How does DHT in torrents work

Question

I m coding a p2p implementation that I would like to make decentralized however I m having some trouble grasping how DHT in protocols like bittorrent work  How does the client know where the peers are if there is no tracker  Are peers stored in the actual torrent file

User · Answer

What happens with bittorrent and a DHT is that at the beginning bittorrent uses information embedded in the torrent file to go to either a tracker or one of a set of nodes from the DHT. Then once it finds one node, it can continue to find others and persist using the DHT without needing a centralized tracker to maintain it.

The original information bootstraps the later use of the DHT.

User · Answer

The general theory can be found in wikipedia s article on Kademlia  The specific protocol specification used in bittorrent is here  http   wiki theory org BitTorrentDraftDHTProtocol

User · Answer

DHT nodes have unique identifiers  termed  Node ID  Node IDs are chosen at random from the same 160-bit space as BitTorrent info-hashes  Closeness is measured by comparing Node ID s routing tables  the closer the Node  the more detailed  resulting in optimal   What then makes them more optimal than it s predecessor  Kademlia  which used simple unsigned integers  distance A B     A xor B  Smaller values are closer  XOR  Besides not being secure  its logic was flawed   If your client supports DHT  there are 8-bytes reserved in which contains 0x09 followed by a 2-byte payload with the UDP Port and DHT node  If the handshake is successful the above will continue

User · Answer

With trackerless DHT torrents  peer IP addresses are stored in the DHT using the BitTorrent infohash as the key   Since all a tracker does  basically  is respond to put get requests  this functionality corresponds exactly to the interface that a DHT  distributed hash table  provides  it allows you to look up and store IP addresses in the DHT by infohash   So a  get  request would look up a BT infohash and return a set of IP addresses   A  put  stores an IP address for a given infohash   This corresponds to the  announce  request you would otherwise make to the tracker to receive a dictionary of peer IP addresses   In a DHT  peers are randomly assigned to store values belonging to a small fraction of the key space  the hashing ensures that keys are distributed randomly across participating peers   The DHT protocol  Kademlia for BitTorrent  ensures that put get requests are routed efficiently to the peers responsible for maintaining a given key s IP address lists

[protocols] How does DHT in torrents work?

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