Internet Engineering Task Force (IETF)
Request for Comments: 7888
Obsoletes: 2088
Category: Standards Track
ISSN: 2070-1721

A. Melnikov, Ed.
Isode Ltd
May 2016

IMAP4 Non-synchronizing Literals

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 it disallows the alternate form of literals unless they are 4096 bytes or less.

This document obsoletes RFC 2088.

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/rfc7888.

Copyright Notice

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.

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Considerations on When to Use and Not to Use Synchronizing
       Literals  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  LITERAL- Capability . . . . . . . . . . . . . . . . . . . . .   5
   6.  Interaction with BINARY Extension . . . . . . . . . . . . . .   6
   7.  Interaction with MULTIAPPEND Extension  . . . . . . . . . . .   6
   8.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Changes since RFC 2088 . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

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 it disallows the alternate form of literals unless they are 4096 bytes or less.

2. 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.

3. Specification

The non-synchronizing literal is added as an alternate form of literal, and it 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 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 preceding the CRLF. This sequence (if found by the receiver) 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 defined in [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; it 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

Understanding of this section is important 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 server implementation that is compliant with LITERAL+ has to make a choice between a couple non- optimal choices:

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 of bandwidth if the literal is large.

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 [RFC7889], 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 a synchronizing literal as specified in RFC 3501. A server that is compliant with LITERAL- and 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 contain the TOOBIG response code [RFC4469]. If responding with an 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-.

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 indicate which one to advertise.)

6. Interaction with BINARY Extension

[RFC4466] updated the non-terminal "literal8" defined in [RFC3516] to allow for non-synchronizing literals if both BINARY [RFC3516] and LITERAL+ extensions are supported by the server.

This document also allows use of this extended "literal8" syntax when both BINARY [RFC3516] and LITERAL- extensions are supported by the server.

7. Interaction with MULTIAPPEND Extension

[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 security concerns beyond those raised by [RFC3501].

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>.

IANA has updated the above registry so that the reference for "LITERAL+" points to this document.

IANA has added the "LITERAL-" capability to the above registry, with this document as the reference.

11. References

11.1. Normative References

   [ABNF]     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>.
   
   [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>.
   
   [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>.

11.2. Informative References

   [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>.
   
   [RFC7889]  SrimushnamBoovaraghamoorthy, J. and N. Bisht, "The IMAP
              APPENDLIMIT Extension", RFC 7889, DOI 10.17487/RFC7889,
              May 2016, <http://www.rfc-editor.org/info/rfc7889>.

Appendix A. Changes since RFC 2088

Added IANA registration.

Updated references. Also updated considerations about interactions of IMAP extensions.

Added implementation considerations based on the IMAP mailing list discussions.

Added description of a new capability: LITERAL-.

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 SrimushnamBoovaraghamoorthy, Jamie Nicolson, Barry Leiba, and SM.

Author's Address

   Alexey Melnikov (editor)
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   United Kingdom
   
   Email: Alexey.Melnikov@isode.com