dccp ccid-3: use per-route RTO or TCP RTO as fallback

This makes RTAX_RTO_MIN also available to CCID-3, replacing the compile-time
RTO lower bound with a per-route tunable value.

The original Kconfig option solved the problem that a very low RTT (in the
order of HZ) can trigger too frequent and unnecessary reductions of the
sending rate.

This tunable does not affect the initial RTO value of 2 seconds specified in
RFC 5348, section 4.2 and Appendix B. But like the hardcoded Kconfig value,
it allows to adapt to network conditions.

The same effect as the original Kconfig option of 100ms is now achieved by

> ip route replace to unicast 192.168.0.0/24 rto_min 100j dev eth0

(assuming HZ=1000).

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 6 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 4340672a817c..278e17069322 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -460,13 +460,12 @@ done_computing_x:
         sk->sk_write_space(sk);
 
         /*
-         * Update timeout interval for the nofeedback timer.
-         * We use a configuration option to increase the lower bound.
-         * This can help avoid triggering the nofeedback timer too
-         * often ('spinning') on LANs with small RTTs.
+         * Update timeout interval for the nofeedback timer. In order to control
+         * rate halving on networks with very low RTTs (<= 1 ms), use per-route
+         * tunable RTAX_RTO_MIN value as the lower bound.
          */
-        hc->tx_t_rto = max_t(u32, 4 * hc->tx_rtt, (CONFIG_IP_DCCP_CCID3_RTO *
-                                                       (USEC_PER_SEC / 1000)));
+        hc->tx_t_rto = max_t(u32, 4 * hc->tx_rtt,
+                                  USEC_PER_SEC/HZ * tcp_rto_min(sk));
         /*
          * Schedule no feedback timer to expire in
          * max(t_RTO, 2 * s/X)  =  max(t_RTO, 2 * t_ipi)