[TCP]: Add highest_sack seqno, points to globally highest SACK

It is guaranteed to be valid only when !tp->sacked_out. In most
cases this seqno is available in the last ACK but there is no
guarantee for that. The new fast recovery loss marking algorithm
needs this as entry point.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -333,6 +333,8 @@ struct tcp_sock {
 
 	struct tcp_sack_block_wire recv_sack_cache[4];
 
+	u32	highest_sack;	/* Start seq of globally highest revd SACK (valid only in slowpath) */
+
 	/* from STCP, retrans queue hinting */
 	struct sk_buff* lost_skb_hint;
 

net/ipv4/tcp_input.c

@@ -979,8 +979,10 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 	int i;
 	int first_sack_index;
 
-	if (!tp->sacked_out)
+	if (!tp->sacked_out) {
 		tp->fackets_out = 0;
+		tp->highest_sack = tp->snd_una;
+	}
 	prior_fackets = tp->fackets_out;
 
 	/* Check for D-SACK. */
@@ -1217,6 +1219,10 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 
 				if (fack_count > tp->fackets_out)
 					tp->fackets_out = fack_count;
+
+				if (after(TCP_SKB_CB(skb)->seq,
+				    tp->highest_sack))
+					tp->highest_sack = TCP_SKB_CB(skb)->seq;
 			} else {
 				if (dup_sack && (sacked&TCPCB_RETRANS))
 					reord = min(fack_count, reord);