How to find the parent network device for a PPPoE device

For a POINTOPOINT [PPPoE](https://en.m.wikipedia.org/wiki/Point-to-Point_Protocol_over_Ethernet) ppp0 device in Linux, how to get the underlying ethernet device in a reliable way? I've looked in the output of ip -d link and in the /sys/class/net/ppp0 directory, but neither mention the parent device, nor the PID of the pppd process. If only one pppd runs on the system, then there is a "workaround":

`
$ cat /proc/net/pppoe
Id       Address           Device
0000516D dc:99:16:c4:58:35 wan.25

` In this case, wan.25 is the wanted info. But that "Id" 0000516D I have not seen other places linking it to the PPP device. So how would I differentiate between several PPPoE interfaces?