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