The following commit has been merged in the batman-adv/next branch: commit aae06bf5f90403554f8d4ff83810a8281aef7f03 Author: Eric Dumazet edumazet@google.com Date: Mon Aug 6 11:04:43 2012 +0000
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 edumazet@google.com Cc: Neal Cardwell ncardwell@google.com Acked-by: Neal Cardwell ncardwell@google.com Signed-off-by: David S. Miller davem@davemloft.net
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 2fd2bc9..fa2c2c2 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;