there is not much incentive for a network to
deploy ingress/egress filtering; when a network deploys ingress/egress filtering,
it has only a minimal effect on how many other networks are able to spoof its
source addresses. But incentive is not the only problem; without nearly 100%
deployment, ingress/egress filtering is ineffective [Park and Lee 2001]. Even
if only a small number of networks do not implement ingress/egress filtering,
hosts within those unprotected networks would still be able to spoof nearly any
source address.