NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-07

Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: 5905 (if approved)                                  A. Malhotra
Intended status: Standards Track                       Boston University
Expires: 21 April 2022                                   18 October 2021

                         NTP Interleaved Modes
                  draft-ietf-ntp-interleaved-modes-07

Abstract

   This document extends the specification of Network Time Protocol
   (NTP) version 4 in RFC 5905 with special modes called the NTP
   interleaved modes, that enable NTP servers to provide their clients
   and peers with more accurate transmit timestamps that are available
   only after transmitting NTP packets.  More specifically, this
   document describes three modes: interleaved client/server,
   interleaved symmetric, and interleaved broadcast.

Status of This Memo

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Interleaved Client/server mode  . . . . . . . . . . . . . . .   4
   3.  Interleaved Symmetric mode  . . . . . . . . . . . . . . . . .   9
   4.  Interleaved Broadcast mode  . . . . . . . . . . . . . . . . .  10
   5.  Protocol Failures . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   RFC 5905 [RFC5905] describes the operations of NTPv4 in a client/
   server, symmetric, and broadcast mode.  The transmit and receive
   timestamps are two of the four timestamps included in every NTPv4
   packet used for time synchronization.

   For a highly accurate and stable synchronization, the transmit and
   receive timestamp should be captured close to the beginning of the
   actual transmission and the end of the reception respectively.  An
   asymmetry in the timestamping causes the offset measured by NTP to
   have an error.

   There are at least four options where a timestamp of an NTP packet
   may be captured with a software NTP implementation running on a
   general-purpose operating system:

   1.  User space (software)

   2.  Network device driver or kernel (software)

   3.  Data link layer (hardware - MAC chip)

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   4.  Physical layer (hardware - PHY chip)

   Software timestamps captured in user space in the NTP implementation
   itself are least accurate.  They do not include system calls used for
   sending and receiving packets, processing and queuing delays in the
   system, network device drivers, and hardware.  Hardware timestamps
   captured at the physical layer are most accurate.

   A transmit timestamp captured in the driver or hardware is more
   accurate than the user-space timestamp, but it is available to the
   NTP implementation only after it sent the packet using a system call.
   The timestamp cannot be included in the packet itself unless the
   driver or hardware supports NTP and can modify the packet before or
   during the actual transmission.

   The protocol described in RFC 5905 does not specify any mechanism for
   a server to provide its clients and peers with a more accurate
   transmit timestamp that is known only after the transmission.  A
   packet that strictly follows RFC 5905, i.e. it contains a transmit
   timestamp corresponding to the packet itself, is said to be in basic
   mode.

   Different mechanisms could be used to exchange timestamps known after
   the transmission.  The server could respond to each request with two
   packets.  The second packet would contain the transmit timestamp
   corresponding to the first packet.  However, such a protocol would
   enable a traffic amplification attack, or it would use packets with
   an asymmetric length, which would cause an asymmetry in the network
   delay and an error in the measured offset.

   This document describes an interleaved client/server, interleaved
   symmetric, and interleaved broadcast mode.  In these modes, the
   server sends a packet which contains a transmit timestamp
   corresponding to the transmission of the previous packet that was
   sent to the client or peer.  This transmit timestamp can be captured
   in any software or hardware component involved in the transmission of
   the packet.  Both servers and clients/peers are required to keep some
   state specific to the interleaved mode.

   An NTPv4 implementation that supports the client/server and broadcast
   interleaved modes interoperates with NTPv4 implementations without
   this capability.  A peer using the symmetric interleaved mode does
   not fully interoperate with a peer which does not support it.  The
   mode needs to be configured specifically for each symmetric
   association.

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   The interleaved modes do not change the NTP packet header format and
   do not use new extension fields.  The negotiation is implicit.  The
   protocol is extended with new values that can be assigned to the
   origin and transmit timestamp.  Servers and peers check the origin
   timestamp to detect requests conforming to the interleaved mode.  A
   response can be valid only in one mode.  If a client or peer that
   does not support interleaved mode received a response conforming to
   the interleaved mode, it would be rejected as bogus.

   An explicit negotiation would require a new extension field.  RFC
   5905 does not specify how servers should handle requests with an
   unknown extension field.  The original use of extension fields was
   authentication with Autokey [RFC5906], which cannot be negotiated.
   Some existing implementations do not respond to requests with unknown
   extension fields.  This behavior would prevent clients from reliably
   detecting support for the interleaved mode.

   Requests and responses cannot always be formed in interleaved mode.
   It cannot be used exclusively.  Servers, clients, and peers that
   support the interleaved mode need to support also the basic mode.

   This document assumes familiarity with RFC 5905.

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.  Interleaved Client/server mode

   The interleaved client/server mode is similar to the basic client/
   server mode.  The difference between the two modes is in the values
   saved to the origin and transmit timestamp fields.

   The origin timestamp is a cookie which is used to detect that a
   received packet is a response to the last packet sent in the other
   direction of the association.  It is a copy of one of the timestamps
   from the packet to which it is responding, or zero if it is not a
   response.  Servers following RFC 5905 ignore the origin timestamp in
   client requests.  A server response which does not have a matching
   origin timestamp is called bogus.

   A client request in the basic mode has an origin timestamp equal to
   the transmit timestamp from the last valid server response, or is
   zero (which indicates the first request of the association).  A

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   server response in the basic mode has an origin timestamp equal to
   the transmit timestamp from the client request.  The transmit
   timestamp in the response corresponds to the transmission of the
   response in which the timestamp is contained.

   A client request in the interleaved mode has an origin timestamp
   equal to the receive timestamp from the last valid server response.
   A server response in the interleaved mode has an origin timestamp
   equal to the receive timestamp from the client request.  The transmit
   timestamp in the response corresponds to the transmission of the
   previous response which had the receive timestamp equal to the origin
   timestamp from the request.

   A server which supports the interleaved mode needs to save pairs of
   local receive and transmit timestamps.  The server SHOULD discard old
   timestamps to limit the amount of memory needed to support clients
   using the interleaved mode.  The server MAY separate the timestamps
   by IP addresses, but it SHOULD NOT separate them by port numbers to
   support clients that change their port between requests, as
   recommended in RFC 9109 [RFC9109].

   The server MAY restrict the interleaved mode to specific IP addresses
   and/or authenticated clients.

   Both servers and clients that support the interleaved mode MUST NOT
   send a packet that has a transmit timestamp equal to the receive
   timestamp in order to reliably detect whether received packets
   conform to the interleaved mode.  One way to ensure that is to
   increment the transmit timestamp by 1 unit (i.e. about 1/4 of a
   nanosecond) if the two timestamps are equal, or a new timestamp can
   be generated.

   The transmit and receive timestamps in server responses need to be
   unique to prevent two different clients from sending requests with
   the same origin timestamp and the server responding in the
   interleaved mode with an incorrect transmit timestamp.  If the
   timestamps are not guaranteed to be monotonically increasing, the
   server SHOULD check that the transmit and receive timestamps are not
   already saved as a receive timestamp of a previous request (from the
   same IP address if the server separates timestamps by addresses), and
   generate a new timestamp if necessary.

   When the server receives a request from a client, it SHOULD respond
   in the interleaved mode if the following conditions are met:

   1.  The request does not have a receive timestamp equal to the
       transmit timestamp.

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   2.  The origin timestamp from the request matches the local receive
       timestamp of a previous request that the server has saved (for
       the IP address if it separates timestamps by addresses).

   A response in the interleaved mode MUST contain the transmit
   timestamp of the response which contained the receive timestamp
   matching the origin timestamp from the request.  The server SHOULD
   drop the timestamps after sending the response.  The receive
   timestamp MUST NOT be used again to detect a request conforming to
   the interleaved mode.

   If the conditions are not met (i.e. the request is not detected to
   conform to the interleaved mode), the server MUST NOT respond in the
   interleaved mode.  The server MAY always respond in the basic mode.
   In any case, the server SHOULD save the new receive and transmit
   timestamps.

   The first request from a client is always in the basic mode and so is
   the server response.  It has a zero origin timestamp and zero receive
   timestamp.  Only when the client receives a valid response from the
   server, it will be able to send a request in the interleaved mode.

   The protocol recovers from packet loss.  When a client request or
   server response is lost, the client will use the same origin
   timestamp in the next request.  The server can respond in the
   interleaved mode if it still has the timestamps corresponding to the
   origin timestamp.  If the server already responded to the timestamp
   in the interleaved mode, or it had to drop the timestamps for other
   reasons, it will respond in the basic mode and save new timestamps,
   which will enable an interleaved response to the subsequent request.
   The client SHOULD limit the number of requests in the interleaved
   mode between server responses to prevent processing of very old
   timestamps in case a large number of consecutive requests is lost.

   An example of packets in a client/server exchange using the
   interleaved mode is shown in Figure 1.  The packets in the basic and
   interleaved mode are indicated with B and I respectively.  The
   timestamps t1~, t3~ and t11~ point to the same transmissions as t1,
   t3 and t11, but they may be less accurate.  The first exchange is in
   the basic mode followed by a second exchange in the interleaved mode.
   For the third exchange, the client request is in the interleaved
   mode, but the server response is in the basic mode, because the
   server did not have the pair of timestamps t6 and t7 (e.g. they were
   dropped to save timestamps for other clients using the interleaved
   mode).

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   Server   t2   t3               t6   t7              t10  t11
       -----+----+----------------+----+----------------+----+-----
           /      \              /      \              /      \
   Client /        \            /        \            /        \
       --+----------+----------+----------+----------+----------+--
         t1         t4         t5         t8         t9        t12

   Mode: B         B           I         I           I         B
       +----+    +----+      +----+    +----+      +----+    +----+
   Org | 0  |    | t1~|      | t2 |    | t4 |      | t6 |    | t5 |
   Rx  | 0  |    | t2 |      | t4 |    | t6 |      | t8 |    |t10 |
   Tx  | t1~|    | t3~|      | t1 |    | t3 |      | t5 |    |t11~|
       +----+    +----+      +----+    +----+      +----+    +----+

       Figure 1: Packet timestamps in interleaved client/server mode

   When the client receives a response from the server, it performs the
   tests described in RFC 5905.  Two of the tests are modified for the
   interleaved mode:

   1.  The check for duplicate packets SHOULD compare both receive and
       transmit timestamps in order to not drop a valid response in the
       interleaved mode if it follows a response in the basic mode and
       they contain the same transmit timestamp.

   2.  The check for bogus packets SHOULD compare the origin timestamp
       with both transmit and receive timestamps from the request.  If
       the origin timestamp is equal to the transmit timestamp, the
       response is in the basic mode.  If the origin timestamp is equal
       to the receive timestamp, the response is in the interleaved
       mode.

   The client SHOULD NOT update its NTP state when an invalid response
   is received, to not lose the timestamps which will be needed to
   complete a measurement when the subsequent response in the
   interleaved mode is received.

   If the packet passed the tests and conforms to the interleaved mode,
   the client can compute the offset and delay using the formulas from
   RFC 5905 and one of two different sets of timestamps.  The first set
   is RECOMMENDED for clients that filter measurements based on the
   delay.  The corresponding timestamps from Figure 1 are written in
   parentheses.

      T1 - local transmit timestamp of the previous request (t1)

      T2 - remote receive timestamp from the previous response (t2)

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      T3 - remote transmit timestamp from the latest response (t3)

      T4 - local receive timestamp of the previous response (t4)

   The second set gives a more accurate measurement of the current
   offset, but the delay is much more sensitive to a frequency error
   between the server and client due to a much longer interval between
   T1 and T4.

      T1 - local transmit timestamp of the latest request (t5)

      T2 - remote receive timestamp from the latest response (t6)

      T3 - remote transmit timestamp from the latest response (t3)

      T4 - local receive timestamp of the previous response (t4)

   Clients MAY filter measurements based on the mode.  The maximum
   number of dropped measurements in the basic mode SHOULD be limited in
   case the server does not support or is not able to respond in the
   interleaved mode.  Clients that filter measurements based on the
   delay will implicitly prefer measurements in the interleaved mode
   over the basic mode, because they have a shorter delay due to a more
   accurate transmit timestamp (T3).

   The server MAY limit saving of the receive and transmit timestamps to
   requests which have an origin timestamp specific to the interleaved
   mode in order to not waste resources on clients using the basic mode.
   Such an optimization will delay the first interleaved response of the
   server to a client by one exchange.

   A check for a non-zero origin timestamp works with SNTP clients that
   always set the timestamp to zero and clients that implement NTP data
   minimization [I-D.ietf-ntp-data-minimization].  From the server's
   point of view, such clients start a new association with each
   request.

   To avoid searching the saved receive timestamps for non-zero origin
   timestamps from requests conforming to the basic mode, the server can
   encode in low-order bits of the receive and transmit timestamps below
   precision of the clock a flag indicating whether the timestamp is a
   receive timestamp.  If the server receives a request with a non-zero
   origin timestamp which does not indicate it is a receive timestamp of
   the server, the request does not conform to the interleaved mode and
   it is not necessary to perform the search and/or save the new receive
   and transmit timestamp.

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3.  Interleaved Symmetric mode

   The interleaved symmetric mode uses the same principles as the
   interleaved client/server mode.  A packet in the interleaved
   symmetric mode has a transmit timestamp which corresponds to the
   transmission of the previous packet sent to the peer and an origin
   timestamp equal to the receive timestamp from the last packet
   received from the peer.

   To enable synchronization in both directions of a symmetric
   association, both peers need to support the interleaved mode.  For
   this reason, it SHOULD be disabled by default and enabled with an
   option in the configuration of the active side of the association.

   In order to prevent the peer from matching the transmit timestamp
   with an incorrect packet when the peers' transmissions do not
   alternate (e.g. they use different polling intervals) and a previous
   packet was lost, the use of the interleaved mode in symmetric
   associations requires additional restrictions.

   Peers which have an association need to count valid packets received
   between their transmissions to determine in which mode a packet
   should be formed.  A valid packet in this context is a packet which
   passed all NTP tests for duplicate, replayed, bogus, and
   unauthenticated packets.  Other received packets may update the NTP
   state to allow the (re)initialization of the association, but they do
   not change the selection of the mode.

   A peer A SHOULD send a peer B a packet in the interleaved mode only
   when all of the following conditions are met:

   1.  The peer A has an active association with the peer B which was
       specified with the option enabling the interleaved mode, OR the
       peer A received at least one valid packet in the interleaved mode
       from the peer B.

   2.  The peer A did not send a packet to the peer B since it received
       the last valid packet from the peer B.

   3.  The previous packet that the peer A sent to the peer B was the
       only response to a packet received from the peer B.

   The first condition is needed for compatibility with implementations
   that do not support or are not configured for the interleaved mode.
   The other conditions prevent a missing response from causing a
   mismatch between the remote transmit (T2) and local receive timestamp
   (T3), which would cause a large error in the measured offset and
   delay.

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   An example of packets exchanged in a symmetric association is shown
   in Figure 2.  The minimum polling interval of the peer A is twice as
   long as the maximum polling interval of the peer B.  The first
   packets sent by the peers are in the basic mode.  The second and
   third packet sent by the peer A is in the interleaved mode.  The
   second packet sent by the peer B is in the interleaved mode, but the
   following packets sent by the peer B are in the basic mode, because
   multiple responses are sent per request.

   Peer A   t2 t3       t6          t8 t9      t12         t14 t15
       -----+--+--------+-----------+--+--------+-----------+--+-----
           /    \      /           /    \      /           /    \
   Peer B /      \    /           /      \    /           /      \
       --+--------+--+-----------+--------+--+-----------+--------+--
         t1       t4 t5          t7      t10 t11        t13      t16

   Mode: B      B      I         B      I      B         B      I
       +----+ +----+ +----+    +----+ +----+ +----+    +----+ +----+
   Org | 0  | | t1~| | t2 |    | t3~| | t4 | | t3 |    | t3 | |t10 |
   Rx  | 0  | | t2 | | t4 |    | t4 | | t8 | |t10 |    |t10 | |t14 |
   Tx  | t1~| | t3~| | t1 |    | t7~| | t3 | |t11~|    |t13~| | t9 |
       +----+ +----+ +----+    +----+ +----+ +----+    +----+ +----+

         Figure 2: Packet timestamps in interleaved symmetric mode

   If the peer A has no association with the peer B and it responds with
   symmetric passive packets, it does not need to count the packets in
   order to meet the restrictions, because each request has at most one
   response.  The peer SHOULD process the requests in the same way as a
   server which supports the interleaved client/server mode.  It MUST
   NOT respond in the interleaved mode if the request was not in the
   interleaved mode.

   The peers SHOULD compute the offset and delay using one of the two
   sets of timestamps specified in the client/server section.  They MAY
   switch between them to minimize the interval between T1 and T4 in
   order to reduce the error in the measured delay.

4.  Interleaved Broadcast mode

   A packet in the interleaved broadcast mode contains two transmit
   timestamps.  One corresponds to the packet itself and is saved in the
   transmit timestamp field.  The other corresponds to the previous
   packet and is saved in the origin timestamp field.  The packet is
   compatible with the basic mode, which uses a zero origin timestamp.

   An example of packets sent in the broadcast mode is shown in
   Figure 3.

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   Server         t1           t3           t5           t7
            ------+------------+------------+------------+---------
                   \            \            \            \
   Client           \            \            \            \
            ---------+------------+------------+------------+------
                     t2           t4           t6           t8

   Mode:           B            I            I            I
                 +----+       +----+       +----+       +----+
   Org           | 0  |       | t1 |       | t3 |       | t5 |
   Rx            | 0  |       | 0  |       | 0  |       | 0  |
   Tx            | t1~|       | t3~|       | t5~|       | t7~|
                 +----+       +----+       +----+       +----+

         Figure 3: Packet timestamps in interleaved broadcast mode

   A client which does not support the interleaved mode ignores the
   origin timestamp and processes all packets as if they were in the
   basic mode.

   A client which supports the interleaved mode SHOULD check if the
   origin timestamp is not zero to detect packets in the interleaved
   mode.  The client SHOULD also compare the origin timestamp with the
   transmit timestamp from the previous packet to detect lost packets.
   If the difference is larger than a specified maximum (e.g. 1 second),
   the packet SHOULD NOT be used for synchronization in the interleaved
   mode.

   The client SHOULD compute the offset using the origin timestamp from
   the received packet and the local receive timestamp of the previous
   packet.  If the client needs to measure the network delay, it SHOULD
   use the interleaved client/server mode.

5.  Protocol Failures

   An incorrect client implementation of the basic mode (RFC 5905) can
   work reliably with servers that implement only the basic mode, but
   the protocol can fail intermittently with servers that implement the
   interleaved mode.

   If the client sets the origin timestamp to other values than the
   transmit timestamp from the last valid server response, or zero, the
   origin timestamp can match a receive timestamp of a previous server
   response (possibly to a different client), causing an unexpected
   interleaved response.  The client is expected to drop the response as
   bogus.  If it did not check for bogus packets, it would be vulnerable
   to off-path attacks.

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   If the client set the origin timestamp to a constant non-zero value,
   this mismatch would be expected to happen once per the NTP era (about
   136 years) if the NTP server was responding at the maximum rate
   needed to go through all timestamp values (about 2 billion responses
   per second).  With lower rates of requests the chance of hitting a
   server timestamp decreases proportionally.

   The worst case of this failure would be a client that specifically
   sets the origin timestamp to the server's receive timestamp, i.e. the
   client accidentally implemented the interleaved mode, but it does not
   accept interleaved responses.  This client would still be able to
   synchronize its clock.  It would drop interleaved responses as bogus
   and set the origin timestamp to the receive timestamp from the last
   valid response in the basic mode.  As servers are required to not
   respond twice to the same origin timestamp in the interleaved mode,
   at least every other response would be in the basic mode and accepted
   by the client.

   Intermittent protocol failures can be caused also by an incorrect
   server implementation of the interleaved mode.  A server which does
   not ensure the receive and transmit timestamps in its responses are
   unique in a sufficiently long interval can misinterpret requests
   formed correctly in the basic mode as interleaved and respond in the
   interleaved mode.  The response would be dropped by the client as
   bogus.

   A duplicated server receive timestamp can cause an expected
   interleaved response to contain a transmit timestamp which does not
   correspond to the transmission of the previous response from which
   the client copied the receive timestamp to the origin timestamp in
   the request, but a different response which contained the same
   receive timestamp.  The response would be accepted by the client with
   a small error in the transmit timestamp equal to the difference
   between the transmit timestamps of the two different responses.  As
   the two requests to which the responses responded were received at
   the same time (according to the server's clock), the two
   transmissions would be expected to be close to each other and the
   difference between them would be comparable to the error a basic
   response normally has in its transmit timestamp.

   One reason for a duplicated server timestamp can be a large backward
   step of the server's clock.  If the timestamps the server has saved
   do not fully cover the second pass of the clock over the repeated
   interval, two requests received in different passes of the clock can
   get the same receive timestamp.  The client which made the first
   request can get the transmit timestamp corresponding to the
   transmission of the second response.  From the server's point of
   view, the error of the transmit timestamp would be still small, but

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   from the client's point of view the server already failed when it
   made the step as it was serving wrong time before or after the step
   with a much larger error than the error caused by the protocol
   failure.

6.  Security Considerations

   The security considerations of time protocols in general are
   discussed in RFC 7384 [RFC7384], and specifically the security
   considerations of NTP are discussed in RFC 5905.

   Security issues that apply to the basic modes apply also to the
   interleaved modes.  They are described in The Security of NTP's
   Datagram Protocol [SECNTP].

   Clients and peers SHOULD NOT leak the receive timestamp in packets
   sent to other peers or clients (e.g. as a reference timestamp) to
   prevent off-path attackers from easily getting the origin timestamp
   needed to make a valid response in the interleaved mode.

   Clients using the interleaved mode SHOULD randomize all bits of both
   receive and transmit timestamps, as recommended for the transmit
   timestamp in the NTP client data minimization
   [I-D.ietf-ntp-data-minimization], to make it more difficult for off-
   path attackers to guess the origin timestamp in the server response.

   The client data minimization cannot be fully implemented in the
   interleaved mode.  The origin timestamp cannot be zeroed out, which
   makes the clients more vulnerable to tracking as they move between
   networks.

   Attackers can force the server to drop its timestamps in order to
   prevent clients from getting an interleaved response.  They can send
   a large number of requests, send requests with a spoofed source
   address, or replay an authenticated request if the interleaved mode
   is enabled only for authenticated clients.  Clients SHOULD NOT rely
   on servers to be able to respond in the interleaved mode.

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   Protecting symmetric associations in the interleaved mode against
   replay attacks is even more difficult than in the basic mode.  In
   both modes, the NTP state needs to be protected between the reception
   of the last non-replayed response and transmission of the next
   request in order for the request to contain the origin timestamp
   expected by the peer.  The difference is in the timestamps needed to
   complete a measurement.  In the basic mode only one valid response is
   needed at a time and it is used as soon as it is received, but the
   interleaved mode needs two consecutive valid responses.  The NTP
   state needs to be protected all the time to not lose the timestamps
   which are needed to complete the measurement when the second response
   is received.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Acknowledgements

   The interleaved modes described in this document are based on the
   implementation written by David Mills in the NTP project
   (http://www.ntp.org).  The specification of the broadcast mode is
   based purely on this implementation.  The specification of the
   symmetric mode has some modifications.  The client/server mode is
   specified as a new mode compatible with the symmetric mode, similarly
   to the basic symmetric and client/server modes.

   The authors would like to thank Theresa Enghardt, Daniel Franke,
   Benjamin Kaduk, Erik Kline, Tal Mizrahi, Steven Sommars, Harlan
   Stenn, and Kristof Teichel for their useful comments.

9.  References

9.1.  Normative References

   [I-D.ietf-ntp-data-minimization]
              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://www.ietf.org/archive/id/draft-ietf-ntp-data-
              minimization-04.txt>.

   [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>.

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   [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>.

   [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>.

9.2.  Informative References

   [RFC5906]  Haberman, B., Ed. and D. Mills, "Network Time Protocol
              Version 4: Autokey Specification", RFC 5906,
              DOI 10.17487/RFC5906, June 2010,
              <https://www.rfc-editor.org/info/rfc5906>.

   [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>.

   [RFC9109]  Gont, F., Gont, G., and M. Lichvar, "Network Time Protocol
              Version 4: Port Randomization", RFC 9109,
              DOI 10.17487/RFC9109, August 2021,
              <https://www.rfc-editor.org/info/rfc9109>.

   [SECNTP]   Malhotra, A., Gundy, M. V., Varia, M., Kennedy, H.,
              Gardner, J., and S. Goldberg, "The Security of NTP's
              Datagram Protocol", 2016,
              <http://eprint.iacr.org/2016/1006>.

Authors' Addresses

   Miroslav Lichvar
   Red Hat
   Purkynova 115
   612 00 Brno
   Czech Republic

   Email: mlichvar@redhat.com

   Aanchal Malhotra
   Boston University
   111 Cummington St
   Boston,  02215
   United States of America

   Email: aanchal4@bu.edu

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