[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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)                              July 26, 2015
Intended status: Standards Track
Expires: January 27, 2016


                    IMAP4 non-synchronizing literals
                 draft-ietf-imapapnd-rfc2088bis-00.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 which does not require this network
   round trip.

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

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
   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."

   This Internet-Draft will expire on January 27, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





Melnikov                Expires January 27, 2016                [Page 1]


Internet-Draft      IMAP4 non-synchronizing literals           July 2015


   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   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
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

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

1.  Specification

   The non-synchronizing literal is added 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




Melnikov                Expires January 27, 2016                [Page 2]


Internet-Draft      IMAP4 non-synchronizing literals           July 2015


   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 which 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 must use
   synchronizing literals instead.  The difference between "LITERAL+"
   and "LITERAL-" extensions is explained in Section 4.

   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
   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 end 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 following line as part of the same command.  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

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.






Melnikov                Expires January 27, 2016                [Page 3]


Internet-Draft      IMAP4 non-synchronizing literals           July 2015


3.  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 difficilt to handle on
   the server side.  When a non synchronizing literal is used by a
   client which is too big for the server to accept, a compliant
   LITERAL+ server implementation has to make a choice between several
   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 size is big.

   2.  Send the untagged BYE response explaining the reason for
       rejecting the literal 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.  [[CREF1: Possibly also
       send the ALERT response code?]]

   The problem described above is most common when using the APPEND
   command, because most of commands don't need to send lots of data
   from the client to the server.  Some server implementations impose
   limits on literal (both synchronizing and non synchronizing) accepted
   from clients in order to protect from 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 4.

   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.

4.  LITERAL- capability

   "LITERAL-" extension is almost identical to "LITERAL+", with one
   exception: when "LITERAL-" is advertised, non synchronizing literals
   used in APPEND (and extensions to APPEND such as MULTIAPPEND
   [RFC3502] and CATENATE [RFC4469]) MUST NOT be larger than 4096 bytes.



Melnikov                Expires January 27, 2016                [Page 4]


Internet-Draft      IMAP4 non-synchronizing literals           July 2015


   When any literal used in APPEND is larger than 4096, RFC 3501
   synchronizing literals MUST be used instead.  A "LITERAL-" compliant
   server which encounters a non synchronizing literal in APPEND larger
   than 4096 bytes MUST reject such APPEND command with a tagged BAD
   response.  [[CREF2: Add a new response code to signal this
   condition?]]

   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.)

5.  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.

6.  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>

7.  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.  Section 3 motivates creation of "LITERAL-" extension that
   partially improves the situation.

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






Melnikov                Expires January 27, 2016                [Page 5]


Internet-Draft      IMAP4 non-synchronizing literals           July 2015


8.  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 updated the above registry to
   include the entry for LITERAL+ capability pointing to this document.

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

9.  To Do

   Exact semantics of LITERAL- is still in flux.

10.  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 and Arnt Gulbrandsen.

11.  References

11.1.  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>.



Melnikov                Expires January 27, 2016                [Page 6]


Internet-Draft      IMAP4 non-synchronizing literals           July 2015


11.2.  Informative References

   [APPENDLIMIT]
              Bisht, N., "The IMAP APPENDLIMIT Extension", draft-ietf-
              imapapnd-appendlimit-extension-00 (work in progress), July
              2015.

   [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>.

   [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>.

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.

   LITERAL- capability description.

Author's Address

   Alexey Melnikov (editor)
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com














Melnikov                Expires January 27, 2016                [Page 7]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/