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<rfc category="std" docName="draft-ietf-ntp-using-nts-for-ntp-07"
     ipr="trust200902" submissionType="IETF">
  <front>
    <title abbrev="NTS4NTP">Using the Network Time Security Specification to
    Secure the Network Time Protocol</title>

    <author fullname="Daniel Fox Franke" initials="D." surname="Franke">
      <organization abbrev="Akamai">Akamai Technologies, Inc.</organization>

      <address>
        <postal>
          <street>150 Broadway</street>

          <city>Cambridge</city>

          <region>MA</region>

          <code>02142</code>

          <country>United States</country>
        </postal>

        <email>dafranke@akamai.com</email>

        <uri>https://www.dfranke.us</uri>
      </address>
    </author>

    <author fullname="Dieter Sibold" initials="D." surname="Sibold">
      <organization abbrev="PTB">Physikalisch-Technische
      Bundesanstalt</organization>

      <address>
        <postal>
          <street>Bundesallee 100</street>

          <city>Braunschweig</city>

          <code>D-38116</code>

          <region/>

          <country>Germany</country>
        </postal>

        <phone>+49-(0)531-592-8420</phone>

        <facsimile>+49-531-592-698420</facsimile>

        <email>dieter.sibold@ptb.de</email>
      </address>
    </author>

    <author fullname="Kristof Teichel" initials="K." surname="Teichel">
      <organization abbrev="PTB">Physikalisch-Technische
      Bundesanstalt</organization>

      <address>
        <postal>
          <street>Bundesallee 100</street>

          <city>Braunschweig</city>

          <region/>

          <code>D-38116</code>

          <country>Germany</country>
        </postal>

        <phone>+49-(0)531-592-8421</phone>

        <facsimile/>

        <email>kristof.teichel@ptb.de</email>

        <uri/>
      </address>
    </author>

    <date day="31" month="October" year="2016"/>

    <area>Internet Area</area>

    <workgroup>NTP Working Group</workgroup>

    <keyword>Integrity</keyword>

    <keyword>Authentication</keyword>

    <keyword>NTP</keyword>

    <keyword>Security</keyword>

    <keyword>DTLS</keyword>

    <abstract>
      <t>This document describes how to reach the objectives described in the
      Network Time Security (NTS) specification when securing time
      synchronization with servers using the Network Time Protocol (NTP).</t>
    </abstract>

    <note title="Requirements Language">
      <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
      "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
      document are to be interpreted as described in <xref
      target="RFC2119">RFC 2119</xref>.</t>
    </note>
  </front>

  <middle>
    <section title="Introduction">
      <t>The Network Time Security (NTS) draft <xref
      target="I-D.ietf-ntp-network-time-security"/> specifies security
      measures which can be used to enable time synchronization protocols to
      verify authenticity of the time server and integrity of the time
      synchronization protocol packets.</t>

      <t>This document provides detail on how to specifically use those
      measures to secure time synchronization between NTP clients and servers.
      In particular, it describes a mechanism for using <xref
      target="RFC6347">Datagram Transport Layer Security</xref> (DTLS) to
      provide cryptographic security for NTP. Certain sections, are not
      inherently NTP-specific and can be taken as guidance on how future work
      may apply the described techniques to other time synchronization
      protocols such as the <xref target="IEC.61588_2009">Precision Time
      Protocol</xref>.</t>
    </section>

    <section title="Objectives">
      <t>The specific objectives for applying the NTS specification to the NTP
      are as follows:<list style="symbols">
          <t>Authenticity: NTS enables an NTP client to authenticate its time
          server(s).</t>

          <t>Integrity: NTS protects the integrity of NTP time synchronization
          protocol packets via a message authentication code (MAC).</t>

          <t>Authorization: NTS optionally enables the server to verify the
          client's authorization.</t>

          <t>Confidentiality: NTS does not provide confidentiality protection
          of the time synchronization data.</t>

          <t>Privacy: NTS preserves unlinkability, i. e. it does not leak data
          that would allow a passive attacker to track mobile NTP clients when
          they move between networks.</t>

          <t>Request-Response-Consistency: NTS enables a client to match an
          incoming response to a request it has sent. NTS also enables the
          client to deduce from the response whether its request to the server
          has arrived without alteration.</t>

          <t>Modes of operation: Both the client-server mode and the symmetric
          peer mode of NTP are supported. The broadcast mode of NTP can NOT be
          secured with measures within this document.</t>

          <t>Hybrid mode: Both secure and insecure communication modes are
          possible for both NTP servers and clients.</t>

          <t>Compatibility:<list style="symbols">
              <t>NTP associations which are not secured by NTS are not
              affected by NTS-secured communication.</t>

              <t>An NTP server that does not support NTS is not affected by
              NTS-secured authentication requests.</t>
            </list></t>
        </list></t>
    </section>

    <section title="Terms and Abbreviations">
      <t><list style="hanging">
          <t hangText="MAC">Message Authentication Code</t>

          <t hangText="NTP  ">Network Time Protocol (<xref
          target="RFC5905">RFC 5905</xref>)</t>

          <t hangText="NTS  ">Network Time Security</t>

          <t hangText="DTLS">Datagram Transport Layer Security</t>

          <t hangText="AEAD">Authenticated Encryption with Associated Data
          (<xref target="RFC5116">RFC 5116</xref>)</t>
        </list></t>
    </section>

    <section title="Overview of NTS-Secured NTP">
      <t>The Network Time Protocol includes many different operating modes to
      support various network topologies. In addition to its best-known and
      most-widely-used client-server mode, it also includes modes for
      synchronization between symmetric peers, a control mode for server
      monitoring and administration and a broadcast mode. These various modes
      have differing and contradictory requirements for security and
      performance. Symmetric and control modes demand mutual authentication
      and mutual replay protection, and for certain message types control mode
      may require confidentiality as well as authentication. Client-server
      mode places more stringent requirements on resource utilization than
      other modes, because servers may have vast number of clients and be
      unable to afford to maintain per-client state. However, client-server
      mode also has more relaxed security needs, because only the client
      requires replay protection: it is harmless for servers to process
      replayed packets. The security demands of symmetric and control modes,
      on the other hand, are in conflict with the resource-utilization demands
      of client-server mode: any scheme which provides replay protection
      inherently involves maintaining some state to keep track of what
      messages have already been seen.</t>

      <t>This document does not discuss how to add security to NTP's broadcast
      mode.</t>

      <section anchor="overview" title="Client-Server Mode">
        <t>The server does not keep a long-term state of the client. NTS
        initially verifies the authenticity of the time server and exchanges
        one or more symmetric keys. The DTLS-based key exchange procedure
        described in <xref target="key-exchange"/> can be used for this
        exchange. An implementation MUST support the use of this procedure. It
        MAY additionally support the use of any alternative secure
        communication for this purpose, as long as it fulfills the
        preconditions given in <xref
        target="I-D.ietf-ntp-network-time-security"/>, Section 6.1.1.</t>

        <t>After the keys have been exchanged, the participants then use them
        to protect the authenticity and the integrity of subsequent
        unicast-type time synchronization packets. In order to do this, the
        server attaches a Message Authentication Code (MAC) to each time
        synchronization packet. The calculation of the MAC includes the whole
        time synchronization packet and the symmetric key which is stored on
        the client side. Therefore, the client can perform a validity check
        for this MAC on reception of a time synchronization packet.</t>
      </section>

      <section title="Symmetric/Peer Mode and Control Modes">
        <t>In the symmetric ("peer") mode as well as in control modes, there
        is no requirement for statelessness on either side. Both sides
        exchange and memorize one or more shared secrets. The shared secrets
        exchanged are then used to secure NTP peer mode or control packets by
        providing at least authenticity and integrity protection and possibly
        also confidentiality. The DTLS-based key exchange procedure described
        in <xref target="dtls-encapsulation"/> can be used for such
        communication. An implementation MUST support the use of this
        procedure.</t>
      </section>
    </section>

    <section anchor="key-exchange" title="Employing DTLS for NTP Security">
      <t>Since (as discussed in <xref target="overview"/>) no single approach
      can simultaneously satisfy the needs of all modes, this specification
      consists of not one protocol but a suite of them: <list style="symbols">
          <t>The "NTS-encapsulated NTPv4" protocol is little more than "NTP
          over DTLS": the two endpoints perform a DTLS handshake and then
          exchange NTP packets encapsulated as DTLS Application Data. It is
          suitable for symmetric and control modes, and is also secure for
          client/server mode but relatively wasteful of server resources.</t>

          <t>The "NTS Key Establishment" protocol (NTS-KE) uses DTLS to
          establish key material and negotiate some additional protocol
          options, but then quickly closes the DTLS channel and does not use
          it for the exchange of time packets. NTS-KE is designed to be
          extensible, and might be extended to support key establishment for
          other protocols such as PTP.</t>

          <t>The "NTS extensions for NTPv4" are a collection of NTP extension
          fields for cryptographically securing NTPv4 using key material
          previously negotiated using NTS-KE. They are suitable for securing
          client/server mode because the server can implement them without
          retaining per-client state, but on the other hand are suitable
          *only* for client/server mode because only the client, and not the
          server, is protected from replay.</t>
        </list></t>

      <section anchor="dtls-profile"
               title="DTLS profile for Network Time Security">
        <t>Since securing time protocols is (as of 2016) a novel application
        of DTLS, no backward-compatibility concerns exist to justify using
        obsolete, insecure, or otherwise broken DTLS features or versions. We
        therefore put forward the following requirements and guidelines,
        roughly representing 2016's best practices.</t>

        <t>Implementations MUST NOT negotiate DTLS versions earlier than
        1.2.</t>

        <t>Implementations willing to negotiate more than one possible version
        of DTLS SHOULD NOT respond to handshake failures by retrying with a
        downgraded protocol version. If they do, they MUST implement <xref
        target="RFC7507"/>.</t>

        <t>DTLS clients MUST NOT offer, and DTLS servers MUST not select, RC4
        cipher suites. <xref target="RFC7465"/></t>

        <t>DTLS clients SHOULD offer, and DTLS servers SHOULD accept, the
        <xref target="RFC5746">TLS Renegotiation Indication Extension</xref>.
        Regardless, they MUST NOT initiate or permit insecure renegotiation.
        (*)</t>

        <t>DTLS clients SHOULD offer, and DTLS servers SHOULD accept, the
        <xref target="RFC7627">TLS Session Hash and Extended Master Secret
        Extension</xref>. (*)</t>

        <t>Use of the <xref target="RFC7301">Application-Layer Protocol
        Negotiation Extension</xref> is integral to NTS and support for it is
        REQUIRED for interoperability.</t>

        <t>(*): Note that DTLS 1.3 or beyond may render the indicated
        recommendations inapplicable.</t>
      </section>

      <section anchor="transport-mechanisms"
               title="Transport mechanisms for DTLS records">
        <t>This section specifies two mechanisms, one REQUIRED and one
        OPTIONAL, for exchanging NTS-related DTLS records. It is intended that
        the choice of transport mechanism be orthogonal to any concerns at the
        application layer: DTLS records SHOULD receive identical disposition
        regardless of which mechanism they arrive by.</t>

        <section title="Transport via NTS port">
          <t>In this transport mechanism, DTLS records, formatted according to
          <xref target="RFC6347">RFC 6347</xref> or a subsequent revision
          thereof, are exchanged directly on UDP port [[TBD]], with one DTLS
          record per UDP packet and no additional layer of encapsulation
          between the UDP header and the DTLS record. Servers which implement
          NTS MUST support this mechanism.</t>
        </section>

        <section title="Transport via NTP extension field">
          <t>In this transport mechanism, DTLS records are exchanged within
          extension fields of specially-formed NTP packets, which are
          themselves exchanged via the usual NTP service port (123/udp). NTP
          packets conveying DTLS records SHALL be formatted as in <xref
          target="dtls-packet-layout"/>. They MUST NOT contain any other
          extensions or a legacy MAC field.</t>

          <figure anchor="dtls-packet-layout"
                  title="Format of NTP packets conveying DTLS records">
            <artwork><![CDATA[
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
.                                                               .
.                     NTP Header (48 octets)                    .
.                                                               .
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Extension Type         |       Extension Length      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
.                                                               .
.                     DTLS Record (variable)                    .
.                                                               .
|                                                               |
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                                                               |
.                                                               .
.                    Padding (1-24 octets)                      .
.                                                               .
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ]]></artwork>
          </figure>

          <t>Within the NTP header, <list style="symbols">
              <t>The Leap Indicator field SHALL be set to 3
              (unsynchronized).</t>

              <t>The Version Number field SHALL be set to 4.</t>

              <t>DTLS clients SHALL set the Mode field to 3, and DTLS servers
              SHALL set the Mode field to 4, even if the DTLS record is being
              used (in the full-encapsulation protocol) to protect some NTP
              mode other than client/server.</t>

              <t>The Stratum field SHALL be set to 0 (unspecified or
              invalid).</t>

              <t>The Reference ID field (conveying a kiss code) SHALL be set
              to "DTLS"</t>

              <t>DTLS servers SHALL set the origin timestamp field from the
              transmit timestamp field of the packet most recently received
              from the client.</t>

              <t>All other header fields MUST be ignored by the receiver, and
              MAY contain arbitrary or bogus values.</t>
            </list></t>

          <t>The Extension Type field SHALL be set to [[TBD]]. The Extension
          Length field SHALL be computed and set as per <xref
          target="RFC7822">RFC 7822</xref>.</t>

          <t>The DTLS Record field SHALL contain a DTLS Record formatted as
          per <xref target="RFC6347">RFC 6347</xref> or a subsequent revision
          thereof.</t>

          <t>The Padding field SHALL contain between 1 and 24 octets of
          padding, with every octet set to the number of padding octets
          included, e.g., "01", "02 02", or "03 03 03". The number of padding
          bytes should be chosen in order to comply with the <xref
          target="RFC7822">RFC 7822</xref> requirement that (in the absence of
          a legacy MAC) extensions have a total length in octets (including
          the four octets for the type and length fields) which is at least 28
          and divisible by 4. Furthermore, since future revisions of DTLS may
          employ record formats that are not self-delimiting, at least one
          octet of padding MUST be included so that receivers can
          unambiguously determine where the DTLS record ends and the padding
          begins. If the length of the DTLS record is already at least 24 and
          a multiple of 4, then the correct amount of padding to include is 4
          octets.</t>

          <t>The NTP header values specified above are selected such that NTP
          implementations which do not understand NTS will interpret the
          packet as an innocuous no-op and not attempt to use it for time
          synchronization. To NTS-aware implementations, however, these
          packets are best understood as not being NTP packets at all, but
          simply a means of "smuggling" arbitrary DTLS records across port
          123/udp. Indeed, these records need not be pertinent to NTP at all
          &mdash; for example, they could be NTS-KE messages eventually
          intended for securing PTP traffic.</t>

          <t>This transport mechanism is intended for use as a fallback in
          situations where firewalls or other middleboxes are preventing
          communication on the NTS port. Support for it is OPTIONAL.</t>
        </section>
      </section>

      <section anchor="dtls-encapsulation"
               title="The NTS-encapsulated NTPv4 protocol">
        <t>The NTS-encapsulated NTPv4 protocol proceeds in two parts. First,
        DTLS handshake records are exchanged using one of the two transport
        mechanisms specified in <xref target="transport-mechanisms"/>. The two
        endpoints carry out a DTLS handshake in conformance with <xref
        target="dtls-profile"/>, with the client offering (via an <xref
        target="RFC7301">ALPN</xref> extension), and the server accepting, an
        application-layer protocol of "ntp/4". Second, once the handshake is
        successfully completed, the two endpoints use the established channel
        to exchange arbitrary NTPv4 packets as DTLS-protected Application
        Data.</t>

        <t>In addition to the requirements specified in <xref
        target="dtls-profile"/>, implementations MUST enforce the anti-replay
        mechanism specified in <xref target="RFC6347"> Section 4.1.2.6 of RFC
        6347</xref> (or an equivalent mechanism specified in a subsequent
        revision of DTLS). Servers wishing to enforce access control SHOULD
        either demand a client certificate or use a PSK-based handshake in
        order to establish the client's identity.</t>

        <t>The NTS-encapsulated NTPv4 protocol is the RECOMMENDED mechanism
        for cryptographically securing mode 1 (symmetric active), 2 (symmetric
        passive), and 6 (control) NTPv4 traffic. It is equally safe for mode
        3/4 (client/server) traffic, but is NOT RECOMMENDED for this purpose
        because it scales poorly compared to using <xref
        target="nts-extensions-for-ntpv4">NTS Extensions for NTPv4</xref>.</t>
      </section>

      <section anchor="dtls-key-establishment"
               title="The NTS Key Establishment protocol">
        <t>The NTS Key Establishment (NTS-KE) protocol is carried out by
        exchanging DTLS records using one of the two transport mechanisms
        specified in <xref target="transport-mechanisms"/>. The two endpoints
        carry out a DTLS handshake in conformance with <xref
        target="dtls-profile"/>, with the client offering (via an <xref
        target="RFC7301">ALPN</xref> extension), and the server accepting, an
        application-layer protocol of "ntske/1". Immediately following a
        successful handshake, the client SHALL send a single request (as
        Application Data encapsulated in the DTLS-protected channel), then the
        server SHALL send a single response followed by a "Close notify" alert
        and then discard the channel state.</t>

        <t>The client's request and the server's response each SHALL consist
        of a sequence of records formatted according to <xref
        target="ntske-record"/>. The sequence SHALL be terminated by a "End of
        Message" record, which has a Record Type of zero and a zero-length
        body. Furthermore, requests and non-error responses each SHALL include
        exactly one NTS Next Protocol Negotiation record.</t>

        <figure anchor="ntske-record">
          <artwork><![CDATA[
0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|         Record Type         |          Body Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
.                                                               .
.                           Record Body                         .
.                                                               .
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ]]></artwork>
        </figure>

        <t>[[Ed. Note: this ad-hoc binary format should be fine as long as we
        continue to keep things very simple. However, if we think there's any
        reasonable probability of wanting to include more complex data
        structures, we should consider using some semi-structured data format
        such as JSON, Protocol Buffers, or (ugh) ASN.1]]</t>

        <t>The requirement that all NTS-KE messages be terminated by an End of
        Message record makes them self-delimiting. One DTLS record MAY, and
        typically will, contain multiple NTS-KE records. NTS-KE records MAY be
        split across DTLS record boundaries. If, likely due to packet loss, an
        incomplete NTS-KE message is received, implementations MUST treat this
        an error, which clients SHOULD handle by restarting with a fresh DTLS
        handshake and trying again.</t>

        <t>The fields of an NTS-KE record are defined as follows: <list
            style="symbols">
            <t>C (Critical Bit): Determines the disposition of unrecognized
            Record Types. Implementations which receive a record with an
            unrecognized Record Type MUST ignore the record if the Critical
            Bit is 0, and MUST treat it as an error if the Critical Bit is
            1.</t>

            <t>Record Type: A 15-bit integer in network byte order (from
            most-to-least significant, its bits are record bits 7&ndash;1 and
            then 15&ndash;8). The semantics of record types 0&ndash;5 are
            specified in this memo; additional type numbers SHALL be tracked
            through the IANA Network Time Security Key Establishment Record
            Types registry.</t>

            <t>Body Length: the length of the Record Body field, in octets, as
            a 16-bit integer in network byte order. Record bodies may have any
            representable length and need not be aligned to a word
            boundary.</t>

            <t>Record Body: the syntax and semantics of this field shall be
            determined by the Record Type.</t>
          </list></t>

        <section title="NTS-KE record types">
          <t>The following NTS-KE Record Types are defined.</t>

          <section title="End of Message">
            <t>The End of Message record has a Record Type number of 0 and an
            zero-length body. It MUST occur exactly once as the final record
            of every NTS-KE request and response. The Critical Bit MUST be
            set.</t>
          </section>

          <section title="NTS Next Protocol Negotiation">
            <t>The NTS Next Protocol Negotiation record has a record type of
            1. It MUST occur exactly once in every NTS-KE request and
            response. Its body consists of a sequence of 16-octet strings.
            Each 16-octet string represents a Protocol Name from the IANA
            Network Time Security Next Protocols registry. The Critical Bit
            MUST be set.</t>

            <t>The Protocol Names listed in the client's NTS Next Protocol
            Negotiation record denote those protocols which the client wishes
            to speak using the key material established through this NTS-KE
            session. The Protocol Names listed in the server's response MUST
            comprise a subset of those listed in the request, and denote those
            protocols which the server is willing and able to speak using the
            key material established through this NTS-KE session. The client
            MAY proceed with one or more of them. The request MUST list at
            least one protocol, but the response MAY be empty.</t>
          </section>

          <section title="Error">
            <t>The Error record has a Record Type number of 2. Its body is
            exactly two octets long, consisting of an unsigned 16-bit integer
            in network byte order, denoting an error code. The Critical Bit
            MUST be set.</t>

            <t>Clients MUST NOT include Error records in their request. If
            clients receive a server response which includes an Error record,
            they MUST discard any negotiated key material and MUST NOT proceed
            to the Next Protocol.</t>

            <t>The following error code are defined. <list style="symbols">
                <t>Error code 0 means "Unrecognized Critical Record". The
                server MUST respond with this error code if the request
                included a record which the server did not understand and
                which had its Critical Bit set. The client SHOULD NOT retry
                its request without modification.</t>

                <t>Error code 1 means "Bad Request". The server MUST respond
                with this error if, upon the expiration of an
                implementation-defined timeout, it has not yet received a
                complete and syntactically well-formed request from the
                client. This error is likely to be the result of a dropped
                packet, so the client SHOULD start over with a new DTLS
                handshake and retry its request.</t>
              </list></t>
          </section>

          <section title="Warning">
            <t>The Warning record has a Record Type number of 3. Its body is
            exactly two octets long, consisting of an unsigned 16-bit integer
            in network byte order, denoting a warning code. The Critical Bit
            MUST be set.</t>

            <t>Clients MUST NOT include Warning records in their request. If
            clients receive a server response which includes an Warning
            record, they MAY discard any negotiated key material and abort
            without proceeding to the Next Protocol. Unrecognized warning
            codes MUST be treated as errors.</t>

            <t>This memo defines no warning codes.</t>
          </section>

          <section title="AEAD Algorithm Negotiation">
            <t>The AEAD Algorithm Negotiation record has a Record Type number
            of 4. Its body consists of a sequence of unsigned 16-bit integers
            in network byte order, denoting Numeric Identifiers from the IANA
            <xref target="RFC5116">AEAD registry</xref>. The Critical Bit MAY
            be set.</t>

            <t>If the NTS Next Protocol Negotiation record offers "ntp/4",this
            record MUST be included exactly once. Other protocols MAY require
            it as well.</t>

            <t>When included in a request, this record denotes which AEAD
            algorithms the client is willing to use to secure the Next
            Protocol, in decreasing preference order. When included in a
            response, this record denotes which algorithm the server chooses
            to use, or is empty if the server supports none of the algorithms
            offered.. In requests, the list MUST include at least one
            algorithm. In responses, it MUST include at most one. Honoring the
            client's preference order is OPTIONAL: servers may select among
            any of the client's offered choices, even if they are able to
            support some other algorithm which the client prefers more.</t>

            <t>Server implementations of <xref
            target="nts-extensions-for-ntpv4">NTS extensions for NTPv4</xref>
            MUST support AEAD_AES_128_GCM (Numeric Identifier 1). That is, if
            the client includes AEAD_AES_128_GCM in its AEAD Algorithm
            Negotiation record, and the server accepts the "ntp/4" protocol in
            its NTS Next Protocol Negotiation record, then the server's AEAD
            Algorithm Negotiation record MUST NOT be empty.</t>
          </section>

          <section title="New Cookie for NTPv4">
            <t>The New Cookie for NTPv4 record has a Record Type number of 5.
            The contents of its body SHALL be implementation-defined and
            clients MUST NOT attempt to interpret them. See [[TODO]] for a
            RECOMMENDED construction.</t>

            <t>Clients MUST NOT send records of this type. Servers MUST send
            at least one record of this type, and SHOULD send eight of them,
            if they accept "ntp/4" as a Next Protocol. The Critical Bit SHOULD
            NOT be set.</t>
          </section>
        </section>

        <section title="Key Extraction (generally)">
          <t>Following a successful run of the NTS-KE protocol, key material
          SHALL be extracted according to <xref target="RFC5705">RFC
          5705</xref>. Inputs to the exporter function are to be constructed
          in a manner specific to the negotiated Next Protocol. However, all
          protocols which utilize NTS-KE MUST conform to the following two
          rules: <list style="symbols">
              <t>The disambiguating label string MUST be
              "EXPORTER-network-time-security/1".</t>

              <t>The per-association context value MUST be provided, and MUST
              begin with the 16-octet Protocol Name which was negotiated as a
              Next Protocol.</t>
            </list></t>
        </section>
      </section>

      <section anchor="nts-extensions-for-ntpv4"
               title="NTS Extensions for NTPv4">
        <section title="Key Extraction (for NTPv4)">
          <t>Following a successful run of the NTS-KE protocol wherein "ntp/4"
          is selected as a Next Protocol, two AEAD keys SHALL be extracted: a
          client-to-server (C2S) key and a server-to-client (S2C) key. These
          keys SHALL be computed according to <xref target="RFC5705">RFC
          5705</xref>, using the following inputs. <list>
              <t>The disambiguating label string SHALL be
              "EXPORTER-network-time-security/1".</t>

              <t>The per-association context value SHALL consist of the
              following 19 octets: <list>
                  <t>The first 16 octets SHALL be (in hexadecimal):<vspace
                  blankLines="1"/>
                  &nbsp;&nbsp;6E&nbsp;74&nbsp;70&nbsp;2F&nbsp;34&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00</t>

                  <t>The next two octets SHALL be the Numeric Identifier of
                  the negotiated AEAD Algorithm, in network byte order.</t>

                  <t>The final octet SHALL be 0x00 for the C2S key and 0x01
                  for the S2C key.</t>
                </list></t>
            </list> Implementations wishing to derive additional keys for
          private or experimental use MUST NOT do so by extending the
          above-specified syntax for per-association context values. Instead,
          they SHOULD use their own disambiguating label string. Note that RFC
          5705 provides that disambiguating label strings beginning with
          "EXPERIMENTAL" MAY be used without IANA registration.</t>
        </section>

        <section title="Packet structure overview">
          <t>In general, an NTS-protected NTPv4 packet consists of: <list>
              <t>The usual 48-octet NTP header, which is authenticated but not
              encrypted.</t>

              <t>Some extensions which are authenticated but not
              encrypted.</t>

              <t>An NTS extension which contains AEAD output (i.e., an
              authentication tag and possible ciphertext). The corresponding
              plaintext, if non-empty, consists of some extensions which
              benefit from both encryption and authentication.</t>

              <t>Possibly, some additional extensions which are neither
              encrypted nor authenticated. These are discarded by the
              receiver. [[Ed. Note: right now there's no good reason for the
              sender to include anything here, but eventually there might be.
              We've seen <xref target="RFC7821">Checksum Complement</xref> and
              LAST-EF as two examples of semantically-void extensions that are
              included to satsify constraints imposed lower on the protocol
              stack, and while there's no reason to use either of these on
              NTS-protected packets, I think we could see similar examples in
              the future. So, rejecting packets with unauthenticated
              extensions could cause interoperability problems, while
              accepting and processing those extensions would of course be a
              security risk. Thus, I think "allow and discard" is the correct
              policy.]]</t>
            </list></t>

          <t>Always included among the authenticated or
          authenticated-and-encrypted extensions are a cookie extension and a
          unique-identifier extension. The purpose of the cookie extension is
          to enable the server to offload storage of session state onto the
          client. The purpose of the unique-identifier extension is to protect
          the client from replay attacks.</t>
        </section>

        <section title="The Unique Identifier extension">
          <t>The Unique Identifier extension has a Field Type of [[TBD]]. When
          the extension is included in a client packet (mode 3), its body
          SHALL consist of a string of octets generated uniformly at random.
          The string SHOULD be 32 octets long. When the extension is included
          in a server packet (mode 4), its body SHALL contain the same octet
          string as was provided in the client packet to which the server is
          responding. Its use in modes other than client/server is not
          defined.</t>

          <t>The Unique Identifier extension provides the client with a
          cryptographically strong means of detecting replayed packets. It may
          also be used standalone, without NTS, in which case it provides the
          client with a means of detecting spoofed packets from off-path
          attackers. Historically, NTP's origin timestamp field has played
          both these roles, but for cryptographic purposes this is suboptimal
          because it is only 64 bits long and, depending on implementation
          details, most of those bits may be predictable. In contrast, the
          Unique Identifier extension enables a degree of unpredictability and
          collision-resistance more consistent with cryptographic best
          practice.</t>

          <t>[[TODO: consider using separate extension types for request and
          response, thus allowing for use in symmetric mode. But proper
          handling in the presence of dropped packets needs to be documented
          and involves a lot of subtlety.]]</t>
        </section>

        <section title="The NTS Cookie extension">
          <t>The NTS Cookie extension has a Field Type of [[TBD]]. Its purpose
          is to carry information which enables the server to recompute keys
          and other session state without having to store any per-client
          state. The contents of its body SHALL be implementation-defined and
          clients MUST NOT attempt to interpret them. See [[TODO]] for a
          RECOMMENDED construction. The NTS Cookie extension MUST NOT be
          included in NTP packets whose mode is other than 3 (client) or 4
          (server).</t>
        </section>

        <section title="The NTS Cookie Placeholder extension">
          <t>The NTS Cookie Placeholder extension has a Field Type of [[TBD]].
          When this extension is included in a client packet (mode 3), it
          communicates to the server that the client wishes it to send
          additional cookies in its response. This extension MUST NOT be
          included in NTP packets whose mode is other than 3.</t>

          <t>Whenever an NTS Cookie Placeholder extension is present, it MUST
          be accompanied by an NTS Cookie extension, and the body length of
          the NTS Cookie Placeholder extension MUST be the same as the body
          length of the NTS Cookie Extension. (This length requirement serves
          to ensure that the response will not be larger than the request, in
          order to improve timekeeping precision and prevent DDoS
          amplification). The contents of the NTS Cookie Placeholder
          extension's body are undefined and, aside from checking its length,
          MUST be ignored by the server.</t>
        </section>

        <section title="The NTS Authenticator and Encrypted Extensions extension">
          <t>The NTS Authenticator and Encrypted Extensions extension is the
          central cryptographic element of an NTS-protected NTP packet. Its
          Field Type is [[TBD]] and the format of its body SHALL be as
          follows: <list>
              <t>Nonce length: two octets in network byte order, giving the
              length of the Nonce field.</t>

              <t>Nonce: a nonce as required by the negotiated AEAD
              Algorithm.</t>

              <t>Ciphertext: the output of the negotiated AEAD Algorithm. The
              structure of this field is determined by the negotiated
              algorithm, but it typically contains an authentication tag in
              addition to the actual ciphertext.</t>

              <t>Padding: between 1 and 24 octets of padding, with every octet
              set to the number of padding octets included, e.g., "01", "02
              02", or "03 03 03". The number of padding bytes should be chosen
              in order to comply with the <xref target="RFC7822">RFC
              7822</xref> requirement that (in the absence of a legacy MAC)
              extensions have a total length in octets (including the four
              octets for the type and length fields) which is at least 28 and
              divisible by 4. At least one octet of padding MUST be included,
              so that implementations can unambiguously delimit the end of the
              ciphertext from the start of the padding.</t>
            </list></t>

          <t>The Ciphertext field SHALL be formed by providing the following
          inputs to the negotiated AEAD Algorithm: <list>
              <t>K: For packets sent from the client to the server, the C2S
              key SHALL be used. For packets sent from the server to the
              client, the S2C key SHALL be used.</t>

              <t>A: The associated data SHALL consist of the portion of the
              NTP packet beginning from the start of the NTP header and ending
              at the end of the last extension which precedes the NTS
              Authenticator and Encrypted Extensions extension.</t>

              <t>P: The plaintext SHALL consist of all (if any) extensions to
              be encrypted.</t>

              <t>N: The nonce SHALL be formed however required by the
              negotiated AEAD Algorithm.</t>
            </list></t>

          <t>The NTS Authenticator and Encrypted Extensions extension MUST NOT
          be included in NTP packets whose mode is other than 3 (client) or 4
          (server).</t>
        </section>

        <section title="Protocol details">
          <t>A client sending an NTS-protected request SHALL include the
          following extensions: <list>
              <t>Exactly one Unique Identifier extension, which MUST be
              authenticated and MUST NOT be encrypted [[Ed. Note: so that if
              the server can't decrypt the request, it can still echo back the
              Unique Identifier in the NTS NAK it sends]]. MUST NOT duplicate
              those of any previous request.</t>

              <t>Exactly one NTS Cookie extension, which MUST be authenticated
              and MUST NOT be encrypted. The cookie MUST be one which the
              server previously provided the client; it may have been provided
              during the NTS-KE handshake or in response to a previous
              NTS-protected NTP request. To protect client's privacy, the same
              cookie SHOULD NOT be included in multiple requests. If the
              client does not have any cookies that it has not already sent,
              it SHOULD re-run the NTS-KE protocol before continuing.</t>

              <t>Exactly one NTS Authenticator and Encrypted Extensions
              extension, generated using an AEAD Algorithm and C2S key
              established through NTS-KE.</t>
            </list></t>

          <t>The client MAY include one or more NTS Cookie Placeholder
          extensions, which MUST be authenticated and MAY be encrypted. The
          number of NTS Cookie Placeholder extensions that the client includes
          SHOULD be such that if the client includes N placeholders and the
          server sends back N+1 cookies, the number of unused cookies stored
          by the client will come to eight. When both the client and server
          adhere to all cookie-management guidance provided in this memo, the
          number of placeholder extensions will equal the number of dropped
          packets since the last successful volley.</t>

          <t>The client MAY include additional (non-NTS-related) extensions,
          which MAY appear prior to the NTS Authenticator and Encrypted
          Extensions extension (therefore authenticated but not encrypted),
          within it (therefore encrypted and authenticated), or after it
          (therefore neither encrypted nor authenticated). In general,
          however, the server MUST discard any unauthenticated extensions and
          process the packet as though they were not present. Servers MAY
          implement exceptions to this requirement for particular extensions
          if their specification explicitly provides for such.</t>

          <t>Upon receiving an NTS-protected request, the server SHALL
          (through some implementation-defined mechanism) use the cookie to
          recover the AEAD Algorithm, C2S key, and S2C key associated with the
          request, and then use the C2S key to authenticate the packet and
          decrypt the ciphertext. If the cookie is valid and authentication
          and decryption succeed, then the server SHALL include the following
          extensions in its response: <list>
              <t>Exactly one Unique Identifier extension, which MUST be
              authenticated, MUST NOT be encrypted, and whose contents SHALL
              echo those provided by the client.</t>

              <t>Exactly one NTS Authenticator and Encrypted Extensions
              extension, generated using the AEAD algorithm and S2C key
              recovered from the cookie provided by the client.</t>

              <t>One or more NTS Cookie extensions, which MUST be
              authenticated and encrypted. The number of NTS Cookie extensions
              included SHOULD be equal to, and MUST NOT exceed, one plus the
              number of valid NTS Cookie Placeholder extensions included in
              the request.</t>
            </list></t>

          <t>The server MAY include additional (non-NTS-related) extensions,
          which MAY appear prior to the NTS Authenticator and Encrypted
          Extensions extension (therefore authenticated but not encrypted),
          within it (therefore encrypted and authenticated), or after it
          (therefore neither encrypted nor authenticated). In general,
          however, the client MUST discard any unauthenticated extensions and
          process the packet as though they were not present. Clients MAY
          implement exceptions to this requirement for particular extensions
          if their specification explicitly provides for such.</t>

          <t>If the server is unable to validate the cookie or authenticate
          the request, it SHOULD respond with a Kiss-o'-Death packet (see
          <xref target="RFC5905">RFC 5905, Section 7.4)</xref>) with kiss code
          "NTSN" (meaning "NTS NAK"). Such a response MUST include exactly one
          Unique Identifier extension whose contents SHALL echo those provided
          by the client. It MUST NOT include any NTS Cookie or NTS
          Authenticator and Encrypted Extensions extension. [[Ed. Note: RFC
          5905 already provides the kiss code "CRYP" meaning "Cryptographic
          authentication or identification failed" but I think this is meant
          to be Autokey-specific.]]</t>

          <t>Upon receiving an NTS-protected response, the client MUST verify
          that the Unique Identifier matches that of an outstanding request,
          and that the packet is authentic under the S2C key associated with
          that request. If either of these checks fails, the packet MUST be
          discarded without further processing.</t>

          <t>Upon receiving an NTS NAK, the client MUST verify that the Unique
          Identifier matches that of an outstanding request. If this check
          fails, the packet MUST be discarded without further processing. If
          this check passes, the client SHOULD discard all cookies and AEAD
          keys associated with the server which sent the NAK and initiate a
          fresh NTS-KE handshake.</t>
        </section>
      </section>

      <section anchor="recommended-format-for-nts-cookies"
               title="Recommended format for NTS cookies">
        <t>This section provides a RECOMMENDED way for servers to construct
        NTS cookies. Clients MUST NOT examine the cookie under the assumption
        that it is constructed according to this section.</t>

        <t>The role of cookies in NTS is closely analagous to that of session
        cookies in TLS. Accordingly, the thematic resemblance of this section
        to <xref target="RFC5077">RFC 5077</xref> is deliberate, and the
        reader should likewise take heed of its security considerations.</t>

        <t>Servers should select an AEAD algorithm which they will use to
        encrypt and authenticate cookies. The chosen algorithm should be one
        such as <xref target="RFC5297">AEAD_AES_SIV_CMAC_256</xref> which
        resists accidential nonce reuse, and it need not be the same as the
        one that was negotiated with the client. Servers should randomly
        generate and store a master AEAD key `K`. Servers should additionally
        choose a non-secret, unique value `I` as key-identifier for `K`.</t>

        <t>Servers should periodically (e.g., once daily) generate a new pair
        (I,K) and immediately switch to using these values for all
        newly-generated cookies. Immediately following each such key rotation,
        servers should securely erase any keys generated two or more rotation
        periods prior. Servers should continue to accept any cookie generated
        using keys that they have not yet erased, even if those keys are no
        longer current. Erasing old keys provides for forward secrecy,
        limiting the scope of what old information can be stolen if a master
        key is somehow compromised. Holding on to a limited number of old keys
        allows clients to seamlessly transition from one generation to the
        next without having to perform a new NTS-KE handshake.</t>

        <t>[[TODO: discuss key management considerations for load-balanced
        servers]]</t>

        <t>To form a cookie, servers should first form a plaintext `P`
        consisting of the following fields: <list>
            <t>The AEAD algorithm negotiated during NTS-KE</t>

            <t>The S2C key</t>

            <t>The C2S key</t>
          </list></t>

        <t>Servers should the generate a nonce `N` uniformly at random, and
        form AEAD output `C` by encrypting `P` under key `K` with nonce `N`
        and no associated data.</t>

        <t>The cookie should consist of the tuple `(I,N,C)`.</t>

        <t>[[TODO: explicitly specify how to verify and decrypt a cookie, not
        just how to form one]]</t>
      </section>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <section title="Field Type Registry">
        <t>Within the "NTP Extensions Field Types" registry table, add the
        field types:</t>

        <figure>
          <artwork><![CDATA[Field Type  Meaning                              References     
----------  ------------------------------------ ----------
TBD1        NTS-Related Content                  [this doc]
TBD2        NTS-Related Content                  [this doc]
TBD3        NTS-Related Content                  [this doc]]]></artwork>
        </figure>
      </section>

      <section title="SMI Security for S/MIME CMS Content Type Registry">
        <t>Within the "SMI Security for S/MIME CMS Content Type
        (1.2.840.113549.1.9.16.1)" table, add one content type identifier:</t>

        <figure>
          <artwork><![CDATA[Decimal  Description                                   References
-------  --------------------------------------------  ----------
TBD4     id-ct-nts-ntsForNtpMessageAuthenticationCode  [this doc]]]></artwork>
        </figure>
      </section>

      <section title="DTLS-Based Key Exchange">
        <t>IANA is requested to allocate an entry in the Service Name and
        Transport Protocol Port Number Registry as follows: <list>
            <t>Service Name: nts</t>

            <t>Transport Protocol: udp</t>

            <t>Assignee: IESG &lt;iesg@ietf.org&gt;</t>

            <t>Contact: IETF Chair &lt;chair@ietf.org&gt;</t>

            <t>Description: Network Time Security</t>

            <t>Reference: [[this memo]]</t>

            <t>Port Number: selected by IANA from the user port range</t>
          </list></t>

        <t>IANA is requested to allocate the following two entries in the
        Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry:
        <list>
            <t>Protocol: Network Time Security Key Establishment, version
            1</t>

            <t>Identification
            Sequence:<vspace/>&nbsp;&nbsp;0x6E&nbsp;0x74&nbsp;0x73&nbsp;0x6B&nbsp;0x65&nbsp;0x2F&nbsp;0x31&nbsp;("ntske/1")</t>

            <t>Reference: [[this memo]]</t>
          </list> <vspace/> <list>
            <t>Protocol: Network Time Protocol, version 4</t>

            <t>Identification
            Sequence:<vspace/>&nbsp;&nbsp;0x6E&nbsp;0x74&nbsp;0x70&nbsp;0x2F&nbsp;0x34&nbsp;("ntp/4")</t>

            <t>Reference: [[this memo]]</t>
          </list></t>

        <t>IANA is requested to allocate the following entry in the TLS
        Exporter Label Registry:</t>

        <texttable>
          <ttcol>Value</ttcol>

          <ttcol>DTLS-OK</ttcol>

          <ttcol>Reference</ttcol>

          <ttcol>Note</ttcol>

          <c>EXPORTER-network-time-security/1</c>

          <c>Y</c>

          <c>[[this memo]]</c>

          <c/>
        </texttable>

        <t>IANA is requested to allocate the following entries in the registry
        of NTP Kiss-o'-Death codes:</t>

        <texttable>
          <ttcol>Code</ttcol>

          <ttcol>Meaning</ttcol>

          <c>DTLS</c>

          <c>Packet conveys a DTLS record</c>

          <c>NTSN</c>

          <c>NTS NAK</c>
        </texttable>

        <t>IANA is requested to allocate the following entries in the NTP
        Extensions Field Types registry:</t>

        <texttable>
          <ttcol>Field Type</ttcol>

          <ttcol>Meaning</ttcol>

          <ttcol>Reference</ttcol>

          <c>[[TBD]]</c>

          <c>DTLS Record</c>

          <c>[[this memo]]</c>

          <c>[[TBD]]</c>

          <c>Unique Identifier</c>

          <c>[[this memo]]</c>

          <c>[[TBD]]</c>

          <c>NTS Cookie</c>

          <c>[[this memo]]</c>

          <c>[[TBD]]</c>

          <c>NTS Authenticator and Encrypted Extensions</c>

          <c>[[this memo]]</c>
        </texttable>

        <t>IANA is requested to create a new registry entitled "Network Time
        Security Key Establishment Record Types". Entries SHALL have the
        following fields: <list>
            <t>Type Number (REQUIRED): An integer in the range 0&ndash;32767
            inclusive</t>

            <t>Description (REQUIRED): short text description of the purpose
            of the field</t>

            <t>Set Critical Bit (REQUIRED): One of "MUST", "SHOULD", "MAY",
            "SHOULD NOT", or "MUST NOT"</t>

            <t>Reference (REQUIRED): A reference to a document specifying the
            semantics of the record.</t>
          </list></t>

        <t>The policy for allocation of new entries in this registry SHALL
        vary by the Type Number, as follows: <list>
            <t>0&ndash;1023: Standards Action</t>

            <t>1024&ndash;16383: Specification Required</t>

            <t>16384&ndash;32767: Private and Experimental Use</t>
          </list></t>

        <t>Applications for new entries SHALL specify the contents of the
        Description, Set Critical Bit and Reference fields and which of the
        above ranges the Type Number should be allocated from. Applicants MAY
        request a specific Type Number, and such requests MAY be granted at
        the registrar's discretion.</t>

        <t>The initial contents of this registry SHALL be as follows:</t>

        <texttable>
          <ttcol>Field Number</ttcol>

          <ttcol>Description</ttcol>

          <ttcol>Critical</ttcol>

          <ttcol>Reference</ttcol>

          <c>0</c>

          <c>End of message</c>

          <c>MUST</c>

          <c>[[this memo]]</c>

          <c>1</c>

          <c>NTS next protocol negotiation</c>

          <c>MUST</c>

          <c>[[this memo]]</c>

          <c>2</c>

          <c>Error</c>

          <c>MUST</c>

          <c>[[this memo]]</c>

          <c>3</c>

          <c>Warning</c>

          <c>MUST</c>

          <c>[[this memo]]</c>

          <c>4</c>

          <c>AEAD algorithm negotation</c>

          <c>MAY</c>

          <c>[[this memo]]</c>

          <c>5</c>

          <c>New cookie for NTPv4</c>

          <c>SHOULD NOT</c>

          <c>[[this memo]]</c>

          <c>16384&ndash;32767</c>

          <c>Reserved for Private &amp; Experimental Use</c>

          <c>MAY</c>

          <c>[[this memo]]</c>
        </texttable>

        <t>IANA is requested to create a new registry entitled "Network Time
        Security Next Protocols". Entries SHALL have the following fields:
        <list>
            <t>Protocol Name (REQUIRED): a sequence of 16 octets. Shorter
            sequences SHALL implicitly be right-padded with null octets
            (0x00).</t>

            <t>Human-Readable Name (OPTIONAL): if the sequence of octets
            making up the protocol name intentionally represent a valid <xref
            target="RFC3629">UTF-8</xref> string, this field SHALL consist of
            that string.</t>

            <t>Reference (RECOMMENDED): a reference to a relevant
            specification document. If no relevant document exists, a
            point-of-contact for questions regarding the entry SHOULD be
            listed here in lieu.</t>
          </list></t>

        <t>Applications for new entries in this registry SHALL specify all
        desired fields, and SHALL be granted on a First Come, First Serve
        basis. Protocol Names beginning with 0x78 0x2D ("x-") SHALL be
        reserved for Private or Experimental Use, and SHALL NOT be registered.
        The reserved entry "ptp/2" may be updated or released by a future
        Standards Action.</t>

        <t>The initial contents of this registry SHALL be as follows:</t>

        <texttable>
          <ttcol>Protocol Name</ttcol>

          <ttcol>Human-Readable Name</ttcol>

          <ttcol>Reference</ttcol>

          <c>&nbsp;&nbsp;0x6E&nbsp;0x74&nbsp;0x70&nbsp;0x2F&nbsp;0x34&nbsp;</c>

          <c>ntp/4</c>

          <c>[[this memo]]</c>

          <c>&nbsp;&nbsp;0x70&nbsp;0x74&nbsp;0x70&nbsp;0x2F&nbsp;0x32&nbsp;</c>

          <c>ptp/2</c>

          <c>Reserved by [[this memo]]</c>
        </texttable>

        <t>IANA is requested to create two new registries entitled "Network
        Time Security Error Codes" and "Network Time Security Warning Codes".
        Entries in each SHALL have the following fields: <list>
            <t>Number (REQUIRED): a 16-bit unsigned integer</t>

            <t>Description (REQUIRED): a short text description of the
            condition.</t>

            <t>Reference (REQUIRED): a reference to a relevant specification
            document.</t>
          </list> The policy for allocation of new entries in these registries
        SHALL vary by their Number, as follows: <list>
            <t>0&ndash;1023: Standards Action</t>

            <t>1024&ndash;32767: Specification Required</t>

            <t>32768&ndash;65535: Private and Experimental Use</t>
          </list></t>

        <t>The initial contents of the Network Time Security Error Codes
        Registry SHALL be as follows:</t>

        <texttable>
          <ttcol>Number</ttcol>

          <ttcol>Description</ttcol>

          <ttcol>Reference</ttcol>

          <c>0</c>

          <c>Unrecognized Critical Extension</c>

          <c>[[this memo]]</c>

          <c>1</c>

          <c>Bad Request</c>

          <c>[[this memo]]</c>
        </texttable>

        <t>The Network Time Security Warning Codes Registry SHALL initially be
        empty.</t>
      </section>
    </section>

    <section title="Security Considerations">
      <t>All security considerations described in <xref
      target="I-D.ietf-ntp-network-time-security"/> have to be taken into
      account. The application of NTS to NTP requires the following additional
      considerations.</t>

      <section title="Usage of NTP Pools">
        <t>The certification-based authentication scheme described in <xref
        target="I-D.ietf-ntp-network-time-security"/> is not applicable to the
        concept of NTP pools. Therefore, NTS is unable to provide secure usage
        of NTP pools.</t>
      </section>

      <section title="Initial Verification of the Server Certificates">
        <t>The client may wish to verify the validity of certificates during
        the initial association phase. Since it generally has no reliable time
        during this initial communication phase, it is impossible to verify
        the period of validity of the certificates.</t>
      </section>

      <section title="Treatment of Initial Messages">
        <t>NTP packets which contains extension fields with key exchange
        messages do not provide integrity and authenticity protection of the
        included time stamps. Therefore these NTP packets MUST NOT be used for
        clock synchronization. Otherwise an initial attack on the client's
        clock <xref target="attacking-ntp"/> can potentially circumvent the
        employed security measures of later messages <xref
        target="delorean"/>.</t>
      </section>

      <section title="DTLS-Related Issues">
        <t>... TBD</t>
      </section>

      <section anchor="DelayAttack" title="Delay Attack">
        <t>In a packet delay attack, an adversary with the ability to act as a
        MITM delays time synchronization packets between client and server
        asymmetrically <xref target="RFC7384"/>. This prevents the client from
        accurately measuring the network delay, and hence its time offset to
        the server <xref target="Mizrahi"/>. The delay attack does not modify
        the content of the exchanged synchronization packets. Therefore,
        cryptographic means do not provide a feasible way to mitigate this
        attack. However, the maximum error that an adversary can introduced is
        bounded by half of the round trip delay. Also, several
        non-cryptographic precautions can be taken in order to detect this
        attack.</t>

        <t><list style="numbers">
            <t>Usage of multiple time servers: this enables the client to
            detect the attack, provided that the adversary is unable to delay
            the synchronization packets between the majority of servers. This
            approach is commonly used in NTP to exclude incorrect time servers
            <xref target="RFC5905"/>.</t>

            <t>Multiple communication paths: The client and server utilize
            different paths for packet exchange as described in the I-D <xref
            target="I-D.ietf-tictoc-multi-path-synchronization"/>. The client
            can detect the attack, provided that the adversary is unable to
            manipulate the majority of the available paths <xref
            target="Shpiner"/>. Note that this approach is not yet available,
            neither for NTP nor for PTP.</t>

            <t>Usage of an encrypted connection: the client exchanges all
            packets with the time server over an encrypted connection (e.g.
            IPsec). This measure does not mitigate the delay attack, but it
            makes it more difficult for the adversary to identify the time
            synchronization packets.</t>

            <t>Introduction of a threshold value for the delay time of the
            synchronization packets. The client can discard a time server if
            the packet delay time of this time server is larger than the
            threshold value.</t>
          </list></t>
      </section>
    </section>

    <section title="Privacy Considerations">
      <section title="Confidentiality">
        <t>The actual time synchronization data in NTP packets does not
        involve any information that needs to be kept secret. There also does
        not seem to be any necessity to disguise the nature of an NTP
        association. This is why content confidentiality is a non-objective
        for this document.</t>
      </section>

      <section title="Unlinkability">
        <t>The scenario that is to be prevented is one where whenever a new
        network address is associated with a device (e.g. because said device
        moved between different networks), a passive attacker is able to link
        said new address with one that was formerly used by the device,
        because of recognizable data that the device persistently sends as
        part of an NTS-secured NTP association. This is the justification for
        continually supplying the client with fresh cookies, so that a cookie
        never represents recognizable data in the sense outlined above.</t>

        <t>Note that the objective of NTS regarding unlinkability is merely to
        not leak any additional data that would cause linkability. NTS does
        not rectify legacy linkability issues that are already present in NTP.
        To minimize the risk of being tracked by a passive adversary the NTP
        client has to minimize the information it transmits within a client
        request (mode 3 packet) as described in the draft
        "I-D.draft-dfranke-ntp-data-minimization".</t>

        <t>Also note that normal NTP clients should not act as NTP servers.
        Otherwise, an active adversary may be able to abuse the client's
        server responses (mode 4 packets) for its tracking. This is done by
        [tbd].</t>
      </section>
    </section>

    <section anchor="Acknowledgements" title="Acknowledgements">
      <t>The authors would like to thank Richard Barnes, Steven Bellovin,
      Sharon Goldberg, Russ Housley, Martin Langer, Miroslav Lichvar, Aanchal
      Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue, Eric K. Rescorla,
      Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich Salz, Brian Sniffen,
      Susan Sons, Douglas Stebila, Harlan Stenn, Martin Thomson, and Richard
      Welty for contributions to this document. on the design of NTS.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include='reference.RFC.2119'?>

      <?rfc include='reference.RFC.3394'?>

      <?rfc include='reference.RFC.4082'?>

      <?rfc include='reference.RFC.4634'?>

      <?rfc include='reference.RFC.5652'?>

      <?rfc include='reference.I-D.draft-ietf-ntp-cms-for-nts-message-06'?>

      <?rfc include='reference.I-D.draft-ietf-ntp-network-time-security-13'?>

      <?rfc include='reference.I-D.draft-ietf-ntp-extension-field-07'?>

      <?rfc include='reference.I-D.draft-ietf-tictoc-multi-path-synchronization-06'?>

      <?rfc include='reference.RFC.3629'?>

      <?rfc include='reference.RFC.5116'?>

      <?rfc include='reference.RFC.5297'?>

      <?rfc include='reference.RFC.5705'?>

      <?rfc include='reference.RFC.5746'?>

      <?rfc include='reference.RFC.5905'?>

      <?rfc include='reference.RFC.6347'?>

      <?rfc include='reference.RFC.7465'?>

      <?rfc include='reference.RFC.7507'?>

      <?rfc include='reference.RFC.7627'?>

      <?rfc include='reference.RFC.7301'?>

      <?rfc include='reference.RFC.7822'?>
    </references>

    <references title="Informative References">
      <?rfc include='reference.RFC.7384'?>

      <?rfc include='reference.RFC.5077'?>

      <?rfc include='reference.RFC.7821'?>

      <reference anchor="IEC.61588_2009"
                 target="http://ieeexplore.ieee.org/servlet/opac?punumber=4839000">
        <front>
          <title>Precision clock synchronization protocol for networked
          measurement and control systems</title>

          <author>
            <organization>IEEE/IEC</organization>
          </author>

          <date day="2" month="February" year="2009"/>

          <abstract>
            <t>A protocol is provided in this standard that enables precise
            synchronization of clocks in measurement and control systems
            implemented with technologies such as network communication, local
            computing, and distributed objects. The protocol is applicable to
            systems communicating via packet networks. Heterogeneous systems
            are enabled that include clocks of various inherent precision,
            resolution, and stability to synchronize. System-wide
            synchronization accuracy and precision in the sub-microsecond
            range are supported with minimal network and local clock computing
            resources. Simple systems are installed and operated without
            requiring the management attention of users because the default
            behavior of the protocol allows for it.</t>
          </abstract>
        </front>

        <seriesInfo name="IEEE" value="1588-2008(E)"/>

        <seriesInfo name="IEC" value="61588:2009(E)"/>

        <seriesInfo name="DOI" value="10.1109/IEEESTD.2009.4839002"/>
      </reference>

      <reference anchor="Mizrahi" target="">
        <front>
          <title>A game theoretic analysis of delay attacks against time
          synchronization protocols</title>

          <author fullname="Tal Mizrahi" initials="T" surname="Mizrahi">
            <organization abbrev=""/>
          </author>

          <date day="" month="September" year="2012"/>
        </front>

        <seriesInfo name="in Proceedings of"
                    value="Precision Clock Synchronization for Measurement Control and Communication, ISPCS 2012, pp. 1-6"/>
      </reference>

      <reference anchor="delorean">
        <front>
          <title>Bypassing HTTP Strict Transport Security</title>

          <author fullname="Jose Selvi">
            <organization/>
          </author>

          <date year="2014"/>

          <abstract>
            <t>For the last few years, some different attacks against SSL/TLS
            have been released. Some of them based on cryptography or protocol
            weaknesses such as BEAST, CRIME, BREACH, etc, and some others,
            such as SSLStrip, based on rewriting HTTPS links into HTTP ones
            and keep user communications always in HTTP. In order to protect
            users against SSLStrip attacks, a new protection called HTTP
            Strict Transport Security (HSTS) has been developed and it&rsquo;s
            currently supported by most widely used browsers. However, under
            certain circumstances, an attacker could exploit an
            inter-operation vulnerability in order to bypass HTTP Strict
            Transport Security protection and use other well-known attack
            techniques such as SSLStrip. In this paper, we review the HSTS
            strengths and weaknesses, and we go in-depth on this
            inter-operation vulnerability and how it could be exploited.</t>
          </abstract>
        </front>
      </reference>

      <reference anchor="Shpiner">
        <front>
          <title>Multi-path Time Protocols</title>

          <author fullname="Alexander Shpiner, Yoram Revah, and Tal Mizrahi">
            <organization/>
          </author>

          <date month="September" year="2013"/>
        </front>

        <seriesInfo name="in Proceedings of"
                    value="IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication (ISPCS)"/>
      </reference>

      <reference anchor="attacking-ntp">
        <front>
          <title>Attacking the Network Time Protocol</title>

          <author fullname="Aanchal Malhotra, Isaac E. Cohen, Erik Brakke, and Sharon Goldberg">
            <organization/>
          </author>

          <date day="21" month="October" year="2015"/>
        </front>
      </reference>
    </references>

    <section title="Flow Diagrams of Client Behaviour">
      <figure anchor="fig_flow_unicast"
              title="The client's behavior in NTS unicast mode.">
        <artwork><![CDATA[+------------------------------>o
|                               |
|                               v
|                        +-------------+
|                        |Key Exchange |
|                        +------+------+
|                               |
|                               o<------------------------------+
|                               |                               |
|                               v                               |
|                     +-------------------+                     |
|                     |Time Sync. Messages|                     |
|                     +---------+---------+                     |
|                               |                               |
|                               v                               |
|                            +-----+                            |
|                            |Check|                            |
|                            +--+--+                            |
|                               |                               |
|            /------------------+------------------\            |
|           v                   v                   v           |
|     .-----------.      .-------------.        .-------.       |
|    ( MAC Failure )    ( Nonce Failure )      ( Success )      |
|     '-----+-----'      '------+------'        '---+---'       |
|           |                   |                   |           |
|           v                   v                   v           |
|    +-------------+     +-------------+     +--------------+   |
|    |Discard Data |     |Discard Data |     |Sync. Process |   |
|    +-------------+     +------+------+     +------+-------+   |
|           |                   |                   |           |
|           |                   |                   v           |
+-----------+                   +------------------>o-----------+]]></artwork>
      </figure>
    </section>
  </back>
</rfc>
