Re: TCP's initial cwnd setting correct?...

[John Heffner <jheffner@psc.edu>]

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@helsinki.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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