Peer-to-Peer Networks

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Peer-to-Peer Networks

• Outline
• Overview
• Gnutella
• Structured Overlays
• BitTorrent

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Peer-to-Peer Networks Overview

• A peer-to-peer (P2P) network allows a community
  of users to pool their resources
• Content
• Storage
• CPU,
• A P2P network is both decentralized and
  self-organizing
• Just like the Internet itself!
• Why do we care about these networks?
• It is challenging to achieve decentralization and
  scalability at the same time.

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Gnutella

• One of the first decentralized P2P networks for
  file sharing
• No central registry of objects.
• Example topology of a Gnutella P2P network
• Edges of the graph correspond to the relationship
  A and B know each other

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Gnutella cont.

• The simple idea in Gnutella is to distribute the
  method for finding data
• Great idea!
• Lots of fun architectural possibilities!
• Gnutella is a distributed search protocol with a
  decentralized model
• Clients can issue/view query results
• Clients can serve/request data
• Clients accept queries and respond with matches
  from their local data store

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Gnutella Protocol

• Protocol defines method of client communication
• Set of descriptors used for communicating data
• Set of rules governing inter-client exchange of
  descriptors
• Descriptors
• Ping active discovery of hosts on a network
• Pong response to Ping includes client address
  and metadata
• Query Ask for an object
• Query Response response to Query includes info
  necessary to get data
• A Gnutella client connects to network by
  establishing a connection with another client on
  the network
• Finding another client is not part of Gnutella
  spec.
• Host cache services are the typical way this is
  done

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Gnutella Protocol

• New client then creates connection to the
  Gnutella client and thereby becomes part of the
  network
• Gnutella client can reject the connect request
• Successful new client can then send/receive
  descriptors
• Pings/pongs are then sent to establish network
• No specification as to how much/often to probe
• Network data can/is cached
• Message routing should be well behaved
• Ping/Query descriptors should be sent to all
  directly connected clients
• Pong/Query Response descriptors should be sent
  back along same path
• TTL is mechanism to limit distance
• File downloads via HTTP/1.0 protocol via direct
  connect

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Gnutella Protocol

• New client then creates connection to the
  Gnutella client and thereby becomes part of the
  network
• Gnutella client can reject the connect request
• Successful new client can then send/receive
  descriptors
• Pings/pongs are then sent to establish network
• No specification as to how much/often to probe
• Network data can/is cached
• Message routing should be well behaved
• Ping/Query descriptors should be sent to all
  directly connected clients
• Pong/Query Response descriptors should be sent
  back along same path
• TTL is mechanism to limit distance
• File downloads via HTTP/1.0 protocol via direct
  connect

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Gnutellas Downside

• Flooding does not scale well!
• Alternatives
• Forward queries randomly or according to
  probability of success based on past results
• Proactively replicate objects to make them easier
  to find
• OR
• Structured Overlays

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Structured Overlays

• Designed to conform to a particular graph
  structure that allows reliable and efficient
  object location.
• However, with the cost of additional complexity
  in overlay construction and maintenance.
• 2 questions to answer
• How do we map objects to nodes that should serve
  them?
• How do we find an object?

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Mapping Objects to Nodes

• Hashing is used to map objects to n nodes
• Regular hashing
• Hash(x,n)
• Return x n
• Challenge
• What if a node joins the network?
• What if a node leaves the network?
• How do we choose the proper n?

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Consistent Hashing

• Hash a set of objects x uniformly across a large
  ID space
• Each object is maintained on the node whose ID is
  closest
• Advantages
• Distributes objects fairly evenly across nodes
• Only a small number of objects have to move when
  a node joins or leaves

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Consistent Hashing cont.

• Example

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How to find an object?

• Idea Route the query message to the node that
  has the object
• Each node maintains a routing table and the IP
  addresses of a small set of numerically larger
  and smaller node IDs.
• Forward the query message to the node that is
  closer than you to the destination node
• This is repeated until you get to destination

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How to find an object Cont.

• Structured Overlays provide a logarithmic bound
  on the number of routing hops required to locate
  an object
• However, Nodes might be arbitrarily far away from
  each other in the Internet!
• Optimizations
• Route to the physically closest node when
  possible
• Replicate data across the nodes

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