Important properties

Thereare severalattributesthatgoverntheperformaneandbehavioroftheIPRTtree

onstrution,andtheymayinueneresoureusagein bothroutingandforwarding:

•

^Theseletion-algorithmusedforpathandylereation

•

^{Run-timeofthealgorithm}

•

^{TheabilitytoprovideQoSproperties}

Themannerofhowtheyleandthearhesareseleted,isofgreatimportanetothe

attributes oftheredundanttrees. Intheoriginal RT algorithms, e.g. Algorithm1, this

wasleftas anopenquestion. However,there hasbeensomeresearhinthisareabyXue

et. al. [27℄[16℄[17℄. Theirresultsshowthatthereationof ylesandarhesisvitalfor

IPRT to meet thedesired performane riteria. Agood examplemay behow thetrees

wouldperformwithashortest-pathversuslongest-pathseletionoftheyleandarhes;

Consideranodethathastwoneighbors,wherealinkhasfailedbetweentherootnodeand

pathbetweenthetwonodesthrougheithertheredorthebluetreewouldthenbeofequal

size to the lengthof the yle- i.e. the lengthof

A ^B 1

^or

A ^R 1

^{in line three in Algorithm}

1. Thus,ifashortestpathwasused,therewouldbeaguaranteethatthisreoverypath

wouldbeofminimallength. Withadepth-rstsearhthatterminatesattherstnode

in-tree,theIPRTmethodwouldnotbeabletogivesuhaguarantee. Iftheinitialylewas

reatedusing alongest-path algorithm, the path-lengthmight besigniant. However,

the inuene thehosenseletion-algorithm exertson the performane and behaviorof

theIPRTmethodisdependantuponthetopology. Forexample,inapurering-topology,

thevariousseletion-algorithmswouldhavenoimpatatallontheperformaneofIPRT.

It hasbeenshownthat anapproah, whereinthelengthoftheyle andthearhesare

kept to a minimum, does show a signiant improvement with respet to the average

reoverypath length[17℄. In addition, it has been shown that this approah generates

treeswithahigherdegreeofoverageinaseofmultipleonurrentfailures.

It isimportantfortheIPRTalgorithmto supportsomelevelof QoS,asarandomly

driven approah may have a negative impat on the performane and resoure usage.

Often,ostsonlinksareusedforQoS,i.e. tomaximizeavailablebandwidthortominimize

the number ofhops. Throughlink ost, the RT method mayalso support someTra

Engineeringaspets,suhastheabilitytosetahighostonlinksknowntofailregularly.

However,advaned requirementsmayintrodueabiggeromputationalost.

Determiningthebest redundanttrees-i.e. QoSorientedIPRT-foratopology,isan

NP-ompleteproblemsomewhatsimilartotheTravellingSalesman-problem. Aninformal

denition of NP problems isalass of problems that anbe veried by adeterministi

Turingmahinein polynomialtime. Furthermore,NP-ompleteproblemsareasublass

of NP problems that has the property that any problem in NP an be polynomially

reduedtoit. Tondthebestsetofredandbluetrees,thealgorithmneedstotryevery

possibleombinationandompositionofirlesandarhesinagiventopology. Tosearh

fortheperfetsolutionwouldbeimpossiblein pratie,asthetimeneededto alulate

allthepossiblesolutionswouldbefartoogreat forthesearhtobepratial. However,

therearemanyoptimizationsandapproximationsavailable,providingseeminglygoodor

probablygood solutions. Forexample,agreedy-algorithmimplementationofAlgorithm

1,usingadepth-rst-searhseletion-algorithmwith aruntime of

O(n + v)

^{, wouldhave}

aruntime of

O(n ² (|n| + |v|))

^{. This is beause linetwowill havea maximumexeution}

time of

O((|n| + |v|))

^{, and will be exeutedexatlyone.} Subsequently, line six will be exeuted at most

O(n)

^{times, as eah iteration will add at least one node to the trees}

andthusterminateat linefour after

O(n)

iterations. Theexeutiontimeofline sixwill beofmagnitude

0(n(n + v))

^{,giventhedepth-rst searhinitiated fromeahnodewith}

aruntime of

O(n + v)

^{. F}urthermore, ifthesearh-method ould beexhangedwith an algorithmwithruntimeof

O(n)

^{,thealgorithmwouldhavearuntimeof}

O(n ³ )

^{[1℄. Other}

algorithms provide morespeialized solutions for QoSIPRT, of whih the best have a

run-timeof only

O(n + v)

^[27℄.

Forareal implementation, afastalgorithm isof greatimportane,aslimited

omputa-tionalresouresmustbeassumed. Inaddition,IPRTshouldompletethetreegeneration

proessasfastaspossibleto havetheneessaryinformationavailabletoanysubsequent

failures. Inthisthesis,therun-timeisasubjetoflessimportaneasthefailure-senarios

maybepre-planned,andthetreesmaybeomputedo-line.Thus,thegreedy-algorithm

approahwould be suient. Thisprovidesa moreversatilesolution, asthe algorithm

is not optimized towardsa single QoSgoal. Tobe able to give an aurate piture of

theperformaneandabilitiesoftheIPRTalgorithm,theQoSpropertiesmustbe

onsid-ered. With thegreedy-algorithmapproah, itwouldbetrivialto exhange thedierent

seletion-algorithms.

Astherun-timeneededtogenerateeahtreemaybebroughtdowntoaminimumof

O(v+n)

^{,ithasbeenshownthatthe}omputationalostofthealgorithmmaybeoptimized enoughtoaommodateforaIPsolution. Furthermore,thealgorithmmayfulllseveral

QoSneedsanddemands,andmaybeversatileenoughtobeusedinexistingIPnetworks.

Thealgorithmmayalsobeusedonavarietyofnetworks,butmightperhapsyieldoptimal

resultsinatwo-vertexonnetednetwork,asthiswouldyieldthebest failureoverage.

5.2 Routing

5.2.1 Enabling IPRT to o-exist in a failure-free environment

TheIPRTmehanismneedstobeabletoo-existwithnormalroutingprotoolsintimes

offailure-freeoperation. ToahievethistheusageoftheoriginalRTreoveryproedure

needstobealtered.

OneoftheoriginalideasforRTroutingwastoletoneofthetwotreesbethe

founda-tionforfailure-freeoperation,i.e. tobeusedasaworkingtree. Inaonnetion-oriented

environmentthisapproahgivesomeadvantages inthereoveryproess,asafailure

re-portedonaonnetionwouldenablearoutertoimmediatelyswithtothebakuppath.

Oneofthemaindisadvantagesofapplying thisapproahto onventionalIP networksis

thelengthofthedefaultpaths. Theonstrutionoftheredundanttreesneedsto follow

stritrulestobeableto providenode-disjointpaths. Thus,itisprobablethat thepaths

generatedbyIPRTdoesnotprovidethebest pathhoiesavailable. Theonstrutionof

theredundanttreesneedstofollowstritrulestobeabletoprovidenode-disjointpaths,

and thus itis probablethat thepathsgenerated byIPRTdo notprovidethebest path

hoiesavailable. Furthermore,thisapproahdoesnotprovideavalidreoveryproedure.

If theIPRTmethod is used asa basisforthe forwardingmehanismduring failure-free

operation, itis alsoasubjetfor theIP re-onvergene proess. Thus, eventhoughthe

methodwouldabletoreovertraattimesoffailure, itdoesnotsolvemiro-loops.

Tobeabletoreovertraat alltimes,additionalvirtualreovery-topologiesould

beused in additionto thenormaltopology duringfailure-free operation. Subsequently,

the FIBs obtainedfrom the additional reovery-topologiesmaybe used to forwardthe

andbeongurablelikeexpetedinanormalnetwork. This,withoutbeinghamperedby

thereoverymehanism,andonlydependingontherestritionsofthepreferredrouting

protool. The FIBs generated from the reovery-topologiesare therefore only used for

forwardinginreoveryoperations.

When the IPRT algorithm is run, the resulting trees are laid asan overlay on the

originaltopology,andsubsequentlyeahofthelinksoutsidethetreeare assignedaost

of veryhigh value,i.e. themaximumavailable. Setting thelink ost withasuiently

high value is thesame asexluding thelink from the topology when theshortest path

algorithmis used. As aresultall thelinks areapartof allthereoverytopologies,but

notusedforpaket forwarding.

WhenIPRTisusedin onjuntionwithanLSroutingprotool,theLSprotoolmay

providethe RT method with the needed topologyinformation and routingmehanism.

Asanexample,inmulti-topologyIS-IS[28℄itispossibletoleteahtopologyeitherhave

their own dediated routingprotool where LSPs aremarkedaording to thetree ID,

orshare theroutingshemewhereLSPsaresharedbetweenthetopologies. Inaddition,

theIPRTreovermehanismmayusetheLSDBtogettheneededtopologyinformation.

SimilaroperationisalsoavailableinOSPF.

In a simulated environment, the routing protool may be represented o-line and

implementedinastatimanner. Bydoingthis,moretimemaybeusedonimplementing

and testingofIPRT,andin addition, thestatipropertyprovidesasimplersenarioto

analyze. Thisapproahalsohelpstoensurethattheroutingisexeutedinadeterministi

manner, and in this manner redue the needed work and potential problems that may

ourwhentesting theIPRTmethod. This approah doesnotloktheimplementation

to aspei routingprotool,but leavethisworkfor futureimplementation anddesign

deisions.

5.2.2 Computation

The omputation of reovery routes an be ahieved in IPRT either by a entralized

sheme, adistributed shemeor aombination of both. The hoie of whih approah

tousedependsonwhatkindofresouresthatareavailableinthenetwork;bandwith or

CPUyles.

The omputation in the original RTshemewasmeant to bedone bya entralized

server. Atonnetion-setup, thenodewereto querytheserver,andobtaintheworking

pathalongwith thereoverypath. Inaonnetionorientedsolution,onnetion-setups

mayberare. Therefore,thedelayassoiatedwithaentralizedshememaybeaeptable,

as long asit is within the order of delay required for setting up the onnetion. In a

onnetionless network, no onnetions are set up prior to initiating a ommuniation.

This does notvoid theuse of aentralized shemein aonnetionlessenvironment, as

thereoveryFIBsortopologiesmaybeomputedataentralizedserveranddistributed

alonganyperiodiortriggeredrouteupdate.

danttrees. ThisispossibleiftheIPRTmethodisusedinonjuntionwithanLSrouting

protool. Insuhasheme,allroutersarerequiredtoomputethepairoftreesforevery

nodein thenetwork. This maybedoneassuming allnodeshaveasynhronizedviewof

theLSDB,theIPRTalgorithmisdeterministi,andusethesamesnapshotoftheLSDB

asinputto thealgorithm.

Itisalsopossibletoutilizeaombinationofdistributedandentralizedomputation

ifIPRTisusedinonjuntionwithaLSroutingprotool. Inthisapproaheahnodein

thenetworkisresponsibleforomputingthepairofredundanttreesofwhihtheyarethe

rootnode. Aswiththedistributed shemethisapproahrequireasynhronizedviewof

theLSDBatthetimethetreesareomputed. Theinformationmaythenbebroadasted

asapartoftheLSrouteupdatemessagesorasaseparatepakagewiththesamedelivery

andsend propertiesasanLSroute update message. Ifthismehanismisto beeetive

in response to a failure the redundant trees should be omputed and broadasted as a

partofthere-onvergeneproess. Thisisbeauseiftheomputationandsubsequently

thebroadastaredelayedtoafter there-onvergenehasnished,themehanismwould

bemorevulnerableto failuresomingin rapid suession. This approah isguaranteed

to work ifoneassumesallLSroute update messagesareguaranteeddeliveredand that

allroutersbroadastLSPs. Furthermore,ifarouterdoesnotdeliverapairofredundant

treesthe routermusthavefailed orbeendisonneted from thenetwork. Sinethere is

nowayorreahingafailed ordisonnetedrouterthereisnoneedforreoverypathsto

thisrouteranyway.

Theentralizedshememayresultinaninreasedamountoftrawhenomparedto

apuredistributedsheme. Thisisbeauseallthetreesneedtobebroadastedto every

routerinthenetworkwhereasadistributedshemeouldusetheLSDBwithoutaddingto

theamountroutingprotoolrelatedtra. Theatualamountofnetworktraneeded

isimplementationdependantbutitwouldneedtorepresent

2 ∗ n

topologies. Inaddition, theshemealsosuersfromthegeneraldrawbaksofhavingaentralizedresponsibility,

e.g. it introdues a single point of failure in the IPRT sheme. However, entralized

approahdoesnot requirethe individual routers to alulate the trees, and thus has a

loweromputationalostateahnode. Withthedistributedshemethesedrawbaksare

not present. This is beausethe only information needed to be broadasted is the LS

route update messages. However,thisapproahimposes ahigheromputationalost at

eah node as the tree pair of everynode needsto be omputed. If the ombination of

entralizedanddistributedomputationisused,theamountofgeneratednetworktra

isstill high. However,theomputationalostis reduedateahnode,and atthesame

timethisapproahdoesnotsuer fromthegeneraldrawbaksofaentralizedapproah.

Thusthedierentapproahesbeomeaquestionofavailableomputationalandnetwork

resoures.

ThisshowsthattheIPRTmethodmaybeexibleandresoureusagemaybeshifted

betweeneither network oromputational usage. However,sine theIPRTmethod is to

berealizedinasimulatorthisresoureusageisnotagoverningriteria. Theentralized

shemeseemsthebesthoiesineitallowstoutilizeaentralizedapproahthatomputes

When a failure ours, it is important that all the nodes in the network are able to

detetthepaketsaetedbythefailureandensurethatallpaketsareforwardedalong

the orret reovery path. In the framework for IP fast reovery, three methods are

presented as possible solutions to represent multi-hop reovery paths, and IPRT may

drawinspirationandoneptualdesignfromtheseapproahes. However,iftheseshemes

aretobeusedinonjuntionwithIPRTtheymayneedsomealteration.

Twomainmehanismsneedtobeaountedfor.

•

^{Tobeabletoforwardapaketalongamulti-hoppaththatdiersfromthedefault}

routesthere isneedforamehanismtorepresentthealternativereoverypathsin

eah router. This is ahieved by introduing an additional reovery FIB at eah

router. In abroad denition, the forward information base in a router is a data

struture that helps the router deide the next hop of a paket, thus the atual

data-strutureof thereoveryFIBsmaytakeonmanyforms.

•

^{The trees, and thus the dierent reovery paths, may be} distinguished by both the root-node and the olor of the tree. The signalingprovides the routers with

amehanismtoidentify areoveredpaket andthusforwardthepaket aording

to thesignaledredundanttreetopology. However,thepathrepresentationandthe

signalingmehanismsmaybeloselyrelatedtoeahotherandasinglesolutionmay

extendtooveralltheneededmehanism.

Path representation Signaling

Table5.1: Possiblepathsandsignalingmehanisms

Path representation

InIPfastrerouteframeworkthereareproposedseveralsolutionstoproperlyrepresentthe

paths. Oneapproahdesribedisto utilizesourerouting. Inthissheme,thereovery

FIBwouldontainaseries ofpre-omputedhains ofintermediateroutersthereovered

tra would needto traverse. Thus, the responsibility to forward thetra alongthe

orret path is assigned to the node initiating the reovery. By using soure routing,

theaetedtra mayfollowthereoverypathbythemeansofthenormalforwarding

proedure. I.e., no intermediate nodes may forward alltra aordingto the normal

routingtableaslongasnoloalfailuresarepresent. Eventhoughtheforward-proedure

doesnotneedtoknowthereoverypaket,thepaketneedstobesignaledasareovered

seondfailure.

Another approah to represent the reovery paths is to reate additional reovery

FIBswhere thestruture andrepresentationis equalto theFIBused in aonventional

IP network. This solution lets all the routers share the responsibility of forwarding a

paket aordingto the seleted reoveryroute. Thus the signaling will need to both

identifyareoveredpaket andwhat reoveryFIBtheintermediateroutersneedstouse

when forwarding said paket. Thus, this method extends the forwarding proedure at

eahrouter, asthe forwardingproedure mustdeide what FIBto useon aperpaket

basis.

Signaling

Aswiththepathrepresentation,thereareseveralapproahestosignaltheexisteneand

seleted path of areoveredpaket. One possibility is for the IPRT solution to utilize

anidentiationapproahsomewhatsimilartotheonefoundinnot-via addresses[24℄.

Bynot assigning speial addresses, but rather assign speial-subnets this sheme ould

fully represent the topologies of the redundant trees. The Internet Assigned Numbers

Authority(IANA) hasreservedalass AIP addressspae forprivate internets[29℄. By

utilizingaaddressingshemewherethedierentlassesofaddressspaesorrespondsto

thedierentuniqueidentiersofaredundanttree,e.g. theroot-node,theolorofthetree

andthenodesrepresentedinthetree,everyrelationofatreeismaintainedinthesignal.

This would allowtheIPRT shemeto identify allnodesin thenetworkand in addition

provideinformation on both theolor and the root of aredundant tree. Furthermore,

theaddressingshemewouldenabletherouterstobeabletoidentify areoveredpaket

based onthe addressof a paket. An exampleould be to letthe dierent root nodes

bemappedtodierentlassBsub-networks,andusethelassCsub-networktoidentify

the red and blue tree within eah lass B network. Furthermore, the higher eight bits

ouldorrespondtothehighereightbits ofallnodesin thenetwork. Thus anodewith

addressonformXXX.XXX.XXX.8 wouldbeassignedtwoaddressesof form10.8.[1,2℄.8

intheredandblue treewhereitwasrootnode. Thisshememayalsoaommodatefor

a lower granularity ifdesired and thus allow a restruturing of the addressassignment

to t aspei implementation. Theshemati of eah reoverytree addressis that a

reoveredpaketmustbedeliveredto thenodeaordingtothetopologyrepresentedin

thereoveryaddress.

Another approah is found in [18℄ thesolution isto markthe paket. This may be

done throughthe type of servie(ToS)eld, oftenused by diserv, of anIPv4 paket.

done throughthe type of servie(ToS)eld, oftenused by diserv, of anIPv4 paket.

In document IP redundant trees for preplanned recovery in connectioning networks (sider 33-0)