Internet DRAFT - draft-ietf-imapapnd-appendlimit-extension
draft-ietf-imapapnd-appendlimit-extension
Network Working Group J. SrimushnamBoovaraghamoorthy
Internet-Draft N. Bisht
Intended status: Standards Track Samsung Electronics America
Expires: July 11, 2016 January 8, 2016
The IMAP APPENDLIMIT Extension
draft-ietf-imapapnd-appendlimit-extension-10.txt
Abstract
This document defines an extension to the IMAP service whereby a
server can inform the client about maximum message upload sizes,
allowing the client to avoid sending APPEND commands that will fail
because the messages are too large.
Status of This Memo
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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 . . . . . . . . . . . . . . . . . . . . . . . 4
5. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
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 message 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 message with huge attachments consumes device
resources like device battery power and mobile data.
The IMAP APPENDLIMIT extension provides an ability to advertise a
maximum upload size allowed by the IMAP server, so that the 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 the APPENDLIMIT extension advertises
this by including the name APPENDLIMIT in its capability list in the
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. An 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
mailboxes. The following example demonstrates the APPENDLIMIT
capability with the 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 can get an APPENDLIMIT value by either
issuing a STATUS or a LIST command.
An IMAP client implementing this extension should be able to parse
both mailbox-specific and global APPENDLIMIT responses. By looking
at the upload size advertised by the IMAP server a client can avoid
trying to APPEND mail more than the advertised limit.
3. Mailbox-specific APPENDLIMIT
An IMAP server can have mailbox-specific APPENDLIMIT values, which
will not be advertised as part of the 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
mailbox. The following subsections describe the changes to the
STATUS and LIST commands in support of this situation.
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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 a STATUS command. An IMAP client
should issue a STATUS command with an 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.
4. APPEND response
If a client uploads a message which exceeds the maximum upload size
set for that mailbox, then the server SHALL reject the APPEND command
with a tagged TOOBIG response code. Refer to [RFC4469] Section 4 for
various APPEND response codes and its handling.
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A client SHOULD avoid use of non-synchronizing literals [LITERAL-],
when the maximum upload size supported by the IMAP server is unknown.
Refer to 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
This extension provides additional information that cooperative
clients can use as an optimization, and does not introduce new
security concerns. This extension does not address abusive clients
that intend to consume server resources, and servers will still have
to take action to disconnect and/or restrict access to clients that
exhibit abusive behavior.
7. IANA Considerations
IANA is asked to add "APPENDLIMIT" to the IMAP Capabilities registry,
using this document as its reference.
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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>.
[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>.
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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 SrimushnamBoovaraghamoorthy
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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