An Investigation into Transport Protocols and Data Transport ...

8.3. Results 167
1200
1000
HTCP 1
HTCP 2
8000
7000
HTCP 1
HTCP 2
6000
cwnd (packets)
800
600
400
cwnd (packets)
5000
4000
3000
2000
200
1000
0
50 100 150 200 250 300 350 400
Time (seconds)
(a) 42msec
0
50 100 150 200 250 300 350 400
Time (seconds)
(b) 324msec
Figure 8.20: H-TCP cwnd dynamics (Symmetric network, 250Mbit/sec, queuesize
20% BDP).
These tests correspond to network buffer sizes of 27, 70 and 342 packets receptively
which correspond approximately to the sum of α for both competing
FAST flows respectively.
At high latencies, H-TCP increases its overhead by almost an order of
magnitude; however at low latencies the results are comparable to those of
Standard TCP. A zoomed figure of the cwnd trace of H-TCP is shown in Figure
8.20 and shows the complex synchronisation between the two competing
flows.
As expected ScalableTCP has the highest overhead due to its very aggressive
exponential algorithm (with the exception of FAST at low speeds).
Although it should be noted that at 10Mbit/sec and 100Mbit/sec with very
long latencies, H-TCP was observed to have a higher overhead than ScalableTCP.
Convergence Time
Figure 8.21 shows the convergence times between the two symmetric New-
TCP flows. The small decrease of cwnd upon loss in ScalableTCP results in a