From ac229dca7e4e582114e1ec9765fda0915aa58468 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Thu, 12 Jan 2017 22:11:40 -0800 Subject: [PATCH] tcp: remove RFC4653 NCR This patch removes the (partial) implementation of the aggressive limited transmit in RFC4653 TCP Non-Congestion Robustness (NCR). NCR is a mitigation to the problem created by the dynamic DUPACK threshold. With the current adaptive DUPACK threshold (tp->reordering) could cause timeouts by preventing fast recovery. For example, if the last packet of a cwnd burst was reordered, the threshold will be set to the size of cwnd. But if next application burst is smaller than threshold and has drops instead of reorderings, the sender would not trigger fast recovery but instead resorts to a timeout recovery. NCR mitigates this issue by checking the number of DUPACKs against the current flight size additionally. The techniqueue is similar to the early retransmit RFC. With RACK loss detection, this mitigation is not needed, because RACK does not use DUPACK threshold to detect losses. RACK arms a reordering timer to fire at most a quarter RTT later to start fast recovery. Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Acked-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 15 --------------- 1 file changed, 15 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 79c819077a59..87315ab1ab1a 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2161,8 +2161,6 @@ static inline int tcp_dupack_heuristics(const struct tcp_sock *tp) static bool tcp_time_to_recover(struct sock *sk, int flag) { struct tcp_sock *tp = tcp_sk(sk); - __u32 packets_out; - int tcp_reordering = sock_net(sk)->ipv4.sysctl_tcp_reordering; /* Trick#1: The loss is proven. */ if (tp->lost_out) @@ -2172,19 +2170,6 @@ static bool tcp_time_to_recover(struct sock *sk, int flag) if (tcp_dupack_heuristics(tp) > tp->reordering) return true; - /* Trick#4: It is still not OK... But will it be useful to delay - * recovery more? - */ - packets_out = tp->packets_out; - if (packets_out <= tp->reordering && - tp->sacked_out >= max_t(__u32, packets_out/2, tcp_reordering) && - !tcp_may_send_now(sk)) { - /* We have nothing to send. This connection is limited - * either by receiver window or by application. - */ - return true; - } - /* If a thin stream is detected, retransmit after first * received dupack. Employ only if SACK is supported in order * to avoid possible corner-case series of spurious retransmissions -- 2.20.1