add HMM RTT jump detector

Author: eli-lebeau

quiche/src/recovery/AGENTS.md

@@ -39,7 +39,7 @@ gcongestion/        Next-gen CC (BBR2)
     probe_bw.rs     ProbeBW mode (bandwidth probing cycles)
     probe_rtt.rs    ProbeRTT mode (min RTT measurement)
     network_model.rs  Bandwidth/RTT model, BbrParams application
-    rtt_jump_detector.rs  Global-min RTT jump detection
+    rtt_jump_detector/  RTT jump detection (global-min + HMM)
   bbr.rs            BBR (v1, not actively used)
 ```
 

quiche/src/recovery/congestion/recovery.rs

@@ -924,9 +924,9 @@ impl RecoveryOps for LegacyRecovery {
     }
 
     fn rtt_persistent_jump_count(&self) -> u64 {
-        // Persistent RTT jump counts are produced by the BBR2 global-min RTT
-        // jump detector. Legacy Reno/CUBIC recovery does not own a BBR2
-        // network model or run that detector, but it still implements
+        // Persistent RTT jump counts are produced by the BBR2 RTT jump
+        // detector. Legacy Reno/CUBIC recovery does not own a BBR2 network
+        // model or run that detector, but it still implements
         // RecoveryOps so PathStats can be populated through one shared
         // interface. Report zero to indicate that no detector is active on
         // this path.

quiche/src/recovery/gcongestion/bbr2/network_model.rs

@@ -193,6 +193,7 @@ pub(super) struct BBRv2NetworkModel {
     /// The most recent ack rate from the BandwidthSampler.
     latest_ack_rate: Option<Bandwidth>,
 
+    /// Detector for persistent RTT jump episodes.
     rtt_jump_detector: RttJumpDetector,
 }
 
@@ -242,7 +243,7 @@ impl BBRv2NetworkModel {
             latest_send_rate: None,
             latest_ack_rate: None,
 
-            rtt_jump_detector: RttJumpDetector::new(),
+            rtt_jump_detector: RttJumpDetector::new(params.rtt_jump_detector),
         }
     }
 
@@ -369,8 +370,7 @@ impl BBRv2NetworkModel {
         if let Some(rtt_sample) = sample.sample_rtt {
             congestion_event.sample_min_rtt = Some(rtt_sample);
 
-            self.rtt_jump_detector.on_rtt_sample_with_mode(
-                params.rtt_jump_detector,
+            self.rtt_jump_detector.on_rtt_sample(
                 rtt_sample,
                 event_time,
                 self.full_bandwidth_reached,
@@ -930,6 +930,28 @@ mod tests {
         assert!(model.last_persistent_jump_time().is_some());
     }
 
+    #[test]
+    fn hmm_detector_can_be_enabled() {
+        let params = &rtt_jump_params(BbrRttJumpDetector::Hmm);
+        let mut model = BBRv2NetworkModel::new(params, RTT);
+        let base = Instant::now();
+
+        for pkt in 1..9u64 {
+            ack_with_rtt(&mut model, params, pkt, base, ms(pkt * 10), RTT);
+        }
+        model.set_full_bandwidth_reached();
+
+        let mut offset = 200u64;
+        for pkt in 9u64..40 {
+            ack_with_rtt(&mut model, params, pkt, base, ms(offset), RTT_3X);
+            offset += 100;
+        }
+
+        assert_eq!(model.rtt_persistent_jump_count(), 1);
+        assert!(model.is_rtt_jump_persistent());
+        assert!(model.last_persistent_jump_time().is_some());
+    }
+
     #[test]
     fn global_min_detector_sustained_step_becomes_persistent() {
         let params = &rtt_jump_params(BbrRttJumpDetector::GlobalMin);
@@ -1012,4 +1034,28 @@ mod tests {
         assert_eq!(model.rtt_persistent_jump_count(), 1);
         assert!(model.is_rtt_jump_persistent());
     }
+
+    #[test]
+    fn hmm_detector_ignores_startup_rtt_jump_until_full_bandwidth() {
+        let params = &rtt_jump_params(BbrRttJumpDetector::Hmm);
+        let mut model = BBRv2NetworkModel::new(params, RTT);
+        let base = Instant::now();
+
+        for pkt in 1..9u64 {
+            ack_with_rtt(&mut model, params, pkt, base, ms(pkt * 10), RTT);
+        }
+
+        for pkt in 9u64..20 {
+            ack_with_rtt(&mut model, params, pkt, base, ms(pkt * 100), RTT_3X);
+        }
+
+        assert_eq!(model.rtt_persistent_jump_count(), 0);
+        assert!(!model.is_rtt_jump_active());
+
+        model.set_full_bandwidth_reached();
+        ack_with_rtt(&mut model, params, 20, base, ms(2100), RTT_3X);
+
+        assert_eq!(model.rtt_persistent_jump_count(), 0);
+        assert!(!model.is_rtt_jump_persistent());
+    }
 }