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Date: Wed, 08 Aug 2007 11:20:05 -0400
From: John Heffner <jheffner@....edu>
To: David Miller <davem@...emloft.net>
CC: ilpo.jarvinen@...sinki.fi, netdev@...r.kernel.org
Subject: Re: TCP's initial cwnd setting correct?...
I believe the current calculation is correct. The RFC specifies a
window of no more than 4380 bytes unless 2*MSS > 4380. If you change
the code in this way, then MSS=1461 will give you an initial window of
3*MSS == 4383, violating the spec. Reading the pseudocode in the RFC
3390 is a bit misleading because they use a clamp at 4380 bytes rather
than use a multiplier in the relevant range.
-John
David Miller wrote:
> From: "Ilpo_Järvinen" <ilpo.jarvinen@...sinki.fi>
> Date: Mon, 6 Aug 2007 15:37:15 +0300 (EEST)
>
>> @@ -805,13 +805,13 @@ void tcp_update_metrics(struct sock *sk)
>> }
>> }
>>
>> -/* Numbers are taken from RFC2414. */
>> +/* Numbers are taken from RFC3390. */
>> __u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst)
>> {
>> __u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);
>>
>> if (!cwnd) {
>> - if (tp->mss_cache > 1460)
>> + if (tp->mss_cache >= 2190)
>> cwnd = 2;
>> else
>> cwnd = (tp->mss_cache > 1095) ? 3 : 4;
>
> I remember suggesting something similar about 5 or 6 years
> ago and Alexey Kuznetsov at the time explained the numbers
> which are there and why they should not be changed.
>
> I forget the reasons though, and I'll try to do the research.
>
> These numbers have been like this forever, FWIW.
> -
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