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rfc:rfc3516

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.

Abstract

 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-
 transfer-encoding.

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

Nerenberg Standards Track [Page 1] RFC 3516 IMAP4 Binary Content Extension April 2003

 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.
    BINARY<section-binary>[<partial>]
       Requests that the specified section be transmitted after
       performing CTE-related decoding.

Nerenberg Standards Track [Page 2] RFC 3516 IMAP4 Binary Content Extension April 2003

       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.
    BINARY.PEEK<section-binary>[<partial>]
       An alternate form of FETCH BINARY that does not implicitly set
       the \Seen flag.
    BINARY.SIZE<section-binary>
       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.
    BINARY<section-binary>[<<number>>]
       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.

Nerenberg Standards Track [Page 3] RFC 3516 IMAP4 Binary Content Extension April 2003

    BINARY.SIZE<section-binary>
       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

Nerenberg Standards Track [Page 4] RFC 3516 IMAP4 Binary Content Extension April 2003

 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
 operations.

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] "]"

Nerenberg Standards Track [Page 5] RFC 3516 IMAP4 Binary Content Extension April 2003

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
 Director.

Nerenberg Standards Track [Page 6] RFC 3516 IMAP4 Binary Content Extension April 2003

11. Author's Address

 Lyndon Nerenberg
 Orthanc Systems
 1606 - 10770 Winterburn Road
 Edmonton, Alberta
 Canada  T5S 1T6
 EMail: lyndon@orthanc.ab.ca

Nerenberg Standards Track [Page 7] RFC 3516 IMAP4 Binary Content Extension April 2003

12. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Nerenberg Standards Track [Page 8]

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