Operating Systems and Networks Network Lecture 12 ... · Operating Systems and Networks Network Lecture 12: Application Layer ... (2) •Application layer messages are often split

OperatingSystemsandNetworks

NetworkLecture12:ApplicationLayer

AdrianPerrigNetworkSecurityGroupETHZürich

2

WhereweareintheCourse• StartingtheApplicationLayer!

– Buildsdistributed“networkservices”(DNS,Web)onTransportservices

PhysicalLink

Application

NetworkTransport

3

Recall• Applicationlayerprotocolsareoftenpartofan“app”

– Butdon’tneedaGUI,e.g.,DNS

TCPIP

802.11

HTTPapp

OS

User-level

(NIC)

4

Recall(2)• Applicationlayermessagesareoftensplitovermultiplepackets– Ormaybeaggregatedinapacket…

802.11 IP TCP HTTP

802.11 IP TCP HTTP

802.11 IP TCP HTTP

HTTP

ApplicationCommunicationNeeds• Varywidelywithapp;mustbuildonTransportservices

5

UDP

DNS

TCP

Seriesofvariablelength,reliablerequest/replyexchanges

Web

UDP

Real-time(unreliable)

streamdelivery

Skype

SeeBook

Short,reliablerequest/replyexchanges

Messagereliability!

OSISession/PresentationLayers• Rememberthis?Tworelevantconcepts…

6

– Providesfunctionsneededbyusers– Convertsdifferentdatarepresentations– Multiplesessionsbetweensamesrc-dst– Providesend-to-enddelivery– Sendspacketsovermultiplelinks– Sendsframesofinformation– Sendsbitsassignals

Butconsiderpartoftheapplication,notstrictlylayered!

7

SessionConcept• Asessionisaseriesofrelatednetworkinteractionsinsupportofanapplicationtask– Ofteninformal,notexplicit

• Examples:– Webpagefetchesmultipleimages– Skypecallinvolvesaudio,video,chat

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PresentationConcept• Appsneedtoidentifythetypeofcontent,andencodeitfortransfer– ThesearePresentationfunctions

• Examples:– Media(MIME)types,e.g.,image/jpeg,identifythetypeofcontent

– Transferencodings,e.g.,gzip,identifycontentencoding– Applicationheadersareoftensimpleandreadableversuspackedforefficiency

9

Topics• EvolvingInternetapplications• DNS(DomainNameSystem)• HTTP(HyperText TransferProtocol)• Webproxiesandcaching• ContentDistributionNetworks• Peer-to-peer(BitTorrent)

• Real-timeapplications(VoIP)

Thislecture

SeeBook

EvolutionofInternetApplications• Alwayschanging,andgrowing…

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20101970 19901980 2000

Traffic

FileTransfer(FTP)Email(SMTP)

News(NTTP)

SecureShell(ssh)Telnet

Email

Web(HTTP)Web(CDNs)

P2P(BitTorrent)Web(Video)

???

EvolutionofInternetApplications(2)• ForapeekatthestateoftheInternet:

– Akamai’sStateoftheInternetReport(quarterly)– Cisco’sVisualNetworkingIndex– MaryMeeker’s InternetReport

• RobustInternetgrowth,esp.video,wirelessandmobile– Mosttrafficisvideo,willbe90%ofInternetinafewyears– Wirelesstrafficwillsoonovertakewiredtraffic– Mobiletrafficisstillasmallportion(15%)ofoverall– GrowingattacktrafficfromChina,alsoU.S.andRussia

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EvolutionoftheWeb

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Source:http://www.evolutionoftheweb.com,Vizzuality,Google,andHyperakt

EvolutionoftheWeb(2)

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Source:http://www.evolutionoftheweb.com,Vizzuality,Google,andHyperakt

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DomainNameSystem(DNS)(§7.1.1-7.1.3)

• TheDNS(DomainNameSystem)– Human-readablehostnames,andmore– Part1:thedistributednamespace

www.uw.edu?

Network

128.94.155.135

NamesandAddresses• Names:higher-level(user-understandable)resourceidentifiers• Addresses: lower-levelresourcelocators

– Multiplelevels,e.g.,fullnameà emailà IPaddressà Ethernetaddress• Resolution (orlookup):mappinganametoanaddress

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Directory

Name,e.g.“AndyTanenbaum,”or“flits.cs.vu.nl”

Address,e.g.“Vrije Universiteit,Amsterdam”

orIPv4“130.30.27.38”Lookup

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BeforetheDNS– HOSTS.TXT• DirectorywasafileHOSTS.TXTregularlyretrievedforallhostsfromacentralmachineattheNIC(NetworkInformationCenter)

• Nameswereinitiallyflat,becamehierarchical(e.g.,lcs.mit.edu)~1985

• NeithermanageablenorefficientastheARPANETgrew…

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DNS• AnamingservicetomapbetweenhostnamesandtheirIP

addresses(andmore)– www.uwa.edu.auà 130.95.128.140

• Goals– Easytomanage(especiallywithmultipleparties)– Efficient(goodperformance,fewresources)

• Approach– Distributeddirectorybasedonahierarchicalnamespace– Automatedprotocoltotiepiecestogether

DNSNamespace• Hierarchical,startingfrom“.”(dot,typicallyomitted)

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robot.cs.washington.edu

TLDs(Top-LevelDomains)• RunbyICANN(InternetCorp.forAssignedNamesandNumbers)

– Startingin‘98;namingisfinancial,political,andinternational

• 22+genericTLDs– Initially.com,.edu ,.gov.,.mil,.org,.net– Added.aero,.info,.museum,etc.from’01through.xxxin’11– DifferentTLDshavedifferentusagepolicies

• ~250countrycodeTLDs– Twoletters,e.g.,“.au”,plusinternationalcharacterssince2010– Widelycommercialized,e.g.,.tv (Tuvalu)– Manydomainhacks,e.g.,instagr.am(Armenia),goo.gl(Greenland)

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DNSZones• Azone isacontiguousportionofthenamespace

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AzoneDelegation

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DNSZones(2)• Zonesarethebasisfordistribution

– EDURegistraradministers.edu– UWadministerswashington.edu– CS&Eadministerscs.washington.edu

• Eachzonehasanameserver tocontactforinformationaboutit– Zonemustincludecontactsfordelegations,e.g.,.eduknowsnameserver forwashington.edu

DNSResourceRecords• AzoneiscomprisedofDNSresourcerecordsthatprovideinformationaboutitsdomainnames

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Type MeaningSOA Start of authority, has main zone parametersA IPv4 address of a hostAAAA (“quad A”) IPv6 address of a hostCNAME Canonical name for an aliasMX Mail exchanger for the domainNS Nameserver of domain or delegated subdomain

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DNSResourceRecords(2)

IPaddressesofcomputers

Nameserver

Mailgateways

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DNSResolution• DNSprotocolletsahostresolveanyhostname(domain)toIPaddress

• Ifunknown,canstartwiththerootnameserver andworkdownzones

• Let’sseeanexamplefirst…

DNSResolution(2)• flits.cs.vu.nlresolvesrobot.cs.washington.edu

25

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Iterativevs.RecursiveQueries• Recursivequery

– Nameserver completesresolutionandreturnsthefinalanswer– E.g.,flitsà localnameserver

• Iterativequery– Nameserver returnstheanswerorwhotocontactnextfortheanswer

– E.g.,localnameserverà allothers

27

Iterativevs.RecursiveQueries(2)• Recursivequery

– Letsserveroffloadclientburden(simpleresolver)formanageability

– Letsservercacheoverapoolofclientsforbetterperformance

• Iterativequery– Letsserver“fileandforget”– Easytobuildhighloadservers

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Caching• Resolutionlatencyshouldbelow

– Addsdelaytowebbrowsing• Cachequery/responsestoanswerfuturequeriesimmediately– Includingpartial(iterative)answers– ResponsescarryaTTLforcaching

Nameserver

query out

responseCache

Caching(2)• flits.cs.vu.nlnowresolveseng.washington.edu

– Andpreviousresolutionscutoutmostoftheprocess

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1:query 2:query

UWnameserver(forwashington.edu)

3:eng.washington.edu4:eng.washington.edu

Localnameserver(forcs.vu.nl)

Iknowtheserverforwashington.edu!

Cache

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LocalNameservers• Localnameservers typicallyrunbyIT(enterprise,ISP)

– ButmaybeyourhostorAP– Oralternativese.g.,GooglepublicDNS

• Clientsneedtobeabletocontacttheirlocalnameservers– TypicallyconfiguredviaDHCP

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RootNameservers• Root(dot)isservedby13servernames

– a.root-servers.nettom.root-servers.net– Allnameservers needrootIPaddresses– Handledviaconfigurationfile(named.ca)

• Thereare>250distributedserverinstances– Highlyreachable,reliableservice– MostserversarereachedbyIPanycast (Multiplelocationsadvertise

sameIP!Routestakeclienttotheclosestone.See§5.2.9)– ServersareIPv4andIPv6reachable

RootServerDeployment

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Source:http://www.root-servers.org.Snapshoton27.02.12.Doesnotrepresentcurrentdeployment.

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DNSProtocol• Queryandresponsemessages

– BuiltonUDPmessages,port53– ARQforreliability;serverisstateless!– Messageslinkedbya16-bitIDfield

Query

Response

Time

Client ServerID=0x1234

ID=0x1234

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DNSProtocol(2)• Servicereliabilityviareplicas

– Runmultiplenameservers fordomain– Returnthelist;clientsuseoneanswer– Helpsdistributeloadtoo

NSforuw.edu?

A

B

C

UseA,BorC

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DNSProtocol(3)• Securityisamajorissue

– Compromiseredirectstowrongsite!– Notpartofinitialprotocols..

• DNSSEC(DNSSecurityExtensions)– Longunderdevelopment,nowpartiallydeployed

Um,security??

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HTTP,theHyperText TransferProtocol(§7.3.1-7.3.4)

• HTTP,(HyperText TransferProtocol)– BasisforfetchingWebpages

requestNetwork

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SirTimBerners-Lee(1955–)• InventoroftheWeb

– DominantInternetappsincemid90s– HenowdirectstheW3C

• DevelopedWebatCERNin‘89– Browser,serverandfirstHTTP– PopularizedviaMosaic(‘93),Netscape– FirstWWWconferencein’94…

Source:ByPaulClarke,CC-BY-2.0,viaWikimediaCommons

WebContext

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HTTPrequest

HTTPresponse

PageasasetofrelatedHTTPtransactions

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WebProtocolContext• HTTPisarequest/responseprotocolforfetchingWebresources– RunsonTCP,typicallyport80– Partofbrowser/serverapp

TCPIP

802.11

browser

HTTPTCPIP

802.11

server

HTTPrequest

response

40

FetchingaWebpagewithHTTP• StartwiththepageURL:

http://de.wikipedia.org/wiki/Chuchichäschtli

• Steps:– ResolvetheservertoIPaddress(DNS)– SetupTCPconnectiontotheserver– SendHTTPrequestforthepage– (AwaitHTTPresponseforthepage)– Execute/fetchembeddedresources/render– CleanupanyidleTCPconnections

Protocol PageonserverServer

**

Staticvs DynamicWebpages• Staticwebpageiscontentofafile,e.g.,image• Dynamicwebpageistheresultofprogramexecution

– Javascript onclient,PHPonserver,orboth

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EvolutionofHTTP• Considersecurity(SSL/TLSforHTTPS)later

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20101990 20001995 2005

1.0developed

1.1developed(persistentconnections)

0.9RFC1945

RFC2068,2109RFC2616

CookiesSSL2.0

SPDY(HTTP2.0)

Proliferationofcontenttypes

andbrowser/server

scriptingtechnologies

RFC2965

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HTTPProtocol• Originallyasimpleprotocol,withmanyoptionsaddedover

time– Text-basedcommands,headers

• Tryityourself:– Asa“browser”fetchingaURL– Run“telnetwww.scion-architecture.net 80”– Type“GET/HTTP/1.1”followedby“Host:www.scion-

architecture.net”followedbyablankline– ServerwillreturnHTTPresponsewiththepagecontents(orother

info)

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HTTPGetResult$telnetwww.scion-architecture.net 80Trying129.132.85.42...Connectedtoscion-architecture.net.Escapecharacteris'^]'.GET/HTTP/1.1Host:www.scion-architecture.net

HTTP/1.1200OKDate:Wed,01Jun201621:04:24GMTServer:Apache/2.2.15(RedHat)X-Powered-By:PHP/5.3.3Transfer-Encoding:chunkedContent-Type:text/html;charset=UTF-8

3565

<!DOCTYPEHTML>

<html>...

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HTTPProtocol(2)• Commandsusedintherequest

Method DescriptionGET ReadaWebpageHEAD ReadaWebpage'sheaderPOST AppendtoaWebpagePUT StoreaWebpageDELETE RemovetheWebpageTRACE EchotheincomingrequestCONNECT ConnectthroughaproxyOPTIONS Queryoptionsforapage

FetchpageUploaddata

HTTPProtocol(3)• Codesreturnedwiththeresponse

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Code Meaning Examples1xx Information 100=serveragreestohandleclient'srequest2xx Success 200=requestsucceeded;204=nocontentpresent3xx Redirection 301=pagemoved;304=cachedpagestillvalid4xx Clienterror 403=forbiddenpage;404=pagenotfound5xx Servererror 500=internalservererror;503=tryagainlater

Yes!

HTTPProtocol(4)• Manyheaderfieldsspecifycapabilitiesandcontent

– E.g.,Content-Type:text/html,Cookie:lect=12-1-http

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Function ExampleHeadersBrowsercapabilities(clientà server)

User-Agent, Accept,Accept-Charset,Accept-Encoding,Accept-Language

Cachingrelated(mixeddirections)

If-Modified-Since, If-None-Match,Date,Last-Modified,Expires,Cache-Control,ETag

Browsercontext(clientà server) Cookie,Referer,Authorization,Host

Contentdelivery(serverà client)

Content-Encoding, Content-Length,Content-Type,Content-Language,Content-Range,Set-Cookie

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HTTPPerformance (§7.3.4)• PerformanceofHTTP

– Parallelandpersistentconnections

requestNetwork

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PLT(PageLoadTime)• PLTisthekeymeasureofwebperformance

– Fromclickuntiluserseespage– SmallincreasesinPLTdecreasesales

• PLTdependsonmanyfactors– Structureofpage/content– HTTP(andTCP!)protocol– NetworkRTTandbandwidth

50

EarlyPerformance(1)• HTTP/1.0usesoneTCPconnectiontofetcheachwebresource– MadeHTTPveryeasytobuild– ButgavefairlypoorPLT…

51

EarlyPerformance(2)• ManyreasonswhyPLTislargerthan

necessary– Sequentialrequest/responses,evenwhen

todifferentservers– MultipleTCPconnectionsetupstothesame

server– MultipleTCPslow-startphases

• Networkisnotusedeffectively– Worsewithmanysmallresources/page

52

WaystoDecreasePLT1. Reducecontentsizefortransfer

– Smallerimages,gzip2. ChangeHTTPtomakebetteruseofavailable

bandwidth(e.g.,avoidTCPslowstart)3. ChangeHTTPtoavoidrepeatedtransfersofthe

samecontent– Caching,andproxies

4. Movecontentclosertoclient– CDNs[later]

53

ParallelConnections• OnesimplewaytoreducePLT

– Browserrunsmultiple(8,say)HTTPinstancesinparallel– Serverisunchanged;alreadyhandlesconcurrentrequestsformanyclients

• Howdoesthishelp?– SingleHTTPwasn’tusingnetworkmuch…– Soparallelconnectionsaren’tslowedmuch– Pullsincompletiontimeoflastfetch

54

PersistentConnections• Parallelconnectionscompetewitheachotherfornetworkresources– 1parallelclient≈8sequentialclients?– Exacerbatesnetworkbursts,andloss

• Persistentconnectionalternative– Make1TCPconnectionto1server– UseitformultipleHTTPrequests

PersistentConnections(2)

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Onerequestperconnection

Sequentialrequestsperconnection

Pipelinedrequestsperconnection

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PersistentConnections(3)• WidelyusedaspartofHTTP/1.1

– Supportsoptionalpipelining– PLTbenefitsdependingonpagestructure,buteasyonnetwork

• Issueswithpersistentconnections– HowlongtokeepTCPconnection?– Canitbeslower?(Yes.Butwhy?)

57

HTTPCachingandProxies (§7.3.4,§7.5.2)

• HTTPcachingandproxies– Enablingcontentreuse

ServerClients

ProxyCache

58

WebCaching• Usersoftenrevisitwebpages

– Bigwinfromreusinglocalcopy!– Thisiscaching

• Keyquestion:– WhenisitOKtoreuselocalcopy?

NetworkCache

Localcopies

Server

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WebCaching(2)• Locallydetermineifcopyisstillvalid

– Basedonexpiryinformationsuchas “Expires”headerfromserver

– Oruseaheuristictoguess(cacheable,freshlyvalid,notmodifiedrecently)

– Contentisthenavailablerightaway

NetworkCacheServer

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WebCaching(3)• Revalidatecopywithremoteserver

– Basedontimestampofcopysuchas“Last-Modified”headerfromserver

– Orbasedoncontentsuchas“ETag”headerfromserver:EntityTag,computedbyserverasauniqueobjectidentifier

– Contentisavailableafter1RTT

NetworkCacheServer

WebCaching(4)• Puttingthepiecestogether:

61

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WebProxies• Placeintermediarybetweenpoolofclientsandexternalwebservers

• Proxycaching– Clientsbenefitfromlarger,sharedcache(otherclientsmayhavealreadyaccessedcontent)

– Benefitslimitedbysecure/dynamicper-clientcontent,aswellas“longtail”dataaccesspattern

– Enablesapplicationoforganizationalaccesspolicies

WebProxies(2)• Clientscontactproxy;proxycontactsserver

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Cache

NearclientFarfromclient

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CDNs (ContentDeliveryNetworks)(§7.5.3)

• CDNs(ContentDeliveryNetworks)– Efficientdistributionofpopularcontent;fasterdeliveryforclients

ContentReplica

Consumers

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Context• Asthewebtookoffinthe90s,trafficvolumesgrewandgrew.This:1. Concentratedloadonpopularservers2. Ledtocongestednetworksandneedtoprovisionmore

bandwidth3. Gaveapooruserexperience

• Idea:– Placepopularcontentnearclients– Helpswithallthreeissuesabove

66

BeforeCDNs• Sendingcontentfromthesourceto4userstakes4x3=12“networkhops”intheexample

Source

User

User

...

67

AfterCDNs• Sendingcontentviareplicastakesonly4+2=6“networkhops”

Source

User

User

...Replica

68

AfterCDNs(2)• Benefitsassumingpopularcontent:

– Reducesserver,networkload– Improvesuserexperience(PLT)

Source

User

User

...Replica

69

PopularityofContent• Zipf’s Law:fewpopularitems,manyunpopularones(“heavytail”ofprobabilitydistribution);bothmatter

Zipf popularity(kth itemis1/k)

RankSource:Wikipedia

GeorgeZipf (1902-1950)

70

Howtoplacecontentnearclients?• Usebrowserandproxycaches

– Helps,butlimitedtooneclientorclientsinoneorganization

• WanttoplacereplicasacrosstheInternetforusebyallnearbyclients– DonebycleveruseofDNS

ContentDeliveryNetwork

71

ContentDeliveryNetwork(2)• DNSresolutionofsitegivesanswerdependingonclient

– Directeachclienttothenearestreplica(usingIPgeolocation)

72

Consumer

site

73

BusinessModel• ClevermodelpioneeredbyAkamai

– PlacingsitereplicaatanISPiswin-win– ImprovessiteexperienceandreducesbandwidthusageofISP

ISPUser

User

...Replica

74

TheFutureofHTTP• TheFutureofHTTP

– Howwillwemakethewebfaster?– Abrieflookatsomeapproaches

requestNetwork

ModernWebPages• Waterfalldiagramshowsprogressionofpageload

75

webpagetest toolforhttp://coursera.org(Firefox,5/1Mbps,fromVA,3/1/13)

76

ModernWebPages(2)

Yikes!-23requests-1Mbdata-2.6secs

webpagetest toolforhttp://coursera.org(Firefox,5/1Mbps,fromVA,3/1/13)

• WaterfallandPLTdependsonmanyfactors– Verydifferentfordifferentbrowsers– Verydifferentforrepeatpageviews– Dependsonlocalcomputationaswellasnetwork

ModernWebPages(3)

77

Yay!(Networkusedwell)

78

RecentworktoreducePLTPagesgrowevermorecomplex!

– Larger,moredynamic,andsecure– HowwillwereducePLT?

1. Betteruseofthenetwork– HTTP/2effortbasedonSPDY

2. Bettercontentstructures– mod_pagespeed serverextension

79

SPDY(“speedy”)• AsetofHTTPimprovements

– Multiplexed(parallel)HTTPrequestsononeTCPconnection– Clientprioritiesforparallelrequests– CompressedHTTPheaders– Serverpushofresources

• Nowbeingtestedandimproved– DefaultinChrome,Firefox– BasisforHTTP/2

80

mod_pagespeed• Observation:

– Thewaypagesarewrittenaffectshowquicklytheyload– Manybooksonbestpracticesforpageauthorsanddevelopers

• Keyidea:– Haveserverre-write(compile)pagestohelpthemloadquickly!

– Apachemod_pagespeed isanexample

81

mod_pagespeed (2)• Apacheserverextension

– Softwareinstalledwithwebserver– Rewritespages“onthefly”withrulesbasedonbestpractices

• Examplerewriterules:– MinifyJavascript– Flattenmulti-levelCSSfiles– Resizeimagesforclient– …andmuchmore(100sofspecificrules)

82

Peer-to-PeerContentDelivery(BitTorrent)(§7.5.4)

• Peer-to-peercontentdelivery– Runswithoutdedicatedinfrastructure– BitTorrent asanexample

Peer

Peer

Peer

PeerPeer

83

Context• Deliverywithclient/serverCDNs:

– Efficient,scalesupforpopularcontent– Reliable,managedforgoodservice

• …butsomedisadvantagestoo:– Needfordedicatedinfrastructure– Centralizedcontrol/oversight

84

P2P(Peer-to-Peer)• Goalisdeliverywithout dedicatedinfrastructureorcentralizedcontrol– Stillefficientatscale,andreliable

• Keyideaistohaveparticipants(orpeers)helpeachother– InitiallyNapster‘99formusic(gone)– NowBitTorrent ‘01onwards(popular!)

85

P2PChallenges• Noserversonwhichtorelyon

– Communicationmustbepeer-to-peer andself-organizing,notclient-server

– Leadstoseveralissuesatscale…

Peer

Peer

Peer

PeerPeer

86

P2PChallenges(2)1. Limitedcapabilities

– Howcanonepeerdelivercontenttoallotherpeers?

2. Participationincentives– Whywouldpeershelpeachother?

3. Decentralization– Howwillpeersfindcontent?

87

OvercomingLimitedCapabilities• Peercansendcontenttoallotherpeersusingadistributiontree– Typicallydonewithreplicasovertime– Self-scalingcapacity(morenodesàmorecapacity)

Source

88

ProvidingParticipationIncentives• Peersplaytworoles:

– Download()tohelpthemselves,andupload()tohelpothers

Source

89

ProvidingParticipationIncentives(2)• Couplethetworoles:

– I’lluploadforyouifyouuploadforme– Encouragescooperation

Source

90

EnablingDecentralization• Peermustlearnwheretogetcontent

– UseDHTs (DistributedHashTables)

• DHTsarefully-decentralized,efficientalgorithmsforadistributedindex– Indexisspreadacrossallpeers– Indexlistspeerstocontactforcontent– Anypeercanlookuptheindex– Startedasacademicworkin2001

91

BitTorrent• MainP2Psysteminusetoday

– DevelopedbyCohenin‘01– Veryrapidgrowth,largetransfers– BigfractionofInternettraffic– Usedforlegalandcopyrightedcontent

• Deliversdatausing“torrents”:– Transfersfilesinpiecesforparallelism– Notablefortreatmentofincentives– Trackerordecentralizedindex(DHT)

ByJacobAppelbaum,CC-BY-SA-2.0,fromWikimediaCommons

BramCohen(1975—)

92

BitTorrent Protocol• Stepstodownloadatorrent:

1. Startwithtorrentdescription2. Contacttrackertojoinandgetlistofpeers(withatleast

seedpeer)2. Or,useDHTindexforpeers3. Tradepieceswithdifferentpeers4. Favorpeersthatuploadtoyourapidly;“choke”peers

thatdon’tbyslowingyouruploadtothem

BitTorrent Protocol(2)• Allpeers(exceptseed)retrievetorrentatthesametime

93

BitTorrent Protocol(3)• Dividingfileintopiecesgivesparallelismforspeed

94

BitTorrent Protocol(4)• Chokingunhelpfulpeersencouragesparticipation

95

STOPSTOP

STOP

XXX

BitTorrent Protocol(5)• DHTindex(spreadoverpeers)isfullydecentralized

96

DHT

DHT

DHTDHT

DHT

DHT

DHT

DHT

97

P2POutlook• AlternativetoCDN-styleclient-servercontentdistribution– Withpotentialadvantages

• P2PandDHTtechnologiesfindingmorewidespreaduseovertime– E.g.,partofskype,Amazoncloudcomputing– Expecthybridsystemsinthefuture