An Investigation into Transport Protocols and Data Transport ...

8.1. Methodology 146
where all network factors such as bandwidth, end-to-end latency and background
loads could be controlled and altered.
This investigation focuses upon the real-life implementation of these algorithms
and as such relies upon real hardware and software on which the
tests are performed. Careful selection of network conditions were selected
such that hardware limitation, such as the lack CPU horse power, did not
dominated the results.
8.1.1 Dummynet
All tests in this Chapter were performed in simulated environments in order
to better understand the performance of these protocols using real equipment.
Experiments were conducted on a dummynet [Riz98] testbed in order
to present results that should be indicative of real-world performance without
the implications on the necessary analysis of real network cross-traffic.
Dummynet provides the facility to emulate the effects of network queues,
bandwidth limitations and communication delays and also enables the repetition
of tests under controllable conditions.
Dummynet utilises the FreeBSD [UCB] IP firewall, ipfw [AKN + ], to allow
the selection of IP packets based on a combination of source and destination
addresses, ports and protocols types (TCP, UDP, ICMP, etc), interface, and
direction (in or out). This selection allows the packets arriving at a dummynet
router to be forwarded onto dummynet pipes which simulate the effects
of a network link.
The topology of the testbed is shown in Figure 7.1 with further details
in Figure C.1. It consists of a simple dumb-bell topology with two pairs of
high-end commodity PCs connected to gigabit ethernet switches which are