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Versions: 00 01 02 03 04 05 RFC 2938

IETF conneg working group                         Graham Klyne, editor
Internet draft                                    Content Technologies
Category: Work-in-progress                              Larry Masinter
                                                                  AT&T
                                                           8 June 2000
                                                Expires: December 2000


                 Identifying composite media features
               <draft-ietf-conneg-feature-hash-05.txt>


Status of this memo

  This document is an Internet-Draft and is in full conformance with
  all provisions of Section 10 of RFC 2026.

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF), its areas, and its working groups.  Note that
  other groups may also distribute working documents as Internet-
  Drafts.

  Internet-Drafts are draft documents valid for a maximum of six
  months and may be updated, replaced, or obsoleted by other
  documents at any time.  It is inappropriate to use Internet-Drafts
  as reference material or to cite them other than as "work in
  progress."


     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
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Copyright Notice

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

Abstract

  In RFC 2533 [1], an expression format is presented for describing
  media feature capabilities as a combination of simple media feature
  tags [2].

  This document describes an abbreviated format for a composite media
  feature set, based upon a hash of the feature expression describing
  that composite.








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Table of contents

  1. Introduction ............................................2
     1.1 Organization of this document                        2
     1.2 Terminology and document conventions                 3
     1.3 Discussion of this document                          3
  2. Motivation and goals ....................................4
  3. Composite feature representation ........................5
     3.1 Feature set hashed reference format                  6
       3.1.1 Hash value calculation                           7
       3.1.2 Base-32 value representation                     7
     3.2 Resolving feature set identifiers                    9
       3.2.1 Query protocol                                   9
       3.2.2 Inline feature set details                       10
  4. Examples ................................................11
  5. Internationalization considerations .....................13
  6. Security considerations .................................14
  7. Acknowledgements ........................................14
  8. References ..............................................14
  9. Authors' addresses ......................................16
  Appendix A: The birthday problem ...........................16
  Appendix B: Revision history ...............................18
  Full copyright statement ...................................19



1. Introduction

  In "A syntax for describing media feature sets" [1], an expression
  format is presented for describing media feature capabilities as a
  combination of simple media feature tags [2].

  This document proposes an abbreviated format for a composite media
  feature set, based upon a hash of the feature expression describing
  that composite.

  This memo extends and builds upon the expression syntax described
  in RFC 2533 [1], and it is assumed that the reader is familiar with
  the interpretation feature set expressions described there.

1.1 Organization of this document

  Section 2 sets out some of the background and goals for feature set
  references.

  Section 3 presents a syntax for feature set references, and
  describes how they are related to feature set expressions.




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1.2 Terminology and document conventions

  This section defines a number of terms and other document
  conventions, which are used with specific meaning in this memo.
  The terms are listed in alphabetical order.

  dereference
            the act of replacing a feature set reference with its
            corresponding feature set expression.  Also called
            "resolution".

  feature set
            some set of media features described by a media feature
            assertion, as described in "A syntax for describing media
            feature sets" [1].  (See that memo for a more formal
            definition of this term.)

  feature set expression
            a string that describes some feature set, formulated
            according to the rules in "A syntax for describing media
            feature sets" [1] (and possibly extended by other
            specifications).

  feature set reference
            a brief construct that references some feature set.
            (See also: "dereference".)

  feature set tag
            a name that conforms to the syntax of a feature tag [1]
            that is used to denote a feature set rather than a single
            feature.

  resolution
            (See "dereference").

  This specification uses syntax notation and conventions described
  in RFC2234 "Augmented BNF for Syntax Specifications: ABNF" [3].

       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.

1.3 Discussion of this document

  Discussion of this document takes place on the content negotiation
  and media feature registration mailing list hosted by the Internet
  Mail Consortium (IMC).



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  Please send comments regarding this document to:

      ietf-medfree@imc.org

  To subscribe to this list, send a message with the body 'subscribe'
  to "ietf-medfree-request@imc.org".

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

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


2. Motivation and goals

  The range of media feature capabilities of a message handling
  system can be quite extensive, and the corresponding feature set
  expression [1] can reach a significant size.

  A requirement has been identified to allow recurring feature sets
  to be identified by a single reference value, which can be combined
  with other elements in a feature set expression.  It is anticipated
  that mechanisms will be provided that allow the recipient of such a
  feature set reference to discover the corresponding feature set
  expression, but any such mechanism is beyond the scope of this
  specification.

  Thus, the goals for this proposal are:

  o  to provide an abbreviated form for referencing an arbitrary
     feature set expression.

  o  the meaning of (i.e. the corresponding feature set expression) a
     feature set reference should be independent of any particular
     mechanism that may be used to dereference it.

  o  to be able to verify whether a given feature set expression
     corresponds to some feature set reference without having to
     perform an explicit dereferencing operation (i.e. without
     incurring additional network traffic).

  o  for protocol processors that conform to RFC 2533 [1] to be able
     to sensibly handle a feature set reference without explicit
     knowledge of its meaning (i.e. the introduction of feature set
     references should not break existing feature expression
     processors).  That is, the applicable interpretation and
     processing rules of RFC 2533 [1] apply equally to expressions
     containing feature set references.




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       NOTE:  This proposal does not attempt to address the
       "override" or "default" problem.  (Where a feature set
       may be referenced and selectively modified.)

  Some circumstances in which such an abbreviated form might be used
  include:

  o  A media feature expression that contains a repeated sub-
     expression.  If the sub-expression is quite large, space can be
     saved by writing it out once, then using the abbreviated form to
     reference it.

  o  A capability that is common to a range of devices, such as a
     given class of fax machine where are large number of feature tags
     are involved, but only a small number of common feature sets.  If
     the recipient understands, or can discover, that some
     abbreviation stands for a given feature set then feature
     expression size can be reduced by using the abbreviation.

     If feature set abbreviations are used in this way, it may be that
     they can be interpreted by a simple table lookup rather than full
     feature expression parsing.  (Making this useful in practice will
     depend on crafting the feature subsets appropriately.)

  Examples of such usage are given in section 4 of this memo.

  This memo does not specify how a program that receives a feature
  set abbreviation should discover the corresponding feature set
  expression:  see section 3.2.


3. Composite feature representation

  This specification hinges on two central ideas:

  o  the use of auxiliary predicates (introduced in RFC 2533 [1]) to
     form the basis of a feature set identifier, and

  o  the use of a token based on a hash function computed over the
     referenced feature set expression.

  A key reason to use a hash function to generate an identifier is to
  define a global name space without requiring a central naming
  authority.  New feature set tags can be introduced by any party
  following the appropriate rules of formulation, without reference
  to any centralized authority.






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  Local resolution services may be needed to map feature set tags to
  their corresponding feature set expressions, but these are not able
  to vary the meaning of any given tag.  Failure of a resolution
  service to return the correct expression is detectable by a calling
  application, which should reject any incorrect value supplied.

       NOTE:  where a feature set reference is used, its meaning
       is defined by substitution of the referenced feature
       expression into the referencing expression.  When all
       references have been thus replaced, the result is
       interpreted as a normal feature expression.

       In particular, if a referenced feature expression
       contains some feature tag that is also constrained by the
       referencing expression, the constraints are interpreted
       per RFC 2533 [1], without regard for their origin.  e.g.
       (using some notation introduced below):
          (& (pix-x=100) (pix-y<=300)
             (h.SBB5REAOMHC09CP2GM4V07PQP0) )
       where (h.SBB5REAOMHC09CP2GM4V07PQP0) resolves to:
          (& (pix-x<=200) (pix-y<=150) )
       yields a result equivalent to:
          (& (pix-x=100) (pix-y<=150) )

3.1 Feature set hashed reference format

  This specification introduces a special form of auxiliary predicate
  name with the following syntax:

     fname       = "h." 1*BASE32DIGIT
     BASE32DIGIT = DIGIT
                 / "A" / "B" / "C" / "D" / "E" / "F" / "G" / "H"
                 / "I" / "J" / "K" / "L" / "M" / "N" / "O" / "P"
                 / "Q" / "R" / "S" / "T" / "U" / "V"

  The sequence of base-32 digits represents the value of a hash
  function calculated over the corresponding feature set expression
  (see following sections).  Note that the above syntax allows upper-
  or lower- case letters for base-32 digits (per RFC 2234 [3]).

  Thus, within a feature set expression, a hashed feature set
  reference would have the following form:

     (h.123456789abcdefghijklmnopq)








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3.1.1 Hash value calculation

  The hash value is calculated using the MD5 algorithm [6] over the
  text of the referenced feature set expression subjected to certain
  normalizations.  The feature expression must conform to the syntax
  given for 'filter' in RFC 2533 [1]:

     filter = "(" filtercomp ")" *( ";" parameter )

  The steps for calculating a hash value are:

  1. Whitespace normalization:  all spaces, CR, LF, TAB and any other
     layout control characters that may be embedded in the feature
     expression string, other than those contained within quoted
     strings, are removed (or ignored for the purpose of hash value
     computation).

  2. Case normalization:  all lower case letters in the feature
     expression, other than those contained within quoted strings, are
     converted to upper case.  That is, unquoted characters with
     values 97 to 122 (decimal) are changed to corresponding
     characters in the range 65 to 90.

  3. Hash computation:  the MD5 algorithm, described in RFC 1321 [6],
     is applied to the normalized feature expression string.

  The result obtained in step 3 is a 128-bit (16 octet) value that is
  converted to a base-32 representation to form the feature set
  reference.

       NOTE:  under some circumstances, removal of ALL
       whitespace may result in an invalid feature expression
       string.  This should not be a problem as this is done
       only for the purpose of calculating a hash value, and
       significantly different feature expressions are expected
       to differ in ways other than their whitespace.

       NOTE:  case normalization is deemed appropriate since
       feature tag and token matching is case insensitive.

3.1.2 Base-32 value representation

  RFC 1321 [6] describes how to calculate an MD5 hash value that is a
  sequence of 16 octets.  This is then required to be coded as a
  base-32 value, which is a sequence of base-32 digit characters.







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  Each successive character in a base-32 value represents 5
  successive bits of the underlying octet sequence.  Thus, each group
  of 8 characters represents a sequence of 5 octets (40 bits):

                 1          2          3
      01234567 89012345 67890123 45678901 23456789
     +--------+--------+--------+--------+--------+
     |< 1 >< 2| >< 3 ><|.4 >< 5.|>< 6 ><.|7 >< 8 >|
     +--------+--------+--------+--------+--------+
                                             <===> 8th character
                                       <====> 7th character
                                  <===> 6th character
                            <====> 5th character
                      <====> 4th character
                 <===> 3rd character
           <====> 2nd character
      <===> 1st character

  The value (i.e. sequence of bits) represented by each base-32 digit
  character is indicated by the following table:
       "0"  0       "A"  10     "K"  20      "U"  30
       "1"  1       "B"  11     "L"  21      "V"  31
       "2"  2       "C"  12     "M"  22
       "3"  3       "D"  13     "N"  23
       "4"  4       "E"  14     "O"  24
       "5"  5       "F"  15     "P"  25
       "6"  6       "G"  16     "Q"  26
       "7"  7       "H"  17     "R"  27
       "8"  8       "I"  18     "S"  28
       "9"  9       "J"  19     "T"  29

  When encoding a base-32 value, each full group of 5 octets is
  represented by a sequence of 8 characters indicated above.  If a
  group of less than 5 octets remain after this, they are encoded
  using as many additional characters as may be needed:  1, 2, 3 or 4
  octets are encoded by 2, 4, 5 or 7 characters respectively.  Any
  spare bits represented by the base-32 digit characters are selected
  to be zero.

  When decoding a base-32 value, the reverse mapping is applied:
  each full group of 8 characters codes a sequence of 5 octets.  A
  final group of 2, 4, 5 or 7 characters codes a sequence of 1, 2, 3
  or 4 octets respectively.  Any spare bits represented by the final
  group of characters are discarded.

  Thus, for a 128-bit (16 octet) MD5 hash value, the first 15 octets
  are coded as 24 base 32 digit characters, and the final octet is
  coded by two characters.




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       NOTE:  Base64 representation (per MIME [4]) would be more
       compact (21 rather than 26 characters for the MD5 128-bit
       hash value), but an auxiliary predicate name is defined
       (by [1]) to have the same syntax as a feature tag, and
       the feature tag matching rules (per [2]) state that
       feature tag matching is case insensitive.

       Base36 representation was considered (i.e. using all
       letters "A"-"Z") but was not used because this would
       require extended precision multiplication and division
       operations to encode and decode the hash values.

3.2 Resolving feature set identifiers

  This memo does not mandate any particular mechanism for
  dereferencing a feature set identifier.  It is expected that
  specific dereferencing mechanisms will be specified for any
  application or protocol that uses them.

  The following sections describe some ways that feature set
  dereferencing information may be incorporated into a feature set
  expression.  These are based on auxiliary predicate definitions
  within a "where" clause [1].

  When a hashed feature set reference is used, conformance to the
  hashing rules takes precedence over any other determination of the
  feature expression.  Any expression, however obtained, may not be
  substituted for the hash-based reference unless it yields the
  correct hash value.

3.2.1 Query protocol

  A protocol providing request/response type queries (e.g. HTTP,
  LDAP, etc.) might be set up to provide a resolution service.

  Thus, a query to a server associated with the capabilities could be
  performed on the feature set identifier.  The response returned
  would be a CONNEG expression; e.g.

     (h.SBB5REAOMHC09CP2GM4V07PQP0)
     where
     (h.SBB5REAOMHC09CP2GM4V07PQP0) :- (& (pix-x<=200) (pix-y<=150) )
     end

  or just:

     (& (pix-x<=200) (pix-y<=150) )

  This result would be combined with the original expression to
  obtain a result not including the hash based predicate.


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  This process might be further enhanced by using URN resolution
  mechanisms (e.g. DNS NAPTR [10]) to discover the resolution
  protocol and server.

3.2.2 Inline feature set details

  In this case, a reference is resolved by including its definition
  inline in an expression.

  The feature set expression associated with a reference value may be
  specified directly in a "where" clause, using the auxiliary
  predicate definition syntax [1];  e.g.

     (& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
     where
     (h.SBB5REAOMHC09CP2GM4V07PQP0) :- (& (pix-x<=200) (pix-y<=150) )
     end

  This form might be used on request (where the request mechanism is
  defined by the invoking application protocol), or when the
  originator believes the recipient may not understand the reference.

  It is an error if the inline feature expression does not yield the
  hash value contained in auxiliary predicate name.

       NOTE:  viewed in isolation, this format does not have any
       obvious value, in that the (h.xxx) form of auxiliary
       predicate could be replaced by any arbitrary name.

       It is anticipated that this form might be used as a
       follow-up response in a sequence along the lines of:
          A> Capabilities are:
            (& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
          B> Do not understand:
            (h.SBB5REAOMHC09CP2GM4V07PQP0)
          A> Capabilities are:
            (& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
            where
            (h.SBB5REAOMHC09CP2GM4V07PQP0) :- (& (pix-x<=200)
            (pix-y<=150) )
            end











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4. Examples

  The following are some examples of feature set expressions
  containing feature set references:

     (& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )

     (& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
     where
     (h.SBB5REAOMHC09CP2GM4V07PQP0) :-
       (& (pix-x<=200) (pix-y<=150) )
     end

     (h.QGEOPMCF02P09QC016CEPU22FO)
     where
     (h.QGEOPMCF02P09QC016CEPU22FO) :-
       (| (& (ua-media=continuous) (dpi=200) (dpi-xyratio=200/100)
             (color=Binary) (paper-size=B4) (image-coding=MH) )
          (& (ua-media=continuous) (dpi=200) (dpi-xyratio=200/100)
             (color=Binary) (paper-size=B4) (image-coding=MR) )
          (& (ua-media=stationery) (dpi=300) (dpi-xyratio=1)
             (color=Binary) (paper-size=A4) (image-coding=JBIG) )
          (& (ua-media=transparency) (dpi=300) (dpi-xyratio=1)
             (color=Binary) (paper-size=A4) (image-coding=JBIG) ) )
     end

  The following examples are based on Internet fax work, and show how
  a feature-hash might be used to express the commonly-used features.
  A form of Internet fax system that is expected to be quite common
  is a so-called "simple mode" system, whose capabilities are
  described by the following feature expression:

     (& (image-file-structure=TIFF-minimal)
        (MRC-mode=0)
        (color=Binary)
        (image-coding=MH) (MRC-mode=0)
        (| (& (dpi=204) (dpi-xyratio=[204/98,204/196]) )
           (& (dpi=200) (dpi-xyratio=[200/100,1]) ) )
        (size-x<=2150/254)
        (paper-size=A4)
        (ua-media=stationery) )

  This might be expressed by the hash-based feature set identifier:

     (h.MSB955PVIRT1QOHET9AJT5JM3O)







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  The following example describes capabilities of a full-colour
  Internet fax system.  Note a number of feature values are
  applicable in common with '(color=grey)' and '(colour=full)':

     (& (image-file-structure=TIFF)
        (MRC-mode=0)
        (| (& (color=Binary)
              (image-coding=[MH,MR,MMR])
              (| (& (dpi=204) (dpi-xyratio=[204/98,204/196]) )
                 (& (dpi=200) (dpi-xyratio=[200/100,1]) )
                 (& (dpi=300) (dpi-xyratio=1) ) ) )
           (& (color=grey)
              (image-coding=JPEG)
              (image-coding-constraint=JPEG-T4E)
              (color-levels<=256)
              (color-space=CIELAB)
              (color-illuminant=D50)
              (CIELAB-L-min>=0)
              (CIELAB-L-max<=100)
              (dpi=[100,200,300]) (dpi-xyratio=1) )
           (& (color=full)
              (image-coding=JPEG)
              (image-coding-constraint=JPEG-T4E)
              (color-subsampling=["1:1:1","4:1:1"])
              (color-levels<=16777216)
              (color-space=CIELAB)
              (color-illuminant=D50)
              (CIELAB-L-min>=0)
              (CIELAB-L-max<=100)
              (CIELAB-a-min>=-85)
              (CIELAB-a-max<=85)
              (CIELAB-b-min>=-75)
              (CIELAB-b-max<=125)
              (dpi=[100,200,300]) (dpi-xyratio=1) ) )
        (size-x<=2150/254)
        (paper-size=[letter,A4,B4]) )
        (ua-media=stationery) )















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  Separating out the common capabilities yields:

     (& (image-file-structure=TIFF)
        (MRC-mode=0)
        (| (& (color=Binary)
              (image-coding=[MH,MR,MMR])
              (| (& (dpi=204) (dpi-xyratio=[204/98,204/196]) )
                 (& (dpi=200) (dpi-xyratio=[200/100,1]) )
                 (& (dpi=300) (dpi-xyratio=1) ) ) )
           (& (color=grey)
              (color-levels<=256)
              (h.QVSEM8V2LMJ8VOR7V682J7079O) )
           (& (color=full)
              (color-subsampling=["1:1:1","4:1:1"])
              (color-levels<=16777216)
              (CIELAB-a-min>=-85)
              (CIELAB-a-max<=85)
              (CIELAB-b-min>=-75)
              (CIELAB-b-max<=125)
              (h.QVSEM8V2LMJ8VOR7V682J7079O) ) )
        (size-x<=2150/254)
        (paper-size=[letter,A4,B4]) )
        (ua-media=stationery) )
     where
     (h.QVSEM8V2LMJ8VOR7V682J7079O) :-
        (& (image-coding=JPEG)
           (image-coding-constraint=JPEG-T4E)
           (color-space=CIELAB)
           (color-illuminant=D50)
           (CIELAB-L-min>=0)
           (CIELAB-L-max<=100)
           (dpi=[100,200,300]) (dpi-xyratio=1) )
     end


5. Internationalization considerations

  Feature set expressions and URI strings are currently defined to
  consist of only characters from the US-ASCII repertoire [1,5];
  under these circumstances this specification is not impacted by
  internationalization considerations (other than any already
  applicable to URIs [5]).










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  But, if future revisions of the feature set syntax permit non-US-
  ASCII characters (e.g. within quoted strings), then some canonical
  representation must be defined for the purposes of calculating hash
  values.  One choice might be to use a UTF-8 equivalent
  representation as the basis for calculating the feature set hash.
  Another choice might be to leave this as an application protocol
  issue (but this could lead to non-interoperable feature sets
  between different protocols).

  Another conceivable issue is that of up-casing the feature
  expression in preparation for computing a hash value.  This does
  not apply to the content of strings so is not likely to be an
  issue.  But if changes are made that do permit non-US-ASCII
  characters in feature tags or token strings, consideration must be
  given to properly defining how case conversion is to be performed.


6. Security considerations

  For the most part, security considerations are the same as those
  that apply for capability identification in general [1,2,9].

  A possible added consideration is that use of a specific feature
  set identifier may reveal more information about a system than is
  necessary for a transaction at hand.


7. Acknowledgements

  Ideas here have been improved by early discussions with Martin
  Duerst, Al Gilman and Ted Hardie.  Useful suggestions for
  improvement were provided by Maurizio Codogno.


8. References

[1]  RFC 2533, "A syntax for describing media feature sets"
     Graham Klyne, 5GM/Content Technologies
     March 1999.

[2]  RFC 2506, "Media Feature Tag Registration Procedure"
     Koen Holtman, TUE
     Andrew Mutz, Hewlett-Packard
     Ted Hardie, Equinix
     March 1999.

[3]  RFC 2234, "Augmented BNF for Syntax Specifications: ABNF"
     D. Crocker (editor), Internet Mail Consortium
     P. Overell, Demon Internet Ltd.
     November 1997.


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[4]  RFC 2045, "Multipurpose Internet Mail Extensions (MIME)
     Part 1: Format of Internet message bodies"
     N. Freed, Innosoft
     N. Borenstein, First Virtual
     November 1996.

[5]  RFC 2396, "Uniform Resource Identifiers (URI): Generic Syntax",
     Tim Berners-Lee, World Wide Web Consortium/MIT
     Roy T. Fielding, University of California, Irvine
     Larry Masinter, Xerox PARC
     August 1998.

[6]  RFC 1321, "The MD5 Message-Digest Algorithm",
     R. Rivest, MIT Laboratory for Computer Science and RSA Data
     Security, Inc.,
     April 1992.

[7]  "Applied Cryptography"
     Bruce Schneier
     John Wiley and Sons, 1996 (second edition)
     ISBN 0-471-12845-7 (cloth)
     ISBN 0-471-11709-9 (paper)

[8]  RFC 2703, "Protocol-independent content negotiation framework"
     Graham Klyne, 5GM/Content Technologies
     September 1999.

[9]  "Numerical Recipes"
     William H Press, Brian P Flannery, Saul A Teukolski and William T
     Vetterling
     Cambridge University Press (1986)
     ISBN 0 521 30811 9
     (The Gamma function approximation is presented in chapter 6 on
     "Special Functions".  There have been several later editions of
     this book published, so the chapter reference may change.)

[10] RFC 2168, "Resolution of Uniform Resource Identifiers using the
     Domain Name System"
     R. Daniel, Los Alamos National Laboratory
     M. Mealling, Network Solutions, Inc.
     June 1997.

[11] Java source code of feature set matching algorithm, with feature
     set hash computation option.
     Linked from <http://www.imc.org/ietf-medfree/>







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9. Authors' addresses

  Graham Klyne
  Content Technologies Ltd.
  1220 Parkview,
  Arlington Business Park
  Theale
  Reading, RG7 4SA
  United Kingdom.
  Telephone: +44 118 930 1300
  Facsimile: +44 118 930 1301
  E-mail:    GK@ACM.ORG

  Larry Masinter
  AT&T
  75 Willow Road
  Menlo Park, CA 94025
  Tel: +1-650-463-7059
  Fax: +1-650-463-7037
  mailto:LM@att.com
  http://larry.masinter.net


Appendix A: The birthday problem

       NOTE:  this entire section is commentary, and does not
       affect the feature set reference specification in any
       way.

  The use of a hash value to represent an arbitrary feature set is
  based on a presumption that no two distinct feature sets will yield
  the same hash value.

  There is a small but distinct possibility that two different
  feature sets will indeed yield the same hash value.

  We assume that the 128-bit hash function distributes hash values
  for feature sets, even those with very small differences, randomly
  and evenly through the range of 2^128 (approximately 3*10^38)
  possible values.  This is a fundamental property of a good digest
  algorithm like MD5.  Thus, the chance that any two distinct feature
  set expressions yield the same hash is less than 1 in 10^38.  This
  is negligible when compared with, say, the probability that a
  receiving system will fail having received data conforming to a
  negotiated feature set.







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  But when the number of distinct feature sets in circulation
  increases, the probability of repeating a hash value increases
  surprisingly.  This is illustrated by the "birthday paradox":
  given a random collection of just 23 people, there is a greater
  than even chance that there exists some pair with the same
  birthday.  This topic is discussed further in sections 7.4 and 7.5
  of Bruce Schneier's "Applied Cryptography" [7].

  The table below shows the "birthday paradox" probabilities that at
  least one pair of feature sets has the same hash value for
  different numbers of feature sets in use.


          Number of feature   Probability of two
          sets in use         sets with the same
                              hash value
          1                   0
          2                   3E-39
          10                  1E-37
          1E3                 1E-33
          1E6                 1E-27
          1E9                 1E-21
          1E12                1E-15
          1E15                1E-9
          1E18                1E-3

       The above probability computations are approximate, being
       performed using logarithms of a Gamma function
       approximation by Lanczos [9].  The probability formula is
       'P=1-(m!/((m-n)! m^n))', where 'm' is the total number of
       possible hash values (2^128) and 'n' is the number of
       feature sets in use.

  If original feature set expressions are generated manually, or only
  in response to some manually constrained process, the total number
  of feature sets in circulation is likely to remain very small in
  relation to the total number of possible hash values.

  The outcome of all this is:  assuming that the feature sets are
  manually generated, even taking account of the birthday paradox
  effect, the probability of incorrectly identifying a feature set
  using a hash value is still negligibly small when compared with
  other possible failure modes.









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Appendix B: Revision history

  [[[RFC editor:  please remove this section on publication]]]

  00a  10-Feb-1999  Initial draft.

  01a  16-Feb-1999  Added pointers to mailing list for discussion.

  01b  25-Mar-1999  Name all authors.  Add some terms to the glossary.
                    Expand on meaning of URI tag used as auxiliary
                    predicate name.  Update references.  Rework
                    section 3 to deal more evenly with both hash and
                    URI based feature set references.  State absolute
                    requirement for hash-based references to be
                    resolved to expressions that yield the correct
                    hash value.

  01c  06-Apr-1999  Define form of URI reference using new '<...>'
                    syntax, and adjust other text accordingly.

  01d  06-Apr-1999  Editorial revisions.  Include values in table of
                    probabilities for hash value clashes.  Remove
                    discussion of algebraic simplification of hash
                    references.  Correct syntax of some examples.

  02a  16-Jun-1999  Move birthday problem to an appendix.  Remove
                    RESCAP citation.  Use base-32 to represent feature
                    hashes;  describe base-32 encoding.

  02b  16-Jun-1999  Add note that the <URI> form of feature reference
                    may not be allowed at arbitrary locations in all
                    contexts.

  03a  19-Jul-1999  Incorporate review comments.  Added some text to
                    emphasize applicability of RFC 2533 rules for
                    interpretation and processing of feature
                    expressions.

  04a  02-Apr-2000  Remove description of use of URIs to identify
                    feature sets;  discuss other techniques for
                    resolution;  update references.

  04b  06-Apr-2000  Update contact address.  Note that whitespace in
                    quoted strings should not be removed by the
                    normalization process.  Update examples to use a
                    proper, calculated MD5 hash value.  Add reference
                    to sample source code.  Move full copyright
                    statement to end.




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  05a  08-Jun-2000  Add text explaining motivation and some uses for
                    hash-based properties (sections 2), and examples
                    of same (section 4).


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