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

8

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…

10

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

11

EvolutionoftheWeb

12

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

EvolutionoftheWeb(2)

13

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

14

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

15

Directory

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

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

orIPv4“130.30.27.38”Lookup

16

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

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

• NeithermanageablenorefficientastheARPANETgrew…

17

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)

18

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)

19

DNSZones• Azone isacontiguousportionofthenamespace

20

AzoneDelegation

21

DNSZones(2)• Zonesarethebasisfordistribution

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

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

DNSResourceRecords• AzoneiscomprisedofDNSresourcerecordsthatprovideinformationaboutitsdomainnames

22

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

23

DNSResourceRecords(2)

IPaddressesofcomputers

Nameserver

Mailgateways

24

DNSResolution• DNSprotocolletsahostresolveanyhostname(domain)toIPaddress

• Ifunknown,canstartwiththerootnameserver andworkdownzones

• Let’sseeanexamplefirst…

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

25

26

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

28

Caching• Resolutionlatencyshouldbelow

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

Nameserver

query out

responseCache

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

– Andpreviousresolutionscutoutmostoftheprocess

29

1:query 2:query

UWnameserver(forwashington.edu)

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

Localnameserver(forcs.vu.nl)

Iknowtheserverforwashington.edu!

Cache

30

LocalNameservers• Localnameservers typicallyrunbyIT(enterprise,ISP)

– ButmaybeyourhostorAP– Oralternativese.g.,GooglepublicDNS

• Clientsneedtobeabletocontacttheirlocalnameservers– TypicallyconfiguredviaDHCP

31

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

32

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

33

DNSProtocol• Queryandresponsemessages

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

Query

Response

Time

Client ServerID=0x1234

ID=0x1234

34

DNSProtocol(2)• Servicereliabilityviareplicas

– Runmultiplenameservers fordomain– Returnthelist;clientsuseoneanswer– Helpsdistributeloadtoo

NSforuw.edu?

A

B

C

UseA,BorC

35

DNSProtocol(3)• Securityisamajorissue

– Compromiseredirectstowrongsite!– Notpartofinitialprotocols..

• DNSSEC(DNSSecurityExtensions)– Longunderdevelopment,nowpartiallydeployed

Um,security??

36

HTTP,theHyperText TransferProtocol(§7.3.1-7.3.4)

• HTTP,(HyperText TransferProtocol)– BasisforfetchingWebpages

requestNetwork

37

SirTimBerners-Lee(1955–)• InventoroftheWeb

– DominantInternetappsincemid90s– HenowdirectstheW3C

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

Source:ByPaulClarke,CC-BY-2.0,viaWikimediaCommons

WebContext

38

HTTPrequest

HTTPresponse

PageasasetofrelatedHTTPtransactions

39

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

41

EvolutionofHTTP• Considersecurity(SSL/TLSforHTTPS)later

42

20101990 20001995 2005

1.0developed

1.1developed(persistentconnections)

0.9RFC1945

RFC2068,2109RFC2616

CookiesSSL2.0

SPDY(HTTP2.0)

Proliferationofcontenttypes

andbrowser/server

scriptingtechnologies

RFC2965

43

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)

44

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>...

45

HTTPProtocol(2)• Commandsusedintherequest

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

FetchpageUploaddata

HTTPProtocol(3)• Codesreturnedwiththeresponse

46

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

47

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

48

HTTPPerformance (§7.3.4)• PerformanceofHTTP

– Parallelandpersistentconnections

requestNetwork

49

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)

55

Onerequestperconnection

Sequentialrequestsperconnection

Pipelinedrequestsperconnection

56

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

59

WebCaching(2)• Locallydetermineifcopyisstillvalid

– Basedonexpiryinformationsuchas “Expires”headerfromserver

– Oruseaheuristictoguess(cacheable,freshlyvalid,notmodifiedrecently)

– Contentisthenavailablerightaway

NetworkCacheServer

60

WebCaching(3)• Revalidatecopywithremoteserver

– Basedontimestampofcopysuchas“Last-Modified”headerfromserver

– Orbasedoncontentsuchas“ETag”headerfromserver:EntityTag,computedbyserverasauniqueobjectidentifier

– Contentisavailableafter1RTT

NetworkCacheServer

WebCaching(4)• Puttingthepiecestogether:

61

62

WebProxies• Placeintermediarybetweenpoolofclientsandexternalwebservers

• Proxycaching– Clientsbenefitfromlarger,sharedcache(otherclientsmayhavealreadyaccessedcontent)

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

– Enablesapplicationoforganizationalaccesspolicies

WebProxies(2)• Clientscontactproxy;proxycontactsserver

63

Cache

NearclientFarfromclient

64

CDNs (ContentDeliveryNetworks)(§7.5.3)

• CDNs(ContentDeliveryNetworks)– Efficientdistributionofpopularcontent;fasterdeliveryforclients

ContentReplica

Consumers

65

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