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Vivaldi: A Decentralized Network Coordinate System. F. Dabak, R. Cox, F. Kaashoek, R. Morris MIT. Outline. Introduction Vivaldi Algorithm Evaluation Coordinate Model Selection Conclusions. Outline. Introduction Vivaldi Algorithm Evaluation Coordinate Model Selection Conclusions. - PowerPoint PPT Presentation
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Vivaldi: A Decentralized Network Coordinate System
F. Dabak, R. Cox,
F. Kaashoek, R. Morris
MIT
Outline
• Introduction
• Vivaldi Algorithm
• Evaluation
• Coordinate Model Selection
• Conclusions
Outline
• Introduction
• Vivaldi Algorithm
• Evaluation
• Coordinate Model Selection
• Conclusions
Motivation
• Large-scale Internet applications can benefit from an ability to predict round-trip times to other hosts without having to contact them first.
Design Goal
• Finding a metric space that embeds the Internet with little error
• Scaling to a large number of hosts
• Decentralizing the implementation
• Minimizing probe traffic
• Adapting to changing network conditions
Contribution of the Paper
• A decentralized, low overhead, adaptive synthetic coordinate system that computes coordinates which predict Internet latencies with low error– Vivaldi is used by the Chord P2P lookup syste
m
• Introduces the notion of a directionless height that improves the prediction accuracy
Outline
• Introduction
• Vivaldi Algorithm
• Evaluation
• Coordinate Model Selection
• Conclusions
Prediction Error
• Let Lij be the actual RTT between nodes i and j, and xi be the coordinates assigned to node i.
• The errors in the coordinates can be characterized using a squared-error function:
The goal is to make this error small.
The simple Vivaldi algorithm
Called for each new RTT measurement
timestep
An Adaptive Timestep
• The rate of convergence is governed by the δ timestep– A small δ causes slow convergence– A large δ causes oscillation
• Vivaldi varies δ depending on how certain the node is about its coordinates
Each node compares each new measured RTT sample with the predicted RTT, and maintains local error
The Vivaldi Algorithm
Outline
• Introduction
• Vivaldi Algorithm
• Evaluation
• Coordinate Model Selection
• Conclusions
Evaluation Environment
• The experiments are conducted using a packet-level network simulator running with RTT data collected from the Internet.– PlanetLab data set: 192 hosts on the PlanetLab
network testbed– King data set: 1740 Internet DNS servers
Evaluation: Convergence
Constant δ
Adaptive δ
Slow convergence
Oscillates
Adaptive δ leads lower error than constant δ
Evaluation: Robustness
The evolution of a stable 200-node network after 200 new nodes join.
Using the constant δ, the initial structure of the system has been destroyed, a result of placing to much faith in young high-error nodes.
Using the adaptive δ preserves the established order.
Evaluation: Communication Patterns
When nodes only contact their neighbors, coordinates at the large scale is not accurate.
The effect of long-distance communication
Even when only 5 % of the samples involve distant nodes, skewed coordinate placements will be avoided.
Evaluation: Adaptation
Increase longer links
Converges after 20 sec.
Go back to shorter links
Performance Comparison
Smallnetwork
Largenetwork
Relative error of Vivaldi is close to that of GNP which requires landmarks.
Outline
• Introduction
• Vivaldi Algorithm
• Evaluation
• Coordinate Model Selection
• Conclusions
Model Selection
• Vivaldi works with any coordinate system that supports the magnitude, addition, and subtraction operations
• We consider a few possible coordinate spaces that might better capture the Internet’s underlying structure
Euclidean Spaces
Increasing dimension decreases error but increases overhead.
Smallnetwork
Largenetwork
Spherical Coordinates
Small network Large network
2D coordinates is better.
Height Vectors
• A height vector consists of a Euclidean coordinate augmented with a height
• The Euclidean portion models a high-speed Internet core with latencies proportional to geographic distance, while the height models the time it takes packets to travel the access link from the node to the core (e.g. queuing delay).
Height Vector Performance
Height vectors perform better than both 2D and 3D Euclidean coordinates.
Conclusions
• Proposed a decentralized, low overhead, adaptive synthetic coordinate system that computes coordinates which predict Internet latencies with low error
• Introduced the notion of a directionless height that improves the prediction accuracy
