Network Working Group L. Nerenberg
Internet Draft: IMAP4 Binary Content Extension Orthanc Systems
Document: draft-nerenberg-imap-binary-07.txt November 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.
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1. Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
2. 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.
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3. 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.
4. 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.
5. 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 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
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.)
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6. Framework for the IMAP4 Binary Extension
This memo defines the following extensions to [IMAP4rev1].
6.1. CAPABILITY Identification
IMAP4 servers that support this extension MUST include "BINARY" in
the response list to the CAPABILITY command.
6.2. 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 may 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.
6.3. 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 character, the server SHOULD return the
data in a <string> instead of a <literal8>; this allows the
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client to determine if the "8bit" data contains the NUL
character without having to explicitly scan the data stream
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.
6.4. 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 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.
7. 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.
8. 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-
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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.
9. 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] "]"
10. Normative References
[ABNF] Crocker, D., P. Overell, "Augmented BNF for Syntax
Specifications: ABNF." RFC2234, November 1997
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[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.
11. Security Considerations
There are no known additional security issues with this extension
beyond those described in the base protocol described in
[IMAP4rev1].
12. 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.
13. Authors' Address
Lyndon Nerenberg
Orthanc Systems
508 - 11025 Jasper Avenue
Edmonton, Alberta
Canada T5K 0K7
Email: lyndon@orthanc.ab.ca
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14. Full Copyright Statement
Copyright (C) The Internet Society (2002). 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 implmentation may be prepared, copied,
published and distributed, in whole or in part, without restriction
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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
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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.
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