An Investigation into Transport Protocols and Data Transport ...

8.2. Test Calibration 153
cwnd and ssthresh (packets)
700
600
500
400
300
200
100
cwnd
ssthresh
cwnd and ssthresh (packets)
3000
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1500
1000
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cwnd
ssthresh
0
0 100 200 300 400 500 600
Time (seconds)
(a) Standard TCP at p = 0.00002
0
0 100 200 300 400 500 600
Time (seconds)
(b) Standard TCP at p = 10 −7
Figure 8.4: Standard TCP cwnd dynamic (250Mbit/sec, 162ms RTT, queue-size
20% BDP).
of H-TCP at 0.2% loss rate. It was observed that there are periods whereby
the cwnd reaches (relative to the test) very high values; where it takes a few
congestion events before the cwnd decreases to a more ‘stable’ value. The
large values of cwnd are compounded by H-TCP’s increase parameter which
is polynomial with respect to time. As a direct consequence of the large cwnd
values, H-TCP is able to achieve high goodput, and the longer the congestion
epochs, the greater the achievable value of cwnd. Whilst the authors of
H-TCP do not provide the calculation of H-TCP’s response function, they
claim that it is comparable to that of HSTCP [LS04a].
As Standard TCP approaches low loss rates, higher values of cwnd are
observed; however, so too is the very slow growth of AIMD. This is observed
in Figure 8.4(b) where the slow increase and the dramatic decrease of cwnd
results in the low utilisation of the link over time-scales less than a single
congestion epoch, after detected loss.
Also, it is this slow growth which
prevents Standard TCP from reaching high values of cwnd under a specific
loss rate compared to the other New-TCP algorithms.
BicTCP argues that the correct response function to balance between