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Versions: 00 01 02                                                      
IETF media feature registration WG                        Graham Klyne
Internet draft                                Content Technologies/5GM
                                                      15 December 1998
                                                    Expires: June 1999

             W3C Composite Capability/Preference Profiles

Status of this memo

  This document is an Internet-Draft.  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

  To view the entire list of current Internet-Drafts, please check
  the "1id-abstracts.txt" listing contained in the Internet-Drafts
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  (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US
  West Coast).

Copyright Notice

  Copyright (C) 1998, The Internet Society


  This document suggests some possible areas for extending the IETF
  'conneg' working group's capability description framework,
  described in [2,3,4].  The suggested areas for extension have been
  motivated by WWW Consortium (W3C) work on Composite
  Capability/Preference Profiles (CCPP) [5] that parallels some
  aspects of IETF 'conneg' work.

  It is presented as a discussion document, with a view to maybe
  integrating some of these ideas into ongoing 'conneg' work.

Klyne                                                         [Page 1]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

Table of contents

1. Introduction.............................................2
  1.1 Structure of this document ...........................3
  1.2 Discussion of this document ..........................3
  1.3 Amendment history ....................................3
2. Tag-independent content negotiation......................4
  2.1 WWW resource transfer scenario .......................4
  2.2 Conclusions ..........................................5
  2.3 Consequences .........................................5
3. Assumed feature values...................................5
  3.1 Unifying "required" and "default" values .............6
  3.2 Message transmission model ...........................7
  3.3. Problem summary .....................................8
  3.4 Proposed extension to 'conneg' framework .............10
     3.4.1 Assumed values construct.........................10
     3.4.2 End of chain indicator...........................10
     3.4.3 Extended form of conjunction.....................10
  3.5 Additional reduction rules ...........................11
  3.6 Extended canonicalization rules ......................12
  3.7 Examples .............................................12
     3.7.1 The "font" problem...............................12
     3.7.2 Resolution dependent font........................14
     3.7.3 Combining default values.........................15
4. XML representation of capability descriptions............17
5. Mapping between RDF and media features...................17
6. Conclusions..............................................21
7. Security considerations..................................21
8. Full copyright statement.................................22
9. Acknowledgements.........................................22
10. References..............................................22
11. Author's address........................................24

1. Introduction

  This document suggests some possible areas for extending the IETF
  'conneg' working group's capability description framework,
  described in [2,3,4].  The suggested areas for extension have been
  motivated by WWW Consortium (W3C) work on Composite
  Capability/Preference Profiles (CCPP) [5] that parallels some
  aspects of IETF 'conneg' work.

  It is presented as a discussion document, with a view to maybe
  integrating some of these ideas into ongoing 'conneg' work.

Klyne                                                         [Page 2]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

1.1 Structure of this document

  The main part of this draft addresses the following main areas:

  Section 2 discusses tag-independent negotiation procedures, one of
  the goals of the 'conneg' work, with particular reference to WWW

  Section 3 suggests a framework for describing "assumed" feature

  Section 4 suggests an approach to using XML to carry capability
  information structured according to the 'conneg' framework.

  Section 5 suggests an approach to mapping between RDF [6] and
  'conneg' media feature registrations.

1.2 Discussion of this document

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

  Please send comments regarding this document to:


  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 subscribed, please see the
  mailing list archive at:


1.3 Amendment history

  00a       19-Oct-1998
            Document initially created.

  00b       20-Oct-1998
            Added sections on tag-independent negotiation, XML syntax
            and RDF mapping.

  00c       31-Oct-1998
            Fixed up section numbers.

  01a       15-Dec-1998
            Minor editorial revisions for re-issue.  Updated CC/PP
            reference to published W3C Note.

Klyne                                                         [Page 3]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

2. Tag-independent content negotiation

  One can imagine two extremes of content negotiation procedure:

  o  One in which the decisions about whether the features in a data
     data resource match some set of capabilities are made in full
     knowledge of the exact meaning of every feature and capability.

  o  The other, in which the matching decision is made without any
     knowledge of the particular features concerned.

  A practical procedure is likely to lie somewhere between these
  extremes.  The purpose of this section is to argue that a procedure
  that minimizes the required knowledge of particular feature tags is
  likely to be more useful than one which depends heavily upon such

  This approach is described here as "tag-independent negotiation":
  negotiation that can proceed without specific knowledge of the tags
  used to describe various media features of the participating

2.1 WWW resource transfer scenario

  Consider a WWW transaction scenario that involves content

     Resource-1 ->-   +--------+         +--------+   ->- Plugin-1
                    \ | Origin |         |        | /
     Resource-2 ->----| Server |--<--->--| Client |---->- Plugin-2
                    / |        |         |        | \
     Resource-3 ->-   +--------+         +--------+   ->- Plugin-3

  In this scenario, the active negotiation takes place between the
  origin server and the receiving client.

  The resources available to the origin server are possibly passive
  data files, with no opportunity for interaction between data
  resource and server.

  The plugins available to the browser may well be dispatched as
  separate programs, with limited opportunity for ongoing interaction
  between the plugin and the client.  Also, there may be a high
  overhead associated with activating a plugin, so it is not
  desirable to activate one or more plugins simply to determine
  whether a data resource is acceptable to that plugin.

Klyne                                                         [Page 4]

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W3C Composite Capability/Preference Profiles

2.2 Conclusions

  In an environment like WWW where new and changed features are
  deployed on a regular basis, it is clearly not desirable that
  detailed knowledge be hard-coded into either the origin server or
  the client software.  This would rather defeat the idea of
  extending functionality through plugins and data resource

  Thus, to avoid early obsolesence of servers and clients, we seek a
  negotiation framwork that permits a data resource to describe
  itself to the origin server, a plugin to describe itself to a
  browser, and allows the server and browser to conduct negotiations
  without any further knowledge.

  There are doubtless some cases where the degree of negotiating
  flexibility indicated here is not essential:  "Internet appliances"
  and other dedicated devices spring to mind.  But even for such
  devices, it can be argued that the flexibility of tag-independent
  negotiation will simplify their configuration.

2.3 Consequences

  Some consequences of the conclusion just outlined are:

  o  The capability description should be able to describe
     dependencies between content features in such a way that they can
     be handled by the negotiation protocol.

  o  The vocabulary used to describe content features should avoid
     having multiple ways to describe the same capabilities.

3. Assumed feature values

  The W3C CC/PP [5] work embodies a concept that is not currently
  supported in the IETF 'conneg' framework [3];  namely the
  construction of a set of capability and preference values from a
  number of separate sources (e.g. hardware platform defaults,
  software platform defaults, user preferences).  Also, there is a
  desire to have a compact way to represent a small number of
  differences from some baseline set of capabilities.  What is not
  covered by this work, that the IETF 'conneg' work can supply, is a
  framework for actually combining the various information sources.

Klyne                                                         [Page 5]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

  Also, there is an identified capability gap in the IETF 'conneg'
  work:  the ability to indicate that some particular feature MUST be
  supplied by a communicating counterparty.  This has been
  charactirized as the "font problem":  how can a sender require that
  some particular font is supported by a recipient?  Currently the
  'conneg' framework operates to effectively ignore a request for an
  unknown font, or other unrecognized feature.

3.1 Unifying "required" and "default" values

  The two situations we wish to describe are:

  (A)  "required" values:  given two feature set descriptions, the
       feature set match is to fail if one of them does not define a
       value for a feature tag tested by the other.  This is
       exemplified by the "fonts" problem:  if a data resource uses
       some font indicated by the feature tag "Font-obscure", and the
       receiver does not explicitly declare an ability to deal with
       that font, then we should assume that the feature sets do not
       match.  As currently defined, the 'conneg' algebraic framework
       [3] would allow a feature set match in the absence of an
       explicit denial of such support.

  (B)  "override" and "default" values:  given two feature set
       descriptions that contain different constraints involving some
       feature tag, the constraint from one of the feature sets is to
       override the constraint in the other.  This is exemplified by
       the scenario outlined in the W3 CC/PP draft [5], in which a
       hardware manufacturer may supply a default profile for a
       device which can then be overridden by local user preferences.

  A mechanism that can address both of these requirements is one
  which allows a feature set description to indicate "assumed" values
  for a given feature.

  Case (A):
       A data resource requires a given font to be available, and its
       feature set description provides "assumed" values for the font
       availability that are used only if the recipient does not
       itself reference that feature.  Thus, if the data resource
          Assume: (Font-obscure=FALSE)
       and the recipient does not supply a constraint for 'Font-
       obscure', then the assumed value is used to force a feature
       set match failure.

Klyne                                                         [Page 6]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

  Case (B):
       A hardware platform capability description consists entirely
       of assumed feature values.  When combined with user
       preferences, these provide values for those features that are
       not mentioned in the user preferences.

  The ideas here can be extended to deal with combinations of more
  than two feature sets.

  The assumed value idea breaks a number of assumptions of the
  current 'conneg' algebraic model [3], which is based pretty much on
  simplified predicate algebra.  The sections that follow reconcile
  this idea with the existing framework.

3.2 Message transmission model

  The following discussion is based on a message transmission model
  that assumes multiple feature set constraints.  The following
  example is from [1], section 3.1:

     The life of a data resource may be viewed as:
            (C)     (T)     (F)
        [A] = author of document
        (C) = original document content
        [S] = message sending system
        (T) = transmitted data file (representation of (C))
        [R] = receiving system
        (F) = formatted (rendered) document data (presentation of (C))
        [U] = user or consumer of a document

     Here, it is [S] and [R] who exchange negotiation metadata to
     decide the form of (T), so these elements are the focus of our

     Negotiation metadata provided by [S] would take account of
     available document content (C) (e.g. availability of resource
     variants) as well as its own possible ability to offer that
     content in variety of formats.

     Negotiation metadata provided by [R] would similarly take account
     of the needs and preferences of its user [U] as well as its own
     capabilities to process and render received data.

Klyne                                                         [Page 7]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

  This example suggests a generic framework within which negotiation
  is conducted: a chain of entities, each of which may impose some
  constraint(s) on the message features that can usefully be

     [.]-->--[E1]-->--[E2]-->-- ... -->--[En]-->--[.]

  '[.]' are used here indicate specific begin and end points of the
  chain, and '[Ei]' are entities that may impose feaure constraints.
  If the feature constraint predicate associated with '[Ei]' is
  'FCi', then the composite feature constraint for the entire chain
  is the conjunction of the individual constraints:

     (& FC1 FC2 ... FCn )

  using the notation of [3].

3.3. Problem summary

  The 'conneg' algebraic framework depends on combining feature sets
  by set intersection (i.e. a logical AND of the predicates that
  describe feature sets).

  For the purposes of this discussion, all feature set expressions
  are reduced to disjunctive normal form:  thus, every feature set is
  described as a disjunction (union) of one or more subsets, each of
  which is described by a conjunction of atomic feature comaprisons;

     (| (& (size=s1) (resolution=r1) (color=c1) )
        (& (size=s2) (resolution=r2) (grey=g2)  ) )

  This is without loss of generality:  the combining framework
  description [3] shows how more complex expressions can be reduced
  to this canonical form.

  Now consider the following observations:

  (1)  all atomic feature comparisons have the form '(ftag relop
       fvalue)', where 'ftag' is a feature tag that uniquely
       identifies a feature, 'relop' is a comparison operator and
       'fvalue' is a particular value associated with the feature

  (2)  The conjunction operator is symmetric:

          (& A B) == (& B A)

Klyne                                                         [Page 8]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

  (3) The conjunction operator is associative:

          (& A (& B C) ) == (& (& A B) C)

       This property is important because it means that the feature
       sets are not required to be combined in any specific order.
       This point is raised in the CC/PP memo [5], and this is a
       property that should be preserved.

  (4)  The assumed value mechanism outlined above is inherrently
       anti-symmetric.  One of the feature sets used takes precedence
       over the other:

          (Assume: A B) != (Assume: B A)

  (5)  The assumed value mechanism, by its very nature, is sensitive
       to the particular feature tags used.

  (6)  Assumed values should be used only when an explicit constraint
       for a feature tag cannot be found.  For example, if a data
       resource has the requirement:


       and also indicates an assumed value:

          Assume: (Font-obscure=FALSE)

       The assumed value must be ignored if the recipient asserts:


       This requirement is captured without imposing an order of
       evaluation on the feature set matching process (because such
       imposition is considered to make the entire process more
       complex and error-prone).

  (7)  A feature set constraint must be reduced to the predicate form
       currently defined by the 'conneg' framework [3] (i.e. all
       default values must be eliminated) before a feature collection
       (i.e. some specific feature values) can be tested with it.

Klyne                                                         [Page 9]

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W3C Composite Capability/Preference Profiles

3.4 Proposed extension to 'conneg' framework

  Two extensions to the 'conneg' algebraic framework are proposed:

  (a)  a "assumed values" construct

  (b)  an "end-of-chain" indicator

  and revision of the conjunction '(&...)' and other processing rules
  to take account of these new constructs.

3.4.1 Assumed values construct

  This is expressed as:

     (+ FC )

  where 'FC' is a list of one or more feature constraint expressions.
  For an expression in the canonical disjunctive normal form, each is
  an atomic feature value constraint (i.e. contains a single feature

3.4.2 End of chain indicator

  This is expressed as:


3.4.3 Extended form of conjunction

  The canonical form of a conjunction is extended to allow:

     (& ... (+ FL) FC (+ FR) ... )

  That is, it contains separate, optional assumed value constructs to
  the left- and right- of any expression in the conjunction.
  Referring to the message transmission chain model, these correspond
  to assumed values that are applied "upstream" and "downstream"
  along the chain.  For example, when feature sets along a chain are

     (& FC1 (& (+ FL2) FC2 (+FR2) ) FC3 )

  the 'FL2' assumed values applied to 'FC1', and values 'FR2' are
  applied to 'FC3'.

  Assumed value constructs can appear anywhere in a conjunction, and
  the additional rules below indicate how to process these into the
  canonical form.

Klyne                                                        [Page 10]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

  The '(.)' construct can appear at the left or right hand end of a

     (& (.) FC (.) )

  It may not appear anywhere else in a conjunction.  Additional
  processing rules (below) show how these can be used to eliminate
  assumed value constructs from a conjunction, thus reducing it to a
  simple predicate form.

3.5 Additional reduction rules

  The aim of these rules is to allow assumed values to propagate
  along the message transfer chain until either (a) it is determined
  they are redundant, or (b) the assumed values are used as actual
  feature constraints which can be evaluated in the normal way.

  The following rules assume that all feature constraints are atomic;
  i.e. they refer to a single feature tag.  The next section contains
  rules for canonicalization of complex predicates to a form with
  just atomic feature constraints.

  Note that if there are any assumed value constructs present in a
  conjunction, re-ordering of the component constraints is allowed
  only according to reordering rule below.

  (a)  Re-ordering of assumed value constraints:
          (& ... (+FC1) FC2 ... )     --->  (& ... FC2 (+FC1) ... )
          (& ... FC1 (+FC2) ... )     --->  (& ... (+FC2) FC1 ... )
          (& ... (+FC1) (+FC2) ... )  --->  (& ... (+FC2) (+FC1) ... )
        ONLY IF: FC1 and FC2 reference different feature tags.

  (b)  Assumed value elimination:
          (& ... FC1 (+FC2) FC3 ... )  --->  (& ... FC1 FC3 ... )
       ONLY IF: FC1, FC2 and FC3 reference the same feature tag,

       Two extended forms of this rule allow multiple assumed values
       to be eliminated:
          (& ... FC1 ... (+FC2) FC3 ... )  --->  (& ... FC1 FC3 ... )
          (& ... FC1 (+FC2) ... FC3 ... )  --->  (& ... FC1 FC3 ... )
       ONLY IF: FC1, FC2 and FC3 reference the same feature tag,

  (c) Default resolution:
          (& (.) (+FC1) ... )  --->  (& FC1 ... )
          (& ... (+FC1) (.) )  --->  (& ... FC1 )

Klyne                                                        [Page 11]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

3.6 Extended canonicalization rules

  The above reduction rules assume a canonical form of predicate
  expressions.  The canonicalization rules described for the 'conneg'
  framework [3] must be extended to handle assumed value constructs.

  The main goal of these additional rules is to push assumed values
  "down" in the expression so that they are contained within all
  disjunctions and conjunctions, and themselves contain only atomic
  feature constraints.

  (a)  Separate default constructs:
          (+ FC1 FC2 ... )  --->  (+FC1) (+FC2) ...
       Following repeated application of this rule, all default
       constructs contain just one feature predicate expression
       (conjunction, disjunction or atomic).

  (b)  Move assumed value into conjunction:
          (+ (& FC1 FC2 ... ) )  --->  (& (+FC1) (+FC2) ... )

  (c)  Move assumed value into disjunction:
          (+ (| FC1 FC2 ... ) )  --->  (| (+FC1) (+FC2) ... )

  (d)  Flatten assumed value nests:
          (+ ... (+FC1) ... )  --->  (+ ... FC1 ... )

3.7 Examples

  Expressions in the following examples assume left is "upstream" in
  the message flow.

3.7.1 The "font" problem

  Sender/data resource description:

     (& (Font-1=TRUE) (Font-2=TRUE)
        (+ (Font-1=FALSE) (Font-2=FALSE) ) )

  Recipient description:

     (& (Font-1=TRUE) (Font-3=TRUE) )

Klyne                                                        [Page 12]

Internet draft                                        15 December 1998
W3C Composite Capability/Preference Profiles

  Combine these to obtain the full transmission path description
  (recall that feature sets are matched by combining them with
  '(&...)', and left is upstream in the transmission path):

     (& (.)
        (& (Font-1=TRUE) (Font-2=TRUE)              // Sender..
           (+ (Font-1=FALSE) (Font-2=FALSE) ) )
        (& (Font-1=TRUE) (Font-3=TRUE) )            // Receiver..
        (.) )

  What follows is a sequence of rewriting rule applications that show
  how this expression is processed to eliminate the (+...) constructs
  and, in the process, the receiver's lack of support for 'Font-2'
  causes the feature set match to fail.

  -->  [expand (+...)]

     (& (.)
        (& (Font-1=TRUE) (Font-2=TRUE)
           (+ (Font-1=FALSE) ) (+ (Font-2=FALSE) ) )
        (& (Font-1-TRUE) (Font-2=TRUE) (Font-4=TRUE) )
        (.) )

  -->  [flatten (&...(&...)...)]

     (& (.)
        (Font-1=TRUE) (Font-2=TRUE)
        (+ (Font-1=FALSE) ) (+ (Font-2=FALSE) )
        (Font-1-TRUE) (Font-2=TRUE) (Font-4=TRUE) )
        (.) )

  -->  [collect features, being careful to preserve ordering of
       constraints with the same feature tag]:

     (& (.)
        (Font-1=TRUE) (+ (Font-1=FALSE) ) (Font-1=TRUE)
        (Font-2=TRUE) (+ (Font-2=FALSE) )
        (Font-3=TRUE) )
     (& (.)

  -->  [eliminate un-needed default]

     (& (.)
        (Font-1=TRUE) (Font-1=TRUE)
        (Font-2=TRUE) (+ (Font-2=FALSE) )
        (Font-3=TRUE) )
     (& (.)

Klyne                                                        [Page 13]

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W3C Composite Capability/Preference Profiles

  -->  [Resolve non-eliminated default:  move to end, then use (+...)
       (.) rewriting rule]

     (& (.)
        (Font-1=TRUE) (Font-1=TRUE)
        (Font-2=TRUE) (Font-2=FALSE)
        (Font-3=TRUE) )
     (& (.)

  -->  [Combine (&...) with same tag:

     (& (.)
        (.) )

  Which leaves a conjunction containing FALSE, indicating that there
  is no feature set matching all the capability constraints.

  Note that the receiver's (Font-3=TRUE) was not eliminated because
  it did not use the (+...) mechanism to say, in effect, that the
  sender must use it.

3.7.2 Resolution dependent font

  A slightly more complex example might be:

  Sender/data resource description:

     (| (& (dpi>=100) (Font-simple=TRUE) (+ (Font-simple=FALSE) ) )
        (& (dpi>=300) (Font-fancy=TRUE)  (+ (Font-fancy=FALSE)  ) ) )

  This would match the recipient capalities:
     (& (dpi=100) (Font-simple=TRUE) )
     (& (dpi=300) (Font-simple=TRUE) (Font-fancy=TRUE) )

  But it would not match the recipient capability:
     (& (dpi=100) (Font-fancy=TRUE) )

Klyne                                                        [Page 14]

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W3C Composite Capability/Preference Profiles

3.7.3 Combining default values

  Finally, note there is a possibility to use nested '(& ... (.) )'
  constructs for defaults which are restricted to a particular entity
  in the transmission chain, for example (which is the type of
  scenario discussed in the CC/PP draft):

     (& <sender>
        (& <system values> (+ <system values> )
           <local values> (.) ) ... )

  Using the "indirect references" example from the CC/PP memo [5], we
  could construct a framework like this:

     (& (& (Hardware-defaults) (+ (Hardware-defaults) )
           (Hardware-platform) (.) )
        (& (Software-defaults) (+ (Software-defaults) )
           (Software-platform) (.) )
        (.) )


     (Hardware-defaults) :-
        (& (Vendor="Nokia")
           (Memory="16Mb) )

     (Hardware-platform) :-

     (Software-defaults) :-
        (& (OS="EPOC1.0")
           (WMLScriptVersion="1.0") )

     (Software-platform) :-
        (& (Sound="Off")
           (Images="Off") )

Klyne                                                        [Page 15]

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W3C Composite Capability/Preference Profiles

     (EpocEmail) :-
        (& (Version="EpocEmail1.0")
           (HTMLVersion="4.0") )

     (EpocCalendar) :-
        (& (Version="EpocCalendar1.0")
           (HTMLVersion="4.0") )


  The repetition of '(Hardware-defaults)' and '(Software-defaults)'
  is needed to force application of the default value through
  reduction rule 2.5(b), "Assumed value elimination".  This is,
  however, a little clumsy and suggests introduction of a new
  construct and reduction rule.

  The new construct, a default end of chain indicator, is expressed


  and its purpose is to force elimination of any unused default
  value, using the following rule

  Default value elimination:
          (& (*) ... (+FC1) FC2 ... )  --->  (& (*) ... FC2 ... )
       ONLY IF: FC1 and FC2 reference the same feature tag.

  Thus, the above feature set expression could be re-written as:

     (& (& (*) (+ (Hardware-defaults) ) (Hardware-platform) (.) )
        (& (*) (+ (Software-defaults) ) (Software-platform) (.) )
        (.) )



Klyne                                                        [Page 16]

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W3C Composite Capability/Preference Profiles

4. XML representation of capability descriptions

  The 'conneg' feature set description framework is well-suited to
  being expressed in XML.  The previously given example:

     (| (& (dpi>=100) (Font-simple=TRUE) (+ (Font-simple=FALSE) ) )
        (& (dpi>=300) (Font-fancy=TRUE)  (+ (Font-fancy=FALSE)  ) ) )

  might be represented (in conjunction with an appropriate XML tag
  definitions) using something like the following XML:

             <GE dpi 100 />
             <EQ Font-simple TRUE />
                 <EQ Font-simple FALSE />
             <GE dpi 300 />
             <EQ Font-fancy TRUE />
                 <EQ Font-fancy FALSE />

  [[[I am not expert in XML, and the above is almost certainly
  flawed.  But I do believe that it is possble to use XML broadly in
  this way, and would welcome further comment from any XML

5. Mapping between RDF and media features

  The following extract is from the RDF Model and Syntax

     "This document introduces a model for representing RDF metadata
     as well as a syntax for encoding and transporting this metadata
     in a manner that maximizes the interoperability of independently
     developed web servers and clients.  The syntax presented here
     uses the Extensible Markup Language [XML]: one of the goals of
     RDF is to make it possible to specify semantics for data based on
     XML in a standardized, interoperable manner. [...] It is also
     important to understand that this XML syntax is only one possible
     syntax for RDF, and that alternate ways to represent the same RDF
     data model may emerge."

Klyne                                                        [Page 17]

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  It is interesting to note that the 'conneg' feature expression
  framework shares with the RDF framework this clear intent to
  separate the data model from its representative syntax.

  In this section, the RDF data model is explored and related to the
  'conneg' feature registration namespace.  The RDF data model is
  very rich compared with that used by 'conneg', so should easily be
  capable of representing the concepts used.

  IETF 'conneg' deals with the following value-sets:

     Feature-tag        ::= Token
     Feature-value      ::= Integer
                          | Rational
                          | Boolean
                          | Token
                          | String
     Feature-collection ::= FINITE MAP Feature-tag TO Feature-value
     Feature-set        ::= SET OF Feature-collection

  Further, a "Feature-set" is described by a collection of feature
  value assertions:

     Feature-assertion  ::= ( "&", SEQUENCE OF Feature-assertion )
                          | ( "|", SEQUENCE OF Feature-assertion )
                          | ( "!", Feature-assertion )
                          | Atomic-assertion
     Atomic-assertion   ::= ( Relation, Feature-tag, Feature-value )
     Relation           ::= "EQ" | "NE" | "GE" | "LE"

  A "Feature-collection" can be represented directly as an RDF
  resource, where each feature tag/value pair is an RDF property name
  and string value.

  A "Feature-set" does not have such an obvious RDF representation.
  However, the characterization of RDF as having "predicates" that
  associate some "object value" with a "resource" suggests a
  representation.  Atomic feature value assertions are also
  predicates that relate a feature tag with a value.

  Thus, to fit the RDF model, we can model a feature as an RDF
  "resource", with a tag and one or more value assertions.  The
  following example models a simple description of a VGA display,
  which using 'conneg' notation would be:

     (& (pix-x<=640) (pix-y<=480) (color<=256) )

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  This display might be described by the following RDF:

       <rdf:Description ID="VGA display">

  In this example, the multiple feature values within a "Feature-
  assertion" are taken to be implicitly ANDed together.  Other
  representations are possible.  All 'conneg'-related tags are
  assumed to be defined in a 'conneg' namespace.

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  To take a slightly more complex 'conneg' example:

     (| (& (pix-x<=800) (pix-y<=600) (color<=256) )
        (& (pix-x<=640) (pix-y<=480) (color<=65536) ) )

  This might be represented as:

       <rdf:Description ID="VGA display">
             <rdf:Description ID="Mode-1">
             <rdf:Description ID="Mode-1">
       </rdf:Description ID="VGA display">

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  [[[I am not expert in RDF, but already I can see ways in which the
  above might to improved.  At this stage, the key ideas I wish to
  test are: (a) that feature collections can be modelled directly as
  RDL resource descriptions, (b) that feature sets must be modelled
  as resources in their own right, and (c) that atomic value
  assertions should use the RDF "predicate" to indicate the kind of
  constraint that is being applied.]]]

6. Conclusions

  One of the goals of these proposals is to allow the 'conneg' work
  to integrate seamlessly with ongoing work that is being undertaken
  by W3C.

  I have suggested some constructs and handling rules that I believe
  capture the required concepts embodied in the W3C work, and provide
  a way to manipulate them consistently within the 'conneg' framework
  already proposed.

  [[[Ideally, the extensions to the conneg algebraic framework should
  be subjected to some mathematical analysis to show that application
  of the rules can never yield contradictory results, and that the
  behaviour is consistent with the concept of assumed values that it
  tries to capture.]]]

7. Security considerations

  In general, these proposals are not believed to raise any security
  considerations not already inherrent in the 'conneg', 'RDF' or
  'CC/PP' work.

  One area can be seen where further consideration of security
  concerns might be required.  The use of URLs, per CC/PP, to locate
  'conneg' feature set descriptions might open up some exposure to
  privacy or denial of service attacks on the referenced description.

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8. Full copyright statement

  Copyright (C) The Internet Society 1998.  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain
  it or assist in its implementation may be prepared, copied,
  published and distributed, in whole or in part, without restriction
  of any kind, provided that the above copyright notice and this
  paragraph are included on all such copies and derivative works.
  However, this document itself may not be modified in any way, such
  as by removing the copyright notice or references to the Internet
  Society or other Internet organizations, except as needed for the
  purpose of developing Internet standards in which case the
  procedures for copyrights defined in the Internet Standards process
  must be followed, or as required to translate it into languages
  other than English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on

9. Acknowledgements


10. References

[1]  "Requirements for protocol-independent content negotiation"
     G. Klyne, Integralis Ltd.
     Internet draft: <draft-ietf-conneg-requirements-00.txt>
     Work in progress, March 1998.

[2]  "Media Feature Tag Registration Procedure"
     Koen Holtman, TUE
     Andrew Mutz, Hewlett-Packard
     Ted Hardie, NASA
     Internet draft: <draft-ietf-conneg-feature-reg-03.txt>
     Work in progress, July 1998.

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[3]  "A syntax for describing media feature sets"
     Graham Klyne, 5GM/Content Technologies
     Internet draft: <draft-ietf-conneg-feature-syntax-00.txt>"
     Work in progress, September 1998.

[4]  "Media Features for Display, Print, and Fax"
     Larry Masinter, Xerox PARC
     Koen Holtman, TUE
     Andrew Mutz, Hewlett-Packard
     Dan Wing, Cisco Systems
     Internet draft: <draft-ietf-conneg-media-features-02.txt>
     Work in progress, September 1998.

[5]  "Composite Capability/Preference Profiles (CC/PP):
     A user side framework for content negotiation"
     Franklin Reynolds, Nokia Research
     W3C Note <http://www.w3.org/TR/NOTE-CCPP/>
     November 1998

[6]  "Resource Description Framework (RDF) Model and Syntax
     Ora Lassila, Nokia Research Centre
     Ralph R Swick, World Wide Web Consortium
     W3C working draft: <http://www.w3.org/TR/WD-rdf-syntax>
     October 1998

[7]  "Resource Description Framework (RDF) Schema Specification"
     Dan Brickley, University of Bristol
     R. V. Guha, Netscape
     Andrew Layman, Microsoft
     W3C working draft: <http://www.w3.org/TR/WD-rdf-syntax>
     August 1998.

[8]  "Extensible Markup Language (XML) 1.0"
     Tim Bray, Textuality and Netscape
     Jean Paoli, Microsoft
     C. M. Sperberg-McQueen, University of Illinois at Chicago
     W3C Recommendation: <http://www.w3.org/TR/REC-xml>
     February 1998.

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11. Author's address

  Graham Klyne
  Content Technologies Ltd.        5th Generation Messaging Ltd.
  Forum 1                          5 Watlington Street
  Station Road                     Nettlebed
  Theale                           Henley-on-Thames
  Reading, RG7 4RA                 RG9 5AB
  United Kingdom                   United Kingdom.

  Telephone: +44 118 930 1300      +44 1491 641 641

  Facsimile: +44 118 930 1301      +44 1491 641 611

  E-mail: GK@ACM.ORG

Klyne                                                        [Page 24]