Flow Control - Engineering Approach to Computer Networking - Lecture Slides, Slides of Computer Science

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Flow Control

An Engineering Approach to Computer Networking An Engineering Approach to Computer Networking

Flow control problem nn

Consider file transferConsider file transfer

n n

Sender sends a stream of packets representing fragments of aSender sends a stream of packets representing fragments of afilefile

n n

Sender should try to match rate at which receiver and networkSender should try to match rate at which receiver and networkcan process datacan process data

n n

Can’t send too slow or too fastCan’t send too slow or too fast

nn

Too slowToo slow

uu

wastes time wastes time

n n

Too fastToo fast

u u

can lead to buffer overflow can lead to buffer overflow

nn

How to find the correct rate?How to find the correct rate?

Where? nn

Usually at transport layerUsually at transport layer

n n

Also, in some cases, in datalinkAlso, in some cases, in

datalink layer

layer

Model nn

Source, sink, server, service rate, bottleneck, round trip timeSource, sink, server, service rate, bottleneck, round trip time

Open loop flow control nn

Two phases to flowTwo phases to flow

u u

Call setup Call setup

u u

Data transmission Data transmission

nn

Call setupCall setup

uu

Network prescribes parameters Network prescribes parameters

u u

User chooses parameter values User chooses parameter values

u u

Network admits or denies call Network admits or denies call

nn

Data transmissionData transmission

uu

User sends within parameter range User sends within parameter range

u u

Network Network

polices polices users

users

u u

Scheduling policies give user Scheduling policies give user QoS

QoS

Hard problems n

Choosing a descriptor at a source

n

Choosing a scheduling discipline at intermediate networkelements

n

Admitting calls so that their performance objectives are met (

call

admission control).

Descriptor requirements nn

RepresentativityRepresentativity

u u

adequately describes flow, so that network does not reserve adequately describes flow, so that network does not reserve

too little or too much resource too little or too much resource

n n

VerifiabilityVerifiability

uu

verify that descriptor holds verify that descriptor holds

nn

PreservabilityPreservability

u u

Doesn’t change inside the network Doesn’t change inside the network

n n

UsabilityUsability

uu

Easy to describe and use for admission control Easy to describe and use for admission control

Examples n

Representative, verifiable, but not useble

u u

Time series of Time series of interarrival

interarrival times

times

n n

Verifiable, preservableVerifiable,

preservable, and useable, but not representative

, and useable, but not representative

uu

peak rate peak rate

Peak rate nn

Highest ‘rate’ at which a source can send dataHighest ‘rate’ at which a source can send data

n n

Two ways to compute itTwo ways to compute it

n n

For networks with fixed-size packetsFor networks with fixed-size packets

uu

min inter-packet spacing min inter-packet spacing

nn

For networks with variable-size packetsFor networks with variable-size packets

u u

highest rate over highest rate over

all all

intervals of a particular duration intervals of a particular duration

n n

Regulator for fixed-size packetsRegulator for fixed-size packets

uu

timer set on packet transmission timer set on packet transmission

uu

if timer expires, send packet, if any if timer expires, send packet, if any

n n

ProblemProblem

u u

sensitive to extremes sensitive to extremes

Average rate nn

Rate over some time period (Rate over some time period (

window window)

n n

Less susceptible to outliersLess susceptible to

outliers

n n

Parameters:Parameters:

t t and

and

a a

nn

Two types: jumping window and moving windowTwo types: jumping window and moving window

nn

Jumping windowJumping window

u u

over consecutive intervals of length over consecutive intervals of length

t t, only

, only

a bits senta

bits sent

u u

regulator reinitializes every interval regulator reinitializes every interval

nn

Moving windowMoving window

uu

over all intervals of length over all intervals of length

t, t, only

only

a a bits sent

bits sent

u u

regulator forgets packet sent more than regulator forgets packet sent more than

t t seconds ago

seconds ago

Leaky bucket nn

A regulator for an LBAPA regulator for an LBAP

n n

Token bucket fills up at rateToken bucket fills up at rate

r r

n n

Largest # tokens <Largest # tokens <

s s

Variants nn

Token and data bucketsToken and data buckets

u u

Sum is what matters Sum is what matters

n n

Peak rate regulatorPeak rate regulator

Choosing minimal parameters nn

Keeping loss rate the sameKeeping loss rate the same

u u

if if

s s is more,

is more,

r r is less (smoothing)

is less (smoothing)

u u

for each for each

r r we have least

we have least

s s

nn

Choose knee of curveChoose knee of curve

LBAP nn

Popular in practice and in academiaPopular in practice and in academia

u u

sort of representative sort of representative

u u

verifiable verifiable

uu

sort of sort of preservable

preservable

uu

sort of usable sort of usable

n n

Problems with multiple time scale trafficProblems with multiple time scale traffic

u u

large burst messes up things large burst messes up things