netdev - Re: [PATCH net-next 2/4] tcp: TCP Fast Open Server - take SYNACK RTT after completing 3WHS

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Date:	Sat, 22 Sep 2012 15:47:50 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ncardwell@...gle.com
Cc:	netdev@...r.kernel.org, edumazet@...gle.com, ycheng@...gle.com,
	hkchu@...gle.com
Subject: Re: [PATCH net-next 2/4] tcp: TCP Fast Open Server - take SYNACK
 RTT after completing 3WHS

From: Neal Cardwell <ncardwell@...gle.com>
Date: Sat, 22 Sep 2012 10:18:55 -0400

> When taking SYNACK RTT samples for servers using TCP Fast Open, fix
> the code to ensure that we only call tcp_valid_rtt_meas() after we
> receive the ACK that completes the 3-way handshake.
> 
> Previously we were always taking an RTT sample in
> tcp_v4_syn_recv_sock(). However, for TCP Fast Open connections
> tcp_v4_conn_req_fastopen() calls tcp_v4_syn_recv_sock() at the time we
> receive the SYN. So for TFO we must wait until tcp_rcv_state_process()
> to take the RTT sample.
> 
> To fix this, we wait until after TFO calls tcp_v4_syn_recv_sock()
> before we set the snt_synack timestamp, since tcp_synack_rtt_meas()
> already ensures that we only take a SYNACK RTT sample if snt_synack is
> non-zero. To be careful, we only take a snt_synack timestamp when
> a SYNACK transmit or retransmit succeeds.
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>

Applied.
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