Network Working Group Arnt Gulbrandsen
Request for Comments: DRAFT Oryx Mail Systems GmbH
draft-gulbrandsen-imap-deflate-02.txt February 2006
The IMAP COMPRESS=DEFLATE extension
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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.
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
(perhaps for patent reasons).
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".
Responses: None
Result: OK The server will compress its responses and expects the
client to compress its commands.
NO The connection already is compressed, or the server doesn't
support the requested mechanism.
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 responses.
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
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the data they send.
The client MUST NOT send additional commands until it has seen the
result of COMPRESS.
If both TLS and COMPRESS are in use, the data should be compressed
before it is encrypted (and decrypted before it is decompressed).
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
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.
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
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compress = "COMPRESS" SP astring
Security considerations
(As for [TLSCOMP] RFC 3749.)
IANA considerations
The IANA is requested to add COMPRESS=DEFLATE to the list of IMAP
extensions.
Credits
(Your name here :)
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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