-
S. Aluru et al. (Eds.): IC3 2011, CCIS 168, pp. 123134, 2011.
Springer-Verlag Berlin Heidelberg 2011
Variable Length Virtual Output Queue Based Fuzzy Adaptive RED
for Congestion Control at Routers
Pramod Kumar Singh and Santosh Kumar Gupta
Computational Intelligence and Data Mining Reasearch Laboratory
ABV-Indian Institute of Information Technology & Management
Gwalior, India
[email protected], [email protected]
Abstract. Internet routers play an important role during the
time of network congestion. All the Internet routers have some
buffer at input and output ports, which hold the packets at the
time of congestion. Many queue management algorithms have been
proposed but they focus on fixed queue limit. Recognizing the fact
that active queue management algorithms have fixed maximum queue
limit, we direct our attention to variable length queue limit for
Combined Input Output Queued (CIOQ) switches. We incorporated our
proposed technique, which is a fuzzy logic control based generic
variable length active queue management scheme in TCP/IP networks,
to the drop-tail and the Adaptive RED (A-RED) algorithm. The
empirical results show low packet loss and high queue utilization
in modified algorithms (augmented with variable length active queue
management scheme) in comparison to the original drop-tail, RED and
A-RED algorithms.
Keywords: Congestion Control, Fuzzy Logic Controller (FLC),
Active Queue Management (AQM), Virtual Output Queue (VOQ), Combined
Input Output Queued (CIOQ) Switch, Adaptive RED (A-RED).
1 Introduction
Congestion is a critical issue as it reduces the overall
throughput of the network and users experience greater delay.
Though the routers play an active role in its resource allocation
to effectively control/prevent congestion, it is still a major
cause of concern because of ever growing Internet and ever growing
number of users; they increase the amount of data to be carried
over the Internet. Todays Internet routers have some buffer at
input and output ports. The buffer size of the router should be
large enough to accommodate the packets during the time of
congestion but, at the same time, should also take care of the
queuing delay. It demands for an optimum size and efficient
management of buffers at the input/output ports. This is known as
active queue management (AQM) [6].
Most of the Internet routers run drop-tail gateways. However,
drop-tail buffer management introduces large queuing delays in
bursty traffic [13]. The RED algorithm [4] [12] [14] manages the
(buffer) queue more effectively and monitors the average queue
length. The average queue length is compared with minimum threshold
(minth) and maximum threshold (maxth). If the queue length is less
than minth then all
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124 P.K. Singh and S.K. Gupta
the incoming packets are accepted. If the queue length is in
between minth and maxth then packets are dropped with a probability
that increases linearly up to maximum drop probability (maxp) and
if the queue length exceeds the maxth then all the incoming packets
are dropped. The most important advantage of RED is that it keeps
the average queue length low to allow occasional burst of packets
in queue. However, it has several shortcomings, e.g., a high degree
of sensitivity towards its operating parameters, unfairness to
flows with different round-trip times, the problem of global
synchronization. A related weakness of RED is that throughput is
also sensitive to the traffic load and the RED parameter. In
particular RED does not perform well when the average queue length
becomes larger than maxth. It results in a significant decrease in
throughput and a significant increase in the drop rate [5]
[17].
The A-RED [11] [13], proposed by one of the authors of RED,
attempts to solve the problem of need for continuously (re)tuning
RED parameters. In particular, A-RED adjusts the value of maximum
drop probability (maxp) to keep average queue length within a
target range half way between the minth and maxth. It is shown in
Figure 1. Though A-RED attempts to tune the RED parameters for a
robust behavior, it fails to do so in various dynamic cases as
A-RED retains REDs basic linear structure.
Every interval seconds:
if (avg > target and maxp 0.5) increase maxp ; maxp maxp+ ;
else if (avg < target and maxp > 0.01)
decrease maxp: maxp maxp * ; Variables: avg: average queue
length Fixed parameters: Interval: time; 0.5 seconds; target :
target for avg ; [minth +0.4*( maxth minth ), minth + 0.6*(maxth -
minth)]. : increment; min (0.01, maxp / 4) : decrease factor;
0.9
Fig. 1. Adaptive RED algorithm
Many other active queue management schemes are also reported in
the literature, e.g., Random Exponential Marking (REM) [2], fuzzy
Proportional Integral (PI) [19], and Adaptive Virtual Queue (AVQ)
[15]. All these existing active queue management schemes are based
on the equation model. These equation models use various control
parameters, which are dependent on different network parameters,
e.g., number of flows, round trip time. However, it is very
difficult to set these parameter as TCP/IP network is dynamic in
nature. Many researchers, e.g., [3], [7], [8], [10], adopted fuzzy
logic controller (FLC) to set these parameters dynamically in the
congestion control algorithms because of its strength in
controlling highly nonlinear, complex systems.
In this paper, we design a fuzzy logic controller (FLC) based on
fuzzy logic set theory [23], [24] for computing the change in
virtual output queue (VOQ) length [1],
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Variable Length Virtual Output Queue Based Fuzzy Adaptive RED
125
[21], [18] according to its instantaneous queue length. It is a
generic variable length active queue management scheme, which may
be incorporated to any VOQ based active queue management congestion
control mechanism to improve its performance while keeping the
basic structure of the original algorithm same. In this paper, we
incorporate our proposed generic method to the drop-tail and the
A-RED and obtained encouraging results.
Rest of the paper is organized as follows. Section 2 discusses
about variable length VOQ. In Section 3, we present our proposed
generic variable length active queue management method and its
application to drop-tail (Fuzzy drop-tail) and A-RED (Fuzzy A-RED).
The rule base design of FLC is presented in Section 4. We discuss
the performance of proposed method through a set of extensive
simulation and compare the obtained results with well-known methods
in Section 5. Finally in section 6, we present our conclusion.
2 Variable Length VOQ
The combined input and output queuing scheme uses buffers at
both input and output modules of a switch, and a switch that
employs this queuing scheme is called a CIOQ switch [9]. Every
input port maintains a virtual queue, known as virtual output queue
(VOQ), at its input buffer for each output port. In other words,
for an NN switch, each input port i (1 i N) maintains a separate
set of FIFO queues for each output port j (1 j N), named as VOQi,j.
Therefore, there are N sets of VOQi,j queues at each input port.
The incoming packets are stored in appropriate VOQ according to
their destination address; the VOQi,j buffers packet at input port
i, which is destined to output port j. The buffer space of an input
port is divided according to the number of VOQs and each VOQ has a
fixed maximum queue limit to store the incoming packets. If there
is no space for incoming packet at VOQ, the packet is dropped.
However, it is desirable and a good strategy to vary (increase or
decrease) the address space of VOQs at run time as per the
requirement for efficient utilization of the queue while the buffer
size at the input port is fixed.
Fig. 2. In Case 1, the maximum queue limit of both VOQs is
equal; in case 2, the maximum queue limit for VOQ11 is less than
the VOQ12 and in case 3, the maximum queue limit for VOQ11 is
greater than the VOQ12
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126 P.K. Singh and S.K. Gupta
Our approach is based on the fact that we can vary the buffer
size of the VOQ during the processing time as per the requirement.
Figure 2 shows three possible cases for 22 switch. We can modify
any VOQ based active queue management algorithm which uses a fixed
queue limit. The amount of variation in the maximum queue limit of
VOQs (while the total buffer size of an input port is fixed) is
calculated by using the fuzzy logic controller.
3 Fuzzy Logic Controller
The fuzzy Controller is a controller which is based on the fuzzy
logic based rules and often contains nonlinear mapping. The idea of
FLC was initially introduced by Zadeh [24] and first applied by
Mamdani [16] in an attempt to control systems that are difficult to
model mathematically. FLC may be viewed as a way of designing
feedback controllers where it is convenient and effective to build
a control algorithm without relying on formal models of the system.
The control algorithm is encapsulated as a set of commonsense
rules. FLC has been applied successfully in control system for
which analytical models are not easily obtained or the model
itself, if available, is too complex and highly non-linear.
Our approach is to design a non-linear fuzzy logic controller,
which operates at each input port of the router. For example, in a
22 switch at input port1 the fuzzy controller has two virtual
queues, namely VOQ11 and VOQ12, which may be in the low, average
and high queue length states (refer, Figure 3). The low, average
and high represent the status of input variables in linguistic
form, which change dynamically over time. In order to determine the
linguistic values of input and output, we partitioned the input and
output space. The controller changes the maximum queue limit
(varies the maximum buffer size of each VOQ) according to status of
the inputs. Each of the input variables is represented by a fuzzy
set.
Fig. 3. Fuzzy Logic Controller for Queue Management System
Model
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Variable Length Virtual Output Queue Based Fuzzy Adaptive RED
127
A notation convention for fuzzy sets [22], when the universe of
discourse, X, is discrete and finite, is shown in Eq. 1. Here, A is
a fuzzy set.
A= { .}={ } (1)
For our algorithm, the fuzzy sets are represented as follows
(refer, Eq. 2):
VOQ11= {
}
VOQ12= {
} (2)
Where VOQ11 and VOQ12 are fuzzy variables; low, average, and
high are the possible values for fuzzy variables, and 1, 2, and 3
are the membership functions of the fuzzy variable.
3.1 Variable Length VOQ for Drop-Tail Algorithm
In modified drop-tail algorithm the maximum queue limit of each
VOQ is not fixed. The FLC uses the instantaneous queue length as
feedback, which is measured periodically, to compute new limit of
the queue length. In other words, the controller varies the maximum
queue length limit (refer, manipulated variable in Figure 3) as per
sampled queue length of each VOQ. Variation in queue limit gives
the low loss rate in comparison to simple drop-tail algorithm while
the average queue length of both the algorithm is approximately
same. It means modified drop-tail algorithm gives higher queue
utilization in comparison to original drop-tail.
3.2 Fuzzy Adaptive RED
The overall guideline for fuzzy A-RED algorithm is same as A-RED
algorithm except the variable target. In A-RED algorithm (refer,
Section 1) the target value is fixed and is calculated as
follows:
Target value: [minth + 0.4*( maxth minth ), minth + 0.6*(maxth -
minth)]
In modified A-RED algorithm, which has the variable length VOQ,
the value of
maxth and minth is not fixed. As a result the target value of
the A-RED algorithm changes in each time interval with the varying
values of minth and maxth according to fuzzy logic controller. For
nth time interval the target value is calculated as follows:
Target value: [minth (n) + 0.4*( maxth (n) minth (n)), minth (n)
+ 0.6*(maxth(n) - minth(n))]
Here the minth(n) and maxth(n) is the minimum and maximum
threshold at n
th time interval respectively. Fuzzy A-RED removes A-REDs
dependence on target value. Adapting the target value results in
low packet loss and low average queue length in comparison to
A-RED.
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128 P.K. Singh and S.K. Gupta
4 Rule Based Design
The FLC uses some linguistic rules for each input port. These
linguistic rules control the system under different operating
conditions. Usually multi-input FLC makes it easier to describe the
system dynamics linguistically. We expect that we can tune the
system and improve the behavior of active queue management
algorithm, e.g., drop-tail, A-RED, by using variable length VOQs.
The fuzzy rule base (IF-THEN rules) for the 22 CIOQ switch is
presented linguistically in Figure 4 and same is represented in
tabular form in Table. 1. The control surface defined by the FLC
rules is shown in Figure 5.
Usually to define the linguistic rules of a fuzzy variable,
Gaussian, triangular or trapezoidal shaped membership function are
used. Since triangular and trapezoidal shaped function offer more
computational simplicity, we have selected them for our rule base.
The rule base is fine-tuned by observing the progress of
simulation, such as packet loss occurrences and number of buffered
packets at each VOQ.
/* linguistic rules for each Input port */ /* initially voq11
and voq12 queue limit is equal and set to maximum queue limit.*/ /*
if voq11 queue limit increases, then the voq12 queue limit
decreases to make input port buffer size fixed */ /* Set of
linguistic rules defining the control surface of FLC */
if voq11 is low and voq12 is low then voq11_modified_queue_
length is equal to max queue limit. if voq11 is average and voq12
is low then voq11_modified _queue_ length is greater than max queue
limit. if voq11 is high and voq12 is low then voq11_modified
_queue_ length is greater than max queue limit. if voq11 is low and
voq12 is average the voq11_modified_queue_length is lesser than max
queue limit. if voq11 is average and voq12 is average then
voq11_modified_queue_length is equal to max queue limit. if voq11
is high and voq12 is average then voq11_modified_queue_length is
greater than the max queue limit. if voq11 is low and voq12 is high
the voq11_modified_queue_length is lesser than max queue limit. if
voq11 is average and voq12 is high then voq11_modified_queue_length
is lesser than max queue limit. if voq11 is high and voq12 is high
then voq11_modified_queue_length is equal to max queue limit.
Fig. 4. Fuzzy rule base for 22 CIOQ switch
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Variable Length Virtual Output Queue Based Fuzzy Adaptive RED
129
Fig. 5. Control decision surface of the Fuzzy Logic Controller
shaped by rule base and linguistic variables
Table 1. Rule base of fuzzy controller
Input variable Output (manipulated variable) VOQ11 VOQ12 VOQ11
Queue limit VOQ12 Queue limit low low equal to max queue limit
equal to max queue limit
average low greater than max queue limit less than max queue
limit
high low greater than max queue limit less than max queue
limit
low average less than max queue limit greater than max queue
limit
average average equal to max queue limit equal to max queue
limit high average greater than max queue limit less than max queue
limit low high less than max queue limit greater than max queue
limit average high less than max queue limit greater than max queue
limit high high equal to max queue limit equal to max queue
limit
5 Simulation Result
In this Section, We compare our scheme with the original
drop-tail, RED and A-RED algorithm.
5.1 Experimental Setup
We perform our simulation on 22 CIOQ [21] switch as shown in
Figure 6. The size of the buffer at input and the output port is
120 and 100 respectively. Buffer size at the input port is
segmented according to the number of VOQs. Each input port carries
multiplexed TCP Reno flows. The TCP flows are generated at separate
source nodes and then multiplexed into the backbone before reaching
at the input port of the switch. In this experiment, we use 4
source nodes at each input port, hence, total 8 source nodes
generates TCP flows in the range of 50 to 500 sessions to the input
ports of the switch. The size of the packets is 1000 bytes. The
queue monitoring interval is set to 0.0001 sec. In A-RED algorithm
is set to 0.01 and is set to 0.9.
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130 P.K. Singh and S.K. Gupta
Fig. 6. 22 CIOQ switch
5.2 Input Queue Length
Figure 7 displays the average input queue length in the queue
management unit when we use the Variable length VOQ based drop-tail
and Fuzzy A-RED. The corresponding simulation results of original
RED, drop-tail and A-RED are also shown for the comparison. For
this simulation, the number of TCP sessions is 250 and speedup
varies from 0.5 to 2.0. For RED, the minimum threshold is set to 19
packets and the maximum threshold is set to 59 packets for each
VOQ. We use drop-tail algorithm at output ports for all the
comparative algorithms.
Fig. 7. Average input queue length v/s speedup for 22 Switch
(load 200 TCP sessions)
On the input port, the drop-tail algorithm has the longest queue
Length as it drops packets only when the buffer overflows. The
suggested change in drop-tail algorithm, i.e., the variable length
VOQs, has the same average queue length as original drop-tail
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Variable Length Virtual Output Queue Based Fuzzy Adaptive RED
131
algorithm whereas the RED algorithm has lower queue length as it
drops packets even before buffer is overflow.
As A-RED keeps the average queue length away from maxth, the
input queue length for A-RED is less than RED algorithm. As
mentioned in section 3, the target value is not fixed in the Fuzzy
A-RED algorithm; hence the result is even better than original
A-RED algorithm.
5.3 Loss Rate
Loss rate is the ratio of the number of packets dropped and the
number of packets sent. In this experiment, the speedup is fixed at
1.1whereas the load varies from 50 to 500 TCP sessions. Figure 8
shows that the loss rate of the fuzzy A-RED algorithm is lowest.
Adjusting the maxp and target value of the Fuzzy A-RED algorithm
avoids the higher packet loss as the average queue length
oscillates near to the target value. We observe that the
performance of fuzzy A-RED is comparatively better than the
drop-tail, RED and the original A-RED because of the efficient
management of the buffer space among the VOQs as per the
requirement and the variation in the target value as well.
Fig. 8. Loss Rate of the 22 Switch at speedup 1.1
5.4 Buffer Size Variation
We investigate the buffer size of drop-tail with fixed length
VOQs (maximum queue limit for drop-tail is set to 60 packets) over
time. Figure 9 shows the buffer size variation of VOQ11 and VOQ12
respectively. The buffer size of the both VOQs is not more than the
60 packets at any time instant.
Figure 10 shows the buffer size variation of proposed algorithm.
The result shows that maximum queue limit of VOQ11 and VOQ12 is
varying from 54-65 packets according to suggested approach, but at
any instant of time the number of packets in
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132 P.K. Singh and S.K. Gupta
Fig. 9. Buffer size variation of VOQ11& VOQ12 over time (for
drop-tail algorithm)
both of the VOQs simultaneously is not more than 60. The reason
is that at that instant of time the other VOQ had relatively less
queue length hence it could relinquish a portion of its unused
address space to share with other heavily loaded VOQ. Therefore,
the proposed variable length VOQ algorithm minimizes the packet
loss and provides efficient management of the buffer space at input
ports.
Fig. 10. Buffer size variation of VOQ11&VOQ12 over time
(variable length VOQ for drop-tail)
As fuzzy A-RED algorithm has the basic structure of the A-RED
algorithm, the size of the buffer oscillates near to the target
value. Figure 11 shows that the number of packets buffered is half
way between maximum and minimum threshold.
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Variable Length Virtual Output Queue Based Fuzzy Adaptive RED
133
Fig. 11. Buffer size variation of VOQ11&VOQ12 over time
(Fuzzy A-RED)
6 Conclusion
Our proposed scheme is a generic concept which may be used to
improve the performance of any active queue management scheme. In
this paper, we have incorporated variable length VOQ with the
active queue management algorithms, e.g., drop-tail, A-RED
algorithm, which improves the performance in terms of queuing
delay, average queue length and loss rate. Variable length VOQ for
drop-tail is the improvement over simple drop-tail algorithm which
utilizes the queue more efficiently. In fuzzy A-RED target value
dynamically change with change in VOQ length which gives
improvement over A-RED algorithm in dynamic network environment. We
formulate an effective and efficient technique for queue management
using the fuzzy logic control, to solve the problem of congestion
in TCP/IP networks. We have demonstrated that in the real world for
the non-linear and complex system the fuzzy logic control gives the
acceptable solution with the help of linguistic models.
Additionally, it does not require any change in the switch
hardware.
In future work, we plan to explore the proposed scheme on other
active queue management algorithms such as REM, AVQ and in other
network scenario like Diff-Serv, and in different types of traffic
condition. We also plan to check the stability and performance of
the proposed method on the larger switches.
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Variable Length Virtual Output Queue Based Fuzzy Adaptive RED
for Congestion Control at RoutersIntroductionVariable Length
VOQFuzzy Logic ControllerVariable Length VOQ for Drop-Tail
AlgorithmFuzzy Adaptive RED
Rule Based DesignSimulation ResultExperimental SetupInput Queue
LengthLoss RateBuffer Size Variation
ConclusionReferences
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