How to read the layer boundaries
Regions group protocols by their role and carrier position in the five-layer teaching model. TLS/DTLS are placed at the lower edge of Application. QUIC is Transport, despite running over UDP. BGP uses TCP and appears in Application, while its purpose is network routing. NDP appears in Network because it uses ICMPv6. Tunnels show their outer carrier only; recursive payloads and uncommon bindings are omitted.
The four-layer Internet model combines Link and Physical. See RFC 1122, QUIC (RFC 9000), and QUIC + TLS (RFC 9001). Individual nodes link to their specifications.