From aae06bf5f90403554f8d4ff83810a8281aef7f03 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 6 Aug 2012 11:04:43 +0000 Subject: [PATCH] tcp: ecn: dont delay ACKS after CE While playing with CoDel and ECN marking, I discovered a non optimal behavior of receiver of CE (Congestion Encountered) segments. In pathological cases, sender has reduced its cwnd to low values, and receiver delays its ACK (by 40 ms). While RFC 3168 6.1.3 (The TCP Receiver) doesn't explicitly recommend to send immediate ACKS, we believe its better to not delay ACKS, because a CE segment should give same signal than a dropped segment, and its quite important to reduce RTT to give ECE/CWR signals as fast as possible. Note we already call tcp_enter_quickack_mode() from TCP_ECN_check_ce() if we receive a retransmit, for the same reason. Signed-off-by: Eric Dumazet Cc: Neal Cardwell Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 2fd2bc9e3c64..fa2c2c2cac2d 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -237,7 +237,11 @@ static inline void TCP_ECN_check_ce(struct tcp_sock *tp, const struct sk_buff *s tcp_enter_quickack_mode((struct sock *)tp); break; case INET_ECN_CE: - tp->ecn_flags |= TCP_ECN_DEMAND_CWR; + if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) { + /* Better not delay acks, sender can have a very low cwnd */ + tcp_enter_quickack_mode((struct sock *)tp); + tp->ecn_flags |= TCP_ECN_DEMAND_CWR; + } /* fallinto */ default: tp->ecn_flags |= TCP_ECN_SEEN;