>I believe the referential security group rules solve this problem (unless I'm not understanding):
I think the disconnect is that you are comparing the way to current mapping driver implements things for the reference implementation with the existing APIs. Under this light, it's not going to look like there is a point to this code being in Neutron since, as you said, the abstraction could happen at a client. However, this changes once new mapping drivers can be added that implement things differently. Let's take the security groups example. Using the security groups API directly is imperative ("put a firewall rule on this port that blocks this IP") compared to a higher level declarative abstraction ("make sure these two endpoints cannot communicate"). With the former, the ports must support security groups and there is nowhere except for the firewall rules on that port to implement it without violating the user's expectation. With the latter, a mapping driver could determine that communication between these two hosts can be prevented by using an ACL on a router or a switch, which doesn't violate the user's intent and buys a performance improvement and works with ports that don't support security groups. Group based policy is trying to move the requests into the declarative abstraction so optimizations like the one above can be made.
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