wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Clock Synchronization in Distributed Systems 18-9
• The second component in the data flow is the so-called peer selection. This unit is included due to
the fact that not only delayed messages can arrive from a node, but also mutually wrong clocks
can present themselves as masters. Reasons for wrong hosts in NTP networks can be broken
server clocks or attacks with the goal to change the time on a network host. The latter can be
tackled by authentication of clock servers, which is nevertheless not sufficient for the first issue
of accidentally wrong clocks. In order to avoid problems resulting from such broken clocks, a
parameter-driven decision of sorting out those clocks is done in peer selection.
• After the peer selection is done—from a message point of view—often contradicting information
regarding the offset and drift of the local clock exist. These data have to be merged in the clock
combining component.
• The last two components, namely the loop filter and voltage-controlled oscillator (VCO), have the
task of conditioning the control signal with respect to control circuit stability and adjusting the
local clock, respectively.
The first element of the NTP signal flow is the data filter. This element is provided to eliminate statistical
errors as well as byzantine nodes. The filter collects data on a per-peer basis and calculates two
measures: the filter dispersion ε σ , and the select dispersion ε ζ . The dispersion is calculated with ϑ i (0 ≤ i ≤ n)
as the offset of the ith sample and the sample difference ε ij = |ϑ i − ϑ j |. The dispersion, ε j is then defined
relative to the jth entry as weighted sum
ε
j
n−1
i+
= ∑ εijw
1
i=
1
Both ε σ and ε ζ are defined by protocol parameters, namely NTP.FILTER and NTP.SELECT, which
are usually set to values less than 0.5 and evaluated relative to the first entry ε 0 . In that context, w
is the weighting factor, which is used to build an exponential average, with a value of w < 0.5. The
second, functional part, the peer selection is most important for the stability of NTP. As soon as new
offsets to a reference clock are estimated, this algorithm decides which peer is used as synchronization
source. Of course, also the unavailability of offset measurements (e.g., due to timeouts) are also
used to (de-)select peers. The actual peer selection is done by assembling of the candidates in a sorted
list. This list represents the available peers sorted by stratum and synchronization dispersion. Also,
several sanity checks to detect defective nodes are a knock-out criterion to remove defective nodes. In
case that the result of this sanity check is an empty list, the local nodes continue running with their
local unadjusted clock frequencies. In the other case, if the list holds more than one node, these statistical
equivalent offsets have to be combined. The clock assembly is done by construction of so-called
final clock correction, η n , for all nodes. Moreover, the synchronization distance, λ n , is also internally
required by the previously described algorithms. The value of 1/λ n is then used as weight of each clock
offset in the list.
The virtual control loop is finally closed by the loop filter and the VCO, respectively. The task of these
two parts is to adapt the calculated settings from NTP to the actual clock of the device. This is done by
adjusting offset and frequency—both algorithms are implemented in a feedback loop. The general structure
of the disciplining is shown in Figure 18.7. In general, this disciplining structure is a subordinate
control loop. The set value ϑ r is taken from the above-described clock selection part of NTP. The value is
compared to the control phase of the variable frequency oscillator, which is the actual driver of the node
clock. The difference between ϑ r and ϑ c is afterward used in the clock filter, which is actually a tapped
delay line, whereas the tap is selected by an algorithm described in [Mills2000].
Afterward, the filtered signal is used in the loop filter (Figure 18.7). This filter adjusts the clock via the
calculation of three variables. First, the phase offset is calculated and directly fed back to the clock adjustment
unit. Second, the frequency offset is calculated via two ways: a phase prediction and a frequency
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