fix(client): invalidate TX correlation after send failures

crates/irtt-client/AGENTS.md

@@ -39,7 +39,10 @@ assumption that failed or nonblocking sends cannot cause that desynchrony.
 The effect is confined to the optional timing enhancement. A kernel timestamp
 whose ID cannot be matched to a pending or retained timed-out probe is
 discarded, as is any implausible timestamp; `sent_at` remains the ordinary
-userspace fallback. This is safe because:
+userspace fallback. **After every failed probe submission, disable kernel-ID
+correlation for the rest of that session.** A gap means an otherwise plausible
+later ID can name the wrong probe, so subsequent error-queue timestamps must
+be discarded rather than attached by ID. This is safe because:
 
 - **Normal successful sends track together.** `wire_seq` starts at 0 on every
   successful Open (`SessionMachine::commit_open` builds a fresh

crates/irtt-client/src/async_client.rs

@@ -608,8 +608,14 @@ impl AsyncClient {
                 // No sent_at measurement, no machine commit, no schedule
                 // commit: the transaction is fully discarded and retried
                 // with a fresh send_anchor on the next loop iteration.
-                Err(error) if error.kind() == io::ErrorKind::WouldBlock => continue,
-                Err(error) => return Poll::Ready(Err(ClientError::Socket(error))),
+                Err(error) if error.kind() == io::ErrorKind::WouldBlock => {
+                    self.machine.invalidate_kernel_tx_correlation();
+                    continue;
+                }
+                Err(error) => {
+                    self.machine.invalidate_kernel_tx_correlation();
+                    return Poll::Ready(Err(ClientError::Socket(error)));
+                }
             };
 
             #[cfg(not(test))]

crates/irtt-client/src/client.rs

@@ -167,9 +167,9 @@ impl Client {
     /// unrelated datagrams are ignored until the current attempt's absolute
     /// deadline, so one attempt may consume several datagrams without
     /// retransmitting. Silence or ignored traffic eventually produces
-    /// [`ClientError::OpenTimeout`]. A malformed or incompatible reply that is
-    /// recognized as coming from the connected peer is terminal; when HMAC is
-    /// configured, that recognition additionally requires authentication.
+    /// [`ClientError::OpenTimeout`]. A structurally recognizable malformed or
+    /// incompatible Open reply from the connected peer is terminal; when HMAC
+    /// is configured, that recognition additionally requires authentication.
     ///
     /// When a trusted reply allocates a token but later negotiation or socket
     /// preparation fails, the client sends a best-effort cleanup close and
@@ -524,11 +524,18 @@ impl Client {
             .unwrap_or_else(Instant::now);
         #[cfg(test)]
         if fail_send {
+            runtime.invalidate_kernel_tx_correlation();
             return Err(ClientError::Socket(io::Error::other(
                 "injected probe send failure",
             )));
         }
-        let bytes = socket.send(&prepared.bytes)?;
+        let bytes = match socket.send(&prepared.bytes) {
+            Ok(bytes) => bytes,
+            Err(error) => {
+                runtime.invalidate_kernel_tx_correlation();
+                return Err(ClientError::Socket(error));
+            }
+        };
         #[cfg(not(test))]
         let send_finished_at = Instant::now();
         #[cfg(test)]

crates/irtt-client/src/probe.rs

@@ -39,7 +39,8 @@ pub(crate) struct PendingProbe {
     /// eligible only for upstream one-way delay after local plausibility
     /// validation against `tx_not_before_wall` (see `compute_one_way`'s
     /// `preferred_send_wall`). `sent_at` remains the userspace fallback for
-    /// upstream one-way delay when this is absent or implausible.
+    /// upstream one-way delay when this is absent, implausible, or kernel-ID
+    /// correlation was invalidated by a failed probe submission.
     pub kernel_tx_timestamp: Option<SystemTime>,
 }
 

crates/irtt-client/src/receive/linux.rs

@@ -17,7 +17,9 @@
 //! A kernel ID can theoretically be consumed by a send that later fails,
 //! desynchronizing that correlation. [`drain_tx_timestamps`] discards
 //! unmatched or implausible records; the userspace `sent_at` timestamp remains
-//! the fallback. It performs a small bounded, nonblocking read of
+//! the fallback. After a probe send failure, correlation is disabled for that
+//! session so later IDs cannot be misattributed. It performs a small bounded,
+//! nonblocking read of
 //! `MSG_ERRQUEUE` so the adapter can opportunistically collect timestamps
 //! without ever waiting for one. The observed kernel TX wall time is retained
 //! as metadata only; it is not consumed by any measurement in this change.

crates/irtt-client/src/session/machine.rs

@@ -60,6 +60,7 @@ struct ActiveSession {
     pending: PendingMap,
     timed_out: TimedOutMap,
     completed: CompletedSet,
+    kernel_tx_correlation_valid: bool,
 }
 
 #[derive(Debug)]
@@ -423,6 +424,18 @@ impl SessionMachine {
         }
     }
 
+    /// Disable Linux kernel TX timestamp correlation after a probe send fails.
+    ///
+    /// A failed submission can consume a kernel `SOF_TIMESTAMPING_OPT_ID`
+    /// without advancing the wire sequence. Existing associated timestamps
+    /// remain valid observations; later error-queue timestamps cannot be
+    /// associated safely and are ignored for the rest of this session.
+    pub(crate) fn invalidate_kernel_tx_correlation(&mut self) {
+        if let MachineState::Open(session) = &mut self.state {
+            session.kernel_tx_correlation_valid = false;
+        }
+    }
+
     /// Record an observed Linux kernel TX timestamp for `wire_seq`, the
     /// automatic `SOF_TIMESTAMPING_OPT_ID` the kernel assigned when the
     /// datagram was submitted.
@@ -443,6 +456,9 @@ impl SessionMachine {
         let MachineState::Open(session) = &mut self.state else {
             return;
         };
+        if !session.kernel_tx_correlation_valid {
+            return;
+        }
         if let Some(probe) = session.pending.get_mut(wire_seq) {
             probe.kernel_tx_timestamp.get_or_insert(timestamp);
             return;
@@ -636,6 +652,7 @@ impl SessionMachine {
             pending: PendingMap::new(self.config.max_pending_probes),
             timed_out: TimedOutMap::new(self.config.max_pending_probes),
             completed: CompletedSet::new(self.config.max_pending_probes),
+            kernel_tx_correlation_valid: true,
         }));
 
         let event = ClientEvent::SessionStarted {
@@ -1231,6 +1248,7 @@ mod tests {
             pending: PendingMap::new(max_pending_probes),
             timed_out: TimedOutMap::new(max_pending_probes),
             completed: CompletedSet::new(max_pending_probes),
+            kernel_tx_correlation_valid: true,
         }));
         machine
     }
@@ -2873,6 +2891,25 @@ mod tests {
         );
     }
 
+    #[test]
+    fn invalidated_kernel_tx_correlation_uses_userspace_send_time() {
+        let mono = Instant::now();
+        let mut machine = upstream_probe(send_wall_ms(1_000), mono);
+
+        // A failed send can consume a kernel ID without advancing wire_seq.
+        // A later timestamp whose ID happens to match a pending wire sequence
+        // must not be associated after that gap.
+        machine.invalidate_kernel_tx_correlation();
+        machine.record_kernel_tx_timestamp(0, send_wall_ms(1_005));
+
+        let events = upstream_reply(&mut machine, 1_020, send_wall_ms(1_040), mono);
+
+        assert_eq!(
+            reply_one_way(&events).unwrap().client_to_server,
+            Some(SignedDuration::from_nanos(20_000_000))
+        );
+    }
+
     #[test]
     fn upstream_one_way_delay_falls_back_without_kernel_tx() {
         let mono = Instant::now();
@@ -3195,6 +3232,7 @@ mod tests {
                 pending: PendingMap::new(MAX_PENDING_PROBES),
                 timed_out: TimedOutMap::new(MAX_PENDING_PROBES),
                 completed: CompletedSet::new(MAX_PENDING_PROBES),
+                kernel_tx_correlation_valid: true,
             }));
             (machine, params)
         }