POP3 Support for UTF-8
RFC 5721

Internet Engineering Task Force (IETF)                        R. Gellens
Request for Comments: 5721                         QUALCOMM Incorporated
Category: Experimental                                         C. Newman
ISSN: 2070-1721                                         Sun Microsystems
                                                           February 2010

                         POP3 Support for UTF-8

Abstract

   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.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see 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/rfc5721.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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.

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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  LANG Capability  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  UTF8 Capability  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  The UTF8 Command . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  USER Argument to UTF8 Capability . . . . . . . . . . . . .  9
   4.  Native UTF-8 Maildrops . . . . . . . . . . . . . . . . . . . .  9
   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

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1.  Introduction

   This document forms part of the Email Address Internationalization
   (EAI) experiment described in the EAI Framework document [RFC4952]
   (for background, please see the charter of the EAI working group) and
   should be evaluated within the context of EAI.  As part of the
   overall EAI work, email messages may be transmitted and delivered
   containing un-encoded UTF-8 characters, and mail drops that are
   accessed using POP3 [RFC1939] might natively store UTF-8.

   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" [RFC5335].  It also
   adds a mechanism to support login names and passwords outside the
   ASCII character set, and a mechanism to support UTF-8 protocol-level
   error strings in a language appropriate for the user.

   This document updates POP3 [RFC1939], and the fact that an
   Experimental specification updates a Standards Track specification
   means that people who participate in the experiment have to consider
   the Standard updated.  In an attempt to reduce confusion, this
   Experimental document does not contain an "Updates" header.  If and
   when a version of this document moves to the Standards Track, an
   "Updates: 1939" header should be added.

   Within this specification, the term "down-conversion" refers to the
   process of modifying a message containing UTF8 headers [RFC5335] or
   body parts with 8bit content-transfer-encoding, as defined in MIME
   Section 2.8 [RFC2045], into conforming 7-bit Internet Message Format
   [RFC5322] 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"
   [RFC5504].

1.1.  Conventions Used in This Document

   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 "Key words for use in
   RFCs to Indicate Requirement Levels" [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.

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   Note that examples always use 7-bit ASCII characters due to
   limitations of this document format; in particular, some examples for
   the "LANG" command may appear silly as a result.

2.  LANG Capability

   Per "POP3 Extension Mechanism" [RFC2449], this document adds a new
   capability response tag to indicate support for a new command: LANG.
   The capability tag and new command are described below.

   CAPA tag:
      LANG

   Arguments with CAPA tag:
      none

   Added Commands:
      LANG

   Standard commands affected:
      All

   Announced states / possible differences:
      both / no

   Commands valid in states:
      AUTHENTICATION, TRANSACTION

   Specification reference:
      this document

   Discussion:

   POP3 allows most +OK and -ERR server responses to include human-
   readable text that, in some cases, might be presented to the user.
   But that text is limited to ASCII by the POP3 specification
   [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 (the "Basic Language Range" as described

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   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 [RFC5646] of the message, optionally followed by a
   single space and a human-readable description of the language in the
   language itself, using the UTF-8 charset.

   Examples:

      < Note that some examples do not include the correct character
      accents due to limitations of this document format. >

      < 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 deprecated MUL language.  Server replies
      with -ERR response. >

      C: LANG MUL
      S: -ERR invalid language MUL

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

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      C: LANG
      S: +OK Language listing follows:
      S: en English
      S: en-boont English Boontling dialect
      S: de Deutsch
      S: it Italiano
      S: es Espanol
      S: sv Svenska
      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. >

      C: LANG es
      S: +OK es Idioma cambiado

      < 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 es Idioma <<UGA>> no es conocido

      C: LANG sv
      S: +OK sv Kommandot "LANG" lyckades

      C: LANG *
      S: +OK es Idioma cambiado

3.  UTF8 Capability

   Per "POP3 Extension Mechanism" [RFC2449], this document adds a new
   capability response tag to indicate support for new server
   functionality, including a new command: UTF8.  The capability tag and
   new command and functionality are described below.

   CAPA tag:
      UTF8

   Arguments with CAPA tag:
      USER

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   Added Commands:
      UTF8

   Standard commands affected:
      USER, PASS, APOP, LIST, TOP, RETR

   Announced states / possible differences:
      both / no

   Commands valid in states:
      AUTHORIZATION

   Specification reference:
      this document

   Discussion:

   This capability adds the "UTF8" command to POP3.  The UTF8 command
   switches the session from ASCII to UTF-8 mode.

3.1.  The UTF8 Command

   The UTF8 command enables UTF-8 mode.  The UTF8 command has no
   parameters.

   Maildrops can natively store UTF-8 or be limited to ASCII.  UTF-8
   mode has no effect on messages in an ASCII-only maildrop.  Messages
   in native UTF-8 maildrops can be ASCII or UTF-8 using
   internationalized headers [RFC5335] and/or 8bit content-transfer-
   encoding, as defined in MIME Section 2.8 [RFC2045].  In UTF-8 mode,
   both UTF-8 and ASCII messages are sent to the client as-is (without
   conversion).  When not in UTF-8 mode, UTF-8 messages in a native
   UTF-8 maildrop MUST be down-converted (downgraded) to comply with
   unextended POP and Internet Mail Format.  POP servers (unlike SMTP
   and Submit servers) are not required to use "Downgrading Mechanism
   for Email Address Internationalization" [RFC5504].

   Discussion: The main argument against a single required mechanism for
   downgrading 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
   UTF-8 and, hence, will issue the UTF8 command in the first place.
   The counter argument to this is that clients that do not support
   UTF-8 might be upgraded in the future; it's desirable for an upgraded
   client to be capable of interpreting prior downgraded messages in the
   local mail store, which is most likely if the messages were
   downgraded using one standardized procedure.

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   Therefore, while POP servers are not required to use "Downgrading
   Mechanism for Email Address Internationalization" [RFC5504], there
   are advantages to them doing so.

   Note that even in UTF-8 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 the actual number of octets sent in a RETR
   response (not counting byte-stuffing).  Sizes reported elsewhere,
   such as in STAT responses and non-standardized, free-form text in
   positive status indicators (following "+OK") need not be accurate,
   but it is preferable if they are.

   Discussion: Mail stores are either ASCII or native UTF-8, and clients
   either issue the UTF8 command or not.  The message needs converting
   only when it is native UTF-8 and the client has not issued the UTF-8
   command, in which case the server must down-convert it.  The down-
   converted message may be larger.  The server may choose various
   strategies regarding down-conversion, which include when to down-
   convert, whether to cache or store the down-converted form of a
   message (and if so, for how long), and whether to calculate or retain
   the size of a down-converted message independently of the down-
   converted content.  If the server does not have immediate access to
   the accurate down-converted size, it may be faster to estimate rather
   than calculate it.  Servers are expected to normally follow the RFC
   1939 [RFC1939] text on using the "exact size" in a scan listing, but
   there may be situations with maildrops containing very large numbers
   of messages in which this might be a problem.  If the server does
   estimate, reporting a scan listing size smaller than what it turns
   out to be could be a problem for some clients.  In summary, it is
   better for servers to report accurate sizes, but if this is not
   possible, high guesses are better than small ones.  Some POP servers
   include the message size in the non-standardized text response
   following "+OK" (the 'text' production of RFC 2449 [RFC2449]), in a
   RETR or TOP response (possibly because some examples in POP3
   [RFC1939] do so).  There has been at least one known case of a client
   relying on this to know when it had received all of the message
   rather than following the POP3 [RFC1939] rule of looking for a line
   consisting of a termination octet (".") and a CRLF pair.  While any
   such client is non-compliant, if a server does include the size in
   such text, it is better if it is accurate.

   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

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   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.

   Servers that include the USER argument in the UTF8 capability
   response SHOULD apply SASLprep [RFC4013] to the arguments of the USER
   and PASS commands.

   A client or server that supports APOP and permits UTF-8 in user names
   or passwords MUST apply SASLprep [RFC4013] to the user name and
   password used to compute the APOP digest.

   When applying SASLprep [RFC4013], servers MUST reject UTF-8 user
   names or passwords that contain a Unicode character listed in Section
   2.3 of SASLprep [RFC4013].  When applying SASLprep to the USER
   argument, the PASS argument, or the APOP username argument, a
   compliant server or client MUST treat them as a query string (i.e.,
   unassigned Unicode codepoints are allowed).  When applying SASLprep
   to the APOP password argument, a compliant server or client MUST
   treat them as a stored string (i.e., unassigned Unicode codepoints
   are prohibited).

   The client does not need to issue the UTF8 command prior to using
   UTF-8 in authentication.  However, clients MUST NOT use UTF-8 in
   USER, PASS, or APOP commands unless the USER argument is included in
   the UTF8 capability response.

   The server MUST reject UTF-8 user names or passwords that fail to
   comply with the formal syntax in UTF-8 [RFC3629].

   Use of UTF-8 in the AUTH command is governed by the POP3 SASL
   [RFC5034] mechanism.

4.  Native UTF-8 Maildrops

   When a POP3 server uses a native UTF-8 maildrop, it is the
   responsibility of the server to comply with the POP3 base
   specification [RFC1939] and Internet Message Format [RFC5322] when
   not in UTF-8 mode.  Mechanisms for 7-bit downgrading to help comply
   with the standards are described in "Downgrading Mechanism for Email
   Address Internationalization" [RFC5504].

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5.  IANA Considerations

   This specification 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 might 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 Transport Layer Security (TLS) [RFC2595]
   can be used to defeat such attacks.

   Modifying server authentication code (in this case, to support UTF-8)
   needs to be done with care to avoid introducing vulnerabilities (for
   example, in string parsing).

   The UTF8 command description (Section 3.1) contains a discussion on
   reporting inaccurate sizes.  An additional risk to doing so is that,
   if a client allocates buffers based on the reported size, it may
   overrun the buffer, crash, or have other problems if the message data
   is larger than reported.

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
              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.

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   [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.

   [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.

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

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5335]  Abel, Y., "Internationalized Email Headers", RFC 5335,
              September 2008.

   [RFC5646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 5646, September 2009.

7.2.  Informative References

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

   [RFC4952]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.

   [RFC5034]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
              (POP3) Simple Authentication and Security Layer (SASL)
              Authentication Mechanism", RFC 5034, July 2007.

   [RFC5504]  Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
              Email Address Internationalization", RFC 5504, March 2009.

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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 UTF-8 is tempting, since this is an
   area that clients often fail to correctly implement.  However, after
   much discussion, the EAI group felt that the benefits did not justify
   the burden.

   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.

   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.

   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 authors for a future revision.

   While down-conversion of native UTF-8 messages is mandatory in the
   absence of the UTF8 command, servers are not required to use
   "Downgrading Mechanism for Email Address Internationalization"
   [RFC5504] to do so.  As clients are upgraded with UTF-8 support and
   the ability to intelligently handle (e.g., display and reply to)
   UTF-8 messages that were downgraded in transit, it is better if they
   are also able to handle messages in the local mail store that were
   downgraded by the POP server.  This is more likely if the POP server
   downgrades messages using the same mechanism as an SMTP server.

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.

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Authors' Addresses

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

   EMail: rg+ietf@qualcomm.com

   Chris Newman
   Sun Microsystems
   800 Royal Oaks
   Monrovia, CA  91016-6347
   US

   EMail: chis.newman@sun.com

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