tcp: record most recent RTT in RACK loss detection

Record the most recent RTT in RACK. It is often identical to the
"ca_rtt_us" values in tcp_clean_rtx_queue. But when the packet has
been retransmitted, RACK choses to believe the ACK is for the
(latest) retransmitted packet if the RTT is over minimum RTT.

This requires passing the arrival time of the most recent ACK to
RACK routines. The timestamp is now recorded in the "ack_time"
in tcp_sacktag_state during the ACK processing.

This patch does not change the RACK algorithm itself. It only adds
the RTT variable to prepare the next main patch.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 23 insertions, 18 deletions

diff --git a/net/ipv4/tcp_recovery.c b/net/ipv4/tcp_recovery.c
index 7ea0377229c0..557363cde58a 100644
--- a/net/ipv4/tcp_recovery.c
+++ b/net/ipv4/tcp_recovery.c
@@ -32,7 +32,7 @@ static void tcp_rack_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
  * The current version is only used after recovery starts but can be
  * easily extended to detect the first loss.
  */
-static void tcp_rack_detect_loss(struct sock *sk)
+static void tcp_rack_detect_loss(struct sock *sk, const struct skb_mstamp *now)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct sk_buff *skb;
@@ -62,13 +62,14 @@ static void tcp_rack_detect_loss(struct sock *sk)
 			continue;
 
 		if (skb_mstamp_after(&tp->rack.mstamp, &skb->skb_mstamp)) {
-
-			if (skb_mstamp_us_delta(&tp->rack.mstamp,
-						&skb->skb_mstamp) <= reo_wnd)
-				continue;
-
-			/* skb is lost if packet sent later is sacked */
-			tcp_rack_mark_skb_lost(sk, skb);
+			/* Step 3 in draft-cheng-tcpm-rack-00.txt:
+			 * A packet is lost if its elapsed time is beyond
+			 * the recent RTT plus the reordering window.
+			 */
+			if (skb_mstamp_us_delta(now, &skb->skb_mstamp) >
+			    tp->rack.rtt_us + reo_wnd) {
+				tcp_rack_mark_skb_lost(sk, skb);
+			}
 		} else if (!(scb->sacked & TCPCB_RETRANS)) {
 			/* Original data are sent sequentially so stop early
 			 * b/c the rest are all sent after rack_sent
@@ -78,7 +79,7 @@ static void tcp_rack_detect_loss(struct sock *sk)
 	}
 }
 
-void tcp_rack_mark_lost(struct sock *sk)
+void tcp_rack_mark_lost(struct sock *sk, const struct skb_mstamp *now)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
@@ -86,20 +87,25 @@ void tcp_rack_mark_lost(struct sock *sk)
 		return;
 	/* Reset the advanced flag to avoid unnecessary queue scanning */
 	tp->rack.advanced = 0;
-	tcp_rack_detect_loss(sk);
+	tcp_rack_detect_loss(sk, now);
 }
 
-/* Record the most recently (re)sent time among the (s)acked packets */
-void tcp_rack_advance(struct tcp_sock *tp,
-		      const struct skb_mstamp *xmit_time, u8 sacked)
+/* Record the most recently (re)sent time among the (s)acked packets
+ * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
+ * draft-cheng-tcpm-rack-00.txt
+ */
+void tcp_rack_advance(struct tcp_sock *tp, u8 sacked,
+		      const struct skb_mstamp *xmit_time,
+		      const struct skb_mstamp *ack_time)
 {
+	u32 rtt_us;
+
 	if (tp->rack.mstamp.v64 &&
 	    !skb_mstamp_after(xmit_time, &tp->rack.mstamp))
 		return;
 
+	rtt_us = skb_mstamp_us_delta(ack_time, xmit_time);
 	if (sacked & TCPCB_RETRANS) {
-		struct skb_mstamp now;
-
 		/* If the sacked packet was retransmitted, it's ambiguous
 		 * whether the retransmission or the original (or the prior
 		 * retransmission) was sacked.
@@ -110,11 +116,10 @@ void tcp_rack_advance(struct tcp_sock *tp,
 		 * so it's at least one RTT (i.e., retransmission is at least
 		 * an RTT later).
 		 */
-		skb_mstamp_get(&now);
-		if (skb_mstamp_us_delta(&now, xmit_time) < tcp_min_rtt(tp))
+		if (rtt_us < tcp_min_rtt(tp))
 			return;
 	}
-
+	tp->rack.rtt_us = rtt_us;
 	tp->rack.mstamp = *xmit_time;
 	tp->rack.advanced = 1;
 }