On 13 jul 2009, at 15:02, Mikael Abrahamsson wrote:
The idea with setting the tunnel MTU is to set a size that is highly unlikely to fragment
That's not good enough. You have to get it right.
as sustained fragmentation is dangerous in any case.
It makes no sense to mess up IPv6 because of an IPv4 limitation. Let them eat fragments as an incentive to upgrade.
I have actually set the MTU of this tunnel to 1350 which means this is the lowest common denominator for PMTUD (is my guess anyway)
Why do you guess this?There are people who set their MTU to 576 in IPv4 (not too many anymore, I hope) while the IPv6 minimum MTU is 1280. You can't win this by guessing.
Also, TCP MSS clamping is not the right solution to limit IPv6 packet sizes. We have the RA MTU option for that, you can use this to advertise the MTU for a subnet, which applies to TCP as well as other protocols.