wdiff draft-ietf-lemonade-compress-03.txt draft-ietf-lemonade-compress-04.txt

Network Working Group Arnt Gulbrandsen
Request for Comments: DRAFT Oryx Mail Systems GmbH
July
September 2006

The IMAP COMPRESS Extension
draft-ietf-lemonade-compress-03.txt
draft-ietf-lemonade-compress-04.txt

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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

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4. Compression Efficiency . . . . . . . . . . . . . . . . . . . . 4
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 5 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
10. Author's Address . . . . . . . . . . . . . . . . . . . . . . 8
11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 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 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

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/MNP compression, COMPRESS offers much better
compression efficiency, and can be used together with TLS, SASL
encryption, VPNs etc. Compared to TLS compression [TLSCOMP],
COMPRESS has the following (dis)advantages:

- COMPRESS can be implemented easily by IMAP servers and clients.
At present, TLS compression is not widely implemented. In the
LEMONADE WG, the general consent is that libraries implementing
TLS compression will not be available soon enough for LEMONADE.

- IMAP compression efficiency benefits from an API that permits
flushing the compressor's dictionary at the right point. This is

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practical for COMPRESS, whereas typical TLS libraries don't
currently allow that.

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- When a TLS librarly implements compression, all protocols that use
TLS automatically are compressed (in LEMONADE's case, SMTP, IMAP,
and some notification protocol), whereas 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 is standard, widely
available, unencumbered by patents and fairly efficient. efficient, so it is
the only algorithm defined by this document.

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 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 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, and COMPRESS. If the server
response was BAD or NO, the client MUST NOT turn on compression.

If the server issues an OK response, the server MUST compress
starting with the first response after the OK. CRLF ending the OK
response. (Responses issued by the server before the OK response
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

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the data they send.

If both COMPRESS and STARTTLS and/or a [SASL] security layer 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:

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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

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 algorith which can
exploit repetition works efficiently. The APPEND command is an
exception; clients which send many APPEND commands may want to send
surround large literals with flushes in the same way that servers do. as is
recommended for server below.

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.

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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.

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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
deflateInit2(), deflate(), inflateInit2() and inflate() suffice to
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.

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 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.

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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

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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

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

algorithm = "DEFLATE"

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

6. Security Considerations

As for [TLSCOMP] RFC 3749.

7. IANA Considerations

The IANA is requested to add COMPRESS=... to COMPRESS=DEFLATE the list of IMAP
extensions.

The IANA is requested to maintain one new registry: IMAP Compression
Algorithms. The registry's purpose is

Note to register compression
algorithms that may be used with this extension. New IMAP
algorithms MUST be defined in a standards track or IESG approved
experimental RFC. New IMAP compression algorithms MUST include IANA: This RFC does not specify the
following information as part creation of their definition:

algorithm identifier
standard commands affected
specification reference
discussion

This a registry is available at URL [RFC-EDITOR NOTE: please insert
URL
for compression mechanisms. The current feeling of registry]

One the IMAP
community is that is is unlikely that another compression algorithm is defined
will be added in the future. However, if this document, with RFC is extended in the
following registration definition:
future by another RFC, and another compression is added at that
time, it would then be appropriate to create a registry.

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algorithm identifier: DEFLATE
standard commands affected: none
specification reference: RFC 1951 and XXXX
discussion: see RFC XXXX

[RFC-EDITOR NOTE: change XXXX to this RFC number]

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.

9.2. Informative References

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

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

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[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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