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School of Computing ScienceSimon Fraser University

CMPT 880: Internet Architectures and CMPT 880: Internet Architectures and ProtocolsProtocols

Introduction to Peer-to-Peer Systems

Instructor: Dr. Mohamed HefeedaInstructor: Dr. Mohamed Hefeeda

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P2P Computing: Definitions

Peers cooperate to achieve desired functions- Peers:

• End-systems (typically, user machines)

• Interconnected through an overlay network

• Peer ≡ Like the others (similar or behave in similar manner)

- Cooperate: • Share resources, e.g., data, CPU cycles, storage, bandwidth

• Participate in protocols, e.g., routing, replication, …

- Functions: • File-sharing, distributed computing, communications,

content distribution, …

Note: the P2P concept is much wider than file sharing

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Overlay Network

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When Did P2P Start?

Napster (Late 1990’s)- Court shut Napster down in 2001

Gnutella (2000) Then the killer FastTrack (Kazaa, ...) BitTorrent, and many others Accompanied by significant research interest Claim

- P2P is much older than Napster!

Proof- The original Internet!

- Remember UUCP (unix-to-unix copy)?

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What IS and IS NOT New in P2P?

What is not new- Concepts!

What is new- The term P2P (may be!)

- New characteristics of • Nodes which constitute the

• System that we build

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What IS NOT New in P2P?

Distributed architectures Distributed resource sharing Node management (join/leave/fail) Group communications Distributed state management ….

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What IS New in P2P?

Nodes (Peers)- Quite heterogeneous

• Several order of magnitudes difference in resources

• Compare the bandwidth of a dial-up peer versus a high-speed LAN peer

- Unreliable• Failure is the norm!

- Offer limited capacity• Load sharing and balancing are critical

- Autonomous• Rational, i.e., maximize their own benefits!

• Motivations should be provided to peers to cooperate in a way that optimizes the system performance

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What IS New in P2P? (cont’d)

System - Scale

• Numerous number of peers (millions)

- Structure and topology• Ad-hoc: No control over peer joining/leaving

• Highly dynamic

- Membership/participation• Typically open

- More security concerns• Trust, privacy, data integrity, …

- Cost of building and running• Small fraction of same-scale centralized systems

• How much would it cost to build/run a super computer with processing power of that 3 Million SETI@Home PCs?

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What IS New in P2P? (cont’d)

So what? We need to design new lighter-weight

algorithms and protocols to scale to millions (or billions!) of nodes given the new characteristics

Question: why now, not two decades ago?- We did not have such abundant (and

underutilized) computing resources back then!

- And, network connectivity was very limited

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Why is it Important to Study P2P?

P2P traffic is a major portion of Internet traffic (50+%), current killer app

P2P traffic has exceeded web traffic (former killer app)!

Direct implications on the design, administration, and use of computer networks and network resources

- Think of ISP designers or campus network administrators

Many potential distributed applications

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Sample P2P Applications

File sharing- Gnutella, Kazaa, Napster, …

Distributed cycle sharing- SETI@home, Gnome@home, …

File and storage systems- OceanStore, CFS, Freenet, Farsite, …

Media streaming and content distribution- PROMISE

- SplitStream, CoopNet, PeerCast, Bullet, Zigzag, NICE, …

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P2P vs its Cousin (Grid Computing)

Common Goal:- Aggregate resources (e.g., storage, CPU

cycles, and data) into a common pool and provide efficient access to them

Differences along five axes [Foster & Imanitchi 03] - Target communities and applications

- Type of shared resources

- Scalability of the system

- Services provided

- Software required

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P2P vs Grid Computing (cont’d)

Issue Grid P2P

Communities and Applications

Established communities, e.g., scientific institutions Computationally-intensive problems

Grass-root communities (anonymous) Mostly, file-swapping

Resources Shared

Powerful and Reliable machines, clusters High-speed connectivity Specialized instruments

PCs with limited capacity and connectivity Unreliable Very diverse

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P2P vs Grid Computing (cont’d)

Issue Grid P2P

System Scalability

Hundreds to thousands of nodes

Hundreds of thousands to Millions of nodes

Services Provided

Sophisticated services: authentication, resources discovery, scheduling, access control, and membership control Members usually trust others

Limited services: resource discovery limited trust among peers

Software required

Sophisticated suit: e.g., Globus, Condor

Simple: (screen saver), e.g., Kazza, SETI@Home

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P2P vs Grid Computing: Discussion

The differences mentioned are based on the traditional view of each paradigm

- In the future, it is conceived that both paradigms will converge and will complement each other [e.g., Butt et al. 03]

Target communities and applications- Grid: is going open

Type of shared resources- P2P: is to include various and more powerful resources

Scalability of the system- Grid: is to increase number of nodes

Services provided- P2P: is to provide authentication, data integrity, trust

management, …

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P2P Systems: Simple Model

P2P Substrate

Operating System

Hardware

Middleware

P2P Application

Software architecture model on a peer

System architecture: Peers form an overlay according to the P2P

Substrate

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Overlay Network

An abstract layer built on top of the physical network

Neighbors in the overlay can be several hops away in the physical network

Why do we need overlays?- Flexibility in

• Choosing neighbors

• Forming and customizing topology to fit application needs (e.g., short delay, reliability, high BW, …)

• Designing communication protocols among nodes

- Get around limitations in legacy networks

- Enable new (and old!) network services

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Overlay Network (cont’d)

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Overlay Network (cont’d)

Some applications that use overlays- Application level multicast, e.g., ESM, Zigzag,

NICE, …

- Reliable inter-domain routing, e.g., RON

- Content Distribution Networks (CDN)

- Peer-to-peer file sharing

Overlay design issues- Select neighbors

- Handle node arrivals, departures

- Detect and handle failures (nodes, links)

- Monitor and adapt to network dynamics

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Overlay Network (cont’d) IP Multicast

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Overlay Network (cont’d)Application Level Multicast (ALM)

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Peer Software Architecture Model

A software client installed on each peer

Three components:

- P2P Substrate

- Middleware

- P2P Application

P2P Substrate

Operating System

Hardware

Middleware

P2P Application

Software architecture model on a peer

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Peer Software Architecture Model (cont’d)

P2P Substrate (key component)- Overlay management

• Construction

• Maintenance (peer join/leave/fail and network dynamics)

- Resource management• Allocation (storage)

• Discovery (routing and lookup)

Can be classified according to the flexibility of placing objects at peers

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P2P Substrates: Classification

Structured (or tightly controlled, DHT) − Objects are rigidly assigned to specific peers

− Looks like as a Distributed Hash Table (DHT)

− Efficient search & guarantee of finding

− Lack of partial name and keyword queries

− Maintenance overhead

− Ex: Chord, CAN, Pastry, Tapestry, Kademila (Overnet)

Unstructured (or loosely controlled)− Objects can be anywhere

− Support partial name and keyword queries

− Inefficient search & no guarantee of finding

− Some heuristics exist to enhance performance

− Ex: Gnutella, Kazaa (super node), GIA [Chawathe et al. 03]

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Peer Software Architecture Model (cont’d)

Middleware- Provides auxiliary services to the P2P

application, e.g., • Peer selection

• Trust management

• Data integrity validation

• Authentication and authorization

• Membership management

• Accounting (Economics and rationality)

• …

- Ex: CollectCast, EigenTrust, Micro payement

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Peer Software Architecture Model (cont’d)

P2P Application- Potentially, there could be multiple applications

running on top of a single P2P substrate

- Applications include• File sharing

• File and storage systems

• Distributed cycle sharing

• Content distribution

- This layer provides some functions and bookkeeping relevant to the target application

• File assembly (file sharing)

• Buffering and rate smoothing (streaming)

Ex: Promise, Bullet, CFS, Gnutella, Kazaa

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Outline of the Rest of the Introduction

P2P Substrates- Structured (DHT)

• Example: CAN

- Unstructured • Example 1: Gnutella

• Example 2: Kazaa

Middleware and P2P Application - Example: CollectCast and Promise

Course Roadmap: - Papers flash overview (1-2 min each!)

Project discussion

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Summary

In P2P computing paradigm:- Peers cooperate to achieve desired functions

Started (or re-discovered) with Napster ’98 Old, well-researched distributed concepts BUT, with new characteristics (e.g., heterogeneity,

unreliability, rationality, scale, ad hoc), new and lighter-weight algorithms are needed

Simple model for P2P systems:- Peers form an abstract layer called overlay

- A peer software client may have three components• P2P substrate, middleware, and P2P application

• Borders between components may be blurred