EE 122 Lecture 22 Overlay Networks Ion Stoica November 27 2001 Motivations Changes in the network happen very slowly Why 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 istoica cs berkeley edu 2 Motivations cont d One size does not fit all Applications need different levels of Reliability Performance latency Security Access control e g who is allowed to join a multicast group istoica cs berkeley edu 3 Goals Make it easy to deploy new functionalities in the network accelerate the pace of innovation Allow users to customize their service istoica cs berkeley edu 4 Solution Deploy processing in the network Have packets processed as they traverse the network AS 1 AS 1 istoica cs berkeley edu IP Overlay Network over IP 5 Examples Overlay multicast Increase robustness and performance Content Distribution Networks CDNs istoica cs berkeley edu 6 Motivations IP Multicast Problems Scalability with number of groups Routers need to maintain per group state Aggregation of multicast addresses is complicated Supporting 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 hard istoica cs berkeley edu 7 Approach Provide IP multicast functionality above the IP layer application level multicast Challenge do this efficiently Projects Narada Overcast Scattercast Yoid istoica cs berkeley edu 8 Narada Yang hua et al 2000 Multi source multicast Involves only end hosts Small group sizes hundreds of nodes Typical application chat istoica cs berkeley edu 9 Narada cont d Gatech Stanford Stan1 Stan2 CMU Berk1 Berk2 Berkeley Overlay Tree Gatech Stan1 Stan2 CMU Berk1 istoica cs berkeley edu Berk2 10 Discussion Scalability of groups Routers do not maintain per group state End systems do but they participate in very few groups Easier to deploy Potentially 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 reliability Scalability of receivers still an open issue Other solutions e g Overcast are scalable but not as flexible typically assume single source multicast trees istoica cs berkeley edu 11 Examples Overlay multicast Increase robustness and performance Content Distribution Networks CDNs istoica cs berkeley edu 12 Motivation Routing 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 Why istoica cs berkeley edu 13 Solution Control routing at the application level Projects Resilient Overlay Networks Detour istoica cs berkeley edu 14 Resilient Overlay Networks Anderson et al 2001 Make the end to end communication more robust Each node monitor the network conditions to every other node by periodically probing the network If node n1 cannot reach n2 directly try to reach it through an intermediate node n3 Intended application robust communication in a small group 50 60 nodes istoica cs berkeley edu 15 Resilient Overlay Networks cont d N1 can no longer communicate directly to N2 N2 N1 istoica cs berkeley edu 16 Resilient Overlay Networks cont d Find a node N3 such that N1 can communicate with N3 and N3 with N2 N2 N1 istoica cs berkeley edu 17 Discussion Find 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 issue istoica cs berkeley edu 18 Examples Overlay multicast Increase robustness and performance Content Distribution Networks CDNs istoica cs berkeley edu 19 Motivations Today s Internet is not optimized for Web traffic Many clients transfer the same information e g CNN front page software downloads Identical files are transferred over and over again IP 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 istoica cs berkeley edu 20 Solution Have nodes inside the network that store and process the documents Examples web caching transcoding istoica cs berkeley edu 21 Base line Solution Many clients transfer same information Generate unnecessary server and network load Clients experience unnecessary latency Server Backbone ISP ISP 1 ISP 2 Clients istoica cs berkeley edu 22 Reverse Caches Cache documents close to server decrease server load Typically done by content providers Server Reverse caches Backbone ISP ISP 1 ISP 2 Clients istoica cs berkeley edu 23 Forward Proxies Cache documents close to clients reduce network traffic and decrease latency Typically done by ISPs or corporate LANs Server Reverse caches Backbone ISP ISP 1 ISP 2 Forward caches Clients istoica cs berkeley edu 24 Content Distribution Networks CDNs Integrate forward and reverse caching functionalities into one overlay network usually administrated by one entity Example Akamai Documents are cached both As a result of clients requests pull Pushed in the expectation of a high access rate Beside caching do processing e g Handle dynamic web pages Transcoding istoica cs berkeley edu 25 CDNs cont d Server CDN Backbone ISP ISP 1 ISP 2 Forward caches Clients istoica cs berkeley edu 26 Discussion CDNs were developed to efficiently handle today s web traffic Relive server and network load Perform load balancing caching Increase client performance Process data according to clients needs A basic technique that makes CDNs possible is redirection see HTTP How istoica cs berkeley edu 27 Conclusions Overlay networks allow to deploy new services in the network today e g Multicast CDNs Can increase network robustness and client perceived performance e g RON CDNs Challenges Efficiency A packet may need to be processed above transport layer before reaching the destination Path followed by the packet might be worse than the direct IP route Scalability istoica cs berkeley edu 28
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