Distributed Systems Concepts and Design Chapter 10: Peer-to-Peer Systems

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Distributed Systems Concepts and Design Chapter
10 Peer-to-Peer Systems

• Bruce Hammer, Steve Wallis, Raymond Ho

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10.1 Introduction

• Peer-to-Peer Systems
• Where data and computational resources are
  contributed by many hosts
• Objective to balance network traffic and reduce
  the load on the primary host
• Management requires knowledge of all hosts, their
  accessibility, (distance in number of hops),
  availability and performance.
• They exploit existing naming, routing, data
  replication and security techniques in new ways

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10.1 Introduction

• Goal of Peer-to-Peer Systems
• Sharing data and resources on a very large scale
• Applications that exploit resources available at
  the edges of the Internet storage, cycles,
  content, human presence (Shirky 2000)
• Uses data and computing resources available in
  the personal computers and workstations

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10.1 Introduction

• Characteristics of Peer-to-Peer Systems
• Each computer contributes resources
• All the nodes have the same functional
  capabilities and responsibilities
• No centrally-administered system
• Offers a limited degree of anonymity
• Algorithm for placing and accessing the data
• Balance workload, ensure availability
• Without adding undue overhead

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10.1 Introduction

• Evolution of Peer-to-Peer Systems
• Napster download music, return address
• Freenet, Gnutella, Kazaa and BitTorrent
• More sophisticated greater scalability,
  anonymity and fault tolerance
• Pastry, Tapestry, CAN, Chord, Kademlia
• Peer-to-peer middleware

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10.1 Introduction

• Evolution (Continued)
• Immutable Files, (music, video)
• GUIDs (Globally Unique Identifiers)
• Middleware to provide better routing algorithms,
  react to outages
• Evolve to mutable files
• Application within one companys intranet

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10.2 Napster and its Legacy

• Napster
• Provided a means for users to share music files
  primarily MP3s
• Launched 1999 several million users
• Not fully peer-to-peer since it used central
  servers to maintain lists of connected systems
  and the files they provided, while actual
  transactions were conducted directly between
  machines
• Proved feasibility of a service using hardware
  and data owned by ordinary Internet users

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10.2 Napster and its Legacy
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10.2 Napster and its Legacy

• Bit Torrent
• Designed and implemented 2001
• Next generation from Napster - true Peer To Peer
  (P2P)
• Can handle large files e.g WAV, DVD, FLAC (e.g
  1CD approx 500KB)
• After the initial pieces transfer from the seed,
  the pieces are individually transferred from
  client to client. The original seeder only needs
  to send out one copy of the file for all the
  clients to receive a copy
• Tracker URL hosted at Bit Torrent site e.g
  Traders Den

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10.2 Napster and its Legacy

• Bit Torrent (contd)
• Many Bit Torrent clients e.g Vuze
• Keep track of seeders and leechers
• Torrent contains metdata about the files to be
  shared and about the tracker
• Tracker - coordinates the file distribution, and
  which controls which other peers to download the
  pieces of the file.

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10.2 Napster and its Legacy
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10.2 Napster and its Legacy
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10.2 Napster and Its Legacy
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10.2 Napster and its Legacy
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10.2 Napster and its Legacy
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10.2 Napster and its Legacy
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10.3 Peer-to-Peer Middleware

• Peer To Peer Middleware
• To provide mechanism to access data resources
  anywhere in network
• Functional Requirements
• Simplify construction of services across many
  hosts in wide network
• Add and remove resources at will
• Add and remove new hosts at will
• Interface to application programmers should be
  simple and independent of types of distributed
  resources

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10.3 Peer-to-Peer Middleware

• Peer To Peer Middleware (contd)
• Non-Functional Requirements
• Global Scalability
• Load Balancing
• Optimization for local interactions between
  neighboring peers
• Accommodation to highly dynamic host availability
  
• Security of data in an environment simplify
  construction of services across many hosts in
  wide network
• Anonymity, deniability and resistance to
  censorship

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10.3 Peer-to-Peer Middleware

• Peer To Peer Middleware (contd)
• Global scalability, dynamic host availability and
  load sharing and balancing across large numbers
  of computers pose major design challenges.
• Design of Middleware layer
• Knowledge of locations of objects must be
  distributed throughout network
• Use of replication to achieve this

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10.4 Routing Overlays

• Routing Overlays
• Sub-systems, APIs, within the peer-to-peer
  middleware
• Responsible for locating nodes and objects
• Implements a routing mechanism in the application
  layer
• Separate from any other routing mechanisms such
  as IP routing
• Ensures that any node can access any object by
  routing each request thru a sequence of nodes
• Exploits knowledge at each node to locate the
  destination

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10.4 Routing Overlays

• GUIDs
• pure names or opaque identifiers
• Reveal nothing about the locations of the objects
• Building blocks for routing overlays
• Computed from all or part of the state of the
  object using a function that deliver a value that
  is very likely to be unique. Uniqueness is then
  checked against all other GUIDs
• Not human readable

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10.4 Routing Overlays

• Tasks of a routing overlay
• Client submits a request including the object
  GUID, routing overlay routes the request to a
  node at which a replica of the object resides
• A node introduces a new object by computing its
  GUID and announces it to the routing overlay
• Clients can remove an object
• Nodes may join and leave the service

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10.4 Routing Overlays

• Types of Routing Overlays
• DHT Distributed Hash Tables
• DOLR Distributed Object Location and Routing
• DOLR is a layer over the DHT that maps GUIDs and
  address of nodes
• DHT GUIDs are stored based on the hash value
• DOLR GUIDs host address is notified using the
  Publish() operation

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10.5 Overlay Case Studies Pastry, Tapestry
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10.6 Application Case Studies Squirrel,
OceanStore, Ivy Squirrel
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10.7 Summary

• Napster immutable data, unsophisticated routing
• Current mutable data, routing overlays,
  sophisticated algorithms
• Internet or company intranet support
• Distributed Computing (SETI)

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10.7 Summary

• Benefits of Peer-to-Peer Systems
• Ability to exploit unused resources (storage,
  processing) in the host computers
• Scalability to support large numbers of clients
  and hosts with load balancing of network links
  and host computer resources
• Self-organizing properties of the middleware
  platforms reduces costs

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10.7 Summary

• Weaknesses of Peer-to-Peer Systems
• Costly for the storage of mutable data compared
  to trusted, centralized service
• Can not yet guarantee anonymity to hosts

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10 Peer-to-Peer Systems

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