GRE tunnel
Generic Routing Encapsulation — Cisco-developed point-to-point tunneling. RFC 2784. Encapsulates arbitrary network traffic (IPv4, IPv6, multicast, IPX) in IP packets. No built-in encryption — often combined with IPsec.
Use cases: routing over IPsec (IPsec doesn't support multicast itself, GRE-over-IPsec solves this), connecting two sites with private IP ranges over the internet, transport for OSPF/EIGRP through NAT. The header is minimal — 24 bytes of overhead. MTU issue: 1476 bytes effective inside a 1500-MTU link, may require PMTUD or mss-clamping. Modern alternatives: WireGuard (encrypted by default), VXLAN (datacenter overlay), STT (stateless transport tunneling).