Using TLS in Applications                                    D. Margolis
Internet-Draft                                                 M. Risher
Intended status: Standards Track                             Google, Inc
Expires: June 18, 2017                                   B. Ramakrishnan
                                                             Yahoo!, Inc
                                                              A. Brotman
                                                            Comcast, Inc
                                                                J. Jones
                                                          Microsoft, Inc
                                                       December 15, 2016

              SMTP MTA Strict Transport Security (MTA-STS)


   SMTP Mail Transfer Agent Strict Transport Security (SMTP STS) is a
   mechanism enabling mail service providers to declare their ability to
   receive TLS-secured connections and an expected validity of
   certificates presented by their MX hosts, and to specify whether
   sending SMTP servers should refuse to deliver to MX hosts that do not
   offer TLS with a trusted server certificate.

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

   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 June 18, 2017.

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

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   ( 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  . . . . . . . . . . . . . . . . . . . .   3
   3.  Policy Discovery  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  MTA-STS TXT Records . . . . . . . . . . . . . . . . . . .   4
     3.2.  MTA-STS Policies  . . . . . . . . . . . . . . . . . . . .   5
     3.3.  HTTPS Policy Fetching . . . . . . . . . . . . . . . . . .   6
     3.4.  Policy Selection for Smart Hosts  . . . . . . . . . . . .   6
   4.  Policy Validation . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  MX Matching . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  MX Certificate Validation . . . . . . . . . . . . . . . .   7
   5.  Policy Application  . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  MX Preference . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Policy Application Control Flow . . . . . . . . . . . . .   8
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
     6.1.  Policy Updates  . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   10. Appendix 1: Domain Owner STS example record . . . . . . . . .  11
     10.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . .  11
   11. Appendix 2: Message delivery pseudocode . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     12.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to negotiate the use of a TLS channel for secure mail

   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

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   (perhaps by overwriting the resolved MX record of the delivery
   domain) can perform downgrade or interception attacks.

   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  expected validity of server certificates presented by the domain's
      MX hosts

   o  what a conforming client should do with messages when TLS cannot
      be successfully negotiated

1.1.  Terminology

   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 committment by the Policy Domain to support PKIX
      authenticated TLS for the specified MX hosts.

   o  Policy Domain: The domain for which an STS Policy is defined.
      (For example, when sending mail to "", the policy
      domain is "".)

   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, unauthenticated encryption into
   required, authenticated encryption.  DANE requires DNSSEC [RFC4033]
   for authentication; the mechanism described here instead relies on
   certificate authorities (CAs) and does not require DNSSEC.  For a
   thorough discussion of this trade-off, see the section _Security_

   In addition, SMTP STS provides an optional report-only mode, enabling
   soft deployments to detect policy failures.

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3.  Policy Discovery

   SMTP STS policies are distributed via HTTPS from a "well-known"
   [RFC5785] path served within the Policy Domain, and their presence
   and current version are indicated by a TXT record at the Policy
   Domain.  These TXT records additionally contain a policy "id" field,
   allowing sending MTAs to check the currency of a cached policy
   without performing an HTTPS request.

   To discover if a recipient domain implements MTA-STS, a sender need
   only resolve a single TXT record.  To see if an updated policy is
   available for a domain for which the sender has a previously cached
   policy, the sender need only check the TXT record's version "id"
   against the cached value.

3.1.  MTA-STS TXT Records

   The MTA-STS TXT record is a TXT record with the name "_mta-sts" at
   the Policy Domain.  For the domain "", this record would
   be "".  MTA-STS TXT records MUST be US-ASCII,
   semicolon-separated key/value pairs containing the following fields:

   o  "v": (plain-text, required).  Currently only "STSv1" is supported.

   o  "id": (plain-text, required).  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 "id" of a previously seen policy.
      There is no implied ordering of "id" fields between revisions.

   An example TXT record is as below:

   "  IN TXT "v=STSv1; id=20160831085700Z;""

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

      sts-text-record = sts-version *WSP %x3B *WSP sts-id [%x3B]

      sts-version     = "v" *WSP "=" *WSP %x53 %x54        ; "STSv1"
                        %x53 %x76 %x31

      sts-id          = "id" *WSP "=" *WSP 1*32(ALPHA / DIGIT)

   If multiple TXT records for "_mta-sts" are returned by the resolver,
   records which do not begin with "v=STSv1;" are discarded.  If the
   number of resulting records is not one, senders MUST assume the

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   recipient domain does not implement MTA STS and skip the remaining
   steps of policy discovery.

3.2.  MTA-STS Policies

   The policy itself is a JSON [RFC4627] object served via the HTTPS GET
   method from the fixed [RFC5785] "well-known" path of ".well-known/
   mta-sts.json" served by the "mta-sts" host at the Policy Domain.
   Thus for "" the path is "

   This JSON object contains the following key/value pairs:

   o  "version": (plain-text, required).  Currently only "STSv1" is

   o  "mode": (plain-text, required).  Either "enforce" or "report",
      indicating the expected behavior of a sending MTA in the case of a
      policy validation failure.

   o  "max_age": Max lifetime of the policy (plain-text non-negative
      integer seconds, required).  Well-behaved clients SHOULD cache a
      policy for up to this value from last policy fetch time.  To
      mitigate the risks of attacks at policy refresh time, it is
      expected that this value typically be in the range of weeks or

   o  "mx": MX patterns (list of plain-text MX match strings, required).
      One or more patterns matching the expected MX for this domain.
      For example, "["*", "*"]" indicates that
      mail for this domain might be handled by any MX with a hostname at
      "" or "".  Valid patterns can be either
      hostname literals (e.g. "") or wildcard matches, so
      long as the wildcard occupies the full left-most label in the
      pattern.  (Thus "*" is valid but "mx*" is

   An example JSON policy is as below:

                        "version": "STSv1",
                        "mode": "enforce",
                        "mx": ["*"],
                        "max_age": 123456

   A lenient parser SHOULD accept TXT records and policy files which are
   syntactically valid (i.e. valid key-value pairs separated by semi-

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   colons for TXT records and valid JSON for policy files) and
   implementing a superset of this specification, in which case unknown
   fields SHALL be ignored.

3.3.  HTTPS Policy Fetching

   When fetching a new policy or updating a policy, the HTTPS endpoint
   MUST present a TLS certificate which is valid for the "mta-sts" host
   (as described in [RFC6125]), chain to a root CA that is trusted by
   the sending MTA, and be non-expired.  It is expected that sending
   MTAs use a set of trusted CAs similar to those in widely deployed Web
   browsers and operating systems.

   HTTP 3xx redirects MUST NOT be followed.

   Senders may wish to rate-limit the frequency of attempts to fetch the
   HTTPS endpoint even if a valid TXT record for the recipient domain
   exists.  In the case that the HTTPS GET fails, we suggest
   implementions may limit further attempts to a period of five minutes
   or longer per version ID, to avoid overwhelming resource-constrained
   recipients with cascading failures.

   Senders MAY impose a timeout on the HTTPS GET to avoid long delays
   imposed by attempted policy updates.  A suggested timeout is one
   minute; policy hosts SHOULD respond to requests with a complete
   policy body within that timeout.

3.4.  Policy Selection for Smart Hosts

   When sending mail via a "smart host"--an intermediate SMTP relay
   rather than the message recipient's server--compliant senders MUST
   treat the smart host domain as the policy domain for the purposes of
   policy discovery and application.

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 STS
   MUST validate:

   1.  That the recipient MX matches the "mx" pattern from the recipient
       domain's policy.

   2.  That the recipient MX supports STARTTLS and offers a valid PKIX
       based TLS certificate.

   This section does not dictate the behavior of sending MTAs when
   policies fail to validate; in particular, validation failures of

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   policies which specify "report" mode MUST NOT be interpreted as
   delivery failures, as described in the section _Policy_

4.1.  MX Matching

   When delivering mail for the Policy Domain to a recipient MX host,
   the sender validates the MX match against the "mx" pattern from the
   applied policy.  The semantics for these patterns are those found in
   section 6.4 of [RFC6125].

   Patterns may contain a wildcard character "*" which matches any
   single domain name component or component fragment, though only as
   the leftmost component in a pattern.  For example, "*" is
   a valid pattern, but "foo.*" is not.  Given the pattern
   "*", "" is a valid MX host, but
   "" is not.

4.2.  MX Certificate Validation

   The certificate presented by the receiving MX MUST be valid for the
   MX hostname 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.

   In the case of an "implicit" MX record (as specified in [RFC2821])
   where no MX RR exists for the recipient domain but there is an A RR,
   the MX hostname is assumed to be that of the A RR and should be
   validated as such.

5.  Policy Application

   When sending to an MX at a domain for which the sender has a valid,
   non-expired STS policy, a sending MTA honoring SMTP STS applies the
   result of a policy validation one of two ways, depending on the value
   of the policy "mode" field:

   1.  "report": In this mode, sending MTAs merely send a report (as
       described in the TLSRPT specification (TODO: add ref)) indicating
       policy application failures.

   2.  "enforce": In this mode, sending MTAs treat STS policy failures
       as a mail delivery error, and MUST NOT deliver the message to
       this host.

   When a message fails to deliver due to an "enforce" policy, a
   compliant MTA MUST check for the presence of an updated policy at the
   Policy Domain before permanently failing to deliver the message.

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   This allows implementing domains to update long-lived policies on the

   Finally, in both "enforce" and "report" modes, failures to deliver in
   compliance with the applied policy result in failure reports to the
   policy domain, as described in the TLSRPT specification (TODO: add

5.1.  MX Preference

   When applying a policy, sending MTAs SHOULD select recipient MXs by
   first eliminating any MXs at lower priority than the current host (if
   in the MX candidate set), then eliminating any non-matching (as
   specified by the STS Policy) MX hosts from the candidate MX set, and
   then attempting delivery to matching hosts as indicated by their MX
   priority, until delivery succeeds or the MX candidate set is empty.

5.2.  Policy Application Control Flow

   An example control flow for a compliant sender consists of the
   following steps:

   1.  Check for a cached policy whose time-since-fetch has not exceeded
       its "max_age".  If none exists, attempt to fetch a new policy.
       (Optionally, sending MTAs may unconditionally check for a new
       policy at this step.)

   2.  Filter candidate MXs against the current policy.

   3.  If no candidate MXs are valid and the policy mode is "enforce",
       temporarily fail the message.  (Otherwise, generate a failure
       report but deliver as though MTA STS were not implemented.)

   4.  For each candidate MX, in order of MX priority, attempt to
       deliver the message, enforcing STARTTLS and the MX host's PKIX
       certificate validation.

   5.  Upon message retries, a message MAY be permanently failed
       following first checking for the presence of a new policy (as
       indicated by the "id" field in the "_mta-sts" TXT record).

6.  Operational Considerations

6.1.  Policy Updates

   Updating the policy requires that the owner make changes in two
   places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and
   at the corresponding HTTPS endpoint.  In the case where the HTTPS

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   endpoint has been updated but the TXT record has not yet been,
   senders will not know there is a new policy released and may thus
   continue to use old, previously cached versions.  Recipients should
   thus expect a policy will continue to be used by senders until both
   the HTTPS and TXT endpoints are updated and the TXT record's TTL has

7.  IANA Considerations

   A new .well-known URI will be registered in the Well-Known URIs
   registry as described below:

   URI Suffix: mta-sts.json Change Controller: IETF

8.  Security Considerations

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

   1.  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.

   2.  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
       intended for the legitimate recipient server (for example, by
       altering BGP routing tables).

   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 able to
   circumvent STS authentication.

   Since we use DNS TXT records 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 sender policy cache is designed to resist this attack.

   We additionally consider the Denial of Service risk posed by an
   attacker who can modify the DNS records for a victim domain.  Absent
   SMTP STS, such an attacker can cause a sending MTA to cache invalid
   MX records for a long TTL.  With SMTP STS, the attacker can

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   additionally advertise a new, long-"max_age" SMTP STS policy with
   "mx" constraints that validate the malicious MX record, causing
   senders to cache the policy and refuse to deliver messages once the
   victim has resecured the MX records.

   This attack is mitigated in part by the ability of a victim domain to
   (at any time) publish a new policy updating the cached, malicious
   policy, though this does require the victim domain to both obtain a
   valid CA-signed certificate and to understand and properly configure

   Similarly, we consider the possibilty of domains that deliberately
   allow untrusted users to serve untrusted content on user-specified
   subdomains.  In some cases (e.g. the service this takes
   the form of providing HTTPS hosting of user-registered subdomains; in
   other cases (e.g. dynamic DNS providers) this takes the form of
   allowing untrusted users to register custom DNS records at the
   provider's domain.

   In these cases, there is a risk that untrusted users would be able to
   serve custom content at the "mta-sts" host, including serving an
   illegitimate SMTP STS policy.  We believe this attack is rendered
   more difficult by the need for the attacker to both inject malicious
   (but temporarily working) MX records and also serve the "_mta-sts"
   TXT record on the same domain--something not, to our knowledge,
   widely provided to untrusted users.  This attack is additionally
   mitigated by the aforementioned ability for a victim domain to update
   an invalid policy at any future date.

   Even if an attacker cannot modify a served policy, the potential
   exists for configurations that allow attackers on the same domain to
   receive mail for that domain.  For example, an easy configuration
   option when authoring an STS Policy for "" is to set the
   "mx" equal to "*"; recipient domains must consider in
   this case the risk that any user possessing a valid hostname and CA-
   signed certificate (for example, "") will, from
   the perspective of STS Policy validation, be a valid MX host for that

9.  Contributors

   Nicolas Lidzborski Google, Inc nlidz (at) google (dot com)

   Wei Chuang Google, Inc weihaw (at) google (dot com)

   Brandon Long Google, Inc blong (at) google (dot com)

   Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com)

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   Klaus Umbach 1&1 Mail & Media Development & Technology GmbH
   klaus.umbach (at) 1und1 (dot de)

   Markus Laber 1&1 Mail & Media Development & Technology GmbH
   markus.laber (at) 1und1 (dot de)

10.  Appendix 1: Domain Owner STS example record

10.1.  Example 1

   The owner of "" wishes to begin using STS with a policy
   that will solicit reports from receivers without affecting how the
   messages are processed, in order to verify the identity of MXs that
   handle mail for "", confirm that TLS is correctly used,
   and ensure that certificates presented by the recipient MX validate.

   STS policy indicator TXT RR:  IN TXT "v=STSv1; id=20160831085700Z;"

   STS Policy JSON served as the response body at [1]

                "version": "STSv1",
                "mode": "report",
                "mx": ["", ""],
                "max_age": 123456

11.  Appendix 2: Message delivery pseudocode

   Below is pseudocode demonstrating the logic of a complaint sending
   MTA.  This implements the "two-pass" approach, first attempting
   delivery with a newly fetched policy (if present) before falling back
   to a cached policy (if present).

func isEnforce(policy) {
  // Return true if the policy mode is "enforce".

func isNonExpired(policy) {
  // Return true if the policy is not expired.

func tryStartTls(mx) {
  // Attempt to open an SMTP connection with STARTTLS with the MX.

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func certMatches(connection, mx) {
  // Return if the server certificate from "connection" matches the "mx" host.

func tryDeliverMail(connection, message) {
  // Attempt to deliver "message" via "connection".

func getMxsForPolicy(domain, policy) {
  // Sort the MXs by priority, filtering out those which are invalid according
  // to "policy".

func tryGetNewPolicy(domain) {
  // Check for an MTA STS TXT record for "domain" in DNS, and return the
  // indicated policy (or a local cache of the unvalidated policy).

func cachePolicy(domain, policy) {
  // Store "policy" as the cached policy for "domain".

func tryGetCachedPolicy(domain, policy) {
  // Return a cached policy for "domain".

func reportError(error) {
  // Report an error via TLSRPT.

func tryMxAccordingTo(message, mx, policy) {
  connection := connect(mx)
  if !connection {
    return false  // Can't connect to the MX so it's not an STS error.
  status := !(tryStartTls(mx, &connection) && certMatches(connection, mx))
  status = true
  if !tryStartTls(mx, &connection) {
    status = false
  } else if certMatches(connection, mx) {
    status = false
  if status || !isEnforce(policy) {
    return tryDeliverMail(connection, message)

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  return false

func tryWithPolicy(message, domain, policy) {
  mxes := getMxesForPolicy(domain, policy)
  if mxs is empty {
  for mx in mxes {
    if tryMxAccordingTo(message, mx, policy) {
      return true
  return false

func handleMessage(message) {
  domain := ... // domain part after '@' from recipient
  oldPolicy := tryGetCachedPolicy(domain)
  newPolicy := tryGetNewPolicy(domain)
  if newPolicy {
    cachePolicy(domain, newPolicy)
    oldPolicy = newPolicy
  if oldPolicy {
    return tryWithPolicy(message, oldPolicy)
  // There is no policy or there's a new policy that did not work.
  // Try to deliver the message normally (i.e. without STS).

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

   [RFC2821]  Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
              2821, DOI 10.17487/RFC2821, April 2001,

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   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <>.

   [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,

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, DOI 10
              .17487/RFC4627, July 2006,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10
              .17487/RFC5785, April 2010,

   [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, <>.

   [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,

12.2.  URIs


Authors' Addresses

   Daniel Margolis
   Google, Inc

   Email: dmargolis (at)

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   Mark Risher
   Google, Inc

   Email: risher (at) google (dot com)

   Binu Ramakrishnan
   Yahoo!, Inc

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

   Alexander Brotman
   Comcast, Inc

   Email: alexander_brotman (at) (dot com)

   Janet Jones
   Microsoft, Inc

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

Margolis, et al.          Expires June 18, 2017                [Page 15]