netdev - Re: [PATCH 1/4] net: arp: introduce arp_evict

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Message-ID: <81cd604b9d44d2058464d90788f43d6803f7eea3.camel@gmail.com>
Date:   Thu, 14 Oct 2021 11:52:00 -0700
From:   James Prestwood <prestwoj@...il.com>
To:     David Ahern <dsahern@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/4] net: arp: introduce arp_evict_nocarrier sysctl
 parameter

Hi David,

On Thu, 2021-10-14 at 12:34 -0600, David Ahern wrote:
> On 10/13/21 4:27 PM, James Prestwood wrote:
> > This change introduces a new sysctl parameter, arp_evict_nocarrier.
> > When set (default) the ARP cache will be cleared on a NOCARRIER
> > event.
> > This new option has been defaulted to '1' which maintains existing
> > behavior.
> > 
> > Clearing the ARP cache on NOCARRIER is relatively new, introduced
> > by:
> > 
> > commit 859bd2ef1fc1110a8031b967ee656c53a6260a76
> > Author: David Ahern <dsahern@...il.com>
> > Date:   Thu Oct 11 20:33:49 2018 -0700
> > 
> >     net: Evict neighbor entries on carrier down
> > 
> > The reason for this changes is to prevent the ARP cache from being
> > cleared when a wireless device roams. Specifically for wireless
> > roams
> > the ARP cache should not be cleared because the underlying network
> > has not
> > changed. Clearing the ARP cache in this case can introduce
> > significant
> > delays sending out packets after a roam.
> 
> how do you know if the existing ARP / ND entries are valid when
> roaming?
> 

I guess there is no way of really knowing, but since your roaming in
the same network I think we can assume the entries are just as valid
prior to the roam as after it right? If there are invalid entries, they
were likely invalid prior to the roam too.

I obviously cant speak to all network configurations, but I think if
your network infrastructure is set up to roam you into a different
subnet you're doing something wrong. Or I can't think of a reason to do
this, it would defeat the purpose of quickly roaming between BSS's
since you would then have to do DHCP aftewards.

Thanks,
James