EE 122: Lecture 22 (Overlay Networks)MotivationsMotivations (cont’d)GoalsSolutionExamplesMotivations: IP Multicast ProblemsApproachNarada [Yang-hua et al, 2000]Narada (cont’d)DiscussionSlide 12MotivationSlide 14Resilient Overlay Networks [Anderson et al, 2001]Resilient Overlay Networks (cont’d)Slide 17Slide 18Slide 19Slide 20Slide 21“Base-line” SolutionReverse CachesForward ProxiesContent Distribution Networks (CDNs)CDNs (cont’d)Slide 27ConclusionsEE 122: Lecture 22(Overlay Networks)Ion StoicaNovember 27, [email protected] 2MotivationsChanges in the network happen very slowlyWhy?-Internet network is a shared infrastructure; need to achieve consensus (IETF)-Many of proposals require to change a large number of routers (e.g., IP Multicast, QoS); otherwise end-users won’t benefit Proposed changes that haven’t happened yet on large scale:-Congestion (RED ‘93); More Addresses (IPv6 ‘91)-Security (IPSEC ‘93); Multi-point (IP multicast ‘90)[email protected] 3Motivations (cont’d)One size does not fit allApplications need different levels of-Reliability-Performance (latency)-Security -Access control (e.g., who is allowed to join a multicast group)-…[email protected] 4GoalsMake it easy to deploy new functionalities in the network accelerate the pace of innovation Allow users to customize their [email protected] 5SolutionDeploy processing in the networkHave packets processed as they traverse the networkAS-1AS-1IPOverlay Network(over IP)[email protected] 6ExamplesOverlay multicastIncrease robustness and performanceContent Distribution Networks (CDNs)[email protected] 7Motivations: IP Multicast ProblemsScalability with number of groups-Routers need to maintain per-group state•Aggregation of multicast addresses is complicatedSupporting higher level functionality is difficult-IP Multicast: best-effort multi-point delivery service-Reliability and congestion control for IP Multicast complicated•Need to deal with heterogeneous receiver negotiation [email protected] 8ApproachProvide IP multicast functionality above the IP layer application level multicastChallenge: do this efficientlyProjects:-Narada-Overcast-Scattercast-Yoid-…[email protected] 9Narada [Yang-hua et al, 2000]Multi-source multicastInvolves only end hostsSmall group sizes <= hundreds of nodesTypical application: [email protected] 10Narada (cont’d)StanfordCMUStan1Stan2Berk2Overlay TreeGatechBerk1BerkeleyGatechStan1Stan2CMUBerk1Berk2istoica@cs.berkeley.edu 11Scalability (# of groups)-Routers do not maintain per-group state-End systems do, but they participate in very few groupsEasier to deployPotentially simplifies support for higher level functionality-Leverage computation and storage of end systems-For example, for buffering packets, transcoding, ACK aggregation-Leverage solutions for unicast congestion control and reliabilityScalability (# of receivers) still an open issue-Other solutions (e.g., Overcast) are scalable but not as flexible: typically assume single-source multicast [email protected] 12ExamplesOverlay multicastIncrease robustness and performanceContent Distribution Networks (CDNs)[email protected] 13MotivationRouting in the Internet is not optimal with respect to -Performance: packets do not necessary propagate along the shortest path-Robustness: two nodes may not be able to communicate although there is a path between them[email protected] 14SolutionControl routing at the application levelProjects-Resilient Overlay [email protected] 15Resilient Overlay Networks [Anderson et al, 2001]Make the end to end communication more robustEach node monitor the network conditions to every other node by periodically probing the networkIf node n1 cannot reach n2 directly, try to reach it through an intermediate node n3Intended application: robust communication in a small group (<= 50, 60 nodes)[email protected] 16Resilient Overlay Networks (cont’d)N1 can no longer communicate directly to [email protected] 17Resilient Overlay Networks (cont’d)Find a node N3 such that N1 can communicate with N3 and N3 with [email protected] 18DiscussionFind an alternate path in most cases when two nodes cannot communicate directly’Can be used to provide better delay and bandwidth than the direct IP route between two nodes Scalability still an open [email protected] 19ExamplesOverlay multicastIncrease robustness and performanceContent Distribution Networks (CDNs)[email protected] 20MotivationsToday’s Internet is not optimized for Web trafficMany clients transfer the same information (e.g., CNN front page, software downloads)Identical files are transferred over and over againIP multicast not a solution:-Users don’t access the same info at the same time-Users have widely different capabilities: •Communication: cable modem vs. dial up modem• Display: high-resolution workstation monitor vs. Palm Pilot•…[email protected] 21SolutionHave nodes inside the network that store and process the documents Examples: web caching, [email protected] 22“Base-line” SolutionMany clients transfer same information -Generate unnecessary server and network load-Clients experience unnecessary latencyServerClientsBackbone [email protected] 23Reverse CachesCache documents close to server decrease server loadTypically done by content providersClientsBackbone ISPISP-1ISP-2ServerReverse [email protected] 24Forward ProxiesCache documents close to clients reduce network traffic and decrease latencyTypically done by ISPs or corporate LANsClientsBackbone ISPISP-1ISP-2ServerReverse cachesForward [email protected] 25Content Distribution Networks (CDNs)Integrate forward and reverse caching functionalities into one overlay network (usually) administrated by one entity-Example: AkamaiDocuments are cached both -As a result of clients’ requests (pull)-Pushed in the expectation of a high access rateBeside caching do processing, e.g.,-Handle dynamic web [email protected] 26CDNs (cont’d)ClientsISP-1ServerForward cachesBackbone [email protected]
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