Content Addressable Network

Abhishek Kedia & Shonit Chamadia

1

What is CAN ?

CAN is an implementation of distributed hash table (DHT).

2

Distributed Hash Table

Hash Table

  • Mapping from keys to values.
  • Use a "hash function" to map keys into a reduced space.
    • For eg.
  • Use this hashed value (       (x) ) to index values into an array.
H(x) = x \text{ mod } 100
H(x)=x mod 100H(x) = x \text{ mod } 100

In DHT the hash table is distributed over a set of inter-connected nodes/peer and is available concurrently to each peer.

H
HH

3

Split and distribute

Approach

Centralized

Distributed

Keys

Values

keys on the same node can be easily accessed

But we need some Routing Mechanism to access keys on other nodes

4

Applications of DHT

Essential for network-wise indexing of resources (files, etc.)

  • BTDigg: BitTorrent search engine

  • cjdns: routing engine for mesh-based networks

  • Freenet: a censorship-resistant anonymous network

  • Hazelcast: Open-source in-memory data grid

  • I2P: An open-source anonymous peer-to-peer network.

  • I2P-Bote: serverless secure anonymous e-mail.

  • YaCy: a distributed search engine

  • Tox: an instant messaging system

  • Twister: a microblogging peer-to-peer platform

5

Implementations

  • CHORD (discussed in class)

  • CAN

  • Pastry/Tapestry

  • SkipNet

  • Symphony
    ...

6

CHORD

  • Structured P2P Network

  • Hash Key Space -> Ring

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Key Space 1-Dimensional Ring

Obvious Extension

d-dimensional torus !

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d-torus

d times

\mathbf{T}^d = S^1 \times \cdots \times S^1
Td=S1×⋯×S1\mathbf{T}^d = S^1 \times \cdots \times S^1

d-torus can be obtained from d-dimensional hypercube by gluing opposite faces together.

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2-torus

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CAN

  • Hash function maps keys onto points on the surface of a d-torus.                                         
  • The surface is divided into zones which are maintained by individual nodes/peers.

Each peer maintains:

  • (Key,Value) pairs corresponding to points residing in its zone.

  • Address of immediate neighboring peers.

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Structure

Peer

Resource

Note - Actually the space wraps onto itself, but that has not been shown for simplicity

12

Routing Mechanism

x

Queries resource with key K

H(k) = (a,b)

(a,b)

Request greedy forwarded neighbor closest to destination

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Average Path Length

If d-dimensional space is partitioned into n equal zones, then

Average Path Length = 

\frac{d}{4} n^{1/d}
d4n1/d\frac{d}{4} n^{1/d}

By choosing                               ,we can achieve similar scaling as in CHORD

d = log_2 n
d=log2nd = log_2 n

CAN Construction

Network State at some point in time

New Node willing to enter the network

Bootstrap Node

Requests Address 

of already existing

node

Responds with address of I

I
II

Bootstrap Response I picks a random point P in space

P
PP

(x,y)

I routes JOIN request to P

On receiving JOIN request zone containing P splits into half

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CAN Construction

I
II

On receiving JOIN request zone containing P splits into half

New Node

One half of the new split is assigned to the new node

Splitting node updates its own neighbor list, and also provides information to the new node 

Finally both old & new nodes' neighbors must be informed about this relocation

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  • These nodes send an update message to their neighbors.

  • Also, every node in the system sends a periodic "heartbeat" to its neighbors

These two ensure that all of the neighbors will quickly learn about the change

  • Addition of a node affects only a small number of nodes in a very small locality 
  • The number of neighbors a node maintains depends only on the dimensionality and independent of number of nodes 

16

Node Departure

Graceful Departure

  • One of the neighbors takes over. The takeover is decided as:
    • If     a neighbor that can be merged to produce a valid zone, then it is the takeover node. 
    • Else, neighbor with the smallest volume temporarily handles both volumes.
  • Departing node hands over its (key,value) pairs to the takeover node.
\exists
∃\exists

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Node failure

Detection

  • Each node periodically sends "heartbeat"/update messages to its neighbors.
  • Prolonged absence of heartbeat signals node failure.

Takeover

  • Each neighbor starts a takeover timer in proportion to its volume.
  • When timer expires, sends TAKEOVER message to other neighbors. They turn off their timers.

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INSA Lyon - IST-4-NET2 - Networks Presentation

By Abhishek Kedia

Source: shonitchamadia

INSA Lyon - IST-4-NET2 - Networks Presentation

Introduction to Content Addressable Network. Review of the paper [http://www.eecs.berkeley.edu/~sylvia/papers/cans.pdf]

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