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Applicability Statement for IETF Core Email Protocols
draft-ietf-emailcore-as-03

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Dr. John C. Klensin , Kenneth Murchison , Ekow Sam
Last updated 2021-08-06 (Latest revision 2021-07-11)
Replaces draft-klensin-email-core-as
RFC stream Internet Engineering Task Force (IETF)
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Nov 2024
Submit Applicability Statment to the IESG for publication at Proposed Standard
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draft-ietf-emailcore-as-03
EMAILCORE                                              J.C. Klensin, Ed.
Internet-Draft                                                          
Intended status: Standards Track                       K. Murchison, Ed.
Expires: 7 February 2022                                        Fastmail
                                                             E. Sam, Ed.
                                                           6 August 2021

         Applicability Statement for IETF Core Email Protocols
                       draft-ietf-emailcore-as-03

Abstract

   Electronic mail is one of the oldest Internet applications that is
   still in very active use.  While the basic protocols and formats for
   mail transport and message formats have evolved slowly over the
   years, events and thinking in more recent years have supplemented
   those core protocols with additional features and suggestions for
   their use.  This Applicability Statement describes the relationship
   among many of those protocols and provides guidance and makes
   recommendations for the use of features of the core protocols.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on 7 February 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Applicability of Some SMTP Provisions . . . . . . . . . . . .   3
     2.1.  Handling of the Domain Argument to the EHLO Command . . .   3
     2.2.  Use of Address Literals . . . . . . . . . . . . . . . . .   4
     2.3.  Use of Addresses in Top-Level Domains . . . . . . . . . .   4
   3.  Applicability of Message Format Provisions  . . . . . . . . .   4
     3.1.  Use of Empty Quoted Strings . . . . . . . . . . . . . . .   4
   4.  MIME and Its Implications . . . . . . . . . . . . . . . . . .   5
   5.  Other Stuff . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   7
     A.1.  Changes from draft-klensin-email-core-as-00 (2020-03-30) to
           draft-ietf-emailcore-as-00  . . . . . . . . . . . . . . .   7
     A.2.  Changes from draft-ietf-emailcore-as-00 (2020-10-06) to
           -01 . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     A.3.  Changes from draft-ietf-emailcore-as-01 (2021-04-09) to
           -02 . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     A.4.  Changes from draft-ietf-emailcore-as-02 (2021-08-06) to
           -03 . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In its current form, this draft is a placeholder and beginning of an
   outline for the Applicability Statement that has been discussed as a
   complement for proposed revisions of the base protocol specifications
   for SMTP [RFC5321] (being revised as [I-D.ietf-emailcore-rfc5321bis])
   and Internet Message Format [RFC5322] (being revised as
   [I-D.ietf-emailcore-rfc5322bis]).  Among other things, it is expected
   to capture topics that a potential WG concludes are important but
   that should not become part of those core documents.

   As discussed in [RFC2026],

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      "An Applicability Statement specifies how, and under what
      circumstances, one or more TSs may be applied to support a
      particular Internet capability."

   That form of a standards track document is appropriate because one of
   the roles of such a document is to explain the relationship among
   technical specifications, describe how they are used together, and
   make statements about what is "required, recommended, or elective".

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119] and
   [RFC8174].

2.  Applicability of Some SMTP Provisions

   Over the years since RFC 5321 was published in October 2008, usage of
   SMTP has evolved, machines and network speeds have increased, and the
   frequency with which SMTP senders and receivers have to be prepared
   to deal with systems that are disconnected from the Internet for long
   periods or that require many hops to reach has decreased.  During the
   same period, the IETF has become much more sensitive to privacy and
   security issues and the need to be more resistant or robust against
   spam and other attacks.  In addition SMTP (and Message Format)
   extensions have been introduced that are expected to evolve the
   Internet's mail system to better accommodate environments in which
   Basic Latin Script is not the norm.

   This section describes adjustments that may be appropriate for SMTP
   under various circumstances and discusses the applicability of other
   protocols that represent newer work or that are intended to deal with
   relatively newer issues.

2.1.  Handling of the Domain Argument to the EHLO Command

   If the "Domain" argument to the EHLO command does not have an address
   record in the DNS that matches the IP address of the client, the SMTP
   server may refuse any mail from the client as part of established
   anti-abuse practice.  Operational experience has demonstrated that
   the lack of a matching address record for the the domain name
   argument is at best an indication of a poorly-configured MTA, and at
   worst that of an abusive host.

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2.2.  Use of Address Literals

   The "address-literal" ABNF non-terminal is used in various places in
   [I-D.ietf-emailcore-rfc5321bis] grammar however, for SMTP connections
   over the public internet, an "address-literal" as the argument to
   EHLO command or the "Domain" part of the "Mailbox" argument to the
   MAIL FROM command is quite likely to result in the message being
   rejected as a matter of policy at many sites, since they are deemed
   to be signs of at best a misconfigured server, and at worst either a
   compromised host or a server that's intentionally configured to hide
   its identity.

2.3.  Use of Addresses in Top-Level Domains

   While addresses in top-level domains (TLDs) are syntactically valid,
   mail to these addresses has never worked reliably.  A handful of
   country code TLDs have top level MX records but they have never been
   widely used nor well supported.  In 2013 [RFC7085] found 18 TLDs with
   MX records, which dropped to 17 in 2021 despite many new TLDs having
   been added.

   Mail sent to addresses with single label domains has typically
   expected the address to be an abbreviation to be completed by a
   search list, so mail to bob@sales would be completed to
   bob@sales.example.com.  This shortcut has led to unfortunate
   consequnces; in one famous case, in 1991 when the .CS domain was
   added to the root, mail in computer science departments started to
   fail as mail to bob@cs was now treated as mail to Czechoslovakia.
   Hence, for reliable service, mail SHOULD NOT use addreses that
   contain single label domains.

3.  Applicability of Message Format Provisions

   This section describes adjustments to the Internet Message Format
   that may be appropriate under various circumstances.

3.1.  Use of Empty Quoted Strings

   The "quoted-string" ABNF non-terminal is used in various places in
   rfc5322bis grammar.  While it allows for empty quoted string, such
   construct is going to cause interoperability issues when used in
   certain header fields.  In particular, use of empty quoted strings is
   NOT RECOMMENDED in "received-token" (a component of a Received header
   field), "keywords" (a component of a Keywords header field) and
   "local-part" (left hand side of email addresses).  Use of empty
   quoted strings is in particular problematic in the "local-part".  For
   example, all of the following email addresses are non interoperable:

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   "".bar@example.com

   foo.""@example.net

   ""@example.com

   Use of empty quoted strings is fine in "display-name".

4.  MIME and Its Implications

   When the work leading to the original version of the MIME
   specification was completed in 1992 [RFC1341], the intention was that
   it be kept separate from the specification for basic mail headers in
   RFC 822 [RFC0822].  That plan was carried forward into RFC 822's
   successors, [RFC2822] and [RFC5322] and the successors of that
   original MIME specification including [RFC2045].  The decision to do
   so was different from the one made for SMTP, for which the core
   specification was changed to allow for the extension mechanism
   [RFC1425] which was then incorporated into RFC 5321 and its
   predecessor [RFC2821].

   Various uses of MIME have become nearly ubiquitous in contemporary
   email while others may have fallen into disuse or been repurposed
   from the intent of their original design.

   It may be appropriate to make some clear statements about the
   applicability of MIME and its features.

5.  Other Stuff

   It is fairly clear that there will be things that do not fit into the
   sections outlined above.  As one example, if the IETF wants to say
   something specific about signatures over headers or what (non-trace)
   headers may reasonably be altered in transit, that may be more
   appropriate to other sections than to any of the three suggested
   above.

6.  Acknowledgments

   The Emailcore group arose out of discussions on the ietf-smtp group
   over changes and additions that should be made to the core email
   protocols.  It was agreed upon that it was time to create a working
   group that would fix many potential errors and opportunities for
   misunderstandings within the RFCs.

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7.  IANA Considerations

   This memo includes no requests to or actions for IANA.  The IANA
   registries associated with the protocol specifications it references
   are specified in their respective documents.

8.  Security Considerations

   All drafts are required to have a security considerations section and
   this one eventually will.

   ... To be supplied ...

9.  References

9.1.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
              <https://www.rfc-editor.org/info/rfc2026>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [I-D.ietf-emailcore-rfc5321bis]
              Klensin, J. C., "Simple Mail Transfer Protocol", Work in
              Progress, Internet-Draft, draft-ietf-emailcore-rfc5321bis-
              03, 10 July 2021, <https://www.ietf.org/archive/id/draft-
              ietf-emailcore-rfc5321bis-03.txt>.

   [I-D.ietf-emailcore-rfc5322bis]
              Resnick, P. W., "Internet Message Format", Work in
              Progress, Internet-Draft, draft-ietf-emailcore-rfc5322bis-
              01, 29 March 2021, <https://www.ietf.org/archive/id/draft-
              ietf-emailcore-rfc5322bis-01.txt>.

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   [RFC0822]  Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET
              TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822,
              August 1982, <https://www.rfc-editor.org/info/rfc822>.

   [RFC1341]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
              Mail Extensions): Mechanisms for Specifying and Describing
              the Format of Internet Message Bodies", RFC 1341,
              DOI 10.17487/RFC1341, June 1992,
              <https://www.rfc-editor.org/info/rfc1341>.

   [RFC1425]  Klensin, J., Freed, N., Ed., Rose, M., Stefferud, E., and
              D. Crocker, "SMTP Service Extensions", February 1993,
              <https://www.rfc-editor.org/info/rfc1425>.

   [RFC2821]  Klensin, J., Ed., "Simple Mail Transfer Protocol",
              RFC 2821, DOI 10.17487/RFC2821, April 2001,
              <https://www.rfc-editor.org/info/rfc2821>.

   [RFC2822]  Resnick, P., Ed., "Internet Message Format", RFC 2822,
              DOI 10.17487/RFC2822, April 2001,
              <https://www.rfc-editor.org/info/rfc2822>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC7085]  Levine, J. and P. Hoffman, "Top-Level Domains That Are
              Already Dotless", RFC 7085, DOI 10.17487/RFC7085, December
              2013, <https://www.rfc-editor.org/info/rfc7085>.

Appendix A.  Change Log

   RFC Editor: Please remove this appendix before publication.

A.1.  Changes from draft-klensin-email-core-as-00 (2020-03-30) to draft-
      ietf-emailcore-as-00

   *  Change of filename, metadata, and date to reflect transition to WG
      document for new emailcore WG.  No other substantive changes

A.2.  Changes from draft-ietf-emailcore-as-00 (2020-10-06) to -01

   *  Added co-authors (list is in alphabetical order for the present).

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   *  Updated references to 5321bis and 5322bis.

   *  Added note at top, "This version is provided as a document
      management convenience to update the author list and make an un-
      expired version available to the WG.  There are no substantive
      changes from the prior version", which should be removed for
      version -02.

A.3.  Changes from draft-ietf-emailcore-as-01 (2021-04-09) to -02

   *  Added new editors and also added some issues the emailcore group
      will be dealing with.

   *  Added reference to RFC 6648.

A.4.  Changes from draft-ietf-emailcore-as-02 (2021-08-06) to -03

   *  Moved discussion of address-literals (issue #1) and domain names
      in EHLO (issue #19) under SMTP Provisions section

   *  Moved discussion of empty quoted-strings under Message Format
      Provisions section

   *  Added text on use of addresses in TLDs (issue #50)

   *  Marked all authors as editors.

   *  Miscellaneous editorial changes.

Authors' Addresses

   John C Klensin (editor)
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA 02140
   United States of America

   Phone: +1 617 245 1457
   Email: john-ietf@jck.com

   Kenneth Murchison (editor)
   Fastmail US LLC
   1429 Walnut Street - Suite 1201
   Philadelphia, PA 19102
   United States of America

   Email: murch@fastmailteam.com

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   E Sam (editor)

   Email: winshell64@gmail.com

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