8/6/2019 13-CongestioninDataNetworks http://slidepdf.com/reader/full/13-congestionindatanetworks 1/49 Data and Computer Data and Computer Communications Communications Eighth Edition Eighth Edition by William Stallings by William Stallings Lecture slides by Lawrie Brown Lecture slides by Lawrie Brown Chapter 13 Chapter 13 ± ± Congestion in Data Congestion in Data Networks Networks
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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 cf. backpressure in blocked fluid pipecf. backpressure in blocked fluid pipe
may mean that other nodes have to apply control onmay mean that other nodes have to apply control on
incoming packet ratesincoming packet rates propagates back to sourcepropagates back to source
can restrict to high traffic logical connectionscan restrict to high traffic logical connections
used in connection oriented nets that allow hopused in connection oriented nets that allow hop
by hop congestion control (eg. X.25)by hop congestion control (eg. X.25) not used in ATM nor frame relaynot used in ATM nor frame relay
only recently developed for IPonly recently developed for IP
transmission delay increases with congestiontransmission delay increases with congestion
hence a packet may be discardedhence a packet may be discarded
source detects this implicit congestion indicationsource detects this implicit congestion indication useful on connectionless (datagram) networksuseful on connectionless (datagram) networks
eg. IP basedeg. IP based
�� (TCP includes congestion and flow control(TCP includes congestion and flow control -- see chapter 17)see chapter 17)
provide equal treatment of various flowsprovide equal treatment of various flows
quality of servicequality of service different treatment for different connectionsdifferent treatment for different connections
reservationsreservations
traffic contract between user and networktraffic contract between user and network carry bestcarry best--effort or discard excess trafficeffort or discard excess traffic
high speed, small cell size, limited overhead bitshigh speed, small cell size, limited overhead bits
still evolvingstill evolving
reasons existing tools are inadequate for ATMreasons existing tools are inadequate for ATM majority of traffic not amenable to flow controlmajority of traffic not amenable to flow control
feedback slow due to reduced transmission timefeedback slow due to reduced transmission timecompared with propagation delaycompared with propagation delay
wide range of application demandswide range of application demands
different traffic patternsdifferent traffic patterns different network servicesdifferent network services
high speed switching and transmission increaseshigh speed switching and transmission increasesvolatilityvolatility
takes ~2.8x10takes ~2.8x10--66 seconds to insert single cellseconds to insert single cell
time to traverse network depends ontime to traverse network depends on
propagation delay and switching delaypropagation delay and switching delay
assume propagation at twoassume propagation at two--thirds speed of lightthirds speed of light
if source and destination on opposite sides of if source and destination on opposite sides of USA, propagation time ~ 48x10USA, propagation time ~ 48x10--33 secondsseconds
given implicit congestion control, by the timegiven implicit congestion control, by the timedropped cell notification has reached source,dropped cell notification has reached source,7.2x107.2x1066 bits have been transmittedbits have been transmitted
this is not a good strategy for ATMthis is not a good strategy for ATM
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
ramework ATM layer traffic and congestion control ATM layer traffic and congestion control
should 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 higher levelnetwork specific, nor higher levelapplication specific protocolsapplication specific protocols
should minimize network and end to endshould minimize network and end to endsystem complexitysystem complexity
timing intervals considered:timing intervals considered: cell insertion timecell insertion time
round trip propagation timeround trip propagation time
connection durationconnection duration long termlong term
traffic control strategy then must:traffic control strategy then must: determine whether a given new connection can bedetermine whether a given new connection can be
accommodatedaccommodated agree performance parameters with subscriber agree performance parameters with subscriber
now review various control techniquesnow review various control techniques
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 meetthe demandthe demand
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 send at min rate with no lossensures application can send at min rate with no loss
if no congestion, higher rates maybe usedif no congestion, higher rates maybe used
two stage filtering processtwo stage filtering process
a frame is tested for conformance to contracta frame is tested for conformance to contract if not, may discard or tagif not, may discard or tag
set upper bound & penalize cells above upper boundset upper bound & penalize cells above upper bound do expect attempt to deliver tagged cellsdo expect 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