Re: Link MTU option ignored in IPv6 router advertisements

[Glen Turner <gdt gdt id au>]

On 04/10/2014, at 2:31 AM, Dan Williams wrote:

> On Fri, 2014-10-03 at 11:36 -0400, Chuck Anderson wrote:
> > I'm not sure if this is a kernel thing or a NetworkManager thing.  Did
> > something change in how IPv6 router advertisements are handled by
> > NetworkManager in Fedora 20?
>
> I think it's a NetworkManager thing.  We'll fix it.

Hi Dan,

I've had a lot of trouble in IPv4 with DHCP in hotel systems setting the MTU to less than the 576 minimum 
(and sometimes less than the size of the IPv4 header). I believe that this was a misguided effort to 
implement fair queuing.

It might be worthwhile anticipating such issues for IPv6, roughly:

/* IPv6 minimum link MTU specified on page 24 of RFC 2460. */
#define IPV6_MINIMUM_MTU 1280

unsigned int enforce_link_mtu_bounds(unsigned int offered_link_mtu,
                                     char *interface_name)
{
  if (offered_link_mtu < IPV6_MINIMUM_MTU) {
    syslog("Offered link MTU for %s is %u, increased to " IPV6_MINIMUM_MTU "\n",
           (interface_name) ? interface_name : "an interface",
           offered_link_mtu);
    return IPV6_MINIMUM_MTU;
  }
  return offered_link_mtu;
}

-glen