diff --git a/src/include/lwip/tcp.h b/src/include/lwip/tcp.h
index f11b4cf3..a09c5ef7 100644
--- a/src/include/lwip/tcp.h
+++ b/src/include/lwip/tcp.h
@@ -320,7 +320,7 @@ void             tcp_sent    (struct tcp_pcb *pcb, tcp_sent_fn sent);
 void             tcp_poll    (struct tcp_pcb *pcb, tcp_poll_fn poll, u8_t interval);
 void             tcp_err     (struct tcp_pcb *pcb, tcp_err_fn err);
 
-#define          tcp_mss(pcb)             ((pcb)->mss)
+#define          tcp_mss(pcb)             (((pcb)->flags & TF_TIMESTAMP) ? ((pcb)->mss - 12)  : (pcb)->mss)
 #define          tcp_sndbuf(pcb)          ((pcb)->snd_buf)
 #define          tcp_sndqueuelen(pcb)     ((pcb)->snd_queuelen)
 #define          tcp_nagle_disable(pcb)   ((pcb)->flags |= TF_NODELAY)