P2P Networking

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What is peer-to-peer (P2P)?

“Peer-to-peer is a way of structuring distributed applications such that the individual nodes have symmetric roles. Rather than being divided into clients and servers each with quite distinct roles, in P2P applications a node may act as both a client and a server.”-- Charter of Peer-to-peer Research Group, IETF/IRTF, June 24, 2004(http://www.irtf.org/charters/p2prg.html)

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Client/Server Architecture

GET /index.html HTTP/1.0

HTTP/1.1 200 OK ...Client Server

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Disadvantages of C/S Architecture

Single point of failure Strong expensive server Dedicated maintenance (a sysadmin) Not scalable - more users, more

servers

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The Client Side

Today’s clients can perform more roles than just forwarding users requests

Today’s clients have:more computing powermore storage space

Thin client Fat client

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Evolution at the Client Side

IBM PC @ 4.77MHz

360k diskettes

PC @ 4-core 4GHz

300GB HD

DEC’S VT100No storage

‘70 ‘80 2008

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What Else Has Changed?

The number of home PCs is increasing rapidly Most of the PCs are “fat clients” As the Internet usage grow, more and more PCs

are connecting to the global net Most of the time PCs are idle

How can we use all this?

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Resources Sharing

What can we share? Computer resources

Shareable computer resources: CPU cycles - seti@home, GIMPS Bandwidth - PPLive, PPStream Storage Space - OceanStore, Murex Data - Napster, Gnutella

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SETI@Home

SETI – Search for Extra-Terrestrial Intelligence

@Home – On your own computer A radio telescope in Puerto Rico scans the

sky for radio signals Fills a DAT tape of 35GB in 15 hours That data have to be analyzed

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SETI@Home (cont.)

The problem – analyzing the data requires a huge amount of computation

Even a supercomputer cannot finish the task on its own

Accessing a supercomputer is expensive What can be done?

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SETI@Home (cont.)

Can we use distributed computing?YEAH

Fortunately, the problem can be solved in parallel - examples:Analyzing different parts of the skyAnalyzing different frequenciesAnalyzing different time slices

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SETI@Home (cont.)

The data can be divided into small segments

A PC is capable of analyzing a segment in a reasonable amount of time

An enthusiastic UFO searcher will lend his spare CPU cycles for the computationWhen? Screensavers

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SETI@Home - Example

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SETI@Home - Summary

SETI reverses the C/S modelClients can also provide servicesServers can be weaker, used mainly for storage

Distributed peers serving the centerNot yet P2P but we’re close

Outcome - great results:Thousands of unused CPU hours tamed for the

mission3+ millions of users

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MUREX: A Mutable Replica Control Scheme for Peer-to-Peer Storage Systems

Murex: Basic Concept

HotOSAttendee

Peer-to-Peer Video Streaming

… …Video stream

Peer-to-Peer Video Streaming

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Napster -- Shawn Fanning

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History of Napster (1/2)

5/99: Shawn Fanning (freshman, Northeastern University) founds Napster Online (supported by Groove)

12/99: First lawsuit 3/00: 25% Univ. of Wisconsin traffic on

Napster

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History of Napster (2/2)

2000: estimated 23M users

7/01: simultaneous online users 160K 6/02: file bankrupt … 10/03: Napster 2 (Supported by Roxio)

(users should pay $9.99/month)

1984~2000, 23M domain names are countedvs.

16 months, 23M Napster-style names are registered at Napster

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Napster Sharing Style: hybrid center+edge

“slashdot”•song5.mp3•song6.mp3•song7.mp3

“kingrook”•song4.mp3•song5.mp3•song6.mp3

•song5.mp3

1. Users launch Napster and connect to Napster server

3. beastieboy enters search criteria

4. Napster displays matches to beastieboy

2. Napster creates dynamic directory from users’ personal .mp3 libraries

Title User Speed song1.mp3 beasiteboy DSLsong2.mp3 beasiteboy DSLsong3.mp3 beasiteboy DSLsong4.mp3 kingrook T1song5.mp3 kingrook T1song5.mp3 slashdot 28.8song6.mp3 kingrook T1song6.mp3 slashdot 28.8song7.mp3 slashdot 28.8

5. beastieboy makes direct connection to kingrook for file transfer

s o n g 5

“beastieboy”•song1.mp3•song2.mp3•song3.mp3

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Gnutella History Gnutella was written by Justin Frankel, the

21-year-old founder of Nullsoft. (Nullsoft acquired by AOL, June 1999) Nullsoft (maker of WinAmp) posted Gnutella

on the Web, March 14, 2000. A day later AOL yanked Gnutella, at the

behest of Time Warner. Too late: 23k users on Gnutella People had already downloaded and shared

the program. Gnutella continues today, run by independent

programmers.

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Gnutella -- Justin Frankel and Tom Pepper

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The ‘Animal’ GNU: Either of two large African antelopes (Connochaetes gnou or C. taurinus) having a drooping mane and beard, a long tufted tail, and curved horns in both sexes. Also called wildebeest.

Gnutella =

GNU: Recursive AcronymGNU’s Not Unix ….

+

Nutella: a hazelnut chocolate spread produced by the Italian

confectioner Ferrero ….

GNU

Nutella

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GNUGNU's Not Unix 1983 Richard Stallman (MIT) established Free Software Foundation and Proposed GNU ProjectFree software is not freewareFree software is open source softwareGPL: GNU General Public License

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About Gnutella No centralized directory servers Pings the net to locate Gnutella friends File requests are broadcasted to friends

Flooding, breadth-first search When provider located, file transferred via

HTTP History:

3/14/00: release by AOL, almost immediately withdrawn

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Peer-to-Peer Overlay Network

Focus at the application layer

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Peer-to-Peer Overlay Network

Internet

End systems

one hop(end-to-end comm.)

a TCP thru the Internet

Gnutella ProtocolScenario: Joining Gnutella Network

A

Gnutella Network The new node connects to a

well known ‘Anchor’ node or ‘Bootstrap’ node.

Then sends a PING message to discover other nodes.

PONG messages are sent in reply from hosts offering new connections with the new node.

Direct connections are then made to the newly discovered nodes.

NewPING

PINGPING

PINGPING

PINGPING

PINGPING

PING

PONG

PONG

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Topology of a Gnutella Network

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xyz.mp3 ?

Gnutella: Issue a Request

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Gnutella: Flood the Request

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xyz.mp3

Gnutella: Reply with the FileFully distributed storage and directory!

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So Far

Centralized :

- Directory size – O(n)

- Number of hops – O(1) Flooded queries:

- Directory size – O(1)

- Number of hops – O(n)

n: number of participating nodes

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We Want

Efficiency : O(log(n)) messages per lookup

Scalability : O(log(n)) state per node Robustness : surviving massive

failures

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How Can It Be Done?

How do you search in O(log(n)) time?Binary search

You need an ordered array How can you order nodes in a network and data

objects? Hash function!

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Example of Hasing

Shark SHA-1

Object ID (key):DE11AC

SHA-1

Object ID (key):AABBCC

194.90.1.5:8080

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Basic Idea

Hash key

Object “y”

Objects have hash keys

Peer “x”Peer nodes also have hash keys in the same hash space

P2P Network

y xH(y) H(x)

Join (H(x))

Publish (H(y))

Place object to the peer with closest hash keys

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Mapping an object to the closest node with a larger key

0 M

- an data object

- a node

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Viewed as a Distributed Hash Table

Hashtable

0 2128-1

Peernode

Internet

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DHT Distributed Hash Table Input: key (file name)

Output: value (file location) Each node is responsible for a range of the hash

table, according to the node’s hash key. Objects’ directories are placed in (managed by) the node with the closest key

It must be adaptive to dynamic node joining and leaving

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How to Find an Object?

Hashtable

0 2128-1

Peernode

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Simple Idea Track peers which allow us to move quickly

across the hash space a peer p tracks those peers responsible for hash keys

(p+2i-1), i=1,..,m

Hashtable

0 2128-1

Peernode

i+22 i+24 i+28i

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Chord Lookup – with finger tableStart Int. node

2+1 [3,4) 3

2+2 [4,6) 7

2+4 [6,10) 7

2+8 [10,2) 10

12

14 2

10

15 1

3

7

I’m node 2.Please find key 14!

Start Int. node

10+1 [11,12) 12

10+2 [12,14) 12

10+4 [14,2) 14

10+8 [2,10) 2

O(log n) hops (messages)for each lookup!!

14 [10,2)∈

14 [14,2)∈

Circular 4-bitID space

O(log n) states per node

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Hybrid P2P – Preserves some of the traditional C/S architecture. A central server links between clients, stores indices tables, etc Napster

Unstructured P2P – no control over topology and file placement Gnutella, Morpheus, Kazaa, etc

Structured P2P – topology is tightly controlled and placement of files are not random Chord, CAN, Pastry, Tornado, etc

Classification of P2P systems

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Q&A