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Versions: (draft-brotman-smtp-tlsrpt) 00 01 02 03

Using TLS in Applications                                    D. Margolis
Internet-Draft                                               Google, Inc
Intended status: Standards Track                              A. Brotman
Expires: August 19, 2017                                    Comcast, Inc
                                                         B. Ramakrishnan
                                                             Yahoo!, Inc
                                                                J. Jones
                                                          Microsoft, Inc
                                                               M. Risher
                                                             Google, Inc
                                                       February 15, 2017


                           SMTP TLS Reporting
                     draft-ietf-uta-smtp-tlsrpt-03

Abstract

   A number of protocols exist for establishing encrypted channels
   between SMTP Mail Transfer Agents, including STARTTLS [RFC3207], DANE
   [RFC6698], and SMTP MTA STS (TODO: Add ref).  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 attackers and
   diagnose unintentional misconfigurations.

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 http://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 August 19, 2017.






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Copyright Notice

   Copyright (c) 2017 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
   (http://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   4
   3.  Reporting Policy  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Example Reporting Policy  . . . . . . . . . . . . . . . .   5
       3.1.1.  Report using MAILTO . . . . . . . . . . . . . . . . .   5
       3.1.2.  Report using HTTPS  . . . . . . . . . . . . . . . . .   5
   4.  Reporting Schema  . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Report Time-frame . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Delivery Summary  . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Success Count . . . . . . . . . . . . . . . . . . . .   7
       4.2.2.  Failure Count . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Result Types  . . . . . . . . . . . . . . . . . . . . . .   7
       4.3.1.  Routing Failures  . . . . . . . . . . . . . . . . . .   7
       4.3.2.  Negotiation Failures  . . . . . . . . . . . . . . . .   7
       4.3.3.  Policy Failures . . . . . . . . . . . . . . . . . . .   8
       4.3.4.  General Failures  . . . . . . . . . . . . . . . . . .   8
       4.3.5.  Transient Failures  . . . . . . . . . . . . . . . . .   8
   5.  Report Delivery . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Report Filename . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Compression . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Email Transport . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  HTTPS Transport . . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Delivery Retry  . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Appendix 1: Example Reporting Policy  . . . . . . . . . . . .  12
     8.1.  Report using MAILTO . . . . . . . . . . . . . . . . . . .  12
     8.2.  Report using HTTPS  . . . . . . . . . . . . . . . . . . .  12
   9.  Appendix 2: JSON Report Schema  . . . . . . . . . . . . . . .  12
   10. Appendix 3: Example JSON Report . . . . . . . . . . . . . . .  15



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   11. Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to establish secure SMTP sessions over TLS.  The protocol
   design is based on "Opportunistic Security" (OS) [RFC7435], which
   provides interoperability for clients that do not support STARTTLS
   but means that any attacker who can delete 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) can perform a downgrade or interception attack.

   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 parts of the MTA-to-MTA
   conversation.

   Recipient domains may also use the mechanisms defined by MTA-STS
   (TODO: Add ref) 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, STARTTLS negotiation, and both DANE [RFC6698]
   and MTA-STS (TODO: Add ref) policy validation errors, and a standard
   TXT record that recipient domains can use to indicate where reports
   in this format should be sent.

   This document is intended as a companion to the specification for
   SMTP MTA Strict Transport Security (MTA-STS, TODO: Add ref).

1.1.  Terminology

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

   We also define the following terms for further use in this document:

   o  STS Policy: A definition of the expected TLS availability,
      behavior, and desired actions for a given domain when a sending
      MTA encounters problems in negotiating a secure channel.  STS is
      defined in [TODO]





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   o  DANE Policy: A mechanism for enabling the administrators of domain
      names to specify the keys used in that domain's TLS servers.  DANE
      is defined in [RFC6698]

   o  TLSRPT Policy: A policy specifying the endpoint to which sending
      MTAs should deliver reports.

   o  Policy Domain: The domain against which an STS or DANE Policy is
      defined.

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

2.  Related Technologies

   o  This document is intended as a companion to the specification for
      SMTP MTA Strict Transport Security (MTA-STS, TODO: Add ref).

   o  The Public Key Pinning Extension for HTTP [RFC7469] contains a
      JSON-based definition for reporting individual pin validation
      failures.

   o  The Domain-based Message Authentication, Reporting, and
      Conformance (DMARC) [RFC7489] contains an XML-based reporting
      format for aggregate and detailed email delivery errors.

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 DMARC
   policies) under the name "smtp-tlsrpt".  For example, for the Policy
   Domain "example.com", the recipient's TLSRPT policy can be retrieved
   from "smtp-tlsrpt.example.com".

   Policies consist of the following directives:

   o  "v": This value MUST be equal to "TLSRPTv1".

   o  "rua": A URI specifying the endpoint to which aggregate
      information about policy failures should be sent (see the section
      _Reporting_ _Schema_ for more information).  Two URI schemes are
      supported: "mailto" and "https".

      *  In the case of "https", reports should be submitted via POST
         ([RFC2818]) to the specified URI.

      *  In the case of "mailto", reports should be submitted to the
         specified email address.  When sending failure reports via



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         SMTP, sending MTAs MUST NOT honor SMTP STS or DANE TLSA
         failures.

   o  "ruf": Future use.  (There may also be a need for enabling more
      detailed "forensic" reporting during initial stages of a
      deployment.  To address this, the authors consider the possibility
      of an optional additional "forensic reporting mode" in which more
      details--such as certificate chains and MTA banners--may be
      reported.)

   The formal definition of the "smtp-tlsrpt" TXT record, defined using
   [RFC5234], is as follows:

    tlsrpt-record    = tlsrpt-version *WSP %x3B tlsrpt-rua

    tlsrpt-version   = "v" *WSP "=" *WSP %x54 %x4C %x53
                       %x52 %x50 %x54 %x76 %x31

    tlsrpt-rua       = "rua" *WSP "=" *WSP tlsrpt-uri

    tlsrpt-uri       = URI
                     ; "URI" is imported from [@!RFC3986]; commas (ASCII
                     ; 0x2C) and exclamation points (ASCII 0x21)
                     ; MUST be encoded; the numeric portion MUST fit
                     ; within an unsigned 64-bit integer

   If multiple TXT records for "smtp-tlsrpt" are returned by the
   resolver, records which 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.

3.1.  Example Reporting Policy

3.1.1.  Report using MAILTO

              smtp-tlsrpt.example.com. IN TXT \
                  "v=TLSRPTv1;rua=mailto:reports@example.com"

3.1.2.  Report using HTTPS

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








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4.  Reporting Schema

   The report is composed as a plain text file encoded in the JSON
   format ([RFC7159]).

   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 SMTP MTA STS policy
         applied (as a string) (2) The DANE TLSA record applied (as a
         string) (3) The literal string "no-policy-found", if neither a
         TLSA nor MTA-STS policy could be found.

      *  The domain for which the policy is applied

      *  The MX host

      *  An identifier for the policy (where applicable)

   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.

4.1.  Report Time-frame

   The report SHOULD cover a full day, from 0000-2400 UTC.  This should
   allow for easier correlation of failure events.







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4.2.  Delivery Summary

4.2.1.  Success Count

   o  "success-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
      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  "failure-count": This indicates that the sending MTA was unable to
      successfully establish a connection with the receiving platform.
      The "Result Types" section 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:

4.3.1.  Routing Failures

   o  "mx-mismatch": This indicates that the MX resolved for the
      recipient domain did not match the MX constraint specified in the
      policy.

4.3.2.  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 STS
      or DANE policy, e.g.  if the CN field did not match the hostname
      of the MX.

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

   o  "certificate-not-trusted": This a label that covers multiple
      certificate related failures that include, but not limited to
      errors such as untrusted/unknown CAs, certificate name contraints,
      certificate chain errors etc.  When using this declaration, the



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      reporting MTA SHOULD utilize the "failure-reason" to provide more
      information to the receiving entity.

   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"
      to provide more information to the receiving entity.

4.3.3.  Policy Failures

4.3.3.1.  DANE-specific Policy Failures

   o  "tlsa-invalid": This indicates a validation error in the TLSA
      record associated with a DANE policy.

   o  "dnssec-invalid": This indicates a failure to authenticate DNS
      records for a Policy Domain with a published TLSA record.

4.3.3.2.  STS-specific Policy Failures

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

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

4.3.4.  General Failures

   When a negotiation failure can not 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 the "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".

4.3.5.  Transient Failures

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

5.  Report Delivery

   Reports can be delivered either as an email message via SMTP or via
   HTTP POST.




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5.1.  Report Filename

   The filename is typically constructed using the following ABNF:

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

       unique-id = 1*(ALPHA / DIGIT)

       sender = 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

       extension = "json" / "json.gz"

   The extension MUST be "json" for a plain JSON file, or "json.gz" for
   a JSON file compressed using GZIP.

   "unique-id" allows an optional unique ID generated by the Sending MTA
   to distinguish among multiple reports generated simultaneously by
   different sources within the same Policy Domain.  For example, this
   is a possible filename for the gzip file of a report to the Policy
   Domain "example.net" from the Sending MTA "mail.sender.example.com":

 `mail.sender.example.com!example.net!1470013207!1470186007!001.json.gz`

5.2.  Compression

   The report SHOULD be subjected to GZIP compression for both email and
   HTTPS transport.  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); compressing the file increases the
   chances of acceptance of the report at some compute cost.








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5.3.  Email Transport

   The report MAY be delivered by email.  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.

   If compressed, the report should use the media type "application/
   gzip" if compressed (see [RFC6713]), and "text/json" otherwise.

   The [RFC5322].Subject field for individual report submissions SHOULD
   conform to the following ABNF:

     tlsrpt-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:"
                      msg-id                          ; from RFC 5322

   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 Sending MTA generating the report.  The
   purpose of the Report-ID: portion of the field is to enable the
   Policy Domain to identify and ignore duplicate reports that might be
   sent by a Sending MTA.

   For instance, this is a possible Subject field for a report to the
   Policy Domain "example.net" from the Sending MTA
   "mail.sender.example.com".  It is line-wrapped as allowed by
   [RFC5322]:

            Subject: Report Domain: example.net
                Submitter: mail.sender.example.com
                Report-ID: <735ff.e317+bf22029@mailexample.net>

   Note that, when sending failure reports via SMTP, sending MTAs MUST
   NOT honor SMTP STS or DANE TLSA failures.

5.4.  HTTPS Transport

   The report MAY be delivered by POST to HTTPS.  If compressed, the
   report should use the media type "application/gzip" (see [RFC6713]),
   and "text/json" otherwise.






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5.5.  Delivery Retry

   In the event of a delivery failure, regardless of the delivery
   method, a sender SHOULD attempt redelivery for up to 24hrs after the
   initial attempt.  As previously stated the reports are optional, so
   while it is ideal to attempt redelivery, it is not required.  If
   multiple retries are attempted, they should be on a logarithmic
   scale.

6.  IANA Considerations

   There are no IANA considerations at this time.

7.  Security Considerations

   SMTP TLS Reporting provides transparency into misconfigurations or
   attempts to intercept or tamper with mail between hosts who support
   STARTTLS.  There are several security risks presented by the
   existence of this reporting channel:

   o  Flooding of the Aggregate report URI (rua) endpoint: An attacker
      could flood the endpoint and prevent the receiving domain from
      accepting additional reports.  This type of Denial-of-Service
      attack would limit visibility into STARTTLS failures, leaving the
      receiving domain blind to an ongoing attack.

   o  Untrusted content: An attacker could inject malicious code into
      the report, opening a vulnerability in the receiving domain.
      Implementers are advised to take precautions against evaluating
      the contents of the report.

   o  Report snooping: An attacker could create a bogus TLSRPT record to
      receive statistics about a domain the attacker does not own.
      Since an attacker able to poison DNS is already able to receive
      counts of SMTP connections (and, absent DANE or MTA-STS policies,
      actual SMTP message payloads), this does not present a significant
      new vulnerability.

   o  Reports as DDoS: TLSRPT allows specifying destinations for the
      reports that are outside the authority of the Policy Domain, which
      allows domains to delegate processing of reports to a partner
      organization.  However, an attacker who controls the Policy Domain
      DNS could also use this mechanism to direct the reports to an
      unwitting victim, flooding that victim with excessive reports.
      DMARC [RFC7489] defines an elegant solution for verifying
      delegation; however, since the attacker had less ability to
      generate large reports than with DMARC failures, and since the




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      reports are generated by the sending MTA, such a delegation
      mechanism is left for a future version of this specification.

8.  Appendix 1: Example Reporting Policy

8.1.  Report using MAILTO

            smtp-tlsrpt.mail.example.com. IN TXT \
                    "v=TLSRPTv1;rua=mailto:reports@example.com"

8.2.  Report using HTTPS

           smtp-tlsrpt.mail.example.com. IN TXT \
                   "v=TLSRPTv1; \
                   rua=https://reporting.example.com/v1/tlsrpt"

9.  Appendix 2: JSON Report Schema

   The JSON schema is derived from the HPKP JSON schema [RFC7469] (cf.
   Section 3)































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         {
           "organization-name": organization-name,
           "date-range": {
             "start-datetime": date-time,
             "end-datetime": date-time
           },
           "contact-info": email-address,
           "report-id": report-id,
           "policy": {
             "policy-type": policy-type,
             "policy-string": policy-string,
             "policy-domain": domain,
             "mx-host": mx-host-pattern
           },
           "summary": {
             "success-aggregate": total-successful-session-count,
             "failure-aggregate:" 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,
               "session-count": failed-session-count,
               "additional-information": additional-info-uri,
               "failure-reason-code": "Text body"
             }
           ]
         }

   Figure: JSON Report Format

   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 Section 5.6, "Internet Date/Time Format", of [RFC3339].  The
      report should be for a full UTC day, 0000-2400.

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



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   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": The string serialization of the policy, whether
      TLSA record or STS policy.  Any linefeeds from the original policy
      MUST be replaced with [SP].  TODO: Help with specifics.

   o  "domain": The Policy Domain upon which the policy was applied.
      For messages sent to "user@example.com" this field would contain
      "example.com".  It is provided as a string.

   o  "mx-host-pattern": The pattern of MX hostnames from the applied
      policy.  It is provided as a string, and is interpreted in the
      same manner as the "Checking of Wildcard Certificates" rules in
      Section 6.4.3 of [RFC6125].

   o  "result-type": A value from the _Result Types_ section 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 or IPv6 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.

   o  "receiving-mx-helo": (optional) The HELO or EHLO string from the
      banner announced during the reported session.

   o  "success-aggregate": The aggregate number (integer) of
      successfully negotiated SSL-enabled connections to the receiving
      site.

   o  "failure-aggregate": The aggregate number (integer) of failures to
      negotiate an SSL-enabled connection to the receiving site.

   o  "session-count": The number of (attempted) sessions that match the
      relevant "result-type" for this section.

   o  "additional-info-uri": An optional URI pointing to additional
      information around the relevant "result-type".  For example, this
      URI might host the complete certificate chain presented during an
      attempted STARTTLS session.





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

10.  Appendix 3: Example JSON Report

{
  "organization-name": "Company-X",
  "date-range": {
    "start-datetime": "2016-04-01T00:00:00Z",
    "end-datetime": "2016-04-01T23:59:59Z"
  },
  "contact-info": "sts-reporting@company-x.com",
  "report-id": "5065427c-23d3-47ca-b6e0-946ea0e8c4be",
  "policy": {
    "policy-type": "sts",
    "policy-string": "{ \"version\": \"STSv1\",\"mode\": \"report\", \"mx\": [\"*.mail.company-y.com\"], \"max_age\": 86400 }",
    "policy-domain": "company-y.com",
    "mx-host": "*.mail.company-y.com"
  },
  "summary": {
    "success-aggregate": 5326,
    "failure-aggregate": 303
  }
  "failure-details": [{
    "result-type": "certificate-expired",
    "sending-mta-ip": "98.136.216.25",
    "receiving-mx-hostname": "mx1.mail.company-y.com",
    "session-count": 100
  }, {
    "result-type": "starttls-not-supported",
    "sending-mta-ip": "98.22.33.99",
    "receiving-mx-hostname": "mx2.mail.company-y.com",
    "session-count": 200,
    "additional-information": "hxxps://reports.company-x.com/
      report_info?id=5065427c-23d3#StarttlsNotSupported"
  }, {
    "result-type: "validation-failure",
    "sending-mta-ip": "47.97.15.2",
    "receiving-mx-hostname: "mx-backup.mail.company-y.com",
    "session-count": 3,
    "failure-error-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED"
  }]
}

   Figure: Example JSON report for a messages from Company-X to
   Company-Y, where 100 sessions were attempted to Company Y servers
   with an expired certificate and 200 sessions were attempted to
   Company Y servers that did not successfully respond to the "STARTTLS"



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   command.  Additionally 3 sessions failed due to
   "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED".

11.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/
              RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <http://www.rfc-editor.org/info/rfc3207>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://www.rfc-editor.org/info/rfc3339>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

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

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <http://www.rfc-editor.org/info/rfc6125>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <http://www.rfc-editor.org/info/rfc6698>.

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





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   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <http://www.rfc-editor.org/info/rfc7435>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

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

Authors' Addresses

   Daniel Margolis
   Google, Inc

   Email: dmargolis (at) google.com


   Alexander Brotman
   Comcast, Inc

   Email: alex_brotman (at) comcast.com


   Binu Ramakrishnan
   Yahoo!, Inc

   Email: rbinu (at) yahoo-inc (dot com)


   Janet Jones
   Microsoft, Inc

   Email: janet.jones (at) microsoft (dot com)


   Mark Risher
   Google, Inc

   Email: risher (at) google (dot com)




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