An Investigation into Transport Protocols and Data Transport ...

6.1. Survey of New-TCP Algorithms 115
3500
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cwnd and ssthresh (packets)
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cwnd
ssthresh
0
0 50 100 150 200 250 300
Time (seconds)
α
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α
β
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Time (seconds)
0.8
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(a) Congestion Window
(b) AIMD Parameters
Figure 6.2: cwnd dynamic of ScalableTCP (Single flow on Dummynet, link capacity
200Mbit/sec, RTT 150ms, queue size 500 packets.)
recommended values of α = 1
100 and β = 7 8
are used.
In order to provide compatibility with Standard TCP, a variable W Switch is
defined to determine the mode switch point from Standard TCP behaviour
to Scalable TCP behaviour.
It is defined in terms of the cwnd, so that
when cwnd ≤ W Switch Standard TCP is invoked. The recommended value of
W Switch = 16 segments is used.
Figure 6.2 shows the cwnd evolution of ScalableTCP as a function of
time. The small decrease of cwnd upon packet loss guarantees high throughput
for individual flows. Meanwhile, the multiplicative increase means that
approximately a constant number of RTTs would be required to reach the
value of cwnd just before congestion.
The scaling property of ScalableTCP means that it implements an exponential
increase in the sending rate when no loss is detected. This kind
of increase, with the associated multiplicative decrease, is known as Multiplicative
Increase Multiplicative Decrease (MIMD). [CJ89] states that MIMD
based algorithms do not converge to fairness in drop-tail environments.