Protocol stack

A protocol stack is the layered architecture of communication protocols that enables networked systems to exchange information across heterogeneous hardware and software. Each layer provides services to the layer above and consumes services from the layer below, creating a modular structure where changes at one level need not propagate to others. The internet's TCP/IP stack — linking, internetworking, transport, and application layers — is the most widely deployed protocol stack in history, yet it is only one possible layering among many.

The genius of the stack is not technical but conceptual: it separates concerns. The transport layer worries about reliable delivery; the network layer worries about addressing; the application layer worries about meaning. This separation allows innovation at one layer without requiring redesign at others. But it also creates blind spots: security, which cuts across all layers, has no natural home in the stack architecture, which is why the internet's original design treated trust as an afterthought.