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RFC 5259 - Internet Message Access Protocol - CONVERT Extension

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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.


   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)

   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",
   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

   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

   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: 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

   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.


   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"
       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"
       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"
       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

   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/

   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
   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:








   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

   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.

     S: * 2 CONVERTED (TAG "a001") (BINARY[3] {2135}
        <the document in .pdf format>

   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

   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"))
     S: * 2 CONVERTED (TAG "c000") (BINARY.SIZE[4] 3135)

   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"))
     S: * 2 CONVERTED (TAG "c001") (BINARY[4]<2001> {135}
          <bytes 2001 - 2135 of the document in text/plain format>

   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: 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: 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: 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: 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-

   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.>

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


   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.

         C: e002 CONVERT 2 (NIL ("PIX-X" "128" "PIX-Y" "96")) (BINARY[2]
             "JPEG" () NIL NIL "8bit" 4182 NIL NIL NIL) BINARY[2]
            <this part of a document is a rescaled image in
             JPEG format with width=128, height=96.>

8.3.  BINARY.SIZE CONVERT Request and Response Item


   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

   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

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
   will not fail.

            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"))

   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

   The server MAY choose to return one ERROR phrase for a single
   conversion if several related data items are requested.  For

     C: b002 CONVERT 2 ("text/plain" ("charset" "us-ascii"))
     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"))
     S: * 2 CONVERTED (tag "b002") (BODYPARTSTRUCTURE[3]
         (ERROR "Source text has non us-ascii" BADPARAMETERS
         "text/html" "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


         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"
         S: b003 NO All conversions failed

                   UNKNOWN-CHARACTER-REPLACEMENT "<badchar>")) BINARY[3]
         S: * 2 CONVERTED (tag "b005") (BINARY[3]
         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

       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-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 = 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

       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) /
         ; '*', 'type/*' or 'type/subtype'.
         ; type-name is defined in [MIME-MTSRP].

       response-payload =/ conversion-data

       conversion-data = "CONVERSION" SP quoted-from-mime-type SP
                         [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

12.1.  Registration of unknown-character-replacement Media Type

   IANA has added the following registration to the registry established
   by RFC 2506.

   To: "Media feature tags mailing list"

   Subject: Registration of media feature tag

   Media feature tag name:

   ASN.1 identifier associated with feature tag:

   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:

   Considerations particular to use in individual applications,
   protocols, services, or negotiation mechanisms:

   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

   Name(s) & email address(es) of person(s) to contact for further
      Alexey Melnikov <alexey.melnikov@isode.com>

   Intended usage:

   Author/Change controller:

   Requested IANA publication delay:

   Other information:

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

   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

   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

   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

   EMail: Alexey.Melnikov@isode.com

   Peter Coates (editor)
   Sun Microsystems
   185 Falcon Drive
   Whitehorse, YT  Y1A 6T2

   EMail: peter.coates@Sun.COM

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