TCP-PCP: A Transport Control Protocol Based on the Prediction of ...

<strong>TCP</strong>-<strong>PCP</strong>: A <strong>Transport</strong> <strong>Control</strong> <strong>Protocol</strong> based on
the Prediction of Congestion Probability over
Wired/Wireless Hybrid Networks
Jin Ye ∗† , Jianxin Wang ∗ , Liang Rong ∗ , and Weijia Jia ‡
∗ School of Information Science and Engineering, Central South University, ChangSha, China, 410083
† College of Information Science and Engineering, Guilin university of Electronic Technology, Guilin, China, 541004
‡ Department of Computer Science, City University of Hong Kong, China
Email: yejin@gliet.edu.cn, jxwang@mail.csu.edu.cn, csulrong@gmail.com, wjia@cs.cityu.edu.hk
Abstract—Many of packet loss as a result of factors other
than congestion impact the performance of <strong>TCP</strong> in wired/wirelss
hybrid networks. Firstly, this paper proposes one concept of
Congestion Probability (CP) and analyzes the correlation of CP
and network state. Then a <strong>Transport</strong> <strong>Control</strong> <strong>Protocol</strong> named
as <strong>TCP</strong>-<strong>PCP</strong> is proposed, which is based on the Prediction of
Congestion Probability instead of single loss event. Depending
on the ECN mechanism, <strong>TCP</strong>-<strong>PCP</strong> calculates the value of CP
by analyzing some latest loss events. The <strong>TCP</strong>-<strong>PCP</strong> sender’s
behavior is controlled by the change of CP value. Simulation
results show that <strong>TCP</strong>-<strong>PCP</strong> can improve <strong>TCP</strong> performance more
efficiently than Westwood and Jersey.
I. INTRODUCTION
Transmission <strong>Control</strong> <strong>Protocol</strong> (<strong>TCP</strong>) was designed to cope
with packet loss due to network congestion. However, packet
loss may be induced also by wireless links with characteristics
of high bit error rate, low bandwidth, long delay, frequent
mobility and so on. Thus, the assumption that packet loss is
an indicator of network congestion may not adapt to wireless
environment, which results in poor performance of traditional
<strong>TCP</strong> in wireless networks [1] [2].
<strong>TCP</strong> enhancements over wired/wireless hybrid networks
have focused on differentiating packet losses. As a most
significant one, <strong>TCP</strong> Westwood [3] adopts available bandwidth
estimation to discriminate the cause of packet loss. The sender
derives the network bandwidth by exploiting the rate and pattern
of the returning ACK stream. Upon experiencing a packet
loss, the sender adjusts the congestion window dynamically
according to the estimation. This is the key innovative idea in
contrary to <strong>TCP</strong> Reno, which “blindly” halves the congestion
window after three duplicate ACKs.
In Westwood, packet losses are differentiated based on the
interval of ACK arrival. However, correct classification based
on end-to-end information has been found to be difficult. For
example, RTT(Round Trip Time) jitter of wireless networks
is more than 10 times larger than that of wired networks [4],
which leads to little correlation between nature of loss and
observable metric. This is similar as other metrics including
that of Westwood. Subsequently, how to effectively estimate
eligible bandwidth in hybrid networks is still an open issue
[5] [6].
As an enhancement to Westwood, <strong>TCP</strong> Jersey [7] uses
the ECN (Explicit Congestion Notification) signal as a tool
to distinguish between congestion loss and corruption loss.
The sender checks the ACK when there is packet dropping.
The loss event will be identified as congestion induced if
its CE(Congestion Explicit) bit is set 1, otherwise it will be
classified to be corruption induced.
There are two problems that should be discussed:
(1)Many simulations demonstrate that ECN mechanism can
not differentiate packet loss efficiently [8] [9]. Actually the
reason is that ECN is a proactive congestion control and its
feedback is a temporary signal. That is to say, network states
reflected by ECN may change quickly. It is incorrect to trigger
congestion congtrol based on single ECN feedback.
Moreover, the signal of a loss event and the loss reason
become now-obsolete because of the large feedback latency
in hybrid network. Therefore, it is pivotal to make better use
of the information on loss events and their classification.
(2)Even if loss events are classified accurately, how to use
the classification result needs to be considered deeply. By
far, most <strong>TCP</strong> variants do not adjust its sending rate if loss
events are caused by non congestion factors. Actually, such
strategy has been questioned because it may lead to congestion
collapse [10]. Therefore, <strong>TCP</strong> variants should focus on how
to trigger congestion control appropriately other than how to
differentiate packet loss accurately.
In fact, reasons of packet dropping are more complicated in
hybrid networks, including link disconnection, routing change
and handoff, and so on. It is becoming more and more difficult
to shield high layer from non-congestion losses. Therefore,
finding a more precise congestion-aware scheme is the best
way to improve <strong>TCP</strong> performance in hybrid networks.
By and large, the protocols like Westwood and Jersey solve
the problem of congestion control over hybrid network by
decoupling congestion loss and corruption loss in different
ways. However, more aspects should be considered on their
solutions.
978-1-4244-2324-8/08/$25.00 © 2008 IEEE.
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.

In order to mitigate the impact of dynamic metric and large
delay, this paper proposes a more precise congestion-aware
scheme, a protocol named <strong>TCP</strong>-<strong>PCP</strong>, in which ECN feedback
is utilized in a novel method. The sender analyzes the network
state from the latest ECN feedback information, which is
termed as Prediction of Congestion Probability. The result is
used to control the sender’s behavior. Simulation results show
that <strong>TCP</strong>-<strong>PCP</strong> outperforms Westwood and Jersey in hybrid
networks.
The rest of the paper is organized as follows. Section II
presents CP and investigates the correlation between CP
and network state. Section III describes the new protocol
in detail. Section IV discusses and analyzes the simulation
results. Finally, concluding remarks for our work are presented
in Section V.
II. PREDICTION OF CONGESTION PROBABILITY
The key idea of CPECN is to analysis the distribution of
CE bit from ECN feedbacks and predict congestion states by
the correlation between feedback serials.
A. Congestion Probability
ECN signal indicates the imminent congestion instead of
congestion already occurring. Since the state of network
changes dynamically, the congestion loss may occur or not
in the next epoch. In other words, the congestion situation
may either continue, or clear up. Therefore, it is not suitable
for <strong>TCP</strong> sender in response to such uncertain signal directly.
Single ECN feedback only reflects the network state at some
earlier point in time, thus several ECN feedbacks can reflect
the dynamic change of network state at several intervals. So
it is more reasonable to respond to several ECN feedbacks
together in an appropriate way.
In order to characterize serial ECN feedbacks, we introduce
the new conception of CP (Congestion Probability), which
can be calculated as follows while packet loss occurring:
CP[i] = 1
m ∗
m−1
wi ∗ ACK[i].ecnecho (1)
i=0
where ACK[i].ecnecho is the value of CE bit in ACK[i],
wi is the weight of ACK[i] and m−1
i=0 wi = 1, CP[i]
is calculated based on ACK series with m amount before
ACK[i], that is ACK[i], ACK[i − 1], ..., ACK[i − m − 1].
To be concise, ACK[i].ecnecho is expressed as ACK[i] in
this paper.
The sender is always gathering CE bit from ACK and calculating
the current CP value by using EWMA (Exponentially
Weighted Moving Average) predictor shown in Eq.(1). We
divide the m ACKs into several segments and each ACK will
be assigned with a weight. The weight of ACKs in the same
segment are equal, which means that the ACKs in the same
segment have the same impact on the judgement of network
state. By this way, the role of one single ACK is decreased
and that of all ACKs in one segment is increased. So the
judgement of network state is build on the change trends of
ECN feedback.
If the last m ACKs are divided into n segments, then CP[i]
can be calculated as following:
where
CP[i] =
n
j=1
wj ∗ n
m ∗
m
n j−1
i= m
n (j−1)
ACK[i] (2)
n
wj =1 (3)
j=1
Generally, the weight of the new ACK is larger than that of
the old one. So the weight of the ACKs in each segment are
assigned as following:
wj = α ∗ wj−1,α∈ (0, 1) (4)
In traditional <strong>TCP</strong> protocol, the sender enters the slowstart
mode after receiving three duplicate ACK packets. So
we choose four ACK packets to compose one segment, i.e:
m =4∗ n (5)
By using the formulas (3),(4),(5), we can rewrite the formula
(2)asfollowing:
CP[i] =
n
j=1
wj ∗ 1
4 ∗
4j−1
i=4(j−1)
The formula ( 6) can be expanded to
CP[i] =w1 ∗ 1
4 ∗
3
i=0
ACK[i]+ w1
α
+ ... + w1
∗
αn−1
1
4 ∗
4n−1
i=4(n−1)
ACK[i],i∈ [1,m] (6)
∗ 1
4 ∗
7
ACK[i]
i=4
ACK[i]
When CP[i +1] >CP[i], the congestion probability is
increasing. Meanwhile, the congestion probability may be
decreasing when CP[i +1]

ECN feedback. On the contrary, if the CE bit of the latest
arriving ACK is 1, thevalueof△CP will belong to [−1, 1].
In this case, it can not be determined that whether the network
is going to become congestion or not. Whatever the CE bit of
the new arriving ACK is, △CP ≤ 0 always indicates that the
network congestion may be relieved.
For simplicity, the last m ACKs are divided into 2 segments,
that is n =2and m =8.Letw1 =0.25 ∗ w2. Byusingthe
formulas (3) and (4), we can get w1 =0.2 and w2 =0.8. All
these parameters are used in the following simulations.
B. Analysis of Congestion Probability
In this subsection, the correlation of CP and network
states is analyzed by simulations. The network topology for
simulation is the same as that used in Ref. [13], which is a
typical topology for wired/wireless hybrid network(which is
also shown in Fig.7). One <strong>TCP</strong> Reno flow traverses along
a bottleneck link and a wireless terminal. When the sender
receives the signal of packet dropping or the ACK packet in
which the CE bit is 1, it regulates the sending window size
according to <strong>TCP</strong> Reno. Packets on routers will be marked if
the average queue length exceeds the given threshold.
In Fig.1, the size of <strong>TCP</strong> sending window, the queue length
of bottleneck link and CP are shown while the loss rate of
wireless link is 0 and 0.001. As shown in Fig.1(a), the size
of <strong>TCP</strong> sending window varies very regularly because of no
loss in wireless link. While there exists packet loss in wireless
link, the size of <strong>TCP</strong> sending window changes frequently and
irregularly, which is shown in Fig.1(b). From Fig.1(a) and
Fig.1(b), it is obvious that the average size of the sender
window without packet loss is larger than that with packet
loss.
The queue length of bottleneck link is shown in Fig.1(c) and
Fig.1(d). As shown in Fig.1(c), there are 4 periods in which
the queue length is larger than the given threshold. In this
case, the router marks the packet and the sender will receive
the ACK packet with CE=1. It is clear that the changes of the
sending window is be consonant with that of the queue length.
In Fig.1(d), there is only one period in which the queue length
is larger than the given threshold. But the sending window
does not change according to the queue length because there
are packets losses in wireless link.
The statistic of CP under different situation are shown in
Fig.1(e) and Fig.1(f) respectively. It is indicated that trend of
CP changing is consistent with that of queue length. So we
can capture network states according to △CP. The detailed
figures show how CP value ranged in one of the period,
which show that CP can sense congestion accurately and
characterize congestion particularly. Compared with end-toend
metrics or single ECN feedback, CP can provide more
accurate and richer information about network states.
III. <strong>TCP</strong>-<strong>PCP</strong> PROTOCOL
A. The description of <strong>TCP</strong>-<strong>PCP</strong> protocol
<strong>TCP</strong>-<strong>PCP</strong> protocol controls <strong>TCP</strong> behavior based on prediction
of CP. Fig.2 gives the pseudo code of <strong>TCP</strong>-<strong>PCP</strong>’s sender
CWND(pkt)
Queue Length(byte)
CP[i]
CP[i]
45
40
35
30
25
20
15
10
5
Loss rate: 0
0
4 6 8 10 12
Time(s)
14 16 18 20
(a) <strong>TCP</strong> window without link loss
5000
4000
3000
2000
1000
Loss rate: 0
0
4 6 8 10 12
Time(s)
14 16 18 20
(c) Queue Length without link loss
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
4 6 8 10 12 14 16 18 20
Time(s)
Loss rate: 0
7.40 7.45 7.50 7.55 7.60 7.65
Time(s)
(e) CP without link loss
CWND(pkt)
Queue length(byte)
CP [i]
CP [i]
40
35
30
25
20
15
10
5
0
5000
4000
3000
2000
1000
Loss rate: 0.001
4 6 8 10 12
Time(s)
14 16 18 20
(b) <strong>TCP</strong> window with link loss
0
4 6 8 10 12 14 16 18 20
Time(s)
Loss rate: 0.001
(d) Queue Length with link loss
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4 6 8 10 12 14 16 18 20
Time(s)
Loss rate: 0.001
7.3 7.4 7.5 7.6 7.7 7.8
Time(s)
(f) CP with link loss
Fig. 1: Correlation between CP and network state
receiving procedure.
While receiving an ACK packet, <strong>TCP</strong>-<strong>PCP</strong> sender will
recalculate new CP value and get △CP.
If the current ACK is new, <strong>TCP</strong>-<strong>PCP</strong> sender operates the
same as <strong>TCP</strong> NewReno does. Since routers support ECN and
<strong>TCP</strong> senders negotiate about ECN, senders will check CE bit
of arrived ACK. If it is set to 1, senders will trigger ecnaction()
and then deal with packets in slow start or congestion
avoidance.
If current ACK is duplicate and △CP > 0, <strong>TCP</strong>-<strong>PCP</strong>
sender concludes that the congestion occurs. The sender will
adjust the sending rate and retransmit the data packet. If
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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.

Recv( ACK[i] ):
CP[i] → CP[i − 1];
calculate CP[i];
CP[i]-CP[i − 1] →△CP ;
if newACK then
if ACK[i].CE=1
ecn-action()
SS() or CA()
else
SS() or CA() //Same as Newreno
endif;
else then
if △CP > 0
Rate<strong>Control</strong>() //Same as Westwood
Retransmit()
else
Retransmit()
endif;
Fig. 2: Pseudo code of <strong>TCP</strong>-<strong>PCP</strong>’s sender receiving procedure
Time t1
Time t2
Ack[ 7]
ecnecho Ack[ 0]
ecnecho
Time t3 A
0 0 0 1 1 1 1 1
1 0 0 0 1 1 1 1
1 1 0 0 0 1 1 1
Time t3 B 0 1 0 0 0 1 1 1
Fig. 3: The first scenario
current ACK is duplicate and △CP ≤ 0, <strong>TCP</strong>-<strong>PCP</strong> sender
only retransmits the data packet. <strong>TCP</strong>-<strong>PCP</strong> modified sender’s
behavior by △CP when there are packet losses. This is the
important distinguishing feature of <strong>TCP</strong>-<strong>PCP</strong> with respect to
other <strong>TCP</strong> variants. For example, the sender of <strong>TCP</strong> Westwood
will trigger Rate<strong>Control</strong> without care about the situation of
mid routers, and that of Jersey will decide whether to do
Rate<strong>Control</strong> just by single ECN signal.
Additionally, <strong>TCP</strong>-<strong>PCP</strong>’s sender estimates the eligible bandwidth
based on ACK packets arriving interval and adjusts
the ssthresh (slow star thresh) and cwnd appropriately. This
method to do Rate<strong>Control</strong> procedure is the same as <strong>TCP</strong>
Westwood, which can be described as follows.
ssthresh =(BWE∗RT Tmin)/seg size;
if(cwnd > ssthresh) cwnd = ssthresh;
Like <strong>TCP</strong> Jersey, <strong>TCP</strong>-<strong>PCP</strong> also employs threshold marking
method in ECN. Concretely, the router marks all the packets
when the EWMA-averaged queue length exceeds a given
threshold, which is set to 1/3 of the link buffer capacity in
experiments.
B. Analysis of <strong>TCP</strong>-<strong>PCP</strong>
In this section, we give two scenarios to show how <strong>TCP</strong>-
<strong>PCP</strong> works in hybrid networks.
As shown in Fig 3, the CE lists at t1, t2 and t3 are
given, where t2 is the time when the sender receives the first
ACK packet after t1, and t3 is after t2. Att1, the CE list
of the latest 8 ACK packets is 00011111. At t2, the sender
Time t1
Time t2
Time t3
Time t4 A
Time t4 B
Ack[ 7]
ecnecho Ack[ 0]
ecnecho
0
0
0 1
1 1
0 0 0 0 1 1 1 1
0 0 0 0 0 1 1 1
1 0 0 0 0 0 1 1
0 0 0 0 0 0 1 1
Fig. 4: The second scenario
receives a duplicate ACK in which CE=1. So the CE list
of the latest 8 ACK packets is 10001111 at t2. In some
protocols, for example, <strong>TCP</strong> Jersey, the sender will trigger
congestion control. However, In <strong>TCP</strong>-<strong>PCP</strong>, calculating result
of CP[1] = CP[2] = 0.4 means that probability of congestion
occurred in time t2 is not increased. So <strong>TCP</strong>-<strong>PCP</strong> sender does
not invoke Rate<strong>Control</strong> procedure at t2.
At t3, if <strong>TCP</strong>-<strong>PCP</strong>’s sender receives a duplicate ACK in
which CE=1 and gets CP[3] = 0.55. DuetoCP[3] >CP[2],
<strong>TCP</strong>-<strong>PCP</strong>’s sender predicts that congestion would occur soon
and triggers the congestion control mechanism at the moment.
On the contrary, if <strong>TCP</strong>-<strong>PCP</strong>’s sender receives a duplicate
ACK in which CE=0 and gets CP[3] = 0.35. Due to
CP[3] CP[3]. On the contrary,
as long as CP[4] is less than CP[3], <strong>TCP</strong>-<strong>PCP</strong>’s sender will
not decrease the sending rate.
It is obvious that in <strong>TCP</strong>-<strong>PCP</strong> protocol, even if the sender
received duplicate ACKs caused by several errors, as long
as the queue length does not exceed the threshold or the
congestion probability does not increase, the sender will not
decrease its sending rate. Contrarily, even if there is only one
duplicate packet arrival, the sending rate might decline because
of CP value being increased.
As a whole, <strong>TCP</strong>-<strong>PCP</strong> is based on ECN mark. By deducing
a congestion probability, <strong>TCP</strong>-<strong>PCP</strong> can protect <strong>TCP</strong> connection
from short-term, recoverable, non-congestive packet
losses, and enhance its performance in hybrid network eventually.
IV. PERFORMANCE EVALUATION
We have done the necessary code modification in NS2 [11]
to simulate <strong>TCP</strong>-<strong>PCP</strong>. The modification includes overwriting
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1
1

S
10Mbps
45ms
BS
2Mbps
1ms
Fig. 5: Network Topology 1
Fig. 6: Comparison of throughput on network topology 1
droptail, downloading Westwood [13] and Jersey [12], appending
<strong>TCP</strong>-<strong>PCP</strong> and so on.
A. Throughput analysis on topology 1
The simulation environment is the same with that used
in Ref. [12]. As shown in Fig.5, the source connects to a
base station via a 10 Mb/s wired link with 45 ms oneway
delay. The base station is linked to the destination, a
wireless mobile node, via a 2 Mb/s loss channel with 1 ms
delay. A single <strong>TCP</strong> connection running a long-lived FTP
application delivers data from the source to the destination.
We run the simulation for <strong>TCP</strong>-<strong>PCP</strong>, Westwood, and Jersey
respectively. In our experiments, the random link error rate at
the wireless bottleneck link varies from 0.001 to 0.1, which
creates by a simple union error model. The simulation time
is 100 seconds. The throughput of different protocols are
shown in Fig.6. Throughput of Westwood falls behind that of
Jersey and <strong>TCP</strong>-<strong>PCP</strong> because Westwood handles corruption by
estimate available bandwidth only. For link error rate smaller
than 0.005, Jersey and <strong>TCP</strong>-<strong>PCP</strong> perform closely to each
other. Beyond that point, <strong>TCP</strong>-<strong>PCP</strong> starts to outperform Jersey.
Especially when the loss rate increases to 0.01, the throughput
of <strong>TCP</strong>-<strong>PCP</strong> outperforms Jersey by 23% and Westwood by
50%.
B. Throughput analysis on topology 2
Simulations of this section are based on the topology shown
in Fig.7,which is the same as that used in Ref. [13]. As shown
in Fig.7, one <strong>TCP</strong> flow is sent from R0 to R3, link R1-R2
emulates a long delay pipe of Internet, and the last hop is a
wireless link with error model. Concretely, the bandwidth of
access link is 100 Mbps with 3 ms delay, the bandwidth of
wireless link is 10 Mbps with 3 ms delay. The bandwidth of
bottleneck is 5 Mbps with 40 ms delay. The buffer is set to
be 100 packets. The simulation lasts for 100 s.
Fig.8 gives the throughput comparison of the three algorithms
under different link. As shown in Fig.8, Jersey’s
throughput is close to Westwood’s, which is different from
D
R0
100 Mbps
3 ms
R1
5 Mbps
40 ms
R2
10 Mbps
3 ms
Fig. 7: Network topology 2
Fig. 8: Comparison of throughput on network topology 2
the simulation results in topology 1. The reason is that there
is an obvious bottleneck in topology 2, which results in
more congestion losses. It is observed that Jersey experiences
performance degradation in the case that congestion losses
and error losses coexist. It is clear that <strong>TCP</strong>-<strong>PCP</strong> can get
higher throughput in this case. Especially, when the packet loss
rate is 0.01, Jersey’s throughput is 1300Kbps and Westwood’s
throughput is 1340Kbps, but <strong>TCP</strong>-<strong>PCP</strong> can get 1444Kbps
throughput.
Then we introduce exponential distributed ON/OFF flow in
topology 2. Its sending rate is 5 Mbps, and there are 25 ms
to be “on” state in 100 ms period. Throughput under such
situation are shown in Table I.
TABLE I: Comparison of throughput with ON/OFF flow in
network topology 2
Loss rate: 0.001 Loss rate: 0.005 Loss rate: 0.01
<strong>Protocol</strong> <strong>TCP</strong> UDP <strong>TCP</strong> UDP <strong>TCP</strong> UDP
(kbps) (kbps) (kbps) (kbps) (kbps) (kbps)
Westwood 1694 936 1457 971 1346 970
Jersey 1437 1114 1393 1041 1359 1039
<strong>TCP</strong>-<strong>PCP</strong> 1679 994 1618 1039 1482 1055
As shown in Table I, the throughput of the three algorithms
are decreasing with the increasing of loss rate. Westwood’s
throughput decreases by 20% when link loss rate varies from
0.001 to 0.01, but <strong>TCP</strong>-<strong>PCP</strong>’s throughput only decreases
by 11%. Moreover, the total throughput of <strong>TCP</strong>-<strong>PCP</strong> is the
highest in the three algorithms. It is anticipated that <strong>TCP</strong>-<strong>PCP</strong>
can cope with error losses better than the other <strong>TCP</strong> variants
by using congestion probability.
C. Fairness analysis
Fairness is an important metric for evaluating <strong>TCP</strong> protocol.
Multiple connections of the same <strong>TCP</strong> scheme must cooperate
fairly. Network topology 3 shown in Fig.9 is used to test this
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R3

S1
Si
Sn
100 Mbps
3 ms
R1
20 Mbps
40 ms
R2
10 Mbps
3 ms
Fig. 9: Network topology 3
metric, in which the network settings are marked. As shown
in Fig.9, 10 same <strong>TCP</strong>-<strong>PCP</strong> flows share a 20 Mbps bottleneck
link.
We use Jain’s fairness index to justify the fairness of
<strong>TCP</strong> variants Ref. [14]. Simulation results are summarized in
Table II, which show that all <strong>TCP</strong> variants including <strong>TCP</strong>-
<strong>PCP</strong> achieve fairly satisfactory fairness index with different
link losses. Meanwhile, it is shown from Table II that <strong>TCP</strong>-
<strong>PCP</strong> still keeps higher throughput than Westwood and Jersey
in this case.
TABLE II: Comparison of fairness and throughput
Westwood Jersey <strong>TCP</strong>-<strong>PCP</strong>
LossRate Thru Fair Thru Fair Thru Fair
(kbps) (-) (kbps) (-) (kbps) (-)
0 1759 0.935 1783 0.962 1795 0.997
0.001 1741 0.998 1757 0.999 1761 0.999
0.005 1612 0.999 1619 1.0 1647 1.0
0.01 1457 0.999 1489 0.998 1532 0.999
D. Friendliness analysis
The experiment of friendliness partitions ten flows into two
parts, one uses <strong>TCP</strong>-<strong>PCP</strong>, the other uses the traditional <strong>TCP</strong>
protocol, such as Westwood.
Table III gives the change of average throughput, which is
calculated by summing up the throughput of the same <strong>TCP</strong>
variant and divided by the number of connections. It is clear
that the bandwidth allocation of <strong>TCP</strong>-<strong>PCP</strong> and Westwood is
close to its fair share without link loss occurring. When the
link loss rate raises from 0.1% to 1%, <strong>TCP</strong>-<strong>PCP</strong> achieves
higher throughput than Westwood. The results are according
with that of one <strong>TCP</strong> flow. But the increase is within a
tolerable range as regard to friendliness performance. It can be
concluded that <strong>TCP</strong>-<strong>PCP</strong> can coexist friendly with Westwood.
V. CONCLUSION
In this paper we proposed a novel <strong>Transport</strong> <strong>Control</strong> <strong>Protocol</strong>
based on the prediction of Congestion Probability. It
can improve <strong>TCP</strong> throughput in hybrid networks because the
sender judges network state by serial ECN feedbacks other
than single ECN feedback or packet dropping.
Actually, the congestion-aware scheme used in <strong>TCP</strong>-<strong>PCP</strong>
can be easily extended to existed <strong>TCP</strong> variants. Our next works
focus on checking its function on other <strong>TCP</strong> variants both on
NS2 simulation and wired/wirelss testbed.
D1
Di
Dn
TABLE III: Friendliness Performance
Loss rate: 0 Loss rate:0.001
Westwood:<strong>TCP</strong>-<strong>PCP</strong> Westwood <strong>TCP</strong>-<strong>PCP</strong> Westwood <strong>TCP</strong>-<strong>PCP</strong>
(kbps) (kbps) (kbps) (kbps)
3:7 1162.00 1174.85 1161.33 1174.14
5:5 1316.80 1331.40 1312.80 1329.40
7:3 1519.28 1536.66 1511.00 1539.00
Loss rate: 0.005 Loss rate:0.01
Westwood:<strong>TCP</strong>-<strong>PCP</strong> Westwood <strong>TCP</strong>-<strong>PCP</strong> Westwood <strong>TCP</strong>-<strong>PCP</strong>
(kbps) (kbps) (kbps) (kbps)
3:7 1289.66 1314.85 1099.00 1186.57
5:5 1294.80 1319.80 1269.80 1352.2
7:3 1485.28 1508.33 1388.14 1506.00
ACKNOWLEDGEMENT
This work was partially supported by the National Natural
Science Foundation of China (60673164), Provincial Natural
Science Foundation of Hunan (06JJ10009), the Specialized
Research Fund for the Doctoral Program of Higher Education
of China (20060533057), Program for New Century Excellent
Talents in University (NCET-05-0683), CityU Strategic
Research Grant No. 7002214.
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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.