Tech-invite3GPPspecsGlossariesIETFRFCsGroupsSIPABNFs   Ti+   SearchTech-invite World Map Symbol

RFC 8460

Proposed STD
Pages: 34
Top     in Index     Prev     Next
in Group Index     Prev in Group     Next in Group     Group: UTA

SMTP TLS Reporting

Part 1 of 2, p. 1 to 15
None       Next Section

 


Top       ToC       Page 1 
Internet Engineering Task Force (IETF)                       D. Margolis
Request for Comments: 8460                                  Google, Inc.
Category: Standards Track                                     A. Brotman
ISSN: 2070-1721                                            Comcast, Inc.
                                                         B. Ramakrishnan
                                                              Oath, Inc.
                                                                J. Jones
                                                         Microsoft, Inc.
                                                               M. Risher
                                                            Google, Inc.
                                                          September 2018


                           SMTP TLS Reporting

Abstract

   A number of protocols exist for establishing encrypted channels
   between SMTP Mail Transfer Agents (MTAs), including STARTTLS, DNS-
   Based Authentication of Named Entities (DANE) TLSA, and MTA Strict
   Transport Security (MTA-STS).  These protocols can fail due to
   misconfiguration or active attack, leading to undelivered messages or
   delivery over unencrypted or unauthenticated channels.  This document
   describes a reporting mechanism and format by which sending systems
   can share statistics and specific information about potential
   failures with recipient domains.  Recipient domains can then use this
   information to both detect potential attacks and diagnose
   unintentional misconfigurations.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8460.

Top      ToC       Page 2 
Copyright Notice

   Copyright (c) 2018 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 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.

Top       Page 3 
Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   5
   3.  Reporting Policy  . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Example Reporting Policy  . . . . . . . . . . . . . . . .   8
       3.1.1.  Report Using MAILTO . . . . . . . . . . . . . . . . .   8
       3.1.2.  Report Using HTTPS  . . . . . . . . . . . . . . . . .   8
   4.  Reporting Schema  . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Report Time Frame . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Delivery Summary  . . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Success Count . . . . . . . . . . . . . . . . . . . .  10
       4.2.2.  Failure Count . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Result Types  . . . . . . . . . . . . . . . . . . . . . .  10
       4.3.1.  Negotiation Failures  . . . . . . . . . . . . . . . .  10
       4.3.2.  Policy Failures . . . . . . . . . . . . . . . . . . .  11
       4.3.3.  General Failures  . . . . . . . . . . . . . . . . . .  11
       4.3.4.  Transient Failures  . . . . . . . . . . . . . . . . .  12
     4.4.  JSON Report Schema  . . . . . . . . . . . . . . . . . . .  12
     4.5.  Policy Samples  . . . . . . . . . . . . . . . . . . . . .  15
   5.  Report Delivery . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Report Filename . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Compression . . . . . . . . . . . . . . . . . . . . . . .  17
     5.3.  Email Transport . . . . . . . . . . . . . . . . . . . . .  17
       5.3.1.  Example Report  . . . . . . . . . . . . . . . . . . .  19
     5.4.  HTTPS Transport . . . . . . . . . . . . . . . . . . . . .  19
     5.5.  Delivery Retry  . . . . . . . . . . . . . . . . . . . . .  20
     5.6.  Metadata Variances  . . . . . . . . . . . . . . . . . . .  20
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  Message Headers . . . . . . . . . . . . . . . . . . . . .  20
     6.2.  Report Type . . . . . . . . . . . . . . . . . . . . . . .  21
     6.3.  +gzip Media Type Suffix . . . . . . . . . . . . . . . . .  22
     6.4.  application/tlsrpt+json Media Type  . . . . . . . . . . .  23
     6.5.  application/tlsrpt+gzip Media Type  . . . . . . . . . . .  24
     6.6.  STARTTLS Validation Result Types  . . . . . . . . . . . .  25
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  27
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Appendix A.  Example Reporting Policy . . . . . . . . . . . . . .  32
     A.1.  Report Using MAILTO . . . . . . . . . . . . . . . . . . .  32
     A.2.  Report Using HTTPS  . . . . . . . . . . . . . . . . . . .  32
   Appendix B.  Example JSON Report  . . . . . . . . . . . . . . . .  32
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

Top      ToC       Page 4 
1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to establish secure SMTP sessions over TLS.  The protocol
   design uses an approach that has come to be known as "Opportunistic
   Security" (OS) [RFC7435].  This method maintains interoperability
   with clients that do not support STARTTLS, but it means that any
   attacker could potentially eavesdrop on a session.  An attacker could
   perform a downgrade or interception attack by deleting parts of the
   SMTP session (such as the "250 STARTTLS" response) or redirect the
   entire SMTP session (perhaps by overwriting the resolved MX record of
   the delivery domain).

   Because such "downgrade attacks" are not necessarily apparent to the
   receiving MTA, this document defines a mechanism for sending domains
   to report on failures at multiple stages of the MTA-to-MTA
   conversation.

   Recipient domains may also use the mechanisms defined by MTA-STS
   [RFC8461] or DANE [RFC6698] to publish additional encryption and
   authentication requirements; this document defines a mechanism for
   sending domains that are compatible with MTA-STS or DANE to share
   success and failure statistics with recipient domains.

   Specifically, this document defines a reporting schema that covers
   failures in routing, DNS resolution, and STARTTLS negotiation; policy
   validation errors for both DANE [RFC6698] and MTA-STS [RFC8461]; and
   a standard TXT record that recipient domains can use to indicate
   where reports in this format should be sent.  The report can also
   serve as a heartbeat to indicate that systems are successfully
   negotiating TLS during sessions as expected.

   This document is intended as a companion to the specification for
   SMTP MTA-STS [RFC8461] and adds reporting abilities for those
   implementing DANE [RFC7672].

1.1.  Terminology

   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.

Top      ToC       Page 5 
   We also define the following terms for further use in this document:

   o  MTA-STS Policy: A mechanism by which administrators can specify
      the expected TLS availability, presented identity, and desired
      actions for a given email recipient domain.  MTA-STS is defined in
      [RFC8461].

   o  DANE Policy: A mechanism by which administrators can use DNSSEC to
      commit an MTA to support STARTTLS and to publish criteria to be
      used to validate its presented certificates.  DANE for SMTP is
      defined in [RFC7672], with the base specification defined in
      [RFC6698] (and updated by [RFC7671]).

   o  TLSRPT (TLS Reporting) Policy: A policy specifying the endpoint to
      which Sending MTAs should deliver reports.

   o  Policy Domain: The domain against which a TLSRPT, an MTA-STS, or a
      DANE policy is defined.  For TLSRPT and MTA-STS, this is typically
      the same as the envelope recipient domain [RFC5321], but when mail
      is routed to a "smarthost" gateway by local policy, the
      "smarthost" domain name is used instead.  For DANE, the Policy
      Domain is the "TLSA base domain" of the receiving SMTP server as
      described in Section 2.2.3 of RFC 7672 and Section 3 of RFC 6698.

   o  Sending MTA: The MTA initiating the relay of an email message.

   o  Aggregate Report URI (rua): A comma-separated list of locations
      where the report is to be submitted.

   o  ABNF: Augmented Backus-Naur Form, a syntax for formally specifying
      syntax, defined in [RFC5234] and [RFC7405].

2.  Related Technologies

   o  This document is intended as a companion to the specification for
      SMTP MTA-STS [RFC8461].

   o  SMTP TLSRPT defines a mechanism for sending domains that are
      compatible with MTA-STS or DANE to share success and failure
      statistics with recipient domains.  DANE is defined in [RFC6698],
      and MTA-STS is defined in [RFC8461].

Top      ToC       Page 6 
3.  Reporting Policy

   A domain publishes a record to its DNS indicating that it wishes to
   receive reports.  These SMTP TLSRPT policies are distributed via DNS
   from the Policy Domain's zone as TXT records (similar to Domain-based
   Message Authentication, Reporting, and Conformance (DMARC) policies)
   under the name "_smtp._tls".  For example, for the Policy Domain
   "example.com", the recipient's TLSRPT policy can be retrieved from
   "_smtp._tls.example.com".

   Policies consist of the following directives:

   o  "v": This document defines version 1 of TLSRPT, for which this
      value MUST be equal to "TLSRPTv1".  Other versions may be defined
      in later documents.

   o  "rua": A URI specifying the endpoint to which aggregate
      information about policy validation results should be sent (see
      Section 4, "Reporting Schema", for more information).  Two URI
      schemes are supported: "mailto" and "https".  As with DMARC
      [RFC7489], the Policy Domain can specify a comma-separated list of
      URIs.

   o  In the case of "https", reports should be submitted via POST
      [RFC7231] to the specified URI.  Report submitters MAY ignore
      certificate validation errors when submitting reports via HTTPS
      POST.

   o  In the case of "mailto", reports should be submitted to the
      specified email address [RFC6068].  When sending failure reports
      via SMTP, Sending MTAs MUST deliver reports despite any TLS-
      related failures and SHOULD NOT include this SMTP session in the
      next report.  This may mean that the reports are delivered
      unencrypted.  Reports sent via SMTP MUST contain a valid
      DomainKeys Identified Mail (DKIM) [RFC6376] signature by the
      reporting domain.  Reports lacking such a signature MUST be
      ignored by the recipient.  DKIM signatures MUST NOT use the "l="
      attribute to limit the body length used in the signature.  This
      ensures attackers cannot append extraneous or misleading data to a
      report without breaking the signature.  The DKIM TXT record SHOULD
      contain the appropriate service type declaration, "s=tlsrpt".  If
      not present, the receiving system MAY ignore reports lacking that
      service type.

   Sample DKIM record:

      dkim_selector._domainkey.example.com TXT
            "v=DKIM1;k=rsa;s=tlsrpt;p=Mlf4qwSZfase4fa=="

Top      ToC       Page 7 
   The formal definition of the "_smtp._tls" TXT record, defined using
   [RFC5234] and [RFC7405], is as follows:

        tlsrpt-record     = tlsrpt-version 1*(field-delim tlsrpt-field)
                            [field-delim]

        field-delim       = *WSP ";" *WSP

        tlsrpt-field      = tlsrpt-rua /        ; Note that the
                            tlsrpt-extension    ; tlsrpt-rua record is
                                                ; required.

        tlsrpt-version    = %s"v=TLSRPTv1"

        tlsrpt-rua        = %s"rua="
                            tlsrpt-uri *(*WSP "," *WSP tlsrpt-uri)

        tlsrpt-uri        = URI
                            ; "URI" is imported from [RFC3986];
                            ; commas (ASCII 0x2C), exclamation
                            ; points (ASCII 0x21), and semicolons
                            ; (ASCII 0x3B) MUST be encoded

        tlsrpt-extension  = tlsrpt-ext-name "=" tlsrpt-ext-value

        tlsrpt-ext-name   = (ALPHA / DIGIT) *31(ALPHA /
                            DIGIT / "_" / "-" / ".")

        tlsrpt-ext-value  = 1*(%x21-3A / %x3C / %x3E-7E)
                            ; chars excluding "=", ";", SP, and control
                            ; chars

   If multiple TXT records for "_smtp._tls" are returned by the
   resolver, records that do not begin with "v=TLSRPTv1;" are discarded.
   If the number of resulting records is not one, senders MUST assume
   the recipient domain does not implement TLSRPT.  If the resulting TXT
   record contains multiple strings (as described in Section 3.3 of
   [RFC7208]), then the record MUST be treated as if those strings are
   concatenated without adding spaces.

   The record supports the ability to declare more than one rua, and if
   there exists more than one, the reporter MAY attempt to deliver to
   each of the supported rua destinations.  A receiver MAY opt to only
   attempt delivery to one of the endpoints; however, the report SHOULD
   NOT be considered successfully delivered until one of the endpoints
   accepts delivery of the report.

Top      ToC       Page 8 
   Parsers MUST accept TXT records that are syntactically valid (i.e.,
   valid key/value pairs separated by semicolons) and implement a
   superset of this specification, in which case unknown fields SHALL be
   ignored.

3.1.  Example Reporting Policy

3.1.1.  Report Using MAILTO

            _smtp._tls.example.com. IN TXT \
                    "v=TLSRPTv1;rua=mailto:reports@example.com"

3.1.2.  Report Using HTTPS

           _smtp._tls.example.com. IN TXT \
                   "v=TLSRPTv1; \
                   rua=https://reporting.example.com/v1/tlsrpt"

4.  Reporting Schema

   The report is composed as a plaintext file encoded in the Internet
   JSON (I-JSON) format [RFC7493].

   Aggregate reports contain the following fields:

   o  Report metadata:

      *  The organization responsible for the report

      *  Contact information for one or more responsible parties for the
         contents of the report

      *  A unique identifier for the report

      *  The reporting date range for the report

   o  Policy, consisting of:

      *  One of the following policy types: (1) the MTA-STS Policy
         applied (as a string), (2) the DANE TLSA record applied (as a
         string, with each RR entry of the RRset listed and separated by
         a semicolon), and (3) the literal string "no-policy-found", if
         neither a DANE nor MTA-STS Policy could be found.

      *  The domain for which the policy is applied

      *  The MX host

Top      ToC       Page 9 
   o  Aggregate counts, comprising result type, Sending MTA IP,
      receiving MTA hostname, session count, and an optional additional
      information field containing a URI for recipients to review
      further information on a failure type.

   Note that the failure types are non-exclusive; an aggregate report
   may contain overlapping "counts" of failure types when a single send
   attempt encountered multiple errors.  Reporters may report multiple
   applied policies (for example, an MTA-STS Policy and a DANE TLSA
   record for the same domain and MX).  Because of this, even in the
   case where only a single policy was applied, the "policies" field of
   the report body MUST be an array and not a singular value.

   In the case of multiple failure types, the "failure-details" array
   would contain multiple entries.  Each entry would have its own set of
   information pertaining to that failure type.

4.1.  Report Time Frame

   The report SHOULD cover a full day, from 00:00-24:00 UTC.  This
   should allow for easier correlation of failure events.  To avoid
   unintentionally overloading the system processing the reports, the
   reports should be delivered after some delay, perhaps several hours.

   As an example, a sending site might want to introduce a random delay
   of up to four hours:

          func generate_sleep_delay() {
            min_delay = 1
            max_delay = 14400
            rand = random(min_delay, max_delay)
            return rand
          }

          func generate_report(policy_domain) {
            do_rpt_work(policy_domain)
            send_rpt(policy_domain)
          }

          func generate_tlsrpt() {
            sleep(generate_sleep_delay())
            for policy_domain in list_of_tlsrpt_enabled_domains {
              generate_report(policy_domain)
            }
          }

Top      ToC       Page 10 
4.2.  Delivery Summary

4.2.1.  Success Count

   o  "total-successful-session-count": This indicates that the Sending
      MTA was able to successfully negotiate a policy-compliant TLS
      connection and serves to provide a "heartbeat" to receiving
      domains that signifies reporting is functional and tabulating
      correctly.  This field contains an aggregate count of successful
      connections for the reporting system.

4.2.2.  Failure Count

   o  "total-failure-session-count": This indicates that the Sending MTA
      was unable to successfully establish a connection with the
      receiving platform.  Section 4.3, "Result Types", will elaborate
      on the failed negotiation attempts.  This field contains an
      aggregate count of failed connections.

4.3.  Result Types

   The list of result types will start with the minimal set below and is
   expected to grow over time based on real-world experience.  The
   initial set is outlined in Sections 4.3.1 to 4.3.4:

4.3.1.  Negotiation Failures

   o  "starttls-not-supported": This indicates that the recipient MX did
      not support STARTTLS.

   o  "certificate-host-mismatch": This indicates that the certificate
      presented did not adhere to the constraints specified in the MTA-
      STS or DANE policy, e.g., if the MX hostname does not match any
      identities listed in the subject alternative name (SAN) [RFC5280].

   o  "certificate-expired": This indicates that the certificate has
      expired.

   o  "certificate-not-trusted": This is a label that covers multiple
      certificate-related failures that include, but are not limited to,
      errors such as untrusted/unknown certification authorities (CAs),
      certificate name constraints, certificate chain errors, etc.  When
      using this declaration, the reporting MTA SHOULD utilize the
      "failure-reason-code" to provide more information to the receiving
      entity.

Top      ToC       Page 11 
   o  "validation-failure": This indicates a general failure for a
      reason not matching a category above.  When using this
      declaration, the reporting MTA SHOULD utilize the "failure-reason-
      code" to provide more information to the receiving entity.

4.3.2.  Policy Failures

4.3.2.1.  DANE-Specific Policy Failures

   o  "tlsa-invalid": This indicates a validation error in the TLSA
      record associated with a DANE policy.  None of the records in the
      RRset were found to be valid.

   o  "dnssec-invalid": This indicates that no valid records were
      returned from the recursive resolver.

   o  "dane-required": This indicates that the sending system is
      configured to require DANE TLSA records for all the MX hosts of
      the destination domain, but no DNSSEC-validated TLSA records were
      present for the MX host that is the subject of the report.
      Mandatory DANE for SMTP is described in Section 6 of [RFC7672].
      Such policies may be created by mutual agreement between two
      organizations that frequently exchange sensitive content via
      email.

4.3.2.2.  MTA-STS-specific Policy Failures

   o  "sts-policy-fetch-error": This indicates a failure to retrieve an
      MTA-STS policy, for example, because the policy host is
      unreachable.

   o  "sts-policy-invalid": This indicates a validation error for the
      overall MTA-STS Policy.

   o  "sts-webpki-invalid": This indicates that the MTA-STS Policy could
      not be authenticated using PKIX validation.

4.3.3.  General Failures

   When a negotiation failure cannot be categorized into one of the
   "Negotiation Failures" stated above, the reporter SHOULD use the
   "validation-failure" category.  As TLS grows and becomes more
   complex, new mechanisms may not be easily categorized.  This allows
   for a generic feedback category.  When this category is used, the
   reporter SHOULD also use "failure-reason-code" to give some feedback
   to the receiving entity.  This is intended to be a short text field,
   and the contents of the field should be an error code or error text,
   such as "X509_V_ERR_UNHANDLED_CRITICAL_CRL_EXTENSION".

Top      ToC       Page 12 
4.3.4.  Transient Failures

   Transient errors due to too-busy networks, TCP timeouts, etc., are
   not required to be reported.

4.4.  JSON Report Schema

   The JSON schema is derived from the HTTP Public Key Pinning (HPKP)
   JSON schema; see Section 3 of [RFC7469].

 {
   "organization-name": organization-name,
   "date-range": {
     "start-datetime": date-time,
     "end-datetime": date-time
   },
   "contact-info": email-address,
   "report-id": report-id,
   "policies": [{
     "policy": {
       "policy-type": policy-type,
       "policy-string": policy-string,
       "policy-domain": domain,
       "mx-host": mx-host-pattern
     },
     "summary": {
       "total-successful-session-count": total-successful-session-count,
       "total-failure-session-count": total-failure-session-count
     },
     "failure-details": [
       {
         "result-type": result-type,
         "sending-mta-ip": ip-address,
         "receiving-mx-hostname": receiving-mx-hostname,
         "receiving-mx-helo": receiving-mx-helo,
         "receiving-ip": receiving-ip,
         "failed-session-count": failed-session-count,
         "additional-information": additional-info-uri,
         "failure-reason-code": failure-reason-code
         }
       ]
     }
   ]
 }


                            JSON Report Format

Top      ToC       Page 13 
   o  "organization-name": The name of the organization responsible for
      the report.  It is provided as a string.

   o  "date-time": The date-time indicates the start and end times for
      the report range.  It is provided as a string formatted according
      to "Internet Date/Time Format", Section 5.6 of [RFC3339].  The
      report should be for a full UTC day, 00:00-24:00.

   o  "email-address": The contact information for the party responsible
      for the report.  It is provided as a string formatted according to
      "Addr-Spec Specification", Section 3.4.1 of [RFC5322].

   o  "report-id": A unique identifier for the report.  Report authors
      may use whatever scheme they prefer to generate a unique
      identifier.  It is provided as a string.

   o  "policy-type": The type of policy that was applied by the sending
      domain.  Presently, the only three valid choices are "tlsa",
      "sts", and the literal string "no-policy-found".  It is provided
      as a string.

   o  "policy-string": An encoding of the applied policy as a JSON array
      of strings, whether it's a TLSA record ([RFC6698], Section 2.3) or
      an MTA-STS Policy.  Examples follow in the next section.

   o  "domain": The Policy Domain against which the MTA-STS or DANE
      policy is defined.  In the case of Internationalized Domain Names
      [RFC5891], the domain MUST consist of the Punycode-encoded
      A-labels [RFC3492] and not the U-labels.

   o  "mx-host-pattern": In the case where "policy-type" is "sts", it's
      the pattern of MX hostnames from the applied policy.  It is
      provided as a JSON array of strings and is interpreted in the same
      manner as the rules in "MX Host Validation"; see Section 4.1 of
      [RFC8461].  In the case of Internationalized Domain Names
      [RFC5891], the domain MUST consist of the Punycode-encoded
      A-labels [RFC3492] and not the U-labels.

   o  "result-type": A value from Section 4.3, "Result Types", above.

   o  "ip-address": The IP address of the Sending MTA that attempted the
      STARTTLS connection.  It is provided as a string representation of
      an IPv4 (see below) or IPv6 [RFC5952] address in dot-decimal or
      colon-hexadecimal notation.

   o  "receiving-mx-hostname": The hostname of the receiving MTA MX
      record with which the Sending MTA attempted to negotiate a
      STARTTLS connection.

Top      ToC       Page 14 
   o  "receiving-mx-helo" (optional): The HELLO (HELO) or Extended HELLO
      (EHLO) string from the banner announced during the reported
      session.

   o  "receiving-ip": The destination IP address that was used when
      creating the outbound session.  It is provided as a string
      representation of an IPv4 (see below) or IPv6 [RFC5952] address in
      dot-decimal or colon-hexadecimal notation.

   o  "total-successful-session-count": The aggregate count (an integer,
      encoded as a JSON number) of successfully negotiated TLS-enabled
      connections to the receiving site.

   o  "total-failure-session-count": The aggregate count (an integer,
      encoded as a JSON number) of failures to negotiate a TLS-enabled
      connection to the receiving site.

   o  "failed-session-count": The number of (attempted) sessions that
      match the relevant "result-type" for this section (an integer,
      encoded as a JSON number).

   o  "additional-info-uri" (optional): A URI [RFC3986] that points to
      additional information around the relevant "result-type".  For
      example, this URI might host the complete certificate chain
      presented during an attempted STARTTLS session.

   o  "failure-reason-code": A text field to include a TLS-related error
      code or error message.

   For report purposes, an IPv4 address is defined via the following
   ABNF:

     IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
     dec-octet     = DIGIT                 ; 0-9
                   / %x31-39 DIGIT         ; 10-99
                   / "1" 2DIGIT            ; 100-199
                   / "2" %x30-34 DIGIT     ; 200-249
                   / "25" %x30-35          ; 250-255

   And an IPv6 address is defined via the following ABNF:


     IPv6address = <as defined in [RFC5954]>

Top      ToC       Page 15 
4.5.  Policy Samples

   Part of the report body includes the policy that is applied when
   attempting relay to the destination.

   For DANE TLSA policies, this is a JSON array of strings each
   representing the RDATA of a single TLSA resource record as a space-
   separated list of its four TLSA fields; the fields are in
   presentation format (defined in [RFC6698], Section 2.2) with no
   internal spaces or grouping parentheses:

       [
       "3 0 1 1F850A337E6DB9C609C522D136A475638CC43E1ED424F8EEC8513
              D747D1D085D",
       "3 0 1 12350A337E6DB9C6123522D136A475638CC43E1ED424F8EEC8513
              D747D1D1234"
       ]

   For MTA-STS policies, this is an array of JSON strings that
   represents the policy that is declared by the receiving site,
   including any errors that may be present.  Note that where there are
   multiple "mx" values, they must be listed as separate "mx" elements
   in the policy array rather than as a single nested "mx" sub-array.

                       [
                       "version: STSv1",
                       "mode: testing",
                       "mx: mx1.example.com",
                       "mx: mx2.example.com",
                       "mx: mx.backup-example.com",
                       "max_age: 604800"
                       ]



(page 15 continued on part 2)

Next Section