rfc2938.txt

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RFC 2938          Identifying Composite Media Features    September 2000


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.

   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.







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   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.

       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.,




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      (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.

   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)




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

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)



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        (ua-media=stationery) )

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

      (h.MSB955PVIRT1QOHET9AJT5JM3O)

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

   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




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RFC 2938          Identifying Composite Media Features    September 2000