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Versions: 00 01 02                                                      
Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Intended status: Standards Track                            Nov 23, 2020
Expires: May 27, 2021


                    Network Time Protocol Version 5
                      draft-mlichvar-ntp-ntpv5-00

Abstract

   This document describes the version 5 of the Network Time Protocol
   (NTP).

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 May 27, 2021.

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
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   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.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Data types  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Message format  . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Extension fields  . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Padding Extension Field . . . . . . . . . . . . . . . . .   8
     4.2.  MAC Extension Field . . . . . . . . . . . . . . . . . . .   8
     4.3.  Reference IDs Extension Field . . . . . . . . . . . . . .   8
     4.4.  Correction Extension Field  . . . . . . . . . . . . . . .   8
     4.5.  Reference timestamp . . . . . . . . . . . . . . . . . . .   8
     4.6.  Monotonic Timestamp Extension Field . . . . . . . . . . .   9
   5.  Client operation  . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Server operation  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Network Time Protocol (NTP) is a protocol which enables computers to
   synchronize their clocks over network.  Time is distributed from
   primary time servers to clients, which can be servers for other
   clients, and so on.  Clients can use multiple servers simultaneously.

   NTPv5 is similar to NTPv4 [RFC5905].  The main differences are:

   1.  The protocol specification (this document) doesn't describe any
       algorithms.

   2.  NTPv5 drops support for the symmetric active, symmetric passive,
       broadcast, control, and private modes.  Only the client and
       server modes are supported.

   3.  Timestamps are clearly separated from values used as cookies.

   4.  NTPv5 messages can be extended only with extension fields.  The
       MAC field is wrapped in an extension field.

   5.  Extension fields can be of any length, even indivisible by 4, but
       are padded to 4 octets.  Extension fields specified for NTPv4 are
       compatible with NTPv5.




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   6.  NTPv5 adds support for other timescales than UTC.

   7.  The NTP era number is exchanged in the protocol, which extends
       the unambiguous interval of the client from 136 years to about
       35000 years.

   8.  NTPv5 adds new measurement modes to provide clients with more
       accurate transmit timestamps.

1.1.  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.

2.  Data types

   NTPv5 uses few different data types.  They are all in the network
   order.  Beside signed and unsigned integers, it has also three fixed-
   point types.

   A 16-bit fixed-point type contains values in seconds.  It has 1
   signed integer bit (i.e. it is just the sign) and 15 fractional bits.
   The minimum value is -1.0, the maximum value is 32767/32768, and the
   resolution is about 30 microseconds.

   A 32-bit fixed-point type contains values in seconds.  It has 4
   unsigned integer bits and 28 fractional bits.  The maximum value is
   16 seconds and the resolution is about 3.7 nanoseconds.

   A 64-bit fixed-point type is used for timestamps.  It has 32 signed
   integer bits and 32 fractional bits.  It spans an interval of about
   136 years and has a resolution of about 0.23 nanoseconds.  It can be
   used in different timescales.  In the UTC timescale it is the number
   of SI seconds since 1 Jan 1972 plus 2272060800, excluding leap
   seconds.  Timestamps in the TAI timescale are the same except they
   include leap seconds and extra 10 seconds for the original difference
   between TAI and UTC in 1972, when leap seconds were introduced.

3.  Message format

   NTPv5 servers and clients exchange messages as UDP datagrams.
   Clients send requests to servers and servers send them back
   responses.  The format of the UDP payload is shown in Figure 1.





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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |LI | VN  |Mode | Scale |Stratum|     Poll      |  Precision    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flags     |      Era      |        Timescale Offset       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Root Delay                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Root Dispersion                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Server Cookie (64)                     +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Client Cookie (64)                     +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      Receive Timestamp (64)                   +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      Transmit Timestamp (64)                  +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                    Extension Field 1 (variable)               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                    Extension Field N (variable)               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1: Format of NTPv5 messages

   Each NTPv5 message has a header containing the following fields:




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   Leap indicator (LI)
      A 2-bit field which can have the following values: 0 (normal), 1
      (leap second inserted at the end of the month), 2 (leap second
      deleted at the end of the month), 3 (not synchronized).  The
      values 1 and 2 are set at most 14 days in advance before the leap
      second.  In requests it is always 0.

   Version Number (VN)
      A 3-bit field containing the value 5.

   Mode
      A 3-bit field containing the value 3 (request) or 4 (response).

   Scale
      A 4-bit identifier of the timescale.  In requests it is the
      requested timescale.  In responses it is the timescale of the
      receive and transmit timestamps.  Defined values are:

         0: UTC

         1: TAI

         2: UT1

         3: Leap-smeared UTC

   Stratum
      A 4-bit field containing the stratum of host.  Primary time
      servers have a stratum of 1, their clients have a stratum of 2,
      and so on.  The value of 0 indicates an unknown or infinite
      stratum.  In requests it is always 0.

   Poll
      An 8-bit signed integer containing the polling interval as a
      rounded log2 value in seconds.  In requests it is the current
      polling interval.  In responses it is the minimum allowed polling
      interval.

   Precision
      An 8-bit signed integer containing the precision of the timestamps
      included in the message as a rounded log2 value in seconds.  In
      requests, which don't contain any timestamps, it is always 0.

   Flags
      An 8-bit integer that can contain the following flags:

      0x1: Multi-message response




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         In requests it is a request for a multi-message response.  In
         responses it indicates the response may, but does not have to,
         have multiple messages.

      0x2: Follow-up message
         In responses it indicates it is not the first message of a
         multi-message response.

      0x4: Interleaved mode
         In requests it is a request for a response in the interleaved
         mode.  In responses it indicates the response is in the
         interleaved mode.

   Era
      An 8-bit unsigned NTP era number corresponding to the receive
      timestamp.  In requests it is always 0.

   Timescale Offset
      A 16-bit value specific to the selected timescale, which is
      referenced to the receive timestamp.  In requests it is always 0.

      *  In the UTC (0) and TAI (1) timescales it is the TAI-UTC offset
         as a signed integer, or 0x8000 if unknown.

      *  In the UT1 timescale (2) it is the UT1-UTC offset as the 16-bit
         fixed-point number, or 0x8000 (-1.0) if unknown.

      *  In the leap-smeared UTC, it is the current offset between the
         leap smeared time and UTC as the 16-bit fixed-point number, or
         0x8000 (-1.0) if unknown.

   Root Delay
      A field using the 32-bit fixed-point type.  In responses it is the
      server's root delay.  In requests it is always 0.

   Root Dispersion
      A field using the 32-bit fixed-point type.  In responses it is the
      server's root dispersion.  In requests it is always 0.

   Server Cookie
      A 32-bit field that enables follow-up messages and interleaved
      mode.  It allows the server to verify that the client is actually
      receiving the responses.  In requests it is either 0 or a copy of
      the server cookie from the last response.

   Client Cookie
      A 32-bit field containing a random number generated by the client.
      Responses contain a copy of the field from the corresponding



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      request, which allows the client to verify that the responses
      arevalid responses to the requests.

   Receive Timestamp
      A field using the 64-bit fixed-point type.  In requests it is
      always 0.  In responses it is the time when the request was
      received.  The timestamp corresponds to the end of the reception.

   Transmit Timestamp
      A field using the 64-bit fixed-point type.  In requests it is
      always 0.  In responses it is the time when a response to the
      client was transmitted.  The specific response depends on the
      selected mode (basic, follow-up, interleaved).  The timestamp
      corresponds to the beginning of the transmission.

   The header has 48 octets, which is the minimum length of a valid
   NTPv5 message.  A message can contain zero, one, or multiple
   extension fields.  The maximum length is not specified, but the
   length is always divisible by 4.

4.  Extension fields

   The format of NTPv5 extension fields is shown in Figure 2.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                           Data (variable)                     .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2: Format of NTPv5 extension fields

   Each extension field has a header which contains a 16-bit type and
   16-bit length.  The length is in octets and it includes the header.
   The minimum length is 4, i.e. an extension field doesn't have to
   contain any data.  If the length is not divisible by 4, the extension
   field is padded with zeroes to 4 octets.

   Generally, if a request contains an extension field, the client is
   asking the server to include the same extension field in the
   response.  Exceptions to this rule are allowed.

   Extension fields specified for NTPv4 can be included in NTPv5
   messages as specified for NTPv4.



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   The rest of this section describes new extension fields specified for
   NTPv5.  Clients are not required to use or support any of these
   extension fields, but servers are required to support some extension
   fields.

4.1.  Padding Extension Field

   This field is used by servers to pad the response to the same length
   as the request if the response doesn't contain all requested
   extension fields, or some have a variable length.  It can have any
   length.

   This field MUST be supported on server.

4.2.  MAC Extension Field

   This field authenticates the NTPv5 message with a symmetric key.
   Implementations SHOULD use the MAC specified in RFC8573 [RFC8573].
   The extension field MUST be the last extension field in the message
   unless an extension field is specifically allowed to be placed after
   a MAC or another authenticator field.

4.3.  Reference IDs Extension Field

   This fields allows servers to prevent synchronization loops, i.e.
   synchronizing to one of its direct or indirect clients.  It contains
   a set (bloom filter) of reference IDs.

   TODO

   This field MUST be supported on server.

4.4.  Correction Extension Field

   This field allows switches and routers to make corrections in NTPv5
   messages to allow clients to compensate for queueing and processing
   delays in the network.

   TODO (reuse draft-mlichvar-ntp-correction-field?)

4.5.  Reference timestamp

   This fields contains the time of the last update of the clock.  It
   has a fixed length of 12 octets.  In requests, the timestamp is
   always 0.

   (Is this really needed?  It was mostly unused in NTPv4.)




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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                      Reference Timestamp (64)                 |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 3: Format of Reference Timestamp Extension Field

4.6.  Monotonic Timestamp Extension Field

   This field contains an extra receive timestamp with a 32-bit step
   counter from a clock which doesn't have adjusted phase and can be
   used for a frequency transfer, e.g. to stabilize synchronization in a
   long chain of servers.  It has a constant length of 16 octets.  In
   requests, the counter and timestamp are always 0.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Step counter                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                  Monotonic Receive Timestamp (64)             |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 4: Format of Monotonic Timestamp Extension Field

5.  Client operation

   An NTPv5 client can use one or multiple servers.  It has a separate
   association with each server.  It makes periodic measurements of its
   offset and delay to the server.  It can filter the measurements and
   compare measurements from different servers to select and combine the
   best servers for synchronization.  It can adjust its clock in order
   to minimize its offset and keep the clock synchronized.  These
   algorithms are not specified in this document.

   The polling interval can be adjusted for the network conditions and
   stability of the clock.  When polling a public server on Internet,
   the client SHOULD use at least a polling interval of 64 seconds,
   increasing up to at least 1024 seconds.



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   The client can make measurements in the following modes: basic,
   follow-up, interleaved.  The follow-up and interleaved modes are two
   different mechanisms with different trade-offs that allow the server
   to provide the client with a more accurate transmit timestamp which
   is available only after the server forms or sends the response.

   Each successful measurement provides the client with an offset, delay
   and dispersion.  When combined with the server's root delay and
   dispersion, it gives the client an estimate of the maximum error.

   On each poll, the client:

   1.  Generates a new random cookie.

   2.  Formats a request with necessary extension fields and the fields
       in the header all zero except:

       *  Version is set to 5.

       *  Mode is set to 3.

       *  Scale is set to the timescale in which the client wants to
          operate.

       *  Poll is set to the rounded log2 value of the current client's
          polling interval in seconds.

       *  Flags are set according to the requested mode.  The Multi-
          message response flag enables the server to respond with
          multiple messages.  The interleaved mode flag requests a
          response in the interleaved mode.

       *  Server cookie is set only in the follow-up and interleaved
          modes.  If a valid response from the server was received
          previously, it is set to the Server Cookie from the previous
          response.

       *  Client cookie is set to the newly generated cookie.

   3.  Sends the request to the server to the UDP port 123 and captures
       a transmit timestamp.

   4.  Waits for a valid response from the server and captures a receive
       timestamp.  A valid response has version 5, mode 4, client cookie
       equal to the cookie from the request, and passes authentication
       if enabled.  The client MUST ignore all invalid responses.  In
       the basic mode and interleaved mode it MUST accept at most one
       valid response.  In the follow-up mode, if the first received



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       response has the "Multi-message response" or "Follow-up message"
       flag set, the client SHOULD wait for some time (e.g. 10
       milliseconds) and process all valid responses that have the same
       receive timestamp as the first received message.  As the local
       receive timestamp, the client SHOULD use the timestamp of the
       message which didn't have the "Follow-up message" flag, or the
       message which it received first.

   5.  Checks whether the response is usable for synchronization of the
       clock.  Such a response has a leap indicator not equal to 3,
       stratum between 0 and 16, root delay and dispersion both smaller
       than a specific value, e.g. 16 seconds, and timescale equal to
       the requested timescale.  If the response is in a different
       timescale, the client can switch to the provided timescale,
       convert the timestamps if the offset between the timescales is
       provided or known, or drop the response.

   6.  Saves the server's receive and transmit timestamps.  In the
       follow-up mode, if multiple messages were received, only the
       latest transmit timestamp is saved.  If the client internally
       counts seconds using a type wider than 32 bits, it SHOULD expand
       the timestamps with the provided NTP era.

   7.  Calculates the offset, delay, and dispersion:

          offset = ((T2 + T3) - (T4 + T1)) / 2

          delay = |(T4 - T1) - (T3 - T2)|

          dispersion = |T4 - T1| * DR

       where

          T1 is the client's transmit timestamp

          T2 is the server's receive timestamp

          T3 is the server's transmit timestamp

          T4 is the client's receive timestamp

          DR is the client's estimated dispersion rate

6.  Server operation

   A server receives requests on the UDP port 123.  The server MUST
   support measurements in the basic mode.  It MAY support the follow-up
   and/or interleaved modes.



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   For the basic and follow-up modes the server doesn't need to keep any
   client-specific state.  For the interleaved mode it needs to save
   transmit timestamps and be able to identify them by a cookie.

   The server maintains its leap indicator, stratum, root delay, and
   root dispersion:

   o  Leap indicator MUST be 3 if the clock is not synchronized or its
      maximum error cannot be estimated with the root delay and
      dispersion.  Otherwise, it MUST be 0, 1, 2, depending on whether a
      leap second will be inserted or deleted in the next 14 days.

   o  Stratum SHOULD be one larger than stratum of the best server it
      uses for its own synchronization.

   o  Root delay SHOULD be the best server's root delay in addition to
      the measured delay to the server.

   o  Root dispersion SHOULD be the best server's root dispersion in
      addition to an estimate of the maximum drift of its own clock
      since the last update of the clock.

   The server has a randomly generated reference ID and it MUST track
   reference IDs of its servers using the Reference IDs Extension Field.

   For each received request, the server:

   1.  Captures a receive timestamp.

   2.  Checks the version in the request.  If it is larger than 5, it
       MUST drop the request.  If it is smaller than 5, it SHOULD either
       drop the request, or handle it according to the corresponding
       specification.

   3.  Drops the request if the format is not valid, mode is not 3, or
       authentication fails if the MAC Extension Field or another
       authenticator field is present.

   4.  Server forms a response with requested extension fields and sets
       the fields in the header as follows:

       *  Leap Indicator, Stratum, Root delay, and Root dispersion, are
          set to the current server's values.

       *  Version is set to 5.






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       *  Scale is set to the client's requested timescale if it is
          supported by the server.  If not, the server SHOULD respond in
          any timescale it supports.

       *  Flags are set according to the requested mode and supported
          mode.  If the follow-up mode was requested, it is supported by
          the server, and the provided server cookie is valid, the
          response may have multiple messages.  They have the same
          length and content except the flags, transmit timestamps
          (captured at different points of the first message's
          transmission), and MAC or authenticator field if present.  The
          first message in the response has the Multi-message response
          flag set and all follow-up messages have the Follow-up flag
          set.  If the interleaved mode is requested and a response in
          the interleaved mode is possible (i.e. a transmit timestamp is
          associated with the server cookie), the Interleaved mode flag
          is set.

       *  Era is set to the NTP era of the receive timestamp.

       *  Timescale Offset is set to the timescale-specific offset, or
          0x8000 if unknown.

       *  Server Cookie is set when the follow-up or interleaved mode is
          requested and it is supported by the server, even if the
          response cannot be in the requested mode yet due to the
          request having an invalid server cookie.  In the follow-up
          mode the cookie SHOULD be derived from the client's address
          and a secret key, which is rotated frequently (e.g. once per
          hour).  It MUST prevent multi-message responses to addresses
          that didn't receive a previous response (other than by
          guessing the 64-bit number).  In the interleaved mode, the
          cookie identifies a more accurate transmit timestamp, which
          can be retrieved by the client later with another request.

       *  Client Cookie is set to the Client Cookie from the request.

       *  Receive Timestamp is set to the server's receive timestamp of
          the request.

       *  Transmit Timestamp is set to a value which depends on the
          measurement mode.  In the basic mode and the first message of
          a multi-message response in the follow-up mode, it is the
          server's current time when the message if formed.  In the
          second and following messages in the follow-up mode it is a
          more accurate transmit timestamp of the first message captured
          after it was formed or sent.  In the interleaved mode it is
          the transmit timestamp of the previous response identified by



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          the server cookie in the request, captured at some point after
          the message was formed.

   5.  Drops the response if it is longer than the request to prevent
       traffic amplification.

   6.  Sends the response.  In the follow-up mode, the follow-up
       messages are sent as soon as more accurate transmit timestamps
       are available.  If the server can predict which transmit
       timestamps it will get, it SHOULD send only one follow-up message
       containing the most accurate timestamp.

   7.  Saves the transmit timestamp and server cookie, if the
       interleaved mode was requested and is supported by the server.

7.  Acknowledgements

   Some ideas were taken from a different NTPv5 design proposed by
   Daniel Franke.

   The follow-up mode was inspired by PTP (IEEE 1588-2008).

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

10.  References

10.1.  Normative References

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

   [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>.

   [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>.






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Internet-Draft       Network Time Protocol Version 5            Nov 2020


10.2.  Informative References

   [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>.

Author's Address

   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic

   Email: mlichvar@redhat.com



































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