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Versions: (draft-jayantheesh-imap-appendlimit-extension) 00 01 02 03 04 05 06 07 08 09 10 RFC 7889

Network Working Group                                        Jayantheesh
Internet-Draft                                                  Narendra
Intended status: Standards Track             Samsung Electronics America
Expires: June 20, 2016                                 December 18, 2015


                     The IMAP APPENDLIMIT Extension
            draft-ietf-imapapnd-appendlimit-extension-07.txt

Abstract

   This document defines an extension to the IMAP service whereby a
   server can inform the client about a maximum mail upload size,
   allowing the client to avoid sending APPEND commands that will fail
   because of the messages are too large.

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
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   This Internet-Draft will expire on June 20, 2016.

Copyright Notice

   Copyright (c) 2015 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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   2
   2.  APPENDLIMIT Extension . . . . . . . . . . . . . . . . . . . .   3
   3.  Mailbox specific APPENDLIMIT  . . . . . . . . . . . . . . . .   3
     3.1.  STATUS response to the STATUS command . . . . . . . . . .   4
     3.2.  STATUS response to the LIST command . . . . . . . . . . .   4
     3.3.  APPENDLIMIT behaviour . . . . . . . . . . . . . . . . . .   4
   4.  APPEND response . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Formal syntax . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Some IMAP servers have limits for mail upload size, and those limits
   are not published to the email client.  When the email client APPENDs
   a message with huge attachments, using non-synchronizing literals,
   the APPEND fails because of the upload limit, but the client has
   already sent the message data anyway.  This results in unnecessary
   resource usage.  Especially in the mobile device environment,
   appending mail with huge attachment consumes device resources like
   device battery power and mobile data.

   The IMAP APPENDLIMIT extension provides an ability to advertise
   maximum upload size allowed by the IMAP server, so that email client
   knows the size limitation beforehand.  By implementing this
   extension, IMAP server side processing of huge attachments above the
   maximum upload size can be avoided.

1.1.  Conventions and Terminology

   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.




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2.  APPENDLIMIT Extension

   An IMAP server that supports APPENDLIMIT extension advertises this by
   including the name APPENDLIMIT in its capability list in
   authenticated state.  The server may also advertise this extension
   before the user has logged in.  If this capability is omitted, no
   information is conveyed about the server's fixed maximum mail upload
   size.  IMAP server can publish the APPENDLIMIT capability in two
   formats.

   (a) APPENDLIMIT=<number>

   This indicates that the IMAP server has the same upload limit for all
   the mailboxes.  The following example, demonstrates the APPENDLIMIT
   capability with same upload limit for all mailboxes.

   C: t1 CAPABILITY
   S: * CAPABILITY IMAP4rev1 ID APPENDLIMIT=257890
   S: t1 OK foo

   (b) APPENDLIMIT

   The APPENDLIMIT capability without any value indicates that the IMAP
   server supports this extension, and that the client will need to
   discover upload limits for each mailbox, which might differ from
   mailbox to mailbox.  The following example, demonstrates the
   APPENDLIMIT capability without any value.

   C: t1 CAPABILITY
   S: * CAPABILITY IMAP4rev1 ID APPENDLIMIT
   S: t1 OK foo

   In this case the client SHOULD get APPENDLIMIT value by issuing
   STATUS or LIST command.  New response code APPENDLIMIT is added to
   get the mailbox specific limit.  Refer section 5 for response code
   syntax.

   IMAP client SHOULD be able to parse both kind of formats.  By looking
   at the upload size advertised by the IMAP server, client MUST NOT try
   to APPEND mail more than advertised limit.

3.  Mailbox specific APPENDLIMIT

   An IMAP server can have mailbox-specific APPENDLIMIT values, which
   will not be advertised as part of CAPABILITY response.  The IMAP
   server can publish specific values for each mailbox, and can publish
   "NIL" for a mailbox to convey that there is no APPENDLIMIT for that




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   mailbox.  The following subsections describe the changes to the
   STATUS and LIST commands in support of this situation.

3.1.  STATUS response to the STATUS command

   A new attribute APPENDLIMIT is added to get the limit set by the
   server for a mailbox as part of STATUS command.  IMAP client should
   issue STATUS command with APPENDLIMIT item to get the mailbox
   specific upload value.  The following example, demonstrates its
   usage.

   C: t1 STATUS INBOX (APPENDLIMIT)
   S: * STATUS INBOX (APPENDLIMIT 257890)
   S: t1 OK STATUS completed

   In the above example, APPENDLIMIT represents the maximum upload size
   for INBOX.

3.2.  STATUS response to the LIST command

   If the server advertises the LIST-STATUS capability [RFC5819], the
   client can issue LIST in combination with the STATUS return option to
   get the mailbox-specific upload value.  The following example,
   demonstrates its usage.

   C: t1 LIST "" % RETURN (STATUS (APPENDLIMIT))
   S: * LIST () "."  "INBOX"
   S: * STATUS "INBOX" (APPENDLIMIT 257890)
   S: t1 OK List completed.

   The IMAP server MUST recognize the APPENDLIMIT attribute and include
   an appropriate STATUS response for each matching mailbox.  Refer to
   Section 5 for the syntax.

   If the server does not support the STATUS return option on the LIST
   command, then the client should use the STATUS command instead.

3.3.  APPENDLIMIT behaviour

   Computing the APPENDLIMIT should be fast, and need not take ACLs,
   quotas, and other such information into account.  The APPENDLIMIT
   specifies one part of the policy, but an APPEND command can still
   fail due to issues related to ACLs and quotas issues, even if the
   message being appended is smaller than the APPENDLIMIT.







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4.  APPEND response

   If client uploads a mail which exceeds the maximum upload size set to
   that mailbox, then server SHALL reject the APPEND command with a
   tagged TOOBIG response code.  Refer [RFC4469] Section (4) for various
   APPEND response codes and its handling.

   Client SHOULD avoid use of non-synchronizing literals [LITERAL-],
   when maximum upload size supported by the IMAP server is unknown.
   Refer section 4.2.2.3 of [RFC4549] for usage of non-synchronizing
   literals and its risk for disconnected IMAP clients.

5.  Formal syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234] including the core
   rules in Appendix B.1.  [RFC3501] defines the non-terminals
   "capability" and "status-att", and [RFC4466] defines "status-att-
   val".

   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.

   capability /= "APPENDLIMIT" ["=" number]
               ;; capability is defined in RFC 3501

   status-att /= "APPENDLIMIT"
               ;; status-att is defined in RFC 3501

   status-att-val /= "APPENDLIMIT" SP (number / nil)
                   ;; status-att-val is defined in RFC 4466


   The number indicates the fixed maximum message size in octets that
   the server will accept.  An APPENDLIMIT number of 0 indicates the
   server will not accept any APPEND commands at all for the affected
   mailboxes.

6.  Security Considerations

   The IMAP APPENDLIMIT extension described in this document can
   conceivably be used to facilitate Denial-of-Service attacks by
   allowing an attacker to home in on a critical value right away.  The
   attacker might want to send a large data block to the server
   repeatedly, forcing the server to process the block, but would not
   want to limit the scope of its attack by filling an actual mailbox



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   with successful appends.  Without this extension, the attacker needs
   to guess: a too-small guess results in an appended message that takes
   up the user's quota, while a far-too-large guess might simply cause
   the server to terminate the connection because of suspected abuse.
   But with this extension, the attacker can immediately choose a value
   that's a little too large, but not so much as to trigger an "abuse"
   response, making it easier to mount such an attack.  To mitigate this
   extension's input to such an attack, a server might take a harder
   line on message sizes that are above the APPENDLIMIT value -- because
   the client knows the limit and should not even be trying to send such
   commands, a server might consider even a single attempt to be
   abusive, and terminate the IMAP connection straight away.

7.  IANA Considerations

   IANA is asked to add "APPENDLIMIT" to the IMAP Capabilities registry,
   using this document as its reference.

8.  Acknowledgements

   Thanks to Alexey Melnikov, Dave Cridland, Adrien de Croy, Michael M
   Slusarz, Timo Sirainen, Chris Newman, Pete Maclean, Jamie Nicolson,
   Stu Brandt, Bron Gondwana, Arnt Gulbrandsen, Cyrus Daboo, Jan
   Kundrat, Brandon Long and Barry Leiba for providing valuable
   comments.

9.  References

9.1.  Normative References

   [LITERAL-]
              Melnikov, A., "IMAP4 non-synchronizing literals,
              https://tools.ietf.org/html/draft-ietf-imapapnd-
              rfc2088bis-01 (work in progress)", October 2015.

   [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>.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5819]  Melnikov, A. and T. Sirainen, "IMAP4 Extension for
              Returning STATUS Information in Extended LIST", RFC 5819,
              DOI 10.17487/RFC5819, March 2010,
              <http://www.rfc-editor.org/info/rfc5819>.

9.2.  Informative References

   [RFC4549]  Melnikov, A., Ed., "Synchronization Operations for
              Disconnected IMAP4 Clients", RFC 4549,
              DOI 10.17487/RFC4549, June 2006,
              <http://www.rfc-editor.org/info/rfc4549>.

Authors' Addresses

   Jayantheesh S B
   Samsung Electronics America
   685 US Highway 202/206
   Bridgewater, NJ  08807
   USA

   Email: jayantheesh.sb@gmail.com


   Narendra Singh Bisht
   Samsung Electronics America
   685 US Highway 202/206
   Bridgewater, NJ  08807
   USA

   Email: narendrasingh.bisht@gmail.com












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