OVN_Kubernetes IPv6 support

Signed-off-by: Aswin Suryanarayanan <asuryana@redhat.com>

submariner/IPV6-OVN.md

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+
+# **Submariner Enhancement for IPv6 Support**
+
+## **Summary**
+This proposal outlines the changes required in Submariner for  OVN Kubernetes CNI to enable IPv6 support, ensuring seamless connectivity between clusters using Submariner. 
+The main proposal has the full design https://github.com/submariner-io/enhancements/blob/devel/submariner/IPV6-datapath.md. This covers only the OVN CNI part of it.
+
+---
+
+## Design Details
+
+The OVNKubernetes handler programs network policies and routes to direct traffic from the gateway and non-gateway nodes to direct the traffic to the remote cluster.
+At present the routes are only programmed for IPv4 for addresses. We need to enhance this to support IPV6 addresses as well.
+
+The handler for creating the Gateway and NonGateway routes needs to be enhanced.
+
+### GatewayRoute CRD:
+
+The GatewayRoute will now populate Ipv4 and Ipv6 addresses for the next hops and remote CIDRs for a dual-stack environment. For Ipv6 only it uses just the IPv6 address
+for these fields. Ipv4 will continue as it is.
+
+The next hop will be the interface IP of ovn-k8s-mp0 interface, which is expected to have two IPs in the case of dual-stack environments.
+
+
+```yaml
+apiVersion: submariner.io/v1alpha1
+kind: GatewayRoute
+metadata:
+  name: remote-cluster-route
+spec:
+  nextHops:
+    - "fd00:abcd::1"
+    - "192.168.1.1"
+  remoteCIDRs:
+    - "fd00:4321::/64"
+    - "10.0.0.0/8"
+```
+
+### NonGatewayRoute CRD:
+
+The NonGatewayRoute will follow the same pattern as GatewayRoute for populating next hops and remotecidrs.
+
+The nexthops will be the transit switch IP of the gateway node.
+
+#### **NonGatewayRoute CRD Example**
+
+```yaml
+apiVersion: submariner.io/v1alpha1
+kind: NonGatewayRoute
+metadata:
+  name: non-gw-route
+spec:
+  nextHops:
+    - "fd00:cafe::1"
+    - "172.16.0.1"
+  remoteCIDRs:
+    - "fd00:5678::/64"
+    - "192.168.0.0/16"
+```
+
+
+### GatewayRoute Handler
+
+The gateway route handler should iterate through the ips configured and must identify the ipv4 remote CIDR and nextHop and Ipv6 remote CIDR pairs from the gateway route.
+It will program one or two logical route policies and logical routes based on the network configuration. For dual-stack, it will be two LRPs. Similarly a logical route will
+be added to route the traffic from the non-gateway nodes and will be resubmitted to hit the above added Logical route policy.
+
+```plaintext
+match: "ip6.dst==fd00:5678::/64"
+action: reroute
+nexthops: ["fd00:abcd::1"]
+priority: 20000
+```
+
+```plaintext
+destination: "fd00:1234::/64"
+nexthop: "fd00:cafe::1"
+priority: 200
+```
+
+### NonGatewayRoute Handler
+
+The NonGatewayRoute  handler should iterate through the ips configured and must identify the ipv4 remote CIDR and nextHop and Ipv6 remote CIDR pairs from the gateway route.
+It will program one or two logical route policies and logical routes based on the network configuration. For dual stack it will be two LRPs.
+
+```plaintext
+match: "ip6.dst==fd00:5678::/64"
+action: reroute
+nexthops: ["fd00:abcd::1"]
+priority: 20000
+```
+
+---