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

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+    /// state of these subsystems is no longer valid or optimal.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 {