IPv6 day and tunnels

Wed Jun 6 01:02:58 UTC 2012

On 6/5/12, Owen DeLong <owen at delong.com> wrote:
> This is a horrible misconfiguration of the devices on that link.
> If your MTU setting on your interface is larger than the smallest MTU
> of any L2 forwarder on the link, then, you have badly misconfigured

Not really;   The network layer and L2 protocols should both be
designed to handle this, it is a design error in the protocol that it
doesn't.    You say it's "misconfiguration",  but if IP handled the
situation reasonably, it shouldn't be necessary to configure anything
in the first place.   Whether the neighbors are LAN or  cross-tunnel,
the issues are similar.

It's only a misconfiguration because of flaws in the protocol.

Just like you expect to plug devices in a typical LAN and it's not a
configuration error to fail to manually find every switch in the LAN
and enter MAC addresses into a forwarding table by hand;  likewise,
you shouldn't expect to key a MTU into every device by hand.

IP should be designed so that devices on the link that _can_  handle
the large transmission unit,  which provides efficiency gains, should
be allowed to fully utilize those capabilities,  without breakage of
connectivity to devices on the same link that  have more limited
capabilities and can only receive the Minimum required frame size
(smaller MTU),   and without separating the subnet or installing
dividing  Proxy ARP servers  to send ICMP TooBig packets.

> Adding probing to compensate for this misconfiguration merely
> serves to perpetuate such errant configurations.

Just like adding MAC address learning to Ethernet switches to
compensate for the misconfiguration of failing to manually enter
hardware addreses into your switches, serves to perpetuate such errant
configurations,   where the state of the forwarding tables
are unreliably left in a non-deterministic state.

>> You've got an issue if there are 100ms between two peers on your LAN.
>> You're right, you don't need to probe for possible MTUs below 1280.
> LAN, sure. However, consider that there are intercontinental L2 links.

Intercontinental multi-access L2  links, perhaps, are a horrible
misconfiguration.

> Owen
--
-JH