Always send updated ack number in `ack_reply()`.

Previously, `ack_reply()` was sending the ack number present in `remote_last_ack`,
which is only updated by `dispatch`. This means the ack replies could contain
a wrong, old ack number.

Most of the time this simply caused wasting an extra roundtrip until the socket state
settles down, but in extreme cases it could lead to an infinite loop of packets, such
as in https://github.com/smoltcp-rs/smoltcp/issues/338

Fixes #338

src/socket/tcp.rs

@@ -858,7 +858,8 @@ impl<'a> TcpSocket<'a> {
         // and an acknowledgment indicating the next sequence number expected
         // to be received.
         reply_repr.seq_number = self.remote_last_seq;
-        reply_repr.ack_number = self.remote_last_ack;
+        reply_repr.ack_number = Some(self.remote_seq_no + self.rx_buffer.len());
+        self.remote_last_ack = reply_repr.ack_number;
 
         // From RFC 1323:
         // The window field [...] of every outgoing segment, with the exception of SYN
@@ -2891,6 +2892,47 @@ mod test {
         }]);
     }
 
+    #[test]
+    fn test_established_rst_bad_seq() {
+        let mut s = socket_established();
+        send!(s, TcpRepr {
+            control: TcpControl::Rst,
+            seq_number: REMOTE_SEQ, // Wrong seq
+            ack_number: None,
+            ..SEND_TEMPL
+        }, Ok(Some(TcpRepr {
+            seq_number: LOCAL_SEQ + 1,
+            ack_number: Some(REMOTE_SEQ + 1),
+            ..RECV_TEMPL
+        })));
+
+        assert_eq!(s.state, State::Established);
+
+        // Send something to advance seq by 1
+        send!(s, TcpRepr {
+            seq_number: REMOTE_SEQ + 1, // correct seq
+            ack_number: Some(LOCAL_SEQ + 1),
+            payload:    &b"a"[..],
+            ..SEND_TEMPL
+        });
+
+        // Send wrong rst again, check that the challenge ack is correctly updated
+        // The ack number must be updated even if we don't call dispatch on the socket
+        // See https://github.com/smoltcp-rs/smoltcp/issues/338
+        send!(s, TcpRepr {
+            control: TcpControl::Rst,
+            seq_number: REMOTE_SEQ, // Wrong seq
+            ack_number: None,
+            ..SEND_TEMPL
+        }, Ok(Some(TcpRepr {
+            seq_number: LOCAL_SEQ + 1,
+            ack_number: Some(REMOTE_SEQ + 2), // this has changed
+            window_len: 63,
+            ..RECV_TEMPL
+        })));
+    }
+
+
     // =========================================================================================//
     // Tests for the FIN-WAIT-1 state.
     // =========================================================================================//