Ping of death is a ping that claims to have a 65536 byte payload: http://en.wikipedia.org/wiki/Ping_of_death

Flashback quite a few years. I was working in IT and my coworker asked me if my WinXP (IIRC) machine was up to date. I said "yes". Next thing I know, it crashed hard. Oops, my buddy just hit me with a ping of death.

Software firewalls can't prevent that problem.

Well, yes they could - if the machine targeted is behind a FW that is blocking that type of traffic.

If the FW itself has that specific NIC, it could be brought down with this attack - but you could prevent SIP traffic from hitting machines which are vulnerable.