fix(probe): preserve measurement through wrap and network outages

src/pinger_icmp_ts.rs

@@ -19,6 +19,20 @@ pub struct PingerICMPTimestampListener {}
 
 pub struct PingerICMPTimestampSender {}
 
+const MS_PER_DAY: i64 = 86_400_000;
+
+// RFC 792 timestamps wrap at midnight UTC; real one-way delays are below half a day.
+fn timestamp_delta(later: i64, earlier: i64) -> i64 {
+    let delta = later - earlier;
+    if delta < -MS_PER_DAY / 2 {
+        delta + MS_PER_DAY
+    } else if delta > MS_PER_DAY / 2 {
+        delta - MS_PER_DAY
+    } else {
+        delta
+    }
+}
+
 impl PingListener for PingerICMPTimestampListener {
     // Result: RTT, down time, up time
     fn parse_packet(
@@ -46,12 +60,19 @@ impl PingListener for PingerICMPTimestampListener {
                         });
                     }
 
+                    // The high bit marks a timestamp that is not milliseconds since midnight UTC.
+                    if receive & 0x8000_0000 != 0 || transmit & 0x8000_0000 != 0 {
+                        return Err(PingError::InvalidPacket(
+                            "non-standard RFC 792 timestamp".into(),
+                        ));
+                    }
+
                     let time_now = Time::new(ClockId::Realtime);
                     let time_since_midnight = time_now.get_time_since_midnight();
 
-                    let rtt = (time_since_midnight - originate as i64) as f64;
-                    let dl_time = (time_since_midnight - transmit as i64) as f64;
-                    let ul_time = (receive as i64 - originate as i64) as f64;
+                    let rtt = timestamp_delta(time_since_midnight, originate as i64) as f64;
+                    let dl_time = timestamp_delta(time_since_midnight, transmit as i64) as f64;
+                    let ul_time = timestamp_delta(receive as i64, originate as i64) as f64;
 
                     Ok(PingReply {
                         reflector,
@@ -88,3 +109,27 @@ impl PingSender for PingerICMPTimestampSender {
         )
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use icmp_socket2::packet::WithTimestampReply;
+
+    #[test]
+    fn timestamps_wrap_at_midnight_in_both_directions() {
+        assert_eq!(timestamp_delta(20, 86_399_900), 120);
+        assert_eq!(timestamp_delta(86_399_900, 20), -120);
+    }
+
+    #[test]
+    fn non_standard_rfc_792_timestamps_are_rejected() {
+        let packet = Icmpv4Packet::with_timestamp_reply(7, 1, 10, 0x8000_0001, 20).unwrap();
+        let result = PingerICMPTimestampListener {}.parse_packet(
+            7,
+            "192.0.2.1".parse().unwrap(),
+            MeasurementType::IcmpTimestamps,
+            packet,
+        );
+        assert!(matches!(result, Err(PingError::InvalidPacket(_))));
+    }
+}