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+Network Working Group L. Nerenberg
+Request for Comments: 3516 Orthanc Systems
+Category: Standards Track April 2003
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+ 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
+
+
+
+
+
+
+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
+ 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.
+
+
+
+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
+ 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.
+
+
+
+
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+RFC 3516 IMAP4 Binary Content Extension April 2003
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+ 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
+
+
+
+
+
+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
+ 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] "]"
+
+
+
+
+
+
+
+
+
+Nerenberg Standards Track [Page 5]
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+RFC 3516 IMAP4 Binary Content Extension April 2003
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+8. Normative References
+
+ [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.
+
+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
+ 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.
+
+
+
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+RFC 3516 IMAP4 Binary Content Extension April 2003
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+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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+12. Full Copyright Statement
+
+ Copyright (C) The Internet Society (2003). 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 implementation 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.
+
+Acknowledgement
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
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projects / uw-imap / commitdiff