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Versions: 00 01 02 03 04 05 06 RFC 8081

Network Working Group                                          C. Lilley
Internet-Draft                                                       W3C
Intended status: Standards Track                        January 29, 2016
Expires: August 1, 2016


                        The font Top Level Type
                    draft-ietf-justfont-toplevel-00

Abstract

   This memo serves to register and document the "font" Top Level Type,
   under which the Internet Media subtypes for representation formats
   for fonts may be registered.  This document also serves as a
   registration application for a set of intended subtypes, which are
   representative of some existing subtypes already registered under the
   "application" tree by their separate registrations.

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 August 1, 2016.

Copyright Notice

   Copyright (c) 2016 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
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   The process of setting type in computer systems and other forms of
   text presentation systems uses fonts in order to provide visual
   representations of the glyphs.  Just as with images, for example,
   there are a number of ways to represent the visual information of the
   glyphs.  Early font formats often used bitmaps, as these could have
   been carefully tuned for maximum readability at a given size on low-
   resolution displays.  More recently, scalable vector outline fonts
   have come into widespread use: in these fonts, the outlines of the
   glyphs are described, and the presentation system renders the outline
   in the desired position and size.

   This document defines a top-level Internet Media Type type "font"
   under which different representation formats of fonts may be
   registered (e.g. a bitmap or outline formats).  It should be
   emphasized that, just as under the "image" top-level type one does
   not find registration for a specific image, for example, "The Night-
   watch" (by Rembrandt) but instead "JPEG" (a compressed image data
   representation format), so, under "font" one will not find "Courier"
   (the name of a popular font) but perhaps "TTF", "OTF" or "SFNT" (the
   names of commonly used TrueType and OpenType font formats as well as
   their higher-level wrapper format).

2.  Background and Justification

   Historically there has not been a registration of formats for fonts.
   Most recently, there have been several representation formats
   registered as MIME subtypes under the "application" top-level type.
   However, with the rapid adoption of web fonts (based on the data from
   HTTP Archive [1] showing a huge increase in web font usage from 1% in
   the end of 2010 to 50% across all sites in the beginning of 2015)
   custom fonts on the web have become a core web resource.  As the in-
   depth analysis [2] shows, the lack of the intuitive top-level font
   type is causing significant confusion among developers - while
   currently defined font subtypes are severely under-utilized there are
   many more sites that already use non-existent (but highly intuitive)
   media types such as "font/woff", "font/ttf" and "font/truetype".  At
   the same time, the majority of sites resort to using generic types
   such as "application/octet-stream", "text/plain" and "text/html"; or
   use unregistrable types such as "application/x-font-ttf".

   Contrary to our expectations, the officially defined IANA subtypes
   such as "application/font-woff" and "application/font-sfnt" see a
   very limited use - their adoption rates trail far behind as the



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   actual use of web fonts continues to increase.  The members of the
   W3C WebFonts WG believe the use of "application" top-level type is
   not ideal.  First, the "application" sub-tree is treated (correctly)
   with great caution with respect to viruses and other active code.
   Secondly, the lack of a top-level type means that there is no
   opportunity to have a common set of optional attributes, such as are
   specified here.  Third, fonts have a unique set of licensing and
   usage restrictions, which makes it worthwhile to identify this
   general category with a unique top-level type.

   The W3C WebFonts WG believes that the situation can be significantly
   improved if a set of font media types is registered using "font" as a
   dedicated top-level type.  Based on the data analysis presented
   above, we believe that it is the presence of simple and highly
   intuitive media types for images that caused the widespread adoption
   of IANA's recommendations, where the correct usage of existing media
   types reaches over 97% for all subtypes in the "image" tree.  The WG
   believes that, considering a rapid adoption of fonts on the web, the
   registration of the top-level media type for fonts along with the
   intuitive set of subtypes that reflect popular and widely used data
   formats would further stimulate the adoption of web fonts,
   significantly simplify web server configuration process and
   facilitate the proper use of IANA media type recommendations.

3.  Security considerations

   Fonts are interpreted data structures that represent collections of
   different tables containing data that represent different types of
   information, including glyph outlines in various formats, hinting
   instructions, metrics and layout information for multiple languages
   and writing systems, rules for glyph substitution and positioning,
   etc.  Depending on the format used to represent the glyph data the
   font may contain TrueType, PostScript or SVG outlines and their
   respective hint instructions, where applicable.  There are many
   existing, already standardized font table tags and formats that allow
   an unspecified number of entries containing predefined data fields
   for storage of variable length binary data.  Many existing (TrueType,
   OpenType and OFF, SIL Graphite, WOFF, etc.) font formats are based on
   the table-based SFNT (scalable font) format which is extremely
   flexible, highly extensible and offers an opportunity to introduce
   additional table structures when needed, in a way that would not
   affect existing font rendering engines and text layout
   implementations.  However, this very extensibility may present
   specific security concerns - the flexibility and ease of adding new
   data structures makes it easy for any arbitrary data to be hidden
   inside a font file.  There is a significant risk that the flexibility
   of font data structures may be exploited to hide malicious binary
   content disguised as a font data component.



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   Fonts may contain 'hints', which are programmatic instructions that
   are executed by the font engine for the alignment of graphical
   elements of glyph outlines with the target display pixel grid.
   Depending on the font technology utilized in the creation of a font
   these hints may represent active code interpreted and executed by the
   font rasterizer.  Even though hints operate within the confines of
   the glyph outline conversion system and have no access outside the
   font rendering engine, hint instructions can be, however, quite
   complex, and a maliciously designed complex font could cause undue
   resource consumption (e.g. memory or CPU cycles) on a machine
   interpreting it.  Indeed, fonts are sufficiently complex, and most
   (if not all) interpreters cannot be completely protected from
   malicious fonts without undue performance penalties.

   Widespread use of fonts as necessary component of visual content
   presentation warrants that a careful attention should be given to
   security considerations whenever a font is either embedded into an
   electronic document or transmitted alongside media content as a
   linked resource.  While many existing font formats provide certain
   levels of protection of data integrity (such mechanisms include e.g.
   checksums and digital signatures), font data formats provide neither
   privacy nor confidentiality protection internally; if needed, such
   protection should be provided externally.

4.  Definition and encoding

   The "font" as the primary media content type indicates that the
   content identified by it requires certain graphic subsystem such as
   font rendering engine (and, in some cases, text layout and shaping
   engine) to process font data, which in turn may require certain level
   of hardware capabilities such as certain levels of CPU performance
   and available memory.  The "font" media type does not provide any
   specific information about the underlying data format and how the
   font information should be interpreted - the subtypes defined within
   a "font" tree will name the specific font formats.  Unrecognized sub-
   types of "font" should be treated as "application/octet-stream".
   Implementations may pass unrecognized subtypes to a common font-
   handling system, if such system is available.

5.  Defined subtypes

   In this section the initial entries under the top-level 'font' MIME
   type are documented.  They also serve as examples for future
   registrations.







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5.1.  Generic SFNT font type

   Type name:  font

   Subtype name:  sfnt

   Required parameters:  None.

   Optional parameters:

      1) Name: Outlines  Value: TTF, CFF, SVG

         This parameter can be used to specify the type of outlines
         supported by the font.  Value "TTF" shall be used when a font
         resource contains glyph outlines in TrueType format, value
         "CFF" shall be used to identify fonts containing PostScript/CFF
         outlines, and value SVG shall be used to identify fonts that
         include SVG outlines.  TTF, CFF or SVG outlines can be present
         in various combinations in the same font file, therefore,
         multiple values for the same optional parameter may be defined.

      2) Name: Layout

         Value: OTF, AAT, SIL

         This parameter identifies the type of implemented support for
         advanced text layout features.  The predefined values "OTF",
         "AAT" and " SIL" respectively indicate support for OpenType
         text layout, Apple Advanced Typography or Graphite SIL.  More
         than one shaping and layout mechanism may be supported by the
         same font file, therefore, multiple values for the same
         optional parameter may be defined.

   Encoding considerations:  Binary.

   Interoperability considerations:  As it was noted in the first
      paragraph of the "Security considerations" section, the same font
      format wrapper can be used to encode fonts with different types of
      glyph data represented as either TrueType or PostScript (CFF)
      outlines.  Existing font rendering engines may not be able to
      process some of the particular outline formats, and downloading a
      font resource that contains unsupported glyph data format would
      result in inability of application to render and display text.
      Therefore, it would be extremely useful to clearly identify the
      format of the glyph outline data within a font using an optional
      parameter, and allow applications to make decisions about
      downloading a particular font resource sooner.  Similar, another
      optional parameter is suggested to identify the type of text



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      shaping and layout mechanism that is supported by a font.  Please
      note that as new outline formats and text shaping mechanisms may
      be defined in the future, the set of allowed values for two
      optional parameters defined by this section may be extended.

   Published specification:  ISO/IEC 14496-22 "Open Font Format" (OFF)
      specification being developed by ISO/IEC SC29/WG11.

   Applications that use this media type:  Any and all applications that
      are able to create, edit or display textual media content.

   Additional information:

      Magic number(s):  The TrueType fonts and OFF / OpenType fonts
         containing TrueType outlines should use 0x00010000 as the
         'sfnt' version number.

         The OFF / OpenType fonts containing CFF data should use the tag
         'OTTO' as 'sfnt' version number.

      File extension(s):  Font file extensions used for OFF / OpenType
         fonts: .ttf, .otf

         Typically, .ttf extension is only used for fonts containing
         TrueType outlines, while .otf extension can be used for any
         OpenType/OFF font, either with TrueType or CFF outlines.

      Macintosh file type code(s):  (no code specified)

      @font-face Format:  none.

      Fragment Identifiers  none.

   Person & email address to contact for further information:  Vladimir
      Levantovsky (vladimir.levantovsky@monotype.com).

   Intended usage:  COMMON

   Restrictions on usage:  None

   Author:  The ISO/IEC 14496-22 "Open Font Format" specification is a
      product of the ISO/IEC JTC1 SC29/WG11.

   Change controller:  The ISO/IEC has change control over this
      specification.






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5.2.  TTF font type

   Type name:  font

   Subtype name:  ttf

   Required parameters:  None.

   Optional parameters:

      Name: Layout  Value: OTF, AAT, SIL

         This parameter identifies the type of support mechanism for
         advanced text layout features.  The predefined values "OTF",
         "AAT" and " SIL" respectively indicate support for OpenType
         text layout, Apple Advanced Typography or Graphite SIL.  More
         than one shaping and layout mechanism may be supported by the
         same font file, therefore, multiple values for the same
         optional parameter may be defined.

   Encoding considerations:  Binary.

   Interoperability considerations:  As was noted in the first paragraph
      of the "Security considerations" section, the same font format can
      be used to encode fonts supporting different types of outlines
      and/or text shaping and layout mechanisms.  Existing font
      rendering engine implementations may not be able to process some
      of the particular layout table formats, and downloading a font
      resource that contains unsupported text shaping mechanism would
      result in inability of applications to display text properly.
      Therefore, it would be extremely useful to clearly identify the
      supported text shaping and layout data within a font using an
      optional parameter, and allow applications to make decisions about
      downloading a particular font resource sooner.  Please note that
      as new text shaping mechanisms may be defined in the future, the
      set of allowed values for the optional parameter defined by this
      section may be extended.

   Published specification:  ISO/IEC 14496-22 "Open Font Format" (OFF)
      specification being developed by ISO/IEC SC29/WG11.

   Applications that use this media type:  Any and all applications that
      are able to create, edit or display textual media content.

   Additional information:






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      Magic number(s):  The TrueType fonts and OFF / OpenType fonts
         containing TrueType outlines should use 0x00010000 as the
         'sfnt' version number.

      File extension(s):  Font file extensions used for TrueType / OFF /
         OpenType fonts: .ttf, .otf

         Typically, .ttf extension is only used for fonts containing
         TrueType outlines, while .otf extension may be used for any
         OpenType/OFF font, either with TrueType or CFF outlines.

      Macintosh file type code(s):  (no code specified)

      @font-face Format:  truetype

      Fragment Identifiers  none.

   Person & email address to contact for further information:  Vladimir
      Levantovsky (vladimir.levantovsky@monotype.com).

   Intended usage:  COMMON

   Restrictions on usage:  None

   Author:  The ISO/IEC 14496-22 "Open Font Format" specification is a
      product of the ISO/IEC JTC1 SC29/WG11.

   Change controller:  The ISO/IEC has change control over this
      specification.

5.3.  OTF font type

   Type name:  font

   Subtype name:  otf

   Required parameters:  None.

   Optional parameters

      Name: Outlines  Value: TTF, CFF, SVG

         This parameter can be used to specify the type of outlines
         supported by the font.  Value "TTF" shall be used when a font
         resource contains glyph outlines in TrueType format, value
         "CFF" shall be used to identify fonts containing PostScript/CFF
         outlines, and value SVG shall be used to identify fonts that
         include SVG outlines.  TTF, CFF or SVG outlines can be present



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         in various combinations in the same font file, therefore,
         multiple values for the same optional parameter may be defined.

   Encoding considerations:  Binary.

   Interoperability considerations:  As it was noted in the first
      paragraph of the "Security considerations" section, the same font
      format can be used to encode fonts with different types of glyph
      data represented as either TrueType, PostScript (CFF) or SVG
      outlines.  Existing font rendering engines may not be able to
      process some of the particular outline formats, and downloading a
      font resource that contains unsupported glyph data format would
      result in inability of application to render and display text.
      Therefore, it would be extremely useful to clearly identify the
      format of the glyph outline data within a font using an optional
      parameter, and allow applications to make decisions about
      downloading a particular font resource sooner.  Please note that
      as new outline formats may be defined in the future, the set of
      allowed values for the optional parameter defined in this section
      may be extended.

   Published specification:  ISO/IEC 14496-22 "Open Font Format" (OFF)
      specification being developed by ISO/IEC SC29/WG11.

   Applications that use this media type:  Any and all applications that
      are able to create, edit or display textual media content.

   Additional information:

      Magic number(s):  The TrueType fonts and OFF / OpenType fonts
         containing TrueType outlines should use 0x00010000 as the
         'sfnt' version number.

         The OFF / OpenType fonts containing CFF data should use the tag
         'OTTO' as 'sfnt' version number.

      File extension(s):  Font file extensions used for OFF / OpenType
         fonts: .ttf, .otf

         Typically, .ttf extension is only used for fonts containing
         TrueType outlines, while .otf extension can be used for any
         OpenType/OFF font, either with TrueType, CFF or SVG outlines.

      Macintosh file type code(s):  (no code specified)

      @font-face Format:  opentype

      Fragment Identifiers  none.



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   Person & email address to contact for further information:  Vladimir
      Levantovsky (vladimir.levantovsky@monotype.com).

   Intended usage:  COMMON

   Restrictions on usage:  None

   Author:  The ISO/IEC 14496-22 "Open Font Format" specification is a
      product of the ISO/IEC JTC1 SC29/WG11.

   Change controller:  The ISO/IEC has change control over this
      specification.

5.4.  WOFF 1.0

   Type name:  font

   Subtype name:  woff

   Required parameters:  None.

   Optional parameters:  None.

   Encoding considerations:  Binary.

   Interoperability considerations:  None.

   Published specification:  This media type registration updates the
      WOFF specification [3] at W3C.

   Applications that use this media type:  WOFF is used by Web browsers,
      often in conjunction with HTML and CSS.

   Additional information:

      Magic number(s):  The signature field in the WOFF header MUST
         contain the "magic number" 0x774F4646

      File extension(s):  woff

      Macintosh file type code(s):  (no code specified)

      Macintosh Universal Type Identifier code:  "org.w3c.woff"

      @font-face Format:  woff

      Fragment Identifiers:  none.




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   Person & email address to contact for further information:  Chris
      Lilley (www-font@w3.org).

   Intended usage:  COMMON

   Restrictions on usage:  None

   Author:  The WOFF specification is a work product of the World Wide
      Web Consortium's WebFonts Working Group.

   Change controller:  The W3C has change control over this
      specification.

5.5.  WOFF 2.0

   Type name:  font

   Subtype name:  woff2

   Required parameters:  None.

   Optional parameters:  None.

   Encoding considerations:  Binary.

   Interoperability considerations:  WOFF 2.0 is an improvement on WOFF
      1.0.  The two formats have different Internet Media Types,
      different @font-face formats, and may be used in parallel.

   Published specification:  This media type registration is extracted
      from the WOFF 2.0 specification [4] at W3C.

   Applications that use this media type:  WOFF 2.0 is used by Web
      browsers, often in conjunction with HTML and CSS.

   Additional information:

      Magic number(s):  The signature field in the WOFF header MUST
         contain the "magic number" 0x774F4632 ('wOF2')

      File extension(s):  woff2

      Macintosh file type code(s):  (no code specified)

      Macintosh Universal Type Identifier code:  "org.w3c.woff2"

      @font-face Format:  woff2




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      Fragment Identifiers  none.

   Person & email address to contact for further information:  Chris
      Lilley (www-font@w3.org).

   Intended usage:  COMMON

   Restrictions on usage:  None

   Author:  The WOFF2 specification is a work product of the World Wide
      Web Consortium's WebFonts Working Group.

   Change controller:  The W3C has change control over this
      specification.

6.  References

6.1.  URIs

   [1] http://httparchive.org/
       trends.php?s=All&minlabel=Nov+15+2010&maxlabel=Feb+15+2015

   [2] http://goo.gl/zbDhUN

   [3] http://www.w3.org/TR/WOFF

   [4] http://www.w3.org/TR/WOFF2

Author's Address

   Chris Lilley
   W3C
   2004 Route des Lucioles
   Sophia Antipolis  06902
   France

   Email: chris@w3.org














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