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, 15 insertions, 0 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0fb84de6da36..685437a16c97 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -252,6 +252,7 @@ extern int sysctl_tcp_max_ssthresh;
 extern int sysctl_tcp_cookie_size;
 extern int sysctl_tcp_thin_linear_timeouts;
 extern int sysctl_tcp_thin_dupack;
+extern int sysctl_tcp_early_retrans;
 
 extern atomic_long_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;
@@ -797,6 +798,20 @@ static inline void tcp_enable_fack(struct tcp_sock *tp)
         tp->rx_opt.sack_ok |= TCP_FACK_ENABLED;
 }
 
+/* TCP early-retransmit (ER) is similar to but more conservative than
+ * the thin-dupack feature.  Enable ER only if thin-dupack is disabled.
+ */
+static inline void tcp_enable_early_retrans(struct tcp_sock *tp)
+{
+        tp->do_early_retrans = sysctl_tcp_early_retrans &&
+                !sysctl_tcp_thin_dupack && sysctl_tcp_reordering == 3;
+}
+
+static inline void tcp_disable_early_retrans(struct tcp_sock *tp)
+{
+        tp->do_early_retrans = 0;
+}
+
 static inline unsigned int tcp_left_out(const struct tcp_sock *tp)
 {
         return tp->sacked_out + tp->lost_out;