TCP Fairness for Uplink and Downlink Flows in WLANs

TCP Fairness for Uplink and Downlink Flows
in WLANs
Juho Ha and Chong-Ho Choi
School of Electrical Engineering and Computer Science, and ASRI
Seoul National University, Seoul, Korea.
{hazo,chchoi}@csl.snu.ac.kr

Abstract— In WLANs, fairness is an important issue because This paper is organized as follows. In section II, we present
the channel is shared by many users. This paper proposes a dual some previous works on this problem. We propose our scheme
queue based scheme in an access point (AP) for TCP fairness in section III, and its performance is evaluated by simulation
among uplink flows and downlink flows. First, we discuss the
unfairness problem of TCP flows due to different responses in section IV. In section V, we conclude the paper.
to data packet drop and TCP ACK packet drop at the AP.
The proposed scheme employs two queues, one for the data II. R ELATED W ORK
packets of downlink TCP flows and another for the ACK packets
corresponding to uplink TCP flows. The performance of the Many MAC protocols have been proposed to achieve
proposed scheme is evaluated by simulation and the results are fairness at the transport layer in the infrastructure mode
presented. The dual queue scheme is simple and effective for
resolving the TCP unfairness problem. WLANs. The researchers showed that fairness at the MAC
layer protocol does not guarantee fairness at the transport layer.
I. I NTRODUCTION In the infrastructure mode, an AP shares wireless channel
Nowadays, Wireless LANs (WLANs) based on the IEEE with its mobile stations. When the AP competes with N
801.11 standard are widely deployed. As the number of mobile stations, it obtains only 1/(N + 1) of transmission
WLAN users has grown rapidly, service fairness among users opportunities. Because all the downlink flows communicates
has become an important issue. The IEEE 802.11 standard through the AP, the downlink flows have to share this portion.
defines two MAC techniques: the distributed coordination This behavior at the MAC layer leads to uplink/downlink
function (DCF) and the point coordination function (PCF). asymmetry at the transport layer.
Currently, most of the WLAN devices employ only DCF due The authors of [2], [3] and [4] proposed MAC protocols,
to its simplicity and efficiency. DCF provides fair channel which leads to TCP fairness among uplink and downlink
access opportunities among mobile stations. flows. In [2], the contention windows of mobile stations are
The most popular network configuration of WLANs is dynamically adjusted. By increasing the contention windows,
the infrastructure mode, in which all the mobile stations the AP has more chances to access the wireless channel, and
communicate through an access point (AP). WLANs in the so the fairness among uplink and downlink flows improves.
infrastructure mode can provide network access at public Similarly, by adjusting DIFS in AP, the AP can get higher
areas, such as campus, cafes and hotels. While DCF allows priority in accessing the channel [3]. In [4], each station defers
all the competing stations equal opportunity for media access, channel access based on the next packet information that is
it does not guarantee fair service provision at the transport collected at AP. All these schemes are based on MAC layer
layer when it is applied to the infrastructure mode. Pilosof et scheduling and give the AP more chances to access the channel
al. [1] pointed out that the interaction between DCF and TCP than the mobile stations. Since these schemes do not consider
can cause unfairness among uplink and downlink TCP flows the interaction between the MAC and TCP protocols, it is
in the infrastructure mode. Because most internet services run difficult to predict the results when they are applied to TCP
over TCP connections, we will restrict our attention to TCP flows. Moreover the MAC layer of mobile stations has to be
flows. modified in [2] and [4].
In this paper, we propose a dual queue based scheme Pilosof et al. [1] showed that the buffer size of AP plays an
that can solve this unfairness problem. In this scheme, an important role in unfairness. They presented a simple solution
AP employs two interface queues, one for the data packets where the AP manipulates the TCP advertised window field
of downlink TCP flows and another for the ACK packets of ACK packets. However this scheme is very complex to
corresponding to uplink TCP flows. The AP serves the queues implement since the AP needs to manipulate TCP headers of
with different probability to control the ratio of TCP data all packets.
sending rate and TCP ACK sending rate. Through analysis and To provide the fairness, the MAC QoS parameters of TCP
simulations, we show that TCP fairness for uplink and down- data packets and ACK packets for uplink and downlink flows
link flows can be achieved by adjusting the queue selection were set to different values [5]. In this scheme, the TCP
probability. The proposed scheme is simple, easy to deploy ACK packets are transmitted with minimal queueing, while the
and only needs minor changes in AP queueing mechanism. downlink TCP data packets are transmitted by TXOP bursting.

1-4244-0357-X/06/$20.00 ©2006 IEEE
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE GLOBECOM 2006 proceedings.

It can be deployed only in the 802.11e networks, and it also congestion window size varies between BD /2m and BD /m.
needs to modify the MAC protocol of mobile stations. If the average round trip time of downlink TCP flows is Td , the
In [6], per-flow queueing was deployed to alleviate the TCP time taken for the congestion window to change from BD /2m
unfairness. It performs well, but it is very complex to manage to BD /m is BD Td /2m. Assuming that the propagation delay
per-flow queues for all the uplink and downlink TCP flows. in wired link is negligible and that there is no congested link
Also Kim [7] proposed a scheme which improves per-station in the wired links, the round trip time can be approximated
fairness of TCP flows based on channel access time of each by the average queueing delay at the AP data queue, i.e.,
mobile station. It works well but requires some computational
3BD
work in AP. Td = .
4pRA
III. P ROPOSED S CHEME We can derive the average downlink TCP data rate per flow
Many researchers found the cause of the uplink and down- Rd as
BD /m
link TCP unfairness at the MAC layer or at the interaction BD /2m
wdw pRA
between the MAC and the transport layer protocols. The Rd = = . (1)
BD Td /2m m
solution for this problem varies depending on how one sees the
cause of the problem. As [1] and [6], we consider that the main When BD ≥ mwr , the downlink TCP data packets can not
cause of the unfairness is the packet dropping mechanism at fill up the data queue, and no dropping occurs at the data
the AP queue. The AP queue has two types of TCP packets. queue. In this case, the average queueing delay of packets at
One type is the data packets for downlink TCP flows and AP corresponds to that of M/M/1 queueing system. Since the
another type is the ACK packets corresponding to uplink TCP round trip time is approximated by the average delay at AP,
flows. These two types of packets are buffered at the same then
1
queue, but the consequence of a packet drop is quite different. Td = , pRA > mRd.
If a data packet for a downlink TCP flow is dropped, the pRA − mRd
congestion window of the TCP flow is reduced to half by The TCP window size of downlink flows increases to wr ,
duplicate ACKs. However if an ACK packet for an uplink flow since no dropping occurs at the AP queue. Then the average
is dropped at the AP queue, the TCP congestion window size downlink TCP data rate per flow is expressed as
may not change due to the cumulative TCP ACK mechanism. wr
When packets are dropped at the AP queue, the congestion Rd = = wr (pRA − mRd ),
Td
window size of a downlink TCP flow tends to decrease,
while the probability for an uplink TCP flow to decrease its and it leads to
congestion window is relatively small. wr pRA pRA
We propose a dual queue based scheme to alleviate this Rd = ≈ ,
1 + mwr m
asymmetric behavior. Two queues are employed at AP; one
queue for the downlink TCP data packets, and another queue which is the same as (1).
for the ACK packets corresponding to the uplink TCP flows. For uplink TCP flows, packet dropping at the AP queue does
The AP can control the service rate of each queue by selecting not affect the congestion window size due to the cumulative
each queue with different probability when MAC service ACK mechanism, and so we assume that the congestion
is ready. To determine the selection probability, we need window size of uplink TCP flows are fixed at wr . Therefore,
to analyze the relation between TCP behavior and the dual the average uplink TCP data rate per flow Ru is
queue. The analysis of the per-flow queue in [6] can easily be wr
applied to the dual queue scheme by examining the similarities Ru = ,
Tu
between the two schemes.
Let BD and BA denote the data queue size and the where Tu is the average round trip time of downlink TCP
ACK queue size, respectively. Also wr denotes the receiver flows. Here Tu can be approximated by the average queueing
advertised window size, and b denotes the number of data time at AP, too.
packets for an uplink TCP flow that are acknowledged by one When BA < nwr /b, the ACK queue of AP is always filled
ACK. If the delayed ACK of TCP is not used, the value of b with ACKs. Therefore every ACK packets should wait for
is 1. The data queue is selected with probability p, while the BA /qRA , i.e. the time taken for BA packets to be transmitted.
ACK queue is selected with probability q (= 1 − p). Let RA Then the average uplink TCP data rate per flow is
denote the bandwidth used by AP. Then pRA is the service wr
Ru = qRA .
rate of the data queue and qRA is the service rate of the ACK BA
queue. Assume that there are m downlink and n uplink TCP
When BA ≥ nwr /b, no packet dropping occurs at the
flows.
AP ACK queue, and the average queueing time at the AP
When BD < mwr , the data queue can be a bottleneck for
corresponds M/M/1 queueing delay.
downlink TCP flows and packet dropping may occur at the
data queue of AP. In the steady state, each of the downlink 1
Tu = , qRA > nRu /b.
TCP flows operates in the congestion avoidance state and the qRA − nRu /b

1-4244-0357-X/06/$20.00 ©2006 IEEE

TABLE I
... S IMULATION PARAMETERS

Parameters Values Parameters Values
Channel Rate 2Mbps Propagation Delay 1µsec
Slot Time 20µsec CWmin 31
SIFS 10µsec CWmax 1023
DIFS 50µsec Packet Size 1500bytes

IV. S IMULATION S TUDY
100Mbps Links
To verify the effectiveness of the proposed scheme, we per-
formed simulations with ns-2 [8]. Figure 1 shows the network
topology used in the simulation. There are m downlink and
n uplink TCP flows at the AP. Each station has only one
Uplink TCP Flows Downlink TCP Flows flow, and each flow shares neither a source nor a destination
node. The capacity of the wired links is 100 Mbps and the
propagation time is 5 ms for each link. Simulation parameters
for the wireless link are given in Table I and are compatible
... with the IEEE 802.11b standard. Since the bandwidth of the
wired link is much higher than that of the wireless link,
the wireless link is the only bottleneck link. The maximum
Fig. 1. Network Topology
window size (wr ) of each flow is set to 42 and the delayed
ACK of TCP is not used (b = 1).
First we ran simulation for the basic AP employing the
Hence the average uplink TCP data rate per flow is expressed droptail queue scheme. In this case, both downlink TCP data
as packets and uplink TCP ACK packets were buffered in a single
Ru = wr (qRA − nRu /b), queue. The queue size was set to 100 packets and there were
two uplink flows and six downlink flows. Figure 2(a) shows
and it becomes the throughput of each flow for this scenario. The unfairness
among the uplink and downlink TCP flows is significant. The
wr qRA bqRA average throughput of uplink flows are 543 kbps while that of
Ru = ≈ .
1 + wr n/b n downlink flows is 167 kbps.
Figure 2(b) shows the result when the dual queue scheme
Then the up/down throughput ratio R is is deployed to the same scenario, where U1 and U2 denote
the uplink TCP flows and D1-D6 denote the downlink TCP
mq wr
if BA < nwr /b flows. In this case we set the size of data queue and ACK
R = Ru /Rd = p BA
(2)
mq
np b if BA ≥ nwr /b. queue to 50 packets each, because the queue was divided into
two. We can see that the fairness among uplink and downlink
In order to achieve the fairness among uplink and downlink TCP flows improves greatly with the dual queue scheme. The
flows, the average uplink and downlink TCP data rates should average throughput of uplink and downlink flows are 166 kbps
be same. From (2) and p + q = 1, we can derive the necessary and 173 kbps respectively. There is little difference among the
values for p and q as throughputs of uplink and downlink flows. If we simulate with
a larger queue size, the fairness will be better.
mwr To show that the queue selection probabilities (3) and (4)
mwr +BA if BA < nwr /b
p= (3)
mb
mb+n if BA ≥ nwr /b, are reasonable, we simulated with various number of flows.
We considered three cases. In the first case, the number of
downlink flows was varied from two to ten while the number
BA
mwr +BA if BA < nwr /b of uplink flows was fixed at two. In the second case, the
q= (4)
n
mb+n if BA ≥ nwr /b. numbers of uplink and downlink flows were equal, and the
numbers were changed from two to ten with an increment of
In the dual queue scheme, the AP selects a queue to transmit two. In the third case, the number of uplink flows was varied
a packet using the queue selection probability p and q. In order from two to ten while the number of downlink flows was fixed
to implement this scheme, AP should classify a packet as a at two. Figure 3 shows the average throughput ratio between
data packet or an ACK packet, and know the number of uplink uplink and downlink flows in log scale. Figure 3(a) is the
and downlink flows. One may classify the packets into two result of the first case, and Fig. 3(b) is the result of the second
categories by simply checking the packet size. The number of case. In the case of the single queue scheme, the throughput
uplink and downlink TCP flows can be counted by reading ratio increases enormously as the number of flows increases.
the TCP header of packets. But the ratio stays near 1 at the dual queue scheme, which

1-4244-0357-X/06/$20.00 ©2006 IEEE

600 250

500
200
Throughput (Kbps)

Throughput (Kbps)
400
150
300
100
200

50
100

0 0
U1 U2 D1 D2 D3 D4 D5 D6 U1 U2 D1 D2 D3 D4 D5 D6
(a) Single Queue (b) Dual Queue

Fig. 2. TCP throughput (2 uplink flows and 6 downlink flows)

1000 1000
single queue single queue
dual queue dual queue
Throughput ratio (up/down)

Throughput ratio (up/down)

100 100

10 10

1 1

0.1 0.1
2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10
Number of downlink flows Number of uplink/downlink flows
(a) Throughput ratio vs. the number of downlink flows (b) Throughput ratio vs. the number of uplink and downlink flows

Fig. 3. TCP throughput ratio

1 1

0.8 0.8
Fairness index

Fairness index

0.6 0.6

0.4 0.4

0.2 0.2
single queue single queue
dual queue dual queue
0 0
2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10
Number of downlink flows Number of uplink/downlink flows
(a) Fairness index vs. the number of downlink flows (b) Fairness index vs. the number of uplink and downlink flows

Fig. 4. Fairness index

1-4244-0357-X/06/$20.00 ©2006 IEEE

TABLE II
FAIRNESS INDEX WITH DIFFERENT PROPAGATION TIME
1
Propagation Time 10ms 20ms 45ms 70ms 95ms
0.8 Group A with Single Queue Scheme 0.508 0.507 0.502 0.505 0.513
Fairness index

Group A with Dual Queue Scheme 0.993 0.959 0.994 0.990 0.938
0.6 Group B with Single Queue Scheme 0.503 0.503 0.502 0.506 0.541
Group B with Dual Queue Scheme 0.996 0.973 0.999 0.999 0.986

0.4

0.2
modified the propagation time of the two wired links from 5ms
single queue to 10-95ms. Each of the modified links connects the router and
dual queue a wired node, one for an uplink TCP flow and the other for
0
10 100 1000 a downlink TCP flow. The fairness indices in Table II were
Queue size (packets) calculated among the six flows with short RTT (group A) and
Fig. 5. Fairness index as the queue size changed also between the two flows with long RTT (group B). The
result shows that the dual queue scheme gives much higher
fairness than the single queue scheme even when there exist
implies excellent fairness among uplink and downlink TCP significant RTT differences among flows.
flows. From the result we can say that the selection probability V. C ONCLUSION
for each queue was chosen properly. Also we calculated the
In this paper, we presented the dual queue scheme to solve
Jain’s fairness index [9] to quantify the fairness for each flow.
the TCP unfairness among uplink and downlink flows. In order
As shown in Fig. 4, in the case of the single queue scheme, the
to solve this problem, we studied the packet dropping behavior
fairness becomes worse as the number of flows increases. For
and derived a simple formula for the throughput ratio between
the dual queue scheme, the fairness index is nearly 1 when
uplink and downlink flows in the dual queue scheme. To make
the number of uplink flows is fixed to two, implying that
the throughput ratio equal to 1, we showed how to set the
high degree of fairness is achieved among the flows. When
queue selection probabilities for the downlink data queue and
the number of uplink flows increases, the fairness index for
the uplink ACK queue. The simulation results show that the
the dual queue scheme drops by 27%, but it is still much
TCP fairness for the dual queue scheme improves significantly
better than the single queue scheme. In the third case, the
compared to that of the single queue scheme. The advantage
average throughput ratio between uplink and downlink flows
of the dual queue scheme is that it is very simple, effective,
increases from 5.66 to 549 as the number of uplink flows
requires only small changes in the AP interface queue and no
increases for the single queue scheme, while it decreases from
changes in the MAC layer.
0.91 to 0.43 for the dual queue scheme. The fairness index
varies between 0.477 and 0.671 for the single queue scheme ACKNOWLEDGEMENT
while it varies between 0.556 and 0.997 for the dual queue This work has been partly supported by POSCO. We thank
scheme. This means that the dual queue scheme shows better Eun-chan Park for the discussions on TCP unfairness problem
performance than the single queue scheme in the third case and the helpful suggestions.
also. The graphs related to this case were not presented here
due to space limitation. R EFERENCES
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TCP Fairness for Uplink and Downlink Flows in WLANs

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