An Investigation into Transport Protocols and Data Transport ...

6.1. Survey of New-TCP Algorithms 122
cwnd and ssthresh (packets)
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(a) Congestion Window
(b) AIMD Parameters
Figure 6.4: cwnd dynamic of BicTCP (Single flow on Dummynet, link capacity
200Mbit/sec, RTT 150ms, queue size 500 packets.)
Figure 6.4 shows the binary search and increase algorithm of BicTCP.
It demonstrates the aggressive probing with the additive increase algorithm
followed by the almost zero increase in cwnd as cwnd approaches w target .
There is then consecutive slow start which induces loss and a subsequent
aggressive additive increase is initiated to equalise cwnd to the value before
loss. As a second probing does not facilitate the discovery of a new w target ,
w target is calculated as the mid-point between the minimum and maximum
cwnd’s during the second probe and the cycle begins again. At the start of
the connection, successful slow start attempts to find a new w target can be
seen where the slow start cycle is initiated until loss is detected.
6.1.5 FAST
TCP Vegas [BP95] uses end-to-end delay as a signal to adapt a source’s
sending rate. Increased end-to-end delay indicates congestion, leading to a
reduction in cwnd; decreased delay is associated with less congestion and
leads to an increase in cwnd. The result is a congestion control which attempts
to avoid packet loss and has bandwidth allocation properties that