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Versions: 00 01 02 03 04 05 06 RFC 3516

Network Working Group                                       L. Nerenberg
Internet Draft: IMAP4 Binary Content Extension             ACI Worldwide
Document: draft-nerenberg-imap-binary-06.txt                January 2002

                     IMAP4 Binary Content Extension

Status of this memo

     This document is an Internet Draft and is in full conformance with
     all provisions of Section 10 of RFC 2026.

     Internet Drafts are working documents of the Internet Engineering
     Task Force (IETF), its areas, and its working groups.  Note that
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     A revised version of this draft document will be submitted to the
     RFC editor as a Proposed Standard for the Internet Community.
     Discussion and suggestions for improvement are requested.
     Distribution of this draft is unlimited.

1.  Abstract

     This memo defines the BINARY extension to the Internet Message
     Access Protocol [IMAP4rev1].  It provides a mechanism for IMAP4
     clients and servers to exchange message body data without using a
     MIME content-transfer-encoding.

2.  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.

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3.  Overview

     The MIME extensions to Internet messaging allow for the transmis-
     sion 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). Reduc-
     ing 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 transmit-
     ting message data to the client.

4.  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 the context of 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 expressable 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 proto-
     col element that is fully 8bit 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.)

5.  Framework for the IMAP4 Binary Extension

     This memo defines the following extensions to [IMAP4rev1].

5.1.  CAPABILITY Identification

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

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5.2.  FETCH Command Extensions

     This extension defines three new FETCH command data items.

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

          When performing a partial FETCH, the offset argument refers to
          the offset into the DECODED section.

          An alternate form of BINARY[<section>] that does not implic-
          itly 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 may be an expen-
          sive 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.

5.3.  FETCH Response Extensions

     This extension defines two new FETCH response data items.

          A <string> or <literal8> expressing the content of the speci-
          fied section after removing any CTE-related encoding.

          If the domain of the decoded data is "8bit" and the data does
          not contain the NUL character, 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 char-
          acter without having to explicitly scan the data stream for

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

          The size of the section after removing any CTE-related encod-
          ing.  The value returned MUST match the <literal> or <lit-
          eral8> size 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 con-
          tains the "UNKNOWN-CTE" extended response code.

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5.4.  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 mod-
     ify the CTE of the appended data, however any such transformation
     MUST NOT result in a loss of data.

     If the specified 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.

6.  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.

7.  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 termina-
     tion 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 (see [IMAP4rev1]
     section 4.3.1). Furthermore, the results of a FETCH BODY MUST
     return the message body content in the format described by the cor-
     responding FETCH BODYSTRUCTURE response.

     While the server is allowed to modify the CTE of APPENDed <lit-
     eral8> data, this should only be done when it is absolutely neces-
     sary. Gratuitous encoding changes will render useless most crypto-
     graphic 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 exten-
     sion SHOULD be prepared to perform their own CTE decoding opera-

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8.  Formal Protocol Syntax

     The following syntax specification uses the augmented Backus-Naur
     Form (ABNF) notation as used in [ABNF], and incorporates by refer-
     ence 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_number
                        ["<" number "." nz_number ">"] /
                        "BINARY.SIZE" section_number

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

     msg_att        =   "(" 1#("ENVELOPE" SP envelope /
                        "FLAGS" SP "(" #(flag / "\Recent") ")" /
                        "INTERNALDATE" SP date_time /
                        "RFC822" [".HEADER" / ".TEXT"] SP nstring /
                        "RFC822.SIZE" SP number /
                        "BODY" ["STRUCTURE"] SP body /
                        "BODY" section ["<" number ">"] SP nstring /
                        "UID" SP uniqueid /
                        "BINARY" section_number ["<" number ">"] SP
                                 (nstring / literal8) /
                        "BINARY.SIZE" section_number SP number) ")"
                        ;; Append BINARY and BINARY.SIZE to <msg_att>.
                        ;; This is not meant to be an absolute list;
                        ;; it was done this way due to ABNF syntax
                        ;; restrictions.

     resp_code_text =/  "UNKNOWN-CTE"

     section_number =   (nz_number *["." nz_number])

9.  References

     [IMAP4rev1] Crispin, M., "Internet Message Access Protocol Version
     4rev1," Work in progress (son-of-RFC 2060)

     [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate
     Requirement Levels," BCP 9, RFC2119, March 1997

     [MIME-IMB] Freed, N., N. Borenstein, "Multipurpose Internet Mail
     Extensions (MIME) Part One: Format of Internet Message Bodies,"
     RFC2045, November 1996.

     [MIME-MHE] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
     Part Three: Message Header Extensions for Non-ASCII Text," RFC2047,
     November 1996.

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10.  Security Considerations

     There are no known security issues with this extension.

11.  Authors' Address

     Lyndon Nerenberg
     ACI Worldwide
     Suite 900
     10117 - Jasper Avenue
     Edmonton, Alberta
     Canada   T5J 1W8

     Email: lyndon@atg.aciworldwide.com

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