The IMAP APPENDLIMIT Extension
An IMAP extension that tells clients the largest message a server will accept before they upload it.
SrimushnamBoovaraghamoorthy, J. and N. Bisht, "The IMAP APPENDLIMIT Extension", RFC 7889, DOI 10.17487/RFC7889, May 2016, <https://www.rfc-editor.org/info/rfc7889>.
@misc{rfc7889,
series = {Request for Comments},
number = 7889,
howpublished = {RFC 7889},
publisher = {RFC Editor},
doi = {10.17487/RFC7889},
url = {https://www.rfc-editor.org/info/rfc7889},
author = {J. SrimushnamBoovaraghamoorthy and N. Bisht},
title = {{The IMAP APPENDLIMIT Extension}},
pagetotal = 7,
year = 2016,
month = may,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. SrimushnamBoovaraghamoorthy
Request for Comments: 7889 N. Bisht
Category: Standards Track Samsung Electronics America
ISSN: 2070-1721 May 2016
The IMAP APPENDLIMIT Extension
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
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7889.
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
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 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 Behavior . . . . . . . . . . . . . . . . . . 4
4. APPEND Response . . . . . . . . . . . . . . . . . . . . . . . 4
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 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 a message with huge attachments consumes
device resources like device battery power and mobile data.
The IMAP APPENDLIMIT extension provides the 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
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.
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 size for
mail uploads. 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, as they 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 that will
not be advertised as part of the CAPABILITY response. The IMAP
server can publish specific values for each mailbox, and it 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.
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 a LIST command 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 Behavior
Computing the APPENDLIMIT should be fast and should not take Access
Control Lists (ACLs), quotas, or 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, even
if the message being appended is smaller than the APPENDLIMIT.
4. APPEND Response
If a client uploads a message that 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 Section 4 of [RFC4469]
for various APPEND response codes and their handling.
A client SHOULD avoid use of non-synchronizing literals [RFC7888]
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 of that document. [RFC3501] defines the non-
terminals "capability" and "status-att", and [RFC4466] defines
"status-att-val".
All alphabetic characters are case insensitive. The use of uppercase
or lowercase 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 has added "APPENDLIMIT" to the "IMAP Capabilities" registry,
using this document as its reference.
8. References
8.1. Normative References
[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>.
[RFC7888] Melnikov, A., Ed., "IMAP4 Non-synchronizing Literals",
RFC 7888, DOI 10.17487/RFC7888, May 2016,
<http://www.rfc-editor.org/info/rfc7888>.
8.2. 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>.
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.
Authors' Addresses
Jayantheesh SrimushnamBoovaraghamoorthy
Samsung Electronics America
685 US Highway 202/206
Bridgewater, NJ 08807
United States
Email: jayantheesh.sb@gmail.com
Narendra Singh Bisht
Samsung Electronics America
685 US Highway 202/206
Bridgewater, NJ 08807
United States
Email: narendrasingh.bisht@gmail.com
AI ELI5
On this page
§1The Problem and the Idea
Some IMAP servers limit message upload size and do not publish the limit. If a client APPENDs a message with huge attachments using non-synchronizing literals, the APPEND fails because of the limit, but the client has already sent the data. That wastes resources, especially on mobile devices where it consumes battery power and mobile data.
The APPENDLIMIT extension lets the server advertise the maximum upload size so the client knows the limit beforehand, and the server can avoid processing attachments above it.
Without the sign, you carry a giant box all the way and only then hear 'too big'. The extension puts up the sign.
§2Advertising the Capability
A server that supports the extension includes APPENDLIMIT in its capability list in the authenticated state, and may also advertise it before login. If the capability is omitted, nothing is conveyed about a fixed maximum size. It comes in two forms.
APPENDLIMIT=<number>means the same upload limit applies to all mailboxes, for exampleS: * CAPABILITY IMAP4rev1 ID APPENDLIMIT=257890.APPENDLIMITwith no value means the extension is supported but limits can differ per mailbox, so the client must discover each one with a STATUS or LIST command.
A client implementing the extension should parse both mailbox-specific and global responses, and avoid trying to APPEND mail larger than the advertised limit.
The sign at the door either says one size for every counter, or says 'ask at each counter'.
§3Per-Mailbox Limits with STATUS and LIST
A server can have mailbox-specific values that are not part of the CAPABILITY response. It can publish a value for each mailbox, or NIL to say that mailbox has no limit. A new STATUS attribute, APPENDLIMIT, retrieves it: C: t1 STATUS INBOX (APPENDLIMIT) is answered by S: * STATUS INBOX (APPENDLIMIT 257890).
If the server advertises LIST-STATUS (RFC 5819), the client can ask for it with LIST "" % RETURN (STATUS (APPENDLIMIT)), and the server MUST recognize the attribute and include a STATUS response for each matching mailbox. If the server does not support the STATUS return option on LIST, the client should use the STATUS command.
Computing the limit should be fast and should not take Access Control Lists (ACLs), quotas or similar information into account. The limit is only part of the policy, so an APPEND smaller than the limit can still fail because of ACLs or quotas.
You can ask each counter its own size limit. A counter may say 'no limit'. Even a small box can still be turned away for other reasons.
- t1 STATUS INBOX (APPENDLIMIT)Ask for the upload limit of one mailbox.
- * STATUS INBOX (APPENDLIMIT 257890)The limit for INBOX.
- t1 OK STATUS completed
§4Rejecting Oversized Messages
If a client uploads a message larger than the limit for that mailbox, the server SHALL reject the APPEND with a tagged TOOBIG response code. Section 4 of RFC 4469 covers APPEND response codes and their handling.
A client SHOULD avoid non-synchronizing literals (RFC 7888) when the server's maximum upload size is unknown. Section 4.2.2.3 of RFC 4549 describes their use and their risk for disconnected IMAP clients.
If you bring a too-big box anyway, the clerk says 'TOOBIG'. When you do not know the limit, do not dump the box on the counter before asking.
§5Formal Syntax
The syntax uses ABNF (RFC 5234) and builds on non-terminals from RFC 3501 (capability, status-att) and RFC 4466 (status-att-val). Strings are case insensitive, and implementations MUST accept them that way.
The additions are capability =/ "APPENDLIMIT" ["=" number], status-att =/ "APPENDLIMIT" and status-att-val =/ "APPENDLIMIT" SP (number / nil). The number is the fixed maximum message size in octets. A value of 0 means the server will not accept any APPEND commands for the affected mailboxes.
The grammar is the rule for how to write the question and answer so both sides read them the same way. Zero means 'nothing is accepted here'.
§6Security and IANA Notes
The extension gives cooperative clients extra information as an optimization and introduces no new security concerns. It does not address abusive clients that try to consume server resources, so servers still have to disconnect or restrict access to clients that behave abusively.
IANA has added APPENDLIMIT to the IMAP Capabilities registry, with this document as the reference.
The sign only helps people who read it. Someone set on causing trouble still has to be shown the door.