| Rfc | 5259 |
|---|
| Title | Internet Message Access Protocol - CONVERT Extension |
|---|
| Author | A. Melnikov,
Ed., P. Coates, Ed. |
|---|
| Date | July 2008 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group A. Melnikov, Ed.
Request for Comments: 5259 Isode Ltd
Category: Standards Track P. Coates, Ed.
Sun Microsystems
July 2008
Internet Message Access Protocol - CONVERT Extension
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.
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 on
server settings.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Relation with Other IMAP Specifications . . . . . . . . . . . 4
3.1. CAPABILITY Response . . . . . . . . . . . . . . . . . . . 4
4. Scope of Conversions . . . . . . . . . . . . . . . . . . . . . 4
5. Discovery of Available Conversions . . . . . . . . . . . . . . 4
5.1. CONVERSIONS Command . . . . . . . . . . . . . . . . . . . 4
5.2. CONVERSION Response . . . . . . . . . . . . . . . . . . . 6
6. CONVERT and UID CONVERT Commands . . . . . . . . . . . . . . . 6
7. CONVERT Conversion Parameters . . . . . . . . . . . . . . . . 12
7.1. Mandatory-to-Implement Conversions and Conversion
Parameters . . . . . . . . . . . . . . . . . . . . . . . . 12
7.2. Additional Features for Mobile Usage . . . . . . . . . . . 13
8. Request/Response Data Items to CONVERT/UID CONVERT Commands . 14
8.1. CONVERTED Untagged Response . . . . . . . . . . . . . . . 14
8.2. BODYPARTSTRUCTURE CONVERT Request and Response Item . . . 14
8.3. BINARY.SIZE CONVERT Request and Response Item . . . . . . 15
8.4. AVAILABLECONVERSIONS CONVERT Request and Response Item . . 16
8.5. Implementation Considerations . . . . . . . . . . . . . . 17
9. Status Responses and Response Code Extensions . . . . . . . . 17
10. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 20
11. Manageability Considerations . . . . . . . . . . . . . . . . . 24
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
12.1. Registration of unknown-character-replacement Media
Type Parameter . . . . . . . . . . . . . . . . . . . . . . 25
13. Security Considerations . . . . . . . . . . . . . . . . . . . 26
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
15.1. Normative References . . . . . . . . . . . . . . . . . . . 28
15.2. Informative References . . . . . . . . . . . . . . . . . . 29
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 the
server's knowledge of the client capabilities, user or administrator
preferences, or server settings.
This extension is primarily intended to be useful to mobile clients.
It satisfies requirements specified in [OMA-ME-RD].
A server that supports CONVERT can convert body parts to other
formats to be viewed (for example) 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 (out of scope for
CONVERT) or can determine them (for example, using a conversion
parameter containing device type), converted body parts can also be
optimized for capabilities of the device (e.g., form factor of
pictures). The client is able to control conversions using optional
conversion (also referred to as "transcoding" in this document)
parameters.
This document relies on the registry of conversion parameters
established by [MEDIAFEAT-REG]. The registry can be used to discover
the underlying legal values that these parameters can take.
Additional conversion parameters, such as those defined by [OMA-STI],
are expected to be registered in the future.
2. Conventions Used in This Document
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 [...] mean that something has been
elided.
When describing the general syntax, some definitions are omitted as
they are defined in [RFC3501]. In particular, the term "session" is
used in this document as defined in Section 1.2 of [RFC3501].
3. Relation with Other IMAP Specifications
Conversion of attachments during streaming is out of scope for the
CONVERT extension and is described in a separate Lemonade WG document
[LEM-STREAMING].
A server claiming compliance with this specification MUST support the
IMAP Binary specification [RFC3516].
3.1. CAPABILITY Response
A server that supports the CONVERT extension MUST return "CONVERT"
and "BINARY" in the CAPABILITY response or response code. (Client
and server authors are reminded that the order of tokens returned in
the CAPABILITY response or response code is arbitrary.)
Example: A server that implements CONVERT.
C: a000 CAPABILITY
S: * CAPABILITY IMAP4rev1 CONVERT BINARY [...]
S: a000 OK CAPABILITY completed
4. 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.
5. Discovery of Available Conversions
5.1. CONVERSIONS Command
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 CONVERSIONS command is allowed in Authenticated and Selected IMAP
states.
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 CONVERSION responses.
If source/target doesn't match any conversion supported by the
server, no CONVERSION response is returned.
Examples:
For conversion information from GIF to JPEG image format (no untagged
CONVERSION response would be returned if no conversion is possible):
C: a CONVERSIONS "image/gif" "image/jpeg"
S: * CONVERSION "image/gif" "image/jpeg" ("pix-y" "pix-x"
"image-interleave")
S: a OK CONVERSIONS completed
For conversion information from GIF image format to anything:
C: b CONVERSIONS "image/gif" "*"
S: * CONVERSION "image/gif" "image/jpeg" ("pix-y" "pix-x"
"image-interleave")
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" ("pix-y" "pix-x"
"image-interleave")
S: * CONVERSION "image/png" "image/jpeg" (...)
[...]
S: c OK CONVERSIONS completed
For conversions from all image formats to all text formats, the
client can issue the following command:
C: d CONVERSIONS "image/*" "text/*"
5.2. CONVERSION Response
Contents: source MIME type
target MIME type
optional list of supported conversion parameters
As a result of executing a CONVERSIONS command, the server can return
one or more CONVERSION responses. Each CONVERSION response specifies
which source MIME type can be converted to the target MIME type, and
also lists supported conversion parameters.
6. CONVERT and UID CONVERT Commands
Arguments: sequence set
conversion parameters
CONVERT data item names
Responses: untagged responses: CONVERTED
Result: OK - convert completed
NO - convert error: can't fetch and/or convert that data
BAD - unrecognized syntax of an argument, unexpected extra
argument, missing argument, etc.
The CONVERT extension defines CONVERT and UID CONVERT commands that
are used to transcode the media type of a MIME part into another
media type, and/or the same media type with different encoding
parameters. These commands are structured and behave similarly to
FETCH/UID FETCH commands as extended by [RFC3516]:
o A successful CONVERT/UID CONVERT command results in one or more
untagged CONVERTED responses (one per message). They are similar
to the untagged FETCH responses. Note that a single CONVERT/ UID
CONVERT command can only perform a single type of conversion as
defined by the conversion parameters. A client that needs to
perform multiple different conversions needs to issue multiple
CONVERT/UID CONVERT commands. Such a client MAY pipeline them.
o BINARY[...] data item requests conversion of a body part or of the
whole message according to conversion parameters and requests that
the converted message/body part be returned as binary.
o BINARY.SIZE data item is similar to RFC822.SIZE, but it requests
size of a converted body part/message.
o BODYPARTSTRUCTURE data item is similar to BODYSTRUCTURE FETCH data
item, but it returns the MIME structure of the converted body
part.
o BODY[...HEADER] encoded words in the requested headers are
converted to the specified charset. The CHARSET parameter is
REQUIRED for this conversion.
o BODY[...MIME] encoded words in the requested headers are converted
to the specified charset. The CHARSET parameter is REQUIRED for
this conversion.
o AVAILABLECONVERSIONS data item requests the list of target MIME
types the specified body part (or the whole message) can be
converted to.
The CONVERT extension also adds one new response code. See Section 9
for more details.
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
exactly the server makes such a choice; however, some basic
guidelines are described in this paragraph. If the server knows
characteristics of the device using an in-band (such as device type
specified in a conversion parameter) or an out-of-band mechanism,
then it should convert the request body part to a media type the
device is likely to support and display/play successfully. Unless
specifically overridden by a conversion parameter, the server MAY
also remove any unnecessary detail that exceeds the capabilities of
the device (e.g., scaling images to just fit on the device's screen).
In the absence of any in-band or out-of-band mechanism for
determining device characteristics, the server should convert the
request body part to the most standard or widely deployed media type
available in that media category, for example, to convert to text/
plain, image/jpeg. In such case, the server should minimize quality
loss. Servers are REQUIRED to support "default conversion" requests.
Server implementations that support conversions to multiple target
MIME types SHOULD make the default conversion configurable. Clients
SHOULD avoid using the default conversion unless they provided a way
(in-band or out-band) to signal their capabilities to the server, as
there is no guaranty that the server would guess their capability
correctly. Client implementors should consider using
AVAILABLECONVERSIONS CONVERT data item or CONVERSIONS command instead
of the default conversion.
CONVERT's command syntax is modeled after the FETCH command syntax in
[RFC3501], as extended by [RFC3516]. CONVERT data items are
generally structured as:
BINARY[section-part]<partial>
BINARY.SIZE[section-part]
BODYPARTSTRUCTURE[section-part]
BODY[HEADER]
BODY[section-part.HEADER]
BODY[section-part.MIME]
AVAILABLECONVERSIONS[section-part]
The semantics of a partial CONVERT BINARY[...] command is the same as
for a partial FETCH BODY[...] command, with the exception that the
<partial> arguments refer to the TRANSCODED and DECODED section data.
Note that unlike the FETCH command, the CONVERT command never sets
the \Seen flag on converted messages. A client wishing to mark a
message with the \Seen flag would need to issue a STORE command
(possibly pipelined with the CONVERT request) to do that.
The UID CONVERT command is different from the CONVERT command in the
same way as the UID FETCH command is different from the FETCH
command:
o UID CONVERT takes as a parameter a sequence of UIDs instead of a
sequence of message numbers.
o UID CONVERT command MUST result in the UID data item in a
corresponding CONVERTED response.
o An EXPUNGE response MUST NOT be sent while responding to a CONVERT
command. This rule is necessary to prevent a loss of
synchronization of message sequence numbers between client and
server. Note that an EXPUNGE response MAY be sent during a UID
CONVERT command.
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 CONVERT 2 ("APPLICATION/PDF") BINARY[3]
S: * 2 CONVERTED (TAG "a001") (BINARY[3] {2135}
<the document in .pdf format>
)
S: a001 OK CONVERT COMPLETED
Example: The client requests for conversion of a text/html body part
to 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 text/html body part, so it fails the request by
returning an ERROR phrase in place of the converted data (see
Section 9).
C: b001 CONVERT 2 ("text/plain" ("charset" "us-ascii")) BINARY[3]
S: * 2 CONVERTED (tag "b001") (BINARY[3]
(ERROR "Source text has non us-ascii" BADPARAMETERS
"text/html" "text/plain" ("charset" "us-ascii")))
S: b001 NO All conversions failed
If the client also specified the "unknown-character-replacement"
conversion parameter (see Section 12.1), the same example can look
like this:
C: b001 CONVERT 2 ("text/plain" ("charset" "us-ascii"
"unknown-character-replacement" "?")) BINARY[3]
S: * 2 CONVERTED (TAG "b001") (BINARY[3] {2135}
<the document in text/plain format with us-ascii
charset>
)
S: b001 OK CONVERT 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 CONVERT 2 ("TEXT/PLAIN" ("CHARSET" "us-ascii"))
BINARY.SIZE[4]
S: * 2 CONVERTED (TAG "c000") (BINARY.SIZE[4] 3135)
S: c000 OK CONVERT COMPLETED
Later on, the client requests 1000 bytes from the converted body
part, starting from byte 2001:
C: c001 CONVERT 2 ("TEXT/PLAIN" ("CHARSET" "us-ascii"))
BINARY[4]<2001.1000>
S: * 2 CONVERTED (TAG "c001") (BINARY[4]<2001> {135}
<bytes 2001 - 2135 of the document in text/plain format>
)
S: c001 OK CONVERT COMPLETED
The server MUST respect the target MIME type and conversion
parameters specified by the client in the transcoding request. Note
that some conversion parameters can restrict what kind of conversion
is possible, while others can remove some restrictions.
It is legal for a client to request conversion of a non-leaf body
part, for example, to request conversion of a multipart/* into a PDF
document. However, servers implementing this extension are not
required to support such conversions. Servers that support such
conversions MUST return one or more CONVERSION responses in response
to a 'CONVERSIONS "multipart/*" "*"' command. See Section 5.1 for
more details.
The client can request header conversions using the BODY[...HEADER]
CONVERT request, for example
C: D001 FETCH 2 BODY[HEADER]
S: * 2 FETCH (BODY[HEADER] {158}
S: Date: Mon, 20 Apr 2007 20:05:43 +0200
S: From: Peter <peter@siroe.example.com>
S: To: Alexey <alexey@siroe.example.com>
S: Subject: =?KOI8-R?Q?why encode this?=
S:
S: )
S: D001 OK
C: D002 CONVERT 2 (NIL ("CHARSET" "utf-8")) BODY[HEADER]
S: * 2 CONVERTED (TAG "d002") (BODY[HEADER] {157}
S: Date: Mon, 20 Apr 2007 20:05:43 +0200
S: From: Peter <peter@siroe.example.com>
S: To: Alexey <alexey@siroe.example.com>
S: Subject: =?UTF-8?Q?why encode this?=
S:
S: )
S: D002 OK
Any such request MUST include the CHARSET parameter. Upon receipt of
the request, the server MUST decode any encoded words (as described
in [RFC2047]) in headers and return them re-encoded in the specified
charset. (Note that encoded-words might not be needed if the result
can be represented entirely in US-ASCII, so the server MAY replace
the resulting encoded-words with their pure US-ASCII representation.)
If the server can't decode any particular encoded word, for example,
if the charset or encoding is not recognized, it MUST leave them as
is. Servers SHOULD also support decoding of any parameters as
described in [RFC2231]. Support for RFC 2231 parameters might
require reformatting of header fields during conversion. Consider
the following
C: D011 FETCH 3 BODY[1.MIME]
S: * 3 FETCH (BODY[1.MIME] {118}
S: Content-Type: text/plain; charset=utf-8;
S: foo*0*=utf-8'fr'tr%c0;
S: foo*1*(very)=%03s_m%c0;
S: foo*2*=(nasty)%09chant
S:
S: D011 OK
The server should preserve the headers during the conversion as much
as possible. In case the characters are split (legally!) between
fragments of an encoded parameter, the server MUST consolidate the
parameter fragments, and convert, emit, and re-fragment them as
necessary in order to keep the line length less than 78. Comments
embedded like this SHOULD be preserved during conversion, but clients
MUST gracefully handle the situation where comments are removed
entirely. If the comments are preserved, they MAY be moved after the
parameter. For example (continuing the previous example):
C: D012 CONVERT 3 (NIL) BODY[1.MIME]
S: * 3 CONVERTED (TAG "D012") (BODY[1.MIME] {109}
S: Content-Type: text/plain; charset=utf-8;
S: foo*0*=utf-8'fr'tr%c0%03s_;
S: foo*1*=%m%c0%09chant (very)(nasty)
S:
S: D012 OK
No destination MIME type MUST be specified with BODY[HEADER],
BODY[section.HEADER], or BODY[section.MIME]. That is, BODY[HEADER],
BODY[section.HEADER], or BODY[section.MIME] can only be used with the
"default conversion". When performing these conversions, the server
SHOULD leave encoded words as encoded words. A failure to do so may
alter the semantics of structured headers.
7. CONVERT Conversion Parameters
The registry established by [MEDIAFEAT-REG] defines names of
conversion parameters that can be used in the CONVERT command.
Support for some conversion parameters is mandatory, as described in
Section 7.1.
According to [MEDIAFEAT-REG], conversion parameter names are case-
insensitive.
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 [DISP-FEATURES].
C: e001 UID CONVERT 100 ("IMAGE/JPEG" ("PIX-X" "128"
"PIX-Y" "96")) BINARY[2]
S: * 2 CONVERTED (TAG "e001") (UID 100 BINARY[2] ~{4182}
<this part of a document is a rescaled image in
JPEG format with width=128, height=96.>
)
S: e001 OK UID CONVERT COMPLETED
7.1. Mandatory-to-Implement Conversions and Conversion Parameters
A server implementing CONVERT MUST support charset conversions for
the text/plain MIME type, and MUST support charset 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 from "text/plain" MIME type (see Section 5.1). A command
'CONVERSIONS "text/plain" "text/plain"' MUST also return "charset"
and "unknown-character-replacement" (see Section 12.1) as supported
conversion parameters in the corresponding CONVERSION response.
IMAP servers implementing the CONVERT extension MUST 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 support any
conversion from the text MIME subtypes specified above, except for
the mandatory-to-implement conversion described above. That is, a
server implementation MUST support the "charset" parameter for text/
csv, only if it supports any conversion from text/csv.
The server MUST support decoding of [RFC2047] headers and their
conversion to UTF-8 as long as the encoded words are in one of the
supported charsets.
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 charset), and the "unknown-character-
replacement" conversion parameter is not specified, then the server
MUST fail the conversion and MUST return the untagged ERROR
BADPARAMETERS response (see Section 9). If the value specified in
the "unknown-character-replacement" conversion itself can't be
represented in the target charset, then the server MUST also fail the
conversion and MUST return the untagged ERROR BADPARAMETERS response
(see Section 9).
7.2. Additional Features for Mobile Usage
This section is informative.
Based on the expected usage of CONVERT in mobile environments, server
implementors should consider support for the following conversions:
o Conversion of HTML and XHTML documents to text/plain in ways that
preserve at the minimum the document structure and tables.
o Image conversions among the types image/gif, image/jpeg, and
image/png for at least the following parameters:
* size limit (i.e., reduce quality)
* width ("pix-x" parameter)
* height ("pix-y" parameter)
* 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.
8. Request/Response Data Items to CONVERT/UID CONVERT Commands
8.1. CONVERTED Untagged Response
Contents: convert correlator
CONVERTED return data items
The CONVERTED response may be sent as a result of a successful,
partially successful, or unsuccessful CONVERT or UID CONVERT command
specified in Section 6.
The CONVERTED response starts with a message number, followed by the
"CONVERTED" label. The label is followed by a convert correlator,
which contains the tag of the command that caused the response to be
returned. This can be used by a client to match a CONVERTED response
against a corresponding CONVERT/UID CONVERT command.
The convert correlator is followed by a list of one or more CONVERT
return data items. If the UID data item is returned, it MUST be
returned as the first data item in the CONVERTED response. This
requirement is to simplify client implementations. See Section 10
and the remainder of Section 8 for more details.
8.2. BODYPARTSTRUCTURE CONVERT Request and Response Item
BODYPARTSTRUCTURE[section-part]
The CONVERT extension defines the BODYPARTSTRUCTURE CONVERT data
item. Data contained in the BODYPARTSTRUCTURE return data item
follows the exact syntax specified in the [RFC3501] BODYSTRUCTURE
data item, but only contains information for 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.
The returned BODYPARTSTRUCTURE data MUST match the BINARY data
returned for exactly the same conversion in the same IMAP "session".
This requirement allows a client to request BODYPARTSTRUCTURE and
BINARY data in separate commands in the same IMAP session.
If the client lists a BODYPARTSTRUCTURE data item for a section-part
before a BINARY data item for the same section-part, then, in the
CONVERTED response, the server MUST return the BODYPARTSTRUCTURE data
prior to the corresponding BINARY data. Also, any BODYSTRUCTURE data
items MUST be after the UID data item if the UID data item is
present. Both requirements are to simplify handling of converted
data in clients.
Example:
C: e002 CONVERT 2 (NIL ("PIX-X" "128" "PIX-Y" "96")) (BINARY[2]
BODYPARTSTRUCTURE[2])
S: * 2 CONVERTED (TAG "e002") (BODYPARTSTRUCTURE[2] ("IMAGE"
"JPEG" () NIL NIL "8bit" 4182 NIL NIL NIL) BINARY[2]
~{4182}
<this part of a document is a rescaled image in
JPEG format with width=128, height=96.>
)
S: e002 OK CONVERT COMPLETED
8.3. BINARY.SIZE CONVERT Request and Response Item
BINARY.SIZE[section-part]
This item requests the converted size of the section (i.e., the size
to expect in a response to the corresponding CONVERT BINARY request).
The returned value MUST be exact and MUST NOT change during a
duration of an IMAP "session", unless the message is expunged in
another session (see below). This allows a client to download a
converted part in chunks (using "<partial>"). This requirement means
that in most cases the server needs to perform conversion of the
requested body part before returning its size.
If the message is expunged in another session, then the server MAY
return the value 0 in response to the BINARY.SIZE request item later
in the same session.
In order to allow for upgrade of server transcoding components,
clients MUST NOT assume that repeating a particular body part
conversion in another IMAP "session" would yield the same result as a
previous conversion of the very same body part -- any characteristics
of the converted body part might be different (format, size, etc.).
In particular, clients MUST NOT cache sizes of converted messages/
body parts beyond duration of any IMAP "session", or use sizes
obtained in one connection in another IMAP connection to the same
server.
Historical note: Previous experience with IMAP servers that returned
estimated RFC822.SIZE value shows that this caused interoperability
problems. If the server returns a value that is smaller than the
actual size, this will result in data truncation if <partial>
download is used. If the server returns a value that is bigger than
the actual size, this might mislead a client to believe that it
doesn't have enough storage to download a body part.
Note for client implementors: client authors are cautioned that this
might be an expensive operation for some server implementations.
Requesting BINARY.SIZE for a large number of converted body parts or
for multiple conversions of the same body part can result in slow
performance and/or excessive server load and is discouraged. Client
implementors should consider implementation approaches that limit
this request to only the most necessary cases and are encouraged to
test the performance impact of BINARY.SIZE with multiple server
implementations.
8.4. AVAILABLECONVERSIONS CONVERT Request and Response Item
AVAILABLECONVERSIONS[section-part] allows the client to request the
list of target MIME types the specified body part of a message or the
whole message can be converted to. This data item is only useful
when the default conversion (see Section 6) is requested.
This data item MUST return a list of target MIME types that is a
subset of the list returned by the CONVERSIONS command for the same
source and target MIME type pairs. If specific conversion is
requested, it MUST return the target MIME type as requested in the
CONVERT command, or the ERROR phrase.
For both specific or default conversion requests, if conversion
parameters are specified, then the server must take them into
consideration when generating the list of target MIME types. For
example, if one or more of the conversion parameters doesn't apply to
a potential target MIME type, then such MIME type MUST be omitted
from the resulting list. If the server only had a single target MIME
type candidate and it was discarded due to the list of conversion
parameters, then the server SHOULD return the ERROR phrase instead of
the empty list of the target MIME types.
The AVAILABLECONVERSIONS request SHOULD be processed quickly if
specified by itself. Note that if a MIME type is returned in
response to the AVAILABLECONVERSIONS, there is no guaranty that the
corresponding BINARY/BINARY.SIZE/BODYPARTSTRUCTURE CONVERT request
will not fail.
Example:
C: f001 CONVERT 2 (NIL) (AVAILABLECONVERSIONS[2])
S: * 2 CONVERTED (TAG "f001") (AVAILABLECONVERSIONS[2]
(("IMAGE/JPEG" "application/PostScript"))
S: f001 OK CONVERT COMPLETED
8.5. Implementation Considerations
Note that this section is normative.
Servers MAY refuse to execute conversion requests that convert
multiple messages and/or body parts at once, e.g., a conversion
request that specifies multiple message numbers/UIDs. If the server
refuses a conversion because the request lists too many messages, the
server MUST return the MAXCONVERTMESSAGES response code (see
Section 9). For example:
C: g001 CONVERT 1:* ("text/plain" ("charset" "us-ascii"))
BINARY[3]
S: g001 NO [MAXCONVERTMESSAGES 1]
If the server refuses a conversion because the request lists too many
body parts, the server MUST return the MAXCONVERTPARTS response code
(see Section 9). For example:
C: h001 CONVERT 1 ("text/plain" ("charset" "us-ascii"))
(BINARY[1] BINARY[2])
S: g001 NO [MAXCONVERTPARTS 1] You can only request 1 body part at
any given time
Note for server implementors: In order to improve performance,
implementations SHOULD cache converted body parts. For example, the
server may perform a body part conversion when it receives the first
BINARY.SIZE[...], BODYPARTSTRUCTURE[...], or BINARY[...] request and
cache it until the client requests conversion/download of another
body part, a different conversion of the same body part, or until the
mailbox is closed. In order to mitigate denial-of-service attacks
from misbehaving or badly-written clients, a server SHOULD limit the
number of converted body parts it can cache. Servers SHOULD be able
to cache at least 2 conversions at any given time.
9. Status Responses and 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 will return an ERROR MISSINGPARAMETERS phrase in place of
the data associated with the data items requested. This is analogous
to the NIL response in FETCH, but with structured data associated
with the failure.
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 that SHOULD contain the TEMPFAIL
response code (see below), or an ERROR TEMPFAIL phrase.
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
CONVERTED response containing a ERROR BADPARAMETERS or ERROR
MISSINGPARAMETERS phrase.
The server MAY choose to return one ERROR phrase for a single
conversion if several related data items are requested. For
instance:
C: b002 CONVERT 2 ("text/plain" ("charset" "us-ascii"))
(BINARY[3] BODYPARTSTRUCTURE[3])
S: * 2 CONVERTED (tag "b002") (BODYPARTSTRUCTURE[3]
(ERROR "Source text has non us-ascii" BADPARAMETERS
"text/html" "text/plain" ("charset" "us-ascii")))
S: b002 NO All conversions failed
If at least one conversion succeeds, the server MUST return an OK
response. If all conversions fail, the server MAY return OK or NO.
For instance:
C: b002 CONVERT 2 ("text/plain" ("charset" "us-ascii"))
(BINARY[3] BODYPARTSTRUCTURE[3] BINARY[4]
BODYPARTSTRUCTURE[4])
S: * 2 CONVERTED (tag "b002") (BODYPARTSTRUCTURE[3]
(ERROR "Source text has non us-ascii" BADPARAMETERS
"text/html" "text/plain" ("charset" "us-ascii"))
BODYSTRUCTURE[4] ("TEXT" "PLAIN" (CHARSET US-ASCII)
NIL NIL "8bit" 4182 NIL NIL NIL) BINARY[4] {4182}
<body in text plain>
)
S: b002 OK Some conversions failed
In general, the client can tell from the BODYPARTSTRUCTURE response
whether or not its request was honored exactly, but may not know the
reasons why.
This document defines the following response codes that can be
returned in the tagged NO response code.
TEMPFAIL - The transcoding request failed temporarily. It might
succeed later, so the client MAY retry.
MAXCONVERTMESSAGES <number> - The server is unable or unwilling to
convert more than <number> messages in any given CONVERT/UID
CONVERT request.
MAXCONVERTPARTS <number> - The server is unable or unwilling to
convert more than <number> body parts of a message at once in
any given CONVERT/UID CONVERT request.
The word ERROR is always followed by an informal human-readable
descriptive text, which is followed by the convert-error-code. The
convert-error-code MUST be one of the following:
TEMPFAIL mm - The transcoding request failed temporarily. It might
succeed later, so the client MAY retry. The client SHOULD wait
for at least mm minutes before retrying.
BADPARAMETERS from-concrete-mime-type to-mime-type
"(" transcoding-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 was specified after the ERROR label. The
transcoding-params can be omitted, in which case, it means that
the conversion from the from-concrete-mime-type to the to-mime-
type is not possible. If the from-concrete-mime-type is NIL,
this means that the specified body part doesn't exist. All
unrecognized or irrelevant parameters MUST be listed in the
transcoding-params. It is not legal behavior to ignore
irrelevant parameters.
Note that if the client requested the "default conversion" (see
Section 6), the to-mime-type contains the destination MIME type
chosen by the server.
MISSINGPARAMETERS from-concrete-mime-type to-mime-type
"(" transcoding-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. Note that if the client
requested the "default conversion" (see Section 6), the to-
mime-type contains the destination MIME type chosen by the
server.
Examples:
C: b002 CONVERT 2 ("APPLICATION/PDF") BINARY[3]
S: b002 NO [TEMPFAIL] All conversions failed
C: b003 CONVERT 2 ("TEXT/PLAIN") BINARY[3]
S: * 2 CONVERTED (tag "b003") (BINARY[3]
(ERROR "CHARSET must be specified for text conversions"
MISSINGPARAMETERS (CHARSET)))
S: b003 NO All conversions failed
C: b005 CONVERT 2 ("TEXT/PLAIN" (CHARSET "US-ASCII"
UNKNOWN-CHARACTER-REPLACEMENT "<badchar>")) BINARY[3]
S: * 2 CONVERTED (tag "b005") (BINARY[3]
(ERROR "UNKNOWN-CHARACTER-REPLACEMENT limited to 4
bytes" BADPARAMETERS (UNKNOWN-CHARACTER-REPLACEMENT
"<badchar>")))
S: b005 NO All conversions failed
10. 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], [IMAPABNF], [MIME-MTSRP], and
[MEDIAFEAT-REG].
command-select =/ convert
uid =/ "UID" SP convert
; Unique identifiers used instead of message
; sequence numbers
convert = "CONVERT" SP sequence-set SP convert-params SP
( convert-att /
"(" convert-att *(SP convert-att) ")" )
convert-att = "UID" /
"BODYPARTSTRUCTURE" section-convert /
"BINARY" section-convert [partial] /
"BINARY.SIZE" section-convert /
"BODY[HEADER]" /
"BODY[" section-part ".HEADER]" /
"BODY[" section-part ".MIME]" /
"AVAILABLECONVERSIONS" section-convert
; <partial> is defined in [RFC3516].
; <section-part> is defined in [RFC3501].
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-names = transcoding-param-name
*(SP transcoding-param-name)
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"
message-data =/ nz-number SP "CONVERTED" SP convert-correlator
SP convert-msg-attrs
convert-correlator = "(" "TAG" SP tag-string ")"
tag-string = string
; tag of the command that caused
; the CONVERTED response, sent as
; a string.
convert-msg-attrs = "(" convert-msg-att *(SP convert-msg-att) ")"
; "UID" MUST be the first data item, if present.
convert-msg-att = msg-att-semistat / msg-att-conv-static
msg-att-conv-static = "UID" SP uniqueid
; MUST NOT change for a message
msg-att-semistat =
( "BINARY" section-convert ["<" number ">"] SP
(nstring / literal8 / converterror-phrase) ) /
( "BINARY.SIZE" section-convert SP
(number / converterror-phrase) ) /
( "BODYPARTSTRUCTURE" section-convert SP
(body / converterror-phrase) ) /
( "AVAILABLECONVERSIONS" section-convert SP
(mimetype-list / converterror-phrase) )
; MUST NOT change during an IMAP "session",
; but not necessarily static in the long term.
section-convert = section-binary
; <section-binary> is defined in [RFC3516].
;
; Note that unlike [RFC3516], conversion
; of a top level multipart/* is allowed.
resp-text-code =/ "TEMPFAIL" /
"MAXCONVERTMESSAGES" SP nz-number /
"MAXCONVERTPARTS" SP nz-number
; <resp-text-code> is defined in [RFC3501].
mimetype-and-params = quoted-to-mime-type
[SP "(" transcoding-params ")"]
; always includes a specific MIME type
mimetype-list = "(" "(" [quoted-to-mime-type
*(SP quoted-to-mime-type)] ")" ")"
; Unordered list of MIME types. It can be empty.
;
; Two levels of parenthesis is needed to distinguish this
; data from <converterror-phrase>.
converterror-phrase = "(" "ERROR" SP
convert-err-descript SP convert-error-code ")"
convert-error-code = "TEMPFAIL" [SP nz-number]
/ bad-params
/ missing-params
convert-err-descript = string
; Human-readable text explaining the conversion error.
; The default charset is US-ASCII, unless
; the LANGUAGE command [IMAP-I18N] is called, when
; the charset changes to UTF-8.
quoted-from-mime-type = DQUOTE from-concrete-mime-type DQUOTE
bad-params = "BADPARAMETERS"
1*(SP (quoted-from-mime-type / nil)
SP mimetype-and-params)
; nil is only returned when the body part doesn't exist.
missing-params = "MISSINGPARAMETERS"
1*(SP quoted-from-mime-type SP
mimetype-and-missing-params)
mimetype-and-missing-params = quoted-to-mime-type
"(" transcoding-param-names ")"
; always includes a specific MIME type
concrete-mime-type = type-name "/" subtype-name
; i.e., "type/subtype".
; type-name and subtype-name
; are defined in [MIME-MTSRP].
from-concrete-mime-type = concrete-mime-type
to-mime-type = concrete-mime-type
command-auth =/ conversions-cmd
conversions-cmd = "CONVERSIONS" SP from-mime-type-req SP
to-mime-type-req
from-mime-type-req = astring
; "mime-type-req" represented as IMAP <atom>,
; <quoted> or <literal>
to-mime-type-req = astring
; <mime-type-req> represented as IMAP <atom>,
; <quoted> or <literal>.
; Note that <mime-type-req> allows for "*",
; which is not allowed in <atom>. Such values must
; be represented as <quoted> or <literal>.
any-mime-type = "*"
mime-type-req = any-mime-type /
(type-name "/" any-mime-type) /
concrete-mime-type
; '*', 'type/*' or 'type/subtype'.
; type-name is defined in [MIME-MTSRP].
response-payload =/ conversion-data
conversion-data = "CONVERSION" SP quoted-from-mime-type SP
quoted-to-mime-type
[SP "(" transcoding-param-name
*(SP transcoding-param-name) ")"]
11. Manageability Considerations
The monitoring of CONVERT operation is similar to monitoring of the
IMAP FETCH operation.
At the time of writing this document, there is no standard IMAP MIB
defined. Similarly, a standard MIB for monitoring CONVERT operations
and their failures does not exist. However, the authors believe that
in the absence of such a MIB, server implementations SHOULD provide
operators with tools to report the following information:
o which conversions (source and target MIME types and possibly
conversion parameters used) are invoked more frequently and how
long they take,
o information about conversion errors and which error condition
caused them (see Section 9), and
o information about users which invoke conversion operation.
This information can help operators to detect client abuse of this
extension and scalability issues that might arise from its use.
Standardizing these tools may be the subject of future work.
12. IANA Considerations
IMAP4 capabilities are registered by publishing a Standards Track or
IESG-approved Experimental RFC. This document defines the CONVERT
IMAP capability. IANA has added this extension to the IANA IMAP
Capability registry.
IANA has performed registrations as defined in the following
subsections.
12.1. Registration of unknown-character-replacement Media Type
Parameter
IANA has added 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
unknown-character-replacement
Media feature tag name:
unknown-character-replacement
ASN.1 identifier associated with feature tag:
1.3.6.1.8.1.33
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 string, such as "?".
This feature tag is also applicable to other conversions
to text, e.g., conversion of images using OCR (optical
character recognition).
Values appropriate for use with this feature tag:
The feature tag contains a UTF-8 string used to replace any
characters from the source media type that can't be
represented in the target media type.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms:
IMAP CONVERT extension [RFC5259]
Examples of typical use:
C: b001 CONVERT 2 BINARY[3 ("text/plain" ("charset"
"us-ascii" "unknown-character-replacement" "?"))]
Related standards or documents:
[RFC5259]
[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
13. 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 a server such as requesting an image be
scaled to extremely large dimensions). Server SHOULD refuse to
execute CPU-expensive conversions. Servers should avoid dangerous
conversions if possible. Whenever possible, servers should perform
verification of the converted attachments before returning them to
the client. Clients should be careful when requesting conversions or
processing transformed attachments. Clients SHOULD use mutual Simple
Authentication and Security Layer (SASL) authentication and the SASL/
TLS integrity layer, to make sure they are talking to trusted
servers.
When the client requests a server-side conversion of a signed body
part (e.g., a part inside multipart/signed), there is no way for the
client to verify that the converted content is authentic. A client
not trusting the server to perform conversion of a signed body part
can download the signed object, verify the signature, and perform the
conversion itself.
A client can create a carefully crafted bad message with the APPEND
command followed by the CONVERT command to attack the server. If the
server's conversion function or library has a security problem (such
as vulnerability to a buffer overflow), this could result in
privilege escalation or denial of service. In order to mitigate such
attacks, servers SHOULD log the client authentication identity on
APPEND and/or CONVERT operations in order to facilitate tracking of
abusive clients. Also server implementors SHOULD isolate the
conversion function or library from the privileged mailstore, perhaps
by running it within a distinct process.
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).
14. Acknowledgments
Stephane H. Maes and Ray Cromwell from Oracle edited several earlier
versions of this document. Their contribution is gratefully
acknowledged.
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), Ted
Hardie (Qualcomm), Larry Masinter (Adobe), Philip Guenther
(Sendmail), Greg Vaudreuil (Alcatel-Lucent), David Harrington
(Comcast), Dave Cridland (Isode), Pasi Eronen (Nokia), Magnus
Westerlund (Ericsson), and Jari Arkko (Ericsson), which improved the
quality of this specification considerably.
The authors would also like to specially thank Dave Cridland for the
MEDIACAPS command proposal and Dan Karp for the CONVERSIONS command
proposal.
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 document: 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)), and Zhao Lijun (China
Mobile Communications Corporation (CMCC)).
15. References
15.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[CHARSET-REG] Hoffman, P., "Registration of Charset and Languages
Media Features Tags", RFC 2987, November 2000.
[IMAPABNF] Melnikov, A. and C. Daboo, "Collected Extensions to
IMAP4 ABNF", RFC 4466, April 2006.
[MEDIAFEAT-REG] Holtman, K., Mutz, A., and T. Hardie, "Media Feature
Tag Registration Procedure", BCP 31, RFC 2506,
March 1999.
[MIME-MTSRP] Freed, N. and J. Klensin, "Media Type Specifications
and Registration Procedures", BCP 13, RFC 4288,
December 2005.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions
for Non-ASCII Text", RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and
Encoded Word Extensions:
Character Sets, Languages, and Continuations",
RFC 2231, November 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[RFC3516] Nerenberg, L., "IMAP4 Binary Content Extension",
RFC 3516, April 2003.
15.2. Informative References
[DISP-FEATURES] Masinter, L., Wing, D., Mutz, A., and K. Holtman,
"Media Features for Display, Print, and Fax",
RFC 2534, March 1999.
[IMAP-I18N] Newman, C., Gulbrandsen, A., and A. Melnikov,
"Internet Message Access Protocol
Internationalization", RFC 5255, June 2008.
[LEM-STREAMING] Cook, N., "Streaming Internet Messaging
Attachments", Work in Progress, June 2008.
[OMA-ME-RD] OMA, "Open Mobile Alliance Mobile Email Requirement
Document", OMA 55.919 3.0.0, December 2007.
[OMA-STI] OMA, "Open Mobile Alliance, Standard Transcoding
Interface Specification", OMA OMA-STI-V1_0,
December 2005.
Authors' Addresses
Alexey Melnikov (editor)
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
Peter Coates (editor)
Sun Microsystems
185 Falcon Drive
Whitehorse, YT Y1A 6T2
Canada
EMail: peter.coates@Sun.COM
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