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Network Working Group                                     A. GulbrandsenRequest for Comments: 4978                        Oryx Mail Systems GmbHCategory: Standards Track                                    August 2007The IMAP COMPRESS ExtensionStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   The COMPRESS extension allows an IMAP connection to be effectively   and efficiently compressed.   Table of Contents1. Introduction and Overview .......................................22. Conventions Used in This Document ...............................23. The COMPRESS Command ............................................34. Compression Efficiency ..........................................45. Formal Syntax ...................................................66. Security Considerations .........................................67. IANA Considerations .............................................68. Acknowledgements ................................................79. References ......................................................79.1. Normative References .......................................79.2. Informative References .....................................7Gulbrandsen                 Standards Track                     [Page 1]

RFC 4978              The IMAP COMPRESS Extension            August 20071.  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 Transport   Security Layer (TLS) [RFC4346], Simple Authentication and Security   layer (SASL) encryption, Virtual Private Networks (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.   - 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 that advertise this   extension SHOULD also advertise the TLS DEFLATE compression mechanism   as defined in [RFC3749].  IMAP clients MAY use either COMPRESS or TLS   compression, however, if the client and server support both, it is   RECOMMENDED that the client choose TLS compression.   The extension adds one new command (COMPRESS) and no new responses.2.  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].Gulbrandsen                 Standards Track                     [Page 2]

RFC 4978              The IMAP COMPRESS Extension            August 2007   In the examples, "C:" and "S:" indicate lines sent by the client and   server respectively. "[...]" denotes elision.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   will, of course, still be uncompressed.)  If the server issues a BAD   or NO response, 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/orGulbrandsen                 Standards Track                     [Page 3]

RFC 4978              The IMAP COMPRESS Extension            August 2007   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 TLS4.  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 that can   exploit repetition works efficiently.  The APPEND command is an   exception; clients that 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.Gulbrandsen                 Standards Track                     [Page 4]

RFC 4978              The IMAP COMPRESS Extension            August 2007   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 type is email headers.  These are equally compressible, and   benefit from using the same dictionary as the IMAP responses.   A third type 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   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 inRFC 3501, page 86).  Small literals are best left alone.  A possible   boundary is 5k.Gulbrandsen                 Standards Track                     [Page 5]

RFC 4978              The IMAP COMPRESS Extension            August 2007   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 incompressible attachments.  A very simple strategy is to   change the level to 0 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"       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 has added COMPRESS=DEFLATE to the list of IMAP capabilities.Gulbrandsen                 Standards Track                     [Page 6]

RFC 4978              The IMAP COMPRESS Extension            August 20078.  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.  References9.1.  Normative References   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification              version 1.3",RFC 1951, May 1996.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION              4rev1",RFC 3501, March 2003.   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF",RFC 4234, October 2005.9.2.  Informative References   [RFC1962]  Rand, D., "The PPP Compression Control Protocol (CCP)",RFC 1962, June 1996.   [RFC3516]  Nerenberg, L., "IMAP4 Binary Content Extension",RFC 3516,              April 2003.   [RFC3749]  Hollenbeck, S., "Transport Layer Security Protocol              Compression Methods",RFC 3749, May 2004.   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.1",RFC 4346, April 2006.   [RFC4422]  Melnikov, A. and  K. Zeilenga, "Simple Authentication and              Security Layer (SASL)",RFC 4422, 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.Gulbrandsen                 Standards Track                     [Page 7]

RFC 4978              The IMAP COMPRESS Extension            August 2007   [MNP]      Gilbert Held, "The Complete Modem Reference", Second              Edition, Wiley Professional Computing, ISBN 0-471-00852-4,              May 1994.Author's Address    Arnt Gulbrandsen    Oryx Mail Systems GmbH    Schweppermannstr. 8    D-81671 Muenchen    Germany    Fax: +49 89 4502 9758    EMail: arnt@oryx.comGulbrandsen                 Standards Track                     [Page 8]

RFC 4978              The IMAP COMPRESS Extension            August 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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, THE IETF TRUST 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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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                 Standards Track                     [Page 9]