Properly ACK duplicate segments

mongoose.c

@@ -9761,9 +9761,9 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
       MG_VERBOSE(("ignoring duplicate pkt"));
     } else {
       MG_VERBOSE(("SEQ != ACK: %x %x %x", seq, s->ack, ack));
-      tx_tcp(c->mgr->ifp, s->mac, &c->loc, &c->rem, c->dscp, TH_ACK,
-             mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
     }
+    tx_tcp(c->mgr->ifp, s->mac, &c->loc, &c->rem, c->dscp, TH_ACK,
+           mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
     return;  // drop it, RFC-9293 3.10.7.4: ignore ACKno
   }
   // Now process the segment for ACK and payload

src/net_builtin.c

@@ -1503,9 +1503,9 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
       MG_VERBOSE(("ignoring duplicate pkt"));
     } else {
       MG_VERBOSE(("SEQ != ACK: %x %x %x", seq, s->ack, ack));
-      tx_tcp(c->mgr->ifp, s->mac, &c->loc, &c->rem, c->dscp, TH_ACK,
-             mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
     }
+    tx_tcp(c->mgr->ifp, s->mac, &c->loc, &c->rem, c->dscp, TH_ACK,
+           mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
     return;  // drop it, RFC-9293 3.10.7.4: ignore ACKno
   }
   // Now process the segment for ACK and payload

test/mip_test.c

@@ -906,11 +906,13 @@ static void test_tcp_retransmit(void) {
     now = mg_millis() - start;
     // we wait enough time for a reply
     if (now > 2 * MG_TCPIP_ACK_MS) {
-      response_recv = false;
+      response_recv = false; // response should have been received by now
       break;
     }
   }
-  ASSERT((!response_recv));  // replies should not be sent for duplicate packets
+  ASSERT((response_recv));  // retransmit the lost ACK for duplicate segs
+  ASSERT((t->flags == TH_ACK));
+  ASSERT((t->ack == mg_htonl(1003)));  // dup ACK
 
   // packet with seq_no = 1003 got lost/delayed, send seq_no = 1005
   create_tcp_simpleseg(&e, &ipp, 1005, 2, TH_PUSH | TH_ACK, 2);