A P2P IRC Network Built on Top of the Kademlia Distributed Hash Table

Overview

History Distributed Hash Tables

IRC

Design

Demo

Distributed Hash Tables (DHTs)

Operations get(key) put(key, value)

How do you find information in a p2p network?

How do peers locate eachother?

Who’s responsible for what?

Generally decentralized

Fault tolerance is important

Must scale well

DHTs– What are they good for?

Web caching and CDNs

Distributed file systems

DNS

Instant messaging

P2P file sharing

History of DHTs - Centralized

Napster

Peers sent local files upon join

Central point of failure

Can only grow as much as central server can

History of DHTs - Gnutella

Flooding

Avoided central point of failure

O(n) look up

Inefficient Extensive modification by devs made it much more

efficient

History of DHTs - Chord

Peers and keys share 160-bit key-space (SHA-1)

Logical ring of keys and peers

successor(k) indicates node responsible for a key k

Each node keeps track of nodes ahead of you in the ring O(n) look up

Finger table improves look up speed Peers keep table of m other peers ith entry contains successor((n+2^(i-1)) mod2^m) O(logn) look up

Kademlia

Peers and keys share 160-bit key-space (SHA-1)

Use XOR to determine distance between peers/keys Symmetric distance measure Satisfies triangle inequality Hashes with long shared prefixes considered close

If local lookup fails, ask peers with IDs closer to key by XOR

O(logn) look up

Kademlia

Peers keep up to 160 buckets of peers called k-buckets Initially one bucket Buckets split into two once they contain k peers

Peers ping peers in k-buckets and remove stale peers to make room for new peers

How do new peers find out about others? Bootstrap nodes

Kademlia

KRPC protocol ping(node_id) find_node(node_id)

If node_id isn’t in your local list, find nodes closer to it than you are (if any) and send find_node(node_id) to them.

get(key) Keep track of peers already queried. Stop the query once

the value is found or no nodes closer to the value is found put(key,value)

Similar to get(key)

IRC

Internet Relay Chat

Servers host channels

Clients connect to servers, join channels and send eachother messages

RFC 2810,2811,2812,2813

RFC 1459