feat(proto): Implement TransportConfig::set_rtt_hint

quinn-proto/src/config/transport.rs

@@ -8,7 +8,7 @@ use std::{
 };
 
 use crate::{
-    ConnectionId, Duration, INITIAL_MTU, Instant, MAX_UDP_PAYLOAD, Side, VarInt,
+    ConnectionId, Duration, FourTuple, INITIAL_MTU, Instant, MAX_UDP_PAYLOAD, Side, VarInt,
     VarIntBoundsExceeded, address_discovery, congestion, connection::qlog::QlogSink,
 };
 #[cfg(feature = "qlog")]
@@ -83,6 +83,8 @@ pub struct TransportConfig {
 
     pub(crate) max_remote_nat_traversal_addresses: Option<NonZeroU8>,
 
+    pub(crate) rtt_hint: Arc<dyn Fn(FourTuple) -> Option<Duration> + Send + Sync>,
+
     #[cfg(feature = "qlog")]
     pub(crate) qlog_factory: Option<Arc<dyn QlogFactory>>,
 }
@@ -467,6 +469,27 @@ impl TransportConfig {
         self
     }
 
+    /// Sets the rtt hint function.
+    ///
+    /// This function is consulted any time a new path is opened on a 4-tuple that we don't
+    /// yet have a good rtt estimate for yet.
+    ///
+    /// If the function returns some RTT, then this RTT is used as the initial RTT in the path's
+    /// rtt estimator.
+    /// If the function returns None, then we use the default rtt estimator's initial RTT value
+    /// (see also [`Self::initial_rtt`]).
+    ///
+    /// Be sure to always use conservative estimates of RTT, as the initial RTT value is used
+    /// for setting the path's timeout. If the RTT estimate is about 9x lower than the actual
+    /// RTT, then the path will be abandoned before a response could even be processed.
+    pub fn set_rtt_hint(
+        &mut self,
+        rtt_hint: impl Fn(FourTuple) -> Option<Duration> + Send + Sync + 'static,
+    ) -> &mut Self {
+        self.rtt_hint = Arc::new(rtt_hint);
+        self
+    }
+
     /// Configures qlog capturing by setting a [`QlogFactory`].
     ///
     /// This assigns a [`QlogFactory`] that produces qlog capture configurations for
@@ -582,6 +605,8 @@ impl Default for TransportConfig {
             // nat traversal disabled by default
             max_remote_nat_traversal_addresses: None,
 
+            rtt_hint: Arc::new(|_| None),
+
             #[cfg(feature = "qlog")]
             qlog_factory: None,
         }
@@ -621,6 +646,7 @@ impl fmt::Debug for TransportConfig {
             default_path_max_idle_timeout,
             default_path_keep_alive_interval,
             max_remote_nat_traversal_addresses,
+            rtt_hint: _,
             #[cfg(feature = "qlog")]
             qlog_factory,
         } = self;

quinn-proto/src/connection/mod.rs

@@ -818,15 +818,6 @@ impl Connection {
         &mut self.paths.get_mut(&path_id).expect("known path").data
     }
 
-    /// Check if the 4-tuple path (as in RFC9000 Path, not multipath path) had already been validated.
-    fn is_path_validated(&self, addresses: FourTuple) -> bool {
-        self.paths
-            .values()
-            .any(|path_state| path_state.data.validated && path_state.data.addresses == addresses)
-        // TODO(@divma): we might want to ensure the path has been recently active to consider the
-        // address validated
-    }
-
     fn ensure_path(
         &mut self,
         path_id: PathId,
@@ -834,7 +825,17 @@ impl Connection {
         now: Instant,
         pn: Option<u64>,
     ) -> &mut PathData {
-        let validated = self.is_path_validated(addresses);
+        let path_on_same_address = self
+            .paths
+            .iter()
+            .find(|(path_id, state)| {
+                addresses.is_probably_same_path(&state.data.addresses)
+                    && state.data.validated
+                    && !self.abandoned_paths.contains(path_id)
+            })
+            .clone();
+        let validated = path_on_same_address.is_some();
+        let initial_rtt = path_on_same_address.map(|(_, state)| state.data.rtt.conservative());
         let vacant_entry = match self.paths.entry(path_id) {
             btree_map::Entry::Vacant(vacant_entry) => vacant_entry,
             btree_map::Entry::Occupied(occupied_entry) => {
@@ -856,6 +857,9 @@ impl Connection {
         );
 
         data.validated = validated;
+        if let Some(rtt_hint) = initial_rtt.or_else(|| (self.config.rtt_hint)(addresses)) {
+            data.rtt.reset_initial_rtt(rtt_hint);
+        }
 
         let pto = self.ack_frequency.max_ack_delay_for_pto() + data.rtt.pto_base();
         self.timers.set(