the content (by removing non-layout
whitespace, non-lossy compression, etc.), add a header and/or footer to
the page, apply content corrections, etc.  It would less desirable to
remove HTML tables, change the size and/or format of an image, etc.
However, if the content returned from the origin server uses features
that the content transformation server "knows" that the client device
does not support (e.g., by examining the User-Agent header sent the
mobile web browser), it is permissible to make more extensive changes to
make the content more suitable for the client device.  For example, if
an origin server returns an image in GIF format to a device that does
not support GIF images, it would be OK for the content transformation
server to transform the image into a different format that the client
device did support.

*       if the response content is not identified as mobile, and there
is no Cache-Control: no-transform header, the content transformation
server should perform all reasonable transformations on the response.


[1]  http://www.w3.org/TR/mobile-bp/

[2]  http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.45

[3]  http://www.w3.org/Protocols/rfc2616/rfc2616.html

[4]  http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html#sec2.1

[5]  http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.8

[6]  http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1

[7]  http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9

[8]  http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.5

[9]  http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.5.2

[10] http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46
Received on Monday, 24 September 2007 21:49:29 UTC

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