Network Time Security for the Network Time Protocol
draft-ietf-ntp-using-nts-for-ntp-14

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12                        
          13 14                                                         
NTP Working Group                                              D. Franke
Internet-Draft
Intended status: Standards Track                               D. Sibold
Expires: March 3, 2019                                        K. Teichel
                                                                     PTB
                                                             M. Dansarie

                                                             R. Sundblad
                                                                  Netnod
                                                         August 30, 2018


          Network Time Security for the Network Time Protocol
                  draft-ietf-ntp-using-nts-for-ntp-13

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
   client-server mode of the Network Time Protocol (NTP).

   NTS is structured as a suite of two loosely coupled sub-protocols:
   the NTS Key Establishment Protocol (NTS-KE) and NTS Extensions for
   NTPv4.  NTS-KE handles NTS service authentication, initial
   handshaking, and key extraction over TLS.  Encryption and
   authentication during NTP time synchronization is performed through
   NTS extension fields in otherwise standard NTP packets.  Except for
   during the initial NTS-KE process, all state required by the protocol
   is held by the client in opaque cookies.

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 https://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 March 3, 2019.




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Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   3.  TLS profile for Network Time Security . . . . . . . . . . . .   7
   4.  The NTS Key Establishment Protocol  . . . . . . . . . . . . .   7
     4.1.  NTS-KE Record Types . . . . . . . . . . . . . . . . . . .   9
       4.1.1.  End of Message  . . . . . . . . . . . . . . . . . . .   9
       4.1.2.  NTS Next Protocol Negotiation . . . . . . . . . . . .  10
       4.1.3.  Error . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.4.  Warning . . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.5.  AEAD Algorithm Negotiation  . . . . . . . . . . . . .  11
       4.1.6.  New Cookie for NTPv4  . . . . . . . . . . . . . . . .  11
       4.1.7.  NTPv4 Server Negotiation  . . . . . . . . . . . . . .  12
       4.1.8.  NTPv4 Port Negotiation  . . . . . . . . . . . . . . .  12
     4.2.  Key Extraction (generally)  . . . . . . . . . . . . . . .  13
   5.  NTS Extension Fields for NTPv4  . . . . . . . . . . . . . . .  13
     5.1.  Key Extraction (for NTPv4)  . . . . . . . . . . . . . . .  13
     5.2.  Packet Structure Overview . . . . . . . . . . . . . . . .  14
     5.3.  The Unique Identifier Extension Field . . . . . . . . . .  14
     5.4.  The NTS Cookie Extension Field  . . . . . . . . . . . . .  15
     5.5.  The NTS Cookie Placeholder Extension Field  . . . . . . .  15
     5.6.  The NTS Authenticator and Encrypted Extension Fields
           Extension Field . . . . . . . . . . . . . . . . . . . . .  15
     5.7.  Protocol Details  . . . . . . . . . . . . . . . . . . . .  17
   6.  Suggested Format for NTS Cookies  . . . . . . . . . . . . . .  22
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Service Name and Transport Protocol Port Number Registry   23
     7.2.  TLS Application-Layer Protocol Negotiation (ALPN)
           Protocol IDs Registry . . . . . . . . . . . . . . . . . .  24
     7.3.  TLS Exporter Labels Registry  . . . . . . . . . . . . . .  24



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     7.4.  NTP Kiss-o'-Death Codes Registry  . . . . . . . . . . . .  24
     7.5.  NTP Extension Field Types Registry  . . . . . . . . . . .  24
     7.6.  Network Time Security Key Establishment Record Types
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.7.  Network Time Security Next Protocols Registry . . . . . .  26
     7.8.  Network Time Security Error and Warning Codes Registries   27
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  28
     8.1.  Implementation PoC 1  . . . . . . . . . . . . . . . . . .  28
       8.1.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  28
       8.1.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  29
       8.1.3.  Contact Information . . . . . . . . . . . . . . . . .  29
       8.1.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  29
     8.2.  Implementation PoC 2  . . . . . . . . . . . . . . . . . .  29
       8.2.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  29
       8.2.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  29
       8.2.3.  Contact Information . . . . . . . . . . . . . . . . .  29
       8.2.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  29
     8.3.  Interoperability  . . . . . . . . . . . . . . . . . . . .  30
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
     9.1.  Sensitivity to DDoS attacks . . . . . . . . . . . . . . .  30
     9.2.  Avoiding DDoS Amplification . . . . . . . . . . . . . . .  30
     9.3.  Initial Verification of Server Certificates . . . . . . .  31
     9.4.  Delay Attacks . . . . . . . . . . . . . . . . . . . . . .  32
     9.5.  Random Number Generation  . . . . . . . . . . . . . . . .  32
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  33
     10.1.  Unlinkability  . . . . . . . . . . . . . . . . . . . . .  33
     10.2.  Confidentiality  . . . . . . . . . . . . . . . . . . . .  33
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  34
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     12.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Appendix A.  Terms and Abbreviations  . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

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 client-server
   mode of the Network Time Protocol (NTP) [RFC5905].

1.1.  Objectives

   The objectives of NTS are as follows:

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



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   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: Client 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 additional to NTP which would permit a passive
      adversary to determine that two packets sent over different
      networks came from the same client.

   o  Non-amplification: Implementations (especially server
      implementations) may avoid acting as distributed denial-of-service
      (DDoS) amplifiers by never responding to a request with a packet
      larger than the request packet.

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

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 partly contradictory requirements for
   security and performance.  Symmetric and control modes demand mutual
   authentication and mutual replay protection.  Additionally, 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 stateless servers to process replayed
   packets.  The security demands of symmetric and control modes, on the



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   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.

   This memo specifies NTS exclusively for the client-server mode of
   NTP.  To this end, NTS is structured as a suite of two protocols:

      The "NTS Extensions for NTPv4" define a collection of NTP
      extension fields for cryptographically securing NTPv4 using
      previously-established key material.  They are suitable for
      securing client-server mode because the server can implement them
      without retaining per-client state.  All state is kept by the
      client and provided to the server in the form of an encrypted
      cookie supplied with each request.  On the other hand, the NTS
      Extension Fields 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 a mechanism for
      establishing key material for use with the NTS Extension Fields
      for NTPv4.  It uses TLS to exchange keys, provide the client with
      an initial supply of cookies, and negotiate some additional
      protocol options.  After this exchange, the TLS channel is closed
      with no per-client state remaining on the server side.

   The typical protocol flow is as follows: The client connects to an
   NTS-KE 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 along with a list of one or more IP addresses to
   NTP servers for which the cookies are valid.  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.  Ideally, the client never needs to connect to the NTS-
   KE server again.

   Time synchronization proceeds with one of the indicated NTP servers
   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 key exchange server)
   and an authentication tag, computed using key material extracted from
   the NTS-KE handshake.  The NTP server uses the cookie to recover this
   key material and send back an authenticated response.  The response
   includes a fresh, encrypted cookie which the client then sends back




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   in the clear in a subsequent request.  (This constant refreshing of
   cookies is necessary in order to achieve NTS's unlinkability goal.)

   Figure 1 provides an overview of the high-level interaction between
   the client, the NTS-KE server, and the NTP server.  Note that the
   cookies' data format and the exchange of secrets between NTS-KE and
   NTP servers are not part of this specification and are implementation
   dependent.  However, a suggested format for NTS cookies is provided
   in Section 6.

                                                        +--------------+
                                                        |              |
                                                    +-> | NTP Server 1 |
                                                    |   |              |
                              Shared cookie         |   +--------------+
   +---------------+      encryption parameters     |   +--------------+
   |               |    (Implementation dependent)  |   |              |
   | NTS-KE Server | <------------------------------+-> | NTP Server 2 |
   |               |                                |   |              |
   +---------------+                                |   +--------------+
          ^                                         |          .
          |                                         |          .
          | 1. Negotiate parameters,                |          .
          |    receive initial cookie               |   +--------------+
          |    supply, generate AEAD keys,          |   |              |
          |    and receive NTP server IP            +-> | NTP Server N |
          |    addresses using "NTS Key                 |              |
          |    Establishment" protocol.                 +--------------+
          |                                                    ^
          |                                                    |
          |             +----------+                           |
          |             |          |                           |
          +-----------> |  Client  | <-------------------------+
                        |          |  2. Perform authenticated
                        +----------+     time synchronization
                                         and generate new
                                         cookies using "NTS
                                         Extension Fields for
                                         NTPv4".

           Figure 1: Overview of High-Level Interactions in NTS

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in




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   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  TLS profile for Network Time Security

   Network Time Security makes use of TLS for NTS key establishment.

   Since the NTS protocol is new as of this publication, no backward-
   compatibility concerns exist to justify using obsolete, insecure, or
   otherwise broken TLS features or versions.  Implementations MUST
   conform with [RFC7525] or with a later revision of BCP 195.
   Furthermore:

   Implementations MUST NOT negotiate TLS versions earlier than 1.2,
   SHOULD negotiate TLS 1.3 [RFC8446] or later when possible, and MAY
   refuse to negotiate any TLS version which has been superseded by a
   later supported version.

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

4.  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 2.  Requests and
   non-error responses each SHALL include exactly one NTS Next Protocol
   Negotiation record.  The sequence SHALL be terminated by a "End of
   Message" record.  The requirement that all NTS-KE messages be
   terminated by an End of Message record makes them self-delimiting.

   Clients and servers MAY enforce length limits on requests and
   responses, however, servers MUST accept requests of at least 1024
   octets and clients SHOULD accept responses of at least 65536 octets.







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    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 2: NTS-KE Record Format

   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 Number: A 15-bit integer in network byte order.  The
      semantics of record types 0-6 are specified in this memo.
      Additional type numbers SHALL be tracked through the IANA Network
      Time Security Key Establishment Record Types registry.

      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.

   For clarity regarding bit-endianness: the Critical Bit is the most-
   significant bit of the first octet.  In C, given a network buffer
   `unsigned char b[]` containing an NTS-KE record, the critical bit is
   `b[0] >> 7` while the record type is `((b[0] & 0x7f) << 8) + b[1]`.

   Figure 3 provides a schematic overview of the key exchange.  It
   displays the protocol steps to be performed by the NTS client and
   server and record types to be exchanged.










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                   +---------------------------------------+
                   | - Verify client request message.      |
                   | - Extract TLS key material.           |
                   | - Generate KE response message.       |
                   |   - Include Record Types:             |
                   |       o NTS Next Protocol Negotiation |
                   |       o AEAD Algorithm Negotiation    |
                   |       o NTP Server Negotiation        |
                   |       o New Cookie for NTPv4          |
                   |       o <New Cookie for NTPv4>        |
                   |       o End of Message                |
                   +-----------------+---------------------+
                                     |
                                     |
   Server -----------+---------------+-----+----------------------->
                     ^                      \
                    /                        \
                   /    TLS application       \
                  /     data                   \
                 /                              \
                /                                V
   Client -----+---------------------------------+---------------->
               |                                 |
               |                                 |
               |                                 |
   +-----------+----------------------+   +------+-----------------+
   |- Generate KE request message.    |   |- Verify server response|
   | - Include Record Types:          |   |  message.              |
   |  o NTS Next Protocol Negotiation |   |- Extract cookie(s).    |
   |  o AEAD Algorithm Negotiation    |   |                        |
   |  o <NTP Server Negotiation>      |   |                        |
   |  o End of Message                |   |                        |
   +----------------------------------+   +------------------------+

                    Figure 3: NTS Key Exchange Messages

4.1.  NTS-KE Record Types

   The following NTS-KE Record Types are defined:

4.1.1.  End of Message

   The End of Message record has a Record Type number of 0 and a 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.






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4.1.2.  NTS Next Protocol Negotiation

   The NTS Next Protocol Negotiation record has a Record Type number 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.

4.1.3.  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 codes are defined:

      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.

4.1.4.  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




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   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 a 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.

4.1.5.  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 Protocol ID 0 (for
   NTPv4), then 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.  It 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.

   Server implementations of NTS extension fields for NTPv4 (Section 5)
   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 Protocol ID 0
   (NTPv4) in its NTS Next Protocol Negotiation record, then the
   server's AEAD Algorithm Negotiation record MUST NOT be empty.

4.1.6.  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 6 for a suggested
   construction.





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   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 the
   Next Protocol Negotiation response record contains Protocol ID 0
   (NTPv4) and the AEAD Algorithm Negotiation response record is not
   empty.  The Critical Bit SHOULD NOT be set.

4.1.7.  NTPv4 Server Negotiation

   The NTPv4 Server Negotiation record has a Record Type number of 6.
   Its body consists of an ASCII-encoded [ANSI.X3-4.1986] string
   conforming to the syntax of the Host subcomponent of a URI
   ([RFC3986]).  IPv6 addresses MUST NOT include zone identifiers
   [RFC6874].

   When NTPv4 is negotiated as a Next Protocol and this record is sent
   by the server, the body specifies the hostname or IP address of the
   NTPv4 server with which the client should associate and which will
   accept the supplied cookies.  If no record of this type is sent, the
   client SHALL interpret this as a directive to associate with an NTPv4
   server at the same IP address as the NTS-KE server.  Servers MUST NOT
   send more than one record of this type.

   When this record is sent by the client, it indicates that the client
   wishes to associate with the specified NTP server.  The NTS-KE server
   MAY incorporate this request when deciding what NTPv4 Server
   Negotiation records to respond with, but honoring the client's
   preference is OPTIONAL.  The client MUST NOT send more than one
   record of this type.

   Servers MAY set the Critical Bit on records of this type; clients
   SHOULD NOT.

4.1.8.  NTPv4 Port Negotiation

   The NTPv4 Port Negotiation record has a Record Type number of 7.  Its
   body consists of a 16-bit unsigned integer in network byte order,
   denoting a UDP port number.

   When NTPv4 is negotiated as a Next Protocol and this record is sent
   by the server, the body specifies the port number of the NTPv4 server
   with which the client should associate and which will accept the
   supplied cookies.  If no record of this type is sent, the client
   SHALL assume a default of 123 (the registered port number for NTP).

   When this record is sent by the client in conjunction with a NTPv4
   Server Negotiation record, it indicates that the client wishes to
   associate with the NTP server at the specified port.  The NTS-KE
   server MAY incorporate this request when deciding what NTPv4 Server



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   Negotiation and NTPv4 Port Negotiation records to respond with, but
   honoring the client's preference is OPTIONAL.

   Servers MAY set the Critical Bit on records of this type; clients
   SHOULD NOT.

4.2.  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.

5.  NTS Extension Fields for NTPv4

5.1.  Key Extraction (for NTPv4)

   Following a successful run of the NTS-KE protocol wherein Protocol ID
   0 (NTPv4) 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.

      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 Protocol ID for NTPv4).

         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



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   their own disambiguating label string.  Note that RFC 5705 [RFC5705]
   provides that disambiguating label strings beginning with
   "EXPERIMENTAL" MAY be used without IANA registration.

5.2.  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 extension fields which are authenticated but not encrypted.

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

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

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

5.3.  The Unique Identifier Extension Field

   The Unique Identifier extension field provides the client with a
   cryptographically strong means of detecting replayed packets.  It has
   a Field Type of [[TBD2]].  When the extension field is included in a
   client packet (mode 3), its body SHALL consist of a string of octets
   generated uniformly at random.  The string MUST be at least 32 octets
   long.  When the extension field 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.  All server
   packets generated by NTS-implementing servers in response to client
   packets containing this extension field MUST also contain this field
   with the same content as in the client's request.  The field's use in
   modes other than client-server is not defined.

   This extension field 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,



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   depending on implementation details, most of those bits may be
   predictable.  In contrast, the Unique Identifier extension field
   enables a degree of unpredictability and collision resistance more
   consistent with cryptographic best practice.

5.4.  The NTS Cookie Extension Field

   The NTS Cookie extension field 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 6 for a
   suggested construction.  The NTS Cookie extension field MUST NOT be
   included in NTP packets whose mode is other than 3 (client) or 4
   (server).

5.5.  The NTS Cookie Placeholder Extension Field

   The NTS Cookie Placeholder extension field has a Field Type of
   [[TBD4]].  When this extension field 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 field MUST
   NOT be included in NTP packets whose mode is other than 3.

   Whenever an NTS Cookie Placeholder extension field is present, it
   MUST be accompanied by an NTS Cookie extension field.  The body
   length of the NTS Cookie Placeholder extension field MUST be the same
   as the body length of the NTS Cookie extension field.  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 field's body are undefined and, aside from
   checking its length, MUST be ignored by the server.

5.6.  The NTS Authenticator and Encrypted Extension Fields Extension
      Field

   The NTS Authenticator and Encrypted Extension Fields extension field
   is the central cryptographic element of an NTS-protected NTP packet.
   Its Field Type is [[TBD5]].  It SHALL be formatted according to
   Figure 4 and include the following fields:

      Nonce length: Two octets in network byte order, giving the length
      of the Nonce field, excluding any padding, interpreted as an
      unsigned integer.






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      Ciphertext Length: Two octets in network byte order, giving the
      length of the Ciphertext field, excluding any padding, interpreted
      as an unsigned integer.

      Nonce: A nonce as required by the negotiated AEAD Algorithm.  The
      field is zero-padded to a word (four octets) boundary.

      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.  The field is zero-padded to a word (four
      octets) boundary.

      Additional Padding: Clients which use a nonce length shorter than
      the maximum allowed by the negotiated AEAD algorithm may be
      required to include additional zero-padding.  The necessary length
      of this field is specified below.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Nonce Length         |      Ciphertext Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .          Nonce, including up to 3 bytes padding               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .        Ciphertext, including up to 3 bytes padding            .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                      Additional Padding                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: NTS Authenticator and Encrypted Extension Fields Extension
                               Field Format

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




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      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 field which precedes the NTS
      Authenticator and Encrypted Extension Fields extension field.

      P: The plaintext SHALL consist of all (if any) NTP extension
      fields to be encrypted.  The format of any such fields SHALL be in
      accordance with RFC 7822 [RFC7822].  If multiple extension fields
      are present they SHALL be joined by concatenation.

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

   The purpose of the Additional Padding field is to ensure that servers
   can always choose a nonce whose length is adequate to ensure its
   uniqueness, even if the client chooses a shorter one, and still
   ensure that the overall length of the server's response packet. does
   not exceed the length of the request.  For mode 4 (server) packets,
   no Additional Padding field is ever required.  For mode 3 (client)
   packets, the length of the Additional Padding field SHALL be computed
   as follows.  Let `N_LEN` be the padded length of the the Nonce field.
   Let `N_MAX` be, as specified by RFC 5116 [RFC5116], the maximum
   permitted nonce length for the negotiated AEAD algorithm.  Let
   `N_REQ` be the lesser of 16 and N_MAX, rounded up to the nearest
   multiple of 4.  If N_LEN is greater than or equal to N_REQ, then no
   Additional Padding field is required.  Otherwise, the Additional
   Padding field SHALL be at least N_REQ - N_LEN octets in length.
   Servers MUST enforce this requirement by discarding any packet which
   does not conform to it.

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

5.7.  Protocol Details

   A client sending an NTS-protected request SHALL include the following
   extension fields as displayed in Figure 5:

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





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      Exactly one NTS Cookie extension field which MUST be authenticated
      and MUST NOT be encrypted.  The cookie MUST be one which has been
      previously provided to the client; either from the key exchange
      server during the NTS-KE handshake or from the NTP server in
      response to a previous NTS-protected NTP request.  To protect the
      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 initiate a re-run the NTS-KE
      protocol.

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

   The client MAY include one or more NTS Cookie Placeholder extension
   fields which MUST be authenticated and MAY be encrypted.  The number
   of NTS Cookie Placeholder extension fields 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.  The client SHOULD NOT include more
   than seven NTS Cookie Placeholder extension fields in a request.
   When both the client and server adhere to all cookie-management
   guidance provided in this memo, the number of placeholder extension
   fields will equal the number of dropped packets since the last
   successful volley.


























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                     +---------------------------------------+
                     | - Verify time request message.        |
                     | - Generate time response message.     |
                     |   - Include NTPv4 extension fields:   |
                     |      o Unique Identifier EF           |
                     |      o NTS Cookie EF                  |
                     |      o <NTS Cookie EF>                |
                     |                                       |
                     | - Generate AEAD tag of NTP message.   |
                     | - Add NTS Authentication and          |
                     |   Encrypted Extension Fields EF.      |
                     | - Transmit time response packet.      |
                     +-----------------+---------------------+
                                       |
                                       |
     Server -----------+---------------+-----+----------------------->
                       ^                      \
                      /                        \
       Time request  /                          \   Time response
       (mode 3)     /                            \  (mode 4)
                   /                              \
                  /                                V
     Client -----+---------------------------------+---------------->
                 |                                 |
                 |                                 |
                 |                                 |
     +-----------+-----------------------+   +-----+------------------+
     |- Generate time request message.   |   |- Verify time response  |
     | - Include NTPv4 extension fields: |   |  message.              |
     |    o Unique Identifier EF         |   |- Extract cookie(s).    |
     |    o NTS Cookie EF                |   |- Time synchronization  |
     |    o <NTS Cookie Placeholder EF>  |   |  processing.           |
     |                                   |   +------------------------+
     |- Generate AEAD tag of NTP message.|
     |- Add NTS Authentication and       |
     |  Encrypted Extension Fields EF.   |
     |- Transmit time request packet.    |
     +-----------------------------------+

                Figure 5: NTS Time Synchronization Messages

   The client MAY include additional (non-NTS-related) extension fields
   which MAY appear prior to the NTS Authenticator and Encrypted
   Extension Fields extension fields (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
   extension fields and process the packet as though they were not



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   present.  Servers MAY implement exceptions to this requirement for
   particular extension fields 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
   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, the server SHALL include the following extension fields in
   its response:

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

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

      One or more NTS Cookie extension fields which MUST be
      authenticated and encrypted.  The number of NTS Cookie extension
      fields included SHOULD be equal to, and MUST NOT exceed, one plus
      the number of valid NTS Cookie Placeholder extension fields
      included in the request.  The cookies returned in those fields
      MUST be valid for use with the NTP server that sent them.  They
      MAY be valid for other NTP servers as well, but there is no way
      for the server to indicate this.

   We emphasize the contrast that NTS Cookie extension fields MUST NOT
   be encrypted when sent from client to server, but MUST be encrypted
   from sent from server to client.  The former is necessary in order
   for the server to be able to recover the C2S and S2C keys, while the
   latter is necessary to satisfy the unlinkability goals discussed in
   Section 10.1.  We emphasize also that "encrypted" means encapsulated
   within the the NTS Authenticator and Encrypted Extensions extension
   field.  While the body of an NTS Cookie extension field will
   generally consist of some sort of AEAD output (regardless of whether
   the recommendations of Section 6 are precisely followed), this is not
   sufficient to make the extension field "encrypted".

   The server MAY include additional (non-NTS-related) extension fields
   which MAY appear prior to the NTS Authenticator and Encrypted
   Extension Fields extension field (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
   extension fields and process the packet as though they were not



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   present.  Clients MAY implement exceptions to this requirement for
   particular extension fields if their specification explicitly
   provides for such.

   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.

   If the server is unable to validate the cookie or authenticate the
   request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see RFC
   5905, Section 7.4 [RFC5905]) with kiss code "NTSN", meaning "NTS
   negative-acknowledgment (NAK)".  It MUST NOT include any NTS Cookie
   or NTS Authenticator and Encrypted Extension Fields extension fields.

   If the NTP server has previously responded with authentic NTS-
   protected NTP packets (i.e., packets containing the NTS Authenticator
   and Encrypted Extension Fields extension field), the client MUST
   verify that any KoD packets received from the server contain the
   Unique Identifier extension field and 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 MUST comply with RFC 5095, Section 7.4 [RFC5905]
   where required.  A client MAY automatically re-run the NTS-KE
   protocol upon forced disassociation from an NTP server.  In that
   case, it MUST be able to detect and stop looping between the NTS-KE
   and NTP servers.

   Upon reception of the NTS NAK kiss code, the client SHOULD wait until
   the next poll for a valid NTS-protected response and if none is
   received, initiate a fresh NTS-KE handshake to try to renegotiate new
   cookies, AEAD keys, and parameters.  If the NTS-KE handshake
   succeeds, the client MUST discard all old cookies and parameters and
   use the new ones instead.  As long as the NTS-KE handshake has not
   succeeded, the client SHOULD continue polling the NTP server using
   the cookies and parameters it has.

   The client MAY reuse cookies in order to prioritize resilience over
   unlinkability.  Which of the two that should be prioritized in any
   particular case is dependent on the application and the user's
   preference.  Section 10.1 describes the privacy considerations of
   this in further detail.

   To allow for NTP session restart when the NTS-KE server is
   unavailable and to reduce NTS-KE server load, the client SHOULD keep
   at least one unused but recent cookie, AEAD keys, negotiated AEAD
   algorithm, and other necessary parameters on persistent storage.



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   This way, the client is able to resume the NTP session without
   performing renewed NTS-KE negotiation.

6.  Suggested Format for NTS Cookies

   This section is non-normative.  It gives a suggested way for servers
   to construct NTS cookies.  All normative requirements are stated in
   Section 4.1.6 and Section 5.4.

   The role of cookies in NTS is closely analogous 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 accidental
   nonce reuse.  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 between NTS-KE and NTP servers as
   well as 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 then 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.

7.  IANA Considerations

7.1.  Service Name and Transport Protocol Port Number Registry

   IANA is requested to allocate the following entry in the Service Name
   and Transport Protocol Port Number Registry [RFC6335]:

      Service Name: ntske

      Transport Protocol: tcp

      Assignee: IESG <iesg@ietf.org>

      Contact: IETF Chair <chair@ietf.org>

      Description: Network Time Security Key Exchange

      Reference: [[this memo]]

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

   [[RFC EDITOR: Replace all instances of [[TBD1]] in this document with
   the IANA port assignment.]]






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7.2.  TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs
      Registry

   IANA is requested to allocate the following entry in the TLS
   Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry
   [RFC7301]:

      Protocol: Network Time Security Key Establishment, version 1

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

      Reference: [[this memo]], Section 4

7.3.  TLS Exporter Labels Registry

   IANA is requested to allocate the following entry in the TLS Exporter
   Labels Registry [RFC5705]:

   +--------------------+---------+-------------+---------------+------+
   | Value              | DTLS-OK | Recommended | Reference     | Note |
   +--------------------+---------+-------------+---------------+------+
   | EXPORTER-network-  | Y       | Y           | [[this        |      |
   | time-security/1    |         |             | memo]],       |      |
   |                    |         |             | Section 4.2   |      |
   +--------------------+---------+-------------+---------------+------+

7.4.  NTP Kiss-o'-Death Codes Registry

   IANA is requested to allocate the following entry in the registry of
   NTP Kiss-o'-Death Codes [RFC5905]:

   +------+---------------------------------------+--------------------+
   | Code | Meaning                               | Reference          |
   +------+---------------------------------------+--------------------+
   | NTSN | Network Time Security (NTS) negative- | [[this memo]],     |
   |      | acknowledgment (NAK)                  | Section 5.7        |
   +------+---------------------------------------+--------------------+

7.5.  NTP Extension Field Types Registry

   IANA is requested to allocate the following entries in the NTP
   Extension Field Types registry [RFC5905]:








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   +----------+----------------------------------+---------------------+
   | Field    | Meaning                          | Reference           |
   | Type     |                                  |                     |
   +----------+----------------------------------+---------------------+
   | [[TBD2]] | Unique Identifier                | [[this memo]],      |
   |          |                                  | Section 5.3         |
   | [[TBD3]] | NTS Cookie                       | [[this memo]],      |
   |          |                                  | Section 5.4         |
   | [[TBD4]] | NTS Cookie Placeholder           | [[this memo]],      |
   |          |                                  | Section 5.5         |
   | [[TBD5]] | NTS Authenticator and Encrypted  | [[this memo]],      |
   |          | Extension Fields                 | Section 5.6         |
   +----------+----------------------------------+---------------------+

   [[RFC EDITOR: Replace all instances of [[TBD2]], [[TBD3]], [[TBD4]],
   and [[TBD5]] in this document with the respective IANA assignments.

7.6.  Network Time Security Key Establishment Record Types Registry

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

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

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

      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 Record 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 as well as which
   of the above ranges the Record Type Number should be allocated from.
   Applicants MAY request a specific Record Type Number and such
   requests MAY be granted at the registrar's discretion.

   The initial contents of this registry SHALL be as follows:



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   +---------------+----------------------------+----------------------+
   | Record Type   | Description                | Reference            |
   | Number        |                            |                      |
   +---------------+----------------------------+----------------------+
   | 0             | End of Message             | [[this memo]],       |
   |               |                            | Section 4.1.1        |
   | 1             | NTS Next Protocol          | [[this memo]],       |
   |               | Negotiation                | Section 4.1.2        |
   | 2             | Error                      | [[this memo]],       |
   |               |                            | Section 4.1.3        |
   | 3             | Warning                    | [[this memo]],       |
   |               |                            | Section 4.1.4        |
   | 4             | AEAD Algorithm Negotiation | [[this memo]],       |
   |               |                            | Section 4.1.5        |
   | 5             | New Cookie for NTPv4       | [[this memo]],       |
   |               |                            | Section 4.1.6        |
   | 6             | NTPv4 Server Negotiation   | [[this memo]],       |
   |               |                            | Section 4.1.7        |
   | 7             | NTPv4 Port Negotiation     | [[this memo]],       |
   |               |                            | Section 4.1.8        |
   | 16384-32767   | Reserved for Private &     | [[this memo]]        |
   |               | Experimental Use           |                      |
   +---------------+----------------------------+----------------------+

7.7.  Network Time Security Next Protocols Registry

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

      Protocol ID (REQUIRED): An integer in the range 0-65535 inclusive,
      functioning as an identifier.

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

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

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

      0-1023: IETF Review

      1024-32767: Specification Required

      32768-65535: Private and Experimental Use

   The initial contents of this registry SHALL be as follows:



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   +-------------+-------------------------------+---------------------+
   | Protocol ID | Protocol Name                 | Reference           |
   +-------------+-------------------------------+---------------------+
   | 0           | Network Time Protocol version | [[this memo]]       |
   |             | 4 (NTPv4)                     |                     |
   | 32768-65535 | Reserved for Private or       | Reserved by [[this  |
   |             | Experimental Use              | memo]]              |
   +-------------+-------------------------------+---------------------+

7.8.  Network Time Security Error and Warning Codes Registries

   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): An integer in the range 0-65535 inclusive

      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

      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        | [[this memo]],       |
   |             | Extension                    | Section 4.1.3        |
   | 1           | Bad Request                  | [[this memo]],       |
   |             |                              | Section 4.1.3        |
   | 32768-65535 | Reserved for Private or      | Reserved by [[this   |
   |             | Experimental Use             | memo]]               |
   +-------------+------------------------------+----------------------+

   The Network Time Security Warning Codes Registry SHALL initally be
   empty except for the reserved range, i.e.:




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   +-------------+-------------------------------+---------------------+
   | Number      | Description                   | Reference           |
   +-------------+-------------------------------+---------------------+
   | 32768-65535 | Reserved for Private or       | Reserved by [[this  |
   |             | Experimental Use              | memo]]              |
   +-------------+-------------------------------+---------------------+

8.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

8.1.  Implementation PoC 1

   Organization: Ostfalia University of Applied Science

   Implementor: Martin Langer

   Maturity: Proof-of-Concept Prototype

   This implementation was used to verify consistency and to ensure
   completeness of this specification.  It also demonstrate
   interoperability with NTP's client-server mode messages.

8.1.1.  Coverage

   This implementation covers the complete specification.







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8.1.2.  Licensing

   The code is released under a Apache License 2.0 license.

   The source code is available at: https://gitlab.com/MLanger/nts/

8.1.3.  Contact Information

   Contact Martin Langer: mart.langer@ostfalia.de

8.1.4.  Last Update

   The implementation was updated 3rd May 2018.

8.2.  Implementation PoC 2

   Organization: tbd

   Implementor: Daniel Fox Franke

   Maturity: Proof-of-Concept Prototype

   This implementation was used to verify consistency and to ensure
   completeness of this specification.

8.2.1.  Coverage

   This implementation provides the client and the server for the
   initial TLS handshake and NTS key exchange.  It provides the the
   client part of the NTS protected NTP messages.

8.2.2.  Licensing

   Public domain.

   The source code is available at: https://github.com/dfoxfranke/nts-
   hackathon

8.2.3.  Contact Information

   Contact Daniel Fox Franke: dfoxfranke@gmail.com

8.2.4.  Last Update

   The implementation was updated 16th March 2018.






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8.3.  Interoperability

   The Interoperability tests distinguished between NTS key exchange and
   NTS time exchange messages.  For the NTS key exchange,
   interoperability between the two implementations has been verified
   successfully.  Interoperability of NTS time exchange messages has
   been verified successfully for the case that PoC 1 represents the
   server and PoC 2 the client.

   These tests successfully demonstrate that there are at least two
   running implementations of this draft which are able to interoperate.

9.  Security Considerations

9.1.  Sensitivity to DDoS attacks

   The introduction of NTS brings with it the introduction of asymmetric
   cryptography to NTP.  Asymmetric cryptography is necessary for
   initial server authentication and AEAD key extraction.  Asymmetric
   cryptosystems are generally orders of magnitude slower than their
   symmetric counterparts.  This makes it much harder to build systems
   that can serve requests at a rate corresponding to the full line
   speed of the network connection.  This, in turn, opens up a new
   possibility for DDoS attacks on NTP services.

   The main protection against these attacks in NTS lies in that the use
   of asymmetric cryptosystems is only necessary in the initial NTS-KE
   phase of the protocol.  Since the protocol design enables separation
   of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE
   server separated from the NTP service it supports will not affect NTP
   users that have already performed initial authentication, AEAD key
   extraction, and cookie exchange.

   NTS users should also consider that they are not fully protected
   against DDoS attacks by on-path adversaries.  In addition to dropping
   packets and attacks such as those described in Section 9.4, an on-
   path attacker can send spoofed kiss-o'-death replies, which are not
   authenticated, in response to NTP requests.  This could result in
   significantly increased load on the NTS-KE server.  Implementers have
   to weigh the user's need for unlinkability against the added
   resilience that comes with cookie reuse in cases of NTS-KE server
   unavailability.

9.2.  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



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   which enable these features can be abused in order to amplify traffic
   volume in 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 extension fields included in server
   responses will be the same size as the NTS-related extension fields
   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 field for every cookie it wants to get back,
   rather than being permitted simply to specify a desired quantity.

   Due to the RFC 7822 [RFC7822] requirement that extensions be padded
   and aligned to four-octet boundaries, response size may still in some
   cases exceed request size by up to three octets.  This is
   sufficiently inconsequential that we have declined to address it.

9.3.  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 NTS-KE server is
   valid and rooted in a trusted certificate authority.  RFC 5280
   [RFC5280] and RFC 6125 [RFC6125] specify 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.  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.



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      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 (RFC 5905 section 
      11.2.1 [RFC5905]) will protect the client from accepting bad time
      from the adversary-controlled servers.

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.

   RFC 5905 [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 field) 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.  Whenever this draft specifies the use of
   random numbers, cryptographically secure random number generation



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   MUST be used.  RFC 4086 [RFC4086] contains 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
   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.

   Note that the unlinkability objective does not prevent a client
   device to be tracked by its time servers.

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



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   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,
   Patrik Faeltstroem (Faltstrom), Scott Fluhrer, 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, Joachim Stroembergsson
   (Strombergsson), Martin Thomson, and Richard Welty for contributions
   to this document and comments on the design of NTS.

12.  References

12.1.  Normative References

   [ANSI.X3-4.1986]
              American National Standards Institute, "Coded Character
              Set - 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://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, <https://www.rfc-editor.org/info/rfc5297>.





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

   [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,
              <https://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, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
              February 2013, <https://www.rfc-editor.org/info/rfc6874>.

   [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, <https://www.rfc-editor.org/info/rfc7301>.

   [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,
              <https://www.rfc-editor.org/info/rfc7507>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

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




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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

12.2.  Informative References

   [I-D.ietf-ntp-data-minimization]
              Franke, D. and A. Malhotra, "NTP Client Data
              Minimization", draft-ietf-ntp-data-minimization-02 (work
              in progress), July 2018.

   [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.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

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

   [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, <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc5869>.



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   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <https://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]

   ALPN    Application-Layer Protocol Negotiation [RFC7301]

   C2S     Client-to-server

   DDoS    Distributed Denial-of-Service

   EF      Extension Field [RFC5905]

   HKDF    Hashed Message Authentication Code-based Key Derivation
      Function [RFC5869]

   IANA    Internet Assigned Numbers Authority

   IP      Internet Protocol

   KoD     Kiss-o'-Death [RFC5905]

   NTP     Network Time Protocol [RFC5905]

   NTS     Network Time Security

   NTS-KE  Network Time Security Key Exchange

   S2C     Server-to-client

   SCSV    Signaling Cipher Suite Value [RFC7507]

   TCP     Transmission Control Protocol [RFC0793]

   TLS     Transport Layer Security [RFC8446]

   UDP     User Datagram Protocol [RFC0768]






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Authors' Addresses

   Daniel Fox Franke

   Email: dfoxfranke@gmail.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)531-592-4471
   Email: kristof.teichel@ptb.de


   Marcus Dansarie

   Email: marcus@dansarie.se


   Ragnar Sundblad
   Netnod

   Email: ragge@netnod.se













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