[PATCH net-next 2/8] tcp: disable RFC6675 loss detection

[Yuchung Cheng]

This patch disables RFC6675 loss detection and make sysctl
net.ipv4.tcp_recovery = 1 controls a binary choice between RACK
(1) or RFC6675 (0).

Signed-off-by: Yuchung Cheng <ych...@google.com>
Signed-off-by: Neal Cardwell <ncardw...@google.com>
Reviewed-by: Eric Dumazet <eduma...@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soh...@google.com>
Reviewed-by: Priyaranjan Jha <priyar...@google.com>
---
 Documentation/networking/ip-sysctl.txt |  3 ++-
 net/ipv4/tcp_input.c                   | 12 ++++++++----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.txt 
b/Documentation/networking/ip-sysctl.txt
index 13bbac50dc8b..ea304a23c8d7 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -449,7 +449,8 @@ tcp_recovery - INTEGER
        features.
 
        RACK: 0x1 enables the RACK loss detection for fast detection of lost
-             retransmissions and tail drops.
+             retransmissions and tail drops. It also subsumes and disables
+             RFC6675 recovery for SACK connections.
        RACK: 0x2 makes RACK's reordering window static (min_rtt/4).
        RACK: 0x4 disables RACK's DUPACK threshold heuristic
 
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index b188e0d75edd..ccbe04f80040 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2035,6 +2035,11 @@ static inline int tcp_dupack_heuristics(const struct 
tcp_sock *tp)
        return tp->sacked_out + 1;
 }
 
+static bool tcp_is_rack(const struct sock *sk)
+{
+       return sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_LOSS_DETECTION;
+}
+
 /* Linux NewReno/SACK/ECN state machine.
  * --------------------------------------
  *
@@ -2141,7 +2146,7 @@ static bool tcp_time_to_recover(struct sock *sk, int flag)
                return true;
 
        /* Not-A-Trick#2 : Classic rule... */
-       if (tcp_dupack_heuristics(tp) > tp->reordering)
+       if (!tcp_is_rack(sk) && tcp_dupack_heuristics(tp) > tp->reordering)
                return true;
 
        return false;
@@ -2722,8 +2727,7 @@ static void tcp_rack_identify_loss(struct sock *sk, int 
*ack_flag)
 {
        struct tcp_sock *tp = tcp_sk(sk);
 
-       /* Use RACK to detect loss */
-       if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_LOSS_DETECTION) {
+       if (tcp_is_rack(sk)) {
                u32 prior_retrans = tp->retrans_out;
 
                tcp_rack_mark_lost(sk);
@@ -2862,7 +2866,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const 
u32 prior_snd_una,
                fast_rexmit = 1;
        }
 
-       if (do_lost)
+       if (!tcp_is_rack(sk) && do_lost)
                tcp_update_scoreboard(sk, fast_rexmit);
        *rexmit = REXMIT_LOST;
 }
-- 
2.17.0.441.gb46fe60e1d-goog