deployment
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... be spoofed.
The reason threat (1) exists is the lack of universal deployment of
IPv4 ingress filtering ...
... will not be able to do a similar verification for the packets it
receives. This may be quite reasonable for most of the deployment
cases, for example, an Internet Service Provider (ISP ...
... able to support the [RFC4301] features described in (1) and (2). If
appropriate, the deployment may choose to use either version of the
security architecture ...