Mask made of ICMP types for which rates are being limited.
Significant bits: IHGFEDCBA9876543210
Default mask: 0000001100000011000 (6168)

Bit definitions (see include/linux/icmp.h):

• 0 Echo Reply
• 3 Destination Unreachable *
• 4 Source Quench *
• 5 Redirect
• 8 Echo Request
• B Time Exceeded *
• C Parameter Problem *
• D Timestamp Request
• E Timestamp Reply
• F Info Request
• G Info Reply
• H Address Mask Request
• I Address Mask Reply

* - rate limited by default (see default mask above)