One document matched: draft-gellens-pop3ext-04.txt
Differences from draft-gellens-pop3ext-03.txt
POP3 Extension Mechanism
Status of this Memo:
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
"working draft" or "work in progress."
To learn the current status of any Internet Draft, please check the
"1id-abstracts.txt" listing contained in the Internet Drafts shadow
directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
ftp.isi.edu (US West Coast).
A version of this draft document will be submitted to the RFC
editor as a Proposed Standard for the Internet Community.
Discussion and suggestions for improvement are requested.
Public comments should be sent to the IETF POP3 Extensions mailing
list, <ietf-pop3ext@imc.org>. To subscribe, send a message
containing SUBSCRIBE to <ietf-pop3ext-request@imc.org>. Private
comments may be sent to the authors.
Copyright Notice
Copyright (C) The Internet Society 1998. All Rights Reserved.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in this Document . . . . . . . . . . . . . . . . 2
3. General Command and Response Grammar . . . . . . . . . . . . . . . 3
4. Parameter and Response Lengths. . . . . . . . . . . . . . . . . . 3
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5. The CAPA Command . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Initial Set of Capabilities . . . . . . . . . . . . . . . . . . . 5
6.1. TOP capability . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.2. USER capability . . . . . . . . . . . . . . . . . . . . . . . . 6
6.3. SASL capability . . . . . . . . . . . . . . . . . . . . . . . . 6
6.4. LOGIN-DELAY capability. . . . . . . . . . . . . . . . . . . . . 7
6.5. PIPELINING capability . . . . . . . . . . . . . . . . . . . . . 7
6.6. EXPIRE capability . . . . . . . . . . . . . . . . . . . . . . . 8
6.7. UIDL capability . . . . . . . . . . . . . . . . . . . . . . . . 9
6.8. IMPLEMENTATION capability. . . . . . . . . . . . . . . . . . . 10
7. Future Extensions to POP3 . . . . . . . . . . . . . . . . . . . . 11
8. Extended POP3 Response Codes . . . . . . . . . . . . . . . . . . 11
8.1. Initial POP3 response codes . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 12
11. References. . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 13
13. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Post Office Protocol version 3 [POP3] is very widely used. However,
while it includes some optional commands (and some useful protocol
extensions have been published), it lacks a mechanism for
advertising support for these extensions or for behavior
variations.
Currently these optional features and extensions can only be
detected by probing, if at all. This is at best inefficient, and
possibly worse. As a result, some clients have manual configuration
options for POP3 server capabilities.
Because one of the most important characteristics of POP3 is its
simplicity, it is desirable that extensions be few in number.
However, some extensions are necessary (such as ones that provide
improved security [POP-AUTH]), while others are very desirable in
certain situations. In addition, a means for discovering server
behavior is needed.
This memo defines a mechanism to announce support for optional
commands, extensions, and unconditional server behavior. Included
is an initial set of currently deployed capabilities which vary
between server implementations. This document also extends POP3
error messages so that machine parsable codes can be provided to
the client. An initial set of response codes is included.
2. Conventions Used in this Document
The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
NOT", and "MAY" in this document are to be interpreted as described
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in "Key words for use in RFCs to Indicate Requirement Levels"
[KEYWORDS].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
3. General Command and Response Grammar
The general form of POP3 commands and responses is described using
[ABNF]:
POP3 commands:
command = keyword *(SP param) CRLF ;255 octets maximum
keyword = 3*4VCHAR
param = 1*VCHAR
POP3 responses:
capa-resp = single-line *capability "." CRLF
capa-tag = 1*cchar
capability = capa-tag *(SP param) ;512 octets maximum
cchar = %x21-2D / %x2F-7F
;printable ASCII, excluding "."
dot-stuffed = *CHAR CRLF ;must be dot-stuffed
gchar = %x21-3B / %x3D-7F
;printable ASCII, excluding "<"
greeting = "+OK" [resp-code] *gchar [timestamp] *gchar
;512 octets maximum
multi-line = single-line *dot-stuffed "." CRLF
rchar = %x21-2E / %x30-5C / %x5E-7F
;printable ASCII, excluding "/" and "]"
resp-code = "[" resp-level *("/" resp-level) "]"
resp-level = 1*rchar
response = greeting / single-line / capa-resp / multi-line
single-line = status SP [text] CRLF ;512 octets maximum
status = "+OK" / "-ERR"
text = [resp-code] *CHAR
timestamp = "<" *VCHAR ">"
;MUST conform to RFC-822 msg-id
4. Parameter and Response Lengths
This specification increases the length restrictions on commands
and parameters imposed by RFC 1939.
The maximum length of a command is increased from 45 characters (4
character command, single space, 40 character argument) to 255
octets.
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Servers which support the CAPA command MUST support commands up to
255 octets. Servers MUST also support the largest maximum command
length specified by any supported capability.
The maximum length of the first line of a command response
(including the initial greeting) is unchanged at 512 octets
(including the terminating CRLF).
5. The CAPA Command
The POP3 CAPA command returns a list of capabilities supported by
the POP3 server. It is available in both the AUTHORIZATION and
TRANSACTION states.
A capability description MUST document in which states the
capability is announced, and in which states the commands are
valid.
Capabilities available in the AUTHORIZATION state MUST be announced
in both states.
(These requirements allow a client to issue only one CAPA command
if it does not use any TRANSACTION-only capabilities. Clients are
also able to repeat the CAPA command after authenticating, if the
authentication step negotiated an integrity protection layer, and
the client wishes to check for active down-negotiation attacks.)
Each capability may enable additional protocol commands, additional
parameters and responses for existing commands, or describe an
aspect of server behavior. These details are specified in the
description of the capability.
Section 3 describes the CAPA response using [ABNF]. When a
capability response describes an optional command, the <capa-tag>
SHOULD be identical to the command keyword. CAPA response tags are
case-insensitive.
CAPA
Arguments:
none
Restrictions:
none
Discussion:
An -ERR response indicates the capability command is not
implemented and the client will have to probe for
capabilities as before.
An +OK response is followed by a list of capabilities, one
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per line. Each capability name MAY be followed by a single
space and a space-separated list of parameters. Each
capability line is limited to 512 octets (including the
CRLF). The capability list is terminated by a line
containing a termination octet (".") and a CRLF pair.
Possible Responses:
+OK -ERR
Examples:
C: CAPA
S: +OK Capability list follows
S: TOP
S: USER
S: SASL CRAM-MD5 KERBEROS_V4
S: LOGIN-DELAY 900
S: PIPELINING
S: EXPIRE 60
S: UIDL
S: IMPLEMENTATION Shlemazle-Plotz-v302
S: .
6. Initial Set of Capabilities
This section defines an initial set of POP3 capabilities. These
include the optional POP3 commands, already published POP3
extensions, and behavior variations between POP3 servers which can
impact clients.
Note that there is no APOP capability, even though APOP is an
optional command in [POP3]. Clients discover server support of
APOP by the presence in the greeting banner of an initial challenge
enclosed in angle brackets ("<>"). Therefore, an APOP capability
would introduce two ways for a server to announce the same thing.
6.1. TOP capability
CAPA tag:
TOP
Arguments:
none
Added commands:
TOP
Standard commands affected:
none
Announced states:
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both
Commands valid in states:
TRANSACTION
Discussion:
The TOP capability indicates the optional TOP command is
available.
6.2. USER capability
CAPA tag:
USER
Arguments:
none
Added commands:
USER PASS
Standard commands affected:
none
Announced states:
both
Commands valid in states:
AUTHENTICATION
Discussion:
The USER capability indicates that the USER and PASS commands
are supported, although they may not be available to all users.
6.3. SASL capability
CAPA tag:
SASL
Arguments:
Supported SASL mechanisms
Added commands:
AUTH
Standard commands affected:
none
Announced states:
both
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Commands valid in states:
AUTHENTICATION
Discussion:
The POP3 AUTHentication command [POP-AUTH] permits the use of
[SASL] authentication mechanisms with POP3. The SASL
capability indicates that the AUTH command is available and
that it supports an optional base64 encoded second argument for
an initial client response as described in the SASL
specification. The argument to the SASL capability is a space
separated list of SASL mechanisms which are supported.
6.4. LOGIN-DELAY capability
CAPA tag:
LOGIN-DELAY
Arguments:
minimum seconds between logins
Added commands:
none
Standard commands affected:
USER APOP AUTH
Announced states:
both
Commands valid in states:
n/a
Discussion:
POP3 clients often login frequently to check for new mail.
Unfortunately, the process of creating a connection,
authenticating the user, and opening the user's maildrop can be
very resource intensive on the server. A number of deployed
POP3 servers try to reduce server load by requiring a delay
between logins. The LOGIN-DELAY capability includes an integer
argument which indicates the number of seconds after an "+OK"
response to a PASS, APOP, or AUTH command before another
authentication will be accepted. Clients which permit the user
to configure a mail check interval SHOULD use this capability
to determine the minimum permissible interval. Servers which
advertise LOGIN-DELAY SHOULD enforce it.
6.5. PIPELINING capability
CAPA tag:
PIPELINING
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Arguments:
none
Added commands:
none
Standard commands affected:
all
Announced states:
both
Commands valid in states:
n/a
Discussion:
The PIPELINING capability indicates the server is capable of
accepting multiple commands at a time; the client does not have
to wait for the response to a command before issuing a
subsequent command. If a server supports PIPELINING, it MUST
process each command in turn. If a client uses PIPELINING, it
MUST keep track of which commands it has outstanding, and match
server responses to commands in order. If either the client or
server uses blocking writes, it MUST not exceed the window size
of the underlying transport layer.
Some POP3 clients have an option to indicate the server
supports "Overlapped POP3 commands." This capability removes
the need to configure this at the client.
This is roughly synonymous with the ESMTP PIPELINING extension
[PIPELINING].
6.6. EXPIRE capability
CAPA tag:
EXPIRE
Arguments:
server-guaranteed minimum retention days, or NEVER
Added commands:
none
Standard commands affected:
none
Announced states:
both
Commands valid in states:
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n/a
Discussion:
While POP3 allows clients to leave messages on the server, RFC
1939 [POP3] warns about the problems that may arise from this,
and allows servers to delete messages based on site policy.
The EXPIRE capability avoids the problems mentioned in RFC
1939, by allowing the server to inform the client as to the
policy in effect. The argument to the EXPIRE capability
indicates the minimum server retention period, in days, for
messages on the server. Zero indicates the server might delete
messages immediately after a POP session ends. "NEVER" asserts
that the server does not delete messages.
A site may have a message expiration policy which treats
messages differently depending on which user actions which have
been performed, or based on other factors. For example, a site
might delete unseen messages after 60 days, and completely- or
partially-seen messages after 15 days.
If a site uses any automatic deletion policy, it SHOULD use the
EXPIRE capability to announce the smallest retention period
used by any category or condition. That is, EXPIRE informs the
user of the minimum number of days messages may remain on the
server under any circumstances. Sites which permit users to
retain messages indefinitely SHOULD announce this with the
EXPIRE NEVER response.
Examples:
EXPIRE 30
EXPIRE NEVER
EXPIRE 0
The first example indicates the server might delete messages
after 30 days. In the second example, the server announces it
does not delete messages. The third example specifies that
there are some cases in which the server deletes messages
immediately after a POP session ends.
6.7. UIDL capability
CAPA tag:
UIDL
Arguments:
none
Added commands:
UIDL
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Standard commands affected:
none
Announced states:
both
Commands valid in states:
TRANSACTION
Discussion:
The UIDL capability indicates that the UIDL command is
supported.
6.8. IMPLEMENTATION capability
CAPA tag:
IMPLEMENTATION
Arguments:
string giving server implementation information
Added commands:
none
Standard commands affected:
none
Announced states:
both
Commands valid in states:
n/a
Discussion:
It is often useful to identify an implementation of a
particular server (for example, when logging). This is
commonly done in the welcome banner, but one must guess if a
string is an implementation ID or not.
The argument to the IMPLEMENTATION capability consists of one
or more tokens which identify the server. Since CAPA response
tag arguments are space-separated, to make the IMPLEMENTATION
capability argument a single token it must not contain spaces.
A server MAY include the implementation identification both in
the welcome banner and in the IMPLEMENTATION capability.
Clients MUST NOT modify their behavior based on the server
implementation. Instead the server and client should agree on
a private extension.
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7. Future Extensions to POP3
Future extensions to POP3 are in general discouraged, as POP3's
usefulness lies in its simplicity. Extensions which offer
capabilities supplied by IMAP [IMAP4] or SMTP [SMTP] are strongly
discouraged and unlikely to be permitted on the IETF standards
track.
Clients MUST NOT require the presence of any extension for basic
functionality.
Capabilities beginning with the letter "X" are reserved for
experimental non-standard extensions and their use is discouraged.
All other capabilities MUST be defined in a standards track or IESG
approved experimental RFC.
8. Extended POP3 Response Codes
POP3 is currently only capable of indicating success or failure to
most commands. Unfortunately, clients often need to know more
information about the cause of a failure in order to gracefully
recover. This is especially important in response to a failed
login (there are widely-deployed clients which attempt to decode
the error text of a PASS command result, to try and distinguish
between "unable to get maildrop lock" and "bad login").
This specification amends the POP3 standard to permit an optional
response code, enclosed in square brackets, at the beginning of the
human readable text portion of an "+OK" or "-ERR" response.
Clients supporting this extension MAY remove any information
enclosed in square brackets prior to displaying human readable text
to the user. Immediately following the open square bracket "["
character is a response code which is interpreted in a
case-insensitive fashion by the client.
The response code is hierarchical, with a "/" separating levels of
detail about the error. Clients MUST ignore unknown hierarchical
detail about the response code. This is important, as it could be
necessary to provide further detail for response codes in the
future.
Section 3 describes response codes using [ABNF].
Examples:
C: USER mrose
S: -ERR [IN-USE] Do you have another POP session running?
8.1. Initial POP3 response codes
This specification defines some POP3 response codes which can be
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used to determine the reason for a failed login. Additional
response codes MAY be defined by publication in an RFC (standards
track or IESG approved experimental RFCs are preferred).
LOGIN-DELAY
This occurs on an -ERR response to an AUTH, USER, PASS or APOP
command and indicates that the user has logged in recently and
will not be allowed to login again until the login delay period
has expired.
IN-USE
This occurs on an -ERR response to an AUTH, APOP, or PASS
command. It indicates the authentication was successful, but
the user's maildrop is currently in use (probably by another
POP3 client).
9. IANA Considerations
This document requests that IANA maintain two new registries: POP3
capabilities and POP3 response codes.
New POP3 capabilities MUST be defined in a standards track or IESG
approved experimental RFC, and MUST NOT begin with the letter "X".
New POP3 capabilities MUST include the following information: CAPA
tag, arguments, added commands, standard commands affected, states
in which the commands are valid, states in which the capability is
announced, and discussion. In addition, new limits for POP3
command and response lengths may need to be included.
New POP3 response codes MUST be defined in an RFC or other
permanent and readily available reference, in sufficient detail so
that interoperability between independent implementations is
possible. (This is the "Specification Required" policy described in
[IANA]).
New POP3 response code specifications MUST include the following
information: the complete response code, for which responses (+OK
or -ERR) and commands it is valid, and a definition of its meaning
and expected client behavior.
10. Security Considerations
A capability list can reveal information about the server's
authentication capabilities which can be used to determine if
certain attacks will be successful. However, allowing clients to
automatically detect availability of stronger mechanisms and alter
their configurations to use them can improve overall security at a
site.
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11. References
[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd.,
November 1997. <ftp://ftp.isi.edu/in-notes/rfc2234.txt>
[IANA] Narten, Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", work in progress.
[IMAP4] Crispin, "Internet Message Access Protocol -- Version
4rev1", RFC 2060, University of Washington, December 1996.
<ftp://ftp.isi.edu/in-notes/rfc2060.txt>
[KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.
<ftp://ftp.isi.edu/in-notes/rfc2119.txt>
[PIPELINING] Freed, "SMTP Service Extension for Command
Pipelining", RFC 2197, Innosoft, September 1997.
<ftp://ftp.isi.edu/in-notes/rfc2197.txt>
[POP3] Myers, Rose, "Post Office Protocol -- Version 3", RFC 1939,
Carnegie Mellon, Dover Beach Consulting, Inc., May 1996.
<ftp://ftp.isi.edu/in-notes/rfc1939.txt>
[POP-AUTH] Myers, "POP3 AUTHentication command", RFC 1734, Carnegie
Mellon, December 1994. <ftp://ftp.isi.edu/in-notes/rfc1734.txt>
[SASL] Myers, "Simple Authentication and Security Layer (SASL)",
RFC 2222, Netscape Communications, October 1997.
<ftp://ftp.isi.edu/in-notes/rfc2222.txt>
[SMTP] Postel, "Simple Mail Transfer Protocol", RFC 821, STD 10,
Information Sciences Institute, August 1982.
<ftp://ftp.isi.edu/in-notes/rfc821.txt>
12. Full Copyright Statement
Copyright (C) The Internet Society 1998. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain
it or assist in its implementation may be prepared, copied,
published and distributed, in whole or in part, without restriction
of any kind, provided that the above copyright notice and this
paragraph are included on all such copies and derivative works.
However, this document itself may not be modified in any way, such
as by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the
purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards process
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must be followed, or as required to translate it into languages
other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
13. Authors' Addresses
Randall Gellens +1 619 651 5115
QUALCOMM, Incorporated +1 619 651 5334 (fax)
6455 Lusk Blvd. randy@qualcomm.com
San Diego, CA 92121-2779
USA
Chris Newman chris.newman@innosoft.com
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790
USA
Laurence Lundblade +1 619 658 3584
QUALCOMM, Incorporated lgl@qualcomm.com
6455 Lusk Blvd.
San Diego, Ca, 92121-2779
USA
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