Re: The IMAP APPENDLIMIT Extension

INTERNET-DRAFT                                           Jayantheesh S B 
Intended status: Standards Track                                 Samsung
Expires: July 2015                                  Narendra Singh Bisht
                                                                 Samsung
                                                        January 21, 2015

                      The IMAP APPENDLIMIT Extension
                draft-jayantheesh-imap-appendlimit-extension-03

Abstract

This memo defines an extension to the IMAP service whereby a server can
advertise its capability, to support maximum mail upload size using
CAPABILITY, SELECT/EXAMINE and LIST command.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1. Conventions and Terminology . . . . . . . . . .  . . . . .   2
   2. APPENDLIMIT Extension . . . . . . . . . . . . . . . . . . . . .  2
   3. Mailbox specific APPENDLIMIT . . . . . . . . . . . . . . . . . . 3
     3.1. SELECT response . . . . . . . . . . . . . . . . . . . . . .  4
     3.2. LIST response . . . . . . . . . . . . . . . . . . . . . . .  4
   4. APPEND response . . . . . . . . . . . . . . . . . . . . . . . .  5
   5. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6. Security Considerations  . . . . . . . . . . . . . . . . . . . . 6
   7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  6
   8. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
    8.1 Normative References . . . . . . . . . . . . . . . . . . . . . 6
    8.2 Informative References . . . . . . . . . . . . . . . . . . . . 7
   9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . .  7
   10. Author's Address . .. . . . . . . . . . . . . . . . . . . . . . 7
   

1. Introduction

Several IMAP server have limitation for mail upload size which is not
published to the email client.  When email client APPEND a mail with
huge attachments, it fails due to size restriction set by the IMAP
server.  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.

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 RFC 2119.

Example lines prefaced by "C:" are sent by the client and ones prefaced
by "S:" by the server.  The five characters [...] means that something
has been elided.

2. APPENDLIMIT Extension

An IMAP server that supports APPENDLIMIT advertises this by including
the word APPENDLIMIT in its capability list.  IMAP server shall publish
the supported mail upload size as part of CAPABILITY response.  The

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advertised upload limit is common across the mailboxes, but client
can still issue SELECT/EXAMINE or LIST command to get the mailbox
specific upload limit set by the IMAP server.  In this case,
APPENDLIMIT value obtained as part of SELECT/EXAMINE or LIST command
takes precedence over the value returned as part of CAPABILITY
response.

The following example, demonstrates the APPENDLIMIT capability with
mailbox limit.

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

If APPENDLIMIT value is omitted in CAPABILITY response, then client
SHOULD issue SELECT/EXAMINE or LIST command to get the mailbox specific
limit set by the server.  New response code APPENDLIMIT is added to get
the mailbox specific limit.  Refer section 5 for response code syntax.

The following example demonstrates, its usage.

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

C: t2 SELECT INBOX
S: * 172 EXISTS
S: * OK [APPENDLIMIT 257890] Maximum upload limit
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
S: t2 OK [READ-WRITE] SELECT completed

By looking at the upload size restriction set by the IMAP server, client
MUST not try to upload mail more than advertised limit in the APPEND
command. 

3. Mailbox specific APPENDLIMIT

IMAP server can still have mailbox specific APPENDLIMIT restriction,
which may not be advertised as part of CAPABILITY response.  In this
case, client can issue SELECT/EXAMINE command to get the per mailbox
specific limit set by the server.  Similarly, if client wish to know
the mailbox specific limit in authenticated state, can be done by
issuing the LIST command in combination with STATUS command.



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3.1 SELECT response

Client can get the per mailbox append limit by issuing the SELECT/
EXAMINE command.  APPENDLIMIT size to this mailbox is obtained as part
of untagged OK response.  In this case, this APPENDLIMIT value will
supersede the value received as part of CAPABILITY response.  If no
per-mailbox APPENDLIMIT is specified for a folder, but the server did
specify a common APPENDLIMIT in the CAPABILITY response, then the
common APPENDLIMIT applies to that folder.

C: t2 SELECT INBOX
S: * 172 EXISTS
S: * OK [APPENDLIMIT 257890] Maximum upload limit
S: [...]
S: t2 OK [READ-WRITE] SELECT completed


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

OK [APPENDLIMIT <n>] Maximum upload limit for this mailbox, in bytes.
                     Refer to section 5 for more information.  If this
                     is missing, the client can always honour the
                     value received as part of CAPABILITY response.


3.2 LIST response

IMAP client can get the mailbox specific APPENDLIMIT in authenticated
state, where it do not need to issue SELECT/EXAMINE command.  LIST
command in combination with STATUS command can be issued to get the per
mailbox specific APPENDLIMIT set by the server.  Refer RFC 5819 for the
usage of LIST command in combination with STATUS command.  Note that a
server implementing this extension, is syntactically compatible with
RFC 5819, however support for RFC 5258 or RFC 5819 is not required,
when implementing this extension.

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.

New attribute APPENDLIMIT is added to get the limit set by the server
for this mailbox as part of STATUS command.  The STATUS response occurs
as a result of an STATUS command.  It returns the mailbox name that

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matches the STATUS specification and the requested mailbox status
information.  IMAP server should recognize an extra "RETURN (STATUS
 (APPENDLIMIT))" at the end of a LIST command and emit an extra STATUS
response for each matching mailbox.  Refer to section 5 for the syntax.

Invoking STATUS command with APPENDLIMIT is also acceptable.  Below
example demonstrates, its usage.

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

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 RFC 4469 Section (4) for various
APPEND response codes and its handling.

Client can avoid use of LITERAL+, when maximum upload size
supported by the IMAP server is unknown. 

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", "resp-text-code" and "status-att".
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.
    
appendlimit-cap = "APPENDLIMIT" ["=" number]
capability /= appendlimit-cap

appendlimit-respcode = "APPENDLIMIT" SP number
resp-text-code /= appendlimit-respcode

appendlimit-status-att = "APPENDLIMIT"
status-att /=appendlimit-status-att

A number indicating the fixed maximum message size in bytes that the
server will accept.  APPENDLIMIT=0 indicates the server SHALL not
accept APPEND command due to size restriction.  The syntax of the
parameter follows the augmented BNF notation of [RFC5234].  If this
capability is omitted, no information is conveyed about the server's
fixed maximum mail upload size.
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6. Security Consideration

It is believed that this extension doesn't add any new security
considerations that are not already present in the base IMAP
protocol [RFC3501].

7. IANA Considerations

The IANA is requested to add APPENDLIMIT to the IMAP4 Capabilities
Registry.  [[Note to RFC-editor: please remove the following before
publication: This registration should take place at the following
location: http://www.iana.org/assignments/imap4-capabilities]]

8. References

8.1 Normative References

The following documents contain definitions or specifications that
are necessary to understand this document properly

[RFC2119]  Bradner, "Key words for use in RFCs to Indicate
           Requirement Levels", RFC 2119, Harvard University, March
           1997.
           
[RFC3501]  Crispin, "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1",
           University of Washington, March 2003

[RFC5234]  Crocker, Overell, "Augmented BNF for Syntax
           Specifications: ABNF", RFC 5234, Brandenburg
           Internetworking, Demon Internet Ltd, January 2008

[RFC5322]  P. Resnick, Ed, "Internet Message Format", RFC 5322,
           Qualcomm Incorporated, October 2008

[RFC2088]  J. Myers, Carnegie Mellon, "IMAP4 non-synchronizing
           literals",  January 1997
		   
[RFC4469]  P. Resnick, "Internet Message Access Protocol (IMAP)
           CATENATE Extension",  April 2006
		   
[RFC5819]  A. Melnikov, T. Sirainen, "IMAP4 Extension for Returning
           STATUS Information in Extended LIST",  March 2010
		   
[RFC5258]  A. Melnikov, B. Leiba, "Internet Message Access Protocol
           version 4 - LIST Command Extensions",  March 2010


		   
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8. 2 Informative References

The following documents describe related protocols:

[RFC2087]  Myers, J., "IMAP4 QUOTA extension", RFC 2087,
           January 1997

[RFC7377]  B. Leiba, A. Melnikov, "IMAP4 Multimailbox SEARCH
           Extension", RFC 7377, October 2014

9. Acknowledgement

TBD


10. Author's Address

Jayantheesh S B
Samsung Telecommunications America,
685 US Highway 202/206,
Bridgewater, NJ 08807.
USA
Email: [email protected]

Narendra Singh Bisht
Samsung Telecommunications America,
685 US Highway 202/206,
Bridgewater, NJ 08807.
USA
Email: [email protected]

















	
	
	
	

	
	
	
	
	
	
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