tcp: early retransmit

This patch implements RFC 5827 early retransmit (ER) for TCP.
It reduces DUPACK threshold (dupthresh) if outstanding packets are
less than 4 to recover losses by fast recovery instead of timeout.

While the algorithm is simple, small but frequent network reordering
makes this feature dangerous: the connection repeatedly enter
false recovery and degrade performance. Therefore we implement
a mitigation suggested in the appendix of the RFC that delays
entering fast recovery by a small interval, i.e., RTT/4. Currently
ER is conservative and is disabled for the rest of the connection
after the first reordering event. A large scale web server
experiment on the performance impact of ER is summarized in
section 6 of the paper "Proportional Rate Reduction for TCP”,
IMC 2011. http://conferences.sigcomm.org/imc/2011/docs/p155.pdf

Note that Linux has a similar feature called THIN_DUPACK. The
differences are THIN_DUPACK do not mitigate reorderings and is only
used after slow start. Currently ER is disabled if THIN_DUPACK is
enabled. I would be happy to merge THIN_DUPACK feature with ER if
people think it's a good idea.

ER is enabled by sysctl_tcp_early_retrans:
  0: Disables ER

  1: Reduce dupthresh to packets_out - 1 when outstanding packets < 4.

  2: (Default) reduce dupthresh like mode 1. In addition, delay
     entering fast recovery by RTT/4.

Note: mode 2 is implemented in the third part of this patch series.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 0 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 9b569a2d9c60..34916e792d9d 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -190,6 +190,20 @@ tcp_cookie_size - INTEGER
 tcp_dsack - BOOLEAN
         Allows TCP to send "duplicate" SACKs.
 
+tcp_early_retrans - INTEGER
+        Enable Early Retransmit (ER), per RFC 5827. ER lowers the threshold
+        for triggering fast retransmit when the amount of outstanding data is
+        small and when no previously unsent data can be transmitted (such
+        that limited transmit could be used).
+        Possible values:
+                0 disables ER
+                1 enables ER
+                2 enables ER but delays fast recovery and fast retransmit
+                  by a fourth of RTT. This mitigates connection falsely
+                  recovers when network has a small degree of reordering
+                  (less than 3 packets).
+        Default: 2
+
 tcp_ecn - INTEGER
         Enable Explicit Congestion Notification (ECN) in TCP. ECN is only
         used when both ends of the TCP flow support it. It is useful to