kern/181388: [route] Routes not updated on mtu change

[John-Mark Gurney]

Joe Holden wrote this message on Mon, Aug 19, 2013 at 09:32 +0100:
> Hm, I hadn't considered that... how do other OSes and vendors handle 
> this?  (eg Linux?)
> 
> Just changing the connected route would probably suffice, or maybe if 
> any routes not added with default interface mtu could/are be flagged, 
> then those could be changed or not depending on what added them?

How do you know which routes are which?  I believe that FreeBSD will
automaticly reduce the MTU if you decrease it, but it won't increase
it..  How do you know the difference between someone increasing the MTU
on the interface to allow a specific host to talk at the larger MTU and
wanting the rest of the hosts to talk at the larger MTU...

At a previous work place, we used this feature so that we could use
MTU 9k to other FreeBSD boxes to get better NFS performance, and but
keep the other windows boxes which didn't have MTU 9k compatible
interfaces talking on the same LAN...

> Perhaps need someone with more experience of the network stack to wade 
> in here...
> 
> Cheers,
> Joe
> 
> On 19/08/2013 08:00, Julian Elischer wrote:
> >The following reply was made to PR kern/181388; it has been noted by GNATS.
> >
> >From: Julian Elischer <julian at elischer.org>
> >To: bug-followup at FreeBSD.org, joe at rewt.org.uk
> >Cc:
> >Subject: Re: kern/181388: [route] Routes not updated on mtu change
> >Date: Mon, 19 Aug 2013 14:57:22 +0800
> >
> >  The problem is that this is not as simple as it seems.
> >  The route MTU MIGHT have been set by something other than the
> >  interface MTU
> >  in the first place.
> >  The interface MTU is a default for the route MTU but is not the only
> >  source.
> >  This actuall bit me a couple of days ago when I was wonderign why my
> >  interface was not sending 9K packets..  turns out you need to do
> >  'ifconfig_xn0="DHCP mtu 9000"' in order to have your dncp
> >  configured interface routes  have the right size.
> >
> >  so, I'm agreeing with you , but noticing that there are complications.

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."