Traffic That Must Not Be Dropped
Error messages that are essential to the establishment and maintenance of communications:
o Destination Unreachable (Type 1) - All codes o Packet Too Big (Type 2) o Time Exceeded (Type 3) - Code 0 only o Parameter Problem (Type 4) - Codes 1 and 2 only
Connectivity checking messages:
o Echo Request (Type 128) o Echo Response (Type 129)
Path MTU
Debugging MTU issues is notoriously hard. Path MTU is not guaranteed to work, and varies by source and destination network.