An Investigation into Transport Protocols and Data Transport ...

9.1. Transfer Tests Across Dedicated Private Wide Area Networks 204
20
15
HSTCP
HTCP
ScalableTCP
Unity
20
15
HSTCP
HTCP
ScalableTCP
Unity
Impact
10
Impact
10
5
5
0
1 10
Number of Standard TCP Flows
0
1 10
Number of Standard TCP Flows
(a) 1 New-TCP Flow
(b) 10 New-TCP Flow
Figure 9.11:
Impact of New-TCP flows on Standard TCP Bulk Traffic on
DataTAG.
tively fairer under the low latency network due to the low cwnd values associated
with the bandwidth delay product of the MB-NG network. H-TCP, on
the other hand, appears to be ‘too fair’ with the competing background traffic,
by actually facilitating a higher goodput of the background traffic. This
was found to be due to the slower cwnd increase of H-TCP while the congestion
epoch time was small combined with the overly aggressive decrease
implementation as a result of cwnd quantisation.
Under DataTAG, it was readily observed that the larger cwnd values associated
with the large bandwidth delay products increased the impact of
the New-TCP flows. Surprisingly, H-TCP appears to be the most aggressive
under single flows. Upon closer inspection of the cwnd traces, it was found
that this was due to the severity of the cwnd decreases due to SACK processing
and moderate_cwnd() which lowered goodput of both ScalableTCP and
HSTCP. H-TCP was aided by the relatively faster convergence times which
facilitate higher goodput, and hence also increased impact.
With a larger number of New-TCP flows, these problems were alleviated
due to smaller bandwidth delay products per flow, and clearly shows the