1 Chapter 13 ATM Traffic & Congestion Control. Chapter 13: ATM Traffic & Congestion Control 2 Introduction ATM congestion problem overview ATM congestion

1

Chapter 13Chapter 13

ATM Traffic & ATM Traffic & Congestion Congestion ControlControl

Chapter 13: ATM Traffic & Congestion Control2

Introduction Introduction

ATM congestion problem overviewATM congestion problem overview ITU-T and ATM Forum framework ITU-T and ATM Forum framework

for for control of delay-sensitive control of delay-sensitive traffictraffic

ATM ATM traffic controltraffic control mechanisms mechanismsATM ATM congestion controlcongestion controlCongestion control schemes for Congestion control schemes for

bursty trafficbursty traffic (ABR and GFR) (ABR and GFR)


ATM Service CategoriesATM Service Categories Constant Bit Rate (CBR)Constant Bit Rate (CBR)

– fixed data rate required at guaranteed fixed data rate required at guaranteed capacitycapacity

Real-Time Variable Bit-Rate (rt-VBR)Real-Time Variable Bit-Rate (rt-VBR)– tightly constrained delay and delay variationtightly constrained delay and delay variation– sustained rate & guaranteed fast burst ratesustained rate & guaranteed fast burst rate

Non-Real-Time Variable Bit-Rate (nrt-VBR)Non-Real-Time Variable Bit-Rate (nrt-VBR)– no delay variation bound, cell loss ratio onlyno delay variation bound, cell loss ratio only

Available Bit Rate (ABR)Available Bit Rate (ABR)– guaranteed minimum capacity, with burstsguaranteed minimum capacity, with bursts

Guaranteed Frame Rate (GFR)Guaranteed Frame Rate (GFR)– like UBR/ABR, expressed in terms of frame ratelike UBR/ABR, expressed in terms of frame rate

Unspecified Bit Rate (UBR)Unspecified Bit Rate (UBR)– best-effort servicebest-effort service


Why Typical Traffic Control Why Typical Traffic Control Schemes Are Inadequate for Schemes Are Inadequate for ATMATM Majority of ATM traffic Majority of ATM traffic sourcessources are time- are time-

sensitive, and not amenable to typical flow sensitive, and not amenable to typical flow control schemes (e.g. CBR, rt-VBR)control schemes (e.g. CBR, rt-VBR)

For long-haul ATM: tFor long-haul ATM: ttranstrans << t << tprop prop … … slow slow feedback (latency/speed effects)feedback (latency/speed effects)

Due to the broad range of ATM application Due to the broad range of ATM application types, flow control may types, flow control may indiscriminately indiscriminately penalizepenalize some some – bandwidth requirements (kbps to Mbps)bandwidth requirements (kbps to Mbps)– traffic patterns (CBR, VBR)traffic patterns (CBR, VBR)– service requirements (delay/loss sensitivity)service requirements (delay/loss sensitivity)

Very high-speed switching increases Very high-speed switching increases volatilityvolatility re: control mechanisms re: control mechanisms


ATM Performance ATM Performance considerationsconsiderationsTwo key issues must be addressed Two key issues must be addressed

for CBR and real-time VBR traffic…for CBR and real-time VBR traffic…– Latency/speed effects for long-haul Latency/speed effects for long-haul

networks: networks: Cause - tCause - ttranstrans << 2 x t << 2 x tpropprop (or, one RTT) (or, one RTT)Approach – fast feedback mechanismsApproach – fast feedback mechanisms

– Cell delay variation:Cell delay variation:Cause - Variation at user-network interface Cause - Variation at user-network interface

and in network coreand in network coreApproach: time reassembly of CBR cells at Approach: time reassembly of CBR cells at

receiverreceiver


Latency/Speed EffectsLatency/Speed Effects Issue: rapid insertion rate of ATM cells vs. Issue: rapid insertion rate of ATM cells vs.

relatively long round-trip delaysrelatively long round-trip delays– small size (53 bytes) of ATM cellsmall size (53 bytes) of ATM cell– high bandwidth links in ATM networks high bandwidth links in ATM networks – small switching delays small switching delays

Simplified ExampleSimplified Example::– 150 Mbps data rate (150 Mbps data rate ( SONET OC-3) SONET OC-3)

– ttinsert insert = t= ttranstrans = = 2.8 x 10 = = 2.8 x 10-6-6 seconds seconds

– U.S. coast-to-coast roundtrip dU.S. coast-to-coast roundtrip dpropprop = 48 msec = 48 msec

– Then the number of cells inserted (N) during RTT =Then the number of cells inserted (N) during RTT =

= 1.7 x 10= 1.7 x 1044 cells = 7.2 million bits cells = 7.2 million bits

53 x 8 bits53 x 8 bits150 x 10150 x 1066 bps bps

48 x 1048 x 10-3-3 seconds seconds2.8 x 102.8 x 10-6-6 seconds seconds


Cell Delay VariationCell Delay Variation General requirement: delay should General requirement: delay should

be shortbe short– ATM designed to ATM designed to minimize delayminimize delay

For some applications, For some applications, rate of rate of deliverydelivery of cells to destination must of cells to destination must be constant (ATM’s CBR service be constant (ATM’s CBR service level)level)

Contributors to cell delay variationContributors to cell delay variation– network contribution: network contribution: queuingqueuing and and

processing variationsprocessing variations– variation at variation at UNIUNI (user network (user network

interface) due to interface) due to cell processingcell processing


Origins of Cell Delay Origins of Cell Delay Variation at UNIVariation at UNI

Per I-371

Interleaving prior to delivery to physical layer

Further delays possible at the physical layer


Cell Delay Variation: CBR Cell Delay Variation: CBR CellsCells

= 1/R = inverse of insertion rate

D(i) = end-to-end delay for the ith cell

V(i) = V(i-1) – [ti – (ti-1 + )]

cell arrives late: discarded

Slope = R cells/sec = 1/ cell insertion rate

V(0) = est. tolerable delay variation


ATM AttributesATM Attributes

How we describe an ATM traffic flowHow we describe an ATM traffic flowTraffic parametersTraffic parametersQoS parametersQoS parametersCongestion (for ABR)Congestion (for ABR)Other (for UBR)Other (for UBR)


Traffic ParametersTraffic ParametersConnection Traffic DescriptorConnection Traffic Descriptor

– Source Traffic DescriptorSource Traffic Descriptor: PCR, SCR, : PCR, SCR, MBS, MCR, MFS (more on next slide)MBS, MCR, MFS (more on next slide)

– Cell Delay Variation Tolerance (Cell Delay Variation Tolerance ()): upper : upper bound on amount of cell delay that is bound on amount of cell delay that is introduced by the network interface and introduced by the network interface and the UNI (due to interleaving, physical the UNI (due to interleaving, physical layer overhead, multiplexing, etc.) layer overhead, multiplexing, etc.)

Note- this is the value V(0) from our earlier Note- this is the value V(0) from our earlier discussion of CDVdiscussion of CDV

– Conformance DefinitionConformance Definition: unambiguous : unambiguous specification of conforming cells of a specification of conforming cells of a connection at the UNI (see connection at the UNI (see GCRAGCRA, later), later)


Traffic ParametersTraffic Parameters SourceSource Traffic Descriptor Traffic Descriptor

– Peak Cell Rate (PCR)Peak Cell Rate (PCR): upper bound on traffic : upper bound on traffic submitted by source (PCR = 1/T, where T = submitted by source (PCR = 1/T, where T = minimum cell spacingminimum cell spacing

– Sustainable Cell Rate (SCR)Sustainable Cell Rate (SCR): upper bound on : upper bound on average rate of traffic submitted by source average rate of traffic submitted by source (over a larger T)(over a larger T)

– Maximum Burst Size (MBS)Maximum Burst Size (MBS): maximum number : maximum number of cells sent continuously at PCRof cells sent continuously at PCR

– Minimum Cell Rate (MCR)Minimum Cell Rate (MCR): used with ABR and : used with ABR and GFR… minimum cell rate requested, access to GFR… minimum cell rate requested, access to unused capacity up to PCR (elastic capacity = unused capacity up to PCR (elastic capacity = PCR-MCR?)PCR-MCR?)

– Maximum Frame Size (MFS)Maximum Frame Size (MFS): maximum size of : maximum size of a frame in cells available for GFR servicea frame in cells available for GFR service


QoS Parameters QoS Parameters (negotiated between user and network during connection (negotiated between user and network during connection set-up, as defined by the ATM Forum)set-up, as defined by the ATM Forum)

Peak-to-peak cell delay variation Peak-to-peak cell delay variation (CDV)(CDV): acceptable delay variation at : acceptable delay variation at destination – the difference between destination – the difference between the best case and worst case CTDthe best case and worst case CTD

Maximum Cell Transfer Delay Maximum Cell Transfer Delay (maxCTD)(maxCTD): maximum time between : maximum time between transmission of first bit of a cell at the transmission of first bit of a cell at the source UNI to receipt of its last bit at source UNI to receipt of its last bit at the destination UNIthe destination UNI

Cell Loss RatioCell Loss Ratio: ratio of lost cells to : ratio of lost cells to total transmitted cells on a connectiontotal transmitted cells on a connection


Cell Transfer Delay Cell Transfer Delay Probability Density (real-time Probability Density (real-time services)services)

Variable component of delay, due to buffering and cell scheduling.

Fraction of cells that exceed Maximum and will be discarded or delivered late.

Propagation throughphysical media.


Congestion Control and Congestion Control and Other Traffic AttributesOther Traffic Attributes Congestion ControlCongestion Control

– defined defined only for ABRonly for ABR service categoryservice category– uses uses network feedback controlsnetwork feedback controls– ABR flow control mechanism (more later)ABR flow control mechanism (more later)

Other Attributes (introduced July 2000)Other Attributes (introduced July 2000)– Behavior class selectorBehavior class selector (BCS): (BCS):

for IP differentiated services (DiffServ)for IP differentiated services (DiffServ) provides for different levels of service among provides for different levels of service among

UBR connectionsUBR connections implementation dependent, no guidance in implementation dependent, no guidance in

specsspecs

– Minimum desired cell rateMinimum desired cell rate (MDCR): (MDCR): UBR application minimum capacity objectiveUBR application minimum capacity objective


Service Category-Attribute Service Category-Attribute RelationshipRelationship


Traffic & Congestion Traffic & Congestion Control Function Control Function Classification – A Classification – A FrameworkFramework

affect more than one connection, effective over long timeframe

determine if/how network can accommodate connection at a given QoS

network responds within the round-trip lifetime of a cell

react immediately to a cell as it is transmitted


Resource Management Resource Management Using Virtual PathsUsing Virtual Paths Multiple VCCs with various QoS Multiple VCCs with various QoS

requirements in same VPC requirements in same VPC Cases to consider:Cases to consider:

– User-to-User applicationUser-to-User application:: VPCs between pairs of VPCs between pairs of UNIs, VCC QoS is user’s responsibility… user UNIs, VCC QoS is user’s responsibility… user must ensure that aggregate of VCCs does not must ensure that aggregate of VCCs does not exceed capacity allocated to VPCexceed capacity allocated to VPC

– User-to-network applicationUser-to-network application:: VPC between UNI VPC between UNI and network node, network must accommodate and network node, network must accommodate QoS of individual VCCsQoS of individual VCCs

– Network-to-network applicationNetwork-to-network application:: network must network must accommodate QoS of individual VCCsaccommodate QoS of individual VCCs


Resource Management Resource Management Using Virtual PathsUsing Virtual Paths Performance (QoS) of a VCC depends on Performance (QoS) of a VCC depends on

resources allocated to the VPC(s) through resources allocated to the VPC(s) through which the VCC extendswhich the VCC extends

Network allocates capacity to each VPCNetwork allocates capacity to each VPC based on performance objectives agreed based on performance objectives agreed between network and subscriber between network and subscriber (contract). Two approaches:(contract). Two approaches:– Aggregate peak demandAggregate peak demand – VPC capacity (e.g. – VPC capacity (e.g.

data rate) set to sum of peak data rates of all data rate) set to sum of peak data rates of all VCCsVCCs

– Statistical multiplexingStatistical multiplexing – VPC capacity set to be – VPC capacity set to be greater than or equal to average demand for all greater than or equal to average demand for all VCCs, but less than aggregate peak demandVCCs, but less than aggregate peak demand


Example VCC/VPC Example VCC/VPC ConfigurationConfiguration


Connection Admission Connection Admission Control (CAC)Control (CAC) Network accepts the connection only Network accepts the connection only

if it can if it can commit resourcescommit resources - in both - in both directions - that satisfy a given directions - that satisfy a given connection requestconnection request::1.1. Service category (CBR, rt-VBR, …)Service category (CBR, rt-VBR, …)2.2. Connection traffic descriptor (PCR, …, Connection traffic descriptor (PCR, …,

CDVT, conformance definition)CDVT, conformance definition)3.3. QoS (peak-to-peak CDV, max CTD, QoS (peak-to-peak CDV, max CTD,

CLR)CLR)4.4. Cell loss priority (CLP bit 0 or 0+1)Cell loss priority (CLP bit 0 or 0+1)

If connection is accepted, a “traffic If connection is accepted, a “traffic contract” is awarded to the usercontract” is awarded to the user


Connection Admission Connection Admission Control (Traffic Contract)Control (Traffic Contract)

(a.k.a. jitter)


Connection Admission Connection Admission Control (CAC)Control (CAC)

Contract

Contract

Contract

• Traffic ParametersPeak cell rateSustainable cell rateBurst toleranceEtc.

• Quality of ServiceDelay JitterCell loss

ATM Network


Usage Parameter ControlUsage Parameter Control

Purpose: after a connection is Purpose: after a connection is established, established, protect the network’sprotect the network’s resources resources from overload/abusefrom overload/abuse by by violating connectionsviolating connections

Monitors connection for Monitors connection for conformanceconformance to the traffic contract to the traffic contract– detect violation of assigned detect violation of assigned

parameters based on conformance parameters based on conformance definition agreed to in contractdefinition agreed to in contract

– take appropriate actiontake appropriate action


Usage Parameter ControlUsage Parameter Control

ATM Network

You areNot in Conformance

with the Contract.What Should the

Penalty Be??

• PASS• MARK CLP BIT• DROP

?DECISION?

Policing

REBELAPPLICATION

Contract


Usage Parameter Control Usage Parameter Control Function LocationFunction Location

Case 2

Case 1


UPC Traffic ManagementUPC Traffic ManagementPeak Cell Rate AlgorithmPeak Cell Rate Algorithm

– Regulates the peak cell rateRegulates the peak cell rate and the and the associated CDVT of a connectionassociated CDVT of a connection

Sustainable Cell Rate AlgorithmSustainable Cell Rate Algorithm– Regulates the sustainable cell rateRegulates the sustainable cell rate

and associated and associated burst toleranceburst tolerance of a of a connectionconnection

Traffic ShapingTraffic Shaping– SmoothesSmoothes out traffic at network out traffic at network

entry points to reduce “clumping”entry points to reduce “clumping”– Reduce delays, ensure Reduce delays, ensure fair resource fair resource

allocationallocation


Generic Cell Rate Algorithm Generic Cell Rate Algorithm (GCRA): Virtual Scheduling(GCRA): Virtual Scheduling

GCRA (I, L):I = IncrementL = Limitta(k) = Time of arrival of a cellTAT = Theoretical arrival time

At time of arrival ta(1) of the first cell of connection, TAT = ta(1)

Algorithm takes two arguments, I and L

With PCR of R, I = T = 1/R

CDVT limit, = L Then peak cell rate

algorithm is expressed as:

GCRA(T, )

I.e., this cell arrived too late.

Late arrival OK

Reset TAT

Early arrival within limit

OK

Early arrival beyond limit

NOT OK

Early arrival TEST


Effect of CDVT on Cell Arrival at UNI Effect of CDVT on Cell Arrival at UNI (example: T=4.5(example: T=4.5))

Ideal ( = 0.5)

Possible ( = 1.5)

Possible ( = 3.5)Note: = T - N = 1 + ( / T-)

Possible ( = 7)Note: > T - N = 1 + ( / T-)

Cell Clumping

Cell Clumping


Sustainable Cell Rate Sustainable Cell Rate AlgorithmAlgorithm

Uses Uses GCRA (TGCRA (Tss, , ss)), where:, where:TTss = 1/R = 1/Rss is the interarrival time at the is the interarrival time at the

sustainable cell rate, Rsustainable cell rate, Rss, and, and

ss is the burst tolerance, or the time is the burst tolerance, or the time scale during which cell rate scale during which cell rate fluctuations (at PCR) are allowedfluctuations (at PCR) are allowed

ss is derived from the burstiness of is derived from the burstiness of the traffic stream:the traffic stream: Burst Tolerance = s = (MBS-1)

-

1 1SCR PCR


Sustainable Cell Rate Sustainable Cell Rate AlgorithmAlgorithm

Note that, if the traffic stream is Note that, if the traffic stream is constrained by both constrained by both GCRA (T, GCRA (T, ) ) and GCRA (Tand GCRA (Tss, , ss) ) , then Maximum , then Maximum Burst Size (MBS) is:Burst Size (MBS) is:

ss is derived from the burstiness of is derived from the burstiness of the traffic stream:the traffic stream: Burst Tolerance = s = (MBS-1)

-

1 1SCR PCR

MBS = 1 +

s

Ts-T


UPC Function: Possible Actions UPC Function: Possible Actions based on CLP bit (dual CLP)based on CLP bit (dual CLP)

P? = Compliance test for parameter P

Forward cell or discard

it?

(see p. 377)


Token Bucket for Traffic Token Bucket for Traffic ShapingShaping

Tokens are generated and fill the bucket at the constant rate, .

To pass, a token is removed from the bucket. If bucket is empty, cell is queued to wait for next token.

Departures rate is “smoothed” to .


ABR Traffic ManagementABR Traffic ManagementCBR, rt-VBR, nrt-VBR: traffic contract CBR, rt-VBR, nrt-VBR: traffic contract

with with open-loop controlopen-loop controlUBR: best effort sharing of unused UBR: best effort sharing of unused

capacitycapacityABR: share unused (available) ABR: share unused (available)

capacity using capacity using closed-loop controlclosed-loop control of of sourcesource– Allowed Cell Rate (ACR):Allowed Cell Rate (ACR): current max. cell current max. cell

transmission ratetransmission rate– Minimum Cell Rate (MCR)Minimum Cell Rate (MCR): network : network

guaranteed minimum cell rateguaranteed minimum cell rate– Peak Cell Rate (PCR):Peak Cell Rate (PCR): max. value for ACR max. value for ACR– Initial Cell Rate (ICR):Initial Cell Rate (ICR): initial value of ACR initial value of ACR


ABR Traffic ManagementABR Traffic Management

ACR is dynamically adjustedACR is dynamically adjusted based on feedback to the source based on feedback to the source in the form of in the form of Resource Resource Management (RM) cellsManagement (RM) cells

RM cells contain three fields:RM cells contain three fields:– Congestion Indication (CI) bitCongestion Indication (CI) bit– No Increase (NI) bitNo Increase (NI) bit– Explicit Cell Rate (ER) fieldExplicit Cell Rate (ER) field


Flow of Data and RM Cells – Flow of Data and RM Cells – ABR ConnectionABR Connection

Nrm parameter usually set to 32

FRM cell flow

BRM Cell flow


ABR Source Reaction RulesABR Source Reaction Rules

NINI CICI ACTIONACTION

00 00 ACR ACR max[MCR,min[ER,PCR,ACR+max[MCR,min[ER,PCR,ACR+RIFRIFxPCR]]xPCR]]

00 11 ACR ACR max[MCR,min[ER, ACR(1- max[MCR,min[ER, ACR(1-RDFRDF)]])]]

11 00 ACR ACR max[MCR,min[ER, ACR]] max[MCR,min[ER, ACR]]

11 11 ACR ACR max[MCR,min[ER, ACR(1-RDF)]] max[MCR,min[ER, ACR(1-RDF)]]

RIF – Fixed rate increase factor (default 1/16)RIF – Fixed rate increase factor (default 1/16)RDF – Fixed rate decrease factor (default RDF – Fixed rate decrease factor (default 1/16)1/16)


Variations in Allowed Cell Variations in Allowed Cell RateRate

RIF = 1/16RDF = 1/4


ABR RM ABR RM Cell Cell FormatFormat


Initial Values of Cell FieldsInitial Values of Cell Fields


ABR ParametersABR Parameters


ABR Capacity AllocationABR Capacity Allocation

Two Functions of ATM SwitchesTwo Functions of ATM Switches– Congestion Control:Congestion Control: throttle back on throttle back on

rates based on buffer dynamicsrates based on buffer dynamics– Fairness:Fairness: throttle back as required to throttle back as required to

ensure fair allocation of available ensure fair allocation of available capacity between connectionscapacity between connections

Two categories of switch Two categories of switch algorithmsalgorithms– BinaryBinary: EFCI, CI and NI bits: EFCI, CI and NI bits– Explicit rateExplicit rate: use of the ER field: use of the ER field


Binary Feedback SchemesBinary Feedback SchemesSingle FIFO queueSingle FIFO queue at each output at each output

port bufferport buffer– switch issues EFCI, CI, NI based on switch issues EFCI, CI, NI based on

threshold(s) in each queuethreshold(s) in each queueMultiple queuesMultiple queues per port - separate per port - separate

queue for each VC, or group of VCsqueue for each VC, or group of VCs– uses threshold levels as aboveuses threshold levels as above

Use selective feedback to Use selective feedback to dynamically allocate dynamically allocate fair share of fair share of capacitycapacity– switch will mark cells that exceed switch will mark cells that exceed

their fair share of buffer capacitytheir fair share of buffer capacity


Explicit Rate Feedback Explicit Rate Feedback SchemesSchemes Basic scheme at switch is:Basic scheme at switch is:

1.1. compute fair sharecompute fair share of capacity for each VC of capacity for each VC2.2. determine the current loaddetermine the current load or degree of or degree of

congestioncongestion3.3. compute an explicit ratecompute an explicit rate (ER) for each VC (ER) for each VC

and send to the source in an RM celland send to the source in an RM cell Several example of this schemeSeveral example of this scheme

– Enhanced proportional rate control algorithm Enhanced proportional rate control algorithm (EPRCA)(EPRCA)

– Explicit rate indication for congestion Explicit rate indication for congestion avoidance (ERICA)avoidance (ERICA)

– Congestion Avoidance using proportional Congestion Avoidance using proportional control (CAPC)control (CAPC)


EPRCAEPRCA Switch calculates mean current load on Switch calculates mean current load on

each connection, called the MACR:each connection, called the MACR:MACR(I) = (1-MACR(I) = (1-) x MACR(I-1) + ) x MACR(I-1) + x CCR(I) x CCR(I)

Note: typical value for Note: typical value for is 1/16 is 1/16

When queue length at an output port When queue length at an output port exceeds the established threshold, exceeds the established threshold, update ER field in RMs for all VCs on that update ER field in RMs for all VCs on that port as:port as:ER ER min[ER, DPF x MACR] min[ER, DPF x MACR]

where DPF is the down pressure factor parameter, where DPF is the down pressure factor parameter, typically set to 7/8.typically set to 7/8.

EffectEffect: lowers ERs of VCs that are : lowers ERs of VCs that are consuming more than fair share of consuming more than fair share of switch capacityswitch capacity


ERICAERICAMakes adjustments to ER based on Makes adjustments to ER based on

switch load factor:switch load factor:Load Factor (LF) = Input rate /Target rateLoad Factor (LF) = Input rate /Target ratewhere where input rateinput rate is averaged over a fixed is averaged over a fixed

interval, and interval, and target ratetarget rate is typically 85-90% of is typically 85-90% of link bandwidthlink bandwidth

When LF > 1, congestion is When LF > 1, congestion is threatened, and ERs are reduced by threatened, and ERs are reduced by VC on a fair share basis:VC on a fair share basis:– Fairshare = target rate/number of VCsFairshare = target rate/number of VCs– Current VCshare = CCR/LF Current VCshare = CCR/LF – newER = min[oldER, max[Fairshare, VCshare]]newER = min[oldER, max[Fairshare, VCshare]]

Documents

1 Chapter 13 ATM Traffic & Congestion Control. Chapter 13: ATM Traffic & Congestion Control 2 Introduction ATM congestion problem overview ATM congestion