Packet

The packets that go on the road first are not HTTP, TLS, or TCP abstractions. They are individual packets that have to cross subnets, routers, carrier paths, and middleboxes before they can reach the destination host. The first thing that decides whether they can keep moving is not the transport layer or the application layer. It is IP.

IP is often described as a list of address and header fields, but the real high-frequency questions in engineering are different: why a packet was lost, rerouted, fragmented, dropped at one hop, or still could not reach the service even though the service itself was healthy. To answer those questions, the boundaries of IP’s responsibility have to be kept sharp.