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draft-nerenberg-imap-binary-06.txt --> draft-nerenberg-imap-binary-07.txt-15307.txt

Network Working Group L. Nerenberg
Internet Draft: IMAP4 Binary Content Extension ACI Worldwide Orthanc Systems
Document: draft-nerenberg-imap-binary-06.txt January draft-nerenberg-imap-binary-07.txt November 2002

IMAP4 Binary Content Extension

Status of this memo 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
other groups may also distribute working documents as Internet
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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. Copyright Notice

2. Abstract

This memo defines the BINARY 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.

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3. Overview

4. Introduction

The MIME extensions to Internet messaging allow for the transmis-
sion
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). Reduc-
ing
Reducing the overhead associated with CTE schemes such as base64
can give a noticeable reduction in resource consumption. The BINARY Binary
extension lets the server perform CTE decoding prior to transmit-
ting
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 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 Binary extension introduces a new type of literal proto-
col
protocol element that is fully 8bit 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].

5.1.

6.1. 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.

6.2. FETCH Command Extensions

This extension defines three new FETCH command data items.

BINARY[<section>]<<partial>>

BINARY<section-binary>[<partial>]

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

When performing

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

BINARY.PEEK[<section>]<<partial>> section data.

BINARY.PEEK<section-binary>[<partial>]

An alternate form of BINARY[<section>] FETCH BINARY that does not implic-
itly implicitly set
the \Seen flag.

BINARY.SIZE[<section>]

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 expen-
sive
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.

5.3.

6.3. FETCH Response Extensions

This extension defines two new FETCH response data items.

BINARY[<section>]<<origin_octet>>
A <string>

BINARY<section-binary>[<<number>>]

An <nstring> or <literal8> expressing the content of the speci-
fied
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 char-
acter
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 con-
tains
contains the "UNKNOWN-CTE" extended response code.

BINARY.SIZE[<section>]

BINARY.SIZE[<section-binary>]

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

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5.4.

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 mod-
ify
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
Binary extension, servers MUST ensure that textual line-oriented
sections are always transmitted using the IMAP4 CRLF line termina-
tion
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 (see [IMAP4rev1]
section 4.3.1). specification. Furthermore, the
results of a FETCH BODY MUST return the message body content in the
format described by the cor-
responding corresponding FETCH BODYSTRUCTURE response.

While the server is allowed to modify the CTE of APPENDed <lit-
eral8>
<literal8> data, this should only be done when it is absolutely neces-
sary.
necessary. Gratuitous encoding changes will render useless most crypto-
graphic
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 exten-
sion
extension SHOULD be prepared to perform their own CTE decoding opera-
tions.

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8.
operations.

9. Formal Protocol Syntax

The following syntax specification uses the augmented Backus-Naur
Form (ABNF) notation as used in [ABNF], and incorporates by refer-
ence
reference the Core Rules defined in that document.

This syntax augments the grammar specified in [IMAP4rev1].

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

fetch_att

fetch-att =/ "BINARY" [".PEEK"] section_number
["<" number "." nz_number ">"] section-binary [partial]
/ "BINARY.SIZE" section_number section-binary

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 /

msg-att-static =/ "BINARY" section_number ["<" number ">"] section-binary SP (nstring / literal8)
/ "BINARY.SIZE" section_number section-binary 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 number

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

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

section_number

section-binary = (nz_number *["." nz_number])

9. "[" [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.

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10.

11. Security Considerations

There are no known additional security issues with this extension.

11. 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
ACI Worldwide
Suite 900
10117
Orthanc Systems
508 - 11025 Jasper Avenue
Edmonton, Alberta
Canada T5J 1W8 T5K 0K7

Email: lyndon@atg.aciworldwide.com lyndon@orthanc.ab.ca

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14. Full Copyright Statement

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
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.

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