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Versions: (draft-margolis-smtp-sts) 00 draft-ietf-uta-mta-sts

Using TLS in Applications                                    D. Margolis
Internet-Draft                                                 M. Risher
Intended status: Standards Track                           N. Lidzborski
Expires: October 23, 2016                                      W. Chuang
                                                                 B. Long
                                                             Google, Inc
                                                         B. Ramakrishnan
                                                             Yahoo!, Inc
                                                              A. Brotman
                                                            Comcast, Inc
                                                                J. Jones
                                                          Microsoft, Inc
                                                               F. Martin
                                                                LinkedIn
                                                               K. Umbach
                                                                M. Laber
                          1&1 Mail & Media Development & Technology GmbH
                                                          April 23, 2016


                   SMTP MTA Strict Transport Security
                       draft-brotman-mta-sts-00

Abstract

   SMTP MTA-STS is a mechanism enabling mail service providers to
   declare their ability to receive TLS-secured connections, to declare
   particular methods for certificate validation, and to request that
   sending SMTP servers report upon and/or refuse to deliver messages
   that cannot be delivered securely.

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 October 20, 2016.




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Copyright Notice

   Copyright (c) 2016 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Differences from DANE . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Advantages of SMTP MTA-STS when compared to DANE  . .   4
       2.1.2.  Advantages of DANE when compared to SMTP MTA-STS  . .   5
   3.  Policy Semantics  . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Formal Definition . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  TXT Record  . . . . . . . . . . . . . . . . . . . . .   6
       3.1.2.  SMTP MTA-STS Policy . . . . . . . . . . . . . . . . .   6
     3.2.  Policy Expirations  . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Policy Updates  . . . . . . . . . . . . . . . . . . .   8
     3.3.  Policy Discovery & Authentication . . . . . . . . . . . .   8
     3.4.  Policy Validation . . . . . . . . . . . . . . . . . . . .   9
     3.5.  Policy Application  . . . . . . . . . . . . . . . . . . .   9
   4.  Failure Reporting . . . . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Appendix 1: Validation Pseudocode . . . . . . . . . . . . . .  12
   9.  Appendix 2: Domain Owner STS example record . . . . . . . . .  12
     9.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . . . .  12
   10. Appendix 3: DEEP Registration Elements  . . . . . . . . . . .  13
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to establish secure SMTP sessions over TLS.  In its current
   form, however, it fails to provide (a) message confidentiality --



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   because opportunistic STARTTLS is subject to downgrade attacks -- and
   (b) server authenticity -- because the trust relationship from email
   domain to MTA server identity is not cryptographically validated.

   While such _opportunistic_ encryption protocols provide a high
   barrier against passive man-in-the-middle traffic interception, any
   attacker who can delete parts of the SMTP session (such as the "250
   STARTTLS" response) or who can redirect the entire SMTP session
   (perhaps by overwriting the resolved MX record of the delivery
   domain) can perform such a downgrade or interception attack.

   This document defines a mechanism for recipient domains to publish
   policies specifying:

   o  whether MTAs sending mail to this domain can expect TLS support

   o  how MTAs can validate the TLS server certificate presented during
      mail delivery

   o  the expected identity of MXs that handle mail for this domain

   o  what an implementing sender should do with messages when TLS
      cannot be be successfully negotiated

   The mechanism described is separated into four logical components:

   1.  policy semantics: whether senders can expect a server for the
       recipient domain to support TLS encryption and how to validate
       the TLS certificate presented

   2.  policy discovery & authentication: how to discover a domain's
       published STS policy and determine the authenticity of that
       policy

   3.  failure report format: a mechanism for informing recipient
       domains about aggregate failure statistics

   4.  failure handling: what sending MTAs should do in the case of
       policy failures

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:




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   o  STS Policy: A definition of the expected TLS availability and
      behavior, as well as the desired actions for a given domain when a
      sending MTA encounters different results.

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

   o  Policy Authentication: Authentication of the STS policy retrieved
      for a recipient domain by the sender.

2.  Related Technologies

   The DANE TLSA record [RFC7672] is similar, in that DANE is also
   designed to upgrade opportunistic encryption into required
   encryption.  DANE requires DNSSEC [RFC4033] for the secure delivery
   of policies; the mechanism described here presents a variant for
   systems not yet supporting DNSSEC.

2.1.  Differences from DANE

   The primary difference between the mechanism described here and DANE
   is that DANE requires the use of DNSSEC to authenticate DANE TLSA
   records, whereas SMTP STS relies on the certificate authority (CA)
   system to avoid interception.  (For a thorough discussion of this
   trade-off, see the section _Security_ _Considerations_.)

   In addition, SMTP MTA-STS introduces a mechanism for failure
   reporting and a report-only mode, enabling offline ("report-only")
   deployments and auditing for compliance.

2.1.1.  Advantages of SMTP MTA-STS when compared to DANE

   SMTP MTA-STS offers the following advantages compared to DANE:

   o  Infrastructure: In comparison to DANE, this proposal does not
      require DNSSEC be deployed on either the sending or receiving
      domain.  In addition, the reporting feature of SMTP MTA-STS can be
      deployed to perform offline analysis of STARTTLS failures,
      enabling mail providers to gain insight into the security of their
      SMTP connections without the need to modify MTA codebases
      directly.

   o  Offline or report-only usage: DANE does not provide a reporting
      mechanism and does not have a concept of "report-only" for
      failures; as a result, a service provider cannot receive metrics
      on TLS acceptability without asking senders to enforce a given
      policy; similarly, senders who cannot enforce DANE constraints at
      send-time have no mechanism to provide recipients such metrics




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      from an offline (and potentially easier-to-deploy) logs-analysis
      batch process.

2.1.2.  Advantages of DANE when compared to SMTP MTA-STS

   o  Infrastructure: DANE may be easier for some providers to deploy.
      In particular, for providers who already support DNSSEC, SMTP MTA-
      STS would additionally require they host a HTTPS webserver and
      obtain a CA-signed X.509 certificate for the recipient domain.

   o  Security: DANE offers an advantage against policy-lookup DoS
      attacks; that is, while a DNSSEC-signed NXDOMAIN response to a
      DANE lookup authoritatively indicates the lack of a DANE record,
      such an option to authenticate policy non-existence does not exist
      when looking up a policy over plain DNS.

3.  Policy Semantics

   SMTP MTA-STS policies are distributed via a "well known" HTTPS
   endpoint in the Policy Domain.

   (Future implementations may move to alternate methods of policy
   discovery or distribution.  See the section _Future_ _Work_ for more
   discussion.)

   Policies MUST specify the following fields in JSON [RFC4627] format:

   o  "version": (plain-text, required).  Currently only "STS1" is
      supported.

   o  "mode": (plain-text, required).  If "enforce", the receiving MTA
      requests that messages be delivered only if they conform to the
      STS policy.  If "report" the receiving MTA requests that failure
      reports be delivered, as specified by the "rua" parameter.

   o  "mx": MX patterns (list of plain-text MX match patterns,
      required).  One or more comma-separated patterns matching the
      expected MX for this domain.  For example, ["_.example.com",
      "_.example.net"] indicates that mail for this domain might be
      handled by any MX whose hostname is a subdomain of "example.com"
      or "example.net."  The semantics for these patterns should be the
      ones found in the "Checking of Wildcard Certificates" rules in
      Section 6.4.3 of [RFC6125].

   o  "max-age": Max lifetime of the policy (plain-text integer
      seconds).  Well-behaved clients SHOULD cache a policy for up to
      this value from last policy fetch time.




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   o  "policy_id": A short string used to track policy updates.  This
      string MUST uniquely identify a given instance of a policy, such
      that senders can determine when the policy has been updated by
      comparing to the "policy_id" of a previously seen policy.

3.1.  Formal Definition

3.1.1.  TXT Record

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

          sts-version     = "v" *WSP "=" *WSP %x53 %x54 %x53 %x31

          sts-id          = "id" *WSP "=" *WSP 1*20VCHAR

3.1.2.  SMTP MTA-STS Policy

   The formal definition of the SMTP MTA-STS policy, using [RFC5234], is
   as follows:































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sts-record      = WSP %x7B WSP  ; { left curly bracket
                  sts-element   ; comma-separated
                  [             ; list
                  WSP %x2c WSP  ; of
                  sts-element   ; sts-elements
                  ]
                  WSP %x7d WSP  ; } right curly bracket
 = %x22 "max
sts-element     = sts-version / sts-mode / sts-id / sts-mx / sts-max-age

sts-version     = %x22 "version" %x22 *WSP %x3a *WSP  ; "version":
                  %x22 %x53 %x54 %x53 %x31            ; "STS1"

sts-mode        = %x22 "mode" %x22 *WSP %x3a *WSP     ; "mode":
                  %x22 ("report" / "enforce") %x22    ; "report"/"enforce"

sts-id          = %x22 "policy_id" %x22 *WSP %x3a *WSP ; "policy_id":
                  %x22 1*20VCHAR %x22                  ; some chars

sts-mx          = %x22 "mx" $x22 *WSP %x3a *WSP       ; "mx":
                  %x5B                                ; [
                  domain-match                        ; comma-separated list
                  [WSP %x2c domain-match WSP]         ; of domain-matches
                  %x5B                                ; ]

sts-max-age     = %x22 "max-age" %x22 $x3a *WSP       ; "max-age":
                  %x22 1*10DIGIT %x22$                ; some digits

domain-match    =  ["*."] 1*dtext *("." 1*dtext)

dtext           =  %d30-39 /          ; 0-9
                   %d41-5A /          ; a-z
                   %61-7A /           ; A-Z
                   %2D                ; "-"

   A size limitation in a sts-uri, if provided, is interpreted as a
   count of units followed by an OPTIONAL unit size ("k" for kilobytes,
   "m" for megabytes, "g" for gigabytes, "t" for terabytes).  Without a
   unit, the number is presumed to be a basic byte count.  Note that the
   units are considered to be powers of two; a kilobyte is 2^10, a
   megabyte is 2^20, etc.

3.2.  Policy Expirations

   In order to resist attackers inserting a fraudulent policy, SMTP MTA-
   STS policies are designed to be long-lived, with an expiry typically
   greater than two weeks.  Policy validity is controlled by two
   separate expiration times: the lifetime indicated in the policy



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   ("max-age=") and the TTL on the DNS record itself.  The policy
   expiration will ordinarily be longer than that of the DNS TTL, and
   senders SHOULD cache a policy (and apply it to all mail to the
   recipient domain) until the policy expiration.

   An important consideration for domains publishing a policy is that
   senders will see a policy expiration as relative to the fetch of a
   policy cached by their recursive resolver.  Consequently, a sender
   MAY treat a policy as valid for up to {expiration time} + {DNS TTL}.
   Publishers SHOULD thus continue to expect senders to apply old
   policies for up to this duration.

3.2.1.  Policy Updates

   Updating the policy requires that the owner make changes in two
   places: the "_mta_sts" RR record in the Policy Domain's DNS zone and
   at the corresponding HTTPS endpoint.  In the case where the HTTPS
   endpoint has been updated but the TXT record has not been, senders
   will not know there is a new policy released and may thus continue to
   use old, previously cached versions.  Recipients thus can expect a
   policy to continue to be used by senders until both the HTTPS and TXT
   endpoints are updated and the TXT record's TTL has passed.

3.3.  Policy Discovery & Authentication

   Senders discover a recipient domain's STS policy, by making an
   attempt to fetch TXT records from the recipient domain's DNS zone
   with the name "_mta_sts".  A valid TXT record presence in
   "_mta_sts.example.com" indicates that the recipent domain supports
   STS.  To allow recipient domains to safely serve new policies, it is
   important that senders are able to authenticate a new policy
   retrieved for a recipient domain.

   Web PKI is the mechanism used for policy authentication.  In this
   mechanism, the sender fetches a HTTPS resource (policy) from a host
   at "policy._mta_sts" in the Policy Domain.  The policy is served from
   a "well known" URI: "https://policy._mta_sts.example.com/current".
   To consider the policy as valid, the "policy_id" field in the policy
   MUST match the "id" field in the DNS TXT record under "_mta_sts".

   When fetching a new policy or updating a policy, the new policy MUST
   be fully authenticated (HTTPS certificate validation + peer
   verification) before use.  A policy which has not ever been
   successfully authenticated MUST not be used to reject mail.







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3.4.  Policy Validation

   When sending to an MX at a domain for which the sender has a valid
   and non-expired SMTP MTA-STS policy, a sending MTA honoring SMTP MTA-
   STS MUST validate that the recipient MX supports STARTTLS, and offers
   a valid PKIX based TLS certificate.  The certificate presented by the
   receiving MX MUST be valid for the MX name and chain to a root CA
   that is trusted by the sending MTA.  The certificate MUST have a CN
   or SAN matching the MX hostname (as described in [RFC6125]) and be
   non-expired.

3.5.  Policy Application

   When sending to an MX at a domain for which the sender has a valid
   non-expired SMTP MTA-STS policy, a sending MTA honoring SMTP MTA-STS
   MAY apply the result of a policy validation one of two ways:

   o  "report": In this mode, sending MTAs merely send a report to the
      designated report address indicating policy application failures.
      This can be done "offline", i.e. based on the MTA logs, and is
      thus a suitable low-risk option for MTAs who wish to enhance
      transparency of TLS tampering without making complicated changes
      to production mail-handling infrastructure.

   o  "enforce": In this mode, sending MTAs SHOULD treat STS policy
      failures, in which the policy action is "reject", as a mail
      delivery error, and SHOULD terminate the SMTP connection, not
      delivering any more mail to the recipient MTA.

   In "enforce" mode, however, sending MTAs MUST first check for a new
   authenticated policy before actually treating a message failure as
   fatal.

   Thus the control flow for a sending MTA that does online policy
   application consists of the following steps:

   1.  Check for cached non-expired policy.  If none exists, fetch the
       latest, authenticate and cache it.

   2.  Validate recipient MTA against policy.  If valid, deliver mail.

   3.  If not valid and the policy specifies reporting, generate report.

   4.  If not valid and policy specifies rejection, perform the
       following steps:

       *  Check for a new (non-cached) authenticated policy.




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       *  If one exists and the new policy is different, update the
          current policy and go to step 2.

       *  If one exists and the new policy is same as the cached policy,
          treat the delivery as a failure.

       *  If none exists and cached policy is not expired, treat the
          delivery as a failure.

   Understanding the details of step 4 is critical to understanding the
   behavior of the system as a whole.

   Remember that each policy has an expiration time (which SHOULD be
   long, on the order of days or months) and a validation method.  With
   these two mechanisms and the procedure specified in step 4,
   recipients who publish a policy have, in effect, a means of updating
   a cached policy at arbitrary intervals, without the risks (of a man-
   in-the-middle attack) they would incur if they were to shorten the
   policy expiration time.

4.  Failure Reporting

   Aggregate statistics on policy failures MAY be reported using the
   "TLSRPT" reporting specification (TODO: Add Ref).

5.  IANA Considerations

   There are no IANA considerations at this time.

6.  Security Considerations

   SMTP Strict Transport Security protects against an active attacker
   who wishes to intercept or tamper with mail between hosts who support
   STARTTLS.  There are two classes of attacks considered:

   o  Foiling TLS negotiation, for example by deleting the "250
      STARTTLS" response from a server or altering TLS session
      negotiation.  This would result in the SMTP session occurring over
      plaintext, despite both parties supporting TLS.

   o  Impersonating the destination mail server, whereby the sender
      might deliver the message to an impostor, who could then monitor
      and/or modify messages despite opportunistic TLS.  This
      impersonation could be accomplished by spoofing the DNS MX record
      for the recipient domain, or by redirecting client connections to
      the legitimate recipient server (for example, by altering BGP
      routing tables).




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   SMTP Strict Transport Security relies on certificate validation via
   PKIX based TLS identity checking [RFC6125].  Attackers who are able
   to obtain a valid certificate for the targeted recipient mail service
   (e.g. by compromising a certificate authority) are thus out of scope
   of this threat model.

   Since we use DNS TXT record for policy discovery, an attacker who is
   able to block DNS responses can suppress the discovery of an STS
   Policy, making the Policy Domain appear not to have an STS Policy.
   The caching model described in _Policy_ _Expirations_ is designed to
   resist this attack, and there is discussion in the _Future_ _Work_
   section around future distribution mechanisms that are robust against
   this attack.

7.  Future Work

   The authors would like to suggest multiple considerations for future
   discussion.

   o  Certificate pinning: One potential improvement in the robustness
      of the certificate validation methods discussed would be the
      deployment of public-key pinning as defined for HTTP in [RFC7469].
      A policy extension supporting these semantics would enable Policy
      Domains to specify certificates that MUST appear in the MX
      certificate chain, thus providing resistence against compromised
      CA or DNSSEC zone keys.

   o  Policy distribution: As with Certificate Transparency ([RFC6962]),
      it may be possible to provide a verifiable log of policy
      _observations_ (meaning which policies have been observed for a
      given Policy Domain).  This would provide insight into policy
      spoofing or faked policy non-existence.  This may be particularly
      useful for Policy Domains not using DNSSEC, since it would provide
      sending MTAs an authoritative source for whether a policy is
      expected for a given domain.

   o  Receive-from restrictions: Policy publishers may wish to also
      indicate to domains _receiving_ mail from the Policy Domain that
      all such mail is expected to be sent via TLS.  This may allow
      policy publishers to receive reports indicating sending MTA
      misconfigurations.  However, the security properties of a
      "receiver-enforced" system differ from those of the current
      design; in particular, an active man-in-the-middle attacker may be
      able to exploit misconfigured sending MTAs in a way that would not
      be possible today with a sender-enforced model.






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   o  Cipher and TLS version restrictions: Policy publishers may also
      wish to restrict TLS negotiation to specific ciphers or TLS
      versions.

8.  Appendix 1: Validation Pseudocode

policy = policy_from_cache()
if not policy or is_expired(policy):
  policy = policy_from_https_endpoint()  // fetch and authenticate!
  update_cache = true
if policy:
  if invalid_mx_or_tls(policy):  // check MX and TLS cert
    if rua:
      generate_report()
    if p_reject():
      policy = policy_from_https_endpoint()  // fetch and authenticate #2!
      update_cache = true
      if invalid_mx_or_tls(policy):
        reject_message()
        update_cache = false
  if update_cache:
    cache(policy)

9.  Appendix 2: Domain Owner STS example record

9.1.  Example 1

   The owner of example.com wishes to begin using STS with a policy that
   will solicit aggregate feedback from receivers without affecting how
   the messages are processed, in order to:

   o  Verify the identity of MXs that handle mail for this domain

   o  Confirm that its legitimate messages are sent over TLS

   o  Verify the validity of the certificates

   o  Determine how many messages would be affected by a strict policy

   DNS STS policy indicator TXT record:

               _mta_sts  IN TXT ( "v=STSv1; id=randomstr;" )

   STS policy served from HTTPS endpoint of the policy (recipient)
   domain, and is authenticated using Web PKI mechanism.






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                      {
                        "version": "STS1",
                        "mode": "report",
                        "policy_id": "randomstr",
                        "mx": ["*.mail.example.com"],
                        "max-age": "123456"
                      }

   The policy is authenticated using Web PKI mechanism.

10.  Appendix 3: DEEP Registration Elements

Name: mx-mismatch
Description: This indicates that the MX resolved for the recipient domain
             did not match the MX constraint specified in the policy.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: certificate-name-mismatch
Description: This indicates that the subject CNAME/SAN in the certificate
             presented by the receiving MX did not match the MX hostname
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: invalid-certificate
Description: This indicates that the certificate presented by the receiving MX
             did not validate according to the policy validation constraint.
             (Either it was not signed by a trusted CA or did not match the
             DANE TLSA record for the recipient MX.)
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: certificate-name-constraints-not-permitted
Description: The certificate request contains a name that is not listed as
             permitted in the name constraints extension of the cert issuer.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: certificate-name-constraints-excluded
Description: The certificate request contains a name that is listed as



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             excluded in the name constraints extension of the issuer.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: expired-certificate
Description: This indicates that the certificate has expired.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: starttls-not-supported
Description: This indicates that the recipient MX did not support STARTTLS.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: tlsa-invalid
Description: This indicates a validation error for Policy Domain specifying
             "tlsa" validation.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: dnssec-invalid
Description: This indicates a failure to validate DNS records for a Policy
             Domain specifying "tlsa" validation or "dnssec" authentication.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG

Name: sender-does-not-support-validation-method
Description: This indicates the sending system can never validate using the
             requested validation mechanism.
Intended Usage:  COMMON
Reference:  RFC XXXX (this document once published)
Submitter:  Authors of this document
Change Controller:  IESG








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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>.

   [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>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, DOI 10.17487/RFC4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, DOI 10
              .17487/RFC4627, July 2006,
              <http://www.rfc-editor.org/info/rfc4627>.

   [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>.

   [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>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <http://www.rfc-editor.org/info/rfc6962>.

   [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>.

   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10
              .17487/RFC7672, October 2015,
              <http://www.rfc-editor.org/info/rfc7672>.





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Authors' Addresses

   Daniel Margolis
   Google, Inc

   Email: dmargolis (at) google.com


   Mark Risher
   Google, Inc

   Email: risher (at) google (dot com)


   Nicolas Lidzborski
   Google, Inc

   Email: nlidz (at) google (dot com)


   Wei Chuang
   Google, Inc

   Email: weihaw (at) google (dot com)


   Brandon Long
   Google, Inc

   Email: blong (at) google (dot com)


   Binu Ramakrishnan
   Yahoo!, Inc

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


   Alexander Brotman
   Comcast, Inc

   Email: alexander_brotman (at) cable.comcast (dot com)


   Janet Jones
   Microsoft, Inc

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



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   Franck Martin
   LinkedIn

   Email: fmartin (at) linkedin (dot com)


   Klaus Umbach
   1&1 Mail & Media Development & Technology GmbH

   Email: klaus.umbach (at) 1und1 (dot de)


   Markus Laber
   1&1 Mail & Media Development & Technology GmbH

   Email: markus.laber (at) 1und1 (dot de)



































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