An Investigation into Transport Protocols and Data Transport ...

8.2. Test Calibration 154
250
cwnd
ssthresh
2500
cwnd
ssthresh
cwnd and ssthresh (packets)
200
150
100
50
cwnd and ssthresh (packets)
2000
1500
1000
500
0
0 100 200 300 400 500 600
Time (seconds)
0
0 100 200 300 400 500 600
Time (seconds)
(a) BicTCP at p = 0.002
(b) BicTCP at p = 10 −4
Figure 8.5: BicTCP cwnd dynamic (250Mbit/sec, 162ms RTT, queue-size 20%
BDP).
friendliness and scalability lies within a region between HSTCP and ScalableTCP
[XHR04]. Figure 8.5(a) shows the cwnd trace of BicTCP at p =
0.002 and p = 10 −4 . It is visible from Figure 8.5(b) that the aggressive additive
increase enables BicTCP to reach large values of cwnd, whilst the low
decrease parameter of 0.8 maintains the high value of cwnd even under consistent
loss. The plateau of cwnd which BicTCP uses to maximise throughput
is less visible at such (relatively) high loss rates. This is especially true in
the case of p = 0.002 where the cwnd growth after loss undergoes additive
increase.
FAST has been excluded from these results due to its dependence upon
network delay rather than network loss.
8.2.2 Bottleneck Queue Size
As bottleneck queue-sizes are important in being able to accommodate packet
bursts, it is important to provide sufficient memory at ingress and egress
queues. The larger values of the congestion avoidance increase α of New-
TCP algorithms are likely to impose a greater need for larger queue-sizes as