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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