Data Communications,Data Networks,computer communications,multiplexing,spread spectrum,protocol architecture,data link protocols,signal encoding techniques,transmission media,asynchronous transfer mode,routing

1. Data and ComputerData and Computer
CommunicationsCommunications
Eighth EditionEighth Edition
by William Stallingsby William Stallings
Lecture slides by Lawrie BrownLecture slides by Lawrie Brown
Chapter 13 – Congestion in DataChapter 13 – Congestion in Data
NetworksNetworks

2. Congestion in Data NetworksCongestion in Data Networks
At St. Paul's a great throng crammed the platform. SheAt St. Paul's a great throng crammed the platform. She
saw a sea of faces, each stamped with a kind ofsaw a sea of faces, each stamped with a kind of
purposeful, hungry urgency, a determination to get intopurposeful, hungry urgency, a determination to get into
this train. As before, when she was on the Northernthis train. As before, when she was on the Northern
Line, she thought there must be some rule, someLine, she thought there must be some rule, some
operating law, that would stop more than a limited,operating law, that would stop more than a limited,
controlled number getting in. Authority would appearcontrolled number getting in. Authority would appear
and stop it.and stop it.
—King Solomon's Carpet, Barbara Vine (Ruth Rendell)

3. What Is Congestion?What Is Congestion?
 congestion occurs when the no of packets beingcongestion occurs when the no of packets being
transmitted through the network approaches thetransmitted through the network approaches the
packet handling capacity of the networkpacket handling capacity of the network
 congestion control aims to keep no of packetscongestion control aims to keep no of packets
below a level at which performance falls offbelow a level at which performance falls off
dramaticallydramatically
 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

4. Queues at a NodeQueues at a Node

5. Interaction of QueuesInteraction of Queues

6. IdealIdeal
NetworkNetwork
UtilizationUtilization

7. Effects ofEffects of
CongestionCongestion
--
No ControlNo Control

8. Mechanisms forMechanisms for
Congestion ControlCongestion Control

9. BackpressureBackpressure
 if node becomes congested it can slow down orif 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

10. Choke PacketChoke Packet
 a control packeta control packet

generated at congested nodegenerated at congested node

sent to source nodesent to source node

eg. ICMP source quencheg. ICMP source quench
• from router or destinationfrom router or destination
• source cuts back until no more source quench messagesource cuts back until no more source quench message
• sent for every discarded packet, or anticipatedsent for every discarded packet, or anticipated
 is a rather crude mechanismis a rather crude mechanism

11. Implicit Congestion SignalingImplicit Congestion Signaling
 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 - see chapter 17)(TCP includes congestion and flow control - see chapter 17)
 used in frame relay LAPFused in frame relay LAPF

12. Explicit Congestion SignalingExplicit Congestion Signaling
 network alerts end systems of increasingnetwork alerts end systems of increasing
congestioncongestion
 end systems take steps to reduce offered loadend systems take steps to reduce offered load
 BackwardsBackwards

congestion avoidance notification in opposite directioncongestion avoidance notification in opposite direction
to packet requiredto packet required
 ForwardsForwards

congestion avoidance notification in same direction ascongestion avoidance notification in same direction as
packet requiredpacket required

13. Explicit Signaling CategoriesExplicit Signaling Categories
 BinaryBinary

a bit set in a packet indicates congestiona bit set in a packet indicates congestion
 Credit basedCredit based

indicates how many packets source may sendindicates how many packets source may send

common for end to end flow controlcommon for end to end flow control
 Rate basedRate based

supply explicit data rate limitsupply explicit data rate limit

nodes along path may request rate reductionnodes along path may request rate reduction

eg. ATMeg. ATM

14. Traffic ManagementTraffic Management
 fairnessfairness

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 best-effort or discard excess trafficcarry best-effort or discard excess traffic

15. 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 congestion info to packets in transitadd congestion info to packets in transit

either backwards or forwardseither backwards or forwards

16. Frame RelayFrame Relay
Congestion ControlCongestion Control
 minimize discardsminimize discards
 maintain agreed QoSmaintain agreed QoS
 minimize probability of one end user monopolyminimize probability of one end user monopoly
 simple to implementsimple to implement
 create minimal additional trafficcreate minimal additional traffic
 distribute resources fairlydistribute resources fairly
 limit spread of congestionlimit spread of congestion
 operate effectively regardless of traffic flowoperate effectively regardless of traffic flow
 minimum impact on other systemsminimum impact on other systems
 minimize variance in QoSminimize variance in QoS

17. FR Control TechniquesFR Control Techniques
 difficult for frame-relaydifficult for frame-relay
 joint network & end-system responsibilityjoint network & end-system responsibility
 techniques:techniques:

discard strategydiscard strategy

congestion avoidancecongestion avoidance

explicit signalingexplicit signaling

congestion recoverycongestion recovery

implicit signaling mechanismimplicit signaling mechanism

18. 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

Committed information rate (CIR)Committed information rate (CIR)
• data in excess of this liable to discarddata in excess of this liable to discard
• not guaranteed in extreme congestion situationsnot guaranteed in extreme congestion situations
• aggregate CIR should not exceed physical data rateaggregate CIR should not exceed physical data rate
 Committed burst sizeCommitted burst size
 Excess burst sizeExcess burst size

19. Operation of CIROperation of CIR

20. RelationshipRelationship
AmongAmong
CongestionCongestion
ParametersParameters

21. Congestion Avoidance usingCongestion Avoidance using
Explicit SignalingExplicit Signaling
 network alerts end systems of growingnetwork alerts end systems of growing
congestion usingcongestion using

backward explicit congestion notificationbackward explicit congestion notification

forward explicit congestion notificationforward explicit congestion notification
 frame handler monitors its queuesframe handler monitors its queues
 may notify some or all logical connectionsmay notify some or all logical connections
 user response reduce rateuser response reduce rate

22. ATM Traffic ManagementATM Traffic Management
 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 time
compared 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 increases
volatilityvolatility

23. Latency/Speed EffectsLatency/Speed Effects
 consider ATM at 150Mbpsconsider ATM at 150Mbps
 takes ~2.8x10takes ~2.8x10-6-6
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 two-thirds speed of lightassume propagation at two-thirds speed of light
 if source and destination on opposite sides ofif source and destination on opposite sides of
USA, propagation time ~ 48x10USA, propagation time ~ 48x10-3-3
secondsseconds
 given implicit congestion control, by the timegiven implicit congestion control, by the time
dropped 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

24. 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

25. Timing of CBR CellsTiming of CBR Cells

26. Network Contribution toNetwork Contribution to
Cell Delay VariationCell Delay Variation
 in packet switched networks is due toin packet switched networks is due to
queuing delays and routing decision timequeuing delays and routing decision time
 in Frame relay networks is similarin Frame relay networks is similar
 in ATM networksin ATM networks

less than frame relayless than frame relay

ATM protocol designed to minimizeATM protocol designed to minimize
processing overheads at switchesprocessing overheads at switches

ATM switches have very high throughputATM 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

27. Cell Delay VariationCell Delay Variation
At The UNIAt The UNI
 application produces data at fixed rateapplication produces data at fixed rate
 3 layers of ATM processing causes delay3 layers of ATM processing causes delay

interleaving cells from different connectionsinterleaving cells from different connections

operation and maintenance cell interleavingoperation and maintenance cell interleaving

if using synchronous digital hierarchy frames,if using synchronous digital hierarchy frames,
these are inserted at physical layerthese are inserted at physical layer
 cannot predict these delayscannot predict these delays

28. Origins of Cell Delay VariationOrigins of Cell Delay Variation

29. Traffic and CongestionTraffic and Congestion
Control FrameworkControl Framework
 ATM layer traffic and congestion controlATM layer traffic and congestion control
should support QoS classes for allshould support QoS classes for all
foreseeable network servicesforeseeable network services
 should not rely on AAL protocols that areshould not rely on AAL protocols that are
network specific, nor higher levelnetwork specific, nor higher level
application specific protocolsapplication specific protocols
 should minimize network and end to endshould minimize network and end to end
system complexitysystem complexity

30. Timings ConsideredTimings Considered
 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 subscriberagree performance parameters with subscriber
 now review various control techniquesnow review various control techniques

31. Resource Management UsingResource Management Using
Virtual PathsVirtual Paths
 separate traffic flow according to serviceseparate traffic flow according to service
characteristics on a virtual pathcharacteristics on a virtual path

user to user applicationuser to user application

user to network applicationuser to network application

network to network applicationnetwork to network application
 QoS parameters concerned with are:QoS parameters concerned with are:

cell loss ratiocell loss ratio

cell transfer delaycell transfer delay

cell delay variationcell delay variation

32. Configuration ofConfiguration of
VCCs and VPCsVCCs and VPCs

33. Allocating VCCs within VPCAllocating VCCs within VPC
 all VCCs within VPC should experienceall VCCs within VPC should experience
similar network performancesimilar network performance
 options for allocation:options for allocation:

aggregate peak demandaggregate peak demand
• set VPC capacity to total of all peak VCC ratesset VPC capacity to total of all peak VCC rates
• will meet peak demands, but often underutilizedwill meet peak demands, but often underutilized

statistical multiplexingstatistical multiplexing
• set VPC capacity to more than average VCC ratesset VPC capacity to more than average VCC rates
• will see greater variation but better utilizationwill see greater variation but better utilization

34. Connection AdmissionConnection Admission
ControlControl
 first line of defensefirst line of defense
 user specifies traffic characteristics for newuser specifies traffic characteristics for new
connection (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 meet
the demandthe demand
 traffic contracttraffic contract

peak cell ratepeak cell rate

cell delay variationcell delay variation

sustainable cell ratesustainable cell rate

burst toleranceburst tolerance

35. Usage Parameter ControlUsage Parameter Control
 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 overload by one connectionfrom overload by one 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 burst tolerancesustainable cell rate and burst tolerance
 UPC discards cells outside traffic contractUPC discards cells outside traffic contract

36. Selective Call DiscardSelective Call Discard
 when network at point beyond UPCwhen network at point beyond UPC
discards (CLP=1) cellsdiscards (CLP=1) cells
 aim to discard lower-priority cells whenaim to discard lower-priority cells when
congested to protect higher-priority cellscongested to protect higher-priority cells

note. can’t distinguish between cells originallynote. can’t distinguish between cells originally
labeled lower priority, verses those tagged bylabeled lower priority, verses those tagged by
UPC functionUPC function

37. Traffic ShapingTraffic Shaping
 UPC provides a form of traffic policingUPC provides a form of traffic policing
 can be desirable to also shape trafficcan be desirable to also shape traffic
 smoothing out traffic flowsmoothing out traffic flow
 reducing cell clumpingreducing cell clumping
 token buckettoken bucket

38. Token BucketToken Bucket forfor
Traffic ShapingTraffic Shaping

39. GFR Traffic ManagementGFR Traffic Management
 guaranteed frame rate (GFR) as simple as UBRguaranteed frame rate (GFR) as simple as UBR
from 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 adaptormay 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

40. Frame RecognitionFrame Recognition
 GFR recognizes frames as well as cellsGFR recognizes frames as well as cells
 when congested, network discards whole framewhen congested, network discards whole frame
rather than individual cellsrather than individual cells
 all cells of a frame have same CLP bit settingall cells of a frame have same CLP bit setting
 CLP=1 AAL5 frames lower priority (best effort)CLP=1 AAL5 frames lower priority (best effort)
 CLP=0 frames minimum guaranteed capacityCLP=0 frames minimum guaranteed capacity

41. GFR Contract ParametersGFR Contract Parameters
 Peak cell rate (PCR)Peak cell rate (PCR)
 Minimum cell rate (MCR)Minimum cell rate (MCR)
 Maximum burst size (MBS)Maximum burst size (MBS)
 Maximum frame size (MFS)Maximum frame size (MFS)
 Cell delay variation tolerance (CDVT)Cell delay variation tolerance (CDVT)

42. Components of GFR SystemComponents of GFR System
Supporting Rate GuaranteesSupporting Rate Guarantees

43. 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 in frame if notset CLP=1 on all cells in frame if not

gives lower prioritygives lower priority
 maybe done by network or sourcemaybe done by network or source
 network may discard CLP=1 cellsnetwork may discard CLP=1 cells

policingpolicing

44. Buffer ManagementBuffer Management
 deals with treatment of buffered cellsdeals with treatment of buffered cells
 congestion indicated by high buffercongestion indicated by high buffer
occupancyoccupancy
 will discard tagged cells in preference towill discard tagged cells in preference to
untagged 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 queue-specificcell discard based on queue-specific
thresholdsthresholds

45. SchedulingScheduling
 preferential treatment to untagged cellspreferential treatment to untagged cells
 separate queues for each VCseparate queues for each VC
 make per-VC scheduling decisionsmake per-VC 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

46. GFC Conformance DefinitionGFC Conformance Definition
 UPC function monitors each active VCUPC function monitors each active VC
 to ensure traffic conforms to contractto ensure traffic conforms to contract
 tag or discard nonconforming cellstag or discard nonconforming cells
 frame conforms if all cells conformframe conforms if all cells conform
 a cell conforms if:a cell conforms if:

rate of cells is within contractrate of cells is within contract

all cells in frame have same CLPall cells in frame have same CLP

frame satisfies MFS parameterframe satisfies MFS parameter
• check if either last cell in frame or cell count < MFScheck if either last cell in frame or cell count < MFS

47. 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

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

48. GFR VC Frame CategoriesGFR VC Frame Categories
 nonconforming framenonconforming frame

cells of this frame will be tagged or discardedcells of this frame will be tagged or discarded
 conforming but ineligible framesconforming but ineligible frames

cells will receive a best-effort servicecells will receive a best-effort service
 conforming and eligible framesconforming and eligible frames

cells will receive a guarantee of deliverycells will receive a guarantee of delivery
 form of cell rate algorithm is usedform of cell rate algorithm is used

49. SummarySummary
 congestion effectscongestion effects
 congestion controlcongestion control
 traffic managementtraffic management
 frame relay congestion controlframe relay congestion control
 ATM congestion controlATM congestion control