An Investigation into Transport Protocols and Data Transport ...

9.2. Preferential Flow Handling Using DiffServ 220
1.05
1
0.95
3
2.5
Standard TCP
HSTCP
ScalableTCP
QEF AF
0.9
0.85
QEF BE
2
1.5
0.8
0.75 Standard TCP
HSTCP
ScalableTCP
0.7
0 20 40 60 80 100
AF Bandwidth Allocation (%)
1
0.5
0 20 40 60 80 100
AF Bandwidth Allocation (%)
(a) TCP (AF)
(b) UDP (BE)
Figure 9.20: QoS Efficiency Factors for various AF:BE allocations.
demonstrated by the over utilisation of the BE class’s traffic when the AF
class is under-provisioned due to the failure of the New-TCP flows in being
able to achieve 100% utilisation.
9.2.3 Discussion
Given the dynamics of cwnd, it would be expected that the performance of
Standard TCP should be poor under high bandwidth, long latency environments.
However, Figure 9.20 clearly indicates that HSTCP and ScalableTCP
appear to have problems under DiffServ environments - not Standard TCP.
As a DiffServ enabled network should provide protection for AF flows, the
goodput performance of New-TCP flows were expected to be greater.
At small AF allocations, i.e. small available bandwidths, it is expected
that the performance differences between algorithms are negligible due to
the sizes of cwnd. However, as the bandwidth, and hence cwnd increases due
to a larger AF provision, one would expect HSTCP and ScalableTCP to be
able to utilise the extra capacity more effectively.
Figure 9.21 shows the cwnd and goodput trace of Standard TCP with
90% AF, 10% BE allocation. Apart from an initial burst of traffic which