An Investigation into Transport Protocols and Data Transport ...

9.2. Preferential Flow Handling Using DiffServ 217
a physically different router output queue. Each queue is then assigned a
minimum guaranteed bandwidth using weighted round robin.
Background CBR UDP traffic was injected into the network using iperf
[TQD + 03] and marked to be treated as Best Effort traffic. A subsequent time
after, a single flow of New-TCP traffic is started using iperf and marked to
the AF class to give higher priority over the BE background traffic.
Various allocations percentages of the AF and BE classes were investigated
and the cwnd dynamic and goodput performances were measured.
Metrics
As DiffServ is only active when there is congestion on the network such that
the traffic breaches the defined DiffServ handling, it is important to measure
how effectively the bandwidth allocations work. This is especially important
for TCP traffic as the congestion avoidance algorithms of TCP cause a lower
utilisation of available resources.
This utilisation, stability and class protection of both the background
BE traffic and the AF TCP flow can be quantified through the computation
of two parameters which are defined as the BE QoS efficiency factor
(QEF BE ) and the AF QoS efficiency factor (QEF AF ) for the BE and AF
class, respectively.
QEF BE = B UDP
B BE
(9.2)
QEF AF = B T CP
B AF
(9.3)
Where, B UDP and B T CP are the achieved (measured) goodputs of the
UDP and TCP flows respectively with allocated bandwidth allocations of
B BE and B AF for the corresponding BE and AF classes. Therefore, QEF s