Interface MTU question...

[George Neville-Neil]

On Jul 11, 2012, at 17:57 , Navdeep Parhar wrote:

> On 07/11/12 14:30, gnn at freebsd.org wrote:
>> Howdy,
>> 
>> Does anyone know the reason for this particular check in
>> ip_output.c?
>> 
>> 	if (rte != NULL && (rte->rt_flags & (RTF_UP|RTF_HOST))) {
>> 		/*
>> 		 * This case can happen if the user changed the MTU
>> 		 * of an interface after enabling IP on it.  Because
>> 		 * most netifs don't keep track of routes pointing to
>> 		 * them, there is no way for one to update all its
>> 		 * routes when the MTU is changed.
>> 		 */
>> 		if (rte->rt_rmx.rmx_mtu > ifp->if_mtu)
>>  			rte->rt_rmx.rmx_mtu = ifp->if_mtu;
>>  		mtu = rte->rt_rmx.rmx_mtu;
>>  	} else {
>> 		mtu = ifp->if_mtu;
>> 	}
>> 
>> To my mind the > ought to be != so that any change, up or down, of the
>> interface MTU is eventually reflected in the route.  Also, this code
>> does not check if it is both a HOST route and UP, but only if it is
>> one other the other, so don't be fooled by that, this check happens
>> for any route we have if it's up.
> 
> I believe rmx_mtu could be low due to some intermediate node between this host and the final destination.  An increase in the MTU of the local interface should not increase the path MTU if the limit was due to someone else along the route.

Yes, it turns out to be complex.  We have several places that store the MTU.  There is the interface,
which knows the MTU of the directly connected link, a route, and the host cache.  All three of these
are used to determine the maximum segment size (MSS) of a TCP packet.  The route and the interface
determine the maximum MTU that the MSS can have, but, if there is an entry in the host cache
then it is preferred over either of the first two.  See tcp_update_mss() in tcp_input.c to
see what I'm talking about.

I believe that the quoted code above has been wrong from the day it was written, in that what it
really says is "if the route is up" and not "if the route is up and is a host route" which is
what I believe people to read that as.  If the belief is that this code is really only there for
hosts routes, then the proper fix is to make the sense of the first if match that belief
and, again, to change the > to != so that when the administrator of the box bumps the MTU in
either direction that the route reflects this.  It is not possible for PMTU on a single link
to a host route to bump the number down if the interface says it's not to be bumped.  And,
even so, any host cache entry will override and avoid this code.

Best,
George