tcp: remove forward retransmit feature

Forward retransmit is an esoteric feature in RFC3517 (condition(3)
in the NextSeg()). Basically if a packet is not considered lost by
the current criteria (# of dupacks etc), but the congestion window
has room for more packets, then retransmit this packet.

However it actually conflicts with the rest of recovery design. For
example, when reordering is detected we want to be conservative
in retransmitting packets but forward-retransmit feature would
break that to force more retransmission. Also the implementation is
fairly complicated inside the retransmission logic inducing extra
iterations in the write queue. With RACK losses are being detected
timely and this heuristic is no longer necessary. There this patch
removes the feature.

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

1 files changed, 0 insertions, 5 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9469ce384d3b..a041a92348ee 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -916,10 +916,6 @@ static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
             before(TCP_SKB_CB(skb)->seq,
                    TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
                 tp->retransmit_skb_hint = skb;
-
-        if (!tp->lost_out ||
-            after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high))
-                tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
 }
 
 /* Sum the number of packets on the wire we have marked as lost.
@@ -1983,7 +1979,6 @@ void tcp_enter_loss(struct sock *sk)
                         TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
                         TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                         tp->lost_out += tcp_skb_pcount(skb);
-                        tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
                 }
         }
         tcp_verify_left_out(tp);