Traffic Control

Prof. Nelson FonsecaState University of Campinas

Traffic

Traffic – bits carried;

Ultimate goal of a network – to transport bits.

Traffic Control

Support of the Quality of Service requirements of the applications;

Efficient use of network resources.

Quality of Services Perception of the quality of information

transport.

Aplicações

Aplicações

AplicaçõesAplicações

QoS

QoS

QoSQoS

REDE

Quality of Services QoS parameters: express numerically

specific aspects of the quality perceived Common parameters:

Mean delay; Loss rate;

REDE

Delay in Packet Switching Networks

Packets expreice delay in their end-to-end path

Four delay components

Node processing checksum table lookup ququeing

Wait for output link Depends on node

congestion

A

B

propagation

transmission

processingqueueing

Delay in Packet Switching Networks

Transmission delay: R=bandwidth (bps) L=packet size (bits) Time to put the bits

in the link = L/R

Propagation delay d = length of the link s = propagation speed

(~2x108 m/sec) Propagation delay=

A

B

propagation

transmission

processingqueueing

Queueing Delay

R=link bandwidth (bps) L=packet size (bits) a=arrival rate of

packets

traffic intensity = La/R

La/R ~ 0: low mean delay La/R -> 1: increasing delay La/R > 1: work arrived exceeds

processing capacity

Jitter

Delay variation Impact on playback of voice

applications Buffer at receiver to ammeliorate jitter

effect

An Example:ATM QoS Parameters

Maximum transfer delay Delay variation peak to peak; Cell Loss rate; Rate of block severiously lost; Rate of cells erroneously inserted.

QoS Parameters

Number of packets consecutively lost; Mena time between faults; Mean time to have access; Mean time to recover from faults.

Traffic Descriptors

Characterizes quantitatively the pattern of the flow of bits

Used to produce estimates of the resource demands of a flow

An Example:ATM Traffic Descriptors

Peak Cell Rate (PCR): Maximum rate of cell transmission;

Sustainable Cell Rate (SCR): upper bound for the transmission rate;

Burst Tolerance – (BT) Maximum Burst Size - maximum time

for transmission at peak rate

An Example:ATM Traffic Descriptors

SCR

BTPCR

Class of Service

Users´choices of agreemet on quality of transport made between service provider

Usually classes of services exists at the network/link layer to support users´expectation of QoS

An Example:ATM Class of Services

CBR; NRT – VBR; RT – VBR; ABR; UBR; GRF.

CBR Constant Bit Rate; Allocation of fixed amount of bandwidth

during the duration of the virtual circuit; Real-time applications are sensitive to

delay and minimum bandwidth; Voice, video, circuit emulation.

CBR

PCR

Cel

l Rat

e

Tempo

RT – VBR Real Time Variable Bit Rate; Time-varying requirements of

bandwidth; Applications which need delay bound Voice and video.

NRT – VBR Non-Real Time Variable Bit Rate; Data-loss sensitive applications; Time-varying bandwidth requirements.

VBR

PCR

Cel

l Rat

e

Tempo

SCR

BT

ABR

Avaliable Bit Rate; Bandwidth allocation depends on

network feedback; Not proper to delay sensitive

applications.

ABR

PCR

Cel

l Rat

e

Tempo

SCR

MCR

UBR

Unspecified Bit Rate; Serviço Best Effort; Ip over ATM; No QoS guarantees.

GRF Generalized Frame Rate; Enhanced Serviço Best Effort; Minimum bandwidth guarantees; Deals with frames intead of cells;

Class of Service

CBR RT-VBR

NRT-VBR

ABR UBR GRB

Cell Loss Ratio (CLR) Y Y Y Y

Cell Delay Variation (CDV) Y Y Y

Cell Transfer Delay (CTD)

Peak Cell Rate (PCR) Y Y Y Y Y Y

CDV Tolerance (CDVT)

Sustainable Cell Rate (SCR) Y Y

Burst Tolerance (BT) Y

Mininum Cell Rate (MCR) Y

Congestion

Network lack of capacity to provide the Quality of Service requirements of applications

QoSC

ell L

oss

Rat

e

Maximum Cell Delay Variation

interactivevideo

Voice

interactivedata

Image filetransfer

10e-4

10e-6

10e-8

10e-10

1 10 100 1000

Congestion Control Congestion control mechanisms work at

different time scales and can be either reactive or pro-active

Congestion Control Mechanisms

Admission control Policing Selective Discard Active queue management Scheduling

Controle de Admissão de Conexão (CAC)

Decision making process. Decides whether or not to accept a flow (connection) into a network domain;

Decisions need to consider the mantainance of QoS requirements of already admitted flows as well as the support of QoS requirements of requesting flows;

Admission Control

My requirements are....

My traffic parameters are...

Service provider

Admission Control

Traffic descriptors

Traffic representation

Estimation of total resource demand

Admission Control

Parametric (analytical models)

Two approaches

Measurement Based

Admission ControlParametric Approach

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log2

Admission ControlParametric Approach

Admission ControlParametric Approach

Admission ControlMeasurement Based

Traffic Envelopes MBAC: Arrival Envelope: describes the peak rate

over defined intervals. Service Envelope: describes the minimum

service received by a traffic class as a function of interval length.

sourcedestination

ingress router

egress router

Admission Control Measurement Based

Admission Control condition: R(t) = mean of the arrival envelope 2 = variance of the arrival envelope S(t) = mean of the service envelope 2 = variance of the service envelope P = peak rate of the new flow D = delay bound = violation probability

Admission Control Measurement Based

Stability condition:

Admission Control Measurement Based

Time-Window/Measured Sum MBAC: The decision algorithm admits a new flow with load f if:

+ f < C *

f: load of the new flow.

: measured load of existing traffic.C: channel capacity.: user-defined utilization target.

Admission Control Measurement Based

Time-Window/Measured Sum MBAC:

+ f < * C

+ f < * B *

B: maximum data rate used by the network.

: estimate of the channel efficiency.

Admissible Region

Admissible Region

Centralized Admission Control

Distributed Admission Control

Emissor

Path

Resv

Path

Resv

PathRoteador 1 Roteador N

Receptor Resv

Interdomain Admission Interdomain Admission ControlControl

Arquiteturas para Provisão de QoS na Internet

BB0

BB1

BB2

BB3

ISP1

ISP2

ISP3

Origem

Destino

BB4

Admission ControlWireless Network

Admission Control aware of:

Signal to interference and noise ratio

Handoff failure

Admission ControlWireless Network

Admission ControlWireless Network

Policing

Keeps track of the transmission of a flow (connection) during its whole duration so that the contract between user and provider can be enforced.

Leaky Bucket

Tokens are generated at a constant rate (leaky rate);

A packet needs to consume a token for entering into the network;

Leaky BucketLeaky Bucket

Chegada dePacotes

Gerador deFichas

taxa média de fichas/segundos

Capacidadedo Balde: fichas

taxamáxima

Rede

SmoothingSmoothing

Chegada dePacotes

Gerador deFichas

taxa média de fichas/segundos

Capacidadedo Balde: fichas

taxamáxima

Redefila de saída

Limitations of the Leaky Bucket Mechanism

Just two parameters to control; Leaky rate controls the mean arrival

rate; Bucket size controls the burst length

The GCRA Algorithm Generic Cell Rate Algorithm

Virtual Scheduling; LB with continuous state;

GCRA (I,L) I: increment; L: Tolerance; TAT: Theoretical arrival time; LCT: Last conforming time – arrival time of the last

conforming cell;

GCRA

TAT = ta(k)

Célula excessiva

TAT = TAT + Ibem-comportada

TAT < ta(k)?

TAT < ta(k) + L

Sim

Sim

Não

Não

GCRA

Y = 0

Célula excessiva

X + Y + ILCT = ta(k)

bem-comportada

Y < 0?

Y > L?

Sim

Sim

Não

Não

Y = X - (ta(k) - LCT)

Selective Discard

Discard packets in congestion situations

Buffer management+

Push out Policy

Selective DiscardBuffer Management

1 1 1 2 1 1 1 2

Compartilhamento total com push-out

1 1 1 1 1

Particionamento total

22

2 1 1 1 1 2 2

Compartilhamento parcial

Selective DiscardPush out Policy

1 2 1 2 1 2

Primeiro-a-chegar-primeiro-a-ser-descartado

1 2 1 2 1 2

Último-a-chegar-primeiro-a-ser-descartado

1 1 1

Randômica

2 2 2

Packet Discard in Cell-switched Networks

3 2 1 3 1

3 2 1

Packet Discard in Cell-switched Networks

5 4 3 2 1

Tail D rop

3 2 1

Early Packet D iscard

Early Packet D iscard with Hysteresis

Active Queue Management Conceived to detect incipient

congestion Discard packets so that TCP

connections reduce their transmission rate and consequently avoid congestion

Drop Tail does not avoid global synchronization and resource monopolization

Active Queue Management

Based on heuristics: RED, FRED, ARED, BLUE

Based on Control Theory: PI-AQM, H2-AQM

Based on Optimization Theory: REM

RED

Random Earlier Detection Standardized by IETF Two threshold: if queue length smaller than first

threshold no packet is discard

RED

If queue length is larger than first thresold and smaller than second threshold packet is discarded with linearly increasing probability

If queue length larger than second threshold all packets are discarded

REDP

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Zona deprevenção de

congestionamento

Zona de controle decongestionamento

RED

Difficulty to set the value of the thresholds

Performance degradation under a large number of flows (burst traffic)

Bias againts connections with small window

Does not promote proportional discard Does not deal with unresponsive flows

ARED

Adaptive Random Earlier Detection Tries more efficient tunning of

parameters Rate of increase of discard

probability is proportional to the variation of the queue length

ARED

Fred Flow Random Early Drop Tries to enforce fairness among flows Keeps variables per flow Each flow is guaranteed a minimum

number of packets in the buffer. This minimum number is adjusted according to the offered load

Each flow cannot exceed a maximum number of packets in queue

Fred Each flow cannot exceed a maximum

number of packets in queue Keeps track of how much each flow tries to

exceed the allowed maximum numbe and penalize those which try to exceed by reducing its maximum allowed number of packets

Prevents non-responsive flows to monopolize the queue

Not scalable

Blue Tries to avoid underutilization of the

queue under low load and excessive loss of packet under high loads

The discard probability changes its values as a function of the load.

The differential value for increasing/decreasing the discard probability as well as the minimum values for the interval between consecutive updates are tunable parameters

SBLue

Stochastic Blue Uses hash scheme to classify flows

and set probability of dicsrad according to blue

Penalizes non-responsive flows

FPQ Flow proportinal Queuing Aim at keeping the queue length proportional

to the number of flow Estimates the number of flows and set discard

probability according to this estimation A minimum number of eight packets is

guaranteed per connection and considers the bandwidth delay product in the computation of the discard probability

AQM based on Optimization Maximize the aggregate utility function

subject to link capacity restrinction Transmission rates are primal variables

and discard probabilities are the dual variables

The Adaptive virtual queue emulates a virtual queue to determine the discard probability, the capacity of the queue is a function of the offered load. The use of virtual queues tends to stabilize the real queue

AQM based on Optimization

REM – price is taken as the congestion level at the link

Exponential RED – discard probability inreases exponentially with the length of a vitual queue which is smaller than the true queue

AQM based on Control Theory Congestion seen as a control problem

AQM based on Control Theory AQM Controllers based on P (Proportional), I

(Integral ), PI (Proportional-Integral ), PD (Proportional-Derivative) ou PID (Proportional-Integral- Derivative) classical controllers.

I component reduces the error of systems state in relation to the equilibrium point

D component decreases response time P & I components work on past error values

and cannot predict future errors D component is able to predict future errors

AQM based on Control Theory RED is a P-type controller Stabilized RED (SRED) stabilizes the

queue independent of the number of active connections. It estimates the number of active connections

DRED uses the difference between the actual queue length and the target one to change the value of the discard probability

VRC Virtual Rate Control aims at a target

virtual rate, similar concept to AVQ but AVQ uses a virtual queue length. VRC aims at target input rate as well as queue length. It uses a PID controller to compensate the difference between the actual and the target input rate

AQM based on Control Theory Yellow controls the difference between

link capacity and network load Receding Horizon AQM policy tries to

compensate the delay element for feedback

SMVSAQM and VS-AQMutilize SMVS - Sliding ModeVariable Structure Control which has an adptive control structure in order to be insentive to congestion control parameters

RIO

Standardized for the DiffServ framework Uses two RED queues: one for In-Profile

traffic and the other for Out-profile traffic

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Scheduling

Defines the transmission order of packets;

Provides performance guarantees, i.e., upper bound on QoS requirements such as delay, jitter and bandwidth.

Isolates traffic from different classes;

Scheduling Discipline First-come-First-Served Priority Virtual Clock Generalized Processor Sharing Weighted Fair Queuing Earliest-Due-Date ….

Scheduling Disciplines

Work conserving disciplines – server is never idle if there are packets in queue;

Non-work-conserving – server can be idle even if there are packets in queue since packets may not be elegible for transmission.

Conservation Law A work-conserving discipline can only reallocate

delays among the flows

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Work Conserving Discipline First Come First Served Priority Virtual Clock Generalized Processor Sharing Weighted Fair Queuing Worst-case weighted fair Queueing Self-clocked Fair Queuing

Virtual Clock

Isloates flows as in TDM systems; Each connection has its own clock with

its own time unit The connection clock advances

according to its time unit Time unit reflects the negotiated

contracted with network provider.

Virtual Clock

1/2

1/5

1/5

Generalized Processor Sharing

Fair Queueing Bandwidth shared according to the

weights of the connections

N

jjii Cg

1

/

m

C

Generalized Processor Sharing

N connections sharing a link with capacity C; Each connection has its own weight i; S(i,, t) bits served from connection i during

the interval [, t];

)(

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),,(

),,(

j

i

tjS

tiS

Generalized Processor Sharing

In a backlogged system, each flow receives a minimum amount of bandwidth proportioned to its weight. Unused bandwidth is distributed among flows according to their weight:

j

Ci

).(

Generalized Processor Sharing

If connection (flow) i is policed by a leaky bucket with parameters [(i),(i)], then an upper bound to the end-to-end delay igual to (i)/(i) is guaranteed.

Generalized Processor Sharing

GPS is an idealized scheduling discipline. It considers that a bit is infinitely divisible.

Packets is the unit of transmission Several disciplines emulate GPS: Weighted Fair Queuing Worst-Case Fair Queuing Self-Clocked Fair Queuing Start-time fair queuing

Weighted Fair Queuing Packet version of GPS

Also called Packet-by-Packet Fair Queueing

Packets are served in order of finishing time of transmission were they served by a GPS discipline

How does WFQ emulates GPS?

Packets are served in increasing order of finishing time, called finish number;

Round number – number of rounds of service a bit-by-bit round-robin scheduler has completed at a given time;

The duration of a round is proportional to the number of active conections

Weighted Fair Queueing In a WFQ a packet can be at most Lmax/C

behind of an equivalent GPS server where Lmax is the size of the largest packet sent in the connection;

The end-to-end delay is bounded by:

m

jj

i

i CLLm

1max

max /)1(

Worst-Case Fair Queueing A packet in a WFQ system can be far ahead

of its equivalent GPS system A W2FQ server considers only those packet

which would have started transmission in a fair queuing system and select the one which would have finish first in a fair queuing system

The difference between W2FQ and FQ can be at most of one packet

Same delay bound of WFQ

Self-Clocked Fair Queuing

Utilizes a more efficient finish number computation.

Packets that arrive to an empty queue utilizes the finish number of the packet in services instead of the round number

Short term unfairness and large worst-case delay

Start-time Fair Queuing Avoids short term unfairness and large

worst-case delay Computes both finish and start number Connections are served in order of start

number The start number is set either to the

finish time of a packet or to the round number (in case of empty server)

Delay-Earliest-Due-Date

Each packet has a deadline to leave the server which is set according to the contract between user and provider

Packets are server in increasing order of their arrival time plus their deadline

Upper bound to delay

Non-Work-Conserving Discipline

Hierarchical Round Robin Jitter-Earliest-Due-Date Rate Controlled Scheduler

Hierarchical Round Robin Multilevel framing strategy The server cycles through different

levels and services packets If it cycles through a slot with no

packet, it will leave the server idle instead of serving a packet in another level

Jitter-Earliest-Due-Date

Eliminates jitter After being served a packet is stamped

with the difference between its deadline and the actual finish time

A regulator at the entrance of the next server holds the packet before is is made eligible

Rate Controlled Scheduler

By coupling a regulator and a scheduler flexibility in providing bandwidth, delay and jitter guarantees is achieved.

Explicit Congestion Notification

Dissociate congestion notification from packet loss

router should be identify flows that use ECN

The two end-points envolved should negotiate at connection setup the use of ECN

Router marks IP datagram

Recipeint receivec notification in IP header and echos it to the sender in the ack packet

ToS fields of IP header used

ECT

CE

0 0 No support to ECN

0 1 Indicates that sender supports ECN

1 0 Indicates that recipient supports ECN

1 1 Indicates congestion