An Investigation into Transport Protocols and Data Transport ...

9.1. Transfer Tests Across Dedicated Private Wide Area Networks 209
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moderate_cwnd()
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moderate_cwnd()
moderate_cwnd() (number)
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100
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moderate_cwnd() (number)
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80
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0
10 12 14 16 18 20
Time (seconds)
(a) 200Mbit/sec CBR Background Traffic
0
10 12 14 16 18 20
Time (seconds)
(b) 600Mbit/sec CBR Background Traffic
Figure 9.15: Number of instance calls of moderate cwnd() of HSTCP with various
CBR Background Traffic Loads on MB-NG.
Figure 9.15 shows the correlation of these calls with HSTCP and 200Mbit/sec
background traffic. When there is only 600Mbit/sec background traffic it was
observed that there is a negligible number of calls of moderate_cwnd() which
in turn results in clean cwnd traces. Similar results were gathered from the
other algorithms.
moderate_cwnd(), when called, sets the current cwnd value to that of
the number of packets in flight. Therefore, under normal operation, cwnd
should not be affected (much) when this function is called as the process
of ack clocking ensures that the number of packets in flight is the same as
the cwnd value [Jac88]. However, when ack clocking is broken, these two
values can stray such that the cwnd value is larger than that of the number
of packets in flight.
This is most noticeable under congestion where ack clocking efficiency
is reduced.
Under such circumstances, the ‘dubious’ conditions whereby
moderate_cwnd() is called, are prevalent. A dubious condition, as defined
by the code of the Linux kernel, is when an ack is received by the sender
which acknowledges more than 3 data packets. Should the TCP receiver not