[TCP] FRTO: RFC4138 allows Nagle override when new data must be sent

This is a corner case where less than MSS sized new data thingie
is awaiting in the send queue. For F-RTO to work correctly, a
new data segment must be sent at certain point or F-RTO cannot
be used at all. RFC4138 allows overriding of Nagle at that
point.

Implementation uses frto_counter states 2 and 3 to distinguish
when Nagle override is needed.

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

2 files changed, 12 insertions, 7 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 6b669898b197..7641b2761a14 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2637,7 +2637,9 @@ static void tcp_undo_spur_to_response(struct sock *sk, int flag)
  *                  algorithm is not part of the F-RTO detection algorithm
  *                  given in RFC4138 but can be selected separately).
  * Otherwise (basically on duplicate ACK), RTO was (likely) caused by a loss
- * and TCP falls back to conventional RTO recovery.
+ * and TCP falls back to conventional RTO recovery. F-RTO allows overriding
+ * of Nagle, this is done using frto_counter states 2 and 3, when a new data
+ * segment of any size sent during F-RTO, state 2 is upgraded to 3.
  *
  * Rationale: if the RTO was spurious, new ACKs should arrive from the
  * original window even after we transmit two new data segments.
@@ -2666,7 +2668,7 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
                 inet_csk(sk)->icsk_retransmits = 0;
 
         if (!before(tp->snd_una, tp->frto_highmark)) {
-                tcp_enter_frto_loss(sk, tp->frto_counter + 1, flag);
+                tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
                 return 1;
         }
 
@@ -2692,7 +2694,7 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
                         return 1;
                 }
 
-                if ((tp->frto_counter == 2) &&
+                if ((tp->frto_counter >= 2) &&
                     (!(flag&FLAG_FORWARD_PROGRESS) ||
                      ((flag&FLAG_DATA_SACKED) && !(flag&FLAG_ONLY_ORIG_SACKED)))) {
                         /* RFC4138 shortcoming (see comment above) */
@@ -2709,14 +2711,15 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
                 if (!tcp_send_head(sk) ||
                     after(TCP_SKB_CB(tcp_send_head(sk))->end_seq,
                                      tp->snd_una + tp->snd_wnd)) {
-                        tcp_enter_frto_loss(sk, tp->frto_counter + 1, flag);
+                        tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3),
+                                            flag);
                         return 1;
                 }
 
                 tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;
                 tp->frto_counter = 2;
                 return 1;
-        } else /* frto_counter == 2 */ {
+        } else {
                 switch (sysctl_tcp_frto_response) {
                 case 2:
                         tcp_undo_spur_to_response(sk, flag);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index b5fa3c19afee..0faacf9c419d 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1035,8 +1035,10 @@ static inline int tcp_nagle_test(struct tcp_sock *tp, struct sk_buff *skb,
         if (nonagle & TCP_NAGLE_PUSH)
                 return 1;
 
-        /* Don't use the nagle rule for urgent data (or for the final FIN).  */
-        if (tp->urg_mode ||
+        /* Don't use the nagle rule for urgent data (or for the final FIN).
+         * Nagle can be ignored during F-RTO too (see RFC4138).
+         */
+        if (tp->urg_mode || (tp->frto_counter == 2) ||
             (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN))
                 return 1;