A Secure Selection and Filtering Mechanism for the Network Time Protocol with Khronos
draft-ietf-ntp-chronos-15

Network Working Group                                    N. Rozen-Schiff
Internet-Draft                                                  D. Dolev
Intended status: Informational            Hebrew University of Jerusalem
Expires: 17 December 2023                                     T. Mizrahi
                                        Huawei Network.IO Innovation Lab
                                                             M. Schapira
                                          Hebrew University of Jerusalem
                                                            15 June 2023

A Secure Selection and Filtering Mechanism for the Network Time Protocol
                              with Khronos
                       draft-ietf-ntp-chronos-15

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

   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 17 December 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 1]
Internet-Draft         NTP Extention with Khronos              June 2023

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
     2.1.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   4
     2.2.  Notations . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Khronos' Design . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Khronos Calibration . . . . . . . . . . . . . . . . . . .   6
     3.2.  Khronos' Poll and System Processes  . . . . . . . . . . .   6
     3.3.  Khronos' Recommended Parameters . . . . . . . . . . . . .   7
   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 . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     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 (shifts) the clock of
   a network device.  Time shifting attacks on NTP clients can be based
   on interfering with 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
   that runs Khronos is interoperable with [RFC5905]-compatible NTPv4

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 2]
Internet-Draft         NTP Extention with Khronos              June 2023

   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
   monitoring client clock (which is updated by NTPv4) and calculating
   an estimated offset which we refer by "Khronos time offset".  When
   the offset exceeds a predefined threshold (specified in Section 4.2),
   this is interpreted as the client experiencing a time shifting
   attack.  In this case, Khronos updates the client's clock.

   When the client is not under attack, Khronos is passive, allowing
   NTPv4 to control the client's 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 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, Khronos client
   randomly selects and samples 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 Khronos 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 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 analysis, if a local
   Khronos pool consists of, for example, 500 servers, 1/7 of whom are
   controlled by an attacker and Khronos queries 15 servers in each

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 3]
Internet-Draft         NTP Extention with Khronos              June 2023

   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 ms from UTC.  The full
   exposition of the formal analysis of this guarantee is available at
   [Khronos_paper].

   Khronos introduces a watchdog mechanism that 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 communicates, 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, Khronos computes "Khronos time offset"
   based on 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.  Terms and Abbreviations

   NTPv4                  Network Time Protocol version 4 [RFC5905].

   System process         Selection Algorithm and the Cluster Algorithm
                          [RFC5905].

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

   NTS                    Network Time Security for the Network Time
                          Protocol [RFC8915].

2.2.  Notations

   Describing Khronos algorithm, the following notation is used.

     +==========+====================================================+
     | Notation | Meaning                                            |
     +==========+====================================================+
     |    n     | The number of candidate servers in Khronos pool    |
     |          | (potentially hundreds).                            |
     +----------+----------------------------------------------------+
     |    m     | The number of servers that Khronos queries in each |
     |          | poll interval (up to tens).                        |

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 4]
Internet-Draft         NTP Extention with Khronos              June 2023

     +----------+----------------------------------------------------+
     |    w     | An upper bound on the distance between any         |
     |          | "truechimer" NTP server (as in [RFC5905]) and UTC. |
     +----------+----------------------------------------------------+
     |    B     | An upper bound on the client's clock error rate    |
     |          | (ms/sec).                                          |
     +----------+----------------------------------------------------+
     |   ERR    | An upper bound on the client's clock error between |
     |          | Khronos polls (ms).                                |
     +----------+----------------------------------------------------+
     |    K     | The number of Khronos pool re-samplings until      |
     |          | reaching "Panic mode".                             |
     +----------+----------------------------------------------------+
     |    H     | Predefined threshold for time offset triggering    |
     |          | clock update by Khronos.                           |
     +----------+----------------------------------------------------+

                         Table 1: Khronos Notations

   The recommended values are discussed in Section 3.3.

3.  Khronos' Design

   Khronos watchdog periodically 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 threshold (denoted
   as H), an attack is indicated.

   Unless an attack is indicated, Khronos uses only one sample from each
   server (avoiding "Clock Filter Algorithm" as defined in Section 10 in
   [RFC5905]).  When under attack, Khronos 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, Khronos discards outliers by executing the
   procedure described in Section 3.2.

   Between consecutive Khronos polls, Khronos keeps track of clock
   offsets, for example by catching clock discipline (as in [RFC5905])
   calls.  The sum of offsets is referred to as "Khronos inter-poll
   offset" (denoted as tk) which is set to zero after each Khronos poll.

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 5]
Internet-Draft         NTP Extention with Khronos              June 2023

3.1.  Khronos Calibration

   Calibration is performed at the first time the Khronos is executed,
   and also periodically, once in a long time (e.g., every few weeks/
   months).  The calibration process generates a local Khronos pool of n
   (up to hundreds) NTP servers the client can synchronize with.  To
   this end, Khronos makes DNS queries to addresses of NTP pools collect
   the union of all received IP addresses.  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 queries general NTP server pools (for
   example pool.org), and not only the pool 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.

   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 Khronos client is less
   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 times can be spread
   uniformly, 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, a new subset of servers is immediately 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 sampled offsets:

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

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 6]
Internet-Draft         NTP Extention with Khronos              June 2023

   *  The distance between the offsets average and Khronos inter-poll
      offset is at most ERR+2w.

   (where w and ERR are as described in Table 1).

   In the event that both of these conditions are satisfied, the average
   of the offsets is set to be the "Khronos time offset".  Otherwise,
   resampling is performed.  This process spreads Khronos client's
   queries across servers thereby improving security against strategic
   and Byzantine attacks (as discussed in Section Section 4.3) and
   mitigating 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".

   If the Khronos time offset exceeds a predetermined threshold (H) 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 until 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).  Moreover, in an
   interval following a panic mode, Khronos executes the same system
   process which starts by querying only m servers (regardless of
   previous panic).

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 ms provides both
   high time accuracy and good security.  Specifically, 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 scenarios, it is recommended to use a higher w value,

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 7]
Internet-Draft         NTP Extention with Khronos              June 2023

   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.  In addition, the probability of an attacker forcing a panic
   mode on a client when K equals 3, 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

   The following man-in-the-middle (MitM) byzantine attacker is
   considered: the attacker is assumed to control a subset of the
   servers in NTP pools and is capable of fully determining the values
   of the time samples returned by 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 security requirements
   discussion in [RFC7384]).

   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., NTS [RFC8915]) to Khronos will enhance its security guarantees
   and enable the detection of various spoofing and modification
   attacks.

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 8]
Internet-Draft         NTP Extention with Khronos              June 2023

4.2.  Attack Detection

   Khronos detects time-shifting attacks by constantly monitoring
   NTPv4's (or potentially any other current or future time protocol)
   clock and the offset computed by Khronos and checking whether the
   offset exceeds a predetermined threshold (H = 30 ms 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 of a Khronos
   client by even a small degree (100 ms), takes many years of effort
   (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.

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

Rozen-Schiff, et al.    Expires 17 December 2023                [Page 9]
Internet-Draft         NTP Extention with Khronos              June 2023

   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 n 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.

Rozen-Schiff, et al.    Expires 17 December 2023               [Page 10]
Internet-Draft         NTP Extention with Khronos              June 2023

     +========+==========+==========+==========+==========+==========+
     | 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 who controls a large number of
   servers in client's 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:

Rozen-Schiff, et al.    Expires 17 December 2023               [Page 11]
Internet-Draft         NTP Extention with Khronos              June 2023

   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) - tk| < 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 at times when no time-shifting attacks
   are detected, the precision and accuracy of a Khronos client are the
   same as NTPv4 at these times.  Under attack, Khronos takes control
   over the client's clock, mitigating the time shift while guaranteeing
   relatively high accuracy (the error is bounded by H).

   Khronos is based on crowdsourcing across servers and regions, 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.  Preliminary experiments demonstrated
   promising results with precision similar to NTPv4.

   Note that in applications, such as multi source media streaming,
   which are highly sensitive to time differences among hosts, it is
   advised to use Khronos at all hosts in order to obtain high precision
   even in the presence of attackers that try to shift each host in a
   different magnitude and/or direction.

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, Marcus Dansarie, Geoff Huston and Roni Even, for
   valuable contributions to this document and helpful discussions and
   comments.

Rozen-Schiff, et al.    Expires 17 December 2023               [Page 12]
Internet-Draft         NTP Extention with Khronos              June 2023

8.  IANA Considerations

   This memo includes no request to IANA.

9.  References

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

Rozen-Schiff, et al.    Expires 17 December 2023               [Page 13]
Internet-Draft         NTP Extention with Khronos              June 2023

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

   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

Rozen-Schiff, et al.    Expires 17 December 2023               [Page 14]