An Investigation into Transport Protocols and Data Transport ...

8.2. Test Calibration 156
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ScalableTCP 1
ScalableTCP 2
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ScalableTCP 1
ScalableTCP 2
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cwnd (packets)
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cwnd (packets)
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Time (seconds)
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Time (seconds)
(a) Queuesize 10% BDP
(b) Queuesize 80% BDP
Figure 8.7: cwnd trace of ScalableTCP at various queue-size provisions under
symmetric network conditions (100Mbit/sec, 82ms RTT, queue-size as shown).
It is worth noting that even though Equation 5.6 specifies that 100% utilisation
of Standard TCP should only occur at 100% BDP queue provision,
the calculations assume that only one flow exists in the system or that all
flows are perfectly synchronised. As such, the experimental results demonstrate
the lack of synchronisation between competing Standard TCP flows
which increases bottleneck utilisation.
Fairness between two flows of the same New-TCP algorithm across the
range of queue provisions is shown in Figure 8.6(b). It shows the difficulty
in being able to achieve consistent fairness results from the New-TCP algorithms,
with relatively large errors shown. However, the results do show
that all of the algorithms, with the exception of ScalableTCP, are reasonably
fair across the difficult queue-size provisions. The problems of achieving fairness
with ScalableTCP is shown in Figure 8.7 where the long convergence
times required for steady state are clear which (combined with the method
of measuring fairness) contribute to low fairness.
As all algorithms share the same SACK code base, the overhead gives an
indication of aggressiveness of the underlying congestion control algorithm