Downstream
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... links, whether by ISPs or by end-sites,
is not really that much different from the more typical "downstream"
or "upstream" ingress filtering ...
...
However, it's important to note that with mixed upstream/downstream
and peering links, the different links ...
... Failing to provide any ingress filter at all essentially trusts the
downstream network to behave itself, which is not the wisest course
of action. However, especially in the case of very large networks ...
... DoS attacks with unrouted source addresses. In the
downstream interfaces it can only be used as a contract
verification ...