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Overcast: Reliable Multicasting with an Overlay Network CS294 Paul Burstein [email protected] 9/15/2003 Outline Goals & Motivation Network Overview Protocols Evaluation Discussion Paul Burstein: Ovarcast, 9/15/2003 2 Motivation Offering bandwidth-intensive content on demand primarily video content Long-running content availability for multiple clients Paul Burstein: Ovarcast, 9/15/2003 3 Goals Maximize Bandwidth Limit repeated usage of physical links No change to existing routers Easy deployment 4 Outline Goals & Motivation Network Overview Protocols Evaluation Discussion Paul Burstein: Ovarcast, 9/15/2003 5 Design Overlay network runs on top of existing infrastructure Central source Distribution Trees Responsive to transient failures and congestion Paul Burstein: Ovarcast, 9/15/2003 6 Why Overlay? Paul Burstein: Ovarcast, 9/15/2003 7 Why Overlay? Pros Incrementally Deployable Adaptable Robust Customizable Standard Paul Burstein: Ovarcast, 9/15/2003 8 Why Overlay? Pros Incrementally Deployable Adaptable Robust Customizable Standard Cons Management “The real world” firewalls, proxies… Inefficiency Information Loss Paul Burstein: Ovarcast, 9/15/2003 9 Why Overlay? Pros Incrementally Deployable Adaptable Robust Customizable Standard Cons Management “The real world” firewalls, proxies… Inefficiency Information Loss Paul Burstein: Ovarcast, 9/15/2003 10 Single-Source Multicast Simplicity Optimization only for one path Extendable to multi-source a clear point of interaction single source forwarding Address Space vs. IP multicast Paul Burstein: Ovarcast, 9/15/2003 11 Deployment & Usage Deployed on unmodified Web browsers via HTTP Final Consumers – HTTP clients HTTP URLs define Overcasts groups Hostname – root Path – network group Paul Burstein: Ovarcast, 9/15/2003 12 Example Video and live stream distribution Studio Appliances The source of content Organize into distribution tree Clients Studio requests get redirected to appliances Paul Burstein: Ovarcast, 9/15/2003 13 Outline Goals & Motivation Network Overview Protocols Evaluation Discussion Paul Burstein: Ovarcast, 9/15/2003 14 Tree Building Protocol Build a deep tree without sacrificing the bandwidth to the root Choose nodes based on bandwidth to root Secondary criteria: proximity (network hops) Dynamic Adaptation vs. Static Configuration Paul Burstein: Ovarcast, 9/15/2003 15 Up/Down Protocol (1/2) Handles joins and departures Periodic status propagation from children to parent nodes “Death Certificates” children that missed report time “Birth Certificates” nodes joining the reporting node Paul Burstein: Ovarcast, 9/15/2003 16 Up/Down Protocol (2/2) Up/Down Race condition Death certificate of a moved node conflicting with its new Birth certificate Associate a sequence number for the number of parent changes Optimization Propagation of certificates for known nodes is unnecessary Paul Burstein: Ovarcast, 9/15/2003 17 Root Replication (1/2) Root Single point of failure Handles join requests Solution 1 Replicate the root Good for joins which are read only Bad for up/down protocol – changing state Paul Burstein: Ovarcast, 9/15/2003 18 Root Replication (2/2) Solution 2 Linearly configured backup nodes Good: consistent through up/down updates Bad: increased latency due to longer initial path Skip extra nodes during distribution Paul Burstein: Ovarcast, 9/15/2003 19 Joining an Group An HTTP request contacts the root and the root selects a server to serve the contents to the client. The selection algorithm is not discussed Paul Burstein: Ovarcast, 9/15/2003 20 Multicasting Data goes down the tree with logs recording the data received A failed node rejoins the tree with up/down protocol and gets the data from the new parent’s log Where’s the reliability? Paul Burstein: Ovarcast, 9/15/2003 21 Outline Goals & Motivation Network Overview Protocols Evaluation Discussion Paul Burstein: Ovarcast, 9/15/2003 22 Evaluation Based on simulations with GT-ITM Five 600-node graphs Bandwidth Averages 3 transit domains (backbone) 8 stub networks per domain 25 nodes per stub 45Mbps, 1.5Mbps, 100Mbps T3, T1, Fast Ethernet One node supports 20 clients (MPEG-1 video) Paul Burstein: Ovarcast, 9/15/2003 23 Bandwidth Utilization Backbone Adds transit nodes first Random All nodes chosen randomly Fraction = Overcast bandwidth/Optimal bandwidth At full participation – distribution trees are different Paul Burstein: Ovarcast, 9/15/2003 24 Tree Convergence Round period Reevaluation period finding new parent Lease period time to get a stable position parent waiting for child’s status Assumption: stable underlying network Paul Burstein: Ovarcast, 9/15/2003 25 Up/Down Protocol (1/2) Simulating node additions topology reconfiguration every parent change results in certificate Certificates Scale Depends on the number of new nodes, not the network size Paul Burstein: Ovarcast, 9/15/2003 26 Up/Down Protocol (2/2) Node Failure handles large networks well scales to number of failures Abnormalities caused by failures near the root and long propagations Paul Burstein: Ovarcast, 9/15/2003 27 Outline Goals & Motivation Network Overview Protocols Evaluation Discussion Paul Burstein: Ovarcast, 9/15/2003 28 What’s the point Adding and using more secondary storage is easier than increasing network bandwidth Is this multicasting or data replication? Paul Burstein: Ovarcast, 9/15/2003 29