Effect of links on DHT routing Effect of links on DHT routing algorithmsalgorithms

Zou FuTaiZou FuTai

zoufutai@cs.sjtu.edu.cnzoufutai@cs.sjtu.edu.cn

Internet Computing Lab Internet Computing Lab

12/5/200312/5/2003

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OutlineOutline

1.What is DHT?1.What is DHT? 2.Motivation and goals2.Motivation and goals 3.Links in current DHT routing algorithms3.Links in current DHT routing algorithms 4.The effect of links on routing performance4.The effect of links on routing performance 5.Improved CAN5.Improved CAN 6.Conclusions6.Conclusions 7.Ongoing works7.Ongoing works

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1.What is DHT?1.What is DHT?

Distributed Hash Table (DHT)Distributed Hash Table (DHT) DHT is structured overlays that offerDHT is structured overlays that offer

– extreme scalabilityextreme scalability – hash-table-like lookup interfacehash-table-like lookup interface

Ideal substrate for many distributed Ideal substrate for many distributed applicationsapplications– file-systems, storage, backups, event-notification, e-mail, file-systems, storage, backups, event-notification, e-mail,

chat, databases, sensor networks ...chat, databases, sensor networks ...

Applications need reliable and efficient DHT Applications need reliable and efficient DHT routingrouting

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2.Motivation2.Motivation

DHT routing algorithms have been DHT routing algorithms have been proposing such as Chord,CAN, Pastry, proposing such as Chord,CAN, Pastry, Tapestry,Koorde,Symphony,…Tapestry,Koorde,Symphony,…

Each has different performances: path hops, Each has different performances: path hops, resilience, load balance…..resilience, load balance…..

What makes them different? What could be What makes them different? What could be the direction for the improvement of the the direction for the improvement of the routing performance?routing performance?

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GoalsGoals

Understand the basic principles of Understand the basic principles of designing DHT routing DHT.designing DHT routing DHT.

Provide the insight into the improvement of Provide the insight into the improvement of the routing performances and the feasible the routing performances and the feasible approach to design new DHT routing approach to design new DHT routing algorithms.algorithms.

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3.Links in DHT3.Links in DHT

A structured geometry presented by DHT routing A structured geometry presented by DHT routing algorithmsalgorithms– Chord: circle, CAN: hypercube, Tapestry:treeChord: circle, CAN: hypercube, Tapestry:tree– Each node can determinedly be routed to any other Each node can determinedly be routed to any other

nodes in the geometry.nodes in the geometry.– However, it is a dynamic geometry, So we use the link However, it is a dynamic geometry, So we use the link

to catch its characteristics. to catch its characteristics. – Link: Link: A link is referred to an edge but it is very

different the edge from the dynamic characteristic. The link can be dynamically adjusted according to the change of peer’s neighbors and it reflects how well the peer senses the system.

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Links in DHTLinks in DHT

Three kinds of links exist in the geometry from the Three kinds of links exist in the geometry from the view of the distance and are maintained by each view of the distance and are maintained by each peer.peer.– Basic short linkBasic short link

– Redundant short linkRedundant short link

– Long linkLong link

The distance:The distance:– Distance d=Xdest - Xsrc (Xdest,Xsrc is node ID)Distance d=Xdest - Xsrc (Xdest,Xsrc is node ID)

– Hamming Distance: h=the number of 1’s in Xdest XOR Hamming Distance: h=the number of 1’s in Xdest XOR XsrcXsrc

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ModelModel

No loss of generality, considering a cycle graph on n nodes where vertices are labeled 1, 2, . . . , n, we use links to express the relation between nodes.

A message can be routed clockwise from a node to any other node according to these links.

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Basic short link: 1Basic short link: 1 Redundant short link: 2, 3Redundant short link: 2, 3 Long link: 4, 5Long link: 4, 5

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4.The effect of links on routing 4.The effect of links on routing performanceperformance

1.1.Average Path Length:the average hops Average Path Length:the average hops between every pair of nodes.It identifies between every pair of nodes.It identifies how quickly a request is forwarded to the how quickly a request is forwarded to the destination.destination.

It is a tradeoff between the number of links It is a tradeoff between the number of links and the maintaining overheads. Average and the maintaining overheads. Average path length can vary from 1 to n. path length can vary from 1 to n.

The key is how to arrange long links. There The key is how to arrange long links. There are diversified approaches.are diversified approaches.

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2.2.Resilience: measure the extent to which Resilience: measure the extent to which DHT can route around failure even without DHT can route around failure even without the aid of recovery mechanisms that fix the the aid of recovery mechanisms that fix the failure.failure.

Short links is the key. . It can improve the Short links is the key. . It can improve the resilience by adding the redundant short resilience by adding the redundant short links.links.

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Current DHT Routing Current DHT Routing AlgorithmsAlgorithms

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5.Improved CAN5.Improved CAN

1.1.Add long linksAdd long links 2.Add redundant short links2.Add redundant short links

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Add long linksAdd long links

Case one: add additional 2 long links to Case one: add additional 2 long links to CAN and compare it with the original CAN.CAN and compare it with the original CAN.

Case two: add additional 4 long links to 4 Case two: add additional 4 long links to 4 dimensional CAN and compare it with 6 dimensional CAN and compare it with 6 dimensional CAN. In this way, both have dimensional CAN. In this way, both have the same number of links.the same number of links.

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Path Hops vs. Long Links (Case One)Path Hops vs. Long Links (Case One)

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Path Hops vs. Long Links (Case Two)Path Hops vs. Long Links (Case Two)

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Add redundant short linksAdd redundant short links

Case one: add redundant short links to 2 Case one: add redundant short links to 2 dimensional CAN. The number of dimensional CAN. The number of additional short links is varied form 0 to 4 .additional short links is varied form 0 to 4 .

Case two: add 2 additional redundant short Case two: add 2 additional redundant short links to 2,4,6 dimensional CAN links to 2,4,6 dimensional CAN respectively. respectively.

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Resilience vs. Redundant Short Links Resilience vs. Redundant Short Links (Case One)(Case One)

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Resilience vs. Redundant Short Links Resilience vs. Redundant Short Links (Case Two)(Case Two)

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6.Conclusions6.Conclusions

1. Links has great effects on the 1. Links has great effects on the performances of DHT routing algorithmsperformances of DHT routing algorithms

2. According to the analysis of links in 2. According to the analysis of links in current DHT routing algorithms, the possible current DHT routing algorithms, the possible improvement of routing performances can be improvement of routing performances can be suggested .suggested .

3. New DHT routing algorithms should take 3. New DHT routing algorithms should take the effect of links into account. the effect of links into account.

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7.Ongoing Works7.Ongoing Works

1.Fixed Links vs. Randomized Links1.Fixed Links vs. Randomized Links 2.More performance arguments need to be 2.More performance arguments need to be

investigated such as load balance, delay….investigated such as load balance, delay….

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Thank You!Thank You!