Rate Based congestion Control Scheme for ABRService in ATM Networks:

A Three-Factor Congestion Detection Approach

BY

Anderson Tiong Ing Heng

A Thesis submitted in full fulfillment of therequirements for the degree of Doctorate of Sciencein Electronics & Telecommunications Engineering

FACULTY OF ENGINEERINGUNIVERSITI MALAYSIA SARAWAK

YEAR 2000

Special dedication to Li chu, Te Ming and Wei En

TABLE OF CONTENTS

I

DEDICATIONTABLE OF CONTENTSLIST OF FIGURELIST OF TABLEACRONYMSABSTRACT

CHAPTERS

1 NETWORK FLOW AND CONGESTION CONTROL

1.1 INTRODUCTION1.2 CONGESTIONANDBUFFERSPACE1.3 CONGESTIONANDHIGHSPEEDLINKS1.4 CONGESTIONANDHIGHSPEEDPROCESSOR1.5 FLOWCONTROL1.6 CONGESTIONCONTROLI .6.I CONGESTIONCONTROLREQUIREMENTS1.7 CONGESTION AVOIDANCE1.7.1 CONGESTION AVOIDANCEANDFEEDBACK SCHEME1.7.2 CONGESTIONAVOIDANCESCHEMECOMPONENTS1.8RELATEDWORKSONCONGESTIONCONTROLOFABRINATMNETWORKS1.9 THESISORGANIZATION

2 FRAMEWORK OF ATM ABR SERVICE

2.1 BACKGROUNDONTHEEVOLUTIONOFATMABRSERVICES2.2ABRRATED-BASEDTRAFFICMANAGEMENTMODEL2.3 ABRPARAMETERS2.4 RM CELL AND ITS FORMATS

3 PERFORMANCE AND CONGESTION CONTROL ISSUES OF

ATM ABR SERVICES

3.1 BASIC BEHAVIOUR OF ABR

3.2 ABR CONGESTION CONTROL FRAMEWORK

3.3 EXISTINGCONGESTIONCONTROLSCHEMES3.3.1 CREDITSCKEME3.3.2 RATESCHEME3.3.2.1 FECNANDBECNRATECONTROL SCHEME

3.3.2.2 PROPORTIONALRATECONTROLALGORITHM(PRCA)3.3.2.3 ENHANCEI) PROPORTIONAL RATE CONTROL

ALGORITHM (EPRCA)

3.3.2.4 MAX-MIN RATE CONTROL ALGORITHM

ii. . .1 1 1

vi

x i

X

xi

11

23445678

1 011

131 31 41 4

1 7181 91 91 92021

2223

,..1 1 1

4 SIMULATION TOOLS AND FRAMEWORK FOR ATM ABR SERVICE 4.1 SIMULATION TOOLS AND FRAMEWORK 4.2 ATM SiMllLATlON MODEL 4 .2 .1 SIMULATION MODEL 4.2.2 EPRCA, MMRCA, NEW CONGESTION CONTROL SCHEME AND ENHANCED NEW CONGESTION CONTROL SCHEMES 4.3 SIMULATION FRAMEWORK 4.4 SIMUALTION PARAMETER

5 STUDY AND ANALYSIS O F SIMULATION RESULTS FOR EPRCA AND MMRCA 5.1 SIMULATION OF THE EPRCA SCHEME 5.2 SIMULATION OF THE MMRCA SCHEME

6 A NEW RATE BASED CONGESTION CONTROL SCHEME 6.1 DESIGN OBJECTIVES OF THE NEW RATE BASED

" - CONGESTION CONTROL SCHEME 6.2 A THREE-FACTOR APPROACH FOR CONGESTION CONTROL ALGORITHM 6.3 NEW ALGORITHM 6.4 MATHEMATICAL ANALYSIS OF THE NEW RATE CONGESTION CONTROL ALGORITHM 6.4.1 NO CONGESTION STATE FOR EPRCA 6.4.2 CONGESTION STATE FOR EPRCA 6.4.3 NO CONGESTION STATE FOR TI-IE NEW RATE BASED CONGESTION CONTROL SCHEME 6.4.4 CONGESTION STATE FOR THE NEW RATE BASED CONGESTION CONTROL SCHEME 6.5 SIMULATION RESULTS FOR NEW RATE BASED CONGESTION CONTROL SCHEME 6.5.1 NO CONGESTION STATE 6.5.2 MILD CONGESTION STATE 6.5.3 SEVERE CONGESTION STATE

7 ENHANCEMENT O F THE NEW RATE BASED CONGESTION CONTROL SCHEME 7.1 ENHANCEMENT OF THE NEW RATE BASED CONGESTION CONTROL SCHEME 7.2 ENHANCED SCHEME ONE

7.3 SIMULATION OF THE ENHANCED SCHEME ONE 7.4 ENHANCED SCHEME TWO 7.5 SIMULATION RESULTS OF THE ENHANCED SCHEME TWO

8 CONCLUSION AND FUTURE WORK 8.1 CONCLUSION '

8.2 FUTURE WORK

BIBLIOGRAPHY APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E

LIST OF FIGURES

Figure

1.1 Queue with long and infinite delay1.2 Congestion at the interconnection of high and low speed links

,1x 1.3 Processor with high speed links1.4 Flow control between source and destination1.5 Congestion control focuses at the network load1.6 Behavior of the system against the control and the feedback

response1.7 Network performance as a function of load1.8 Congestion avoidance scheme model2.1 Bandwidth prioritization for CBR, VBR and ABR2.2 Forward and backward resource management cell2.3 RM cell format3.1 Behavior of allowed cell rate

a\ rx 3.2 Credit flow control scheme3.3 Rate Based flow control scheme4.1 ATM simulation platform4.2 ATM simulation configuration4.3 ATM simulation model4.4 Traffic intensity to simulate no, mild and severe congestion state5.1 RM cell return rate simulated for ERPCA (no congestion state)5.2 CCRs of VCs simulated for EPRCA (no congestion state)5.3 Throughput & queue occupancy of Atom SW 1 simulated for

EPRCA (no congest ion s tate)

Page

5710141516181920282828303233

345.4 RM cell return rate simulated for ERPCA (mild congestion state) 345.5 CCRs of VCs simulated for EPRCA (mild congestion state) 355.6 Throughput & queue occupancy of Atom SW 1 simulated for

EPRCA (mild congestion state) 35

5.7 RM cell return rate simulated for ERPCA (severe congestion state) 365.8 CCRs of VCs simulated for EPRCA (severe congestion state) 365.9 Throughput & queue occupancy of Atom SW 1 simulated for

EPRCA (severe congestion state) 375.10 Throughput of link1 simulated for EPRCA

(severe congestion state) 375.11 Estimated & computed fairshare 38

5.12 RM cell return rate simulated for MMRCA (no congestion state) 385.13 CCRs of VCs simulated for MMRCA (no congestion state) 395.14 Throughput & queue occupancy of link 1 simulated for MMRCA

(no congestion state) 395.14aThroughput & queue length oElink 1 simulated for MMRCA

at s teady s ta te 40

vi

5.15 RM cell return rate simulated for ERPCA (mild congestion state) 40 5.16 CCRs of VCs simulated for MMRCA (mild congestion state) 4 1 5.17 Throughput & queue occupancy of Atom Sw 1 simulated for

MMRCA (mild congestion state) 4 1

5.18 RM cell return rate simulated for MMRCA (severe congestion state) 42

5.19 CCRs of VCs simulated for MMRCA (severe congestion state) 42 5.20 Throughput & queue occupancy of Atom Swl simulated for

MMRCA(severe congestion state) 43

5.2 1 Throughput of linkl simulated for MMRCA (severe congestion state) 43

5.22 Comparison of cell loss for EPRCA and MMRCA 43 6.1 Network throughput against load 44 6.2 Queue length against load 45 6.3 Simulation model 48 6.4 RM cell return rate simulated for New Scheme

(no congestion state) 5 3 6.5 CCRs of VCs simulated for New Scheme (no congestion state) 53 6.6 Throughput & queue occupancy of Atom Swl simulated for New

Scheme (no congestion state) 54 6.7 Throughput and Maximum queue length (no congestion state) 54 6.8 RM cell return rate simulated for New Scheme

(mild congestion state) 5 5 6.9 CCRs of VCs simulated for New Scheme (mild congestion state) 55 6.10 Throughput & queue occupancy of link 1 simulated for

New Scheme (mild congestion state) 56 6.1 1 Throughput for EPRCA, MMRCA and the New Scheme

at no and mild congestion states 5 6 6.12 RM cell return rate simulated for New Scheme

(severe congestion state) 5 7 6.13 CCRs of VCs simulated for New Scheme

(severe congestion state) 57 6.14 Throughput gL queue occupancy of linkl simulated for

New Scheme (severe congestion state) 58 6.15 Thr-oughput of linkl simulated for New Scheme

(severe congestion state) 58 7.1 Throughput & queue occupancy of Atom Sw 1 simulated

for Enhanced Scheme. One (no congestion state) 62 7.2 Throughput & queue occupancy of Atom Swl simulated

for Enhanced Scheme One (mild congestion state) 63 7.3 Comparison of queue length (at severe congestion state) 61 7.4 Compar~son of throughput (at severe no congestion state) 64

vii

7.5 Queue Length & Throughput & of Atom Swl simulated for Enhanced Scheme Two (at no congestion state) 66

7.6 Queue Length & Throughput & of Atom Swl simulated for Enhanced Scheme Two(mi1d congestion state) 66

7.7 Queue occupancy of Atom Swl simulated for Enhanced Scheme Two (severe congestion state) 66

7.5 Load against cell lose for Enhanced Scheme One & Two 67 7.9 Cell lose against load for different rate based congestion ' control scheme 67 7.10 Cell lose against load for different rate based congestion

control scheme under heavy load 65 8.1 Cell Loss (under severe congestion state) 69

LIST OF TABLES

Figure Page

4.1 Simulation parameters for no and mild congestion state 30 4.2 Simulation parameters for severe congestion state 30 6.1 Cell loss and load factor threshold 59 6.2 Cell loss and dfactor 59

LIST OF ACRONYMS

AAL '' ABR

ACR AIR ATM BECN CBR CCR CI DPF EFCI

' - EPRCA ER FECN ICR IPF LB MACR MMRCA MCR PCR PRCA RIF RDF RM

ATM Adaptation Layer Available Bit Rate Allowed Cell Rate Additive Increase Rate Asynchronous Transfer Mode Backward Explicit Congestion Notification Constant Bit Rate Current Cell Rate Congestion Indicator Decrease Pressure Factor Explicit Forward Congestion Indication Enhanced Proportional Rate Control Algorithm Explicit Rate Forward Explicit Congestion Notification Initial Cell Rate Increase Pressure Factor Link Bandwidth Mean Allowed Cell Rate Maximum and Minimum Rate Control Algorithm Minimum Cell Rate Peak Cell Rate Proportional Rate Control Algorithm Rate Increase Factor Rate Decrease Factor Resource Management

Abstract

The convergence of data, audio and video has made Asynchronous Transfer Mode (ATM) a goodplatjorm for supporting multimedia applications. ATM network has the ability to support and deliver data, audio and video trafic on the same shared network. The available bit rate (ABR) service o f ATM is designed to efficiently support data trafic. The ATM trafic management 4.0 involves the design of a set of mechanisms which ensure that the network bandwidth, buffer and other resources are eficiently utilized as well as meeting the quality of service (QOS) guarantees given to the source as part of the traffic contract. Considerable standardization eflorts in the past years had been centered on the trafic management of ABR service. In this thesis, we examine the two leading rate based congestion control schemes, EPRCA and MMRCA, adopted by ATM Forum as standards. The EPRCA and MMRCA schemes have two deficiencies. First, they produce low throughput and low queue occupancy at nort congestion state. Secondly, they suffer cell lost at severe congestion state. These deficiencies caused an ineficient use of the network resources. A new rate based control congestion scheme is introduced to rectifL these deficiencies. The new scheme uses three metrics, namely; the load factor, queue length and the early congestion indicator to better detect and determine congestion. Its objectives are to keep the network operating at its optimum state with no cell lost and to minimize cell lost when the network inevitably falls into severe congestion state. The new scheme has shown a rnajor improvement of network throughput or utilization at nort congestion state and a low cell lost at severe congestion state. Two enhancement schemes were made to further reduce cell lost so as to achieve better perjbrmance at severe congestion state. Both enhanced scheme one and two have achieved a no cell lost at severe congestion state. However, when the network is stressed further, it is observed that enhanced scheme two is more robust and resilient than that of enhanced scheme one in operating under heavy congestion state. The two enhunced schemes are able to operate in and maintain at their optimum states in all the three states with the condition that the maximum input traflc is six times or less than the link bandwidth.

“. Abstract - -

Abstrak

Pertemuan antara data, audio dun video teloh membuat ‘Asynchronous TransferMode (ATM)’ sebagai suafu pelantaran untuk menyokong apiikasi multimedia.Rongkaian ATM mempunyai keupayaan untuk menyokong dan menghantur tru$kdata, audio dan video pada rangkaian yang satna. Perkhidmatan ’ available bitrate (ABR)’ ATM direka untuk menyokong trafik data dengan cekap. PengurusantraJik ATM 4.0 merangkumi rekaan suatu set mekanisme yang menentukan lebarjalur (bandwidth) rangkaian, penimbal (buffer) dan sumber lain digclnakansecekapnya dan memenuhi kualiti perkhidmatan (quality of service (QOS)) yangdijaminkan kepada sumber sebugai sebahagian daripadu kontrak trafik. Usahapempiawaian pada tahun-tahun yang lepas telah tertumpu kepada pengurusantraJik bugi perkhidmatan ABR. Dalam tesis ini, kami mengkaji dua skim kawalankesesakan, EPRCA dan MMRCA yang diterima oleh Forum ATM sebagaipiawaian. Skim EPRCA dan MMRCA mempunyai dua kekurangan. Pertama.pengeluarannya adalah rendah dan giliran penghunian yang rendah padakeadaan yang tidak sesak. Kedua, ia menghadapi kehilangan se1 pada keadaankesesakan yang teruk. Kekurangan ini menyebabkan penggunaan sumberrangkaian yang tidak cekap. Suatu skim kawalan kesesakan diperkenalkan untukmembetulkan kekurangan ini. Skim baru menggunakan tiga metrik, faktor muatan(load), panjang giliran dan penunjuk kesesakan yang awal untuk mengesan danmenentukan kesesakan dengan Iebih baik. Tujuannya ialah supaya rangkaianberoperasi pada keadaan yang optimum dengan tiada kehilangan se1 danmengurangkan kehilangan se1 apabila rangkaian berada dalam keadaankesesakan yang teruk. Skim yang baru telah menunjukkan peningkatan yang baikkepada pengeluaran atau penggunaan rangkuian pada keadaan t idak sesak dankehilangan se1 yang kurang pada keadaan kesesakan yang teruk. Dua skimpenambahan dibuat untuk mengurangkan lagi kehilungan se1 bagi mendapatpencapaian yang lebih baik pada keadaan kesesakan yang teruk. Kedua-dua skimpenambahan satu dan dua mencapai tiada kehilangan se1 pada keadaankesesakan yang teruk. Walau bagaimanapun, apabila tekanan rangkaiandilanjutkan, adalah didapati skim penambahan dua lebih tegap dan tahandaripada skim penambahan satu apabila beroperasi pada keadaan kesesakan.Kedua-dua skim penambahan berupaya beroperasi dan berada pada keadaanoptimum pada kesemua tiga kecldaan dengan syarat trafik kemasukan maksimaadalah enam kali atau kurang daripada pautan lebar jalur (link bandwidth).

xii

Chapter 1

Network Flow and Congestion Control

1.1 IntroductionNetwork flow and congestion have been a hot topic of researchamong the network experts. Network congestion exists in allnetworks regardless of their sizes and complexities. Whencongestion occurs in a network, its performance starts to decline. Ifno action is taken to remedy the congestion [ 11, its performance willdecline further until the network collapses where all the bandwidthwill be used unproductively for sending and re-sending of discardedpackets arising from buffer overflows. When that happens, allmission critical applications are not getting the requiredperformance from the underlying service provider. This has a directimpact on the application response time. Ones would always believethat with the advent of switching technologies, powerful processorsand bigger pipe of bandwidth, network congestion would become aproblem of the past. In fact, most network designers have found theopposite to be true. Congestion occurs when the demand is greaterthan the available resources. However, it is generally believed thatcongestion is caused by shortage of buffer space, slow links or slowprocessors. With the fulfillment of the three requirements,congestion will go away. This has led to the following three mythsabout congestion control as described by [34]:-

a ) Congestion is caused by shortage of buffer space and when thememory becomes cheap enough to allow infinite buffer space,congest ion wil l be solved.

b) Congestion is caused by slow links. When high speed linksbecome available, the problem will be rectif ied.

c ) Congestion is caused by slow processors. The problem will besolved when the speed of the processors are improved.

1.2 Congestion and Buffer SpaceI t has been found that congest ion problem cannot be solved with largebuffer space and it has also found out that networks with infinitememory switch are as susceptible to congestion as those with lowmemory as described by [34]. With too l i t t le memory, packets may bediscarded unnecessarily arising from the undesirable buffer overflowof the switch with a mere short burst of traffic. However, a switchwith an infinite memory space will have a queue with long or infinitedelay and thus will cause the system to time out as shown in Figure

Network Flow and Coneestion Control

1.1. and initiate for retransmission of packets repeatedly. In either case, precious network bandwidth is being wasted and the performance of applications are affected. In fact, too much memory is more harmful than too little memory for the packets are dropped after they have consumed the precious network resources.

+ Timed Out

Figure 1.1 Queue with long or infinite delay

1.3 Congestion and High Speed Links b

Congestion problem cannot be solved with high speed links. When a high speed LAN is connected to a low speed link, congestion at the interconnection becomes a problem. Figure 1.2 shows four nodes that are connected in series with 19.2 kbps links. The time to transfer a file was five minutes. After it was replaced with a faster link of IMbps, the transfer time was increased to seven hours. The packet arrival rate of the first router is higher than its departure rate. As such, queue is built up which leads to buffer overflows and packets lose that caused the transfer time to increase. It is pointed out that high speed link cannot stay in isolation. The slow speed link does not go away as high speed links are added to the network. The protocol has to be designed specifically to ensure that the increase of the low speed links do not degrade the network performance.

Network Flow and b y e s t i o n Control

19.2 kbps, Time to transfer a file = 5

1 Mbps 19.2 kbps, Time to transfer a file = 7

Figure 1.2 Congestion at the interconnection of high and low speed links

1.4 Congestion and High Speed Processor Congestion problem cannot be solved with high speed processors. The arguments for processors is similar to that of for links. The introduction of a high speed processor to an existing network will cause a mismatch of speeds and thus caused congestion. Figure 1.3 shows four processors, A,B,C and D which have a data delivery capacity of lGbps each. A and B simultaneously send lGbps to C which leads to a 2Gbps input to router. However, the output of the router is only lGbps and thereby causing congestion.

All links are 1 Gb/s

Figure 1.3 Processor with high speed links

The conclusion is that congestion is a dynamic problem that cannot be solved with just static solutions alone. A re-design has to be done to the protocol to protect the network in the event of congestion.

-, Network Flow and Congestion Control

1.5 Flow Control

Flow control regulates the traffic between the source and the destination so that the destination will not be swamped by packets arriving at a rate faster than it can receive as shown in Figure 1.4. When that happens, buffer starts to build up and eventually it overflows which will then lead to packet losses, re-transmission and degraded performance.

Figure 1.4 Flow control between source and destination

There are two types of flow control scheme, namely; window based and rate based. In a window-based scheme, the destination specifies the number of packets that a source can send. This will minimize or prevent buffer overflow at the destination. The source can further reduce the window upon receipt of congestion signal from the network. In the rate based scheme, the destination specifies a maximum rate in terms of packets per second or bit per second that a source is allowed to send. The current trend of control flow is towards rate based scheme.

Source

1.6 Congestion Control

a .- -

When the source or destination send packets at rate faster than the network can handle, network queues start to build up. Without additional control on the source/destination, packets will start to drop leading retransmission and performance degradation. Congestion control deals with the control of traffic so as to reduce network load as shown in Figure 1.6. Flow control solves the problem of destination resource being the bottleneck and whereas congestion control solves the problem of intermediate routers and links being the bottleneck.

Destination A

buffer 7

- a

Pletwork Flow and Co-1

Figure 1.5 Congestion control focuses at the network load

Congestion control scheme consists of a feedback and control mechanism. It is well known in control theory that the control frequency should be equal to the feedback frequency as shown in Figure 1.5. If the control is faster than the feedback, the system will oscillate and becomes unstable. However, if the feedback is faster than the control, the system will be tardy or sluggish and is slow in response to changes. Another principle of control theory is that no scheme can solve congestion that last less than its feedback delay.

A

Fast State

Time

Figure 1.6 Behavior of the system against the control and the feedback response.

1.6.1 Congestion Control Requirements Despite the fact that a number of schemes have been proposed for congestion control, the search for new scheme continues. The following requirements for congestion control as discussed by [34] make it difficult to provide a satisfactory solution:-

a) Low overhead. The scheme must have a low overhead. It should not increase network traffic during congestion. This is one of the reasons why explicit feedback message scheme is considered undesirable. Some network researchers even suggested that feedback be sent only during low load, thus in the absence of feedback would automatically indicate

.?a

*I Network Flow and Congestion Control

congestion. Even such scheme is not desirable as networkresource can be better used by other applications.

b) Fair. Fairness may not be important during low load when allthe demands can be met. However, during congestion when theresource is less than the demand, it is imperative that resourcebe allocated fairly among the contending sources. Thedefinition of fairness varies from one source to another andthere is no one universally accepted definition for fairness.

c) Responsive. The available bandwidth or capacity on a networkis a constantly changing quantity. The available capacity of thenetwork is increased or decreased as the nodes or links goes upor down. The demand for network resources changes as thenumber of users or sessions is increased or decreased. As such,the congestion control scheme is required to match thedemands dynamically to the available network capacity.Therefore, it should ask the users to increase the demand ifthere is additional capacity available and to decrease it whenthe demand exceed the capacity.

d ) Exhaustive environment. When the network is under stress orcongestion, the rate of transmission error, deadlocks and lostpackets increases considerably. The scheme should be able towork and function under these conditions.

1.7 Congestion Avoidance

A scheme that allows network to operate at knee as shown in Figure1.7 [38] is called congestion avoidance scheme. It ensures that usersare encouraged to increase their load as long as this does not affectthe response time. If it does, then users are to decrease their loadand re-adjust the network to operate at its optimal state. Withoutnetwork congestion control, the network may cease operating,whereas network can operate without congestion avoidance for along time. Congestion control is a curing process whereascongestion avoidance is a prevention process.

6

- - . , - . - Network Flow and C o w n control ~JNlV4~,RSI'I'I MA! ,AJr7IA Y1'4.1) t

L o n 6

Figure 1.7 Network performance as a function of load [38]

1.7.1 Congestion Avoidance Feedback Scheme. Congestion control and congestion avoidance are dynamic control systems which according to [35] can consist of a feedback and a control mechanisms. The feedback mechanism informs the source of the current state of the network. Whereas the control mechanism allows the source to adjust its load on the network. The feedback signal of a congestion avoidance scheme tells the source whether the network is operating below or above the knee. The feedback signal of a congestion control scheme tells the source whether the network is operating below or above the cliff. The feedback mechanism for congestion avoidance scheme has the following schemes:-

a) Congestion feedback via packet sent from routers to sources. This scheme is also known as choke packet whereby a control packet is generated at a congested node and transmitted back to the source to restrict traffic flow. An example of choke packet is the Internet Control Message Protocol (ICMP) source quench message. Upon receipt of a source quench message, the source is to cut back its sending rate until it no longer receives the source quench messages. This scheme introduces additional traffic into the network during congestion and is thus considered undesirable.

b) Feedback included in the routing messages exchanged among routers.

c) End-to-end probe packets sent by the sources.

d) Each packet contains a congestion feedback field that is filled in by the routers in packets going in the reverse direction.

e) A congestion feedback field is filled in by the routers in packets in the forward direction.

Netwmk Flow and Conzestion - Control

1.7.2 Congestion Avoidance Scheme Components The two keys component of congestion avoidance scheme are the feedback mechanism and the control mechanism. They are belng referred to as network policies and user policies respectively. The network policy consists of three algorithms: congestion detection, feedback filter and feedback selector. The user policy also consists three algorithms: signal filter, decision function and increaseldecrease

" A algorithm as presented in Figure 1.8:

a) Congestion Detection: Before the network can feedback any information to the source, it nust determine its state or load level. It may be under-utilized (below the knee) or over- utilized (above the cliff). This can be determined based on link utilization, buffer availability , or queue length. It is found out that queue length is the best mechanism to determine if the network is operating above or below the knee.

* a*

b) Feedback Filter: After the network has determined its load level, its feedback is only useful to the source if the state lasts long enough for the source to act based on it. A state that changes very fast may lead to confusion because by the time the source receives the feedback, the state no longer holds. Therefore, a low pass filter function is needed to pass those states that are expected to last long enough for the user action to be meaningful. The exponential weighted average is example of a low pass filter function.

c) Feedback Selector: It is used to determine the set of users to be notified. The network may want all users to reduce the traffic or it may selectively ask some users to reduce and some others to increase the traffic.

d) Signal Filter: The sources receiving the feedback signals from the network need to interpret the signal as all the signals may not be identical. Some may indicate that the network is under loaded while other may indicated that it over loaded. The sources need to combine these signals to decide its action.

e) Decision Function: Once the source knows the network load level, it has to decide either to increase the load or to decrease it load. The function can be broken into two part; the first part determines the direction and the other determines the amount. They are being referred to as the decision function and the increaseldecrease function. The decision function decides how often should the rate be changed. Changing it too often leads to

Network Flow and C-trol

unnecessary oscillations, on the other hands, changing it infrequently leads to a system that takes a long time to adapt.

f ) Increase/Decrease Algorithm: The purpose of the algorithm is to determine the amount by which the rate should be changed once a decision is made to adjust it. The search for the increaseldecrease function is limited to the first order linear function.

Increase: W,, = aWold + b Decrease: W,, = cW,, - d

where W,,, and W,, are the old and new rates of the source respectively and a,b,c and d are the rate increase or decrease variables.

The four alternatives are as follows; i) Multiplicative Increase, Additive decrease (b = 0, c =1 ) ii) Multiplicative Increase, Multiplicative Decrease (b = 0, d

= 0) iii) Additive Increase, Additive Decrease (a = 1, c = 1) iv) Additive Increase Multiplicative Decrease ( a = 1, d = 0)

The choices of the alternatives and parameter values are governed by the following goals:

a) Efficiency: The system should be operating at the knee. b) Fairness: The sources sharing a common bottleneck should get

the same throughput c) Minimum Convergence Time: The system should reach its

optimal state as soon as possible. d) Minimum Oscillation: Once the system reaches its optimal

state, the source windows oscillate continuously below and above this state. The parameters should be chosen such that the oscillation size is minimal.

s* - Network Flow and Conpestion Control

SourceIDestination Switch

Control Feedback

Figure 1.8 Congestion avoidance scheme components

* ** 1.8 Related Works on Congestion Control of ABR in ATM Networks

One of the success factors for ATM networks is the resolution of a scheme that is able to adequately control the flow of data into the ATM networks so as to avoid congestion at the network switching point [6]. Two types of control mechanism have been proposed by [ l l ] for the ATM networks: preventive and reactive mechanism. Preventative mechanism deals with traffic control whereas reactive mechanism deals with congestion control. In spite of these preventive control measures, congestion can still occur. Therefore, reactive actions are necessary to minimize the intensity, spread and duration of the congestion [12]. ABR service also specifies a rate based feedback mechanism for congestion control. A number of different rate based congestion schemes have been proposed in the literature [30]. ,

Congestion control requires extensive exchange of information among the nodes of the networks. The exchange occurs either through the header of the data cells or through the resource management cells. Two types of notification are used, forward and backward congestion notifications [23]. Several congestion control schemes of ATM ABR service have been proposed based on the these two congestion notification techniques [l 11, namely the proportional rate control algorithm (PRCA) and the enhanced proportional rate control algorithm (EPRCA).

Over the years, several proposals have been made to further enhance the EPRCA scheme. In [ll], a threshold based control algorithm scheme is proposed to enhance that of EPRCA. The scheme depends solely on the predefined queue threshold and the queue occupancy. The scheme also allows cells with cell loss priority (CLP) = 1 to be

Network Flow and -n C-1

discarded by any immediate node even if it is not congested. In addition, when the threshold is reached, all cells are discarded regardless of their quality of service (QOS) requirements. Modified enhanced proportional rate control algorithm (M-EPRCA) proposed by [15] also uses queue length and length gradient policy (LGP) as a basis to detect congestion. If the queue exceeds the threshold or when input buffer exceeds a pre-assigned threshold, the switch is said to be entering a congestion state. The scheme has claimed to produce better performance characteristic than the EPRCA scheme.

Modified enhanced proportional rate control algorithm (EPRCAM) proposed by [21] also uses queue thresholds to determine congestion and the exponential weighted average to estimate fairshare. EPRCAM is different fiom EPRCA in that the rate of increase during no congestion state is calculated as

ER = MIN (ER, MACR * (1 + RIFs)

where RIFS is the rate increase factor of the switch. The scheme has shown improvement in both performance as well as fairshare over that of EPRCA scheme.

All the above schemes including the maximum and minimum rate control algorithm (MMRCA) proposed by [14] are mainly using the queue thresholds to determine congestion. It is discussed in [48] that queue threshold detection scheme is slow in detecting congestion. In fact, congestion is only detected after the queue has been built up, in which case the network is already in the stage of congestion.

The past researches on EPRCA scheme are mainly focused on the enhancement of EPRCA's performance at the steady state [6, 7, 9, 11, 15,20, 21,27,30,46]. None of the research works has been done on the enhancement of performance for EPRCA scheme at no and severe congestion states. The objective of this thesis is therefore, to concentrate on the enhancement of the network performance of the two leading rate based congestion control algorithms, EPRCA and Maximum and Minimum Rate Control Algorithm (MMRCA), at no and severe congestion states. A new rate based congestion control scheme follows by two of the other enhancement schemes will be proposed to enhance the network performance of EPRCA in terms of network throughput and cell loss at no and severe congestion states.

1.9 Thesis organization The rest of the thesis will be arranged as follows;

Chapter 2 will cover the framework of the ATM ABR service.Chapter 3 will discuss the performance and the congestion of ABRservice in the ATM networks. Chapter 4 will present the ATMsimulation model as well as its framework. Chapter 5 will analyzeand discuss the simulation results of the two leading rate basedcongestion control schemes, EPRCA and MMRCA, which wereadopted by the ATM Forum as standards. Chapter 6 wil l present thedesign objectives of the new rate based congestion control schemeand discuss and compare its simulation results with that of EPRCAand Mh4RCA. Chapter 7 will present two enhancements that aremade to the new scheme to achieve better network performance andlast ly concluding summary in chapter 8.

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** Network Flow and Coneestion Control