diff --git a/src/core/tcp.c b/src/core/tcp.c
index 40f4c1d3..ec2e1f92 100644
--- a/src/core/tcp.c
+++ b/src/core/tcp.c
@@ -1013,11 +1013,11 @@ tcp_slowtmr_start:
     pcb_remove = 0;
     pcb_reset = 0;
 
-    if (pcb->state == SYN_SENT && pcb->nrtx == TCP_SYNMAXRTX) {
+    if (pcb->state == SYN_SENT && pcb->nrtx >= TCP_SYNMAXRTX) {
       ++pcb_remove;
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: max SYN retries reached\n"));
     }
-    else if (pcb->nrtx == TCP_MAXRTX) {
+    else if (pcb->nrtx >= TCP_MAXRTX) {
       ++pcb_remove;
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: max DATA retries reached\n"));
     } else {
@@ -1051,7 +1051,8 @@ tcp_slowtmr_start:
           /* Double retransmission time-out unless we are trying to
            * connect to somebody (i.e., we are in SYN_SENT). */
           if (pcb->state != SYN_SENT) {
-            pcb->rto = ((pcb->sa >> 3) + pcb->sv) << tcp_backoff[pcb->nrtx];
+            u8_t backoff_idx = LWIP_MIN(pcb->nrtx, sizeof(tcp_backoff)-1);
+            pcb->rto = ((pcb->sa >> 3) + pcb->sv) << tcp_backoff[backoff_idx];
           }
 
           /* Reset the retransmission timer. */
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index 1a8a2245..2435408a 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -1420,7 +1420,9 @@ tcp_rexmit_rto(struct tcp_pcb *pcb)
   pcb->unacked = NULL;
 
   /* increment number of retransmissions */
-  ++pcb->nrtx;
+  if (pcb->nrtx < 0xFF) {
+    ++pcb->nrtx;
+  }
 
   /* Don't take any RTT measurements after retransmitting. */
   pcb->rttest = 0;
@@ -1465,7 +1467,9 @@ tcp_rexmit(struct tcp_pcb *pcb)
   }
 #endif /* TCP_OVERSIZE */
 
-  ++pcb->nrtx;
+  if (pcb->nrtx < 0xFF) {
+    ++pcb->nrtx;
+  }
 
   /* Don't take any rtt measurements after retransmitting. */
   pcb->rttest = 0;