NaminginNetworking

JenniferRexford

COS316GuestLecture

Names

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TypeofName ExampleUniform Resource Locator http://www.cs.princeton.edu/

~jrex/foo.htmlE-mail jrex@cs.princeton.eduHostname www.cs.princeton.eduInternet Protocol 128.112.7.156Media Access Control 00:15:C5:49:04:A9

Today’s lecture focuses on the last three!

InternetProtocolLayers

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Best-effort local packet delivery

Best-effort global packet delivery

Reliable streams

Applications

Messages

Link

Network

Transport

Application Hostname

IP address

MAC address

InternetProtocolStack

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HTTP

TCP

IP

Ethernetinterface

HTTP

TCP

IP

Ethernetinterface

IP IP

Ethernetinterface

Ethernetinterface

Opticalinterface

Opticalinterface

host host

router router

HTTP message

TCP segment

IP packet IP packetIP packet

Ethernet frame Ethernet frame

What’sinaName?• Humanreadable?– Ifendusersinteractwiththenames

• Fixedlength?– Ifnamesmustbeprocessedathighspeed

• Largenamespace?– Ifmanynodesneeduniquenames

• Hierarchicalnames?– Ifthesystemisverylargeand/orfederated

• Self-certifying?– Ifpreventing“spoofing”isimportant 5

DifferentLayers,DifferentNames

• Hostname (e.g.,www.cs.princeton.edu)– Mnemonic,variable-length,appreciatedbyhumans– Hierarchical,basedonorganizations

• IPaddress (e.g.,128.112.7.156)– Numerical32-bitaddressappreciatedbyrouters– Hierarchical,basedonorganizationsandtopology

• MACaddress (e.g.,00:15:C5:49:04:A9)– Numerical48-bitaddressappreciated byadapters– Non-hierarchical,unrelatedtonetworktopology

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HierarchicalAllocationProcesses

• Hostname: www.cs.princeton.edu– Domain:registrarforeachtop-leveldomain(e.g.,.edu)– Hostname:localadministratorassignstoeachhost

• IPaddresses: 128.112.7.156– Prefixes:ICANN,regionalInternetregistries,andISPs– Hosts:staticconfiguration,ordynamicusingDHCP

• MACaddresses: 00:15:C5:49:04:A9– Blocks:assignedtoequipmentvendorsbytheIEEE– Adapters:assignedbythevendorfromitsblock

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HostNamesvs.IPAddresses

• Namesareeasier(forus!)toremember– www.cnn.comvs.64.236.16.20

• IPaddressescanchangeunderneath– E.g.,renumberingwhenchangingproviders

• NamecouldmaptomultipleIPaddresses– www.cnn.comtomultiplereplicasoftheWebsite

• Maptodifferentaddressesindifferentplaces– E.g.,toreducelatency,orreturndifferentcontent

• Multiplenamesforthesameaddress– E.g.,aliaseslikeee.mit.eduandcs.mit.edu

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IPvs.MACAddresses

• LANsdesignedforarbitrarynetworkprotocols– NotjustforIP(e.g.,IPX,Appletalk,X.25,…)– DifferentLANsmayhavedifferentaddressschemes

• Ahostmaymovetoanewlocation– So,cannotsimplyassignastaticIPaddress– Instead,mustreconfiguretheadapter

• Mustidentifytheadapterduringbootstrap– NeedtotalktotheadaptertoassignitanIPaddress

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Hostname,IP,andMACHostname IP Address MAC Address

Example www.cs.princeton.edu 128.112.7.156 00:15:C5:49:04:A9

Size Hierarchical, human readable, variable length

Hierarchical, machine readable, 32 bits (in IPv4)

Flat, machine readable, 48 bits

Read by Humans, hosts Internet routers LAN switches

Allocation, top-level

Domain name assigned by registrar (e.g., for .edu)

Variable-length prefixes, assigned by ICANN, RIR, or ISP

Fixed-sized blocks, assigned by IEEE to vendors (e.g., Dell)

Allocation, low-level

Host name assigned by local administrator

Interface, by DHCPor local administrator

Interface, by equipment vendor

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Directory:TranslateNametoAddress

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linksession

path

name

address

Directory

• Akey-valuestore– Key:name,value:address(es)– Answerqueries:givenname,returnaddress(es)

• Cachingtheresponse– Reusetheresponse,foraperiodoftime– Betterperformanceandloweroverhead

• Allowentriestochange– Updatingtheaddress(es)associatedwithaname– Invalidatingorexpiringcachedresponses

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DirectoryDesign:ThreeExtremes• Floodthequery(e.g.,ARP)– Thenamednoderespondswithitsownaddress– But,highoverheadinlargenetworks

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AddressResolutionProtocol(ARP)

• EveryhostinaLANmaintainsanARPtable– (IPaddress,MACaddress)pair

• Consultthetablewhensendingapacket–MapdestinationIPaddresstodest MACaddress– TransmittheIPpacketwithinanEthernetframe

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1.2.3.4 1.2.3.19

00:15:C5:49:04:A9 78:9A:B5:23:5D:98

LocalAreaNetwork

AddressResolutionProtocol(ARP)

• But,whatifthekeyisnotinthetable?– Senderbroadcast:“WhohasIPaddress1.2.3.19?”– Receiveranswer:“MACaddress78:9A:B5:23:5D:98”– SendercachestheresultinitslocalARPcache

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1.2.3.4 1.2.3.19

00:15:C5:49:04:A9 78:9A:B5:23:5D:98

LocalAreaNetwork

AddressResolutionProtocol(ARP)

• ManagingtheARPcache– Storingallkey-valuepairsintroducesoverhead– Entriesbecomestale(e.g.,IPassignedtonewhost)– Removeanentryifnotusedforsomeperiodoftime

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1.2.3.4 1.2.3.19

00:15:C5:49:04:A9 78:9A:B5:23:5D:98

LocalAreaNetwork

DirectoryDesign:ThreeExtremes• Floodthequery(e.g.,ARP)– Thenamednoderespondswithitsaddress– But,highoverheadinlargenetworks

• Pushdatatoallnodes(e.g.,/etc/hosts)– Allnodesstoreafullcopyofthedirectory– But,highoverheadformanynamesandupdates

• Centraldirectoryserver– Alldataandquerieshandledbyonenode– But,poorperformance,scalability,andreliability

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DistributedDirectoryDesign

• Hierarchicaldirectory(e.g.,DNS)– Followthehierarchyofthenamespace– Distributethedirectory,distributethequeries– Enabledecentralizedupdatestothedirectory

• DistributedHashTable(e.g.,P2Papplications)– Directoryasahashtablewithflatnames– Eachdirectorynodehandlesrangeofhashoutputs– Usehashtodirectquerytothedirectorynode

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DomainNameSystem(DNS)

• PropertiesofDNS– Hierarchicalnamespacedividedintozones– DistributedoveracollectionofDNSservers

• HierarchyofDNSservers– Rootservers– Top-leveldomain(TLD)servers– AuthoritativeDNSservers

• Performingthetranslations– LocalDNSserversandclientresolvers

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DistributedHierarchicalDatabase

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com edu org ac uk zw arpa

unnamed root

bar

west east

foo my

ac

cam

usr

in-addr

12

34

56

generic domains country domains

my.east.bar.edu usr.cam.ac.uk

12.34.56.0/24

DNSRootServers

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• 13rootservers(seehttp://www.root-servers.org/)• LabeledAthroughM

B USC-ISI Marina del Rey, CAL ICANN Los Angeles, CA

E NASA Mt View, CAF Internet Software C. PaloAlto, CA (and 17 other locations)

I Autonomica, Stockholm (plus 3 other locations)

K RIPE London (also Amsterdam, Frankfurt)

m WIDE Tokyo

A Verisign, Dulles, VAC Cogent, Herndon, VA (also Los Angeles)D U Maryland College Park, MDG US DoD Vienna, VAH ARL Aberdeen, MDJ Verisign, ( 11 locations)

TLDandAuthoritativeDNSServers

• GlobalTop-leveldomain(gTLD)servers– Genericdomains(e.g.,.com,.org,.edu)– Countrydomains(e.g.,.uk,.fr,.ca,.jp)– Managedprofessionally(e.g.,Verisignfor.com.net)

• AuthoritativeDNSservers– Providepublicrecordsforhostsatanorganization– Fortheorganization’sservers(e.g.,Webandmail)– Canbemaintainedlocallyorbyaserviceprovider

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UsingDNS

• LocalDNSserver(“defaultnameserver”)– Usuallyneartheendhostswhouseit– Localhostsconfiguredwithlocalserver(e.g.,/etc/resolv.conf)orlearntheserverviaDHCP

• Clientapplication– Extractservername(e.g.,fromtheURL)– Dogethostbyname() orgetaddrinfo()togetaddress

• Serverapplication– ExtractclientIPaddressfromsocket– Optionalgethostbyaddr() totranslateintoname

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DNSQueries

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Hostatcis.poly.edu wantsIPaddressforgaia.cs.umass.edu

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

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TLD DNS server

Recursive query: #1Iterative queries: #2, 4, 6

Recursivevs.IterativeQueries

• Recursivequery– Askservertogetanswerforyou

– E.g.,request1andresponse8

• Iterativequery– Askserverwhotoasknext

– E.g.,allotherrequest-responsepairs

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requesting hostcis.poly.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

DNSCaching• DNSquerylatency

– E.g.,1seclatencybeforestartingadownload

• Cachingtoreduceoverheadanddelay– Small#oftop-levelservers,thatchangerarely

– Popularsitesvisitedoften

• Wheretocache?– LocalDNSserver– Browser

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requesting hostcis.poly.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

DNSCacheConsistency

• Cacheconsistency– Ensuringcacheddataisuptodate

• DNSdesignconsiderations– Cacheddatais“readonly”– Explicitinvalidationwouldbeexpensive

• Avoidingstaleinformation– Responsesincludea“timetolive”(TTL)field– DeletethecachedentryafterTTLexpires

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SettingtheTimeToLive(TTL)

• TTLtrade-offs– SmallTTL:fastresponsetochange– LargeTTL:highercachehitrate

• Followingthehierarchy– Topofthehierarchy:daysorweeks– Bottomofthehierarchy:secondstohours

• Tensioninpractice– SetlowTTLsforloadbalancingandfailover– Browserscachefor15-60seconds

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NegativeCaching

• Brokendomainnamesareslowtoresolve–Misspellingslikewww.cnn.comm andwww.cnnn.com

– Thesecantakealongtimetofailthefirsttime• Rememberthingsthatdon’twork– Goodtorememberthattheydon’twork– …sothefailuretakeslesstimeinthefuture

• Butdon’trememberfortoolong– Useatime-to-livetoexpire

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DNSReliability

• DNSserversarereplicated– Nameserviceavailableifatleastone replicaisup– Queriescanbeloadbalancedbetweenreplicas

• Retransmissionoflostqueries– Noresponsetoaquery?Tryagain!

• Tryalternateserversontimeout– Exponentialback-offwhenretryingsameserver

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Conclusions

• Networknames– Toidentifyremoteend-points– Readability?Format?Length?Hierarchy?– Hostnames,IPaddresses,andMACaddresses

• Networkdirectories– Key-valuestorestomapnametoaddress– Flooding(ARP),localcopy,centralserver– Hierarchical(DNS)ornon-hierarchical(DHT)

• Moreonprotocollayersinafewweeks!

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