peer_store: Increase peer ban time until escapes banned threshold (#4031)

This is a tiny PR to increase the time a peer remains banned.

A peer is banned when the reputation drops below a threshold.
With every second, the peer reputation is exponentially decayed towards
zero.

For the previous setup:
- decaying to zero from (i32::MAX or i32::MIN) would take 948 seconds
(15mins 48seconds)
- from i32::MIN to escaping the banned threshold would take 10 seconds
This means we are decaying reputation a bit too aggressive and
misbehaving peers can misbehave again in 10 seconds.
Another side effect of this is that we have encountered multiple
warnings caused by a few misbehaving peers.

In the new setup:
- decaying to zero from (i32::MAX or i32::MIN) would take 3544 seconds
(59 minutes)
- from i32::MIN to escaping the banned threshold would take ~69 seconds

This is a followup of:
- https://github.com/paritytech/polkadot-sdk/pull/4000.

### Testing Done
- Created a misbehaving client with
[subp2p-explorer](https://github.com/lexnv/subp2p-explorer), the client
is banned for approx 69seconds until it is allowed to connect again.

cc @paritytech/networking

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

substrate/client/network/src/peer_store.rs

@@ -38,15 +38,22 @@ use wasm_timer::Delay;
 pub const LOG_TARGET: &str = "peerset";
 
 /// We don't accept nodes whose reputation is under this value.
-pub const BANNED_THRESHOLD: i32 = 82 * (i32::MIN / 100);
+pub const BANNED_THRESHOLD: i32 = 71 * (i32::MIN / 100);
 /// Reputation change for a node when we get disconnected from it.
 const DISCONNECT_REPUTATION_CHANGE: i32 = -256;
 /// Relative decrement of a reputation value that is applied every second. I.e., for inverse
-/// decrement of 50 we decrease absolute value of the reputation by 1/50. This corresponds to a
-/// factor of `k = 0.98`. It takes ~ `ln(0.5) / ln(k)` seconds to reduce the reputation by half,
-/// or 34.3 seconds for the values above. In this setup the maximum allowed absolute value of
-/// `i32::MAX` becomes 0 in ~1100 seconds (actually less due to integer arithmetic).
-const INVERSE_DECREMENT: i32 = 50;
+/// decrement of 200 we decrease absolute value of the reputation by 1/200.
+///
+/// This corresponds to a factor of `k = 0.955`, where k = 1 - 1 / INVERSE_DECREMENT.
+///
+/// It takes ~ `ln(0.5) / ln(k)` seconds to reduce the reputation by half, or 138.63 seconds for the
+/// values above.
+///
+/// In this setup:
+/// - `i32::MAX` becomes 0 in exactly 3544 seconds, or approximately 59 minutes
+/// - `i32::MIN` becomes 0 in exactly 3544 seconds, or approximately 59 minutes
+/// - `i32::MIN` escapes the banned threshold in 69 seconds
+const INVERSE_DECREMENT: i32 = 200;
 /// Amount of time between the moment we last updated the [`PeerStore`] entry and the moment we
 /// remove it, once the reputation value reaches 0.
 const FORGET_AFTER: Duration = Duration::from_secs(3600);
@@ -455,7 +462,7 @@ mod tests {
 	#[test]
 	fn decaying_max_reputation_finally_yields_zero() {
 		const INITIAL_REPUTATION: i32 = i32::MAX;
-		const SECONDS: u64 = 1000;
+		const SECONDS: u64 = 3544;
 
 		let mut peer_info = PeerInfo::default();
 		peer_info.reputation = INITIAL_REPUTATION;
@@ -470,7 +477,7 @@ mod tests {
 	#[test]
 	fn decaying_min_reputation_finally_yields_zero() {
 		const INITIAL_REPUTATION: i32 = i32::MIN;
-		const SECONDS: u64 = 1000;
+		const SECONDS: u64 = 3544;
 
 		let mut peer_info = PeerInfo::default();
 		peer_info.reputation = INITIAL_REPUTATION;