Internet DRAFT - draft-ietf-msgtrk-smtpext


Internet Draft                                               E. Allman
draft-ietf-msgtrk-smtpext-05.txt                        Sendmail, Inc.
Valid for six months                                         T. Hansen
Updates: RFC 1891                                    AT&T Laboratories
                                                        March 19, 2003

                        SMTP Service Extension
                         for Message Tracking


Status of This Memo

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Internet Draft     Message Tracking ESMTP Extension     March 19, 2003

     This document is a submission by the MSGTRK Working Group of the
Internet Engineering Task Force (IETF).  Comments should be submitted
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     Distribution of this memo is unlimited.

1.  Abstract

        This memo defines an extension to the SMTP service whereby a
   client may mark a message for future tracking.

2.  Other Documents and Conformance

        The model used for Message Tracking is described in [DRAFT-

        Doing a Message Tracking query is intended as a "last resort"
   mechanism.  Normally, Delivery Status Notifications (DSNs) [RFC-
   DSN-SMTP] and Message Disposition Notifications (MDNs) [RFC-MDN]
   would provide the primary delivery status.  Only if the message is
   not received, or there is no response from either of these
   mechanisms should a Message Tracking query be issued.

        The definition of the base64 token is imported from section
   6.8 of [RFC-MIME].  Formally,

       base64 =  %2b / %2f / %x30-39 / %x41-5a / %x61-7a

        The definition of the DIGIT token is imported from [RFC-
   MSGFMT].  Formally,

       DIGIT =        %x30-39

        Syntax notation in this document conforms to [RFC-ABNF].

        The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   in this document are to be interpreted as described in RFC 2119

3.  SMTP Extension Overview

        The Message Tracking SMTP service extension uses the SMTP
   service extension mechanism described in [RFC-ESMTP].  The
   following service extension is hereby defined:

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    (1)   The name of the SMTP service extension is "Message

    (2)   The EHLO keyword value associated with this extension is

    (3)   No parameters are allowed with this EHLO keyword value.
          Future documents may extend this specification by specifying
          parameters to this keyword value.

    (4)   One optional parameter using the keyword "MTRK" is added to
          the MAIL command.  In addition, the ENVID parameter of the
          MAIL command (as defined in RFC 1891 sections 5.4) MUST be
          supported, with extensions as described below.  The ORCPT
          parameter of the RCPT command (as defined in RFC 1891
          section 5.2) MUST also be supported.  All semantics
          associated with ENVID and ORCPT described in RFC 1891 MUST
          be supported as part of this extension.

    (5)   The maximum length of a MAIL command line is increased by 40
          characters by the possible addition of the MTRK keyword and
          value.  Note that the 507 character extension of RCPT
          commands for the ORCPT parameter and the 107 character
          extension of MAIL commands for the ENVID parameter as
          mandated by RFC 1891 [RFC-DSN-SMTP] must also be included.

    (6)   No SMTP verbs are defined by this extension.

4.  The Extended MAIL Command

        The extended MAIL command is issued by an SMTP client when it
   wishes to inform an SMTP server that message tracking information
   should be retained for future querying.  The extended MAIL command
   is identical to the MAIL command as defined in [RFC-SMTP], except
   that MTRK, ORCPT, and ENVID parameters appear after the address.

   4.1.  The MTRK parameter to the ESMTP MAIL command

           Any sender wishing to request the retention of data for
      further tracking of message must first tag that message as
      trackable by creating two values A and B:

          A = some-large-random-number
          B = SHA1(A)

      The large random number A is calculated on a host-dependent
      basis.  See [RFC-RANDOM] for a discussion of choosing good
      random numbers.  This random number MUST be at least 128 bits
      but MUST NOT be more than 1024 bits.

           The 128-bit hash B of A is then computed using the SHA-1
      algorithm as described in [NIST-SHA1].

           The sender then base64 encodes value B and passes that
      value as the mtrk-certifier on the MAIL command:

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          mtrk-parameter  = "MTRK=" mtrk-certifier [ ":" mtrk-timeout ]
          mtrk-certifier  = base64        ; authenticator
          mtrk-timeout    = 1*9DIGIT      ; seconds until timeout

           A is stored in the originator's tracking database to
      validate future tracking requests as described in [DRAFT-MTRK-
      MTQP].  B is stored in tracking databases of compliant receiver
      MTAs and used to authenticate future tracking requests.

           The mtrk-timeout field indicates the number of seconds that
      the client requests that this tracking information be retained
      on intermediate servers, as measured from the initial receipt of
      the message at that server.  Servers MAY ignore this value if it
      violates local policy.  In particular, servers MAY silently
      enforce an upper limit to how long they will retain tracking
      data; this limit MUST be at least one day.

           If no mtrk-timeout field is specified then the server
      should use a local default.  This default SHOULD be 8-10 days
      and MUST be at least one day.  Notwithstanding this clause, the
      information MUST NOT be expired while the message remains in the
      queue for this server: that is, an MTQP server MUST NOT deny
      knowledge of a message while that same message sits in the MTA

           If the message is relayed to another compliant SMTP server,
      the MTA acting as the client SHOULD pass an mtrk-timeout field
      equal to the remaining life of that message tracking
      information.  Specifically, the tracking timeout is decremented
      by the number of seconds the message has lingered at this MTA
      and then passed to the next MTA.  If the decremented tracking
      timeout is less than or equal to zero, the entire MTRK parameter
      MUST NOT be passed to the next MTA; essentially, the entire
      tracking path is considered to be lost at that point.

           See [RFC-DELIVERYBY] section 4 for an explanation of why a
      timeout is used instead of an absolute time.

   4.2.  Use of ENVID

           To function properly, Message Tracking requires that each
      message have a unique identifier that is never reused by any
      other message.  For that purpose, if the MTRK parameter is
      given, an ENVID parameter MUST be included, and the syntax of
      ENVID from RFC 1891 section 5.4 is extended as follows:

          envid-parameter = "ENVID=" unique-envid
          unique-envid    = local-envid "@" fqhn
          local-envid     = xtext
          fqhn            = xtext

      The unique-envid MUST be chosen in such a way that the same
      ENVID will never be used by any other message sent from this
      system or any other system.  In most cases, this means setting
      fqhn to be the fully qualified host name of the system

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      generating this ENVID, and local-envid to an identifier that is
      never re-used by that host.

           In some cases, the total length of (local-envid + fqhn + 1)
      (for the `@' sign) may exceed the total acceptable length of
      ENVID (100).  In this case, the fqhn SHOULD be replaced by the
      SHA1(fqhn) encoded into BASE64.  After encoding, the 160 bit
      SHA-1 will be a 27 octet string, which limits local-envid to 72
      octets.  Implementors are encouraged to use an algorithm for the
      local-envid that is reasonably unique.  For example, sequential
      integers have a high probability of intersecting with sequential
      integers generated by a different host, but a SHA-1 of the
      current time of day concatenated with the host's IP address and
      a random number are unlikely to intersect with the same
      algorithm generated by a different host.

           Any resubmissions of this message into the message
      transmission system MUST assign a new ENVID.  In this context,
      "resubmission" includes forwarding or resending a message from a
      user agent, but does not include MTA-level aliasing or
      forwarding where the message does not leave and re-enter the
      message transmission system.

   4.3.  Forwarding Tracking Certifiers

           MTAs SHOULD forward unexpired tracking certifiers to
      compliant mailers as the mail is transferred during regular hop-
      to-hop transfers.  If the "downstream" MTA is not MTRK-
      compliant, then the MTRK= parameter MUST be deleted.  If the
      downstream MTA is DSN-compliant, then the ENVID and ORCPT
      parameters MUST NOT be deleted.

           If aliasing, forwarding, or other redirection of a
      recipient occurs, and the result of the redirection is exactly
      one recipient, then the MTA SHOULD treat this as an ordinary
      hop-to-hop transfer and forward the MTRK=, ENVID=, and ORCPT=
      values; these values MUST NOT be modified except for
      decrementing the mtrk-timeout field of the MTRK= value, which
      MUST be modified as described in section 4.1 above.

           MTAs MUST NOT copy MTRK certifiers when a recipient is
      aliased, forwarded, or otherwise redirected and the redirection
      results in more than one recipient.  However, an MTA MAY
      designate one of the multiple recipients as the "primary"
      recipient to which tracking requests shall be forwarded; other
      addresses MUST NOT receive tracking certifiers.  MTAs MUST NOT
      forward MTRK certifiers when doing mailing list expansion.

5.  Security Considerations

   5.1.  Denial of service

           An attacker could attempt to flood the database of a server
      by submitting large numbers of small, tracked messages.  In this
      case, a site may elect to lower its maximum retention period

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   5.2.  Confidentiality

           The mtrk-authenticator value (``A'') must be hard to
      predict and not reused.

           The originating client must take reasonable precautions to
      protect the secret.  For example, if the secret is stored in a
      message store (e.g., a "Sent" folder), the client must make sure
      the secret isn't accessible by attackers, particularly on a
      shared store.

           Many site administrators believe that concealing names and
      topologies of internal systems and networks is an important
      security feature.  MTAs need to balance such desires with the
      need to provide adequate tracking information.

           In some cases site administrators may want to treat
      delivery to an alias as final delivery in order to separate
      roles from individuals.  For example, sites implementing
      ``postmaster'' or ``webmaster'' as aliases may not wish to
      expose the identity of those individuals by permitting tracking
      through those aliases.  In other cases, providing the tracking
      information for an alias is important, such as when the alias
      points to the user's preferred public address.

           Therefore, implementors are encouraged to provide
      mechanisms by which site administrators can choose between these

6.  IANA Considerations

        IANA is to register the SMTP extension defined in section 3.

7.  Acknowledgements

        Several individuals have commented on and enhanced this draft,
   including Philip Hazel, Alexey Melnikov, Lyndon Nerenberg, Chris
   Newman, and Gregory Neil Shapiro.

8.  Normative References

        T. Hansen, ``Message Tracking Model and Requirements.''
        draft-ietf-msgtrk-model-03.txt.  November 2000.

        T. Hansen, ``Message Tracking Query Protocol.''  draft-ietf-
        msgtrk-mtqp-01.txt.  November 2000.

        Crocker, D., Editor, and P. Overell, ``Augmented BNF for

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        Syntax Specifications: ABNF'', RFC 2234, November 1997.

        Rose, M., Stefferud, E., Crocker, D., Klensin, J. and N.
        Freed, ``SMTP Service Extensions.''  STD 10, RFC 1869.
        November 1995.

        S. Bradner, ``Key words for use in RFCs to Indicate
        Requirement Levels.''  RFC 2119.  March 1997.

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

        NIST FIPS PUB 180-1, ``Secure Hash Standard.''  National
        Institute of Standards and Technology, U.S. Department of
        Commerce.  May 1994.  DRAFT.

        J. Klensin, editor, ``Simple Mail Transfer Protocol.''  RFC
        2821.  April 2001.

9.  Informational References

        D. Newman, ``Deliver By SMTP Service Extension.''  RFC 2852.
        June 2000.

        K. Moore, ``SMTP Service Extension for Delivery Status
        Notifications.''  RFC 1891.  January 1996.

        R. Fajman, ``An Extensible Message Format for Message
        Disposition Notifications.''  RFC 2298.  March 1998.

        D. Eastlake, S. Crocker, and J. Schiller, ``Randomness
        Recommendations for Security.''  RFC 1750.  December 1994.

10.  Authors' Addresses

       Eric Allman
       Sendmail, Inc.
       6425 Christie Ave, 4th Floor
       Emeryville, CA  94608

       E-Mail: eric@Sendmail.COM
       Phone: +1 510 594 5501
       Fax: +1 510 594 5429

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       Tony Hansen
       AT&T Laboratories
       Middletown, NJ 07748

       Phone: +1 732 420 8934

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