rfc2879.txt

<VC++网络游戏建摸与实现>源代码

   Procedure" [1] (i.e. these feature tags are subject to the "IETF
   Consensus" policies described in RFC 2434 [21]).

   Appendix section A.5 introduces one new feature tag (color-
   illuminant) to be registered according to the same procedure.  An
   ASN.1 identifier should be assigned for this new tag 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.  Implementers and users should take care that the
   mechanisms employed ensure that capabilities are revealed only to
   appropriate persons, systems and agents.

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.



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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", 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 T.30 Feature Mapping",
        RFC 2880, July 2000.

   [5]  Masinter, L. and D. Wing, RFC 2532, "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 (1999),
        International Telecommunications Union, March 1999

   [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)

   [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)

   [12] Klyne, G., "Protocol-independent Content Negotiation Framework",
        RFC 2703, September 1999.




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   [13] "Standardization of Group 3 facsimile terminals for document
        transmission", ITU-T Recommendation T.4 (1999), 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].)

   [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.

   [22] Klyne, G. and L. McIntyre, "Content feature schema for Internet
        fax", RFC 2531, March 1999.








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9. Authors' Addresses

   Graham Klyne
   Content Technologies Ltd.
   1220 Parkview,
   Arlington Business Park
   Theale
   Reading, RG7 4SA
   United Kingdom.

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]


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

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]



























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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:

        size-x:      1.3.6.1.8.1.7
        size-y:      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)







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   -  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








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   -  Author/Change controller:

        IETF

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) )





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   -  Related standards or documents: