An Investigation into Transport Protocols and Data Transport ...

8.4. Discussion of Results 186
order of hundreds of seconds for a symmetric latency of 162ms. In HSTCP’s
case, even though the decrease factor is not static, but dependent upon the
cwnd value, a high capacity flow has smaller a back-off per congestion, and
therefore many more congestion epochs are required for the flows to converge
to fairness. Meanwhile, HSTCP flows with small cwnds experience large
decreases upon congestion, and therefore are much more likely to take a
longer time under periodic losses to reach a large value of cwnd. [NY04]
specifies an adaptation to the back-off times of HSTCP such that the decrease
factor is the same as that of Standard TCP (β = 0.5) under a detected
downward trend of cwnd at consecutive congestion events.
Slow convergence is also evident with BicTCP, where even though the
decrease parameter is static, the time between congestion epochs is dependent
upon the increase parameter. Under low BDP’s, BicTCP reverts to an almost
‘additive increase only’ regime which aids short congestion epoch times and
therefore improves convergence times. However, as larger cwnd values are
reached by BicTCP, the result of this plateau is that the congestion epoch
time is larger, and as such the convergence time increases.
FAST, unlike all of the loss-based algorithms, always results in the perturbing
flow achieving larger cwnd values upon start-up than that of the
existing flow. As a result, the convergence times for FAST is very small -
however, this does not include the time required to actually reach stability
between competing flows.
H-TCP takes the approach whereby under highly variable environments,
the back-off factor is reduced to that of Standard TCP. Also, as H-TCP’s
α is polynomial in time, the period between congestion epochs is always
small. The combination of these two factors results in a fast convergence
between competing H-TCP flows where for long latency environments, the