Samsara:%20Honor%20Among%20Thieves%20in%20Peer-to-Peer%20Storage

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Samsara Honor Among Thieves in Peer-to-Peer
Storage

• Landon P. Cox and Brian D. Noble
• University of Michigan

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Samsara

• From Wikipedia, the free encyclopedia
• Sa?sara or Sa?sara (Sanskrit ?????)
• Literally means "continuous flow"
• Is the cycle of birth, life, death, rebirth or
  reincarnation within many Eastern religions

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Paper overview

• Proposes an incentive mechanism motivating
  participants in a P2P distributed file system to
  contribute as much space as they consume
• Addresses the tragedy of the commons
• Requires each peer that requests storage from
  another peer to hold a claim for same amount of
  storage
• Claims can be exchanged

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The tragedy of the commons

• Assume a group of herders that a common pasture,
  on which they are entitled to let their cows
  graze
• To maximize his/her personal benefit, each herder
  will put as many cows as it can on the common
  pasture
• As a result, the common pasture becomes
  overgrazed and useless
• Happened to the Boston Common

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Boston common
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Introduction

• P2P file systems have many advantages
• Require users to consume storage according to
  their contribution
• Otherwise system will collapse
• Solution is a mechanism enforcing"storage
  fairness"
• Incentive mechanism

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Extant solutions

• A trusted third-party enforcing quotas
• Requires a centralized administration
• Letting people buy and sell storage space
• Requires a trusted clearance infrastructure
• Using certified identities and trusted keys
• Requires a trusted certification authority
• Enforcing total symmetry within pairs of peers
• Unpractical

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Samsara key idea (I)

• Manufacture symmetric relations
• through claim forwarding
• All exchanges of data for claims form symmetric
  contracts
• Each node periodically checks the other for
  compliance
• Done in a probabilistic fashion
• When a node breaches the contract, other node is
  free to drop the data of its partner

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Samsara key idea (II)

• Nodes can forward claims rather than honoring
  them
• Still remain responsible for the claims they have
  forwarded
• Mechanism penalizes unresponsive nodes in a
  probabilistic fashion
• A node suffering a short outage may lose some
  replicas of its data

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Background

• Samsara is an add-on to Pastiche a P2P
  cooperative backup system
• To be discussed later
• Built itself on top of Pastry network

Pastiche Pastiche
Samsara Pastry
OS Disks OS Disks
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Overall design

• Objective is equal exchange
• If A stores data for B then B must store an
  equal-size claim for B
• If B discards As claim then A can discard Bs
  data
• Equal exchange is enforced by periodic queries
• Not answering a query is a sufficient reason to
  have you data dropped

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The problem

• This simple claim model punishes nodes too
  severely for transient failures
• New approach
• Is probabilistic
• Takes into account transient failures
• When a node fails to answer a query, each of is
  replica sites drops data with some probability

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Claim construction (I)

• Claims are incompressible placeholders
• Computing a claim requires
• a secret passphrase P
• a secret symmetric key K
• and a location in storage space

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Claim construction (II)

• Assuming we have 512-byte claims
• The first claim C0 would contain
• Twenty-five 20-bit hashes hi SHA1(P, i) where
  P is the secret pass phrase and i the hash
  index
• First 12 bits of next hash in sequence
• all encrypted with the symmetric key K
• C0 h0, h1, , first 12 bits of h25K

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Claim construction (III)

• Successive claims are built using repeating the
  process
• C1 h26, h27, , first 12 bits of h51K
• Ci hj, hj1, , first 12 bits of hj25K
• where j 26i

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Answering claim queries

• Can be done with a single SHA1 hash
• Querying party provides
• Unique value h0
• List of objects to verify
• Responding party
• Append h0 to first object O0 in list and compute
  h1 SHA1(O0, h0)
• Recursively computes hi1 SHA1(Oi, hi)
• Returns last hj

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Example (I)
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Example
B has claim ß1 on A and B has claim ?1 on B
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Example
Node B does not have enough space to hold claim
?1
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Example
Node B forwards its claim for space on node A to
node C
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Claim forwarding

• If a node X
• has a claim ? on another node Y and
• owns a claim ? to a third node Z
• It can forward its claim ? to node Y
• Everything works fine until a node fail

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Failures in dependency chains
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Failures in dependency chains

• Before failure,
• B stores data for A,
• C stores data for B
• E stores data for D and hold a claim e1 on A
• When C fails and stop answering queries from B,
• B uses it storage rights on A and replaces claim
  e1 by its own claim ß1

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Failures in dependency chains

• After that we have a cascade of damaging actions
• A fails to answer queries from E
• E holds D responsible for loss of claim e1 and
  discards the data it had stored for D
• D loses its backup data on E even though it had
  always operated in a correct fashion
• Forwarding claims increases the risk of data
  losses

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Failures in dependency cycles
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Failures in dependency cycles

• The effect of a failure is much less dramatic
  when we have a dependency cycle, where
• B stores data for A,
• C stores data for B
• E stores data for D
• A stores data for E

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Failures in dependency cycles

• When C fails and stop answering queries from B,
• B uses it storage rights on A and requests it to
  store its claim ß1
• Since A stores data for E, it can forward claim
  ß1 to E
• Since E stores data for D, it can forward claim
  ß1 to E
• E keeps claim ß1 because it has data on E

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Evaluation

• Samsara is faster than scp
• Most chain are short as long as there is free
  space
• Great news!
• Nodes should forward claims in a very
  conservative fashion to minimize data losses