ADVANCED COMPUTER NETWORKS

Peer-Peer (P2P) Networks

1

What is P2P?

“the sharing of computer resources and services by direct exchange of information”

2

What is P2P?

Various definitions seem to agree on sharing of resources direct communication between equals

(peers) no centralized control

3

What is a peer?

“…an entity with capabilities similar to other entities in the system.”

4

Client/Server Architecture

Well known, powerful, reliable server is a data source

Clients request data from server

Very successful model WWW (HTTP), FTP,

Web services, etc.

Server

Client

Client Client

Client

Internet

5

Client/Server Limitations

Scalability is hard to achieve Presents a single point of failure Requires administration

P2P systems try to address these limitations

6

P2P Architecture

All nodes are both clients and servers Provide and consume

data Any node can initiate a

connection

No centralized data source “The ultimate form of

democracy on the Internet”

“The ultimate threat to copy-right protection on the Internet”

Node

Node

Node Node

Node

Internet

7

P2P Network Characteristics

Clients are also servers and routers Nodes contribute content, storage, memory, CPU

Nodes are autonomous (no administrative authority)

Network is dynamic: nodes enter and leave the network “frequently”

Nodes collaborate directly with each other (not through well-known servers)

8

P2P Applications

File sharing (BitTorrent, Napster, Kazaa)

Multiplayer games (Counter Strike, Unreal Tournament, DOOM)

Collaborative applications (ICQ, shared whiteboard)

9

Popular file sharing P2P Systems

BitTorrent

Large scale sharing of files. User A makes files (music, video, etc.) on

their computer available to others User B connects to the network, searches

for files and downloads files directly from user A

Issues of copyright violation

10

Approaches

Structured Unstructured

11

Structured

original “Napster” design

- when peer connects, it informs central server: IP address content

2) User B searches for file

3) User B requests file from User A

centralizeddirectory server

peers

User B

User A

1

1

1

12

3

12

Centralized model

A B

DC

file transfer is decentralized, but locating content is highly centralized

13

Napster: how does it work

Four steps: Connect to Napster server Upload your list of files to server. Give server keywords to search the full list with. Select “best” of correct answers.

14

Napster

napster.com

users

File list is uploaded

1.

15

Napster

napster.com

user

Requestand

results

User requests search at server.

2.

16

Napster

napster.com

user

pings

User pings hosts that apparently have data.

3.

17

Napster

napster.com

user

Retrievesfile

User retrieves file

4.

18

Unstructured

Fully distributed no central server

Used by KaZaa

Each peer indexes the files it makes available for sharing (and no other files)

19

KaZaa: Ping flooding

Query

QueryHit

Query

Query

QueryHit

Query

Query

QueryHit

File transfer:HTTP

Ping messagesent over existing TCPconnections

peers forwardping message

Pong sent over reversepath

20

KaZaa

file divided into 256KB chunks.

while downloading, peer uploads chunks to other peers.

peers may come and go once peer has entire file, it may leave or remain

21

P2P : Skype

Skype clients (SC)

Supernode (SN)

22

Issues with P2P

Free Riding Downloading but not sharing any data

On BitTorrent 15% of users contribute 94% of content 63% of users never responded to a query

No ranking: what is a trusted source?

23