policy-based routing in openstack virtual router

General routing is only performed based on destination IP of a packet, while policy-based routing can be performed based on not only destination IP but also source IP, transport layer protocol, transport layer ports including source port and destination port, and input port of router, in which case it provisions great capability and much more flexibilty to control a network flow, e.g. we can direct a specific flow to a defined nexthop before reaching the destination.