-8) i?,i?,NAK (encoded as zero)
-
-If the link-layer can't work out what to do with a packet (session has
-gone away, etc.) it can transmit a NAK back to the sender. The sender
-can then try to verify whether the session is alive by sending ping
-packets, and forget the key if it isn't. Potential denial-of-service
-if the attacker can stop the ping/pong packets getting through (the
-key will be forgotten and another key setup must take place), but if
-they can delete packets then we've lost anyway...