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Commit 7a269ffa authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

tcp: ECN blackhole should not force quickack mode 



While playing with a new ADSL box at home, I discovered that ECN
blackhole can trigger suboptimal quickack mode on linux : We send one
ACK for each incoming data frame, without any delay and eventual
piggyback.

This is because TCP_ECN_check_ce() considers that if no ECT is seen on a
segment, this is because this segment was a retransmit.

Refine this heuristic and apply it only if we seen ECT in a previous
segment, to detect ECN blackhole at IP level.

Signed-off-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
CC: Jamal Hadi Salim <jhs@mojatatu.com>
CC: Jerry Chu <hkchu@google.com>
CC: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
CC: Jim Gettys <jg@freedesktop.org>
CC: Dave Taht <dave.taht@gmail.com>
Acked-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 0bdb0bd0

include/net/tcp.h

+1 −0
Original line number Diff line number Diff line
@@ -356,6 +356,7 @@ static inline void tcp_dec_quickack_mode(struct sock *sk, 
#define	TCP_ECN_OK		1

#define	TCP_ECN_QUEUE_CWR	2

#define	TCP_ECN_DEMAND_CWR	4

#define	TCP_ECN_SEEN		8



static __inline__ void

TCP_ECN_create_request(struct request_sock *req, struct tcphdr *th)

net/ipv4/tcp_input.c

+16 −7
Original line number Diff line number Diff line
@@ -217,16 +217,25 @@ static inline void TCP_ECN_withdraw_cwr(struct tcp_sock *tp) 
	tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;

}



static inline void TCP_ECN_check_ce(struct tcp_sock *tp, struct sk_buff *skb)

static inline void TCP_ECN_check_ce(struct tcp_sock *tp, const struct sk_buff *skb)

{

	if (tp->ecn_flags & TCP_ECN_OK) {

		if (INET_ECN_is_ce(TCP_SKB_CB(skb)->flags))

			tp->ecn_flags |= TCP_ECN_DEMAND_CWR;

	if (!(tp->ecn_flags & TCP_ECN_OK))

		return;



	switch (TCP_SKB_CB(skb)->flags & INET_ECN_MASK) {

	case INET_ECN_NOT_ECT:

		/* Funny extension: if ECT is not set on a segment,

		 * it is surely retransmit. It is not in ECN RFC,

		 * but Linux follows this rule. */

		else if (INET_ECN_is_not_ect((TCP_SKB_CB(skb)->flags)))

		 * and we already seen ECT on a previous segment,

		 * it is probably a retransmit.

		 */

		if (tp->ecn_flags & TCP_ECN_SEEN)

			tcp_enter_quickack_mode((struct sock *)tp);

		break;

	case INET_ECN_CE:

		tp->ecn_flags |= TCP_ECN_DEMAND_CWR;

		/* fallinto */

	default:

		tp->ecn_flags |= TCP_ECN_SEEN;

	}

}