Subject: Re: ARP problems.
To: None <>
From: Charles M. Hannum <mycroft@MIT.EDU>
List: tech-net
Date: 03/19/2004 13:17:35
On Fri, Mar 19, 2004 at 07:02:06PM +0100, wrote:
> The ARP implementation in NetBSD will delete an existing ARP entry
> after arpt_keep seconds (20 minutes), regardless whether there are 
> traffic on the interface or not, and do a new ARP request if a new 
> packet is sent out for that route.  While this is OK for slow links,
> it is a disaster for high-speed long-distance communications.
> As an example, when the ARP entry on the path of a TCP flow running
> at 4Gbit over a distance of > 10000km times out, it may take 5-10ms
> for the entry to be renewed, and during that time ~1500 packets may
> be lost, which will effectively stop the transfer.
> A solution to this is to send out a new ARP request some time before
> the entry is removed (if the routing entry has been active since the
> last ARP update), and let the reply refresh the ARP entry, hence 
> avoid the arp entry update delay time.
> I plan to implement this and send out patches unless someone else
> have another better way of solving the ARP problem.

Given that we already waste time tracking "use" values, checking whether
the "use" value has changed since the last check, before deleting the
ARP entry would be sufficient.  However, this would bloat the routing

I would STRONGLY oppose any solution that puts additional overhead in
any of the critical paths.