tcp: send loss probe after 1s if no RTT available (f9b99582) · Commits · 戴 / test

This patch makes TLP to use 1 sec timer by default when RTT is not available due to SYN/ACK retransmission or SYN cookies. Prior to this change, the lack of RTT prevents TLP so the first data packets sent can only be recovered by fast recovery or RTO. If the fast recovery fails to trigger the RTO is 3 second when SYN/ACK is retransmitted. With this patch we can trigger fast recovery in 1sec instead. Note that we need to check Fast Open more properly. A Fast Open connection could be (accepted then) closed before it receives the final ACK of 3WHS so the state is FIN_WAIT_1. Without the new check, TLP will retransmit FIN instead of SYN/ACK. Signed-off-by:

Yuchung Cheng <ycheng@google.com> Signed-off-by:

Nandita Dukkipati <nanditad@google.com> Signed-off-by:

Neal Cardwell <ncardwell@google.com> Signed-off-by:

Eric Dumazet <edumazet@google.com> Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

+5 −4

Original line number	Diff line number	Diff line
		@@ -2165,7 +2165,7 @@ bool tcp_schedule_loss_probe(struct sock *sk)
		/* Don't do any loss probe on a Fast Open connection before 3WHS
		* finishes.
		*/
		if (sk->sk_state == TCP_SYN_RECV)
		if (tp->fastopen_rsk)
		return false;

		/* TLP is only scheduled when next timer event is RTO. */
		@@ -2175,7 +2175,7 @@ bool tcp_schedule_loss_probe(struct sock *sk)
		/* Schedule a loss probe in 2*RTT for SACK capable connections
		* in Open state, that are either limited by cwnd or application.
		*/
		if (sysctl_tcp_early_retrans < 3 \|\| !tp->srtt_us \|\| !tp->packets_out \|\|
		if (sysctl_tcp_early_retrans < 3 \|\| !tp->packets_out \|\|
		!tcp_is_sack(tp) \|\| inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
		return false;

		@@ -2184,9 +2184,10 @@ bool tcp_schedule_loss_probe(struct sock *sk)
		return false;

		/* Probe timeout is at least 1.5*rtt + TCP_DELACK_MAX to account
		* for delayed ack when there's one outstanding packet.
		* for delayed ack when there's one outstanding packet. If no RTT
		* sample is available then probe after TCP_TIMEOUT_INIT.
		*/
		timeout = rtt << 1;
		timeout = rtt << 1 ? : TCP_TIMEOUT_INIT;
		if (tp->packets_out == 1)
		timeout = max_t(u32, timeout,
		(rtt + (rtt >> 1) + TCP_DELACK_MAX));

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