fix(irtt): use effective RTT for latency fallback

src/baseliner.rs

@@ -289,6 +289,7 @@ mod tests {
             },
             started_at: received_at,
             observed_at: received_at,
+            round_trip: None,
             one_way: Some(OneWayLatency {
                 uplink: SignedDuration::from_millis(up_time as i64),
                 downlink: SignedDuration::from_millis(down_time as i64),

src/measurement/icmp.rs

@@ -326,6 +326,7 @@ impl IcmpEngine {
                     stream: probe.stream,
                     started_at: probe.sent_at,
                     observed_at,
+                    round_trip: None,
                     one_way: None,
                 })
             }
@@ -385,6 +386,7 @@ impl IcmpEngine {
                     stream: probe.stream,
                     started_at: probe.sent_at,
                     observed_at,
+                    round_trip: None,
                     one_way: Some(OneWayLatency {
                         uplink: SignedDuration::from_millis(uplink_ms),
                         downlink: SignedDuration::from_millis(downlink_ms),

src/measurement/irtt.rs

@@ -255,6 +255,7 @@ impl IrttEngine {
                                     sent_at,
                                     received_at,
                                     one_way,
+                                    rtt,
                                     ..
                                 } => {
                                     let Some(stream) = stream else {
@@ -265,6 +266,8 @@ impl IrttEngine {
                                         stream,
                                         started_at: sent_at.mono,
                                         observed_at: received_at.mono,
+                                        round_trip: i64::try_from(rtt.effective.as_nanos())
+                                            .ok().map(SignedDuration::from_nanos),
                                         one_way: one_way.and_then(map_one_way),
                                     };
 

src/measurement/model.rs

@@ -121,6 +121,9 @@ pub struct MeasurementObservation {
     /// Local monotonic time at which the result was observed.
     pub observed_at: Instant,
 
+    /// Measured round-trip latency when it differs from elapsed local time.
+    pub round_trip: Option<SignedDuration>,
+
     /// Directional timing, when the measurement mechanism can provide it.
     pub one_way: Option<OneWayLatency>,
 }
@@ -131,7 +134,10 @@ impl MeasurementObservation {
     }
 
     pub fn rtt_ms(&self) -> f64 {
-        self.rtt().as_secs_f64() * 1000.0
+        self.round_trip
+            .map(SignedDuration::as_secs_f64)
+            .unwrap_or_else(|| self.rtt().as_secs_f64())
+            * 1000.0
     }
 }