





Network Working Group                                         G. Klyne
Request for Comments: 2531                    5GM/Content Technologies
Category: Standards Track                                  L. McIntyre
                                                     Xerox Corporation
                                                            March 1999


                Content Feature Schema for Internet Fax

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This document defines a content feature schema that is a profile of
   the media feature registration mechanisms [1,2,3] for use in
   performing capability identification between extended Internet fax
   systems [5].

   This document does not describe any specific mechanisms for
   communicating capability information, but does presume that any such
   mechanisms will transfer textual values.  It specifies a textual
   format to be used for describing Internet fax capability information.



















Klyne & McIntyre            Standards Track                     [Page 1]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


Table of Contents

   1. Introduction .............................................3
   1.1 Organization of this document............................3
   1.2 Terminology and document conventions.....................3
   2. Fax feature schema syntax ................................4
   3. Internet fax feature tags ................................4
   3.1 Image size...............................................5
   3.2 Resolution...............................................5
   3.3 Media type...............................................6
   3.4 Paper Size...............................................6
   3.5 Color capability.........................................6
   3.6 Color model..............................................8
   3.7 Image coding............................................10
   4. Examples ................................................12
   4.1 Simple mode Internet fax system.........................12
   4.2 High-end black-and-white Internet fax system............12
   4.3 Grey-scale Internet fax system..........................13
   4.4 Full-color Internet fax system..........................13
   4.5 Full-color Internet fax system (MRC)....................14
   4.6 Sender and receiver feature matching....................15
   5. IANA Considerations .....................................17
   6. Security Considerations .................................17
   6.1 Capability descriptions and mechanisms..................17
   6.2 Specific threats........................................18
   7. Acknowledgements ........................................18
   8. References ..............................................18
   9. Authors' Addresses ......................................21
   Appendix A: Feature registrations ..........................22
   A.1 Image size..............................................22
   A.2 Resolution aspect ratio.................................24
   A.3 Color levels............................................25
   A.4 Color space.............................................27
   A.5 CIELAB color depth......................................30
   A.6 CIELAB color gamut......................................32
   A.7 Image file structure....................................34
   A.8 Image data coding.......................................36
   A.9 Image coding constraint.................................38
   A.10 JBIG stripe size.......................................39
   A.11 Image interleave.......................................41
   A.12 Color subsampling......................................42
   A.13 MRC availability and mode..............................43
   A.14 MRC maximum stripe size................................45
   Appendix B: TIFF mode descriptions .........................47
   Appendix C: Revision history ...............................49
   Full Copyright Statement ...................................51





Klyne & McIntyre            Standards Track                     [Page 2]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


1. Introduction

   This document defines a content feature schema that is a profile of
   the media feature registration mechanisms [1,2,3] for use in
   performing capability identification between extended Internet fax
   systems [5].

   This document does not describe any specific mechanisms for
   communicating capability information, but does presume that any such
   mechanisms will transfer textual values.  It specifies a textual
   format to be used for describing Internet fax capability information.

   The range of capabilities that can be indicated are based on those
   covered by the TIFF file format for Internet fax [7] and Group 3
   facsimile [6].  A companion document [4] describes the relationship
   and mapping between this schema and Group 3 fax capabilities.

1.1 Organization of this document

   Section 2 specifies the overall syntax for fax feature descriptions
   by reference to the media feature registration and syntax documents
   [1,2].

   Section 3 enumerates the feature tags that are to be recognized and
   processed by extended Internet fax systems, according to their
   capabilities.

   Appendix A contains additional feature tag registrations for media
   features that are specific to fax and for which no applicable
   registration already exists.  These are presented in the form
   prescribed by the media feature registration procedure [1].

1.2 Terminology and document conventions

   The term "extended Internet fax system" is used to describe any
   software, device or combination of these that conforms to the
   specification "Extended Facsimile Using Internet Mail" [5].

   "capability exchange" describes any transfer of information between
   communicating systems that is used to indicate system capabilities
   and hence determine the form of data transferred.  This term covers
   both one-way and two-way transfers of capability information.

   "capability identification" is a particular form of capability
   exchange in which a receiving system provides capability information
   to a sending system.





Klyne & McIntyre            Standards Track                     [Page 3]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   "capability description" is a collection of data presented in some
   specific format that describes the capabilities of some communicating
   entity.  It may exist separately from any specific capability
   exchange mechanism.

      NOTE: Comments like this provide additional nonessential
      information about the rationale behind this document.  Such
      information is not needed for building a conformant
      implementation, but may help those who wish to understand the
      design in greater depth.

2. Fax feature schema syntax

   The syntax for the fax feature schema is described by "A syntax for
   describing media feature sets" [2].  This in turn calls upon media
   feature tags that may be registered according to the procedure
   described in "Media Feature Tag Registration Procedure" [1].

      NOTE: Media feature registration provides a base vocabulary of
      features that correspond to media handling capabilities.  The
      feature set syntax provides a mechanism and format for combining
      these to describe combinations of features.  This memo indicates
      those features that may be associated with extended Internet fax
      systems.

3. Internet fax feature tags

   This section enumerates and briefly describes a number of feature
   tags that are defined for use with extended Internet fax systems and
   applications.  These tags may be used also by other systems and
   applications that support corresponding capabilities.

   The feature tags presented below are those that an extended Internet
   fax system is expected to recognize its ability or non-ability to
   handle.

   Definitive descriptions of feature tags are indicated by reference to
   their registration per the media feature registration procedure [1]
   (some of which are appended to this document)

      NOTE: The presence of a feature tag in this list does not mean
      that an extended Internet fax system must have that capability;
      rather, it must recognize the feature tag and deal with it
      according to the capabilities that it does have.







Klyne & McIntyre            Standards Track                     [Page 4]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      Further, an extended Internet fax system is not prevented from
      recognizing and offering additional feature tags.  The list below
      is intended to provide a basic vocabulary that all extended
      Internet fax systems can use in a consistent fashion.

      If an unrecognized or unused feature tag is received, the feature
      set matching rule (described in RFC2533 [2]) operates so that tag
      is effectively ignored.

3.1 Image size

      Feature tag name    Legal values
      ----------------    ------------
      size-x              <Rational> (>0)
      size-y              <Rational> (>0)

   Reference:  this document, Appendix A.

   These feature values indicate a rendered document size in inches.

   Where the actual size is measured in millimetres, a conversion
   factor of 10/254 may be applied to yield an exact inch-based value.

3.2 Resolution

      Feature tag name    Legal values
      ----------------    ------------
      dpi                 <Integer> (>0)
      dpi-xyratio         <Rational> (>0)

   Reference: "Media Features for Display, Print, and Fax" [3], and this
   document appendix A.

   If 'dpi-xyratio' is present and not equal to 1 then the horizontal
   resolution (x-axis) is indicated by the 'dpi' feature value, and the
   vertical resolution (y-axis) is the value of 'dpi' divided by 'dpi-
   xyratio'.

   For example, the basic Group 3 fax resolution of 200*100dpi might be
   indicated as:

         (& (dpi=200) (dpi-xyratio=200/100) )

   When describing resolutions for an MRC format document, the complete
   set of usable resolutions is listed.  However, there are some
   restrictions on their use: (a) 100dpi resolution can be used only





Klyne & McIntyre            Standards Track                     [Page 5]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   with multi-level images, and (b) any multi-level image resolution is
   required to be an integral sub-multiple of the applicable mask
   resolution.

3.3 Media type

      Feature tag name    Legal values
      ----------------    ------------
      ua-media            screen
                          screen-paged
                          stationery
                          transparency
                          envelope
                          envelope-plain
                          continuous

   Reference: "Media Features for Display, Print, and Fax" [3].

      NOTE: Where the recipient indicates specific support for hard copy
      or soft copy media type, a sender of color image data may wish to
      adjust the color components (e.g.  per the related rules of ITU
      recommendation T.42 [9]) to improve rendered image quality on that
      medium.

3.4 Paper Size

      Feature tag name    Legal values
      ----------------    ------------
      paper-size          A4
                          A3
                          B4
                          letter
                          legal

   Reference: "Media Features for Display, Print, and Fax" [3].

3.5 Color capability

   Feature tag name    Legal values
   ----------------    ------------
   color               Binary  (bi-level only)
                       Limited (a limited number of colors)
                       Mapped  (palette or otherwise mapped color)
                       Grey    (grey-scale only)
                       Full    (full continuous-tone color)


   Reference: "Media Features for Display, Print, and Fax" [3].



Klyne & McIntyre            Standards Track                     [Page 6]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   The intention here is to give a broad indication of color handling
   capabilities that might be used, for example, to select among a small
   number of available data resources.

   The value of this feature also gives an indication of the more
   detailed color handling features that might be applicable (see next
   section).

   'Binary' indicates black-and-white, or other bi-level capability.  No
   further qualifying feature tags are required.

   'Limited' indicates a small number of distinct fixed colors, such as
   might be provided by a highlight printer, pen plotter or limited
   color display.  The 'color-levels' tag should be used to indicate the
   number of distinct colors available.

      NOTE: No ability to indicate any specific or named color is
      implied by this option.

   Some devices might use different intensity levels rather than
   different hues for distinction.

   'Mapped' indicates that pixel color values are mapped in some
   specifiable way to a multi-component color space.  The 'color-levels'
   tag may be used to indicate the number of distinct colors available;
   in its absence, sufficient levels to display a photographic image
   should be assumed.

   'Grey' indicates a continuous tone grey-scale capability.

   'Full' indicates full continuous tone color capability.

   For 'Mapped', 'Grey' and 'Full' color, additional feature tags
   (section 3.6) may be used to further qualify the color reproduction.

















Klyne & McIntyre            Standards Track                     [Page 7]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


3.6 Color model

      Feature tag name    Legal values
      ----------------    ------------
      color-levels        <integer>   (>2)
      color-space         Device-RGB  (device RGB)
                          Device-CMY  (device CMY)
                          Device-CMYK (device CMYK)
                          CIELAB      (LAB per T.42 [9])
                          (may be extended by further registrations)

      CIELAB-L-depth      <integer>   (>0)
      CIELAB-a-depth
      CIELAB-b-depth
      CIELAB-L-min        <integer>
      CIELAB-L-max
      CIELAB-a-min
      CIELAB-a-max
      CIELAB-b-min
      CIELAB-b-max

   Reference: this document, appendix A.

   The general model for image handling (both color and non-color) is
   described here from a receiver's perspective; a similar model
   operates in the reverse direction for a scan/send perspective:

          raw bit        pixel         color         physical
          stream  -(A)-> values -(B)-> values -(C)-> rendition

      -   "raw bit stream" is a stream of coded bits

     (A)  indicates image coding/decoding (MH,MR,MMR,JPEG,JBIG,etc.)

      -   "pixel values" are a single numeric value per picture element
          that designates the color of that element.

     (B)  indicates pixel-to-color value mapping

      -   "color values" have a separate numeric value for each color
          component (i.e. L*, a*, b* in the case of CIELAB indicated
          above.)

     (C)  indicates how the color values are related to a physical
          color.  This involves interpretation of the color value with
          respect to a color model (e.g. RGB, L*a*b*, CMY, CMYK) and a
          color space (which is typically recipient-dependent).




Klyne & McIntyre            Standards Track                     [Page 8]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      -   "physical rendition" is a color value physically realized on a
          display, printer or other device.

   There are many variables that can be applied at each stage of the
   processing of a color image, and any may be critical to meaningful
   handling of that image in some circumstances.  In other circumstances
   many of the variables may be implied (to some level of approximation)
   in the application that uses them (e.g. color images published on a
   Web page).

   The color feature framework described here is intended to allow
   capability description at a range of granularity: feature tags which
   correspond to implied (or "don't care" or "unknown") feature values
   may simply be omitted from a capability description.

   Grey scale and bi-level images are handled within this framework as a
   special case, having a 1-component color model.  The following
   features are used for describing color capabilities:

   'color-levels' indicates the number of distinct values for each
   picture element, and applies to all but bi-level images.  For bi-
   level images, a value of 2 is implied.

   'color-space' is used mainly with 'Mapped' and 'Full', but could be
   used with other modes if the exact color used is significant.  Two
   kinds of color space can be distinguished: device-dependent and
   calibrated.  Device dependent spaces are named here as 'Device-xxx',
   and are used to indicate a color space that is defined by the
   receiving device.  Calibrated color spaces presume the existence of a
   rendering system that is calibrated with respect to an indicated
   definition, and is capable of processing the device-independent color
   information accordingly.

   A color-handling receiver should indicate any appropriate device
   color space capability in addition to any calibrated color spaces
   that it may support.  A calibrated color space should be used when
   precise color matching is required in the absence of specific
   knowledge of the receiving system.

      NOTE: In practice, although they appear to be separate concepts,
      the color model and color space cannot be separated.  In the final
      analysis, a color model (RGB, CMY, etc.) must be defined with
      respect to some color space.

   'CIELAB-L-depth', 'CIELAB-a-depth' and 'CIELAB-b-depth' indicate the
   number of different values that are possible for the L*, a* and b*
   color components respectively, and are significant only when colors




Klyne & McIntyre            Standards Track                     [Page 9]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   are represented in a CIELAB color space.  These features would be
   used with palettized color, or with full color where each color
   component has a different number of possible values.

   The 'CIELAB-x-min' and 'CIELAB-x-max' values indicate a color gamut
   (i.e. a range of color values that are used or may be rendered).  A
   gamut may be indicated in terms of the CIELAB color space even when
   colors are represented in some other space.

3.7 Image coding

      Feature tag name    Legal values
      ----------------    ------------
      image-file-         TIFF-S
      structure           TIFF-F
                          TIFF-J
                          TIFF-C
                          TIFF-L
                          TIFF-M
                          (may be extended by further registrations,
                          to cover non-TIFF image file structures)
      image-coding        MH
                          MR
                          MMR
                          JBIG
                          JPEG
                          (may be extended by further registrations)
      image-coding-       JBIG-T85    (bi-level, per ITU T.85)
      constraint          JBIG-T43    (multi-level, per ITU T.43)
                          JPEG-T4E    (per ITU T.4, Annex E)
                          (may be extended by further registrations)
      JBIG-stripe-size    <Integer>
      image-interleave    Stripe
                          Plane
      color-subsampling   "1:1:1"     (no color subsampling)
                          "4:1:1"     (4:1:1 color subsampling)
      MRC-mode            <Integer> (0..7)   (per ITU T.44 [15])
      MRC-max-stripe-size <Integer>

   Reference: this document, appendix A.

   'image-file-structure' defines how the coded image data is wrapped
   and formatted.  Options defined here are the various profiles of
   TIFF-FX, per RFC 2301 [7].  These options apply to overall formatting
   of the image data (TIFF file format, byte ordering, bit ordering,
   etc.) and do not define specific image coding issues that are covered
   by other aspects of the TIFF-FX profile specifications.




Klyne & McIntyre            Standards Track                    [Page 10]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   'image-coding' describes how the raw image data is compressed and
   coded as a sequence of bits.  These are generic tags that may apply
   to a range of file formats and usage environments.

   'image-coding-constraint' describes how the raw image data coding
   method is constrained to meet a particular operating environment.
   Options defined here are JBIG and JPEG coding constraints that apply
   in typical Group 3 fax environments.

   The 'JBIG-stripe-size' feature may be used with JBIG image coding,
   and indicates the number of scan lines in each stripe except the last
   in an image.  The legal constraints are:

      (JBIG-stripe-size=128)
      (JBIG-stripe-size>=0)

   The latter being equivalent to no restriction.

   The 'MRC-mode' feature is used to indicate the availability of MRC
   (mixed raster content) image format capability, and also the MRC mode
   available.  A zero value indicates MRC is not available, a non-zero
   value indicates the available MRC mode number.

   An MRC formatted document is actually a collection of several images,
   each of which is described by a separate feature collection.  An
   MRC-capable receiver is presumed to be capable of accepting any
   combination of contained images that conform to the MRC construction
   rules and declared image-coding capabilities.

   Within an MRC-formatted document, multi-level coders are used for
   foreground and background images (i.e. odd-numbered layers: 1, 3, 5,
   etc.) and bi-level coders are used for mask layers (i.e. even
   numbered layers 2, 4, 6, etc.).

      NOTE: an MRC formatted document may appear within a TIFF image
      file structure, so this separate feature is needed to capture the
      full range of possible capabilities.

   The 'MRC-max-stripe-size' feature may be used with MRC coding, and
   indicates the maximum number of scan lines in each MRC stripe.  The
   legal constraints are:

      (MRC-max-stripe-size=[0..256])
      (MRC-max-stripe-size>=0)

   These values indicate upper bounds on the stripe size.  The actual
   value may vary between stripes, and the actual size for each stripe
   is indicated in the image data.



Klyne & McIntyre            Standards Track                    [Page 11]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      NOTE: there are many image coding options here, and not all are
      required in all circumstances.

      Specification of the image-file-structure tag value alone is not
      normally sufficient to describe the capabilities of a recipient.
      A general rule is that sufficient detail should be provided to
      exclude any unsupported features.

      For extended Internet fax, image-file-structure and image-coding
      should always be specified, together with additional values
      described above as needed to clearly indicate which feature tag
      values are supported and which are not.  (See also the examples in
      section 4.)

4. Examples

   Some of the examples contain comments introduced by '--...'.  These
   are not part of the allowed capability description syntax.  They are
   included here to explain some of the constructs used.

   The level of detail captured here reflects that used for capability
   identification in Group 3 facsimile.

4.1 Simple mode Internet fax system

   This example describes the capabilities of a typical simple mode
   Internet fax system.  Note that TIFF application S is required to be
   supported by such a system.

      (& (color=Binary)
         (image-file-structure=TIFF-S)
         (dpi=200)
         (dpi-xyratio=[200/100,200/200])
         (paper-size=A4)
         (image-coding=MH) (MRC-mode=0)
         (ua-media=stationery) )

4.2 High-end black-and-white Internet fax system

   This would include support for B/W JBIG and be equivalent to what is
   sometimes called "Super G3", except that Internet fax functionality
   would be added.









Klyne & McIntyre            Standards Track                    [Page 12]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      (& (color=Binary)
         (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
         (| (& (dpi=200) (dpi-xyratio=200/100) )    -- 200*100
            (& (dpi=200) (dpi-xyratio=1) )          -- 200*200
            (& (dpi=204) (dpi-xyratio=204/391) )    -- 204*391
            (& (dpi=300) (dpi-xyratio=1) ) )        -- 300*300
         (| (image-coding=[MH,MR,MMR])
            (& (image-coding=JBIG)
               (image-coding-constraint=JBIG-T85)
               (JBIG-stripe-size=128) ) )
         (MRC-mode=0)
         (paper-size=[A4,B4]) )

4.3 Grey-scale Internet fax system

   This is the previous example extended to handle grey scale multi-
   level images.  In keeping with Group 3 fax, this example requires
   equal x- and y- resolutions for a multi-level image.

      (& (| (& (color=Binary)
               (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
               (| (image-coding=[MH,MR,MMR])
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T85)
                     (JBIG-stripe-size=128) ) )
               (| (& (dpi=200) (dpi-xyratio=200/100) )
                  (& (dpi=200) (dpi-xyratio=1) )
                  (& (dpi=204) (dpi-xyratio=204/391) )
                  (& (dpi=300) (dpi-xyratio=1) ) ) )
            (& (color=Grey)
               (image-file-structure=[TIFF-C,TIFF-L])
               (color-levels<=256)
               (color-space-CIELAB)
               (| (& (image-coding=JPEG)
                     (image-coding-constraint=JPEG-T4E) )
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T43)
                     (JBIG-stripe-size=128)
                     (image-interleave=stripe) ) )
               (dpi=[100,200,300])
               (dpi-xyratio=1) ) )
         (MRC-mode=0)
         (paper-size=[A4,B4]) )

4.4 Full-color Internet fax system

   This adds 16-bit full-color to the previous example.




Klyne & McIntyre            Standards Track                    [Page 13]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      (& (| (& (color=Binary)
               (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
               (| (image-coding=[MH,MR,MMR])
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T85)
                     (JBIG-stripe-size=128) ) )
               (| (& (dpi=200) (dpi-xyratio=200/100) )
                  (& (dpi=200) (dpi-xyratio=1) )
                  (& (dpi=204) (dpi-xyratio=204/391) )
                  (& (dpi=300) (dpi-xyratio=1) ) ) )
            (& (| (& (color=Grey) (color-levels<=256) )
                  (& (color=Full) (color-levels<=65536)
                     (color-subsampling=["1:1:1","4:1:1"]) ) )
               (image-file-structure=[TIFF-C,TIFF-L])
               (color-space=CIELAB)
               (| (& (image-coding=JPEG)
                     (image-coding-constraint=JPEG-T4E) )
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T43)
                     (JBIG-stripe-size=128)
                     (image-interleave=stripe) ) )
               (dpi=[100,200,300])
               (dpi-xyratio=1) ) )
         (MRC-mode=0)
         (paper-size=[A4,B4]) )

4.5 Full-color Internet fax system (MRC)

      (& (| (& (color=Binary)
               (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
               (MRC-mode=0)
               (image-coding=[MH,MMR])
               (| (& (dpi=200) (dpi-xyratio=[200/100,1]) )
                  (& (dpi=204) (dpi-xyratio=204/391) )
                  (& (dpi=300) (dpi-xyratio=1) )
                  (& (dpi=400) (dpi-xyratio=1) ) ) )
            (& (image-file-structure=[TIFF-C,TIFF-L])
               (| (& (color=Grey) (color-levels<=256) )
                  (& (color=Full) (color-levels<=65536)
                     (color-subsampling=["1:1:1","4:1:1"]) ) )
               (color-space=CIELAB)
               (MRC-mode=0)
               (image-coding=JPEG)
               (image-coding-constraint=JPEG-T4E)
               (dpi=[100,200,300,400])
               (dpi-xyratio=1) )
            (& (image-file-structure=TIFF-M)
               (MRC-mode=1) (MRC-max-stripe-size=[0..256])



Klyne & McIntyre            Standards Track                    [Page 14]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


               (image-coding=[MH,MMR,JPEG])
               (| (color=Binary)
                  (& (color=Grey) (color-levels<=256) )
                  (& (color=Full) (color-levels<=65536)
                     (color-subsampling=["1:1:1","4:1:1"]) ) )
               (color-space=CIELAB)
               (dpi=[100,200,300,400])
               (dpi-xyratio=1) ) )
         (paper-size=[A4,B4]) )

4.6 Sender and receiver feature matching

   This example considers sending a document to a high-end black-and-
   white fax system with the following receiver capabilities:

      (& (| (& (dpi=200) (dpi-xyratio=200/100) )    -- 200*100
            (& (dpi=200) (dpi-xyratio=1) )          -- 200*200
            (& (dpi=300) (dpi-xyratio=1) )          -- 300*300
            (& (dpi=400) (dpi-xyratio=1) ) )        -- 400*400
         (color=Binary)
         (| (& (paper-size=A4) (ua-media=[stationery,transparency]) )
            (& (paper-size=B4) (ua-media=continuous) ) )
         (image-coding=[MH,MR,JBIG]) )

   Turning to the document itself, assume it is available to the sender
   in three possible formats, A4 high resolution, B4 low resolution and
   A4 high resolution color, described by:

      (& (dpi=300) (dpi-xyratio=1)
         (color=Binary)
         (paper-size=A4)
         (image-coding=[MMR,JBIG]) )

      (& (dpi=200) (dpi-xyratio=200/100)
         (color=Binary)
         (paper-size=B4)
         (image-coding=[MH,MR]) )

      (& (dpi=300) (dpi-xyratio=1)
         (color=Mapped) (color-levels<=256)
         (paper-size=A4)
         (image-coding=JPEG) )

   These three image formats can be combined into a composite capability
   statement by a logical-OR operation (to describe format-1 OR format-2
   OR format-3):





Klyne & McIntyre            Standards Track                    [Page 15]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      (& (dpi=300) (dpi-xyratio=1)
         (color=Binary)
         (paper-size=A4)
         (image-coding=[MMR,JBIG]) )
      (& (dpi=200) (dpi-xyratio=200/100)
         (color=Binary)
         (paper-size=B4)
         (image-coding=[MH,MR]) )
      (& (dpi=300) (dpi-xyratio=1)
         (color=Mapped) (color-levels=42)
         (paper-size=A4)
         (image-coding=JPEG) ) )

   This could be simplified, but there is little gain in doing so at
   this point.

   The composite document description can be matched with the receiver
   capability description, according to the rules in [2], to yield the
   result:

      (& (dpi=300) (dpi-xyratio=1)
         (color=Binary)
         (paper-size=A4)
         (ua-media=[stationery,transparency])
         (image-coding=JBIG) )
      (& (dpi=200) (dpi-xyratio=200/100)
         (color=Binary)
         (paper-size=B4)
         (ua-media=continuous)
         (image-coding=[MH,MR]) ) )

   Points to note about the feature matching process:

   o  The color document option is eliminated because the receiver
      cannot handle either color (indicated by '(color=Mapped)') or JPEG
      coding (indicated by '(image-coding=JPEG)').

   o  The high resolution version of the document with '(dpi=300)' must
      be send using '(image-coding=JBIG)' because this is the only
      available coding of the image data that the receiver can use for
      high resolution documents.  (The available 300dpi document codings
      here are MMR and JBIG, and the receiver capabilities are MH, MR
      and JBIG.)

   o  The low-resolution version of the document can be sent with either
      MH or MR coding as the receiver can deal with either of these for
      low resolution documents.




Klyne & McIntyre            Standards Track                    [Page 16]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   o  The high resolution variant of the document is available only for
      A4, so that is the paper-size used in that case.  Similarly the
      low resolution version is sent for B4 paper.

   o  Even though the sender may not understand the 'ua-media' feature
      tag, and does not mention it, the matching rules preserve the
      constraint that the B4 document is rendered with '(ua-
      media=continuous)', and the A4 document may be rendered with '
      (ua-media=[stationery,transparency])'.

   Finally, note that when matching an MRC document description, the
   description of each component sub-image must match the capabilities
   of the intended receiver.

5. IANA Considerations

   Appendix A of this document calls for registrations of feature tags
   in the "IETF tree", as defined in section 3.1.1 of "Media Feature Tag
   Registration Procedure" [1] (i.e. these feature tags are subject to
   the "IETF Consensus" policies described in RFC 2434 [21]).

   ASN.1 identifiers should be assigned for each of these registered
   feature tags and replaced in the body of the registration.

6. Security Considerations

   The points raised below are in addition to the general security
   considerations for extended Internet fax [5], and others discussed in
   [2,8,11,12,13]

6.1 Capability descriptions and mechanisms

   Negotiation mechanisms reveal information about one party to other
   parties.  This may raise privacy concerns, and may allow a malicious
   party to make better guesses about the presence of specific security
   holes.

   Most of these concerns pertain to capability information getting into
   the hands of someone who may abuse it.  This document specifies
   capabilities that help a sender to determine what image
   characteristics can be processed by the recipient, not mechanisms for
   their publication.  Implementors and users should take care that the
   mechanisms employed ensure that capabilities are revealed only to
   appropriate persons, systems and agents.







Klyne & McIntyre            Standards Track                    [Page 17]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


6.2 Specific threats

   1.  Unsolicited bulk mail:  if it is known that a recipient can
       process certain types of images, they may be targeted by bulk
       mailers that want to send such images.

7. Acknowledgements

   The authors gratefully acknowledge the contributions of the following
   persons who commented on earlier versions of this memo: James
   Rafferty, Dan Wing, Robert Buckley, Mr Ryuji Iwazaki.  The following
   contributed ideas upon which some of the features described here have
   been based: Larry Masinter, Al Gilman, Koen Holtman.

8. References

   [1]  Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
        Registration Procedure", BCP 31, RFC 2506, March 1999.

   [2]  Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
        2533, March 1999.

   [3]  Masinter, L., Holtman, K., Mutz, A. and D. Wing, "Media Features
        for Display, Print, and Fax", RFC 2534, March 1999.

   [4]  McIntyre, L. and G. Klyne, "Internet fax feature mapping from
        Group 3 fax", Work in Progress.

   [5]  Masinter, L. and D. Wing, "Extended Facsimile Using Internet
        Mail", RFC 2532, March 1999.

   [6]  "Procedures for document facsimile transmission in the general
        switched telephone network", ITU-T Recommendation T.30 (1996),
        International Telecommunications Union, July 1996.

   [7]  McIntyre, L., Buckley, R., Venable, D., Zilles, S., Parsons, G.
        and J. Rafferty, "File format for Internet fax", RFC 2301, March
        1998.

   [8]  Toyoda, K., Ohno, H., Murai, J. and D. Wing, "A Simple Mode of
        Facsimile Using Internet Mail", RFC 2305, March 1998.

   [9]  "Continuous-tone color representation method for facsimile"
        ITU-T Recommendation T.42 (1996), International
        Telecommunications Union, (Covers custom illuminant, gamut).






Klyne & McIntyre            Standards Track                    [Page 18]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   [10] "Colour and gray-scale image representation using lossless
        coding scheme for facsimile", ITU-T Recommendation T.43 (1997),
        International Telecommunications Union. (Covers JBIG for
        colour/grey images).

   [11] Hardie, T., "Scenarios for the Delivery of Negotiated Content",
        Work in Progress.

   [12] Klyne, G., "Requirements for protocol-independent content
        negotiation", Work in Progress.

   [13] "Standardization of Group 3 facsimile terminals for document
        transmission", ITU-T Recommendation T.4 (1996), International
        Telecommunications Union, (Covers basic fax coding formats: MH,
        MR).

   [14] "Facsimile coding schemes and coding control functions for Group
        4 facsimile apparatus", ITU Recommendation T.6, International
        Telecommunications Union, (Commonly referred to as the MMR
        standard; covers extended 2-D fax coding format).

   [15] "Mixed Raster Content (MRC)", ITU-T Recommendation T.44,
        International Telecommunications Union.

   [16] "Information technology - Digital compression and coding of
        continuous-tone still image - Requirements and guidelines",
        ITU-T Recommendation T.81 (1992) | ISO/IEC 10918-1:1993,
        International Telecommunications Union, (Commonly referred to as
        JPEG standard).

   [17] "Information technology - Coded representation of picture and
        audio information - Progressive bi-level image compression",
        ITU-T Recommendation T.82 (1993) | ISO/IEC 11544:1993,
        International Telecommunications Union, (Commonly referred to as
        JBIG1 standard).

   [18] "Application profile for Recommendation T.82 - Progressive bi-
        level image compression (JBIG1 coding scheme for facsimile
        apparatus)", ITU-T Recommendation T.85 (1995), International
        Telecommunications Union, (Covers bi-level JBIG).

   [19] "Colorimeter, 2nd ed.", CIE Publication No. 15.2, 1986.
        (Defines CIELAB color space;  use with fax is further
        constrained by T.42 [9].)







Klyne & McIntyre            Standards Track                    [Page 19]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   [20] Tag Image File Format, Revision 6.0, Adobe Developers
        Association,
        <ftp://ftp.adobe.com/pub/adobe/devrelations/devtechnotes
        /pdffiles/tiff6.pdf>, June 1992.

   [21] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.












































Klyne & McIntyre            Standards Track                    [Page 20]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


9. Authors' Addresses

   Graham Klyne
   5th Generation Messaging Ltd.    Content Technologies Ltd.
   5 Watlington Street              Forum 1, Station Road
   Nettlebed                        Theale
   Henley-on-Thames, RG9 5AB        Reading, RG7 4RA
   United Kingdom                   United Kingdom.

   Phone:     +44 1491 641 641      +44 118 930 1300
   Facsimile: +44 1491 641 611      +44 118 930 1301
   EMail:     GK@ACM.ORG


   Lloyd McIntyre
   Xerox Corporation
   Mailstop PAHV-121
   3400 Hillview Ave.
   Palo Alto, CA 94304 USA

   Phone:     +1-650-813-6762
   Facsimile: +1-650-845-2340
   EMail: Lloyd.McIntyre@pahv.xerox.com




























Klyne & McIntyre            Standards Track                    [Page 21]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


Appendix A: Feature registrations

A.1 Image size

   -  Media Feature tag name(s):

         size-x
         size-y

   -  ASN.1 identifiers associated with these feature tags:

         1.3.6.1.8.1.7
         1.3.6.1.8.1.8

   -  Summary of the media features indicated:

         These feature tags indicate the size of a displayed, printed or
         otherwise rendered document image;  they indicate horizontal
         (size-x) and vertical (size-y) dimensions.

         The unit of measure is inches (to be consistent with the
         measure of resolution defined by the feature tag 'dpi').

         Where the actual size is available in millimetres, a conversion
         factor of 10/254 may be applied to yield an exact inch-based
         value.

   -  Values appropriate for use with these feature tags:

         Rational (>0)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Print and display applications where different media choices
         will be made depending on the size of the recipient device.

   -  Examples of typical use:

         This example describes the maximum scanned image width and
         height for Group 3 fax: 215x297 mm (8.46x11.69 inches):

         (size-x<=2150/254)
         (size-y<=2970/254)







Klyne & McIntyre            Standards Track                    [Page 22]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related standards or documents:

         The memo "Media Features for Display, Print, and Fax" [3]
         describes features (pix-x, pix-y) for measuring document size
         in pixels.

         Fax applications should declare physical dimensions using the
         features defined here.

   -  Considerations particular to use in individual applications,
      protocols, services, or negotiation mechanisms:

         Where no physical size is known or available, but a pixel size
         is known, a notional size should be declared based upon known
         pixel dimensions and a notional resolution of (say) 100dpi

         For example, to describe a 640x480 pixel display:

            (& (size-x<=640/100) (size-y<=480/100) (dpi=100) )

         The notional 100dpi resolution is used as it represents a
         fairly typical resolution for a pixel-limited display.
         Reducing the rational numbers to canonical form gives the
         following equivalent expression:

            (& (size-x<=32/5) (size-y<=24/5) (dpi=100) )

   -  Interoperability considerations:

         For interoperability with other (non-fax) applications that use
         only pixel-based measurements, pixel dimensions (pix-x, pix-y)
         may be declared in addition to physical measurements.

   -  Related feature tags:

         pix-x                 [3]
         pix-y                 [3]
         dpi                   [3]
         dpi-xyratio           [this document]

   -  Intended usage:

         Common

   -  Author/Change controller:

         IETF




Klyne & McIntyre            Standards Track                    [Page 23]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


A.2 Resolution aspect ratio

   -  Media Feature tag name(s):

         dpi-xyratio

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.9

   -  Summary of the media features indicated:

         This feature is used to indicate differential horizontal and
         vertical resolution capability.  In the absence of this
         feature, horizontal and vertical resolutions are presumed to be
         the same.

         When this feature tag is specified, any declared resolution
         (dpi) is presumed to apply to the horizontal axis, and the
         vertical resolution is obtained by dividing that declared
         resolution by the resolution ratio.

         The value of this feature is a pure number, since it represents
         the ratio of two resolution values.

   -  Values appropriate for use with this feature tag:

         Rational (>0)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other print or display applications that must
         handle differential horizontal and vertical resolution values.

   -  Examples of typical use:

         The following example describes a fax resolution of 204 dpi
         horizontally by 391 dpi vertically:

            (& (dpi=204) (dpi-xyratio=204/391) )

   -  Related standards or documents:

         The memo "Media Features for Display, Print, and Fax" [3]
         describes a feature (dpi) for measuring document resolution.





Klyne & McIntyre            Standards Track                    [Page 24]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Interoperability considerations:

         When interoperating with an application that does not recognize
         the differential resolution feature, resolution matching may be
         performed on the basis of the horizontal resolution only, so
         aspect ratio information may be lost.

   -  Related feature tags:

         dpi                   [3]
         size-x                [this document]
         size-y                [this document]

   -  Intended usage:

         Internet fax

   -  Author/Change controller:

         IETF

A.3 Color levels

   -  Media Feature tag name(s):

         color-levels

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.10

   -  Summary of the media features indicated:

         This feature tag is used to indicate a number of different
         image data pixel color values.

         When mapped (palettized) color is used, this is generally
         different from the number of different colors that can be
         represented through the color mapping function.

         This feature tag is used in conjunction with a 'color' feature
         having a value other than 'Binary'.

   -  Values appropriate for use with this feature tag:

         Integer  (>=2)





Klyne & McIntyre            Standards Track                    [Page 25]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Color image printing or display applications where the data
         resource used may depend upon color handling capabilities of
         the recipient.

   -  Examples of typical use:

         To describe recipient capabilities:
         (& (color=limited) (color-levels<=6) )
         (& (color=grey)    (color-levels<=64) )
         (& (color=mapped)  (color-levels<=240) )
         (& (color=full)    (color-levels<=16777216) )

         To describe capabilities used by a document:
         (& (color=limited) (color-levels=4) )
         (& (color=grey)    (color-levels=48) )
         (& (color=mapped)  (color-levels=100) )
         (& (color=full)    (color-levels=32768) )

   -  Related standards or documents:

         The memo "Media Features for Display, Print, and Fax" [3]
         describes a feature (color) for indicating basic color
         capabilities.

   -  Interoperability considerations:

         The actual number of color values used by a document does not,
         in general, exactly match the number that can be handled by a
         recipient.  To achieve a feature match, at least one must be
         declared as an inequality.

         It is recommended that a recipient declares the number of color
         values that it can handle as an inequality (<=), and a data
         resource declares the number of colors that it uses with an
         equality, as shown in the examples above.

   -  Security considerations:

      - Privacy concerns, related to exposure of personal information:
        Where feature matching is used to select content applicable to
        the physical abilities of a user, unusual values for this
        feature tag might give an indication of a user's restricted
        abilities.





Klyne & McIntyre            Standards Track                    [Page 26]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related feature tags:

         color                 [3]
         color-space           [this document]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.4 Color space

   -  Media Feature tag name(s):

         color-space

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.11

   -  Summary of the media features indicated:

         This feature indicates a color space.

         A color space value provides two types of information:
         o  the color model used to represent a color value, including
         the number of color components
         o  a mapping between color values and their physical
         realizations

         Device color space values are defined for applications where
         the general color representation used is significant, but exact
         color rendering is left to the device used.  Device color
         spaces defined here have values of the form 'Device- xxx'.

         Calibrated color space values are provided for use with a
         rendering system that is calibrated with respect to some
         indicated definition, and capable of processing device-
         independent color information accordingly.

   -  Values appropriate for use with this feature tag:

         Token




Klyne & McIntyre            Standards Track                    [Page 27]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


         Device color      Device-RGB   (device dependent RGB)
         spaces:           Device-CMY   (device dependent CMY)
                           Device-CMYK  (device dependent CMYK)

         Calibrated color  CIELAB       (per T.42 [9])
         space:

                           (may be extended by further registrations)

         'Color-space=CIELAB' indicates the CIE L*a*b* colour space,
         using CIED50 illuminant and its perfectly diffuse reflecting
         white point (per T.42 [9]).

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Color image printing and display applications where the data
         resource used may depend upon color handling capabilities of
         the recipient.

         Scanning applications where the data transferred may depend
         upon the image generation capabilities of the originator.

   -  Examples of typical use:

         To describe rendering or scanning capabilities:

         (color-space=[Device-RGB,CIELAB])

         To describe capabilities assumed by a document for which
         approximate color reproduction is required:

         (color-space=Device-RGB)

         To describe capabilities assumed by a document for which exact
         color reproduction is required:

         (color-space=CIELAB)

   -  Related standards or documents:

         CIELAB color space is defined in [19]

         CIELAB use for fax is described in ITU T.42 [9]







Klyne & McIntyre            Standards Track                    [Page 28]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Interoperability considerations:

         A color-handling receiver should indicate at any appropriate
         device color space capability, in addition to any calibrated
         color spaces that it may support.

         Calibrated color spaces are intended to be used when precise
         color matching is required; otherwise, if applicable, a device
         color space (color-space=Device-xxx) should be indicated.

         Documents for which exact color matching is not important
         should indicate a device color space capability, if applicable.

         These principles allow sender/receiver feature matching to be
         achieved when exact color matching is not required.

   -  Security considerations:

      - Privacy concerns, related to exposure of personal
        information:
            Where feature matching is used to select content applicable
            to the physical abilities of a user, unusual values for this
            feature tag might give an indication of a user's restricted
            abilities.

      - Denial of service concerns related to consequences of
        specifying incorrect values:
            Failure to indicate a generic color space capability for a
            device may lead to failure to match color space for an
            application or document that does not require an exact color
            match.

   -  Related feature tags:

         color                 [3]

   -  Related media types or data formats:

         TIFF-FX               [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF



Klyne & McIntyre            Standards Track                    [Page 29]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


A.5 CIELAB color depth

   -  Media Feature tag name(s):

         CIELAB-L-depth
         CIELAB-A-depth
         CIELAB-B-depth

   -  ASN.1 identifiers associated with these feature tags:

         1.3.6.1.8.1.12
         1.3.6.1.8.1.13
         1.3.6.1.8.1.14

   -  Summary of the media features indicated:

         These feature tags indicate a color depth capability; i.e.  the
         level of detail to which an individual CIELAB color component
         can be specified.  They define the number of distinct values
         possible for each of the color components L*, a* and b*.

         Typically, this feature would be used with 'color=mapped', and
         possibly 'color=grey' or 'color=full', to indicate the number
         of distinct colors that can be realized.

   -  Values appropriate for use with these feature tags:

         Integer (>0)

   -  These feature tags are intended primarily for use in the
      following applications, protocols, services, or negotiation
      mechanisms:

         Color image printing and display applications where the data
         resource used may depend upon color handling capabilities of
         the recipient.

         Scanning applications where the data transferred may depend
         upon the image generation capabilities of the originator.

   -  Examples of typical use:

         To describe rendering or scanning capabilities:

         (& (color=mapped) (color-levels<=240)
            (CIELAB-L-depth<=128)
            (CIELAB-a-depth<=128)
            (CIELAB-b-depth<=128) )



Klyne & McIntyre            Standards Track                    [Page 30]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


         (& (color=full) (color-levels<=16777216)
            (CIELAB-L-depth<=256)
            (CIELAB-a-depth<=128)
            (CIELAB-b-depth<=128) )

         To describe capabilities assumed by a document:

         (& (color=mapped) (color-levels=200)
            (CIELAB-L-depth=32)
            (CIELAB-a-depth=32)
            (CIELAB-b-depth=32) )
         (& (color=full) (color-levels=32768)
            (CIELAB-L-depth=128)
            (CIELAB-a-depth=32)
            (CIELAB-b-depth=32) )

   -  Related standards or documents:

         The memo "Media Features for Display, Print, and Fax" [3]
         defines a feature (color) for indicating basic color
         capabilities.

         CIELAB color space is defined in [19]

         CIELAB use for fax is described in ITU T.42 [9]

   -  Related feature tags:

         color                 [3]
         color-levels          [this document]
         color-space           [this document]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF











Klyne & McIntyre            Standards Track                    [Page 31]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


A.6 CIELAB color gamut

   -  Media Feature tag name(s):

         CIELAB-L-min
         CIELAB-L-max
         CIELAB-a-min
         CIELAB-a-max
         CIELAB-b-min
         CIELAB-b-max

   -  ASN.1 identifiers associated with these feature tags:

         1.3.6.1.8.1.15
         1.3.6.1.8.1.16
         1.3.6.1.8.1.17
         1.3.6.1.8.1.18
         1.3.6.1.8.1.19
         1.3.6.1.8.1.20

   -  Summary of the media features indicated:

         These feature indicate a supported range of color values, by
         indicating minimum and maximum values used for each color
         component in a CIELAB color space.

         'CIELAB-L-min' and 'CIELAB-L-max' are the minimum and maximum
         values of the L* component.

         'CIELAB-a-min' and 'CIELAB-a-max' are the minimum and maximum
         values of the a* component.

         'CIELAB-b-min' and 'CIELAB-b-max' are the minimum and maximum
         values of the b* component.

   -  Values appropriate for use with this feature tag:

         Rational

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Color image printing and display applications where the data
         resource used may depend upon detailed color handling
         capabilities of the recipient.






Klyne & McIntyre            Standards Track                    [Page 32]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


         Scanning applications where the data transferred may depend
         upon the detailed color image generation capabilities of the
         originator.

   -  Examples of typical use:

         To describe rendering or scanning capabilities:

         (& (CIELAB-L-min>=0)
            (CIELAB-L-max<=100)
            (CIELAB-a-min>=-75)
            (CIELAB-a-max<=+75)
            (CIELAB-b-min>=-85)
            (CIELAB-b-max<=+85) )

         To describe capabilities required by a document:

         (& (CIELAB-L-min=20)
            (CIELAB-L-max=80)
            (CIELAB-L-min=-35)
            (CIELAB-L-max=+55)
            (CIELAB-L-min=-45)
            (CIELAB-L-max=+65) )

   -  Related standards or documents:

         CIELAB color space is defined in [19]

         CIELAB use for fax is described in ITU T.42 [9]

   -  Interoperability considerations:

         When describing a recipient's capabilities, the minimum and
         maximum color component values that can be rendered should be
         indicated by inequalities as shown in the examples above.

         When describing a document, the actual minimum and maximum
         color component values used should be indicated, as shown
         above.

   -  Security considerations:

         - Privacy concerns, related to exposure of personal
            information:
            Where feature matching is used to select content applicable
            to the physical abilities of a user, unusual values for this
            feature tag might give an indication of a user's restricted
            abilities.



Klyne & McIntyre            Standards Track                    [Page 33]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related feature tags:

         color                 [3]
         color-space           [this document]

   -  Related media types or data formats:

         TIFF-FX               [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.7 Image file structure

   -  Media Feature tag name(s):

         image-file-structure

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.21

   -  Summary of the media features indicated:

         This feature indicates a file structure used for transfer and
         presentation of image data.

         It does not indicate image data coding:  that is described by
         separate feature tags (image-coding, etc.).

   -  Values appropriate for use with this feature tag:

         Token












Klyne & McIntyre            Standards Track                    [Page 34]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


         TIFF-FX profiles  TIFF-S
         [7]:              TIFF-F
                           TIFF-J
                           TIFF-C
                           TIFF-L
                           TIFF-M

                           (may be extended by further registrations,
                           to cover non-TIFF image file structures)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other print or display applications that
         transfer image data.

   -  Examples of typical use:

         See Appendix B of this memo.

   -  Considerations particular to use in individual applications,
      protocols, services, or negotiation mechanisms:

         This tag is intended to provide information about an image file
         structure.  Information about image data coding is provided by
         other tags.

         In the case of TIFF-FX image data, there are a number of image
         file format constraints that are imposed by the various usage
         profiles defined in RFC 2301 [7].  The purpose of the 'image-
         file-structure' feature tag is to capture those file format
         constraints.

         Registration of additional image file structure tags should
         focus similarly on image file structure issues, not raw image
         data compression and coding.  As a guide, an image file
         structure may contain image data coded in a variety of ways,
         and carries information to describe that coding separately from
         MIME content-type labelling, etc.

   -  Related feature tags:

         image-coding          [this document]

   -  Related media types or data formats:

         TIFF-FX               [7]
         TIFF V6.0 (Adobe)     [20]



Klyne & McIntyre            Standards Track                    [Page 35]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Intended usage:

         Internet fax
         Image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.8 Image data coding

   -  Media Feature tag name(s):

         image-coding

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.22

   -  Summary of the media features indicated:

         This feature tag indicates a form of image data compression and
         coding used.

         It identifies a generic image coding technique used, without
         regard to any specific profiling of that technique that may be
         applied.  Values for this feature are generally applicable
         across a wide range of image transfer applications.

         This information is distinct from the image file structure and
         MRC information conveyed by the 'image-file-structure' tags.

   -  Values appropriate for use with this feature tag:

         Token             MH
                           MR
                           MMR
                           JBIG
                           JPEG

                           (may be extended by further registrations)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other applications that transfer image data.





Klyne & McIntyre            Standards Track                    [Page 36]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Examples of typical use:

         See Appendix B of this memo.

   -  Related standards or documents:

         MH, MR:     ITU T.4 [13]
         MMR:        ITU T.6 [14]
         JPEG:       ITU T.81 [16]
         JBIG:       ITU T.82 [17]

   -  Interoperability considerations:

         To establish the correct conditions for interoperability
         between systems, capabilities to handle the generic image
         coding technique and the specific image coding constraints must
         be established.

   -  Related feature tags:

         image-coding-constraint  [this document]
         JBIG-stripe-size         [this document]
         image-interleave         [this document]

   -  Related media types or data formats:


         TIFF-FX                  [7]

   -  Intended usage:

         Internet fax
         Image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.9 Image coding constraint

   -  Media Feature tag name(s):

         image-coding-constraint

   -  ASN.1 identifier associated with these feature tags:

         1.3.6.1.8.1.23




Klyne & McIntyre            Standards Track                    [Page 37]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Summary of the media features indicated:

         This feature tag qualifies the 'image-coding' feature with a
         specific profile or usage constraints.

         Values for this feature are generally specific to some given
         value of 'image-coding' and also to some restricted application
         or class of applications.

   -  Values appropriate for use with this feature tag:

         Token             JBIG-T85    (bi-level, per ITU T.85)
                           JBIG-T43    (multi-level, per ITU T.43)
                           JPEG-T4E    (per ITU T.4, Annex E)

                           (may be extended by further registrations)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other applications that transfer image data.

         The specific values for this feature indicated above are
         intended for use with Internet fax.

   -  Examples of typical use:

         See Appendix B of this memo.

   -  Related standards or documents:

         JBIG-T85:   ITU T.85 [18]
         JBIG-T43:   ITU T.43 [10]
         JPEG-T4E:   ITU T.4 Annex E [13]

   -  Interoperability considerations:

         To establish the correct conditions for interoperability
         between systems, capabilities to handle the generic image
         coding technique and the specific image coding constraints must
         be established.

   -  Related feature tags:

         image-coding             [this document]
         JBIG-stripe-size         [this document]
         image-interleave         [this document]




Klyne & McIntyre            Standards Track                    [Page 38]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related media types or data formats:

         TIFF-FX                  [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.10 JBIG stripe size

   -  Media Feature tag name(s):

         JBIG-stripe-size

   -  ASN.1 identifier associated with these feature tags:

         1.3.6.1.8.1.24

   -  Summary of the media features indicated:

         This feature is a specific usage constraint that is applied to
         JBIG image coding (image-coding=JBIG), and indicates the
         allowable size for each stripe of an image, except the last.

         A stripe of a JBIG image is a delimited horizontal band of
         compressed image data that can be decompressed separately from
         the surrounding data.

   -  Values appropriate for use with this feature tag:

         Integer  (>0)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other applications that transfer image data.

   -  Examples of typical use:

         (JBIG-stripe-size=128)
         (JBIG-stripe-size>0)





Klyne & McIntyre            Standards Track                    [Page 39]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related standards or documents:

         JBIG:       ITU T.82 [17]
         JBIG-T85:   ITU T.85 [18]
         JBIG-T43:   ITU T.43 [10]

   -  Considerations particular to use in individual applications,
      protocols, services, or negotiation mechanisms:

         In the case of Internet fax, the specific constraints allowed
         for a receiver are those given as examples above.

         Specifying a stripe size that is not limited (JBIG-stripe-
         size>0) means that an entire page of image data is encoded as a
         single unit.  This may place considerable demands on the memory
         of a receiving system, as the entire stripe needs to be
         buffered in memory.

   -  Interoperability considerations:

         To establish the correct conditions for interoperability
         between systems, capabilities to handle the generic image
         coding technique and the specific image coding constraints must
         be established.

   -  Related feature tags:

         image-coding             [this document]
         image-coding-constraint  [this document]
         image-interleave         [this document]

   -  Related media types or data formats:

         TIFF-FX                  [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF








Klyne & McIntyre            Standards Track                    [Page 40]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


A.11 Image interleave

   -  Media Feature tag name(s):

         image-interleave

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.25

   -  Summary of the media features indicated:

         This feature indicates an image interleave capability.

         It may be used with JBIG images (image-coding=JBIG) to indicate
         color plane interleaving of either stripes or entire image
         planes.

   -  Values appropriate for use with this feature tag:

         Token             Stripe
                           Plane

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other applications that transfer image data.

   -  Examples of typical use:

         (image-interleave=stripe)
         (image-interleave=[stripe,plane])

   -  Considerations particular to use in individual applications,
      protocols, services, or negotiation mechanisms:

         Specifying a plane interleave means that an entire page of
         image data must be buffered in order to generate render the
         image.  This may place considerable demands on the memory of a
         sending or receiving system.

   -  Related feature tags:

         image-coding             [this document]
         JBIG-stripe-size         [this document]






Klyne & McIntyre            Standards Track                    [Page 41]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related media types or data formats:

         TIFF-FX                  [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.12 Color subsampling

   -  Media Feature tag name(s):

         color-subsampling

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.26

   -  Summary of the media features indicated:

         This feature tag indicates whether color information may be
         subsampled with respect to luminance data.

         It is used with continuous color images (color=full), color
         spaces that use separate luminance and color components (e.g.
         color-space=LAB), and image file structures that support color
         subsampling.

   -  Values appropriate for use with this feature tag:

         String            "1:1:1"
                           This value indicates a full set of color
                           component samples for each luminance
                           component sample.

                           "4:1:1"
                           This value indicates a set of color samples
                           for each luminance sample.

                           (may be extended by further registrations)






Klyne & McIntyre            Standards Track                    [Page 42]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Color image printing and display applications where the data
         resource used may depend upon color handling capabilities of
         the recipient.

         Scanning applications where the data transferred may depend
         upon the image generation capabilities of the originator.

   -  Examples of typical use:

         (& (color=full) (color-space=[Device-RGB,CIELAB])
            (color-subsampling=["1:1:1","4:1:1"]) )

   -  Related feature tags:

         color                 [3]
         color-space           [this document]
         image-file-structure  [this document]

   -  Related media types or data formats:

         TIFF-FX               [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.13 MRC availability and mode

   -  Media Feature tag name(s):

         MRC-mode

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.27








Klyne & McIntyre            Standards Track                    [Page 43]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Summary of the media features indicated:

         This feature is used to indicate the availability of MRC (mixed
         raster content) image format capability, and also the MRC mode
         available.  A zero value indicates MRC is not available, a
         non-zero value (in the range 1..7) indicates the available MRC
         mode number.

         An MRC formatted document is actually a collection of several
         images, each of which is described by a separate feature
         collection.  An MRC-capable receiver is presumed to be capable
         of accepting any combination of contained images that conform
         to the MRC construction rules, where each such image matches
         the separately declared resolution, color capability, color
         model, image coding, and any other capabilities.

            NOTE: an MRC formatted document may appear within a TIFF
            image file structure.

            Within an MRC-formatted document, multi-level coders are
            used for foreground and background images (i.e.  odd-
            numbered layers: 1, 3, 5, etc.) and bi-level coders are used
            for mask layers (i.e. even numbered layers 2, 4, 6, etc.).

   -  Values appropriate for use with this feature tag:

         Integer (0..7)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other applications that transfer image data.

   -  Examples of typical use:

         See Appendix B of this document.

   -  Related standards or documents:

         ITU T.44 [15]

   -  Interoperability considerations:

         To establish the correct conditions for interoperability
         between systems, capabilities to handle the MRC mode and any
         contained image coding techniques must be established.





Klyne & McIntyre            Standards Track                    [Page 44]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related feature tags:

         image-coding             [this document]
         MRC-max-stripe-size      [this document]

   -  Related media types or data formats:

         TIFF-FX                  [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.14 MRC maximum stripe size

   -  Media Feature tag name(s):

         MRC-max-stripe-size

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.28

   -  Summary of the media features indicated:

         This feature may be used with MRC coding (MRC-mode>=1), and
         indicates the maximum number of scan lines in each MRC stripe.

         The value given indicates an upper bound on the stripe size.
         The actual value may vary between stripes, and the actual size
         for each stripe is indicated in the image data.

   -  Values appropriate for use with this feature tag:

         Integer (>0)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other applications that transfer image data.






Klyne & McIntyre            Standards Track                    [Page 45]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Examples of typical use:

         (MRC-max-stripe-size=[0..256])
         (MRC-max-stripe-size>=0)

   -  Considerations particular to use in individual applications,
      protocols, services, or negotiation mechanisms:

         For Internet fax, the legal constraints for an image receiver
         are those given as examples above.

   -  Related feature tags:

         MRC-mode              [this document]

   -  Related media types or data formats:

         TIFF-FX               [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF
























Klyne & McIntyre            Standards Track                    [Page 46]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


Appendix B: TIFF mode descriptions

   This appendix contains descriptions of the TIFF modes defined by RFC
   2301 [7], presented as feature set expressions in the form defined by
   "A syntax for describing media feature sets" [2] and using the
   feature schema introduced by this document.

   These may be taken as illustrations of the feature set combinations
   that are required for the corresponding TIFF profiles described by
   RFC 2301.

      (Tiff-S) :-
           (& (image-file-structure=TIFF-S)
              (color=Binary)
              (image-coding=MH) (MRC-mode=0) )

      (Tiff-F) :-
           (& (image-file-structure=TIFF-F)
              (color=Binary)
              (image-coding=MH) (MRC-mode=0) )

      (TIFF-J) :-
           (& (image-file-structure=TIFF-J)
              (color=Binary)
              (image-coding=JBIG) (MRC-mode=0) )

      (TIFF-C) :-
           (& (image-file-structure=TIFF-C)
              (color=Grey)
              (image-coding=JPEG) (MRC-mode=0) )

      (TIFF-L) :-
           (& (image-file-structure=TIFF-L)
              (color=Grey)
              (image-coding=JBIG) (MRC-mode=0) )

      (TIFF-M) :-
           (& (image-file-structure=TIFF-M)
              (color=[Binary,Grey])
              (image-coding=[MH,JPEG]) (MRC-mode>=1) )

   The feature sets described above are minimum requirements for the
   corresponding TIFF modes.  Thus, MR and MMR image coding are not
   mandatory with TIFF mode F, and would be indicated by combining the
   expression for (TIFF-F) with (image-coding=MR) and/or (image-
   coding=MMR).





Klyne & McIntyre            Standards Track                    [Page 47]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   Similarly, limited, mapped or full color are not mandatory with the
   grey/color TIFF modes (C, L and M), and would be indicated by
   combining the corresponding expression with (color=limited),
   (color=mapped) and/or (color=full).

   TIFF profile M is a composite structure that can combine image data
   coding options from other profiles: the description above indicates
   mandatory features; other options may be indicated by combining
   TIFF-M with other options (e.g. color= limited, mapped or full, and
   image-coding= MR, MMR or JBIG).

   Support for multiple TIFF profiles may be indicated by combining
   their expressions with the OR operator; e.g.

        (| (TIFF-F) (TIFF-S) (TIFF-J) )

   indicates support for all black-and-white modes.


































Klyne & McIntyre            Standards Track                    [Page 48]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


Appendix C: Revision history

   00a  28-Sep-1998  Initial draft.

   01a  12-Oct-1998  Incorporated review comments.  Described feature
                     tag for differential x/y resolution ratio.  Added
                     some examples.

   01b  19-Oct-1998  Updated section 3.6 on image coding.  Added
                     Appendix B containing feature expressions for the
                     TIFF modes from RFC 2301.

   02a  26-Oct-1998  Update examples.  Add separate stripe size features
                     for JBIG and MRC.

   02b  30-Oct-1998  Update examples.  Add text clarifying the
                     description of MRC documents (as a set of feature
                     collections describing multiple contained images).
                     Add text describing constrains on resolution and
                     image coding usage within an MRC document.

   02c  11-Nov-1998  Add ITU references.  Added terminology: "capability
                     exchange", "capability identification" and
                     "capability description".  Update JBIG and MRC
                     stripe size tags.  Move subsampling to colour
                     section.  Remove preferred-unit tag.  Add T.4, T.6,
                     T.44 and T.81 references.

   02d  16-Nov-1998  Update colour handling features, reflecting
                     proposed changes to the media features memo [3].
                     Update the image coding capability framework.
                     Updated TIFF mode descriptions in Appendix B.

   03a  17-Nov-1998 Replace use of 'pix-x', 'pix-y' with 'size-x', '
                     size-y'.  Add registrations in Appendix A.

   03b  08-Dec-1998  Remove normative language and reference to RFC2119
                     (normative statements will be in the main fax
                     protocol draft).  Revise structure of colour
                     features, and removed color-palette feature. Define
                     colour feature tags specific to CIELAB model and
                     colour space.

   04a  14-Dec-1998  Update examples to reflect revised feature tags.
                     Revise description of MRC document in section 3.7.
                     Clarified interpretation of 'color=fixed'.  Change
                     feature value 'color=fixed' to 'color=limited'.




Klyne & McIntyre            Standards Track                    [Page 49]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   05a  04-Jan-1999  Incorporate WG last-call comments:  change
                     references to MRC-stripe-size to MRC-max-stripe-
                     size;  similarly references to MRC-maximum-stripe-
                     size.  Change "eifax" to "extended Internet fax".
                     Added guidance note for image coding feature usage.
                     Added IANA consideration comments to Appendix A.

   05b  08-Jan-1999  Added new section for IANA considerations; removed
                     references to fax working group from registration
                     change control sections.  Remove JPEG from TIFF-L
                     auxiliary predicate.  Clarify description of MRC
                     receiver capabilities in section A.13.  Remove '
                     color=full' from (TIFF-C) and (TIFF-M) predicates,
                     and add some explanatory text.  Remove
                     'color=limited' from (TIFF-L) predicate.

   05c  08-Jan-1999  Minor revisions to TIFF profile illustrations and
                     descripions in Appendix B.  Reformatted description
                     of 'color=limited' in section 3.5 to clarify that
                     this does not indicate support for specific named
                     colors.






























Klyne & McIntyre            Standards Track                    [Page 50]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























Klyne & McIntyre            Standards Track                    [Page 51]

