Introduction to Computer NetworksProfessor of Computer Science & Engineering ... CDNs (Content...
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David Wetherall ([email protected]) Professor of Computer Science & Engineering Introduction to Computer Networks CDNs (Content Delivery Networks) (§7.5.3)
Transcript of Introduction to Computer NetworksProfessor of Computer Science & Engineering ... CDNs (Content...
DavidWetherall([email protected])ProfessorofComputerScience&Engineering
IntroductiontoComputerNetworks
CDNs(ContentDeliveryNetworks)(§7.5.3)
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Topic• CDNs(ContentDeliveryNetworks)– Efficientdistributionofpopularcontent;fasterdeliveryforclients
ContentReplica
Consumers
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Context• Asthewebtookoffinthe90s,traffic
volumesgrewandgrew.This:1. Concentratedloadonpopularservers2. Ledtocongestednetworksandneed
toprovisionmorebandwidth3. Gaveapooruserexperience
• Idea:– Placepopularcontentnearclients– Helpswithallthreeissuesabove
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BeforeCDNs• Sendingcontentfromthesourceto4userstakes4x3=12“networkhops”intheexample
Source
User
User
...
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AfterCDNs• Sendingcontentviareplicastakesonly4+2=6“networkhops”
Source
User
User
...Replica
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AfterCDNs(2)• Benefitsassumingpopularcontent:– Reducesserver,networkload– Improvesuserexperience(PLT)
Source
User
User
...Replica
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PopularityofContent• Zipf’sLaw:fewpopularitems,manyunpopularones;bothmatter
Zipfpopularity(kthitemis1/k)
Rank Source:Wikipedia
GeorgeZipf(1902-1950)
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Howtoplacecontentnearclients?• Usebrowserandproxycaches– Helps,butlimitedtooneclientorclientsinoneorganization
• WanttoplacereplicasacrosstheInternetforusebyallnearbyclients– DonebycleveruseofDNS
ContentDeliveryNetwork
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ContentDeliveryNetwork(2)• DNSresolutionofsitegivesdifferentanswerstoclients– Telleachclientthesiteisthenearestreplica(mapclientIP)
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Consumersite
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BusinessModel• ClevermodelpioneeredbyAkamai– PlacingsitereplicaatanISPiswin-win– ImprovessiteexperienceandreducesbandwidthusageofISP
ISP
User
User
...Replica
Whatarecompaniesdoingwithnetworking?
QUIC• Why?
• Whatdoesitdo?
QUIC:Why?• ChangingTCPisapainandunlikelytohappen
• Ifyoucancontroltheclient/serverthenimplementyourownreliabilityoverUDP
QUIC:What?• GooglecontrolstheChromebrowser,Search/Youtubeserver
• QUICisatransportlayerprotocolimplementedoverUDP
QUIC:Whatdoesitdo?• lower-latencyconnectionestablishment
• improvedcongestioncontrolandbetterlossrecovery
Zero-latencyconnectionestablishment
Improvedcongestioncontrolandbetterlossrecovery
• PacketsequencenumbersareneverreusedàNoconfusion
• UseNACKtoexplicitlytellthesenderwhichpacketsareloss
• Retransmitlostpacketsfasteratthesender
Improvedcongestioncontrolandbetterlossrecovery
• QUICoutshinesTCPunderpoornetworkconditions,shavingafullsecondofftheGoogleSearchpageloadtimefortheslowest1%ofconnections
• Usersreport30%fewerrebufferswhenwatchingvideosoverQUIC.
• In2015,roughlyhalfofallrequestsfromChrometoGoogleserversareservedoverQUIC
