Counts ICMP Parameter Problem messages received, which routers send back when they can't process a packet's IP header field. On a gateway, a rise here often points at malformed or nonstandard packets from a specific peer, worth checking whether it's a buggy implementation, such as some third-party VPN stacks, versus deliberately malformed packets used to probe how the firewall handles bad headers. Example: icmpInParmProbs increasing only from one particular VPN peer's address range suggests that peer's device is sending headers this gateway's IP stack objects to, worth raising with that peer's admin.