Network Working Group                                          C. Newman
Request for Comments: 3848                              Sun Microsystems
Category: Standards Track                                      July 2004


           ESMTP and LMTP Transmission Types Registration

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2004).

Abstract

  This registers seven new mail transmission types (ESMTPA, ESMTPS,
  ESMTPSA, LMTP, LMTPA, LMTPS, LMTPSA) for use in the "with" clause of
  a Received header in an Internet message.

1.  IANA Considerations

  As directed by SMTP [2], IANA maintains a registry [7] of "WITH
  protocol types" for use in the "with" clause of the Received header
  in an Internet message.  This registry presently includes SMTP [6],
  and ESMTP [2].  This specification updates the registry as follows:

  o  The new keyword "ESMTPA" indicates the use of ESMTP when the SMTP
     AUTH [3] extension is also used and authentication is successfully
     achieved.

  o  The new keyword "ESMTPS" indicates the use of ESMTP when STARTTLS
     [1] is also successfully negotiated to provide a strong transport
     encryption layer.

  o  The new keyword "ESMTPSA" indicates the use of ESMTP when both
     STARTTLS and SMTP AUTH are successfully negotiated (the
     combination of ESMTPS and ESMTPA).

  o  The new keyword "LMTP" indicates the use of LMTP [4].






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  o  The new keyword "LMTPA" indicates the use of LMTP when the SMTP
     AUTH extension is also used and authentication is successfully
     achieved.

  o  The new keyword "LMTPS" indicates the use of LMTP when STARTTLS is
     also successfully negotiated to provide a strong transport
     encryption layer.

  o  The new keyword "LMTPSA" indicates the use of LMTP when both
     STARTTLS and SMTP AUTH are successfully negotiated (the
     combination of LSMTPS and LSMTPA).

  o  The references for the ESMTP and SMTP entries in the registry
     should be updated to the latest specification [2] since both RFC
     821 and RFC 1869 [5] are obsoleted by RFC 2821.

2.  Implementation Experience

  The ESMTPA, ESMTPS and ESMTPSA keywords have been implemented in
  deployed email server software for several years and no problems have
  been reported with their use.

3.  Security Considerations

  Use of these additional keywords provides trace information to
  indicate when various high-level security framing protocols are used
  for hop-to-hop transport of email without exposing details of the
  specifics of the security mechanism.  This trace information provides
  an informal way to track the deployment of these mechanisms on the
  Internet and can assist after-the-fact diagnosis of email abuse.

  These keywords are not normally protected in transport which means
  they can be modified by an active attacker.  They also do not
  indicate the specifics of the mechanism used, and therefore do not
  provide any real-world security assurance.  They should not be used
  for mail filtering or relaying decisions except in very controlled
  environments.  As they are both cryptic and hidden in trace headers
  used primarily to diagnose email problems, it is not expected they
  will mislead end users with a false sense of security.  Information
  with a higher degree of reliability can be obtained by correlating
  the Received headers with the logs of the various Mail Transfer
  Agents through which the message passed.

  The trace information provided by these keywords and other parts of
  the Received header provide a significant benefit when doing after-
  the-fact diagnosis of email abuse or problems.  Unfortunately, some
  people in a misguided attempt to hide information about their
  internal servers will strip Received headers of useful information



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  and reduce their ability to correct security abuses after they
  happen.  The result of such misguided efforts is usually a reduction
  of the overall security of the systems.

4.  References

4.1.  Normative References

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

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

  [3]  Myers, J., "SMTP Service Extension for Authentication", RFC
       2554, March 1999.

  [4]  Myers, J., "Local Mail Transfer Protocol", RFC 2033, October
       1996.

4.2.  Informative References

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

  [6]  Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
       August 1982.

4.3.  URIs

  [7]  <http://www.iana.org/assignments/mail-parameters>

Author's Address

  Chris Newman
  Sun Microsystems
  1050 Lakes Drive
  West Covina, CA  91790
  US

  EMail: [email protected]










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RFC 3848     ESMTP and LMTP Transmission Types Registration    July 2004


Full Copyright Statement

  Copyright (C) The Internet Society (2004).  This document is subject
  to the rights, licenses and restrictions contained in BCP 78, and
  except as set forth therein, the authors retain all their rights.

  This document and the information contained herein are provided on an
  "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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Acknowledgement

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









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