Network Working Group                                     A. GulbrandsenRequest for Comments: 4978                        Oryx Mail Systems GmbHCategory: Standards Track                                    August 2007                      The 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 Contents   1. Introduction and Overview .......................................2   2. Conventions Used in This Document ...............................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 .....................................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