fix(dtls): retransmit the last flight when the peer repeats its own (RFC 6347 4.2.4)

rtc-dtls/src/conn/mod.rs

@@ -103,6 +103,10 @@ pub struct DTLSConn {
     pub(crate) handshake_config: Arc<HandshakeConfig>,
     pub(crate) retransmit: bool,
     pub(crate) handshake_rx: Option<()>,
+    /// Whether the datagram most recently passed to [`Self::read`] carried a handshake record.
+    /// Reset on every `read`, unlike the sticky `handshake_rx`, so a completed association can tell
+    /// an RFC 6347 §4.2.4 last-flight repeat from ordinary post-handshake application data.
+    pub(crate) received_handshake_in_last_datagram: bool,
 }
 
 impl DTLSConn {
@@ -167,6 +171,7 @@ impl DTLSConn {
             handshake_config,
             retransmit: false,
             handshake_rx: None,
+            received_handshake_in_last_datagram: false,
         }
     }
 
@@ -467,13 +472,18 @@ impl DTLSConn {
 
     /// Feeds one received datagram into the connection.
     ///
+    /// Records whether this datagram carried a handshake record in
+    /// `received_handshake_in_last_datagram`, which a completed association uses to tell an
+    /// RFC 6347 §4.2.4 last-flight repeat from ordinary post-handshake application data.
+    ///
     /// # Errors
     ///
     /// Fails if the record is malformed or fails authentication.
     pub fn read(&mut self, buf: &[u8]) -> Result<()> {
         // Per RFC 6347: buffer future-epoch packets only until Finished is received
         // (i.e. until handshake completes). After that, discard them.
         let enqueue = !self.is_handshake_completed();
+        self.received_handshake_in_last_datagram = false;
         for pkt in unpack_datagram(buf)? {
             let (hs, alert, err) = self.handle_incoming_packet(pkt, enqueue);
             if let Some(alert) = alert {
@@ -502,6 +512,7 @@ impl DTLSConn {
             }
 
             if hs {
+                self.received_handshake_in_last_datagram = true;
                 self.handshake_rx = Some(());
             }
         }