A Method to Estimate the Timestamp Accuracy of Measurement Hardware and Software Tools Patrik Arlos Blekinge Institute of Technology
Inter-arrival time Mean: 1230 μ s Min: 1110 μ s Max: 1623 μ s Std.dev: 44.8 μ s Mean: 1230 μ s Min: 1230 μ s Max: 1721 μ s Std.dev: 2.2 μ s 2
Accuracy The comparison between a measured value and the correct value To what level can we trust a timestamp? � 1117564161.171961925000 (Hardware) 1113647109.171036000000 (Software) � Many digits do not mean high accuracy 3
Timestamp Resolution � T = 0.2, 0.2, 0.4, 0.5 4
Timestamp Resolution � Clock Value Implementation � 0 GetTimestamp() 1 t request 2 t response GetTimestamp()=2 3 4 5
Timestamp Resolution � Implementation � Synchronization � 6
Timestamp Accuracy Method 1. Generate traffic, fixed-size PDUs back-to-back I = T L / C 2. Collect the traffic (PDUs) and their timestamps 3. Verify that the trace is correct, i.e., no loss or reordering has occurred ˆ 4. Calculate the inter-arrival time, T , I i ε = − ˆ T i T 5. I , I i ⎧ ε + ε max( ) min( ) ⎪ Case 1 6. = ⎨ T 2 Δ ⎪ ε + ε max( ) min( ) Case 2 ⎩ 7
8 Setup
Results Snapshots for the evaluated systems! � Vanilla systems, i.e. no optimization efforts were � made. ntpd used the default ntp.conf of the OS/distribution. � ntpq was used to verify that stability was reached, before starting � the session. Link: 10Mbps Full duplex Ethernet. � MP was a Pentium-4 2.4GHz/1GByte RAM. � 250 000 PDUs ~ 300s � 9
Timestamp Accuracy Estimations T Δ T Δ System System DAG 3.5 60 ns RAW-ioctl Agilent J6800 100 ns Linux 2.4 300 μ s PCAP Linux 2.6 410 μ s Linux 2.4 346 μ s RAW-TSC Linux 2.6 374 μ s Linux 2.4 573 μ s FreeBSD 5.1 3.24 ms Linux 2.6 410 μ s PCAP-PFRING Linux 2.4 320 μ s Linux 2.6 440 μ s Pentium-4 2.4 GHz PCAP-MMAP Synchronized using NTP Linux 2.4 340 μ s 250000 PDUs ~300 s. Linux 2.6 460 μ s 10
11 Software Based systems ε [ns]
Conclusions Timestamp accuracy influences ALL subsequent � analysis. Synchronization is a major problem. � Timestamp accuracy depends on the entire � system, hence the entire system needs to be evaluated, before use. HW based systems, ~1us � SW based systems, ~1ms � � W ithout synchronization events occurring! PCAP seems to be timestamping at the � application level. 12
Future work Evaluation of tools in real-time while in use. � � Get a accuracy value for the collected trace, upon completion of the trace. H/W implementation of a reference traffic � generator. 13
A Method to Estimate the Timestamp Accuracy of Measurement Hardware and Software Tools Questions? Patrik.Arlos@bth.se Blekinge Institute of Technology 14
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