Network Working Group                                          C. Newman
Internet-Draft                                          Sun Microsystems
Updates: 1939 (if approved)                                   R. Gellens
Intended status: Experimental                      QUALCOMM Incorporated
Expires: January 14, 2009                                  July 13, 2008

                         POP3 Support for UTF-8

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This specification extends the Post Office Protocol version 3 (POP3)
   to support un-encoded international characters in user names,
   passwords, mail addresses, message headers, and protocol-level
   textual error strings.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in this Document  . . . . . . . . . . . .  3
     1.2.  Change History . . . . . . . . . . . . . . . . . . . . . .  3
       1.2.1.  Changes from -03 to -04  . . . . . . . . . . . . . . .  3
       1.2.2.  Changes from -02 to -03  . . . . . . . . . . . . . . .  4
       1.2.3.  Changes from -01 to -02  . . . . . . . . . . . . . . .  4
       1.2.4.  Changes from -00 to -01  . . . . . . . . . . . . . . .  4
       1.2.5.  Changes from draft-newman-ima-pop  . . . . . . . . . .  5
     1.3.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  LANG Capability  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  UTF8 Capability  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  The UTF8 Command . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  USER Argument to UTF8 Capability . . . . . . . . . . . . .  9
   4.  Issues with UTF-8 Header maildrop  . . . . . . . . . . . . . . 10
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Design Rationale  . . . . . . . . . . . . . . . . . . 12
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Introduction

   This specification extends POP3 [RFC1939] using the POP3 Extension
   Mechanism [RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers
   as described in Internationalized Email Headers
   [I-D.ietf-eai-utf8headers].  It also adds a mechanism to support
   login names outside the ASCII character set, and a mechanism to
   support UTF-8 protocol-level error strings in a language appropriate
   for the user.

   Within this specification, the term down-conversion refers to the
   process of modifying a message containing UTF8 headers
   [I-D.ietf-eai-utf8headers] or body parts with 8bit content-transfer-
   encoding as defined in MIME section 2.8 [RFC2045] into conforming
   7-bit Internet message format [RFC2822] with Message Header
   Extensions for Non-ASCII Text [RFC2047] and other 7-bit encodings.
   Down-conversion is specified by Downgrading mechanism for Email
   Address Internationalization [I-D.ietf-eai-downgrade].

1.1.  Conventions Used in this Document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [RFC2119].

   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC4234] notation including the core rules defined in Appendix B of
   RFC 4234.

   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

1.2.  Change History

   This section describes the change history of this Internet draft and
   will be removed when/if this is published as an RFC.

1.2.1.  Changes from -03 to -04

   o  Specified that it is an error to issue STLS after UTF8.

   o  Removed prior open issues.

   o  Downgrading added as open issue.

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1.2.2.  Changes from -02 to -03

   o  Updated references.

   o  Replaced US-ASCII with ASCII.

   o  Added comment to language listing failure example.

   o  Replaced RET8, LST8, and TOP8 commands with a single mode-switch
      UTF8 command issued before authentication.  This simplifies the
      protocol, and allows servers to optionally down-convert a cache of
      the maildrop prior to issuing the +OK response entering
      TRANSACTION state.

   o  Removed most up-conversion material.

   o  Removed definition of up-conversion.

   o  Removed IMAP4 reference.

   o  Added AUTH command to those affected by UTF8 capability.

   o  Removed LST8 and TOP8 capability parameters and commands.

   o  Removed NO-RETR capability.  POP servers are now unconditionally
      required to support down-conversion of UTF8-native maildrops.

   o  Added sentence about modifying authentication code to Security

   o  eai-downgrade draft is now normative and required.

   o  Deleted references to RFCs 1341, 1847, 2049, 2183, 3501, 3516, and

1.2.3.  Changes from -01 to -02

   o  Minor grammatical tweaks.

   o  Add passwords to Abstract.

   o  Removed new editor's name from Acknowledgments.

1.2.4.  Changes from -00 to -01

   o  Update references

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1.2.5.  Changes from draft-newman-ima-pop

   o  Change title to make this a WG document.

   o  Add LANG command and extension.

   o  Rename RET8 capability to UTF8 and add sub-sections for arguments.

   o  Add TOP8 command.

   o  Add definition of up-conversion and down-conversion.

   o  Some grammar fix-ups and section re-ordering based on RFC editor

1.3.  Open Issues

   1.   How should downgrading (Section 3.1) be handled?

2.  LANG Capability

   CAPA tag:


   Added Commands:

   Standard commands affected:

   Announced states / possible differences:
      both / no

   Commands valid in states:

   Specification reference:
      this document


   POP3 allows most +OK and -ERR server responses to include human-
   readable text that in some cases needs to be presented to the user.
   But that text is limited to ASCII by the POP3 specification

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   [RFC1939].  The LANG capability and command permit a POP3 client to
   negotiate which language the server should use when sending human-
   readable text.

   A server that advertises the LANG extension MUST use the language
   "i-default" as described in [RFC2277] as its default language until
   another supported language is negotiated by the client.  A server
   MUST include "i-default" as one of its supported languages.

   The LANG command requests that human-readable text included in all
   subsequent +OK and -ERR responses be localized to a language matching
   the language range argument as described by [RFC4647].  If the
   command succeeds, the server returns a +OK response followed by a
   single space, the exact language tag selected, another space, and the
   rest of the line is human-readable text in the appropriate language.
   This and subsequent protocol-level human readable text is encoded in
   the UTF-8 charset.

   If the command fails, the server returns an -ERR response and
   subsequent human-readable response text continues to use the language
   that was previously active (typically i-default).

   The special "*" language range argument indicates a request to use a
   language designated as preferred by the server administrator.  The
   preferred language MAY vary based on the currently active user.

   If no argument is given and the POP3 server issues a positive
   response, then the response given is multi-line.  After the initial
   +OK, for each language tag the server supports, the POP3 server
   responds with a line for that language.  This line is called a
   "language listing".

   In order to simplify parsing, all POP3 servers are required to use a
   certain format for language listings.  A language listing consists of
   the language tag [RFC4646] of the message, optionally followed by a
   single space and a human readable description of that language using
   the UTF-8 charset.

      < The server defaults to using English i-default responses until
      the client explicitly changes the language. >

      C: USER karen
      S: +OK Hello, karen
      C: PASS password
      S: +OK karen's maildrop contains 2 messages (320 octets)

      < Client requests depricated MUL language. Server replies
      with -ERR response >

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      C: LANG MUL
      S: -ERR invalid language MUL

      < A LANG command with no arguments is a request for
      a language listing. >

      C: LANG
      S: +OK Language listing follows:
      S: en English
      S: en-boont English Boontling dialect
      S: de German
      S: it Italian
      S: i-default Default language
      S: .

      < A request for a language listing might fail >

      C: LANG
      S: -ERR Server is unable to list languages

      < Once the client changes the language, all responses will be in
      that language starting with the response to the LANG command.
      Note: the example does not include the correct character accents
      due to limitations of this document format. >

      C: LANG fr
      S: +OK fr La Language commande a ete execute avec success

      < If a server does not support the requested primary language,
      responses will continue to be returned in the current language
      the server is using. >

      C: LANG uga
      S: -ERR Ce Language n'est pas supporte

      C: LANG fr-ca
      S: +OK fr La Language commande a ete execute avec success

      C: LANG *
      S: +OK fr La Language commande a ete execute avec success


3.  UTF8 Capability

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   CAPA tag:


   Added Commands:

   Standard commands affected:

   Announced states / possible differences:
      both / no

   Commands valid in states:

   Specification reference:
      this document


   This capability adds the "UTF8" command to POP3.  The UTF8 command
   switches the session from ASCII to UTF8 mode.

3.1.  The UTF8 Command

   The UTF8 command enables UTF8 mode.  Maildrops can natively store
   UTF8 or be limited to ASCII.  UTF8 mode has no effect on messages in
   an ACII-only maildrop.  Messages in native-UTF8 maildrops can be
   ASCII or UTF8 using internationalized headers
   [I-D.ietf-eai-utf8headers] and/or 8bit content-transfer-encoding as
   defined in MIME section 2.8 [RFC2045].  In UTF8 mode, both UTF8 and
   ASCII messages are sent to the client as-is (without conversion).
   When not in UTF8 mode, UTF8 messages in a native UTF8 maildrop MUST
   be downconverted using the Downgrading mechanism for Email Address
   Internationalization [I-D.ietf-eai-downgrade].

   [[Downgrading: How should downgrading and the reference to
   draft-ietf-eai-downgrade be handled?]]


   1  MUST downgrade according to eai-downgrade,

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   2  SHOULD downgrade according to eai-downgrade,

   3  "might" downgrade according to eai-downgrade-.  In this case,
      "might" is just a note that "many people like to do it this way",
      and has no normative force.

   The main argument against a prescribed mechanism for downgrade by a
   POP server is that the only clients that have any use for a
   standardized downgraded message (because they wish to interpret
   downgrade headers, for example) are ones that can support UTF8 and
   hence will issue the UTF8 command in the first place.  The counter
   argument is that non-UTF8 clients might be upgraded to be capable of
   interpreting prior downgraded messages if they were in a standard
   format. [[ (end of downgrade open issue): ]]

   Note that even in UTF8 mode, MIME binary content-transfer-encoding is
   still not permitted.

   The octet count (size) of a message reported in a response to the
   LIST command SHOULD match what the server sends in a RETR response.
   Sizes reported elsewhere, such as in STAT responses and free-form
   text in positive status indicators (following "+OK") need not be
   accurate, but it is preferable if they are.

   Clients MUST NOT issue the STLS command [RFC2595] after issuing UTF8;
   servers MAY (but are not required to) enforce this by rejecting with
   an "-ERR" response an STLS command issued subsequent to a successful
   UTF8 command.  (Because this is a protocol error as opposed to a
   failure based on conditions, an extended response code [RFC2449] is
   not specified.)

3.2.  USER Argument to UTF8 Capability

   If the USER argument is included with this capability, it indicates
   that the server accepts UTF-8 user names and passwords and applies
   SASLprep [RFC4013] to the arguments of the AUTH, USER, PASS and APOP
   commands.  A client that supports APOP and permits UTF-8 in user
   names or passwords MUST also implement SASLprep [RFC4013] on the user
   name and password used to compute the APOP digest.

   The client does not need to issue the UTF8 command prior to using
   UTF8 in authentication.  However, clients MUST NOT use UTF8 in USER,
   PASS, or APOP commands unless the USER argument is included with the
   UTF8 capability.

   Use of UTF8 in the AUTH command is governed by the SASL mechanism.

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4.  Issues with UTF-8 Header maildrop

   When a POP3 server uses a UTF8-native maildrop, it is the
   responsibility of the server to comply with the POP3 base
   specification [RFC1939] and RFC 2822 [RFC2822] when not in UTF8 mode.
   Mechanisms for 7-bit downgrading to help comply with the standards
   are specified in Downgrading mechanism for Email Address
   Internationalization [I-D.ietf-eai-downgrade].

5.  IANA Considerations

   This adds two new capabilities ("UTF8" and "LANG") to the POP3
   capability registry [RFC2449].

6.  Security Considerations

   The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013]
   apply to this specification, particularly with respect to use of
   UTF-8 in user names and passwords.

   The "LANG *" command can reveal the existence and preferred language
   of a user to an active attacker probing the system if the active
   language changes in response to the USER, PASS, or APOP commands
   prior to validating the user's credentials.  Servers MUST implement a
   configuration to prevent this exposure.

   It is possible for a man-in-the-middle attacker to insert a LANG
   command in the command stream thus making protocol-level diagnostic
   responses unintelligible to the user.  A mechanism to integrity
   protect the session, such as TLS [RFC2595] can be used to defeat such

   Modifying server authentication code (in this case, to support UTF8)
   needs to be done with care to avoid introducing vulnerabilities (for
   example, in string parsing).

7.  References

7.1.  Normative References

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message

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              Bodies", RFC 2045, November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [RFC2449]  Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
              Mechanism", RFC 2449, November 1998.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC4646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 4646, September 2006.

   [RFC4647]  Phillips, A. and M. Davis, "Matching of Language Tags",
              BCP 47, RFC 4647, September 2006.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

              Yeh, J., "Internationalized Email Headers",
              draft-ietf-eai-utf8headers-09 (work in progress),
              Feb 2008.

7.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

              Yoneya, Y. and K. Fujiwara, "Downgrading mechanism for
              Email Address Internationalization (EAI)",
              draft-ietf-eai-downgrade-06 (work in progress), Feb 2008.

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Appendix A.  Design Rationale

   This non-normative section discusses the reasons behind some of the
   design choices in the above specification.

   Having servers perform up-conversion so that, at a minimum, RFC2047-
   encoded words are decoded into UTF8 is tempting, since this is an
   area that clients often fail to correctly implement.  However,
   modifying messages breaks digital signatures, and would require
   servers to support arbitrary charset conversion.

   USER is optional because the implementation burden of SASLprep
   [RFC4013] is not well understood and mandating such support in all
   cases could negatively impact deployment.

   Due to interoperability problems with RFC 2047 and limited deployment
   of RFC 2231, it is hoped these 7-bit encoding mechanisms can be
   deprecated in the future when UTF-8 header support becomes prevalent.

   While it is possible to provide useful examples for language
   negotiation without support for non-ASCII characters, it is difficult
   to provide useful examples for commands specifically designed to use
   the UTF-8 charset un-encoded when the document format is limited to
   ASCII.  As a result, there are no plans to provide examples for that
   part of the specification as long as this remains an experimental
   proposal.  However, implementers of this specification are encouraged
   to provide examples to the document author for a future revision.

Appendix B.  Acknowledgments

   Thanks to John Klensin, Tony Hansen and other EAI working group
   participants who provided helpful suggestions and interesting debate
   that improved this specification.

Authors' Addresses

   Chris Newman
   Sun Microsystems
   3401 Centrelake Dr., Suite 410
   Ontario, CA  91761


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   Randall Gellens
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92651


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Full Copyright Statement

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