netdev - Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is received

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Date:	Thu, 8 Jan 2015 17:00:24 +0100
From:	Sébastien Barré <sebastien.barre@...ouvain.be>
To:	Neal Cardwell <ncardwell@...gle.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Gregory Detal <gregory.detal@...ouvain.be>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is
 received


Le 08/01/2015 16:43, Neal Cardwell a écrit :
>> Le 08/01/2015 16:07, Eric Dumazet a écrit :
>>> Do you have at hand a packetdrill test to demonstrate that the patch
>>> works ?
> I cooked up the packetdrill test below when Sebastien sent out his v1
> a few weeks ago. It fails on a kernel without his patch, and passes on
> a kernel with his patch.
Thanks a lot for this ! I will include it in our test suite then.
I understand that there is convergence on having a
bool called is_probe_rcvd with the whole logic.
Eric, is this ok for you ?

Thanks,

Sébastien.
>
> The code change looks fine to me, but if Eric prefers that the
> expression be assigned to a bool before the check, that also sounds
> fine to me.
>
> neal
>
>
> ------------
> // Establish a connection.
> 0     socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
> +0     setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> +0    bind(3, ..., ...) = 0
> +0    listen(3, 1) = 0
>
> +0    < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
> +0    > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 6>
> +.020 < . 1:1(0) ack 1 win 257
> +0    accept(3, ..., ...) = 4
>
> // Send 1 packet.
> +0    write(4, ..., 1000) = 1000
> +0    > P. 1:1001(1000) ack 1
>
> // Loss probe retransmission.
> // packets_out == 1 => schedule PTO in max(2*RTT, 1.5*RTT + 200ms)
> // In this case, this means: 1.5*RTT + 200ms = 230ms
> +.230 > P. 1:1001(1000) ack 1
> +0    %{ assert tcpi_snd_cwnd == 10 }%
>
> // Receiver ACKs at tlp_high_seq with a DSACK,
> // indicating they received the original packet and probe.
> +.020 < . 1:1(0) ack 1001 win 257 <sack 1:1001,nop,nop>
> +0    %{ assert tcpi_snd_cwnd == 10 }%
>
> // Send another packet.
> +0    write(4, ..., 1000) = 1000
> +0    > P. 1001:2001(1000) ack 1
>
> // Receiver ACKs above tlp_high_seq, which should end the TLP episode
> // if we haven't already. We should not reduce cwnd.
> +.020 < . 1:1(0) ack 2001 win 257
> +0    %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%

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