Merge branch 'tcp-rack'

Yuchung Cheng says:

====================
RACK loss detection

RACK (Recent ACK) loss recovery uses the notion of time instead of
packet sequence (FACK) or counts (dupthresh).

It's inspired by the FACK heuristic in tcp_mark_lost_retrans(): when a
limited transmit (new data packet) is sacked in recovery, then any
retransmission sent before that newly sacked packet was sent must have
been lost, since at least one round trip time has elapsed.

But that existing heuristic from tcp_mark_lost_retrans()
has several limitations:
  1) it can't detect tail drops since it depends on limited transmit
  2) it's disabled upon reordering (assumes no reordering)
  3) it's only enabled in fast recovery but not timeout recovery

RACK addresses these limitations with a core idea: an unacknowledged
packet P1 is deemed lost if a packet P2 that was sent later is is
s/acked, since at least one round trip has passed.

Since RACK cares about the time sequence instead of the data sequence
of packets, it can detect tail drops when a later retransmission is
s/acked, while FACK or dupthresh can't. For reordering RACK uses a
dynamically adjusted reordering window ("reo_wnd") to reduce false
positives on ever (small) degree of reordering, similar to the delayed
Early Retransmit.

In the current patch set RACK is only a supplemental loss detection
and does not trigger fast recovery. However we are developing RACK
to replace or consolidate FACK/dupthresh, early retransmit, and
thin-dupack. These heuristics all implicitly bear the time notion.
For example, the delayed Early Retransmit is simply applying RACK
to trigger the fast recovery with small inflight.

RACK requires measuring the minimum RTT. Tracking a global min is less
robust due to traffic engineering pathing changes. Therefore it uses a
windowed filter by Kathleen Nichols. The min RTT can also be useful
for various other purposes like congestion control or stat monitoring.

This patch has been used on Google servers for well over 1 year. RACK
has also been implemented in the QUIC protocol. We are submitting an
IETF draft as well.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 109 insertions, 0 deletions

diff --git a/net/ipv4/tcp_recovery.c b/net/ipv4/tcp_recovery.c
new file mode 100644
index 000000000000..5353085fd0b2
--- /dev/null
+++ b/net/ipv4/tcp_recovery.c
@@ -0,0 +1,109 @@
+#include <linux/tcp.h>
+#include <net/tcp.h>
+
+int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOST_RETRANS;
+
+/* Marks a packet lost, if some packet sent later has been (s)acked.
+ * The underlying idea is similar to the traditional dupthresh and FACK
+ * but they look at different metrics:
+ *
+ * dupthresh: 3 OOO packets delivered (packet count)
+ * FACK: sequence delta to highest sacked sequence (sequence space)
+ * RACK: sent time delta to the latest delivered packet (time domain)
+ *
+ * The advantage of RACK is it applies to both original and retransmitted
+ * packet and therefore is robust against tail losses. Another advantage
+ * is being more resilient to reordering by simply allowing some
+ * "settling delay", instead of tweaking the dupthresh.
+ *
+ * The current version is only used after recovery starts but can be
+ * easily extended to detect the first loss.
+ */
+int tcp_rack_mark_lost(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+	u32 reo_wnd, prior_retrans = tp->retrans_out;
+
+	if (inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery || !tp->rack.advanced)
+		return 0;
+
+	/* Reset the advanced flag to avoid unnecessary queue scanning */
+	tp->rack.advanced = 0;
+
+	/* To be more reordering resilient, allow min_rtt/4 settling delay
+	 * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
+	 * RTT because reordering is often a path property and less related
+	 * to queuing or delayed ACKs.
+	 *
+	 * TODO: measure and adapt to the observed reordering delay, and
+	 * use a timer to retransmit like the delayed early retransmit.
+	 */
+	reo_wnd = 1000;
+	if (tp->rack.reord && tcp_min_rtt(tp) != ~0U)
+		reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
+
+	tcp_for_write_queue(skb, sk) {
+		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
+
+		if (skb == tcp_send_head(sk))
+			break;
+
+		/* Skip ones already (s)acked */
+		if (!after(scb->end_seq, tp->snd_una) ||
+		    scb->sacked & TCPCB_SACKED_ACKED)
+			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_skb_mark_lost_uncond_verify(tp, skb);
+			if (scb->sacked & TCPCB_SACKED_RETRANS) {
+				scb->sacked &= ~TCPCB_SACKED_RETRANS;
+				tp->retrans_out -= tcp_skb_pcount(skb);
+				NET_INC_STATS_BH(sock_net(sk),
+						 LINUX_MIB_TCPLOSTRETRANSMIT);
+			}
+		} else if (!(scb->sacked & TCPCB_RETRANS)) {
+			/* Original data are sent sequentially so stop early
+			 * b/c the rest are all sent after rack_sent
+			 */
+			break;
+		}
+	}
+	return prior_retrans - tp->retrans_out;
+}
+
+/* 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)
+{
+	if (tp->rack.mstamp.v64 &&
+	    !skb_mstamp_after(xmit_time, &tp->rack.mstamp))
+		return;
+
+	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.
+		 *
+		 * If the original is lost, there is no ambiguity. Otherwise
+		 * we assume the original can be delayed up to aRTT + min_rtt.
+		 * the aRTT term is bounded by the fast recovery or timeout,
+		 * 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))
+			return;
+	}
+
+	tp->rack.mstamp = *xmit_time;
+	tp->rack.advanced = 1;
+}