efficient content sharing taking account of updating replicas in hybrid peer-to-peer networks...

Efficient Content Sharing Taking Account of Updating

Replicas in Hybrid Peer-to-Peer

Networks

Tatsuru Kato, Shinji Sugawara, Yutaka Ishibashi

Nagoya Institute of Technology

Nagoya, Japan

2011 Annual IEEE CQR International WorkshopMay 10-12,2011, The Naples Beach Hotel & Golf Club

Background

• In date sharing using Peer-to-Peer (P2P) system

• Increase of storage load in each peer• Possibility to access the obsolete information

： Peer

： Contents before the update

Up-date

Req-

uest

Problems

： Contents after the update

Objective

• In data sharing using hybrid P2P networks, replicas’ relocation strategies to reduce consumed storage resources are proposed• Content update is not considered

Actually, the content update occurs on networks

A control to keep a consistency with contents

We propose a content sharing strategy that has a consistency control responding to content updates

This study

Previous study

Formulation of the problem (1/2)

• A P2P network topology that consists of many peers and their links is given

Assumption

• A server for managing all the peers is prepared, and it is aware of what content item is stored in each peer on the network • Content updating occurs only on peers having original content items, and each original content item exists on a single peer at most in the network

Formulation of the problem (2/2)

We achieve a content sharing strategy that minimizes the weighted sum of the

costs

• Network costThe load that accrues in the network when some replicas are downloaded, transferred, replicated, and updated informationis propagated

• Storage cost

• Content loss costThe cost that accrues when a peer cannot get the requested content item because no peer possesses a replica of the con-tent item in the network

The capacity to store a replica to be shared

Proposed method

• Content Request Procedure Replication, or relocation of a replica are executed

• Content Update Procedure Propagation of the latest replica is working

Restrain storage cost by restricting unnece-ssary replication, and propagation of the latest replica

Proposed method

• Content Request Procedure

• Content Update Procedure

Replication, or relocation of a replica are executed

Propagation of the latest replica is working

Restrain storage cost by restricting unnece-ssary replication, and propagation of the latest replica

Each replica has a distance range within a hop thre-shold(Hth) from the peer in which the replica is stored. This range is called the “referable range’’ referable rangeExample ： Hth = 2

Referable range

： peer

： replica

When the requesting peer does not exist within the referable range, a replica of the content is

replicated on the requesting peer from the peer possessing the

contentExample ： Hth = 2

Referable range

： peer

： replica

：requesting peer

Replication

Example ： Hth = 2

Referable range

： peer

： replica

：requesting peer

RelocationReference

When the requesting peer exists within the referable

range, content replication is not executed and just

referred the content from the requesting peer

Relocation of the replica (1/5)

(1)Select one of the peers inside the referable range,

and suppose that the referred replica is relocated.

On all peers inside the referable range, calculate CR .

CR is an expected value of the cost to access the

replica from the other peers.

Calculate sum of CR

： peer

： replica

： updating peer：requesting peer

Relocation of replica (2/5)

(2) Change the peer to be supposed that the referred replica is relocated, calculate sum of CR on all peers inside the referable range

： peer

： replica


Calculate sum of CR

Calculate sum of CR


Calculate CU ： peer

： replica


(3) On all peers inside the referable range, calculate CU .

CU is an expected value of the cost to update the replica.


： peer

： replica


(4) Change the peer to be supposed that the referred

replica is relocated, calculate sum of CU on all

peers inside the referable rangeCalculate CU


(5) Calculate C = CR +WU ・ CU

Relocate the replica to the peer which has the least C WU is a weight to adjust the importance of CR

andCU

The peer which has the least C

Relocate the replica

： peer

： replica


Proposed method

• Content Request Procedure

• Content Update Procedure

Restrain storage cost by restricting replica-tion, and propagation of the latest replica

Replication, or relocation of a replica are executed

Propagation of the latest replica is working

Propagation of the replica (1/2)

Occurrence of update

The latest replica is sent only to the peers that hold the same content item’s obsolete replicas which have not referred for more than T1 units of time

： peer

： replica

： updating peer

Propagation of the replica (2/2)

Propagation of the replica

The latest replica is sent only to the peers that hold the same content item’s obsolete replicas which have not referred for more than T1 units of time

： peer

： replica

： updating peer

Evaluation method

E ： Total cost

EN ： Network cost (sum of the number of hops of data movement per total elapsed time)

ES ： Storage cost (sum of the number of replicas in each unit time per total elapsed time)

EL ： Content loss cost (sum of the number of contents loss times per total elapsed time)

• Evaluate the methods by computer simulation

WN ,WS ,WL 　： Weight (the relative importance of each of costs)

E =WN ・ EN +WS ・ ES +WL ・ EL

Methods for comparison

Requested content is regularly replicated on the requesting peer

RCT

Owner replication

When the number of hops between the peer possessing the content and the requesting peer is larger than threshold Hth ,replicate the content on the requesting peer.Otherwise, the requesting peer only refers to the replica.

Simulation conditions

Request of contents ・・・ Poisson distribution(λreq =0.5)Peers joining and dropping out ・・・ Poisson distribution 　　 (λmov =0.1)

Network topology BA model

Initial number of peers

100

Total number of contents

30

Threshold Hth 1 ～ 6

ThresholdT1 200

Weight (WN, WS, WL) (2, 1, 5), (1, 2, 5)

An example of BA model

Simulation result (1/2)I :95% confidence interval

Tota

l co

st E

Total costE (WN =2,WS=1,WL =5)

Proposed(Hth=1) RCT(Hth=1)Owner

Simulation result (2/2)


RCT(Hth=1)Owner

I :95% confidence interval

Tota

l co

st E

Proposed(Hth=2)

Conclusions

Proposed an efficient content replication method taking account of updating replicas for Hybrid P2P

• When the network cost is more expensive, the proposed method succeeded in reducing total cost as well as owner replication

• When the storage cost is more expensive, the proposed method reduced total cost more than owner replication and RCT

Future works

• Investigate effectiveness of the proposed method in much more various network environments• Improve the proposed method for further efficiency of relocation of replicas

Formulation of the problem

• Each peer can possess shared content items that can be replicated and downloaded by other peers

• There are many different shared content items in the network, and each of them are replicated and held by some peers for redundancy• Each peer (actually, a user on the peer) requests content from time to time, and this request preference is biased depending on the combination of each content item and the requesting peer

Assumption

• Each of the peers can drop off the network randomly once in a while and join in again with a certain probability

• All content items have the same size

• Storage capacity of each peer is not limited

Assumption

Formulation of the problem

1. Select a dropping out peer randomly

2. Select one of the neighboring peer of the dropping out peer randomly, change link of the others to the selected peer

Dropping out peer Selected peer

Drop out of a peer

BA (Barabasi-Albert) model topology

scale-free network

• A peer is added to the network one at a time •A peer is preferentially connected to the peer which has many neighbor peers

Zipf distribution

Zipf distribution (S=1 、 N=100)

N1n

s

s

nk

P(k)S : Zipf’s coefficient N: Total number of dates

Poisson distribution

Unit of time k

even

t pro

bab

ility

P(k

)

Poisson distribution (λ= 0.5)

!k

e k 　λλ-P(k)

λ: Average frequency of occurrence of an event

Simulation resultN

etw

ork

cost

EN

Network cost EN

Threshold Hth

Sto

rag

e c

ost

ES

Storage cost ES

Simulation result

Threshold Hth


Con

tent

loss

cost

EL

Content loss cost EL

Simulation result

Threshold Hth




Tota

l co

st E


efficient content sharing taking account of updating replicas in hybrid peer-to-peer networks...

Documents

content replication

network content

peer replication slide

requesting peer slide

hybrid peer

peer relocationreference

peer possibility

single peer