"unknown", "notpresent", "down", "lowerlayerdown", "testing",
"dormant", "up".
+What: /sys/class/net/<iface>/phys_port_id
+Date: July 2013
+KernelVersion: 3.12
+Contact: netdev@vger.kernel.org
+Description:
+ Indicates the interface unique physical port identifier within
+ the NIC, as a string.
+
What: /sys/class/net/<iface>/speed
Date: October 2009
KernelVersion: 2.6.33