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Table 1: Hybrid Topology Transfer Time (minutes).
Algorithm
1
Download
1
Upload
Crossed 2 Downloads 2 Uploads
Down Up Down#1 Down#2 Up#1 Up#2
BIC 12.15 6.0 17.0 16.0 10.0 10.0 11.0 10.30
New Reno 7.0 6.40 12.0 7.0 11.0 10.0 10.0 10.0
Vegas 10.0 5.10 13.10 5.20 13.30 14.10 10.0 9.30
Westwood+ 11.30 12.10 47.0 16.0 67.30 68.0 15.10 14.50
H-TCP 8.30 5.45 13.0 6.10 10.0 9.30 10.10 10.30
focusing on the bandwidth percentage they manage
to exploit.
Therefore, it is evident that, regarding the data
transferring scenario, in the crossed data flow
scenario Westwood+ algorithm exhibited the poorest
bandwidth utilization as both flows captured only
approximately 1Mbyte/sec and completed the
transfer on 47 min and 16 min for download and
upload respectively. On the other hand Vegas and
Reno delivered a considerably higher 3Mbyte/sec
aggregated throughout, followed by HTCP
2,5Mbyte/sec and BIC recording 1,2Mbyte/sec.
Valuable information can be extracted observing
that after the completion of each upload, on the
crossed scenario, the corresponding download flow
probes the channel in order to occupy higher
percentage of the available bandwidth. However,
Westwood+ was unable to take advantage of the
absence of the competing flow as it managed to
increase its throughput by only 200Kbyte/sec in the
remaining 31 min whereas H-TCP and Vegas
improved their throughput by 800Kbyte/sec in 7min
and 8min till session termination respectively, Reno
by 600Kbyte/sec in 5min and BIC by 200Kbyte/sec
in 1min.
The clear performance degradation of the
network when using Westwood+ algorithm, as
opposed to the rest of the considered TCP
algorithms when operating on the hybrid topology,
is represented by high required transfer periods and
low measured throughput for all the transport
scenarios. This fact raised some concerns about the
effectiveness of its bandwidth estimation
mechanism. Especially on the crossed scenario
where data and ACKs flow in the same direction, the
result was very high RTT values leading to even
lower bandwidth estimation. Therefore, based on
understanding the theoretical approach of
Westwood+ TCP algorithm and presented
measurements, in the following section
modifications in key parameters and respective
performance evaluation are presented towards
improving Westwood+ performance in the specific
scenarios.
5 PROPOSED MODIFICATIONS
AND EVALUATION
The proposed Westwood+ algorithm modifications
focus on the respective bandwidth estimation
mechanism. Two parameters of the Linux
implementation are going to be modified each one
toward a different goal. The main objectives are,
tackling poor bandwidth estimation due to high
measured RTT values, and slow response of the
algorithm following the absence of the competing
data flow. Both parameters are modified in the
“tcp_westwood.c” code file of the Linux kernel.
The first modification concerns the value of the
constant parameter
TCP_WESTWOOD_RTT_MIN. The
initial value of this parameter is 50msec while the
value considered in experiments presented in this
section is 40msec. The observation triggering this
modification was the comparative analysis of the
average measured value of RTT for Westwood+ as
opposed to the rest algorithms on the hybrid
topology “Fig. 7”. This parameter is used by the
function
westwood_update_window in order to
decide whether the
westwood_filter function that
evaluates the bandwidth samples should be called.
Our goal is to reduce the values of the measured
RTT samples so as the algorithm to achieve better
utilization of the channel’s bandwidth.
The second modification aims to affect the
sensitivity of the Low pass filter (Grieco, 2004),
implemented by the function
westwood_do_filter
using constant coefficients. We are going to present
experiments configured to two different values for
the factor C=7 that is used in the function. The
objective of these changes is to make the algorithm
more aggressive when it comes to probing for
available bandwidth.
PERFORMANCE EVALUATION OF TCP ALGORITHMS ON HYBRID WIRED/WIRELESS LAN TEST-BED
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