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SubjectRe: IPSEC transport mode w/2.2.x kernels and large packets

> Set the MTU to 64K
> Set the MSS on the routers to the estimated path mtu

Yup. And to add code to calculate MSS from estimated path mtu,
taking into account IPsec overhead.

> Now TCP generates frames the right size for optimal performance, everything
> else hands down full datagrams and the world is a happier place.

Alan, the world is not such happy place 8). IPv6 has no DF flag,
so that path mtu discovery is not an option.

> You can't run path MTU discovery with IPsec. The DF could be faked and aimed
> at dropping your link to unusably low speeds. Ignoring the DF could equally
> be a complete link failure. So you don't run mtu discovery.

Path MTU discovery is run to IMPROVE things. If we do not use
path MTU discovery we must use mtu of 576. The period.

The only security problem of pmtu discovery is when someone
blocks icmp. Well, it is curable. And if without IPsec we may
blame on stupid network admins, blocking them, with IPsec we will
have to work it around.


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