OVS driver and agent for symmetric Port Chains

This specification describes an OVS driver and agent enhancements to support symmetric port chains in which SFs in the reverse path (from destination to source) are traversed in reverse order to the SFs in the forward path (from source to destination). Currently the networking-sfc OVS agent does not support symmetric chain paths.

The steering of chain traffic in the data-plane ensures symmetry:
• The source Logical Port in the flow-classifier is used to install Open vSwitch (OVS) rules to match traffic for the forward path. The destination Logical Port in the flow-classifier is used to install OVS rules to match traffic for the reverse path.
• Rules must be installed so that the SFs in the reverse path are traversed in reverse order from the forward path. Forward traffic is steered from the egress port of SFn to the ingress port of the SFn+1. In addition, reverse traffic must be steered from the ingress port of a SFn+1 to the egress port of SFn.
• Each SF Group must have a Load Balancer pair: one for the forward direction and the other for the reverse direction. In addition, to ensure that traffic in the forward and reverse directions is delivered to the same SF in a SF Group, these LB pairs must use symmetric hash functions.