13 congestion in data networks real

8/6/2019 13 Congestion in Data Networks REAL

http://slidepdf.com/reader/full/13-congestion-in-data-networks-real 1/47

Data and Computer Data and Computer

CommunicationsCommunications

Ninth EditionNinth Edition

by William Stallingsby William Stallings

Chapter 13Chapter 13 ± ± Congestion in DataCongestion in DataNetworksNetworks

Data and Computer Communications, NinthData and Computer Communications, NinthEdition by William Stallings, (c) PearsonEdition by William Stallings, (c) Pearson

EducationEducation -- Prentice Hall, 2011Prentice Hall, 2011



Congestion in Data NetworksCongestion in Data Networks

We were doing very well, up to the kind of sum when a bath is filling at theWe were doing very well, up to the kind of sum when a bath is filling at therate of so many gallons and two holes are letting the water out, and pleaserate of so many gallons and two holes are letting the water out, and please

to say how long it will take to fill the bath, when my mother put down theto say how long it will take to fill the bath, when my mother put down the

socks she was darning and clicked her tongue in impatience.socks she was darning and clicked her tongue in impatience.

"Filling up an old bath with holes in it, indeed. Who would be such a fool?" "Filling up an old bath with holes in it, indeed. Who would be such a fool?"

"A sum it is, girl," my father said. "A sum. A problem for the mind." "A sum it is, girl," my father said. "A sum. A problem for the mind." "Filling the boy with old nonsense," Mama said."Filling the boy with old nonsense," Mama said.

"Not nonsense, Beth," my father said. "A sum, it is. The water pours in and "Not nonsense, Beth," my father said. "A sum, it is. The water pours in and

takes so long. It pours out and takes so long. How long to fill? That is all." takes so long. It pours out and takes so long. How long to fill? That is all."

"But who would pour water into an old bath with holes?" my mother said."But who would pour water into an old bath with holes?" my mother said.

"Who would think to do it, but a lunatic?" "Who would think to do it, but a lunatic?"

²²How Green Was My Valley,How Green Was My Valley,

Richard LlewellynRichard Llewellyn



What Is Congestion?What Is Congestion?

congestion occurs when thecongestion occurs when the number number of packetsof packets

being transmitted through the networkbeing transmitted through the network

approaches the packet handling capacity of theapproaches the packet handling capacity of the

networknetwork

oobjective of congestionbjective of congestion controlicontroliss to keepto keep thethe

number number of packets below a level at whichof packets below a level at which

performance falls off dramaticallyperformance falls off dramatically a data network is a network of queuesa data network is a network of queues

generally 80% utilization is criticalgenerally 80% utilization is critical

finite queues mean data may be lostfinite queues mean data may be lost



Queues at a NodeQueues at a Node



Interaction of QueuesInteraction of Queues



IdealIdeal

NetworkNetworkUtilizationUtilization



Effects of Effects of

CongestionCongestion--

No ControlNo Control



Mechanisms for Mechanisms for

Congestion ControlCongestion Control



BackpressureBackpressure

if node becomes congested it can slow down or if node becomes congested it can slow down or halt flow of packets from other nodeshalt flow of packets from other nodes same effect as backpressure in a blocked fluid pipesame effect as backpressure in a blocked fluid pipe

other nodes have to apply control on incoming packetother nodes have to apply control on incoming packet

ratesrates propagates back to sourcepropagates back to source

can restrict to high traffic logical connectionscan restrict to high traffic logical connections

used in connection oriented networks that allowused in connection oriented networks that allow

hophop--byby--hop congestion control (eg. X.25)hop congestion control (eg. X.25) not used in ATM or frame relaynot used in ATM or frame relay

only recently developed for IPonly recently developed for IP



Choke PacketChoke Packet

a controla control packetpacket

generated at congested nodegenerated at congested node

sent back to source nodesent back to source node

Internet Control Message Protocol (ICMP) SourceInternet Control Message Protocol (ICMP) Source

Quench packetQuench packet

�� from router or destination end systemfrom router or destination end system

�� source cuts back until it no longer receives quench messagessource cuts back until it no longer receives quench messages

�� message is issued for every discarded packetmessage is issued for every discarded packet�� message may also be issued for anticipated congestionmessage may also be issued for anticipated congestion

crude control techniquecrude control technique



Implicit Congestion SignalingImplicit Congestion Signaling

with network congestion:with network congestion:

ttransmission delay increasesransmission delay increases

ppackets may be discardedackets may be discarded

source can detect congestion and reduce flowsource can detect congestion and reduce flow

r responsibility of end systemsesponsibility of end systems

effectiveeffective on connectionless (datagram)on connectionless (datagram) networksnetworks

aalso used in connectionlso used in connection--oriented networksoriented networks LAPF control is capable of detecting lost framesLAPF control is capable of detecting lost frames



Expli it ti i liExpli it ti i li

BackwardsBackwards

congestion avoidancecongestion avoidance

notification in oppositenotification in opposite

direction to packetdirection to packetrequiredrequired

ForwardsForwards

congestion avoidancecongestion avoidance

notification in samenotification in samedirection as packetdirection as packet

requiredrequired



Tr ffi M tTr ffi M t

fairness

� provide equal treatment of various flows

quality of service� different treatment for different connections

reservations� traffic contract between user and network

� e cess traffic discarded or handled on a best-effort basis



Congestion Control in PacketCongestion Control in Packet

Switched NetworksSwitched Networks send control packet to some or all sourcesend control packet to some or all source

nodesnodes

requires additional traffic during congestionrequires additional traffic during congestion

rely on routing informationrely on routing information

may react too quicklymay react too quickly

end to end probe packetsend to end probe packets

adds to overheadadds to overhead

add congestionadd congestion informationinformation to packets into packets intransittransit

either backwards or forwardseither backwards or forwards



A M Traffic ManagementATM Traffic Management

high speed, small cell size, limited overhead bitshigh speed, small cell size, limited overhead bits

stillstill evolvingevolving

reasonsreasonse istinge istingtools aretools areinadequateinadequatefor ATMfor ATM

majority of traffic not amenable to flow control

feedback is slow due to reduced transmission time comparedwith propagation delay

wide range of application demands

different traffic patterns

different network services

high speed switching and transmission increases volatility



Latency/Speed EffectsLatency/Speed Effects� consider ATM at

150Mbps

� takes ~2.8x10-6

seconds to insert asingle cell

� time to traversenetwork dependson propagationdelay andswitching delay

� assume propagationat two-thirds speedof light

� if source anddestination on

opposite sides of USA, propagationtime ~ 48x10-3seconds

� given implicitcongestion control, by the timedropped cellnotification has

reached source, 7.2x106 bits havebeen transmitted



Cell Delay VariationCell Delay Variation

for ATM voice/video, data is a stream of cellsfor ATM voice/video, data is a stream of cells

delay across network must be shortdelay across network must be short

rate of delivery must be constantrate of delivery must be constant there will always be some variation in transitthere will always be some variation in transit

delay cell delivery to application so that constantdelay cell delivery to application so that constant

bit rate can be maintained to applicationbit rate can be maintained to application



Timing of CBR CellsTiming of CBR Cells



Network Contribution toNetwork Contribution to

Cell Delay VariationCell Delay Variation in packet switched networks is due toin packet switched networks is due to

queuingqueuing delaysdelays and routing decisionand routing decision timetime

in ATM networksin ATM networks less than frame relayless than frame relay

ATM protocol designed to minimize ATM protocol designed to minimizeprocessingprocessing overheadoverhead at switchesat switches

ATM switches have very high throughput ATM switches have very high throughput

only noticeable delay is from congestiononly noticeable delay is from congestion

must not accept load that causes congestionmust not accept load that causes congestion



Cell Delay VariationCell Delay Variation

At The UNIAt The UNI

cannot predict these delayscannot predict these delays

3 layers of ATM processing causes delay

� interleaving cells from different connections

� operation and maintenance cell interleaving

� if using synchronous digital hierarchy frames,these are inserted at physical layer



Origins of Cell Delay VariationOrigins of Cell Delay Variation



Traffic and CongestionTraffic and Congestion

Control FrameworkControl Framework

ATM layer traffic and congestion control ATM layer traffic and congestion controlshould support QoS classes for allshould support QoS classes for allforeseeable network servicesforeseeable network services

should not rely on AAL protocols that areshould not rely on AAL protocols that arenetwork specific, nor on higher levelnetwork specific, nor on higher level

application specific protocolsapplication specific protocols should minimize network and end to endshould minimize network and end to end

system comple itysystem comple ity



Traffic Control andTraffic Control and

Congestion Control FunctionsCongestion Control Functions



Traffic Management andTraffic Management and

Congestion ControlCongestion Control

TechniquesTechniques

resource management using virtual pathsresource management using virtual paths

connection admission controlconnection admission control usage parameter controlusage parameter control

selective cell discardselective cell discard

traffic shapingtraffic shaping

ATM traffic ATM traffic

techniques:techniques:



Resource Management UsingResource Management Using

Virtual PathsVirtual Paths QoS parametersQoS parameters

concerned with are:concerned with are: cell loss ratiocell loss ratio cell transfer delaycell transfer delay

cell delay variationcell delay variation



Configuration of Configuration of

VCCs and VPCsVCCs and VPCs



Allocating VCCs within VPCAllocating VCCs within VPC

all VCCs within VPC should e perienceall VCCs within VPC should e periencesimilar network performancesimilar network performance

options for allocationoptions for allocation::

aggregatepeak

demand

� set VPC capacity to total of all peakVCC rates

� will meet peak demands, but oftenunderutilized

statisticalmultiple ing

� set VPC capacity to more thanaverage VCC rates

� will see greater variation but better utilization



Connection AdmissionConnection Admission

ControlControl first line of defensefirst line of defense

user specifies traffic characteristics for newuser specifies traffic characteristics for newconnection (VCC or VPC) by selecting a QoSconnection (VCC or VPC) by selecting a QoS

network accepts connection only if it can meetnetwork accepts connection only if it can meetthethe demand and maintain QoSdemand and maintain QoS



Traffic Parameters Used inTraffic Parameters Used in

Defining VCC/VPC Quality of Defining VCC/VPC Quality of

ServiceService



Procedures Used to Set Values of Procedures Used to Set Values of

Traffic Contract ParametersTraffic Contract Parameters



Traffic Rate ManagementTraffic Rate Management

must discard frames to cope with congestionmust discard frames to cope with congestion

arbitrarily, no regard for sourcearbitrarily, no regard for source

no reward for restraint so end systems transmit asno reward for restraint so end systems transmit as

fast as possiblefast as possible ccommittedommitted information rate (CIR)information rate (CIR)

�� data in e cess of this liable to discarddata in e cess of this liable to discard

�� not guaranteed in e treme congestion situationsnot guaranteed in e treme congestion situations

�� aggregate CIR should not e ceed physical data rateaggregate CIR should not e ceed physical data rate

ccommittedommitted burst sizeburst size

ee cesscess burst sizeburst size



Usage Parameter Usage Parameter ControlControl

(UPC)(UPC)

UPC function monitors a connection toUPC function monitors a connection to

ensure traffic obeys contractensure traffic obeys contract

purpose is to protect network resourcespurpose is to protect network resources

from overloadfrom overload onon one connectionone connection

done on VCC and VPCdone on VCC and VPC

peak cell rate and cell delay variationpeak cell rate and cell delay variation

sustainable cell rate and burstsustainable cell rate and burst tolerancetolerance



Cell R te Al ritCell R te Al rit

ttrafficraffic flow is compliant if the peak rate of flow is compliant if the peak rate of

cell transmission does not e ceed thecell transmission does not e ceed the

agreed peak cell rateagreed peak cell rate ccellell rate algorithm:rate algorithm:

known as theleaky bucket

algorithm

defines trafficcompliance

compliantcells are

passed on

noncompliantcells are

discarded



SelectiveSelective CellCell DiscardDiscard

comes in to play when network at a pointcomes in to play when network at a point

beyond UPC discards (CLP=1) cellsbeyond UPC discards (CLP=1) cells

object is to discard lower object is to discard lower--priority cellspriority cellswhen congested to protect higher when congested to protect higher--prioritypriority

cellscells

network has no way to discriminatenetwork has no way to discriminate

between cells that were labeled as lower between cells that were labeled as lower

priority by the source and that were taggedpriority by the source and that were tagged

by the UPC functionby the UPC function



Traffic ShapingTraffic Shaping

UPC referred to as traffic policingUPC referred to as traffic policing

cells that e ceed a certain performance levelcells that e ceed a certain performance level

are discarded or taggedare discarded or tagged

can be desirable to also shape trafficcan be desirable to also shape traffic

traffic shaping reduces cell clumpingtraffic shaping reduces cell clumping

token buckettoken bucket

controls flow of compliant cellscontrols flow of compliant cells

smoothes out bursts of cellssmoothes out bursts of cells



Token BucketToken Bucket for for

Traffic ShapingTraffic Shaping



GFR Traffic ManagementGFR Traffic Management

guaranteed frame rate (GFR) as simple as UBRguaranteed frame rate (GFR) as simple as UBRfrom end system viewpointfrom end system viewpoint

places modest requirements on ATM networkplaces modest requirements on ATM network

end system does no policing or shaping of trafficend system does no policing or shaping of traffic may transmit at line rate of ATM adaptor may transmit at line rate of ATM adaptor

no guarantee of deliveryno guarantee of delivery so higher layer (eg. TCP) must do congestion controlso higher layer (eg. TCP) must do congestion control

user can reserve capacity for each VCuser can reserve capacity for each VC ensures application can transmit at minimum rate withensures application can transmit at minimum rate with

no lossno loss

if no congestion, higher rates may be usedif no congestion, higher rates may be used



GFR Contract ParametersGFR Contract Parameters

Peak cell rate (PCR)Peak cell rate (PCR)

Minimum cell rate (MCR)Minimum cell rate (MCR)

Ma imum burst size (MBS)Ma imum burst size (MBS)Ma imum frame size (MFS)Ma imum frame size (MFS)

Cell delay variation tolerance (CDVT)Cell delay variation tolerance (CDVT)



Components of GFR SystemComponents of GFR System

SupportingR

ateG

uaranteesSupportingR

ateG

uarantees



Tagging and PolicingTagging and Policing

discriminates between frames thatdiscriminates between frames that

conform to contract and those that don¶tconform to contract and those that don¶t

set CLP=1 on all cells of each frame thatset CLP=1 on all cells of each frame thatdoes not conformdoes not conform

gives lower prioritygives lower priority

tagging can be done by network or sourcetagging can be done by network or source network may discard CLP=1 cellsnetwork may discard CLP=1 cells

ccellell discard considered a policing functiondiscard considered a policing function



Buffer ManagementBuffer Management

deals with treatment of buffered cellsdeals with treatment of buffered cells

congestion indicated by high buffer congestion indicated by high buffer occupancyoccupancy

will discard tagged cells in preference towill discard tagged cells in preference tountagged cellsuntagged cells

including ones already in buffer to make roomincluding ones already in buffer to make room

may do per VC buffering for fairnessmay do per VC buffering for fairness cell discard based on queuecell discard based on queue--specificspecific

thresholdsthresholds



SchedulingScheduling

preferential treatment to untagged cellspreferential treatment to untagged cells

separate queues for each VCseparate queues for each VC

make per make per--VC scheduling decisionsVC scheduling decisions enables control of outgoing rate of VCsenables control of outgoing rate of VCs

VCs get fair capacity allocationVCs get fair capacity allocation

still meet contractstill meet contract



GFC Conf or nce efinitionGFC Conf or nce efinition

UPC functionUPC function

monitors each activemonitors each active

VC to ensure trafficVC to ensure traffic

conforms to contractconforms to contractand tags or discardsand tags or discards

nonconforming cellsnonconforming cells

frame conforms if allframe conforms if all

cells conformcells conform

all cells in framehave same CLP

rate of cells iswithin contract

frame satisfiesMFS parameter

� check if either

last cell in frameor cell count <MFS

a cell conforms



QoS Eligibility TestQoS Eligibility Test

two stage filtering processtwo stage filtering process

a frame is tested for conformance to contracta frame is tested for conformance to contract may discard or tagmay discard or tag

set upper bound & penalize cells above upper boundset upper bound & penalize cells above upper bound do e pect attempt to deliver tagged cellsdo e pect attempt to deliver tagged cells

determine frames eligible for QoS guaranteesdetermine frames eligible for QoS guarantees under GFR contract for VCunder GFR contract for VC

set lower bound on trafficset lower bound on traffic frames in traffic flow below threshold are eligibleframes in traffic flow below threshold are eligible



SummarySummary

congestion effectscongestion effects ideal and practical performanceideal and practical performance

congestioncongestion controlcontrol

bbackpressure, choke packet, implicit/e plicitackpressure, choke packet, implicit/e plicit traffictraffic managementmanagement

f fairness, QoS, reservationsairness, QoS, reservations

ATM traffic management ATM traffic management ATM ATM--GFR traffic managementGFR traffic management

tagging, policing, buffer, schedulingtagging, policing, buffer, scheduling

QoS eligibility testingQoS eligibility testing

13 congestion in data networks real

Documents