Bump versions.

[gsasl.git] / doc / specification / draft-siemborski-rfc2554bis-03.txt

Network Working Group                                  Robert Siemborski
INTERNET-DRAFT                                Carnegie Mellon University
Intended Category: Proposed Standard                         April, 2004


               SMTP Service Extension for Authentication
                  <draft-siemborski-rfc2554bis-03.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.









Siemborski                Expires October, 2004                 [Page 1]





SMTP Service Extension for Authentication                    April, 2004


Table of Contents


1. How to Read This Document . . . . . . . . . . . . . . . . . . . .   3
2. The Authentication Service Extension  . . . . . . . . . . . . . .   3
3. The AUTH Command  . . . . . . . . . . . . . . . . . . . . . . . .   3
3.1. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
4. The AUTH Parameter to the MAIL FROM command . . . . . . . . . . .   8
4.1. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
5. Status Codes  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  11
7. Security Considerations . . . . . . . . . . . . . . . . . . . . .  12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .  14
9. Protocol Actions  . . . . . . . . . . . . . . . . . . . . . . . .  14
10. Intellectual Property Rights . . . . . . . . . . . . . . . . . .  14
11. Copyright  . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
13. Changes Since RFC 2554 . . . . . . . . . . . . . . . . . . . . .  17
14. Author's Address . . . . . . . . . . . . . . . . . . . . . . . .  17
15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  18































Siemborski                Expires October, 2004                 [Page 2]





SMTP Service Extension for Authentication                    April, 2004


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 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 encoded as described in Section
          3 of [BASE64].

      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.



Siemborski                Expires October, 2004                 [Page 3]





SMTP Service Extension for Authentication                    April, 2004


          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
          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 invalid (e.g. is not
          supported or requires an encrpytion layer), the server rejects
          the AUTH command with a 504 reply, and if it supports the
          [ESMTP-CODES] extension it SHOULD return a 5.5.4 enhanced
          response code.

          The SASL authentication 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 complete response line is "334 ".

          Note that the AUTH command is still subject to the line length
          limitations defined in [SMTP]. If use of the initial response
          argument would cause the AUTH command to exceed this length,
          the client MUST NOT use the initial response parameter (and
          instead proceed as defined in section 5.1 of [SASL]).

          If the client is transmitting an initial response of zero



Siemborski                Expires October, 2004                 [Page 4]





SMTP Service Extension for Authentication                    April, 2004


          length, it MUST instead 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
          501 reply.  Servers using the enhanced status codes extension
          [ESMTP-CODES] SHOULD return an enhanced status code of 5.7.0
          in this case.

          If the server cannot [BASE64] decode any client response, it
          MUST reject the AUTH command with a 501 reply (and an enhanced
          status code of 5.5.2).  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) 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 can be much longer than
          normal SMTP commands.  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.

          The authorization identity generated by this [SASL] exchange
          is a simple username, and both client and server MUST use the
          [SASLprep] profile of the [StringPrep] algorithm to prepare
          these names for transmission or comparison.  If preparation of
          the authorization identity fails or results in an empty string
          (unless it was transmitted as the empty string), the server
          MUST fail the authentication.

          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 is appropriate.  Should the client
          successfully complete the exchange, the SMTP server issues a
          235 reply.  These status codes, along with others defined by
          this extension, are discussed in Section 5 of this document.

          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



Siemborski                Expires October, 2004                 [Page 5]





SMTP Service Extension for Authentication                    April, 2004


          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".  This service name is also to be used for the
          [SUBMIT] protocol.

          If an AUTH command fails, the client MAY proceed without
          authentication, Alternatively, the client MAY try another
          authentication mechanism or present different credentials by
          issuing another AUTH command.

          To ensure interoperability, client and server implementations
          of this extension MUST implement the [DIGEST-MD5] SASL
          mechanism.


3.1.    Examples

    Here is an example of a client attempting AUTH using the [PLAIN]
    SASL mechanism under a TLS layer, and making use of the initial
    client response:









Siemborski                Expires October, 2004                 [Page 6]





SMTP Service Extension for Authentication                    April, 2004


     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-ENHANCEDSTATUSCODES
     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 2.7.0 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: there is a single space following the 334 on the following line)
     S: 334
     C: dGVzdAB0ZXN0ADEyMzQ=
     S: 235 2.7.0 Authentication successful

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

     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 CRAM-MD5
     S: 250-ENHANCEDSTATUSCODES
     S: 250 STARTTLS
     C: AUTH CRAM-MD5
     S: 334 PDQxOTI5NDIzNDEuMTI4Mjg0NzJAc291cmNlZm91ci5hbmRyZXcuY211LmVkdT4=
     C: cmpzMyBlYzNhNTlmZWQzOTVhYmExZWM2MzY3YzRmNGI0MWFjMA==
     S: 235 2.7.0 Authentication successful

    Here is an example of a client attempting AUTH EXTERNAL under TLS,
    using the derived authorization ID (and thus a zero-length initial
    client send).





Siemborski                Expires October, 2004                 [Page 7]





SMTP Service Extension for Authentication                    April, 2004


     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-ENHANCEDSTATUSCODES
     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 EXTERNAL GSSAPI PLAIN
     C: AUTH EXTERNAL =
     S: 235 2.7.0 Authentication successful


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 the authorization
        identity of 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:
        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



Siemborski                Expires October, 2004                 [Page 8]





SMTP Service Extension for Authentication                    April, 2004


        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
        authenticated identity which resulted from the AUTH command.

        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 generate an
        addr-spec from the user's authenticated identity for use in an
        AUTH parameter when relaying the message to any server which
        supports the AUTH extension.  The generated addr-spec is
        implementation specific, but it MUST conform to the syntax of
        [RFC2822].  If the implementation cannot generate a valid addr-
        spec, it MUST transmit AUTH=<> when relaying this message.

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




Siemborski                Expires October, 2004                 [Page 9]





SMTP Service Extension for Authentication                    April, 2004


4.1.    Examples

    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.  Status Codes

    The following error codes may be used to indicate various success or
    failure conditions.  Servers that return enhanced status codes
    [ESMTP-CODES] SHOULD use the enhanced codes suggested here.

    235 2.7.0 Authentication Succeeded

    This response to the AUTH command indicates that the authentication
    was successful.

    432 4.7.12 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.

    454 4.7.0 Temporary authentication failure

    This response to the AUTH command indicates that the authentication
    failed due to a temporary server failure.  The client SHOULD NOT
    prompt the user for another password in this case, and instead
    notify the user of server failure.

    534 5.7.9 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.  The client SHOULD retry with a new authentication
    mechanism.




Siemborski                Expires October, 2004                [Page 10]





SMTP Service Extension for Authentication                    April, 2004


    535 5.7.8 Authentication credentials invalid

    This response to the AUTH command indicates that the authentication
    failed due to invalid or insufficient authentication credentials.
    In this case, the client SHOULD ask the user to supply new
    credentials (such as by presenting a password dialog box).

    530 5.7.0 Authentication required

    This response SHOULD be returned by any command other than AUTH,
    EHLO, HELO, NOOP, RSET, or QUIT when server policy requires
    authentication in order to perform the requested action and
    authentication is not currently in force.

    538 5.7.11 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.

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




Siemborski                Expires October, 2004                [Page 11]





SMTP Service Extension for Authentication                    April, 2004


        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

        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.



Siemborski                Expires October, 2004                [Page 12]





SMTP Service Extension for Authentication                    April, 2004


    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
    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.

    Servers MAY implement a policy whereby the connection is dropped
    after a number of failed authentication attempts.  If they do so,
    they SHOULD NOT drop the connection until atleast 3 attempts to
    authenticate have failed.

    Implementations MUST support a configuration where SASL mechanisms
    that are vulnerable to passive eavesdropping attacks (such as
    [PLAIN]) are not advertised or used without the presence of an
    external security layer such as [TLS].

    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.



Siemborski                Expires October, 2004                [Page 13]





SMTP Service Extension for Authentication                    April, 2004


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.  Protocol Actions

    [RFC Editor: Remove this section prior to publication]

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

10.  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
    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.

11.  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



Siemborski                Expires October, 2004                [Page 14]





SMTP Service Extension for Authentication                    April, 2004


    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 October, 2004                [Page 15]





SMTP Service Extension for Authentication                    April, 2004


12.  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.

[DIGEST-MD5]
            Leach, P., Melnikov, A., and Newman C., "Using Digest
            Authentication as a SASL Mechanism", draft-ietf-sasl-
            rfc2831bis-*.txt, a work in progress.

[ESMTP-CODES]
            Freed, N., "SMTP Service Extension for Returning Enhanced
            Error Codes", RFC 2034, October 1996.

[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

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

[SASL]      Melnikov, A., "Simple Authentication and Security Layer
            (SASL)", draft-ietf-sasl-rfc2222bis-*.txt, a work in
            progress.

[SASLprep]  Zeilega, K., "SASLprep: Stringprep profile for user names
            and passwords", draft-ietf-sasl-saslprep-*.txt, a work in
            progress

[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.

[StringPrep]
            Hoffman, P., Blanchet, M., "Preparation of Internationalized
            Strings ("stringprep")", draft-hoffman-rfc3454bis-*.txt, a
            work in progress




Siemborski                Expires October, 2004                [Page 16]





SMTP Service Extension for Authentication                    April, 2004


[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.

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

[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.

13.  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 DIGEST-MD5.

            5.   Clarify that the mechanism list can change.

            6.   Deprecate the use of the 538 response code.

            7.   Add the use of the SASLprep profile for preparing
                 authorization identities.

            8.   Substantial cleanup of response codes and indicate
                 suggested enhanced response codes.  Also indicate what
                 response codes should result in a client prompting the
                 user for new credentials.

            9.   General other editorial clarifications.







Siemborski                Expires October, 2004                [Page 17]





SMTP Service Extension for Authentication                    April, 2004


14.  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

15.  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, Mark Crispin, and
    Chris Newman for the time they devoted to reviewing early drafts of
    this document.
































Siemborski                Expires October, 2004                [Page 18]