From b4b7ff4b08e691656c9d77c758fc355833128ac0 Mon Sep 17 00:00:00 2001
From: André Fabian Silva Delgado <emulatorman@parabola.nu>
Date: Wed, 20 Jan 2016 14:01:31 -0300
Subject: Linux-libre 4.4-gnu

---
 net/ipv4/tcp_recovery.c | 109 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 net/ipv4/tcp_recovery.c

(limited to 'net/ipv4/tcp_recovery.c')

diff --git a/net/ipv4/tcp_recovery.c b/net/ipv4/tcp_recovery.c
new file mode 100644
index 000000000..5353085fd
--- /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;
+}
-- 
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