fix(client): timestamp probes after successful send

Moves the client userspace probe sent_at measurement to immediately
after a successful UDP send, improving raw RTT and the userspace
upstream-OWD fallback by excluding the socket send-call interval and
the small amount of local work preceding it.

All fallible probe-commit preflight (timeout deadline checked_add,
capacity/collision checks) still runs before the send, from a private
pre-send anchor (SessionMachine::finalize_probe_commit). The public
sent_at is now supplied afterward to an infallible
SessionMachine::commit_probe_sent, so DurationOverflow still guarantees
nothing was transmitted and a WouldBlock try_send still commits
nothing.

The operational probe timeout deadline stays anchored to the pre-send
timestamp on purpose, not to the new post-send sent_at, preserving
existing timeout/loss timing. Linux kernel TX_SOFTWARE plausibility
keeps its own pre-send wall-clock lower bound
(PendingProbe::tx_not_before_wall) instead of the post-send sent_at,
since a legitimate TX_SOFTWARE timestamp can be generated while the
send syscall is still in flight and would otherwise be wrongly
rejected.

Also flags (without touching production code) that irtt-stats'
ReplySample::from_reply_parts feeds the public sent_at directly into
IpdvSample.client_send_mono/client_send_wall_ns, so the upstream/send
IPDV component naturally shifts along with the sent_at semantics
change; per scope this is reported rather than compensated for.

crates/irtt-client/AGENTS.md

@@ -96,33 +96,67 @@ socket.
 ## Retained state
 
 `PendingProbe` carries an optional `kernel_tx_timestamp: Option<SystemTime>`
-alongside the authoritative userspace `sent_at`. It survives pending ->
-timed-out retention (`SessionMachine::record_kernel_tx_timestamp` updates
-whichever of `PendingMap`/`TimedOutMap` still holds the probe) and is
-otherwise bounded by the same limits those maps already enforce — no new
-unbounded state. First valid observation wins on duplicates.
+alongside the userspace `sent_at`. It survives pending -> timed-out retention
+(`SessionMachine::record_kernel_tx_timestamp` updates whichever of
+`PendingMap`/`TimedOutMap` still holds the probe) and is otherwise bounded by
+the same limits those maps already enforce — no new unbounded state. First
+valid observation wins on duplicates.
 
-**It is eligible for exactly one measurement: the upstream (`client_to_server`)
-one-way delay send endpoint.** `compute_one_way`'s private `preferred_send_wall`
-helper prefers it over the userspace `sent_at.wall` sample when
+### Pre-send anchor vs. post-send `sent_at` (F-04)
 
-    sent_at.wall <= kernel_tx_timestamp <= reply_received_at.wall
+The probe send transaction splits into two timestamps around the socket
+send, not one:
+
+- A **private pre-send anchor**, sampled immediately before the fallible
+  commit preflight (`SessionMachine::finalize_probe_commit`) that must
+  happen before the socket send: timeout deadline `checked_add` arithmetic
+  (so `DurationOverflow` still guarantees nothing was transmitted) and the
+  kernel TX lower-plausibility bound. Its monotonic half becomes
+  `PendingProbe::timeout_at`'s basis; its wall half becomes
+  `PendingProbe::tx_not_before_wall`. It is never exposed publicly.
+- The **public `sent_at`**, a paired wall/monotonic `ClientTimestamp`
+  captured immediately after the socket send returns successfully
+  (`SessionMachine::commit_probe_sent`, which is infallible — all fallible
+  work already happened against the pre-send anchor). This is the RTT
+  endpoint, the userspace upstream-OWD fallback endpoint, `timer_error`'s
+  basis, and the value surfaced on `EchoSent`/`EchoReply`/`EchoLoss`/
+  `LateReply`.
+
+So `timeout_at` is deliberately **not** derived from `sent_at.mono` — it
+remains anchored to before the send, on purpose, so the existing pre-send
+timeout/overflow/no-send contract is unchanged. Only the *measurement*
+`sent_at` moved to after the send. A `WouldBlock` `try_send` (Tokio) never
+samples a `sent_at`, never calls `finalize_probe_commit`'s matching
+`commit_probe_sent`, and never advances the machine or schedule; each retry
+gets its own fresh pre-send anchor, and only a successful attempt's anchor
+and `sent_at` are ever retained.
+
+**The kernel TX timestamp is eligible for exactly one measurement: the
+upstream (`client_to_server`) one-way delay send endpoint.**
+`compute_one_way`'s private `preferred_send_wall` helper prefers it over the
+userspace `sent_at.wall` sample when
+
+    tx_not_before_wall <= kernel_tx_timestamp <= reply_received_at.wall
 
 (inclusive, compared only against the client's own wall clock — never
-against the remote server's wall timestamp). Otherwise, including when no
-kernel timestamp was observed, the upstream endpoint falls back to
-`sent_at.wall`; a rejected observation is never erased from `PendingProbe`,
-only skipped for that measurement. There is no maximum-lag bound: unlike the
-kernel receive side, legitimate client send-to-reply time can exceed a
-second.
+against the remote server's wall timestamp). The lower bound is the
+pre-send `tx_not_before_wall`, not the post-send `sent_at.wall`: a
+legitimate `TX_SOFTWARE` timestamp can be generated while the send syscall
+is still in flight, so it can legitimately precede the post-send `sent_at`
+sample without being implausible. Otherwise, including when no kernel
+timestamp was observed, the upstream endpoint falls back to `sent_at.wall`
+(the post-send sample); a rejected observation is never erased from
+`PendingProbe`, only skipped for that measurement. There is no
+maximum-lag bound: unlike the kernel receive side, legitimate client
+send-to-reply time can exceed a second.
 
-`sent_at` remains the sole input to RTT, timeout/scheduling, and the
-downstream (`server_to_client`) one-way delay, exactly as before this
-capability existed; `TX_SOFTWARE` is still a software kernel timestamp near
-the driver handoff, not a NIC hardware transmit time. IPDV is likewise
-unaffected — it is computed from the existing userspace timestamp fields
-only. A normal reply and a measurable late reply (one whose `PendingProbe`
-is still retained in `TimedOutMap`) apply the identical selection policy; an
+`sent_at` remains the sole input to RTT and the downstream
+(`server_to_client`) one-way delay, exactly as before this capability
+existed; `TX_SOFTWARE` is still a software kernel timestamp near the driver
+handoff, not a NIC hardware transmit time. IPDV is likewise unaffected — it
+is computed from the existing userspace timestamp fields only. A normal
+reply and a measurable late reply (one whose `PendingProbe` is still
+retained in `TimedOutMap`) apply the identical selection policy; an
 untracked late reply has no retained probe and stays unmeasurable, and a
 kernel timestamp that arrives after a reply has already been processed is
 simply never observed by that reply.

crates/irtt-client/src/async_client.rs

@@ -528,15 +528,20 @@ impl AsyncClient {
 
             let expected_bytes = prepared.bytes.len();
             let scheduled_at = schedule_commit.scheduled_at;
+            // Private pre-send anchor: finalizes all fallible commit work
+            // (timeout deadline arithmetic, kernel TX lower bound) before
+            // the send attempt. A WouldBlock retry samples a fresh one on
+            // the next loop iteration; only the successful attempt's anchor
+            // is retained (via `machine_commit`/`commit_probe_sent` below).
             #[cfg(not(test))]
-            let sent_at = ClientTimestamp::now();
+            let send_anchor = ClientTimestamp::now();
             #[cfg(test)]
-            let sent_at = test_timestamps
-                .map(|timestamps| timestamps.sent_at)
+            let send_anchor = test_timestamps
+                .map(|timestamps| timestamps.send_anchor)
                 .unwrap_or_else(ClientTimestamp::now);
             let machine_commit = match self
                 .machine
-                .finalize_probe_commit(machine_preflight, sent_at)
+                .finalize_probe_commit(machine_preflight, send_anchor)
             {
                 Ok(commit) => commit,
                 Err(error) => return Poll::Ready(Err(error)),
@@ -553,6 +558,9 @@ impl AsyncClient {
             let send_result = self.socket.try_send(&prepared.bytes);
             let bytes = match send_result {
                 Ok(bytes) => bytes,
+                // 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))),
             };
@@ -563,7 +571,18 @@ impl AsyncClient {
             let send_finished_at = test_timestamps
                 .map(|timestamps| timestamps.send_finished_at)
                 .unwrap_or_else(Instant::now);
-            let sent = self.machine.commit_probe_sent(machine_commit, bytes);
+            // Public post-send measurement, captured only after the
+            // successful send above. The subsequent machine/schedule commit
+            // is infallible.
+            #[cfg(not(test))]
+            let sent_at = ClientTimestamp::now();
+            #[cfg(test)]
+            let sent_at = test_timestamps
+                .map(|timestamps| timestamps.sent_at)
+                .unwrap_or_else(ClientTimestamp::now);
+            let sent = self
+                .machine
+                .commit_probe_sent(machine_commit, sent_at, bytes);
             schedule.commit(schedule_commit);
             self.prepared_probe = None;
 

crates/irtt-client/src/async_client/tests.rs

@@ -689,12 +689,14 @@ fn probe_false_readiness_resamples_all_timestamps() {
         client.test_hooks.probe_timestamps.borrow_mut().extend([
             ProbeSendTimestamps {
                 permission_at: scheduled_at + Duration::from_millis(1),
+                send_anchor: first_sent,
                 sent_at: first_sent,
                 send_call_start: scheduled_at + Duration::from_millis(4),
                 send_finished_at: scheduled_at + Duration::from_millis(6),
             },
             ProbeSendTimestamps {
                 permission_at: scheduled_at + Duration::from_millis(8),
+                send_anchor: second_sent,
                 sent_at: second_sent,
                 send_call_start: scheduled_at + Duration::from_millis(12),
                 send_finished_at: scheduled_at + Duration::from_millis(17),
@@ -741,6 +743,7 @@ fn public_timeout_polling_remains_exhaustive() {
                     wall: SystemTime::UNIX_EPOCH,
                 },
                 timeout_at: now - Duration::from_secs(1),
+                tx_not_before_wall: SystemTime::UNIX_EPOCH,
                 kernel_tx_timestamp: None,
             });
         }
@@ -803,7 +806,7 @@ fn stale_retained_probe_is_rejected_before_send() {
             .unwrap();
         client
             .machine
-            .commit_probe_sent(commit, advanced.bytes.len());
+            .commit_probe_sent(commit, ClientTimestamp::now(), advanced.bytes.len());
         let attempts = client.test_hooks.send_attempts.get();
 
         assert!(matches!(
@@ -832,6 +835,10 @@ fn finite_duration_boundary_clears_preparation_without_send() {
             .borrow_mut()
             .push_back(ProbeSendTimestamps {
                 permission_at: start + Duration::from_millis(20),
+                send_anchor: ClientTimestamp {
+                    wall: SystemTime::UNIX_EPOCH,
+                    mono: start + Duration::from_millis(21),
+                },
                 sent_at: ClientTimestamp {
                     wall: SystemTime::UNIX_EPOCH,
                     mono: start + Duration::from_millis(21),

crates/irtt-client/src/client.rs

@@ -75,9 +75,12 @@ struct ClientTestHooks {
 #[derive(Debug, Clone, Copy)]
 pub(crate) struct ProbeSendTimestamps {
     pub(crate) permission_at: Instant,
-    pub(crate) sent_at: ClientTimestamp,
+    /// Private pre-send anchor: timeout deadline / kernel TX lower bound.
+    pub(crate) send_anchor: ClientTimestamp,
     pub(crate) send_call_start: Instant,
     pub(crate) send_finished_at: Instant,
+    /// Public post-send measurement: RTT / OWD fallback / timer_error.
+    pub(crate) sent_at: ClientTimestamp,
 }
 
 #[cfg(test)]
@@ -410,13 +413,17 @@ impl Client {
         let scheduled_at = schedule_commit.scheduled_at;
         #[cfg(test)]
         let reported_bytes = self.probe_reported_len.take();
+        // All fallible probe-commit preflight (timeout deadline arithmetic,
+        // capacity/collision checks already done above) is finalized from
+        // this PRIVATE pre-send anchor, before the socket send. It is not
+        // the public measurement `sent_at` captured further below.
         #[cfg(not(test))]
-        let sent_at = ClientTimestamp::now();
+        let send_anchor = ClientTimestamp::now();
         #[cfg(test)]
-        let sent_at = test_timestamps
-            .map(|timestamps| timestamps.sent_at)
+        let send_anchor = test_timestamps
+            .map(|timestamps| timestamps.send_anchor)
             .unwrap_or_else(ClientTimestamp::now);
-        let machine_commit = runtime.finalize_probe_commit(machine_preflight, sent_at)?;
+        let machine_commit = runtime.finalize_probe_commit(machine_preflight, send_anchor)?;
         #[cfg(not(test))]
         let send_call_start = Instant::now();
         #[cfg(test)]
@@ -436,7 +443,17 @@ impl Client {
         let send_finished_at = test_timestamps
             .map(|timestamps| timestamps.send_finished_at)
             .unwrap_or_else(Instant::now);
-        let sent = runtime.commit_probe_sent(machine_commit, bytes);
+        // The public measurement timestamp: paired wall/monotonic sample
+        // captured immediately after the successful socket send completed.
+        // This, not `send_anchor` above, is the RTT/OWD-fallback/timer_error
+        // endpoint. The infallible machine/schedule commit below never fails.
+        #[cfg(not(test))]
+        let sent_at = ClientTimestamp::now();
+        #[cfg(test)]
+        let sent_at = test_timestamps
+            .map(|timestamps| timestamps.sent_at)
+            .unwrap_or_else(ClientTimestamp::now);
+        let sent = runtime.commit_probe_sent(machine_commit, sent_at, bytes);
         schedule.commit(schedule_commit);
 
         let send_call = send_finished_at.saturating_duration_since(send_call_start);

crates/irtt-client/src/client/tests/probes_replies/probe_schedule.rs

@@ -3,6 +3,7 @@ use super::*;
 fn probe_timestamps(permission_at: Instant, sent_at: ClientTimestamp) -> ProbeSendTimestamps {
     ProbeSendTimestamps {
         permission_at,
+        send_anchor: sent_at,
         sent_at,
         send_call_start: sent_at.mono,
         send_finished_at: sent_at.mono,
@@ -176,12 +177,21 @@ fn blocking_probe_separates_permission_committed_send_and_send_call_timing() {
         .unwrap(),
     );
     let permission_at = opened_mono + Duration::from_millis(500);
-    let sent_at = ClientTimestamp {
+    // Distinct pre-send anchor and post-send `sent_at`: the anchor drives
+    // the operational timeout deadline, while `sent_at` (captured after the
+    // send call) is the public measurement/timer_error endpoint. They are
+    // deliberately decoupled by this PR.
+    let send_anchor = ClientTimestamp {
         wall: SystemTime::now(),
         mono: opened_mono + Duration::from_millis(525),
     };
+    let sent_at = ClientTimestamp {
+        wall: SystemTime::now(),
+        mono: opened_mono + Duration::from_millis(534),
+    };
     let timestamps = ProbeSendTimestamps {
         permission_at,
+        send_anchor,
         sent_at,
         send_call_start: opened_mono + Duration::from_millis(526),
         send_finished_at: opened_mono + Duration::from_millis(533),
@@ -200,11 +210,14 @@ fn blocking_probe_separates_permission_committed_send_and_send_call_timing() {
         }] if *scheduled_at == opened_mono
             && *event_sent_at == sent_at
             && *send_call == Duration::from_millis(7)
-            && *timer_error == Duration::from_millis(525)
+            && *timer_error == Duration::from_millis(534)
     ));
     assert_eq!(client.next_send_deadline(), Some(opened_mono + interval));
     assert_eq!(client.runtime.packets_sent(), 1);
-    let timeout_at = sent_at.mono + client.probe_timeout();
+    // The timeout deadline is anchored to the pre-send `send_anchor`, not
+    // to the post-send `sent_at` captured 9ms later.
+    let timeout_at = send_anchor.mono + client.probe_timeout();
+    assert_ne!(timeout_at, sent_at.mono + client.probe_timeout());
     assert!(client
         .poll_timeouts_at(timeout_at - Duration::from_nanos(1))
         .unwrap()
@@ -236,6 +249,7 @@ fn blocking_send_call_excludes_wrapped_history_cleanup() {
     client.runtime.seed_wrapped_probe_history_for_test(sent_at);
     let timestamps = ProbeSendTimestamps {
         permission_at: scheduled_at,
+        send_anchor: sent_at,
         sent_at,
         send_call_start: scheduled_at + Duration::from_millis(6),
         send_finished_at: scheduled_at + Duration::from_millis(13),
@@ -343,6 +357,7 @@ fn managed_probe_skips_missed_schedule_slots() {
     };
     let timestamps = ProbeSendTimestamps {
         permission_at,
+        send_anchor: delayed_send,
         sent_at: delayed_send,
         send_call_start: first_deadline + Duration::from_millis(49),
         send_finished_at: first_deadline + Duration::from_millis(55),

crates/irtt-client/src/client/tests/probes_replies/timing_math.rs

@@ -72,7 +72,14 @@ fn compute_one_way_omits_samples_when_client_wall_time_overflows_i64() {
     };
 
     assert_eq!(
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts),
+        compute_one_way(
+            &sent_at,
+            sent_at.wall,
+            &received_at,
+            None,
+            &ReceiveMeta::default(),
+            &ts
+        ),
         None
     );
 }
@@ -93,8 +100,15 @@ fn compute_one_way_returns_available_direction_samples() {
         ..Default::default()
     };
 
-    let sample =
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
+    let sample = compute_one_way(
+        &sent_at,
+        sent_at.wall,
+        &received_at,
+        None,
+        &ReceiveMeta::default(),
+        &ts,
+    )
+    .unwrap();
     assert_eq!(
         sample.client_to_server,
         Some(SignedDuration::from_nanos(15_000_000))
@@ -121,8 +135,15 @@ fn compute_one_way_preserves_negative_client_to_server_delay() {
         ..Default::default()
     };
 
-    let sample =
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
+    let sample = compute_one_way(
+        &sent_at,
+        sent_at.wall,
+        &received_at,
+        None,
+        &ReceiveMeta::default(),
+        &ts,
+    )
+    .unwrap();
 
     assert_eq!(
         sample.client_to_server,
@@ -150,8 +171,15 @@ fn compute_one_way_preserves_negative_server_to_client_delay() {
         ..Default::default()
     };
 
-    let sample =
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
+    let sample = compute_one_way(
+        &sent_at,
+        sent_at.wall,
+        &received_at,
+        None,
+        &ReceiveMeta::default(),
+        &ts,
+    )
+    .unwrap();
 
     assert_eq!(
         sample.client_to_server,

crates/irtt-client/src/event.rs

@@ -94,16 +94,19 @@ pub enum ClientEvent {
         remote: SocketAddr,
         /// Monotonic deadline at which the probe was scheduled to be sent.
         scheduled_at: Instant,
-        /// Client wall/monotonic timestamp captured after probe preparation,
-        /// immediately before timeout finalization and the socket send.
+        /// Paired client wall/monotonic timestamp captured immediately after
+        /// the successful UDP send completed. This is the RTT and userspace
+        /// upstream one-way-delay fallback endpoint. The operational probe
+        /// timeout deadline is computed from an earlier, private, pre-send
+        /// timestamp instead — see the client crate's `AGENTS.md`.
         sent_at: ClientTimestamp,
         /// Number of bytes passed to the UDP socket.
         bytes: usize,
         /// Elapsed time spent in the socket send call, excluding protocol and
         /// scheduler commit work.
         send_call: Duration,
-        /// Absolute difference between the scheduled send time and actual send
-        /// timestamp.
+        /// Absolute difference between the scheduled send time and the
+        /// post-send `sent_at` timestamp above.
         timer_error: Duration,
     },
 
@@ -117,7 +120,8 @@ pub enum ClientEvent {
         seq: u32,
         /// Resolved remote socket address.
         remote: SocketAddr,
-        /// Client timestamp recorded when the matching request was sent.
+        /// Client timestamp recorded immediately after the matching request
+        /// finished sending (see `ClientEvent::EchoSent::sent_at`).
         sent_at: ClientTimestamp,
         /// Client timestamp recorded when the reply was received.
         received_at: ClientTimestamp,
@@ -143,7 +147,10 @@ pub enum ClientEvent {
     EchoLoss {
         /// Wire sequence number that timed out.
         seq: u32,
-        /// Client timestamp recorded when the request was sent.
+        /// Client timestamp recorded immediately after the request finished
+        /// sending (see `ClientEvent::EchoSent::sent_at`). The timeout
+        /// deadline below is anchored to an earlier, private, pre-send
+        /// timestamp, not to this one.
         sent_at: ClientTimestamp,
         /// Monotonic deadline at which the probe was declared lost.
         timeout_at: std::time::Instant,
@@ -178,7 +185,8 @@ pub enum ClientEvent {
         highest_seen: u32,
         /// Resolved remote socket address.
         remote: SocketAddr,
-        /// Original client send timestamp, when still retained.
+        /// Original post-send client timestamp, when still retained (see
+        /// `ClientEvent::EchoSent::sent_at`).
         sent_at: Option<ClientTimestamp>,
         /// Client timestamp recorded when the late reply was received.
         received_at: ClientTimestamp,

crates/irtt-client/src/managed/tests.rs

@@ -354,6 +354,7 @@ fn timeout_probe(seq: u32, sent_at: Instant, timeout_at: Instant) -> PendingProb
             wall: SystemTime::UNIX_EPOCH,
         },
         timeout_at,
+        tx_not_before_wall: SystemTime::UNIX_EPOCH,
         kernel_tx_timestamp: None,
     }
 }

crates/irtt-client/src/probe.rs

@@ -8,15 +8,38 @@ use crate::{error::ClientError, timing::ClientTimestamp};
 #[derive(Debug, Clone)]
 pub(crate) struct PendingProbe {
     pub wire_seq: u32,
+    /// Paired client wall/monotonic timestamp captured immediately after the
+    /// successful UDP send completed. This is the RTT endpoint (via
+    /// `compute_rtt`), the userspace fallback endpoint for upstream one-way
+    /// delay, `timer_error`'s basis, and the value surfaced on the public
+    /// `EchoSent`/`EchoReply`/`EchoLoss`/`LateReply` events. It is *not* the
+    /// input to `timeout_at` below — see that field's doc.
     pub sent_at: ClientTimestamp,
+    /// Operational timeout deadline, computed from the pre-send send
+    /// anchor's monotonic timestamp plus the configured probe timeout
+    /// (`SessionMachine::finalize_probe_commit`), not from `sent_at.mono`
+    /// above. Timeout semantics deliberately stay anchored to before the
+    /// socket send so that fallible deadline arithmetic (and its overflow
+    /// check) still runs, and a `PendingProbe` is only ever created, before
+    /// the datagram is transmitted.
     pub timeout_at: Instant,
+    /// Local wall-clock lower bound for validating an asynchronous Linux
+    /// kernel `TX_SOFTWARE` timestamp, sampled at the same pre-send send
+    /// anchor as `timeout_at` — i.e. before the socket send, not from
+    /// `sent_at.wall` above. A legitimate `TX_SOFTWARE` timestamp can be
+    /// generated during the send path and therefore observed earlier than
+    /// the post-send `sent_at.wall` sample; using this earlier pre-send
+    /// bound instead of `sent_at.wall` avoids rejecting such a timestamp
+    /// merely because userspace samples `sent_at` after `send()` returns.
+    /// See `compute_one_way`'s `preferred_send_wall`.
+    pub tx_not_before_wall: SystemTime,
     /// Observed Linux kernel TX_SOFTWARE wall timestamp for this probe's
     /// send, when the socket has TX timestamping enabled and a matching
     /// `MSG_ERRQUEUE` record has been drained. Optional observed metadata,
     /// eligible only for upstream one-way delay after local plausibility
-    /// validation (see `compute_one_way`'s `preferred_send_wall`). `sent_at`
-    /// remains authoritative for RTT, timeout, pacing/scheduling semantics,
-    /// and the userspace fallback for upstream one-way delay.
+    /// 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.
     pub kernel_tx_timestamp: Option<SystemTime>,
 }
 
@@ -386,10 +409,12 @@ mod tests {
     }
 
     fn pending(seq: u32, timeout_at: Instant) -> PendingProbe {
+        let sent_at = ts(timeout_at - Duration::from_secs(1));
         PendingProbe {
             wire_seq: seq,
-            sent_at: ts(timeout_at - Duration::from_secs(1)),
+            sent_at,
             timeout_at,
+            tx_not_before_wall: sent_at.wall,
             kernel_tx_timestamp: None,
         }
     }

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

@@ -152,7 +152,13 @@ pub(crate) struct ProbeCommitPreflight {
 
 #[derive(Debug)]
 pub(crate) struct ProbeCommit {
-    pending: PendingProbe,
+    wire_seq: u32,
+    /// Operational timeout deadline, derived from the pre-send send anchor.
+    /// See [`PendingProbe::timeout_at`].
+    timeout_at: Instant,
+    /// Pre-send wall-clock lower bound for kernel TX plausibility. See
+    /// [`PendingProbe::tx_not_before_wall`].
+    tx_not_before_wall: SystemTime,
     next_wire_seq: u32,
     next_packets_sent: u64,
 }
@@ -351,38 +357,62 @@ impl SessionMachine {
         })
     }
 
+    /// Finalizes all fallible probe-commit preflight work that must happen
+    /// before the socket send: the timeout deadline arithmetic (which can
+    /// overflow) and capturing the pre-send wall-clock lower bound used to
+    /// validate a later asynchronous kernel TX timestamp.
+    ///
+    /// `send_anchor` is a *private* pre-send timestamp, not the public
+    /// measurement `sent_at` — it is sampled immediately before this call,
+    /// before the socket send. Its `mono` field anchors the operational
+    /// timeout deadline and its `wall` field becomes
+    /// [`PendingProbe::tx_not_before_wall`]. See the client crate's
+    /// `AGENTS.md` for why timeout semantics deliberately stay pre-send
+    /// anchored while the public `sent_at` measurement moves after the send.
     pub(crate) fn finalize_probe_commit(
         &self,
         preflight: ProbeCommitPreflight,
-        sent_at: ClientTimestamp,
+        send_anchor: ClientTimestamp,
     ) -> Result<ProbeCommit, ClientError> {
-        let timeout_at = sent_at
+        let timeout_at = send_anchor
             .mono
             .checked_add(self.config.probe_timeout)
             .ok_or(ClientError::DurationOverflow)?;
 
         Ok(ProbeCommit {
-            pending: PendingProbe {
-                wire_seq: preflight.seq,
-                sent_at,
-                timeout_at,
-                kernel_tx_timestamp: None,
-            },
+            wire_seq: preflight.seq,
+            timeout_at,
+            tx_not_before_wall: send_anchor.wall,
             next_wire_seq: preflight.next_wire_seq,
             next_packets_sent: preflight.next_packets_sent,
         })
     }
 
-    pub(crate) fn commit_probe_sent(&mut self, commit: ProbeCommit, bytes: usize) -> ProbeSent {
+    /// Infallibly commits a probe as sent, using the post-send measurement
+    /// `sent_at` captured immediately after the successful socket send
+    /// returned. All fallible work already happened in
+    /// [`Self::finalize_probe_commit`]; nothing here can fail.
+    pub(crate) fn commit_probe_sent(
+        &mut self,
+        commit: ProbeCommit,
+        sent_at: ClientTimestamp,
+        bytes: usize,
+    ) -> ProbeSent {
         let session = match &mut self.state {
             MachineState::Open(session) => session,
             _ => unreachable!("probe commits are only created for an open session"),
         };
-        let seq = commit.pending.wire_seq;
-        let sent_at = commit.pending.sent_at;
+        let seq = commit.wire_seq;
+        let pending = PendingProbe {
+            wire_seq: seq,
+            sent_at,
+            timeout_at: commit.timeout_at,
+            tx_not_before_wall: commit.tx_not_before_wall,
+            kernel_tx_timestamp: None,
+        };
         session.timed_out.remove(seq);
         session.completed.remove(seq);
-        session.pending.commit_insert(commit.pending);
+        session.pending.commit_insert(pending);
         session.next_wire_seq = commit.next_wire_seq;
         session.packets_sent = commit.next_packets_sent;
 
@@ -701,6 +731,7 @@ impl SessionMachine {
                 let server_timing = build_server_timing(&reply.timestamps);
                 let one_way = compute_one_way(
                     &pending.sent_at,
+                    pending.tx_not_before_wall,
                     &now,
                     pending.kernel_tx_timestamp,
                     &meta,
@@ -756,6 +787,7 @@ impl SessionMachine {
                 let server_timing = build_server_timing(&reply.timestamps);
                 let one_way = compute_one_way(
                     &timed_out.sent_at,
+                    timed_out.tx_not_before_wall,
                     &now,
                     timed_out.kernel_tx_timestamp,
                     &meta,
@@ -984,39 +1016,49 @@ fn build_server_timing(ts: &TimestampFields) -> Option<ServerTiming> {
 ///
 /// Prefers the correlated Linux kernel `TX_SOFTWARE` timestamp — a software
 /// timestamp near the driver handoff, not physical NIC departure — when it
-/// is locally plausible: it must not precede `sent_at` and must not follow
-/// `received_at`, both on the client's own wall clock. Comparing only these
-/// two same-client anchors means the check detects local causal-ordering
-/// inconsistency (e.g. a backward wall-clock step), not every possible
-/// wall-clock discontinuity, and it deliberately does not compare against
-/// the remote server's wall clock: cross-host clock offset is a
-/// synchronization property of the resulting delay, not evidence that this
-/// endpoint is invalid. There is no maximum lag bound — unlike scheduler
-/// wakeup delay on the receive side, legitimate send-to-receive time can
-/// exceed a second. Falls back to `sent_at` when the kernel timestamp is
-/// absent or fails this check; the raw observation on `PendingProbe` is
-/// never mutated by a fallback here.
+/// is locally plausible: it must not precede `tx_not_before_wall` and must
+/// not follow `received_at`, both on the client's own wall clock.
+/// `tx_not_before_wall` is the pre-send send-anchor sample, not the
+/// post-send `sent_at_wall` fallback below — a legitimate `TX_SOFTWARE`
+/// timestamp can be generated while the send syscall is still in flight and
+/// therefore legitimately precede the post-send `sent_at` sample, so the
+/// lower bound must stay anchored before the send rather than after it.
+/// Comparing only these two same-client anchors means the check detects
+/// local causal-ordering inconsistency (e.g. a backward wall-clock step),
+/// not every possible wall-clock discontinuity, and it deliberately does
+/// not compare against the remote server's wall clock: cross-host clock
+/// offset is a synchronization property of the resulting delay, not
+/// evidence that this endpoint is invalid. There is no maximum lag bound —
+/// unlike scheduler wakeup delay on the receive side, legitimate
+/// send-to-receive time can exceed a second. Falls back to `sent_at_wall`
+/// (the post-send measurement) when the kernel timestamp is absent or fails
+/// this check; the raw observation on `PendingProbe` is never mutated by a
+/// fallback here.
 fn preferred_send_wall(
-    sent_at: SystemTime,
+    tx_not_before_wall: SystemTime,
+    sent_at_wall: SystemTime,
     kernel_tx_timestamp: Option<SystemTime>,
     received_at: SystemTime,
 ) -> SystemTime {
     match kernel_tx_timestamp {
-        Some(kernel_tx) if sent_at <= kernel_tx && kernel_tx <= received_at => kernel_tx,
-        _ => sent_at,
+        Some(kernel_tx) if tx_not_before_wall <= kernel_tx && kernel_tx <= received_at => kernel_tx,
+        _ => sent_at_wall,
     }
 }
 
 /// Computes both one-way delay directions from wall-clock endpoints.
 ///
 /// The upstream direction prefers a locally plausible correlated kernel
-/// `TX_SOFTWARE` send timestamp over the userspace pre-send sample; see
-/// [`preferred_send_wall`]. The downstream direction uses
+/// `TX_SOFTWARE` send timestamp over the userspace post-send `sent_at`
+/// sample; see [`preferred_send_wall`]. `tx_not_before_wall` is the private
+/// pre-send lower bound used only for that plausibility check — it is never
+/// itself a candidate endpoint. The downstream direction uses
 /// [`ReceiveMeta::preferred_receive_wall`], which prefers a plausible kernel
 /// receive timestamp over the userspace receive wall sample. Neither kernel
 /// timestamp is ever used for RTT.
 pub(crate) fn compute_one_way(
     sent_at: &ClientTimestamp,
+    tx_not_before_wall: SystemTime,
     received_at: &ClientTimestamp,
     kernel_tx_timestamp: Option<SystemTime>,
     meta: &ReceiveMeta,
@@ -1025,7 +1067,12 @@ pub(crate) fn compute_one_way(
     let server_recv_wall = ts.recv_wall.or(ts.midpoint_wall);
     let server_send_wall = ts.send_wall.or(ts.midpoint_wall);
 
-    let client_send_wall = preferred_send_wall(sent_at.wall, kernel_tx_timestamp, received_at.wall);
+    let client_send_wall = preferred_send_wall(
+        tx_not_before_wall,
+        sent_at.wall,
+        kernel_tx_timestamp,
+        received_at.wall,
+    );
     let client_send_ns = unix_epoch_ns_i64(client_send_wall);
     let client_recv_ns = unix_epoch_ns_i64(meta.preferred_receive_wall(received_at.wall));
 
@@ -1125,6 +1172,7 @@ impl SessionMachine {
             wire_seq: 0,
             sent_at: now,
             timeout_at: now.mono,
+            tx_not_before_wall: now.wall,
             kernel_tx_timestamp: None,
         });
         session.completed.insert(0);
@@ -1707,7 +1755,7 @@ mod tests {
         let sent_at = timestamp(Instant::now());
         let preflight = machine.preflight_probe_commit(&accepted).unwrap();
         let commit = machine.finalize_probe_commit(preflight, sent_at).unwrap();
-        machine.commit_probe_sent(commit, accepted.bytes.len());
+        machine.commit_probe_sent(commit, sent_at, accepted.bytes.len());
 
         let capacity = active(&machine).pending.capacity();
         assert!(matches!(
@@ -1737,7 +1785,7 @@ mod tests {
         }
         let preflight = machine.preflight_probe_commit(&prepared).unwrap();
         let commit = machine.finalize_probe_commit(preflight, sent_at).unwrap();
-        let sent = machine.commit_probe_sent(commit, prepared.bytes.len());
+        let sent = machine.commit_probe_sent(commit, sent_at, prepared.bytes.len());
 
         assert_eq!(sent.seq, 0);
         let session = active(&machine);
@@ -1750,16 +1798,20 @@ mod tests {
     fn probe_commit_does_not_require_presentation_timing() {
         let mut machine = open_machine(4, Duration::from_secs(1));
         let prepared = machine.prepare_probe().unwrap().unwrap();
-        let sent_at = timestamp(Instant::now());
+        let send_anchor = timestamp(Instant::now());
         let preflight = machine.preflight_probe_commit(&prepared).unwrap();
-        let commit = machine.finalize_probe_commit(preflight, sent_at).unwrap();
-        assert_eq!(commit.pending.sent_at, sent_at);
-        assert_eq!(
-            commit.pending.timeout_at,
-            sent_at.mono + Duration::from_secs(1)
-        );
+        let commit = machine
+            .finalize_probe_commit(preflight, send_anchor)
+            .unwrap();
+        // Fallible work (timeout deadline arithmetic, tx lower bound) is
+        // finalized from the pre-send anchor, not any post-send sample.
+        assert_eq!(commit.timeout_at, send_anchor.mono + Duration::from_secs(1));
+        assert_eq!(commit.tx_not_before_wall, send_anchor.wall);
 
-        let sent = machine.commit_probe_sent(commit, prepared.bytes.len());
+        // The post-send measurement `sent_at` is supplied afterward and can
+        // legitimately differ from the pre-send anchor.
+        let sent_at = timestamp(Instant::now());
+        let sent = machine.commit_probe_sent(commit, sent_at, prepared.bytes.len());
 
         assert_eq!(sent.seq, prepared.seq);
         assert_eq!(sent.sent_at, sent_at);
@@ -1778,7 +1830,7 @@ mod tests {
             .finalize_probe_commit(preflight, timestamp(Instant::now()))
             .unwrap();
 
-        machine.commit_probe_sent(commit, prepared.bytes.len());
+        machine.commit_probe_sent(commit, timestamp(Instant::now()), prepared.bytes.len());
 
         let session = active(&machine);
         assert_eq!(session.pending.capacity(), reserved_capacity);
@@ -1794,7 +1846,7 @@ mod tests {
         let sent_at = timestamp(Instant::now());
         let preflight = machine.preflight_probe_commit(&first).unwrap();
         let commit = machine.finalize_probe_commit(preflight, sent_at).unwrap();
-        machine.commit_probe_sent(commit, first.bytes.len());
+        machine.commit_probe_sent(commit, sent_at, first.bytes.len());
 
         let second = machine.prepare_probe().unwrap().unwrap();
         assert!(matches!(
@@ -1811,7 +1863,7 @@ mod tests {
         let sent_at = timestamp(Instant::now());
         let preflight = machine.preflight_probe_commit(&first).unwrap();
         let commit = machine.finalize_probe_commit(preflight, sent_at).unwrap();
-        machine.commit_probe_sent(commit, first.bytes.len());
+        machine.commit_probe_sent(commit, sent_at, first.bytes.len());
         active_mut(&mut machine).next_wire_seq = 0;
 
         let reused = machine.prepare_probe().unwrap().unwrap();
@@ -1862,6 +1914,7 @@ mod tests {
                 wire_seq: seq,
                 sent_at,
                 timeout_at: sent_at.mono + Duration::from_secs(1),
+                tx_not_before_wall: sent_at.wall,
                 kernel_tx_timestamp: None,
             });
         }
@@ -1888,7 +1941,7 @@ mod tests {
         let commit = machine
             .finalize_probe_commit(preflight, timestamp(Instant::now()))
             .unwrap();
-        machine.commit_probe_sent(commit, prepared.bytes.len());
+        machine.commit_probe_sent(commit, timestamp(Instant::now()), prepared.bytes.len());
 
         assert_eq!(active(&machine).next_wire_seq, 0);
         assert_eq!(machine.prepare_probe().unwrap().unwrap().seq, 0);
@@ -1898,10 +1951,12 @@ mod tests {
     fn successful_wrapped_reuse_purges_obsolete_history_only_on_commit() {
         let mut machine = open_machine(3, Duration::from_secs(1));
         let now = Instant::now();
+        let obsolete_sent_at = timestamp(now - Duration::from_secs(1));
         let obsolete = PendingProbe {
             wire_seq: 0,
-            sent_at: timestamp(now - Duration::from_secs(1)),
+            sent_at: obsolete_sent_at,
             timeout_at: now,
+            tx_not_before_wall: obsolete_sent_at.wall,
             kernel_tx_timestamp: None,
         };
         let session = active_mut(&mut machine);
@@ -1917,7 +1972,7 @@ mod tests {
         assert!(active(&machine).timed_out.contains(0));
         assert!(active(&machine).completed.contains(0));
 
-        machine.commit_probe_sent(commit, prepared.bytes.len());
+        machine.commit_probe_sent(commit, timestamp(now), prepared.bytes.len());
         let session = active(&machine);
         assert!(!session.timed_out.contains(0));
         assert!(!session.completed.contains(0));
@@ -1982,8 +2037,21 @@ mod tests {
         }
     }
 
-    /// Machine with one outstanding probe for sequence 0.
+    /// Machine with one outstanding probe for sequence 0. The pre-send
+    /// `tx_not_before_wall` bound defaults to `sent_at.wall`, i.e. this
+    /// helper does not exercise the pre-/post-send distinction; use
+    /// [`machine_with_pending_probe_anchored`] where that distinction
+    /// matters.
     fn machine_with_pending_probe(sent_at: ClientTimestamp) -> SessionMachine {
+        machine_with_pending_probe_anchored(sent_at.wall, sent_at)
+    }
+
+    /// Machine with one outstanding probe for sequence 0, with an explicit
+    /// pre-send `tx_not_before_wall` bound independent of `sent_at.wall`.
+    fn machine_with_pending_probe_anchored(
+        tx_not_before_wall: SystemTime,
+        sent_at: ClientTimestamp,
+    ) -> SessionMachine {
         let mut machine = open_machine(4, Duration::from_secs(1));
         let session = active_mut(&mut machine);
         session.pending.preflight_insert(0).unwrap();
@@ -1991,6 +2059,7 @@ mod tests {
             wire_seq: 0,
             sent_at,
             timeout_at: sent_at.mono + Duration::from_secs(1),
+            tx_not_before_wall,
             kernel_tx_timestamp: None,
         });
         session.next_wire_seq = 1;
@@ -2006,6 +2075,7 @@ mod tests {
             wire_seq: 0,
             sent_at,
             timeout_at: sent_at.mono + Duration::from_secs(1),
+            tx_not_before_wall: sent_at.wall,
             kernel_tx_timestamp: None,
         });
         session.next_wire_seq = 1;
@@ -2310,7 +2380,9 @@ mod tests {
         let prepared = machine.prepare_probe().unwrap().unwrap();
         let preflight = machine.preflight_probe_commit(&prepared).unwrap();
         let commit = machine.finalize_probe_commit(preflight, sent_at).unwrap();
-        machine.commit_probe_sent(commit, prepared.bytes.len()).seq
+        machine
+            .commit_probe_sent(commit, sent_at, prepared.bytes.len())
+            .seq
     }
 
     #[test]
@@ -2635,85 +2707,132 @@ mod tests {
 
     #[test]
     fn preferred_send_wall_without_kernel_timestamp_uses_sent_at() {
-        let sent_at = send_wall_ms(1_000);
+        let anchor = send_wall_ms(1_000);
         let received_at = send_wall_ms(1_040);
-        assert_eq!(preferred_send_wall(sent_at, None, received_at), sent_at);
+        assert_eq!(
+            preferred_send_wall(anchor, anchor, None, received_at),
+            anchor
+        );
     }
 
     #[test]
     fn preferred_send_wall_uses_kernel_timestamp_strictly_between_bounds() {
-        let sent_at = send_wall_ms(1_000);
+        let anchor = send_wall_ms(1_000);
         let kernel_tx = send_wall_ms(1_005);
         let received_at = send_wall_ms(1_040);
         assert_eq!(
-            preferred_send_wall(sent_at, Some(kernel_tx), received_at),
+            preferred_send_wall(anchor, anchor, Some(kernel_tx), received_at),
             kernel_tx
         );
     }
 
     #[test]
-    fn preferred_send_wall_accepts_kernel_timestamp_equal_to_sent_at() {
-        let sent_at = send_wall_ms(1_000);
+    fn preferred_send_wall_accepts_kernel_timestamp_equal_to_lower_bound() {
+        let anchor = send_wall_ms(1_000);
         let received_at = send_wall_ms(1_040);
         assert_eq!(
-            preferred_send_wall(sent_at, Some(sent_at), received_at),
-            sent_at
+            preferred_send_wall(anchor, anchor, Some(anchor), received_at),
+            anchor
         );
     }
 
     #[test]
     fn preferred_send_wall_accepts_kernel_timestamp_equal_to_received_at() {
-        let sent_at = send_wall_ms(1_000);
+        let anchor = send_wall_ms(1_000);
         let received_at = send_wall_ms(1_040);
         assert_eq!(
-            preferred_send_wall(sent_at, Some(received_at), received_at),
+            preferred_send_wall(anchor, anchor, Some(received_at), received_at),
             received_at
         );
     }
 
     #[test]
-    fn preferred_send_wall_rejects_kernel_timestamp_earlier_than_sent_at() {
-        let sent_at = send_wall_ms(1_000);
-        let kernel_tx = sent_at - Duration::from_nanos(1);
+    fn preferred_send_wall_rejects_kernel_timestamp_earlier_than_lower_bound() {
+        let anchor = send_wall_ms(1_000);
+        let kernel_tx = anchor - Duration::from_nanos(1);
         let received_at = send_wall_ms(1_040);
         assert_eq!(
-            preferred_send_wall(sent_at, Some(kernel_tx), received_at),
-            sent_at
+            preferred_send_wall(anchor, anchor, Some(kernel_tx), received_at),
+            anchor
         );
     }
 
     #[test]
     fn preferred_send_wall_rejects_kernel_timestamp_later_than_received_at() {
-        let sent_at = send_wall_ms(1_000);
+        let anchor = send_wall_ms(1_000);
         let received_at = send_wall_ms(1_040);
         let kernel_tx = received_at + Duration::from_nanos(1);
         assert_eq!(
-            preferred_send_wall(sent_at, Some(kernel_tx), received_at),
-            sent_at
+            preferred_send_wall(anchor, anchor, Some(kernel_tx), received_at),
+            anchor
         );
     }
 
     #[test]
     fn preferred_send_wall_handles_extreme_timestamps_without_panicking() {
-        let sent_at = send_wall_ms(1_000);
+        let anchor = send_wall_ms(1_000);
         let received_at = send_wall_ms(1_040);
         let far_future = UNIX_EPOCH + Duration::from_secs(u64::from(u32::MAX)) * 4;
         let before_epoch = UNIX_EPOCH - Duration::from_secs(u64::from(u32::MAX));
 
         assert_eq!(
-            preferred_send_wall(sent_at, Some(far_future), received_at),
-            sent_at
+            preferred_send_wall(anchor, anchor, Some(far_future), received_at),
+            anchor
         );
         assert_eq!(
-            preferred_send_wall(sent_at, Some(before_epoch), received_at),
-            sent_at
+            preferred_send_wall(anchor, anchor, Some(before_epoch), received_at),
+            anchor
         );
         assert_eq!(
-            preferred_send_wall(before_epoch, None, received_at),
+            preferred_send_wall(before_epoch, before_epoch, None, received_at),
             before_epoch
         );
     }
 
+    // Pre-/post-send decoupling (F-04): the plausibility lower bound is the
+    // pre-send anchor, but the fallback endpoint is the post-send sample,
+    // and the two can legitimately differ.
+
+    #[test]
+    fn preferred_send_wall_accepts_kernel_timestamp_before_post_send_sample() {
+        let tx_not_before_wall = send_wall_ms(1_000);
+        let post_send_sent_at = send_wall_ms(1_008);
+        let kernel_tx = send_wall_ms(1_005);
+        let received_at = send_wall_ms(1_040);
+
+        // The kernel timestamp precedes the post-send sample but follows
+        // the pre-send anchor: it must still be accepted.
+        assert_eq!(
+            preferred_send_wall(
+                tx_not_before_wall,
+                post_send_sent_at,
+                Some(kernel_tx),
+                received_at
+            ),
+            kernel_tx
+        );
+    }
+
+    #[test]
+    fn preferred_send_wall_rejected_kernel_timestamp_falls_back_to_post_send_sample() {
+        let tx_not_before_wall = send_wall_ms(1_000);
+        let post_send_sent_at = send_wall_ms(1_008);
+        let kernel_tx = send_wall_ms(999);
+        let received_at = send_wall_ms(1_040);
+
+        // Below the pre-send lower bound: rejected, and the fallback is the
+        // post-send sample, not the pre-send anchor.
+        assert_eq!(
+            preferred_send_wall(
+                tx_not_before_wall,
+                post_send_sent_at,
+                Some(kernel_tx),
+                received_at
+            ),
+            post_send_sent_at
+        );
+    }
+
     fn upstream_probe(sent_wall: SystemTime, mono: Instant) -> SessionMachine {
         machine_with_pending_probe(ClientTimestamp {
             mono,
@@ -2877,4 +2996,105 @@ mod tests {
 
         assert_eq!(reply_one_way(&events).unwrap().client_to_server, None);
     }
+
+    // F-04: post-send `sent_at` measurement timing.
+    //
+    // `sent_at` moved from immediately before timeout finalization/socket
+    // send to immediately after a successful send. These tests pin the
+    // resulting end-to-end behavior: raw RTT and the userspace upstream OWD
+    // fallback both shrink because they no longer include the send-call
+    // interval, while kernel TX plausibility keeps a separate pre-send
+    // lower bound so a legitimate `TX_SOFTWARE` timestamp generated during
+    // the send path is not rejected merely for preceding the post-send
+    // sample.
+
+    #[test]
+    fn raw_rtt_uses_post_send_sent_at_not_pre_send_anchor() {
+        let base = Instant::now();
+        // A pre-send anchor at base+100ms is deliberately not used here:
+        // raw RTT must be measured from the post-send `sent_at` at
+        // base+110ms, giving 50ms, not the 60ms a pre-send anchor would
+        // have produced.
+        let sent_at = ClientTimestamp {
+            mono: base + Duration::from_millis(110),
+            wall: SystemTime::now(),
+        };
+        let received_at = ClientTimestamp {
+            mono: base + Duration::from_millis(160),
+            wall: SystemTime::now(),
+        };
+
+        let rtt = compute_rtt(&sent_at, &received_at, &TimestampFields::default());
+
+        assert_eq!(rtt.raw, Duration::from_millis(50));
+        assert_eq!(rtt.effective, SignedDuration::from_duration(rtt.raw));
+    }
+
+    #[test]
+    fn userspace_upstream_owd_fallback_uses_post_send_sent_at() {
+        let mono = Instant::now();
+        let mut machine = machine_with_pending_probe_anchored(
+            send_wall_ms(1_000), // pre-send anchor
+            ClientTimestamp {
+                mono,
+                wall: send_wall_ms(1_008), // post-send sent_at
+            },
+        );
+
+        let events = upstream_reply(&mut machine, 1_020, send_wall_ms(1_040), mono);
+
+        // Old behavior would have used the pre-send 1000ms sample, giving
+        // 20ms. The new fallback uses the post-send 1008ms sample.
+        assert_eq!(
+            reply_one_way(&events).unwrap().client_to_server,
+            Some(SignedDuration::from_nanos(12_000_000))
+        );
+    }
+
+    #[test]
+    fn kernel_tx_accepted_even_though_earlier_than_post_send_sent_at() {
+        let mono = Instant::now();
+        let mut machine = machine_with_pending_probe_anchored(
+            send_wall_ms(1_000), // pre-send anchor / tx_not_before_wall
+            ClientTimestamp {
+                mono,
+                wall: send_wall_ms(1_008), // post-send sent_at
+            },
+        );
+        // The kernel TX timestamp is after the pre-send anchor and before
+        // the reply, but before the post-send `sent_at` sample. It must
+        // remain accepted: rejecting it here would be the regression this
+        // test guards against.
+        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(15_000_000))
+        );
+    }
+
+    #[test]
+    fn kernel_tx_below_pre_send_anchor_falls_back_to_post_send_not_pre_send() {
+        let mono = Instant::now();
+        let mut machine = machine_with_pending_probe_anchored(
+            send_wall_ms(1_000), // pre-send anchor / tx_not_before_wall
+            ClientTimestamp {
+                mono,
+                wall: send_wall_ms(1_008), // post-send sent_at
+            },
+        );
+        // Earlier than even the pre-send anchor: implausible, rejected.
+        machine.record_kernel_tx_timestamp(0, send_wall_ms(999));
+
+        let events = upstream_reply(&mut machine, 1_020, send_wall_ms(1_040), mono);
+
+        // Fallback is the post-send 1008ms sample (12ms), not the pre-send
+        // 1000ms anchor (which would have given 20ms).
+        assert_eq!(
+            reply_one_way(&events).unwrap().client_to_server,
+            Some(SignedDuration::from_nanos(12_000_000))
+        );
+    }
 }