This document describes some of the ways in which parts of the MIME system, originally designed for electronic mail, have been used in the web, and some of the ways in which those uses have resulted in difficulties. This informational document is intended as background and justification for a companion Best Current Practice which makes some changes to the registry of Internet Media Types and other specifications and practices, in order to facilitate Web application design and standardization.
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2.1. Origins of MIME
2.2. Introducing MIME into the Web
2.3. Distributed Extensibility
3. Problems with application to the Web
3.1. Differences between email and web delivery
3.2. The Rules Weren't Quite Followed
3.4. The Down Side of Extensibility
4. Additional considerations
4.1. There are related problems with charsets
4.2. Embedded, downloaded, launch independent application
4.3. Additional Use Cases: Polyglot and Multiview
4.4. Evolution, Versioning, Forking
4.5. Content Negotiation
4.6. Fragment identifiers
5. Where we need to go
6. Specific recommendations
6.1. Internet Media Type registration
6.3. Other specifications and BCPs
8. IANA Considerations
9. Security Considerations
10. Informative References
§ Author's Address
This document was prompted by a set of discussions in the W3C Technical Architecture Group about web architecture and the difficulties surrounding evolution of the web, Internet Media types, multiple specifications for a single media type, and related discussions. The goal of the document is to prompt an evolution within W3C and IETF over the use of MIME (and in particular Internet Media Types) to fix some of the outstanding problems. This is an initial version review and update. The goal is to initially survey the current situation and then make a set of recommendation to the definition and use MIME components (and specifically, Internet Media Types and charset declarations) to facilitate their standardization across Web and Web-related technologies with other Internet applications. Discussion of this document is suggested on the mailing list firstname.lastname@example.org, a mailing list open for subscription to all, archives at http://lists.w3.org/Archives/Public/www-tag/.
MIME was invented originally for email, based on general principles of 'messaging', a foundational architecture framework. The role of MIME was to extend Internet email messaging from ASCII-only plain text, to include other character sets, images, rich documents, etc.) The basic architecture of complex content messaging is:
MIME is a "tagging and bagging" specification:
- how to label content so the intent of how the content should be interpreted is known
- how to wrap the content so the label is clear, or, if there are multiple parts to a single message, how to combine them.
"MIME types" (renamed "Internet Media Types") were part of the tagging -- a name space for describing how to initiate interpretation of a message. The "Internet Media Type registry" (MIME type registry) is where someone can tell the world what a particular label means, as far as the sender's intent of how recipients should process a message of that type, and the description of a recipients capability and ability for senders.
The original World Wide Web (the 0.9 version of HTTP) didn't have "tagging and bagging" -- everything sent via HTTP was assumed to be HTML. However, at the time (early 1990's) other distributed information access systems, including Gopher (distributed menu system) and WAIS (remote access to document databases) were adding capabilities for accessing many things other text and hypertext and the WWW folks were considering type tagging. It was agreed that HTTP should use MIME as the vocabulary for talking about file types and character sets. The result was that HTTP 1.0 added the "content-type" header, following (more or less) MIME. Later, for content negotiation, additional uses of this technology (in 'Accept' headers) were also added.
The differences between the use of Internet Media Types between email and HTTP were minor:
These minor differences have caused a lot of trouble.
The real advantage of using Internet Media Types to label content meant that the web was no longer restricted to a single format. This one addition meant expanding from Global Hypertext to Global Hypermedia (as suggested in a 1992 email (Connolly, D., “Global Hypermedia,” Oct 1992.) [connolly92])
|The Internet currently serves as the backbone for a global hypertext. FTP and email provided a good start, and the gopher, WWW, or WAIS clients and servers make wide area information browsing simple. These systems even interoperate, with email servers talking to FTP servers, WWW clients talking to gopher servers, on and on.|
|This currently works quite well for text. But what should WWW clients do as Gopher and WAIS servers begin to serve up pictures, sounds, movies, spreadsheet templates, postscript files, etc.? It would be a shame for each to adopt its own multimedia typing system.|
|If they all adopt the MIME typing system (and as many other features from MIME as are appropriate), we can step from global hypertext to global hypermedia that much easier.|
The fact that HTTP could reliably transport images of different formats, for example, allowed NCSA to add <img> to HTML. MIME allowed other document formats (Word, PDF, Postscript) and other kinds of hypermedia, as well as other applications, to be part of the web. MIME was arguably the most important extensibility mechanism in the web.
Unfortunately, while the use of Internet Media Types for the web added incredible power, several problems have arisen.
Some of the differences between the application contexts of email and web delivery determine different requirements:
Operating systems use using, and continued to evolve to use, different systems to determine the 'type' of something, different from the MIME tagging and bagging:
Information about these other ways of determining type (rather than by the content-type label) were gathered for the Internet Media Type registry; those registering types are encouraged to also describe 'magic numbers', Mac file type, common file extensions. However, since there was no formal use of that information, the quality of that information in the registry is haphazard.
Finally, there was the fact that tagging and bagging might be OK for unilaterally initiated (one-way) messaging, you might want to know whether you could handle the data before reading it in and interpreting it, but the Internet Media Types weren't enough to tell.
The behavior of the community when the Internet Media Type registry was designed haven't matched expectations:
In particular, web implementations of Internet Media Types diverged from expected behavior:
Incorrect senders coupled with liberal readers wind up feeding a negative feedback loop based on the robustness principle.
The result, alas, is that the web is unreliable, in that
This ambiguity and 'sniffing' also applies to packaged content in webapps ('bagging' but using ZIP rather than MIME multipart). (NOTE: NEEDS EXPANSION)
Extensibility adds great power, and allows the web to evolve without committee approval of every extension. For some (those who want to extend and their clients who want those extensions), this is power! For others (those who are building web components or infrastructure), extensibility is a drawback -- it adds to the unreliability and difference of the web experience. When senders use extensions recipients aren't aware of, implement incorrectly or incompletely, then communication often fails. With messaging, this is a serious problem, although most 'rich text' documents are still delivered in multiple forms (using multipart/alternative).
If your job is to support users of a popular browser, however, where each user has installed a different configuration of file handlers and extensibility mechanisms, MIME may appear to add unnecessary complexity and variable experience for users of all but the most popular types.
This section notes some additional considerations.
MIME includes provisions not only for file 'types', but also, importantly the "character encoding" used by text types: for example, simple US ASCII, Western European ISO-8859-1, Unicode UTF8. A similar vicious cycle also happened with character set labels: mislabeled content happily processed correctly by liberal browsers encouraged more and more sites to proliferate text with mis-labeled character sets, to the point where browsers feel they *have* to guess the wrong label. (NEEDS EXPANSION)
There are sites that intentionally label content as iso-2022-jp or euc-jp when it is in fact one of the Microsoft extension charsets (e.g., for access to circled digits. This is an intentional misuse of the definitions of the charsets themselves -- definitions which originated at the national standards body level.
The type of a document might be determined not only for entire documents "HTML" vs "Word" vs "PDF", but also to embedded components of documents, "JPEG image" vs. "PNG image". However, the use cases, requirements and likely operational impact of MIME handling is likely different for those use cases.
There are some interesting additional use cases which add to the design requirements:
Formats and their specifications evolve over time -- some times compatibly, some times not. It is part of the responsibility of the designer of a new version of a file type to try to insure both forward and backward compatibility: new documents work reasonably (with some fallback) with old viewers and that old documents work reasonably with new viewers. In some cases this is accomplished, others not; in some cases, "works reasonably" is softened to "either works reasonably or gives clear warning about nature of problem (version mismatch)."
In MIME, the 'tag', the Internet Media Type, corresponds to the versioned series. Internet Media Types do not identify a particular version of a file format. Rather, the general idea is that the Internet Media Type identifies the family, and also how you're supposed to otherwise find version information on a per-format basis. Many (most) file formats have an internal version indicator, with the idea that you only need a new Internet Media Type to designate a completely incompatible format. The notion of an "Internet Media Type" is very course-grained. The general approach to this has been that the actual Media Type includes provisions for version indicator(s) embedded in the content itself to determine more precisely the nature of how the data is to be interpreted. That is, the message itself contains further information.
Unfortunately, lots has gone wrong in this scenario as well -- processors ignoring version indicators encouraging content creators to not be careful to supply correct version indicators, leading to lots of content with wrong version indicators.
Those updating an existing Internet Media Type registration to account for new versions are admonished to not make previously conforming documents non-conforming. This is harder to enforce than would seem, because the previous specifications are not always accurate to what the Internet Media Type was used for in practice.
(NOTE: MULTIPLE INCOMPATIBLE AUTHORITATIVE SPECS)
The general idea of content negotiation is when party A communicates to party B, and the message can be delivered in more than one format (or version, or configuration), there can be some way of allowing some negotiation, some way for A to communication to B the available options, and for B to be able to accept or indicate preferences.
Content negotiation happens all over. When one fax machine twirps to another when initially connecting, they are negotiating resolution, compression methods and so forth. In Internet mail, which is a one-way communication, the "negotiation" consists of the sender preparing and sending multiple versions of the message, one in text/html, one in text/plain, for example, in sender-preference order. The recipient then chooses the first version it can understand.
HTTP added "Accept" and "Accept-language" to allow content negotiation in HTTP GET, based on Internet Media Types, and there are other methods explained in the HTTP spec.
The web added the notion of being able to address part of a content and not the whole content by adding a 'fragment identifier' to the URL that addressed the data. Of course, this originally made sense for the original web with just HTML, but how would it apply to other content. The URL spec glibly noted that "the definition of the fragment identifier meaning depends on the Internet Media Type", but unfortunately, few of the Internet Media Type definitions included this information, and practices diverged greatly.
If the interpretation of fragment identifiers depends on the MIME type, though, this really crimps the style of using fragment identifiers differently if content negotiation is wanted.
Many people are confused about the purpose of MIME in the web, its uses, the meaning of Internet Media Types. Many W3C specifications TAG findings and Internet Media Type registrations make what are (IMHO) incorrect assumptions about the meaning and purposes of a Internet Media Type registration.
We need a clear direction on how to make the web more reliable, not less. We need a realistic transition plan from the unreliable web to the more reliable one. Part of this is to encourage senders (web servers) to mean what they say, and encourage recipients (browsers) to give preference to what the senders are sending.
We should try to create specifications for protocols and best practices that will lead the web to more reliable and secure communication. To this end, we give an overall architectural approach to use of MIME, and then specific specifications, for HTTP clients and servers, Web Browsers in general, proxies and intermediaries, which encourage behavior which, on the one hand, continues to work with the already deployed infrastructure (of servers, browsers, and intermediaries), but which advice, if followed, also improves the operability, reliability and security of the web.
NOTE: This section should be elaborated to include requirements for changes to MIME and Internet Media Type registrations to improve the situation.
NOTE: We should try to get agreement on the background, problem statement and requirements, before sending out any more about possible solutions. The intention is that recommendations for changes to IETF-specified processes and registries would be moved into a new BCP-track document.
However, the following is a partial list of documents that should be reviewed and updated, or new documents written.
Update the Internet Media Type registration process (via a new IETF BCP document):
Various new specifications promote the use of 'sniffing' -- using the content of the data to supplement or even override the declared content-type or charset. Update these specifications:
This document is the result of discussions among many individuals in the IETF and W3C.
This memo includes no request to IANA.
This document discusses some of the security issues resulting from use (and mis-use) of MIME content types in the web.
|[connolly92]||Connolly, D., “Global Hypermedia,” Oct 1992.|
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