[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: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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. */