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

Alan Cox (alan@lxorguk.ukuu.org.uk)
Sat, 7 Aug 1999 10:20:24 +0100 (BST)

> > Alan Cox suggested setting the MTU to the maximum allowed:0xfff0, then
> > fragmenting after encryption to the MTU of the physical device.
>
> So when one fragment gets dropped the whole packet is lost ? This is the
> "NFS trap" and works badly over WANs, because you would disturb path
> MTU discovery. It is important that packets with DF get a icmp error
> when they exceed the effective size of the tunnel.

What I suggested is a bit cleverer than that

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

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

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.

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