CS 554. PeertoPeer Systems Faithfulness in Internet Algorithms

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CS 554. Peer-to-Peer SystemsFaithfulness in
Internet Algorithms

Haridimos Kondylakis (kondylak_at_csd.uoc.gr)
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Presentation Outline

• Introduction to Faithfulness
• Software as a strategy
• Faithfulness Caveats
• Faithfulness in BitTorrent
• Strategies and Specifics
• Finding Flaws in Faithfulness
• Backtracking BitTorrent
• Open Questions

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1. Introduction to Faithfulness

• Definition A system is faithful with respect to
  some knowledge assumption if a rational (selfish,
  utility maximizing) node follows an algorithm
  that always has the same externally visible
  effects as following the suggested strategy, for
  all states of the world.
• Given some assumptions about the type of nodes in
  a system, a node will follow the default
  strategy, because no other strategy yields a
  higher utility for this node.

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2. Software as a strategy (1/3)

• Software executed by a node can be seen as a
  representation of that nodes strategy.
• Software start in an initial state.
• Then receives input that determine worlds state
• Then software moves to a new state possibly
  emitting an external action.
• In economics all internal state transitions and
  internal action, wrapped together as part of a
  strategy that generates external observable
  action.

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2. Software as a strategy (2/3)

• The strategy space is restricted to those
  strategies that can be expressed in a finite
  multiset of possible execution of machine
  instructions.
• Often strategies expressions given up
• In favor of ease of interaction (CPU, OS)
• By software provided by system designer
• Software can provide a default suggested strategy
  from the restricted strategy space.
• If utility gained from software modification gt
  cost of modification the rational node will
  expend this cost.

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2. Software as a strategy (3/3)

• When those modifications harm the other users?
• Do we lose some mechanism functionality?
• A system is robust
• Feel free to modify this client as you see fit.
  One can prove that the best strategy you can
  follow is the strategy that I have given you.

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3. Faithfulness Caveats

• When making a faithfulness claim one should
  consider strategy manipulations at all parts of
  the system and declare the assumed obedience.
  (e.g alter ethernet cards firmware)
• If expected utility gain gt cost of manipulation
  then a rational person wont make it
• But in P2P what about community-endowed prestige
  or respect for useful work?
• One user can do the manipulations and then spread
  the software all around.

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4. Faithfulness in BitTorrent

• BitTorrent incents users by linking the receipt
  of pieces to serving of other pieces
• Interesting protocol to investigate since
• The system was designed with rational behavior in
  mind
• There is some idea if incenting good strategic
  behavior from a rational user.
• The system designer has provided a suggested
  strategy
• The client is released in source and executable
  form
• The cost of changing the strategy space
  potentially small

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4. Faithfulness in BitTorrent

• 3. The system is simple enough to anticipate and
  model several classes of rational users
• 4. The BitTorrent protocol is popular
• Nodes in BitTorrent
• Tracker Maintains a list of peers that are
  active with respect to a file
• Complete peers that hold the entire file
• Incomplete peers that are likely in the process
  of downloading the file tracked

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4.1 BitTorrent strategy specifics

• After initialization, a k subset of nodes is
  internally marked as being unchoked. (Requests
  will be fulfilled)
• Peers send special messages to each other to
  update their claim about which pieces they
  posses.
• The peer is always capable of receiving a file
  piece from any connection regardless the
  internally marked choke state.
• Any active peer that is complete is altruistic
• But an incomplete peer will selfishly unchoke
  peers that provide pieces at the highest
  throughput.

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4.1 BitTorrent strategy specifics

• All peers will optimistically unchoke a new peer
  in an attempt to find a better trading partner
  with higher throughput.
• K fastest partners kept in the unchoked list
• BitTorrent used by
• Obedient users bounded rational nodes
• Speed Critical Places a high utility on
  receiving a file as fast as possible
• Free Rider Places a high utility on receiving
  the file using as little bandwidth as possible.
• Anti-social, malicious e.t.c
• A node aims to maximize the benefit it receives

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5. Finding flaws in Faithfulness

• Backtracking
• For each type of rational node in the system
• For each communication configuration
• For each goal state
• Trace backwards through logical steps (code)
  in interaction with others. Mark branches and
  neighbor interactions as candidate
  manipulation points.
• Examine and classify point.
• Each manipulation point can be classified as
• False Alarm
• Beneficial Manipulation
• Harmful Manipulation

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5. Finding flaws in Faithfulness
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5.1 Backtracking BitTorrent

• Identity Register multiple times in tracker
• A node will be optimistic unchoked proportional
  to its declared identities.
• Upload Bandwidth If a node unchokes the fastest
  k nodes out of a list of k1, this peer must
  upload at rate just above k-th fastest competitor
• The reported upload rate can be less than this
  peer true speed
• Unchoke Next The protocol adds a new connection
  halfway into the unchoke_next list of the
  connection recipient.
• A node newly choked may be better off closing and
  reopening a connection to achieve a better place
  on list

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5.1 Backtracking BitTorrent

• Garbage upload A client can claim to have all
  the pieces of a file, sending garbage.
• The transmited garbage count as valid incoming
  traffic and a high bandwidth node can bootstrap
  very quickly

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6. Open Questions

• But what happens with antisocial and malicious
  peers.
• A user could publish a file with a Trojan horse,
  and then generate a bootstrap list of probable
  affected targets from trackers ip list.
• Sybil attack could slow down distribution by
  providing garbage or no service
• Open Question 1 For cases of agreement in the
  presence of f faults, 3f1 machines are
  requirement to reach consensus.
• What if some of the faulty nodes are actually
  rational nodes that are unable to express
  themselves?

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6. Open Questions

• Open Question 2 How a faulty node acting
  arbitrarily can change faithfulness properties?
• Can one design faithful mechanisms in the
  presence of faulty nodes?
• What about a network with n-1 obedient nodes and
  1 rational node
• Open Question 3 Can program slicing ideas/tools
  be applied in finding manipulation points in real
  systems?

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Thanks !!!