An Investigation into Transport Protocols and Data Transport ...

8.2. Test Calibration 150
In order to obtain a good representation of the range of performance
metrics, all tests were repeated 5 times and the arithmetic mean taken. The
error on each measurement was taken as the standard error from this mean
[Boa83] and is presented in all plots.
Furthermore, due to the prevalence of FIFO queuing mechanisms in use on
the Internet, the study is limited to drop-tail environments to gauge the worse
case scenario whereby network feedback is limited only by buffer overflows.
8.2 Test Calibration
8.2.1 Response Function
The validation of the altAIMD algorithm and the representative New-TCP
algorithms is shown in Figure 8.1. It shows the theoretical and measured
response function of a single New-TCP flow (See Section 5.4.2) under the
dummynet test network. The loss rate was controlled by the dummynet p
variable that defines the average random loss rate experienced through a
dummynet pipe.
It can be seen that the measured results match very well to the theoretical
predictions as specified by the respective New-TCP authors. The most
notable deviation is that for both ScalableTCP and HSTCP, where there
is a gradual, rather than immediate switch from their low speed modes to
their high speed modes; this is represented by their increased goodputs over
the regions of the switch. This was found to be due to the mode switching
from Standard TCP into their representative high speed modes (this does not
apply to FAST or H-TCP) as cwnd values approach their relevant thresholds.
For HSTCP (Figure 8.2(a)), it was observed that the actual average cwnd