[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>

1 files changed, 4 insertions, 2 deletions

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;