feat(kad): add config option for bootstrap interval

[mxinden]

When using libp2p-kad it is recommended to regularly call Kademlia::bootstrap to ensure a healthy routing table.

rust-libp2p/protocols/kad/src/behaviour.rs

Lines 846 to 865 in fc6efaf

	/// Bootstraps the local node to join the DHT.
	///
	/// Bootstrapping is a multi-step operation that starts with a lookup of the local node's
	/// own ID in the DHT. This introduces the local node to the other nodes
	/// in the DHT and populates its routing table with the closest neighbours.
	///
	/// Subsequently, all buckets farther from the bucket of the closest neighbour are
	/// refreshed by initiating an additional bootstrapping query for each such
	/// bucket with random keys.
	///
	/// Returns `Ok` if bootstrapping has been initiated with a self-lookup, providing the
	/// `QueryId` for the entire bootstrapping process. The progress of bootstrapping is
	/// reported via [`Event::OutboundQueryProgressed{QueryResult::Bootstrap}`] events,
	/// with one such event per bootstrapping query.
	///
	/// Returns `Err` if bootstrapping is impossible due an empty routing table.
	///
	/// > **Note**: Bootstrapping requires at least one node of the DHT to be known.
	/// > See [`Behaviour::add_address`].
	pub fn bootstrap(&mut self) -> Result<QueryId, NoKnownPeers> {

See e.g. libp2p-server calling Kademlia::bootstrap on an interval.

rust-libp2p/misc/server/src/main.rs

Line 26 in fc6efaf

const BOOTSTRAP_INTERVAL: Duration = Duration::from_secs(5 * 60);

Instead of requiring each user to setup their own interval on which to call Kademlia::bootstrap, provide a new option in libp2p_kad::Config. Something along the lines of:

  /// The configuration for the `Kademlia` behaviour.
  ///
  /// The configuration is consumed by [`Behaviour::new`].
  #[derive(Debug, Clone)]
  pub struct Config {
+    bootstrap_interval(Option<Duration>), 

[thomaseizinger]

Is "bootstrap" a kademlia term (from the paper)? Maybe we should call it "refresh"?

[PanGan21]

Hi, I was looking at this issue. Since a new refresh_interval will be added in the Config of Kademlia, isn't it required to move the following logic from the server in the kademlia behaviour?

if let Poll::Ready(()) = futures::poll!(&mut bootstrap_timer) {
            bootstrap_timer.reset(BOOTSTRAP_INTERVAL);
            let _ = swarm.behaviour_mut().kademlia.as_mut().map(|k| {
                k.bootstrap();
            });
        }

Otherwise the interval should be exposed from kademlia in a way to the client code I think

[thomaseizinger]

Yes correct. We will have to have a timer within the libp2p_kad::Behaviour struct that is polled as part of Behaviour::poll (and calls self.bootstrap() when the timer fires). Does that make sense?

[PanGan21]

Makes sense yes. Can I give it a try?