tcp: ECN blackhole should not force quickack mode (7a269ffa) · Commits · 戴 / test

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:

Eric 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:

Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> Signed-off-by:

David S. Miller <davem@davemloft.net>

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;
		}
		}

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