An Investigation into Transport Protocols and Data Transport ...

9.1. Transfer Tests Across Dedicated Private Wide Area Networks 195
25
ScalableTCP
20
cwnd (packets)
15
10
5
0
15 15.5 16 16.5 17 17.5 18
Time (seconds)
Figure 9.2: Quantisation of cwnd.
latency environment of MB-NG. The exception is the introduction of a slight
glitch just before the mode switch of ScalableTCP at approximately p = 0.02
and the slightly higher goodput of HSTCP at approximately p = 0.001. In
both cases, this was due to the switch between the respective low and high
speed modes, which due to the constant packet loss rate results in larger
values of cwnd and hence high goodput (See Section 8.2.1).
The slightly higher goodputs experienced in the experimental tests compared
to the theory for MB-NG for ScalableTCP and HSTCP are due to
quantisation effects as the Linux kernel stores the value of cwnd as integers 1 .
As the implementation of the decrease calculation of cwnd upon congestion
in Linux is cwnd ← cwnd − b × cwnd, the calculation of cwnd upon loss is
such that the rounding error is less than what it should be and hence the
final value of cwnd is in fact higher.
This is shown for ScalableTCP at 0.02 p in Figure 9.2 where the maximum
value of cwnd is 18 packets. As the back-off b of ScalableTCP is 1/8, the
corresponding value of cwnd upon packet loss should be 15.75; however, due
to the quantisation of cwnd, the actual value used is 16. This results in the
1 In order to determine when to update cwnd, a separate integer counter cwnd cnt
which is increased per valid ack and checked to see if it is larger than cwnd before cwnd
is increased by a whole number. Thus maintaining an increase of 1 cwnd per RTT under
Standard TCP.