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Versions: (draft-melnikov-rfc2088bis) 00 01 02 03 04 RFC 7888

Network Working Group                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Obsoletes: 2088 (if approved)                              March 6, 2016
Intended status: Standards Track
Expires: September 7, 2016


                    IMAP4 non-synchronizing literals
                 draft-ietf-imapapnd-rfc2088bis-04.txt

Abstract

   The Internet Message Access Protocol (RFC 3501) contains the
   "literal" syntactic construct for communicating strings.  When
   sending a literal from client to server, IMAP requires the client to
   wait for the server to send a command continuation request between
   sending the octet count and the string data.  This document specifies
   an alternate form of literal that does not require this network round
   trip.

   This document specifies 2 IMAP extensions: LITERAL+ and LITERAL-.
   LITERAL+ allows the alternate form of literals in all IMAP commands.
   LITERAL- is the same as LITERAL+, but disallows the alternate form of
   literals unless they are 4096 bytes or less.

   This document obsoletes RFC 2088.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 7, 2016.








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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   3
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Considerations on when to use and not to use synchronizing
       literals  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  LITERAL- capability . . . . . . . . . . . . . . . . . . . . .   5
   6.  Interaction with BINARY extension . . . . . . . . . . . . . .   6
   7.  Interaction with MULTIAPPEND extension  . . . . . . . . . . .   6
   8.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     12.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Changes since RFC 2088 . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8





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1.  Introduction

   The Internet Message Access Protocol [RFC3501] contains the "literal"
   syntactic construct for communicating strings.  When sending a
   literal from client to server, IMAP requires the client to wait for
   the server to send a command continuation request between sending the
   octet count and the string data.  This document specifies an
   alternate form of literal that does not require this network round
   trip.

   This document specifies 2 IMAP extensions: LITERAL+ and LITERAL-.
   LITERAL+ allows the alternate form of literals (called "non-
   synchronized literals" below) in all IMAP commands.  LITERAL- is the
   same as LITERAL+, but disallows the alternate form of literals unless
   they are 4096 bytes or less.

2.  Requirements Notation

   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].

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.  If a single "C:" or "S:" label applies to
   multiple lines, then the line breaks between those lines are for
   editorial clarity only and are not part of the actual protocol
   exchange.

3.  Specification

   The non-synchronizing literal is added as an alternate form of
   literal, and may appear in communication from client to server
   instead of the IMAP [RFC3501] form of literal.  The IMAP form of
   literal, used in communication from client to server, is referred to
   as a synchronizing literal.  The non-synchronizing literal form MUST
   NOT be sent from server to client.

   Non-synchronizing literals may be used with any IMAP server
   implementation that returns "LITERAL+" or "LITERAL-" as one of the
   supported capabilities to the CAPABILITY command.  If the server does
   not advertise either of the above capabilities, the client can only
   use synchronizing literals.  The difference between "LITERAL+" and
   "LITERAL-" extensions is explained in Section 5.

   The non-synchronizing literal is distinguished from the original
   synchronizing literal by having a plus ('+') between the octet count
   and the closing brace ('}').  The server does not generate a command
   continuation request in response to a non-synchronizing literal, and



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   clients are not required to wait before sending the octets of a non-
   synchronizing literal.

   The protocol receiver of an IMAP server MUST check the end of every
   received line (a sequence of octets that ends with a CRLF) for an
   open brace ('{') followed by an octet count, a plus ('+'), and a
   close brace ('}') immediately preceeding the CRLF.  If it finds this
   sequence, it is the octet count of a non-synchronizing literal and
   the server MUST treat the specified number of following octets and
   the the following line ([RFC3501]) as part of the same command.

   It's important to note that the literal is not delimited by CRLF.  It
   ends after the number of bytes specified by the octet count, and the
   current command continues from there.  There might be a CRLF
   immediately after, which ends the command.  Or there might be more
   octets, specifying other command parameters, before the CRLF.  If a
   SP (space) character is needed between parameters, it's important
   that the SP appear after the literal, in its appropriate place.

   A server MAY still process commands and reject errors on a line-by-
   line basis, as long as it checks for non-synchronizing literals at
   the end of each line.

   Example:

   C: A001 LOGIN {11+}
   C: FRED FOOBAR {7+}
   C: fat man
   S: A001 OK LOGIN completed

   This is semantically equivalent to this version that uses quoted
   strings instead of literals:

   C: A001 LOGIN "FRED FOOBAR" "fat man"
   S: A001 OK LOGIN completed

   Note that the space after FOOBAR in the first version corresponds
   to the space between the two quoted strings in the second.

4.  Considerations on when to use and not to use synchronizing literals

   This section is important to understand for both client and server
   developers of this IMAP extension.

   While non-synchronizing literals have clear advantages for clients,
   such as simplicity of use, they might be more difficult to handle on
   the server side.  When a client uses a non-synchronizing literal that
   is too big for the server to accept, a compliant LITERAL+ server



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   implementation has to make a choice between couple non-optimal
   choices:

   1.  Read the number of bytes specified in the non-synchronizing
       literal and reject the command that included the literal anyway.
       (The server is allowed to send the tagged BAD/NO response before
       reading the whole non-synchronizing literal.)  This is quite
       wasteful on bandwidth if the literal is large.

   2.  Send an untagged BYE response explaining the reason for rejecting
       the literal (possibly accompanied by an ALERT response code in
       another response) and close the connection.  This will force the
       client to reconnect or report the error to the user.  In the
       latter case the error is unlikely to be understandable to the
       user.  Additionally, some naive clients are known to blindly
       reconnect in this case and repeat the operation that caused the
       problem, introducing an infinite loop.

   The problem described above is most common when using the APPEND
   command, because most commands don't need to send lots of data from
   the client to the server.  Some server implementations impose limits
   on literals (both synchronizing and non-synchronizing) accepted from
   clients in order to defend against denial-of-service attacks.
   Implementations can generally impose much lower limits on literal
   sizes for all commands other than APPEND.  In order to address
   literal size issue in APPEND, this document introduces a new
   extension "LITERAL-", described in Section 5.

   The situation can also be improved by implementing support for the
   APPENDLIMIT extension [APPENDLIMIT], which allows a server to
   advertise its APPEND limit, so that well behaved clients can check it
   and avoid uploading big messages in the first place.

5.  LITERAL- capability

   The "LITERAL-" extension is almost identical to "LITERAL+", with one
   exception: when "LITERAL-" is advertised, non-synchronizing literals
   used in any command MUST NOT be larger than 4096 bytes.  Any literal
   larger than 4096 bytes MUST be sent as an RFC 3501 synchronizing
   literal.  A "LITERAL-" compliant server that encounters a non-
   synchronizing literal larger than 4096 bytes proceeds as described in
   Section 4.  If responding to an APPEND command, the tagged BAD
   response MUST contains the TOOBIG response code [RFC4469].  If
   responding with untagged BYE response, it SHOULD include the TOOBIG
   response code.  Note that the form of the non-synchronizing literal
   does not change: it still uses the "+" in the literal itself, even if
   the applicable extension is "LITERAL-".




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   Because "LITERAL-" is a more restricted version of "LITERAL+", IMAP
   servers MUST NOT advertise both of these capabilities at the same
   time.  (A server implementation can choose to have a configuration
   option to pick which one to advertise.)

6.  Interaction with BINARY extension

   RFC 4466 [RFC4466] updated the non-terminal "literal8" defined in
   [RFC3516] to allow for non-synchronizing literals if both [RFC3516]
   and "LITERAL+" extensions are supported by the server.

   This document also allows use of this extended "literal8" syntax when
   both [RFC3516] and "LITERAL-" extensions are supported by the server.

7.  Interaction with MULTIAPPEND extension

   RFC 3502 [RFC3502] describes MULTIAPPEND extension and how it can be
   used with LITERAL+.  The LITERAL- extension can be used with the
   MULTIAPPEND extension in the same way.

8.  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
   [RFC3501].

     literal = "{" number ["+"] "}" CRLF *CHAR8
                ; Number represents the number of CHAR8 octets

     CHAR8   = <defined in RFC 3501>

     literal8 = <defined in RFC 4466>

9.  Security Considerations

   Use of non-synchronizing literals can consume extra resources (e.g.
   memory) on IMAP servers and can be used for denial-of-service
   attacks.  The "LITERAL-" extension partially improved this situation.

   This document doesn't raise any other security concerns not already
   raised by [RFC3501].








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10.  IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located
   at:

      http://www.iana.org/assignments/imap-capabilities

   This document requests that IANA update the above registry replace
   the reference for LITERAL+ to point to this document.

   This document also requests that IANA add "LITERAL-" capability
   pointing to this document to the above registry.

11.  Acknowledgments

   John G.  Myers edited the original LITERAL+ extension.

   Valuable comments, both in agreement and in dissent, were received
   from Dave Cridland, Michael M Slusarz, Arnt Gulbrandsen, Jayantheesh
   S B., Jamie Nicolson, Barry Leiba and SM.

12.  References

12.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <http://www.rfc-editor.org/info/rfc3501>.

   [RFC3516]  Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
              DOI 10.17487/RFC3516, April 2003,
              <http://www.rfc-editor.org/info/rfc3516>.

   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006,
              <http://www.rfc-editor.org/info/rfc4466>.






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   [RFC4469]  Resnick, P., "Internet Message Access Protocol (IMAP)
              CATENATE Extension", RFC 4469, DOI 10.17487/RFC4469, April
              2006, <http://www.rfc-editor.org/info/rfc4469>.

12.2.  Informative References

   [APPENDLIMIT]
              SrimushnamBoovaraghamoorthy, J. and N. Bisht, "The IMAP
              APPENDLIMIT Extension", draft-ietf-imapapnd-appendlimit-
              extension-10 (work in progress), January 2016.

   [RFC3502]  Crispin, M., "Internet Message Access Protocol (IMAP) -
              MULTIAPPEND Extension", RFC 3502, DOI 10.17487/RFC3502,
              March 2003, <http://www.rfc-editor.org/info/rfc3502>.

Appendix A.  Changes since RFC 2088

   Added IANA registration.

   Updated references.  Also updated considerations about interactions
   of IMAP extensions.

   Additional implementation considerations based on the IMAP mailing
   list discussions.

   Added description of a new capability: LITERAL- .

Author's Address

   Alexey Melnikov (editor)
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   UK

   Email: Alexey.Melnikov@isode.com















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