Bump versions.

[gsasl.git] / doc / specification / draft-siemborski-rfc2554bis-00.txt

Network Working Group                                  Robert Siemborski
INTERNET-DRAFT                                Carnegie Mellon University
Intended Category: Proposed Standard                       October, 2003


               SMTP Service Extension for Authentication
                  <draft-siemborski-rfc2554bis-00.txt>


Status of this Memo
    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet 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 and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet Drafts as
    reference material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.

    Distribution of this memo is unlimited.

Abstract

    This document defines a Simple Mail Transport Protocol (SMTP)
    extension whereby an SMTP client may indicate an authentication
    mechanism to the server, perform an authentication protocol
    exchange, and optionally negotiate a security layer for subsequent
    protocol interactions during this session.  This extension includes
    a profile of the Simple Authentication and Security Layer (SASL) for
    SMTP.

    This document obsoletes RFC 2554 and replaces it as a Proposed
    Standard.

1.  How to Read This Document

    The key words "MUST, "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
    "RECOMMENDED", and "MAY" in this document are to be interpreted as
    defined in "Key words for use in RFCs to Indicate Requirement



Siemborski                 Expires April, 2004                  [Page 1]





SMTP Service Extension for Authentication                  October, 2003


Table of Contents


1. How to Read This Document . . . . . . . . . . . . . . . . . . . .   1
2. The Authentication Service Extension  . . . . . . . . . . . . . .   3
3. The AUTH Command  . . . . . . . . . . . . . . . . . . . . . . . .   3
3.1. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
4. The AUTH Parameter to the MAIL FROM command . . . . . . . . . . .   7
4.1. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
5. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  10
7. Security Considerations . . . . . . . . . . . . . . . . . . . . .  11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .  12
9. Intellectual Property Rights  . . . . . . . . . . . . . . . . . .  12
10. Copyright  . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
12. Changes Since RFC 2554 . . . . . . . . . . . . . . . . . . . . .  15
13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . .  15
14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  15
































Siemborski                 Expires April, 2004                  [Page 2]





SMTP Service Extension for Authentication                  October, 2003


    Levels" [KEYWORDS]

    In examples, "C:" and "S:" indicate lines sent by the client and
    server, respectively.

2.  The Authentication Service Extension

     1.   The name of this [SMTP] service extension is "Authentication"

     2.   The EHLO keyword value associated with this extension is
          "AUTH"

     3.   The AUTH EHLO keyword contains as a parameter a space
          separated list of the names of available [SASL] mechanisms.
          The list of available mechanisms MAY change after a successful
          STARTTLS command [SMTP-TLS].

     4.   A new [SMTP] verb "AUTH" is defined.

     5.   An optional parameter using the keyword "AUTH" is added to the
          MAIL FROM command, and extends the maximum line length of the
          MAIL FROM command by 500 characters.

     6.   This extension is appropriate for the submission protocol
          [SUBMIT].

3.  The AUTH Command

    AUTH mechanism [initial-response]

      Arguments:
          mechanism: A string identifying a [SASL] authentication
          mechanism.

          initial-response: An optional initial client response.  If
          present, this response MUST be [BASE64] encoded.

      Restrictions:
          After an AUTH command has been successfully completed, no more
          AUTH commands may be issued in the same session.  After a
          successful AUTH command completes, a server MUST reject any
          further AUTH commands with a 503 reply.

          The AUTH command is not permitted during a mail transaction.

      Discussion:
          The AUTH command initiates a [SASL] authentication exchange
          between the client and the server.  The client identifies the



Siemborski                 Expires April, 2004                  [Page 3]





SMTP Service Extension for Authentication                  October, 2003


          SASL mechanism to use with the first parameter of the AUTH
          command.  If the server supports the requested authentication
          mechanism, it performs the SASL exchange to authenticate the
          user.  Optionally, it also negotiates a security layer for
          subsequent protocol interactions during this session.  If the
          requested authentication mechanism is not supported, the
          server rejects the AUTH command with a 504 reply.

          The authentication protocol exchange consists of a series of
          server challenges and client responses that are specific to
          the chosen [SASL] mechanism.

          A server challenge is sent as a 334 reply with the text part
          containing the [BASE64] encoded string supplied by the SASL
          mechanism.  This challenge MUST NOT contain any text other
          than the BASE64 encoded challenge.

          A client response consists of a line containing a [BASE64]
          encoded string.  If the client wishes to cancel the
          authentication exchange, it issues a line with a single "*".
          If the server receives such a response, it MUST reject the
          AUTH command by sending a 501 reply.

          The optional initial response argument to the AUTH command is
          used to save a round trip when using authentication mechanisms
          that support an initial client response.  If the initial
          response argument is omitted and the chosen mechanism requires
          an initial client response, the server MUST proceed as defined
          in section 5.1 of [SASL].  In SMTP, a server challenge that
          contains no data is defined as a 334 reply with no text part.
          Note that there is still a space following the reply code, so
          the full response line is "334 ".

          If the client needs to send a zero length initial response,
          the client MUST transmit the response as a single equals sign
          ("=").  This indicates that the response is present, but
          contains no data.

          If the client uses an initial-response argument to the AUTH
          command with a SASL mechanism that does not support an initial
          client send, the server MUST reject the AUTH command with a
          535 reply.

          If the server cannot [BASE64] decode any client response, it
          MUST reject the AUTH command with a 501 reply.  If the client
          cannot BASE64 decode any of the server's challenges, it MUST
          cancel the authentication using the "*" response.  In
          particular, servers and clients MUST reject (and not ignore)



Siemborski                 Expires April, 2004                  [Page 4]





SMTP Service Extension for Authentication                  October, 2003


          any character not explicitly allowed by the BASE64 alphabet,
          and MUST reject any sequence of BASE64 characters that
          contains the pad character ('=') anywhere other than the end
          of the string (e.g. "=AAA" and "AAA=BBB" are not allowed).

          Note that these [BASE64] strings may be of arbitrarily length.
          Clients and servers MUST be able to handle the maximum encoded
          size of challenges and responses generated by their supported
          authentication mechanisms.  This requirement is independent of
          any line length limitations the client or server may have in
          other parts of its protocol implementation.

          If the server is unable to authenticate the client, it SHOULD
          reject the AUTH command with a 535 reply unless a more
          specific error code, such as one listed in Section 5, is
          appropriate.  Should the client successfully complete the
          exchange, the SMTP server issues a 235 reply.

          If a security layer is negotiated during the SASL exchange, it
          takes effect for the client on the octet immediately following
          the CRLF that concludes the last response generated by the
          client.  For the server, it takes effect immediately following
          the CRLF of its success reply.

          When a security layer takes effect, the SMTP protocol is reset
          to the initial state (the state in SMTP after a server issues
          a 220 service ready greeting).  The server MUST discard any
          knowledge obtained from the client, such as the EHLO argument,
          which was not obtained from the SASL negotiation itself.
          Likewise, the client MUST discard any knowledge obtained from
          the server, such as the list of SMTP service extensions, which
          was not obtained from the SASL negotiation itself (Note that a
          client MAY compare the advertised SASL mechanisms before and
          after authentication in order to detect an active down-
          negotiation attack).

          The client SHOULD send an EHLO command as the first command
          after a successful SASL negotiation which results in the
          enabling of a security layer.

          When both [TLS] and SASL security layers are in effect, the
          TLS encoding MUST be applied after the SASL encoding,
          regardless of the order in which the layers were negotiated.

          The service name specified by this protocol's profile of SASL
          is "smtp".

          If an AUTH command fails, the client may try another



Siemborski                 Expires April, 2004                  [Page 5]





SMTP Service Extension for Authentication                  October, 2003


          authentication mechanism or present different credentials by
          issuing another AUTH command.  Likewise, the server MUST
          behave as if the client had not issued the AUTH command.

          To ensure interoperability, client and server implementations
          of this extension MUST implement the STARTTLS Extension [SMTP-
          TLS], and the [PLAIN] SASL mechanism.  Implementations MUST
          support a configuration where SASL mechanisms that are
          vulnerable to passive eavesdropping attacks are not advertised
          or used without the presence of an external security layer
          such as [TLS].

3.1.    Examples

    Here is an example of a client attempting AUTH PLAIN under TLS and
    making use of the initial client response:

     S: 220-smtp.example.com ESMTP Server
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com, pleased to meet you
     S: 250-AUTH GSSAPI
     S: 250-STARTTLS
     C: STARTTLS
     S: 220 Ready to start TLS
       ... TLS negotiation proceeds, further commands protected by TLS layer ...
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com, pleased to meet you
     S: 250-AUTH KERBEROS_V4 GSSAPI PLAIN
     C: AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ=
     S: 235 Authentication successful

    Here is another client that is attempting AUTH PLAIN under a TLS
    layer, this time without the initial response.  Parts of the
    negotiation before the TLS layer was established have been omitted:

       ... TLS negotiation proceeds, further commands protected by TLS layer ...
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com, pleased to meet you
     S: 250-AUTH KERBEROS_V4 GSSAPI PLAIN
     C: AUTH PLAIN
      (note that there is a single space following the 334 on the following line)
     S: 334
     C: dGVzdAB0ZXN0ADEyMzQ=
     S: 235 Authentication successful

    Here is an example using a mechanism which does not support an
    initial client send, and includes server challenges:




Siemborski                 Expires April, 2004                  [Page 6]





SMTP Service Extension for Authentication                  October, 2003


     S: 220-smtp.example.com ESMTP Server
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com, pleased to meet you
     S: 250-AUTH KERBEROS_V4
     S: 250-STARTTLS
     C: AUTH KERBEROS_V4
     S: 334 asYNKQ==
        (the following lines are broken for editorial clarity only)
     C: BAYFQU5EUkVXLkNNVS5FRFUAOCCNPBDBLMcPZH4tgQ5hEI4XrsVr0xzDubFTzrKypQ
        oxLifPkfRootCpSkFvF+4K3DSnAaok9mQLYW+3MqyH5XmxyKraLHYKtyeRXX0u/X1V
        O1eJBGBZLmVE
     S: 334 /HEaZgpK4VA=
     C: 8YDkh5Lu3cB09Pso3dIRmg==
     S: 235 Authentication successful
        ... a security layer has been established, so the client re-issues
            the EHLO command ...
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com, pleased to meet you
     S: 250-AUTH KERBEROS_V4
     S: 250-STARTTLS


4.  The AUTH Parameter to the MAIL FROM command

    AUTH=addr-spec

    Arguments:
        An addr-spec (see section 3.4.1 or [RFC2822]) that names the
        identity which submitted the message to the delivery system, or
        the two character sequence "<>" indicating such an identity is
        unknown or insufficiently authenticated.  To comply with
        restrictions imposed on ESMTP parameters, the addr-spec is
        encoded inside an xtext.  The syntax of an xtext is described in
        Section 5 of [ESMTP-DSN].

    Note:
        For the purposes of this discussion, "authenticated identity"
        refers to the identity (if any) derived from a previous AUTH
        command, while the terms "authorized identity" and "supplied
        addr-spec" refer to the sender identity that is being associated
        with a particular message.  Note that one authenticated identity
        may be able to identify messages as being sent by any number of
        authorized identities within a single session.  For example,
        this may be the case when an SMTP server (one authenticated
        identity) is processing its queue (many messages with distinct
        authorized identities).

    Discussion:



Siemborski                 Expires April, 2004                  [Page 7]





SMTP Service Extension for Authentication                  October, 2003


        The optional AUTH parameter to the MAIL FROM command allows
        cooperating agents in a trusted environment to communicate the
        authorization identity associated with individual messages.

        If the server trusts the authenticated identity of the client to
        assert that the message was originally submitted by the supplied
        addr-spec, then the server SHOULD supply the same addr-spec in
        an AUTH parameter when relaying the message to any other server
        which supports the AUTH extension.

        For this reason, servers that advertise support for this
        extension MUST support the AUTH parameter to the MAIL FROM
        command even when the client has not authenticated itself to the
        server.

        A MAIL FROM parameter of AUTH=<> indicates that the original
        submitter of the message is not known.  The server MUST NOT
        treat the message as having been originally submitted by the
        current authenticated identity.

        If the AUTH parameter to the MAIL FROM command is not supplied,
        the client has authenticated, and the server believes the
        message is an original submission, the server MAY supply the
        client's identity in the addr-spec in an AUTH parameter when
        relaying the message to any server which supports the AUTH
        extension.

        If the server does not sufficiently trust the authenticated
        identity of the client, or if the client is not authenticated,
        then the server MUST behave as if the AUTH=<> parameter was
        supplied.  The server MAY, however, write the value of any
        supplied AUTH parameter to a log file.

        If an AUTH=<> parameter was supplied, either explicitly or due
        to the requirement in the previous paragraph, then the server
        MUST supply the AUTH=<> parameter when relaying the message to
        any server which it has authenticated to using the AUTH
        extension.

        A server MAY treat expansion of a mailing list as a new
        submission, setting the AUTH parameter to the mailing list
        address or mailing list administration address when relaying the
        message to list subscribers.

        Note that an implementation which is hard-coded to treat all
        clients as being insufficiently trusted is compliant with this
        specification.  In that case, the implementation does nothing
        more than parse and discard syntactically valid AUTH parameters



Siemborski                 Expires April, 2004                  [Page 8]





SMTP Service Extension for Authentication                  October, 2003


        to the MAIL FROM command, and supply AUTH=<> parameters to any
        servers which it authenticates to.


4.1.    Example

    An example where the original identity of the sender is trusted and
    known:

     C: MAIL FROM:<e=mc2@example.com> AUTH=e+3Dmc2@example.com
     S: 250 OK

    One example where the identity of the sender is not trusted or is
    otherwise being suppressed by the client:

     C: MAIL FROM:<john+@example.org> AUTH=<>
     S: 250 OK


5.  Error Codes

    The following error codes may be used to indicate various failure
    conditions:

    432 A password transition is needed

    This response to the AUTH command indicates that the user needs to
    transition to the selected authentication mechanism.  This is
    typically done by authenticating once using the [PLAIN]
    authentication mechanism.  The selected mechanism SHOULD then work
    for authentications in subsequent sessions.

    534 Authentication mechanism is too weak

    This response to the AUTH command indicates that the selected
    authentication mechanism is weaker than server policy permits for
    that user.

    538 Encryption required for requested authentication mechanism

    This response to the AUTH command indicates that the selected
    authentication mechanism may only be used when the underlying SMTP
    connection is encrypted. Note that this response code is documented
    here for historical purposes only.  Modern implementations SHOULD
    NOT advertise mechanisms that are not permitted due to lack of
    encryption, unless an encryption layer of sufficient strength is
    currently being employed.




Siemborski                 Expires April, 2004                  [Page 9]





SMTP Service Extension for Authentication                  October, 2003


    454 Temporary authentication failure

    This response to the AUTH command indicates that the authentication
    failed due to a temporary server failure.

    530 Authentication required

    This response may be returned by any command other than AUTH, EHLO,
    HELO, NOOP, RSET, or QUIT.  It indicates that server policy requires
    authentication in order to perform the requested action.

6.  Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form notation as specified in [ABNF].

    Except as noted otherwise, all alphabetic characters are case-
    insensitive.  The use of upper or lower case characters to define
    token strings is for editorial clarity only.  Implementations MUST
    accept these strings in a case-insensitive fashion.


        UPALPHA         = %x41-5A            ;; Uppercase: A-Z

        LOALPHA         = %x61-7A            ;; Lowercase: a-z

        ALPHA           = UPALPHA / LOALPHA  ;; case insensitive

        DIGIT           = %x30-39            ;; Digits 0-9

        HEXDIGIT        = %x41-46 / DIGIT    ;; hexidecimal digit (uppercase)

        hexchar         = "+" HEXDIGIT HEXDIGIT

        xchar           = %x21-2A / %x2C-3C / %x3E-7E
                          ;; US-ASCII except for "+", "=", SPACE and CTL

        xtext           = *(xchar / hexchar)

        AUTH_CHAR       = ALPHA / DIGIT / "-" / "_"

        auth_type       = 1*20AUTH_CHAR

        auth_command    = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
                          *(CRLF [base64]) CRLF






Siemborski                 Expires April, 2004                 [Page 10]





SMTP Service Extension for Authentication                  October, 2003


        auth_param      = "AUTH=" xtext
                            ;; The decoded form of the xtext MUST be either
                            ;; an addr-spec or the two characters "<>"

        base64          = base64_terminal /
                          ( 1*(4base64_CHAR) [base64_terminal] )

        base64_char     = UPALPHA / LOALPHA / DIGIT / "+" / "/"
                          ;; Case-sensitive

        base64_terminal = (2base64_char "==") / (3base64_char "=")

        continue_req    = "334" SPACE [base64] CRLF

        CR              = %x0C           ;; ASCII CR, carriage return

        CRLF            = CR LF

        CTL             = %x00-1F / %x7F ;; any ASCII control character and DEL

        LF              = %x0A           ;; ASCII LF, line feed

        SPACE           = %x20           ;; ASCII SP, space


7.  Security Considerations

    Security issues are discussed throughout this memo.

    If a client uses this extension to get an encrypted tunnel through
    an insecure network to a cooperating server, it needs to be
    configured to never send mail to that server when the connection is
    not mutually authenticated and encrypted.  Otherwise, an attacker
    could steal the client's mail by hijacking the [SMTP] connection and
    either pretending the server does not support the Authentication
    extension or causing all AUTH commands to fail.

    Before the [SASL] negotiation has begun, any protocol interactions
    are performed in the clear and may be modified by an active
    attacker.  For this reason, clients and servers MUST discard any
    knowledge obtained prior to the start of the SASL negotiation upon
    the establishment of a security layer.

    This mechanism does not protect the TCP port, so an active attacker
    may redirect a relay connection attempt (i.e. a connection between
    two MTAs) to the submission port [SUBMIT].  The AUTH=<> parameter
    prevents such an attack from causing a relayed message, in the
    absence of other envelope authentication, from picking up the



Siemborski                 Expires April, 2004                 [Page 11]





SMTP Service Extension for Authentication                  October, 2003


    authentication of the relay client.

    A message submission client may require the user to authenticate
    whenever a suitable [SASL] mechanism is advertised.  Therefore, it
    may not be desirable for a submission server [SUBMIT] to advertise a
    SASL mechanism when use of that mechanism grants the clients no
    benefits over anonymous submission.

    This extension is not intended to replace or be used instead of end-
    to-end message signature and encryption systems such as [S/MIME] or
    [PGP].  This extension addresses a different problem than end-to-end
    systems; it has the following key differences:


     1.   It is generally useful only within a trusted enclave.

     2.   It protects the entire envelope of a message, not just the
          message's body.

     3.   It authenticates the message submission, not authorship of the
          message content.

     4.   When mutual authentication is used along with a security
          layer, it can give the sender some assurance that the message
          was successfully delivered to the next hop.

Additional security considerations are mentioned in the [SASL]
specification.


8.  IANA Considerations

    This document requests that the IANA update the entry for the "smtp"
    SASL protocol name to point at this document.

    This document requests that the IANA register the Authentication
    SMTP service extension as defined in Section 2 of this document.

9.  Intellectual Property Rights

    The IETF takes no position regarding the validity or scope of any
    intellectual property or other rights that might be claimed to
    pertain to the implementation or use of the technology described in
    this document or the extent to which any license under such rights
    might or might not be available; neither does it represent that it
    has made any effort to identify any such rights.  Information on the
    IETF's procedures with respect to rights in standards-track and
    standards-related documentation can be found in BCP-11.  Copies of



Siemborski                 Expires April, 2004                 [Page 12]





SMTP Service Extension for Authentication                  October, 2003


    claims of rights made available for publication and any assurances
    of licenses to be made available, or the result of an attempt made
    to obtain a general license or permission for the use of such
    proprietary rights by implementors or users of this specification
    can be obtained from the IETF Secretariat.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights which may cover technology that may be required to practice
    this standard.  Please address the information to the IETF Executive
    Director.

10.  Copyright

    Copyright (C) The Internet Society (2003). 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 must be
    followed, or as required to translate it into languages other than
    English.

    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.















Siemborski                 Expires April, 2004                 [Page 13]





SMTP Service Extension for Authentication                  October, 2003


11.  References

     The following documents contain normative definitions or
specifications that are necessary for correct understanding of this
protocol:

[ABNF]      Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 2234, November 1997.

[BASE64]    Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 3548, July 2003.

[ESMTP-DSN] Moore, K., "SMTP Service Extension for Delivery Status
            Notifications", RFC 1891, January 1996.

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

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

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

[SASL]      Myers, J., "Simple Authentication and Security Layer
            (SASL)", RFC 2222, October 1997.

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

[SMTP-TLS]  Hoffman, P. "SMTP Service Extension for Secure SMTP over
            Transport Layer Security", RFC 3207, February 2002.

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

The following references are for informational purposes only:

[PGP]       Elkins, M., "MIME Security with Pretty Good Privacy (PGP)",
            RFC 2015, October 1996.

[S/MIME]    Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
            2633, June 1999.

[TLS]       Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
            2246, January 1999.






Siemborski                 Expires April, 2004                 [Page 14]





SMTP Service Extension for Authentication                  October, 2003


12.  Changes Since RFC 2554

            1.   Clarify that servers MUST support the use of the
                 AUTH=addr-spec parameter to MAIL FROM, even when the
                 client is not authenticated.

            2.   Clarify the initial-client-send requirements, and give
                 additional examples.

            3.   Update references to newer versions of various
                 specifications.

            4.   Require the minimum implementation of TLS+PLAIN.

            5.   Clarify that the mechanism list can change.

            6.   Deprecate the use of the 538 response code.

            7.   General other editorial clarifications.

13.  Author's Address:

    Robert Siemborski
    Carnegie Mellon, Andrew Systems Group
    Cyert Hall 207
    5000 Forbes Avenue
    Pittsburgh, PA  15213
    +1 412 268 7456
    rjs3+@andrew.cmu.edu

14.  Acknowledgments:

    The author would like to acknowledge the contributions of John Myers
    and other contributors to RFC 2554, on which this document draws
    from heavily.

    The author would also like to thank Ken Murchison and Mark Crispin
    for the time they devoted to reviewing early drafts of this
    document.












Siemborski                 Expires April, 2004                 [Page 15]