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PingER measurements are made by ~60 MAs in 23 countries. They make measurements to over 700 targets in ~ 160 countries containing more than 99% of the world's connected population. The measurement cycle is scheduled at roughly 30 minute intervals. The actual scheduled timing of a measurement is deliberately randomized so measurements from one MA are not synchronized with another MA. Typical absolute separation of the timestamp of a measurement from say pinger.slac.stanford.edu to sitka.triumf.ca versus pinger-raspberry.slac.stanford.edu to sitka.triumf.ca is several minutes (e.g. ~ 8 mins for measurements during the time frame June 17 to July 14, 2015), see spreadsheet. At each measurement cycle, each MA issues a set of 100 Byte pings and a set of 1000 Byte ping requests to each target in the MA’s list of targets, stopping when the MA receives 10 ping responses or it has issued 30 ping requests. The number of ping responses is referred to as N and is in the range 0 - 10. The data recorded for each set of pings consists of: the MA and target names and IP addresses; a time-stamp; the number of bytes in the ping request; the number of ping requests and responses (N); the minimum Round Trip Time (RTT) (Min_RTT), the average RTT (Avg_RTT) and maximum RTT (Max_RTT) of the N ping responses; followed by the N ping sequence numbers, followed by the N RTTs. From the N RTTs we derive various metrics including: the minimum ping RTT; average RTT; maximum RTT; standard deviation (stdev) of RTTs, 25% probability (first quartile) of RTT; 75% probability (third quartile) of RTT; Inter Quartile Range (IQR); loss; and reachability (host is unreachable if it gets 100 % loss). We also derive the Inter Packet delay (IPD) and the Inter Packet Delay Variability (IPDV) as the IQR of the IPDs.
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To try and determine more closely the impact of the host on the measurements, we eliminated the effect of the network by making measurements to the host's localhost NIC port from August 9th to August 19th. Looking . Looking at the analyzed data it is apparent that:
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Looking at the RTT frequency distributions it is apparent that:
- the pinger and raspberry host RTT frequency distributions hardly overlap at all (one sample (0.04%) of the raspberry pi distribution overlaps ~ 7% of the pinger distribution).
- the pinger host's Median RTT is 7 times smaller (0.03ms vs 0.2 ms) than that of the raspberry-pi
- the pinger host's maximum outlier (0.174ms) is ~ factor 60 smaller than the raspberry-pi.
- the pinger host exhibits a pronounced bimodaility not seen for the raspberry-pi.
- the IQRs are very similar (0.025 vs 0.020ms)
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Though there are large relative differences in the distributions, the absolute differences of the aggregated statistics (medians, IQRs) are sub millisecond and so should not noticeably affect PingER wide area network results.
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- The factor of 7 difference in the median RTTs is probably at least partially related to the factor ten in the NICs (1Gbps vs 100Mbps) and the factor of 5 difference in the clock speeds (3Ghz vs 600MHz).
- Looking at the minimum RTTs for each set of pings it is seen that pinger has a larger ratio of median RTT / IQR compared to the raspberry-pi (0.33 vs 0.096).
- This may be related to the fact that the pinger host is less dedicated to acting as a PinGER PingER MA since in addition it runs lost lots of cronjobs to gather, archive and analyze the data. This may also account for some of the more pronounced multi-modality of the pinger host's RTT distributions.
- Currently we do not have a rationale for the reduced outliers on pinger vs the raspberry-pi.
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