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Network Working Group                                          C. NewmanRequest for Comments: 3848                              Sun MicrosystemsCategory: Standards Track                                      July 2004ESMTP and LMTP Transmission Types RegistrationStatus 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].Newman                      Standards Track                     [Page 1]

RFC 3848     ESMTP and LMTP Transmission Types Registration    July 2004   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 bothRFC821 andRFC 1869 [5] are obsoleted byRFC 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 informationNewman                      Standards Track                     [Page 2]

RFC 3848     ESMTP and LMTP Transmission Types Registration    July 2004   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.  References4.1.  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",RFC2554, 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: chris.newman@sun.comNewman                      Standards Track                     [Page 3]

RFC 3848     ESMTP and LMTP Transmission Types Registration    July 2004Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 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   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights 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; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Newman                      Standards Track                     [Page 4]