Revert Backoff [v3]: Revert RTO on ICMP destination unreachable

Here, an ICMP host/network unreachable message, whose payload fits to
TCP's SND.UNA, is taken as an indication that the RTO retransmission has
not been lost due to congestion, but because of a route failure
somewhere along the path.
With true congestion, a router won't trigger such a message and the
patched TCP will operate as standard TCP.

This patch reverts one RTO backoff, if an ICMP host/network unreachable
message, whose payload fits to TCP's SND.UNA, arrives.
Based on the new RTO, the retransmission timer is reset to reflect the
remaining time, or - if the revert clocked out the timer - a retransmission
is sent out immediately.
Backoffs are only reverted, if TCP is in RTO loss recovery, i.e. if
there have been retransmissions and reversible backoffs, already.

Changes from v2:
1) Renaming of skb in tcp_v4_err() moved to another patch.
2) Reintroduced tcp_bound_rto() and __tcp_set_rto().
3) Fixed code comments.

Signed-off-by: Damian Lukowski <damian@tvk.rwth-aachen.de>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 3 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2bdb0da237e6..af6d6fa00db1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -685,7 +685,7 @@ static inline void tcp_set_rto(struct sock *sk)
          *    is invisible. Actually, Linux-2.4 also generates erratic
          *    ACKs in some circumstances.
          */
-        inet_csk(sk)->icsk_rto = (tp->srtt >> 3) + tp->rttvar;
+        inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);
 
         /* 2. Fixups made earlier cannot be right.
          *    If we do not estimate RTO correctly without them,
@@ -696,8 +696,7 @@ static inline void tcp_set_rto(struct sock *sk)
         /* NOTE: clamping at TCP_RTO_MIN is not required, current algo
          * guarantees that rto is higher.
          */
-        if (inet_csk(sk)->icsk_rto > TCP_RTO_MAX)
-                inet_csk(sk)->icsk_rto = TCP_RTO_MAX;
+        tcp_bound_rto(sk);
 }
 
 /* Save metrics learned by this TCP session.