You are designing a VPN solution to connect multiple branch offices to the headquarters' network. The solution needs to support the transmission of multiple types of protocols and must be compatible with existing network infrastructure, which includes intermediate routers that only inspect outer packet headers for forwarding decisions.

Which tunneling protocol is MOST suitable for your VPN solution?

Datagram Transport Layer Security $($DTLS$)$

Generic Routing Encapsulation $($GRE$)$

Point$-$to$-$Point Protocol $($PPP$)$

Internet Protocol Security $($IPSec$)$