kademlia | Dead simple C11 kademlia distributed hash table library | Hashing library

In order to get going initially, geth uses a set of bootstrap nodes whose endpoints are recorded in the source code.

Source: Geth docs

Here's the list of the bootstrap nodes hardcoded in the Geth source code: https://github.com/ethereum/go-ethereum/blob/v1.10.11/params/bootnodes.go#L23

The --bootnodes option allows you to overwrite this list with your own. Example from the above linked docs:

how the sibling list works?

It seems like it replaces the refinement of the bucket splitting for unbalanced trees with a a list of the closest known nodes relative to the local node ID. Unlike the bucket splitting approach it uses a different parameter instead of the bucket size K.

The details don't seem to be spelled out but it seems logical that one simply computes whether a node would be inserted in that list based on the currently furthest node in that list (assuming the maximum size based on the new parameter has been reached) and otherwise spilling it into the main routing table which is still based on buckets.

how can it solve highly unbalanced tree problem?

Pretty much the same way that kademlia does with the refined splitting approach (which many implementations fail to consider!), but in a way that's easier to reason about and can be parametrized separately.

I guess this is simply under the assumption that α ≤ k. Under that condition you will get the k closest nodes automatically from the closest bucket, or if the bucket contains fewer than α nodes the bracketed condition will apply

(or, if that bucket has fewer than α entries, it just takes the α closest nodes it knows of)

Also note that you're looking at the pre-proceedings version of the paper, which does not contain the full kademlia description. You can find the full paper here.

Does this flow look all right?

It seems roughly correct, but your wording is not very precise.

Each node has a routing table by which it organizes the neighbors it knows about and another table in which it organizes the data it is asked to store by others. Nodes have a quasi-random ID that determines their position in the routing keyspace. The hashes of keys for stored data don't precisely match any particular node ID, so the data is stored on the nodes whose ID is closest to the hash, as determined by the distance metric. That's how node IDs and key hashes are used for both.

When you perform a lookup for data (i.e. find_value) you ask the remote nodes for the k-closest neighbor set they have in their routing table, which will allow you to home in on the k-closest set for a particular target key. The same query also asks the remote node to return any data they have matching that target ID.

When you perform a find_node on the other hand you're only asking them for the closest neighbors but not for data. This is primarily used for routing table maintenance where you're not looking for any data.

Those are the abstract operations, if needed an actual implementation could separate the lookup from the data retrieval, i.e. first perform a find_node and then use the result set to perform one or more separate get operations that don't involve additional neighbor lookups (similar to the store operation).

Since kademlia is UDP-based you can't really serve arbitrary files because those could easily exceed reasonable UDP packet sizes. So in practice kademlia usually just serves as a hash table for small binary values (e.g. contact information, public keys and such). Bulk operations are either performed by other protocols bootstrapped off those values or by additional operations beyond those mentioned in the kademlia paper.

What the paper describes is only the basic functionality for a routing algorithm and most basic key value storage. It is a spherical cow in a vacuum. Actual implementations usually need additional features or work around security and reliability problems faced on the public internet.

But then what's bootstrapping node? Is it another process that's always running? What if the bootstrapping node gets turned off?

That's covered in this question (by example of the bittorrent DHT)