Overcast: Reliable Multicasting with an Overlay Network

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Overcast Reliable Multicasting with an Overlay
Network

• CS294
• Paul Burstein
• burst_at_cs.berkeley.edu
• 9/15/2003

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Outline

• Goals Motivation
• Network Overview
• Protocols
• Evaluation
• Discussion

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Motivation

• Offering bandwidth-intensive content on demand
• primarily video content
• Long-running content availability for multiple
  clients

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Goals

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

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Outline

• Goals Motivation
• Network Overview
• Protocols
• Evaluation
• Discussion

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Design

• Overlay network
• runs on top of existing infrastructure
• Central source
• Distribution Trees
• Responsive to transient failures and congestion

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Why Overlay?
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Why Overlay?

• Pros
• Incrementally Deployable
• Adaptable
• Robust
• Customizable
• Standard

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Why Overlay?

• Pros
• Incrementally Deployable
• Adaptable
• Robust
• Customizable
• Standard

• Cons
• Management
• The real world
• firewalls, proxies
• Inefficiency
• Information Loss

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Why Overlay?

• Pros
• Incrementally Deployable
• Adaptable
• Robust
• Customizable
• Standard

• Cons
• Management
• The real world
• firewalls, proxies
• Inefficiency
• Information Loss

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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

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Deployment Usage

• Deployed on unmodified Web browsers via HTTP
• Final Consumers HTTP clients
• HTTP URLs define Overcasts groups
• Hostname root
• Path network group

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Example

• Video and live stream distribution
• Studio
• The source of content
• Appliances
• Organize into distribution tree
• Clients
• Studio requests get redirected to appliances

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Outline

• Goals Motivation
• Network Overview
• Protocols
• Evaluation
• Discussion

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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

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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

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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

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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

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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

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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

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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 parents
  log
• Wheres the reliability?

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Outline

• Goals Motivation
• Network Overview
• Protocols
• Evaluation
• Discussion

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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)

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Bandwidth Utilization

• Backbone
• Adds transit nodes first
• Random
• All nodes chosen randomly
• Fraction Overcast bandwidth/Optimal bandwidth
• At full participation distribution trees are
  different

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Tree Convergence

• Round period
• time to get a stable position
• Reevaluation period
• finding new parent
• Lease period
• parent waiting for childs status
• Assumption stable underlying network

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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

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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

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Outline

• Goals Motivation
• Network Overview
• Protocols
• Evaluation
• Discussion

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Whats the point

• Adding and using more secondary storage is easier
  than increasing network bandwidth
• Is this multicasting or data replication?