Overcast: Reliable Multicasting with an Overlay Network CS294 Paul Burstein 9/15/2003

Paul Burstein: Ovarcast, 9/15/20032 Outline Goals & Motivation Network Overview Protocols Evaluation Discussion

Paul Burstein: Ovarcast, 9/15/20033 Motivation Offering bandwidth-intensive content on demand primarily video content Long-running content availability for multiple clients

4 Goals Maximize Bandwidth Limit repeated usage of physical links No change to existing routers Easy deployment

Paul Burstein: Ovarcast, 9/15/20035 Outline Goals & Motivation Network Overview Protocols Evaluation Discussion

Paul Burstein: Ovarcast, 9/15/20036 Design Overlay network runs on top of existing infrastructure Central source Distribution Trees Responsive to transient failures and congestion

Paul Burstein: Ovarcast, 9/15/20037 Why Overlay?

Paul Burstein: Ovarcast, 9/15/20038 Why Overlay? Pros Incrementally Deployable Adaptable Robust Customizable Standard

Paul Burstein: Ovarcast, 9/15/20039 Why Overlay? Pros Incrementally Deployable Adaptable Robust Customizable Standard Cons Management “The real world” firewalls, proxies… Inefficiency Information Loss

Paul Burstein: Ovarcast, 9/15/ Why Overlay? Pros Incrementally Deployable Adaptable Robust Customizable Standard Cons Management “The real world” firewalls, proxies… Inefficiency Information Loss

Paul Burstein: Ovarcast, 9/15/ Single-Source Multicast Simplicity a clear point of interaction Optimization only for one path Extendable to multi-source single source forwarding Address Space vs. IP multicast

Paul Burstein: Ovarcast, 9/15/ 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/ Example Video and live stream distribution Studio The source of content Appliances Organize into distribution tree Clients Studio requests get redirected to appliances

Paul Burstein: Ovarcast, 9/15/ Outline Goals & Motivation Network Overview Protocols Evaluation Discussion

Paul Burstein: Ovarcast, 9/15/ 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/ 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/ 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/ 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/ 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/ 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/ 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/ Outline Goals & Motivation Network Overview Protocols Evaluation Discussion

Paul Burstein: Ovarcast, 9/15/ Evaluation Based on simulations with GT-ITM Five 600-node graphs 3 transit domains (backbone) 8 stub networks per domain 25 nodes per stub Bandwidth Averages 45Mbps, 1.5Mbps, 100Mbps T3, T1, Fast Ethernet One node supports 20 clients (MPEG-1 video)

Paul Burstein: Ovarcast, 9/15/ 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/ Tree Convergence Round period time to get a stable position Reevaluation period finding new parent Lease period parent waiting for child’s status Assumption: stable underlying network

Paul Burstein: Ovarcast, 9/15/ 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/ 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/ Outline Goals & Motivation Network Overview Protocols Evaluation Discussion

Paul Burstein: Ovarcast, 9/15/ What’s the point Adding and using more secondary storage is easier than increasing network bandwidth Is this multicasting or data replication?