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P2P Computing

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What is P2P?

Server-Client model

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What is P2P?Traditional network architecture

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What is P2P?

Problem with Server-Client Model Scalability

As the number of users increases, there is a higher demand for computing power, storage space, and bandwidth associated with the server-side

Reliability The whole network will depend on the highly loaded server

to function properly

Why P2P?

The Internet has three valuable fundamental assets Information Computing resources Bandwidth

Why P2P?

Hot Spots become hotter,

Yet Cold spots keep cold

Internet

Personal Computers80% idle CPU time

Computers in our Lab99% idle CPU time

!

Yildiray’s Laptop90% idle CPU time

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What is P2P? A peer in P2P network acts as both a client and

a server in traditional client/server architecture Typical P2P file sharing application

Users publish the file they want to share Users search the needed file by providing a query

Query is sent to other online peers in the network A peer that has local media files matching the query

will return information on how to retrieve the files Users may receive multiple successful responses and

can select the files they want to retrieve The files are down loaded from the remote peer.

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querymusiccategory

bannerad

3 million users online

sharing 4 PetaBytes of data

KazaaNativeWindowsApplication

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What is P2P?A pure P2P network architecture

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What is P2P? A hybrid P2P architecture

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Peer-to-Peer (P2P) Peer-to-Peer computing is inspired by the controversial music-

sharing service Napster Instead of Internet information being held in a few central locations,

Peer-to-Peer computing makes it theoretically possible to access the files and data residing on every personal computer connected to the Internet

P2P Not P2P

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Napster, first widely used p2p-applicationThe application: A p2p application for the distribution of mp3 files

Each user can contribute its own content

How it works: Central index server

Maintains list of all active peers and their available content

Distributed storage and download Client nodes also act as file servers All downloaded content is shared

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File Sharing: Napster

Here is how Napster used to work: It is first necessary to download and install Napster software

and to sign up for a free account at Napster By clicking the Napster icon, a connection is established with

Napster Web site, where the name of the music searched can be entered in a search window

The Napster software tracks all users who are online at that particular time and provides access to tracks stored on users' hard drives

When the song requested is found, Napster establishes a connection between the two computers so that it could be downloaded

However, after the Recording Industry Association of America sued Napster for copyright infringements, the court ruled that Napster was indeed violating copyrights and it is shut down

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History, motivation and evolution - Napster (cont’d)

Central index server…

peers

Initial join Peers connect to Napster server Transmit current listing of shared

files to server

join

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History, motivation and evolution - Napster (cont’d)

1) query

2) answer

Content search Peers sends song request to

Napster server Napster server checks song

database and returns list of matched peers

…

peers

Central index server

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History, motivation and evolution - Napster (cont’d)

1) request2) download…

peers

File retrieval The requesting peer contacts the

peer having the file directly and downloads it

Central index server

1) 2)

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Napster was the first simple but successful P2P-appliciation. Many others followed…

P2P File Download Protocols: 1999: Napster 2000: Gnutella, eDonkey 2001: Kazaa 2002: eMule, BitTorrent

History, motivation and evolution - File Download

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P2P is not restricted to file download!

P2P Protocols: 1999: Napster, End System Multicast (ESM) 2000: Gnutella, eDonkey 2001: Kazaa 2002: eMule, BitTorrent 2003: Skype 2004: PPLive Today:TVAnts, PPStream, SopCast… Next: Video-on-Demand, Gaming

File Download

Streaming

Telephony

Video-on-Demand

Gaming

Application type:

History, motivation and evolution - Applications

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Why is P2P so successful? Scalable – It’s all about sharing resources

No need to provision servers or bandwidth Each user brings its own resource

Eliminating the single-source bottleneck

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Why is P2P so successful? (cont’d) Cheap - No infrastructure needed Everybody can bring its own content

Homemade content Illegal content But also legal content …

High availability – Content accessible most of time

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Problems

Some of the providers of leading P2P applications earn revenue from third parties by embedding spyware into the applications

A large amount of polluted or corrupted content has been published in file sharing systems

Copyright problem Free-rider problem

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P2P-Overlay

Build graph at application layer, and forward packet at the application layer

It is a virtual graph Underlying physical graph is transparent to the

user Edges are TCP connection or simply a entry of an

neighboring node’s IP address The graph has to be continuously maintained

(e.g. check if nodes are still alive)

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P2P-Overlay (cont’d)

Underlay

Overlay

Source

Source

Basic P2P elements

Peers Each peer has a unique Peer ID Client Server Router

Operations Join/leave File publish File search

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Principles of the P2P Paradigm Symmetric roles

Peers are function equally Scalable

The network and computing resources used at each peer exhibit a growth rate as a function of overlay size that is less than linear

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Principles of the P2P Paradigm Autonomous

each peer determines when it joins/makes requests/leaves the overlay

Resource sharing Resource contribution should be mutually

beneficial Resilient

P2P overlays are resilient in the fact of dynamic peer membership

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The P2P enabling technologies Unstructured p2p-overlays

Generally random overlay Structured p2p-overlays

Distributed Hash Tables (DHTs) Super nodes architecture

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Unstructured p2p-overlays Unstructured p2p-overlays do not really care

how the overlay is constructed Peers are organized in a random graph topology

E.g., new node randomly chooses three existing nodes as neighbors

Build your p2p-service based on this graph Several proposals

Gnutella KaZaA/FastTrack BitTorrent

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One Example of usage of unstructured overlays

Typical problem in unstructured overlays: How to do content search and query? Flooding

Limited Scope, send only to a subset of your neighbors

Time-To-Live, limit the number of hops per messages

Search “Britney Spears”Example of flooding:(similar to Gnutella)

Found entry!

NotifyUpload

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Unstructured p2p-overlays (cont’d) Unstructured p2p-overlays are just a framework, you

can build many applications on top of it

Unstructured p2p-overlays pros & cons Pros

Very flexible: copes with node churn Supports complex queries (conversely to structured overlays)

Cons There is a tradeoff between generated traffic (overhead) and

the horizon of the partial view

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Structured p2p-overlays

Motivation Locate content efficiently

Solution – DHT (Distributed Hash Table) Particular nodes hold particular keys Locate a key: route search request to a particular

node that holds the key Representative solutions

Chord, CAN, Pastry/Tapestry, etc.

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Challenges to Structured p2p-overlays Load balance

spreading keys evenly over the nodes. Decentralization

no node is more important than any other. Scalability

Lookup must be efficient even with large systems Peer dynamics

Nodes may come and go, may fail Ensure the node responsible for a key can always

be found.