CHORD

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CHORD peer to peer lookup protocol
Shankar Karthik Vaithianathan Aravind Sivaraman
University of Central Florida
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Outline

• History
• General definition of a DHT system
• Chord
• Applications
• Implementation
• Demo/Open questions

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History client-server model
query
data transferring
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History peer-to-peer model (Napster)
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History peer-to-peer model (Gnutella)
query
data transferring
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History peer-to-peer model (DHT systems)
query
data transferring
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Distributed Hash Table
What is normal hashing ? Putting items into say n
buckets, often solved by a static solution(e.g
H(key)mod N) Problems N changes ? We must move
every thing (simple solution) or could we move
just (items) / (buckets) on an average (optimal
solution) what if we do not know the number of
buckets ?
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Distributed Hash Table
Consistent Hashing

• Let A B be Hashing. They send an object k to
  two
• different buckets, say y and z, respectively.
  Then either
• A cannot see bucket z or B cannot see bucket y.
• Each bucket is a assigned a number in (0,1), and
• each item gets a value (0,1). You could then be
  in
• according to least distance or always round
  down(or
• up) and wrap at the edges.

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Distributed Hash Table
Consistent Hashing ctd
If a new bucket is added, we therefore need to
move the Contents of bucket immediately before
it. If we let each bucket have m elements and (M
log n) then each bucket gets a fair share of
load and on an average we move only the average
contents of the bucket around. If we let each
bucket have m elements and (M log n) then each
bucket gets a fair share of load and on an
average we move only the average contents of the
bucket around.
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Distributed Hash Table

• a new class of peer-to-peer routing
  infrastructures.
• support a hash table-like functionality on
  Internet-like scale
• Consider the buckets to be computers on the
  internet and that given a key, they should find
  the value(IP address)
• The challenges with such a system are
• Load balancing
• Scalability
• dynamic nature
• no critical point
• deterministic.

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Basic DHT components

• an overlay space and its partition approach
• a routing protocol
• local routing table
• next-hop decision
• a base hash function
• variants proximity-aware, locality-preserving,
  etc.

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Consistent hashing
data
server
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Chord Stoica et al. Sigcomm2001

• Overlay space
• - one-dimensional unidirectional key space 0
  2m-1.
• Given two m-bit identifiers x and y, d(x,
  y)(y-x2m) 2m
• - Each node is assigned a random m-bit
  identifier.
• - Each node is responsible for all keys equal to
  or before its identifier until its predecessor
  nodes identifier in the key space.

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Chord routing protocol

• Routing table (finger table)
• - (at most) m entries. The ith entry of node n
  contains the pointer to the first node that
  succeeds n by at least 2(i-1) on the key space, 1
  ? i ? m.
• Next-hop decision
• For the given target key k, find the closest
  finger before (to) k and forward the request to
  it.
• Ending condition The request terminates when k
  lies between the ID range of current node and its
  successor node.
• - The routing path length is O(log n) for a
  n-nodes network with high probability (w.h.p.).

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Chord an example (m8)
Network node
A Chord network with 8 nodes and 8-bit key space
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Chord routing table setup
Network node
Pointer
0
255
A Chord network with 8 nodes and 8-bit key space
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Chord a lookup for key 120
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How to look up a key quickly ?

• We need finger table

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How to look up a key quickly ?(cont.)

• finger table for node 1

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How to look up a key quickly ?(cont.)
Predecessor(1) ? successor(1)
Value of key 1 ?
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Node joins

• Two challenges
• Each nodes finger table is correctly filled
• Each key k is stored at node successor(k)

• Three operations
• Initialize the predecessor and fingers of the new
  node n
• Update the fingers and predecessors of existing
  nodes
• Copy to n all keys for which node n has became
  their successor

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Initialize the predecessor and fingers of node n

• Idea Ask an existing node for information needed

Join in
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Update the fingers and predecessors of existing
nodes

• Observation when node n joins the network, n
  will become the ith finger of a node p when the
  following two conditions meet
• P proceeds n by at least 2i-1
• The ith finger of node p succeeds n

• Solution Try to find predecessor(n- 2i-1) for
  all 1ltiltm and check whether n is their ith
  finger, and whether n is their predecessors ith
  finger.

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Update the fingers and predecessors of existing
nodes (cont.)
Predecessor(6-21-1) 3, update
Join in
?6
?6
?6
?6
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Copy to n all keys for which node n has became
their successor

• Idea Node n can become the successor only for
  keys stored by the node immediately following n

Join in
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Extended Chord protocol

• Concurrent joins
• Failures and replication
• Beyond are project scope. Ref. Stoica et, all
  sigcomm 2001

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• Example applications
• Co-operative Mirroring
• Time-shared storage
• Distributed indexes
• Large-Scale combinatorial search

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Implementation Our current implementation is
simulation of the Chord protocol in O(log n)
steps in java. The framework can be shown as
below
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Methods Implemented
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Conclusion Questions ? Reference Chord A
Scalable Peer to peer Lookup Service for
Internet Applications Ion Stoica , Robert Morris,
David Karger, M. Frans Kaashoek, Hari
Balakrishnan MIT Laboratory for Computer
Science chord_at_lcs.mit.edu http//pdos.lcs.mit.edu/
chord/
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THANK YOU