StackPi [Yaar et al. 2006], essentially an improved version of Pi that functions
better than Pi with incremental deployment, also added mechanisms to
protect against spoofing packets. To protect against IP spoofing, an end-host
can remember markings it sees in legitimate packets from different source addresses.
Then, when under attack, the end-host can filter out any packet which
has a marking that does not match the stored marking for the source of that
packet. Note, there is no technical reason end-hosts in the Pi system cannot
perform a similar comparison of markings; the authors simply did not evaluate
such usage in the original Pi paper.