How do BitTorrent magnet links work?

Question

For the first time I used a magnet link. Curious about how it works, I looked up the specs and didn\'t find any answers. The wiki says xt means \"exact topic\"

Answer 1

Peer discovery and resource discovery (files in your case) are two different things.

I am more familiar with JXTA but all peer to peer networks work on the same basic principles.

The first thing that needs to happen is peer discovery.

Peer Discovery

Most p2p networks are "seeded" networks: when first starting a peer will connect to a well-known (hard-coded) address to retrieve a list of running peers. It can be direct seeding like connecting to dht.transmissionbt.com as mentioned in another post or indirect seeding as usually done with JXTA where the peer connects to an address that only delivers a plain text list of other peers network addresses.

Once connection is established with the first (few) peer(s), the connecting peer performs a discovery of other peers (by sending requests out) and maintains a table of them. Since the number of other peers can be huge, the connecting peer only maintains part of a Distributed Hash Table (DHT) of the peers. The algorithm to determine which part of the table the connecting peer should maintain varies depending on Network. BitTorrent uses Kademlia with 160 bit identifiers/keys.

Resource Discovery

Once a few peers have been discovered by the connecting peer, the latter sends a few requests out for discovery of resources to them. Magnet links identifies those resources and are built in such a way that they are a "signature" for a resource and guarantee that they uniquely identify the requested content among all the peers. The connecting peer will then send a discovery request for the magnet link/resource to peers around it. The DHT is built in such a way that it helps determine which peers should be asked first for the resource (read on Kademlia in Wikipedia for more). If the requested peer does not hold the requested resource it will usually "pass on" the query to additional peers fetched from its own DHT.

The number of "hops" the query can be passed on is usually limited; 4 is an usual number with JXTA type networks.

When a peer holds the resource, it replies with its full details. The connecting peer can then connect to the peer holding the resource (directly or via a relay - I won't go into details here) and start fetching it.

Resources/Services in P2P networks are not directly attached to network addresses: they are distributed and that is the beauty of these highly scalable networks.