IPv6

Until yesterday I was running a cable modem with DHCP on the wan side and a HE tunnel for IPv6 and everything was working fine.  Now I'm running DSL with PPPoE and if I don't manually set my MTU lower I can't connect to some IPv6 sites anymore.  That's under Windows, under Linux it's much worse with most sites taking forever to load.  An MTU of 1452 seems to work good and I've put that into my m0n0wall in the LAN configuration page but it seems like my OSs are ignoring it.  Why am I having these issues now when I never had to mess with MTU settings before?

The problem is that it is the device that initiates a connection determines the value of MTU to be used. That happens on your PCs, which have an ethernet default value of 1500 for MTU.

You need to lower MTU on all your PC ethernet adapters to be the same or lower that what you have set on m0n0wall.

The reason you never had problems before is that your DHCP cable modem was compatible with PCs set to MTU 1500.

From what checking I've done isn't Path MTU discovery supposed to take care of issues like this, especially with IPv6?

Also, I need to figure out why my computers are ignoring the MTU I set in the RA.

There are things that also break Path MTU discovery, see: http://www.netheaven.com/pmtu.html

If you want to solve this, lower MTU on your PC ethernet adapters.

I figured out why my computer was ignoring the RA MTU.  It's because m0n0wall wasn't sending it.  I had to check "Other stateful configuration" to get m0n0wall to send the MTU with the RA even though I'm not using DHCPv6 at all.

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There are things that also break Path MTU discovery, see: http://www.netheaven.com/pmtu.html

If you want to solve this, lower MTU on your PC ethernet adapters.

Do you think that mis-configured web-servers (e.g. Google) is a likely cause of Path MTU Discovery failure?

• my ethernet adapter defaults to 9k MTU (i.e. Jumbo Frames, although 1500 byte frames is also a problem)
• if i try to send a packet larger than the link size, m0n0wall will send me an IPv6 ICMP "Packet too big" notification
• my IPv6 firewall rules are set to allow everything from the outside in (e.g. ICMP packets)
• i cannot receive large downloads (i.e. a web-page) from nearly every site

i have no way to trace back too Youtube's servers, but does it sound reasonable that they are blocking ICMP messages that my router would be sending them?

Is it likely that getting the world to transition to use Path MTU Discovery will be more of a challenge than getting the world to transition to IPv6?

PMTUD isn't something that can be relied upon to function. MSS clamping is the best solution, as your LAN can be whatever you want it to be, and your firewall MSS clamps to the required value. With a 1500 MTU Internet connection and same LAN, there's no need. But when you're tunneling, whether PPPoE, or IPv6 tunneled, you need MSS clamping. m0n0wall handles that via the MTU value on WAN, but I'm not sure offhand whether that works for v6 too.