feat(quinn-proto): Allow notifying of network path changes

This gives the proto state the ability to be externally notified about
network path changes it might not be aware off.  In this case it will
re-set the RTT, Congestion Controller and MTU settings to the initial
states based on the TransportConfig.

This addresses the quinn-proto part of #2018.

quinn-proto/src/connection/mod.rs

@@ -1314,6 +1314,20 @@ impl Connection {
         self.path.congestion.as_ref()
     }
 
+    /// Resets path-specific settings.
+    ///
+    /// This will force-reset several subsystems related to a specific network path.
+    /// Currently this is the congestion controller, round-trip estimator, and the MTU
+    /// discovery.
+    ///
+    /// This is useful when it is known the underlying network path has changed and the old
+    /// state of these subsystems is no longer valid or optimal. In this case it might be
+    /// faster or reduce loss to settle on optimal values by restarting from the initial
+    /// configuration in the [`TransportConfig`].
+    pub fn path_changed(&mut self, now: Instant) {
+        self.path.reset(now, &self.config);
+    }
+
     /// Modify the number of remotely initiated streams that may be concurrently open
     ///
     /// No streams may be opened by the peer unless fewer than `count` are already open. Large

quinn-proto/src/connection/mtud.rs

@@ -56,6 +56,15 @@ impl MtuDiscovery {
         }
     }
 
+    pub(super) fn reset(&mut self, current_mtu: u16, min_mtu: u16) {
+        self.current_mtu = current_mtu;
+        if let Some(state) = self.state.take() {
+            self.state = Some(EnabledMtuDiscovery::new(state.config));
+            self.on_peer_max_udp_payload_size_received(state.peer_max_udp_payload_size);
+        }
+        self.black_hole_detector = BlackHoleDetector::new(min_mtu);
+    }
+
     /// Returns the current MTU
     pub(crate) fn current_mtu(&self) -> u16 {
         self.current_mtu

quinn-proto/src/connection/paths.rs

@@ -115,6 +115,18 @@ impl PathData {
         }
     }
 
+    /// Resets RTT, congestion control and MTU states.
+    ///
+    /// This is useful when it is known the underlying path has changed.
+    pub(super) fn reset(&mut self, now: Instant, config: &TransportConfig) {
+        self.rtt = RttEstimator::new(config.initial_rtt);
+        self.congestion = config
+            .congestion_controller_factory
+            .clone()
+            .build(now, config.get_initial_mtu());
+        self.mtud.reset(config.get_initial_mtu(), config.min_mtu);
+    }
+
     /// Indicates whether we're a server that hasn't validated the peer's address and hasn't
     /// received enough data from the peer to permit sending `bytes_to_send` additional bytes
     pub(super) fn anti_amplification_blocked(&self, bytes_to_send: u64) -> bool {