Re: IPv6 day and tunnels

[Owen DeLong <owen () delong com>]

On Jun 5, 2012, at 10:15 AM, Jimmy Hess wrote:

> On 6/5/12, Owen DeLong <owen () delong com> wrote:
> [snip]
> > But that's a whole lot more packets than working PMTU-D to get there and
> > you're also waiting for all those round trips, not just the 4 timeouts.
> > The round trips add up if you're dealing with a 100ms+ RTT. 22 RTTs at
> > 100ms is 2.2 seconds. That's a long time to go without first data packet
>
> I'm only suggesting probing to discover the MTU between  neighboring
> endpoints directly connected to the same subnet -- Layer 2
> interconnect.   PTMU doesn't work, because devices know the MTU of
> _their link_,  but not necessarily the MTU of every intervening bridge
> or L2 tunnel,   Not for IP end-end-to-end MTU discovery.

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
your system(s).

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

> The "too big packet"  forwarded is just discarded by the L2 bridge,
> there's no ICMP packet that can result from that,  the L2 bridge might
> not even have an IP address to source one from,  so the PMTU  method
> which relies on ICMP alone cannot possibly work.

Sure... PMTU-D isn't designed to compensate for misconfigured links,
it's designed to detect the path MTU based on the smallest correctly
configured L3 MTU in the path.

> The router after discovering the local MTU constraints to its
> neighbors would then be responsible for sending TooBig messages as
> needed or passing on the MTU constraint.

So you want to add an MTU setting for each L2 destination to the
L2 adjacency table? This seems like a really bad idea. The better
solution would be to correctly configure your link MTUs in the first
place.

> 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.

Owen