* It should not be client-specific.
  * In Istio sidecars, historically we had a lot of client-specific xDS. For example, putting the xDS-client's IP back into the xDS response. This makes efficient control plane implementation (most notably, caching), extremely challenging.
  * In practice, this largely means that references are fully qualified in the API. IP Addresses (generally) have a network associated with them, node names have a cluster associated with them, etc.