Network Working Group Arnt Gulbrandsen
Request for Comments: DRAFT Oryx Mail Systems GmbH
October 2006
The IMAP COMPRESS Extension
draft-ietf-lemonade-compress-05.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-
Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society 2006.
Abstract
The COMPRESS extension allows an IMAP connection to be effectively
and efficiently compressed.
Gulbrandsen Expires January 2007 [Page 1]
Internet-draft August 2006
Table of Contents
1. Conventions Used in This Document . . . . . . . . . . . . . . 2
2. Introduction and Overview . . . . . . . . . . . . . . . . . . 2
3. The COMPRESS Command . . . . . . . . . . . . . . . . . . . . . 3
4. Compression Efficiency . . . . . . . . . . . . . . . . . . . . 4
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 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
Gulbrandsen Expires January 2007 [Page 2]
Internet-draft August 2006
practical for COMPRESS, whereas typical TLS libraries don't
currently allow that.
- 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, 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. 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 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
Gulbrandsen Expires January 2007 [Page 3]
Internet-draft August 2006
the data they send.
When COMPRESS is combined with TLS or [SASL] layers, the order of
processing data to be sent SHALL be to first COMPRESS, then SASL,
and finally TLS. When receiving data, the processing order MUST be
reversed. This ensures that 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 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
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.
Gulbrandsen Expires January 2007 [Page 4]
Internet-draft August 2006
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
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
Gulbrandsen Expires January 2007 [Page 5]
Internet-draft August 2006
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
capability =/ "COMPRESS=" algorithm
;; multiple COMPRESS capabilities allowed
algorithm = "DEFLATE"
Note that due the syntax of capability 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=DEFLATE the list of IMAP
extensions.
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
Gulbrandsen Expires January 2007 [Page 6]
Internet-draft August 2006
algorithm will be added in the future. However, if this RFC is
extended in the future by another RFC, and another compression 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, 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
[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] Melnikov, Zeilenga, "Simple Authentication and Security
Layer (SASL)", RFC 4422, Isode Limited, June 2006
[BINARY] Nerenberg, "IMAP4 Binary Content Extension", Orthanc
Gulbrandsen Expires January 2007 [Page 7]
Internet-draft August 2006
Systems, April 2003.
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.
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Gulbrandsen Expires January 2007 [Page 8]
Internet-draft August 2006
Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
Disclaimer of Validity
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Gulbrandsen Expires January 2007 [Page 9]
