Hi,
using kernel 2.4.17 I run into the following problem.
Using an ethernet device with ipv6 I cannot increase the mtu
size.
Lowering the mtu size is no problem.
BTW: After increasing the mtu size, ifconfig will display the
correct new size. But using tcpdump or a network sniffer, you can
see that the really used mtu size is the old one.
This IMHO buggy behavior is caused by the following code fragment
in file net/ipv6/route.c:
static int rt6_mtu_change_route(struct rt6_info *rt, void *p_arg)
{
struct rt6_mtu_change_arg *arg = (struct rt6_mtu_change_arg *)
p_arg;
/* In IPv6 pmtu discovery is not optional,
so that RTAX_MTU lock cannot disable it.
We still use this lock to block changes
caused by addrconf/ndisc.
*/
if (rt->rt6i_dev == arg->dev &&
rt->u.dst.pmtu > arg->mtu &&
!(rt->u.dst.mxlock&(1<<RTAX_MTU)))
rt->u.dst.pmtu = arg->mtu;
[snip]
}
This function is called, if mtu size is changed using ifconfig.
(e.g. "ifconfig ethx mtu 5000" and mtu size was previously different
from 5000)
Because the if condition (rt->u.dst.pmtu > arg->mtu) must hold when
changing the mtu size, I am not able to increase the mtu size for a
network path for which the routing entry is already written.
To increase the mtu size for a device in ipv6 I have the following
choices:
(1) o change mtu size using ifconfig
o delete all (relevant) entries in ipv6 routing table for the
device
o recreate all (relevant) entries in ipv6 routing table for the
device
(now all those routes will use the new mtu size)
(2) o change mtu size using ifconfig
o ifconfig <dev> down
o ifconfig <dev> up
o add all relevant routing entries for that device
(now all those routes will use the new mtu size)
(3) o set mtu size to desired value before any entry in the routing
table is created
I don't have a patch to eliminate this messy behavior. (Because I am not
much familiar with this kernel network code and don't know much about
possible side effects when changing some if conditions here and
there ... ;-(
But maybe someone has an idea (solutions?) regarding this problem?
Thanks,
Andreas
--
Linux for eServer Development
Tel : +49-7031-16-4640
Notes mail : Andreas Herrmann/GERMANY/IBM@IBMDE
email : aherrman@de.ibm.com
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