An Investigation into Transport Protocols and Data Transport ...

8.2. Test Calibration 158
25000
20000
ScalableTCP 1
ScalableTCP 2
Retransmitted (bytes)
15000
10000
5000
0
100 150 200 250 300 350 400
Time (seconds)
Figure 8.9: Instantaneous retransmissions of ScalableTCP every 100ms at 80%
queue size provision under symmetric network conditions. (100Mbit/sec, 82ms
RTT).
It can be seen that the cwnd of FAST is flat, suggesting that it is able to
utilise its RTT calculation effectively to settle into a constant cwnd value. As
such, there are almost no losses and retransmissions during the course of the
test. However, upon start-up, FAST’s slow start algorithm (which is different
from standard slow start) induces much loss and hence retransmissions. This
effect is much more pronounced in the small queue size provisions (where
Figure 8.8(d) shows almost no retransmissions) than in the large queue size
provisions, which result in the much lower overhead of FAST with larger
queue sizes.
In contrast, loss based algorithms, such as ScalableTCP as shown in Figure
8.9 have to induce loss continuously for the entire duration of the test.
Figure 8.9 corresponds to the cwnd trace presented in Figure 8.7(b) and
it can be seen that the larger cwnd flow (Scalable 1) experiences greater
loss than that of the second flow. This is due to the aggressive nature of
ScalableTCP’s algorithm with its decrease value β which favours the higher
throughput (and hence larger cwnd) flow. This is common to all loss-based
algorithms, and implies that with large capacities, much more data will have
to be retransmitted in order to maintain congestion avoidance.