unit ii-nws

8/13/2019 Unit II-NWs

1/85

www.jabez.webnode.com


2/85

WHY WIT HIN ?

Whentwoormorecomputerownedbythesamecompany

it

is

often

easier

just

to

run

a

cable

between

them LANsworkthisway. (PointtoPoint)/mesh/star

. topassthroughapublicpremises.

The

cost

of

running

private

cables

are

usually

prohibitive..

ur ermore n us a ou oacoun ry n ewor ,

stringingpublictransmissionlinesacross(orunderneath)publicpropertyisalsoillegal.

Consequently,network

designers

must

rely

on

switching.



3/85

Imagine

a

network

of

six

devices.

A,B,C,D,E

&

F WhenA&Bareconnected,thelinksconnectingAto

eachoftheotherdevicesareidleandwasted.

.

Aswitched

network

consists

of

aseries

of

interlinked

nodescalledasswitches.

Switchesare hardwareorsoftwaredevicescapableofcreatingtemporaryconnectionsbetweentwoormoredevices

linked

to

the

switch

but

not

to

each

other.

Inaswitchednetworksomeofthenodesareconnectedto thecommunicationdevices.

. Threemethodsofswitching1.CircuitSwitching2.

PacketSwitching3.MessageSwitching.



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5/85

DeviceD



6/85

phonesorcomputer.

Inaboveitisnotusingpointtopointconnectionbetweenev ces.

aconnection

called

acircuit

is

set

up

between

two

devices.

Thisconnectionisusedforthewholecommunication.

itmaybeacircuitthatiscreatedonanasneededbasis

Evenif

many

potential

paths

through

intermediate

devices

y v , ywillbeusedforanygivendialog.

Herebeforecommunicationcanoccurbetweentwodevices,

acircuit

is

established

between

them.

thickblueline fromDeviceAtoDeviceB

.



7/85

Eg:thetelephonesystem.

If oucallsomeonethe answer

establishedacircuit

connection

pass

data

between ou. ma beman intermediate

devicesareusedtocarryyourvoiceafteruse

terminate

the

circuit

next

time

ou

call

ou

getanewcircuitmay(probablywill)use

differenthardwarethanthefirstcircuitdid

dependingon

what's

available

at

that

time

in

thenetwork



8/85

CircuitSwitch

:

isadevicewithninputsandmoutputs

thatcreates

temporary

connection

between

amin utlinkandout utlink



9/85

Folded

Switch Annbynfoldedswitchcanconnectnlinesin

fulldu lexmode.

Eg)

itcan

connect

ntelephones

in

such

away

otherphone.



10/85

Connectsninputstomoutputsinagrid,usingelectronic

microswitches(transistors)ateachcrosspoint.

Connectingninputs

to

m

outputs

requires

m

Xncross

points.

Eg)1000inputsto1000outputsrequires1,000,000cross

po nts.

Thismakessizeofthecrossbarhuge.

crosspointareinuseatagiventime.

Restisidle.



11/85



12/85



13/85

MultistageSwitches

.



14/85



15/85

Belowfigureshowstwowaysoftrafficcanmovefromaninputtoanoutputusingtheswitch



16/85

TimeDivisionSwitches

usestimedivisionmultiplexingtoachieveswitching.

Twopopularmethods

mes o n erc angean e us.

TimeSlot

Interchange

(TSI)



17/85

Inabovefigasystemconnecting4inputand4outputlines.

1 3 24 31 42

Figashowsordinarytimedivisionmultiplexing.

Heredesiredtaskisnotaccomplished.

Dataare

output

in

the

same

order

as

they

are

input.

Ie 11 22 33 44

Infigb.anewcalledtimeslotinterchangeisinserted.

TSIchanges

the

ordering

of

the

slots

based

on

the

desired

.

InthiscaseitchangestheorderofdatafromA,B,C,DtoC,D,A,B.

en

e

emu p exer separa es

es o s,

passes

em

toproperoutputs.

TSIworksasinthefollowingfigure.



18/85

TSIconsistsofRAMwithseveral

Memorylocations.

Size of each location is same as the

sizeofasingletimeslot.

No.ofmemorylocationisthesame

asno.ofinputs.

TheRAMfillswithupwith

incomingdatafromtimeslotsinthe

orderreceived.

Slotsarethensentoutindesired

orderbasedonthedecisionof

controlunit.



19/85



20/85

Aneg forCircuitSwitchedtelephonenetworkisPSTNinNorth

mer ca. Switchingcentersareorganizedintofiveclasses.

Regionaloffices(class1),Sectionaloffices(class2),Primaryoffices(C3),tolloffices(c4),endoffices(c5).

Subscribertelephones

are

connected

through

local

loops

to

end

offices.

Smalltownmayhave1endoffice,largecitymayhaveseveralendoffice

Manyend

offices

are

connected

to

one

toll

office.

Severaltollofficesmayconnectedtoaprimaryoffices.

Severalprimaryofficesmayconnectedtoasectionaloffice,whichnormall serves more than one state.

Finallyall

sectional

offices

are

connected

to

one

regional

office.

Allregionalofficesareconnectedusing meshtopology.



21/85

throughthetouchtonetechnique.

twosmall

burst

analog

signals

calleddualtone.

T e requencyo t es gna ssentdependsontherowandcolumn

ofthe

pressed

pad.

Eg)Whenauserdialthenumber6,twoburstsofanalogsignalwithfrequencies770and1477

Hzare

sent

to

end

office.



22/85

PacketSwitching

Circuitswitchisdesignedforvoicecommunication.

Inatelephoneconversation,onceacircuitisestablished,itremains.

Circuit

switching

creates

temporary

(dialed)

or

permanent

(leased)

dedicatedlinkstocommunication.

.

Nonvoicetransmissionstendtobebursty.

Ie)

data

come

with

idle

gaps

between

them. .

Circuitswitchingseesalltransmissionequal.

Anyrequestisgrantedtowhateverlinkisavailable.

.

X>cangoanytime,zmustgoimmediately.

Abettersolutionfordatatransmissionispacketswitching.



23/85

Inpacketswitching,

.

Thedataischopped upintosmallpiecescalledpackets

Eachpacketcontainsnotonlydatabutalsoaheader.

Thesepackets

are

sent

over

the

network

node

to

node.

Ateachnodethepacketisroutedaccordingtotheinformationpresentinheader.

Thedataisreadfromthepacketsandreassembledinto

theform

of

the

original

data

in

receiving

end.

Inapacketswitchednetwork,nocircuitissetuppriortosendingdatabetweendevices.

maytake

any

number

of

paths

as

it

journeys

from

one

devicetoanother.

VirtualCircuit.



24/85


25/85

DatagramApproach:



26/85

DatagramApproach:

Indatagramapproach,eachpacketistreatedasindependent

soifthepacketsareforthesamedestination,eachpacketmaytakethedifferentrulestoreachthesamedestination.

Thisis ossiblebecauseeach acketconsistoffullsourceanddestinationaddress.

Packetin

this

approach

is

known

as

datagram.

Letustakeanexamplewherethereare3packetswhichbelongtothesamesourceanddestination.

butmaygobydifferentpathtoreachtheirdestination.

Thisapproach

can

cause

the

datagram

to

arrive

at

their

destination

outo or er.

Thisishappenedbecausethedatagrammaytakethedifferentroutes.

nmos pro oco , s erespons yo anupper ayerpro oco(transport

layer)

to

reorder

the

datagram

before

passing

them

to

thecorrectprocess.


l h


27/85

VirtualCircuitApproach

B



28/85

VirtualCircuitApproach

Singlerouteestablishedbeforeanypackets

sentbetweensenderandreceiver.

Whenthe

data

are

sent

all

packets

of

the

thatroute.

wo orma so : w c e v r ua c rcu

(SVC)andpermanentvirtualcircuit(PVC).



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SVC DialupLines

existfor

certain

duration

of

the

specific

.

ConnectionEstablishment

DataTrans er

ConnectionRelease



30/85

Leasedline

The

circuit

is

dedicated

between

two

users

on

continuousbasis.

Nooneelsecanuseit.

establishmentandconnectiontermination.

users

may

ge

eren

rou e

every

me.



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MessageSwitching



32/85

descriptivetermasstoreandforward.

,

appropriateroute

is

free,

the

sends

it

along.

econ ary s nee e

Requirementof

large

capacity

of

storage

at

eachnodemakeitunpopular.



33/85



34/85

Twoofmoredevicesconnectedforthepurpose

o

s ar ng

ataor

resources

.

LANneedtocovermoredistance.

Number

of

stations

may

be

too

great

for

efficient

framedeliveryormanagementofNW.

NWmayneedtobesubdivided.

Infirst

case

aRepeater

is

inserted

to

increase

coverabledistance.

management.



35/85

InternetworkWhentwoormoreseparatenetworks

. LinkingnumberofLANsintoaninternetrequires

gateways. Thesedevicesaredesi nedtoovercomeobstaclesto

interconnectionwithoutdisruptingtheindependent

functioning

of

NW. Aninternetisaninterconnectionofindividual

networks.

Tocreate

nternet

we

nee

nternetwor ng

ev ces

calledroutersandgateways.



36/85

internet(lowercasei) interconnectionof

. Internet(UppercaseI) Worldwidenetwork.

.

Fourcategories

of

networking

and

, ,RoutersandGateways.

Eachof

these

four

device

t es

interacts

with

protocolsatdifferentlayersoftheOSImodel.

HowNetworkcanbeconnected: Networks

can

be

interconnected

by

different

devices



37/85



38/85

Repeaters actonlyupontheelectrical

componen so

as gna

ac ve

upon

p ys ca

layer.

affectthe

flow

control

of

asingle

LAN.

Activeat datalinklayer.

Routers providelinkbetweentwoseparatebut

same

type

LANs. Activeat networklayer.

Gatewaysprovidetranslationservicesbetween.

Activeinalllayers.



39/85

ConnectingDevices&OSImodel



40/85

Repeaters: Is

an

electronic

device

that

operates

on

physical

layer

oftheOSImodel.

travelafixeddistancebeforeandbecomesweak.

Repeaterinstalledonalinkreceivesthesignalbeforeit.

Regeneratestheoriginalbitpatternandputsthe

refreshedcopy

back

onto

the

link.

Allowsustoextendthephysicallengthofanetwork.

Itdoesnotchangethefunctionalityofthenetworkin.

Twosections

are

connected

by

the

repeater

in

below

fig.



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42/85



43/85

RepeaterisnotAmplifier

Anamplifiercannotdiscriminatebetweentheintendedsignalandnoise.

Itamplifies

equally

everything

fed

into

it.

Itregeneratesit.

signal,itcreatesacopybitforbitattheori inal stren th.

Arepeater

must

be

placed

before

signal

gets



44/85

Bridges: Operates

in

both

physical

&

Data

Link

Layers

of

OSI

Models.

segments. Canalsorelayframesbetweentwooriginallyseparated

s.

Unlikerepeaters,bridgescontainlogicthatallowsthem

tokeep

the

traffic

for

each

segment

separate.

Theyfiltertraffic.

Makesthemusefulforcontrollingcongestionand.

Canalso

provide

security

through

this

partitioning

of

traffic.



45/85



46/85

Bridgeoperatesatdatalinklayer,givingitaccessto

theto

the

physical

addresses

of

all

stations

connectedtoit.

Whenaframeentersabridge,thenotonly

regeneratesthe

signal

but

also

check

the

address

of

e es na onan orwar s enewcopyon y o

thesegmentwhichthat addressbelongsto.

addresscontainedintheframeandaddresswitha

Whenit

finds

amatch,

it

discovers

to

which

segment

thestationbelon sandrela sthe acketonl tothat

segment.www.jabez.webnode.com


47/85


48/85

Inabovefig twosegmentsjoinedbythe

bridge.

A acketfromstationAaddressedtostationD

arrivesat

the

bridge.

.

Thereforethepacketisblockedfromcrossing

n oo er

segmen .



49/85



50/85

stationG.

e r gesa ow epac e ocrossan

relaysit

to

entire

segment.

WhereitisreceivedbystationG.



51/85

TypesofBridges: Toselectbetweensegments,abridgemusthavealookup

tablethatcontainsthephysicaladdressesofeverystation

.

Thetable

indicates

to

which

segments

each

station

belongs.

Mostprimitiveandleastexpensive.

addressesofallthestationsincludedineachofthem.

Whatmakesitprimitiveisthattheaddressesmustbeentered

manually. Beforeasimplebridgecanbeusedanoperatormust sitdown

.

Whenevernewstationisadded,thetablemustbemodified.www.jabez.webnode.com

If a station is removed the newly invalid address must be deleted


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Ifastationisremovedthenewlyinvalidaddressmustbedeleted.

moretroublethancostsaving.

MultiportBridge: UsedtoconnectmorethantwoLANs.

Inthis

figure,

the

bridge

has

three

tables,

each

one

holding

the

physicaladdressesofstationsreachablethroughcorresponding

port.


TransparentBridge:


53/85

Itbuildsitstableofstationaddressesonitsown.

Duringfirst

installation,

its

table

is

empty.

Asitencounterseachpacketitlooksatboththedestination

an sourcea ress.

Itchecks

the

destination

to

decide

where

to

send

the

packet.

,

thepackettoallofthestationsonbothsegments.

Ituses

the

source

address

to

build

its

table.

Asitreadsthesourceaddressitnoteswhichsidethepacket

camefromandassociatestheaddresswithsegmenttowhich

.

Eg)When

station

A

sends

it

packet

to

station

G,

bridge

learns

thatpacketscomingfromAanditssegment.

Nowwhenever,thebridgeencounterspacketsaddressedtoA,

itknows

to

relay

them

into

which

segment.www.jabez.webnode.com


54/85

Bridgelearns

the

segment

associated

with

that

stationwhenthefirstpackettransmittedbyeach

.

Eventuallyit

has

acomplete

table

of

station

address

.

Byrepeatingthisprocessevenaftertableis

. Itisselfupdating.

.

Ifstored

locations

for

both

station

are

wrong.

.



55/85

BridgesconnectingdifferentLANs:

FrameFormat

AddressBit

order

Payloadsize:sizeofdatathecanbe

encapsulatedinaframevariesfromprotocol

toprotocol.

Howevertherearebridgestodaythatcan

handleall

of

these

problems

&

can

connect

onetypeofLANtoother



56/85

Routers: Repeaters

&

Bridges

are

simple

hardware

devices

capable

of

executingspecifictasks.

Routersaremoresop st cate .

Theyhave

access

to

network

layer

addresses.

amongseveralpossiblepaths.

Routersoperates

in

physical,

Data

Link

and

Network

layers

of

theOSImodel.

Routersrelaypacketsamongmultipleinterconnectednetworks.

eyroutepac ets romonenetwor toanyo anum ero

potentialdestination

networks

on

ainternet.

The below fi ure shows a ossible internetwork of five

networks.



57/85



58/85


Apacketsentfromastationononenetworktoastationona


59/85

neighboringnetworkgoesfirstrouter.

Whichswitch

it

over

to

the

destination

network.

ifthereisnoonerouterconnectedtoboththesendingand

receivingnetworks.

Sendingrouter

transfers

the

packet

to

one

of

its

neighbor

.

ThatNRforwardsthepacketuntilitreach destination.

Routersacts

like

stations

on

anetwork.

Butunlikemoststations,whicharemembersofonlyone

network.

Routers ave in stotwoormorenetwor att esametime.

Theyreceivepacketsfromoneconnectednetworkandpass

Considersthebestrouteforthepacketandpassesittothe

destinationnetwork. www.jabez.webnode.com

GATEWAYS


60/85

GATEWAYS: Potentially

operate

in

all

seven

layers

of

OSI

model.

Agatewayisaprotocolconverter.

Arouterbyitselftransfers,acceptsandrelays packetsonly

acrossnetworks

using

similar

protocols.

(Eg.AppleTalk)andconvertittoapacketformattedfor

anotherprotocol(TCC/IP)before forwardingit.

Agatewayisgenerallyasoftwareinstalledwithinarouter.

Thegatewayunderstandstheprotocolsusedbyeach

fromone

to

another.

Headerandtrailermodification(insomecases)

Thegatewaymustadjustdatarate,sizeandformataswell.



61/85


Internetworking:T t k t d t f i t t


62/85

Twoormorenetworksareconnectedtoforminternet.

Networkscan

differ

in

many

ways.

Packetshastocrossthroughdifferentnetworksbeforeit

reac es e .

Differentmodulation

techniques

or

frame

formats

are

in

h sical and data link la ers.

Serviceoffered connectionorientedversusconnectionless

Protocols IP,IPX,SNA,ATM,MPLS,AppleTalketc.

Multicasting PresentorAbsent

PacketSize

Errorhandling

Flowcontrol

Security privacyrules,encryption

Parameters

differenttimeouts,

flow

specifications.www.jabez.webnode.com

Manynetworktypesareinoperationtoday


63/85

. AnexampleofdifferentnetworksinterconnectionisinbelowFig


ConcatenatedVirtualCircuits:


64/85

Twostylesof

internetworkingare

common:

Aconnectionorientedconcatenationofvirtualcircuit

su nets. guarteequa tyo serv ce

Adatagram

internet

style.

(mistake

to

think)


In the concatenated virtual circuit model (above Fig ) a


65/85

Intheconcatenatedvirtualcircuitmodel(aboveFig.)a

connectionto

ahost

in

adistant

network

is

set

up

in

away

similartothewayconnectionsarenormallyestablished.

gatewaysalongthewayfromthesourcenodetothedestination

node.

Eachgatewaymaintainstables

Tellingwhichvirtualcircuitspassthroughit.

w ere

eyare

o

erou e .

whatthenewvirtualcircuitnumberis.


ConnectionlessInternetworking


66/85

.

Inthismodel,thenetworklayerofferstothetransportlayer

justtheabilitytoinjectdatagramintothesubnetandhopeit

willgettothedestination.

Notall

packet

from

asource

to

the

same

destination

traverse

.

Aroutingdecisionismadeseparatelyforeachpacketpossibly

depending

on

the

traffic

at

the

moment

the

packet

is

sent.


Tunneling


67/85

an ng

e

genera

case

o

ma ng

wo

eren

ne wor s

interworkisexceedinglydifficult.

However,inthes ecialcase,whenthesourceanddestination

hostsareonthesametypeofnetwork,

butthere

is

adifferent

network

in

between,

the

situation

s managea e.

EginternationalbankwithTCP/IPbasedEthernetinParisand

London.

nonIP

wide

area

NW

in

between

Thesolutiontothisproblemisatechniquecalledtunneling.

InthebelowFig.tosendanIPpackettohost2,

host

1

constructs

the

packet

containing

the

IP

address

of

host

2,

insertsitintoanEthernetframeaddressedtotheParis

mu pro oco rou er,an pu s on e erne .


ThemultiprotocolrouterremovestheIPpacket,insertsitinthe

payload field of the WAN network layer packet.


68/85

payloadfieldoftheWANnetworklayerpacket.

addressesthe

later

to

the

WAN

address

of

the

London

multiprotocolrouter.

,

and

sends

it

to

host

2

inside

an

Ethernet

frame. SotheWANcanbeseenasabi tunnelextendin fromone

multiprotocolroutertotheother.



69/85



70/85

routingtablesentry.

Ifthatroutercanbereachedusingthepacket'snativenetworkprotocol,itis

Otherwise

it

is

tunneled

there,

encapsulated

in

the

protocol

required

by

the

interveningnetwork.

This rocessisre eateduntilthe acketreachesthedestinationnetwork.


Fragmentation Each network im oses some maximum size on its ackets These


71/85

Eachnetworkim osessomemaximumsizeonits ackets.These

limitshave

various

causes,

among

them:

Hardware(e.g.,thewidthofaTDMtransmissionslot).

O eratin s stem e. . all buffers are 512 b tes .

Protocols(e.g.,thenumberofbitsinthepacketlengthfield).

Compliancewith

some

(inter)national

standards.

.

Desiretopreventonepacketfromoccupyingthechanneltoolong.

Maximumpayloadrangefrom48bytes(ATMcells)to65515

. Aproblemappearswhenalargepacketwantstotravelthrough

anetworkwhosemaximumpacketsizeistoosmall.

Theonlysolutiontotheproblemistoallowgatewaystobreakpackets

into

fragments


Butthenanewproblemarises:howtoputthefragments


72/85

Twoopposingstrategiesexistsforrecombiningthe

fragmentsbackintotheoriginalpacket:

Thefragments

are

recombined

in

the

next

gateway

so

the

fra mentationismadetrans arenttoan subse uentnetwork Fi .5

41(a)).

Onceapackethasbeenfragmented,eachfragmentistreatedas

thou hit

were

an

ori inal

acket.

Recombination

occurs

onl

at

the

destinationhost(Fig.541(B)).



73/85

, u u u w y

theoriginaldatastreamcanbereconstructed.


Firewalls

Forindividualsathome,wanderingaroundtheInternetislotsoffun.


74/85

Forcorporate

security

managers,

it

is

anightmare.

Mechanismsareneededtoprotectsystemsasmuchaspossibleagainstthe

unauthorizedaccess.

Firewallsareabletoaccomplishthisgoal.

Firewallisan

electronic

drawbridge,

all

traffic

to

or

from

acompany's

LAN

is

forcedto othrou h Fi .543 .

Thefirewallinthisconfigurationhastwocomponents:tworoutersthatdo

packetfilteringandanapplicationgateway.

,

everypacketmusttransittwofiltersandanapplicationgatewaytogoinor

out.

allowingevery

incoming

or

outgoing

packet

to

be

inspected.

Packetsmeetingsomecriterionareforwardednormally.

.

thepacketfilterontheinsideLANcouldcheckoutgoingpacketsandtheone

onthe

outside

LAN

checks

incoming

packets.



75/85



76/85


http://www.freesoft.org/CIE/RFC/791/12.htm


77/85

AnIP

datagram

consists

of

aheader

part

and

a

textpart.

The

header

has

a

20

byte

fixed

part

and

a

Itistransmittedfromlefttoright,withthe

AllIPpacketsarestructuredthesameway



78/85

IPV4Headerwww.jabez.webnode.com

Version: 4bits

.


79/85

alwaysset

to

the

value

4in

the

current

version

of

IP

(IPV4).

IPHeaderLength : .

thuspointstothebeginningofthedata.

Notethat

the

minimum

value

for

acorrect

header

is

5.

8bits.

TheTypeofServiceprovidesanindicationoftheabstractparametersofthequalityofservicedesired.

Severalnetworksofferserviceprecedence,whichsomehowtreatshighprecedencetrafficasmoreimportantthanothertraffic .

Themajorchoiceisathreewaytradeoffbetweenlowdelay,high, .

The

use

of

the

Delay,

Throughput,

and

Reliability

indications

may

increase

thecost(insomesense)oftheservice.

Inmanynetworksbetterperformanceforoneoftheseparametersiscoup e

AboveareDifferentialServicewww.jabez.webnode.com


80/85


TotalLength:


81/85

TotalLength

is

the

length

of

the

datagram,

measured

in

octets,

including

internetheaderanddata.

16bits

Anidentifying

value

assigned

by

the

sender

to

aid

in

assembling

the

ragmen so a a agram.

Thisisuniqueduringthetimedatagramwillremainintheinternet.

Flags:

3bits

VariousControlFlags.



82/85

Dont

Fragment

Flag

bit

prohibitsfragmentationwhen

set.

Usefulifitisknownthat

destination

does

not

have

the

capabilityofreassemble

fragments.

Thedatagramwillbe

discarded if it exceeds themaximumsizeofanenroute

network.


FragmentOffset: (13bits)


83/85

Thisfield

indicates

where

in

the

datagram

this

fragment

belongs.

.

Thefirstfragmenthasoffsetzero.

TimetoLive:(8bits)

Thisfieldindicatesthemaximumtimethedatagramis

allowedtoremainintheinternet.

Ifthis

field

contains

the

value

zero,

then

the

datagram

must

bedestroyed.

modulethat

processes

adatagram

must

decrease

the

TTL

by

atleastone

Theintentionistocauseundeliverabledatagramstobe

discardedwww.jabez.webnode.com

Protocol: (8bits)


84/85

s e n ca es e nex eve pro oco use n e a aportionoftheinternetdatagram.

Thevaluesforvarious rotocolsares ecifiedin"Assi ned

Numbers.

Examplevalues

are

TCP

=6;

UDP

=17;

ICMP

=1.

HeaderChecksum:

16bits.

c ec sumon

e

ea eron y.

Sincesomeheaderfieldschange(e.g.,timetolive),thisis

recom utedandverifiedateach ointthattheinternet

header

is.


SourceAddress:

.


85/85

Thesourceaddress.

DestinationAddress:

32bits.

Thedestinationaddress.

Options:

Variablelength

Encodesthe

options

requested

by

the

sending

user.

Insomeenvironmentsthesecurityoptionmayberequiredinall

.

Padding: Theinternetheaderpaddingisusedtoensurethattheinternet

headerendsona32bitboundary.

The

padding

is

zero. www.jabez.webnode.com

unit ii-nws

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