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: ietf-fax@imc.org To subscribe to this list, send a message with the body 'subscribe' to "ietf-fax-request@imc.org". 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 (>0) size-y (>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 (>0) dpi-xyratio (>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 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. Klyne & McIntyre Standards Track [Page 7] RFC 2879 Content Feature Schema for Internet Fax (V2) August 2000 For 'Mapped', 'Grey' and 'Full' color, additional feature tags (section 3.6) may be used to further qualify the color reproduction. 3.6 Color model Feature tag name Legal values ---------------- ------------ color-levels (>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) color-illuminant (per ITU T.4 [13], E.6.7) D50 D65 D75 SA SC F2 F7 F11 CTnnnn (see below) CIELAB-L-depth (>0) CIELAB-a-depth " CIELAB-b-depth " CIELAB-L-min 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 a