On delivery you'd need to decide what security scheme is actually
available on the packet and in what order to interpret any inbound
security data. If you had an MLS scheme that uses CIPSO, an integrity
mechanism using IPSEC and a DAC scheme that assigns user ids by
host address getting the ordering right and every domain registered
properly in the networking stack would be a trick. Plus, making sure
that any state the security scheme requires is tricky. Maybe it's not
actually worse if the schemes agree on what qualifies as a security
element, but if one scheme does access control outbound while another
does inbound it will get hairy.
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