An Investigation into Transport Protocols and Data Transport ...

6.1. Survey of New-TCP Algorithms 121
⎧
B
(δ ≤ 1, cwnd < w
σ target )or(w target ≤ cwnd < w target + B)
⎪⎨ δ 1 < δ ≤ S max , cwnd < w target
α(δ, cwnd) =
(6.17)
w target
B ≤ cwnd − w
B−1 target < S max (B − 1)
⎪⎩ otherwise
S max
where the parameter B is set to 4.
If BicTCP is successful in maintaining a larger cwnd than w target (as after
a while the difference between w target and cwnd will be larger than S max if
no losses are experienced), BicTCP initiates slow-start to probe for new parameters
to maximise throughput utilisation. During this timed regime, the
pre-calculated value of w target may be wrong, and it is therefore recalculated
as the maximum cwnd value at the time of packet loss. Therefore, upon the
next iteration, cwnd will rise quickly to the new value of w target at the point
of loss.
[XHR04] argue that as the design specifies a logarithmic increase in cwnd
as it approaches the targeting window sizes, the need to retransmit packets
due to losses from aggressive probing is reduced.
Similar to the regimes utilised by HSTCP and ScalableTCP, BicTCP
imposes fairness with Standard TCP by utilising the standard AIMD parameters
when cwnd is less than 14 packets.
Many varying versions of the BicTCP implementation exist [XHR04,
LL04] where β = 7 or 0.8 and other features such as ‘Low Utilisation Detection’
whereby upon detection of low network utilisation, a more aggressive
8
algorithm is implemented to increase utilisation. The version used throughout
this study is based on version 1.1 of the official BicTCP implementation
and uses a value of β = 0.8 with Low Utilisation Detection.