netdev - Re: [PATCH] sctp: make the max

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Date:	Mon, 2 Dec 2013 14:08:21 +0100
From:	Michael Tuexen <Michael.Tuexen@...chi.franken.de>
To:	Neil Horman <nhorman@...driver.com>
Cc:	Wang Weidong <wangweidong1@...wei.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	David Miller <davem@...emloft.net>, linux-sctp@...r.kernel.org,
	netdev@...r.kernel.org, dingtianhong@...wei.com
Subject: Re: [PATCH] sctp: make the max_burst min value to 1

On Dec 2, 2013, at 12:56 PM, Neil Horman <nhorman@...driver.com> wrote:

> On Mon, Dec 02, 2013 at 02:51:45PM +0800, Wang Weidong wrote:
>> From: Wang Weidong <wangweidong1@...wei.com>
>> 
>> when I setted the max_burst to 0, do the lksctp-tools I got hang.
>> I found sctp_transport_burst_limited would make the cwnd to 0.
>> so I make the max_burst min value to 1.
>> Signed-off-by: Wang Weidong <wangweidong1@...wei.com>
>> ---
>> net/sctp/sysctl.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
>> index 7637e8e..46832d3 100644
>> --- a/net/sctp/sysctl.c
>> +++ b/net/sctp/sysctl.c
>> @@ -135,7 +135,7 @@ static struct ctl_table sctp_net_table[] = {
>> 		.maxlen		= sizeof(int),
>> 		.mode		= 0644,
>> 		.proc_handler	= proc_dointvec_minmax,
>> -		.extra1		= &zero,
>> +		.extra1		= &one,
>> 		.extra2		= &int_max
>> 	},
>> 	{
>> -- 
>> 1.7.12
>> 
>> 
> 
> 
> This seems like a band-aid to me.  There are a few things wrong:
> 
> 1) You can also set the the max_burst via setsockopt, and so this would need to
> be checked in that path as well.
> 
> 2) I don't see how having a cwnd of zero would cause a hang.  It looks like a
> cwnd of zero would perpetually place the association in a slow start state,
> which is silly but not illegal.  
> 
> Please investigate the acutally root cause of the problem before just avoiding
> it like this.
I second this.

Please note the using max_burst of  in the socket option means that you
deactivate max_burst, not that you actually use the value 0. See
http://tools.ietf.org/html/rfc6458#section-8.1.24

I FreeBSD we use the sysctl variable as the default for sockets. Therefore
max_burst is a valid value, it just disables max_burst.

Best regards
Michael
> 
> Thanks!
> Neil
> 
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