According to RFC 1700, some protocol types in IP header are
reserved and unassigned at this time. Precisely because these
protocols are undefined, there is no way to know in advance
if a particular unknown protocol is benign or malicious. Unless
your network makes use of a non-standard protocol with reserved
or unassigned protocol number, a cautious stance is to block
such unknown elements from entering your protected network.
When the Unknown Protocol Protection SCREEN option is enabled,
the security device drops packets when the protocol field
contains a protocol ID number of 137 or greater by default.
This attribute records the detection of Unknown protocol
IP packets.