rfc2879.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                             G. Klyne
Request for Comments: 2879                            Content Technologies
Obsoletes: 2531                                                L. McIntyre
Category: Standards Track                                Xerox Corporation
                                                               August 2000

              Content Feature Schema for Internet Fax (V2)

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 (2000).  All Rights Reserved.

Abstract

   This document defines a content media feature schema for Internet
   fax.

   It is a profile of the media feature registration mechanisms [1,2,3]
   for use in performing capability identification between extended
   Internet fax systems [5].  It replaces and updates the feature schema
   defined in RFC 2531.

Table of Contents

   1. Introduction .............................................2
      1.1 Organization of this document ........................3
      1.2 Terminology and document conventions .................3
      1.3 Discussion of this document ..........................4
   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 .....................................7
      3.6 Color model ..........................................8
      3.7 Image coding ........................................11
      3.8 MRC mode ............................................12
   4. Examples ................................................13
      4.1 Simple mode Internet fax system  ....................13



Klyne & McIntyre            Standards Track                     [Page 1]

RFC 2879      Content Feature Schema for Internet Fax (V2)   August 2000


      4.2 High-end black-and-white Internet fax system.........14
      4.3 Grey-scale Internet fax system ......................14
      4.4 Full-color Internet fax system (JPEG only) ..........15
      4.5 Full-color Internet fax system (JPEG and JBIG) ......16
      4.6 Full-color Internet fax system (MRC) ................17
      4.7 Sender and receiver feature matching ................19
   5. IANA Considerations .....................................21
   6. Security Considerations .................................21
      6.1 Capability descriptions and mechanisms ..............21
      6.2 Specific threats ....................................21
   7. Acknowledgements ........................................22
   8. References ..............................................22
   9. Authors' Addresses ......................................24
   Appendix A: Feature registrations ..........................25
      A.1 Image size ..........................................25
      A.2 Resolution aspect ratio .............................27
      A.3 Color levels ........................................28
      A.4 Color space  ........................................30
      A.5 CIELAB color illuminant .............................33
      A.6 CIELAB color depth ..................................35
      A.7 CIELAB color gamut ..................................37
      A.8 Image file structure ................................39
      A.9 Image data coding ...................................41
      A.10 Image coding constraint ............................43
      A.11 JBIG stripe size .................................. 44
      A.12 Image interleave ...................................46
      A.13 Color subsampling ..................................47
      A.14 MRC availability and mode ..........................49
      A.15 MRC maximum stripe size ............................50
   Appendix B: TIFF mode descriptions .........................52
   Appendix C: Changes from RFC 2531 ..........................57
   Full Copyright Statement ...................................58

1. Introduction

   This document defines a content media feature schema for Internet
   fax.

   It is a profile of the media feature registration mechanisms [1,2,3]
   for use in performing capability identification between extended
   Internet fax systems [5].  It replaces and updates the feature schema
   defined in RFC 2531.

   The media feature description mechanisms do not describe any specific
   mechanisms for communicating capability information, but do presume
   that any such mechanisms will transfer textual values.  In
   conjunction with this feature schema, they specify a textual format
   to be used for describing Internet Fax capability information.



Klyne & McIntyre            Standards Track                     [Page 2]

RFC 2879      Content Feature Schema for Internet Fax (V2)   August 2000


   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.

   "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.






Klyne & McIntyre            Standards Track                     [Page 3]

RFC 2879      Content Feature Schema for Internet Fax (V2)   August 2000


1.3 Discussion of this document

   Discussion of this document should take place on the Internet fax
   mailing list hosted by the Internet Mail Consortium (IMC).  Please
   send comments regarding this document to:

       [e-mail]

   To subscribe to this list, send a message with the body 'subscribe'
   to "[e-mail]".

   To see what has gone on before you subscribed, please see the mailing
   list archive at:

       http://www.imc.org/ietf-fax/

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 according to the media feature registration
   procedure [1] (some of which are appended to this document).






Klyne & McIntyre            Standards Track                     [Page 4]

RFC 2879      Content Feature Schema for Internet Fax (V2)   August 2000


        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.

        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 [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'.







Klyne & McIntyre            Standards Track                     [Page 5]

RFC 2879      Content Feature Schema for Internet Fax (V2)   August 2000


   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
   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].








Klyne & McIntyre            Standards Track                     [Page 6]

RFC 2879      Content Feature Schema for Internet Fax (V2)   August 2000


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].

   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 blank-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.

   In the context of Internet fax, 'limited' is interpreted as one-bit-
   per-color-sample (RGB, CMY or CMYK), depending on the color space
   used.

   'Mapped' indicates that pixel color values are mapped in some