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Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                               P. Coates
Expires: July 27, 2008                                  Sun Microsystems
                                                        January 24, 2008


                    Discovery of CONVERT parameters
            draft-melnikov-lemonade-convert-discovery-00.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This is a companion document to the Lemonade CONVERT
   (draft-ietf-lemonade-convert-XX.txt) extension.  It summarizes
   various proposals for CONVERT MIME type and conversion parameter
   discovery.






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Table of Contents

   1.  Requirements notation . . . . . . . . . . . . . . . . . . . . . 3
   2.  Discovery of available conversions and controlling default
       conversions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     2.1.  Client preferences regarding default conversions:
           MEDIACAPS Command . . . . . . . . . . . . . . . . . . . . . 3
     2.2.  Discovery of available conversions  . . . . . . . . . . . . 4
       2.2.1.  GETMETADATA . . . . . . . . . . . . . . . . . . . . . . 4
       2.2.2.  CONVERSIONS command . . . . . . . . . . . . . . . . . . 6
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9



































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1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.  If a single "C:" or "S:" label applies to
   multiple lines, then the line breaks between those lines are for
   editorial clarity only and are not part of the actual protocol
   exchange.  The five characters [...] means that something has been
   elided.

   [[anchor2: Editorial comments and questions are marked like this.]]


2.  Discovery of available conversions and controlling default
    conversions

2.1.  Client preferences regarding default conversions: MEDIACAPS
      Command

   Arguments:  list of supported MIME types and corresponding conversion
      parameters

   Responses:  none

   Result:   OK - MEDIACAPS command completed
             BAD - unrecognized syntax of an argument

   The client list MIME types and corresponding conversion parameters in
   the order of preference, starting with the most prefered MIME media
   type(s).

   Servers MUST ignore conversion parameters and MIME types that they
   don't recognize.

   If a MEDIACAPS command was issued on a connection and the client has
   requested to perform the default conversion (see section 5 for more
   details), the server MUST use one of the MIME types specified by the
   client in this command as the target MIME type.  The server SHOULD
   use the first MIME type from the ordered list that it supports.

     Example:

     C: A01 MEDIACAPS ("TEXT" "HTML") ("TEXT" "PLAIN" "FORMAT" ("FLOWED"
         "FIXED")) ("TEXT" "*" "CHARSET" ("UTF-8" "US-ASCII")) ("IMAGE"
         ("JPEG" "PNG" "GIF") "PIX-X" "240" "PIX-Y" "320")



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   [[anchor5: The FORMAT conversion parameter is not registered with
   IANA]]

   With such command the client is saying (each parenthesized list
   converted to a sentence): "I do text/html.  I will also do text/
   plain, preferably with format=flowed, but I can handle format=fixed
   too.  For all text media types I do, I can handle a charset of either
   UTF-8 or US-ASCII.  I can handle image/jpeg, image/png, and, least
   preferred, image/gif, and my ideal resolution is 240x320."

   ABNF for this command is as follows:

      mediacaps-cmd = "MEDIACAPS" 1*(SP mediacap)

      mediacap = "(" media-type SP media-subtype
                     *( SP media-param SP  media-param-values ) ")"

      media-type = astring

      media-subtype = DQUOTE "*" DQUOTE /
                      astring /
                      "(" astring *( SP  astring ) ")"
        ;; "*" means all subtypes for the media-type specified
        ;; in the command.
        ;; Otherwise, either the specific subtype or a list of them.

      media-param = astring

      media-param-values = astring / "(" astring *(SP astring) ")"
        ;; Either a single acceptable value or a list of
        ;; acceptable values.

2.2.  Discovery of available conversions

   [[anchor7: Note that only one of the proposals specified in
   subsections of this section will be standardized.]]

2.2.1.  GETMETADATA

   [[anchor9: Proposal # 1]]

   To determine which conversions are supported, server annotations are
   used.  For each MIME format (<type>/<subtype> [MIME-IMT]) that can be
   converted, an annotation with the name "/convert/<type>/<subtype>/
   types" SHOULD exist.  The "value.shared" attribute of this annotation
   contains a semicolon separated list of type/subtype output formats.

   The selection of available conversions MAY be adjustable by the



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   server administrator, and MAY be sensitive to the current user.  The
   selection of available conversions MAY also depend on information
   about the client obtained through a different mechanism outside the
   scope of CONVERT (e.g. dynamically through device description
   mechanisms or when the device was associated to the account).

   For each source MIME type that the client is interested in, it SHOULD
   determine which target conversions are supported by reading the
   "value.shared" attribute.

   In addition to the subtype-specific annotations, a special "wildcard"
   annotation named "/convert/<type>/@/types" MAY be used to reference
   any subtype of <type> media type.  A client that doesn't find an
   "/convert/<type>/<subtype>/types" annotation SHOULD check the value
   of the "/convert/<type>/@/types" annotation.

   Note that names of server annotations are case-sensitive (see
   [METADATA]).  In order to guaranty interoperability, clients and
   servers MUST use the lowercased version of <type> and <subtype> when
   constructing an annotation name described above.

      Example: Discover all image conversions

         C: a GETMETADATA "/convert/image/@/types" value.shared
         S: * METADATA "/convert/image/@/types"
             (value.shared "image/jpeg;image/png;image/gif")
         S: a OK GETMETADATA complete

   The above example shows that the server supports one kind of input
   image transcoding, from image/jpeg to three different outputs: JPEG,
   PNG, and GIF.

   For a given conversion, optional transcoding parameters MAY be
   present.  These are mapped into the "value.shared" attribute in the
   "/convert/<srctype>/<srcsubtype>/<desttype>/<destsubtype>/params"
   annotation.  A client wishing to use a conversion parameter SHOULD
   check if the server will accept it by reading the "value.shared"
   attribute.

      Example: Discover optional parameters for image/jpeg -> image/gif.

         C: a GETMETADATA /convert/image/jpeg/image/gif/params
             "value.shared"
         S: * METADATA /convert/image/jpeg/image/gif/params
               ("value.shared" "pix-x;pix-y")
         S: a OK GETMETADATA complete

   The above example shows that to convert from image/jpeg to image/gif,



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   the transcoding supports the following types of optional parameters:
   pix-x (width), pix-y (height).

   As with conversion types, some "wildcarding" is permitted.  Thus if
   the same parameters are allowed for all conversions to image/gif,
   then the server can store the one metadata value "/convert/@/@/image/
   gif/parameters".

   A client MAY use these values to check whether or not a desired
   conversion is possible, or it might, for example, present the
   parameters as a GUI preferences pane for the user to customize.

   If the client is going to check which conversion parameters are
   available, it MUST read the "value.shared" attribute from the
   following annotations in the following order:

       "/convert/<srctype>/<srcsubtype>/<desttype>/<destsubtype>/params"
       "/convert/<srctype>/@/<desttype>/<destsubtype>/params"
       "/convert/@/@/<desttype>/<destsubtype>/params"

   The client MUST use the "value.shared" attribute value from the first
   existing annotation in the list specified above.

2.2.2.  CONVERSIONS command

   [[anchor11: Proposal # 2]]

   Arguments:  source MIME type
             target MIME type

   Responses:  untagged responses: CONVERSION

   Result:   OK - CONVERSIONS command completed
             BAD - unrecognized syntax of an argument,
                 unexpected extra argument, missing argument, etc.

   The first parameter to the CONVERSIONS command is a source MIME type,
   the second parameter is the target MIME type.  Both parameters are
   partially (e.g. "text/*") or completely ("*") wildcardable.

   Conversions matching the source/target pair and their associated
   conversion parameters are returned in untagged CONVERSIONS responses.
   If source/target doesn't match any conversion supported by the
   server, no CONVERSIONS response is returned.

   Examples:

   For conversion info from GIF to JPEG (no untagged CONVERT would be



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   returned if no conversion was possible):

       C: a CONVERSIONS "image/gif" "image/jpeg"
       S: * CONVERSION "image/gif" "image/jpeg" ("width" "height"
           "depth" "interlaced")
       S: a OK CONVERSIONS completed

   For conversion info from GIF to anything:

       C: b CONVERSIONS "image/gif" *
       S: * CONVERSION "image/gif" "image/jpeg" ("width" "height"
           "depth" "interlaced")
       S: * CONVERSION "image/gif" "image/png" (...)
       [...]
       S: b OK CONVERSIONS completed

   For conversion of anything to JPEG:

       C: c CONVERSIONS * "image/jpeg"
       S: * CONVERSION "image/gif" "image/jpeg" ("width" "height"
           "depth" "interlaced")
       S: * CONVERSION "image/png" "image/jpeg" (...)
       [...]
       S: c OK CONVERSIONS completed

   For conversions from all image formats to all text formats (maybe via
   OCR?):

       C: d CONVERSIONS "image/*" "text/*"
       S: d OK CONVERSIONS completed

   [[anchor12: ABNF is missing for this proposal.]]


3.  IANA Considerations

   TBD if needed.


4.  Security Considerations

   [[anchor15: TBD]]


5.  Acknowledgments

   Authors would also like to thank Dave Cridland for the MEDIACAPS
   command proposal and Dan Karp for the CONVERSIONS command proposal.



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6.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for
              Syntax Specifications: ABNF", RFC 4234, October 2005.

   [METADATA]
              Daboo, C., "IMAP METADATA Extension",
              draft-daboo-imap-annotatemore (work in progress),
              December 2007.

   [MIME-IMT]
              Freed, N. and N. Borenstein, "MIME (Multipurpose Internet
              Mail Extensions) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.


Authors' Addresses

   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com


   Peter Coates
   Sun Microsystems
   185 Falcon Drive
   Whitehorse, YT  Y1A 6T2
   Canada

   Email: peter.coates@Sun.COM










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