[TCP] FRTO: Consecutive RTOs keep prior_ssthresh and ssthresh

In case a latency spike causes more than one RTO, the later should not
cause the already reduced ssthresh to propagate into the prior_ssthresh
since FRTO declares all such RTOs spurious at once or none of them. In
treating of ssthresh, we mimic what tcp_enter_loss() does.

The previous state (in frto_counter) must be available until we have
checked it in tcp_enter_frto(), and also ACK information flag in
process_frto().

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 6 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 561e5d40498..194e880af51 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1252,6 +1252,10 @@ int tcp_use_frto(const struct sock *sk)
 /* RTO occurred, but do not yet enter Loss state. Instead, defer RTO
  * recovery a bit and use heuristics in tcp_process_frto() to detect if
  * the RTO was spurious.
+ *
+ * Do like tcp_enter_loss() would; when RTO expires the second time it
+ * does:
+ *  "Reduce ssthresh if it has not yet been made inside this window."
  */
 void tcp_enter_frto(struct sock *sk)
 {
@@ -1259,11 +1263,10 @@ void tcp_enter_frto(struct sock *sk)
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb;
 
-        tp->frto_counter = 1;
-
-        if (icsk->icsk_ca_state <= TCP_CA_Disorder ||
+        if ((!tp->frto_counter && icsk->icsk_ca_state <= TCP_CA_Disorder) ||
             tp->snd_una == tp->high_seq ||
-            (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
+            ((icsk->icsk_ca_state == TCP_CA_Loss || tp->frto_counter) &&
+             !icsk->icsk_retransmits)) {
                 tp->prior_ssthresh = tcp_current_ssthresh(sk);
                 tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                 tcp_ca_event(sk, CA_EVENT_FRTO);
@@ -1285,6 +1288,7 @@ void tcp_enter_frto(struct sock *sk)
 
         tcp_set_ca_state(sk, TCP_CA_Open);
         tp->frto_highmark = tp->snd_nxt;
+        tp->frto_counter = 1;
 }
 
 /* Enter Loss state after F-RTO was applied. Dupack arrived after RTO,
@@ -2513,12 +2517,16 @@ static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
  *     to prove that the RTO is indeed spurious. It transfers the control
  *     from F-RTO to the conventional RTO recovery
  */
-static void tcp_process_frto(struct sock *sk, u32 prior_snd_una)
+static void tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         tcp_sync_left_out(tp);
 
+        /* Duplicate the behavior from Loss state (fastretrans_alert) */
+        if (flag&FLAG_DATA_ACKED)
+                inet_csk(sk)->icsk_retransmits = 0;
+
         if (tp->snd_una == prior_snd_una ||
             !before(tp->snd_una, tp->frto_highmark)) {
                 tcp_enter_frto_loss(sk);
@@ -2607,7 +2615,7 @@ static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
         flag |= tcp_clean_rtx_queue(sk, &seq_rtt);
 
         if (tp->frto_counter)
-                tcp_process_frto(sk, prior_snd_una);
+                tcp_process_frto(sk, prior_snd_una, flag);
 
         if (tcp_ack_is_dubious(sk, flag)) {
                 /* Advance CWND, if state allows this. */