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An Investigation into Transport Protocols and Data Transport ...

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8.5. Summary 189<br />

The importance of analysing the performance of each New-TCP with<br />

more than just a single metric was demonstrated. Whilst ScalableTCP <strong>and</strong><br />

BicTCP show the highest goodput of all of the New-TCP algorithms, they<br />

have problems with slow convergence which results in short term unfairness<br />

between competing flows.<br />

Also, whilst FAST appears to have problems competing with another<br />

FAST flow, it shows benefits in short term deployability with its reactive approach<br />

to congestion by ensuring high levels of friendliness with a competing<br />

St<strong>and</strong>ard TCP flow.<br />

One problem which is exhibited by all New-TCP algorithms, except for<br />

H-TCP, is RTT unfairness. At lower capacities, the level of unfairness is<br />

much lower. However, at much higher capacities the increased aggressiveness<br />

of most of the algorithms means that lower latency flows are much more<br />

aggressive <strong>and</strong> therefore cause almost complete lock-out of the longer latency<br />

flow.<br />

H-TCP also has the advantage of fast convergence between competing<br />

flows, exhibiting almost constant convergence times at longer latencies. However,<br />

H-TCP also has problems with increased overhead with long latencies<br />

due to its time based approach to congestion control.

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