ICMP Attacks against TCP

"ICMP Attacks against TCP", Fernando Gont (2005)

Reset the connection with ICMP "Hard Errors"
Stall  /  slow down the connection with PMTU games

MD5 doesn't help... but randomising the source port does.

There are other countermeasures, like checking the sequence number in the ICMP payload... BSD's have all implemented this for some time.