feat: kad-dht (#290)

Author: salmad3

content/concepts/introduction/protocols/dht.md

@@ -1,9 +1,74 @@
 ---
 title: "Kademlia DHT"
-description: ""
+description: "The libp2p Kad-DHT subsystem is an implementation of the Kademlia
+DHT, a distributed hash table."
 weight: 25
 ---
 
-Coming soon!
+## Overview
 
-For now, check out the following [Launchpad guide on the DHT](https://curriculum.pl-launchpad.io/curriculum/libp2p/dht/).
+The Kademlia Distributed Hash Table (DHT), or Kad-DHT, is a distributed hash table
+that is designed for P2P networks.
+
+Kad-DHT in libp2p is a subsystem based on the
+[Kademlia whitepaper](https://pdos.csail.mit.edu/~petar/papers/maymounkov-kademlia-lncs.pdf).
+
+Kad-DHT offers a way to find nodes and data on the network by using a
+[routing table](https://en.wikipedia.org/wiki/Routing_table) that organizes peers based
+on how similar their keys are.
+
+<details>
+  <summary>A deeper look</summary>
+
+  The routing table is organized based on a prefix length and a distance metric.
+  The prefix length helps to group similar keys, and the distance metric helps to
+  find the closest peers to a specific key in the routing table. The table maintains
+  a list of `k` closest peers for each possible prefix length between `0` and `L-1`,
+  where `L` is the length of the keyspace, determined by the length of the hash
+  function used. **Kad-DHT uses SHA-256**, with a keyspace of 256 bits, trying to maintain
+  `k` peers with a shared key prefix for every prefix length between `0` and `255` in
+  its routing table.
+
+  The prefix length measures the proximity of two keys in the routing table and
+  divides the keyspace into smaller subspaces, called "buckets", each containing nodes
+  that share a common prefix of bits in their SHA-256 hash. The prefix length is the
+  number of bits that are the same in the two keys' SHA-256 hash. The more leading bits
+  that are the same, the longer the prefix length and the closer the proximity of the
+  two keys are considered to be.
+
+  The distance metric is a way to calculate the distance between two keys by
+  taking the bitwise exclusive-or (XOR) of the SHA-256 hash of the two keys. The
+  result is a measure of the distance between the two keys, where a distance of
+  `0` means the keys are identical, and a distance of `1` means that only one
+  bit is different, meaning the two keys are close to each other (i.e. their
+  SHA-256 hashes are similar).
+
+  This design allows for efficient and effective lookups in the routing table when
+  trying to find nodes or data that share similar prefixes.
+
+</details>
+
+## Peer routing
+
+The Kad-DHT uses a process called "peer routing" to discover nodes in the
+network. When looking for a peer, the local node contacts the `k` closest nodes to
+the remote peer's ID asking them for closer nodes. The local node repeats the
+process until it finds the peer or determines that it is not in the network.
+
+## Content provider routing
+
+Kad-DHT also includes a feature for content provider discovery, where nodes can
+look up providers for a given key. The local node again contacts the `k` closest
+nodes to the key asking them for either providers of the key and/or closer nodes
+to the key. The local node repeats the process until it finds providers for the
+key or determines that it is not in the network.
+
+## Bootstrap process
+
+To maintain a healthy routing table and discover new nodes, the Kad-DHT includes
+a bootstrap process that runs periodically. The process starts by generating a random peer
+ID and looking it up via the peer routing process. The node then adds the closest peers it
+discovers to its routing table and repeats the process multiple times. This process also
+includes looking up its own peer ID to improve awareness of nodes close to itself.
+
+{{< alert icon="💡" context="note" text="See the Kademlia DHT <a class=\"text-muted\" href=\"https://github.com/libp2p/specs/tree/master/kad-dht\">technical specification</a> for more details." />}}