An Investigation into Transport Protocols and Data Transport ...

8.4. Discussion of Results 184
sive probing of FAST causes severe unfairness against Standard TCP and
other FAST flows. However, with sufficiently provisioned buffers, sized to
the sum of α value(s) of the FAST flow(s), long term fairness is optimal,
even when competing against Standard TCP. The term optimal is used as
it is not aggressive against the competing flow, but facilitates the usage of
all of the spare bandwidth on the link. It is therefore optimal in the sense
that it attempts to not induce loss on the competing flow and hence enables
the perturbative flow to achieve as much goodput as it can without
inducing losses on the network path that could result in unfairness. This
is especially important in high-speed, high-delay networks as there is plenty
of spare bandwidth, and FAST is capable of using the throughput without
being too aggressive.
H-TCP also shows high friendliness at low latencies, however becomes
less friendly with high latencies due to the increased congestion epoch times.
More revealing are the asymmetric tests which show that all algorithms
exhibit lower fairness than that of Standard TCP with the exception for
H-TCP. The implication of RTT unfairness upon all of the loss-based algorithms
is that short latency flow almost completely prevents the long latency
flow from achieving any throughput. The way in which fairness scales is of
vital importance as higher network capacities (and BDPs) result in a greater
degrees of unfairness between flows of different latencies as the high throughput
flow becomes more aggressive. This is most evident with ScalableTCP
and BicTCP which almost consistently prevent another flow from attaining
sufficiently large values of cwnd to enable fairness.
More interestingly, whilst ScalableTCP and BicTCP perform the best
with regards to goodput performance, they are the worst performers under
the fairness metrics.