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Network Working Group                                       L. NerenbergRequest for Comments: 3516                               Orthanc SystemsCategory: Standards Track                                     April 2003IMAP4 Binary Content ExtensionStatus 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 base64Nerenberg                   Standards Track                     [Page 1]

RFC 3516             IMAP4 Binary Content Extension           April 2003   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]

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.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.Nerenberg                   Standards Track                     [Page 3]

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.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 CTENerenberg                   Standards Track                     [Page 4]

RFC 3516             IMAP4 Binary Content Extension           April 2003   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]

RFC 3516             IMAP4 Binary Content Extension           April 20038.  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 inBCP-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.Nerenberg                   Standards Track                     [Page 6]

RFC 3516             IMAP4 Binary Content Extension           April 200311.  Author's Address   Lyndon Nerenberg   Orthanc Systems   1606 - 10770 Winterburn Road   Edmonton, Alberta   Canada  T5S 1T6   EMail: lyndon@orthanc.ab.caNerenberg                   Standards Track                     [Page 7]

RFC 3516             IMAP4 Binary Content Extension           April 200312.  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.Nerenberg                   Standards Track                     [Page 8]