Computer Networks

Lecture23:LANConnectivity

Repeaters

Ethernetsegmentislimitedto500mduetosignalattenuation

Arepeater:

• ananalogelectronicdevice

•  continuouslymonitorselectricalsignalsoneachLAN

•  repeatsandstrengthens/amplifiessignal

Ethernetonlyallows4repeaters:max2.5km.Why?

Repeater

Hubs

Hubsjoinsmultipleinputlineselectrically

• bitscomingfromonelinkgooutallotherlinks

• atthesamerate

• noframebuffering

• donotnecessarilyamplifysignal

• extendsmaxdistancebetweennodes

NoCSMA/CDathub:

•  collisiondetectionlefttohostadaptors•  individualsegmentcollision

domainsbecomeonelarge

collisiondomain

hub

hub

hub

hub

LimitationsofRepeatersandHubs

Onelargesharedlink•  eachbitispropagatedtothewholenetwork

•  aggregatethroughputislimited

•  e.g.,threedepartmentseachhasa10MbpsLAN

•  ifconnectedviaahub,theymustsharethe10Mbps

CannotsupportmultipleLANtechnologies•  doesnotbufferorinterpretframes

•  can’tinterconnectbetweendifferentratesorformats

•  e.g.,can’tinterconnect10BaseT&100BaseT

Limitationsonmaximum#nodesanddistances•  sharedmediumimposeslengthlimits

•  e.g.,cannotgobeyond2500metersonEthernet

Switches/Bridges

Linklayerrouter-equivalent:

•  connectLANsatthelinklayer

•  unlikerouters,onlyknowwhether

anodeisinasegment

•  canconnectsegmentswithdifferentMACprotocols

•  canalsoconnectdirectlytohost,atfullduplex

Storeandforwardframesbetweensegments

•  extractsdestinationaddressfromtheframe

•  looksupthedestinationinatable

hub hub

hub

switch

1 2

3

Bridges/SwitchesSupportconcurrentcommunication(A�C,B�D)

• doesnotpropagateinterferenceandcollisions�mustbuffer

• whenaframeistobeforwardedonasegment,

usesCSMA/CDtoaccesssegment

•  increaseeffective/aggregatebandwidthofaLANby

takingadvantageofspatiallocality

host host host host host

host host host host host

host

host

Bridge

A

C

B

D

TransparentBridges/Switches

Transparent:hostsareunawareofthepresenceofswitches

Eachswitchhasaswitchtable

Entryinswitchtable:

•  <MACaddress,interface,timestamp>

•  staleentriesintabledropped(TTLcanbe60mins)

Plug-and-play:self-learningswitches

donotneedtobeconfigured

hub hub

hub

switch

1 2

3

BackwardLearning

Howdoesaswitchknowatwhichsegmentanodeislocated?

Backwardlearning:

• whenaframeisreceived,switch“learns”theincominginterfacethroughwhichasendermaybereached

•  recordssender/interfacepairinswitchtable

FrameFiltering/ForwardingWhenaswitchreceivesaframe:

LookfortheMACdestinationaddressinswitchtable

ifentryfoundfordestination{

ifdestinationisonthesamesegmentfromwhichframearrived{

droptheframe

}else{

forwardtheframeoninterfaceindicated

}

}else{

flood//forwardtoallinterfacesexcepttheincominginterface

}

FloodingExample

SupposeCsendsaframetoD

SwitchreceivesframefromC recordsinswitchtablethatCisoninterface1becauseDisnotintable,switchforwards

frametointerfaces2and3framereceivedbyD

hub hub hub

switch

A

B CD

EF G H

I

address interface

ABEG

1123

1 2 3

BackwardLearningExample

SupposeDnowsendsaframetoC

SwitchreceivesframefromD recordsinswitchtablethatDisoninterface2becauseCisintable,switchforwardsframe

onlytointerface1framereceivedbyC

hub hub hub

switch

A

B CD

EF G H

I

address interface

ABEGCD

112312

1 2 3

Switch:TrafficIsolationSwitchbreakssubnetintoLANsegments

Switchfilterspackets:•  same-LAN-segmentframesarenotusuallyforwardedontootherLANsegments

•  segmentsbecomeseparatecollisiondomains

hub hubhub

switch

collisiondomain collisiondomain

collisiondomain

Switches:DedicatedAccess

Hostscanhavedirectconnectiontoswitch•  fullduplex:dedicatedtransmissionline

ineachdirection,stillCSMA/CD,

butnochanceofcollision

Switching:A-to-DandB-to-Esimultaneously,nocollisions

Switchescansupportcombinationsofshared/dedicatedand

10/100/1000Mbpsinterfaces

switch

A

D

B

E

C

F

Cut-ThroughSwitching

Bufferingdelaycanbeahighfractionoftotaldelay•  receivingaframeoflengthLfromalinkwithtransmission

rateRtakesL/Rtimeunits

•  overshortdistancespropagationdelayissmall

•  andbufferingdelaycanbecomealargefractionoftotal

Cut-throughswitching:streamingtransmission•  inspecttheframeheaderanddothetablelook-up

•  ifoutgoinglinkisidle,immediatelystartforwardingthe

headoftheframetotheoutgoinglink• whilestillreceivingthetailviatheincominglink

A B

switches

ExampleEnterpriseNetworkSwitch/HubInstallment

hub hubhub

switch

toexternalnetwork

router

IPsubnet

mailserver

webserver

CyclesandBroadcastStorm

LANsmayformcycles• eitheraccidentally,orbydesign,forhigherreliability• useoffloodingcanleadtoforwardingloops

• causing“broadcaststorm”

Topreventbroadcaststorm,

switchesneedtoavoidsome

linkswhenflooding,soasnottoformaloop

Howtodecidewhichlinktoavoid?

SpanningTreeWhatisaspanningtreeofagraph?• asub-graphthatcoversallnodes,butcontainsnocycle

Toavoidloops,linksnotinthespanningtreedonot

forwardframes

Needadistributedalgorithmtocomputespanningtree•  switchescooperatetobuildthespanningtree

•  andadaptautomaticallywhenfailuresoccur[afterRexford]

ConstructingaSpanningTreeKeyingredientsofthealgorithm

•  switchesneedtoelecta“root”•  root::=theswitchwiththesmallestidentifier

•  “rootmessages”oftheform(X, R, d)isbroadcast•  XistheIDofthenodesending/forwardingtherootmessage•  Risthecurrentroot(smallestIDseen)•  disX’scost/distancetoR

•  eachswitchcheckswhetheritsinterfaceisonthe

shortestpathfromtheroot•  excludefromthespanningtreeinterfacesnotontheshortestpathfromroot,breaktiebyID

•  eachLANhasadesignatedswitch•  multipleswitcheselectonewithshortestrootpath,breaktiebyID

[afterRexford]

StepsinSpanningTreeAlgorithm

Initially,eachswitchthinksitistheroot•  switchsendsarootmessageouteveryinterface

•  identifyingitselfastherootwithdistance0•  example:switchXannounces(X, X, 0)

Switchesupdatetheir“rootview”•  uponreceivingarootmessage,checktherootid

•  ifthenewidissmaller,startviewingthatswitchasroot

Switchescomputetheirdistancefromtheroot•  add1tothedistancereceivedfromaneighbor

•  identifyinterfacesnotonashortestpathtotheroot•  andexcludethemfromthespanningtree

•  floodanupdatedrootmessage

[afterRexford]

ExamplefromSwitch4’sViewpoint

Switch4thinksitistheroot•  sends(4, 4, 0)rootmessageto2and7

Then,switch4hearsfromswitch2•  receives(2, 2, 0)rootmessagefrom2•  andthinksthatswitch2istheroot•  atdistanceonehopaway

Then,switch4hearsfromswitch7•  receives(7, 2, 1)from7•  realizesthatthisisalongerpath•  so,prefersitsown1-hoppath(onrootport)•  andremoves4-7linkfromthetree

1

2

3

4

5

67

[afterRexford]

Switch2hearsaboutswitch1•  switch2hears(3, 1, 1)from3•  switch2startstreating1asroot•  andsends(2, 1, 2)toneighbors

Switch4hearsfromswitch2•  switch4startstreating1asroot•  andsends(4, 1, 3)toneighbors

Switch4hearsfromswitch7•  switch4receives(7, 1, 3)from7•  andrealizesthatthisisalongerpath•  prefersitsown3-hoppath(onrootport)•  andremoves4-7Iinkfromthetree

ExamplefromSwitch4’sViewpoint

1

2

3

4

5

67

[afterRexford]

RobustSpanning-TreeAlgorithm

Algorithmmustreacttofailures•  failureoftherootnode•  needtoelectanewroot,withthenextlowestidentifier

•  failureofotherswitchesandlinks•  needtore-computethespanningtree

Rootswitchcontinuestosendrootmessages•  periodicallyre-announcesitselfastheroot(1, 1, 0)•  otherswitchescontinuetoforwardrootmessages

Detectfailuresthroughtimeout(softstate)•  aswitchwaitstohearfromothers•  eventuallytimesoutandclaimstobetheroot,and

restartsthedistributedalgorithmalloveragain

[afterRexford]

ForwardingonSpanningTreeSummaryofdistributedspanningtreecomputation:• switchwithlowestIDbecomesrootoftree

• allswitches(exceptroot)determinerootport(porttoroot)

• thespanningtreeconsistsofswitchesandroot-portlinks

• designated-portlinksconnectdesignatedswitchestoLANs

Forwardingonthetree:

•  forwardframesonlyonroot-portanddesignated-portlinks

•  treedoesnotprovideshortestpath,e.g.,AtoCdoesnotgothroughB3

Peterson&Davie

AdvantagesofSwitchesoverHubs/Repeaters

Onlyforwardsframesasneeded•  filtersframestoavoidunnecessaryloadonsegments

•  sendsframesonlytosegmentsthatneedtoseethem

Extendsthegeographicspanofthenetwork•  separatesegmentsallowlongerdistances

Improvesprivacybylimitingscopeofframes•  hostscan“snoop”onlythetraffictraversingtheirsegment

Canjoinsegmentsusingdifferenttechnologies

[afterRexford]

DisadvantagesofSwitchesoverHubs/RepeatersDelayinforwardingframes•  bridge/switchmustreceiveandparsetheframe

•  andperformalook-uptodecidewheretoforward

•  storingandforwardingthepacketintroducesdelay

•  solution:cut-throughswitching

Needtolearnwheretoforwardframes•  bridge/switchneedstoconstructaforwardingtable

•  ideally,withoutinterventionfromnetworkadministrators

•  solution:self-learning

Highercost• morecomplicateddevicescostmoremoney

[afterRexford]

Switchesvs.Routers

Bothstore-and-forwarddevices

Givenbridges/switches,whydowestillneedrouters?

•  routersarenetworklayerdevices(whatdoesthismean?)

•  routersmaintainroutingtables,implementroutingalgorithms

•  switchesarelinklayerdevices

•  switchesmaintainswitchtables,implementfiltering,backwardlearningalgorithms

Switch

Segmentvs.Subnet

Acommonlyuseddifferentiator:

• segment:alayer-2collisiondomain

• subnet:alayer-3broadcastdomain

Asubnetmaycontainmultiplesegments

Asegmentmaycontainmultiplesubnets(notrecommended)

“Segment”isalsooftenusedtosimplymean“partofanetwork”notalwaysaccordingtoaprecisetechnicaldefinition

MovingFromSwitchestoRouters

Advantagesofswitchesoverrouters•  plug-and-play•  fastfilteringandforwardingofframes

Disadvantagesofswitchesoverrouters•  topologyisrestrictedtoaspanningtree•  largenetworksrequirelargeARPtables

•  broadcaststormscancausenetworkcollapse

[afterRexford]

ComparingHubs,Switches,Routers

Hub/Repeater

Bridge/Switch

Router

Trafficisolation ✗ ✔ ✔

PlugandPlay ✔ ✔ ✗

Efficientrouting ✗ ✗ ✔

Cutthrough ✔ ✔ ✗

[afterRexford]

EvolutionTowardVirtualLANs

WhenbeingpartofaLANmeanstappingintoacablethatpasses

throughone’soffice•  peopleinadjacentofficeswereputonthesameLAN

•  regardlessoftheirfunctionalrole

Withhubsandswitchessittingincentralwiringclosets,oftenwith

multipleLANs(khubs)connectedbyswitches•  adjacentofficescanbe

mappedtodifferentLANs

[afterRexford]

hubhub

switch

wiringcloset

WhyGroupbyOrganizationalStructure?Security•  Ethernetisasharedmedia

•  anyinterfacecardcanbeputinto“promiscuous”mode

•  andgetacopyofallofthetraffic(e.g.,midtermexam)

•  so,isolatingtrafficonseparateLANsimprovessecurity

Load•  someLANsegmentsaremoreheavilyusedthanothers•  e.g.,researchersrunningexperimentsthatgetoutofhandcansaturatetheirownsegmentandnottheothers

•  plus,theremaybenaturallocalityofcommunication•  e.g.,trafficbetweenpeopleinthesameresearchgroup

[afterRexford]

LANReconfiguration

Organizationalchangesarefrequent•  administrativeofficebecomesamarketingoffice

•  technicalsupportpersonnelbecomesanadministrativepersonnel

•  aspeoplechangerole,theirmachinesmovefromoneLANto

another

Physicalrewiringisamajorpain•  requiresunpluggingthecablefromoneport

•  andpluggingitintoanother•  andhopingthecableislongenoughtoreach•  andhopingyoudon’tmakeamistake

Wouldliketo“rewire”thebuildinginsoftware•  theresultingconceptisaVirtualLAN(VLAN)

[afterRexford]

VLANsImplementationsAddconfigurationtablesatbridges/switches•  sayingwhichVLANsareaccessibleviawhichinterfaces

ApproachestoVLANmapping:•  giveeachinterfaceaVLAN“color”•  onlyworksifallhostsonthesamesegmentbelongtothesameVLAN

•  giveeachMACaddressaVLAN“color”•  usefulwhenhostsonthesamesegmentbelongtodifferentVLANs•  usefulwhenhostsmovefromonephysicallocationtoanother

ChangeEthernetheader•  addafieldforVLANtag

•  recognizedbybridges/switchesonly•  ignoredbyoldEthernetcards

[afterRexford]

Example:TwoVirtualLANs

RedVLANandOrangeVLANSwitchesforwardtrafficasneeded

R

O

RO

R

R

R

OOOR OR R R

O

O

O

[Rexford]

RedVLANandOrangeVLANBridgesforwardtrafficasneeded

RRO RO

ORO

Example:TwoVirtualLANs

[Rexford]

EthernetSwitches

Independent• followtheirownrules

• determinetheirownforwardingpath

• responsibleforVLANandotherservices

• communicatetopologyinformationwiththeirpeers

Onceaperson/hostgetsonanEthernet

network,itcandoanything

Whatifwewanttohavefinercontrolofwhata

host/personcandoonaLAN?

Ethane:aPrototypeSoftware-DefinedNetwork(SDN)

CentralizedNetworkControl• networkrulesenforcedbynetworkcontroller

• controllermonitorsandapprovesalltraffic• allowsforcompletepolicy-basedcontrolofthenetwork

• accesscontrolsbuiltin• networkunderstandsusers,hardware,topology,andpolicies

• controllerresponsiblefordamage-routing

[C+5]

FlowSetupProcess1. UserAtriestoconnecttoUserB

2. UserAtoUserBflowisn’tinSwitch1’sflowtable,

sothepacketisforwardedtotheController

3. Controllereitherapprovesordeniesroute

4. Ifapproved,Switch1andSwitch2establishaflowfromUserAtoUserB

[C+5]

Ethane’sAssumptions

Policydeterminespacketflow

Networkshouldmaintainastrong

connectionbetweenusersandtraffic

Bakesecurityintonetworkpolicy

Policyshouldbesimpletoimplement

Incrementaldeployability• shouldworkwithEthernet

EthanePolicyConfiguration

TheconfigurationlanguageforEthane:

• compiledintocontroller

• individualrulesareANDedofsimplestatements

• allowsforuser-basedrules

• rulesprioritydetermined

byorderinfile

• veryhuman-readable

[C+5]

Dependentoncontroller• requiresconnectiontocontrollertoroutenewtraffic

• communicateswithcontrolleroverasecurechannel

Simple• minimalon-boardlogic•  “flow”tablelookuponly• onlystoresactiveflows• nounderstandingofnetworktopology• noNATknowledge• noVLANsupport

SDNSwitchesFlowTableEntry �

Type0OpenFlowSwitch

Switchport

MACsrc

MACdst

Ethtype

VLANID

IPsrc

IPdst

IPprot

TCPsport

TCPdport

Rule Action Stats

1.  Forwardpackettoport(s)2.  Encapsulateandforwardtocontroller3.  Droppacket4.  Sendtonormalprocessingpipeline

+mask

Packet+bytecounters

TheNetworkController

SwitchesreportnetworktopologytoNetworkController(NC)• NCusesthistocreateflowrules

Controlsallroutesbetweenhosts• allowsforprioritization

• NChandlescongestion• canrestrictclientmovement

HandlesAuthentication• users,devices,switches• understandswhereauserisphysicallyconnectedtothenetwork

TheNetworkController

Informedoflinkfailuresandupdatesflowrules

Cancutoffmisbehavinghostsattheswitch,

completelydenyingnetworkaccess

Supportsresourcelimitsonclients

Handlesbroadcastrequests

Allowsforverydetailednetworkusagelogs• usefulforfailurepost-mortems

• presentssomethingofaprivacyrisk [C+5]

SDNnotLimitedtoLANB4:Google’sWAN

• connectsafewdozenWANdatacenters

• hasbeenindeploymentsinceJuly2010• mosttrafficcarried:synchronizinglargedatasets

• usesSDNandOpenFlowtoimplementTrafficEngineering

•  controlofedgesitesandapplications:•  re-routetraffictolesscongestedpath

•  schedulebackuptraffictoquiettime