CONGESTION CONTROL WITH DYNAMIC THRESHOLD

ADAPTATION AND CROSS-LAYER RESPONSE FOR TCP

OVER IEEE 80.11 WIRELESS NETWORKS

Rung-Shiang Cheng

, Der-Jiunn Deng

and Han-Chieh Chao

Department of Computer and Communication, Kun Shan University, Yung Kang, Taiwan

Department of Computer Science and Information Engineering, National Changhua University of Education

Changhua City, Taiwan

Department of Electronic Engineering, National Ilan University, Ilan City, Taiwan

Keywords: TCP, Congestion control, IEEE 802.11, Cross-layer response.

Abstract: Wireless technologies provide mobile access and enable rapid and cost-effective network deployment. But a

wireless link is generally accompanied by high interference, transmission errors and a varying latency. The

erratic packet losses usually lead to a curbing of the flow of segments on the TCP connection, and thus limit

TCP performance. This paper presents a threshold control mechanism with cross-layer response approach

for improving TCP Vegas performance in IEEE 802.11 wireless networks. By making slight modifications

to the legacy IEEE 802.11 MAC and TCP, the numerical results reveal that the proposed scheme provides a

significant improvement in TCP performance under IEEE 802.11 wireless environments.

1 INTRODUCTION

The proliferation of mobile computing devices in

recent years has led to the extensive deployment of

wireless communication networks. More and more

devices (such as PCs, Laptops, Smart phones,

Tablets) are equipped with network interface cards

that integrate wireless technologies (such as

3G/UMTS/GPRS and Wi-Fi) to connect to the

networks. New standards for wireless

communications (IEEE Standard 802.11, 1999)

(IEEE Standard 802.16-2004, 2004) have greatly

improved data transmission rate and prompted the

development of high-speed wireless communication

services capable of supporting the anticipated

introduction of a wide range of new applications

over the coming years.

Recently, TCP and IEEE 802.11 have become

the most commonly applied transport protocol for

Internet applications and the standard of choice for

wireless local area networks, respectively. In IEEE

802.11, wireless communication applications are

commonly constrained by scalability problems.

which fail to achieve a reasonable throughput as the

number of hosts increases since CSMA/CA access

schemes have extremely low spatial-reuse

efficiency. As a result, the link layer reliability

deteriorates rapidly as the scale of the network

increases, and thus the number of link layer

contentions increases significantly.

TCP Reno (Paxson, 1999) is currently the most

widely applied transport layer protocol for Internet

congestion control. Although Reno-based TCP (e.g.

NewReno, SACK) (Floyd, 1999) (Mathis, 1996) is

the most commonly used TCP, however, its

aggressive behavior causes a severe oscillation of

the congestion window size, and the resulting

periodic congestion leads to a limited throughput.

Hence, several bandwidth estimation schemes for

TCP have been proposed (Jain, 2002). Some TCP

bandwidth estimation schemes utilize the end-to-end

delay information derived from the measured packet

round-trip time (RTT). The delay-based approach,

originally described by Jain (Jain, 1989), is best

represented by TCP Vegas (Brakmo, 1995).

Unlike Reno-based TCP, TCP Vegas implements

a more precise congestion avoidance mechanism

based on packet delay rather than the packet loss.

Although TCP Vegas yields both a higher

throughput and a lower packet-loss ratio than TCP

Reno does, however, TCP Vegas fails to obtain a

fair share of the bandwidth when competing with

TCP Reno in heterogeneous networks. Therefore, in

(Cheng, 2010), we proposed an adaptive TCP

Cheng R., Deng D. and Chao H..

CONGESTION CONTROL WITH DYNAMIC THRESHOLD ADAPTATION AND CROSS-LAYER RESPONSE FOR TCP OVER IEEE 80.11 WIRELESS

NETWORKS.

DOI: 10.5220/0003677100950100

In Proceedings of the International Conference on Wireless Information Networks and Systems (WINSYS-2011), pages 95-100

ISBN: 978-989-8425-73-7

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

congestion control scheme which dynamically

adjusts the threshold parameters being used to define

the congestion window size in such a way as to

improve the buffer occupancy of TCP Vegas when

competing with TCP Reno.

Since the performance of data transfer not only

relies on the support of the end-to-end transport

layer, but also on the quality of the hop-by-hop

communication link, the present study attempts to

enhance the legacy IEEE 802.11 MAC and TCP

protocols such that TCP is rendered capable of

differentiating between corruption and congestion

losses by using information received from the MAC

layer. In this paper, the current study proposes a

collaborative cross-layer approach based on the

threshold adjustment mechanism and negative-ACK

scheme for enhancing the performance of TCP

Vegas in wireless networks.

The remainder of this paper is organized as

follows. Section 2 briefly describes the IEEE 802.11

media access control scheme and the TCP protocol.

Section 3 describes the implementation of the cross-

layer TCP Vegas and IEEE 802.11 MAC protocol.

Section 4 compares the performance of the cross-

layer scheme with that of TCP Vegas by performing

a series of numerical simulations. Finally, Section 5

presents some brief conclusions.

2 RELATED WORKS

2.1 Media Access in IEEE 802.11 MAC

The IEEE 802.11 standard defines two types of

service, namely a contention-free polling-based

point coordination function (PCF) and a contention-

based distributed coordination function (DCF). In

infrastructure-based networks, DCF can operate

either alone or in conjunction with PCF. However,

in ad hoc networks, DCF operates alone. DCF is the

basic access method for the 802.11 standard and is

based on the conventional carrier sense multiple

access with collision avoidance (CSMA/CA)

scheme. DCF comprises both a basic access method

and an optional channel access method based on

RTS/CTS exchanges.

In 802.11, priority access to the wireless medium

is controlled by applying an inter-frame space (IFS)

time between the frame transmissions. To prevent

collisions, the transmitter is obliged to wait for the

availability of the channel for a specified interval of

time, designated as the distributed inter-frame space

(DIFS) before sending a frame. If the medium is

currently busy or becomes busy during this interval,

the transmitter defers the frame transmission until it

detects a DIFS.

Due to the half-duplex nature of wireless

interfaces, stations in the network are unable to

detect a collision simply by listening to their own

transmissions. Therefore, an immediate positive

acknowledgment technique is employed to confirm

the successful reception of a frame. Specifically,

having received a frame, the receiver waits for a

short inter-frame space (SIFS) and then transmits a

positive MAC acknowledgment to the transmitter

confirming that the frame has been correctly

received. The SIFS is deliberately assigned a shorter

interval than the DIFS in order to assign the

receiving station a higher priority than any other

stations waiting for making a transmission. The

ACK is only transmitted when the frame is received

correctly, and hence if the transmitter does not

receive an ACK, it assumes that the data frame must

have been lost and therefore schedules a

retransmission. Figure 1(a) illustrates the basic

operations involved in 802.11 DCF.

To alleviate the hidden-station problem (Kim,

1997), 802.11 DCF also provides an optional

channel access method using a virtual carrier sensing

mechanism based on two special control frames,

namely request-to-send (RTS) and clear-to-send

(CTS). As shown in Figure 1(b), before transmitting

a frame, the transmitter transmits an RTS frame

asking the receiver if the medium in its vicinity is

free. Once the receiver receives this RTS frame and

assumes that no interfering transmission is present, it

waits for the specified SIFS interval and then sends a

CTS frame to the transmitter. Both transmitter and

receiver neighbors overhear these frames and

consider the medium reserved for the transmission

duration.

(a) Basic access mechanism.

(b) Virtual carrier sensing mechanism.

Figure 1: Media access mechanisms in IEEE 802.11 DCF.

The IEEE 802.11 MAC layer provides a reliable

communication link by handling packet delivery

using the MAC positive ACK and retransmission

WINSYS 2011 - International Conference on Wireless Information Networks and Systems

mechanisms (i.e. an automatic repeat request,

(ARQ)). However, if several transmission attempts

of a packet (e.g. 7 times for the basic access

mechanism and 4 times for the virtual carrier

sensing mechanism) by a sender in a wireless

network fail, the sender simply assumes that the

receiver is no longer within its transmission range

and gives up its transmission attempt.

As described above, the stop-and-wait schemes

such as the IEEE 802.11 ARQ scheme are highly

inefficient due to the idle time spent by the sender

waiting for the receiver's positive ACK message to

arrive. When the end-to-end path comprises multiple

wireless links, the accumulated backoff delays and

losses further reduce the network throughput, and

thus increasing the underutilization of the wireless

links. Hence, it is desirable to enhance the

interaction between the MAC layer and the transport

layer in order to achieve the level of transmission

throughput demanded by communication protocols

such as TCP.

2.2 Congestion Control in TCP Vegas

TCP Vegas (Brakmo, 1995) adopts the measured

RTT to calculate the number of data packets which

the sender can inject into the network without

incurring packet loss. While doing this, Vegas

attempts to maintain the number of the packets

queued in the router buffer within two limits,

denoted as α and β, respectively. If the measured

RTT increases, Vegas recognizes that the network is

becoming congested and throttles the congestion

window down. Conversely, if the RTT reduces,

Vegas recognizes the availability of more bandwidth

and therefore increases the congestion window size

accordingly. Each time a new ACK is received, the

sender updates the congestion window size in

accordance with the difference (δ) between the

expected throughput and the actual throughput.

According to the definition in (Chengrs, 2010), δ is

represented as the number of the packets queued in

the buffer of end-to-end path and can be expressed

()

minRTT

TmeasuredRT

minRTT













−=

×−= t throughpuActualt throughpuExpected

(1)

Here, the expected throughput represents the

available bandwidth for the current TCP connection

in the absence of network congestion, while the

actual throughput represents the bandwidth currently

used by the TCP connection. Finally, minRTT repre-

sents the minimum value of the measured RTT.

Vegas utilizes two threshold parameters α and β

to control the maximum difference between the

expected and actual throughputs. The congestion

window size, W, is increased by one packet if δ < α

and is decreased by one packet if α > β (the default

values of α and β are 1 and 3, respectively). In other

words, having calculated

, the sender updates W in

accordance with











>−

otherwise,

if,1

βδ

αδ

(2)

To detect and avoid congestion during the slow-start

phase, Vegas doubles the window size every two

RTT intervals rather than every RTT interval as it

does in Reno. Furthermore, when receiving a

duplicate ACK, the sender checks whether the

difference between the current time and the sending

time plus the minRTT of the relevant packet is

greater than the timeout value. If it is, Vegas

retransmits the packet immediately without waiting

for the arrival of three duplicate ACKs. This

modification resolves the problem that the sender

never actually receives three duplicate ACKs and

must therefore rely on the occurrence of the coarse-

grain timeout and therefore yields a significant

improvement in the throughput.

3 DESCRIPTION OF PROPOSED

METHOD

The IEEE 802.11 link layer protocol supports

reliable data to transfer locally while the TCP

transport layer protocol supports reliable data to

transfer end-to-end. This section proposes a

collaborative approach for TCP Vegas enhancement.

3.1 Extension to IEEE 802.11 MAC

In standard IEEE 802.11 DCF schemes, whenever a

node fails to transmit a frame (e.g. channel

collisions), it retransmits that frame and then

increases the value of the retry limit parameter by

one. However, if the retry transmission exceeds a

specified retry threshold, the IEEE 802.11 MAC

protocol reports a link breakage and then discards

the transmitting frame. To reduce the effects of link

layer packet corrupt on the performance of TCP, this

study extends the IEEE 802.11 DCF. Whenever a

frame transmission failure occurs, a negative-ACK

option is triggered by modified link layer protocol in

accordance with the following function:

CONGESTION CONTROL WITH DYNAMIC THRESHOLD ADAPTATION AND CROSS-LAYER RESPONSE FOR

TCP OVER IEEE 80.11 WIRELESS NETWORKS

If the number of retry limit parameter exceeds a

specified threshold {

If receiving of TCP ACK within the same flow

{

Forward the negative-ACK option piggybacks

using the reverse TCP ACK along the end-to-end

path. }

}

Else {

Increase the contention window and then launch

the backoff procedure.

}

In the extended protocol, if a TCP data frame is

discarded after several retransmission attempts, the

modified link layer protocol triggers a negative-

ACK as the sequence number associates with the

dropped packet and then piggybacks this option by a

reverse TCP ACK in order to avoid increasing

contention in the link layer. This cross-layer support

from the link layer protocol ensures that the

transport layer TCP protocol is aware of the

transmission error in the link layer and can then

react to this error in accordance with the wireless

corruption information.

The extended link layer protocol provides

additional information for natively wired TCP

protocols to the wireless environment and for

improving their performance. A major advantage of

the proposed scheme is that it eliminates the

requirement of forward nodes to cache

unacknowledged packets for every TCP connection

passing through it and is required to be fully

supported at all of the wireless stations in the

wireless network.

3.2 Modification to Vegas Congestion

Control

To estimate the buffer currently available in each

round-trip time interval, the expected bandwidth for

TCP at the bottleneck link in the network is first

derived. The queue length at the bottleneck is then

approximately measured in each round-trip time

interval from the inter-arrival times of the returned

ACK messages. The available buffer size along the

path between the sender and the receiver is then

estimated from the derived value of the expected

bandwidth and the measured value of the queue

length. Finally, the TCP Vegas threshold parameters

and

are adjusted in accordance with the

estimated buffer length, thereby, changing the size

of the congestion window indirectly and

consequently improving the network utilization. The

details of the proposed Vegas transmission control

scheme are described in the following.

Let maxRTT be the maximum measured RTT,

and λ denote the bottleneck link bandwidth as

calculated by the packet-pair scheme (i.e. the

number of the acknowledged packets divided by the

average ACK inter-arrival time) (Keshav, 1991). Let

represent the measured buffer size in last RTT.

According to the information obtained from last

RTT, Q

can then be calculated as:

()

WW W

minRTT maxRTT maxRTT

minRTT

ρρλ





=⋅ − +−⋅−









(3)

where

is a smoothing factor, and Q

is obtained by

the exponentially weighted moving average (EWMA)

method.

Let Q

denote the average observed buffer size

which can be held in the end-to-end path. To prevent

the estimated queue length from wild fluctuation, Q

is configured by using the EWMA as:

· Q

t-1

()

−1 ·

(4)

where the parameter,

indicates the ratio of TCP

sender transmission time to its measured round-trip

delay time. The value of

varies with the bandwidth

utilization of the sender in the previous round-trip.

According to Eq. (4), the threshold values α and

β are then adjusted as follows:

2-1 where

timeoutgrainedcoarseif),default(1

losspacket if,2

durationpreset

a beyond status stable in staying continueif,2

status stable in staying coutinueif,

≤≤











(5)

1.5. is of luedefault va theand ,3 where

timeoutgrainedcoarseif),default(3

≤≤







⋅

(6)

When receive an ACK, the modified Vegas

regulates its transmission rate in accordance with the

following function:

If receiving a new ACK {

Adjust the threshold parameters α and β according to

Eqs. (5) and (6) and then regulates the congestion

window size based on the following function that

mentioned in Eqs. (1) and (2):











>−

otherwise,

if,1

βδ

αδ

}

Else {

If receiving a triple-duplicate ACK, and the

WINSYS 2011 - International Conference on Wireless Information Networks and Systems

next expected packet is not a "corrupted

packet" {

Retransmit the next expected packet and set W

= W/2 and W = W

. Then modified Vegas enter

the regular TCP Fast recovery phase. }

}

If receiving the ACK which contains a negative-

ACK option {

Recording sequence number of the "corrupted

packet" obtained from negative-ACK, and then

retransmitting the corrupted packet.

}

In the proposed approach, the negative-ACK number

of the corrupted packet is tracked using a circular

queue. By inspecting the historical negative-ACK

information, the modified TCP can clearly identify

corruption losses in the wireless links and therefore

determine whether it is necessary to invoke the

regular congestion control mechanisms. Hence, if

retransmitted packets are lost repeatedly as a result

of transmission errors, the original packets can be

retransmitted immediately upon receipt of an ACK

with a negative-ACK option without waiting for

timeout expiry or for the "round-trip" time to elapse

as in conventional TCP error recovery schemes.

4 NUMERICAL RESULTS

In the simulations, the ns-2 (Network Simulator, NS-

2) 802.11 MAC layer and transport layer modules

were both extended to model the proposed cross-

layer functionality. The parameter values used in the

simulations are summarized in Table 1. The default

sizes of RTS, CTS and ACK packets are assigned to

20 bytes, 14 bytes and 14 bytes, respectively.

Furthermore, the two-ray ground reflection model

(Rappaport, 1960) was used to simulate signal

propagation in the wireless network.

Table 1: Default parameters for MAC and physical layers.

Parameter 802.11g

SLOT 9 μsec

SIFS 10 μsec

DIFS 28 μsec

PHY

hdr

192 bits

min

max

1024

In the following simulations, packet errors are

introduced into the wireless link (indicated by dotted

line) using the two-state Markov error model

(Gilbert, 1960) (see Figure 2). The packet corruption

rates P

and P

are set as 0.0005 and 0.001,

respectively, and P

, P

and P

are set to

0.9, 0.1, 0.85 and 0.15, respectively. The rate at

which errors occur in the GE model depends on

channel conditions.

Figure 2: Two-state Markov error model.

Figure 3 gives the simulation topology when two

TCP flavors (Reno and Vegas) coexist.

Figure 3: Simulation model with link-level errors.

Table 2 compares the proposed Vegas

modification with the original Vegas. As shown in

Table 2, packets transmitted on wireless channels

are frequently affected by burst errors which cause

the MAC layer to discard the frame, resulting in the

loss of the corresponding packet in the TCP

protocol. In this case, although Reno still has a better

performance, it can be seen that the variation

between mechanisms is decreased. Transmission

rate of Reno is not able to increase stably because of

bit error and provide Vegas more opportunities to

obtain more bandwidth. From results shown in Table

2, it can be seen that the proposed Vegas

modification decreases the variation between the

two mechanisms and achieves better bandwidth

utilization.

Table 2: Interaction of TCP in heterogeneous

environments (with link-level bit error).

TCP flavors Average goodput

Reno:Vegas 23.59 Mbps:16.53 Mbps

Reno:Modieied Vegas 19.04 Mbps:19.76 Mbps

To further examine the performance of the

proposed scheme under a more realistic and

complicated network environment, a series of

simulations is conducted on randomly generated ad

hoc network topologies. In these simulations, the

CONGESTION CONTROL WITH DYNAMIC THRESHOLD ADAPTATION AND CROSS-LAYER RESPONSE FOR

TCP OVER IEEE 80.11 WIRELESS NETWORKS

network region is 100 m × 100 m with 20 wireless

stations, and TX

range

and PCS

range

are set to 40 m and

85 m, respectively. As illustrated in Figure 4, at the

beginning of each simulation, wireless stations are

randomly placed within this region, and a random ad

hoc topology is constructed. The number of short-

lived TCP connections varies from two to nine.

Moreover, the TCP source and the TCP destination

of each connection are randomly selected.

Figure 4: Illustrative random topology.

Table 3 presents the TCP goodputs of the

proposed mechanism and other TCP flavors when

the path lengths are 2-hop and 3-hop, respectively.

As shown in Table 3, the TCP goodput is inversely

proportional to the number of connections. This

phenomenon is caused by the competition for the

bandwidth among TCP connections and the carrier

sense dependencies over wireless stations.

Comparing the results in Table 3, it is obvious that

the proposed Modified Vegas achieves a better

performance than those of other TCP schemes.

Table 3: TCP average goodput comparison (Only the TCP

goodput with established connection is calculated).

Number of

connection

Reno Vegas Modified Vegas

2 11.58Mbps 11.51Mbps 14.52Mbps

3 9.76 Mbps 10.32Mbps 12.51Mbps

4 8.75 Mbps 8.74 Mbps 10.82Mbps

5 7.24 Mbps 8.20 Mbps 9.32 Mbps

6 6.53 Mbps 7.11 Mbps 8.90 Mbps

7 6.50 Mbps 7.04 Mbps 8.37 Mbps

5 CONCLUSIONS

This paper has proposed a cross-layer approach

designated to enhance the performance of TCP

Vegas in wireless networks. In the proposed

approach, a cross-layer mechanism is employed in

both the transport layer and the MAC layer to

provide explicit corruption loss information. The

results confirm that the proposed scheme has a

number of key advantages compared to the

conventional TCP, including a more efficient

treatment on frequent transmission losses, a faster

reaction to corruption losses, and the ability to

distinguish between congestion errors and

transmission errors and to take appropriate remedial

action which is particularly advantageous for the

deployment in heterogeneous wireless environments.

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