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 (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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