>>8696
Hi IHM,
A BitTorrent magnet link identifies a torrent using1 a SHA-1 or truncated SHA-256 hash value known as the "infohash". This is the same value that peers (clients) use to identify torrents when communicating with trackers or other peers. A traditional .torrent file contains a data structure with two top-level keys: announce, identifying the tracker(s) to use for the download, and info, containing the filenames and hashes for the torrent. The "infohash" is the hash of the encoded info data.
Some magnet links include trackers or web seeds, but they often don't. Your client may know nothing about the torrent except for its infohash. The first thing it needs to is find other peers who are downloading the torrent. It does this using a separate peer-to-peer network2 operating a "distributed hash table" (DHT). A DHT is a big distributed index which maps torrents (identified by infohashes) to lists of peers (identified by IP address and ports) who are participating in a swarm for that torrent (uploading/downloading data or metadata).
The first time a client joins the DHT network it generates a random 160-bit ID from the same space as infohashes. It then bootstraps its connection to the DHT network using either hard-coded addresses of clients controlled by the client developer, or DHT-supporting clients previously encountered in a torrent swarm. When it wants to participate in a swarm for a given torrent, it searches the DHT network for several other clients whose IDs are as close3 as possible to the infohash. It notifies these clients that it would like to participate in the swarm, and asks them for the connection information of any peers they already know of who are participating in the swarm.
When peers are uploading/downloading a particular torrent, they try to tell each other about all of the other peers they know of that are participating in the same torrent swarm. This lets peers know of each other quickly, without subjecting a tracker or DHT to constant requests. Once you've learned of a few peers from the DHT, your client will be able to ask those peers for the connection information of yet more peers in the torrent swarm, until you have all of the peers you need.
Finally, we can ask these peers for the torrent's info metadata, containing the filenames and hash list. Once we've downloaded this information and verified that it's correct using the known infohash, we're in practically the same position as a client that started with a regular .torrent file and got a list of peers from the included tracker.
The download may begin.
Hope this helps!!1!