An Investigation into Transport Protocols and Data Transport ...

10.3. Future Directions 254
mial increase of cwnd facilitates fast utilisation of network resources, whilst
the adaptive backoff and resetting of β to half upon consecutive drops in
throughput due to competing traffic greatly aids fast convergence and fairness.
However, the polynomial increases also results in a large overhead -
especially at long latencies (unsurprising as the polynomial increase is time
based). As such, some tuning of this parameter and matching to real networks
will be beneficial.
The delayed-based FAST algorithm also demonstrates similar attributes
to H-TCP. However, the major disadvantage of such delayed-based algorithms
is the requirement of network buffering to be proportional to the number
of FAST flows sharing the bottleneck link. As demonstrated throughput
Chapter 8, when the network buffers are small, FAST has some very undesirable
throughput, fairness and friendliness performances. Therefore, further
research into the choice of γ is required. Also a mechanism whereby FAST
does not cause these large fluctuations in goodput when there is insufficient
buffering should be developed.
Several other TCP congestion control algorithms have been proposed but
were chosen to either not be within the scope of the research or were not
incorporated due to time constraints.
• TCP Limited Slow-Start [Flo04] is an adaptation of the slow start algorithm
to prevent large numbers of retransmits due to the exponential
growth of cwnd by switching to an aggressive linear increase of cwnd
when cwnd is larger than a threshold value.
• TCP Westwood [CGM + 02, GM, WVS + 02] utilises RTT to determine
the appropriate bandwidth delay product in order to determine the
appropriate cwnd size for the TCP flow. Unlike FAST, it still incor-