livello rete i protocolli di routing: rip, ospf,...

Reti(già “Reti di Calcolatori”)

Livello ReteI protocolli di Routing: RIP, OSPF, BGP

RenatoLoCigno

http://disi.unitn.it/locigno/index.php/teaching-duties/computer-networks

Acknowledgement

• Credits– Partof thematerialis based onslides provided by thefollowing authors• JimKurose,KeithRoss,“ComputerNetworking:ATopDownApproach,”4thedition,Addison-Wesley,July2007

• DouglasComer,“ComputerNetworksandInternets,”5thedition,PrenticeHall

• Behrouz A.Forouzan,Sophia Chung Fegan,“TCP/IPProtocol Suite,”McGraw-Hill,January 2005

• Latraduzione,sepresente,èingeneraleopera(eresponsabilità)deldocente

[email protected] LivelloRete- Protocollidirouting 2

Contenuto e temi

• Spaziodiindirizzamento

• IndirizziIPelorouso

• Consegnadeipacchetti

• ConfigurazionedeiPCedellereti

• InstradamentoeRouting


Routing: What is it?

• Processoffindingapathfromasourcetoeverydestinationinthenetwork

• SupposeyouwanttoconnecttoAntarcticafromyourdesktop– whatrouteshouldyoutake?– doesashorterrouteexist?– whatifalinkalongtheroutegoesdown?– whatifyou’reonamobilewirelesslink?

• Routingdealswiththesetypesofissues


Basics

• Aroutingprotocolsetsuparoutingtable inrouters– internaltablethatsays,foreachdestination,whichisthenextoutputtotake

• Anodemakesalocalchoicedependingonglobaltopology:thisisthefundamentalproblem


Key problem

• Howtomakecorrectlocaldecisions?– eachroutermustknowsomethingaboutglobalstate

• Globalstate– inherentlylarge– dynamic– hardtocollect

• Aroutingprotocolmustintelligentlysummarizerelevantinformation


Requirements

• Minimizeroutingtablespace– fasttolookup– lesstoexchange

• Minimizenumberandfrequencyofcontrolmessages• Robustness:avoid

– blackholes– loops– oscillations

• Useoptimalpath


Different degrees of freedom

• Centralizedvs.distributedrouting– centralizedissimpler,butpronetofailureandcongestion

• Globalvslocalinformationexchange– conveyglobalinformationisexpensive

• Staticvsdynamic– staticmayworkattheedge,notinthecore

• Stochasticvs.deterministic– stochasticspreadsload,avoidingoscillations,butmisorders

• Singlevs.multiplepath– primaryandalternativepaths(comparewithstochastic)

• State-dependentvs.state-independent– doroutesdependoncurrentnetworkstate(e.g.delay)


Dynamic Routing And Routers

• Toensurethatallroutersmaintaininformationabouthowtoreacheachpossibledestination– eachrouterusesaroutepropagationprotocol

• toexchangeinformationwithotherrouters– whenitlearnsaboutchangesinroutes

• updatesthelocalroutingtable• Becauseroutersexchangeinformationperiodically

– thelocalroutingtableisupdatedcontinuously


1

23

0111

valore nell’intestaz.del pacchetto entrante

Algoritmo d’instradamento

Tabella d’instrad. localeVal. intestaz. link in usc.

0100010101111001

3221

Routing e Forwarding


Grafo: G = (N,E)

N = insieme di nodi (router) = { u, v, w, x, y, z }

E = insieme di archi (collegamenti) ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Grafo di una rete di calcolatori

N.B.:Ilgrafoèun’astrazioneutileancheinaltricontestidirete

Esempio:P2P,doveNèuninsiemedipeeredEèuninsiemedicollegamentiTCP

u

yx

wv

z2

21

3

1

1

2

53

5


Grafo di una rete: costi

u

yx

wv

z2

21

3

1

1

2

53

5 • c(x,x’)=costodelcollegamento(x,x’)

- es.,c(w,z)=5

• ilcostodiuncamminoèsemplicementelasommadituttiicostidegliarchilungoilcammino

Costodiuncammino(x1,x2,x3,…,xp)=c(x1,x2)+c(x2,x3)+…+c(xp-1,xp)

Domanda:Qualèilcamminoacostominimotrauez?

Algoritmod’instradamento:determinailcamminoacostominimoProtocollod’instradamento:procuraall’algoritmoidatinecessarialcalcoloediffondeirisultati


Algoritmi e protocolli Distance Vector

Monotonia e coerenza dei camminiminimi

Definec(i,k)>=0:costfromi tok(directconnection)D(i,j):costofleast-costpathfromi toj

è Thesubsetofashortestpathisalsotheshortestpathbetweenthetwointermediatenodes

• Then,iftheshortestpathfromnodei tonodej,withdistanceD(i,j),passesthroughneighbork,withlinkcostc(i,k),wehave:– D(i,j)=c(i,k)+D(k,j)

ik j


Distance Vector (DV) algorithm

• Initialdistancevalues(iteration1):– D(i,i)=0– D(i,k)=c(i,k)ifkisaneighbor(kisone-hopaway)– D(i,j)=INFINITYforallothernon-neighborsj

• ThesetofvaluesD(i,*)isadistancevectoratnodei• Thealgorithmmaintainsanext-hopvalue(forwardingtable)for

everydestinationj,initializedas:– next-hop(i)=i;– next-hop(k)=kifkisaneighbor,and– next-hop(j)=UNKNOWN ifjisanon-neighbor.


Distance Vector (DV) algorithm

• Aftereveryiterationeachnodei exchangesitsdistancevectorsD(i,*)withallitsimmediateneighbors.

Foranyneighborkifc(i,k)+D(k,j)<D(i,j)then{D(i,j)=c(i,k)+D(k,j)next-hop(j)=k

}


In summary

Basicidea:• Fromtime-to-time,eachnodesendsitsowndistancevector

estimatetoneighborsAsynchronous• WhenanodexreceivesnewDVestimatefromneighbor,itupdates

itsownDVusingB-Fequation:D(x,y)←minv{c(x,v)+D(v,y)}foreachnodey∊ N

• Underminor,naturalconditions,theestimateD(x,y)convergestotheactualleastcost


In summary

• Iterative,asynchronous:eachlocaliterationcausedby:– locallinkcostchange– DVupdatemessagefromneighbor

• Distributed:eachnodenotifiesneighborsonlywhenitsDVchanges– neighborsthennotifytheirneighborsifnecessary

wait for (change in local link cost or msg from neighbor)

recompute estimates

if DV to any dest has changed, notify neighbors

Eachnode:


Distance Vector: example (initialization)

A

B C

D

7

1

1

2

25

E

A

àAàBàCàDàE

dist

07--1

NH

-B--E

B

àAàBàCàDàE

dist

701-5

NH

A-C-E

C

àAàBàCàDàE

dist

-102-

NH

-B-D-

NH

--C-E

D

àAàBàCàDàE

dist

--202

NH

AB-D-

E

àAàBàCàDàE

dist


Distance Vector: example (running)

A

B C

D

7

1

1

2

25

E

C

àAàBàCàDàE

dist

-102-

NH

-B-D-

D

àAàBàCàDàE

dist

--202

NH

--C-E

E

àAàBàCàDàE

dist

15-20

NH



A

B C

D

7

1

1

2

25

E

C

àAàBàCàDàE

dist

-1024

thr.

-B-DD

NH

ABDD-

E

àAàBàCàDàE

dist

15420

NH

--C-E

D

àAàBàCàDàE

dist



A

B C

D

7

1

1

2

25

E

NH

ABDD-

E

àAàBàCàDàE

dist

15420

NH

--C-E

D

àAàBàCàDàE

dist

--202

A

àAàBàCàDàE

dist

07--1

NH

-B--E

B

àAàBàCàDàE

dist

701-5

NH

A-C-E



A

B C

D

7

1

1

2

25

E

NH

ABDD-

E

àAàBàCàDàE

dist

15420

NH

EEC-E

D

àAàBàCàDàE

dist

37202

A

àAàBàCàDàE

dist

06531

NH

-EEEE

B

àAàBàCàDàE

dist

60175

NH

E-CEE



Distance Vector: example (final point)

A

B C

D

7

1

1

2

25

E

A

àAàBàCàDàE

dist

06531

NH

-EEEE

B

àAàBàCàDàE

dist

60135

NH

E-CCE

C

àAàBàCàDàE

dist

51024

NH

DB-DD

NH

ECC-E

D

àAàBàCàDàE

dist

33202

NH

ABDD-

E

àAàBàCàDàE

dist

15420

Problem: “counting to infinity”

• ConsidertheentriesineachroutingtablefornetworkNTW_1

• RouterDisdirectlyconnectedtoNTW_1

B

AC

D

1 1

110

NTW_1

Router ADest Next Metric

NTW_1 D 2

Router BDest Next Metric

NTW_1 A 3

Router CDest Next Metric

NTW_1 A 3

Router DDest Next Metric

NTW_1 dir 1

1




NTW_1 Unr. -


NTW_1 A 3


NTW_1 A 3


NTW_1 dir 1

B

AC

D

1 1

110

NTW_1

1

Link between B and D fails

time


NTW_1 C 4


NTW_1 C 4


NTW_1 B 4


NTW_1 dir 1


NTW_1 C 5


NTW_1 C 5


NTW_1 B 5


NTW_1 dir 1



B

AC

D

1 1

10

NTW_1

1

time


NTW_1 C 11


NTW_1 C 11


NTW_1 B 11


NTW_1 dir 1


NTW_1 C 12


NTW_1 C 12


NTW_1 D 11


NTW_1 dir 1

…


Solutions to “counting to infinity”

• Maximumnumberofhopsboundedto15– thislimitstheconvergencetime

• SplitHorizon– simple

• eachnodeomits routeslearnedfromoneneighborinupdatesenttothatneighbor

– withpoisonedreverse• eachnodeinclude routeslearnedfromoneneighborinupdatesenttothatneighbor,settingtheirmetricstoinfinity

– drawback:routingmessagesizegreaterthansimpleSplitHorizon


Distance Vector: link cost changes

• Iflinkcostchanges:– goodnewstravelsfast

• good=costdecreases– badnewstravelsslow

• bad=costincreases• Exercise

– trytoapplythealgorithminthesimplescenariodepictedabovewhen• thecostofthelinkAà Bchangesfrom4to1• thecostofthelinkAà Bchangesfrom4to60

A

CB4 1

50


RIP at a glance

• RoutingInformationProtocol• Asimpleintradomainprotocol• StraightforwardimplementationofDistanceVectorRouting…

– DistributedversionofBellman-Ford(DBF)…withwellknownissues– slowconvergence– workswithlimitednetworksize

• Strengths– simpletoimplement– simplemanagement– widespreaduse


RIP at a glance

• Metricbasedonhopcount– maximumhopcountis15,with“16”equalto“infinity”

• imposedtolimittheconvergencetime– thenetworkadministratorcanalsoassignvalueshigherthan1toasinglehop

• Eachrouteradvertisesitsdistancevectorevery30seconds(orwheneveritsroutingtablechanges)toallofitsneighbors– RIPusesUDP,port520,MulticastGroup224.0.0.9

• Changesarepropagatedacrossnetwork• Routesaretimeout(setto16)after3minutesiftheyarenot

updated


RIP procedures: introduction

• RIProutingtablesaremanagedbyapplication-levelprocess– e.g.,routed onUNIXmachines

• AdvertisementsaresentinUDPpackets(port520)• RIPmaintains3differenttimerstosupportitsoperations

– Periodicupdatetimer(25-30sec)• usedtosentoutupdatemessages

– Invalidtimer(180sec)• Ifupdateforaparticularentryisnotreceivedfor180sec,routeisinvalidated

– Garbagecollectiontimer(120sec)• Aninvalidrouteinmarked,notimmediatelydeleted• Fornext120s.therouteradvertisesthisroutewithdistanceinfinity

IP

TCP UDP

route

Data Link

Physical


RIP procedures: input processing

• RequestMessages– generatedbyroutersthatjustcomeup– action:therouterrespondsdirectlytotherequestor’saddressandport• requestisprocessedentrybyentry

• ResponseMessages– routersthatperformregularupdates,triggeredupdatesorrespondtoaspecificquery

– action:therouterupdatesitsroutingtable• incaseofnewrouteorchangedroutes,therouterstartsatriggeredupdateprocedure


RIP procedures: output processing

• Outputaregenerated– whentheroutercomesupinthenetwork– ifrequiredbytheinputprocessingprocedures– byregularroutingupdate

• Action:theroutergeneratesthemessagesaccordingtothecommandsreceived– themessagescontainentriesfromtheroutingtable

timers

input output

request response

timers

input output

response response

timers

input output

response


ALGORITMI E PROTOCOLLI LINK-STATE

Unapprocciodiversoe“centralizzato”pertrovareicamminiminimi


Link State (LS) Approach

• Thelinkstate(Dijkstra)approachisiterative,butitpivotsarounddestinationsj,andtheirpredecessorsk=p(j)– Observethatanalternativeversionoftheconsistencyconditionholdsfor

thiscase:D(i,j)=D(i,k)+c(k,j)

• Eachnodei collectsalllinkstatesc(*,*)firstandrunsthecompleteDijkstraalgorithmlocally

ik

j


Link State (LS) Approach…

• Linkstatesaredistributedtoallnodes,whichbuildagraphG(N,E)describingtheentirenetwork

• EachnodeappliesDijkstra:minimumshortestpathsfromitselftoallnodes

• Ateachiteration,thealgorithmfindsanewdestinationjandashortestpathtoit

• Aftermiterationsthealgorithmhasexploredpaths,whicharemhopsorsmallerfromnodei

• TheDijkstra algorithmatnodei maintainstwosets:

– setNthatcontainsnodestowhichtheshortestpathshavebeenfound

– setMthatcontainsallothernodes

– Forallnodesk,twovaluesaremaintained:

• D(i,k):currentvalueofdistancefromi tok

• p(k):thepredecessornodetokontheshortestknownpathfromi


Dijkstra: Initialization

• Initialization:– D(i,i)=0andp(i)=i;– D(i,k)=c(i,k)andp(k)=i ifkisaneighborofi– D(i,k)=INFINITYandp(k)=UNKNOWN ifkisnotneighborofi– SetN={i },andnext-hop(i)=i– SetM={j|jisnoti}

• InitiallysetNhasonlythenodei andsetMhastherestofthenodes• Attheendofthealgorithm,setNcontainsallthenodes,andsetM

isempty


Dijkstra: Iteration

• Ineachiteration,anewnodejismovedfromsetMintothesetN.

– NodejhastheminimumdistanceamongallcurrentnodesinMD(i,j)=min(kinM) D(i,k).

– Ifmultiplenodeshavethesameminimumdistance,anyoneofthemischosenasj

– Next-hop(j)=theneighborofi ontheshortestpath,orjitself

– ThedistancevaluesofanyneighborkofjinsetMisrecomputedas:IfD(i,k)> (D(i,j)+c(j,k))

D(i,k)=D(i,j)+c(j,k);p(k)=j

• Thisoperationiscalled“relaxing”theedgesofnodej


Dijkstra’s algorithm: example

Step012345

set NA

ADADE

ADEBADEBC

ADEBCF

D(B),p(B)2,A2,A2,A

D(C),p(C)5,A4,D3,E3,E

D(D),p(D)1,A

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F2

21

3

1

1

2

53

5

The shortest-paths spanning tree rooted at A is called an [email protected] LivelloRete- Protocollidirouting 40

Dijkstra’s algorithm, discussion

Algorithmcomplexity: nnodes• eachiteration:needtocheckallnodes,w,notinN• n(n+1)/2comparisons:O(n2)• moreefficientimplementationspossible:O(nlog(n))Oscillationspossible:• e.g.,linkcost=amountofcarriedtraffic

AD

CB

1 1+e

e0

e1 1

0 0

AD

CB

2+e 0

001+e 1

AD

CB

0 2+e

1+e10 0

AD

CB

2+e 0

e01+e 1

initially … recomputerouting

… recompute … recompute


Open Shortest Path First (OSPF)

• InalternativaalprotocolloRIPditipo DistanceVectorinInternetesiste ilprotocolloOSPFditipo LinkState

• Itre principali criteri diprogettazione delprotocollo OSPFsono:– distinzione tra hosterouter– reti broadcast– suddivisione delle reti digrandi dimensioni

• Hli hostsono collocati nelle aree periferiche della reteasottoreti locali connessealla attraverso router(defaultgateway)

• Ilmodello linkstateprevede che il databaselinkstate includa una entryperognilinktra hosterouter

• OSPFassocia il linkdiaccesso aduna stubnetwork– una stubnetworkè una sottorete terminale che nonfornisce servizio di

transito– il linkdiaccesso viene identificatodall’indirizzo della sottorete


Il protocollo OSPF

• IlprotocolloOSPFutilizza 3procedure,chiamati ancora `protocolli’,persvolgere leproprie funzioni

– HelloProtocol

– ExchangeProtocol

– FloodingProtocol


Messaggi OSPF (1)

• Imessaggi OSPFsono trasportati direttamente all’internodei pacchetti IP– nonviene utilizzato il livello ditrasporto– viene usato l’indirizzomulticast224.0.0.5

• Tutti i messaggi OSPFcondividono lostesso header

Packet lengthTypeVersion #Router IDArea ID

Checksum Auth TypeAuthenticationAuthentication


Messaggi OSPF (2)

• Version#=2• Type:indicailtipodimessaggio• PacketLength:numerodibytedelmessaggio• RouterID:indirizzoIPdelrouterdiriferimento




Messaggi OSPF (3)

• AreaID:identificativo dell’area– OSPFconsente una divisione della retein“aree”perridurre lacomplessità

delcalcolo dei percorsi eperconsentire uninstradamentogerarchico.• Auth Type:tipo diautenticazione

– 0noautenticazione,1autenticazione conpasswd• Authentication:password




Il protocollo Hello

• Funzioni:– verificarel’operativitàdeilink

• Messaggi:– Hello

PriorityOptionsDead interval

Designated router

Common header (type = 1, hello)Network mask

Hello interval

Backup Designated routerNeighbor


Il protocollo Exchange

• Funzioni:– sincronizzazionedeidatabaselinkstate(bringupadjacencies)traduerouterchehannoappenaverificatol’operativitàbidirezionaledellinkcheliconnette

– protocolloclient-server– messaggi:

• DatabaseDescriptionPackets• LinkStateRequest• LinkStateUpdate

– N.B.ilmessaggioLinkStateUpdatevienedistribuitoinflooding


Exchange Protocol: messaggi (1)

• DatabaseDescription

0OptionsDD sequence number

Link State Type

Common header (type = 2, db description)

Link State IDAdvertising router

00

Link State Sequence NumberLink State Checksum Link State Age


Il protocollo di Flooding

• Funzioni:– aggiornare il databaselinkstatedell’autonomoussystemaseguito delcambiamento distato diunlink

– Garantisce laconsegna ditutti Imessaggi atutti,acosto diparecchie repliche

• Messaggi:– LinkStateUpdate

Number of link state advertisementLink state advertisement #1Link state advertisement #2

Common header (type = 4, link state update)


Controlled Flooding

• Tutti i nodi inviano i pacchetti ricevuti dauna interfaccia su tutte lealtre

• Inuna reteconlinkpunto-punto il numerodipacchetti è pari alnumero dilink

• Inuna retecondomini dibroadcast,il numero dipacchetti è pari ilnumero didomini boradcast


8repliche 2repliche

Summary

• Topologyinformationisfloodedwithintheroutingdomain

• Bestend-to-endpathsarecomputedlocallyateachrouter

• Bestend-to-endpathsdeterminenext-hops

• Basedonminimizingsomenotionofdistance

• Worksonlyifpolicyissharedanduniform

• Examples:OSPF

• Eachrouterknowslittleaboutnetworktopology

• Onlybestnext-hopsarechosenbyeachrouterforeachdestinationnetwork.

• Bestend-to-endpathsresultfromcompositionofallnext-hopchoices

• Doesnotrequireanynotionofdistance

• Doesnotrequireuniformpoliciesatallrouters

• Examples:RIP

Link State Distance Vector


Comparison of LS and DV algorithms

Messagecomplexity• LS:withnnodes,Elinks,O(nE)msgs

sent• DV:exchangebetweenneighbors

only– convergencetimevaries

SpeedofConvergence• LS:O(n2)algorithmrequiresO(nE)

msgs– mayhaveoscillations

• DV:convergencetimevaries– mayberouting loops– count-to-infinity problem

Robustness:whathappensifroutermalfunctions?

• LS:– nodecanadvertiseincorrectlink

cost– eachnodecomputesonlyitsown

table• DV:

– DVnodecanadvertiseincorrectpathcost

– eachnode’s tableusedbyothers• errorpropagatethrunetwork


Instradamento e Topologia Globaliin Internet

Comesi effettua il trasferimento el’instradamento complessivo intutta lareteQualche considerazione sulle proprietàglobali delsistema

Instradamento gerarchico

Scala: conmilionididestinazioni:r Archiviareleinformazioni

d’instradamentosuciascunasottoreterichiederebbeun’enormequantitàdimemoria

r IltrafficogeneratodagliaggiornamentiLSnonlascerebberobandaperipacchettididati!

Autonomiaamministrativa:r Internet=laretedelleretir Daunpuntodivistaideale,

ciascunodovrebbeessereingradodiamministrarelapropriaretenelmododesiderato,purmantenendolapossibilitàdiconnetterlaalleretiesterne

Finquiabbiamovistolaretecomeunacollezionedirouterinterconnessir Ciascunroutereraindistinguibiledaglialtrir Visioneomogeneadellarete

…nellapraticalecosenonsonocosìsemplici

Instradamento gerarchico

r Organizzazionedirouterin sistemiautonomi(AS,autonomous system).

r Irouterdiungruppoautonomoeseguonolostessoalgoritmod’instradamento.mProtocollod’instradamentointernoalsistemaautonomo(intra-AS).

mIrouterappartenentiadifferentiASpossonoeseguireprotocollid’instradamentointra-ASdiversi

BorderRoutersr Hannoilcompito

aggiuntivod’inoltrarepacchettiadestinazioniesterne.

3b

1d

3a

1c

2a

AS3

AS1

AS2

1a

2c2b

1b

Algoritmod’instradamento

(intra-AS)

Algoritmod’instradamento

(intra-AS)

Tabellad’inoltro

3c

Sistemi autonomi interconnessi

r Ciascunsistemaautonomosacomeinoltrarepacchettilungoilpercorsoottimoversoqualsiasidestinazioneinternaalgruppom IsistemiAS2eAS3hanno

trerouterciascunom Iprotocollid’instradamento

deitresistemiautonominonsononecessariamenteglistessi

m Irouter1b,1c,2ae3asonogateway

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Instradamento tra sistemi autonomi

r SupponiamocheunrouterinAS1ricevaundatagrammalacuidestinazionericadealdifuoridiAS1m Ilrouterdovrebbeinoltrare

ilpacchettoversounodeiduegateway.Maquale??

AS1deve:1. Saperequalidestinazionisono

raggiungibiliattraversoAS2equaliattraversoAS3

2. Informaretuttiirouterall’internodelsistemainmodocheciascunopossaconfigurarelapropriatabellad’inoltropergestiredestinazioniesterne

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3cx

Esempio: scegliere fra più AS

r SupponiamoinoltrecheAS1apprendadalprotocollod’instradamentotrasistemiautonomichelasottoretex èraggiungibiledaAS2e daAS3

r Alfinediconfigurarelapropriatabellad’inoltro,ilrouter1Ddovrebbedeterminareaqualegateway,1bo1c,indirizzareipacchettidestinatiallasottoretex

r Anchequestoèuncompitochespettaalprotocollod’instradamentointer-AS!

r Lasceltaingeneresibasasudecisionilocaliditipoeconomico,commerciale,dipolitica(policy)

Border Gateway Protocol (BGP)

• Adifferenzadeiprotocolliintra-ASesisteunsoloprotocollointer-AS:BGPappunto

• Loscopodelprotocolloèdistribuireinformazionidiraggiungibilitàditutteledestinazioni

• BGPnoncercaunpercorsoottimo,lasciacheidiversiASprendanolelorodecisioniinbasealall’elencodiASchebisognaattraversareperraggiungereunadestinazione

• BGPsidivideindueparti• eBGP:(exterior),ilprotocollodirouting veroeproprioche

distribuiscelaraggiungibilitàditutte ledestinazioni• iBGP:(interior),tramite il qualeunrouterche partecipa aeBGP

comunica atutti i nodi delsuo ASdoveinstradare i pacchetti perlevarie destinazioni


Fondamenti di BGP

r BGPsiappoggiaaunamagliadiconnessioniTCP:duerouterconnessisonopeer BGP,elaconnessioneèdettasessioneBGP

r QuandoAS2annunciaunprefissoaAS1,AS2sta promettendocheinoltreràidatagrammisuunpercorsoversoilprefissocuisonodestinati.mAS2puòaggregarepiùprefissinelsuoannuncio

m E`sufficientecheunrouterperASpartecipiaeBGP

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

Sessione eBGP

Sessione iBGP

Attributi del percorso e rotte BGP

r QuandounrouterannunciaunprefissoperunasessioneBGP,includeancheuncertonumerodiattributiBGPmprefisso+attributi=“rotta”

r Duedeipiùimportantiattributisono:mAS-PATH: elencaisistemiautonomiattraversoiqualièpassatol’annunciodelprefisso:es.AS67-AS17-AS21

mNEXT-HOP: indirizzodelrouter(interfaccia0dausareperaccedereadAS-PATH

r Quandounborder routerriceveunannunciodirotta,utilizzalepropriepolitiched’importazione perdecidereseaccettareofiltrarelarotta

Selezione dei percorsi BGP

r Unrouterpuòricavarepiùdiunarottaversoundeterminatoprefisso,edevequindisceglierneuna

r Regoledieliminazione:1. Allerottevieneassegnatocomeattributounvaloredi

preferenzalocale.Siselezionanoquindilerotteconipiùaltivaloridipreferenzalocale

2. SiselezionalarottaconvaloreAS-PATHpiùbreveescludendoquellechecontengonoASconcuinoncisonoaccordicommerciali

3. SiselezionaquellailcuirouterdiNEXT-HOPèpiùvicino4. Serimaneancorapiùdiunarotta,ilrouterfaunascelta

casuale

Messaggi BGP

r ImessaggiBGPvengonoscambiatiattraversoTCP.

rMessaggiBGP:

mOPEN: aprelaconnessioneTCPeautenticailmittente

mUPDATE: annunciailnuovopercorso(ocancellaquellovecchio)

mKEEPALIVE mantienelaconnessioneattivainmancanzadiUPDATE

mNOTIFICATION: riportaglierroridelprecedentemessaggio;usatoancheperchiudereilcollegamento.

Politiche d’instradamento BGP

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer

network:

provider

network

r A,B,Csonoretidiproviderdidorsaler X,W,Ysonoretistubr Xèunaretestub apiùdomicili

m XnonvuolecheiltrafficodaBaClepassiattraversom ...ecosìXnonannunceràaBlarottaversoC

Retedelprovider

Retedelcliente

Legenda

Politiche d’instradamento BGP

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer

network:

provider

network

r AannunciaaBilpercorsoAW.r BannunciaaXilpercorsoBAW.r BdeveannunciareaCdelpercorsoBAW?

m Certocheno!Bnonhanessun“interesse”nellarottaCBAWpoichénéW néCsonoclientidiB

m BvuolecostringereCadinstradareversoW attraversoAm Bvuoleinstradaresoloda/versoisuoiclienti!

Retedelprovider

Retedelcliente

Legenda

inter-ASvsintra-AS

Politiche:r Inter-AS:ilcontrolloamministrativodesideraavereilcontrollosu

comeiltrafficovieneinstradatoesuchiinstradaattraversolesuereti.

r Intra-AS:unicocontrolloamministrativo,ediconseguenzalequestionidipoliticahannounruolomoltomenoimportantenellosceglierelerotteinternealsistema

Scala:r L’instradamentogerarchicofa“risparmiare”sulletabelle

d’instradamento,eriduceiltrafficodovutoalloroaggiornamento

Prestazioni:r Intra-AS:orientatoalleprestazionir Inter-AS:lepolitichepossonoprevaleresulleprestazioni

Topologia di Internet

• Troppocomplessaperpoterla“disegnare”– AnchesologliASsonooltre50.000,iprefissididestinazioneoltre1milione(alivelloBGP,quindiaggregati)

• Caratteristiche“smallworld”– Pochinodicontantissimeconnessioni– Tantissiminodiconpocheconnessionie“stub”– Pochi“hop”(alivelloAS)perarrivareovunque

• Proprietàdelgrafointeressantiecomplesse– esisteunaintera“scienza”dianalisidellecaratteristichedellatopologiadiInternet


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