ICMP Normalizations

#	ICMP Type	Normalization Performed
1	Echo request	Drop if destination is a multicast or broadcast address.
2	Echo request	Optionally drop if ping checksum incorrect.
3	Echo request	Zero ``code'' field.
4	Echo reply	Optionally drop if ping checksum incorrect.
5	Echo reply	Drop if no matching request.$\dagger$
6	Echo reply	Zero ``code'' field.
7	Source quench	Optionally drop to prevent DoS.$\dagger$
8	Destination Unreachable	Unscramble embedded scrambled IP identifier.$\dagger$
9	other	Drop.$\dagger$

The following ``transport'' protocols are recognized, but currently passed through unnormalized: IGMP, IP-in-IP, RSVP, IGRP, PGM.