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The information below is for an old version of the document that is already published as an RFC.
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This is an older version of an Internet-Draft that was ultimately published as RFC 4978.
Author Arnt Gulbrandsen
Last updated 2020-01-21 (Latest revision 2007-04-27)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
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IESG IESG state Became RFC 4978 (Proposed Standard)
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Responsible AD Chris Newman
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Network Working Group                                   Arnt Gulbrandsen
Request for Comments: DRAFT                       Oryx Mail Systems GmbH
Intended Status: Proposed Standard                            April 2007

                      The IMAP COMPRESS Extension

Status of this Memo

    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as Internet-

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet-Drafts as
    reference material or to cite them other than as "work in progress".

    The list of current Internet-Drafts can be accessed at  The list of Internet-
    Draft Shadow Directories can be accessed at

Copyright Notice

    Copyright (C) The IETF Trust (2007).


    The COMPRESS extension allows an IMAP connection to be effectively
    and efficiently compressed.

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Table of Contents

    1. Conventions Used in This Document  . . . . . . . . . . . . . .  2
    2. Introduction and Overview  . . . . . . . . . . . . . . . . . .  2
    3. The COMPRESS Command . . . . . . . . . . . . . . . . . . . . .  3
    4. Compression Efficiency . . . . . . . . . . . . . . . . . . . .  5
    5. Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . .  6
    6. Security Considerations  . . . . . . . . . . . . . . . . . . .  7
    7. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
    8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
    9. References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     9.1. Normative References  . . . . . . . . . . . . . . . . . . .  7
     9.2. Informative References  . . . . . . . . . . . . . . . . . .  8
    10. Author's Address  . . . . . . . . . . . . . . . . . . . . . .  8

1.  Conventions Used in This Document

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in [RFC2119].

    Formal syntax is defined by [RFC4234] as modified by [RFC3501].

    In the examples, "C:" and "S:" indicate lines sent by the client and
    server respectively. "[...]" denotes elision.

2.  Introduction and Overview

    A server which supports the COMPRESS extension indicates this with
    one or more capability names consisting of "COMPRESS=" followed by a
    supported compression algorithm name as described in this document.

    The goal of COMPRESS is to reduce the bandwidth usage of IMAP.

    Compared to PPP compression (see [RFC1962]) and modem-based
    compression (see [MNP] and [V42BIS]), COMPRESS offers much better
    compression efficiency.  COMPRESS can be used together with TLS
    [RFC4346], SASL encryption, VPNs etc. Compared to TLS compression
    [RFC3749], COMPRESS has the following (dis)advantages:

    - COMPRESS can be implemented easily both by IMAP servers and

    - IMAP COMPRESS benefits from an intimate knowledge of the IMAP
      protocol's state machine, allowing for dynamic and aggressive
      optimization of the underlying compression algorithm's parameters.

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    - When the TLS layer implements compression, any protocol using that
      layer can transparently benefit from that compression (e.g. SMTP
      and IMAP). COMPRESS is specific to IMAP.

    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 (defined in [RFC1951]) is
    standard, widely available and fairly efficient, so it is the only
    algorithm defined by this document.

    In order to increase interoperation, IMAP servers which advertise
    this extension SHOULD also advertise the TLS DEFLATE compression
    mechanism as defined in [RFC3749]. IMAP clients MAY use either
    COMPRESS or TLS compression.

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

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 Compression is already active via another layer.
           BAD Command unknown, invalid or unknown 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 further commands until it has seen the
    result of COMPRESS. If the response was OK, the client MUST compress
    starting with the first command after COMPRESS. If the server
    response was BAD or NO, the client MUST NOT turn on compression.

    If the server responds NO because it knows that the same mechanism
    is active already (e.g. because TLS has negotiated the same
    mechanism), it MUST send COMPRESSIONACTIVE as resp-text-code (see
    [RFC3501] section 7.1), and the resp-text SHOULD say which layer

    If the server issues an OK response, the server MUST compress
    starting immediately after the CRLF which ends the tagged OK
    response.  (Responses issued by the server before the OK response

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    will, of course, still be uncompressed.)  If the server issues a BAD
    or NO respnose, the server MUST NOT turn on compression.

    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.

    When COMPRESS is combined with TLS (see [RFC4346]) or SASL (see
    [RFC4422]) security layers, the sending order of the three
    extensions MUST be first COMPRESS, then SASL, and finally TLS.  That
    is, before data is transmitted it is first compressed.  Second, if a
    SASL security layer has been negotiated, the compressed data is then
    signed and/or encrypted accordingly. Third, if a TLS security layer
    has been negotiated, the data from the previous step is signed
    and/or encrypted accordingly. When receiving data, the processing
    order MUST be reversed.  This ensures that before sending, data is
    compressed before it is encrypted, 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:

         C: a starttls
         S: a OK TLS active

             From this point on, everything is encrypted.

         C: b login arnt tnra
         S: b OK Logged in as arnt
         C: c compress deflate
         S: d OK DEFLATE active

             From this point on, everything is compressed before being

    The following example demonstrates how a server may refuse to
    compress twice:

         C: c compress deflate

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4.  Compression Efficiency

    This section is informative, not normative.

    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 algorithm which can
    exploit repetition works efficiently. The APPEND command is an
    exception; clients which send many APPEND commands may want to
    surround large literals with flushes in the same way as is
    recommended for servers later in this section.

    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

    When using the zlib library (see [RFC1951]), the functions
    deflateInit2(), deflate(), inflateInit2() and inflate() suffice to

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    implement this extension. The windowBits value must be in the range
    -8 to -15, or else deflateInit2() uses the wrong format.
    deflateParams() can be used to improve compression rate and resource
    use. The Z_FULL_FLUSH argument to deflate() can be used to clear the
    dictionary (the receiving peer does not need to do anything).

    A client can improve downstream compression by implementing BINARY
    (defined in [RFC3516]) 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 possible
    boundary is 5k.

    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, to avoid trying
    to compress uncompressible attachments. A very simple strategy is to
    change the level to 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. More complex strategies are possible.

5.  Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form (ABNF) notation as specified in [RFC4234]. This syntax augments
    the grammar specified in [RFC3501]. [RFC4234] defines SP and
    [RFC3501] defines command-auth, capability and resp-text-code.

    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-auth =/ compress

        compress    = "COMPRESS" SP algorithm

        capability  =/ "COMPRESS=" algorithm
                      ;; multiple COMPRESS capabilities allowed

        algorithm   = "DEFLATE"

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        resp-text-code =/ "COMPRESSIONACTIVE"

    Note that due the syntax of capability names, future algorithm names
    must be atoms.

6.  Security Considerations

    As for TLS compression [RFC3749].

7.  IANA Considerations

    The IANA is requested to add COMPRESS=DEFLATE the list of IMAP
    capabilities. [Note to IANA: This is at]

    Note to IANA: This RFC does not specify the creation of a registry
    for compression mechanisms. The current feeling of the IMAP
    community is that is is unlikely that another compression mechanism
    will be added in the future. However, if this RFC is extended in the
    future by another RFC, and another compression mechanism is added at
    that time, it would then be appropriate to create a registry.

8.  Acknowledgements

    Eric Burger, Dave Cridland, Tony Finch, Ned Freed, Philip Guenther,
    Randall Gellens, Tony Hansen, Cullen Jennings, Stephane Maes, 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

9.  References

9.1. Normative References

    [RFC1951]  Deutsch, "DEFLATE Compressed Data Format Specification
               version 1.3", RFC 1951, Aladdin Enterprises, May 1996.

    [RFC2119]  Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119, Harvard University, March

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    [RFC3501]  Crispin, "Internet Message Access Protocol - Version
               4rev1", RFC 3501, University of Washington, June 2003.

    [RFC4234]  Crocker, Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 4234, Brandenburg
               Internetworking, Demon Internet Ltd, October 2005.

9.2. Informative References

    [RFC1962]  Rand, "The PPP Compression Control Protocol (CCP)", RFC
               1962, June 1996.

    [RFC3516]  Nerenberg, "IMAP4 Binary Content Extension", RFC 3516,
               Orthanc Systems, April 2003.

    [RFC3749]  Hollenbeck, "Transport Layer Security Protocol
               Compression Methods", RFC 3749, VeriSign, May 2004.

    [RFC4346]  Dierks, Rescorla, "The Transport Layer Security (TLS)
               Protocol, Version 1.1", RFC 4346, April 2006.

    [RFC4422]  Melnikov, Zeilenga, "Simple Authentication and Security
               Layer (SASL)", RFC 4422, Isode Limited, June 2006.

    [V42BIS]   ITU, "V.42bis: Data compression procedures for data
               circuit-terminating equipment (DCE) using error
               correction procedures",
               V.42bis, January 1990.

    [MNP]      Gilbert Held, "The Complete Modem Reference", Second
               Edition, Wiley Professional Computing, ISBN
               0-471-00852-4, May 1994.

10. Author's Address

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

    Fax: +49 89 4502 9758


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