Commit aae06bf5 authored Aug 06, 2012 by Eric Dumazet Committed by David S. Miller Aug 06, 2012

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>

parent a9e050f4

net/ipv4/tcp_input.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -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:
		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;

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