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RFC 3516 - IMAP4 Binary Content Extension

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Network Working Group                                       L. Nerenberg
Request for Comments: 3516                               Orthanc Systems
Category: Standards Track                                     April 2003

                     IMAP4 Binary Content 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.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This memo defines the Binary extension to the Internet Message Access
   Protocol (IMAP4).  It provides a mechanism for IMAP4 clients and
   servers to exchange message body data without using a MIME content-

1.   Conventions Used in this Document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as described in [KEYWORD].

   The abbreviation "CTE" means content-transfer-encoding.

2.   Introduction

   The MIME extensions to Internet messaging allow for the transmission
   of non-textual (binary) message content [MIME-IMB].  Since the
   traditional transports for messaging are not always capable of
   passing binary data transparently, MIME provides encoding schemes
   that allow binary content to be transmitted over transports that are
   not otherwise able to do so.

   The overhead of MIME-encoding this content can be considerable in
   some contexts (e.g., slow radio links, streaming multimedia).
   Reducing the overhead associated with CTE schemes such as base64

   can give a noticeable reduction in resource consumption.  The Binary
   extension lets the server perform CTE decoding prior to transmitting
   message data to the client.

3.  Content-Transfer-Encoding Considerations

   Every IMAP4 body section has a MIME content-transfer-encoding.
   (Those without an explicit Content-Transfer-Encoding header are
   implicitly labeled as "7bit" content.)  In the terminology of [MIME-
   IMB], the CTE specifies both a decoding algorithm and the domain of
   the decoded data.  In this memo, "decoding" refers to the CTE
   decoding step described in [MIME-IMB].

   Certain CTEs use an identity encoding transformation.  For these CTEs
   there is no decoding required, however the domain of the underlying
   data may not be expressible in the IMAP4 protocol (e.g., MIME
   "binary" content containing NUL octets).  To accommodate these cases
   the Binary extension introduces a new type of literal protocol
   element that is fully eight bit transparent.

   Thus, server  processing of the FETCH BINARY command involves two
   logical steps:

   1)  perform any CTE-related decoding

   2)  determine the domain of the decoded data

   Step 2 is necessary to determine which protocol element should be
   used to transmit the decoded data.  (See FETCH Response Extensions
   for further details.)

4.  Framework for the IMAP4 Binary Extension

   This memo defines the following extensions to [IMAP4rev1].

4.1.  CAPABILITY Identification

   IMAP4 servers that support this extension MUST include "BINARY" in
   the response list to the CAPABILITY command.

4.2.  FETCH Command Extensions

   This extension defines three new FETCH command data items.


         Requests that the specified section be transmitted after
         performing CTE-related decoding.

         The <partial> argument, if present, requests that a subset of
         the data be returned.  The semantics of a partial FETCH BINARY
         command are the same as for a partial FETCH BODY command, with
         the exception that the <partial> arguments refer to the DECODED
         section data.


         An alternate form of FETCH BINARY that does not implicitly set
         the \Seen flag.


         Requests the decoded size of the section (i.e., the size to
         expect in response to the corresponding FETCH BINARY request).

         Note: client authors are cautioned that this might be an
         expensive operation for some server implementations.
         Needlessly issuing this request could result in degraded
         performance due to servers having to calculate the value every
         time the request is issued.

4.3.  FETCH Response Extensions

   This extension defines two new FETCH response data items.


         An <nstring> or <literal8> expressing the content of the
         specified section after removing any CTE-related encoding.  If
         <number> is present it refers to the offset within the DECODED
         section data.

         If the domain of the decoded data is "8bit" and the data does
         not contain the NUL octet, the server SHOULD return the data in
         a <string> instead of a <literal8>; this allows the client to
         determine if the "8bit" data contains the NUL octet without
         having to explicitly scan the data stream for for NULs.

         If the server does not know how to decode the section's CTE, it
         MUST fail the request and issue a "NO" response that contains
         the "UNKNOWN-CTE" extended response code.


         The size of the section after removing any CTE-related
         encoding.  The value returned MUST match the size of the
         <nstring> or <literal8> that will be returned by the
         corresponding FETCH BINARY request.

         If the server does not know how to decode the section's CTE, it
         MUST fail the request and issue a "NO" response that contains
         the "UNKNOWN-CTE" extended response code.

4.4.  APPEND Command Extensions

   The APPEND command is extended to allow the client to append data
   containing NULs by using the <literal8> syntax.  The server MAY
   modify the CTE of the appended data, however any such transformation
   MUST NOT result in a loss of data.

   If the destination mailbox does not support the storage of binary
   content, the server MUST fail the request and issue a "NO" response
   that contains the "UNKNOWN-CTE" extended response code.

5.  MIME Encoded Headers

   [MIME-MHE] defines an encoding that allows for non-US-ASCII text in
   message headers.  This encoding is not the same as the content-
   transfer-encoding applied to message bodies, and the decoding
   transformations described in this memo do not apply to [MIME-MHE]
   encoded header text.  A server MUST NOT perform any conversion of
   [MIME-MHE] encoded header text in response to any binary FETCH or
   APPEND request.

6.  Implementation Considerations

   Messaging clients and servers have been notoriously lax in their
   adherence to the Internet CRLF convention for terminating lines of
   textual data in Internet protocols.  When sending data using the
   Binary extension, servers MUST ensure that textual line-oriented
   sections are always transmitted using the IMAP4 CRLF line termination
   syntax, regardless of the underlying storage representation of the
   data on the server.

   A server may choose to store message body binary content in a non-
   encoded format.  Regardless of the internal storage representation
   used, the server MUST issue BODYSTRUCTURE responses that describe the
   message as though the binary-encoded sections are encoded in a CTE

   acceptable to the IMAP4 base specification.  Furthermore, the results
   of a FETCH BODY MUST return the message body content in the format
   described by the corresponding FETCH BODYSTRUCTURE response.

   While the server is allowed to modify the CTE of APPENDed <literal8>
   data, this should only be done when it is absolutely necessary.
   Gratuitous encoding changes will render useless most cryptographic
   operations that have been performed on the message.

   This extension provides an optimization that is useful in certain
   specific situations.  It does not absolve clients from providing
   basic functionality (content transfer decoding) that should be
   available in all messaging clients.  Clients supporting this
   extension SHOULD be prepared to perform their own CTE decoding

7.  Formal Protocol 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 [IMAP4rev1].

   append         =/  "APPEND" SP mailbox [SP flag-list]
                      [SP date-time] SP literal8

   fetch-att      =/  "BINARY" [".PEEK"] section-binary [partial]
                      / "BINARY.SIZE" section-binary

   literal8       =   "~{" number "}" CRLF *OCTET
                      ; <number> represents the number of OCTETs
                      ; in the response string.

   msg-att-static =/  "BINARY" section-binary SP (nstring / literal8)
                      / "BINARY.SIZE" section-binary SP number

   partial        =   "<" number "." nz-number ">"

   resp-text-code =/  "UNKNOWN-CTE"

   section-binary =   "[" [section-part] "]"

8.  Normative References

   [ABNF]      Crocker, D., Editor, and P. Overell, "Augmented BNF for
               Syntax Specifications: ABNF", RFC 2234, November 1997.

   [IMAP4rev1] Crispin, M., "Internet Message Access Protocol Version
               4rev1", RFC 3501, March 2003.

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

   [MIME-IMB]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, November 1996.

   [MIME-MHE]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
               Part Three: Message Header Extensions for Non-ASCII
               Text", RFC 2047, November 1996.

9.  Security Considerations

   There are no known additional security issues with this extension
   beyond those described in the base protocol described in [IMAP4rev1].

10.  Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

11.  Author's Address

   Lyndon Nerenberg
   Orthanc Systems
   1606 - 10770 Winterburn Road
   Edmonton, Alberta
   Canada  T5S 1T6

   EMail: lyndon@orthanc.ab.ca

12.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

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