Network Layer

1. I NETWORK LAYER
The NetworkLayeristhe thirdlayerof the OSI model.Ithandlesthe service requestsfromthe
transportlayerand furtherforwardsthe service requesttothe data linklayer.The networklayer
translatesthe logical addressesinto physical addresses.
Functionsof Network Layer
DeviceswhichworkonNetworkLayermainlyfocusonrouting.Routingmayinclude varioustasks
aimedtoachieve a single goal.These canbe:
 Addressingdevicesandnetworks.
 Populatingroutingtablesorstaticroutes.
 Queuingincomingandoutgoingdataandthenforwardingthemaccordingtoqualityof
service constraintssetforthose packets.
 Internetworkingbetweentwodifferentsubnets.
 Deliveringpacketstodestinationwithbestefforts.
 Providesconnectionorientedandconnectionlessmechanism.
Network Layer Features
Withits standardfunctionalities,Layer3can provide variousfeaturesas:
 Qualityof service management
 Load balancingandlinkmanagement
 Security
 Interrelationof differentprotocolsand subnetswithdifferentschema.
 Differentlogical networkdesignoverthe physical networkdesign.
 L3 VPN andtunnelscanbe usedtoprovide endtoenddedicated
connectivity.
1.1ROUTING ALGORITHMS:
Types ofRouting

2. Routingisa processwhichisperformedbylayer3 (or networklayer) devicesinorderto
deliverthe packetbychoosinganoptimal pathfromone networktoanother.
There are 3 typesof routing:
1. Static Routing
2. DefaultRouting
3. DynamicRouting
1. Static routing
Staticroutingis a processinwhichwe have to manuallyaddroutesinroutingtable.
Advantages
 No routingoverheadforrouterCPUwhichmeansa cheaperroutercan be usedto
do routing.
 It addssecuritybecause onlyadministratorcanallow routingtoparticular networks
only.
 No bandwidthusage betweenrouters.
Disadvantage
 For a large network,itisa hectictaskfor administratortomanuallyaddeachroute
for the networkinthe routingtable oneach router.
 The administratorshouldhave goodknowledgeof the topology.If anew
administratorcomes,thenhe hasto manuallyaddeachroute so he shouldhave
verygoodknowledge of the routesof the topology.
2. DefaultRouting
 Thisis the methodwhere the routerisconfiguredtosendall packetstowardsasingle router
(nexthop).
 It doesn’tmattertowhichnetworkthe packetbelongs,itisforwardedouttorouterwhichis
configuredfordefaultrouting.
 It isgenerallyusedwithstubrouters.A stubrouterisa routerwhichhas onlyone route to
reach all othernetworks.
3. DynamicRouting–
Dynamicroutingmakesautomaticadjustmentof the routesaccordingtothe current state of the
route inthe routingtable.Dynamicroutingusesprotocolstodiscovernetworkdestinationsandthe
routesto reachit. RIPand OSPFare the bestexamplesof dynamicroutingprotocol.Automatic
adjustmentwillbe made toreach the networkdestinationif one route goesdown.
3.Dynamic Protocol

3. A dynamicprotocol have followingfeatures:
 The routersshouldhave the same dynamicprotocol runninginorderto exchange routes.
 Whena router findsachange in the topologythenrouteradvertisesittoall otherrouters.
Advantages
 Easy to configure.
 More effectiveatselectingthe bestroute toa destinationremote networkandalsofor
discoveringremote network.
Disadvantage
 Consumesmore bandwidthforcommunicatingwithotherneighbors.
 Lesssecure than staticrouting.
RoutingProtocols:
The Protocolsare
RIP RoutingInformationProtocol
OSPFOpenShortestPath
BGPBorderGatewayProtocol
RIP:

4. III CONGESTION CONTROL ALGORITHMS:
What is congestion?
A state occurringin networklayerwhenthe message trafficissoheavythatit slowsdown
networkresponse time.
Effectsof Congestion
 As delayincreases,performance decreases.
 If delayincreases,retransmissionoccurs,makingsituationworse.
 Congestioncontrol algorithms
CongestionControl Technique:
OpenLoop CongestionControl- Openloopcongestioncontrol policiesare appliedtoprevent
congestionbefore ithappens.The congestioncontrol ishandledeitherbythe source or the
destination.
ClosedLoop CongestionControl- Closed-loopcongestioncontrol mechanismstrytoalleviate
congestionafterithappens.Several mechanismshave beenusedbydifferentprotocols.

5. Leaky Bucket Algorithm
Let usconsideran example tounderstand
Imagine abucketwitha small hole inthe bottom.Nomatterat whatrate waterentersthe
bucket,the outflowisatconstantrate.When the bucketisfull withwateradditional waterentering
spillsoverthe sidesandislost.
Figure(A)
Leaky Bucket
Similarly,eachnetworkinterface containsaleakybucketandthe followingstepsare involvedin
 leaky bucketalgorithm:
Whenhost wantsto sendpacket,packetisthrownintothe bucket.

6. The bucketleaksata constantrate,meaningthe networkinterface transmitspacketsata constant
rate.
Bursty trafficisconvertedtoa uniformtrafficbythe leakybucket.
In practice the bucketisa finite queuethatoutputsata finite rate.
 Token bucketAlgorithm
Needoftoken bucket Algorithm:-
The leakybucketalgorithmenforcesoutputpatternatthe average rate,nomatterhow burstythe
trafficis.So inorder todeal withthe burstytrafficwe needa flexible algorithmsothatthe data is
not lost.One suchalgorithmistokenbucketalgorithm.
 Stepsof this algorithm can be describedas follows:
 In regularintervalstokensare thrownintothe bucket.ƒ
 The buckethas a maximum capacity.ƒ
 If there is a readypacket,a tokenisremovedfromthe bucket,andthe packetis sent.
 If there is notokeninthe bucket,the packetcannotbe sent.
 Ways inwhich token bucket issuperior to leakybucket:
The leakybucketalgorithmcontrols the rate at whichthe packetsare introducedinthe network,but
it isveryconservative innature.Some flexibilityisintroducedinthe tokenbucketalgorithm.Inthe
tokenbucket,algorithmtokensare generatedateachtick(up to a certainlimit).For anincoming
packetto be transmitted,itmustcapture a tokenandthe transmissiontakesplace atthe same rate.
Hence some of the bustypacketsare transmittedatthe same rate if tokensare available andthus
introducessome amountof flexibilityinthe system.
Formula:M * s = C + ρ * s
where S – istime taken
M – Maximumoutputrate
ρ – Tokenarrival rate
C – Capacityof the tokenbucketinbyte
Figure(b)
Let’sunderstandwithanexample,
In figure (A) we see abucketholdingthree tokens,withfive packetswaitingtobe transmitted.Fora
packetto be transmitted,itmustcapture and destroyone token.Infigure (B) We see thatthree of

7. the five packetshave gottenthrough,butthe othertwoare stuck waitingformore tokenstobe
generated.
Transport Layer
 The OSI model consistsof sevenprotocol layersandeachlayerperformsasupportive
communicationtask.
 The Transport layeristhe fourthlayerin the OSImodel,whichprovidescommunication
servicesbetweenthe computersconnectedinthe network.
 For example:The transportlayerprovidesanerrorcheckingservice duringthe transmission
of data packetsfromsource computerto destinationcomputer.
 Process-to-ProcessDelivery
 The data linklayerhelpstodeliverthe framesbetweentwoneighboringnodesoveralink.
Thisprocessis calledasnode-to-node delivery.
 The networklayerhelpstodeliverthe datagramsbetweentwohosts.Thisprocessiscalled
as host-to-hostdelivery.
 Several processesare carriedoutonthe source host and destinationhost.Some mechanism
isneededtocomplete the deliveryprocessbetweenthe source hostanddestinationhost.
 The transport layerhelpstocarry out the process-to-processdeliveryi.e.the deliveryof a
packetor part of message fromone processto anotherprocess.