Do not send past the right edge of the peer window

src/stream/tcb.rs

@@ -453,10 +453,15 @@ impl Tcb {
         self.inflight_packets.values().collect::<Vec<_>>()
     }
 
+    /// Bytes that can still be sent before reaching the right edge of the peer window or the cap on
+    /// unacknowledged bytes, whichever is nearer. Simplified version: min(cwnd, rwnd) - in flight.
+    pub(super) fn get_usable_send_window(&self) -> u32 {
+        let in_flight = self.seq.distance(self.get_last_received_ack());
+        self.max_unacked_bytes.min(self.get_send_window()).saturating_sub(in_flight)
+    }
+
     pub fn is_send_buffer_full(&self) -> bool {
-        // To respect the receiver's window (remote_window) size and avoid sending too many unacknowledged packets, which may cause packet loss
-        // Simplified version: min(cwnd, rwnd)
-        self.seq.distance(self.get_last_received_ack()) >= self.max_unacked_bytes.min(self.get_send_window())
+        self.get_usable_send_window() == 0
     }
 }
 

src/stream/tcp.rs

@@ -391,6 +391,8 @@ impl AsyncWrite for IpStackTcpStream {
             return Poll::Pending;
         }
 
+        // A segment past the right edge of the peer window is trimmed by the peer (RFC 9293 §3.10.7.4).
+        let buf = &buf[..buf.len().min(tcb.get_usable_send_window() as usize)];
         let sender = &self.up_packet_sender;
         let payload_len = write_packet_to_device(sender, nt, &tcb, None, ACK | PSH, None, Some(buf.to_vec()))?;
         let was_empty = tcb.is_inflight_queue_empty();
@@ -1470,4 +1472,28 @@ mod tests {
         assert!(matches!(Pin::new(&mut stream).poll_write(&mut cx, b"sent"), Poll::Ready(Ok(4))));
         assert_eq!(up_rx.recv().await.unwrap().payload.as_deref(), Some(&b"sent"[..]));
     }
+
+    /// A write that would pass the right edge of the peer window is cut at the edge, and the next
+    /// one waits for an ACK instead of sending past it.
+    #[tokio::test]
+    async fn writer_stops_at_the_right_edge_of_the_peer_window() {
+        let (mut stream, syn_ack, mut up_rx) = open(2000, &[]).await;
+        let peer_seq = SeqNum(syn_ack.acknowledgment_number);
+        feed(&stream, &syn_ack, peer_seq, 2000, &[1; 4]);
+        up_rx.recv().await.unwrap();
+
+        let (_, waker) = recording_waker();
+        let mut cx = Context::from_waker(&waker);
+        let chunk = [7; 1460];
+        assert!(matches!(Pin::new(&mut stream).poll_write(&mut cx, &chunk), Poll::Ready(Ok(1460))));
+        assert_eq!(up_rx.recv().await.unwrap().payload.map(|p| p.len()), Some(1460));
+
+        assert!(
+            matches!(Pin::new(&mut stream).poll_write(&mut cx, &chunk), Poll::Ready(Ok(540))),
+            "the second write is not cut at the right edge of the 2000-byte window"
+        );
+        assert_eq!(up_rx.recv().await.unwrap().payload.map(|p| p.len()), Some(540));
+
+        assert!(Pin::new(&mut stream).poll_write(&mut cx, &chunk).is_pending());
+    }
 }