How does BitTorrent work without an (external) tracker?

[lgillis]

• Does BitTorrent work without an external tracker?

• What protocols must be used for communication?

• What does a corresponding torrent file look like?

• Is it also possible without a torrent file, for example only with a checksum?

Hi folks,

I have some idea of how BitTorrent might work without resorting to an external tracker. After all, BiglyBT even has its own tracker built in. But of course, I could have completely misunderstood, because the absolute majority still use the resident tackers …

I2P currently works with BitTorrent clients such as I2PSnark, BiglyBT and the newly added clients based on libtorrent-rasterbar. These clients all have to be reconciled, so they have to speak a common protocol with which they can communicate.

It would be nice if someone has answers that even the new participants can understand, and at best could give some practical tips.

(By the way, in this forum you don't need a fixed account under a pseudonym, after solving a puzzle you can already participate completely anonymously.)

[Guest]

Ever heard of DHT and magnet links?

[lgillis]

Ever heard of DHT and magnet links?

Es ist selbstverständlich jedermanns gutes Recht mit rhetorischen Gegenfragen aus der untersten Schublade die Angelegenheit vom Tisch wischen zu wollen. Und klar ist auch, dass sich jeder im Internet informieren kann.

https://youtu.be/QFZ_RjXyjq0 John Wick: Ruled by Consequence (die Erklärung für Fische)

[anikey]

so basically torrents without trackers work by replacing usual centralized tracker with DHT (distributed hash table). (these two mechanisms can be combined together).

Torrent with regular trackers works by telling the tracker that you are downloading/seeding (this is called announcing). Tracker then returns you some other peers that announced to it.

DHT works just like a tracker, but instead of sending to a centralized server, it sends to some of the DHT participants (i.e. all of the clients that are currently online and support DHT). (i think choosing whom exactly to send is based on torrent's infohash). The details of DHT are i guess beyond the scope of a post, but in summary i can say that I2P itself also uses such a mechanism for storing and getting leasesets and routerinfos.

Although i should note that not all clients support DHT, especially if we are talking specifically about in-I2P torrenting. Most notably, libtorrent-rasterbar (which is what qbittorrent is based on), which just recently got basic I2P support, does not yet support more advanced torrenting features such as DHT and PEX (peer exchange). (and yes, i would consider DHT to be a more advanced feature).

tl;dr: without a tracker, torrent will use DHT

[lgillis]

That's a good explanation, anikey.

Although i should note that not all clients support DHT, especially if we are talking specifically about in-I2P torrenting […]

BiglyBT has installed a tracker server. This should make it possible to close the gap.

[anikey]

BiglyBT has installed a tracker server. This should make it possible to close the gap.

If i understand you correctly, it has an embedded/built-in tracker server?

A tracker that stands alone is not as useful as DHT. You need some shared space to store and share peers. DHT provides that, a unified space. Standalone trackers do not provide that, they are fragmented (one set of peers might be using one tracker, while another set of peers using another tracker).

Something better to close the gap would be public, well-known open trackers; there are multiple known of them. So that a user without DHT can just specify one of these well-known trackers and hopefully discover peers.
A fragmented system of trackers, i think, would provide less of a chance to find peers. (Unless you somehow make the trackers federate/share peers between each other, like irc shares messages between servers).

Another idea i've had (that i don't know is possible or not) is a tracker that works as a proxy to dht. Client announces to proxy => proxy announces to DHT. Client asks for peers => proxy asks DHT => DHT answers to proxy => proxy answers to client. Again, i don't know whether that is possible. But if it is, it would be another way to close the gap.

[lgillis]

If i understand you correctly, it has an embedded/built-in tracker server?

That's how it is. But the hurdle of setting this up and managing it relatively safely is far too high when viewed in the light of day. You basically have to meet people where they are. As with PaT, there are no technical hurdles to overcome and a simple click is enough to get to your destination with the included rudimentary BitTorrent client. In addition, the good feeling that you can transfer responsibility for your own actions to the tacker operators.