Peer-to-Peer Systems

1
Peer-to-Peer Systems

• Rodrigo Rodrigues
• Peter Druschel
• Max Planck Institute for Software Systems

2
Paper Overview

• Survey paper
• Long list of references
• Presents a high view of various aspects of P2P
  systems
• Describes distributed hash tables in more detail

3
In the beginning

• 1999
• Napster music sharing system
• Gave a bad reputation to P2P systems
• Freenet anonymous data store
• SETI_at_home volunteer-based distributed
  computational project

4
Now

• BitTorrent
• Skype P2P telephony system
• Skinkers enterprise communication management
  system
• P2PLive, CoolStreaming, BBCs iPlayer

5
What I think

• Previous list mentioned commercial products
• Does it mean all major issues have been solved?

6
Email from Skype

• To our valued customersAs 2010 draws to a
  close, I would like to take a moment to thank
  each of you for your patience, understanding, and
  support during Skypes recent outage.
• Kind regards, Tony BatesCEOSkype

7
Defining propertiesof P2P systems

• High degree of decentralization
• Few or no dedicated central nodes
• Multiple administrative domains
• Low barriers to deployment
• Organic growth
• Resilience to faults and attacks
• Abundance and diversity of resources

8
What I think

• P2P systems are
• Very cheap
• Very easy to deploy
• Highly scalable

9
Applications (I)

• Sharing and distributing files
• Napster (quickly shutdown)
• Gnutella, FastTrack aka Kazaa(all
  decentralized)
• eDonkey, BitTorrent (faster)
• Streaming media
• PPLive, Coolstreaming (academia)
• BBCs iPlayer, Skinkers Livestation (industry)

10
Applications (II)

• Telephony
• Skype
• Scientific Computing
• SETI_at_home
• BOINC
• Other
• Distributed storage systems (Freenet)
• Content-delivery networks(CoralCDN, CoDeeN)

11
Typology (I)

• Degree of centralization
• Partly decentralized
• BitTorrent tracker,
• Skype billing subsystem
• Fully decentralized
• More scalable
• Resilient to failure, attacks and legal
  challenges
• Can have supernodes

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Typology (II)

• Overlay maintenance
• Overlay is graph G (N, E) describing set of
  links E among members of set N of participating
  nodes
• If there is a link in E between two nodes, they
  are aware of each other
• Overlays can be structured or unstructured

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Unstructured overlays

• When a node joins, it acquires a set of
  "neighbors" by
• Contacting the tracker (BitTorrent)
• Contacting a system participant
• Must have a mechanism advertising these nodes

14
Structured overlays (I)

• Use key-based routing
• Each node has a unique identifier
• 160-bit integer
• Identifiers are uniformly distributed
• Addressing is based on keys
• Each key is mapped into exactly one of the
  current overlay nodes
• Smallest integer "larger" than key value make
  identifier space circular

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Structured overlays (II)

• Key-based routing implements primitiveKBR(no, k)
  that produces a path going from a node no to the
  node holding key k
• Big tradeoff is between
• Keeping paths short
• Minimizing state information kept by nodes

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Typology (III)

• Distributed state
• In partly decentralized systems state is
  maintained by
• The central node(s)
• The peers assigned by it/them to each node
• In decentralized systems, state is kept by
• The content providers
• Individual peers

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Locating data

• In unstructured systems, nodes wanting to access
  a specific object flood their neighbors, which
  flood their neighbors and so on
• Structured systems use distributed hash tables
• All data have keys
• Stored at node responsible for key value and
  replicated at its successors

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Typology (IV)

• Distributed control
• In unstructured systems , it is typically done by
  epidemic techniques
• Can also build a spanning tree among the nodes if
  membership is fairly stable
• In structured systems, it is much easier to
  build spanning trees

19
Content distribution

• Tree-based protocols
• Main disadvantage is that leave nodes ado not
  contribute anything
• Full binary tree of height n has 2n1- 1 nodes
  and 2n leaves
• Swarm-based protocols
• BitTorrent
• All nodes can participate

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Challenges (I)

• Controlling membership
• Preventing Sybil attacks
• One node pretending to be many
• Can require proof of work or use trusted
  identities (FARSITE)

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Challenges (II)

• Protecting data
• Integrity and Authenticity
• Can use digital signatures
• Data stored in DHTs can be self-certifying by
  making DHT keys function of data themselves
• Can use voting (LOCKSS)
• Availability and Durability
• Replicate data and keep system alive

22
Challenges (III)

• Incentives
• Fighting Free riding
• Big problem
• BitTorrent tit-for-tat
• Not always feasible
• Managing P2P Systems
• Lack of centralized control can make system hard
  to manage
• Skype collapses

23
P2P and ISPs

• P2P systems consume a lot of bandwidth
• Current ISP billing models assume that customers
  send much less bits than they receive
• Flat-rate pricing for residential customers
• Bandwidth-based pricing for information providers
• ISPs have no way to bill anyone for P2P traffic

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Conclusions

• P2P is a disruptive technology with great
  potential
• Major strength is lack of centralized control
• Also creates new challenges that can be
• Technical
• Commercial
• Legal