Resend: [PATCH] TCP Early Retransmit: reduce required dupacks for triggering fast retrans

[Christian Samsel <Christian.Samsel@rwth-aachen.de>]

This patch implements draft-ietf-tcpm-early-rexmt. The early retransmit 
mechanism allows the transport to reduce the number of duplicate
acknowledgments required to trigger a fast retransmission in case we
don't expect enough dupacks, (e.g. because there are not enough
packets inflight and nothing to send). This allows the transport to use
fast retransmit to recover packet losses that would otherwise require
a lengthy retransmission timeout.

See: http://tools.ietf.org/html/draft-ietf-tcpm-early-rexmt-01

Signed-off-by: Christian Samsel <christian.samsel@rwth-aachen.de>

---
 net/ipv4/tcp_input.c |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index af6d6fa..c0cc4fd 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2913,6 +2913,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
  int do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
                                     (tcp_fackets_out(tp) > tp->reordering));
  int fast_rexmit = 0, mib_idx;
+ u32 in_flight;
 
  if (WARN_ON(!tp->packets_out && tp->sacked_out))
          tp->sacked_out = 0;
@@ -3062,6 +3063,21 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
  if (do_lost || (tcp_is_fack(tp) && tcp_head_timedout(sk)))
          tcp_update_scoreboard(sk, fast_rexmit);
  tcp_cwnd_down(sk, flag);
+       
+
+ /* draft-ietf-tcpm-early-rexmt: lowers dup ack threshold to prevent rto
+         * in case we don't expect enough dup ack. if number of outstanding
+         * packets is less than four and there is either no unsent data ready
+         * for transmission or the advertised window does not permit new
+         * segments.
+         */
+ in_flight = tcp_packets_in_flight(tp);
+ if ( in_flight < 4 && (skb_queue_empty(&sk->sk_write_queue) ||
+         tcp_may_send_now(sk) == 0) )
+         tp->reordering = in_flight - 1;
+ else if (tp->reordering != sysctl_tcp_reordering)
+         tp->reordering = sysctl_tcp_reordering;
+
  tcp_xmit_retransmit_queue(sk);
 }
 
-- 
1.6.4.1