Overlay Networks Reading: 9.4Goals of Today’s LectureTwo-Tiered Routing SystemEnd-to-End Paths are a CompositionDelivering Packets in Stub NetworksInterdomain Routing With BGPIntradomain RoutingRouting Policy Constrains PathsSingle-Path Routing is RestrictiveRouting Doesn’t Consider PerformanceAll Traffic Follows the Same PathsDisruptions During ConvergenceDoes IP Routing Really Stink?Overlay NetworksSlide 15IP Tunneling to Build Overlay LinksTunnels Between End HostsSlide 18Circumventing Policy RestrictionsAdapting to Network ConditionsCustomizing to ApplicationsRON: Resilient Overlay NetworksHow Does RON Work?How Does Ron Work?RON Works in PracticeRON Limited to Small DeploymentsShould All This Bother ISPs?Using Overlays to Evolve the Internet6Bone: Deploying IPv6 over IP4Secure Communication Over Insecure LinksCommunicating With Mobile UsersIP MulticastMBone: Multicast BackboneMulticast TodayDiscussionConclusions1Overlay NetworksReading: 9.4COS 461: Computer NetworksSpring 2007 (MW 1:30-2:50 in Friend 004)Jennifer RexfordTeaching Assistant: Ioannis Avramopoulos http://www.cs.princeton.edu/courses/archive/spring07/cos461/2Goals of Today’s Lecture•Limitations of IP routing and forwarding–Same paths used for all kinds of traffic–Routing protocols are oblivious to performance–Different ASes have different objectives–Routing changes lead to transient disruptions•Motivations for overlay networks–Customized routing and forwarding solutions–Incremental deployment of new protocols•Example overlay networks–Robust routing (e.g., Resilient Overlay Networks)–6Bone, Mbone, security, mobility3Two-Tiered Routing SystemIntradomain InterdomainObjectives Efficiency, performance, robustnessBusiness relationshipsScale Tens to hundreds of routersTens of thousands of ASesTrust All routers run by the same entityASes run by different entitiesProtocols Metric-based (e.g., OSPF)Policy-based (e.g., BGP)4End-to-End Paths are a Composition•Between the end hosts and the Internet•Interdomain AS path across multiple ASes•Intradomain path inside each transit ASes1234567Client Web server5Delivering Packets in Stub Networks•Inside the stub network–End hosts–Gateway routersgatewayroutercampusInternetGateway router picks from a small set of paths…6Interdomain Routing With BGP•Each AS picks a “best path” to the destination•Among the choices advertised by its neighbors•Based on each ASes’ local policy objectives1234567Client Web server7Intradomain Routing•Routers compute shortest paths•Based on configurable link weights•Operators set weights to satisfy network goals32211315348Routing Policy Constrains Paths•Paths that violate policy cannot be used•Some failures may disconnect hostsAT&TUSLECPUPUYaleAT&TSprintLevel39Single-Path Routing is Restrictive•BGP routers pick a single best path•Shortest-path protocols use only shortest pathsAT&TPU322113153410Routing Doesn’t Consider Performance•Routing protocols do not react to load–Routing based on routing policies or link weights–Static configuration that changes infrequently•Routers have limited visibility –Routers cannot see the topology in other ASes–Routers do not keep state about performance•Network operators weigh many objectives–Minimizing cost or maximizing revenue –Balancing load in the network–Not just the end-to-end performance11All Traffic Follows the Same Paths•IP does destination-based forwarding–All traffic follows the same paths–Independent of the application requirements•Yet, applications have different needs–Voice and gaming: low latency and loss–File sharing: high bandwidthlow latency,but low throughputHigh throughput,but high latency12Disruptions During Convergence•Changes to the network are disruptive–Topology changes, due to failures and recovery–Configuration changes, e.g., tweak link weights•Routers have to reach agreement again–Detect the change in the network–Propagate new information among themselves•In the meantime, performance suffers–Blackholes: packets dropped on the floor–Loops: packets spin around in a loop–Delays: packets take a circuitous path13Does IP Routing Really Stink?•Some improvements would help–Multi-path routing–Adaptation to changes in load–Faster routing convergence•But, IP routing is solving a hard problem–Decentralized control with common protocols–Different, sometime competing, objectives–Large scale (200,000 prefixes and 20,000 ASes)•IP routing does an okay job for everyone–Rather than an optimal job for anyone–And leaves everything else to the end hosts…14Overlay Networks15Overlay NetworksFocus at the application level16IP Tunneling to Build Overlay Links•IP tunnel is a virtual point-to-point link–Illusion of a direct link between two separated nodes•Encapsulation of the packet inside an IP datagram–Node B sends a packet to node E–… containing another packet as the payloadABEFtunnelLogical view:Physical view:ABEF17Tunnels Between End HostsACBSrc: ADest: BSrc: ADest: BSrc: ADest: CSrc: ADest: BSrc: CDest: B18Overlay Networks•A logical network built on top of a physical network–Overlay links are tunnels through the underlying network•Many logical networks may coexist at once–Over the same underlying network–And providing its own particular service•Nodes are often end hosts–Acting as intermediate nodes that forward traffic–Providing a service, such as access to files•Who controls the nodes providing service?–The party providing the service –Distributed collection of end users19Circumventing Policy Restrictions•IP routing depends on AS routing policies–But hosts may pick paths that circumvent policiesAT&TPUPatriotISPmeMy buddy’s computer20Adapting to Network Conditions•Start experiencing bad performance–Then, start forwarding through intermediate hostACB21Customizing to Applications•VoIP traffic: low-latency path•File sharing: high-bandwidth pathACBvoicefile sharing22RON: Resilient Overlay NetworksPremise: by building application overlay network, can increase performance and reliability of routingTwo-hop (application-level)Berkeley-to-Princeton routeapplication-layer routerPrincetonYaleBerkeleyhttp://nms.csail.mit.edu/ron/23How Does RON Work?•Keeping it small to avoid scaling problems–A few friends who want better service–Just for their communication with each other–E.g., VoIP, gaming, collaborative work, etc.•Send probes
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