fix(client): handle sequence wrap in reply ordering

crates/irtt-client/src/client.rs

@@ -417,7 +417,9 @@ impl Client {
             let server_timing = build_server_timing(&reply.timestamps);
             let one_way = compute_one_way(&pending.sent_at, &now, &reply.timestamps);
             let received_stats = build_received_stats(&reply);
-            let is_late = session.highest_received_seq.is_some_and(|h| wire_seq < h);
+            let is_late = session
+                .highest_received_seq
+                .is_some_and(|h| sequence_is_before(wire_seq, h));
             let highest_seen = session.highest_received_seq.unwrap_or(wire_seq);
 
             update_highest_received(session, wire_seq);
@@ -486,9 +488,10 @@ impl Client {
                 bytes: packet.len(),
                 packet_meta: meta.into(),
             }])
-        } else if session.highest_received_seq.is_some_and(|h| wire_seq < h) {
+        } else if session
-            // TODO: u32 ordering is acceptable for finite tests but continuous
+            .highest_received_seq
-            // probing will eventually require wrap-aware comparison.
+            .is_some_and(|h| sequence_is_before(wire_seq, h))
+        {
             Ok(vec![ClientEvent::LateReply {
                 seq: wire_seq,
                 logical_seq: None,
@@ -635,11 +638,21 @@ impl Client {
 }
 
 fn update_highest_received(session: &mut ActiveSession, wire_seq: u32) {
-    session.highest_received_seq = Some(
+    session.highest_received_seq = Some(session.highest_received_seq.map_or(wire_seq, |h| {
-        session
+        if sequence_is_after(wire_seq, h) {
-            .highest_received_seq
+            wire_seq
-            .map_or(wire_seq, |h| h.max(wire_seq)),
+        } else {
-    );
+            h
+        }
+    }));
+}
+
+fn sequence_is_after(candidate: u32, current: u32) -> bool {
+    candidate != current && candidate.wrapping_sub(current) < (1 << 31)
+}
+
+fn sequence_is_before(candidate: u32, current: u32) -> bool {
+    current != candidate && current.wrapping_sub(candidate) < (1 << 31)
 }
 
 fn instant_abs_diff(left: Instant, right: Instant) -> Duration {

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

@@ -17,4 +17,5 @@ mod negotiation;
 mod no_test;
 mod open;
 mod probes_replies;
+mod sequence;
 mod ttl;

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

@@ -1584,6 +1584,66 @@ fn send_probe_wraps_wire_sequence_at_u32_max() {
     server.join();
 }
 
+#[test]
+fn wrapped_reply_after_u32_max_is_not_late() {
+    let params = default_params();
+    let server = start_fake_server(move |socket, tx| {
+        let (_, peer) = recv_request(&socket, &tx);
+        let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, None);
+        socket.send_to(&reply, peer).unwrap();
+
+        socket
+            .set_read_timeout(Some(Duration::from_secs(2)))
+            .unwrap();
+        let mut seqs = Vec::new();
+        for _ in 0..2 {
+            let mut buf = [0_u8; 2048];
+            let (size, _) = socket.recv_from(&mut buf).unwrap();
+            tx.send(buf[..size].to_vec()).unwrap();
+            seqs.push(u32::from_le_bytes(buf[12..16].try_into().unwrap()));
+        }
+        assert_eq!(seqs, vec![u32::MAX, 0]);
+
+        let ts = TimestampFields::default();
+        let reply_max = echo_reply_packet(TOKEN, u32::MAX, &params, &ts, None);
+        socket.send_to(&reply_max, peer).unwrap();
+        thread::sleep(Duration::from_millis(10));
+        let reply_zero = echo_reply_packet(TOKEN, 0, &params, &ts, None);
+        socket.send_to(&reply_zero, peer).unwrap();
+    });
+    let config = ClientConfig {
+        socket_config: crate::SocketConfig {
+            recv_timeout: Some(Duration::from_millis(200)),
+            ..Default::default()
+        },
+        ..default_test_config(server.addr)
+    };
+    let mut client = Client::connect(config).unwrap();
+    assert_open_started(client.open(ClientTimestamp::now()).unwrap());
+    let session = client.session.as_mut().unwrap();
+    session.next_wire_seq = u32::MAX;
+    session.next_logical_seq = 41;
+
+    client.send_probe().unwrap();
+    client.send_probe().unwrap();
+    thread::sleep(Duration::from_millis(50));
+
+    let first = client.recv_once().unwrap();
+    assert!(matches!(
+        &first[0],
+        ClientEvent::EchoReply { seq: u32::MAX, .. }
+    ));
+
+    thread::sleep(Duration::from_millis(30));
+    let second = client.recv_once().unwrap();
+    assert!(
+        matches!(&second[0], ClientEvent::EchoReply { seq: 0, .. }),
+        "freshly wrapped reply should not be late, got {:?}",
+        second[0]
+    );
+    server.join();
+}
+
 #[test]
 fn send_probe_after_sending_done_is_noop() {
     let params = default_params();

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

@@ -0,0 +1,45 @@
+use super::*;
+use std::time::Instant;
+
+use crate::{
+    probe::{CompletedSet, PendingMap, TimedOutMap},
+    session::ActiveSession,
+};
+
+#[test]
+fn sequence_ordering_handles_normal_and_wrapped_values() {
+    assert!(sequence_is_after(11, 10));
+    assert!(sequence_is_before(9, 10));
+    assert!(!sequence_is_after(10, 10));
+    assert!(!sequence_is_before(10, 10));
+
+    assert!(sequence_is_after(0, u32::MAX - 1));
+    assert!(!sequence_is_before(0, u32::MAX - 1));
+    assert!(sequence_is_after(1, u32::MAX));
+    assert!(!sequence_is_before(1, u32::MAX));
+
+    assert!(sequence_is_before(u32::MAX, 1));
+    assert!(!sequence_is_after(u32::MAX, 1));
+}
+
+#[test]
+fn highest_received_seq_updates_across_wrap() {
+    let mut session = ActiveSession {
+        next_wire_seq: 0,
+        next_logical_seq: 0,
+        highest_received_seq: Some(u32::MAX),
+        packets_sent: 0,
+        start_mono: Instant::now(),
+        end_mono: None,
+        next_send_at: Instant::now(),
+        pending: PendingMap::new(8),
+        timed_out: TimedOutMap::new(8),
+        completed: CompletedSet::new(8),
+        sending_done: false,
+    };
+
+    update_highest_received(&mut session, 0);
+    assert_eq!(session.highest_received_seq, Some(0));
+    update_highest_received(&mut session, u32::MAX);
+    assert_eq!(session.highest_received_seq, Some(0));
+}