tcp: use RACK to detect losses

This patch implements the second half of RACK that uses the the most
recent transmit time among all delivered packets to detect losses.

tcp_rack_mark_lost() is called upon receiving a dubious ACK.
It then checks if an not-yet-sacked packet was sent at least
"reo_wnd" prior to the sent time of the most recently delivered.
If so the packet is deemed lost.

The "reo_wnd" reordering window starts with 1msec for fast loss
detection and changes to min-RTT/4 when reordering is observed.
We found 1msec accommodates well on tiny degree of reordering
(<3 pkts) on faster links. We use min-RTT instead of SRTT because
reordering is more of a path property but SRTT can be inflated by
self-inflicated congestion. The factor of 4 is borrowed from the
delayed early retransmit and seems to work reasonably well.

Since RACK is still experimental, it is now used as a supplemental
loss detection on top of existing algorithms. It is only effective
after the fast recovery starts or after the timeout occurs. The
fast recovery is still triggered by FACK and/or dupack threshold
instead of RACK.

We introduce a new sysctl net.ipv4.tcp_recovery for future
experiments of loss recoveries. For now RACK can be disabled by
setting it to 0.

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

1 files changed, 9 insertions, 0 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 3c3a9fe057d3..11e320412216 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -567,6 +567,7 @@ void tcp_resume_early_retransmit(struct sock *sk);
 void tcp_rearm_rto(struct sock *sk);
 void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req);
 void tcp_reset(struct sock *sk);
+void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb);
 
 /* tcp_timer.c */
 void tcp_init_xmit_timers(struct sock *);
@@ -1752,6 +1753,14 @@ void tcp_init(void);
 
 /* tcp_recovery.c */
 
+/* Flags to enable various loss recovery features. See below */
+extern int sysctl_tcp_recovery;
+
+/* Use TCP RACK to detect (some) tail and retransmit losses */
+#define TCP_RACK_LOST_RETRANS  0x1
+
+extern int tcp_rack_mark_lost(struct sock *sk);
+
 extern void tcp_rack_advance(struct tcp_sock *tp,
 			     const struct skb_mstamp *xmit_time, u8 sacked);