On Fri, Sep 27, 2002 at 12:34:43PM +0800, Yuan Huailin-w19410 wrote:
> So, My question is how does the network device driver tell the IP layer its MTU in linux 2.4.17?
the ip layer gets an pointer to the device structure as a result of a
routing cache or database lookup. One field of this device structure is the
mtu of the target device.
ip_forward is called with an skb which contains the ip packet and the
decided destination. The route to the destionation is retrieved:
rt = (struct rtable*)skb->dst;
... and the mtu of this route entry is retrieved
mtu = rt->u.dst.pmtu;
The initial path mtu of the route is read somewhere from the device's mtu.
The mtu of a device is set by
case SIOCGIFMTU: /* Get the MTU of a device */
ifr->ifr_mtu = dev->mtu;
in dev.c, where dev is a struct of the type net_device. I think this is dine
in route.c in the function rt_set_nexthop:
if (fi->fib_mtu == 0) {
rt->u.dst.pmtu = rt->u.dst.dev->mtu;
if (rt->u.dst.mxlock & (1 << RTAX_MTU) &&
rt->rt_gateway != rt->rt_dst &&
rt->u.dst.pmtu > 576)
rt->u.dst.pmtu = 576;
}
Greetings
Bernd
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