Internet DRAFT - draft-ietf-imapapnd-rfc2088bis
draft-ietf-imapapnd-rfc2088bis
Network Working Group A. Melnikov, Ed.
Internet-Draft Isode Ltd
Obsoletes: 2088 (if approved) March 6, 2016
Intended status: Standards Track
Expires: September 7, 2016
IMAP4 non-synchronizing literals
draft-ietf-imapapnd-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 that does not require this network round
trip.
This document specifies 2 IMAP extensions: LITERAL+ and LITERAL-.
LITERAL+ allows the alternate form of literals in all IMAP commands.
LITERAL- is the same as LITERAL+, but disallows the alternate form of
literals unless they are 4096 bytes or less.
This document obsoletes RFC 2088.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 7, 2016.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Considerations on when to use and not to use synchronizing
literals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. LITERAL- capability . . . . . . . . . . . . . . . . . . . . . 5
6. Interaction with BINARY extension . . . . . . . . . . . . . . 6
7. Interaction with MULTIAPPEND extension . . . . . . . . . . . 6
8. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
12.1. Normative References . . . . . . . . . . . . . . . . . . 7
12.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Changes since RFC 2088 . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The Internet Message Access Protocol [RFC3501] 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 that does not require this network round
trip.
This document specifies 2 IMAP extensions: LITERAL+ and LITERAL-.
LITERAL+ allows the alternate form of literals (called "non-
synchronized literals" below) in all IMAP commands. LITERAL- is the
same as LITERAL+, but disallows the alternate form of literals unless
they are 4096 bytes or less.
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.
3. Specification
The non-synchronizing literal is added as 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 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 that 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 can only
use synchronizing literals. The difference between "LITERAL+" and
"LITERAL-" extensions is explained in Section 5.
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
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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 (a sequence of octets that ends with a CRLF) 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 the following line ([RFC3501]) as part of the same command.
It's important to note that the literal is not delimited by CRLF. It
ends after the number of bytes specified by the octet count, and the
current command continues from there. There might be a CRLF
immediately after, which ends the command. Or there might be more
octets, specifying other command parameters, before the CRLF. If a
SP (space) character is needed between parameters, it's important
that the SP appear after the literal, in its appropriate place.
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
This is semantically equivalent to this version that uses quoted
strings instead of literals:
C: A001 LOGIN "FRED FOOBAR" "fat man"
S: A001 OK LOGIN completed
Note that the space after FOOBAR in the first version corresponds
to the space between the two quoted strings in the second.
4. 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 difficult to handle on
the server side. When a client uses a non-synchronizing literal that
is too big for the server to accept, a compliant LITERAL+ server
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implementation has to make a choice between couple 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 is large.
2. Send an untagged BYE response explaining the reason for rejecting
the literal (possibly accompanied by an ALERT response code in
another response) 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, introducing an infinite loop.
The problem described above is most common when using the APPEND
command, because most commands don't need to send lots of data from
the client to the server. Some server implementations impose limits
on literals (both synchronizing and non-synchronizing) accepted from
clients in order to defend against 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 5.
The situation can also be improved by implementing support for the
APPENDLIMIT extension [APPENDLIMIT], 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.
5. LITERAL- capability
The "LITERAL-" extension is almost identical to "LITERAL+", with one
exception: when "LITERAL-" is advertised, non-synchronizing literals
used in any command MUST NOT be larger than 4096 bytes. Any literal
larger than 4096 bytes MUST be sent as an RFC 3501 synchronizing
literal. A "LITERAL-" compliant server that encounters a non-
synchronizing literal larger than 4096 bytes proceeds as described in
Section 4. If responding to an APPEND command, the tagged BAD
response MUST contains the TOOBIG response code [RFC4469]. If
responding with untagged BYE response, it SHOULD include the TOOBIG
response code. Note that the form of the non-synchronizing literal
does not change: it still uses the "+" in the literal itself, even if
the applicable extension is "LITERAL-".
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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.)
6. 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+" extensions are supported by the server.
This document also allows use of this extended "literal8" syntax when
both [RFC3516] and "LITERAL-" extensions are supported by the server.
7. Interaction with MULTIAPPEND extension
RFC 3502 [RFC3502] describes MULTIAPPEND extension and how it can be
used with LITERAL+. The LITERAL- extension can be used with the
MULTIAPPEND extension in the same way.
8. 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>
9. Security Considerations
Use of non-synchronizing literals can consume extra resources (e.g.
memory) on IMAP servers and can be used for denial-of-service
attacks. The "LITERAL-" extension partially improved this situation.
This document doesn't raise any other security concerns not already
raised by [RFC3501].
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10. 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 update the above registry replace
the reference for LITERAL+ to point to this document.
This document also requests that IANA add "LITERAL-" capability
pointing to this document to the above registry.
11. Acknowledgments
John G. Myers edited the original LITERAL+ extension.
Valuable comments, both in agreement and in dissent, were received
from Dave Cridland, Michael M Slusarz, Arnt Gulbrandsen, Jayantheesh
S B., Jamie Nicolson, Barry Leiba and SM.
12. References
12.1. 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,
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>.
[RFC3516] Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
DOI 10.17487/RFC3516, April 2003,
<http://www.rfc-editor.org/info/rfc3516>.
[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>.
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[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>.
12.2. Informative References
[APPENDLIMIT]
SrimushnamBoovaraghamoorthy, J. and N. Bisht, "The IMAP
APPENDLIMIT Extension", draft-ietf-imapapnd-appendlimit-
extension-10 (work in progress), January 2016.
[RFC3502] Crispin, M., "Internet Message Access Protocol (IMAP) -
MULTIAPPEND Extension", RFC 3502, DOI 10.17487/RFC3502,
March 2003, <http://www.rfc-editor.org/info/rfc3502>.
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.
Added description of a new capability: LITERAL- .
Author's Address
Alexey Melnikov (editor)
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
Email: Alexey.Melnikov@isode.com
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