Content Transformation Guidelines 1g

1.1 Purpose

From the point of view of this document Content Transformation is the manipulation in various ways, by proxies, of content as delivered by an origin server with a view to making it more suitable for mobile presentation.

The W3C MWI BPWG neither approves nor disapproves of Content Transformation, but recognizes that is being deployed widely across mobile data access networks. The deployments are widely divergent to each other, with many non-standard HTTP implications, and no well-understood means either of identifying the presence of such transforming proxies, nor of controlling their actions.

This document establishes a framework to allow that to happen.

A more extensive discussion of the requirements for these guidelines is discussed in "Content Transformation Landscape" [CT-Landscape].

1.2 Scope

Rev 1g: New in this rev

The recommendations in this document refer only to "Web browsing" - i.e. access by user agents that are intended primarily for access to HTML based Web pages (Web browsers).

The BPWG is not chartered to create new technology, its role is to advise on best practice for use of existing technology. It is therefore not in scope to propose the creation of new HTTP headers. Thus although interactions between user agents and content transformation proxies might benefit from an enhanced repertoire of HTTP headers this document does not define any such headers, and discusses in only a limited way such interactions using existing headers.

1.3 Audience

Rev 1g: New in this rev

The audience for this document is creators of content transformation proxies, purchasers and operators of such proxies and content providers whose services may be accessed by means of such proxies.

2.1 Summary of Requirements

Rev 1g: This requires revision before publication.

The purpose of this section is to summarize the communication requirements of actors (transforming proxies, origin servers, and to some extent users) to communicate with each other. The relevant scenario involving a content transformation proxy is as follows:

user <------> user agent <---HTTP---> one or more content transformation proxies <---HTTP---> origin server

The needs of these actors are as follows:

1. The user agent needs to be able to tell the content transformation proxy [@@and the origin server]:

   1. what type of mobile device and what user agent is being used;

   2. what media-type (presentation format e.g. desktop, handheld) the user desires;

   3. that all content transformation should be avoided, or that reformatting is allowed/desired.

2. The content transformation proxy needs to be able to tell the origin server:

   1. that some degree of content transformation (re-coding and reformatting) can be performed;

   2. that content transformation will be carried out unless instructed not to;

   3. that content is being requested on behalf of something else [@@?? and what that something else is];

   4. that the request headers have been altered (e.g. additional content types inserted).

3. The origin server needs to be able to tell the content transformation proxy:

   1. that it varies its presentation according to device type and other factors;

   2. that it's permissible or otherwise to perform content transformation of various kinds;

   3. that it has media-specific representations;

   4. that is unable or unwilling to deal with the request in its present form.

4. The content transformation proxy needs to be able to tell the client browser:

   1. that it has applied transformations of various kinds to the content;

   2. how to access the untransformed representation of the content.

5. The content transformation proxy needs to be able to interact with the user:

   1. to allow the user to disable its features;

   2. to alert the user to the fact that it has transformed content and to allow access to an untransformed representation of the content.

Rev 1g: Moved from the following section

In satisfying content transformation requirements existing HTTP headers, directives and behaviors must be respected, and as far as is practical, no extensions to HTTP [HTTP] are to be used.

2.2 Objectives

Rev 1g The overall objective of this document is to provide a means, as far as is practical, for users to be provided with at least a [Device Independence Glossary] "functional user experience" of the Web, taking into account the fact that an increasing number of content providers create experiences specially tailored to the mobile context which they do not wish to be altered by third parties. Equally it takes into account the fact that there remain a very large number of Web sites that do not provide a functional user experience when perceived on many mobile devices.

Rev 1g The document describes how the origin server may request conforming transforming proxies not to alter HTTP requests and responses, as well as describing control options that a transforming proxy should offer users.

Rev 1g There is the possibility for conflict between the desires regarding content transformation of the content provider and the desires of users of that content. This document sets out to provide a framework within which, for matters of presentation, the desires of the content provider are usually accommodated but, where necessary, the user may override those desires with their preferences.

Rev 1g: Is this where the output of Sean Patterson's ACTION-678 might reasonably go? [What is the difference between a CT proxy and a Opera Mini?]

Rev 1g: Can the following go now?

[@@ other principles behind what we are trying to do - e.g. noting Sean's point that there is a wide diversity of different devices that all fall under the simple appellation of "handheld".]

2.3 Types of Proxy

Alteration of HTTP requests and responses is not prohibited by HTTP other than in the circumstances referred to in [HTTP] Section 13.5.2.

HTTP defines two types of proxy: transparent proxies and non-transparent proxies. As discussed in [HTTP] Section 1.3, Terminology:

"A [Definition: transparent proxy] is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. A [Definition: non transparent proxy] is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies."

This document elaborates the behavior of non-transparent proxies, when used for content transformation in the context discussed in [CT-Landscape].

2.4 Types of Transformation

Transforming proxies can carry out a wide variety of operations. In this document we categorize this rich vocabulary of possible operations into two types:

1. Alteration of Requests;

2. Alteration of Responses.

Alteration of responses is further sub-categorized into

1. Restructuring content including rewriting URIs;

2. Recoding content;

3. Optimizing content.

Restructuring content is a process whereby the original layout is altered so that content is added or removed or where the spatial or navigational relationship of parts of content is altered, e.g. by linearization or pagination. It includes also rewriting of URIs so that subsequent requests route via the proxy.

Recoding content is a process whereby the layout of the content remains the same, but details of its encoding may be altered. Examples include re-encoding HTML as XHTML, correcting invalid markup in HTML, conversion of images between formats (but not, for example, reducing animations to static images).

Optimizing content means removing redundant white space, re-compressing images (without loss of fidelity), zipping for transfer ...

2.5 Control of the Behavior of the Proxy

A transforming proxy as described in this document must offer a level of control to users and to origin servers with which it communicates.

Ref 1g: The following per Aaron's ACTION-666 and agreed on call on 2008-03-11

3.1 Proxy Treatment of Request

e.g. Accept: image/jpeg, image/gif, image/png;q=0.7 

3.2 Server Response to Proxy

Rev 1f: what are we saying the server should do with the POWDER received from the proxy?

If the server varies its presentation according to examination of received HTTP Headers then it must include a Vary HTTP header indicating this to be the case. If, in addition to, or instead of HTTP headers, the server varies its presentation on other factors (source IP Address ...) then it must include a * as one of the fields in the Vary response.

Rev 1f

The server should indicate, using [@@ref POWDER] and the vocabulary defined in Appendix [@@] its capabilities and its defaults in respect of content it serves. [@@ note that you should be able to get all the info for a server by following links from a single resource]

The server must include a no-transform directive if one is received from the user agent. If it is capable of varying its presentation it should take account of user agent capabilities [@@as derived from a DDR etc.] and formulate an appropriate experience according to those criteria.

If the server has distinct presentations according to its perception of the presentation media, then the medium for which the presentation is intended should be indicated [@@using the ...] This is going to be something like the link headers, I think

If the server creates a specific user experience for certain presentation media types it should inhibit transformation of the response by including a no-transform directive.

Servers may base their actions on a priori knowledge of behavior of transforming proxies, when they are identified in a Via header.

The server should not choose a Content-Type for its response based on its assumptions about the heuristic behavior of any intermediaries. (e.g. it should not choose content-type: application/vnd.wap.xhtml+xml solely on the basis that it suspects that transforming proxies will apply heuristics that make them not restructure it).

[@@ Vary headers in 406 response - restrict to the one(s) that have caused the 406.??]

Servers should respond with a 406 not a 200 if they can't handle the request. Servers should provide information about alternative representations by using the Vary header (if the alternatives are available from the same URI) or using link information if alternative representations are handled by different URIs. [This restricts to HTML for now. If link headers a reinstated in HTTP then this becomes a more universal mechanism. Open question as to whether it SVG or WICD etc. support any such notion]

[@@300 Response - could this be used as a signal from the server to say that it understands the protocol? A la RFC 2295]

3.3 Proxy Receipt and Forwarding of Response from Server

Rev 1g: the following is rather wobbly. Need a clearer steer on how much it is legitimate for a server to infer from earlier responses. Either way the reference to Vary needs to be adjusted because it doesn't quite make sense in the present version of the document.

If the proxy has altered any of the HTTP request headers, and it receives a Vary response from the server it should re-make the request with the original headers and forward the subsequent response without restructuring it, irrespective of the contents of the subsequent response. The proxy should take note of this and should not vary headers for subsequent requests, unless requests are subsequently received with no Vary header

[@@note that loop detection and elimination is needed here]

3.4 Proxy Response to User Agent

If the response includes a Warning: 214 Transformation Applied the proxy must not apply further transformation.

If the response includes a Cache-Control: no-transform directive then the response must be forwarded to the user agent unaltered other than in the respects noted for transparent operation of HTTP proxies as specified in RFC2616, and in particular the addition of a Via HTTP header.

Rev 1f: Follows from saying the server should label itself

If the response contains references to one or more Web Description Resource indicating server preferences for the treatment of its content the proxy should retrieve those WDRs, analyze them as to how they pertain to this resource, and act in accordance with the server's expressed preferences. Proxies should retain such WDRs for future reference in accordance with the policies, if any, described in those WDRs.

In the absence of a Vary or no-transform directive or of a WDR the proxy should apply heuristics to the content to determine whether it is appropriate to restructure or recode it (in the presence of such directives, heuristics should not be used.)

If the proxy alters the content then it must add a Warning: 214 Transformation Applied HTTP Header. [@@ should this be elaborated to say what kind of transformation?]

Rev 1g: The following is an adaptation (sic) of Andrew's ACTION-633

If the response contains links whose URIs have the scheme https the proxy may rewrite them so that it can transform the content. If the proxy rewrites such links, it must advise the end user of the security implications of doing so and must provide the option to avoid transformation of the resources the links refer to.

If the proxy has transformed (reformatted) the content but not rewritten https links, it should annotate those links to indicate that transformation service is not available on them.

A proxy should strive for the best possible user experience that the user agent supports. It should only alter the format, layout, dimensions etc. to match the specific capabilities of the user agent. For example, when resizing images, they should only be reduced so that they are suitable for the specific user agent, and this should not be done on a generic basis.

Rev 1e: Did we say we would remain silent on this?

Rev 1g: Francois made some proposals under ACTION-625 for dangerous content but no one has responded

If the proxy determines that the resource as currently represented is likely to cause serious mis-operation of the user agent then the proxy may transform the resource but only sufficiently to alter the specific aspect of the content that is likely to cause mis-operation. Proxies must not exhibit this behavior unless this has been specifically allowed by both the server and the user. [@@ either by persistent registration of preferences, or by use of the [@@correct dangerous content] directive.]