An Investigation into Transport Protocols and Data Transport ...

8.1. Methodology 149
Various other constraints on TCP network performance were also removed,
such as the default low limits on the TCP socket buffer sizes. txqueuelen
and max_backlog values were left at acceptable default values (under Linux
2.6) of 1, 000 and 300 respectively. Default device driver settings were also
used which were found to be sufficiently provisioned for these tests.
8.1.3 Overview
In order to minimise the effects of local hosts’ queues and flow interactions,
only a single TCP flow was injected from each source machine into the testbed
through the dummynet router using iperf [TQD + 03] . All flows were run
with slow-start enabled upon the start of each transfer to give a representative
test of real users competing for bandwidth when conducting bulk transport
along the network path.
A second flow was initiated at a random time after the first flow such
that the perturbation of the second flow’s slow start covered a range of times
within a congestion epoch of the first flow. This was to ensure that a representative
range of convergence times and cwnd dynamics was captured.
As defined in [Cla82b], all TCP receivers were run with delayed acking on.
As Linux 2.6 kernels were used, the effects of quick-acking (See Section 5.2.4)
were left enabled. All TCP senders were configured with Appropriate Byte
Sizing (See Section 5.2.4).
Each individual test was run for at least ten minutes. This duration gave
a good representation of the number of congestion epochs required for all
of the New-TCP algorithms. In the case of tests involving Standard TCP,
individual tests were run for up to an hour as the long epoch time of Standard
TCP, especially at large BDPs, requires more time to reach steady state.