How can we download torrents even when the tracker is down ?
Every torrent client uses a DHT (Distributed Hash Table) algorithm Kademlia invented by Petar Maymounkov & David Mazières.
Lets Begin :-)
- Every torrent client has a DHT node with it. (Mostly enabled by default)
- The node is assigned a 160 bit id.
- Every torrent also has an "infohash" - 160 bits.
The nodes are arranged as the leaves in a giant binary tree. The id corresponds to the nodes position.
If we only had 3 bit ids the below would be the entire network.
Distance between nodes
- XOR is used to measure the distance between to id's or nodes.
Why XOR ?
- Has a lot of interesting properties.
- d(x,y) = d(y,x), unidirectional, d(x,x) =0 etc
- Unidirectional - there is only one point with a given distance from a given point.
- Originally 3 RPC's - Find Node, Find Value, Store Value.
- Ping list of known nodes that are closest to the node we want to find.
- We then recursively ping those nodes in the same way till we find our node.
- Closeness is measured using the "distance" defined by the XOR.
- Similar to find node.
- But in this case we get the list of IP's instead who have our torrent.
- We search for the infohash - 1E69917FBAA2C767BCA463A96B5572785C6D8A12 - (Pride & Prejudice)
Routing Table - List of nodes with us.
- Stored in K-buckets
- Gives preference to older active nodes.