Applicability Statement for IETF Core Email Protocols
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
Klensin, et al. Expires 7 February 2022 [Page 1]
Internet-Draft Core Email A/S August 2021
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],
Klensin, et al. Expires 7 February 2022 [Page 2]
Internet-Draft Core Email A/S August 2021
"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.
Klensin, et al. Expires 7 February 2022 [Page 3]
Internet-Draft Core Email A/S August 2021
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:
Klensin, et al. Expires 7 February 2022 [Page 4]
Internet-Draft Core Email A/S August 2021
"".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.
Klensin, et al. Expires 7 February 2022 [Page 5]
Internet-Draft Core Email A/S August 2021
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>.
Klensin, et al. Expires 7 February 2022 [Page 6]
Internet-Draft Core Email A/S August 2021
[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).
Klensin, et al. Expires 7 February 2022 [Page 7]
Internet-Draft Core Email A/S August 2021
* 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
Klensin, et al. Expires 7 February 2022 [Page 8]
Internet-Draft Core Email A/S August 2021
E Sam (editor)
Email: winshell64@gmail.com
Klensin, et al. Expires 7 February 2022 [Page 9]