Network Working Group Arnt Gulbrandsen
Request for Comments: DRAFT Oryx Mail Systems GmbH
draft-ietf-lemonade-compress-01.txt June 2006
The IMAP COMPRESS=DEFLATE Extension
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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.
Conventions Used in This Document
The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
NOT", and "MAY" in this document are to be interpreted as described
in "Key words for use in RFCs to Indicate Requirement Levels"
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[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.
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 few or no implementations do so in practice.
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.
The COMPRESS Command
Arguments: Name of compression mechanism: "DEFLATE".
Direction: "UP", "DOWN" or "BOTH".
Responses: None
Result: OK The server will compress its responses (if the direction
is DOWN or BOTH) and expects the client to compress its
commands (if the direction is UP or BOTH).
NO The connection already is compressed, or the server
doesn't support the requested mechanism, or the direction
specified is unknown.
BAD Command unknown or invalid argument.
The COMPRESS command instructs the server to use the named
compression mechanism ("DEFLATE" is the only one defined) for future
commands and/or responses. If the direction specified is "UP", only
commands are compressed. If the direction specified is "DOWN", only
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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.
The client MUST NOT send additional commands until it has seen the
result of COMPRESS.
If both SASL/TLS 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.
Example
This example shows a simple login sequence. The client uses TLS for
privacy and [DEFLATE] for compression.
S: * OK [CAPABILITY IMAP4REV1 STARTTLS COMPRESS=DEFLATE]
C: a starttls
S: a OK
C: b compress deflate
S: b OK
C: c login arnt tnra
S: c OK
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 quotes works efficiently. The APPEND command is an
exception; clients which send many APPEND commands may want to take
special care.
Downstream has the unusual property that 3-4 kinds of data are sent,
confusing all dictionary-based compression algorithms.
The first 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.
The second is email headers. These are equally compressible, and
benefit from using the same dictionary as the IMAP responses.
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The 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 from IMAP to the attachment's
format, and then back.
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 a
6-bit bytes in an 8-bit format.
A few file formats aren't compressible using deflate, e.g. .gz, .zip
and .jpg files.
According to the author's measurements, the compression level used
makes little difference. zlib's level 1 compresses IMAP almost as
well as level 9, and for the receiver, level 1 seems to require
(just a tiny bit) pmore CPU than level 9. Independent verification
is strongly desired.
Implementation Notes
When using the zlib library (see [DEFLATE]), the functions
deflateInit(), deflate(), inflateInit() and inflate() suffice to
implement this extension.
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.
A client can improve downstream compression by implementing [BINARY]
and using FETCH BINARY instead of FETCH BODY.
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
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literals (before and after '*CHAR8' in the definition of literal in
RFC 3501, page 86).
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.
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 SP ( "UP" / "DOWN" /
"BOTH" )
algorithm = "DEFLATE"
Security considerations
(As for [TLSCOMP] RFC 3749.)
IANA Considerations
The IANA is requested to add COMPRESS=DEFLATE to the list of IMAP
extensions.
Credits
Quite a few people on the LEMONADE mailing list have offered
comments, including Dave Cridland, Ned Freed and Tony Hansen. And
various people in the rooms at meetings. Send me mail, I'll add you.
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Open Issues
Both ends can already disable compression at any point by calling
deflateParams(). The only missing feature is for the client to
request that the server stop compressing - are there use-cases for
that? It requires adding more server-side state, so I'm wary.
What text and numbers are needed wrt. compression levels? A bit of
solid information is not amiss.
Normative References
[ABNF] Crocker, Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, Internet Mail
Consortium, Demon Internet Ltd, November 1997.
[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.
Informative References
[TLSCOMP] Hollenbeck, "Transport Layer Security Protocol
Compression Methods", RFC 3749, VeriSign, May 2004.
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Author's Address
Arnt Gulbrandsen
Oryx Mail Systems GmbH
Schweppermannstr. 8
D-81671 Muenchen
Germany
Fax: +49 89 4502 9758
Email: arnt@oryx.com
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