Network Working Group                                          C. Newman
Internet-Draft                                          Sun Microsystems
Updates: 1939 (if approved)                            February 27, 2006
Expires: August 31, 2006

                         POP3 Support for UTF-8

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This specification extends the Post Office Protocol version 3 (POP3)
   to support unencoded international characters in user names, mail
   addresses and message headers.  This is an early draft and intended
   as a framework for discussion.  Please do not deploy implementations
   of this draft.

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Table of Contents

   1.  Conventions Used in this Document  . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  RET8 Capability  . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  NO-RETR Capability . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Up-Conversion Server Requirements  . . . . . . . . . . . . . .  5
   6.  Issues with UTF-8 Header Mail Drop . . . . . . . . . . . . . .  6
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     9.1   Normative References . . . . . . . . . . . . . . . . . . .  6
     9.2   Informative References . . . . . . . . . . . . . . . . . .  7
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  8
   A.  Design Rationale . . . . . . . . . . . . . . . . . . . . . . .  8
   B.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10

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

2.  Introduction

   This specification extends POP3 [RFC1939] using the POP3 Extension
   Mechanism [RFC2449] to permit unencoded UTF-8 [RFC3629] in headers as
   described in Transmission of Email Headers in UTF-8 Encoding
   [I-D.yeh-ima-utf8headers].  It also adds a mechanism to support login
   names outside the US-ASCII character set.

3.  RET8 Capability

   CAPA tag:

      USER, LST8

   Added Commands:
      RET8, LST8

   Standard commands affected:

   Announced states / possible differences:
      both / no

   Commands valid in states:

   Specification reference:
      this document


   This capability adds UTF-8 support to POP3.  This capability always
   adds the "RET8" command to POP3.  The RET8 command is identical to
   the RETR command, except that the retrieved message uses UTF-8 in
   headers [I-D.yeh-ima-utf8headers].  In addition, the 8bit content-
   transfer-encoding as defined in MIME section 2.8 [RFC2045] is
   explicitly permitted.  The retrieved message MUST still be textual

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   and otherwise formatted according to RFC 2822 [RFC2822] and MIME
   [RFC2045].  The MIME binary content-transfer-encoding is not
   permitted.  Clients wishing to use binary MIME should implement IMAP4
   [RFC3501] with the IMAP4 Binary Content Extension [RFC3516].

   If the USER argument is included with this capability, that indicates
   the server accepts UTF-8 user names and passwords and applies
   SASLprep [RFC4013] to the arguments of the USER, PASS and APOP
   commands.  A client which 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.

   If the LST8 argument is included with this capability, that indicates
   the server implements the LST8 command.  The LST8 command is
   identical to the LIST command except that the octet counts are the
   exact octet counts returned by the RET8 command.  A POP3 client which
   uses RET8 MUST use LST8 instead of LIST if LST8 is advertised.

4.  NO-RETR Capability

   CAPA tag:


   Added Commands:

   Standard commands affected:

   Announced states / possible differences:
      both / no

   Commands valid in states:

   Specification reference:
      this document


   This capability permits a POP3 server to advertise that it does not
   support the RETR, LIST or TOP commands.  Any attempt to use any of
   these three commands will result in an error response.  As this is an
   incompatible change to POP3, a clear warning is necessary.  POP3
   clients which find implementation of the UTF8 capability problematic

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   are encouraged to at least detect the NO-RETR capability and provide
   an informative error message to the end-user.

   When a POP3 server runs on a UTF-8 header native mail drop, the down-
   conversion step necessary to implement RETR in a backwards compatible
   fashion will become more difficult to support.  Although it is hoped
   deployed POP3 servers do not advertise NO-RETR for some years, this
   capability is intended to minimize the disruption when legacy support
   finally goes away.

   A server which advertises NO-RETR MUST advertise UTF8 with at least
   the LST8 argument and MUST NOT advertise TOP.

5.  Up-Conversion Server Requirements

   When a POP3 server uses a traditional mail drop that supports only
   7-bit headers, it MUST support message header up-conversion for the
   RET8 and LST8 commands.  As POP3 clients are best when simple, the
   more up-conversion the server performs, the better.  Minimal up-
   conversion is described in this section.

   The server MUST support up-conversion of the following address
   header-fields in the message header: From, Sender, To, CC, Bcc,
   Resent-From, Resent-Sender, Resent-To, Resent-CC, Resent-Bcc, and
   Reply-To.  This up-conversion MUST include address local-parts
   encoded according to [TBD], address domains encoded according to IDNA
   [RFC3490], and MIME header encoding [RFC2047] of display-names and
   any RFC 2822 comments.

   The following charsets MUST be supported for up-conversion of MIME
   header encoding [RFC2047]: UTF-8, US-ASCII, ISO-8859-1, ISO-8859-2,
   ISO-8859-3, ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7,
   ISO-8859-8, ISO-8859-9, ISO-8859-10, ISO-8859-14, and ISO-8859-15.
   Other widely deployed MIME charsets SHOULD be supported.

   Up-conversion of MIME header encoding of the following headers MUST
   also be implemented: Subject, Date (RFC 2822 comments only),
   Comments, Keywords, Content-Description.

   While this specification does not require it, server implementations
   are encouraged to up-convert all MIME body headers, and particularly
   the deprecated (and misused) name parameter [RFC1341] on Content-Type
   and the Content-Disposition filename parameter.  These may be encoded
   using the standard MIME parameter encoding [RFC2231] mechanism, or
   via non-standard use of MIME header encoding [RFC2047] in quoted

   The POP server MUST NOT perform up-conversion of headers and content

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   of multipart/signed, as well as Original-Recipient and Return-Path.

6.  Issues with UTF-8 Header Mail Drop

   When a POP3 server uses a mail drop that supports UTF-8 headers and
   it does not advertise the NO-RETR capability, it is the
   responsibility of the server to comply with the POP3 base
   specification [RFC1939] and RFC 2822 [RFC2822] with respect to the
   RETR, LIST and TOP commands.  Mechanisms for 7-bit downgrading to
   help comply with the standards are discussed in Downgrading mechanism
   for Internationalized eMail Address (IMA) [I-D.yoneya-ima-downgrade].

   A POP3 server with a mail drop that supports UTF-8 headers MUST
   comply with the RET8 protocol requirements implicit from Section 5.
   However, the code necessary for such compliance need not be part of
   the POP3 server itself in this case.  For example, the minimal
   required up-conversion could be performed when a message is inserted
   into the POP3-accessible mail drop.

7.  IANA Considerations

   This adds two new capabilities ("UTF8" and "NO-RETR") to the POP3
   capability registry [RFC2449].

8.  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 server.  Otherwise, this is not
   believed to alter the security considerations of POP3.

9.  References

9.1  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
              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.

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   [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.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [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. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

              Yeh, J., "Transmission of Email Headers in UTF-8
              Encoding", draft-yeh-ima-utf8headers-00 (work in
              progress), September 2005.

9.2  Informative References

   [RFC1341]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
              Mail Extensions): Mechanisms for Specifying and Describing
              the Format of Internet Message Bodies", RFC 1341,
              June 1992.

   [RFC1847]  Galvin, J., Murphy, S., Crocker, S., and N. Freed,
              "Security Multiparts for MIME: Multipart/Signed and
              Multipart/Encrypted", RFC 1847, October 1995.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, November 1996.

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2231, November 1997.

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and

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              Languages", BCP 18, RFC 2277, January 1998.

              4rev1", RFC 3501, March 2003.

   [RFC3516]  Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
              April 2003.

              Yoneya, Y. and K. Fujiwara, "Downgrading mechanism for
              Internationalized eMail Address (IMA)",
              draft-yoneya-ima-downgrade-00 (work in progress),
              October 2005.

Author's Address

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


Appendix A.  Design Rationale

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

   The basic approach of advertising a parallel command set and
   permitting graceful migration of both client and server with minimal
   disruption is a deliberate choice.  While a mechanism that makes RETR
   "just-send-UTF-8" might deploy faster, it would also create
   interoperability problems.  The approach used prevents
   interoperability problems until the NO-RETR mechanism is deployed.  A
   client command to cause a model switch could also work, but the
   parallel command approach is cleaner given the small number of

   The choice to make RET8 nearly identical to RETR is important to
   minimize the code changes necessary in a client.  An alternative
   approach which permits binary MIME and uses a length-counted argument
   would be architecturally superior but is dismissed due to the
   migration problems it would cause.  The IMAP4 Binary extension should
   be sufficient for cases where binary MIME support is deemed

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   LST8 is optional to minimize the cost of deploying UTF-8 support on a
   legacy mail drop.  The server load necessary to perform up-conversion
   on every message in the mail drop to determine the LST8 octet-counts
   would be prohibitively expensive when there's no way to cache those
   counts.  The octet counts from the LIST command should be close
   enough to the RET8 size for most POP3 user interfaces, and robust
   POP3 clients already have to deal with LIST octet counts that don't
   match the actual size of the RETR result.

   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.

   The NO-RETR mechanism simplifies diagnosis of interoperability
   problems when legacy support goes away.  In the situation where
   backwards compatibility is broken anyway, just-send-8 RETR has the
   advantage that it might work with some legacy clients.  However, the
   difficulty of diagnosing interoperability problems caused by a just-
   send-8 RETR mechanism is the reason the NO-RETR mechanism was chosen.

   This specification deliberately deprecates the optional TOP command
   by not providing a TOP8 command.  TOP is a crude partial fetch
   mechanism, especially now that MIME support is widespread.  IMAP4rev1
   [RFC3501] now has complete partial fetch functionality.  As a result
   it is preferable to error on the side of simplicity in this case.

   The up-conversion requirements are designed to balance the desire to
   deprecate and eventually eliminate complicated encodings (like MIME
   header encodings) without creating a significant deployment burden
   for servers.  While it would be desirable to require up-conversion of
   attachment file names, the erroneous perception that MIME parsing is
   difficult in combination with multiple deployed mechanisms for such
   file names tip the balance.

   The set of mandatory charsets comes from two sources: MIME
   requirements [RFC2049] and IETF Policy on Character Sets [RFC2277].
   Including a requirement to up-convert widely deployed encoded
   ideographic charsets to UTF-8 would be reasonable for most scenarios,
   but may require unacceptable table sizes for some embedded devices.
   The open-ended recommendation to support widely deployed charsets
   avoids the political ramifications of attempting to list such
   charsets.  The author believes market forces, existing open-source
   software, and public conversion tables are sufficient to deploy the
   appropriate charsets.

Appendix B.  Acknowledgments


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