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Versions: 00 01 02                                                      
Internet Engineering Task Force                              L. Masinter
Internet-Draft                                                     Adobe
Intended status: Informational                        September 23, 2010
Expires: March 27, 2011

                    Internet Media Types and the Web


   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.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 27, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  History  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Origins of MIME  . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Introducing MIME into the Web  . . . . . . . . . . . . . .  4
     2.3.  Distributed Extensibility  . . . . . . . . . . . . . . . .  4
   3.  Problems with application to the Web . . . . . . . . . . . . .  5
     3.1.  Differences between email and web delivery . . . . . . . .  5
     3.2.  The Rules Weren't Quite Followed . . . . . . . . . . . . .  6
     3.3.  Consequences . . . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  The Down Side of Extensibility . . . . . . . . . . . . . .  7
   4.  Additional considerations  . . . . . . . . . . . . . . . . . .  8
     4.1.  There are related problems with charsets . . . . . . . . .  8
     4.2.  Embedded, downloaded, launch independent application . . .  8
     4.3.  Additional Use Cases: Polyglot and Multiview . . . . . . .  8
     4.4.  Evolution, Versioning, Forking . . . . . . . . . . . . . .  9
     4.5.  Content Negotiation  . . . . . . . . . . . . . . . . . . . 10
     4.6.  Fragment identifiers . . . . . . . . . . . . . . . . . . . 10
   5.  Where we need to go  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Specific recommendations . . . . . . . . . . . . . . . . . . . 11
     6.1.  Internet Media Type registration . . . . . . . . . . . . . 11
     6.2.  Sniffing . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.3.  Other specifications and BCPs  . . . . . . . . . . . . . . 12
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   10. Informative References . . . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   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 www-tag@w3c.org, a mailing list open for subscription to
   all, archives at http://lists.w3.org/Archives/Public/www-tag/.

2.  History

2.1.  Origins of MIME

   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:

   o  Message sent from A to B.

   o  Message includes some data.  Sender A includes standard 'headers'
      telling recipient B enough information that recipient B knows how
      sender A intends the message to be interpreted.

   o  Recipient B gets the message, interprets the headers for the data
      and uses it as information on how to interpret the data.

   MIME is a "tagging and bagging" specification:

   tagging  how to label content so the intent of how the content should
      be interpreted is known

   bagging  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

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   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.

2.2.  Introducing MIME into the Web

   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:

   o  default charset

   o  requirement for CRLF in plain text.

   These minor differences have caused a lot of trouble.

2.3.  Distributed Extensibility

     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 [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.                                               |

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   | 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.

3.  Problems with application to the Web

   Unfortunately, while the use of Internet Media Types for the web
   added incredible power, several problems have arisen.

3.1.  Differences between email and web delivery

   Some of the differences between the application contexts of email and
   web delivery determine different requirements:

   o  web "messages" are generally HTTP responses to a specific request;
      this means you know more about the data before you receive it.  In
      particular, the data really does have a 'name' (mainly, the URL
      used to access the data), while in messaging, the messages were

   o  You would like to know more about the content before you retrieve
      it.  The "tagging" is often not sufficient to know, for example,
      "can I interpret this if I retrieve it", because of versioning,
      capabilities, or dependencies on things like screen size or
      interaction capabilities of the recipient.

   o  Some content isn't delivered over the HTTP (files on local file
      system), or there is no opportunity for tagging (data delivered
      over FTP) and in those cases, some other ways are needed for
      determining file type.

   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:

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   o  'magic numbers': in many contexts, file types could be guessed
      pretty reliably by looking for headers.

   o  Originally MAC OS had a 4 character 'file type' and another 4
      character 'creator code' for file types.

   o  Windows evolved to use the "file extension" -- 3 letters (and then
      more) at the end of the file name

   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.

3.2.  The Rules Weren't Quite Followed

   The behavior of the community when the Internet Media Type registry
   was designed haven't matched expectations:

   o  Lots of file types aren't registered (no entry in IANA for file

   o  Those that are, the registration is incomplete or incorrect
      (people doing registration didn't understand 'magic number' or
      other fields).

   o  The actual content deployed or created by deployed software
      doesn't match the registration.

   In particular, web implementations of Internet Media Types diverged
   from expected behavior:

   o  Browser implementors would be liberal in what they accepted, and
      use file extension and/or magic number or other 'sniffing'
      techniques to decide file type, without assuming content-label was
      authoritative.  This was necessary anyway for files that weren't
      delivered by HTTP.

   o  HTTP server implementors and administrators didn't supply ways of
      easily associating the 'intended' file type label with the file,
      resulting in files frequently being delivered with a label other

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      than the one they would have chosen if they'd thought about it,
      and if browsers *had* assumed content-type was authoritative.
      Some popular servers had default configuration files that treated
      any unknown type as "text/plain" (plain ext in ASCII).  Since it
      didn't matter (the browsers worked anyway), it was hard to get
      this fixed.

   Incorrect senders coupled with liberal readers wind up feeding a
   negative feedback loop based on the robustness principle.

3.3.  Consequences

   The result, alas, is that the web is unreliable, in that

   o  servers sending responses to browsers don't have a good guarantee
      that the browser won't "sniff" the content and decide to do
      something other than treat it as it is labeled

   o  browsers receiving content don't have a good guarantee that the
      content isn't mis-labeled

   o  intermediaries (gateways, proxies, caches, and other pieces of the
      web infrastructure) don't have a good way of telling what the
      conversation means.

   This ambiguity and 'sniffing' also applies to packaged content in
   webapps ('bagging' but using ZIP rather than MIME multipart).  (NOTE:

3.4.  The Down Side of Extensibility

   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.

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4.  Additional considerations

   This section notes some additional considerations.

4.1.  There are related problems with charsets

   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.

4.2.  Embedded, downloaded, launch independent application

   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.

4.3.  Additional Use Cases: Polyglot and Multiview

   There are some interesting additional use cases which add to the
   design requirements:

   o  "Polyglot" documents: A 'polyglot' document is one which is some
      data which can be treated as two different Internet Media Types,
      in the case where the meaning of the data is the same.  This is
      part of a transition strategy to allow content providers (senders)
      to manage, produce, store, deliver the same data, but with two
      different labels, and have it work equivalently with two different
      kinds of receivers (one of which knows one Internet Media Type,
      and another which knows a second one.)  This use case was part of
      the transition strategy from HTML to an XML-based XHTML, and also
      as a way of a single service offering both HTML-based and XML-
      based processing (e.g., same content useful for news articles and
      web pages.

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   o  "Multiview" documents: This use case seems similar but it's quite
      different.  In this case, the same data has very different meaning
      when served as two different content-types, but that difference is
      intentional; for example, the same data served as text/html is a
      document, and served as an RDFa type is some specific data.

4.4.  Evolution, Versioning, Forking

   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.


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4.5.  Content Negotiation

   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.

4.6.  Fragment identifiers

   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.

5.  Where we need to go

   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)

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   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

6.  Specific recommendations

   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.

6.1.  Internet Media Type registration

   Update the Internet Media Type registration process (via a new IETF
   BCP document):

   o  Allow commenting or easier update; not all Internet Media Type
      owners need or have all the information the internet needs.  Wiki
      for Internet Media Types as well as formal registry?  Ability to
      add comments about deployed senders, deployed content, deployed
      recievers for new recievers or senders.

   o  Be clearer about relationship of 'magic numbers' to sniffing;
      review Internet Media Types already registered and update.

   o  Be clearer about requiring Security Considerations to address
      risks of sniffing

   o  require definition of fragment identifier applicability

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   o  ask the 'applications that use this type' section to be clearer
      about whether the file type is suitable for embedding (plug-in) or
      as a separate document with auto-launch (MIME handler), or should
      always be donwloaded.

   o  Be clearer about file extension use and relationship of file
      extensions to MIME handlers

6.2.  Sniffing

   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:

   o  Sniffing uses MIME registry for 'magic numbers'

   o  all sniffing can be a priviledge upgrade, if there is a buggy
      recipient, although bugs can be fixed.

   o  discourage sniffing unless there is no type label:

      *  malformed content-type: error

      *  no knowledge that given content-type isn't better than guessed

6.3.  Other specifications and BCPs

   o  FTP specifications: do FTP clients also change rules about
      guessing file types based on OS of FTP server?

   o  update Tag finding on authoritative metadata: is it possible to
      remove 'authority'?

   o  new: MIME and Internet Media Type section to WebArch, referencing
      this memo

   o  New: Add a W3C web architecture material on MIME in HTML to W3C
      web site, referencing this memo

   o  Reconsider other extensibility mechanisms (namespaces, for
      example): should they use MIME or something like it?

7.  Acknowledgements

   This document is the result of discussions among many individuals in
   the IETF and W3C.

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8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   This document discusses some of the security issues resulting from
   use (and mis-use) of MIME content types in the web.

10.  Informative References

              Connolly, D., "Global Hypermedia", Oct 1992, <http://

Author's Address

   Larry Masinter
   345 Park Ave.
   San Jose,   95110

   Phone: +1 408 536 3024
   Email: masinter@adobe.com
   URI:   http://larry.masinter.net

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