https://www.ibm.com/docs/pl/i/7.1?topic=concepts-what-is-ppp
https://www.ibm.com/docs/pl/i/7.1?topic=alternatives-l2tp-tunneling-support-ppp-connections
https://www.ibm.com/docs/pl/i/7.1?topic=concepts-layer-2-tunnel-protocol
https://eia.pg.edu.pl/documents/1113028/0/PSI%20-%20VLAN%20i%20tunelowanie%202015.pdf
https://home.agh.edu.pl/~pmarynow/pliki/sieci2019/Tunelowanie.pdf
https://www.expressvpn.com/pl/what-is-vpn/vpn-tunnel