Network Working Group Arnt Gulbrandsen
Request for Comments: DRAFT Oryx Mail Systems GmbH
Intended Status: Proposed Standard April 2007
The IMAP COMPRESS Extension
draft-ietf-lemonade-compress-08.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
clients.
- 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
compresses.
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:
S: * OK [CAPABILITY IMAP4REV1 STARTTLS COMPRESS=DEFLATE]
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
encrypted.
The following example demonstrates how a server may refuse to
compress twice:
S: * OK [CAPABILITY IMAP4REV1 STARTTLS COMPRESS=DEFLATE]
[...]
C: c compress deflate
S: c NO [COMPRESSIONACTIVE] DEFLATE active via TLS
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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
compressor.
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
http://www.iana.org/assignments/imap4-capabilities]
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
mailbox.
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
1997.
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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", http://www.itu.int/rec/T-REC-
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
Germany
Fax: +49 89 4502 9758
Email: arnt@oryx.com
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