On Thu, Jul 12, 2012 at 10:55:16AM -0400, George Neville-Neil wrote:
>
> On Jul 11, 2012, at 17:57 , Navdeep Parhar wrote:
>
> > On 07/11/12 14:30, g...@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.
>
Something else to look into ...
# ifconfig lagg0 mtu 1492
ifconfig: ioctl (set mtu): Invalid argument
This is on stable/8 r238264 when the interface was up/up and down/down
Also attempted on the member interfaces dc0 and dc1
--
- (2^(N-1))
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