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DMARC Aggregate Reporting
draft-ietf-dmarc-aggregate-reporting-07

Document Type Active Internet-Draft (dmarc WG)
Author Alex Brotman
Last updated 2022-12-22
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draft-ietf-dmarc-aggregate-reporting-07
DMARC                                                    A. Brotman (ed)
Internet-Draft                                             Comcast, Inc.
Obsoletes: 7489 (if approved)                           22 December 2022
Intended status: Standards Track                                        
Expires: 25 June 2023

                       DMARC Aggregate Reporting
                draft-ietf-dmarc-aggregate-reporting-07

Abstract

   DMARC allows for domain holders to request aggregate reports from
   receivers.  This report is an XML document, and contains extensible
   elements that allow for other types of data to be specified later.
   The aggregate reports can be submitted to the domain holder's
   specified destination as supported by the receiver.

   This document (along with others) obsoletes RFC7489.

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 25 June 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  DMARC Feedback  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Aggregate Reports . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Handling Domains in Reports . . . . . . . . . . . . .   6
       2.1.2.  DKIM Signatures in Aggregate Report . . . . . . . . .   7
       2.1.3.  Unique Identifiers in Aggregate Reporting . . . . . .   7
     2.2.  Extensions  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Changes in Policy During Reporting Period . . . . . . . .   8
     2.4.  Recommended Reporting Periods . . . . . . . . . . . . . .   8
     2.5.  Report Request Discovery  . . . . . . . . . . . . . . . .   8
     2.6.  Transport . . . . . . . . . . . . . . . . . . . . . . . .   9
       2.6.1.  Email . . . . . . . . . . . . . . . . . . . . . . . .   9
       2.6.2.  Definition of Report-ID . . . . . . . . . . . . . . .  11
       2.6.3.  Other Methods . . . . . . . . . . . . . . . . . . . .  12
       2.6.4.  Handling of Duplicates  . . . . . . . . . . . . . . .  12
   3.  Verifying External Destinations . . . . . . . . . . . . . . .  12
   4.  Extensible Reporting  . . . . . . . . . . . . . . . . . . . .  14
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Registration request for the DMARC namespace: . . . . . .  16
     5.2.  Registration request for the DMARC XML schema:  . . . . .  16
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
     6.1.  Data Exposure Considerations  . . . . . . . . . . . . . .  16
     6.2.  Report Recipients . . . . . . . . . . . . . . . . . . . .  17
     6.3.  Data Contained Within Reports (Tkt64) . . . . . . . . . .  17
     6.4.  Feedback Leakage  . . . . . . . . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  Appendix A.  DMARC XML Schema . . . . . . . . . . . . . . . .  19
   9.  Appendix B.  Sample Report  . . . . . . . . . . . . . . . . .  26
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  27
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  28

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1.  Introduction

   A key component of DMARC is the ability for domain holders to request
   that receivers provide various types of reports.  These reports allow
   domain holders to have insight into which IP addresses are sending on
   their behalf, and some insight into whether or not the volume may be
   legitimate.  These reports expose information relating to the DMARC
   policy, as well as the outcome of SPF [RFC7208] & DKIM [RFC6376]
   validation.

1.1.  Terminology

   In many IETF documents, several words, when they are in all capitals
   as shown below, are used to signify the requirements in the
   specification.  These capitalized words can bring significant clarity
   and consistency to documents because their meanings are well defined.
   This document defines how those words are interpreted in IETF
   documents when the words are in all capitals.

   *  These words can be used as defined here, but using them is not
      required.  Specifically, normative text does not require the use
      of these key words.  They are used for clarity and consistency
      when that is what's wanted, but a lot of normative text does not
      use them and is still normative.

   *  The words have the meanings specified herein only when they are in
      all capitals.

   *  When these words are not capitalized, they have their normal
      English meanings and are not affected by this document.

   Authors who follow these guidelines should incorporate this phrase
   near the beginning of their document:

      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
      "MAY", and "OPTIONAL" in this document are to be interpreted as
      described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
      appear in all capitals, as shown here.

2.  DMARC Feedback

   Providing Domain Owners with visibility into how Mail Receivers
   implement and enforce the DMARC mechanism in the form of feedback is
   critical to establishing and maintaining accurate authentication
   deployments.  When Domain Owners can see what effect their policies
   and practices are having, they are better willing and able to use
   quarantine and reject policies.

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2.1.  Aggregate Reports

   The DMARC aggregate feedback report is designed to provide Domain
   Owners with precise insight into:

   *  authentication results,

   *  corrective action that needs to be taken by Domain Owners, and

   *  the effect of Domain Owner DMARC policy on email streams processed
      by Mail Receivers.

   Aggregate DMARC feedback provides visibility into real-world email
   streams that Domain Owners need to make informed decisions regarding
   the publication of DMARC policy.  When Domain Owners know what
   legitimate mail they are sending, what the authentication results are
   on that mail, and what forged mail receivers are getting, they can
   make better decisions about the policies they need and the steps they
   need to take to enable those policies.  When Domain Owners set
   policies appropriately and understand their effects, Mail Receivers
   can act on them confidently.

   Visibility comes in the form of daily (or more frequent) Mail
   Receiver-originated feedback reports that contain aggregate data on
   message streams relevant to the Domain Owner.  This information
   includes data about messages that passed DMARC authentication as well
   as those that did not.

   The report may include the following data:

   *  The DMARC policy discovered and applied, if any

   *  The selected message disposition

   *  The identifier evaluated by SPF and the SPF result, if any

   *  The identifier evaluated by DKIM and the DKIM result, if any

   *  For both DKIM and SPF, an indication of whether the identifier was
      in alignment

   *  A separate report should be generated for each Policy Domain
      encountered during the reporting period.  If there are multiple
      5322.From domains that are included, those should result in
      distinct records within the report.  See below for further
      explanation in "Handling Domains in Reports".

   *  Sending and receiving domains

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   *  The policy requested by the Domain Owner and the policy actually
      applied (if different)

   *  The number of successful authentications

   *  The counts of messages based on all messages received, even if
      their delivery is ultimately blocked by other filtering agents.

   The format for these reports is defined in Appendix A.

   DMARC Aggregate Reports MUST contain three primary sections; one
   consisting of descriptive information (with two subsections), and the
   other a set of IP-focused row-based data.  Each report MUST contain
   data for only one Author Domain.  A single report MUST contain data
   for one policy configuration.  If multiple configurations were
   observed during a single reporting period, a reporting entity MAY
   choose to send multiple reports, otherwise the reporting entity
   SHOULD note only the final configuration observed during the period.
   See below for further information.

   The informative section MUST contain two sub-sections.  One will be
   the metadata section which MUST contain the fields related to
   org_name, email, report_id, and date_range.  Optional fields MAY
   include extra_contact_info, an error field, and an optional version
   field.  The version field, if present, MUST contain a 1.0 [RFC7489]
   or 2.0 [RefNeeded], noting to which version of the aggregate
   reporting specification the report adheres.  The date_range section
   which will note begin and end values as epoch timestamps.  The other
   sub-section will be the policy_published, and record the policy
   configuration observed by the receiving system.  Mandatory fields are
   domain, p, sp.  Optional fields are fo, adkim, aspf, testing, and
   version_published.  There MAY be an optional third section for
   extensions.

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   Within the data section, the report will contain row(s) of data
   stating which IPs were seen to have delivered messages for the Author
   Domain to the receiving system.  For each IP that is being reported,
   there will be at least one record element, which will then have each
   of a row, identifiers, and auth_results sub-element.  Within the row
   element, there MUST be source_ip and count.  There MUST also exist a
   policy_evaluated, with sub-elements of disposition, dkim, and spf.
   There MAY be an element for reason, meant to include any notes the
   reporter might want to include as to why the disposition policy does
   not match the policy_published, such as a Local Policy override
   (possible values listed in Appendix A).  The dkim and spf elements
   MUST be the evaluated values as they relate to DMARC, not the values
   the receiver may have used when overriding the policy.  Within the
   identifiers element, there MUST exist the data that was used to apply
   policy for the given IP.  In most cases, this will be a header_from
   element, which will contain the 5322.From domain from the message.

   There MUST be an auth_results element within the 'record' element.
   This will contain the data related to authenticating the messages
   associated with this sending IP.  The dkim sub-element is optional as
   not all messages are signed, while there MUST be one spf sub-element.
   These elements MUST have a domain that was used during validation, as
   well as result.  The dkim element MUST include a selector element
   that was observed during validation.  For the spf element, the result
   element MUST contain a lower-case string where the value is one of
   none/neutral/pass/fail/softfail/temperror/permerror.  The dkim result
   MUST contain a lower-case string where the value is one of
   none/pass/fail/policy/neutral/temperror/permerror.  Both the spf and
   dkim results may optionally include a human_readable field meant for
   the report to convey more descriptive information back to the domain
   holder relating to evaluation failures.  There MAY exist an optional
   section for extensions.

2.1.1.  Handling Domains in Reports

   In the same report, there MUST be a single Policy Domain, though
   there could be multiple 5322.From Domains.  Each 5322.From domain
   will create its own record within the report.  Consider the case
   where there are three domains with traffic volume to report:
   example.com, foo.example.com, and bar.example.com.  There will be
   explicit DMARC records for example.com and bar.example.com, with
   distinct policies.  There is no explicit DMARC record for
   foo.example.com, so it will be reliant on the policy described for
   example.com.  For a report period, there would now be two reports.
   The first will be for bar.example.com, and contain only one record,
   for bar.example.com.  The second report would be for example domain
   contain multiple record elements, one for example.com and one for
   foo.example.com (and extensibly, other record elements for subdomains

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   which likewise did not have an explicit DMARC policy declared).

2.1.2.  DKIM Signatures in Aggregate Report

   Within a single message, the possibility exists that there could be
   multiple DKIM signatures.  When validation of the message occurs,
   some signatures may pass, while some may not.  As these pertain to
   DMARC, and especially to aggregate reporting, reporters may not find
   it clear which DKIM signatures they should include in a report.
   Signatures, regardless of outcome, could help the report ingester
   determine the source of a message.  However, there is a preference as
   to which signatures are included.

   1.  A signature that passes DKIM, in strict alignment with the
       5322.From domain

   2.  A signature that passes DKIM, in relaxed alignment with the
       5322.From domain

   3.  Any other DKIM signatures that pass

   4.  DKIM signatures that do not pass

   A report SHOULD contain no more than 100 signatures for a given row,
   in decreasing priority.

2.1.3.  Unique Identifiers in Aggregate Reporting

   There are a few places where a unique identifier is specified as part
   of the body of the report, the subject, and so on.  These unique
   identifiers should be consistent per each report.  Specified below,
   the reader will see a msg-id, Report-ID, unique-id.  These are the
   fields that MUST be identical when used.

   NOTE, this no longer seems to be the case given changes to the
   Report-ID now used in the subject?  If we allow "<>" in the Subject,
   those characters are illegal in Windows filnames I believe.  We could
   say that the "unique-id" and the "ridtxt" are identical, other than
   the "<>" characters.  Should/Could we add a MUST to the format?
   Could we just point at RFC4122 (is it still used/valid)?

2.2.  Extensions

   There MAY be optional sections for extensions within the document.
   The absence or existence of this section SHOULD NOT create an error
   when processing reports.  This will be covered in a separate section.

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2.3.  Changes in Policy During Reporting Period

   Note that Domain Owners or their agents may change the published
   DMARC policy for a domain or subdomain at any time.  From a Mail
   Receiver's perspective, this will occur during a reporting period and
   may be noticed during that period, at the end of that period when
   reports are generated, or during a subsequent reporting period, all
   depending on the Mail Receiver's implementation.  Under these
   conditions, it is possible that a Mail Receiver could do any of the
   following:

   *  generate for such a reporting period a single aggregate report
      that includes message dispositions based on the old policy, or a
      mix of the two policies, even though the report only contains a
      single "policy_published" element;

   *  generate multiple reports for the same period, one for each
      published policy occurring during the reporting period;

   *  generate a report whose end time occurs when the updated policy
      was detected, regardless of any requested report interval.

   Such policy changes are expected to be infrequent for any given
   domain, whereas more stringent policy monitoring requirements on the
   Mail Receiver would produce a very large burden at Internet scale.
   Therefore, it is the responsibility of report consumers and Domain
   Owners to be aware of this situation and allow for such mixed reports
   during the propagation of the new policy to Mail Receivers.

2.4.  Recommended Reporting Periods

   Aggregate reports are most useful when they all cover a common time
   period.  By contrast, correlation of these reports from multiple
   generators when they cover incongruent time periods is difficult or
   impossible.  Report generators SHOULD, wherever possible, adhere to
   hour boundaries for the reporting period they are using.  For
   example, starting a per-day report at 00:00; starting per-hour
   reports at 00:00, 01:00, 02:00; etc.  Report generators using a
   24-hour report period are strongly encouraged to begin that period at
   00:00 UTC, regardless of local timezone or time of report production,
   in order to facilitate correlation.

2.5.  Report Request Discovery

   A Mail Receiver discovers reporting requests when it looks up a DMARC
   policy record that corresponds to an RFC5322.From domain on received
   mail.  The presence of the "rua" tag specifies where to send
   feedback.

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2.6.  Transport

   Where the URI specified in a "rua" tag does not specify otherwise, a
   Mail Receiver generating a feedback report SHOULD employ a secure
   transport mechanism.

   The Mail Receiver, after preparing a report, MUST evaluate the
   provided reporting URIs in the order given.  Any reporting URI that
   includes a size limitation exceeded by the generated report (after
   compression and after any encoding required by the particular
   transport mechanism) MUST NOT be used.  An attempt MUST be made to
   deliver an aggregate report to every remaining URI, up to the
   Receiver's limits on supported URIs.

   If transport is not possible because the services advertised by the
   published URIs are not able to accept reports (e.g., the URI refers
   to a service that is unreachable, or all provided URIs specify size
   limits exceeded by the generated record), the Mail Receiver MAY send
   a short report (see Section 7.2.2) indicating that a report is
   available but could not be sent.  The Mail Receiver MAY cache that
   data and try again later, or MAY discard data that could not be sent.

2.6.1.  Email

   The message generated by the Mail Receiver MUST be a [RFC5322]
   message formatted per [RFC2045].  The aggregate report itself MUST be
   included in one of the parts of the message.  A human-readable
   portion MAY be included as a MIME part (such as a text/plain part).

   The aggregate data MUST be an XML file that SHOULD be subjected to
   GZIP compression.  Declining to apply compression can cause the
   report to be too large for a receiver to process (a commonly observed
   receiver limit is ten megabytes); doing the compression increases the
   chances of acceptance of the report at some compute cost.  The
   aggregate data MUST be present using the media type "application/
   gzip" if compressed (see [RFC6713]), and "text/xml" otherwise.  The
   filename MUST be constructed using the following ABNF:

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    filename = receiver "!" policy-domain "!" begin-timestamp
               "!" end-timestamp [ "!" unique-id ] "." extension

    receiver = domain
               ; imported from [@!RFC5322]

    policy-domain   = domain

    begin-timestamp = 1*DIGIT
                      ; seconds since 00:00:00 UTC January 1, 1970
                      ; indicating start of the time range contained
                      ; in the report

    end-timestamp = 1*DIGIT
                    ; seconds since 00:00:00 UTC January 1, 1970
                    ; indicating end of the time range contained
                    ; in the report

    unique-id = 1*(ALPHA / DIGIT)

    extension = "xml" / "xml.gz"

   The extension MUST be "xml" for a plain XML file, or "xml.gz" for an
   XML file compressed using GZIP.

   "unique-id" allows an optional unique ID generated by the Mail
   Receiver to distinguish among multiple reports generated
   simultaneously by different sources within the same Domain Owner.

   If a report generator needs to re-send a report, the system MUST use
   the same filename as the original report.  This would allow the
   receiver to overwrite the data from the original, or discard second
   instance of the report.

   For example, this is a sample filename for the gzip file of a report
   to the Domain Owner "example.com" from the Mail Receiver
   "mail.receiver.example":

    mail.receiver.example!example.com!1013662812!1013749130.gz

   No specific MIME message structure is required.  It is presumed that
   the aggregate reporting address will be equipped to extract MIME
   parts with the prescribed media type and filename and ignore the
   rest.

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   Email streams carrying DMARC feedback data MUST conform to the DMARC
   mechanism, thereby resulting in an aligned "pass" (see Section 3.1).
   This practice minimizes the risk of report consumers processing
   fraudulent reports.

   The RFC5322.Subject field for individual report submissions MUST
   conform to the following ABNF:

    dmarc-subject = %x52.65.70.6f.72.74 1*FWS       ; "Report"
                    %x44.6f.6d.61.69.6e.3a 1*FWS    ; "Domain:"
                    domain-name 1*FWS               ; from RFC 6376
                    %x53.75.62.6d.69.74.74.65.72.3a ; "Submitter:"
                    1*FWS domain-name 1*FWS
                    %x52.65.70.6f.72.74.2d.49.44.3a ; "Report-ID:"
                    ridtxt                          ; defined below

   The first domain-name indicates the DNS domain name about which the
   report was generated.  The second domain-name indicates the DNS
   domain name representing the Mail Receiver generating the report.
   The purpose of the Report-ID: portion of the field is to enable the
   Domain Owner to identify and ignore duplicate reports that might be
   sent by a Mail Receiver.

   For instance, this is a possible Subject field for a report to the
   Domain Owner "example.com" from the Mail Receiver
   "mail.receiver.example".  It is line-wrapped as allowed by [RFC5322]:

    Subject: Report Domain: example.com
        Submitter: mail.receiver.example
        Report-ID: <2002.02.15.1>

   This transport mechanism potentially encounters a problem when
   feedback data size exceeds maximum allowable attachment sizes for
   either the generator or the consumer.  See Section 7.2.2 for further
   discussion.

   Optionally, the report sender MAY choose to use the same ridtxt as a
   part or whole of the 5322.Message-Id header included with the report.
   Doing so may help receivers distinguish when a message is a re-
   transmission or duplicate report.

2.6.2.  Definition of Report-ID

   As noted elsewhere, this identifier MUST be unique among reports to
   the same domain to aid receivers in identifying duplicate reports
   should they happen.

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   ridtxt = ("<" *(ALPHA / DIGIT / "." / "-") ["@" _(ALPHA / DIGIT / "."
   / "-")] ">") / (_(ALPHA / DIGIT / "." / "-") ["@" *(ALPHA / DIGIT /
   "." / "-")])

   The format specified here is not very strict as the key goal is
   uniqueness.

2.6.3.  Other Methods

   The specification as written allows for the addition of other
   registered URI schemes to be supported in later versions.

2.6.4.  Handling of Duplicates

   There may be a situation where the report generator attempts to
   deliver duplicate information to the receiver.  This may manifest as
   an exact duplicate of the report, or as duplicate information between
   two reports.  In these situations, the decision of how to handle the
   duplicate data lies with the receiver.  As noted above, the sender
   MUST use the same unique identifiers when sending the report.  This
   allows the receiver to better understand when duplicates happen.  A
   few options on how to handle that duplicate information:

   *  Reject back to sender, ideally with a permfail error noting the
      duplicate receipt

   *  Discard upon receipt

   *  Inspect the contents to evaluate the timestamps and reported data,
      act as appropriate

   *  Accept the duplicate data

   When accepting the data, that's likely in a situation where it's not
   yet noticed, or a one-off experience.  Long term, duplicate data is
   not ideal.  In the situation of a partial time frame overlap, there
   is no clear way to distinguish the impact of the overlap.  The
   receiver would need to accept or reject the duplicate data in whole.

3.  Verifying External Destinations

   It is possible to specify destinations for the different reports that
   are outside the authority of the Domain Owner making the request.
   This allows domains that do not operate mail servers to request
   reports and have them go someplace that is able to receive and
   process them.

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   Without checks, this would allow a bad actor to publish a DMARC
   policy record that requests that reports be sent to a victim address,
   and then send a large volume of mail that will fail both DKIM and SPF
   checks to a wide variety of destinations; the victim will in turn be
   flooded with unwanted reports.  Therefore, a verification mechanism
   is included.

   When a Mail Receiver discovers a DMARC policy in the DNS, and the
   Organizational Domain at which that record was discovered is not
   identical to the Organizational Domain of the host part of the
   authority component of a [RFC3986] specified in the "rua" or "ruf"
   tag, the following verification steps MUST be taken:

   1.  Extract the host portion of the authority component of the URI.
       Call this the "destination host", as it refers to a Report
       Receiver.

   2.  Prepend the string "_report._dmarc".

   3.  Prepend the domain name from which the policy was retrieved,
       after conversion to an A-label if needed.

   4.  Query the DNS for a TXT record at the constructed name.  If the
       result of this request is a temporary DNS error of some kind
       (e.g., a timeout), the Mail Receiver MAY elect to temporarily
       fail the delivery so the verification test can be repeated later.

   5.  For each record returned, parse the result as a series of
       "tag=value" pairs, i.e., the same overall format as the policy
       record (see Section 6.4).  In particular, the "v=DMARC1" tag is
       mandatory and MUST appear first in the list.  Discard any that do
       not pass this test.

   6.  If the result includes no TXT resource records that pass basic
       parsing, a positive determination of the external reporting
       relationship cannot be made; stop.

   7.  If at least one TXT resource record remains in the set after
       parsing, then the external reporting arrangement was authorized
       by the Report Receiver.

   8.  If a "rua" or "ruf" tag is thus discovered, replace the
       corresponding value extracted from the domain's DMARC policy
       record with the one found in this record.  This permits the
       Report Receiver to override the report destination.  However, to
       prevent loops or indirect abuse, the overriding URI MUST use the
       same destination host from the first step.

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   For example, if a DMARC policy query for "blue.example.com" contained
   "rua=mailto:reports@red.example.net", the host extracted from the
   latter ("red.example.net") does not match "blue.example.com", so this
   procedure is enacted.  A TXT query for
   "blue.example.com._report._dmarc.red.example.net" is issued.  If a
   single reply comes back containing a tag of "v=DMARC1", then the
   relationship between the two is confirmed.  Moreover,
   "red.example.net" has the opportunity to override the report
   destination requested by "blue.example.com" if needed.

   Where the above algorithm fails to confirm that the external
   reporting was authorized by the Report Receiver, the URI MUST be
   ignored by the Mail Receiver generating the report.  Further, if the
   confirming record includes a URI whose host is again different than
   the domain publishing that override, the Mail Receiver generating the
   report MUST NOT generate a report to either the original or the
   override URI.  A Report Receiver publishes such a record in its DNS
   if it wishes to receive reports for other domains.

   A Report Receiver that is willing to receive reports for any domain
   can use a wildcard DNS record.  For example, a TXT resource record at
   "*._report._dmarc.example.com" containing at least "v=DMARC1"
   confirms that example.com is willing to receive DMARC reports for any
   domain.

   If the Report Receiver is overcome by volume, it can simply remove
   the confirming DNS record.  However, due to positive caching, the
   change could take as long as the time-to-live (TTL) on the record to
   go into effect.

   A Mail Receiver might decide not to enact this procedure if, for
   example, it relies on a local list of domains for which external
   reporting addresses are permitted.

4.  Extensible Reporting

   DMARC reports allow for some extensibility, as defined by future
   documents that utilize DMARC as a foundation.  These extensions MUST
   be properly formatted XML and meant to exist within the structure of
   a DMARC report.  Two positions of type <any> are provided in the
   existing DMARC structure, one at file level, in an <extension>
   element after <policy_published> and one at record level, after
   <auth_results>.  In either case, the extensions MUST contain a URI to
   the definition of the extension so that the receiver understands how
   to interpret the data.

   At file level:

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   <feedback xmlns="urn:ietf:params:xml:ns:dmarc-2.0"
             xmlns:ext="URI for an extension-supplied name space">
     ...
     <policy_published>
       <domain>example.com</domain>
       <p>quarantine</p>
       <sp>none</sp>
       <testing>n</testing>
     </policy_published>
     <extension>
       <ext:arc-override>never</ext:arc-override>
     </extension>

   Within the record element:

     <record>
       <row>
          ...
       </row>
       <identifiers>
          ...
       </identifiers>
       <auth_results>
          ...
       </auth_results>
       <ext:arc-results>
          ...
       </ext:arc-results>
     </record>
     <record>
        ...

   Here "arc-override" and "arc-results" are hypothetical element names
   defined in the extension's name space.

   Extension elements are optional.  Any number of extensions is
   allowed.  If a processor is unable to handle an extension in a
   report, it SHOULD ignore the data and continue to the next extension.

5.  IANA Considerations

   This document uses URNs to describe XML namespaces and XML schemas
   conforming to a registry mechanism described in [RFC3688].  Two URI
   assignments will be registered by the IANA.

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5.1.  Registration request for the DMARC namespace:

   URI: urn:ietf:params:xml:ns:dmarc-2.0

   Registrant Contact: See the "Author's Address" section of this
   document.

   XML: None.  Namespace URIs do not represent an XML specification.

5.2.  Registration request for the DMARC XML schema:

   URI: urn:ietf:params:xml:schema:dmarc-2.0

   Registrant Contact: See the "Author's Address" section of this
   document.

   XML: See Appendix A.  DMARC XML Schema in this document.

6.  Privacy Considerations

   This section will discuss exposure related to DMARC aggregate
   reporting.

6.1.  Data Exposure Considerations

   Aggregate reports are limited in scope to DMARC policy and
   disposition results, to information pertaining to the underlying
   authentication mechanisms, and to the identifiers involved in DMARC
   validation.

   Aggregate report may expose sender and recipient identifiers,
   specifically the RFC5322.From addresses.

   Domain Owners requesting reports will receive information about mail
   claiming to be from them, which includes mail that was not, in fact,
   from them.  Information about the final destination of mail where it
   might otherwise be obscured by intermediate systems will therefore be
   exposed.

   When message-forwarding arrangements exist, Domain Owners requesting
   reports will also receive information about mail forwarded to domains
   that were not originally part of their messages' recipient lists.
   This means that destination domains previously unknown to the Domain
   Owner may now become visible.

   Disclosure of information about the messages is being requested by
   the entity generating the email in the first place, i.e., the Domain
   Owner and not the Mail Receiver, so this may not fit squarely within

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   existing privacy policy provisions.  For some providers, aggregate
   reporting is viewed as a function similar to complaint reporting
   about spamming or phishing and are treated similarly under the
   privacy policy.  Report generators (i.e., Mail Receivers) are
   encouraged to review their reporting limitations under such policies
   before enabling DMARC reporting.

6.2.  Report Recipients

   A DMARC record can specify that reports should be sent to an
   intermediary operating on behalf of the Domain Owner.  This is done
   when the Domain Owner contracts with an entity to monitor mail
   streams for abuse and performance issues.  Receipt by third parties
   of such data may or may not be permitted by the Mail Receiver's
   privacy policy, terms of use, or other similar governing document.
   Domain Owners and Mail Receivers should both review and understand if
   their own internal policies constrain the use and transmission of
   DMARC reporting.

   Some potential exists for report recipients to perform traffic
   analysis, making it possible to obtain metadata about the Receiver's
   traffic.  In addition to verifying compliance with policies,
   Receivers need to consider that before sending reports to a third
   party.

6.3.  Data Contained Within Reports (Tkt64)

   Aggregate feedback reports contain aggregated data relating to
   messages purportedly originating from the Domain Owner.  The data
   does not contain any identifying characteristics about individual
   users.  No personal information such as individual email addresses,
   IP addresses of individuals, or the content of any messages, is
   included in reports.

   Mail Receivers should have no concerns in sending reports as they do
   not contain personal information.  In all cases, the data within the
   reports relates to the domain-level authentication information
   provided by mail servers sending messages on behalf of the Domain
   Owner.  This information is necessary to assist Domain Owners in
   implementing and maintaining DMARC.

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   Domain Owners should have no concerns in receiving reports as they do
   not contain personal information.  The reports only contain
   aggregated data related to the domain-level authentication details of
   messages claiming to originate from their domain.  This information
   is essential for the proper implementation and operation of DMARC.
   Domain Owners who are unable to receive reports for organizational
   reasons, can choose to exclusively direct the reports to an external
   processor.

6.4.  Feedback Leakage

   Providing feedback reporting to PSOs can, in some cases, cause
   information to leak out of an organization to the PSO.  This leakage
   could potentially be utilized as part of a program of pervasive
   surveillance (see [RFC7624]]).  There are roughly three cases to
   consider:

   Single Organization PSDs (e.g., ".mil"):  RUA 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.

   Multi-organization PSDs that require DMARC usage (e.g., ".bank"):  Re
      ports based on PSD DMARC 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 of the organization (or hosts) will
      be used.  The only direct risk of feedback leakage 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.

   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.  Any non-DMARC Organizational Domain would have its 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 MUST be limited to Aggregate Reports.  Feedback
   Reports carry more detailed information and present a greater risk.

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   Due to the inherent privacy and security risks associated with DMARC
   at the PSD level for Organizational Domains in multi-organization
   PSDs that do not participate in DMARC, any feedback reporting related
   to multi- organizational PSDs MUST be limited to non-existent domains
   except in cases where the reporter knows that PSO requires use of
   DMARC.

7.  Security Considerations

   TBD

8.  Appendix A.  DMARC XML Schema

   <?xml version="1.0"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
    targetNamespace="urn:ietf:params:xml:ns:dmarc-2.0"
    xmlns="urn:ietf:params:xml:ns:dmarc-2.0"
    elementFormDefault="qualified">

   <!-- The time range in UTC covered by messages in this report,
      specified in seconds since epoch. -->
   <xs:complexType name="DateRangeType">
    <xs:all>
     <xs:element name="begin" type="xs:integer"
           minOccurs="1" maxOccurs="1"/>
     <xs:element name="end" type="xs:integer"
           minOccurs="1" maxOccurs="1"/>
    </xs:all>
   </xs:complexType>

   <!-- Report generator metadata. -->
   <!--
    org_name: Reporting Organization
    email: Contact to be used when contacting
        the Reporting Organization
    extra_contact_info: Additional contact details
    report_id: UUID, specified elsewhere
    date_range: Timestamps used when forming report data
    error: ?
   -->
   <xs:complexType name="ReportMetadataType">
    <xs:all>
     <xs:element name="org_name" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
     <xs:element name="email" type="xs:string"
           minOccurs="1" maxOccurs="1"/>

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     <xs:element name="extra_contact_info" type="xs:string"
           minOccurs="0" maxOccurs="1"/>
     <xs:element name="report_id" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
     <xs:element name="date_range" type="DateRangeType"
           minOccurs="1" maxOccurs="1"/>
     <xs:element name="error" type="xs:string"
           minOccurs="0" maxOccurs="unbounded"/>
    </xs:all>
   </xs:complexType>

   <!-- Alignment mode (relaxed or strict) for DKIM and SPF. -->
   <xs:simpleType name="AlignmentType">
    <xs:restriction base="xs:string">
     <xs:enumeration value="r"/>
     <xs:enumeration value="s"/>
    </xs:restriction>
   </xs:simpleType>

   <!-- The policy actions specified by p and sp in the
      DMARC record. -->
   <xs:simpleType name="DispositionType">
    <xs:restriction base="xs:string">
     <xs:enumeration value="none"/>
     <xs:enumeration value="quarantine"/>
     <xs:enumeration value="reject"/>
    </xs:restriction>
   </xs:simpleType>

   <!-- The policy actions utilized on messages for this record. -->
   <!--
      "none": No action taken
      "pass": No action, passing DMARC w/enforcing policy
      "quarantine": Failed DMARC, message marked for quarantine
      "reject": Failed DMARC, marked as reject
   -->
   <xs:simpleType name="ActionDispositionType">
    <xs:restriction base="xs:string">
     <xs:enumeration value="none"/>
     <xs:enumeration value="pass"/>
     <xs:enumeration value="quarantine"/>
     <xs:enumeration value="reject"/>
    </xs:restriction>
   </xs:simpleType>

   <!-- The published DMARC policy.  Unspecified tags have their
        default values. -->
   <xs:complexType name="PolicyPublishedType">

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    <xs:all>
     <!-- The domain at which the DMARC record was found. -->
     <xs:element name="domain" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
     <!-- The version declared in the DMARC record found. -->
     <xs:element name="version_published" type="xs:decimal"
           minOccurs="0" maxOccurs="1"/>
     <!-- The DKIM alignment mode. -->
     <xs:element name="adkim" type="AlignmentType"
           minOccurs="0" maxOccurs="1"/>
     <!-- The SPF alignment mode. -->
     <xs:element name="aspf" type="AlignmentType"
           minOccurs="0" maxOccurs="1"/>
     <!-- The policy published for messages from the domain. -->
     <xs:element name="p" type="DispositionType"
           minOccurs="1" maxOccurs="1"/>
     <!-- The policy published for messages from subdomains. -->
     <xs:element name="sp" type="DispositionType"
           minOccurs="1" maxOccurs="1"/>
     <!-- The percent declared in the DMARC record -->
     <xs:element name="testing" type="TestingType"
           minOccurs="0" maxOccurs="1"/>
     <!-- Failure reporting options in effect. -->
     <xs:element name="fo" type="xs:string"
           minOccurs="0" maxOccurs="1"/>
    </xs:all>
   </xs:complexType>

   <!-- Values for Testing mode attached to Policy -->
   <xs:simpleType name="TestingType">
    <xs:restriction base="xs:string">
     <xs:enumeration value="n"/>
     <xs:enumeration value="y"/>
    </xs:restriction>
   </xs:simpleType>

   <!-- The DMARC-aligned authentication result. -->
   <xs:simpleType name="DMARCResultType">
    <xs:restriction base="xs:string">
     <xs:enumeration value="pass"/>
     <xs:enumeration value="fail"/>
    </xs:restriction>
   </xs:simpleType>

   <!-- Reasons that may affect DMARC disposition or execution
      thereof. -->
   <xs:simpleType name="PolicyOverrideType">
    <xs:restriction base="xs:string">

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     <xs:enumeration value="forwarded"/>
     <xs:enumeration value="sampled_out"/>
     <xs:enumeration value="trusted_forwarder"/>
     <xs:enumeration value="mailing_list"/>
     <xs:enumeration value="local_policy"/>
     <xs:enumeration value="other"/>
    </xs:restriction>
   </xs:simpleType>

   <!-- Override reason consists of pre-defined override type and
        free-text comment. -->
   <xs:complexType name="PolicyOverrideReason">
    <xs:all>
     <xs:element name="type" type="PolicyOverrideType"
           minOccurs="1" maxOccurs="1"/>
     <xs:element name="comment" type="xs:string"
           minOccurs="0" maxOccurs="1"/>
    </xs:all>
   </xs:complexType>

   <!-- Taking into account everything else in the record,
    the results of applying DMARC. If alignment fails
    and the policy applied does not match the domain's
    configured policy, the reason element MUST be specified  -->

   <xs:complexType name="PolicyEvaluatedType">
    <xs:sequence>
     <xs:element name="disposition" type="ActionDispositionType"/>
     <xs:element name="dkim" type="DMARCResultType"/>
     <xs:element name="spf" type="DMARCResultType"/>
     <xs:element name="reason" type="PolicyOverrideReason"
           minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
   </xs:complexType>

   <!-- The Internet Protocol Address
        from which messages were received -->
   <xs:simpleType name="IPAddress">
    <xs:restriction base="xs:string">
     <!-- IPv4 -->
     <xs:pattern value=
     "((1?\d?\d|2[0-4]\d|25[0-5])\.){3}(1?\d?\d|2[0-4]\d|25[0-5])"/>
     <!-- IPv6 -->
     <xs:pattern value="([A-Fa-f0-9]{1,4}:){7}[A-Fa-f0-9]{1,4}"/>
     <!-- RFC 5952 zero compression IPv6 (lax) -->
     <xs:pattern value="([A-Fa-f0-9]{1,4}:){1,7}:"/>
     <xs:pattern value="([A-Fa-f0-9]{1,4}:){1,6}:[A-Fa-f0-9]{1,4}"/>
     <xs:pattern value=

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     "([A-Fa-f0-9]{1,4}:){1,5}:[A-Fa-f\d]{1,4}:[A-Fa-f\d]{1,4}"/>
     <xs:pattern value=
     "([A-Fa-f\d]{1,4}:){1,4}:([A-Fa-f\d]{1,4}:){1,2}[A-Fa-f\d]{1,4}"/>
     <xs:pattern value=
     "([A-Fa-f\d]{1,4}:){1,3}:([A-Fa-f\d]{1,4}:){1,3}[A-Fa-f\d]{1,4}"/>
     <xs:pattern value=
     "([A-Fa-f\d]{1,4}:){1,2}:([A-Fa-f\d]{1,4}:){1,4}[A-Fa-f\d]{1,4}"/>
     <xs:pattern value=
     "[A-Fa-f\d]{1,4}::([A-Fa-f\d]{1,4}:){1,5}[A-Fa-f\d]{1,4}"/>
     <xs:pattern value="::([A-Fa-f\d]{1,4}:){1,6}[A-Fa-f\d]{1,4}"/>
     <xs:pattern value="::[A-Fa-f\d]{1,4}"/>
    </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="RowType">
    <xs:sequence>
     <!-- The connecting IP. -->
     <xs:element name="source_ip" type="IPAddress"
           minOccurs="1" maxOccurs="1"/>
     <!-- The number of messages for which the
     PolicyEvaluatedType was applied. -->
     <xs:element name="count" type="xs:integer"
           minOccurs="1" maxOccurs="1"/>
     <!-- The DMARC disposition applied to matching messages. -->
     <xs:element name="policy_evaluated"
           type="PolicyEvaluatedType"
           minOccurs="1" maxOccurs="1"/>
    </xs:sequence>
   </xs:complexType>

   <xs:complexType name="IdentifierType">
    <xs:all>
     <!-- The envelope recipient domain. -->
     <xs:element name="envelope_to" type="xs:string"
           minOccurs="0" maxOccurs="1"/>
     <!-- The RFC5321.MailFrom domain or, if that value is empty,
          the RFC5321.Helo domain. -->
     <xs:element name="envelope_from" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
     <!-- The RFC5322.From domain. -->
     <xs:element name="header_from" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
    </xs:all>
   </xs:complexType>

   <!-- DKIM verification result, according to RFC 8601
      Section 2.7.1. -->
   <xs:simpleType name="DKIMResultType">

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    <xs:restriction base="xs:string">
     <xs:enumeration value="none"/>
     <xs:enumeration value="pass"/>
     <xs:enumeration value="fail"/>
     <xs:enumeration value="policy"/>
     <xs:enumeration value="neutral"/>
     <xs:enumeration value="temperror"/>
     <xs:enumeration value="permerror"/>
    </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="DKIMAuthResultType">
    <xs:all>
     <!-- The "d=" parameter in the signature. -->
     <xs:element name="domain" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
     <!-- The "s=" parameter in the signature. -->
     <xs:element name="selector" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
     <!-- The DKIM verification result. -->
     <xs:element name="result" type="DKIMResultType"
           minOccurs="1" maxOccurs="1"/>
     <!-- Any extra information (e.g., from
        Authentication-Results). -->
     <xs:element name="human_result" type="xs:string"
           minOccurs="0" maxOccurs="1"/>
    </xs:all>
   </xs:complexType>

   <!-- SPF domain scope. -->
   <xs:simpleType name="SPFDomainScope">
    <xs:restriction base="xs:string">
     <xs:enumeration value="helo"/>
     <xs:enumeration value="mfrom"/>
    </xs:restriction>
   </xs:simpleType>

   <!-- SPF result. -->
   <xs:simpleType name="SPFResultType">
    <xs:restriction base="xs:string">
     <xs:enumeration value="none"/>
     <xs:enumeration value="neutral"/>
     <xs:enumeration value="pass"/>
     <xs:enumeration value="fail"/>
     <xs:enumeration value="softfail"/>
     <!-- "TempError" commonly implemented as "unknown". -->
     <xs:enumeration value="temperror"/>
     <!-- "PermError" commonly implemented as "error". -->

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     <xs:enumeration value="permerror"/>
    </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="SPFAuthResultType">
    <xs:all>
     <!-- The checked domain. -->
     <xs:element name="domain" type="xs:string"
           minOccurs="1" maxOccurs="1"/>
     <!-- The scope of the checked domain. -->
     <xs:element name="scope" type="SPFDomainScope"
           minOccurs="0" maxOccurs="1"/>
     <!-- The SPF verification result. -->
     <xs:element name="result" type="SPFResultType"
           minOccurs="1" maxOccurs="1"/>
     <!-- Any extra information
       (e.g., from Authentication-Results).
     The information in the field below should be for a
      person to be provided with additional information
      that may be useful when debugging SPF authentication
      issues.  This could include broken records, invalid
      DNS responses, etc.
     -->
     <xs:element name="human_result" type="xs:string"
           minOccurs="0" maxOccurs="1"/>
    </xs:all>
   </xs:complexType>

   <!-- This element contains DKIM and SPF results, uninterpreted
      with respect to DMARC. -->
   <xs:complexType name="AuthResultType">
    <xs:sequence>
     <!-- There may be no DKIM signatures, or multiple DKIM
        signatures. -->
     <xs:element name="dkim" type="DKIMAuthResultType"
           minOccurs="0" maxOccurs="unbounded"/>
     <!-- There will always be at least one SPF result. -->
     <xs:element name="spf" type="SPFAuthResultType" minOccurs="1"
           maxOccurs="unbounded"/>
    </xs:sequence>
   </xs:complexType>

   <!-- This element contains all the authentication results that
      were evaluated by the receiving system for the given set of
      messages. -->
   <xs:complexType name="RecordType">
    <xs:sequence>
     <xs:element name="row" type="RowType"/>

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     <xs:element name="identifiers" type="IdentifierType"
           minOccurs="1" maxOccurs="1"/>
     <xs:element name="auth_results" type="AuthResultType"
           minOccurs="1" maxOccurs="1"/>
     <!-- Extension at row level-->
     <xs:any processContents="lax"
           minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
   </xs:complexType>

   <xs:complexType name="ExtensionType">
    <xs:sequence>
     <xs:any processContents="lax"
           minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
   </xs:complexType>

   <!-- Parent -->
   <xs:element name="feedback">
    <xs:complexType>
     <xs:sequence>
      <xs:element name="version"
            minOccurs="0" maxOccurs="1" type="xs:decimal"/>
      <xs:element name="report_metadata"
            minOccurs="1" maxOccurs="1"
            type="ReportMetadataType"/>
      <xs:element name="policy_published"
            minOccurs="1" maxOccurs="1"
            type="PolicyPublishedType"/>
      <!-- Extension at top level -->
      <xs:element name="extension" type="ExtensionType"
            minOccurs="0" maxOccurs="1"/>
      <!-- One record per (IP, result, IDs Auths) tuples -->
      <xs:element name="record" type="RecordType"
            minOccurs="1" maxOccurs="unbounded"/>
     </xs:sequence>
    </xs:complexType>
   </xs:element>
   </xs:schema>

9.  Appendix B.  Sample Report

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   <feedback xmlns="http://dmarc.org/dmarc-xml/0.2">
     <version>2.0</version>
     <report_metadata>
       <org_name>Sample Reporter</org_name>
       <email>report_sender@example-reporter.com</email>
       <extra_contact_info>...</extra_contact_info>
       <report_id>3v98abbp8ya9n3va8yr8oa3ya</report_id>
       <date_range>
         <begin>161212415</begin>
         <end>161221511</end>
       </date_range>
     </report_metadata>
     <policy_published>
       <domain>example.com</domain>
       <p>quarantine</p>
       <sp>none</sp>
       <testing>n</testing>
     </policy_published>
     <record>
       <row>
         <source_ip>192.168.4.4</source_ip>
         <count>123</count>
         <policy_evaluated>
           <disposition>quarantine</disposition>
           <dkim>pass</dkim>
           <spf>fail</spf>
         </policy_evaluated>
       </row>
       <identifiers>
         <envelope_from>example.com</envelope_from>
         <header_from>example.com</header_from>
       </identifiers>
       <auth_results>
         <dkim>
           <domain>example.com</domain>
           <result>pass</result>
           <selector>abc123</selector>
         </dkim>
         <spf>
           <domain>example.com</domain>
           <result>fail</result>
         </spf>
       </auth_results>
     </record>
   </feedback>

10.  Normative References

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   [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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

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

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC6713]  Levine, J., "The 'application/zlib' and 'application/gzip'
              Media Types", RFC 6713, DOI 10.17487/RFC6713, August 2012,
              <https://www.rfc-editor.org/info/rfc6713>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [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>.

   [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>.

Author's Address

   Alex Brotman
   Comcast, Inc.

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   Email: alex_brotman@comcast.com

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