vive en los países de régimen fascista”
4.1. Análisis de la crónica “Alemania bajo el poder de Hitler”
In this report we have examined the field of active network performance meas-urement. We have presented notions and metrics for measuring the quality of IP networks and we have implemented parts of the upcoming TWAMP standard. The implementation presented in this work has been evaluated by comparing it to three other active measurement tools. The evaluation was performed in a separated net-work environment where the properties latency, loss, reorder and duplication were emulated. Test sessions where traffic was routed over the Internet have also been performed.
The results from the experiments in the closed network environment show that the latency reported by the TWAMP software deviated approximately 0.8 milli-second from the emulated values. The true deviation in each direction can however be computed to about 0.4 millisecond, since the actual link delay of 0.8 millisecond is not taken into account by the network emulator. The relative latency between the different measurement tools differed by less than 0.7 millisecond. The remaining metrics reported by the TWAMP implementation showed a loss deviation of 0.04 percentage points, a duplicate deviation of 0.03 pp and a reorder deviation of 0.56 pp, compared to the emulated reference values. As explained in this report, the net-work emulator uses statistical distributions, and deviations of this small magnitude are difficult to avoid.
The evaluation over the Internet did also include some uncertainties; the band-width at one of the end-points was not solely dedicated to test traffic, nor did this session allow a preset reference value. Multiple measurements were performed and a 95% confidence interval was calculated. With available bandwidth, the variance within the confidence interval was less than 0.2 millisecond for each measurement tool, and no more than 0.5 millisecond among themselves. The variance of the TWAMP implementation was 0.16 respective 0.13 millisecond in each direction.
When traffic was deliberately injected into the network to fill up any available
CHAPTER 5. SUMMARY AND CONCLUSION
bandwidth, the variance increased to approximately 3 seconds. This shows the importance of available bandwidth as the accuracy of the measurement was consid-erably deteriorated. If NTP is used for synchronization, link capacity is even more important, as delayed NTP packets may be a source of error.
The TWAMP implementation presented in this report has been proven useful for measuring network performance metrics. Although it is difficult to determine the exact one-way delay, the time reported by our application under stressed conditions has differed by no more than 6 milliseconds from the reference methods. Under more favourable conditions when bandwidth capacity was available, the measurements within the 95% confidence interval deviated in the sub millisecond range. Along with the high correctness of the measurements in the emulated environment, the TWAMP implementation must be considered successful, and the TWAMP protocol can be established as a competitive alternative for network performance measurements.
34
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Appendix A
Format of Test Packets
This document describes the different types of test packets transmitted by the OWAMP[44] and TWAMP[49] test protocols in unauthenticated mode.
Test Packet from the Session-sender
Figure A.1 shows the format of a test packet transmitted by OWAMP’s and TWAMP’s session-sender. The packet contains a 32 bit sequence number, a 64 bit timestamp, a 16 bit error estimate of the synchronization and an optional packet padding of variable length.
Sequence Number
Timestamp
Error Estimate
0 15 31
Packet Padding
Figure A.1. Test packet transmitted by OWAMP’s and TWAMP’s session-sender.
APPENDIX A. FORMAT OF TEST PACKETS
Test Packet from the Session-reflector
Figure A.2 shows the format of a test packet transmitted by the TWAMP session-reflector. The sender sequence number, the sender timestamp and sender error estimate are copied from the received packet. The receive timestamp represents the time the arriving test packet was received by the reflector, the error estimate is the responder’s estimate of synchronization error, the timestamp indicates when the test packet was reflected back to the session-sender and the sequence number is generated independently from the identifier of the arriving packets. If TWAMP Light is used, the sender sequence number is used as sequence number. This information along with 8 bits indicating the TTL of the arriving packet and an optional packet padding of variable length constitute the test packet. The bits labelled MBZ (must be zero) are set to zero and must be ignored by receivers.
Sequence Number
Timestamp
Error Estimate
0 15 31
MBZ
Receive Timestamp
Sender Sequence Number
Sender Timestamp
Sender Error Estimate MBZ Sender
TTL
Packet Padding
Figure A.2. Test packet transmitted by TWAMP’s session-reflector.
40
Appendix B
Internet Test Sessions
This document presents the delay measurements from the Internet test sessions.
1w refers to the delay of the forward path, 2w refers to the delay of the reverse path.
Table B.1. Delay (ms) from Internet test session 1.
TWAMP OWAMP PTAnalyzer Ping
1w 2w 1w 2w 1w 2w RTT
6.30 6.31 6.56 6.29 5.76 6.26 13.08 6.51 6.17 6.73 6.22 6.39 5.94 13.06 6.47 6.28 6.73 6.29 6.46 6.41 13.21 6.11 6.59 6.37 6.56 6.63 5.89 13.11 6.35 6.39 6.62 6.39 6.11 6.16 13.16 6.53 6.27 6.83 6.28 6.41 6.01 13.25 6.56 6.22 6.86 6.19 6.37 6.06 13.23 6.68 6.08 6.94 6.06 6.20 6.48 13.19 6.85 6.27 7.10 6.27 6.02 6.64 13.55 6.76 5.91 7.04 5.87 6.20 6.01 13.11
APPENDIX B. INTERNET TEST SESSIONS
Table B.2. Delay (ms) from Internet test session 2.
TWAMP OWAMP PTAnalyzer Ping
1w 2w 1w 2w 1w 2w RTT
65.46 6.51 65.93 6.66 65.66 6.54 72.60 67.05 6.54 67.68 6.59 70.84 6.22 73.67 68.04 6.25 68.00 6.30 68.84 6.70 76.32 67.02 6.21 67.20 6.27 71.01 6.35 73.83 77.12 6.62 79.79 6.71 76.52 6.41 85.84 66.76 6.48 67.26 6.52 72.10 6.89 75.49 73.85 6.62 74.20 6.66 77.61 6.94 80.23 66.75 6.23 67.31 6.30 68.05 6.57 73.56 67.05 6.51 68.15 6.54 68.69 6.62 73.80 68.07 6.44 68.39 6.47 70.10 6.03 76.30
Table B.3. Delay (ms) from Internet test session 3.
TWAMP OWAMP PTAnalyzer Ping
1w 2w 1w 2w 1w 2w RTT
6.58 58.68 6.83 59.23 6.80 59.01 65.72 6.63 60.35 6.85 60.80 6.22 60.96 66.82 6.62 59.53 6.89 59.99 6.05 59.65 66.43 6.80 61.36 7.00 61.96 5.73 60.12 68.49 6.61 60.74 6.02 60.05 5.87 60.24 67.38 6.65 58.71 6.93 59.21 6.25 59.88 65.88 6.74 58.90 6.97 59.07 6.61 58.64 65.81 6.81 59.27 7.07 59.54 6.07 59.48 66.16 6.77 58.67 7.04 59.30 6.77 60.31 65.94 6.73 60.34 6.92 60.62 6.04 60.54 67.18
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TRITA-CSC-E 2009:038 ISRN-KTH/CSC/E--09/038--SE ISSN-1653-5715