Network Working Group                                        J. Yao, Ed.
Internet-Draft                                               W. Mao, Ed.
Expires: November 13, 2006                                         CNNIC
                                                            May 12, 2006

           SMTP extension for internationalized email address

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   Internationalized eMail Address (IMA) includes two parts, the local
   part and the domain part.  The way email addresses are used by
   protocols are different from the way domain names are used.  The most
   critical difference is that emails are delivered through a chain of
   peering clients and servers while domain names are resolved by name
   servers by looking up their own tables.  In addition to this, email
   transport protocols SMTP and ESMTP provide a negotiation mechanism
   through which clients can make decisions for further processing.  So

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   IMA is different from the internationalized domain name (IDN).  IMA
   can be solved by exploiting the negotiation mechanism while IDN can
   not use the negotiation mechanism.  So IMA should be solved in the
   mail transport-level using the negotiation mechanism, which is an
   architecturally desirable approach.  This document specifies the use
   of SMTP extension for IMA delivery.  It also mentions the backward
   compatible mechanism for downgrade procedure, as specified in an
   associated specification.  The protocol proposed here is MTA-level
   solution which is feasible, architecturally more elegant, and not as
   difficult to deploy in relevant communities.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of this specification . . . . . . . . . . . . . . . .  3
     1.2.  Proposal Context . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Mail Transport-level Protocol  . . . . . . . . . . . . . . . .  4
     2.1.  Framework for the Internationalization Extension . . . . .  4
     2.2.  The Address Internationalization Service Extension . . . .  4
     2.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  5
     2.4.  The ALT-ADDRESS and ATOMIC parameter . . . . . . . . . . .  6
     2.5.  Additional ESMTP Changes and Clarifications  . . . . . . .  8
       2.5.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . .  8
       2.5.2.  Trace Fields . . . . . . . . . . . . . . . . . . . . .  8
       2.5.3.  Mailing List Question  . . . . . . . . . . . . . . . .  8
       2.5.4.  Message Header Label . . . . . . . . . . . . . . . . .  8
   3.  Potential problems . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Impact to IRI  . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Impact to RFC 2476 and many email related RFC  . . . . . .  9
   4.  Implementation Advice  . . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Introduction

1.1.  Role of this specification

   An overview document [IMA-overview] specifies the requirements for,
   and components of, full internationalization of electronic mail.
   This document specifies an element of that work, specifically the
   definition of an SMTP extension [RFC1869] for IMA transport delivery.

1.2.  Proposal Context

   In order to use internationalized email addresses, we need to
   internationalize both the domain part and the local part of the email
   address.  Domain part of the email address has been internationalized
   through IDNA [RFC3490].  But the local part of the email address
   still remains as non-internationalized.

   The syntax of Internet email addresses is restricted to a subset of
   7-bit ASCII for the domain-part, with a less-restricted subset for
   the local-part.  These restrictions are specified in RFC 2821
   [RFC2821].  To be able to deliver internationalized email through
   SMTP servers, we need to upgrade SMTP server to be able to carry IMA.
   Since older SMTP servers and the mail-reading clients and other
   systems that are downstream from them may not be prepared to handle
   these extended addresses, an SMTP extension is specified to identify
   and protect the addressing mechanism.

   This specification describes a change to the email transport
   mechanism that permits IMA in both the envelope and header fields of
   messages.  The context for the change is described in [IMA-overview]
   and the details of the header changes are described in [IMA-

1.3.  Terminology

   The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
   and "MAY" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

   All specialized terms used in this specification are defined in the
   IMA overview [IMA-overview] or in [RFC2821] and [RFC2822].

   This document is being discussed on the IMA mailing list.  See for information about
   subscribing.  The list's archive is at

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2.  Mail Transport-level Protocol

2.1.  Framework for the Internationalization Extension

   The following service extension is defined:

   1.  The name of the SMTP service extension is "Internationalized
       Email and Extensions";
   2.  The EHLO keyword value associated with this extension is
   3.  No parameter values are defined for this EHLO keyword value.  In
       order to permit future (although unanticipated) extensions, the
       EHLO response MUST NOT contain any parameters for that keyword.
       If a parameter appears, the SMTP client that is conformant to
       this version of this specification MUST treat the ESMTP response
       as if the IMA keyword did not appear.
   4.  Two optional parameters are added to the SMTP MAIL and RCPT
       commands.  The first parameter is named as ALT-ADDRESS.  The
       second is ATOMIC.  The "ALT-ADDRESS" requires an all-ASCII
       address as a substitute for the internationalized (UTF-8 coded)
       address that we call the primary address; you can learn more in
       [IMA-overview] or [IMA-downgrading].  The value of "ALT-ADDRESS"
       may be set by sender or be gotten by using some algorithmic
       transformation according to the value of "ATOMIC".  The "ATOMIC"
       has one of two values: y or n.  The parameter "ATOMIC" is
       designed to assert whether the address is atomic, which means
       that the primary address(IMA) can be safely transformed or
       converted to the respect ASCII email address via ACE (ASCII
       Compatible Encoding) if the value is 'y' or not if the value is
   5.  No additional SMTP verbs are defined by this extension.
   6.  Servers offering this extension MUST provide support for, and
       announce, the 8BITMIME extension [RFC1652].

2.2.  The Address Internationalization Service Extension

   An SMTP Server that announces this extension MUST be prepared to
   accept a UTF-8 string [RFC3629] in any position in which RFC 2821
   specifies that a "mailbox" may appear.  That string must be parsed
   only as specified in RFC 2821, i.e., by separating the mailbox into
   source route, local part and domain part, using only the characters
   colon (U+003A), comma (U+002C), and at-sign (U+0040) as specified
   there.  Once isolated by this parsing process, the local part MUST be
   treated as opaque unless the SMTP Server is the final delivery MTA.
   Any domain names that are to be looked up in the DNS MUST be
   processed into the form as specified in IDNA [RFC3490] by means of
   the ToASCII() operation unless they are already in that form.  Any
   domain names that are to be compared to local strings SHOULD be

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   checked for validity and then MUST be compared as specified in
   section 3.4 of IDNA.

   An SMTP Client that receives the IMA extension keyword MAY transmit a
   mailbox name as an internationalized string in UTF-8 form and MAY
   send an internationalized mail header [IMA-utf8header].  It MAY
   transmit the domain part of that string in either punycode (derived
   from the IDNA process) or UTF-8 form.  If it sends the domain in
   UTF-8 form, the original SMTP client SHOULD first verify that the
   string is valid for a domain name according to IDNA rules.  As
   required by RFC 2821, it MUST not attempt to parse, evaluate, or
   transform the local part in any way if the IMA SMTP extension is
   offered by the server.  If the IMA SMTP extension is not offered by
   the Server, the SMTP Client MUST NOT transmit an internationalized
   address and MUST NOT transmit a mail body which contains
   internationalized mail headers [IMA-utf8header].  Instead, it MUST
   either return the message to the user as undeliverable or replace it
   with the alternate ASCII address.  If it is replaced, the replacement
   MUST be either the ASCII-only address specified with the ALT-ADDRESS
   parameter or with an address obtained from some algorithmic
   conversions of the primary address that conforms to the syntax rules
   of RFC 2821, which is defined in [IMA-downgrading].

2.3.  Extended Mailbox Address Syntax

   RFC 2821, section 4.1.2, defines the syntax of a mailbox as

         Mailbox = Local-part "@" Domain

         Local-part = Dot-string / Quoted-string
               ; MAY be case-sensitive

         Dot-string = Atom *("." Atom)

         Atom = 1*atext

         Quoted-string = DQUOTE *qcontent DQUOTE

         Domain = (sub-domain 1*("." sub-domain)) / address-literal
         sub-domain = Let-dig [Ldh-str]

   The key changes made by this specification are, informally, to

   o  Change the definition of "sub-domain" to permit either the
      definition above or a UTF-8 string representing a DNS label that
      is conformant with IDNA [RFC3490].  That label MUST NOT contain

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      the characters "@" or ".", even though those characters can
      normally be inserted into a DNS label.
   o  Change the definition of "Atom" to permit either the definition
      above or a UTF-8 string.  That string MUST NOT contain any of the
      ASCII characters (either graphics or controls) that are not
      permitted in "atext"; it is otherwise unrestricted.

   According to the description above, define the syntax of an IMA
   mailbox with ABNF [RFC4234] as

         Mailbox = Local-part "@" Domain

         Local-part = Dot-string / Quoted-string
               ; MAY be case-sensitive

         Dot-string = Atom *("." Atom)

         Atom = 1*Ucharacter
         Ucharacter = atext / UTF8-2 / UTF8-3 / UTF8-4

         Quoted-string = DQUOTE *qcontent DQUOTE

         Domain = (sub-domain 1*("." sub-domain)) / address-literal
         sub-domain = ULet-dig [ULdh-str]

         ULet-dig = Let-dig / Non-ASCII

         ULdh-str = *( ALPHA / DIGIT / "-" / Non-ASCII) ULet-dig

         Non-ASCII = UTF8-2 / UTF8-3 / UTF8-4
                       ; UTF-8 characters prohibited by nameprep
                       ; MUST NOT be used.

   Where "atext", "qcontent" and "DQUOTE" are defined in [RFC2822],
   "Let-dig", "Ldh-str" and "address-literal" are defined in [RFC2821]
   and UTF8-2, UTF8-3 and UTF8-4 are defined in [RFC3629].  The value of
   "Local-part" should pass Stringprep [RFC3454]; The value of "domain"
   should be verified with [RFC3490]; If failed, The value of "Local-
   part" and "domain", the email address can not be regarded as the
   valid email address.

2.4.  The ALT-ADDRESS and ATOMIC parameter

   If the IMA extension is offered, the syntax of the SMTP MAIL and RCPT
   commands is extended to support both the optional "ALT-ADDRESS" and
   "ATOMIC" parameter.

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   The "ALT-ADDRESS" requires an all-ASCII address, which may set by the
   sender or some algorithmic transformation.

   The big problem with applying an ACE to all local-parts is that the
   sending or converting system doesn't know if there are some specific
   data or instructions embedded in the address that the ACE process
   would hide.  Some SMTP servers may depend on these specific data or
   instructions to do some operations while the local parts applied with
   ACE will lose or hide these data or instructions.  SMTP [RFC2821]
   prohibits SMTP relays from converting local parts because the level
   of SMTP relays' knowledge on the structure of local parts is assumed
   to be zero.  However, we can raise the knowledge level by supplying
   additional information.  Many human users' email addresses do not
   have any embedded structure processed by the final delivery MTA.  In
   that case, the sender can specify that these email addresses are safe
   to be converted in predefined way.  The final delivery SMTP server
   can revert the addresses even though they are as in all ASCII form.
   In such cases, a potential recipient might be able to tell someone to
   whom the address is given "it is ok, there is no embedded information
   here and you can convert it to an ACE address without danger".  If
   the recipient says that, then if the sender can pass that assertion
   along to his or her own (originator) MTA and the MTA can pass it down
   the line, then an MTA that needs to do downgrading would know that
   ACE-encoding is safe.  The "ATOMIC" parameter is designed for the
   above aim.  Transmission of local-parts of UTF-8 avoids having to
   deal with the problem.

   The use of the ALT-ADDRESS will be according to the following
   priority if SMTP servers can not support IMA capability.  If the
   sender has already set the ALT-ADDRESS value in spite of the value of
   ATOMIC, the client SMTP server will use this address as the email
   address when the SMTP server does the subsequent operations.  If the
   ALT-ADDRESS value is not set by the sender but the value of ATOMIC is
   'y', the sender SMTP server should apply some algorithmic
   transformation such as punycode to the entire local part of IMA; IDNA
   should also be applied to the domain part of IMA; these operations
   will get an ASCII email address for the subsequent SMTP operations
   related to the email address.  If the ALT-ADDRESS value is not set by
   the sender and the value of ATOMIC is 'n' which means that the local
   part of IMA can not be converted to the ASCII email address safely,
   the email must be bounced to the original sender.

   The suggested algorithmic transformation is punycode if the value of
   ALT-ADDRESS is not set by sender and the value of ATOMIC is 'y' when
   SMTP servers can not support IMA.  Since the prefix "xn--" had been
   used for IDNA, it is better that other prefix such as "bq--" is used
   for the local part of converted version of the primary address to
   avoid the potential confusion.

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2.5.  Additional ESMTP Changes and Clarifications

   The mail transport process involves addresses ("mailboxes") and
   domain names in contexts in addition to the MAIL and RCPT commands
   and extended alternatives to them.  In general, the rule is that,
   when RFC 2821 specifies a mailbox, this document expects UTF-8 to be
   used for the entire string; when RFC 2821 specifies a domain name,
   the name should be in punycode form if its raw form is non-ASCII.

   The following subsections list and discuss all of the relevant cases.

   Support and use of this extension requires support for 8BITMIME.  It
   means that 8BITMIME must be advertised by the IMA capability SMTP

2.5.1.  The Initial SMTP Exchange

   When an SMTP or ESMTP connection is opened, the server sends a
   "banner" response consisting of the 220 reply code and some
   information.  The client then sends the EHLO command.  Since the
   client cannot know whether the server supports IMA until after it
   receives the response from EHLO, any domain names that appear in this
   dialogue, or in responses to EHLO, must be in hostname form, i.e.,
   internationalized ones must be in punycode form.

2.5.2.  Trace Fields

   Internationalized domain names in Received fields must be transmitted
   in the punycode form.  Addresses in "for" clauses need further
   examination and might be treated differently depending on [IMA-
   utf8header].  The reasoning in the introductory portion of [IMA-
   overview] strongly suggests that these addresses be in UTF-8 form,
   rather than some specialized encoding.

2.5.3.  Mailing List Question

   How a mixture of traditional and internationalized addresses on a
   mailing list will impact message flows, error reports, and delivery
   notifications in all plausible combinations of IMA capability and un-
   capability servers is still not clear.  This is an issue, which we
   can delve into in detail in the future discussion.  We will proposed
   the detail solution to it in another document, and do some
   experiments to find the best solution to it.

2.5.4.  Message Header Label

   There is a hot discussion about message header label when SMTP
   messages are transmitted on wire.  How to identify them and

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   distinguish them from the normal message.  Many referred the famous
   "MIME-Version:1.0" as the example.  In order to get the robustness in
   the absence of context, we should consider the issue whether or not
   we need a mechanism(such as self-label) or some indicator to
   distinguish or recognize the format of a "stored" message: new
   format(i.e.  IMA compliant) or old one (i.e.  RFC 822 compliant).
   [Note in draft: The detail discussion of this issue will be available
   in [IMA-utf8header].]

3.  Potential problems

3.1.  Impact to IRI

   The mailto: schema in IRI [RFC3987] may need to be modified when IMA
   is standardized.

3.2.  POP and IMAP

   While SMTP mainly takes care of the transportation of messages and
   the header fields on wire, POP essentially handles the retrieval of
   mail objects from the server by a client.  In order to use
   internationalized user names based on IMA for the retrieval of
   messages from a mail server using the POP protocol, a new capability
   should be introduced following the POP3 extension mechanism

   IMAP [RFC3501] uses the traditional user name which is based on
   ASCII.  IMAP should be updated to support the internationalized user
   names based on IMA for the retrieval of messages from a mail server.

3.3.  Impact to RFC 2476 and many email related RFC

   The IMA protocol will impact on many email related RFC such as
   Message Submission [RFC2476] and SMTP Service Extension for DSNs
   [RFC3461].  These protocol should be considered when implementing the
   IMA protocol.

4.  Implementation Advice

   In the absence of this extension, SMTP clients and servers are
   constrained to using only those addresses permitted by RFC 2821.  The
   local parts of those addresses may be made up of any ASCII
   characters, although certain of them must be quoted as specified
   there.  It is notable in an internationalization context that there
   is a long history on some systems of using overstruck ASCII
   characters (a character, a backspace, and another character) within a

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   quoted string to approximate non-ASCII characters.  This form of
   internationalization should be phased out as this extension becomes
   widely deployed but backward-compatibility considerations require
   that it continue to be supported.

5.  IANA Considerations

   IANA is requested to add "IEmail" to the SMTP extensions registry
   with the entry pointing to this specification for its definition.

6.  Security considerations

   See the extended security considerations discussion in [IMA-overview]

7.  Acknowledgements

   Much of the text in the initial version of this document was derived
   or copied from [Klensin-emailaddr] with the permission of the author.
   Significant comments and suggestions were received from Xiaodong LEE,
   Nai-Wen Hsu, Yangwoo KO, Yoshiro YONEYA, and other members of the JET
   team and were incorporated into the document.  Special thanks to
   those contributors for this version of document, those includes (but
   not limited to) John C Klensin, Charles Lindsey, Dave Crocker, Harald
   Tveit Alvestrand, Marcos Sanz, Chris Newman, Martin Duerst, Edmon

8.  References

8.1.  Normative References

   [ASCII]    American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange", ANSI X3.4-1968, 1968.

              ANSI X3.4-1968 has been replaced by newer versions with
              slight modifications, but the 1968 version remains
              definitive for the Internet.

              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", draft-klensin-ima-framework-01
              (work in progress), February 2006.


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

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

   [RFC1869]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
              November 1995.

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

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

   [RFC2476]  Gellens, R. and J. Klensin, "Message Submission",
              RFC 2476, December 1998.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

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

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, January 2003.

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

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

              4rev1", RFC 3501, March 2003.

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

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   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

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

8.2.  Informative References

              YONEYA, Y. and K. Fujiwara, "Downgrade Mechanism for
              Internationalized Email Address (IMA)",
              draft-yoneya-ima-downgrade-00 (work in progress),
              October 2005.

              Klensin, J., "Internationalization of Email Addresses",
              draft-klensin-emailaddr-i18n-03 (work in progress),
              July 2005.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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Authors' Addresses

   Jiankang YAO (editor)
   No.4 South 4th Street, Zhongguancun

   Phone: +86 10 58813007

   Wei MAO (editor)
   No.4 South 4th Street, Zhongguancun

   Phone: +86 10 58813055

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Intellectual Property Statement

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Copyright Statement

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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