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. 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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]