An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 94
1400
cwnd
ssthresh
1400
cwnd
ssthresh
cwnd and ssthresh (packets)
1200
1000
800
600
400
cwnd and ssthresh (packets)
1200
1000
800
600
400
200
200
0
0 5 10 15 20 25 30
Time (seconds)
(a) Normal
0
0 5 10 15 20 25 30
Time (seconds)
(b) DRS
Figure 5.7: Effect on cwnd with Dynamic Right Sizing.
This behaviour is especially important for long-distance, high throughput
environments where the Multiplicative Decrease of TCP upon congestion/loss
causes a halving in throughput. Another benefit of using DRS is
that with appropriate socket buffer sizing (see Section 5.1.1) it also keeps the
amount of memory utilised to the minimum required for each flow.
Figure 5.7 shows the behaviour of the sender cwnd with and without
DRS along the MB-NG testbed (See Appendix C.2) with periodic loss on the
forward path. MB-NG has a RTT of 6.2ms with 1Gb/sec connectivity. As
such, the Bandwidth Delay Product (BDP - Equation 4.1) is approximately
535 packets.
Figure 5.7(a) shows the effect of queueing resulting in the curved cwnd
graph as the router queues begin to fill 2 . The initial plateau of cwnd in
Figure 5.7(a) is caused by socket buffer limitations at the sender which enable
flow control by capping the maximum value of cwnd. Figure 5.7(b) shows the
same network and dropping probability, but this time with DRS enabled at
the receiver. It can be clearly seen that the cwnd value is capped to that of
2 Note that in this case, the actual loss is imposed by the receiver selectively dropping
packets rather than the network.