Content Transformation Guidelines 1.0

1.1 Purpose

From the point of view of this document, Content Transformation is the manipulation in various ways, by proxies, of requests made to and content 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.

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 "functional user experience" [Device Independence Glossary] of the Web, when mobile, 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.

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 offers users.

A more extensive discussion of the requirements for these guidelines can be found in "Content Transformation Landscape" [CT Landscape].

1.2 Audience

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.

1.3 Scope

The recommendations in this document refer only to "Web browsing" - i.e. access by user agents that are intended primarily for interaction by users with HTML 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. In satisfying Content Transformation requirements, existing HTTP headers, directives and behaviors must be respected, and as far as is practical, no extensions to [RFC 2616 HTTP] are to be used.

2.1 Types of Proxy

Alteration of HTTP requests and responses is not prohibited by HTTP other than in the circumstances referred to in [RFC 2616 HTTP] Section 13.5.2.

HTTP defines two types of proxy: transparent proxies and non-transparent proxies. As discussed in [RFC 2616 HTTP] Section 1.3, Terminology:

[Definition: "A transparent proxy is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification."]

[Definition: "A 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].

Editorial Note: The BPWG requests feedback on the degree to which it is necessary to distinguish between Content Transformation proxies that interact with user agents using HTTP, and other types of arrangements where a (proprietary) client application interacts with an in-network component using other techniques.

2.2 Types of Transformation

Transforming proxies can carry out a wide variety of operations. In this document we categorize these operations as follows:

1. Alteration of Requests

   Transforming proxies process requests in a number of ways, especially replacement of various request headers to avoid HTTP 406 Status responses (the server can not provide content in the format requested) and at user request.

2. Alteration of Responses

   There are three classes of operation on responses:

   1. Restructuring content

      [Definition: 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.]

   2. Recoding content

      [Definition: 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). ]

   3. Optimizing content

      [Definition: Optimizing content includes removing redundant white space, re-compressing images (without loss of fidelity) and compressing for transfer.]

2.3 Normative Language for Conformance Requirements

Individual conformance requirements or testable statements are identified in this document by the use of specific key words. In particular, the key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional in this Recommendation are to be interpreted as described in [RFC 2119] .

2.4 Normative and Informative Parts of this Document

Each section of this document contains an indication of whether it is normative or not.

3.1 Summary of Requirements

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:

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. that all Content Transformation should be avoided.

2. The Content Transformation proxy needs to be able to tell the origin server:

   1. that some degree of Content Transformation (restructuring and recoding) can be performed;

   2. that Content Transformation will be carried out unless requested 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 is permissible (or not) 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 user agent:

   1. that it has applied transformations of various kinds to 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.

3.2 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.

4.1 Proxy Treatment of Request

4.2 Server Response to Proxy

Servers should respond with a 406 HTTP Status if a request can not be satisfied with content that meets the criteria specified by values of request HTTP headers (and not a 200 Status).

If it is capable of varying its presentation it should take account of user agent capabilities and formulate an appropriate experience according to those criteria.

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, in accordance with [RFC 2616 HTTP], include a Vary header containing the value '*'.

If the server has distinct presentations that vary according to presentation media, then the medium for which the presentation is intended should be indicated.

Editorial Note: The BPWG is studying the use of the link element of HTML which is used for this purpose. It is noted that the link element is not available in formats other than HTML, and it is noted that there is currently active discussion about the use of the Link HTTP header, which would serve this purpose well.

If the server creates a specific user experience according to device characteristics or presentation media types it should inhibit transformation of the response by including a no-transform directive.

The server must include a no-transform directive if one is received from the user agent.

Servers may base their actions on knowledge of behavior of specific transforming proxies, as identified in a Via header, but should not choose a Content-Type for their response based on their assumptions about the heuristic behavior of any intermediaries. (e.g. a server should not choose content-type: application/vnd.wap.xhtml+xml solely on the basis that it suspects that proxies will not transform content of this type).

4.3 Proxy Receipt and Forwarding of Response from Server

If HTTP header fields were altered in the request then the proxy must be prepared to re-issue the request in an unaltered form on receipt of a Vary header in the response indicating that the server offers variants of its presentation according to any of the HTTP header fields that have been modified.

Editorial Note: The BPWG is aware that more precision may be needed in the above statement. If a transforming proxy has followed the guidelines in this document, then it should not receive a response with a Vary header if it has not already received such a response to a request with unaltered headers.

4.4 Proxy Response to User Agent

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

In the absence of a Vary or no-transform directive 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.)

Examples of heuristics:

• The server has previously shown that it is contextually aware, even if the present response does not indicate this;

• the Content-Type or other aspects of the response are known to be specific to the device or class of device e.g. application/vnd.wap.xhtml+xml or the Document Type is DOCTYPE XHTML-MP;

• the response is an HTML response and it includes <link> elements specifying alternatives according to presentation media type.

Editorial Note: The BPWG May include further example heuristics.

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.

If the proxy alters the content then it must add a Warning 214 Transformation Applied HTTP Header.

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

If the response includes a Cache-Control: no-transform directive then the response must remain unaltered other than to comply with transparent HTTP behavior and other than as noted below.

If the proxy determines that the resource as currently represented is likely to cause serious mis-operation of the user agent then it may, with the users explicit prior consent, warn the user and provide links to both transformed and unaltered versions of the resource.