rfc2077.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                        S. Nelson
Request for Comments: 2077                                        LLNL
Category: Standards Track                                     C. Parks
                                                                  NIST
                                                                 Mitra
                                                            WorldMaker
                                                          January 1997


                   The Model Primary Content Type for
                 Multipurpose Internet Mail Extensions

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Introduction

   The purpose of this memo is to propose an update to Internet RFC 2045
   to include a new primary content-type to be known as "model". RFC
   2045 [1] describes mechanisms for specifying and describing the
   format of Internet Message Bodies via content-type/subtype pairs. We
   believe that "model" defines a fundamental type of content with
   unique presentational, hardware, and processing aspects.  Various
   subtypes of this primary type are immediately anticipated but will be
   covered under separate documents.

Table of Contents

      1. Overview.............................................  2
      2. Definition...........................................  2
      3. Consultation Mechanisms..............................  4
      4. Encoding and Transport...............................  5
      5. Security Considerations Section......................  6
      6. Authors' Addresses...................................  7
      7. Expected subtypes....................................  7
      8. Appendix.............................................  9
      9. Acknowledgements..................................... 13









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RFC 2077                Model Primary MIME Types            January 1997


1. Overview

   This document will outline what a model is, show examples of models,
   and discuss the benefits of grouping models together.  This document
   will not directly deal with the intended subtypes since those will be
   covered by their separate registrations.  Some immediately expected
   subtypes are listed in section 7.

   This document is a discussion document for an agreed definition,
   intended eventually to form a standard accepted extension to RFC
   2045.  We are also targeting developers of input/output filters,
   viewer software and hardware, those involved in MIME transport, and
   decoders.

2. Definition of a model

   A model primary MIME type is an electronically exchangeable
   behavioral or physical representation within a given domain.  Each
   subtype in the model structure has unique features, just as does each
   subtype in the other primary types.  The important fact is that these
   various subtypes can be converted between each other with less loss
   of information then to that of other primary types.  This fact groups
   these subtypes together into the model primary type.  All of the
   expected subtypes have several features in common and that are unique
   to this primary type.

   To loosely summarize: models are multidimensional structures composed
   of one or more objects.  If there are multiple objects then one
   object defines the arrangement/setting/relationship of the others.
   These objects all have calibrated coordinate systems but these
   systems need not be in the same units nor need they have the same
   dimensionality.  In detail:

   1. have 3 or more dimensions which are bases of the system and
      form an orthogonal system (any orthogonal system is sufficient).

      This system is specifically defined in terms of an orthogonal
      set of basis functions [for a subspace of the L^2 function space]
      over a coordinate system of dimension 3 or more. Note that this
      does not preclude regular skewed systems, elliptical coordinates,
      different vector spaces, etc.

   2. contain a structural relationship between model elements.

   3. have scaling or calibration factors which are related to physical
      units (force, momentum, time, velocity, acceleration, size, etc.).
      Thus, an IGES file will specify a building of non-arbitrary size,
      computational meshes and VRML models will have real spatial/



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RFC 2077                Model Primary MIME Types            January 1997


      temporal units. This allows for differing elements to be combined
      non-arbitrarily.

   4. Models can be single objects or composed of a collection of
      objects.  These normally independent objects are arranged
      in a master/slave scenario so that one object acts as the
      reference, or primary object, which defines how the other
      objects interrelate and behave.  This allows for the creation
      of mathematical, physical, economic, behavioral, etc. models
      which typically are composed of different elements.  The key is
      in the description: these types describe how something
      "behaves"; contrasted to typical data types which describe
      how something "is".

      The inclusion of this "collective" system works similar to the
      Email system's multipart/related type which defines the actions
      of the individual parts.  Further specification of the model/*
      subtypes utilizing these properties is left to the subtype
      authors.

   With these assumptions:

   a. the default dimensionality will be spatial and temporal (but
      any are allowed).

   b. it is presumed that models will contain underlying structure
      which may or may not be immediately available to the
      user. (fluid dynamics vector fields, electromagnetic
      propagation, interrelated IGES dimensional specifiers, VRML
      materials and operators, etc.)

   c. it is assumed that basis set conversion between model domains
      is lossless.  The interpretation of the data may change but
      the specification will not.  i.e. convert the model of the
      U.S.A.  Gross Domestic Product into a VRML model and navigate
      it to explore the variances and interrelationships.  The model
      has many dimensions but also "passages" and "corridors"
      linking different parts of it.  A similar situation is true
      for meshes and CAD files. The key is identifying the basis set
      conversion which makes sense.

   d. models are grouped to assure LESS loss of information between
      the model subtypes than to subtypes of other primary
      types. (i.e.  converting a chemical model into an image is
      more lossy than concerting it into a VRML model).






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RFC 2077                Model Primary MIME Types            January 1997


   Items c and d above define the grouping for model similar to the way
   that "images" and "videos" are grouped together; to assure less loss
   of information.  Obviously converting from a GIF image to a JPEG
   image looses less information than converting from a GIF image to an
   AU audio file.

3.  Consultation Mechanisms

   Before proposing a subtype for the model/* primary type, it is
   suggested that the subtype author examine the definition (above) of
   what a model/* is and the listing (below) of what a model/* is not.
   Additional consultations with the authors of the existing model/*
   subtypes is also suggested.

   Copies of RFCs are available on:

                        ftp://ftp.isi.edu/in-notes/

   Copies of Internet-Drafts are available on:

                    ftp://ftp.ietf.org/internet-drafts/

   Similarly, the VRML discussion list has been archived as:

                        http://vrml.wired.com/arch/

   and discussions on the comp.mail.mime group may be of interest.
   Discussion digests for the existing model/* subtypes may be
   referenced in the respective documents.

   The mesh community presently has numerous different mesh geometries
   as part of different packages.  Freely available libraries need to be
   advertised more than they have been in the past to spur the
   development of interoperable packages.  It is hoped that by following
   the example of the VRML community and creating a freely available
   comprehensive library of input/output functions for meshes [11] that
   this problem will be alleviated for the mesh community.  A freely
   available mesh viewer conforming to these standards is available now
   for various platforms.  Consulations with the authors of the mesh
   system,

            http://www-dsed.llnl.gov/documents/tests/mesh.html

   will be beneficial.

   The IGES community has a suite of tests and conformance utilities to
   gauge the conformance to specifications and software authors are
   encouraged to seek those out from NIST [14].



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RFC 2077                Model Primary MIME Types            January 1997


4. Encoding and Transport

   a. Unrecognized subtypes of model should at a minimum be treated
      as "application/octet-stream".  Implementations may optionally
      elect to pass subtypes of model that they do not specifically
      recognize to a robust general-purpose model viewing
      application, if such an application is available.

   b. Different subtypes of model may be encoded as textual
      representations or as binary data.  Unless noted in the
      subtype registration, subtypes of model should be assumed to
      contain binary data, implying a content encoding of base64 for
      email and binary transfer for ftp and http.

   c. The formal syntax for the subtypes of the model primary type
      should look like this:

      Media type name:          model
      Media subtype name:       xxxxxxxx
      Required parameters:      none
      Optional parameters:      dimensionality, state
                                (see below)
      Encoding considerations:  base64 encoding is recommended when
                                transmitting model/* documents through
                                MIME electronic mail.
      Security considerations:  see section 5 below
      Published specification:  This document.
                                See Appendix B for references to some of
                                the expected subtypes.
      Person and email address to contact for further information:
                                Scott D. Nelson <[e-mail]>
                                7000 East Ave.
                                Lawrence Livermore National Laboratory
                                Livermore, CA  94550

   The optional parameters consist of starting conditions and variable
   values used as part of the subtypes.  A base set is listed here for
   illustration purposes only and will be covered in detail as part of
   the respective subtypes:

  dimension := string ; a number indicating the number of dimensions.
                        This is used as a "hint" in selecting
                        applicable viewer programs.








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RFC 2077                Model Primary MIME Types            January 1997


  state     := string ; "static" or "dynamic".  In "static", the
                        observer may move about, thus effecting
                        translations, rotations, pans, zooms, etc.
                        but the data does not change.  In "dynamic",
                        the data itself is manipulated via
                        skews, elongations, scales, etc.  Note that
                        time evolution is still a static operation
                        since it is just a translation along one of
                        the principal dimensions while the elongation
                        of a cube or object deformation are dynamic
                        operations.

      Note that this optional parameter list does not limit those
      specified by the various subtypes.

   d. The specific issues relating to the various subtypes are covered
      as part of the description of those specific subtypes.  The
      following is an example of a typical MIME header used for mail
      transport purposes:

         To:   [e-mail]
         From: [e-mail]
         Date: Fri, 30 Aug 96 13:33:19 -0700
         Content-Type: model/mesh; dimension="4"; state="static"
         Content-Transfer-Encoding: base64
         MIME-Version: 1.0
         Subject: model data file

         I1ZSTUwgVjEuMCBhc2NpaQojIFRoaXMgZmlsZSB3YXMgIGdlbmVyY...
         byBDb21tdW5pY2F0aW9ucwojIGh0dHA6Ly93d3cuY2hhY28uY29tC...
         IyB1c2VkIGluIHJvb20gMTkyICh0ZXN0IHJvb20pCiAgIAojIFRvc...
         .
         .
         .

5.  Security Considerations Section

   Note that the data files are "read-only" and do not contain file
   system modifiers or batch/macro commands.  The transported data is
   not self-modifying but may contain interrelationships.  The data
   files may however contain a "default view" which is added by the
   author at file creation time.  This "default view" may manipulate
   viewer variables, default look angle, lighting, visualization
   options, etc.  This visualization may also involve the computation of
   variables or values for display based on the given raw data.  For
   motorized equipment, this may change the position from the hardware's
   rest state to the object's starting orientation.




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RFC 2077                Model Primary MIME Types            January 1997


   The internal structure of the data files may direct agents to access
   additional data from the network (i.e. inclusions); the security
   limits of whom are not pre-supposed.  Actions based on these
   inclusions are left to the security definitions of the inclusions.
   Further comments about the security considerations for the subtypes
   will be contained in each subtype's registration.

6. Authors' Addresses

      S. D. Nelson
      Lawrence Livermore National Laboratory,
      7000 East Ave., L-153,
      Livermore CA 94550, USA.
      E-Mail: [e-mail]

      C. Parks
      National Institute of Standards & Technology
      Bldg 220, Room B-344
      Gaithersburg, MD 20899, USA.
      E-Mail: [e-mail]

      Mitra
      WorldMaker
      1056 Noe