A Secure Selection and Filtering Mechanism for the Network Time Protocol with Khronos
draft-ietf-ntp-chronos-10

Network Working Group                                    N. Rozen-Schiff
Internet-Draft                                                  D. Dolev
Intended status: Informational            Hebrew University of Jerusalem
Expires: 25 April 2023                                        T. Mizrahi
                                        Huawei Network.IO Innovation Lab
                                                             M. Schapira
                                          Hebrew University of Jerusalem
                                                         22 October 2022

A Secure Selection and Filtering Mechanism for the Network Time Protocol
                              with Khronos
                       draft-ietf-ntp-chronos-10

Abstract

   The Network Time Protocol version 4 (NTPv4), as defined in RFC 5905,
   is the mechanism used by NTP clients to synchronize with NTP servers
   across the Internet.  This document specifies an extension to the
   NTPv4 client, named Khronos, which is used as a "watchdog" alongside
   NTPv4, and provides improved security against time shifting attacks.
   Khronos involves changes to the NTP client's system process only.
   Since it does not affect the wire protocol, the Khronos mechanism is
   applicable to any current or future time protocol.

Status of This Memo

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   This Internet-Draft will expire on 25 April 2023.

Copyright Notice

   Copyright (c) 2022 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
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   4
     2.3.  Notations . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Khronos' Design . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Khronos Calibration . . . . . . . . . . . . . . . . . . .   6
     3.2.  Khronos' Poll and System Processes  . . . . . . . . . . .   7
     3.3.  Khronos' Recommended Parameters . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     4.1.  Threat Model  . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Attack Detection  . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Security Analysis Overview  . . . . . . . . . . . . . . .   9
   5.  Khronos' Pseudocode . . . . . . . . . . . . . . . . . . . . .  11
   6.  Precision vs. Security  . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   NTPv4, as defined in RFC 5905 [RFC5905], is vulnerable to time
   shifting attacks, in which the attacker changes (shift) the clock of
   a network device.  Time shifting attacks on NTP clients can be based
   on interfering the communication between the NTP clients and servers
   or compromising the servers themselves.  Time shifting attacks on NTP
   are possible even if NTP communication is encrypted and
   authenticated.  A weaker man-in-the-middle (MitM) attacker can shift
   time simply by dropping or delaying packets, whereas a powerful
   attacker, who has full control over an NTP server, can do so by
   explicitly determining the NTP response content.  This document
   introduces a time shifting mitigation mechanism called Khronos.
   Khronos can be integrated into NTPv4-compatible servers as an NTPv4
   client's "watchdog" against time shifting attacks.  An NTP client

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   that runs Khronos is interoperable with [RFC5905]-compatible NTPv4
   servers.  The Khronos mechanism does not affect the wire mechanism
   and is therefore applicable to any current or future time protocol.

   Khronos is a mechanism that runs in the background, continuously
   maintains a virtual "Khronos" clock, and compares this clock's
   reading to NTPv4's clock updates.  When the gap between the two
   clocks exceeds a certain threshold (specified in Section 4), this is
   interpreted as the client experiencing a time shifting attack.  In
   this case, Khronos is used to update the client's clock, and the
   conventional NTPv4 client time-synchronization algorithm is run in
   the background until the gap between the two algorithms is again
   below this threshold, and hence the conventional NTPv4 client
   algorithm is deemed safe to use again.

   When the client is not under attack, Khronos is passive, allowing
   NTPv4 to control the client clock and providing the ordinary high
   precision and accuracy of NTPv4.  When under attack, Khronos takes
   control over the client's clock, mitigating the time shift, while
   guaranteeing relatively high accuracy with respect to UTC (error is
   bounded by 100 ms when using the recommended parameters) and
   precision, as discussed in Section 6.

   By leveraging techniques from distributed computing theory for time-
   synchronization in the presence of Byzantine attackers, Khronos
   achieves accurate synchronization even in the presence of powerful
   attackers who are in direct control of a large number of NTP servers.
   Khronos will prevent shifting the clock when the ratio of compromised
   time samples is below 2/3.  In each polling interval, Khorons client
   randomly selects and samples (regardless of precision) a few NTP
   servers out of a local pool of hundreds of servers.  Khronos is
   carefully engineered to minimize the load on NTP servers and the
   communication overhead.  In contrast, NTPv4, employs an algorithm
   which typically relies on a small subset of the NTP server pool
   (e.g., 4 servers) for time synchronization, and is much more
   vulnerable to time shifting attacks.  Configuring NTPv4 to use
   several hundreds of servers will increase its security, but will
   incur very high network and computational overhead compared to
   Khronos and will be bounded by compromised ratio of half of the time
   samples.

   A Khronos client iteratively "crowdsources" time queries across NTP
   servers and applies a provably secure algorithm for eliminating
   "suspicious" responses and for averaging over the remaining
   responses.  In each poll interval, the Khronos client selects,
   uniformly at random, a small subset (e.g., 10-15 servers) of a large
   server pool (containing hundreds of servers).  To minimize the load
   on NTP servers and the communication overhead, the frequency of

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   Khronos poll intervals should be much less dense than that of
   standard NTPv4 clock updates, e.g., the Khronos clock can be updated
   once every 10 NTPv4 clock updates.  Khronos' security was evaluated
   both theoretically and experimentally with a prototype
   implementation.  According to this security analyses, if a local
   Khronos pool consists of, for example, 500 servers, 1/7 of whom are
   controlled by a man-in-the-middle, attacker and Khronos queries 15
   servers in each Khronos poll interval (around 10 times the NTPv4 poll
   interval), then over 20 years of effort are required (in expectation)
   to successfully shift time at a Khronos client by over 100
   milliseconds from UTC.  The full exposition of the formal analysis of
   this guarantee is available at [Khronos_paper].

   Khronos introduces a watchdog mechanism that is added to the client's
   system process and maintains a time offset value that is used as a
   reference for detecting attacks.  The time offset value computation
   differs from the current NTPv4 in two key aspects.  First, Khronos
   periodically synchronizes, in each Khronos poll interval, with only a
   few (tens) randomly selected servers out of a pool consisting of a
   large number (e.g., hundreds) of NTP servers, thereby providing high
   security while minimizing the load on the NTP servers.  Second, the
   selection algorithm of the time offset uses an approximate agreement
   technique to remove outliers, thus limiting the attacker's ability to
   contaminate the "time samples" (offsets) derived from the queried NTP
   servers.  These two elements of Khronos' design provide provable
   security guarantees against both man-in-the-middle attackers and
   attackers capable of compromising a large number of NTP servers.

2.  Conventions Used in This Document

2.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.2.  Terms and Abbreviations

   NTPv4                  Network Time Protocol version 4 [RFC5905].

   Selection process      Clock filter algorithm and system process
                          [RFC5905].

   Selection process      Security Requirements of Time Protocols in
                          Packet Switched Networks [RFC7384].

   Selection process      Network Time Security for the Network Time
                          Protocol [RFC8915].

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2.3.  Notations

   Describing Khronos algorithm, the following notation is used.

      +==========+=================================================+
      | Notation | Meaning                                         |
      +==========+=================================================+
      |    n     | The number of candidate servers in Khronos pool |
      |          | that Khronos can query (potentially hundreds)   |
      +----------+-------------------------------------------------+
      |    m     | The number of servers that Khronos queries in   |
      |          | each poll interval (up to tens)                 |
      +----------+-------------------------------------------------+
      |    w     | An upper bound on the distance of the local     |
      |          | time from any NTP server with an accurate clock |
      |          | (termed "truechimer" in [RFC5905])              |
      +----------+-------------------------------------------------+
      |   Cest   | The client's estimate of the time that has      |
      |          | passed since its last synchronization with the  |
      |          | Khronos pool (sec)                              |
      +----------+-------------------------------------------------+
      |    B     | An upper bound on the client's time estimation  |
      |          | error rate (ms/sec)                             |
      +----------+-------------------------------------------------+
      |   ERR    | An upper bound on the client's error regarding  |
      |          | its estimate of the time that elapsed from the  |
      |          | last update, which equals to B*Cest (ms)        |
      +----------+-------------------------------------------------+
      |    K     | Panic trigger - the number of Khronos pool re-  |
      |          | samplings until reaching "Panic mode"           |
      +----------+-------------------------------------------------+
      |    tc    | The current time [sec], as indicated by the     |
      |          | time offset value that is computed by Khronos   |
      +----------+-------------------------------------------------+

                        Table 1: Khronos Notations

   The recommended values are discussed in Section 3.3.

3.  Khronos' Design

   A client that runs Khronos as a watchdog uses NTPv4 as in [RFC5905]
   and in the background runs a modification to the elements of the
   system process described in Section 11.2.1 and 11.2.2 in [RFC5905]
   (namely, the Selection Algorithm and the Cluster Algorithm).  The
   NTPv4 conventional protocol periodically queries 3-4 servers in each
   poll interval.  In parallel, the Khronos watchdog periodically

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   queries a set of m (tens) servers from a large (hundreds) server pool
   in each Khronos poll interval, where the m servers are selected from
   the server pool at random.  Based on empirical analyses, to minimize
   the load on NTP servers while providing high security, the Khronos
   poll interval should be around 10 times the NTPv4 poll interval
   (i.e., a Khronos clock update occurs once every 10 NTPv4 clock
   updates).  In each Khronos poll interval, if the Khronos time offset
   exceeds a predetermined value, an attack is detected.

   Under Khronos, unless an attack is detected, only one sample from
   each server is used (avoiding "Clock Filter Algorithm" as defined in
   Section 10 in [RFC5905]).  When under attack, Chornos uses several
   samples from each server, and executes the "Clock Filter Algorithm"
   for choosing the best sample from each server, with low jitter.
   Then, given a sample from each server, the client discards outliers
   by executing the procedure described in this section and the next.
   Then, the NTPv4 "Combine Algorithm" is used for computing the system
   peer offset, as specified in Section 11.2.3 in [RFC5905].

3.1.  Khronos Calibration

   At the first time the Khronos system process is executed, calibration
   is needed.  The calibration process generates a local Khronos pool of
   servers the client can synchronize with, consisting of n servers (up
   to hundreds).  To this end, the NTP client executes the "Peer
   Process" and "Clock Filter Algorithm" as in Sections 9,10 in
   [RFC5905] (respectively), on an hourly basis, for 24 consecutive
   hours, and generates the union of all received NTP servers' IP
   addresses.  Importantly, this process can also be executed in the
   background periodically, once in a long time (e.g., every few weeks/
   months).  The servers in the Khronos pool should be scattered across
   different regions to make it harder for an attacker to compromise, or
   gain man-in-the-middle capabilities, with respect to a large fraction
   of the Khronos pool.  Therefore, Khronos calibration is with respect
   to the general NTP server pool (for example pool.org), and not only
   with respect to the servers in the client's state or region.

   The first Khronos update requires m servers, which can be found in
   several minutes.  Moreover, it is possible to query several DNS pool
   names (for example 0.pool.ntp.org, 1.pool.ntp.org etc. and regional
   pools) to vastly accelerate the calibration and the first update.  By
   storing the retrieved addresses to permanent storage a recalibration
   will avoided in case of system restart.

   The calibration is the only Khronos part where DNS traffic is
   generated.  Around 250 DNS queries are required by Khronos to obtain
   a pool of 1000 NTP servers.  Assuming the calibration period is one
   month, the expected DNS traffic generated by Khorons client is less

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   than 10 DNS queries per day, which is usually several orders of
   magnitude lower than the total daily number of DNS queries per
   machine.

3.2.  Khronos' Poll and System Processes

   In each Khronos poll interval the Khronos system process randomly
   chooses a set of m (tens) servers out of the Khronos pool of n
   (hundreds) servers.  Khronos server polling is allowed to spread
   normally, similar to NTPv4.  Servers which do not respond during the
   Khronos poll are filtered out.  If less than 1/3 of the m servers are
   left, immediately, a new subset of servers is sampled, in the exact
   same manner (called "resampling" process).

   Next, out of the time-samples received from this chosen subset of
   servers, the lowest third of the samples' offset values and highest
   third of the samples' offset values are discarded.

   Khronos checks that the following two conditions hold for the
   remaining samples:

   *  The maximal distance between every two time samples does not
      exceed 2w (can be verified by considering just the minimum and the
      maximum samples).

   *  The average value of the remaining samples is at distance at most
      ERR+2w from the client's local clock.

   (where w, ERR are as described in Table 1.

   In the event that both of these conditions are satisfied, the average
   of the remaining samples is set to be the "Khronos time offset".
   Otherwise, resampling is performed.  This process ensures that the
   Khronos client's queries are spread across servers so as to both
   yield improved security against strategic and Byzantine attacks (as
   discussed in Section Section 4.3) and to mitigate the effect of a DoS
   attack on NTP servers that renders them non-responsive.  This
   resampling process continues in subsequent Khronos poll intervals
   until the two conditions are both satisfied or the number of times
   the servers are re-sampled exceeds a "Panic Trigger" (K in Table 1),
   in which case Khronos enters a "Panic Mode".  Note that whether the
   client allows panic mode or not is configurable.

   In panic mode, Khronos queries all the servers in its local Khronos
   pool, orders the collected time samples from lowest to highest and
   eliminates the lowest third and the highest third of the samples.
   The client then averages over the remaining samples, and sets this
   average to be the new "Khronos time offset".

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   If the Khronos time offset exceeds a predetermined value it is passed
   on to the clock discipline algorithm in order to steer the system
   time (as in [RFC5905]).

   Note that resampling follows immediately the previous sampling since
   waiting untill the next polling interval may increase the time shift
   in face of attack.  This shouldn't generate high overhead since the
   number of resamples is bounded by K (after K resamplings, "Panic
   mode" is in place) and the chances to arrive to repeated resampling
   are low (see Section Section 4 for more details).

3.3.  Khronos' Recommended Parameters

   According to empirical observations (presented in [Khronos_paper]),
   querying 15 servers at each poll interval (i.e., m=15) out of 500
   servers (i.e., n=500), and setting w to be around 25 milliseconds
   provides both high time accuracy and good security.  Moreover,
   empirical analyses showed that, on average, when selecting w=25ms,
   approximately 83% of the servers' clocks are at most w-away from UTC,
   and within 2w from each other, satisfying the first condition of
   Khronos' system process.  However, in order to support congested
   links senarios, we recommend to use a higher w value, such as 1 sec.

   Furthermore, according to Khronos security analysis, setting K to be
   3 (i.e., if after 3 re-samplings the two conditions are not satisfied
   then Khronos enters "panic mode") is safe when facing time shifting
   attacks.  Moreover, when setting K to 3, the probability of an
   attacker forcing a panic mode on a client is negligible (less than
   0.000002 for each polling interval).

   Khronos' effect on precision and accuracy are discussed in Section 6
   and Section 4.

4.  Security Considerations

4.1.  Threat Model

   Khronos repeatedly gathers time samples from small subsets of a large
   local Khronos pool of NTP servers.  The following man-in-the-middle
   (MitM) byzantine attacker is considered: the attacker is assumed to
   control a subset of the servers in the Khronos pool and is capable of
   fully determining the values of the time samples gathered from these
   NTP servers.  The threat model encompasses a broad spectrum of MitM
   attackers, ranging from fairly weak (yet dangerous) MitM attackers
   only capable of delaying and dropping packets (for example using the
   Bufferbloat attack) to extremely powerful MitM attackers who are in
   control of (even authenticated) NTP servers (see detailed discussion
   in [RFC7384]).

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   MitM attackers covered by this model might be, for example, (1) in
   direct control of a fraction of the NTP servers (e.g., by exploiting
   a software vulnerability), (2) an ISP (or other Autonomous-System-
   level attacker) on the default BGP paths from the NTP client to a
   fraction of the available servers, (3) a nation state with authority
   over the owners of NTP servers in its jurisdiction, or (4) an
   attacker capable of hijacking (e.g., through DNS cache poisoning or
   BGP prefix hijacking) traffic to some of the available NTP servers.
   The details of the specific attack scenario are abstracted by
   reasoning about MitM attackers in terms of the fraction of servers
   with respect to which the attacker has MitM capabilities.

   Notably, Khronos provides protection from MitM attacks that cannot be
   achieved by cryptographic authentication protocols since even with
   such measures in place an attacker can still influence time by
   dropping/delaying packets.  However, adding an authentication layer
   (e.g., [RFC8915]) to Khronos will enhance its security guarantees and
   enable the detection of various spoofing and modification attacks.

4.2.  Attack Detection

   Khronos detects time-shifting attacks by constantly monitoring
   NTPv4's (or potentially any other current or future time protocol)
   offset and the offset computed by Khronos and checking whether the
   difference between the two exceeds a certain threshold (10
   milliseconds by default).  Unless an attack was detected, NTPv4
   controls the client's clock.  Under attack, Khronos takes control
   over the clients clock in order to prevent its shift.

   Analytical results (in [Khronos_paper]) indicate that if a local
   Khronos pool consists of 500 servers, 1/7 of whom are controlled by a
   man-in-the-middle attacker, and 15 servers are queried in each
   Khronos poll interval, then success in shifting time at a Khronos
   client by even a short time (e.g., 100 milliseconds), takes many
   years of effort (e.g., over 20 years in expectation).  See a brief
   overview of Khronos' security analysis below.

   Khronos' security analysis is briefly described next.

4.3.  Security Analysis Overview

   Time-samples that are at most w away from UTC are considered "good",
   whereas other samples are considered "malicious".  Two scenarios are
   considered:

   *  Less than 2/3 of the queried servers are under the attacker's
      control.

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   *  The attacker controls more than 2/3 of the queried servers.

   The first scenario, where there are more than 1/3 good samples,
   consists of two sub-cases: (i) there is at least one good sample in
   the set of samples not eliminated by Khronos (in the middle third of
   samples), and (ii) there are no good samples in the remaining set of
   samples.  In the first of these two cases (at least one good sample
   in the set of samples that was not eliminated by Khronos), the other
   remaining samples, including those provided by the attacker, must be
   close to a good sample (for otherwise, the first condition of
   Khronos' system process in Section 3.2 is violated and a new set of
   servers is chosen).  This implies that the average of the remaining
   samples must be close to UTC.  In the second sub-case (where there
   are no good samples in the set of remaining samples), since more than
   a third of the initial samples were good, both the (discarded) third
   lowest-value samples and the (discarded) third highest-value samples
   must each contain a good sample.  Hence, all the remaining samples
   are bounded from both above and below by good samples, and so is
   their average value, implying that this value is close to UTC
   [RFC5905].

   In the second scenario, where the attacker controls more than 2/3 of
   the queried servers, the worst possibility for the client is that all
   remaining samples are malicious (i.e., more than w away from UTC).
   However, as proved in [Khronos_paper], the probability of this
   scenario is extremely low even if the attacker controls a large
   fraction (e.g., 1/4) of the servers in the local Khronos pool.
   Therefore, the probability that the attacker repeatedly reach this
   scenario decreases exponentially, rendering the probability of a
   significant time shift negligible.  We can express the improvement
   ratio of Khronos over NTPv4 by the ratios of their single shift
   probabilities.  Such ratios are provided in Table Table 2, where
   higher values indicate higher improvement of Khronos over NTPv4 and
   are also proportional to the expected time till a time shift attack
   succeeds once.

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     +========+==========+==========+==========+==========+==========+
     | Attack |    6     |    12    |    18    |    24    |    30    |
     | Ratio  | samples  | samples  | samples  | samples  | samples  |
     +========+==========+==========+==========+==========+==========+
     |  1/3   | 1.93e+01 | 3.85e+02 | 7.66e+03 | 1.52e+05 | 3.03e+06 |
     +--------+----------+----------+----------+----------+----------+
     |  1/5   | 1.25e+01 | 1.59e+02 | 2.01e+03 | 2.54e+04 | 3.22e+05 |
     +--------+----------+----------+----------+----------+----------+
     |  1/7   | 1.13e+01 | 1.29e+02 | 1.47e+03 | 1.67e+04 | 1.90e+05 |
     +--------+----------+----------+----------+----------+----------+
     |  1/9   | 8.54e+00 | 7.32e+01 | 6.25e+02 | 5.32e+03 | 4.52e+04 |
     +--------+----------+----------+----------+----------+----------+
     |  1/10  | 5.83e+00 | 3.34e+01 | 1.89e+02 | 1.07e+03 | 6.04e+03 |
     +--------+----------+----------+----------+----------+----------+
     |  1/15  | 3.21e+00 | 9.57e+00 | 2.79e+01 | 8.05e+01 | 2.31e+02 |
     +--------+----------+----------+----------+----------+----------+

                        Table 2: Khronos Improvement

   In addition to evaluating the probability of an attacker successfully
   shifting time at the client's clock, we also evaluated the
   probability that the attacker succeeds in launching a DoS attack on
   the servers by causing many clients to enter a panic mode (and query
   all the servers in their local Khronos pools).  This probability
   (with the previous parameters of n=500, m=15, w=25 and k=3) is
   negligible even for an attacker in control of a large number of
   servers in clients' local Khronos pools, and it is expected to take
   decades to force panic mode.

   Further details about Khronos's security guarantees can be found in
   [Khronos_paper].

5.  Khronos' Pseudocode

   The pseudocode for Khronos' Time Sampling Scheme, which is invoked in
   each Khronos poll interval is as follows:

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   counter := 0
   S = []
   T = []
   While counter < K do
      S := sample(m) //gather samples from (tens of) randomly chosen servers
      T := bi_side_trim(S,1/3) //trim the third lowest and highest values
      if (max(T) - min(T) <= 2w) and (|avg(T)-tc| < ERR + 2w) Then
          return avg(T) // Normal case
      end
      counter ++
   end
   // panic mode
   S := sample(n)
   T := bi-sided-trim(S,1/3) //trim lowest and highest thirds;
   return avg(T)

6.  Precision vs. Security

   Since NTPv4 updates the clock as long as time-shifting attacks are
   not detected, the precision and accuracy of a Khronos client are the
   same as NTPv4's when not under attack.  Under attack, Khronos takes
   control over the client's clock, mitigating the time shift while
   guaranteeing relatively high accuracy (error is bounded by (Err+2w),
   which is 100 ms for the recommended parameters specified in
   Section 3.3).  Khronos is based on crowdsourcing across servers,
   changes the set of queried servers more frequently than NTPv4
   [Khronos_paper], and avoids some of the filters in NTPv4's system
   process.  These factors can potentially harm its precision.
   Therefore, a smoothing mechanism can be used, where instead of a
   simple average of the remaining samples, the smallest (in absolute
   value) offset is used unless its distance from the average is higher
   than a predefined value Y.  Setting Y to 1 millisecond, was
   impractically demonstrated to result with precision similar to NTPv4.

7.  Acknowledgements

   The authors would like to thank Erik Kline, Miroslav Lichvar, Danny
   Mayer, Karen O'Donoghue, Dieter Sibold, Yaakov.  J.  Stein, Harlan
   Stenn, Hal Murray and Marcus Dansarie, for valuable contributions to
   this document and helpful discussions and comments.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  References

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9.1.  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>.

   [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>.

   [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>.

   [RFC8915]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
              <https://www.rfc-editor.org/info/rfc8915>.

9.2.  Informative References

   [Khronos_paper]
              Deutsch, O., Schiff, N.R., Dolev, D., and M. Schapira,
              "Preventing (Network) Time Travel with Chronos", 2018,
              <https://www.ndss-symposium.org/wp-
              content/uploads/2018/02/ndss2018_02A-2_Deutsch_paper.pdf>.

Authors' Addresses

   Neta Rozen-Schiff
   Hebrew University of Jerusalem
   Jerusalem
   Israel
   Phone: +972 2 549 4599
   Email: neta.r.schiff@gmail.com

   Danny Dolev
   Hebrew University of Jerusalem
   Jerusalem
   Israel
   Phone: +972 2 549 4588
   Email: danny.dolev@mail.huji.ac.il

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   Tal Mizrahi
   Huawei Network.IO Innovation Lab
   Israel
   Email: tal.mizrahi.phd@gmail.com

   Michael Schapira
   Hebrew University of Jerusalem
   Jerusalem
   Israel
   Phone: +972 2 549 4570
   Email: schapiram@huji.ac.il

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