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

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.

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.

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

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] "]"

8. Normative References

[ABNF] Crocker, D., Editor, and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC2234, November 1997.

[IMAP4rev1] Crispin, M., "Internet Message Access Protocol Version
4rev1", RFC3501, March 2003.

[KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[MIME-IMB] Freed, N. and 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.

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
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proprietary rights by implementors or users of this specification can
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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.

11. Author's Address

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

EMail: lyndon@orthanc.ab.ca

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