An Investigation into Transport Protocols and Data Transport ...

9.2. Preferential Flow Handling Using DiffServ 227
cwnd and ssthresh (packets)
30000
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15000
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cwnd
ssthresh
Throughput (mbit/sec)
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UDP BE
TCP AF
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0 50 100 150 200 250 300
Time (seconds)
(a) CWND
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Time (seconds)
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Figure 9.26: Effect of a gentle WRED upon ScalableTCP with 90% AF allocation.
Congestion moderation was kept disabled during the test to simplify the
interpretation of the SACK processing results.
Results
The improvement of cwnd dynamic when WRED2 is employed is dramatic
as shown in Figure 9.26 which demonstrates that the cwnd dynamic of ScalableTCP
no longer exhibits the large drop of cwnd values that result in low
goodput performance as seen without WRED.
This result validates that gently redistributing the loss pattern in order to
avoid the large flux of SACK Blocks is beneficial to the goodput performance
of TCP flow at high speeds. This was necessitated from the processing burden
of large bursts of SACKs which cripple the sending host.
9.2.6 Summary
This section has shown that the performance penalties due to software and
hardware limitations on the transport of TCP data across large bandwidth
delay networks can be alleviated with both Linux TCP stack modifications
and hardware intervention from network nodes.