Remote port for L2 communicate outside the cloud

In some scenes, the instance in OpenStack need to communicate with some server outside the cloud(e.g. an application in traditionally network). We need a port named remote port，which represent the server outside the cloud. We coud made overlay L2 networks across OpenStack instance by this kind of ports.
In this kind of ports, some attributes are needed(e.g. tunnel ip, another device owner, mac address etc).
By using these ports, we can implicate the application migration from traditional network to neutron network.