[TCP]: Zero initial timestamps are valid, Windows XP emits these.

net/ipv4/tcp_ipv4.c

@@ -1465,15 +1465,6 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 		tp.saw_tstamp = 0;
 	}
 
-	if (tp.saw_tstamp && !tp.rcv_tsval) {
-		/* Some OSes (unknown ones, but I see them on web server, which
-		 * contains information interesting only for windows'
-		 * users) do not send their stamp in SYN. It is easy case.
-		 * We simply do not advertise TS support.
-		 */
-		tp.saw_tstamp = 0;
-		tp.tstamp_ok  = 0;
-	}
 	tp.tstamp_ok = tp.saw_tstamp;
 
 	tcp_openreq_init(req, &tp, skb);