Internet DRAFT - draft-ietf-ntp-chronos
draft-ietf-ntp-chronos
Network Working Group N. Rozen-Schiff
Internet-Draft D. Dolev
Intended status: Informational Hebrew University of Jerusalem
Expires: 1 March 2024 T. Mizrahi
Huawei Network.IO Innovation Lab
M. Schapira
Hebrew University of Jerusalem
29 August 2023
A Secure Selection and Filtering Mechanism for the Network Time Protocol
with Khronos
draft-ietf-ntp-chronos-25
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 describes a companion application
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 current and future time protocols.
Status of This Memo
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This Internet-Draft will expire on 1 March 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
2.1. Terms and Abbreviations . . . . . . . . . . . . . . . . . 5
2.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Khronos Design . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Khronos Calibration - Gathering the Khronos Pool . . . . 6
3.2. Khronos's Poll and System Processes . . . . . . . . . . . 7
3.3. Khronos's Recommended Parameters . . . . . . . . . . . . 8
4. Operational Considerations . . . . . . . . . . . . . . . . . 9
4.1. Load considerations . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Attack Detection . . . . . . . . . . . . . . . . . . . . 11
5.3. Security Analysis Overview . . . . . . . . . . . . . . . 11
6. Khronos Pseudocode . . . . . . . . . . . . . . . . . . . . . 13
7. Precision vs. Security . . . . . . . . . . . . . . . . . . . 13
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 14
8.1. Implementation 1 . . . . . . . . . . . . . . . . . . . . 14
8.1.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 14
8.1.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 15
8.1.3. Contact Information . . . . . . . . . . . . . . . . . 15
8.1.4. Last Update . . . . . . . . . . . . . . . . . . . . . 15
8.2. Implementation 2 . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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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 machine-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 as a background monitoring application
("watchdog") that guard against time shifting attacks in any NTP
client. An NTP client 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
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 5.2),
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 7.
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By leveraging techniques from distributed computing theory for time-
synchronization, Khronos achieves accurate time 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). While Khronos
queries around 3 times more servers per polling interval than NTP,
Khronos's polling interval can be longer (e.g., 10 times longer) than
NTPv4, thereby, minimizing the load on NTP servers and the
communication overhead. Moreover, Khronos's random server selection
may even help to distribute queries across the whole pool.
Khronos's 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 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. 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 aspects allow
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Khronos to minimize the load on the NTP servers and to provide
provable security guarantees against both MITM attackers and
attackers capable of compromising a large number of NTP servers.
We note that, to some extent, NTS [RFC8915] could make it more
challenging for attackers to perform MITM attacks, but is of little
impact if the servers themselves are compromised.
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). |
+----------+----------------------------------------------------+
| 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". |
+----------+----------------------------------------------------+
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| 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.
3.1. Khronos Calibration - Gathering the Khronos Pool
Calibration is performed at the first time the Khronos is executed,
and also periodically, once in a long time (every two weeks). 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 machine-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.ntp.org), and not only the pool in the client's state or region.
In addition, servers can be selected to Khronos pool manually or by
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using other NTP pools (such as NIST internet time servers).
The first Khronos update requires m servers, which can be found in
several minutes. Moreover, it is possible to query several DNS pool
names to vastly accelerate the calibration and the first update.
The calibration is the only Khronos part where DNS traffic is
generated. Around 125 DNS queries are required by Khronos to obtain
addresses of 500 NTP servers which is higher than Khronos pool size
(n). Assuming the calibration period is two weeks, 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's 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 and samples them. Note that the randomness of the
server selection is crucial for the security of the scheme and
therefore any Khronos implementation must use secure randomness
implementation such as used for encryption key generation.
Khronos's polling times of different servers may spread uniformly
within its poll interval, similar to NTPv4. Servers which do not
respond during the Khronos poll interval 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).
* 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).
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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 powerful
attackers (as discussed in Section 5.3) 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".
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]). In this case the user and/or admin of
the client machine should be notified about the detected time
shifting attack, for example by a message written to a relevant event
log or displayed on screen.
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 5 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's 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's system process. For similar reason, the
threshold for time offset triggering clock update by Khronos (H)
should be between w to 2w and is selected on default to 30ms. Note
that in order to support congested links scenarios, it is recommended
to use a higher w value, such as 1 sec.
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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's effect on precision and accuracy are discussed in Section 7
and Section 5.
4. Operational Considerations
Khronos is designed in order to defend NTP clients from time shifting
attacks while using public NTP servers. As such, Khronos is not
applicable for datacenters and enterprises which synchronize with
local atomic clocks, GPS devices or a dedicated NTP server (for
example due to regulations).
Khronos can be used for devices that require and depend upon time
keeping withing a configurable constant distance from UTC.
4.1. Load considerations
One requirement from Khronos is thus not to induce excessive load on
NTP servers beyond that of NTPv4, even if widely integrated into NTP
clients. We discuss below the possible causes for Khronos-induced
load on servers and how this can be mitigated.
Servers in pool.ntp.org are weighted differently by the NTP server
pool when assigned to NTP clients. Specifically, server owners
define a ``server weight'' (the ``netspeed'' parameter) and servers
are assigned to clients probabilistically according to their
proportional weight. Khronos (watchdog mode) queries are equally
distributed across a pool of servers. To avoid overloading servers,
Khronos queries servers less frequently than NTPv4, with Khronos
query interval set to 10 times the default NTPv4 maxpoll (interval)
parameter. Hence, if Khronos queries are targeted at servers in
pool.ntp.org, any target increase in server load (in terms of
multiplicative increase in queries or number of bytes per second) is
controlled by the poll interval configuration which was analyzed in
[Ananke_paper].
Consider the scenario where an attacker attempts to generate
significant load on NTP servers by triggering multiple consecutive
panic modes at multiple NTP clients. We note that to accomplish
this, the attacker must have man-in-the-middle capabilities with
respect to the communication between each and every client in a large
group of clients and a large fraction of all NTP servers in the
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queried pool. This implies that the attacker must either be
physically located at a central location (e.g., at the egress of a
large ISP) or launch a wide scale attack (e.g., on BGP or DNS) and
thereby capable to carry similar and even higher impact attacks
regardless of Khronos clients.
5. Security Considerations
5.1. Threat Model
The following powerful attacker, including MitM, is considered: the
attacker is assumed to control a subset (e.g., third) 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 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 attackers who are in control of (even authenticated) NTP
servers (see detailed security requirements discussion in [RFC7384]).
The 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 attackers in terms of the fraction of servers with
respect to which the attacker has adversarial capabilities.
Attackers that can impact communications with (or control) higher
fraction of the servers, for example all servers, are out of scope.
Considering pool size to be thousands across the world, such
attackers will most probably be capable of performing far worst
damage than time shifting.
Notably, Khronos provides protection from MitM and powerful 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.
Moreover, Khronos uses randomness to independently select the queried
servers in each poll interval, preventing attackers from exploiting
observations of past server selections.
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5.2. 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). 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
machine-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's
security analysis below.
Khronos's security analysis is briefly described next.
5.3. 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.
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's 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].
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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 reaches 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.
+========+==========+==========+==========+==========+==========+
| 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].
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6. Khronos Pseudocode
The pseudocode for Khronos Time Sampling Scheme, which is invoked in
each Khronos poll interval is as follows:
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 lowest and highest thirds
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)
7. 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. Khronos is proved to maintain an
accurate estimation of the UTC with high probability. Therefore when
Khronos detects that client's clock error exceeds a threshold (H), it
considers it as an attack and takes control over the client's clock.
As a result, the time shift is mitigated and high accuracy is
guaranteed (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.
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Note that in applications such as multi source media streaming, which
are highly sensitive to time differences among hosts, it is advisable
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. Another more efficient approach for this
cases may be to allow direct time synchronization between one host
who runs Khronos to others.
8. Implementation Status
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
8.1. Implementation 1
Organization: Hebrew University of Jerusalem
Implementers: Neta Rozen-Schiff, May Yaaron, Noam Caspi and Shahar
Cohen
Maturity: Proof-of-Concept Prototype
This implementation was used to check consistency and to ensure
completeness of this specification.
8.1.1. Coverage
This implementation covers the complete specification.
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8.1.2. Licensing
The code is released under the MIT license.
The source code is available at: https://github.com/netars/chronos
8.1.3. Contact Information
Contact Martin Langer: neta.r.schiff@gmail.com
8.1.4. Last Update
The implementation was updated in June 2022.
8.2. Implementation 2
Organization: Network Time Foundation (NTF)
Implementers: Neta Rozen-Schiff, Danny Mayer, juergen perlinger and
Harlan Stenn.
Contact Martin Langer: neta.r.schiff@gmail.com
Maturity: in progress (https://khronos.nwtime.org/).
9. 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, Roni Even, Benjamin
Schwartz, Tommy Pauly, Rob Sayre, Dave Hart and Ask Bjorn Hansen for
valuable contributions to this document and helpful discussions and
comments.
10. IANA Considerations
This memo includes no request to IANA.
11. References
11.1. Normative References
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
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[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>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[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>.
11.2. Informative References
[Ananke_paper]
Perry, Y., Rozen-Schiff, N., and M. Schapira, "Preventing
(Network) Time Travel with Chronos", 2021,
<https://www.ndss-symposium.org/wp-content/uploads/
ndss2021_1A-2_24302_paper.pdf>.
[Khronos_paper]
Deutsch, O., Rozen-Schiff, N., 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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