µtorrent - DHT?

It's not just related to file sharing. In fact, a DHT allows you to find anything (not just a shared file) on the Internet that has a solid digital fingerprint (SHA1 or more, and even MD5). This can be used to secure cryptographic key exchanges without knowing the location on the network of the person to whom we are sending the key, and even completely anonymously: instead of searching for the public key of the recipient, we will use their key to encrypt our own private key and the unique identifier of the service for which we are sending the decryption key to secure a connection. We hash it all with a reliable digital fingerprint (SHA512) and send the search for this fingerprint on the DHT, with the recipient also participating in the DHT and listening for an inverse key on the DHT. The principle of the DHT is to form a connected network of the highest possible degree (maximizing the routing length before looping and creating redundant paths between the DHT participants). For that, each participant in the DHT has a unique random identifier, connects to an initial DHT tracker which will give them (for example) 64 addresses of DHT peers respecting at least a certain number of bits of difference in their unique random identifier. We browse the DHT for a certain time to see if we can find no duplicate network addresses, neither on ourselves nor our own network address; for the same degree, we compare the number of common neighbors to determine which of our peers has the least in common with the others of the same degree and keep the list sorted according to this criterion; if we build a DHT with 64 degrees, the list cannot exceed 64 peers, we choose to eliminate in priority the peers with the lowest degree and otherwise those with the most common neighbors in duplicate); if we do not find a duplicate, we progress by successively querying the neighbors. The method of constructing the DHT is quite fast and consumes little resources, quickly achieving a tree-like network of almost exponential size (each "degree" of the DHT doubles the number of neighbors).
The goal then is to construct a graph that allows querying from close to close (using all the peers we are "connected" to (generally no more than 64) by transmitting our key searches: the DHT is designed so that, after the peers have queried each of their own DHT peers, we will have transmitted minimal duplicate searches to the same members of the global DHT via different paths. However, the DHT allows for multiple paths (DHT members can disconnect or become inaccessible or overloaded). It thus creates a global graph, very robust, stable, allowing querying of a network containing a plethora of disparate data. A DHT by itself does not allow searching for a specific file, just a digital fingerprint: all peers connected to the DHT will ultimately query their immediate peers to see if they know a fingerprint: the responses will be returned directly to the one who requested it by the reverse path (we can have a duplicate response through several paths, but there will generally be very few).
Thus, we end up with a list of hosts on the Internet that know a key and where we can connect to search for it explicitly: here we can check for example a real identity (perform a PKI key exchange, verify public signatures, PGP keys, etc...), connect securely once the identity is verified, then ask for what we really wanted with the searched fingerprint on the DHT: the host will say if it actually has what we requested with this anonymous fingerprint. We can also use the DHT to search for a non-anonymous file (only by its known and public digital fingerprint of its content: this is what is used for file sharing for example on Torrents where we generally search for a file with a public SHA1 fingerprint and we check that we indeed have the file we wanted once connected by asking for a longer but more secure key like another SHA512 fingerprint).
The DHT can be used for many things, including consolidating a blockchain and preventing a takeover and fragmentation of the network into several large islands and many isolated islets, as it would be very complicated to block 100% of the Internet (at worst, a country can block all Internet addresses of other countries to cut off bridges and create an islet, but it's actually very difficult because of VPNs and various means of transmitting a routing address or a mandatory service allowing interconnection, and there are many encryption processes that prevent seeing what we do on a private connection transiting over the Internet through various intermediaries depending on the load of the routers, etc.)
A DHT is therefore quite generic. With it, one can consolidate any peer-to-peer protocol, and even consolidate the routing of the internet itself (the links allowing several distinct networks to locate a routing path through announcements, which are themselves digitally signed): indeed, the Internet itself, already IPv4 alone, is a peer-to-peer network, just like the entire DNS system (everything works through delegations of trust and multiple possible paths, aiming to consolidate the structure so that if one of the network links fails, we can still find alternative routes, assess them, see which ones work best, provoke the least duplicate transmissions through the possible alternative paths).
The concept of the DHT is therefore very valuable: it is difficult to break the network and isolate parts because alternative routes can easily be found. And the DHT network optimizes itself (even though the DHT is not very fast to respond: the paths followed in the DHT searches make many "hops" but in fact 64 hops each covered in ~50ms gives a very acceptable response time of just over 3 seconds to search for something precious because unique in a dataset comprising millions (I should say billions of billions) of very different elements of different types known over billions of connected objects to the Net, and it requires very little bandwidth because we are not obliged to query every billion hosts on the network by connecting to each (which takes much longer).
In principle, the DHT is thus much faster and more reliable and does not require any permanent session (no need for TCP to interconnect its peers: queries are made with very small isolated datagrams: DHT members who receive a datagram can directly ignore it without doing anything else if this datagram does not "match" enough common bits with the unique keys searched, and if they respond, they do not need to wait for an acknowledgment from the one who was searching for the key; they can however respond multiple times to a search if a response has been lost along the way and if the same pair from the DHT does not request too frequently, for example with a minimum of 30 seconds to 1 minute before retransmitting the same response)
Based on a DHT, one can therefore build a protocol as reliable as TCP, with very good delivery guarantees, even across a giant network that is very unstable and heterogeneous in the speed and throughput of its individual connections. The DHT itself does not require cryptography; anyone can enter it (even a hacker wanting to sabotage it will not sabotage it, on the contrary, it will strengthen it and we do not need to know "who we are talking to" on a DHT, all lies eventually isolate and relegate far in the network those who do not respect the normal rules allowing the DHT to be constructed: the DHT easily detects "spoofing," for example, which is "man-in-the-middle" type attacks because it constrains each member to respect the highest degrees and otherwise will see themselves relegated to a greater distance and quickly become increasingly inaccessible and unused: the DHT requires trust but first and foremost enforces honesty, which is not easy to achieve with traditional P2P including IP routing protocols and possible routing announcements where someone can still easily divert traffic and "silence" certain parts of the IP network).
The DHT is an excellent building block in terms of efficiency; it is very economical. One can create a DHT connected simultaneously in IPv4 and IPv6 or any other protocol; there will always be an intermediary that understands and adapts the protocols (although the DHT is normally designed to be connected in "datagram" mode, generally UDP, it may have some peers using reliable transits like TCP or a serial port, a USB hub, a PCI bus, shared memory addresses in a host, no matter what, the DHT functions over a very heterogeneous network).