netdev - Re: [net-next][PATCH] tcp: probe timer MUST not less than 5 minuter for tcp PMTU

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Message-ID: <dc9f1ba2-b0f4-e187-5270-f2b37d6a2ac3@gmail.com>
Date:   Sat, 2 Jun 2018 06:19:22 -0400
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Li RongQing <lirongqing@...du.com>, netdev@...r.kernel.org
Subject: Re: [net-next][PATCH] tcp: probe timer MUST not less than 5 minuter
 for tcp PMTU



On 06/01/2018 07:03 AM, Li RongQing wrote:
> RFC4821 say: The value for this timer MUST NOT be less than
> 5 minutes and is recommended to be 10 minutes, per RFC 1981.
> 
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>  net/ipv4/sysctl_net_ipv4.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index d2eed3ddcb0a..ed8952bb6874 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -47,6 +47,7 @@ static int tcp_syn_retries_max = MAX_TCP_SYNCNT;
>  static int ip_ping_group_range_min[] = { 0, 0 };
>  static int ip_ping_group_range_max[] = { GID_T_MAX, GID_T_MAX };
>  static int comp_sack_nr_max = 255;
> +static int tcp_probe_interval_min = 300;
>  
>  /* obsolete */
>  static int sysctl_tcp_low_latency __read_mostly;
> @@ -711,7 +712,8 @@ static struct ctl_table ipv4_net_table[] = {
>  		.data		= &init_net.ipv4.sysctl_tcp_probe_interval,
>  		.maxlen		= sizeof(int),
>  		.mode		= 0644,
> -		.proc_handler	= proc_dointvec,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &tcp_probe_interval_min,
>  	},
>  	{
>  		.procname	= "igmp_link_local_mcast_reports",
> 

Note that this change would stop people from being able to have packetdrill
tests which would run in a reasonable amount of time.

I do not believe linux kernel must enforce such a limit.

It is up to the admin to set a value here really, depending on the environment
the host is running in.