path: root/doc/rfc/rfc8915.txt

Internet Engineering Task Force (IETF)                         D. Franke
Request for Comments: 8915                                        Akamai
Category: Standards Track                                      D. Sibold
ISSN: 2070-1721                                               K. Teichel
                                                                     PTB
                                                             M. Dansarie
                                                                        
                                                             R. Sundblad
                                                                  Netnod
                                                          September 2020


          Network Time Security for the Network Time Protocol

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 first (NTS Key Establishment (NTS-KE)) handles initial
   authentication and key establishment over TLS.  The second (NTS
   Extension Fields for NTPv4) handles encryption and authentication
   during NTP time synchronization via extension fields in the NTP
   packets, and holds all required state only on the client via opaque
   cookies.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8915.

Copyright Notice

   Copyright (c) 2020 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
     1.1.  Objectives
     1.2.  Terms and Abbreviations
     1.3.  Protocol Overview
   2.  Requirements Language
   3.  TLS Profile for Network Time Security
   4.  The NTS Key Establishment Protocol
     4.1.  NTS-KE Record Types
       4.1.1.  End of Message
       4.1.2.  NTS Next Protocol Negotiation
       4.1.3.  Error
       4.1.4.  Warning
       4.1.5.  AEAD Algorithm Negotiation
       4.1.6.  New Cookie for NTPv4
       4.1.7.  NTPv4 Server Negotiation
       4.1.8.  NTPv4 Port Negotiation
     4.2.  Retry Intervals
     4.3.  Key Extraction (Generally)
   5.  NTS Extension Fields for NTPv4
     5.1.  Key Extraction (for NTPv4)
     5.2.  Packet Structure Overview
     5.3.  The Unique Identifier Extension Field
     5.4.  The NTS Cookie Extension Field
     5.5.  The NTS Cookie Placeholder Extension Field
     5.6.  The NTS Authenticator and Encrypted Extension Fields
           Extension Field
     5.7.  Protocol Details
   6.  Suggested Format for NTS Cookies
   7.  IANA Considerations
     7.1.  Service Name and Transport Protocol Port Number Registry
     7.2.  TLS Application-Layer Protocol Negotiation (ALPN) Protocol
           IDs Registry
     7.3.  TLS Exporter Labels Registry
     7.4.  NTP Kiss-o'-Death Codes Registry
     7.5.  NTP Extension Field Types Registry
     7.6.  Network Time Security Key Establishment Record Types
           Registry
     7.7.  Network Time Security Next Protocols Registry
     7.8.  Network Time Security Error and Warning Codes Registries
   8.  Security Considerations
     8.1.  Protected Modes
     8.2.  Cookie Encryption Key Compromise
     8.3.  Sensitivity to DDoS Attacks
     8.4.  Avoiding DDoS Amplification
     8.5.  Initial Verification of Server Certificates
     8.6.  Delay Attacks
     8.7.  NTS Stripping
   9.  Privacy Considerations
     9.1.  Unlinkability
     9.2.  Confidentiality
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Authors' Addresses

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:

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

   *  Authentication: Implementations can 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.

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

   *  Replay prevention: Client implementations can detect when a
      received time synchronization packet is a replay of a previous
      packet.

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

   *  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.

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

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

   *  Performance: NTS must not significantly degrade the quality of the
      time transfer.  The encryption and authentication used when
      actually transferring time should be lightweight (see Section 5.7
      of RFC 7384 [RFC7384]).

1.2.  Terms and Abbreviations

   AEAD       Authenticated Encryption with Associated Data [RFC5116]

   ALPN       Application-Layer Protocol Negotiation [RFC7301]

   C2S        Client-to-server

   DoS        Denial-of-Service

   DDoS       Distributed Denial-of-Service

   EF         Extension Field [RFC5905]

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

   KoD        Kiss-o'-Death [RFC5905]

   NTP        Network Time Protocol [RFC5905]

   NTS        Network Time Security

   NTS NAK    NTS negative-acknowledgment

   NTS-KE     Network Time Security Key Establishment

   S2C        Server-to-client

   TLS        Transport Layer Security [RFC8446]

1.3.  Protocol Overview

   The Network Time Protocol includes many different operating modes to
   support various network topologies (see Section 3 of RFC 5905
   [RFC5905]).  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, the 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 a 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 other hand,
   are in conflict with the resource-utilization demands of client-
   server mode: any scheme that provides replay protection inherently
   involves maintaining some state to keep track of which 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 Extension Fields 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 establish keys, to provide the client
      with an initial supply of cookies, and to negotiate some
      additional protocol options.  After this, 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 an address and port of an NTP server 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 the indicated NTP server.  The
   client sends the server an NTP client packet that includes several
   extension fields.  Included among these fields are a cookie
   (previously provided by the key establishment 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 that the client then sends back 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
   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 RFC 7525 [RFC7525] or with a later revision of BCP 195.

   Implementations MUST NOT negotiate TLS versions earlier than 1.3
   [RFC8446] and MAY refuse to negotiate any TLS version that 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.

   Implementations MUST follow the rules in RFC 5280 [RFC5280] and RFC
   6125 [RFC6125] for the representation and verification of the
   application's service identity.  When NTS-KE service discovery (out
   of scope for this document) produces one or more host names, use of
   the DNS-ID identifier type [RFC6125] is RECOMMENDED; specifications
   for service discovery mechanisms can provide additional guidance for
   certificate validation based on the results of discovery.
   Section 8.5 of this memo discusses particular considerations for
   certificate verification in the context of NTS.

4.  The NTS Key Establishment Protocol

   The NTS key establishment protocol is conducted via TCP port 4460.
   The two endpoints carry out a TLS handshake in conformance with
   Section 3, with the client offering (via an ALPN extension
   [RFC7301]), 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.  After sending their respective request and response, the
   client and server SHALL send TLS "close_notify" alerts in accordance
   with Section 6.1 of RFC 8446 [RFC8446].

   The client's request and the server's response each SHALL consist of
   a sequence of records formatted according to Figure 2.  The request
   and a non-error response 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.

    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
      (see Section 4.1.3).

   Record Type Number:  A 15-bit integer in network byte order.  The
      semantics of Record Types 0-7 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 the C programming language,
   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]'.

   Note that, although the Type-Length-Body format of an NTS-KE record
   is similar to that of an NTP extension field, the semantics of the
   length field differ.  While the length subfield of an NTP extension
   field gives the length of the entire extension field including the
   type and length subfields, the length field of an NTS-KE record gives
   just the length of the body.

   Figure 3 provides a schematic overview of the key establishment.  It
   displays the protocol steps to be performed by the NTS client and
   server and Record Types to be exchanged.

                   +---------------------------------------+
                   | - 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 <NTPv4 Server Negotiation>    |
                   |       o <NTPv4 Port 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 <NTPv4 Server Negotiation>    |
   |  o <NTPv4 Port Negotiation>      |
   |  o End of Message                |
   +----------------------------------+

                  Figure 3: NTS Key Establishment 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.

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 (Section 7.7).
   The Critical Bit MUST be set.

   The Protocol IDs listed in the client's NTS Next Protocol Negotiation
   record denote those protocols that the client wishes to speak using
   the key material established through this NTS-KE session.  Protocol
   IDs listed in the NTS-KE server's response MUST comprise a subset of
   those listed in the request and denote those protocols that the NTP
   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 that includes an Error record, they MUST
   discard any key material negotiated during the initial TLS exchange
   and MUST NOT proceed to the Next Protocol.  Requirements for retry
   intervals are described in Section 4.2.

   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
      that the server did not understand and that 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 the request is not complete and syntactically well-
      formed, or, upon the expiration of an implementation-defined
      timeout, it has not yet received such a request.  The client
      SHOULD NOT retry its request without modification.

      Error code 2 means "Internal Server Error".  The server MUST
      respond with this error if it is unable to respond properly due to
      an internal condition.  The client MAY retry its request.

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
   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 that 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
   Algorithms" registry [IANA-AEAD].  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 that 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.

   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 [RFC0020] string.  The contents
   of the string SHALL be either an IPv4 address, an IPv6 address, or a
   fully qualified domain name (FQDN).  IPv4 addresses MUST be in dotted
   decimal notation.  IPv6 addresses MUST conform to the "Text
   Representation of Addresses" as specified in RFC 4291 [RFC4291] and
   MUST NOT include zone identifiers [RFC6874].  If a label contains at
   least one non-ASCII character, it is an internationalized domain
   name, and an A-LABEL MUST be used as defined in Section 2.3.2.1 of
   RFC 5890 [RFC5890].  If the record contains a domain name, the
   recipient MUST treat it as a FQDN, e.g., by making sure it ends with
   a dot.

   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 that 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 which 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.

   If the client has sent a record of this type, the NTS-KE server
   SHOULD reply with the same record if it is valid and the server is
   able to supply cookies for it.  If the client has not sent any record
   of this type, the NTS-KE server SHOULD respond with either an NTP
   server address in the same family as the NTS-KE session or a FQDN
   that can be resolved to an address in that family, if such
   alternatives are available.

   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 that 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
   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.  Retry Intervals

   A mechanism for not unnecessarily overloading the NTS-KE server is
   REQUIRED when retrying the key establishment process due to protocol,
   communication, or other errors.  The exact workings of this will be
   dependent on the application and operational experience gathered over
   time.  Until such experience is available, this memo provides the
   following suggestion.

   Clients SHOULD use exponential backoff, with an initial and minimum
   retry interval of 10 seconds, a maximum retry interval of 5 days, and
   a base of 1.5.  Thus, the minimum interval in seconds, 't', for the
   nth retry is calculated with the following:

      t = min(10 * 1.5^(n-1), 432000).

   Clients MUST NOT reset the retry interval until they have performed a
   successful key establishment with the NTS-KE server, followed by a
   successful use of the negotiated Next Protocol with the keys and data
   established during that transaction.

4.3.  Key Extraction (Generally)

   Following a successful run of the NTS-KE protocol, key material SHALL
   be extracted using the HMAC-based Extract-and-Expand Key Derivation
   Function (HKDF) [RFC5869] in accordance with Section 7.5 of RFC 8446
   [RFC8446].  Inputs to the exporter function are to be constructed in
   a manner specific to the negotiated Next Protocol.  However, all
   protocols that utilize NTS-KE MUST conform to the following two
   rules:

      The disambiguating label string [RFC5705] MUST be "EXPORTER-
      network-time-security".

      The per-association context value [RFC5705] MUST be provided and
      MUST begin with the two-octet Protocol ID that 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 with the HKDF defined in
   Section 7.5 of RFC 8446 [RFC8446] using the following inputs:

      The disambiguating label string [RFC5705] SHALL be "EXPORTER-
      network-time-security".

      The per-association context value [RFC5705] 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
   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 following:

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

      Some extension fields, which are authenticated but not encrypted.

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

      Possibly, some additional extension fields that are neither
      encrypted nor authenticated.  In general, 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, as described in Section 5.7.  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 0x0104.  When the extension field is included in a
   client packet (mode 3), its body SHALL consist of a string of octets
   generated by a cryptographically secure random number generator
   [RFC4086].  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 this is suboptimal
   for cryptographic purposes 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 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 0x0204.  Its
   purpose is to carry information that 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
   0x0304.  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 SHOULD be all zeros 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 0x0404.  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.

   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
      end of 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 end of the field is zero-padded to a word
      (four octets) boundary.

   Additional Padding:  Clients that 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 octets padding              .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .        Ciphertext, including up to 3 octets 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:

   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 that 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; if multiple extension fields are present, they
       SHALL be joined by concatenation.  Each such field SHALL be
       formatted in accordance with RFC 7822 [RFC7822], except that,
       contrary to the RFC 7822 requirement that fields have a minimum
       length of 16 or 28 octets, encrypted extension fields MAY be
       arbitrarily short (but still MUST be a multiple of 4 octets in
       length).

   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 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 that
   does not conform to it.

   Senders are always free to include more Additional Padding than
   mandated by the above paragraph.  Theoretically, it could be
   necessary to do so in order to bring the extension field to the
   minimum length required by RFC 7822 [RFC7822].  This should never
   happen in practice because any reasonable AEAD algorithm will have a
   nonce and an authenticator long enough to bring the extension field
   to its required length already.  Nonetheless, implementers are
   advised to explicitly handle this case and ensure that the extension
   field they emit is of legal length.

   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 that MUST be
      authenticated, MUST NOT be encrypted, and whose contents MUST be
      the output of a cryptographically secure random number generator
      [RFC4086].

      Exactly one NTS Cookie extension field that 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
      establishment server during the NTS-KE handshake or from the NTP
      server in response to a previous NTS-protected NTP request.

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

   To protect the client's privacy, the client SHOULD avoid reusing a
   cookie.  If the client does not have any cookies that it has not
   already sent, it SHOULD initiate a rerun of the NTS-KE protocol.  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 9.1 describes the privacy considerations of this
   in further detail.

   The client MAY include one or more NTS Cookie Placeholder extension
   fields that 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.

   In rare circumstances, it may be necessary to include fewer NTS
   Cookie Placeholder extensions than recommended above in order to
   prevent datagram fragmentation.  When cookies adhere to the format
   recommended in Section 6 and the AEAD in use is the mandatory-to-
   implement AEAD_AES_SIV_CMAC_256, senders can include a cookie and
   seven placeholders and still have packet size fall comfortably below
   1280 octets if no non-NTS-related extensions are used; 1280 octets is
   the minimum prescribed MTU for IPv6 and is generally safe for
   avoiding IPv4 fragmentation.  Nonetheless, senders SHOULD include
   fewer cookies and placeholders than otherwise indicated if doing so
   is necessary to prevent fragmentation.

                   +---------------------------------------+
                   | - Verify time request message.        |
                   | - Generate time response message.     |
                   |   - Included NTPv4 extension fields:  |
                   |      o Unique Identifier EF           |
                   |      o NTS Authentication and         |
                   |        Encrypted Extension Fields EF  |
                   |        - NTS Cookie EF                |
                   |        - <NTS Cookie EF>              |
                   | - Transmit time request 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-Protected NTP Time Synchronization Messages

   The client MAY include additional (non-NTS-related) extension fields
   that 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).  The server
   MUST discard any unauthenticated extension fields.  Future
   specifications of extension fields MAY provide exceptions to this
   rule.

   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 that 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 that 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
   when 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 9.1.  We emphasize also that "encrypted" means encapsulated
   within 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
   that 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).  The client
   MUST discard any unauthenticated extension fields.  Future
   specifications of extension fields MAY provide exceptions to this
   rule.

   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.  In particular, the client MUST
   discard unprotected responses to NTS-protected requests.

   If the server is unable to validate the cookie or authenticate the
   request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see
   Section 7.4 of RFC 5905 [RFC5905]) with kiss code "NTSN", meaning
   "NTS NAK" (NTS negative-acknowledgment).  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, 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 Section 7.4 of RFC 5905 [RFC5905] where required.

   A client MAY automatically rerun the NTS-KE protocol upon forced
   disassociation from an NTP server.  In that case, it MUST avoid
   quickly looping between the NTS-KE and NTP servers by rate limiting
   the retries.  Requirements for retry intervals in NTS-KE are
   described in Section 4.2.

   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.

   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 in persistent storage.
   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
   tickets 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 that 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 secret
   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.  Following each such key rotation, servers should
   securely erase any previously generated keys that should now be
   expired.  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 pseudorandom
   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:

      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 has allocated the following entry in the "Service Name and
   Transport Protocol Port Number Registry" [RFC6335]:

   Service Name:  ntske

   Port Number:  4460

   Transport Protocol:  tcp

   Description:  Network Time Security Key Establishment

   Assignee:  IESG <iesg@ietf.org>

   Contact:  IETF Chair <chair@ietf.org>

   Registration Date:  2020-04-07

   Reference:  RFC 8915

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

   IANA has allocated 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:  RFC 8915, Section 4

7.3.  TLS Exporter Labels Registry

   IANA has allocated the following entry in the TLS Exporter Labels
   registry [RFC5705]:

   +================================+=======+===========+=========+====+
   | Value                          |DTLS-OK|Recommended|Reference|Note|
   +================================+=======+===========+=========+====+
   | EXPORTER-network-time-security |Y      |Y          |RFC 8915,|    |
   |                                |       |           |Section  |    |
   |                                |       |           |4.3      |    |
   +--------------------------------+-------+-----------+---------+----+

                                  Table 1

7.4.  NTP Kiss-o'-Death Codes Registry

   IANA has allocated the following entry in the "NTP Kiss-o'-Death
   Codes" registry [RFC5905]:

          +======+===============================+=============+
          | Code | Meaning                       | Reference   |
          +======+===============================+=============+
          | NTSN | Network Time Security (NTS)   | RFC 8915,   |
          |      | negative-acknowledgment (NAK) | Section 5.7 |
          +------+-------------------------------+-------------+

                                 Table 2

7.5.  NTP Extension Field Types Registry

   IANA has allocated the following entries in the "NTP Extension Field
   Types" registry [RFC5905]:

    +============+============================+=======================+
    | Field Type | Meaning                    | Reference             |
    +============+============================+=======================+
    | 0x0104     | Unique Identifier          | RFC 8915, Section 5.3 |
    +------------+----------------------------+-----------------------+
    | 0x0204     | NTS Cookie                 | RFC 8915, Section 5.4 |
    +------------+----------------------------+-----------------------+
    | 0x0304     | NTS Cookie Placeholder     | RFC 8915, Section 5.5 |
    +------------+----------------------------+-----------------------+
    | 0x0404     | NTS Authenticator and      | RFC 8915, Section 5.6 |
    |            | Encrypted Extension Fields |                       |
    +------------+----------------------------+-----------------------+

                                  Table 3

7.6.  Network Time Security Key Establishment Record Types Registry

   IANA has created a new registry entitled "Network Time Security Key
   Establishment Record Types".  Entries 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 registration policy varies by Record Type Number, as follows:

   0-1023:  IETF Review

   1024-16383:  Specification Required

   16384-32767:  Private or Experimental Use

   The initial contents of this registry are as follows:

       +====================+======================+===============+
       | Record Type Number | Description          | Reference     |
       +====================+======================+===============+
       | 0                  | End of Message       | RFC 8915,     |
       |                    |                      | Section 4.1.1 |
       +--------------------+----------------------+---------------+
       | 1                  | NTS Next Protocol    | RFC 8915,     |
       |                    | Negotiation          | Section 4.1.2 |
       +--------------------+----------------------+---------------+
       | 2                  | Error                | RFC 8915,     |
       |                    |                      | Section 4.1.3 |
       +--------------------+----------------------+---------------+
       | 3                  | Warning              | RFC 8915,     |
       |                    |                      | Section 4.1.4 |
       +--------------------+----------------------+---------------+
       | 4                  | AEAD Algorithm       | RFC 8915,     |
       |                    | Negotiation          | Section 4.1.5 |
       +--------------------+----------------------+---------------+
       | 5                  | New Cookie for NTPv4 | RFC 8915,     |
       |                    |                      | Section 4.1.6 |
       +--------------------+----------------------+---------------+
       | 6                  | NTPv4 Server         | RFC 8915,     |
       |                    | Negotiation          | Section 4.1.7 |
       +--------------------+----------------------+---------------+
       | 7                  | NTPv4 Port           | RFC 8915,     |
       |                    | Negotiation          | Section 4.1.8 |
       +--------------------+----------------------+---------------+
       | 8-16383            | Unassigned           |               |
       +--------------------+----------------------+---------------+
       | 16384-32767        | Reserved for Private | RFC 8915      |
       |                    | or Experimental Use  |               |
       +--------------------+----------------------+---------------+

                                  Table 4

7.7.  Network Time Security Next Protocols Registry

   IANA has created a new registry entitled "Network Time Security Next
   Protocols".  Entries 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 registration policy varies by Protocol ID, as follows:

   0-1023:  IETF Review

   1024-32767:  Specification Required

   32768-65535:  Private or Experimental Use

   The initial contents of this registry are as follows:

   +=============+=========================================+===========+
   | Protocol ID | Protocol Name                           | Reference |
   +=============+=========================================+===========+
   | 0           | Network Time Protocol                   | RFC 8915  |
   |             | version 4 (NTPv4)                       |           |
   +-------------+-----------------------------------------+-----------+
   | 1-32767     | Unassigned                              |           |
   +-------------+-----------------------------------------+-----------+
   | 32768-65535 | Reserved for Private                    | RFC 8915  |
   |             | or Experimental Use                     |           |
   +-------------+-----------------------------------------+-----------+

                                  Table 5

7.8.  Network Time Security Error and Warning Codes Registries

   IANA has created two new registries entitled "Network Time Security
   Error Codes" and "Network Time Security Warning Codes".  Entries in
   each 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 registration policy varies by Number, as follows:

   0-1023:  IETF Review

   1024-32767:  Specification Required

   32768-65535:  Private or Experimental Use

   The initial contents of the "Network Time Security Error Codes"
   registry are as follows:

      +=============+==============================+===============+
      | Number      | Description                  | Reference     |
      +=============+==============================+===============+
      | 0           | Unrecognized Critical Record | RFC 8915,     |
      |             |                              | Section 4.1.3 |
      +-------------+------------------------------+---------------+
      | 1           | Bad Request                  | RFC 8915,     |
      |             |                              | Section 4.1.3 |
      +-------------+------------------------------+---------------+
      | 2           | Internal Server Error        | RFC 8915,     |
      |             |                              | Section 4.1.3 |
      +-------------+------------------------------+---------------+
      | 3-32767     | Unassigned                   |               |
      +-------------+------------------------------+---------------+
      | 32768-65535 | Reserved for Private or      | RFC 8915      |
      |             | Experimental Use             |               |
      +-------------+------------------------------+---------------+

                                 Table 6

   The "Network Time Security Warning Codes" registry is initially empty
   except for the reserved range, i.e.:

            +=============+======================+===========+
            | Number      | Description          | Reference |
            +=============+======================+===========+
            | 0-32767     | Unassigned           |           |
            +-------------+----------------------+-----------+
            | 32768-65535 | Reserved for Private | RFC 8915  |
            |             | or Experimental Use  |           |
            +-------------+----------------------+-----------+

                                 Table 7

8.  Security Considerations

8.1.  Protected Modes

   NTP provides many different operating modes in order to support
   different network topologies and to adapt to various requirements.
   This memo only specifies NTS for NTP modes 3 (client) and 4 (server)
   (see Section 1.3).  The best current practice for authenticating the
   other NTP modes is using the symmetric message authentication code
   feature as described in RFC 5905 [RFC5905] and RFC 8573 [RFC8573].

8.2.  Cookie Encryption Key Compromise

   If the suggested format for NTS cookies in Section 6 of this document
   is used, an attacker who has gained access to the secret cookie
   encryption key 'K' can impersonate the NTP server, including
   generating new cookies.  NTP and NTS-KE server operators SHOULD
   remove compromised keys as soon as the compromise is discovered.
   This will cause the NTP servers to respond with NTS NAK, thus forcing
   key renegotiation.  Note that this measure does not protect against
   MITM attacks where the attacker has access to a compromised cookie
   encryption key.  If another cookie scheme is used, there are likely
   similar considerations for that particular scheme.

8.3.  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 DoS attacks by on-path adversaries.  In addition to dropping
   packets and attacks such as those described in Section 8.6, 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.

8.4.  Avoiding DDoS Amplification

   Certain nonstandard and/or deprecated features of the Network Time
   Protocol enable clients to send a request to a server that causes the
   server to send a response much larger than the request.  Servers that
   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 address.  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.

8.5.  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 that 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.  On systems
      with the ntp_adjtime() system call, a return code other than
      TIME_ERROR indicates that some trusted software has already set
      the time 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 that have a battery-backed clock or that
      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.

      NTP time replies are expected to be consistent with the NTS-KE TLS
      certificate validity period, i.e. time replies received
      immediately after an NTS-KE handshake are expected to lie within
      the certificate validity period.  Implementations are recommended
      to check that this is the case.  Performing a new NTS-KE handshake
      based solely on the fact that the certificate used by the NTS-KE
      server in a previous handshake has expired is normally not
      necessary.  Clients that still wish to do this must take care not
      to cause an inadvertent denial-of-service attack on the NTS-KE
      server, for example by picking a random time in the week preceding
      certificate expiry to perform the new handshake.

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

8.6.  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 that 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.

8.7.  NTS Stripping

   Implementers must be aware of the possibility of "NTS stripping"
   attacks, where an attacker attempts to trick clients into reverting
   to plain NTP.  Naive client implementations might, for example,
   revert automatically to plain NTP if the NTS-KE handshake fails.  A
   man-in-the-middle attacker can easily cause this to happen.  Even
   clients that already hold valid cookies can be vulnerable, since an
   attacker can force a client to repeat the NTS-KE handshake by sending
   faked NTP mode 4 replies with the NTS NAK kiss code.  Forcing a
   client to repeat the NTS-KE handshake can also be the first step in
   more advanced attacks.

   For the reasons described here, implementations SHOULD NOT revert
   from NTS-protected to unprotected NTP with any server without
   explicit user action.

9.  Privacy Considerations

9.1.  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 document NTP Client Data Minimization
   [NTP-DATA-MIN].

   The unlinkability objective only holds for time synchronization
   traffic, as opposed to key establishment 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 linkable data, such as the TLS
   certificate).

   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 from being tracked by its time servers.

9.2.  Confidentiality

   NTS does not protect the confidentiality of information in NTP's
   header fields.  When clients implement NTP Client Data Minimization
   [NTP-DATA-MIN], client packet headers do not contain any information
   that 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.

10.  References

10.1.  Normative References

   [IANA-AEAD]
              IANA, "Authenticated Encryption with Associated Data
              (AEAD) Parameters",
              <https://www.iana.org/assignments/aead-parameters/>.

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

   [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>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [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>.

   [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>.

   [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>.

10.2.  Informative References

   [Mizrahi]  Mizrahi, T., "A game theoretic analysis of delay attacks
              against time synchronization protocols", 2012 IEEE
              International Symposium on Precision Clock Synchronization
              for Measurement, Control and Communication Proceedings,
              pp. 1-6, DOI 10.1109/ISPCS.2012.6336612, September 2012,
              <https://doi.org/10.1109/ISPCS.2012.6336612>.

   [NTP-DATA-MIN]
              Franke, D. F. and A. Malhotra, "NTP Client Data
              Minimization", Work in Progress, Internet-Draft, draft-
              ietf-ntp-data-minimization-04, 25 March 2019,
              <https://tools.ietf.org/html/draft-ietf-ntp-data-
              minimization-04>.

   [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>.

   [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>.

   [RFC8573]  Malhotra, A. and S. Goldberg, "Message Authentication Code
              for the Network Time Protocol", RFC 8573,
              DOI 10.17487/RFC8573, June 2019,
              <https://www.rfc-editor.org/info/rfc8573>.

   [Shpiner]  Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
              Protocols", 2013 IEEE International Symposium on Precision
              Clock Synchronization for Measurement, Control and
              Communication (ISPCS) Proceedings, pp. 1-6,
              DOI 10.1109/ISPCS.2013.6644754, September 2013,
              <https://doi.org/10.1109/ISPCS.2013.6644754>.

Acknowledgments

   The authors would like to thank Richard Barnes, Steven Bellovin,
   Scott Fluhrer, Patrik Fältström, Sharon Goldberg, Russ Housley,
   Benjamin Kaduk, Suresh Krishnan, Mirja Kühlewind, Martin Langer,
   Barry Leiba, Miroslav Lichvar, Aanchal Malhotra, Danny Mayer, Dave
   Mills, Sandra Murphy, Hal Murray, Karen O'Donoghue, Eric K. Rescorla,
   Kurt Roeckx, Stephen Roettger, Dan Romascanu, Kyle Rose, Rich Salz,
   Brian Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn, Joachim
   Strömbergsson, Martin Thomson, Éric Vyncke, Richard Welty, Christer
   Weinigel, and Magnus Westerlund for contributions to this document
   and comments on the design of NTS.

Authors' Addresses

   Daniel Fox Franke
   Akamai Technologies
   145 Broadway
   Cambridge, MA 02142
   United States of America

   Email: dafranke@akamai.com


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

   Phone: +49-(0)531-592-8462
   Email: dieter.sibold@ptb.de


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

   Phone: +49-(0)531-592-4471
   Email: kristof.teichel@ptb.de


   Marcus Dansarie
   Sweden

   Email: marcus@dansarie.se
   URI:   https://orcid.org/0000-0001-9246-0263


   Ragnar Sundblad
   Netnod
   Sweden

   Email: ragge@netnod.se