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NTP Working Group                                              D. Franke
Internet-Draft                                                    Akamai
Intended status: Standards Track                               D. Sibold
Expires: December 28, 2017                                    K. Teichel
                                                                     PTB
                                                           June 26, 2017


          Network Time Security for the Network Time Protocol
                  draft-ietf-ntp-using-nts-for-ntp-09

Abstract

   This memo specifies Network Time Security (NTS), a mechanism for
   using Transport Layer Security (TLS) and Authenticated Encryption
   with Associated Data (AEAD) to provide cryptographic security for the
   Network Time Protocol.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 28, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Protocol overview . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  TLS profile for Network Time Security . . . . . . . . . . . .   5
   4.  The NTS-encapsulated NTPv4 protocol . . . . . . . . . . . . .   6
   5.  The NTS Key Establishment protocol  . . . . . . . . . . . . .   8
     5.1.  NTS-KE record types . . . . . . . . . . . . . . . . . . .   9
       5.1.1.  End of Message  . . . . . . . . . . . . . . . . . . .   9
       5.1.2.  NTS Next Protocol Negotiation . . . . . . . . . . . .   9
       5.1.3.  Error . . . . . . . . . . . . . . . . . . . . . . . .   9
       5.1.4.  Warning . . . . . . . . . . . . . . . . . . . . . . .  10
       5.1.5.  AEAD Algorithm Negotiation  . . . . . . . . . . . . .  10
       5.1.6.  New Cookie for NTPv4  . . . . . . . . . . . . . . . .  11
     5.2.  Key Extraction (generally)  . . . . . . . . . . . . . . .  11
   6.  NTS Extensions for NTPv4  . . . . . . . . . . . . . . . . . .  11
     6.1.  Key Extraction (for NTPv4)  . . . . . . . . . . . . . . .  11
     6.2.  Packet structure overview . . . . . . . . . . . . . . . .  12
     6.3.  The Unique Identifier extension . . . . . . . . . . . . .  13
     6.4.  The NTS Cookie extension  . . . . . . . . . . . . . . . .  13
     6.5.  The NTS Cookie Placeholder extension  . . . . . . . . . .  13
     6.6.  The NTS Authenticator and Encrypted Extensions extension   14
     6.7.  Protocol details  . . . . . . . . . . . . . . . . . . . .  15
   7.  Recommended format for NTS cookies  . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  22
     9.1.  Avoiding DDoS amplification . . . . . . . . . . . . . . .  22
     9.2.  Initial verification of server certificates . . . . . . .  22
     9.3.  Usage of NTP pools  . . . . . . . . . . . . . . . . . . .  23
     9.4.  Delay attacks . . . . . . . . . . . . . . . . . . . . . .  24
     9.5.  Random number generation  . . . . . . . . . . . . . . . .  24
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  24
     10.1.  Unlinkability  . . . . . . . . . . . . . . . . . . . . .  24
     10.2.  Confidentiality  . . . . . . . . . . . . . . . . . . . .  25
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     12.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Terms and Abbreviations  . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28






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1.  Introduction

   This memo specifies Network Time Security (NTS), a cryptographic
   security mechanism for network time synchronization.  A complete
   specification is provided for application of NTS to the Network Time
   Protocol (NTP) [RFC5905].  However, certain sections of this memo are
   not inherently NTP-specific, and enable future work to apply them to
   other time synchronization protocols such as the Precision Time
   Protocol (PTP) [IEC.61588_2009].

1.1.  Objectives

   The objectives of NTS are as follows:

   o  Identity: Through the use of the X.509 PKI, implementations may
      cryptographically establish the identity of the parties they are
      communicating with

   o  Authentication: Implementations may cryptographically verify that
      any time synchronization packets are authentic, i.e., that they
      were produced by an identified party and have not been modified in
      transit.

   o  Confidentiality: Although basic time synchronization data is
      considered non-confidential and sent in the clear, NTS includes
      support for encrypting NTP extension fields.

   o  Replay prevention: Implementations may detect when a received time
      synchronization packet is a replay of a previous packet.

   o  Request-response consistency: Client implementations may verify
      that a time synchronization packet received from a server was sent
      in response to a particular request from the client.

   o  Unlinkability: For mobile clients, NTS will not leak any
      information which would permit a passive adversary to determine
      that two packets sent over different networks came from the same
      client.

   o  Non-amplification: implementations may avoid acting as DDoS
      amplifiers by never responding to a request with a packet larger
      than the request packet.

   o  Scalability: Servers implementations may serve large numbers of
      clients without having to retain any client-specific state.






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1.2.  Protocol overview

   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.

   In order to simulatenously serve these conflicting requirements, NTS
   is structured as a suite of three protocols:

      The "DTLS-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
      provides mutual replay protection and is suitable for symmetric
      and control modes, and is also secure for client/server mode but
      relatively wasteful of server resources.

      The "NTS Extensions for NTPv4" are a collection of NTP extension
      fields for cryptographically securing NTPv4 using prevoiously-
      established key material.  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.

      The "NTS Key Establishment" protocol (NTS-KE) is mechanism for
      establishing key material for use with the NTS extensions for
      NTPv4.  It uses TLS to exchange keys and negotiate some additional
      protocol options, but then quickly closes the TLS channel and
      permits the server to discard all associated state.  NTS-KE is not
      NTP-specific; it is designed to be extensible, and might be




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      extended to support key establishment for other protocols such as
      PTP.

   It is intended that NTP implementations will use DTLS-encapsulated
   NTPv4 to secure symmetric mode and control mode, and use NTS-KE
   followed by NTS Extensions for NTPv4 to secure client/server mode.
   NTS does not support NTP's broadcast mode.

   As previously stated, DTLS-encapsulated NTPv4 is trivial.  The
   communicating parties establish a DTLS session and then exchange
   arbitrary NTP packets as DTLS Application Data.

   The typical protocol flow for client/server mode is as follows.  The
   client connects to the server on the NTS TCP port and the two parties
   perform a TLS handshake.  Via the TLS channel, the parties negotiate
   some additional protocol parameters and the server sends the client a
   supply of cookies.  The parties use TLS key export [RFC5705] to
   extract key material which will be used in the next phase of the
   protocol.  This negotiation takes only a single round trip, after
   which the server closes the connection and discards all associated
   state.  At this point the NTS-KE phase of the protocol is complete.

   Time synchronization proceeds over the NTP UDP port.  The client
   sends the server an NTP client packet which includes several
   extension fields.  Included among these fields are a cookie
   (previously provided by the server), and an authentication tag,
   computed using key material extracted from the NTS-KE handshake.  The
   server uses the cookie to recover this key material (previously
   discarded to avoid maintaining state) and send back an authenticated
   response.  The response includes a fresh, encrypted cookie which the
   client then sends back in the clear with its next request.  (This
   constant refreshing of cookies is necessary in order to achieve NTS's
   unlinkability goal.)

2.  Requirements Language

   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 RFC 2119 [RFC2119].

3.  TLS profile for Network Time Security

   Network Time Security makes use of both TLS (for NTS Key
   Establishment) and DTLS (for NTS-encapsulated NTPv4).  In either
   case, the requirements and recommendations of this section are
   similar.  The notation "(D)TLS" refers to both TLS and DTLS.





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   Since securing time protocols is (as of 2017) a novel application of
   (D)TLS, no backward-compatibility concerns exist to justify using
   obsolete, insecure, or otherwise broken TLS features or versions.  We
   therefore put forward the following requirements and guidelines,
   roughly representing 2017's best practices.

   Implementations MUST NOT negotiate (D)TLS versions earlier than 1.2.

   Implementations willing to negotiate more than one possible version
   of (D)TLS SHOULD NOT respond to handshake failures by retrying with a
   downgraded protocol version.  If they do, they MUST implement
   [RFC7507].

   (D)TLS clients MUST NOT offer, and (D)TLS servers MUST not select,
   RC4 cipher suites.  [RFC7465]

   (D)TLS clients SHOULD offer, and (D)TLS servers SHOULD accept, the
   TLS Renegotiation Indication Extension [RFC5746].  Regardless, they
   MUST NOT initiate or permit insecure renegotiation. (*)

   (D)TLS clients SHOULD offer, and (D)TLS servers SHOULD accept, the
   TLS Session Hash and Extended Master Secret Extension [RFC7627]. (*)

   Use of the Application-Layer Protocol Negotation Extension [RFC7301]
   is integral to NTS and support for it is REQUIRED for
   interoperability.

   (*): Note that (D)TLS 1.3 or beyond may render the indicated
   recommendations inapplicable.

4.  The NTS-encapsulated NTPv4 protocol

   The NTS-encapsulated NTPv4 protocol proceeds in two parts.  The two
   endpoints carry out a DTLS handshake in conformance with Section 3,
   with the client offering (via an ALPN [RFC7301] 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.

   In addition to the requirements specified in Section 3,
   implementations MUST enforce the anti-replay mechanism specified in
   Section 4.1.2.6 of RFC 6347 [RFC6347] (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.




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   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 NTS Extensions for NTPv4
   (Section 6).

   Since DTLS-encapsulated NTPv4 sessions may carry arbitrary NTP
   packets, there is no prescribed implication from an implementation's
   role as a DTLS client vs. DTLS server, to its role in the
   application-level Network Time Protocol.  For example, it is entirely
   permissible for an implementation to initiate a DTLS handshake (thus
   acting in the role of DTLS client), and then once the handshake is
   completed, act as an NTP server with the DTLS server acting as an NTP
   client.  The following guidelines are offered as sensible default
   behavior.  Implementations may depart from this guidance if the user
   configures them to do so.

   Implementations typically should not use DTLS-encapsulated NTPv4 for
   client/server mode, instead preferring to use NTS-KE and NTS
   Extensions for NTPv4.  If DTLS-encapsulated NTPv4 is used for client/
   server mode, then the NTP client (mode 3) should be the DTLS client
   and the NTP server (mode 4) should be the DTLS server.

   For control mode (6), the party sending queries should be the DTLS
   client and the party responding to the queries should be the DTLS
   server.

   For symmetric operation between an active (mode 1) and passive (mode
   2) peer, the active peer should be the DTLS client and the passive
   peer should be the DTLS server.

   For symmetric operation between two active (mode 1) peers, both
   parties should attempt to initiate a DTLS session with their peer.
   If one handshake fails and the other succeeds, the successfully-
   established session should be used for traffic in both directions.
   If both handshakes succeed, either session may be used and packets
   should receive identical dispositon regardless of which of the two
   sessions they arrived over.  Inactive sessions may be timed out but
   the redundant session should not be proactively closed.

   If, likely as a result of user error, party A is configured as a
   symmetry active peer of party B, but party B is neither accepting
   DTLS handshakes from party A nor initiating one with it, then after a
   suitable number of failed attempts, party A may fall back to acting
   as an NTP client (mode 3) of party B using NTS-KE and NTS Extensions
   for NTPv4.




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5.  The NTS Key Establishment protocol

   The NTS key establishment protocol is conducted via TCP port
   [[TBD1]].  The two endpoints carry out a TLS handshake in conformance
   with Section 3, with the client offering (via an ALPN [RFC7301]
   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 TLS-protected channel), then the server SHALL send a single
   response followed by a TLS "Close notify" alert and then discard the
   channel state.

   The client's request and the server's response each SHALL consist of
   a sequence of records formatted according to Figure 1.  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.

   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                         .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

   The requirement that all NTS-KE messages be terminated by an End of
   Message record makes them self-delimiting.

   The fields of an NTS-KE record are defined as follows:

      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.

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



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      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.

      Record Body: the syntax and semantics of this field shall be
      determined by the Record Type.

5.1.  NTS-KE record types

   The following NTS-KE Record Types are defined.

5.1.1.  End of Message

   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.

5.1.2.  NTS Next Protocol Negotiation

   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-bit unsigned integers in network
   byte order.  Each integer represents a Protocol ID from the IANA
   Network Time Security Next Protocols registry.  The Critical Bit MUST
   be set.

   The Protocol IDs 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 IDs 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.

5.1.3.  Error

   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.

   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.

   The following error code are defined.



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      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.

      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 TLS handshake and retry its request.

5.1.4.  Warning

   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.

   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.

   This memo defines no warning codes.

5.1.5.  AEAD Algorithm Negotiation

   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 AEAD
   registry [RFC5116].  The Critical Bit MAY be set.

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

   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




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   choices, even if they are able to support some other algorithm which
   the client prefers more.

   Server implementations of NTS extensions for NTPv4 (Section 6) MUST
   support AEAD_AES_SIV_CMAC_256 [RFC5297] (Numeric Identifier 15).
   That is, if the client includes AEAD_AES_SIV_CMAC_256 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 Negotation record MUST NOT be empty.

5.1.6.  New Cookie for NTPv4

   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 Section 7 for a RECOMMENDED
   construction.

   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.

5.2.  Key Extraction (generally)

   Following a successful run of the NTS-KE protocol, key material SHALL
   be extracted according to RFC 5705 [RFC5705].  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:

      The disambiguating label string MUST be "EXPORTER-network-time-
      security/1".

      The per-association context value MUST be provided, and MUST begin
      with the two-octet Protocol ID which was negotiated as a Next
      Protocol.

6.  NTS Extensions for NTPv4

6.1.  Key Extraction (for NTPv4)

   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 RFC 5705 [RFC5705], using the
   following inputs.





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      The disambiguating label string SHALL be "EXPORTER-network-time-
      security/1".

      The per-association context value SHALL consist of the following
      five octets:

         The first two octets SHALL be zero.

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

         The final octet SHALL be 0x00 for the C2S key and 0x01 for the
         S2C key.

   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.

6.2.  Packet structure overview

   In general, an NTS-protected NTPv4 packet consists of:

      The usual 48-octet NTP header, which is authenticated but not
      encrypted.

      Some extensions which are authenticated but not encrypted.

      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.

      Possibly, some additional extensions which are neither encrypted
      nor authenticated.  These are discarded by the receiver.

   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.







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6.3.  The Unique Identifier extension

   The Unique Identifier extension has a Field Type of [[TBD2]].  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.

   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.

6.4.  The NTS Cookie extension

   The NTS Cookie extension has a Field Type of [[TBD3]].  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 Section 7 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).

6.5.  The NTS Cookie Placeholder extension

   The NTS Cookie Placeholder extension has a Field Type of [[TBD4]].
   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.

   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



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   extension's body are undefined and, aside from checking its length,
   MUST be ignored by the server.

6.6.  The NTS Authenticator and Encrypted Extensions extension

   The NTS Authenticator and Encrypted Extensions extension is the
   central cryptographic element of an NTS-protected NTP packet.  Its
   Field Type is [[TBD5]] and the format of its body SHALL be as
   follows:

      Nonce length: two octets in network byte order, giving the length
      of the Nonce field.

      Nonce: a nonce as required by the negotiated AEAD Algorithm.

      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.

      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 RFC 7822 [RFC7822] 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.

   The Ciphertext field SHALL be formed by providing the following
   inputs to the negotiated AEAD Algorithm:

      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.

      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.

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

      N: The nonce SHALL be formed however required by the negotiated
      AEAD Algorithm.




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

6.7.  Protocol details

   A client sending an NTS-protected request SHALL include the following
   extensions:

      Exactly one Unique Identifier extension, which MUST be
      authenticated, MUST NOT be encrypted, and whose contents MUST NOT
      duplicate those of any previous request.

      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.

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

   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.

   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.

   Upon receiving an NTS-protected request, the server SHALL (through
   some implementation-defined mechanism) use the cookie to recover the



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   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:

      Exactly one Unique Identifier extension, which MUST be
      authenticated, MUST NOT be encrypted, and whose contents SHALL
      echo those provided by the client.

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

      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.

   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.

   If the server is unable to validate the cookie or authenticate the
   request, it SHOULD respond with a Kiss-o'-Death packet (see RFC 5905,
   Section 7.4) [RFC5905]) 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.

   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.

   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 wait until the next poll for a
   valid NTS-protected response and if none is received, discard all



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   cookies and AEAD keys associated with the server which sent the NAK
   and initiate a fresh NTS-KE handshake.

7.  Recommended format for NTS cookies

   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.

   The role of cookies in NTS is closely analagous to that of session
   cookies in TLS.  Accordingly, the thematic resemblance of this
   section to RFC 5077 [RFC5077] is deliberate, and the reader should
   likewise take heed of its security considerations.

   Servers should select an AEAD algorithm which they will use to
   encrypt and authenticate cookies.  The chosen algorithm should be one
   such as AEAD_AES_SIV_CMAC_256 [RFC5297] 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`.

   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.

   The need to keep keys synchronized across load-balanced clusters can
   make automatic key rotation challenging.  However, the task can be
   accomplished without the need for central key-management
   infrastructure by using a ratchet, i.e., making each new key a
   deterministic, cryptographically pseudo-random function of its
   predecessor.  A recommended concrete implementation of this approach
   is to use HKDF [RFC5869] to derive new keys, using the key's
   predecessor as Input Keying Material and its key identifier as a
   salt.

   To form a cookie, servers should first form a plaintext `P`
   consisting of the following fields:




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      The AEAD algorithm negotiated during NTS-KE

      The S2C key

      The C2S key

   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.

   The cookie should consist of the tuple `(I,N,C)`.

   To verify and decrypt a cookie provided by the client, first parse it
   into its components `I`, `N`, and `C`. Use `I` to look up its
   decryption key `K`. If the key whose identifier is `I` has been
   erased or never existed, decryption fails; reply with an NTS NAK.
   Otherwise, attempt to decrypt and verify ciphertext `C` using key `K`
   and nonce `N` with no associated data.  If decryption or verification
   fails, reply with an NTS NAK.  Otherwise, parse out the contents of
   the resulting plaintext `P` to obtain the negotiated AEAD algorithm,
   S2C key, and C2S key.

8.  IANA Considerations

   IANA is requested to allocate two entries, identical except for the
   Transport Protocol, in the Service Name and Transport Protocol Port
   Number Registry as follows:

      Service Name: nts

      Transport Protocol: tcp, udp

      Assignee: IESG <iesg@ietf.org>

      Contact: IETF Chair <chair@ietf.org>

      Description: Network Time Security

      Reference: [[this memo]]

      Port Number: [[TBD1]], selected by IANA from the user port range

   IANA is requested to allocate the following two entries in the
   Application-Layer Protocol Negotation (ALPN) Protocol IDs registry:

      Protocol: Network Time Security Key Establishment, version 1

      Identification Sequence:



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      0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1")

      Reference: [[this memo]]

      Protocol: Network Time Protocol, version 4

      Identification Sequence:
      0x6E 0x74 0x70 0x2F 0x34 ("ntp/4")

      Reference: [[this memo]]

   IANA is requested to allocate the following entry in the TLS Exporter
   Label Registry:

   +----------------------------------+---------+---------------+------+
   | Value                            | DTLS-OK | Reference     | Note |
   +----------------------------------+---------+---------------+------+
   | EXPORTER-network-time-security/1 | Y       | [[this memo]] |      |
   +----------------------------------+---------+---------------+------+

   IANA is requested to allocate the following entry in the registry of
   NTP Kiss-o'-Death codes:

                            +------+---------+
                            | Code | Meaning |
                            +------+---------+
                            | NTSN | NTS NAK |
                            +------+---------+

   IANA is requested to allocate the following entries in the NTP
   Extensions Field Types registry:

   +------------+---------------------------------------+--------------+
   | Field Type | Meaning                               | Reference    |
   +------------+---------------------------------------+--------------+
   | [[TBD2]]   | Unique Identifier                     | [[this       |
   |            |                                       | memo]]       |
   | [[TBD3]]   | NTS Cookie                            | [[this       |
   |            |                                       | memo]]       |
   | [[TBD4]]   | NTS Cookie Placeholder                | [[this       |
   |            |                                       | memo]]       |
   | [[TBD5]]   | NTS Authenticator and Encrypted       | [[this       |
   |            | Extensions                            | memo]]       |
   +------------+---------------------------------------+--------------+

   IANA is requested to create a new registry entitled "Network Time
   Security Key Establishment Record Types".  Entries SHALL have the
   following fields:



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      Type Number (REQUIRED): An integer in the range 0-32767 inclusive

      Description (REQUIRED): short text description of the purpose of
      the field

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

      Reference (REQUIRED): A reference to a document specifying the
      semantics of the record.

   The policy for allocation of new entries in this registry SHALL vary
   by the Type Number, as follows:

      0-1023: IETF Review

      1024-16383: Specification Required

      16384-32767: Private and Experimental Use

   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.

   The initial contents of this registry SHALL be as follows:

   +-------------+-----------------------------+----------+------------+
   | Field       | Description                 | Critical | Reference  |
   | Number      |                             |          |            |
   +-------------+-----------------------------+----------+------------+
   | 0           | End of message              | MUST     | [[this     |
   |             |                             |          | memo]]     |
   | 1           | NTS next protocol           | MUST     | [[this     |
   |             | negotiation                 |          | memo]]     |
   | 2           | Error                       | MUST     | [[this     |
   |             |                             |          | memo]]     |
   | 3           | Warning                     | MUST     | [[this     |
   |             |                             |          | memo]]     |
   | 4           | AEAD algorithm negotiation  | MAY      | [[this     |
   |             |                             |          | memo]]     |
   | 5           | New cookie for NTPv4        | SHOULD   | [[this     |
   |             |                             | NOT      | memo]]     |
   | 16384-32767 | Reserved for Private &      | MAY      | [[this     |
   |             | Experimental Use            |          | memo]]     |
   +-------------+-----------------------------+----------+------------+




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   IANA is requested to create a new registry entitled "Network Time
   Security Next Protocols".  Entries SHALL have the following fields:

      Protocol ID (REQUIRED): a 16-bit unsigned integer functioning as
      an identifier.

      Protocol Name (REQUIRED): a short text string naming the protocol
      being identified.

      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.

   Applications for new entries in this registry SHALL specify all
   desired fields, and SHALL be granted upon approval by a Designated
   Expert.  Protocol IDs 32768-65535 SHALL be reserved for Private or
   Experimental Use, and SHALL NOT be registered.

   The initial contents of this registry SHALL be as follows:

   +-------------+-------------------------------+---------------------+
   | Protocol    | Human-Readable Name           | Reference           |
   | Name        |                               |                     |
   +-------------+-------------------------------+---------------------+
   | 0           | Network Time Protocol version | [[this memo]]       |
   |             | 4                             |                     |
   | 1           | Precision Time Protocol       | Reserved by [[this  |
   |             | version 2                     | memo]]              |
   | 32768-65535 | Reserved for Private or       | Reserved by [[this  |
   |             | Experimental Use              | memo]]              |
   +-------------+-------------------------------+---------------------+

   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:

      Number (REQUIRED): a 16-bit unsigned integer

      Description (REQUIRED): a short text description of the condition.

      Reference (REQUIRED): a reference to a relevant specification
      document.

   The policy for allocation of new entries in these registries SHALL
   vary by their Number, as follows:

      0-1023: IETF Review




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      1024-32767: Specification Required

      32768-65535: Private and Experimental Use

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

       +--------+---------------------------------+---------------+
       | Number | Description                     | Reference     |
       +--------+---------------------------------+---------------+
       | 0      | Unrecognized Critical Extension | [[this memo]] |
       | 1      | Bad Request                     | [[this memo]] |
       +--------+---------------------------------+---------------+

   The Network Time Security Warning Codes Registry SHALL initially be
   empty.

9.  Security considerations

9.1.  Avoiding DDoS amplification

   Certain non-standard and/or deprecated features of the Network Time
   Protocol enable clients to send a request to a server which causes
   the server to send a response much larger than the request.  Servers
   which enable these features can be abused in order to amplify traffic
   volume in distributed denial-of-service (DDoS) attacks by sending
   them a request with a spoofed source IP.  In recent years, attacks of
   this nature have become an endemic nuisance.

   NTS is designed to avoid contributing any further to this problem by
   ensuring that NTS-related extensions included in server responses
   will be the same size as the NTS-related extensions sent by the
   client.  In particular, this is why the client is required to send a
   separate and appropriately padded-out NTS Cookie Placeholder
   extension for every cookie it wants to get back, rather than being
   permitted simply to specify a desired quantity.

9.2.  Initial verification of server certificates

   NTS's security goals are undermined if the client fails to verify
   that the X.509 certificate chain presented by the server is valid and
   rooted in a trusted certificate authority.  [RFC5280] and [RFC6125]
   specifies how such verification is to be performed in general.
   However, the expectation that the client does not yet have a
   correctly-set system clock at the time of certificate verification
   presents difficulties with verifying that the certificate is within
   its validity period, i.e., that the current time lies between the
   times specified in the certificate's notBefore and notAfter fields,



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   and it may be operationally necessary in some cases for a client to
   accept a certificate which appears to be expired or not yet valid.
   While there is no perfect solution to this problem, there are several
   mitigations the client can implement to make it more difficult for an
   adversary to successfully present an expired certificate:

      Check whether the system time is in fact unreliable.  If the
      system clock has previously been synchronized since last boot,
      then on operating systems which implement a kernel-based phase-
      locked-loop API, a call to ntp_gettime() should show a maximum
      error less than NTP_PHASE_MAX.  In this case, the clock should be
      considered reliable and certificates can be strictly validated.

      Allow the system administrator to specify that certificates should
      *always* be strictly validated.  Such a configuration is
      appropriate on systems which have a battery-backed clock and which
      can reasonably prompt the user to manually set an approximately-
      correct time if it appears to be needed.

      Once the clock has been synchronized, periodically write the
      current system time to persistent storage.  Do not accept any
      certificate whose notAfter field is earlier than the last recorded
      time.

      Do not process time packets from servers if the time computed from
      them falls outside the validity period of the server's
      certificate.

      Use multiple time sources.  The ability to pass off an expired
      certificate is only useful to an adversary who has compromised the
      corresponding private key.  If the adversary has compromised only
      a minority of servers, NTP's selection algorithm ([RFC5905]
      section 11.2.1) will protect the client from accepting bad time
      from the adversary-controlled servers.

9.3.  Usage of NTP pools

   Additional standardization work and infrastructure development is
   necessary before NTS can be used with public NTP server pools.
   First, a scheme needs to be specified for determining what
   constitutes an acceptable certificate for a pool server, such as
   establishing a value required to be contained in its Extended Key
   Usage attribute, and how to determine, given the DNS name of a pool,
   what Subject Alternative Name to expect in the certificates of its
   members.  A more important matter, however, is that pool operators
   need procedures for establishing and maintaining trust in their
   members.  Pools in existence as of 2017 are volunteer-run, with
   minimal requirements for admission and no organized effort to monitor



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   pool servers for misbehavior.  Without any sort of policing in place,
   there is nothing to prevent an adversary from going through normal
   channels to obtain a valid certificate for participation in a pool
   and then proceeding to serve maliciously inaccurate time.

9.4.  Delay attacks

   In a packet delay attack, an adversary with the ability to act as a
   man-in-the-middle delays time synchronization packets between client
   and server asymmetrically [RFC7384].  Since NTP's formula for
   computing time offset relies on the assumption that network latency
   is roughly symmetrical, this leads to the client to compute an
   inaccurate value [Mizrahi].  The delay attack does not reorder or
   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 introduce is bounded by half of the round trip delay.

   [RFC5905] specifies a parameter called MAXDIST which denotes the
   maximum round-trip latency (including not only the immediate round
   trip between client and server but the whole distance back to the
   reference clock as reported in the Root Delay filed) that a client
   will tolerate before concluding that the server is unsuitable for
   synchronization.  The standard value for MAXDIST is one second,
   although some implementations use larger values.  Whatever value a
   client chooses, the maximum error which can be introduced by a delay
   attack is MAXDIST/2.

   Usage of multiple time sources, or multiple network paths to a given
   time source [Shpiner], may also serve to mitigate delay attacks if
   the adversary is in control of only some of the paths.

9.5.  Random number generation

   At various points in NTS, the generation of cryptographically secure
   random numbers is required.  See [RFC4086] for guidelines concerning
   this topic.

10.  Privacy Considerations

10.1.  Unlinkability

   Unlinkability prevents a device from being tracked when it changes
   network addresses (e.g. because said device moved between different
   networks).  In other words, unlinkability thwarts an attacker that
   seeks to link a new network address used by a device with a network
   address that it was formerly using, because of recognizable data that
   the device persistently sends as part of an NTS-secured NTP



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   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.

   NTS's unlinkability objective is merely to not leak any additional
   data that could be used to link a device's network address.  NTS does
   not rectify legacy linkability issues that are already present in
   NTP.  Thus, a client that requires unlinkability MUST also minimize
   information transmitted in a client query (mode 3) packet as
   described in the draft [I-D.ietf-ntp-data-minimization].

   The unlinkability objective only holds for time synchronization
   traffic, as opposed to key exchange traffic.  This implies that it
   cannot be guaranteed for devices that function not only as time
   clients, but also as time servers (because the latter can be
   externally triggered to send authentication data).

   It should also be noted that it could be possible to link devices
   that operate as time servers from their time synchronization traffic,
   using information exposed in (mode 4) server response packets (e.g.
   reference ID, reference time, stratum, poll).  Also, devices that
   respond to NTP control queries could be linked using the information
   revealed by control queries.

10.2.  Confidentiality

   NTS does not protect the confidentiality of information in NTP's
   header fields.  When clients implement
   [I-D.ietf-ntp-data-minimization], client packet headers do not
   contain any information which the client could conceivably wish to
   keep secret: one field is random, and all others are fixed.
   Information in server packet headers is likewise public: the origin
   timestamp is copied from the client's (random) transmit timestamp,
   and all other fields are set the same regardless of the identity of
   the client making the request.

   Future extension fields could hypothetically contain sensitive
   information, in which case NTS provides a mechanism for encrypting
   them.

11.  Acknowledgements

   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,




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   and Richard Welty for contributions to this document. on the design
   of NTS.

12.  References

12.1.  Normative References

   [I-D.ietf-ntp-data-minimization]
              Franke, D. and A. Malhotra, "NTP Client Data
              Minimization", draft-ietf-ntp-data-minimization-00 (work
              in progress), May 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5297]  Harkins, D., "Synthetic Initialization Vector (SIV)
              Authenticated Encryption Using the Advanced Encryption
              Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
              2008, <http://www.rfc-editor.org/info/rfc5297>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <http://www.rfc-editor.org/info/rfc5705>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <http://www.rfc-editor.org/info/rfc5746>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

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





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   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <http://www.rfc-editor.org/info/rfc7301>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <http://www.rfc-editor.org/info/rfc7465>.

   [RFC7507]  Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
              <http://www.rfc-editor.org/info/rfc7507>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.

   [RFC7822]  Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
              (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
              March 2016, <http://www.rfc-editor.org/info/rfc7822>.

12.2.  Informative References

   [IEC.61588_2009]
              IEEE/IEC, "Precision clock synchronization protocol for
              networked measurement and control systems",
              IEEE 1588-2008(E), IEC 61588:2009(E),
              DOI 10.1109/IEEESTD.2009.4839002, February 2009,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=4839000>.

   [Mizrahi]  Mizrahi, T., "A game theoretic analysis of delay attacks
              against time synchronization protocols", in Proceedings
              of Precision Clock Synchronization for Measurement Control
              and Communication, ISPCS 2012, pp. 1-6, September 2012.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.




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   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <http://www.rfc-editor.org/info/rfc5869>.

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <http://www.rfc-editor.org/info/rfc7384>.

   [Shpiner]  "Multi-path Time Protocols", in Proceedings of IEEE
              International Symposium on Precision Clock Synchronization
              for Measurement, Control and Communication (ISPCS),
              September 2013.

Appendix A.  Terms and Abbreviations

   AEAD  Authenticated Encryption with Associated Data [RFC5116]

   DDoS  Distributed Denial of Service

   DTLS  Datagram Transport Layer Security

   NTP    Network Time Protocol [RFC5905]

   NTS    Network Time Security

   PTP    Precision Time Protocol

   TLS  Transport Layer Security

Authors' Addresses









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   Daniel Fox Franke
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA  02142
   United States

   Email: dafranke@akamai.com
   URI:   https://www.dfranke.us


   Dieter Sibold
   Physikalisch-Technische Bundesanstalt
   Bundesallee 100
   Braunschweig  D-38116
   Germany

   Phone: +49-(0)531-592-8420
   Fax:   +49-531-592-698420
   Email: dieter.sibold@ptb.de


   Kristof Teichel
   Physikalisch-Technische Bundesanstalt
   Bundesallee 100
   Braunschweig  D-38116
   Germany

   Phone: +49-(0)531-592-8421
   Email: kristof.teichel@ptb.de






















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