linux-kernel - Re: [PATCH] tcp: Fix RFC reference in comment

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Message-ID: <D0DB041A.44371%dbanerje@akamai.com>
Date:	Tue, 13 Jan 2015 18:17:50 -0500
From:	"Banerjee, Debabrata" <dbanerje@...mai.com>
To:	John Heffner <johnwheffner@...il.com>
CC:	Yuchung Cheng <ycheng@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tcp: Fix RFC reference in comment

On 1/13/15, 5:01 PM, "John Heffner" <johnwheffner@...il.com> wrote:

>On Tue, Jan 13, 2015 at 4:42 PM, Banerjee, Debabrata
><dbanerje@...mai.com> wrote:
>> On 1/13/15, 4:36 PM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>>
>>>RFC2861 resets the cwnd like in RFC2581, but the rest of the code
>>>implements RFC2861. So I think the current comment is fine.
>>
>>
>> No RFC2861 is an experimental RFC that's implemented in
>> tcp_cwnd_application_limited(). RFC2861 Recommends reducing the cwnd by
>> averaging the current cwnd and the used cwnd as the new cwnd.
>>
>>
>> RFC2581 4.1 Says to set cwnd to initial cwnd if more than one rto has
>> passed since the last send. This is what is implemented in the function
>> above.
>
>Look at the code a little closer -- it's decaying cwnd based on number
>of timeouts as described in 2861, not resetting to IW as recommended
>in 2581.
>
>  -John


You're right it's not RFC2581 I was partially misled by the comment
(reset/restart window), but it doesn't appear to be doing what rfc2861 3.2
says either:

           For i=1  To (tcpnow - T_last)/RTO
	win =  min(cwnd, receiver's declared max window)
	cwnd =  max(win/2, MSS)


Versus:

u32 restart_cwnd = tcp_init_cwnd(tp, dst);

restart_cwnd = min(restart_cwnd, cwnd);

	while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
	cwnd >>= 1;
	tp->snd_cwnd = max(cwnd, restart_cwnd);


It's not using receiver window, it's using cwnd/init_cwnd, it should at
least be using tp->snd_wnd, no?.


I stumbled onto this because it looks like tcp_cwnd_application_limited()
doesn't execute when it should, because tp->snd_cwnd_stamp is being
touched much more often than in rfc2861 3.2. Something seems not right
here...

-Debabrata

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