MEASUREMENTS OF TCPW ABSE FAIRNESS AND
FRIENDLINESS
Ilhem Lengliz, Haifa Touati, Fehmi Sanàa, Farouk Kamoun
RAMSIS/CRISTAL Laboratory, National School of Computer Science
Complexe Universitaire La Manouba, La Manouba 2010, Tunisia
Medy Yahia Sanadidi
UCLA, Computer Science Department
4732 Boelter Hall, Los Angeles, CA 90095-1596
Keywords: TCP Westwood ABSE, Internet, congestion control, experimentation.
Abstract: TCP Westwood (TCPW), is a TCP protocol with a sender-side modification of the window congestion
control scheme. This protocol is intended to act in packet lossy environments. It relies on a continuous
estimation, by the traffic source, of the connection packet rate based on the ACK reception rate. And this in
order to compute the congestion window and the slow start threshold settings after a congestion episode.
Given that it has been yet established through experimental and simulation studies that TCPW exhibits
significant improvements in throughput performance over Reno in various environments, we are focusing in
this paper on TCPW performance measurements with respect to throughput, fairness and friendliness
towards TCP New Reno in a wired LAN and in the Internet. Which constitutes a proceeding of a set of
measurements achieved on TCPW in similar environment. In this paper we present the results of some
experimentations carried out in the CRSITAL Laboratory with a FreeBSD TCPW ABSE protocol
implementation.
1 INTRODUCTION
TCP Westwood has been initially designed in
(Casetti, 2000). This new protocol relies on a simple
modification of the TCP source protocol behavior
for a faster recovery. This latter mechanism consists
in choosing both a slow start threshold and a
congestion window that result from the effective
connection while congestion is experienced. Hence,
TCPW attempts to make a more “informed”
decision, in contrast with TCP Reno, which
automatically halves the congestion window after
three duplicate ACKs.
Like TCP Reno, TCPW cannot distinguish between
buffer overflow loss and random loss. However, in
presence of random loss, TCP Reno overreacts and
reduces the window by half. Whereas, TCPW after
packet loss and retransmission timeout, resumes
with the previous window as long as the bottleneck
is not yet saturated (i.e., there is no buffer overflow).
To prevent the unnecessary window reduction of
TCP Reno in case of random packet loss, and more
precisely the loss caused when there is wireless
links, several schemes have been proposed
(Balakrishnan, 1997). All of these schemes require
the cooperation of intermediate routers/proxies.
However, TCPW does not require any specific
intervention/support from intermediate routers, thus
preserving the original “end to end design” principle
(Gerla, 2001).
One of the TCPW refinements is proposed in
(Wang, 2002), and named TCPW RE (Rate
Estimation) to address TCP Reno Friendliness. The
other is TCPW+, described and studied in (Ferorelli,
2002), it is intended to improve TCPW performance
in the case of Internet transmissions. The TCPW
ABSE protocol we used in our work to carry out the
experimentations has been proposed in (Casetti,
2002). It palliates TCP efficiency degradation in
packet loss environment.
175
Lengliz I., Touati H., Sanàa F., Kamoun F. and Yahia Sanadidi M. (2004).
MEASUREMENTS OF TCPW ABSE FAIRNESS AND FRIENDLINESS.
In Proceedings of the First International Conference on E-Business and Telecommunication Networks, pages 175-182
DOI: 10.5220/0001398801750182
Copyright
c
 SciTePress

In this paper we present the results of preliminary
experimental measurements on TCPW performances
when used in a LAN or in the Internet.
The remaining of this paper is organized as follows.
In Section 2, we describe the TCPW ABSE protocol.
In Section 3, we present the measurement results
when TCPW is applied in a wired LAN. Section 4
gives a synthesis of TCPW performance
measurements in the Internet. Finally section 5 gives
the conclusion of the paper and draws some
perspectives for this work.
2 TCPW ABSE PROTOCOL
TCPW ABSE (Adaptive Bandwidth Share
Estimation), initially proposed in (Wang, 2002), is a
sender-only modification of TCP NewReno. The
TCP sender Adaptively determines a Bandwidth
Share Estimate. This estimate is based on
information carried in the ACKs, and on the rate at
which the ACKs are received. After a packet loss
indication, which could be due to either congestion
or link errors, the sender uses the estimated
bandwidth to properly set the congestion window
and the slow start threshold. Further details
regarding bandwidth estimation are provided in the
following sections. For now, let us assume that a
sender has determined the connection bandwidth
estimate as mentioned above, and let us describe
how the estimate is used to properly set cwin and
ssthresh after a packet loss indication.
In TCPW, congestion window dynamics during slow
start and congestion avoidance are unchanged; that
is, they increase exponentially and linearly,
respectively, as in current TCP NewReno.
A packet loss is indicated by :
(a)the reception of 3 DUPACKs,
or
(b) a coarse timeout expiration.
In case (a), TCPW sets cwin and ssthresh as follows
:
if (3 DUPACKs are received)
ssthresh = (ABSE*RTTmin)/seg_size;
if (cwin > ssthresh) /*congestion avoid.*/
cwin = ssthresh;
endif
endif
In case a packet loss is indicated by a timeout
expiration, cwin and ssthresh are set as follows:
if (coarse timeout expires)
cwin = 1;
ssthresh = (ABSE * RTTmin)/seg_size;
if (ssthresh < 2)
ssthresh = 2;
endif
endif
The rationale of the algorithm above is that after a
timeout, cwin and the ssthresh are set equal to 1 and
ABSE, respectively. Thus, the basic Reno behavior
is still captured, while a reasonably speedy recovery
is ensured by setting ssthresh to the value of the
ABSE.
2.1 Adaptive Bandwidth Share
Estimation
The ABSE algorithm adapts to the congestion level
in performing its bandwidth sampling, and employs
a filter that adapts to the round trip time and to the
rate of network conditions change. The bandwidth
share estimation is computed using a time varying
coefficient, exponentially-weighted moving average
(EWMA) filter, which has both adaptive gain and
adaptive sampling. Let t
k
be the time instant at which
the k
th
ACK is received at the sender. Let s
k
be the
bandwidth share sample, and
k
s
ˆ
the filtered estimate
of the bandwidth share at time t
k
.. Let α
k
be the time-
varying coefficient at t
k
. The ABSE filter is then
given by :
kk
kk
k
kk
kkkk
t
t
where
ss
ss
∆+
∆−
=
+
−+=
−
−
τ
τ
α
αα
2
2
)
2
)(1(
ˆ
.
ˆ
1
1
(1)
and
k
τ
a filter parameter which determines the filter
gain, and varies over time adapting to path
conditions. In the filter formula above, the
bandwidth sample at time k is :
k
Ttt
j
k
T
d
s
kkj
∑
−>
=
(2)
where d
j
is the number of bytes that have been
reported delivered by the j
th
ACK, and T
k
is an
interval over which the bandwidth sample is
calculated.
2.1.1 ABSE adaptive sampling
ABSE provides an adaptive sampling scheme, in
which the time interval T
k
associated with the k
th
received ACK is appropriately chosen, depending on
the network congestion level. To determine the
network congestion level, a simple throughput filter
is proposed to estimate the recent throughput
achieved. By comparing this estimate with the
instantaneous sending rate obtained from cwin, the
path congestion level is determined.
When the k
th
ACK arrives, a sample of throughput
during the previous RTT is calculated as :
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176

R
TT
d
h
ˆ
T
RTTtt
j
k
kj
∑
−>
=
where d
j
is the amount of data reported by ACK j.
In (Wang, 2002), the value (ε = 0.6) has been
employed for the constant-gain filter to calculate the
recent throughput as :
kkk
ThhThT )1(
ˆˆ
1
εε −+=
−
(3)
When
RTThT
k
*
ˆ
is larger than the current cwin
value, indicating a path without congestion, T
k
is set
to T
min
. Otherwise, T
k
is set to :
cwin
RTThTcwin
RTTT
k
k
)*
ˆ
(
*
min
−
=
(4)
2.1.2 Filter Gain Adaptation
When τ
k
is larger, α
k
will be larger and the filter
tends to be more stable and less agile. After a certain
point, α
k
basically stays unchanged as the value of τ
k
increases. The parameter
τ
k
adapts to network
conditions to dampen estimates when the network
exhibits very unstable behavior, and reacts quickly
to persistent changes. A stability detection filter can
be used to dynamically change the value of
τ
k
. The
network instability U is measured with a time-
constant EWMA filter (Wang, 2002)(Casetti, 2002) :
11
)1(
−−
−−+=
kkkk
ssUU ββ
(5)
In (Ferorelli, 2002), s
k
is the k
th
sample, and β is the
gain of this filter, which is set to be 0.6 in (Wang,
2002). When the network exhibits high instability,
the consecutive observations diverge from each
other, as a result, U
k
increases. Under this condition,
increasing the value of
τ
k
makes the ABSE filter
defined by eq. (1) more stable. When a TCP
connection is operating normally, the interval
between the consecutive acknowledgements are
likely to vary between the smallest the bottleneck
capacity allows, and one RTT. Therefore, τ
k
should
be larger than one RTT, thus τ
min
= RTT. In (Wang,
2002) τ
k
is set to be :
max
*
U
U
RTTNRTT
k
k
+= τ
(6)
The value of RTT can be obtained from the
smoothed RTT estimated in TCP (Wang, 2002).
2.2 Accounting for delayed and
cumulative ACKs in bandwidth
measurement
This issue has been addressed in (Wang, 2002).
DUPACKs should count towards the bandwidth
estimation since their arrival indicates a successfully
received segment, albeit in the wrong order. As a
consequence, a cumulative ACK should only count
as one segment’s worth of data since duplicate
ACKs ought to have been already taken into
account. Further complications result from delayed
ACKs. The standard TCP implementation provides
for an ACK being sent back once every other in-
sequence segment received, or if a 200 ms timeout
expires after the reception of a single segment
(Casetti, 2002). The combination of delayed and
cumulative ACKs can potentially disrupt the
bandwidth estimation process. Therefore, in (Casetti,
2002) two important aspects of the bandwidth
estimation process are stressed :
(a) the source must keep track of the DUPACKs
number it has received before new data is
acknowledged;
(b) the source should be able to detect delayed
ACKs and act accordingly.
The approach chosen to take care of these two issues
can be found in the AckedCount procedure,
detailed below, showing the set of actions to be
undertaken upon the reception of an ACK, for a
correct determination of the number of packets that
should be accounted for by the bandwidth estimation
procedure, indicated by the variable acked in the
pseudocode. The key variable is accounted,
which keeps track of the received DUPACKs.
When an ACK is received, the number of segments
it acknowledges is first determined (cumul_ack).
If it is equal to 0, then the received ACK is clearly a
DUPACK and counts as 1 segment towards the
BWE; the DUPACK count is also updated. If
cumul_ack is larger than 1, the received ACK is
either a delayed ACK or a cumulative ACK
following a retransmission event; in that case, the
number of ACKed segments is to be checked against
the number of segments already accounted for
(accounted_for). If the received ACK
acknowledges fewer or the same number of
segments than expected, it means that the “missing”
segments were already accounted for when
DUPACKs were received, and they should not be
counted twice. If the received ACK acknowledges
more segments than expected, it means that although
part of them were already accounted for by way of
DUPACKs, the rest are cumulatively acknowledged
by the currentACK; therefore, the currentACK
should only count as the cumulatively acknowledged
segments. It should be noted that the last condition
correctly estimates the delayed ACKs.
cumul_ack = 2 and accounted_for = 0):
PROCEDURE AckedCount
cumul_ack=current_ack_seqno-
last_ack_seqno;
if (cumul_ack = 0)
last_ack_seqno = current_ack_seqno;
acked = cumul_ack;
return(acked);
MEASUREMENTS OF TCPW ABSE FAIRNESS AND FRIENDLINESS
177

END PROCEDURE
accounted_for = accounted_for + 1;
cumul_ack = 1;
endif
if (cumul_ack > 1)
if (accounted_for >= cumul_ack)
accounted_for=accounted_for- cumul_ack;
cumul_ack = 1;
else if (accounted_for < cumul_ack)
cumul_ack = cumul_ack - accounted_for;
accounted_for = 0;
endif
endif
In the next two sections we are presenting a
performance evaluation on TCPW in a wired
environment, by mean of experimental
measurements. The first section deals with
measurements on an Ethernet LAN at CRISTAL
Laboratory in the ENSI (Ecole Nationale des
Sciences de l'Informatique). The second section is
concerned with an outdoor experimentations. On one
hand between the ENSI and the CCK (Centre de
Calcul ElKhawarizmi) and on the other hand on a
connection between the ENSI and UCLA (UCLA
Computer Science Department). We focus usually
on the fairness and friendliness of TCPW ABSE in
comparison to TCP New Reno, since the latter is
shown to be fair.
3 TCPW ABSE in a lan
3.1 The LAN environment
As illustrated by the figure 1, this configuration
consists of two traffic sources, the first is a TCPW
ABSE one, whereas the second is a TCP New Reno
one.
Figure 1: The testbed layout
They share a connection to a same destination,
passing through a router. Hence this connection is
the bottleneck. The RTT is of 2.2 ms and the data is
generated via ftp.
3.2 TCPW Rate
From figure 2, it appears readily that TCPW and
TCP New Reno connections share the available
bandwidth in a similar manner. In fact, the measured
values for each of these protocols rate have the same
behavior regarding the number of parallel ftp
sessions.
We consider in particular the point 1, where the
measurement is made for only one connection. If we
compare the TCW rate to that of TCP New Reno,
we can observe that New Reno gives a best rate than
Westwood. Hence, TCPW doesn't offer, in this case,
any gain in the rate compared to TCP New Reno.
This is due to the fact that the LAN doesn't induce
any random losses because of congestion states.
Figure 2: Rate per connection versus number of
connection
Besides, the propagation delay is very small, about
2.2 ms. In such a situation, it is preferable not to use
TCPW, because it isn't a massive packet lossy
environment and the congestion control mechanism
isn't invoked usually. This is very insufficient to get
a considerable gain in the rate.
3.3 TCPW Fairness
TCP New Reno insures fairness in bandwidth share
by nature. In fact, the congestion mechanism
accelerates the convergence to the fair share, using
the dividing of the congestion window. TCPW
ABSE offers a fairness index comparable to those
obtained with TCP New Reno. This index is
obtained using the following equation :
Router
TCPW
TCPN Reno
Destination
10 Mbps
2,2 ms
Rate (bps)
Connection number
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178

∑
∑
=
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
n
i
i
n
i
i
bN
b
index fairness
1
2
2
1
i
b
: rate of the flow i
n : the total number of active connections
The figure 3 exhibits the variation of this index
versus the number of competing connections and
this using the test bed configuration of figure 1. We
can note that the results performed by TCPW are
close to those of TCP New Reno. This consolidates
the results yet established in (Wang, 2002). In the
first test, only one source is active at once, TCPW
ABSE or TCPW. But, a source is generating
multiple ftp sessions toward the destination. The
number of competing connections ranges from 2 to
19. Each measurement is performed twenty once.
Note that each of these TCP protocols has been
tested separately. For example, the measure
corresponding to the label 10 on the x axis stands
for the mean rate of a TCPW or a TCP new Reno
connection in the case of 10 ftp sessions transmitting
simultaneously.
Figure 3: Fairness index versus number of competing
connections
3.4 TCPW ABSE friendliness
towards TCP New Reno
The figure 4 gives the mean rate achieved by each of
the protocols when the number of connections
changes (in this case we have 5 TCP New Reno
connections against 5 TCPW ABSE connections).
Besides, the friendliness towards TCPNReno has
been a goal to reach since the BE version of TCPW.
Figure 4: TCPW ABSE/ New Reno rates
Using the same configuration of figure 1, both of the
sources are initiated simultaneously. The total
number of connections is 10, but for each test we
vary the proportion of TCP connections of each
protocol (i.e. n TCPNReno vs (10-n) TCPW, n=1..9)
in order to study the effect of one protocol on the
other.
Figure 5: New Reno vs. TCPW ABSE
The results plotted on figure 5 show that TCPW
exhibits a friendly behavior towards TCP New
Reno. Thus, the new estimation technique used in
TCPW improves the friendliness of this protocol
towards TCP New Reno. From an other point of
view, we can observe the effect of these protocols
each on other. This figure shows the rate reached by
each of the protocols versus the number of the other
competing flows. As an illustration, the measure 1
shows that the connection TCP New Reno reaches
225.53 Ko/s and this with 9 competing TCPW
connections, whereas a TCPW ABSE connection
reaches only 179.15 Ko/s in the presence of 9 TCP
New Reno connections. All of these results
F
a
irn
ess
in
de
x
R
ate
(bps)
R
a
t
e
(bps)
Number of TCPNReno connections
Test numbe
r
Number of Connections
MEASUREMENTS OF TCPW ABSE FAIRNESS AND FRIENDLINESS
179

consolidates once again the conclusions yet
established by simulations in (Wang, 2002).
4 TCPW ABSE ON THE
INTERNET
4.1 ENSI-CCK Connection
4.1.1 Test scenario
We are comparing here the rate performed by
TCPW to that of TCP New Reno over an Internet
connection between the ENSI and the CCK. The test
platform is shown in figure 6. It consists of two
stations located in CRISTAL Laboratory at the ENSI
and a third station situated in the CCK. TCPW
ABSE is installed on the first station
: cristal1.ensi.rnu.tn, whereas the second :
cristal2.ensi.rnu.tn, supports TCP New Reno. The
test scenario consists in starting two ftp sessions
from one of the two sources at the ENSI to the
destination at the CCK. The files transferred are of
10 Mo. In order to collect the results, we used
tcpdump (www.tcpdump.org), then we analyzed
them by tcptrace (www.tcptrace.org). We repeat
these measurements over two days.
Figure 6: Test platform ENSI-CCK
4.1.2 Results
Figure 7: TCPW ABSE/TCP New Reno rates
ENSI-CCK (first day)
The measurements are plotted on figures 7 and 8 for
the first and the second day respectively. Once
again, these results agree with those established for
TCPW in (Wang, 2002). As one can see, TCPW
ABSE achieves a better rate than TCP New Reno. In
fact, during the first day of experimentation, the
mean TCPW rate is 490.87 kbps whereas that of
TCP NewReno is 119.88 kbps. The second set of
measurements shows a TCPW mean rate of 319.04
kbps, while TCP New Reno achieves only a 61.821
kbps rate.
Figure 8: TCPW ABSE/TCP New Reno rates
ENSI-CCK (second day)
TCPW/TCP New Reno rates (second day)
R
ate
(bps)
Rate (bps)
time (ms)
time (ms)
ENSI - CRISTAL
Router
ensi.rnu.tn
Cristal1.ensi.rnu.tn
Cristal2.ensi.rnu.tn
CCK
TCPW
TCPNReno
Internet
TCPW/TCP New Reno rates (first day)
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180

4.2 ENSI-UCLA Connection
4.2.1 Test scenario
The second experimentation on the TCPW
performance in the Internet has been carried on two
connections between the CRISTAL Laboratory and
the UCLA Computer Science Department. The test
platform is presented in figure 9.
In this configuration, the station at the CRISTAL
Laboratory : Cristal1.ensi.rnu.tn, stands for the
destination, while the two stations in UCLA are the
sources. Indeed, the first station : Orion.cs.ucla.edu,
implements TCPW ABSE and the second :
Mvalla.cs.ucla.edu keeps its default TCP package
i.e. TCP New Reno.
We used iperf data transfer to measure and compare
the rates of both TCPW and TCP New Reno on this
connection. The file is of 4 Mo.
We execute so tcpdump to collect the measures, and
tcptrace for the analysis. The connections TCPW
and TCP New Reno are activated alternatively. We
repeat the test thirty once for each TCP protocol in
different periods of the day.
Figure 9: Test platform ENSI-UCLA
4.2.2 Results
During the tests, we noted that the RTT varies on a
large scale, so we classed all the measurements into
three groups. The results are plotted on figure 10.
Figure 10: TCPW/TCP New Reno rate versus RTT
Once again, these results matche those yet retrieved
in (Wang, 2002), showing that TCPW achieves a
better rate than TCP New Reno on Internet
connections. We can also observe that greater is the
RTT better will be the enhancement given by
TCPW. Hence, for an RTT ranging in (202-267 ms)
this enhancement is about 7,9%, it is about 17,64 %
for an RTT between 292 and 383 ms and it
reaches 20,88% for the last range (391-488 ms).
5 CONCLUSION AND FUTURE
WORK
We have presented an exhaustive study of TCPW
ABSE performance in a LAN environment and in
the Internet. In the first case, we have shown that
this new protocol doesn't perform better than TCP
New Reno as far as the transmission rate is
concerned. This is an expected result, because
TCPW is designed for lossy networks. Nevertheless,
the fairness and friendliness of this protocol have
been once again proved. In the Internet, both of the
test configurations allowed us to verify the rate
enhancement that TCPW achieves compared to TCP
New Reno, especially when the RTT is large.
In a future work, we will be concerned with a large
simulation tests on TCPW ABSE, so that we can
study this protocol on larger RTT scales and loss
rates. Besides, we will investigate the enhancement
of the bandwidth estimation process.
R
a
t
e
(bps)
RTT range (ms)
TCPW/TCP New Reno rates versus RTT
ENSI - CRISTAL
Router
ensi.rnu.tn
Cristal1.ensi.rnu.tn
Cristal2.ensi.rnu.tn
UCLA
TCPW ABSE
Orion.cs.ucla.edu
TCPW New Reno
Mvalla.cs.ucla.edu
Cabernet.cs.ucla.edu
Saruman.cs.ucla.edu
Internet
MEASUREMENTS OF TCPW ABSE FAIRNESS AND FRIENDLINESS
181

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