[Openswan Users] NAT-T really needed?

[Alessandro Macuz]

I have been trying to start a VPN between a PIX 525 and a Linux box.
PIX is behind a router doing NAT.
I enabled NAT-T on both sides but Linux box tells me

Apr 12 23:05:10 testvpn kernel: klips_debug:ipsec_rcv: suspected 
ESPinUDP packet (NAT-Traversal) [1].

Why does it tell "suspected ESPinUDP packet (NAT-Traversal)". Do I 
really need NAT-T?
What does it mean?
Obviously the tunnel doesn't work.

Alex.