Internet Draft: CONVERT A. Melnikov (Ed.)
Document: draft-ietf-lemonade-convert-08 Isode Limited
Intended status: Standard Track R. Cromwell (Ed.)
S. H. Maes (Ed.)
Oracle
Expires: November 2007 May 2007
IMAP CONVERT extension
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
CONVERT defines extensions to IMAP allowing clients to request
adaptation and/or transcoding of attachments. Clients can specify the
conversion details or allow servers to decide based on knowledge of
client capabilities, on user or administrator preferences or its
settings.
Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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].
When describing the general syntax, some definitions are omitted as
they are defined in [RFC3501].
Table of Contents
Status of this Memo ....................................... 1
Copyright Notice........................................... 1
Abstract................................................... 1
Conventions used in this document.......................... 1
Table of Contents.......................................... 2
1. Introduction............................................ 3
2. Relation with other E-mail specifications............... 3
3. Scope of Conversions.................................... 4
4. Discovery with the CAPABILITY and GETMETADATA Commands.. 4
4.1. CAPABILITY......................................... 4
4.2. GETMETADATA ....................................... 4
5. CONVERT extension to BODY and BINARY FETCH data items... 5
6. CONVERT transcoding parameters.......................... 7
6.1. Mandatory Transcoding support...................... 7
6.1.1. Additional features for mobile usage.......... 8
7. FETCH response extensions............................... 8
8. Status responses, Response code extensions.............. 8
9. Formal Syntax........................................... 9
10. IANA Considerations.................................... 11
11. IANA Entry and Attribute registrations ................ 11
11.1. IANA Entry "/convert"............................. 11
11.2. IANA Attribute "types"............................ 12
11.3. IANA Attribute "params"........................... 12
Security Considerations.................................... 12
Normative References....................................... 13
Informative References..................................... 14
Version History............................................ 14
Acknowledgments............................................ 15
Authors' Addresses........................................ 15
Intellectual Property Statement........................... 16
Disclaimer of Validity.................................... 16
Copyright Statement .......................................16
1. Introduction
This document defines the CONVERT extension to IMAP4 [RFC3501].
CONVERT provides adaptation and transcoding of body parts as
needed by the client. Conversion (adaptation,
transcoding) may be requested by the client and performed by the
server on a best effort basis or, when requested by the client,
decided by the server based on server's
knowledge of the client capabilities, user or administrator
preferences or servers settings.
This extension is primarily intended to be useful to mobile clients.
It satisfies requirements specified in [MEMAIL] and [OMA-ME-RD].
A server that supports CONVERT can convert body parts to other
formats to be viewed on a mobile device. The client can explicitly
request a particular conversion or ask the server to select the best
available conversion. When allowed by the client, the server
determines how to convert based on its own strategy (e.g. based on
knowledge of the client as discussed hereafter). If the server
knows the characteristics of the device or can determine them (out of
scope for CONVERT), the attachments can also be optimized for the
capabilities of the devices (e.g. form factor of pictures).
2. Relation with other E-mail specifications
The CONVERT extension does not address conversion during streaming
of attachments.
CONVERT depends on the METADATA extension [METADATA] to support
discovery of supported conversion formats. In addition, to use
CONVERT, the server MUST support the IMAP Binary specification
[RFC3516].
3. Scope of Conversions
Conversions only affect what is sent to the client; the original data
in the message store MUST NOT be altered. This document does not
specify how the server performs conversions.
Note: The requirement that original data be unaltered allows such
data to remain accessible by other clients, permits replies or
forwards of the original documents, permits signature verification
(the converted body parts are not likely to contain any signatures),
and preserves BODYSTRUCTURE and related information.
4. Discovery with the CAPABILITY and GETMETADATA Commands
4.1. CAPABILITY
A server that supports the CONVERT extension MUST return "CONVERT",
"METADATA", and "BINARY" in the CAPABILITY response.
Example: A server that implements CONVERT
C: a001 CAPABILITY
S: * CAPABILITY IMAP4rev1 CONVERT BINARY METADATA [...]
S: a001 OK CAPABILITY completed
4.2. GETMETADATA
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
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 lowercases 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;image/wbmp")
S: a OK GETMETADATA complete
The above example shows that the server supports one kind of input
image transcoding, from image/jpeg to four different outputs: JPEG,
PNG, GIF, and WBMP.
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,
the transcoding supports the following types of option parameters:
pix-x (width), pix-y (height).
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.
This document relies on registry of transcoding parameters
established by [MEDIAFEAT-REG]. The registry can be used to
discover the underlying legal values that these parameters may take.
Additional transcoding parameters, such as defined by [OMA-STI],
are expected to be standardized in the future.
5. CONVERT extension to BODY and BINARY FETCH data items
CONVERT defines a FETCH extension used to transcode
the media type of a MIME part into another media type, and/or the
same media type with different encoding parameters. It adds new
options to the section-spec part of the BODY data item, a new
FETCH data item CONVERT.SIZE, a new FETCH
response data item BODYPARTSTRUCTURE, and new response codes. It is
also expected to work with the IMAP BINARY data item extension, whose
grammar is modified as well. The response to a CONVERT request
always includes a BODYPARTSTRUCTURE.
Each request for a BODY or BINARY FETCH data item that contains
CONVERT MUST result in a FETCH response containing BODY/BINARY
and BODYPARTSTRUCTURE data items containing the same section
specifier (including the CONVERT keyword and its parameters).
This is needed so that the client can match the requested
data items with the received ones. This also allows the client
to request multiple conversions of the same body part in a single
request.
Typically clients will request conversion of leaf body parts. In
addition to support of leaf body part conversion, servers MAY offer
conversion of non-leaf body parts (e.g. conversion from
multipart/related).
Instead of specifying the exact target MIME media type the client
wants to convert to, the client MAY use a special marker NIL (also
known as "default conversion") to request the server to pick a
suitable target media type. This document doesn't describe how the
server makes such choice. For example, the server can know
characteristics of the device through a device description
mechanism, or it can have a prioritized lists of MIME types based
on how widespread they are, how difficult their rendering is, etc.
Note that servers are REQUIRED to support "default conversion"
requests.
CONVERT's syntax is modeled after the HEADER.FIELDS syntax in
[RFC3501], and is generally structured as:
BODY[section-part.CONVERT ("media type/subtype"
(parameters))]
BODY.PEEK[section-part.CONVERT ("media type/subtype"
(parameters))]<partial>
BINARY[section-part.CONVERT ("media type/subtype"
(parameters))]<partial>
BINARY.PEEK[section-part.CONVERT ("media type/subtype"
(parameters))]<partial>
BINARY.SIZE[section-part.CONVERT ("media type/subtype"
(parameters))]<partial>
CONVERT.SIZE[section-part ("media type/subtype"
(parameters))]
Example: The client fetches body part section 3 in the message with
the message sequence number of 2 and asks to have that attachment
converted to pdf format.
C: a001 FETCH 2 BODY[3.CONVERT ("APPLICATION/PDF")]
S: * 2 FETCH (BODYPARTSTRUCTURE[3.CONVERT ("APPLICATION/PDF")]
("APPLICATION" "PDF" () NIL NIL "Base64" 2135 NIL NIL NIL)
BODY[3.CONVERT ("APPLICATION/PDF")] {2135}
<the document in .pdf format>
)
S: a001 OK FETCH COMPLETED
Example: The client requests for conversion of a text/html section
as text/plain and asks for a charset of us-ascii. The server cannot
respect the charset conversion request because there are non-us-ascii
characters in the html code, so it fails the request with tagged NO
response, containing the BADPARAMETERS response code (see section 8).
C: b001 FETCH 2 BODY[3.CONVERT ("text/plain" ("charset"
"us-ascii"))]
S: b001 NO [BADPARAMETERS text/html text/plain (charset)] Source
text has non us-ascii
If the client also specified the "replace-unknown-character"
conversion parameter (see Section 10.2), the same example can look
like this:
C: b001 FETCH 2 BODY[3.CONVERT ("text/plain" ("charset"
"us-ascii" "replace-unknown-character" "TRUE"))]
S: * 2 FETCH (BODYPARTSTRUCTURE[3.CONVERT ("text/plain" ("charset"
"us-ascii" "replace-unknown-character" "TRUE"))] ("TEXT"
"PLAIN" ("charset" "us-ascii") NIL NIL
"Base64" 2135 181 NIL NIL NIL) BODY
[3.CONVERT ("text/plain" ("charset" "us-ascii"
"replace-unknown-character" "TRUE"))] {2135}
<the document in text/plain format with us-ascii
character set>
)
S: b001 OK FETCH COMPLETED
The server replaced non-us-ascii characters with a us-ascii
character such as "?".
Example: The client first requests the converted size of a text/html
body part converted to text/plain:
C: c000 FETCH 2 CONVERT.SIZE[4 ("TEXT/PLAIN" ("CHARSET"
"us-ascii"))]
S: * 2 FETCH (CONVERT.SIZE[4 ("TEXT/PLAIN" ("CHARSET"
"us-ascii"))] 3135)
S: c000 OK FETCH COMPLETED
Later on the client requests 1000 bytes from the converted bodypart,
starting from byte 2001:
C: c001 FETCH 2 BODY[4.CONVERT ("TEXT/PLAIN" ("CHARSET"
"us-ascii"))]<2001.1000>
S: * 2 FETCH (BODYPARTSTRUCTURE[4.CONVERT ("TEXT/PLAIN"
("CHARSET" "us-ascii"))] ("TEXT" "PLAIN" ("charset"
"us-ascii") NIL NIL "7bit" 2135 181 NIL NIL NIL)
BODY[4.CONVERT ("TEXT/PLAIN" ("CHARSET" "us-ascii"))]
<2001>{135}
<bytes 2001 - 2135 of the document in text/plain format>
)
S: c001 OK FETCH COMPLETED
The server MUST respect the target MIME type and transcoding
parameters specified by the client in the transcoding request.
Note that some transcoding parameters can restrict what kind of
conversion is possible, while others can remove some restrictions.
6. CONVERT transcoding parameters
IMAP servers which support CONVERT MAY support additional transcoding
parameters for each media type, as defined by the registry
established by [MEDIAFEAT-REG]. All such servers MUST minally
support recognition of the "charset" [CHARSET-REG] parameter for
text/plain, text/html, text/css, text/csv, text/enriched and
text/xml MIME types.
(Note, a server implementation is not required to implement any
conversion from the text MIME subtypes specified above, except for
the mandatory to implement conversion described in section 6.1.
I.e., a server implementation MUST support the "charset" parameter
for text/csv, only if it supports any conversion from text/csv.)
The following example illustrates how target picture dimensions can
be specified in a CONVERT request using the PIX-X and PIX-Y
parameters defined in [DISPLAY-FEATURES].
C: d001 UID FETCH 100 BINARY[2.CONVERT ("IMAGE/JPG" (
"PIX-X" "128" "PIX-Y" "96"))]
S: * 2 FETCH (UID 100 BODYPARTSTRUCTURE[2.CONVERT (
"IMAGE/JPG" ("PIX-X" "128" "PIX-Y" "96"))] ("IMAGE/JPG"
() NIL NIL "8bit" 4182 NIL NIL NIL)
BINARY[2.CONVERT ("IMAGE/JPG" ("PIX-X" "128" "PIX-Y"
"96"))] ~{4182}
<this part of a document is a rescaled image in
JPG format with width=128, height=96.>
)
S: d001 OK UID FETCH COMPLETED
6.1. Mandatory Transcoding support
A server implementing CONVERT MUST support character set conversions
for the text/plain MIME type, and MUST support character set
conversions from iso-8859-1, iso-8859-2, iso-8859-3, iso-8859-4,
iso-8859-5, iso-8859-6, iso-8859-7, iso-8859-8 and iso-8859-15 to
utf-8.
The server MUST list "text/plain" as an allowed destination
conversion in the "/convert/text/plain/types" annotation. A request
for annotation "/convert/text/plain/text/plain/params" MUST return
"charset" and "replace-unknown-character" (see Section 10.2) as a
supported transcoding parameter.
Servers SHOULD offer additional character encoding conversions where
they make sense as character conversion libraries are generally
available on many platforms.
If the server cannot carry out the charset conversion while
preserving all the characters (i.e. a source character can't be
represented in the target character set), and the
"replace-unknown-character" conversion parameter is not specified
or specified with the value "FALSE", then the server MUST fail the
conversion and MUST return the BADPARAMETERS response code (see
Section 8).
6.1.1. Additional features for mobile usage
This section is non normative.
Based on the expected usage of convert in mobile environments,
server implementors should consider support for the following
conversions:
- Conversion of HTML and XHTML documents to
text/plain in ways that preserve at the minimum the document
structure and tables.
- Image conversions among the types image/gif,
image/jpeg and image/png for at least the following parameters:
o Size limit (i.e. reduce quality),
o width,
o height,
o resize directive (crop, stretch, aspect ratio)
The support for "depth" may also be of interest.
Audio conversion is also of interest but the relevant formats
depend significantly on the usage context.
7. FETCH request/response extensions
7.1. BODYPARTSTRUCTURE FETCH response item
The BODYPARTSTRUCTURE data item is introduced when using the CONVERT
extension. Its data follows the exact syntax specified in the
[RFC3501] BODYSTRUCTURE data item, but contains information for only
the converted part. All information contained in BODYPARTSTRUCTURE
pertains to the state of the part after it is converted, such as the
converted MIME type, sub-type, size, or charset.
Note that the client can expect the returned MIME type to match
the one it requested (as the server is required to obey the requested
MIME type) and can treat mismatch as an error.
7.2. CONVERT.SIZE FETCH request and response item
CONVERT.SIZE[section-part ("media type/subtype" (parameters))]
Requests the converted size of the section (i.e., the size to
expect in response to the corresponding FETCH BODY request).
Note: client authors are cautioned that this might be an
expensive operation for some server implementations.
Needlessly issuing this request could result in degraded
performance due to servers having to calculate the value every
time the request is issued.
8. Status responses, Response code extensions
A syntactically invalid MIME media type SHOULD
generate a BAD tagged response from the server. An unrecognized MIME
media type generates a NO tagged response.
Some transcodings may require parameters. If a transcoding request
with no parameters is sent for a format which requires parameters,
the server MUST reply with a tagged NO response that contains the
MISSINGPARAMETERS response code.
If the server is unable to perform the requested conversion because
a resource is temporary unavailable (e.g., lack of disk space,
temporary internal error, transcoding service down) then the server
MUST return a tagged NO response. The response SHOULD contain the
TEMPFAIL response code (see below).
If the requested conversion cannot be performed because of a
permanent error, for example if a proprietary document format has no
existing transcoding implementation, the server MUST return a tagged
NO response.
Otherwise, the server returns an OK response. The client in
general can tell from the BODYPARTSTRUCTURE response whether or not
its request was honored exactly, but may not know the reasons why.
The following extension response codes are provided for OK and NO
responses to disambiguate those situations:
TEMPFAIL - the transcoding request failed temporarily. It might
succeed later, so the client may retry.
BADPARAMETERS from-concrete-mime-type to-mime-type
"(" convert-params ")" -
the listed parameters were not understood, not valid for the
source/destination MIME type pair, had invalid values or could not be
honored for another reason noted in the human readable text that
follows the response code.
MISSINGPARAMETERS from-concrete-mime-type to-mime-type
"(" convert-params ")" -
the listed parameters are required for conversion of the specified
source MIME type to the destination MIME type, but were not seen in
the request.
9. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (ABNF) notation as used in [ABNF], and incorporates by reference
the Core Rules defined in that document.
This syntax augments the grammar specified in [RFC3501] and
[RFC3516]. Non-terminals not defined in this document can be found
in [RFC3501], [RFC3516], [MIME-MTSRP] and [MEDIAFEAT-REG].
In the ABNF section-msgtext grammar in section 9 of [RFC3501],
Section-msgtext is hereby amended to read:
section-convert = "CONVERT" SP convert-params
section-msgtext =/ section-convert
convert-params = "(" (quoted-to-mime-type / default-conversion)
[SP "(" transcoding-params ")"] ")"
quoted-to-mime-type = DQUOTE to-mime-type DQUOTE
transcoding-params = transcoding-param
*(SP transcoding-param)
transcoding-param = transcoding-param-name SP
transcoding-param-value
transcoding-param-name = astring
; <transcod-param-name-nq> represented as a quoted,
; literal or atom. Note that
; <transcod-param-name-nq> allows for "%" which is
; not allowed in atoms. Such values must be
; represented as quoted or literal.
transcod-param-name-nq = Feature-tag
; Feature-tag is defined in [MEDIAFEAT-REG].
transcoding-param-value = astring
default-conversion = "NIL"
fetch-att =/ "CONVERT.SIZE" convert-size-section
msg-att-static =/ "CONVERT.SIZE" convert-size-section SP number
convert-size-section = "[" section-part SP convert-params "]"
In the ABNF syntax "section-binary" of [RFC3516], is amended to:
section-binary = "[" [section-binary-spec] "]"
section-binary-spec = section-part ["." section-convert] /
section-convert
; Note that conversion of a top level
; multipart/* is allowed.
In the ABNF syntax "msg-att-static" of [RFC3501], is amended to:
msg-att-static =/ "BODYPARTSTRUCTURE" section SP body
In the ABNF syntax "resp-text-code" of [RFC3501], is amended to:
resp-text-code =/ "TEMPFAIL" /
bad-params-resp-code /
missing-params-resp-code /
mimetype-and-params = from-concrete-mime-type SP to-mime-type
SP "(" transcoding-params ")"
; The values can't include the ']' character, as this
; non-terminal is returned in an IMAP response code.
<<Does this include param values? If not, then the value in ()
must be changed to "transcod-param-name-nq *(SP transcod-param-name-nq)"
>>
bad-params-resp-code = "BADPARAMETERS"
1*(SP mimetype-and-params)
missing-params-resp-code = "MISSINGPARAMETERS" SP
1*(SP mimetype-and-params)
In addition, the following ABNF describes the syntax of the
GETMETADATA entries in Section 4.2
convert-entry-req = available-conversions /
available-transcoding-parameters
available-conversions = "/convert/" from-mime-type "/types"
any-mime-type = "@"
from-mime-type = (type-name "/" any-mime-type) /
concrete-mime-type
; "type/@" or "type/subtype"
; type-name is defined in [MIME-MTSRP].
concrete-mime-type = type-name "/" subtype-name
; i.e. "type/subtype".
; type-name and subtype-name
; are defined in [MIME-MTSRP].
<<Are '.' allowed in annotation names? Yes, as long
as the name doesn't contain "priv" or "shared" component,
e.g. foo.priv.bar is disallowed.>>
from-concrete-mime-type = concrete-mime-type
to-mime-type = concrete-mime-type
available-transcoding-parameters = "/convert/"
from-concrete-mime-type "/"
to-mime-type "/params"
; Name of an annotation containing transcoding parameters.
; i.e. /convert/frmtype/frmsubtype/totype/tosubtype/params.
The "value.shared" syntax of any "/convert/<type>/<subtype>/types"
annotation has the following syntax:
types-shared-value = from-concrete-mime-type
*(";" from-concrete-mime-type)
The "value.shared" syntax of any "/convert/<fromtype>/<fromsubtype>/
<totype>/<tosubtype>/params" annotation has the following syntax:
params-shared-value = transcoding-param-name
*(";" transcoding-param-name)
10. IANA Considerations
IMAP4 capabilities are registered by publishing a standards track or
IESG approved experimental RFC. The registry is currently located at
<http://www.iana.org/assignments/imap4-capabilities>. This document
defines the CONVERT IMAP capability. IANA is requested to add this
extension to the IANA IMAP Capability registry.
IANA is also requested to perform registrations as defined in
sections 10.1 and 10.2 of this document.
10.1. IANA Entry and Attribute registrations
The following sections specify IANA registrations for entries and
attributes used in this document.
10.1.1. IANA Entry "/convert"
To: iana@iana.org
Subject: IMAP METADATA Registration
Please register the following IMAP METADATA item:
[x] Entry [ ] Attribute
[ ] Mailbox [x] Server
Name: /convert
Description: All annotations below this one are reserved
for use by [this RFC] and its extensions.
Content-type: text/plain; charset=utf-8
Contact person: Alexey Melnikov
email: alexey.melnikov@isode.com
10.1.2. IANA Entry "/convert/<type>/<subtype>/types"
To: iana@iana.org
Subject: IMAP METADATA Registration
Please register the following IMAP METADATA item:
[x] Entry [ ] Attribute
[ ] Mailbox [x] Server
Name: /convert/<type>/<subtype>/types
Description: Defined in [this RFC], Section 4.2.
Content-type: text/plain; charset=us-ascii
Contact person: Alexey Melnikov
email: alexey.melnikov@isode.com
10.1.3. IANA Entry "/convert/.../params"
To: iana@iana.org
Subject: IMAP METADATA Registration
Please register the following IMAP METADATA item:
[x] Entry [ ] Attribute
[ ] Mailbox [x] Server
Name: /convert/<fromtype>/<fromsubtype>/<totype>/
<tosubtype>/params
Description: Defined in [this RFC], Section 4.2.
Content-type: text/plain; charset=utf-8
Contact person: Alexey Melnikov
email: alexey.melnikov@isode.com
10.2. Registration of replace-unknown-character media type parameter
IANA is requested to add the following registration to the registry
established by RFC 2506.
To: "Media feature tags mailing list"
<media-feature-tags@apps.ietf.org>
Subject: Registration of media feature tag replace-unknown-character
Media feature tag name:
replace-unknown-character
ASN.1 identifier associated with feature tag:
New assignment by IANA
Summary of the media feature indicated by this feature tag:
Allows servers that can perform charset conversion for text/plain
text/html, text/css, text/csv, text/enriched and text/xml MIME
types to replace characters not supported by the target charset
with a fixed character, such as '?'.
Values appropriate for use with this feature tag:
The feature tag is Boolean and may have values of TRUE or FALSE.
When this media feature is not specified in an IMAP CONVERT
request, the default of FALSE is assumed.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms:
IMAP CONVERT extension [RFC XXXX]
Examples of typical use:
C: b001 FETCH 2 BODY[3.CONVERT ("text/plain" ("charset"
"us-ascii" "replace-unknown-character" "TRUE"))]
Related standards or documents:
[RFC XXXX]
[CHARSET-REG]
Considerations particular to use in individual applications,
protocols, services, or negotiation mechanisms:
None
Interoperability considerations: None
Security considerations: None
Additional information:
This media feature only make sense for MIME types that
also support the "charset" media type parameter
[CHARSET-REG].
Name(s) & email address(es) of person(s) to contact for further
information:
Alexey Melnikov <alexey.melnikov@isode.com>
Intended usage:
COMMON
Author/Change controller:
IETF
Requested IANA publication delay:
None
Other information:
None
11. Security Considerations
It is to be noted that some conversions may present security threats
(e.g. converting a document to a damaging executable, exploiting a
buffer overflow in a media codec/parser, or a denial of service
attack against a client or server such as requesting an image be
scaled to extremely large dimensions). Clients should be careful when
requesting conversions or processing transformed attachments. Servers
should avoid dangerous conversions if possible. Whenever possible,
servers should perform verification of the converted
attachments before returning them to the client.
A client can create a carefully crafted bad message with the APPEND
command followed by the FETCH command to attack the server. If the
server's conversion function or library has a security problem,
this could result in provilege escalation or Denial of Service.
On bandwidth limited mobile networks where users pay per data
volumes, spam may become an important issue. It can be mitigated with
appropriate filters and server-side spam prevention tools. These are
of course outside the scope of CONVERT.
Deployments in which the actual transcoding is done outside the IMAP
server in a separate server are recommended to keep the servers in
the same trusted domain (e.g. subnet)
12. References
12.1. Normative References
[METADATA] Daboo, C., "IMAP METADATA Extension",
work in progress, draft-daboo-imap-annotatemore-11, 2007.
[ABNF] D. Crocker, et al. "Augmented BNF for Syntax Specifications:
ABNF", RFC 4234, October 2005.
http://www.ietf.org/rfc/rfc4234
[RFC2119] Brader, S. "Keywords for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
http://www.ietf.org/rfc/rfc2119
[RFC3501] Crispin, M. "IMAP4, Internet Message Access Protocol
Version 4 rev1", RFC 3501, March 2003.
http://www.ietf.org/rfc/rfc3501
[RFC3516] Nerenberg, L. "IMAP4 Binary Content Extension", RFC 3516,
April 2003.
http://www.ietf.org/rfc/rfc3516.txt
[MIME-IMT] Freed, N. and N. Borenstein, "MIME (Multipurpose
Internet Mail Extensions) Part Two: Media Types", RFC 2046,
November 1996.
[MIME-MTSRP] Freed, N. and J. Klensin, "Media Type Specifications
and Registration Procedures", RFC 4288, December 2005.
[MEDIAFEAT-REG] Holtman, K., Mutz, A. and T. Hardie, "Media Feature
Tag Registration Procedure", RFC 2506, BCP 31, March 1999.
[CHARSET-REG] Hoffman, P., "Registration of Charset and Languages
Media Features Tags", RFC 2987, November 2000.
12.2. Informative References
[MEMAIL] Maes, S.H., "Lemonade and Mobile e-mail", draft-maes-
lemonade-mobile-email-xx.txt, (work in progress).
[OMA-ME-RD] Open Mobile Alliance Mobile Email Requirement Document,
work in progress, http://www.openmobilealliance.org/
[OMA-STI] Open Mobile Alliance, Standard Transcoding Interface
Specification, version 1.0, work in progress,
<http://member.openmobilealliance.org/ftp/Public_documents/BAC/STI
/Permanent_documents/OMA-STI-V1_0-20050209-D.zip>.
[DISPLAY-FEATURES] Masinter, L., Holtman, K., Mutz, A. and D. Wing,
"Media Features for Display, Print, and Fax", RFC 2534, March 1999.
13. Acknowledgments
The authors want to specifically acknowledge the excellent criticism
and comments received from Randall Gellens (Qualcomm), Arnt
Gulbrandsen (Oryx), Zoltan Ordogh (Nokia), Ben Last (Emccsoft),
Dan Karp (Zimbra), Pete Resnick (Qualcomm), Chris Newman (Sun) which
improved the quality of the CONVERT specification considerably.
The authors also want to thank all who have contributed key insight
and extensively reviewed and discussed the concepts of CONVERT and
its predecessor P-IMAP. In particular, this includes
the authors of the LCONVERT draft: Rafiul Ahad (Oracle Corporation),
Eugene Chiu (Oracle Corporation), Ray Cromwell (Oracle Corporation),
Jia-der Day (Oracle Corporation), Vi Ha (Oracle Corporation), Wook-
Hyun Jeong (Samsung Electronics Co. LTF), Chang Kuang (Oracle
Corporation), Rodrigo Lima (Oracle Corporation), Stephane H. Maes
(Oracle Corporation), Gustaf Rosell (Sony Ericsson), Jean Sini
(Symbol Technologies), Sung-Mu Son (LG Electronics), Fan Xiaohui
(CHINA MOBILE COMMUNICATIONS CORPORATION (CMCC)), Zhao Lijun (CHINA
MOBILE COMMUNICATIONS CORPORATION (CMCC)).
14. Authors' Addresses
Stephane H. Maes (Editor)
Oracle Corporation
500 Oracle Parkway
M/S 4op634
Redwood Shores, CA 94065
USA
Phone: +1-650-607-6296
Email: stephane.maes@oracle.com
Ray Cromwell (Editor)
Oracle Corporation
500 Oracle Parkway
Redwood Shores, CA 94065
USA
Alexey Melnikov (Editor)
Isode Limited
5 Castle Business Village, 36 Station Road,
Hampton, Middlesex, TW12 2BX, UK
Email: Alexey.Melnikov@isode.com
Version History
Note to RFC-editor: Please delete this section before publication
Release 08
- Updated the document to use the media feature IANA registry
established by RFC 2506
- Allow for conversion to non-leaf body parts
- Removed .STRICT (all conversion is strict now)
- Added replace-unknown-character media feature tag
Release 07
- Made default conversion mandatory for servers to support
- Added CONVERT.SIZE FETCH data item
- Removed INFORMATIONLOSS and SERVEROVERRIDE response codes
- Added TEMPFAIL and MISSINGPARAMETERS response codes
- Addressed editorial comments from Randy Gellens
- Updated examples and ABNF
Release 06
- Allow conversion of non-leaf body parts.
- Clarified that the target MIME type must be obeyed.
- Changed from using new annotation attributes to standard ones
- Major corrections to the ABNF section.
- Disallow /convert/* annotation entry.
- The * character is not allowed in annotation names, so the @
character is used instead.
- Clarified handling of default conversion.
- Updated examples to match ABNF.
- Updated or added missing references.
Release 05
- Client not mandated to support BINARY
- Misc syntax and spelling fixes
- New abstract contributed by Randall Gellens
Release 04
- Remove compression and encryption
- Update to use latest METADATA draft
- Add IANA registrations
Release 03
- Add mandatory character set conversions.
- Add object level compression
- Add object level encryption
Release 02
Fixed a normative example to be informative. Added formal syntax
for BODYPARTSTUCTURE, response text codes, and formal structure
of composite GETANNOTATE values.
Release 01
Corrected some grammatical mistakes. Clarified meaning of
GETTANNOTATION response properties. Changed CONVERT grammar to merge
media type and subtype into a single parameter instead of two
parameters. Clarified that BODYSTRUCTURERESPONSE is always returned
for CONVERT requests. Moved transcoding parameter discussion to main
body from appendix.
Release 00
Initial release published in October 2005 based on draft-maes-
lemonade-lconvert-00 and the comments received at the London face to
face meeting end of September 2005.
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