Overcast: Reliable Multicasting with an Overlay Network

CS294Paul Burstein

burst@cs.berkeley.edu9/15/2003

Paul Burstein: Ovarcast, 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

Paul Burstein: Ovarcast, 9/15/2003

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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

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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 parent’s log

Where’s 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

child’s 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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What’s the point

Adding and using more secondary storage is easier than increasing network bandwidth

Is this multicasting or data replication?