An Investigation into Transport Protocols and Data Transport ...

8.4. Discussion of Results 185
New-TCP Convergence Time
Standard TCP 32.8±12.3
BicTCP 45.2±5.1
FAST 4.7±0.2
HSTCP 60.2±3.7
H-TCP 29.6±2.7
ScalableTCP -
Table 8.3: Summary convergence times (seconds) of two competing New-TCP flows
at 250Mbit/sec bottleneck and 82ms RTT under Symmetric network conditions
(20% BDP queue-size).
HSTCP also shows RTT unfairness which is caused by the relatively low
increase parameters and small decrease factors of low cwnd flows. This results
in a flow that is not as aggressive as the low latency flow (which gain large
cwnd values quickly) and therefore incapable of achieving high throughput.
H-TCP, with its RTT Scaling, enables fairer sharing of asymmetric links;
so much so that it is actually fairer than Standard TCP. This therefore
prevents lock-out and will ensure that even very long latency flows will not
be starved of goodput.
8.4.3 Responsiveness/Convergence Time
It can be seen in Figure 8.13 that the convergence times of ScalableTCP are
very slow (it does not converge to fairness as shown in Table 8.3).
Even
though ScalableTCP has a small congestion epoch time, the decrease factor
of 0.875 means that the drop in throughput of the first flow at the moment
of congestion is small, and is therefore slow to react to sudden decreases in
available bandwidth.
This effect is also evident with HSTCP during the time of the test (see
Figure 8.21) and can readily be seen that the convergence time is in the