OSI model.
also: OSI layers · OSIA seven-layer vocabulary for networking, from physical links up to applications; system design mostly talks about layers 3, 4 and 7.
the two-minute refresher
- Layer 7 application (HTTP, gRPC, DNS); layer 4 transport (TCP, UDP); layer 3 network (IP).
- TLS sits around layer 6, presentation: encryption and encoding.
- The practical TCP/IP model folds these into fewer layers; what matters is where a tool works.
- 'An L4 load balancer' or 'an L7 firewall' tells you exactly what it can see.