Subject: [dmarc-ietf] draft-ietf-dmarc-psd review

From: "Murray S. Kucherawy" <[email-masked]>
Date: Mon, 22 Jul 2019 09:23:15 -0700

Reviewing as the document shepherd:

Abstract

[...]
   organization can use to improve mail handling.  DMARC policies can be
   applied at the individual domain level or for a set of domains at the
   organizational level.

I think the abstract is a bit too abstract.  Which set of domains?

   The design of DMARC precludes grouping
   policies for a set of domains above the organizational level, such as
   TLDs (Top Level Domains).

Is that the same set?

   These types of domains (which are not all
   at the top level of the DNS tree) can be collectively referred to as
   Public Suffix Domains (PSDs).

What "types" are there?

   For the subset of PSDs that require
   DMARC usage, this memo describes an extension to DMARC to enable
   DMARC functionality for such domains.

What's the requirement?

1.  Introduction

   DMARC [RFC7489] provides a mechanism for publishing organizational
   policy information to email receivers.  DMARC [RFC7489] allows policy
   to be specified for both individual domains and sets of domains

Which sets?

   within a single organization.  For domains above the organizational
   level in the DNS tree, policy can only be published for the exact
   domain.  There is no method available to such domains to express
   lower level policy or receive feedback reporting for sets of domains.

Still too abstract.  I don't know what sort of set groupings you might
be after here.

   This prevents policy application to non-existent domains and
   identification of domain abuse in email, which can be important for
   brand and consumer protection.

   As an example, imagine a country code TLD (ccTLD) which has public
   subdomains for government and commercial use (.gov.example and
   .com.example).  Within the .gov.example public suffix, use of DMARC
   [RFC7489] has been mandated and .gov.example has published its own
   DMARC [RFC7489] record:

   "v=DMARC1;p=reject;rua=mailto:[email-masked]"

Can we add spaces after the semicolons? I know this is legal format
but it would be more readable.

   This memo provides a simple extension to DMARC [RFC7489] to allow

s/memo/document/g

   operators of Public Suffix Domains (PSDs) to express policy for
   groups of subdomains, extends the DMARC [RFC7489] policy query

"groups of subdomains" suggests the capability of creating a policy
that applies to parts of the subtree only, or different policies for
different parts of the subtree.  If that's not what we're actually
defining here, this should be reworded.

   As an additional benefit, the PSD DMARC extension will clarify

s/will clarify/clarifies/

   existing requirements.  Based on the requirements of DMARC [RFC7489],
   DMARC should function above the organizational level for exact domain
   matches (i.e. if a DMARC record were published for 'example', then
   mail from example@example should be subject to DMARC processing).
   Testing had revealed that this is not consistently applied in
   different implementations.  PSD DMARC will help clarify that DMARC is

s/will help clarify/clarifies/

...and saying it twice in the same paragraph is probably not necessary.

   not limited to organizational domains and their sub-domains.

   There are two types of Public Suffix Operators (PSOs) for which this
   extension would be useful and appropriate:

   o  Branded PSDs (e.g., ".google"): These domains are effectively
      Organizational Domains as discussed in DMARC [RFC7489].  They
      control all subdomains of the tree.  These are effectively private
      domains, but listed in the Public Suffix List.  They are treated
      as Public for DMARC [RFC7489] purposes.  They require the same
      protections as DMARC [RFC7489] Organizational Domains, but are
      currently excluded.

How are they current excluded?  Is this because they're on the PSL, so
DMARC treats them differently?

   o  Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
      Because existing Organizational Domains using this PSD have their
      own DMARC policy, the applicability of this extension is for non-
      existent domains.  The extension allows the brand protection
      benefits of DMARC [RFC7489] to extend to the entire PSD, including
      cousin domains of registered organizations.

An example would be useful here.

   Due to the design of DMARC [RFC7489] and the nature of the Internet
   email architecture [RFC5598], there are interoperability issues
   associated with DMARC [RFC7489] deployment.  These are discussed in
   Interoperability Issues between DMARC and Indirect Email Flows
   [RFC7960].  These issues are not applicable to PSDs, since they
   (e.g., the ".gov.example" used above) do not send mail.

DMARC doesn't need to be followed by its RFC number every time.

2.2.  Public Suffix Domain (PSD)

   The global Internet Domain Name System (DNS) is documented in
   numerous Requests for Comment (RFC).  It defines a tree of names
   starting with root, ".", immediately below which are Top Level Domain
   names such as ".com" and ".us".  They are not available for private
   registration.  In many cases the public portion of the DNS tree is
   more than one level deep.  PSD DMARC includes all public domains
   above the organizational level in the tree, e.g., ".gov.uk".

I don't know what that last sentence means.  PSD DMARC is an
extension; how does it include domains?

2.4.  Public Suffix Operator (PSO)

   A Public Suffix Operator manages operations within their PSD.

s/their/its/; "operator" is singular.

2.6.  Non-existent Domains

   For DMARC [RFC7489] purposes, a non-existent domain is a domain name
   that publishes none of A, AAAA, or MX records that the receiver is
   willing to accept.  This is a broader definition than that in
   NXDOMAIN [RFC8020].

Is there any discussion that could be referenced about DNS RRs that
one is not willing to accept?

3.2.  Section 6.1 DMARC Policy Record

   PSD DMARC records are published as a subdomain of the PSD.  For the
   PSD ".example", the PSO would post DMARC policy in a TXT record at
   "_dmarc.example".

Don't you mean the PSD DMARC record is a record in the zone of the
PSD?  It's not a subdomain.

3.3.  Section 6.5.  Domain Owner Actions

   In addition to the DMARC [RFC7489] domain owner actions, PSOs that
   require use of DMARC ought to make that information available to
   receivers.
By what mechanism?

3.4.  Section 6.6.3.  Policy Discovery

   A new step between step 3 and 4 is added:

   3A.  If the set is now empty and the longest PSD (Section 2.3) of the
      Organizational Domain is one that the receiver has determined is
      acceptable for PSD DMARC, the Mail Receiver MUST query the DNS for
      a DMARC TXT record at the DNS domain matching the longest PSD
      (Section 2.3) in place of the RFC5322.From domain in the message
      (if different).  A possibly empty set of records is returned.

Section 6.6.3 of DMARC doesn't talk about "acceptable for DMARC", so I
don't know what "acceptable for PSD DMARC" might mean.

Section numbers in the prose of this section should make clear which
document they're referencing.

3.5.  Section 7.  DMARC Feedback

   Operational note for PSD DMARC: For PSOs, feedback for non-existent
   domains is desired and useful.  See Section 4 for discussion of
   Privacy Considerations.

Section 4 of which document?

4.  Privacy Considerations

   These privacy considerations are developed based on the requiremetns

typo

   of [RFC6973].  The Privacy Considerations of [RFC7489] apply to this
   document.

4.1.  Feedback leakage

   Providing feedback reporting to PSOs can, in some cases, create
   leakage of information outside of an organization to the PSO.  This
   leakage could be potentially be utilized as part of a program of

Remove one of the "be"s.

   pervasive surveillance (See [RFC7624]).  There are roughly three
   cases to consider:

   o  Single Organization PSDs (e.g., ".google"), RUA and RUF reports
      based on PSD DMARC have the potential to contain information about
      emails related to entities managed by the organization.  Since
      both the PSO and the Organizational Domain owners are common,
      there is no additional privacy risk for either normal or non-
      existent Domain reporting due to PSD DMARC.

   o  Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
      PSD DMARC based reports will only be generated for domains that do
      not publish a DMARC policy at the organizational or host level.
      For domains that do publish the required DMARC policy records, the
      feedback reporting addresses (RUA and RUF) of the organization (or
      hosts) will be used.  The only direct feedback leakage risk for
      these PSDs are for Organizational Domains that are out of
      compliance with PSD policy.  Data on non-existent cousin domains
      would be sent to the PSO.

This is the second use of "cousin domain".  An example here or at the
first use might be a good idea.

   o  Multi-organization PSDs (e.g., ".com") that do not mandate DMARC
      usage: Privacy risks for Organizational Domains that have not
      deployed DMARC within such PSDs are significant.  For non-DMARC
      Organizational Domains, all DMARC feedback will be directed to the
      PSO.  PSD DMARC is opt-out (by publishing a DMARC record at the
      Organizational Domain level) vice opt-in, which would be the more
      desirable characteristic.  This means that any non-DMARC
      organizational domain would have it's feedback reports redirected
      to the PSO.  The content of such reports, particularly for
      existing domains, is privacy sensitive.

   PSOs will receive feedback on non-existent domains, which may be
   similar to existing Organizational Domains.  Feedback related to such
   cousin domains have a small risk of carrying information related to
   an actual Organizational Domain.  To minimize this potential concern,
   PSD DMARC feedback is best limited to Aggregate Reports.  Feedback
   Reports carry more detailed information and present a greater risk.

   Due to the inherent Privacy and Security risks associated with PSD
   DMARC for Organizational Domains in multi-organization PSDs that do
   not particpate in DMARC, any Feedback Reporting related to multi-
   organizational PSDs ought to be limited to non-existent domains
   except in cases where the reporter knows that PSO requires use of
   DMARC.

5.  Security Considerations

   This document does not change the Security Considerations of
   [RFC7489] and [RFC7960].

   The risks of the issues identified in [RFC7489], Section 12.5,
   External Reporting Addresses, are amplified by PSD DMARC.  By design,
   PSD DMARC causes unrequested reporting of feedback to entities
   external to the Organizational Domain.  This is discussed in more
   detail in Section 4.

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6.  IANA Considerations

   This document does not require any IANA actions.

7.  References

7.1.  Normative References

   [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>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

   [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>.

7.2.  Informative References

   [psddmarc.org]
              multiple, "PSD DMARC Web Site", April 2019,
              <https://psddmarc.org/>.

   [PSL]      multiple, "Public Suffix List", April 2019,
              <https://publicsuffix.org/>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <https://www.rfc-editor.org/info/rfc5598>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

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   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.

   [RFC7960]  Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky,
              E., Ed., and K. Andersen, Ed., "Interoperability Issues
              between Domain-based Message Authentication, Reporting,
              and Conformance (DMARC) and Indirect Email Flows",
              RFC 7960, DOI 10.17487/RFC7960, September 2016,
              <https://www.rfc-editor.org/info/rfc7960>.

   [RFC8020]  Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
              Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
              November 2016, <https://www.rfc-editor.org/info/rfc8020>.

Appendix A.  The Experiment

   To mitigate the privacy concerns associated with Multi-organization
   PSDs that do not mandate DMARC usage, see Section 4.1, a mechanism to
   indicate which PSDs do not present this privacy risk is appropriate.
   There are multiple approaches that are possible.

   The experiment is to evaluate different possible approaches.  The
   experiment will be complete when there is rough consensus on a
   technical approach that is demonstrated to be operationally usable
   and effective at mitigating the privacy concern.

   The mechanism needs the following attributes:

   o  Be reliably, publicly accessible

   o  Be under configuration control based on a public set of criteria

   o  List PSDs that either mandate DMARC for their registrants or for
      which all lower level domains are controlled by the PSO and that
      the relevant PSO has indicated a desire for the PSD to participate
      in PSD DMARC

   o  Have a small operational footprint (e.g. provide a documented,
      lightweight mechanism for developers and operators to retrieve the
      list of PSD DMARC participants)

   o  Not allow PSO to add PSDs to the PSD DMARC participants list
      without third party review

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   As of this writing, three approaches have been proposed.  None of
   them are ideal:

   o  An extension to the Public Suffix List at [PSL]

   o  A dedicated registry queried via DNS - an example of such a
      service is described in Appendix B.1 below

   o  An IANA registry

Appendix B.  DMARC PSD Registry Examples

   To faciliate experimentation around data leakage mitigation, samples
   of the DNS based and IANA like registries are available at
   [psddmarc.org].

B.1.  DMARC PSD DNS Query Service

   A sample stand-alone DNS query service is available at
   [psddmarc.org].  It was developed based on the contents suggested for
   an IANA registry in an earlier revision of this draft.  Usage of the
   service is described on the web site.

B.2.  DMARC Public Suffix Domain (PSD) Registry

   [psddmarc.org] provides an IANA like DMARC Public Suffix Domain (PSD)
   Registry as a stand-alone DNS query service.  It follows the contents
   and structure described below.  There is a Comma Separated Value
   (CSV) version of the listed PSD domains which is suitable for use in
   build updates for PSD DMARC capable software.

   Names of PSDs participating in PSD DMARC must be registered this new
   registry.  New entries are assigned only for PSDs that require use of
   DMARC.  The requirement has to be documented in a manner that
   satisfies the terms of Expert Review,per [RFC5226].  The Designated
   Expert needs to confirm that provided documentation adequately
   describes PSD policy to require domain owners to use DMARC or that
   all domain owners are part of a single organization with the PSO.

   The initial set of entries in this registry is as follows:

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   +-------------+---------------+
   |    PSD      | Status        |
   +-------------+---------------+
   | .bank       | current       |
   +-------------+---------------+
   | .insurance  | current       |
   +-------------+---------------+
   | .gov.uk     | current       |
   +-------------+---------------+

Appendix C.  Implementation

   There is one known implementation of PSD DMARC available for testing.

C.1.  Authheaders Module

   The authheaders Python module and command line tool is available for
   download or installation from Pypi (Python Packaging Index).

   It supports both use of the DNS based query service and download of
   the CSV registry file from [psddmarc.org].

Acknowledgements

   Thanks to the following individuals for their contributions (both
   public and private) to improving this document.  Special shout out to
   Dave Crocker for naming the beast.

   Kurt Andersen, Seth Blank, Dave Crocker, Heather Diaz, Tim Draegen,
   Zeke Hendrickson, Andrew Kennedy, John Levine, Dr Ian Levy, Craig
   Schwartz, Alessandro Vesely, and Tim Wicinski

Author's Address

   Scott Kitterman
   fTLD Registry Services
   600 13th Street, NW, Suite 400
   Washington, DC  20005
   United States of America

   Phone: +1 301 325-5475
   Email: [email-masked]

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---

From: "Murray S. Kucherawy" <[email-masked]>
Date: Mon, 22 Jul 2019 09:29:43 -0700

Sorry, I hit "send" when I didn't mean to.  Finishing up:

>       would be sent to the PSO.
 This is the second use of "cousin domain". An example here or at the first
use might be a good idea.

> "vice" should be "versus"?

>    except in cases where the reporter knows that PSO requires use of
Feedback about non-existent domains is generated by virtue of the fact that
such mail never passes SPF or bears a valid signature, correct?  If so,
that should probably be stated explicitly somewhere.

>    This document does not change the Security Considerations of
s/and/or/

typo

>    registry.  New entries are assigned only for PSDs that require use of
Missing verb in the first sentence.

-MSK

---

From: "Murray S. Kucherawy" <[email-masked]>
Date: Mon, 22 Jul 2019 09:42:07 -0700

One (hopefully) last thing:

On Mon, Jul 22, 2019 at 9:23 AM Murray S. Kucherawy <[email-masked]>
wrote:

> Section 6.6.3 of DMARC doesn't talk about "acceptable for DMARC", so I don't know what "acceptable for PSD DMARC" might mean.
>
> Section numbers in the prose of this section should make clear which document they're referencing.
>
>
I think an example in an appendix that shows an evaluation without PSD and
then with PSD, and then some prose about what it enables (i.e., what the
benefit is), would be extremely helpful.

-MSK

---

From: Scott Kitterman <[email-masked]>
Date: Fri, 26 Jul 2019 13:14:30 -0400

> Reviewing as the document shepherd:

Thanks.  I've been working on document updates based on the last call 
discussion as I have time (xml is in the WG github repo).  I think some of 
these comments will be OBE to those changes, so my plan is to finish LC based 
updates, review your comments, make additional changes as needed, and then ask 
for further discussion/clarification if I'm unsure how to proceed.  I expect to 
have that done today.

Scott K

---

From: Scott Kitterman <[email-masked]>
Date: Sat, 27 Jul 2019 16:44:58 -0400

>    TLDs (Top Level Domains).

I updated the paragraph to start:

   DMARC (Domain-based Message Authentication, Reporting, and
   Conformance) is a scalable mechanism by which a mail-originating
   organization can express domain-level policies and preferences for
   message validation, disposition, and reporting, that a mail-receiving
   organization can use to improve mail handling.  DMARC policies can be
   applied to individual domains or to all domains within an
   organization.  The design of DMARC precludes grouping policies for
   domains based on policy published above the organizational level,
   such as TLDs (Top Level Domains).  

Does that clear those comments up?

>    Public Suffix Domains (PSDs).

Changed to:

   organization.  The design of DMARC precludes grouping policies for
   domains based on policy published above the organizational level,
   such as TLDs (Top Level Domains).  Domains at this higher level of
   the DNS tree (but not necessarily at the top of the DNS tree) can be
   collectively referred to as Public Suffix Domains (PSDs). 

>    DMARC functionality for such domains.

Given that the document no longer contains such a constraint, I've updated my 
working copy to say:

 This memo describes an extension to DMARC to enable DMARC functionality PSDs.

>    to be specified for both individual domains and sets of domains
Comment is OBE based on previous changes.

> Still too abstract.  I don't know what sort of set groupings you might

I think the new text is better.  Please check and let me know if it's there 
yet or not.

> but it would be more readable.

Changed.

>    This memo provides a simple extension to DMARC [RFC7489] to allow

Changed throughout.

> defining here, this should be reworded.

OBE to previous comments.

> s/will clarify/clarifies/

Changed.

> s/will help clarify/clarifies/

Changed.

> ....and saying it twice in the same paragraph is probably not necessary.

And then deleted the sentence.
> DMARC treats them differently?

Yes.  Per the DMARC definition, they are not organizational domains.

> An example would be useful here.

I've added this:

   As an example, take the ".gov.example" described above and add that
   for this entity emails about tax returns are sent from
   tax.gov.example.  It would not be surprising if fraudulent emails
   were sent purporting to be from taxes.gov.example (taxes is a cousin
   of tax - different enough to evade DMARC protection, but similar
   enough to potentially confuse users).  As defined in DMARC [RFC7489],
   a DMARC record could be published for tax.gov.example, but there is
   no general solution to publishing a DMARC policy to defend against
   abuse of non-existent cousins such as taxes.gov.example.  While an
   explicit record could be published for this one domain, the universe
   of possible cousins is such that this approach does not scale.

How's that?

> DMARC doesn't need to be followed by its RFC number every time.

I went through and removed most of these.  I tried to remove them so that it's 
mostly there when talking about DMARC the RFC, not DMARC the protocol.

> extension; how does it include domains?

Deleted the sentence.  I don't think it is necessary for the definition, so 
let's not confuse things.

> s/their/its/; "operator" is singular.

Changed.

> one is not willing to accept?

OBE.  Removed that from the definition based on the WGLC discussion about the 
definition.

> PSD?  It's not a subdomain.

Here's what's in RFC 7489 about DMARC record publishing:

6.1.  DMARC Policy Record

   Domain Owner DMARC preferences are stored as DNS TXT records in
   subdomains named "_dmarc".  For example, the Domain Owner of
   "example.com" would post DMARC preferences in a TXT record at
   "_dmarc.example.com".

I think our 3.2 is equivalent and adequate given this section is a modification 
to RFC 7489 Section 6.1.  I did not make a change.

>    require use of DMARC ought to make that information available to

Changed to:

   In addition to the DMARC domain owner actions, PSOs that require use
   of DMARC and participate in PSD DMARC ought to make that information
   available to receivers.  The mechanism for doing so is one of the
   experimental elements of this document.  See the experiment
   description (Appendix A).

> don't know what "acceptable for PSD DMARC" might mean.

Changed to:

   3A.  If the set is now empty and the longest PSD (Section 2.3) of the
      Organizational Domain is one that the receiver has determined is
      acceptable for PSD DMARC (discussed in the experiment description
      (Appendix A)), the Mail Receiver MUST query the DNS for a DMARC
      TXT record at the DNS domain matching the longest PSD
      (Section 2.3) in place of the RFC5322.From domain in the message
      (if different).  A possibly empty set of records is returned.

> document they're referencing.

I checked and any reference to an RFC7489 paragraph number that's in the text 
is labeled as such.
> Section 4 of which document?

Added 'of this document'.

>    leakage could be potentially be utilized as part of a program of

Fixed.

> first use might be a good idea.

Added at first use.

And that's the last comment.

I intend to post a revision shortly so the group can more easily review my 
proposed changes based on WGLC.

Scott K

---

From: "Stan Kalisch" <[email-masked]>
Date: Sat, 27 Jul 2019 18:00:30 -0400

Scott, I have two editorial nits that account for a grand total of three characters. It seemed less complicated to just send a PR in Github, so I did.

Stan

On Sat, Jul 27, 2019, at 4:45 PM, Scott Kitterman wrote:
> _______________________________________________
> dmarc mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/dmarc
>

---

From: Scott Kitterman <[email-masked]>
Date: Sun, 28 Jul 2019 01:02:34 +0000

On July 27, 2019 10:00:30 PM UTC, Stan Kalisch <[email-masked]> wrote:
>Scott, I have two editorial nits that account for a grand total of
>three characters. It seemed less complicated to just send a PR in
>Github, so I did.
>
Thanks.  That looks correct.

Scott K

---

From: "Murray S. Kucherawy" <[email-masked]>
Date: Wed, 31 Jul 2019 17:28:17 -0700

Thanks for this, much better.  Some additional feedback.

Please note (everyone) that I'm offering these as clarifying contributions
to text; I'm not prescribing or proscribing anything.  If the text I'm
proposing isn't worthy of consensus, please speak up.

> Does that clear those comments up?

That last sentence still puzzles me, and I think it's the term "grouping"
that gives me trouble.

How's this?

   DMARC (Domain-based Message Authentication, Reporting, and
   Conformance) is a scalable mechanism by which a mail-originating
   organization can express domain-level policies and preferences for
   message validation, disposition, and reporting, that a mail-receiving
   organization can use to improve mail handling.  The design of DMARC
   presumes that domain names represent either nodes in the tree below
   which registrations occur, or nodes where registrations have occurred;
   it does not permit a domain name to have both of these properties
   simultaneously.  Since its deployment in 2015, use of DMARC has shown

    a clear need for the ability to express policy for these domains as well.

   Domains at which registrations can occur are referred to as Public
   Suffix Domains (PSDs).  This document describes an extension to DMARC
   to enable DMARC functionality for PSDs.

   This document also seeks to address implementations that consider a domain

   on the Public Suffix List (PSL) to be ineligible for DMARC enforcement.

If we like this, I suggest it or language like it can also be used to
rework the first paragraph of the Introduction section in -05.  To
import DMARC definitions into the introduction, you could say that
DMARC presumes that a domain is either on the PSL or is an OD, and
never both,.

> > but it would be more readable.

Thanks.  Now for the text right before the example; I suggest:

As an example, ...  Suppose there exists a registered domain
"tax.gov.example" that is responsible for taxation in this imagined
country.  However, by exploiting the typically unauthenticated nature of
email, there are regular malicious campaigns to impersonate this
organization that use similar-looking ("cousin") domains such as
"t4x.gov.example".  These domains are not registered.  Within the
".gov.example" public suffix, use of DMARC has been mandated, so
"gov.example" publishes the following DMARC record:

...

Defensively registering all variants of "tax" is obviously not a scalable
strategy.  The intent of this specification, therefore, is to enhance the
DMARC algorithm by enabling an agent receiving such a message to be able to
determine that a relevant policy is present at "gov.example", which is
precluded by the current DMARC algorithm.

> Yes.  Per the DMARC definition, they are not organizational domains.

I suggest "unable to benefit from DMARC" rather than "excluded".
"Excluded" makes me think DMARC specifically targeted them as not worthy of
or appropriate for inclusion, which isn't the case.

>    of possible cousins is such that this approach does not scale.

Great, and it's also pretty much what I proposed above.  (Apologies; my
read-ahead failed.)  I suggest picking one or the other, but I think it's
of benefit to put this higher up where my suggestion is to set context.

> mostly there when talking about DMARC the RFC, not DMARC the protocol.

Yeah I think you got most of them but there are a few I would still
reduce.  Not a big deal unless others concur; the RFC Editor will probably
chime in as well.

> let's not confuse things.

The text present now for 2.2 defines the name/label structure of the DNS,
but doesn't actually say what a PSD is.

This is an interesting problem, because we are attempting to describe (or
ascribe) semantics of the DNS that simply do not exist.  The DNS is a
hierarchical naming structure and related database, plain and simple; we
are attempting to ascribe to it boundaries that are not there in DNS-land.
But we happen to do lookups in it using DNS APIs.

So let's try removing the DNS from the definition:

The domain name structure consists of a tree of names, each of which is
made of a sequence of words ("labels") separated by period characters.  The
root of the tree is simply called ".".  The Internet community at large,
through processes and policies external to this work, selects points in
this tree at which to register domain names "owned" by independent
organizations.  Real-world examples are ".com", ".org", ".us", and ".gov.uk".
Names at which such registrations occur are called Public Suffix Domains
(PSDs), and a registration consists of a label selected by the registrant
to which a desirable PSD is appended.  For example, "ietf.org" is a
registered domain name, and ".org" is its PSD.

While I'm here, I suggest 2.3 is a little terse.  Maybe:

The longest PSD is the PSD matching more labels in the domain name under
evaluation than any other PSL entry.

> to RFC 7489 Section 6.1.  I did not make a change.

Embarrassing.  RFC7489 6.1's text you cited is unfortunate.  I would change
"in subdomains named" to "at labels named".

But also, why do we need this section?  The PSD DMARC record is in the same
place as one would expect it having read only RFC7489, correct?

> I checked and any reference to an RFC7489 paragraph number that's in the

I'm talking about the "Section 2.3" here, and the "Section x.y" strings in
the section headings in Section 3.  Since you are pointing back to RFC7489
frequently, I thought it might be best to say "Section 2.3 of this
document", or "Section x.y of RFC7489", as appropriate.

Also in the example in the paragraph below here in the document, you're
using ".cctld"; should that be ".example" or ".cctld.example"?

One final thing: I added a sentence at the end of my proposed Abstract that
tries to highlight that domains on the PSL also need to be checked and not
special-cased when one receives mail from a PSL.  In order to tie it to the
experiment, I suggest this (as a new section at the end; reorder at your
discretion):

3.7.  PSDs Are Eligible For DMARC

Experience with DMARC has shown that some implementations short-circuit
messages, bypassing DMARC policy application, when the domain name
extracted by the receiver (from the RFC5322.From) is on the PSL.  This
prevents the capability being created by this specification.  Therefore,
the following paragraph is appended to Section 6.6.1 of RFC7489:

Note that domain names that appear on the Public Suffix List are not exempt
from DMARC policy application and reporting.

-MSK