Ch. 7 – Distance Vector Routing Protocols Part 1 of 2: Distance Vector Routing and RIP
Distance Vector Routing - Moodle 2017-2018 · PDF fileDistance Vector Routing ... (e.g....
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1 Distance Vector Routing Jean‐Yves Le Boudec 2014 ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE
Transcript of Distance Vector Routing - Moodle 2017-2018 · PDF fileDistance Vector Routing ... (e.g....
1
Distance Vector Routing
Jean‐YvesLeBoudec2014
ÉCOLE POLYTECHNIQUEFÉDÉRALE DE LAUSANNE
2
Contents1. RoutinginGeneral2. Distancevector:theory3. Distancevector:practice(RIP)
TextbookSection5.1.1,Thecontrolplane
1. IntroductionWhy were routing protocols invented
IPassumesroutingtablesaremaintainedathostsandroutersusedbyPacketForwardingRouting=controlmethod
maintainroutingtablesautomaticallyinrouters
PacketForwardingforeverypacketdone inrealtime
Routingcomputationofroutingtablesordatastructuresforunicastandmulticastnormallyonlybetweenroutersnon‐realtime:latencyupto2minutesusesdedicatedprotocols(RIP,OSPF,EIGRP(Cisco)forunicastandDVMRP,M‐OSPF,PIM)Routingtablesinhostsarenormallyderivedfromconfigurationinfo
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4
What a routing protocol does
findreachabledestinationsfindbestpathstowardsdestinations
best=distancetodestinationisminimalDistance=computedusingsomemetricStatic metric ‐ does notdepend onthenetworkstate;forexample:
number ofhopslinkcapacityandstaticdelayAdministrativecost
Dynamic metric‐ depend onthenetworkstatelinkloadcurrentdelayseeendofsection
How routing protocols workTherearemanydifferentways,whichcanbeclassifiedasDistanceVector:
Everyrouterknowsonly:itsneighbours +distancestoalldestination(usedinsidedomains– interiorrouting)– thismoduleandlab4
LinkState:Everyrouterhasadetailedmapofentirenetwork(usedinsidedomains– interiorrouting)– module“linkstate”
PathvectorEveryrouterknowsonly:itsneighbours +pathstoalldestination(usedbetweendomains– exteriorrouting)‐‐ moduleBGPandlab6
SourceroutingAsanalternativetorouting protocols it is also possibletoconfigurerouting tablesusing manual configuration(see lab1)oroutofbandmanagementsoftware(e.g.OpenFlow)
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Source Routing
sourcewritesrouteintopacketheader(IPheaderextension)routerreadsnexthopfrompacketheader,movespointerroutediscoveredbythesource,floodingwithIP,thisiscalled“strictsourcerouting”:notusedintheInternetbutusedinsomead‐hocnetworks
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A B 2.2.4IS
BISIS
IS
A
SA DA RI data
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3
1 2
1 3
1
3
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A B 2.2.4A B 2.2.4
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2. Distance Vector: Theory
What itdoes:itcomputesshortestpathstoalldestinations
Costofapath=sumofthecostofnetworklinksalongthepathCostofalinkissetupbyconfigurationShortestpath=pathwhosecostisminimalDistancefromnodeAtonodeB=costofashortestpathfromAtoB
How itworksusesthe“DistributedBellman‐Ford”algorithmFullydistributedUsingasonlyinformationthedistancesfromselftoalldestinations
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2.1 The Centralized Bellman‐Ford Algorithm
Algorithm BF‐Cinput: a directed graph with links costs ( , ); assume ( , )>0 and when nodes i and j are not connected.output: vector s.t. ( )= cost of best path from node to node 1
for
for dofor
untilreturn( )
Example: shortest paths to node 1: run BF‐C for
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1 0∞
1BC
DE
1
51
1
6
2 2
k=0
0∞∞
∞ ∞
1BC
DE
1
51
1
6
2 2
k=1
01∞
∞ 5
1BC
DE
1
51
1
6
2 2
k=2
1BC
DE
1
51
1
6
2 2
k=3
1BC
DE
1
51
1
6
2 2
k=4
1BC
DE
1
51
1
6
2 2
k=5
1BF‐C
Message Passing Interpretation of Bellman‐Ford
WithBF‐Citisasif:atiteration ,node receivesfromneighbors theirpreviouscosts andupdatesowncostbypickingthebestBF‐Ccanbeinterpretedasasynchronousmessagepassingalgorithm
synchronous=nodeswaituntiliteration 1 isfinishedbeforestartingiteration
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1BC
DE
1
51
1
6
2 2
k=1
01∞
∞ 5
1BC
DE
1
51
1
6
2 2
k=2
0
∞1
3 5
Example: shortest paths to node 1: run BF‐C,
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1 0∞
1BC
DE
1
51
1
6
2 2
k=0
0∞∞
∞ ∞
1BC
DE
1
51
1
6
2 2
k=1
01∞
∞ 5
1BC
DE
1
51
1
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2 2
k=2
017
3 3
1BC
DE
1
51
1
6
2 2
k=3
1BC
DE
1
51
1
6
2 2
k=4
1BC
DE
1
51
1
6
2 2
k=5
1BF‐C
At which does the algorithm stop ?(i.e. what is the smallest such that ?)
1. k=12. k=23. k=34. k=45. k=56. k 67. Idon’t
know
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k = 1
k = 2
k = 3
k = 4
k = 5
I don’t know
0% 0%
6%2%
8%
60%
24%
Solution: algorithm stops at
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0 0∞
1BC
DE
1
51
1
6
2 2
k=0
0∞∞
∞ ∞
1BC
DE
1
51
1
6
2 2
k=1
01∞
∞ 5
1BC
DE
1
51
1
6
2 2
k=2
017
3 3
1BC
DE
1
51
1
6
2 2
k=3
014
3 3
1BC
DE
1
51
1
6
2 2
k=4
014
3 3
1BC
DE
1
51
1
6
2 2
p(A)=0p(B)=1p(C)=4
p(E)=3 p(E)=3
Correctness of BF‐C
Theorem11. Ifthenetworkisfullyconnected,BF‐Cstopsatthelatestat
(where numberofnodes)andoutputsthedistancealongshortestpathfrom to1,forall
isthedistancefrom to1inatmost hops3. Thenexthopalongashortestpathfrom to1isfoundbynextHop(i) Argminji
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Computation of shortest path tree
Whenalgorithmhasconverged,thenexthopofnodeCisobtainedbylookingfor thatminimizes
1BC
DE
1
51
1
6
2 2
k=4
014
3 3
2.2 Assume we start fromother initial conditions. What will happen ?
1. Thealgorithm convergestothecorrectdistances
2. Thealgorithm does notconverge3. Thealgorithm convergesbutnotto
thecorrectdistances4. Idon’t know
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1BC
DE
1
51
1
6
2 2
k=0
0101
1 1
replaced by values on graph →
The algorithm
converges to
The algorithm
does n
ot co
.The algorithm
converges b
I don’t know
19%
2%
64%
15%
Run BF‐C with bizarre initial conditions
the value of is…
1. 12. 23. 34. 45. 5
7. Idon’tknow
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0 010
1BC
DE
1
51
1
6
2 2
k=0
0101
1 1
1BC
DE
1
51
1
6
2 2
k=1
?
1 2 3 4 5
I don’t know
38%
30%
4%2%
21%
0%
4%
Solution:Run BF‐C with bizarre initial
conditions
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Onthisexample,algorithmBF‐Cconvergesto
thecorrectvaluesinspiteofabnormalinitialconditions
0 010
1BC
DE
1
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1
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2 2
k=0
0101
1 1
1BC
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1
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1
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2 2
k=1
012
2 2
1BC
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1
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1
6
2 2
k=2
013
3 3
1BC
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1
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1
6
2 2
k=3
014
3 3
1BC
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1
51
1
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2 2
k=4
014
3 3
1BF‐C
Assume we break some links and start from these initial
conditions. What will happen ?
1. Thealgorithm convergestothecorrectdistances
2. Thealgorithm does notconverge3. Thealgorithm convergesbutnotto
thecorrectdistances4. Idon’t know
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replaced by values on graph →
1BC
DE
1
51
1
6
2 2
014
3 3
∞∞∞
The a
lgorit
hm co
nverge
s to..
The a
lgorit
hm doe
s not co
...
The a
lgorit
hm co
nverge
s b.
I don
’t kn
ow
35%
2%
27%
35%
Run BF‐C with disconnected network
the value of is…
1. 12. 23. 34. 45. 5
7. Idon’tknow
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0 010
1BC
DE
1
51 2
k=0
014
3 3
1BC
DE
1
51 2
k=1
?
1 2 3 4 5
I don’t know
8%
4%2%
12%
45%
10%
20%
Solution:BF‐C with disconnected
network
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Onthisexample,algorithmBF‐Cconvergesto
thecorrectvaluesat BandDandcountsto∞ atCandE
1BC
DE
1
51 2
k=0
014
3 3
1BC
DE
1
51 2
k=1
014
5 3
1BC
DE
1
51 2
k=2
016
5 3
1BC
DE
1
51 2
k=3
016
7 3
1BC
DE
1
51 2
k=4
018
7 3
BF‐C
Correctness of BF‐C
with arbitrary initial
condition and arbitrary network
Theorem21. Ifthereisapathfrom to1,thenfor largeenough
∀ where isthedistancefrom to1.Itfollowsthatifthenetworkisfullyconnected,BF‐Cwitharbitraryinitialconditionsterminatesandeventuallycomputesthecorrectdistances.However,ingeneral,when issmall isnot thedistancefrom to1in hops.
2. Ifthereisnopathfrom to1then lim→
∞.Itfollowsthatifthenetworkisnotfullyconnected,BF‐Cdoesnotterminate(“counttoinfinity”).
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any nonnegative value
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ProofWe dotheproofassuming allnodes areconnected.1. Letpk bethevectorpk[i],i=2,….LetBbethemappingthattransformsanarrayx[i]i=2…into
thearrayBx definedfori 1 byBx[i]=minj i,j 1[A(i,j)+x(j)]
Letbbethearraydefinedfori 1 byb[i]=A(i,1)
Thealgorithmcanberewritteninvectorformas(1)pk =Bpk‐1 bwhere isthepointwise minimum
2. Eq (1)isamin‐pluslinearequationandtheoperatorBsatisfiesB(xy)=Bx By.Thus,Eq(1) canbesolvedusingmin‐plusalgebrainto(2)pk =Bkp0 Bk‐1b… Bb b
3. Definethearrayefori 1bye[i]=.Letp0=e.Eq (2)becomes(3)pk =Bk‐1b… Bb b.NowwehavetheBellmanFordalgorithmwithclassicalinitialconditions,thus,byTheorem1:(4)fork n‐1:Bk‐1b… Bb b=qwhereq[i]isthedistancefromi to1.
4. WecanrewriteEq(2)fork n‐1as(5)pk =Bkp0 q
5. Bkp0[i]canbewrittenasA[i,i1]+A[i1,i2]+…+A[ik‐1,ik]+p[ik]thus(6)Bkp0[i] ka,whereaistheminimumofallA[i,j].ThusBkp0[i]tendstowhenkgrows.Thusforklargeenough,Bkp0islargerthanqandcanbeignoredinEq(5).Inotherwords,forklargeenough:(6)pk =q
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2.3 Distributed Bellman Ford
BF‐Ccanbeusedtocomputep(i)i.e.findtheshortestpath.However,thisisnotitsmaininterest,becausethereisabetteralgorithm(Dijkstra)thatcanbeusedinacentralizedway.But:itcanbedistributed asfollows.
Thisisasynchronousmessagepassing,i.e.nodesdonotwaitforcompleteroundsofiterationtobeperformed.
DistributedBellman‐FordAlgorithm,BF‐prelimnode maintainsanestimate ofthetruedistance tonode1;node alsokeepsarecordoflatestvalues forallneighborsinitialconditionsarearbitrarybut 1 0 atallsteps;
fromtimetotime,i sendsitsvalue toallneighbours
when receivesanupdatedvalue fromneighbor ,noderecomputes :eq (1) ← min
A i, j q j
nextHop(i) issettoavalueofj thatachievestheminineq(1)
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Correctness of BF‐prelim
Theorem:Assumeatleastonemessageisreliablysenttoallneighborsbyeverynodeevery timeunitsandthenetworkisfullyconnected.BF‐prelimconvergestothesameresultasthecentralizedversionBF‐C(i.e.tothecorrectdistances)in timeunits,where isthenumberofstepsperformedbyBF‐Cwithsameinitialconditions.
DistributedBellman‐FordAlgorithm,BF‐prelimnode maintainsanestimate ofthetruedistance tonode1;node alsokeepsarecordoflatestvalues forallneighborsinitialconditionsarearbitrarybut 1 0 atallsteps;
fromtimetotime,i sendsitsvalue toallneighbours
when receivesanupdatedvalue fromneighbor ,noderecomputes :eq (1) ← min
A i, j q j
nextHop(i) issettoavalueofj thatachievestheminineq(1)
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Distributed Bellman‐Ford BF‐prelim
Apossiblerun ofalgorithm BF‐prelim.Thetableshowsthesuccessivevaluesof
q(i) 1 B C D E0 0 1 0 1 3 0 1 7 3 0 1 7 3 80 1 7 3 40 1 7 3 4
1 ‐> BB ‐> DB ‐> CC ‐> ED ‐> E1 ‐> DB ‐> E
473∞
1BC
DE
1
5
1
1
6
2 2
017
3
Converged !
0 1 7 3 3E ‐> BE ‐> C
0 1 7 3 30 1 4 3 3
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Super‐Duper Bellman‐Ford
BF‐prelimrequiresanodetorememberallpreviouslyreceivedestimatesq(j) forallneighbors,eveniftheyarenotthebestones;canwedobetter?Apossiblemodificationwouldbetoreplaceeq(1)by
Super‐DuperBellman‐Ford:
Nodei nowneedstostoreonlyitsownestimate
Q. doesthiswork?
← min , ,
when receives an updated value from neighbor , node recomputes :eq (1s) ← min
, ,
Is Super‐Duper Bellman‐Ford correct ?
1. Yes,regardless ofinitialcondition
2. Only ifinitialconditionsareand
at time03. Itmay fail,even with initial
conditionsasin2.4. Idon’t know
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← min , ,
when receives an updated value from neighbor , node recomputes :eq (1s) ← min
, ,
Yes,
rega
rdles
s of in
itial c
o...
Only
if init
ial co
nditio
ns ar
...
It may
fail,
even
with
initia
l...
I don
’t kno
w
54%
7%15%
24%
Solution
Super‐Dupercanonlydecrease .Soifwestartfrominitialconditionsasinexample« ImpactofInitialConditions »,thealgorithmwillnotconvergetotherightvalue.Itgets« stuck »withalowvalue.Itispossibletoshowthatitworksifallinitialconditionsareabovethefinalvalues,forexample andinitially.Buteventhen,itwillnotworkifthereisatopologychange,sincethisisequivalenttostartingfromdifferentinitialconditions.Indeed,requiringthatinitialconditionsarecorrectmeansthatweareabletoresettheentirenetworkwheneverthereisatopologychange(whichisnotfeasibleinpractice).
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Distributed Bellman‐Ford, the true version
Requiresonlytorememberdistancefromselftodestination+thebestneighbor(nextHop( ))andworksforallinitialconditions
DistributedBellman‐FordAlgorithm,BF‐Dnode maintainsanestimate ofthedistance tonode1;node remembersthebestneighbornextHop(i)
initialconditionsarearbitrarybut 1 0 atallsteps;
fromtimetotime,i sendsitsvalue toallneighbors
when receivesanupdatedvalue from ,node recomputes :eq (2) if nextHop
then ← ,else ← min , ,
ifeq(2)causes tobemodified,nextHop ←
1BC
DE
1
5
1
1
6
2 2
017
3
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Distributed Bellman‐Ford BF‐D
Apossiblerun ofalgorithm BF‐D.Thetableshowsthesuccessivevaluesof
1 B C D E 0 10 1 6 10 1 1 6 10 1 1 3 10 1 1 3 10 1 1 3 10 1 1 3 1
1 ‐> BB ‐> DB ‐> CB ‐> ED ‐> EC ‐> E
1nextHop
0 1 1 3 2
1BC
DE
1
5
1
1
6
2 2
017
3
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Distributed Bellman‐Ford BF‐D, cont’d
Apossiblerun ofalgorithm BF‐D.Thetableshowsthesuccessivevaluesof
1 B C D E 0 10 1 6 10 1 1 6 10 1 1 3 10 1 1 3 10 1 1 3 10 1 1 3 1
1 ‐> BB ‐> DB ‐> CB ‐> ED ‐> EC ‐> EE ‐> CC ‐> EE ‐> CC ‐> EE ‐> CB ‐> E
4nextHop
0 1 1 3 20 1 2 3 2 0 1 1 3 30 1 3 3 30 1 3 3 40 1 4 3 40 1 4 3 3
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Correctness of BF‐D
Theorem:Assumeatleastonemessageisreliablysenttoallneighborsbyeverynodeevery timeunitsandthenetworkisfullyconnected.BF‐DconvergestothesameresultasthecentralizedversionBF‐C(i.e.tothecorrectdistances)intimeunits,where isthenumberofstepsperformedbyBF‐Cwithsameinitialconditions.Comment:ThemaindifferencewithBF‐prelimisthateq(2)replaceseq(1).AssumeweuseBF‐D,andwestartfromaconditionsuchthatq(i)isindeedequaltotheminimumgivenbyeq (1)(whichiswhat,intuitively,istruemostofthetime).
WhenjisnotequaltonextHop(i),botheq(1)andeq(2)havethesameeffect:thenewvalueofq(i)isthesameinbothcases.Incontrast,ifj==nextHop(i),theneq (2)setsq(i)tothenewvalueA(i,j)+q(j),whereaseq(1)setsittominj neighbor (A(i,j)+q(j)).Eq(2)providesanupperboundoneq(1),inthiscase.Itturnsoutthatthealgorithmstillworks,bythesamemechanismthatmakesthealgorithmworkevenwhentheinitialconditionsarearbitrary.Indeed,nodeiwillsenditsnewvaluetoallremainingneighbors,whowillinturndoanupdateandeventually,nodei willreceivevaluesofq(j)thatwillcorrecttheproblem.Inotherwords,ifthenewvalueofq(i)istoohigh(comparedtowhatwouldbeobtainedwitheq (1)),thisisrepairedinoneroundofexchangeswiththeneighbors.
Take‐Home Message
Bellman‐Ford,Distributed(BF‐D),allowsanodetocomputedistancestoonedestinationStoredinformationis:
NexthoptodestinationDistancetodestination
BF‐Dworksbymessagepassing:everynodeinformsitsneighborsofitscurrentdistance
BF‐Dworksevenwheninitialconditionsarenotasexpected(whichoccursduetotopologychanges)
Ifdestinationisunreachable(distance )thenthealgorithmscountstoinfinity.
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3. Distance Vector routing in practice
Distancevectorrouting=basedonBF‐DBF‐DisruninallroutersforallpossibledestinationprefixesRouteri computesshortestpathandnexthopforallnetworkprefixesn thatitheardof.Initially:D(i,n) =1ifi directlyconnectedton andimplicitlyD(i,n) =+ foranyn thatwasneverheardof.Nodei receivesfromneighbour k latestvaluesofD(k,n) foralln(thisisthedistancevector).Nodei computesthebestestimatesaccordingtoalgorithmBF‐D
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c(i,m)
c(i,1) D(1,n)
c(i,k) D(k,n)
D(m,n)
i n
1
k
m
Periodic Updates
Updatesaresentperiodically(e.g.every30sec)Ifneighbour k isnolongerpresent,nodeiwillnolongerreceivehellomessages,andafteratimeout,thishasthesameeffectasifnodeiwouldreceivethemessagefromk: foralln.ThenalgorithmBF‐Disrun:Itfollowsthatif was thenext hopto ,then declaresunreachable.
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c(i,m)
c(i,1) D(1,n)
c(i,k) D(k,n)
D(m,n)
i n
1
k
m
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Example 1
n1A B
n3D C
n2n4
net dist nxt
n1 1 n1,An4 1 n4,A
net dist nxt
n1 1 n1,Bn2 1 n2,B
net dist nxt
n3 1 n3,Dn4 1 n4,Dm3 1 m3,D
net dist nxt
n2 1 n2,Cn3 1 n3,Cm1 1 m1,Cm2 1 m2,C
A B
CDm1
m2
m3
All link costs = 1
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Example 1
n1A B
n3D C
n2n4
net dist nxt
n1 1 n1,An4 1 n4,A
net dist nxt
n1 1 n1,Bn2 1 n2,Bn4 2 n1,A
net dist nxt
n3 1 n3,Dn4 1 n4,Dm3 1 m3,D
net dist nxtn2 1 n2,Cn3 1 n3,Cm1 1 m1,Cm2 1 m2,Cn4 2 n3,Dm3 2 n3,D
from An1 1 n4 1
A B
CDm1
m2
m3
from Dn3 0 n4 0m3 0
All link costs = 1
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Example 1
n1A B
n3D C
n2n4
net dist nxt
n1 1 n1,An4 1 n4,A
net dist nxt
n3 1 n3,Dn4 1 n4,Dm3 1 m3,D
net dist nxtn2 1 n2,Cn3 1 n3,Cm1 1 m1,Cm2 1 m2,Cn4 2 n3,Dm3 2 n3,D
A
CDm1
m2
m3
from Cn2 1 n3 1m1 1m2 1 n4 2m3 2
net dist nxt
n1 1 n1,Bn2 1 n2,Bn3 2 n2,Cn4 2 n1,Am1 2 n2,Cm2 2 n2,Cm3 3 n2,C
B
All link costs = 1
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Example 1 ‐ Final
n1A B
n3D C
n2n4
net dist nxt
n1 1 n1,An2 2 n1,Bn3 2 n4,Dn4 1 n4,Am1 3 n4,Dm2 3 n4,Dm3 2 n4,D
net dist nxt
n1 2 n4,An2 2 n3,Cn3 1 n3,Dn4 2 n4,Dm1 2 n3,Cm2 2 n3,Cm3 1 m3,D
A
C
D
m1
m2
m3
net dist nxt
n1 2 n2,Bn2 1 n2,Cn3 1 n3,Cm1 1 m1,Cm2 1 m2,Cn4 2 n3,Dm3 2 n3,D
net dist nxt
n1 1 n1,Bn2 1 n2,Bn3 2 n2,Cn4 2 n1,Am1 2 n2,Cm2 2 n2,Cm3 3 n2,C
B
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Example 1 ‐ Failure
n1A B
n3D C
n2n4
m1
m2
m3
net dist nxt
n1 2 An2 2 Cn3 1 Dn4 1 Dm1 2 Cm2 2 Cm3 1 D
D
C
net dist nxt
n1 2 Bn2 1 Cn3 1 Cm1 1 Cm2 1 Cn4 2 Dm3 2 D
net dist nxt
n1 1 Bn2 1 Bn3 2 Cn4 2 Am1 2 Cm2 2 Cm3 3 C
B
We show only the router in the next hop field
All link costs = 1
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Example 1 ‐ Failure
n1A B
n3D C
n2n4
m1
m2
m3
timeout
net dist nxt
n1 1 Bn2 1 Bn3 2 Cn4 2 Am1 2 Cm2 2 Cm3 3 C
B
C
net dist nxt
n1 2 Bn2 1 Cn3 1 Cm1 1 Cm2 1 Cn4 2 Dm3 2 D
net dist nxt
n1 2 An2 2 Cn3 1 Dn4 1 Dm1 2 Cm2 2 Cm3 1 D
Dtimeout
All link costs = 1
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Example 1 ‐ Failure
n1A B
n3D C
n2n4
m1
m2
m3
net dist nxt
n1 1 Bn2 1 Bn3 2 C
m1 2 Cm2 2 Cm3 3 C
B
C
net dist nxt
n1 2 Bn2 1 Cn3 1 Cm1 1 Cm2 1 Cn4 2 Dm3 2 D
net dist nxt
n2 2 Cn3 1 Dn4 1 Dm1 2 Cm2 2 Cm3 1 D
D
From C:n1 2n2 1n3 1m1 1m2 1n4 2m3 2
All link costs = 1
n1 3 C
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Example 1 ‐ After Failure
n1A B
n3D C
n2n4
m1
m2
m3
net dist nxt
n1 1 Bn2 1 Bn3 2 Cn4 3 Cm1 2 Cm2 2 Cm3 3 C
B
C
net dist nxt
n1 2 Bn2 1 Cn3 1 Cm1 1 Cm2 1 Cn4 2 Dm3 2 D
net dist nxt
n2 2 Cn3 1 Dn4 1 Dm1 2 Cm2 2 Cm3 1 D
D
All link costs = 1
n1 3 C
Example 1: Conclusion
Example1illustrateshowBellmanFordismappedtothenetworkconceptshowtopologychangesaretakenintoaccount
mostrecentannouncementreplacespreviousonesnonrefreshedannouncementsbecomeobsolete
howdistancevectorcarriesreachabilityinformation
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Example 2Assumealgorithmhasconverged
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A B
dest link cost
a local 1b l1 2c l1 3d l3 2e l3 3
A B
D E
l4l3 C l5
l2
dest link cost
a l1 2b local 1c l2 2d l4 3e l4 2
dest link cost
a l2 3b l2 2c local 1 d l5 3e l5 2
C
dest link cost
a l3 2b l3 3c l6 3d local 1e l6 2
D
dest link cost
a l6 3b l4 2c l5 2d l6 2e local 1
E
All link costs = 1
Example 2Linksl1andl6failsimultaneously,Ddetectsfailure
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A B
dest link cost
a local 1b l1 2c l1 3d l3 2e l3 3
A B
D E
l4l3 C l5
l2
dest link cost
a l1 2b local 1c l2 2d l4 3e l4 2
dest link cost
a l2 3b l2 2c local 1 d l5 3e l5 2
C
dest link cost
a l3 2b l3 3c NR ∞d local 1e NR ∞
D
dest link cost
a l6 3b l4 2c l5 2d l6 2e local 1
E
All link costs = 1
Example 2 : Zoom on A and B
48
dest link cost
a local 1b l1 2c l1 3d l3 2e l3 3
A
A
D
l3
dest link cost
a l3 2b l3 3c l3 4d local 1e l3 4
D
from A:dest costa 1b,d 2c,e 3
dest link cost
a local 1b NR ∞c NR ∞d l3 2e l3 3
A
A
D
l3
dest link cost
a l3 2b l3 3c l3 4d local 1e l3 4
D
A detects failure of dest link cost
a local 1b l3 c l3 d l3 2e l3 5
A
A
D
l3
dest link cost
a l3 2b l3 3c l3 d local 1e l3
D
from D:dest costd 1a 2b 3c,e 4
All link costs = 1
After processing this update, what is the distance at D to e ?
1. 12. 23. 34. 45. 56. 67. Idon’t know
49
dest link cost
a local 1b l3 c l3 d l3 2e l3 5
A
A
D
l3
dest link cost
a l3 2b l3 3c l3 d local 1e l3
D
from A:dest costa 1d 2b 4c,e 5
All link costs = 1
1 2 3 4 5 6I d
on’t k
now
3% 3% 3%
51%
30%
8%
3%
Solution
AtD:DistributedBellman‐Ford(BF‐D)updateisreceivedfromnexthoptoethereforenewdistanceisacceptedDsetsdistancetoeto
50
dest link cost
a local 1b l3 c l3 d l3 2e l3 5
A
A
D
l3
dest link cost
a l3 2b l3 3c l3 d local 1e l3
D
from A:dest costa 1d 2b 4c,e 5
Count to Infinity
51
dest link cost
a local 1b l3 c l3 d l3 2e l3 5
A
A
D
l3
dest link cost
a l3 2b l3 5c l3 6d local 1e l3 6
D
from A:dest costa 1d 2b 4c,e 5
dest link cost
a local 1b l3 6c l3 7d l3 2e l3 7
A
A
D
l3
dest link cost
a l3 2b l3 5c l3 6d local 1e l3 6
D
from D:dest costa 1d 2b 4c,e 5
dest link cost
a local 1b l3 6c l3 7d l3 2e l3 7
A
A
D
l3
dest link cost
a l3 2b l3 7c l3 8d local 1e l3 8
D
from A:dest costa 1d 2b 6c,e 7
All link costs = 1
Conclusion from Example 2
Thecoststob,c,e growunbounded“CounttoInfinity”thetruecostsareinfiniteThisistheexpectedbehaviour ofBellman‐Ford
Inpractice,weenforceconvergencebysettinglargenumber
e.g.RIP:∞ 16
Butwecandobetter…
52
Route poisoning and Split Horizon
TwopracticalheuristicsinordertoPreventcounttoinfinityPreventping‐pong loops
Routepoisoning:ifAdetectsthatroutetoxisnotreachable,Asends«distancetox= »toneighbours
Further,when adistance= isreceivedforx,anyfurtherupdateaboutxisignoredforsometime(holddown timer)
SplitHorizon:ifAroutespacketstoxviaB,AdoesnotannouncethisroutetoB
53
Example 2with Route Poisoning and Split Horizon
A B
dest link cost
a local 1b l1 2c l1 3d l3 2e l3 3
A B
D E
l4l3 C l5
l2
dest link cost
a l1 2b local 1c l2 2d l4 3e l4 2
dest link cost
a l2 3b l2 2c local 1 d l5 3e l5 2
C
dest link cost
a l3 2b l3 3c NR ∞d local 1e NR ∞
D
dest link cost
a l6 3b l4 2c l5 2d l6 2e local 1
E
D detects failureD immediately sends update to A with poisoned routes (∞ distances)Routes to a and b are not advertized (split horizon)
from B:d 1c, e: ∞
All link costs = 1
After processing this update, what are the distances at A to c and e ?
5. Noneoftheabove6. Idon’t know
55
dest link cost
a local 1b l1 2c l1 3d l3 2e l3 3
A
A
D
l3
dest link cost
a l3 2b l3 3c NR ∞d local 1e NR ∞
D
from B:d 1c, e: ∞
All link costs = 1
$$ $$ =3, $$ $$ =
None of the abov
I don’t know
12%
7%10%
0%
44%
27%
Solution
AappliesBF‐D:
Disnotthenexthoptoc,thereforeAcomputes
distanceis3,nochange
Disthenexthoptoe,thereforeAcomputes
distanceis ,i.e.eisunreachable
56
dest link cost
a local 1b l1 2c l1 3d l3 2e NR ∞
A
A
D
l3
dest link cost
a l3 2b l3 3c NR ∞d local 1e NR ∞
D
from B:d 1c, e: ∞
Example 2 with Route poisoning and Split Horizon
NocounttoinfinityAfter sometime,thedistancesareremoved bytimeout
57
A detects failuredest link cost
a local 1b NR ∞c NR ∞d l3 2e NR ∞
A
A
D
l3
dest link cost
a l3 2b NR ∞c NR ∞d local 1e NR ∞
D
from A:a 1b,c,e ∞
dest link cost
a local 1b l1 2c l1 3d l3 2e NR ∞
A
A
D
l3
dest link cost
a l3 2b l3 3c NR ∞d local 1e NR ∞
D
from B:d 1c, e: ∞
All link costs = 1
58
RIP (Routing Information Protocol)ImplementsBF‐DCostofonelinkis1bydefaultCostsareunidirectional,itispossibletohave , ,∞ 16;thereforenetworkspanlimitedto15SplithorizonandroutepoisoningUDPport520andmulticastaddress224.0.0.9/ff02::9Sendsdistancevectorupdateevery30seconds(bydefault)orwhenchangeisdetected(“triggeredupdate”)
Routenotannouncedduring3minutesbecomesinvalidRIPkeepsadatabaseofallbestpaths,called“RoutingInformationBase”(RIB),whichisdifferent(supersetof)routingtable
unlikeslidesinpart“theory”AuthenticationinRIPv2byMD5(sharedsecret)RIPforIPv4,RIPng forIPv6
59
EIGRP (Enhanced Interior Gateway Routing Protocol)
ProprietaryprotocolbyCISCOMetricthatestimatestheglobaldelayMaintainsseveralroutesofsimilarcost
loadsharingNolimitof15
numberofroutersincludedinmessagesBroadcastevery90sec
TypicallyoneprocessforIPv4andoneprocessforIPv6
60
Metric example
MetricTrans=10000000/Bandwidth(timetosend10Kb)delay=(sumofDelay)/10m=[K1*Trans+(K2*Trans)/(256‐load)+K3*delay]default:K1=1,K2=0,K3=1,K4=0,K5=0ifK5 0,m=m*[K5/(Reliability+K4)]
BandwidthinKb/s,DelayinsAtVenus:Routefor172.17/16:Metric=10000000/784+(20000+1000)/10=14855AtSaturn:Routefor12./8:Metric=10000000/224+(20000+1000)/10=46742
In a fully connected network, say what is true :
1. BF‐Cconvergestothecorrectdistanceswhatevertheinitialconditions
2. BF‐Dconvergestothecorrectdistanceswhatevertheinitialconditions
3. 1and24. None5. Idon’tknow
61
BF‐C
conv
erge
s to the c
orre
..
BF‐D
conv
erge
s to the c
orre..
1 and
2
None
I d
on’t kn
ow
0% 0%0%0%0%
62
Conclusion
DistancevectorissmartFullydistributed,littleinformationstored
SimpleBut:relativelyslow
NotsuitedforlargeandcomplexnetworksLinkStateprotocolsshouldbeusedinstead
![IPv6 Routing Header Security. - secdevsecdev.org/conf/IPv6_RH_security-csw07.pdf · IPv6 specification [RFC2460] defines Routing Header extension as a mean for a source to list](https://static.fdocuments.us/doc/165x107/5e9be7f5841a3045af1febf3/ipv6-routing-header-security-ipv6-speciication-rfc2460-deines-routing-header.jpg)
