An Investigation into Transport Protocols and Data Transport ...

8.1. Methodology 147
fed into the dummynet router via two separate NIC’s (Intel e1000’s). The
dummynet router is of a similar hardware specification to the testbed PCs
(See Table A.5), but runs FreeBSD 4.8 instead of Linux 2.6.
The dummynet set-up is an idealised network in the sense that all aspects
of the network variables are configurable and is perfect for validating the
performance of New-TCP algorithms against the theoretical performance.
The chosen latency values 1 of 16ms to 162ms give a good indication of
the trans-European to trans-Atlantic transmission times on today’s Internet.
Bottleneck capacities of upto 250Mbit/sec were chosen as hardware limitations
resulted in unrepeatable results at higher capacities. Relatively small
bottleneck link speeds of 10Mbit/sec were used to investigate the algorithmic
switching of various New-TCP algorithms into their ‘high-speed’ modes.
8.1.2 altAIMD Kernel
The various New-TCP algorithms have their independent patches publicly
available. However, certain deficiencies in the Linux networking stack [Lei04]
suggest that testing of these algorithms using their publicly available implementations
would most likely yield the testing of the various Linux performance
patches rather than the actual New-TCP algorithms. As the various
patches either do not address the issues at all, or do so in different ways, it was
necessary to build the congestion control algorithms into a common kernel
for the purpose of this investigation so that only the algorithmic differences
of New-TCP algorithms were comparatively analysed.
This kernel will hereby be referred to as the altAIMD kernel. It is based
on the Linux 2.6.6 kernel and incorporates the following features:
1 The minimal limit of 16ms is imposed by Dummynet as tests performed at lower
latencies exhibited strange unaccountable effects upon throughput.