Correct Operation of BGP (nice, but ...)
Neighbor that sent the UPDATE, correctly applied BGP rules, local policies, etc.
BGP speaker that received the UPDATE, correctly applied BGP rules, local policies, etc.
Subscriber traffic forwarded by a BGP speaker is valid (not spoofed, duplicated, etc.)
We can’t enforce these aspects of correct operation because BGP affords speakers considerable latitude with regard to local policy, ASes do not tend to make public their local policies, and because validation and tracking of subscriber traffic is impractical