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.
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
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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.
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.)
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.
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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.
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.
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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.
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.
[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.
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
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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.
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].
[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.
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RFC 3516 IMAP4 Binary Content Extension April 2003
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