Internet DRAFT - draft-melnikov-rfc2088bis
draft-melnikov-rfc2088bis
Network Working Group J. Myers
Internet-Draft
Obsoletes: 2088 (if approved) A. Melnikov, Ed.
Intended status: Standards Track Isode Ltd
Expires: September 10, 2015 March 9, 2015
IMAP4 non-synchronizing literals
draft-melnikov-rfc2088bis-04.txt
Abstract
The Internet Message Access Protocol (RFC 3501) contains the
"literal" syntactic construct for communicating strings. When
sending a literal from client to server, IMAP requires the client to
wait for the server to send a command continuation request between
sending the octet count and the string data. This document specifies
an alternate form of literal which does not require this network
round trip.
This document specifies 2 IMAP extensions: LITERAL+ and LITERAL-.
The former allows the alternate form of literals in all IMAP command.
The latter is the same as LITERAL+, but disallow the alternate form
in IMAP APPEND, unless they are 4096 bytes or less.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
Myers & Melnikov Expires September 10, 2015 [Page 1]
Internet-Draft IMAP4 non-synchronizing literals March 2015
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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Specification . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
3. Considerations on when to use and not to use synchronizing
literals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. LITERAL- capability . . . . . . . . . . . . . . . . . . . . . 4
5. Interaction with BINARY extension . . . . . . . . . . . . . . 5
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. To Do . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
11.1. Normative References . . . . . . . . . . . . . . . . . . 6
11.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Changes since RFC 2088 . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Specification
The non-synchronizing literal is added an alternate form of literal,
and may appear in communication from client to server instead of the
IMAP [RFC3501] form of literal. The IMAP form of literal, used in
communication from client to server, is referred to as a
Myers & Melnikov Expires September 10, 2015 [Page 2]
Internet-Draft IMAP4 non-synchronizing literals March 2015
synchronizing literal. The non-synchronizing literal form MUST NOT
be sent from server to client.
Non-synchronizing literals may be used with any IMAP server
implementation which returns "LITERAL+" or "LITERAL-" as one of the
supported capabilities to the CAPABILITY command. If the server does
not advertise either of the above capabilities, the client must use
synchronizing literals instead. The difference between "LITERAL+"
and "LITERAL-" extensions is explained in Section 4.
The non-synchronizing literal is distinguished from the original
synchronizing literal by having a plus ('+') between the octet count
and the closing brace ('}'). The server does not generate a command
continuation request in response to a non-synchronizing literal, and
clients are not required to wait before sending the octets of a non-
synchronizing literal.
The protocol receiver of an IMAP server must check the end of every
received line for an open brace ('{') followed by an octet count, a
plus ('+'), and a close brace ('}') immediately preceeding the CRLF.
If it finds this sequence, it is the octet count of a non-
synchronizing literal and the server MUST treat the specified number
of following octets and the following line as part of the same
command. A server MAY still process commands and reject errors on a
line-by-line basis, as long as it checks for non-synchronizing
literals at the end of each line.
Example:
C: A001 LOGIN {11+}
C: FRED FOOBAR {7+}
C: fat man
S: A001 OK LOGIN completed
2. Requirements Notation
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.
Myers & Melnikov Expires September 10, 2015 [Page 3]
Internet-Draft IMAP4 non-synchronizing literals March 2015
3. Considerations on when to use and not to use synchronizing literals
This section is important to understand for both client and server
developers of this IMAP extension.
While non-synchronizing literals have clear advantages for clients,
such as simplicity of use, they might be more difficilt to handle on
the server side. When a non synchronizing literal is used by a
client which is too big for the server to accept, a compliant
LITERAL+ server implementation has to make a choice between several
non optimal choices:
1. Read the number of bytes specified in the non synchronizing
literal and reject the command that included the literal anyway.
(The server is allowed to send the tagged BAD/NO response before
reading the whole non synchronizing literal.) This is quite
wasteful on bandwidth if the literal size is big.
2. Send the untagged BYE response explaining the reason for
rejecting the literal and close the connection. This will force
the client to reconnect or report the error to the user. In the
latter case the error is unlikely to be understandable to the
user. Additionally, some naive clients are known to blindly
reconnect in this case and repeat the operation that caused the
problem. [[CREF1: Possibly also send the ALERT response code?]]
The situation can be improved by implementing support for the
APPENDLIMIT extension [I-D.jayantheesh-imap-appendlimit-extension],
which allows a server to advertise its APPEND limit, so that well
behaved clients can check it and avoid uploading big messages in the
first place.
The problem described above is most common when using the APPEND
command, because most of commands don't need to send lots of data
from the client to the server. Some server implementations impose
limits on literal (both synchronizing and non synchronizing) accepted
from clients in order to protect from Denial Of Service attacks.
Implementations can generally impose much lower limits on literal
sizes for all commands other than APPEND. In order to address
literal size issue in APPEND, this document introduces a new
extension "LITERAL-", described in Section 4.
4. LITERAL- capability
"LITERAL-" extension is almost identical to "LITERAL+", with one
exception: when "LITERAL-" is advertised, non synchronizing literals
used in APPEND (and extensions to APPEND such as MULTIAPPEND
[RFC3502] and CATENATE [RFC4469]) MUST NOT be larger than 4096 bytes.
Myers & Melnikov Expires September 10, 2015 [Page 4]
Internet-Draft IMAP4 non-synchronizing literals March 2015
When any literal used in APPEND is larger than 4096, RFC 3501
synchronizing literals MUST be used instead. A "LITERAL-" compliant
server which encounters a non synchronizing literal in APPEND larger
than 4096 bytes MUST reject such APPEND command with a tagged BAD
response.
Because "LITERAL-" is a more restricted version of "LITERAL+", IMAP
servers MUST NOT advertise both of these capabilities at the same
time. (A server implementation can choose to have a configuration
option to pick which one to advertise.)
5. Interaction with BINARY extension
RFC 4466 [RFC4466] updated the non-terminal "literal8" defined in
[RFC3516] to allow for non-synchronizing literals if both [RFC3516]
and "LITERAL+" (or "LITERAL-") extensions are supported by the
server.
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF].
Non-terminals referenced but not defined below are as defined by
[RFC3501].
literal = "{" number ["+"] "}" CRLF *CHAR8
; Number represents the number of CHAR8 octets
CHAR8 = <defined in RFC 3501>
literal8 = <defined in RFC 4466>
7. Security Considerations
This document doesn't raise any new security concerns not already
raised by [RFC3501].
8. IANA Considerations
IMAP4 capabilities are registered by publishing a standards track or
IESG approved experimental RFC. The registry is currently located
at:
http://www.iana.org/assignments/imap-capabilities
This document requests that IANA updated the above registry to
include the entry for LITERAL+ capability pointing to this document.
Myers & Melnikov Expires September 10, 2015 [Page 5]
Internet-Draft IMAP4 non-synchronizing literals March 2015
This document also requests that IANA adds "LITERAL-" capability
pointing to this document to the above registry.
9. To Do
Exact semantics of LITERAL- is still in flux.
10. Acknowledgments
Valuable comments, both in agreement and in dissent, were received
from Dave Cridland, Michael M Slusarz and Arnt Gulbrandsen.
11. References
11.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC3516] Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
April 2003.
[RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
ABNF", RFC 4466, April 2006.
11.2. Informative References
[I-D.jayantheesh-imap-appendlimit-extension]
Bisht, N., "The IMAP APPENDLIMIT Extension", draft-
jayantheesh-imap-appendlimit-extension-04 (work in
progress), February 2015.
[RFC3502] Crispin, M., "Internet Message Access Protocol (IMAP) -
MULTIAPPEND Extension", RFC 3502, March 2003.
[RFC4469] Resnick, P., "Internet Message Access Protocol (IMAP)
CATENATE Extension", RFC 4469, April 2006.
Myers & Melnikov Expires September 10, 2015 [Page 6]
Internet-Draft IMAP4 non-synchronizing literals March 2015
Appendix A. Changes since RFC 2088
Added IANA registration.
Updated references. Also updated considerations about interactions
of IMAP extensions.
Additional implementation considerations based on the IMAP mailing
list discussions.
LITERAL- capability description.
Authors' Addresses
John G. Myers
Email: jgm+@cmu.edu
Alexey Melnikov (editor)
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
Myers & Melnikov Expires September 10, 2015 [Page 7]
