RFC1519CIDR.docx

RFC1519CIDR.docx

《RFC1519CIDR.docx》由会员分享，可在线阅读，更多相关《RFC1519CIDR.docx（33页珍藏版）》请在冰豆网上搜索。

RFC1519CIDR

datatracker.ietf.org

SignIn

ClasslessInter-DomainRouting（CIDR）:

anAddressAssignmentandAggregationStrategy

RFC1519

ClasslessInter-DomainRouting（CIDR）:

anAddressAssignmentandAggregationStrategy

StatusofthisMemo

ThisRFCspecifiesanInternetstandardstrackprotocolforthe

Internetcommunity,andrequestsdiscussionandsuggestionsfor

improvements.Pleaserefertothecurrenteditionofthe"Internet

OfficialProtocolStandards"forthestandardizationstateandstatus

ofthisprotocol.Distributionofthismemoisunlimited.

Abstract

Thismemodiscussesstrategiesforaddressassignmentoftheexisting

IPaddressspacewithaviewtoconservetheaddressspaceandstem

theexplosivegrowthofroutingtablesindefault-route-freerouters.

TableofContents

Acknowledgements.................................................2

1.Problem,Goal,andMotivation................................2

2.CIDRaddressallocation......................................3

2.1Aggregationanditslimitations.............................3

2.2Distributednetworknumberallocation.......................5

3.Cost-benefitanalysis........................................6

3.1Presentallocationfigures..................................7

3.2Historicgrowthrates.......................................8

3.3Detailedanalysis...........................................8

3.3.1Benefitsofnewaddressingplan...........................9

3.3.2Growthrateprojections...................................9

4.Changestointer-domainroutingprotocolsandpractices......11

4.1Protocol-independentsemanticchanges.......................11

4.2Rulesforrouteadvertisement...............................11

4.3Howtheruleswork..........................................13

4.4Responsibilityforandconfigurationofaggregation.........14

4.5Intra-domainprotocolconsiderations........................15

5.Exampleofnewallocationandrouting........................15

Fuller,Li,Yu&Varadhan[Page1]

RFC1519CIDRAddressStrategySeptember1993

5.1Addressallocation..........................................15

5.2Routingadvertisements......................................17

6.ExtendingCIDRtoclassAaddresses..........................18

7.DomainNamingServiceconsiderations.........................20

7.1Proceduralchangesforclass-C"supernets"...................20

7.2Proceduralchangesforclass-Asubnetting....................21

8.Transitioningtoalongtermsolution........................22

9.Conclusions..................................................22

10.Recommendations.............................................22

11.References..................................................23

12.SecurityConsiderations.....................................23

13.Authors'Addresses..........................................24

Acknowledgements

Theauthorswishtoexpresstheirappreciationtothemembersofthe

ROADgroupwithwhommanyoftheideascontainedinthisdocument

wereinspiredanddeveloped.

1.Problem,Goal,andMotivation

AstheInternethasevolvedandgrownoverinrecentyears,ithas

becomeevidentthatitissoontofaceseveralseriousscaling

problems.Theseinclude:

1.ExhaustionoftheclassBnetworkaddressspace.One

fundamentalcauseofthisproblemisthelackofanetwork

classofasizewhichisappropriateformid-sized

organization;classC,withamaximumof254host

addresses,istoosmall,whileclassB,whichallowsupto

65534addresses,istoolargeformostorganizations.

2.GrowthofroutingtablesinInternetroutersbeyondthe

abilityofcurrentsoftware,hardware,andpeopleto

effectivelymanage.

3.Eventualexhaustionofthe32-bitIPaddressspace.

Ithasbecomeclearthatthefirsttwooftheseproblemsarelikely

tobecomecriticalwithinthenextonetothreeyears.Thismemo

attemptstodealwiththeseproblemsbyproposingamechanismtoslow

thegrowthoftheroutingtableandtheneedforallocatingnewIP

networknumbers.Itdoesnotattempttosolvethethirdproblem,

whichisofamorelong-termnature,butinsteadendeavorstoease

enoughoftheshorttomid-termdifficultiestoallowtheInternetto

continuetofunctionefficientlywhileprogressismadeonalonger-

termsolution.

Fuller,Li,Yu&Varadhan[Page2]

RFC1519CIDRAddressStrategySeptember1993

TheproposedsolutionistotopologicallyallocatefutureIPaddress

assignment,byallocatingsegmentsoftheIPaddressspacetothe

transitroutingdomains.

ThisplanforallocatingIPaddressesshouldbeundertakenassoonas

possible.Webelievethatthiswillsufficeasashortterm

strategy,tofillthegapbetweennowandthetimewhenaviablelong

termplancanbeputintoplaceanddeployedeffectively.Thisplan

shouldbeviableforatleastthree（3）years,afterwhichtime,

deploymentofasuitablelongtermsolutionisexpectedtooccur.

Thisplanisprimarilydirectedatthefirsttwoproblemslisted

above.Webelievethatthejudicioususeofvariable-length

subnettingtechniquesshouldhelpdefertheonsetofthelastproblem

problem,theexhaustionofthe32-bitaddressspace.Notealsothat

improvedtoolsforperformingaddressallocationina"supernetted"

andvariably-subnettedworldwouldgreatlyhelptheusercommunityin

acceptingthesesometimesconfusingtechniques.Effortstocreate

somesimpletoolsforthispurposeshouldbeencouragedbythe

Internetcommunity.

Notethatthisplanneitherrequiresnorassumesthatalready

assignedaddresseswillbereassigned,thoughifdoingsowere

possible,itwouldfurtherreduceroutingtablesizes.Itisassumed

thatroutingtechnologywillbecapableofdealingwiththecurrent

routingtablesizeandwithsomereasonablysmallrateofgrowth.

Theemphasisofthisplanisonsignificantlyslowingtherateof

thisgrowth.

Notethatthisplandoesnotrequiredomainstorenumberifthey

changetheirattachedtransitroutingdomain.Domainsareencouraged

torenumbersothattheirindividualaddressallocationsdonotneed

tobeadvertised.

Thisplanwillnotaffectthedeploymentofanyspecificlongterm

plan,andtherefore,thisdocumentwillnotdiscussanylongterm

plansforroutingandaddressarchitectures.

2.CIDRaddressallocation

Therearetwobasiccomponentsofthisaddressingandroutingplan:

one,todistributetheallocationofInternetaddressspaceandtwo,

toprovideamechanismfortheaggregationofroutinginformation.

2.1Aggregationanditslimitations

OnemajorgoalofthisaddressingplanistoallocateInternet

addressspaceinsuchamannerastoallowaggregationofrouting

Fuller,Li,Yu&Varadhan[Page3]

RFC1519CIDRAddressStrategySeptember1993

informationalongtopologicallines.Forsimple,single-homed

clients,theallocationoftheiraddressspaceoutofatransit

routingdomain'sspacewillaccomplishthisautomatically-rather

thanadvertiseaseparaterouteforeachsuchclient,thetransit

domainmayadvertiseasingleaggregateroutewhichdescribesallof

thedestinationsconnectedtoit.Unfortunately,notallsitesare

singly-connectedtothenetwork,sosomelossofabilitytoaggregate

isrealizedforthenon-trivialcases.

Therearetwosituationsthatcausealossofaggregationefficiency.

oOrganizationswhicharemulti-homed.Becausemulti-homed

organizationsmustbeadvertisedintothesystembyeachof

theirserviceproviders,itisoftennotfeasibleto

aggregatetheirroutinginformationintotheaddressspace

anyoneofthoseproviders.Notethattheystillmayreceive

theiraddressallocationoutofatransitdomain'saddress

space（whichhasotheradvantages）,buttheirrouting

informationmuststillbeexplicitlyadvertisedbymostof

theirserviceproviders（theexceptionbeingthatifthe

site'sallocationcomesoutofitsleast-preferableservice

provider,thenthatserviceproviderneednotadvertisethe

explicitroute-longest-matchwillinsurethatits

aggregatedrouteisusedtogettothesiteonabackup

basis）.Forthisreason,theroutingcostforthese

organizationswilltypicallybeaboutthesameasitis

today.

oOrganizationswhichchangeserviceproviderbutdonot

renumber.Thishastheeffectof"punchingahole"inthe

aggregationoftheoriginalserviceprovider'sadvertisement.

Thisplanwillhandlethesituationbyrequiringthenewer

serviceprovidertoadvertiseaspecificadvertisementfor

thenewclient,whichispreferredbyvirtueofbeingthe

longestmatch.Tomaintainefficiencyofaggregation,itis

recommendedthatorganizationswhichdochangeservice

providersplantoeventuallymigratetheiraddress

assignmentsfromtheoldprovider'sspacetothatofthenew

provider.Tothisend,itisrecommendedthatmechanismsto

facilitatesuchmigration,includingimprovedprotocolsand

proceduresfordynamichostaddressassignment,bedeveloped.

Notethatsomeaggregationefficiencygaincanstillbehadfor

multi-homedsites（and,ingeneral,foranysitecomposedof

multiple,logicalIPnetworknumbers）-byallocatingacontiguous

power-of-twoblockofnetworknumberstotheclient（asopposedto

multiple,independentnetworknumbers）theclient'srouting

informationmaybeaggregatedintoasingle（net,mask）pair.Also,

Fuller,Li,Yu&Varadhan[Page4]

RFC1519CIDRAddressStrategySeptember1993

sincetheroutingcostassociatedwithassigningamulti-homedsite

outofaserviceprovider'saddressspaceisnogreaterthanthe

currentmethodofa