Added comments to clarify the LWIP_TCP_TIMESTAMPS option.

src/core/tcp_in.c

@@ -1660,10 +1660,12 @@ tcp_parseopt(struct tcp_pcb *pcb)
           return;
         }
         /* TCP timestamp option with valid length */
-        tsval = (opts[c+2]) | (opts[c+3] << 8) | 
+        tsval = (opts[c+2]) | (opts[c+3] << 8) |
           (opts[c+4] << 16) | (opts[c+5] << 24);
         if (flags & TCP_SYN) {
           pcb->ts_recent = ntohl(tsval);
+          /* Enable sending timestamps in every segment now that we know
+             the remote host supports it. */
           pcb->flags |= TF_TIMESTAMP;
         } else if (TCP_SEQ_BETWEEN(pcb->ts_lastacksent, seqno, seqno+tcplen)) {
           pcb->ts_recent = ntohl(tsval);

src/core/tcp_out.c

@@ -393,6 +393,8 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
 
 #if LWIP_TCP_TIMESTAMPS
   if ((pcb->flags & TF_TIMESTAMP)) {
+    /* Make sure the timestamp option is only included in data segments if we
+       agreed about it with the remote host. */
     optflags = TF_SEG_OPTS_TS;
     optlen = LWIP_TCP_OPT_LENGTH(TF_SEG_OPTS_TS);
   }
@@ -754,7 +756,9 @@ tcp_enqueue_flags(struct tcp_pcb *pcb, u8_t flags)
 #endif /* LWIP_WND_SCALE */
   }
 #if LWIP_TCP_TIMESTAMPS
-  if ((pcb->flags & TF_TIMESTAMP)) {
+  if ((pcb->flags & TF_TIMESTAMP) {
+    /* Make sure the timestamp option is only included in data segments if we
+       agreed about it with the remote host. */
     optflags |= TF_SEG_OPTS_TS;
   }
 #endif /* LWIP_TCP_TIMESTAMPS */

src/include/lwip/opt.h

@@ -1055,6 +1055,9 @@
 
 /**
  * LWIP_TCP_TIMESTAMPS==1: support the TCP timestamp option.
+ * The timestamp option is currently only used to help remote hosts, it is not
+ * really used locally. Therefore, it is only enabled when a TS option is
+ * received in the initial SYN packet from a remote host.
  */
 #ifndef LWIP_TCP_TIMESTAMPS
 #define LWIP_TCP_TIMESTAMPS             0