Authoritative Metadata

TAG Finding 25 February 2004

This version:
Latest version:
http://www.w3.org/2001/tag/doc/mime-respect (XML)
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18 February 2004 Draft, 27 January 2004 Draft, 10 December 2003 Draft, 9 July 2003 Draft, 25 June 2003 Draft
Ian Jacobs, W3C


Web architecture depends on applications having a shared understanding of the messages exchanged between agents (for example, clients, servers, and intermediaries) and a shared expectation of how the payload of a message -- a representation -- will be interpreted by the recipient. The Web architecture uses representation metadata, when supported by the communication protocol, to indicate the sender's intentions to the recipient. In particular, dispatching and security-related decisions regarding the processing of a message are often based on values provided in representation metadata fields, such as the "Content-Type" field of HTTP and MIME. In this finding, we review the architectural design choice that metadata provided by a sender be authoritative. We also examine why recipient behavior that misrepresents information provided by the sender can be harmful if it is done without consent from the user. Finally, we consider how specification authors should incorporate these points into their work.

Status of this Document

This document has been developed for discussion by the W3C Technical Architecture Group. This finding addresses issue contentTypeOverride-24 and partly addresses issue errorHandling-20. The TAG finding "Internet Media Type registration, consistency of use" also includes material related to this issue.

At their 23 February 2004 teleconference, the TAG approved this finding. Publication of this finding 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.

Additional TAG findings, both approved and in draft state, may also be available. The TAG expects to incorporate this and other findings into a Web Architecture Document that will be published according to the process of the W3C Recommendation Track.

The terms MUST, SHOULD, and SHOULD NOT are used in this document in accordance with [RFC2119].

Please send comments on this finding to the publicly archived TAG mailing list www-tag@w3.org (archive).

Table of Contents

1 Summary of Key Points
2 Scenarios
    2.1 Scenario 1: Silent recovery from error
    2.2 Scenario 2: Server misconfiguration
    2.3 Scenario 3: Metadata hints in specifications
3 Why the sender is the authoritative source of representation metadata
    3.1 The Role of Metadata
    3.2 Sources of Metadata
    3.3 Risks Associated with Overriding Authoritative Metadata
4 Inconsistency between representation data and metadata
    4.1 Recipient Handling of Inconsistency
    4.2 Self-describing data and Risk of Inconsistency
    4.3 Reducing the Risk of Inconsistency
5 Metadata Hints in Specifications
6 Future Work
7 References
8 Acknowledgments

1 Summary of Key Points

The following are the key architectural points of this finding:

  1. Representation metadata, when provided by the sender of a representation, is authoritative in defining the nature of the representation being sent.
  2. It is an error for an agent to ignore or override authoritative metadata without the consent of the party the agent represents.
  3. Inconsistency between representation data and metadata is an error that should be discovered and corrected rather than silently ignored.
  4. Format specifications SHOULD NOT work against the Web architecture by requiring or suggesting that a recipient override sender-provided metadata without user consent.

This finding addresses the following issues that are raised by these architectural points:

2 Scenarios

The scenarios in this section illustrate some issues that arise when the architectural points described in this finding are ignored. The remainder of the finding examines these issues in more detail.

2.1 Scenario 1: Silent recovery from error

Stuart runs his own Web server at "http://www.example.org/". He creates an HTML page and means to serve it as "text/html", but misconfigures the Web server so that the content is served via HTTP/1.1 [RFC2616] as "text/plain". Tim's browser looks inside the page, detects some markup that suggests that this is an HTML document (e.g., a <!DOCTYPE declaration or <title> element), and, without informing Tim, proceeds as though the content were "text/html", rendering it according to the HTML and CSS specifications. Janet's browser displays the content as plain text.

Which party has neglected a principle of Web architecture: Stuart for the server misconfiguration, Tim's browser for silently overriding the HTTP headers from the server, or Janet's browser for not detecting that the content looked like HTML?

Answer: By silently overriding metadata from the representation provider in the HTTP headers, Tim's browser did not respect Web architecture principles that promote shared understanding and security.

2.2 Scenario 2: Server misconfiguration

Norm publishes an XHTML document that includes this link:

<link href="cool-style" type="text/css" rel="stylesheet"/>

Although the link refers to an XSLT style sheet, Norm has set the type attribute to "text/css". Stuart has configured the Web server so that the style sheet is served via HTTP/1.1 as "application/xslt+xml". With a user agent that understands XSLT but not CSS, Janet requests the content that includes this link. As it interprets the representation data, Janet's user agent reads the type hint and does not fetch the style sheet."

Which party is responsible for the fact that Janet did not receive content she should have: Stuart for the server configuration, Norm for stating that the style sheet is served as "text/css" when in fact it's served with a different media type, or Janet's user agent for not double-checking the media type with the server?

Answer: Though not a violation of principles of Web architecture, Norm's mislabeling of content deprived Janet of content she should have received.

2.3 Scenario 3: Metadata hints in specifications

The MyFormat specification specifies a type attribute and that, when type is present, a receiving agent must use its value and ignore conflicting metadata provided by the sender. The MyFormat specification designers explain that such a definition of the type attribute allows content authors to work around misconfigured servers. They contend that this is necessary because in many environments content authors do not have sufficient access to server managers to affect server configuration.

Should the MyFormat specification designers ignore a principle of Web architecture or define type this way to remedy this social problem?

Answer: The TAG does not believe that author-specified overrides in representation data offer the proper solution to social problems such as interactions with server managers. An agent that silently overrides server-provided metadata can create security risks and prevent errors from being detected and corrected.

3 Why the sender is the authoritative source of representation metadata

Successful communication between two parties using a piece of information relies on shared understanding. In the Web architecture, agents identify resources with URIs and they communicate state information for these resources. Below we examine the role of metadata in that exchange and make the case for the sender as the authoritative source of metadata.

3.1 The Role of Metadata

The sequence of numbers "324033" might be a license plate number in the state of Arkansas or an old-style telephone number in Italy. In general, one cannot determine the nature of data in the absence of context. One way to provide a context for interpretation is through metadata. On the Web, examples of important metadata include the Internet media type, which explains what data format specification can be used to interpret the data, and the character encoding, which explains how octets map to characters.

In The Web architecture, data and metadata are distinguished during an exchange between agents (in the protocol used to carry out the exchange). Agents exchange resource state information through a "representation," which consists of two parts:

  1. Representation data, expressed in one or more formats (e.g., XHTML, SVG, PNG) used separately or in combination.
  2. Representation metadata, expressed in the protocol. This metadata includes the Internet media type (e.g., "text/html" or "image/jpeg") of the representation data.

Not only does this separation promote shared understanding, it enables more efficient processing. It is far easier to dispatch behavior on the basis of inspecting metadata (typically short strings) than it is to invoke a generic document parser and try to divine the purpose of data by inspecting the data itself (with no guarantee of success). Separating data from metadata also increases the Web's flexibility as data formats rise, evolve, and fall.

3.2 Sources of Metadata

One can imagine different (competing) sources of metadata:

  • From the agent providing the representation data.
  • From the agent receiving the representation data.
  • From a third party referring to a resource.

To enable the greatest number of independent agents to interpret representation data in a consistent manner (i.e., according to a common set of specifications), the Web architecture adopts the first choice: representation metadata, when provided by the sender of a representation, is authoritative in defining the nature of the representation being sent.

Thus, if the sender asserts through a protocol that "the following representation data has the Internet media type text/html", that assertion is authoritative. The IANA media type registry maps these short strings such as "text/html" or "image/png" to data format specifications (e.g., XHTML, CSS, PNG, XLink, RDF/XML, etc.) via intermediate media type registrations. For instance, in the IANA registry, the content type "text/html" is associated with [RFC2854], which in turn states that:

The text/html media type is now defined by W3C Recommendations; the latest published version is [HTML401].

The Internet media type asserts "this is X", not "process this as follows." Representation metadata does not constrain the receiving agent to process the representation data in one particular way. It does allow the designer of the receiving agent to create applications that correctly interpret the sender's intentions, while taking into account the desires of the party employing the agent (e.g., expressed through configuration and user choices).

3.3 Risks Associated with Overriding Authoritative Metadata

A user agent represents the user for protocol-level interactions with representation providers. A user agent that does not respect protocol specifications can violate user privacy, produce security holes, and otherwise create confusion. For example, a user agent can create a security problem by ignoring a "Content-Type" header with value "text/plain", guessing that representation data is a shell script, and executing the script on the user's machine without the user's knowledge. Because of such risks, it is an error for an agent to ignore or override authoritative metadata without the consent of the party employing the agent. For this reason, the HTTP/1.1 specification states, "If and only if the media type is not given by a "Content-Type" field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource."

In scenario 1, in terms of Web architecture, Stuart is innocent; misconfiguration of the server is not an architectural error, it's just a human error. Instead, Tim's browser is the culprit since it misrepresents the resource provider by ignoring the authoritative metadata, without Tim's consent. Janet's browser respected the "Content-Type" header field, and by doing so, helps Janet and Stuart detect a server misconfiguration.

Note the difference between an agent taking authoritative metadata into account and an agent ignoring the metadata without the consent of the user. The first scenario below is an error, the second is not:

  • The agent silently ignores the authoritative Internet media type "text/plain" and renders (without user consent) the representation data as though the Internet media type were "text/html".
  • The agent recognizes the authoritative Internet media type of "text/plain", and because the user has so requested, tries to detect any URIs in the content and then dereferences them to determine whether representations are available for the identified resources.

4 Inconsistency between representation data and metadata

Although there are benefits to separating representation metadata from data, there are risks as well. In particular, the representation provider may create inconsistencies by misassigning metadata. Inconsistency between representation data and metadata is an error. Examples of inconsistencies between headers and representation data that have been observed on the Web include:

Recipients SHOULD detect inconsistencies between representation data and metadata but MUST NOT resolve them without the consent of the user.

4.1 Recipient Handling of Inconsistency

In Scenario 2, Norm is responsible for Janet not having access to representation data she was meant to receive. The HTML 4.01 Recommendation states that "Authors who use [the type] attribute take responsibility to manage the risk that it may become inconsistent with the content available at the link target address." Janet's client could have done more than merely read the type hint and decide to skip the style sheet" Users benefit from clients that allow different configurations for handling hints, including:

  • Query the server, and when there is an inconsistency, choose the authoritative metadata, or
  • Query the server, and when there is an inconsistency, prompt the user for instructions on how to proceed.

4.2 Self-describing data and Risk of Inconsistency

Data is "self-describing" if it includes enough information to allow two parties to establish a consistent interpretation without additional clues. If the representation provider intends for the data to be interpreted in a manner other than what is self-described (e.g., "treat this XML content as plain text"), then clarifying metadata is required (e.g., in protocol headers). As illustrated above, providing redundant metadata for data that is self-describing can lead to inconsistencies.

Representation providers SHOULD NOT in general specify the character encoding for XML data in protocol headers since the data is self-describing.

4.3 Reducing the Risk of Inconsistency

Representation providers can help reduce the risk of inconsistency through careful assignment of representation metadata (especially that which applies across representations). In particular:

  • Server software designers SHOULD NOT specify a default Internet media type in the default configuration shipped with the server.
  • Server managers SHOULD be wary of specifying a default Internet media type.
  • Server managers SHOULD NOT specify an arbitrary Internet media type (e.g., "text/plain" or "application/octet-stream") when the Internet media type is unknown.
  • Content authors SHOULD inform server managers of metadata misconfigurations.
  • Server managers SHOULD provide authors with a means to override a metadata configuration when it is inappropriate for a specific representation. This does not mean that the representation data should override the representation metadata, only that the author should have a way to supply correct metadata.

5 Metadata Hints in Specifications

Some format specifications allow content authors to provide metadata hints for servers and clients. For instance, the http-equiv attribute of the HTML meta element was intended for servers (not clients). In HTML 2.0 [RFC1866], section 5.2.5, the attribute is specified as follows:

HTTP servers may read the content of the document <head> to generate header fields corresponding to any elements defining a value for the attribute HTTP-EQUIV.

The HTML 4.01 attribute type for the link element (used in Scenario 2) gives clients a hint about what the media type of a representation of the linked resource is likely to be.

A format specification that includes metadata hints for clients should make clear that when these hints interact with server metadata, they are advisory only. Format specifications SHOULD NOT include requirements for clients to override server metadata without user consent; this is the error of Scenario 3. An architecturally sound description of an advisory attribute might read:

The author may provides a hint to the client about the likely Internet media type of representations of the designated resource. Although the client MUST treat server metadata (including that provided by the file system) as authoritative, the client MAY use the hint in a number of ways, including as a preference when negotiating with the server, as input to a decision to retrieve a representation, or to recover from a misconfigured server. However, the client MUST NOT override the server's headers (by using the hint or any other mechanism) without the consent of the user.

Section 2.2.2 of the W3C Proposed Recommendation Speech Recognition Grammar Specification Version 1.0 [SRGS10] describes agent behavior that is consistent with this finding.

The W3C Recommendation SMIL 2.0 [SMIL20] is inconsistent with the current finding in this regard since the definition of the type attribute (section 7.3.1) specifies circumstances in which type is supposed to take precedence over server metadata.

6 Future Work

  1. The TAG is working with the authors of [RFC3023] to revise section 7.1 of that RFC, which suggests behavior regarding character encoding metadata that is inconsistent with the current finding.
  2. Reviewers of this finding asked whether similar architectural principles apply to headers sent in the direction of client to the server. This is the TAG's issue putMediaType-38: "Relation of HTTP PUT to GET, and whether client headers to server are authoritative."

7 References

Internet Assigned Numbers Authority (IANA) (See http://www.iana.org/.)
T. Berners-Lee, D. Connolly Hypertext Markup Language - 2.0, RFC1866, November 1995. (See http://www.ietf.org/rfc/rfc1866.)
N. Freed, N. Borenstein Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types, RFC2046, November 1996. (See http://www.ietf.org/rfc/rfc2046.txt.)
S. Bradner Key words for use in RFCs to Indicate Requirement Levels, RFC2119, March 1997. (See http://www.ietf.org/rfc/rfc2119.txt.)
R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee Hypertext Transfer Protocol -- HTTP/1.1, RFC2616, June 1999. (See http://www.ietf.org/rfc/rfc2616.txt.)
The Text/Plain Format Parameter, RFC2646, August 1999. (See http://www.ietf.org/rfc/rfc2646.txt.)
D. Connolly, L. Masinter The 'text/html' Media Type, RFC2854, June 2000. (See http://www.ietf.org/rfc/rfc2854.txt.)
M. Murata, S. St. Laurent, D. Kohn XML Media Types, RFC3023, January 2001. (See http://www.ietf.org/rfc/rfc3023.txt.)
J. Ayars et al. Synchronized Multimedia Integration Language (SMIL 2.0), W3C Recommendation, 7 August 2001. (See http://www.w3.org/TR/2001/REC-smil20-20010807/.)
A. Hunt, S. McGlashan eds. Speech Recognition Grammar Specification Version 1.0 , W3C Proposed Recommendation, 18 December 2003. (See http://www.w3.org/TR/2003/PR-speech-grammar-20031218/.)

8 Acknowledgments

Roy Fielding and Stuart Williams scrutinized this finding and provided substantial amounts of text. Dan Connolly generously provided valuable input as well. Many thanks to Martin Dürst, Philipp Hoschka, Rob Lanphier, and Norm Walsh for their reviews. Thanks to all reviewers for their contributions to this finding.