Content Transformation Guidelines 1.0

W3C Working Draft 14 April 2008

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Jo Rabin, mTLD Top Level Domain (dotMobi)


This document provides guidance to content transformation proxies and content providers as to how inter-work when delivering Web content.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is the First Public Working Draft of the Content Transformation Guidelines; it has been produced by the Content Transformation Task Force of the Mobile Web Best Practices Working Group as part of the Mobile Web Initiative.

This document is the Guidelines document referred to in the Deliverables section of the Charter of the W3C Mobile Web Initiative Best Practices Working Group Content Transformation Task Force. It is based on the issues identified by the Content Transformation Landscape document.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

Please send comments on this document to the Working Group's public email list public-bpwg-comments@w3.org, a publicly archived mailing list. The Working Group is especially looking for feedback on the guidelines that control the Behavior of Components of the delivery chain (see part 4 below). The editorial notes underline the issues that still need to be addressed. Suggestions in these areas would be most appreciated as well.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. This document is informative only. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Revision Description

Table of Contents

1 Introduction
    1.1 Purpose
    1.2 Audience
    1.3 Scope
2 Terminology
    2.1 Types of Proxy
    2.2 Types of Transformation
    2.3 Normative Language for Conformance Requirements
    2.4 Normative and Informative Parts of this Document
3 Requirements
    3.1 Summary of Requirements
    3.2 Control of the Behavior of the Proxy
        3.2.1 Control by the User
        3.2.2 Control by Server
        3.2.3 Control by Administrative or Other Arrangements.
        3.2.4 Resolving Conflicting Instructions
4 Behavior of Components
    4.1 Proxy Treatment of Request
        4.1.1 no-transform directive in Request
        4.1.2 Proxy Decision to Transform
        4.1.3 Proxy Indication of its Presence to Server
        4.1.4 Altering Header Values
    4.2 Server Response to Proxy
    4.3 Proxy Receipt and Forwarding of Response from Server
    4.4 Proxy Response to User Agent
5 Testing
6 Conformance


A References
B Scope for Future Work (Non-Normative)
C Acknowledgments (Non-Normative)

1 Introduction

This section is informative.

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].

1.3 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 Terminology

This section is normative.

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.]

3 Requirements

This section is informative.

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:

Interactions between actors

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.

3.2.2 Control by Server

Transforming proxies must allow origin servers to control the Content Transformation process. The control mechanisms include use of HTTP conventions as discussed in the following section (4 Behavior of Components).

Editorial Note: The BPWG is also considering the possible use of other techniques, such as [POWDER] to allow origin servers to express preferences regarding transformation of their content and also to provide information about the availability of different representations of their content.

4 Behavior of Components

This section is normative.

4.1 Proxy Treatment of Request

4.1.1 no-transform directive in Request

If the request contains a Cache-Control: no-transform directive the proxy must forward the request unaltered to the server, other than to comply with transparent HTTP behavior and in particular to add a Via HTTP header.

Irrespective of the presence of the no-transform directive, the proxy must behave transparently unless it is able positively to determine that the user agent is a Web browser. The mechanism by which the proxy recognizes the user agent as a Web browser should use evidence from the HTTP request, in particular the User-Agent and Accept headers.

4.1.2 Proxy Decision to Transform

If there is no no-transform directive present in the request the proxy should analyze whether it intends to offer transformation services by referring to:

Editorial Note: POWDER would provide an additional and potentially more definitive means ascertaining server preferences.

Proxies should not alter HTTP requests unless:

  1. unaltered headers would result in the user's request being rejected by the origin server;

  2. the user has specifically requested a restructured version of a desktop presentation.


Rejection of a request by a server is taken to mean both a HTTP 406 Status as well as HTTP 200 Status, with content indicating that the request cannot be handled - e.g. "Your browser is not supported"

Editorial Note: The BPWG is studying heuristics for determining when a response with a 200 Status should be treated as a response with a 406 Status.

Proxies should not intervene in methods other than GET, POST, HEAD and PUT.

User agents sometimes issue HTTP HEAD requests in order to determine if a resource is of a type and/or size that they are capable of handling. A transforming proxy may convert a HEAD request into a GET request if it requires the response body to determine the characteristics of the transformed response that it would return were the user agent subsequently to issue a GET request for that content.

In this case, the proxy should (providing such action is in accordance with normal HTTP caching rules) cache the response so that it is not required to send a second GET request to the server.

Editorial Note: The BPWG is aware of a number of circumstances in which request bodies are altered by transforming proxies (for example: transformation of character set from one supported by the device to one required by the server, carrying out a POST of a form that has been delivered in a paginated form to the device to avoid memory limitations, changing multipart/form-data to application/x-www-form-urlencoded where the device does not support multipart/form-data). Further study is required as to what prescriptions, if any, should be made about transformation of request bodies.

Knowing that the browser has available a linearization or zoom capability and/or supports a broad range of content formats the proxy should not restructure or recode content.

Editorial Note: Further discussion is needed on the subject of "session", in which consistency of user experience needs to be maintained across a sequence of related requests. This affects the proxy decision to transform and whether it should alter request headers.

If, as a result of carrying out this analysis the proxy remains unaware of the server's preferences and capabilities it should:

  • Issue a request with unaltered headers and examine the response (see 4.4 Proxy Response to User Agent);

  • If it is still in doubt, issue a request with altered headers

Editorial Note: Further discussion is needed on the fact that this technique can cause two requests for the same resource. It is noted that GET is supposed to be "idempotent" and not change the state of the server, however, it is also noted that there are circumstances in which GET is often used for such purposes, e.g. following a link from email.

Editorial Note: The BPWG notes that some transforming proxies issue duplicate requests as a matter of course in order to compare responses. This practice may specifically be deprecated in a future revision of this document.

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.


Including a no-transform directive can disrupt the behavior of WAP/WML proxies, because it can inhibit such proxies from converting WML to WMLC.

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).

4.4 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.

5 Testing

This section is normative.

Operators of transforming proxies should make available interfaces that facilitate testing of Web sites accessed through them and should make such interfaces available through normal Internet access paths.

6 Conformance

This section is normative.

A conforming implementation of this Recommendation must implement all the sections marked as normative.

A References

CT Landscape
Content Transformation Landscape 1.0, Jo Rabin, Andrew Swainston (eds), W3C Working Draft 25 October 2007 (See http://www.w3.org/TR/ct-landscape/)
RFC 2119
Key words for use in RFCs to Indicate Requirement Levels, S. Bradner, March 1997 (See http://www.ietf.org/rfc/rfc2119.txt)
Hypertext Transfer Protocol -- HTTP/1.1 Request for Comments: 2616, R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee, June 1999 (See http://tools.ietf.org/html/rfc2616)
Device Independence Glossary
W3C Glossary of Terms for Device Independence, Rhys Lewis (ed), W3C Working Draft 18 January 2005
Best Practices
Mobile Web Best Practices 1.0 Basic Guidelines, Jo Rabin, Charles McCathieNevile (eds), W3C Proposed Recommendation, 2 November 2006 (See http://www.w3.org/TR/mobile-bp/)
W3C Protocol for Web Description Resources (POWDER) Working Group Home Page (See http://www.w3.org/2007/powder/)

B Scope for Future Work (Non-Normative)

A placeholder.

C Acknowledgments (Non-Normative)

The editors acknowledge contributions of various kinds from members of the MWI BPWG Content Transformation Task Force.

The editor acknowledges significant written contributions from: