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Versions: 00 01 02 03 04 05 06 07 08 RFC 4978

Network Working Group                                   Arnt Gulbrandsen
Request for Comments: DRAFT                       Oryx Mail Systems GmbH
                                                               July 2006


                  The IMAP COMPRESS=DEFLATE Extension
                  draft-ietf-lemonade-compress-02.txt


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Copyright Notice

    Copyright (C) The Internet Society 2006.


Abstract

    The COMPRESS=DEFLATE extension allows an IMAP connection to be
    compressed using the DEFLATE algorithm, such that effective
    compression is available even when TLS is used.


Table of Contents

    1. Conventions Used in This Document  . . . . . . . . . . . . . .  2
    2. Introduction and Overview  . . . . . . . . . . . . . . . . . .  2



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    3. The COMPRESS Command . . . . . . . . . . . . . . . . . . . . .  3
    4. Compression Efficiency . . . . . . . . . . . . . . . . . . . .  4
    5. Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . .  5
    6. Security Considerations  . . . . . . . . . . . . . . . . . . .  6
    7. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
    8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
    9. References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     9.1. Normative References  . . . . . . . . . . . . . . . . . . .  6
     9.2. Informative References  . . . . . . . . . . . . . . . . . .  6
    10. Author's Address  . . . . . . . . . . . . . . . . . . . . . .  7
    11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  7


1.  Conventions Used in This Document

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in RFC 2119 [KEYWORDS].
    Formal syntax is defined by [ABNF] as modified by [IMAP].

    In the example, "C:" and "S:" indicate lines sent by the client and
    server respectively.


2.  Introduction and Overview

    An IMAP server that supports this extension announces
    "COMPRESS=DEFLATE" as one of its capabilities.

    The goal of COMPRESS=DEFLATE is to reduce the bandwidth usage of
    IMAP.  On regular IMAP connections, the PPP or MNP compression used
    with many low-bandwidth links compresses IMAP well. However, when
    TLS is used, PPP/MNP compression is ineffective. TLS too may provide
    compression, but a careful IMAP implementation can do much better.

    In order to increase interoperation, it is desirable to have as few
    different compression algorithms as possible, so this document
    specifies only one.  The DEFLATE algorithm is standard, widely
    available, unencumbered by patents and fairly efficient.  Hopefully
    it will not be necessary to define additional algorithms.

    The extension adds one new command (COMPRESS) and no new responses.









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3.  The COMPRESS Command

    Arguments: Name of compression mechanism: "DEFLATE".

    Responses: None

    Result: OK The server will compress its responses and expects the
               client to compress its commands.
            NO The server doesn't support the requested mechanism.
           BAD Command unknown, invalid argument, or COMPRESS already
               active.

    The COMPRESS command instructs the server to use the named
    compression mechanism ("DEFLATE" is the only one defined) for all
    commands and/or responses after COMPRESS.

    The client MUST NOT send any commands until it has seen the result
    of COMPRESS.

    For DEFLATE (as for many other compression mechanisms), the
    compressor can trade speed against quality.  When decompressing
    there isn't much of a tradeoff.  Consequently, the client and server
    are both free to pick the best reasonable rate of compression for
    the data they send.

    If both [STARTTLS] and COMPRESS are in use, the data should be
    compressed before it is encrypted (and decrypted before it is
    decompressed), independent of the order in which the client issues
    COMPRESS, AUTHENTICATE and STARTTLS.

    The following example illustrates how commands and responses are
    compressed during a simple login sequence:

         S: * OK [CAPABILITY IMAP4REV1 STARTTLS COMPRESS=DEFLATE]
         C: a starttls
         S: a OK TLS active

         From this point on, everything is encrypted.

         C: b compress deflate
         S: b OK DEFLATE active

         From this point on, everything is compressed before being
         encrypted.

         C: c login arnt tnra
         S: c OK Logged in as arnt




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4.  Compression Efficiency

    IMAP poses some unusual problems for a compression layer.

    Upstream is fairly simple. Most IMAP clients send the same few
    commands again and again, so any compression algorith which can
    exploit repetition works efficiently. The APPEND command is an
    exception; clients which send many APPEND commands may want to take
    special care of literals, in the same way that servers do.

    Downstream has the unusual property that several kinds of data are
    sent, confusing all dictionary-based compression algorithms.

    One type is IMAP responses. These are highly compressible; zlib
    using its least CPU-intensive setting compresses typical responses
    to 25-40% of their original size.

    Another is email headers. These are equally compressible, and
    benefit from using the same dictionary as the IMAP responses.

    A third is email body text. Text is usually fairly short and
    includes much ASCII, so the same compression dictionary will do a
    good job here, too. When multiple messages in the same thread are
    read at the same time, quoted lines etc. can often be compressed
    almost to zero.

    Finally, attachments (non-text email bodies) are transmitted, either
    in [BINARY] form or encoded with base-64.

    When attachments are retrieved in [BINARY] form, DEFLATE may be able
    to compress them, but the format of the attachment is usually not
    IMAP-like, so the dictionary built while compressing IMAP does not
    help. The compressor has to adapt its dictionary from IMAP to the
    attachment's format, and then back. A few file formats aren't
    compressible at all using deflate, e.g. .gz, .zip and .jpg files.

    When attachments are retrieved in base-64 form, the same problems
    apply, but the base-64 encoding adds another problem. 8-bit
    compression algorithms such as deflate work well on 8-bit file
    formats, however base-64 turns a file into something resembling
    6-bit bytes, hiding most of the 8-bit file format from the
    compressor.

    When using the zlib library (see [DEFLATE]), the functions
    deflateInit(), deflate(), inflateInit() and inflate() suffice to
    implement this extension. deflateParams() can be used to improve
    compression rate and resource use.




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    A client can improve downstream compression by implementing [BINARY]
    and using FETCH BINARY instead of FETCH BODY. In the author's
    experience, the improvement ranges from 5% to 40% depending on the
    attachment being downloaded.

    A server can improve downstream compression if it hints to the
    compressor that the data type is about to change strongly, e.g. by
    sending a Z_FULL_FLUSH at the start and end of large non-text
    literals (before and after '*CHAR8' in the definition of literal in
    RFC 3501, page 86). Small literals are best left alone.

    A server can improve the CPU efficiency both of the server and the
    client if it adjusts the compression level (e.g. using the
    deflateParams() function in zlib) at these points. A very simple
    strategy is to change the level 0 to at the start of a literal
    provided the first two bytes are either 0x1F 0x8B (as in deflate-
    compressed files) or 0xFF 0xD8 (JPEG), and to keep it at 1-5 the
    rest of the time.

    Note that when using TLS, compression may actually decrease the CPU
    usage, depending on which algorithms are used in TLS. This is
    because fewer bytes need to be encrypted, and encryption is
    generally more expensive than compression.


5.  Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form (ABNF) notation as specified in [ABNF]. Non-terminals
    referenced but not defined below are as defined by [ABNF] (SP, CRLF)
    or [IMAP] (all others).

    Except as noted otherwise, all alphabetic characters are case-
    insensitive.  The use of upper or lower case characters to define
    token strings is for editorial clarity only.  Implementations MUST
    accept these strings in a case-insensitive fashion.

        command-any =/ compress

        compress    = "COMPRESS" SP algorithm

        algorithm   = "DEFLATE"


6.  Security Considerations

    As for [TLSCOMP] RFC 3749.




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7.  IANA Considerations

    The IANA is requested to add COMPRESS=DEFLATE to the list of IMAP
    extensions.


8.  Acknowledgements

    Eric Burger, Dave Cridland, Tony Finch, Ned Freed, Philip Guenther,
    Randall Gellens, Tony Hansen, Alexey Melnikov, Lyndon Nerenberg and
    Zoltan Ordogh have all helped with this document.

    The author would also like to thank various people in the rooms at
    meetings, whose help is real, but not reflected in the author's
    mailbox.


9.  References


9.1. Normative References

    [ABNF]     Crocker, Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 4234, Brandenburg
               Internetworking, Demon Internet Ltd, October 2005.

    [IMAP]     Crispin, "Internet Message Access Protocol - Version
               4rev1", RFC 3501, University of Washington, June 2003.

    [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119, Harvard University, March
               1997.

    [DEFLATE]  Deutsch, "DEFLATE Compressed Data Format Specification
               version 1.3", RFC 1951, Aladdin Enterprises, May 1996.

    [STARTTLS] Newman, C. "Using TLS with IMAP, POP3 and ACAP", RFC
               2595, June 1999.


9.2. Informative References

    [TLSCOMP]  Hollenbeck, "Transport Layer Security Protocol
               Compression Methods", RFC 3749, VeriSign, May 2004.

    [BINARY]   Nerenberg, "IMAP4 Binary Content Extension", Orthanc
               Systems, April 2003.




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10. Author's Address

    Arnt Gulbrandsen
    Oryx Mail Systems GmbH
    Schweppermannstr. 8
    D-81671 Muenchen
    Germany

    Fax: +49 89 4502 9758

    Email: arnt@oryx.com


11. Open Issues

    What text and numbers are needed wrt. compression levels? A bit of
    solid information is not amiss.


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    Copyright (C) The Internet Society (2006).




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