INTERNET-DRAFT Jayantheesh S B
Intended status: Standards Track Samsung
Expires: May 2015 Narendra Singh Bisht
Samsung
November 18, 2014
The IMAP APPENDLIMIT Extension
draft-jayantheesh-imap-appendlimit-extension-00
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 and SELECT/EXAMINE command.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 2
2. APPENDLIMIT Extension . . . . . . . . . . . . . . . . . . . . . 2
3. SELECT response . . . . . . . . . . . . . . . . . . . . . . . . 3
4. APPEND response . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1 Normative References . . . . . . . . . . . . . . . . . . . . . 5
8.2 Informative References . . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . 6
10. Author's Address . .. . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Several IMAP server have limitation in 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
advertised upload limit is common across the mailboxes, but client
can still issue SELECT/EXAMINE command to get the mailbox specific
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upload limit set by the IMAP server. In this case, APPENDLIMIT value
obtained as part of SELECT/EXAMINE 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 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
SHALL not try to upload mail more than advertised limit in the APPEND
command.
3. SELECT response
IMAP server may have mailbox specific maximum upload limit. Client can
get the 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.
C: t2 SELECT INBOX
S: * 172 EXISTS
S: * OK [APPENDLIMIT 257890] Maximum upload limit
S: [...]
S: t2 OK [READ-WRITE] SELECT completed
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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.
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 NO response with maximum allowed size set by the server.
The following example demonstrates, its usage.
C: t1 APPEND "Draft" (\SEEN \DRAFT) {98994952}
S: t1 NO [APPENDLIMIT 257890] Exceeds the maximum allowed size
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" and "resp-text-code".
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" ["=" nz-number]
capability /= appendlimit-cap
appendlimit-respcode = "APPENDLIMIT" SP nz-number
resp-text-code /= appendlimit-respcode
A non-zero number indicating the fixed maximum message size in bytes
that the server will accept. The syntax of the parameter follows the
augmented BNF notation of [RFC 5234]. 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
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
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9. Acknowledgement
TBD
10. Author's Address
Jayantheesh S B
Samsung Telecommunications America,
685 US Highway 202/206,
Bridgewater, NJ 08807.
USA
Email: jayantheesh.sb@gmail.com
Narendra Singh Bisht
Samsung Telecommunications America,
685 US Highway 202/206,
Bridgewater, NJ 08807.
USA
Email: narendrasingh.bisht@gmail.com
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