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A Method to Estimate the Timestamp Accuracy of Measurement Hardware - - PowerPoint PPT Presentation
A Method to Estimate the Timestamp Accuracy of Measurement Hardware - - PowerPoint PPT Presentation
A Method to Estimate the Timestamp Accuracy of Measurement Hardware and Software Tools Patrik Arlos Blekinge Institute of Technology Inter-arrival time Mean: 1230 s Min: 1110 s Max: 1623 s Std.dev: 44.8 s Mean: 1230 s Min: 1230
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Accuracy
- To what level can we trust a timestamp?
1117564161.171961925000 (Hardware) 1113647109.171036000000 (Software)
Many digits do not mean high accuracy
The comparison between a measured value and the correct value
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Timestamp
- Resolution
T = 0.2, 0.2, 0.4, 0.5
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Timestamp
- Resolution
- Implementation
GetTimestamp() GetTimestamp()=2 trequest tresponse Clock Value 1 2 3 4
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Timestamp
- Resolution
- Implementation
- Synchronization
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Timestamp Accuracy Method
- 1. Generate traffic, fixed-size PDUs back-to-back
- 2. Collect the traffic (PDUs) and their timestamps
- 3. Verify that the trace is correct, i.e., no loss or
reordering has occurred
- 4. Calculate the inter-arrival time,
5. 6.
i I
T , ˆ
C L T /
I =
I , I
ˆ T T i
i
− = ε
⎪ ⎩ ⎪ ⎨ ⎧ + + =
Δ
2 Case ) min( ) max( 1 Case 2 ) min( ) max( ε ε ε ε T
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Setup
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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
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Timestamp Accuracy Estimations
410 μs 573 μs Linux 2.6 Linux 2.4 410 μs RAW-TSC 300 μs Linux 2.6 Linux 2.4 RAW-ioctl System 460 μs 340 μs 440 μs 320 μs 3.24 ms 374 μs 346 μs 100 ns 60 ns TΔ TΔ System Linux 2.6 Linux 2.4 PCAP-MMAP Linux 2.6 Linux 2.4 FreeBSD 5.1 PCAP-PFRING Linux 2.4 Linux 2.6 PCAP Agilent J6800 DAG 3.5 Pentium-4 2.4 GHz Synchronized using NTP 250000 PDUs ~300 s.
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Software Based systems
ε [ns]
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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.
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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.
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