[TCP]: Allow minimum RTO to be configurable via routing metrics.

Cell phone networks do link layer retransmissions and other
things that cause unnecessary timeout retransmits.  So allow
the minimum RTO to be inflated per-route to deal with this.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/rtnetlink.h

@@ -351,6 +351,8 @@ enum
 #define RTAX_INITCWND RTAX_INITCWND
 	RTAX_FEATURES,
 #define RTAX_FEATURES RTAX_FEATURES
+	RTAX_RTO_MIN,
+#define RTAX_RTO_MIN RTAX_RTO_MIN
 	__RTAX_MAX
 };
 

net/ipv4/tcp_input.c

@@ -555,6 +555,16 @@ static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
 		tcp_grow_window(sk, skb);
 }
 
+static u32 tcp_rto_min(struct sock *sk)
+{
+	struct dst_entry *dst = __sk_dst_get(sk);
+	u32 rto_min = TCP_RTO_MIN;
+
+	if (dst_metric_locked(dst, RTAX_RTO_MIN))
+		rto_min = dst->metrics[RTAX_RTO_MIN-1];
+	return rto_min;
+}
+
 /* Called to compute a smoothed rtt estimate. The data fed to this
  * routine either comes from timestamps, or from segments that were
  * known _not_ to have been retransmitted [see Karn/Partridge
@@ -616,13 +626,13 @@ static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt)
 			if (tp->mdev_max < tp->rttvar)
 				tp->rttvar -= (tp->rttvar-tp->mdev_max)>>2;
 			tp->rtt_seq = tp->snd_nxt;
-			tp->mdev_max = TCP_RTO_MIN;
+			tp->mdev_max = tcp_rto_min(sk);
 		}
 	} else {
 		/* no previous measure. */
 		tp->srtt = m<<3;	/* take the measured time to be rtt */
 		tp->mdev = m<<1;	/* make sure rto = 3*rtt */
-		tp->mdev_max = tp->rttvar = max(tp->mdev, TCP_RTO_MIN);
+		tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
 		tp->rtt_seq = tp->snd_nxt;
 	}
 }