Network Working Group Arnt Gulbrandsen Request for Comments: DRAFT Oryx Mail Systems GmbH draft-gulbrandsen-imap-deflate-00.txt September 2003 The IMAP COMPRESS=DEFLATE extension Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 2003. All Rights Reserved. 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" [KEYWORDS]. Formal syntax is defined by [ABNF] as modified by [IMAP]. Gulbrandsen Expires March 2004 [Page 1]
Internet-draft September 2003 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 for patent reasons 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". 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 the data they send. Gulbrandsen Expires March 2004 [Page 2]
Internet-draft September 2003 The client MUST NOT send additional commands until it has seen the result of COMPRESS. If the client wants to use both TLS and compression, it SHOULD send STARTTLS before COMPRESS. 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 compress = "COMPRESS" SP astring Gulbrandsen Expires March 2004 [Page 3]
Internet-draft September 2003 Security considerations The proposed extension does not cause any security problems. It may marginally reduce the scope for plaintext attacks when used together with [TLS]. Credits (Your name here :) References [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd, November 1997. [IMAP] Crispin, M., "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, P., "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. Author's Address: Arnt Gulbrandsen Oryx Mail Systems GmbH Joseph-Dollinger-Bogen 14 D-80807 Muenchen Germany Phone: +49 89 32356-401 Fax: +49 89 32356-409 Email: arnt@oryx.com Gulbrandsen Expires March 2004 [Page 4]