recovery: support maximum PTO

quiche/src/lib.rs

@@ -720,6 +720,8 @@
     max_stream_window: u64,
 
     disable_dcid_reuse: bool,
+
+    max_pto: time::Duration,
 }
 
 // See https://quicwg.org/base-drafts/rfc9000.html#section-15
@@ -779,6 +781,7 @@
             max_stream_window: stream::MAX_STREAM_WINDOW,
 
             disable_dcid_reuse: false,
+            max_pto: time::Duration::MAX,
         })
     }
 
@@ -1209,6 +1212,16 @@
     pub fn set_disable_dcid_reuse(&mut self, v: bool) {
         self.disable_dcid_reuse = v;
     }
+
+    /// Sets the maximum Probe Timeout
+    ///
+    /// PTO is a function of RTT and can get large. Configure a max PTO to limit
+    /// how large it can get.
+    ///
+    /// The default value is [`Duration::MAX`]
+    pub fn set_max_pto(&mut self, v: time::Duration) {
+        self.max_pto = v;
+    }
 }
 
 /// A QUIC connection.

quiche/src/recovery/mod.rs

@@ -80,6 +80,7 @@ pub struct Recovery {
     loss_detection_timer: Option<Instant>,
 
     pto_count: u32,
+    max_pto: Duration,
 
     time_of_last_sent_ack_eliciting_pkt:
         [Option<Instant>; packet::Epoch::count()],
@@ -177,6 +178,7 @@ pub struct RecoveryConfig {
     hystart: bool,
     pacing: bool,
     max_pacing_rate: Option<u64>,
+    max_pto: Duration,
 }
 
 impl RecoveryConfig {
@@ -188,6 +190,7 @@ impl RecoveryConfig {
             hystart: config.hystart,
             pacing: config.pacing,
             max_pacing_rate: config.max_pacing_rate,
+            max_pto: config.max_pto,
         }
     }
 }
@@ -201,6 +204,7 @@ impl Recovery {
             loss_detection_timer: None,
 
             pto_count: 0,
+            max_pto: recovery_config.max_pto,
 
             time_of_last_sent_ack_eliciting_pkt: [None; packet::Epoch::count()],
 
@@ -723,7 +727,10 @@ impl Recovery {
     }
 
     pub fn pto(&self) -> Duration {
-        self.rtt() + cmp::max(self.rttvar * 4, GRANULARITY)
+        cmp::min(
+            self.rtt() + cmp::max(self.rttvar * 4, GRANULARITY),
+            self.max_pto,
+        )
     }
 
     pub fn delivery_rate(&self) -> u64 {
@@ -2184,6 +2191,23 @@ mod tests {
             now + Duration::from_secs_f64(12000.0 / pacing_rate as f64)
         );
     }
+
+    #[test]
+    fn max_pto() {
+        let mut cfg = crate::Config::new(crate::PROTOCOL_VERSION).unwrap();
+        cfg.set_max_pto(Duration::from_secs(10));
+
+        let mut r = Recovery::new(&cfg);
+        r.update_rtt(Duration::from_millis(100), Duration::ZERO, Instant::now());
+        assert!(
+            r.pto() < Duration::from_secs(10),
+            "pto not unnecessarily limited"
+        );
+
+        r.update_rtt(Duration::from_secs(60), Duration::ZERO, Instant::now());
+
+        assert_eq!(r.pto(), Duration::from_secs(10), "pto limited");
+    }
 }
 
 mod bbr;