Performance and security issues of TCP bulk data transfer in a last mile wireless scenario investigations and solutions

PERFORMANCE AND SECURITY ISSUES OF TCP BULK DATA
TRANSFER IN A LAST MILE WIRELESS SCENARIO:
INVESTIGATIONS AND SOLUTIONS

VENKATESH S OBANAIK

NATIONAL UNIVERSITY OF SINGAPORE

2003


PERFORMANCE AND SECURITY ISSUES OF TCP BULK DATA
TRANSFER IN A LAST MILE WIRELESS SCENARIO:
INVESTIGATIONS AND SOLUTIONS

VENKATESH S OBANAIK
(B.Tech Electronics and Communication Engineering)

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
(COMPUTER SCIENCE)

DEPARTMENT OF COMPUTER SCIENCE
NATIONAL UNIVERSITY OF SINGAPORE
2003


Acknowledgements

It is often said that “It takes an artist to turn a piece of stone into a work of art”.
Fortunately, I had the privilege of rubbing shoulders with many such artists who

contributed in chiselling my ideas into this thesis work. This thesis would not have
been possible without the:
Invaluable suggestions and encouragement of my supervisor Dr. Lillykutty Jacob,
Constructive criticisms and support of my co-supervisor Dr. Ananda A L,
Useful interactions with the research community on various mailing lists like IPv6,
Iperf, Tcpdump and Linux Users Group,
Ample infrastructure and a good research environment in CIR,
Fruitful discussions with my former colleagues Saravanan, Srijith and Michael,
Lively ambience and encouraging atmosphere at CIR due to my friends
Sudharshan, Rahul, Sridhar and Aurbind,
Timely help by Sridhar with LA TE X,

ii


Acknowledgements

iii

Soothing songs on Gold 90 FM which accompanied me on lonely nights in CIR,
And wonderful episodes of Seinfeld that kept me going till the end.
Finally, I extend my gratitude to everybody, who in one way or the other rendered
their support and help.


Summary

TCP was designed nearly three decades ago with some inherent assumptions. Over
the years many fixes and solutions have been proposed to make TCP cope with
changing network conditions. This research work investigates some of the proposed

solutions, studies their applicability and/or limitations in the last mile wireless scenario and proposes novel solutions. Two specific issues are addressed in this thesis:
(a) The effect of algorithms that improve the fairness of TCP congestion avoidance
on slow links and long thin networks, (b) The combined issue of performance and
security in a wired-cum-wireless scenario.

The first part of the thesis demonstrates that fairness algorithms have a detrimental effect on connections traversing slow links and long thin networks. Simulations and test-bed experiments substantiate this claim. Some solutions are suggested
to overcome the performance degradation.

iv


SUMMARY

v

The second part of the thesis explores the limitations of existing solutions for
improving TCP performance in hybrid wired-wireless networks. The thesis proposes
an integrated solution for IP security and TCP performance in hybrid wired-wireless
networks, traditionally dealt with in a mutually exclusive manner. The novel scheme
called the SPEP (Secure Performance Enhancing Proxy) ensures end-to-end security, enhances TCP performance, and offers multifarious benefits over the existing
schemes. The SPEP scheme was implemented in FreeBSD 4.5 and performance
tests were conducted in a controlled test-bed setup. The results show remarkable
improvement in TCP performance in a “last mile wireless” scenario.


Contents

Acknowledgements

ii

Summary

iv

Contents

vi

List of Figures

ix

List of Tables

xi

1 Introduction

1

1.1

Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

1.2

Research Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

1.3

Thesis Contribution . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

1.4

Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

2 Background Work

6

2.1

TCP Congestion Avoidance and Control . . . . . . . . . . . . . . . .

6

2.2

Issues with TCP Congestion Avoidance and Control . . . . . . . . . .

7

2.2.1

Unfairness of TCP Congestion Avoidance . . . . . . . . . . . .

7

2.2.2

Inability to Identify the Nature of Loss . . . . . . . . . . . . .

8

Solutions Proposed to Address the Issues . . . . . . . . . . . . . . . .

9

2.3

2.3.1
2.3.2

Algorithms That Improve Fairness of TCP Congestion Avoidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

Performance Enhancing Schemes for TCP over Wireless . . . . 11

2.4

Limitations of the Proposed Solutions . . . . . . . . . . . . . . . . . . 13

2.5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
vi


CONTENTS

vii

3 Fairness Algorithms and Performance Implications

15

3.1

Simulation Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2

Simulation Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.3

3.2.1

Behaviour of IBK, CR and CANIT Policies on Connections
that Traverse Slow Links and Long Thin Networks . . . . . . 20

3.2.2

Impact of Last-hop Router Buffer Size on Performance . . . . 23

3.2.3

Impact of Selectively Disabling the Policies on Performance . . 25

3.2.4

Impact of Advertising a Limited Receive Window on Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Test-bed Experiments

. . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.3.1

Test Configuration . . . . . . . . . . . . . . . . . . . . . . . . 29

3.3.2

Impact of IBK, CANIT and CR Policies on Slow Link and
LTN Connections . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.3.3

Impact of Last Hop Router Buffer Size on Performance . . . . 33

3.3.4

Impact of Receiver’s Advertised Window on Performance . . . 34

3.3.5

Impact of Selectively Disabling Fairness Policies on Slow Links
and Long Thin Networks . . . . . . . . . . . . . . . . . . . . . 35

3.4

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4 SPEP: Secure Performance Enhancing Proxy

38

4.1

Related Work and Issues . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.2

The SPEP Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.3

4.4

4.2.1

SPEP Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.2.2

SPEP Design Considerations . . . . . . . . . . . . . . . . . . . 50

4.2.3

SPEP Implementation Description . . . . . . . . . . . . . . . 52

Behavior of SPEP under Different Conditions . . . . . . . . . . . . . 54
4.3.1

Presence of Packet Reordering . . . . . . . . . . . . . . . . . . 55

4.3.2

SPEP Mobile Handoff Scenario . . . . . . . . . . . . . . . . . 56

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58


CONTENTS

viii

5 SPEP: Test Methodology and Performance Evalulation

59

5.1

Test Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.2

Performance Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.3

SPEP Approach: Merits . . . . . . . . . . . . . . . . . . . . . . . . . 64

5.4

Problems Encountered . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

6 Conclusion

69

6.1

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

6.2

Review of thesis objectives . . . . . . . . . . . . . . . . . . . . . . . . 71

6.3

Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Bibliography

73

A Appendix I

80

A.1 Papers published related to thesis . . . . . . . . . . . . . . . . . . . . 80
List of Abbreviations

80


List of Figures

3.1

Simulation topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.2

Congestion window variation with arrival of ACKs for slow link connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3

Congestion window variation with arrival of ACKs for LTN connection 22

3.4

Goodput and loss for slow link connection . . . . . . . . . . . . . . . 24

3.5

Goodput and loss for LTN connection . . . . . . . . . . . . . . . . . . 24

3.6

RTT variation for different buffer size . . . . . . . . . . . . . . . . . . 25

3.7

Test configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.8

Nokia D211 PCMCIA multimode radio card . . . . . . . . . . . . . . 30

3.9

Variation of congestion window for slow link connection . . . . . . . . 31

3.10 Variation of congestion window for LTN connection . . . . . . . . . . 32
4.1

Split-Connection approach . . . . . . . . . . . . . . . . . . . . . . . . 40

4.2

Snoop approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.3

Freeze-TCP approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.4

The SPEP approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.5

IPv6 header . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1

SPEP test configuration . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.2

Congestion window variation for LAN scenario (1 error in every 32KB) 61

5.3

Time-Sequence graph LAN scenario (1 error in every 32KB) . . . . . 62

5.4

Throughput of New Reno with and without SPEP for LAN scenario
ix

62


LIST OF FIGURES

x

5.5

Congestion window variation for WAN scenario (1 error in every
32KB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.6

Time-Sequence graph WAN scenario (1 error in every 32KB)

5.7

Throughput of New Reno with and without SPEP for WAN scenario

5.8

Throughput of SPEP with NewReno V/s SPEP . . . . . . . . . . . . 65

. . . . 64
64


List of Tables

3.1

Performance comparison for test configuration 1 . . . . . . . . . . . . 23

3.2

Performance comparison for test configuration 2 . . . . . . . . . . . . 23

3.3

Performance of slow link connection when policies are selectively disabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.4

Performance of LTN connection when policies are selectively disabled

3.5

Performance of slow link connection with a limited receive window . . 28

3.6

Performance of LTN connection with a limited receive window . . . . 28

3.7

Performance of slow link and LTN for various congestion avoidance
policies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.8

Effect of buffer size on goodput of the slow link connection . . . . . . 33

3.9

Effect of buffer size on goodput of the LTN connection . . . . . . . . 34

27

3.10 Effect of receiver window on slow link connection . . . . . . . . . . . 34
3.11 Effect of receiver window on LTN connection . . . . . . . . . . . . . . 34
3.12 Performance of slow link connection when policies are selectively disabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.13 Performance of LTN link connection when policies are selectively disabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.1

SPEP loss detection and distinction algorithm . . . . . . . . . . . . . 53

4.2

SPEP behavior in presence of packet reordering . . . . . . . . . . . . 55

4.3

SPEP before handoff operation . . . . . . . . . . . . . . . . . . . . . 57

4.4

SPEP after handoff operation . . . . . . . . . . . . . . . . . . . . . . 57

xi


“The time to begin writing an article is when you have finished it to your
satisfaction. By that time you begin to clearly and logically perceive what it
is you really want to say.”
- Mark Twain

1
Introduction

T

he thesis studies the existing solutions for improving the performance
of TCP, investigates the applicability and/or limitations of the existing

schemes in “the last mile” wireless scenario and proposes efficient solutions for it.
The access network is colloquially known as “the last mile”. The connection from
local service provider to the consumers is referred to as the “local loop” or “the last
mile” [1]. Originally developed to support telephony traffic, the “local loop” of the

telecommunications network now supports both voice and Internet traffic. Different
media can be used to provide “the last mile” connectivity, such as telephone wire,
coaxial cable, fibre optics, satellite communications and wireless RF [1]. Wireless
access to the Internet is referred to as the “last mile wireless” scenario. The “last mile
wireless” is seen as a viable and cost-effective solution for last mile connectivity [2].
Hence, it becomes essential to take a second look at the existing solutions in the
1


Chapter 1. Introduction

2

context of the last mile wireless scenario. The following sections of this chapter
describe the motivation for the research work, research objectives, contribution and
the organization of the thesis, respectively.

1.1

Motivation

TCP was designed nearly three decades ago and fine tuned over the years for traditional networks comprising of wired networks and fixed hosts. However, the networks have changed over the years from wired to wireless, low bandwidth to very
high bandwidth, stationary host to mobile host, and infrastructure based networks
to ad-hoc networks. Meanwhile, Internet applications have become more demanding
and versatile. Among today’s applications are interactive applications demanding a
quick response time, bulk data transfer applications requiring high throughput and
multimedia applications sensitive to jitter. Nevertheless, TCP continues to be the
most widely used transport layer protocol. In an attempt to equip TCP to make
better use of the network and meet the demands of the user applications, many solutions have been proposed. The thesis investigates two specific solutions proposed to
fine tune TCP and discusses the limitations of such schemes in the last mile wireless

scenario and proposes solutions to circumvent the encountered problems.


Chapter 1. Introduction

1.2

3

Research Objectives

The thesis aims to study the existing solutions for improving the performance of
TCP and to identify specific issues for further study. Investigations into the issues
concerning the applicability and/or limitations of existing schemes in the context of
the last mile wireless scenario and providing efficient solutions constitute the over-all
objectives of the thesis.

1.3

Thesis Contribution

The thesis identifies two issues with the existing schemes designed to improve the
performance of TCP: (i) The detrimental effects of fairness algorithms on the performance of slow links [3] and Long Thin Network [4](LTN); (ii) The limitations of
the existing performance enhancement schemes for TCP over wireless to function in
an end-to-end IPSEC environment.

Simulation and test bed experiments were conducted as part of the detailed
investigations to study the effect of algorithms that improve the fairness of TCP
congestion avoidance on the performance of slow links and LTN. Our results show
that the fairness algorithms have adverse effects on connections traversing either

slow links or LTN. We argue that it is not appropriate to apply the fairness algorithms for connections that traverse slow links or LTN. We have studied some of the
possible solutions (increasing the last-hop router buffer size, reducing the advertised


Chapter 1. Introduction

4

window of the receiver and selectively disabling the fairness policies) in order to circumvent the adverse effects of fairness algorithms in the last mile scenario. We show
that the impact can be reduced by selectively turning off the policies for slow link
or LTN connections. In the second part of the thesis, we present a detailed survey
of the co-existence of security and performance enhancing schemes in the last mile
wireless scenario. We expose the limitations of the existing solutions in providing
both end-to-end security and improved transport layer performance. We propose an
innovative mechanism, which we call Secure Performance Enhancing Proxy (SPEP)
to address the seemingly arduous problem of enhancing TCP performance over wireless networks, preserving end-to-end TCP semantics as well as ensuring end-to-end
security. We have implemented the proposed scheme in FreeBSD 4.5 and conducted
experiments in a controlled test bed setup. Our results show improved TCP performance in a secured environment with introduction of about 7 % overhead when
compared to the end-to-end ELN scheme in a WAN scenario with high error rates
of 1 error in every 16KB of data.

1.4

Thesis Organization

The thesis is organized as follows. Chapter 2 describes related work for enhancing
the performance of TCP data transfer in a last mile wireless scenario and identifies two specific issues: (i) The effect of algorithms that improve the fairness of
TCP congestion avoidance on the performance of slow links and LTN; (ii) The combined issue of performance and security in a last mile wireless scenario. Chapter 3

Chapter 1. Introduction

5

presents the setup for simulation and the test-bed experiments conducted to study
the effect of fairness algorithms on performance of slow links and LTN and provides
recommendations. Chapter 4 presents our novel approach called SPEP which provides a solution for the co-existence of IPSEC and performance enhancing solutions.
Chapter 5 describes our test methodology to evaluate the performance of SPEP
and presents performance results. We conclude with an indication of future work in
Chapter 6.


“To look backward for a while is to refresh the eye, to restore it, and to render
it more fit for its prime function of looking forward”
- Margaret Fairless Barber

2
Background Work

T

his chapter introduces the reader to the prior work related to improving
the performance of TCP and describes the specific issues addressed by

the thesis. The following sections discuss congestion control mechanisms of TCP,
the issues with TCP congestion avoidance and control, and solutions proposed to
address the issues.

2.1

TCP Congestion Avoidance and Control

TCP is an end-to-end connection-oriented transport layer protocol which ensures
reliable transfer of data. TCP uses a window based congestion control algorithm to
reduce congestion in the network. It is a self-clocking protocol and automatically
6


Chapter 2. Background Work

7

adjusts to the bandwidth of the network. At the start of the connection, TCP
probes the network capacity by sending out packets at an increasingly exponential
rate. This is the slow start phase and it continues until the slow start threshold is
reached or a packet is lost. TCP then enters the congestion avoidance phase and
sends out packets at a linear rate.

2.2

Issues with TCP Congestion Avoidance and
Control

TCP congestion avoidance and control [5] was originally proposed by Van Jacobson
in one of the seminal papers. It was proposed to solve a series of ‘congestion collapses’
that occurred during 1986. The congestion avoidance and control mechanism, later
supplemented with fast recovery and fast retransmit mechanisms became the de
facto standard [6] for TCP. However, there are some issues with TCP congestion
control. Two specific issues are discussed in the following subsections.

2.2.1

Unfairness of TCP Congestion Avoidance

Fairness is an important criterion in the design of congestion control mechanisms.
One way to define fairness is that if multiple TCP connections share a bottleneck
link, the available bandwidth is shared equally among all the connections. However,
it is seen that when a bottleneck link is shared by multiple connections with short
and long round trip times (RTTs), the short RTT connections get a greater share
of the bottleneck bandwidth [7, 8]. TCP uses the slow start mechanism to probe


Chapter 2. Background Work

8

the network at the start of a connection, time spent in the slow start phase is
directly proportional to the RTT. And for a long RTT connection, it means that
TCP stays in the slow start phase for a longer time when compared to a short
RTT connection. This drastically reduces the throughput of short duration TCP
connections. Furthermore, following each packet loss, TCP enters the congestion
avoidance phase or even the slow start (in case of retransmission timeout). During
the congestion avoidance phase, the TCP sender increases its congestion window by
atmost 1 segment after each RTT [5], thus the connections with long RTT open up
their congestion windows relatively slower when compared to the connections with
short RTT. In an attempt to counter the bias of the congestion avoidance mechanism
against long RTT connections, and in effect, to improve the fairness, many policies
have been proposed [7, 9, 10] which will be discussed in Section 2.3.1. The policies
were designed to enable long RTT connections to open up their congestion windows
relatively fast.

2.2.2

Inability to Identify the Nature of Loss

TCP was designed for wired networks with an inherent assumption that packet loss
caused by damage in the network is very small and that the loss of a packet always
signals congestion [5]. TCP congestion avoidance and control procedures are invoked
on detection of a packet loss. The occurrence of packet loss is indicated by either
a retransmission timer timeout or the receipt of duplicate acknowledgements [6, 11].
However, packet loss can occur for reasons other than congestion. Communication
over wireless links is affected by high bit error rate, temporary disconnections, high


Chapter 2. Background Work

9

latencies and low bandwidth. Losses due to bit-error rate and mobility of devices
has a significant effect on the dynamics of TCP resulting in sub-optimal performance
and reduced throughput for the connection.

2.3

Solutions Proposed to Address the Issues

In this section, we discuss the various solutions proposed to address and resolve the
issues mentioned in Section 2.2.1 and Section 2.2.2.

2.3.1

Algorithms That Improve Fairness of TCP Congestion
Avoidance

In an attempt to counter the bias of TCP congestion avoidance against long RTT
connections, various alternate congestion avoidance policies have been proposed.
The “Constant Rate” [9] algorithm was one of the proposed solutions. In this
scheme it is suggested that congestion window be increased by ‘c ∗ r2 ’ segments
for each RTT, where ‘c’ is some fixed constant and ‘r’ is the average round trip
time. In the standard congestion avoidance algorithm, the congestion window is
increased at the rate of approximately 1 segment every RTT. If ‘r’ is the average
RTT of the connection, the increase in throughput of the connection would be
‘1/r’ segment/s every ‘r’ seconds. This means the rate of increase in throughput is
‘1/r2 ’ segments/s/s. Therefore the long RTT connections suffer. The suggestions
in [8,9] was to modify the additive increase policy so that all connections, irrespective
of their RTT, increase their sending rate similarly. Hence, it was referred to as


Chapter 2. Background Work

10

“Constant-Rate” window increase algorithm. However, the choice of a proper value
for the constant ‘c’ is an open problem. According to the studies conducted in [7],
the fairness properties were best at values of ‘c’ less than 100 and large values of
‘c’ made the connections very aggressive. However, smaller values of ‘c’ resulted in
under utilization of the link. As mentioned in [7], in reality, the choice of a proper
value for ‘c’ is not possible.

“Increase-by-K” (IBK) [7] was another policy that was suggested. The IBK policy was designed so that long RTT connections could increase their own throughput

without co-operation from other connections. The IBK policy suggested that the
congestion window should be increased by ‘K’ segments every RTT. The policy was
to be selectively enabled, only on the long RTT connections. The values of ‘K’ up
to 4 was recommended for good performance [7].

Another algorithm that was proposed was Congestion Avoidance with Normalized Interval of Time (CANIT) [10]. In the long RTT connections the arrival of
ACK packets is relatively slow, compared to short RTT connections. CANIT addresses the fairness problem in this perspective. CANIT introduces a new parameter
called Normalized Interval of Time (NIT). The congestion avoidance mechanism is
modified to increase the congestion window at the rate of

RT T
N IT

·

1
Cwnd

on receipt of

every ACK. Thus, all the connections increase their congestion window by the same
amount after each interval NIT. CANIT with NIT value of 30ms was considered to
be most fair.


Chapter 2. Background Work

2.3.2

11

Performance Enhancing Schemes for TCP over Wireless

The performance enhancing schemes can be broadly classified into three categories:

• Link-layer schemes
• Split Connection schemes
• End-to-End Schemes

A. Link-layer Schemes

The link-layer schemes address the problem from the perspective that the cause of
suboptimal performance of TCP over wireless is the transmission error that occurs on
the wireless link. Hence, the link-layer schemes propose reliable link-layer protocols
to address the problem. The link-layer protocols employ two classes of techniques (i)
error correction using forward error correction (FEC), and (ii) retransmission of lost
packets using automatic repeat request (ARQ). The link-layer protocols for digital
cellular techniques CDMA and TDMA primarily use ARQ. The AIRMAIL [12]
protocol uses a combination of FEC and ARQ for loss recovery.

B. Split Connection Schemes

Split connection schemes attribute the performance degradation of TCP over wireless to the inability of TCP to cope with the dynamics of wireless link. The split
connection schemes as the name suggests, splits the TCP connection from sender


Chapter 2. Background Work

12

to receiver at the base station referred to as the mobility support node. One connection is established between the sender and the base station and the other from
the base station to the receiver. Split connection schemes such as I-TCP [13]use
regular TCP for its connection over the wireless link, while other schemes such as
MTCP [14], recommend a protocol optimized for wireless links to be used for the
connection between the base station and the receiver.

C. End-to-End Schemes

End-to-end schemes abide by the principle that end-to-end argument is one of the
architectural principles in the design of the Internet. Hence, all problems with
TCP have to be solved end-to-end. The schemes which maintain the end-to-end
semantics are Freeze-TCP [15], TCP HACK [16], SACK [17]. Freeze-TCP proposes
an end-to-end solution to enable TCP to cope with long periods of disconnection
due to degraded wireless link. The TCP receiver in Freeze-TCP advertises a zero
window in case of an imminent link failure. The sender reacts to the zero window
advertisement by freezing all retransmit timers and entering persist mode. TCP
HACK [16] proposes a scheme, in which the receiver can distinguish between the
nature of loss congestion or corruption. The information about the nature of loss is
conveyed to the sender. The sender retransmits the packets lost due to corruption
without invoking congestion control while the packets lost due to congestion are
handled normally. SACK [17] provides a mechanism which enables the TCP sender
to recover from multiple losses in a window of data transmitted. The SACK enabled
receiver uses the TCP SACK option to acknowledge the blocks of data received in


Chapter 2. Background Work

13

sequence. The sender retransmits the lost segments, thereby reducing the number

of probable retransmission timeouts.

D. Other Schemes

The Snoop protocol [18] proposed by Hari et al is another performance enhancement
scheme, which was designed with the intent that local problems should be solved
locally. Therefore, Snoop suppresses the duplicate acknowledgements that signal loss
in wireless link and locally retransmit the lost segments. Thereby, Snoop achieves a
remarkable improvement in the performance of TCP over wireless links.

2.4

Limitations of the Proposed Solutions

The solutions proposed in Section 2.3 overcome the shortcomings of TCP mentioned
in Section 2.2. However, the solutions take a myopic view of the problem. Although
the proposed solutions attempt to resolve the issue at hand, they have some limitations which restrict their applicability. In the following chapters, the drawbacks are
exposed and solutions are proposed to counter the same.

2.5

Summary

This chapter presents a brief description of TCP congestion avoidance and control
and the issues concerning it. Two specific issues with TCP congestion avoidance and
control mechanism are identified for further examination, namely: (i) the unfairness