Content Transformation Guidelines 1l (Rev 12)

1 Introduction

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.

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) using HTTP. Clients that interact with proxies using mechanisms other than HTTP (and that typically involve the download of a special client) are out of scope, and are considered to be a distributed user agent. Proxies which are operated in the control of or under the direction of the operator of an origin server are similarly considered to be a distributed origin server and hence out of scope.

Note:

The document is not intended as guidelines for delivery of WAP/WML. Some of its recommendations may, in some circumstances, disrupt the delivery of WML.

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.

1.4 Summary of Requirements

This section summarizes the communication requirements of actors (users, user agents, transforming proxies and origin servers) to communicate with each other. It is recognised that several transformation proxies may be present but their interactions are not discussed in detail. The relevant scenario is as follows:

The needs of these actors are as follows:

The user agent needs to be able to tell the Content Transformation proxy and the origin server:
1. what type of mobile device and which user agent is being used;
2. that all Content Transformation should be avoided.
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 is being requested on behalf of something else and what that something else is;
3. that the request headers have been altered and what the original ones were.
The origin server needs to be able to tell the Content Transformation proxy:
1. that it varies the representation of its responses according to device type and other factors;
2. that it is not permissible to perform Content Transformation;
3. that it has media-specific representations;
4. that is unable or unwilling to deal with the request in its present form.
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.
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.

Note:

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

2 Terminology

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].

2.2 Types of Transformation

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

Alteration of Requests
Transforming proxies process requests in a number of ways, especially replacement of various request headers to avoid HTTP 406 Status responses (if a server can not provide content that is compatible with the original HTTP request headers) and at user request.
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 handling this response.]
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 Interpretation of RFC 2119 Key Words

The key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional in this Recommendation have the meaning defined in [RFC 2119].

Editorial Note1l: @@TODO need to have a conformance statement and re-introduce text where sections are non-normative?

3 Behavior of Components

3.1 Proxy Forwarding of Request

3.1.1 Applicable HTTP Methods

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.

If the HTTP method is altered from HEAD to GET, proxies should (providing such action is in accordance with normal HTTP caching rules) cache the response so that a second GET request for the same content is not required.

3.1.2 `no-transform` directive in Request

If the request contains a Cache-Control: no-transform directive proxies must forward the request unaltered to the server, other than to comply with transparent HTTP behavior and as noted below (see 3.1.6 Additional HTTP Headers).

Note:

An example of the use of Cache-Control: no-transform is the issuing of asynchronous HTTP requests, perhaps by means of XMLHTTPRequest [XHR], which may include such a directive in order to prevent transformation of both the request and the response.

3.1.3 Treatment of Requesters that are not Web browsers

Proxies must act as though a no-transform directive is present (see 3.1.2 no-transform directive in Request unless they are able positively to determine that the user agent is a Web browser. The mechanism by which a proxy recognizes the user agent as a Web browser should use evidence from the HTTP request, in particular the User-Agent and Accept headers.

3.1.4 Serving Cached Responses

Proxies should follow standard HTTP procedures in respect of caching and should use cached copies of resources where this is in accordance with those procedures

In some circumstances, proxies may paginate responses and where this is the case a request may be for a subsequent page of a previously requested resource.

Editorial Note1l: This hasn't been discussed before, what should we say about serving subsequent parts of a response that is set to "no-cache" or whose expiry time has elapsed?

3.1.5 Alteration of HTTP Header Values

Other than to comply with transparent HTTP operation, proxies should not modify request headers unless:

the user would be prohibited from accessing content as a result of the server responding that the request is "unacceptable" (see 3.3.3 Server Rejection of HTTP Request;
the user has specifically requested a restructured desktop experience;
the request is part of a sequence of requests to the same Web site and either it is technically infeasible not to adjust the request because of earlier interaction, or because doing so preserves consistency of user experience.

Note:

In this section, the concept of "Web site" is used (rather than "origin server") as some origin servers host many different Web sites. Since the concept of "Web site" is not strictly defined, proxies should use heuristics including comparisons of domain name to assess whether resources form part of the same "Web site".

These circumstances are detailed in the following sections.

3.1.5.1 Content Tasting

The theoretical idempotency of GET requests is not always respected by servers. In order, as far as possible, to avoid mis-operation of such content, proxies should avoid issuing duplicate requests and specifically should not issue duplicate requests for comparison purposes.

3.1.5.2 Avoiding "Unacceptable" Responses

A proxy may reissue a request with altered HTTP header values if a previous request with unaltered values resulted in the origin server rejecting the request as "unacceptable" (see 3.3.3 Server Rejection of HTTP Request). A proxy may apply heuristics of various kinds to assess, in advance of sending unaltered header values, whether the request is likely to cause a "request unacceptable" response. If it determines that this is likely then it may alter header values without sending unaltered values in advance, providing that it subsequently assesses the response as described under (@@@ below) and is prepared to reissue the request with unaltered headers, and alter its subsequent behavior in respect of the Web site so that unaltered headers are sent.

A proxy must not issue a POST/PUT request with altered headers when the response to the unaltered POST/PUT request has HTTP status code 200 (in other words, it may only send the altered request for a POST/PUT request when the unaltered one resulted in an HTTP 406 response, and not a "request unacceptable" response).

According to [RFC 2616 HTTP] Section 14.45 Via header comments "may be removed by any recipient prior to forwarding the message". However, the justification for removing such comments is based on memory limitations of early proxies, most modern proxies do not suffer such limitations.

3.2 Server Response to Proxy

3.2.1 Use of HTTP 406 Status

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

3.2.2 Server Origination of `Cache-Control: no-transform`

Servers must include a Cache-Control: no-transform directive if one is received in the HTTP request.

Servers should include a Cache-Control: no-transform directive if, for any reason, they wish to inhibit transformation of the response.

Note:

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

Note:

If a server is unable to add a Cache-Control HTTP header, in HTML documents it should add a meta HTTP-Equiv element containing Cache-Control: no-transform.

Editorial Note1l: I think this is here to be symmetrical with the section below on proxies taking account of it, however, this really applies to legacy servers that can do nothing else, so does it belong here? How much could a legacy server actually comply with this spec? Perhaps, if anywhere, this needs to be moved to an appendix on "legacy servers" to accompany the other "things that out of scope components should do anyway?"

3.2.3 Varying Representations

Servers should take account of user agent capabilities and formulate an appropriate experience according to those capabilities. Servers should provide a means for users to select among available representations, should default to the last selected representation and should provide a means of changing the selection.

Editorial Note1l: Above wording changed and elaborated from 1k, to be consistent with discussion at F2F and to proposed Best Practice in BP2. We did not resolve a mechanism by which servers should indicate that they are offering user choice in this way.

3.2.3.1 Use of `Vary` HTTP Header

If a server varies its representation 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, a server varies its representation based on other factors (e.g. source IP Address) then it must, in accordance with [RFC 2616 HTTP], include a Vary header containing the value '*'.

Servers may base their actions on knowledge of behavior of specific transforming proxies, as identified in a Via header, but should not choose an Internet content type for a response based on an assumption or heuristics about 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).

3.2.3.2 Indication of Intended Presentation Media Type of Representation

If a server has distinct representations that vary according to the target presentation media type, it should inhibit transformation of the response by including a Cache-Control: no-transform directive (see 3.2.2 Server Origination of Cache-Control: no-transform).

In HTML content it should indicate the medium for which the representation is intended by including link elements as follows:

Include a link element identifying the target presentation media types of this representation by setting the media attribute and set the href attribute to a valid local reference (i.e. use the fragment identifier added to the URI of the document being served to point to a valid target within the document);
Include link elements identifying the target presentation media types of other available representations by setting the media attribute to indicate those representations and set the href attribute to a URI without a fragment identifier.

Note:

The presence of the second usage of the link element discussed above in the absence of a link element conforming to the first usage does not indicate that this representation is or is not formatted for the presentation media types listed.

Note:

Some examples of the use of the link element are included below in B Example Transformation Interactions.

3.3 Proxy Forwarding of Response to User Agent

3.3.1 Receipt of Cache-Control: no-transform

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 follows.

If a proxy determines that the resource as currently represented is likely to cause serious mis-operation of the user agent then it may advise the user that this is the case and must provide the option for the user to continue with unaltered content.

3.3.2 Receipt of Warning: 214 Transformation Applied

If the response includes a Warning: 214 Transformation Applied HTTP header, proxies must not apply further transformation.

3.3.3 Server Rejection of HTTP Request

For compatibility with servers that do not implement this Recommendation (see 3.2.1 Use of HTTP 406 Status), a proxy may treat responses with an HTTP 200 Status as though they were responses with an HTTP 406 Status if it has determined that the content (e.g. "Your browser is not supported") is equivalent to one with an HTTP 406 Status.

3.3.4 Receipt of `Vary` HTTP Header

If, in response to an HTTP request with altered headers, that was not preceded by an HTTP request with unaltered headers, a proxy receives a response containing a Vary header referring to one of the altered headers then it should request the resource again and update whatever heuristics it uses so that unaltered headers are always presented first for this resource.

3.3.5 Link to "handheld" Representation

If the response is an HTML response and it contains a <link rel="alternate" media="handheld" /> element, the CT-proxy SHOULD request and process the referenced resource, unless the resource referenced is the current resource as determined by the presence of link elements as discussed under 3.2.3.2 Indication of Intended Presentation Media Type of Representation.

3.3.6 Proxy Decision to Transform

In the absence of a Vary or no-transform directive (or a meta HTTP-Equiv element containing Cache-Control: no-transform) proxies should apply heuristics to the response 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 Web site (@@sic) has previously shown that it is contextually aware, even if the present response does not indicate this;
a claim of mobileOK Basic™ ??? conformance is indicated;
the Content-Type or other aspects of the response are known to be specific to the device or class of device (e.g. for HTML documents the DOCTYPE is "-//OMA//DTD XHTML Mobile 1.2//EN", "-//WAPFORUM//DTD XHTML Mobile 1.1//EN" "-//WAPFORUM//DTD XHTML Mobile 1.0//EN" "-//W3C//DTD XHTML Basic 1.1//EN" and "-//W3C//DTD XHTML Basic 1.0//EN");
the user agent has linearization or zoom capabilities or other features which allow it to present the content unaltered;
the URI of the response (following redirection or as indicated by the Content-Location HTTP header) indicates that the resource is intended for mobile use (e.g. the domain is *.mobi, wap.*, m.*, mobile.* or the leading portion of the path is /m/ or /mobile/);
the response contains client-side scripts that may mis-operate if the resource is restructured;
the response is an HTML response and it includes <link> elements specifying alternatives according to presentation media type.

3.3.6.1 Alteration of Response

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 a proxy alters the response then it must add a Warning 214 Transformation Applied HTTP header.

If a proxy alters a response body then the altered content should validate according to an appropriate published formal grammar.

3.3.6.2 HTTPS Link Re-writing

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 a 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.

ACTION-732If the proxy re-writes HTTPS links, replacement links must have the scheme https.

4 Testing

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.

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