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452 lines
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Network Working Group L. Nerenberg
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Request for Comments: 3516 Orthanc Systems
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Category: Standards Track April 2003
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IMAP4 Binary Content Extension
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Status of this Memo
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice
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Copyright (C) The Internet Society (2003). All Rights Reserved.
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Abstract
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This memo defines the Binary extension to the Internet Message Access
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Protocol (IMAP4). It provides a mechanism for IMAP4 clients and
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servers to exchange message body data without using a MIME content-
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transfer-encoding.
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1. Conventions Used in this Document
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The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
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in this document are to be interpreted as described in [KEYWORD].
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The abbreviation "CTE" means content-transfer-encoding.
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2. Introduction
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The MIME extensions to Internet messaging allow for the transmission
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of non-textual (binary) message content [MIME-IMB]. Since the
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traditional transports for messaging are not always capable of
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passing binary data transparently, MIME provides encoding schemes
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that allow binary content to be transmitted over transports that are
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not otherwise able to do so.
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The overhead of MIME-encoding this content can be considerable in
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some contexts (e.g., slow radio links, streaming multimedia).
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Reducing the overhead associated with CTE schemes such as base64
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Nerenberg Standards Track [Page 1]
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RFC 3516 IMAP4 Binary Content Extension April 2003
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can give a noticeable reduction in resource consumption. The Binary
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extension lets the server perform CTE decoding prior to transmitting
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message data to the client.
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3. Content-Transfer-Encoding Considerations
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Every IMAP4 body section has a MIME content-transfer-encoding.
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(Those without an explicit Content-Transfer-Encoding header are
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implicitly labeled as "7bit" content.) In the terminology of [MIME-
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IMB], the CTE specifies both a decoding algorithm and the domain of
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the decoded data. In this memo, "decoding" refers to the CTE
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decoding step described in [MIME-IMB].
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Certain CTEs use an identity encoding transformation. For these CTEs
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there is no decoding required, however the domain of the underlying
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data may not be expressible in the IMAP4 protocol (e.g., MIME
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"binary" content containing NUL octets). To accommodate these cases
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the Binary extension introduces a new type of literal protocol
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element that is fully eight bit transparent.
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Thus, server processing of the FETCH BINARY command involves two
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logical steps:
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1) perform any CTE-related decoding
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2) determine the domain of the decoded data
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Step 2 is necessary to determine which protocol element should be
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used to transmit the decoded data. (See FETCH Response Extensions
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for further details.)
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4. Framework for the IMAP4 Binary Extension
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This memo defines the following extensions to [IMAP4rev1].
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4.1. CAPABILITY Identification
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IMAP4 servers that support this extension MUST include "BINARY" in
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the response list to the CAPABILITY command.
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4.2. FETCH Command Extensions
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This extension defines three new FETCH command data items.
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BINARY<section-binary>[<partial>]
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Requests that the specified section be transmitted after
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performing CTE-related decoding.
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Nerenberg Standards Track [Page 2]
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RFC 3516 IMAP4 Binary Content Extension April 2003
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The <partial> argument, if present, requests that a subset of
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the data be returned. The semantics of a partial FETCH BINARY
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command are the same as for a partial FETCH BODY command, with
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the exception that the <partial> arguments refer to the DECODED
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section data.
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BINARY.PEEK<section-binary>[<partial>]
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An alternate form of FETCH BINARY that does not implicitly set
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the \Seen flag.
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BINARY.SIZE<section-binary>
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Requests the decoded size of the section (i.e., the size to
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expect in response to the corresponding FETCH BINARY request).
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Note: client authors are cautioned that this might be an
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expensive operation for some server implementations.
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Needlessly issuing this request could result in degraded
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performance due to servers having to calculate the value every
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time the request is issued.
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4.3. FETCH Response Extensions
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This extension defines two new FETCH response data items.
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BINARY<section-binary>[<<number>>]
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An <nstring> or <literal8> expressing the content of the
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specified section after removing any CTE-related encoding. If
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<number> is present it refers to the offset within the DECODED
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section data.
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If the domain of the decoded data is "8bit" and the data does
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not contain the NUL octet, the server SHOULD return the data in
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a <string> instead of a <literal8>; this allows the client to
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determine if the "8bit" data contains the NUL octet without
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having to explicitly scan the data stream for for NULs.
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If the server does not know how to decode the section's CTE, it
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MUST fail the request and issue a "NO" response that contains
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the "UNKNOWN-CTE" extended response code.
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Nerenberg Standards Track [Page 3]
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RFC 3516 IMAP4 Binary Content Extension April 2003
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BINARY.SIZE<section-binary>
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The size of the section after removing any CTE-related
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encoding. The value returned MUST match the size of the
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<nstring> or <literal8> that will be returned by the
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corresponding FETCH BINARY request.
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If the server does not know how to decode the section's CTE, it
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MUST fail the request and issue a "NO" response that contains
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the "UNKNOWN-CTE" extended response code.
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4.4. APPEND Command Extensions
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The APPEND command is extended to allow the client to append data
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containing NULs by using the <literal8> syntax. The server MAY
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modify the CTE of the appended data, however any such transformation
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MUST NOT result in a loss of data.
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If the destination mailbox does not support the storage of binary
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content, the server MUST fail the request and issue a "NO" response
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that contains the "UNKNOWN-CTE" extended response code.
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5. MIME Encoded Headers
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[MIME-MHE] defines an encoding that allows for non-US-ASCII text in
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message headers. This encoding is not the same as the content-
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transfer-encoding applied to message bodies, and the decoding
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transformations described in this memo do not apply to [MIME-MHE]
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encoded header text. A server MUST NOT perform any conversion of
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[MIME-MHE] encoded header text in response to any binary FETCH or
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APPEND request.
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6. Implementation Considerations
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Messaging clients and servers have been notoriously lax in their
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adherence to the Internet CRLF convention for terminating lines of
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textual data in Internet protocols. When sending data using the
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Binary extension, servers MUST ensure that textual line-oriented
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sections are always transmitted using the IMAP4 CRLF line termination
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syntax, regardless of the underlying storage representation of the
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data on the server.
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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
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used, the server MUST issue BODYSTRUCTURE responses that describe the
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message as though the binary-encoded sections are encoded in a CTE
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Nerenberg Standards Track [Page 4]
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RFC 3516 IMAP4 Binary Content Extension April 2003
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acceptable to the IMAP4 base specification. Furthermore, the results
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of a FETCH BODY MUST return the message body content in the format
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described by the corresponding FETCH BODYSTRUCTURE response.
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While the server is allowed to modify the CTE of APPENDed <literal8>
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data, this should only be done when it is absolutely necessary.
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Gratuitous encoding changes will render useless most cryptographic
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operations that have been performed on the message.
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This extension provides an optimization that is useful in certain
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specific situations. It does not absolve clients from providing
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basic functionality (content transfer decoding) that should be
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available in all messaging clients. Clients supporting this
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extension SHOULD be prepared to perform their own CTE decoding
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operations.
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7. Formal Protocol Syntax
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The following syntax specification uses the augmented Backus-Naur
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Form (ABNF) notation as used in [ABNF], and incorporates by reference
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the Core Rules defined in that document.
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This syntax augments the grammar specified in [IMAP4rev1].
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append =/ "APPEND" SP mailbox [SP flag-list]
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[SP date-time] SP literal8
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fetch-att =/ "BINARY" [".PEEK"] section-binary [partial]
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/ "BINARY.SIZE" section-binary
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literal8 = "~{" number "}" CRLF *OCTET
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; <number> represents the number of OCTETs
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; in the response string.
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msg-att-static =/ "BINARY" section-binary SP (nstring / literal8)
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/ "BINARY.SIZE" section-binary SP number
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partial = "<" number "." nz-number ">"
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resp-text-code =/ "UNKNOWN-CTE"
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section-binary = "[" [section-part] "]"
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Nerenberg Standards Track [Page 5]
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RFC 3516 IMAP4 Binary Content Extension April 2003
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8. Normative References
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[ABNF] Crocker, D., Editor, and P. Overell, "Augmented BNF for
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Syntax Specifications: ABNF", RFC 2234, November 1997.
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[IMAP4rev1] Crispin, M., "Internet Message Access Protocol Version
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4rev1", RFC 3501, March 2003.
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[KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
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[MIME-IMB] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
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Extensions (MIME) Part One: Format of Internet Message
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Bodies", RFC 2045, November 1996.
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[MIME-MHE] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
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Part Three: Message Header Extensions for Non-ASCII
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Text", RFC 2047, November 1996.
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9. Security Considerations
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There are no known additional security issues with this extension
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beyond those described in the base protocol described in [IMAP4rev1].
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10. Intellectual Property
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The IETF takes no position regarding the validity or scope of any
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intellectual property or other rights that might be claimed to
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pertain to the implementation or use of the technology described in
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this document or the extent to which any license under such rights
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might or might not be available; neither does it represent that it
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has made any effort to identify any such rights. Information on the
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IETF's procedures with respect to rights in standards-track and
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standards-related documentation can be found in BCP-11. Copies of
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claims of rights made available for publication and any assurances of
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licenses to be made available, or the result of an attempt made to
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obtain a general license or permission for the use of such
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proprietary rights by implementors or users of this specification can
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be obtained from the IETF Secretariat.
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The IETF invites any interested party to bring to its attention any
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copyrights, patents or patent applications, or other proprietary
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rights which may cover technology that may be required to practice
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this standard. Please address the information to the IETF Executive
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Director.
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Nerenberg Standards Track [Page 6]
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RFC 3516 IMAP4 Binary Content Extension April 2003
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11. Author's Address
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Lyndon Nerenberg
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Orthanc Systems
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1606 - 10770 Winterburn Road
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Edmonton, Alberta
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Canada T5S 1T6
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EMail: lyndon@orthanc.ab.ca
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Nerenberg Standards Track [Page 7]
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RFC 3516 IMAP4 Binary Content Extension April 2003
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12. Full Copyright Statement
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Copyright (C) The Internet Society (2003). All Rights Reserved.
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This document and translations of it may be copied and furnished to
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others, and derivative works that comment on or otherwise explain it
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or assist in its implementation may be prepared, copied, published
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and distributed, in whole or in part, without restriction of any
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kind, provided that the above copyright notice and this paragraph are
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included on all such copies and derivative works. However, this
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document itself may not be modified in any way, such as by removing
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the copyright notice or references to the Internet Society or other
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Internet organizations, except as needed for the purpose of
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developing Internet standards in which case the procedures for
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copyrights defined in the Internet Standards process must be
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followed, or as required to translate it into languages other than
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English.
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The limited permissions granted above are perpetual and will not be
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revoked by the Internet Society or its successors or assigns.
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This document and the information contained herein is provided on an
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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Acknowledgement
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Funding for the RFC Editor function is currently provided by the
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Internet Society.
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Nerenberg Standards Track [Page 8]
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