An Investigation into Transport Protocols and Data Transport ...

6.1. Survey of New-TCP Algorithms 113
additive increase and multiplicative decrease parameters which change according
to the current value of cwnd. These translate to new additive increase
and multiplicative decrease parameters α and β as such:
ACK: cwnd ← cwnd + α(cwnd)
cwnd
LOSS: cwnd ← β(cwnd)
cwnd
(6.1)
(6.2)
where α(cwnd) is a function of cwnd and α = 1 when cwnd ≤ W Switch
and α > 1 when cwnd > W Switch . β(cwnd) is also a function of cwnd and
β = 0.5 1 when cwnd < W Switch and α > 1 when cwnd ≥ W Switch with
0.5 ≤ β < 1:
α(w) = W High 2 × P × 2 × (1 − β(w))
(6.3)
2 − (1 − β(w))
β(w) = 0.5 − (H decr − 0.5) × (log(w) − log(W Switch ))
log(W High ) − log(W Switch )
(6.4)
The recommended settings of W Switch , W High , P and H decr are 38, 83,000,
10 −7 and 0.1 respectively. The use of these settings for HighSpeed TCP will
be henceforth be referred to as HSTCP.
Figure 6.1 shows a typical cwnd trace of a single flow of HSTCP. Due
to the compute cost of calculating a continuous function for the values of
α(cwnd) and β(cwnd), it is suggested that implementations should use a
look-up table to determine the appropriate values [Flo03]. The dynamic
change of the AIMD parameters is shown in Figure 6.1(b) - it clearly shows
the adaptation of the increase and decrease parameters to make the HSTCP
algorithm more aggressive as the value of cwnd increases.
1 Traditionally, represented as cwnd ← cwnd − b(cwnd)
cwnd
. However, for consistent comparision
against other New-TCP algorithms, define β(cwnd) = 1 − b(cwnd).