NM handles master/slave relationships well already.

NM doesn't handle parent/child device relationships well.

By parent/child, we mean kernel interfaces that are logically a "child" of some other device, like a WWAN modem's ethernet port is logically a child of the WWAN device itself, since it is controlled by the WWAN device and cannot be used separately.  Same for PPP interfaces for WWAN devices, ATM devices, or PPPoE connections.

Similarly, VPN tunnel interfaces are really children of their parent NMVPNConnection, and if/when we create a fake NMDevice for each VPN connection, the tunnel interface should be attached to the parent VPN connection device.

NetworkManager currently allows these child devices to be manipulated separately from their parents, which is wrong and error-prone.  It's also confusing for clients because they don't know that these child devices cannot be manipulated from the API.

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NMDevice should grow a "parent" property that points to the object path of the parent NMDevice.  Any NMDevice that has the "parent" property set would no longer be activatable or disconnectable on it's own, because it is controlled by the parent and not separately useful.  Each child device should still provide it's own IP4Config/IP6Config though.

D-Bus clients like gnome-shell or nm-applet can use the "parent" property to hide child devices and ensure that only the parent device is shown to the user in a UI.

Next, the parent NMDevice's IP4/IP6 Config objects should be a *composite* of the child device's configs, to maintain backwards compatibility, and to provide a simpler API for clients that don't care about the specific parent/child relationships.