OSI and TCP/IP Models

Why layers?

Networking is split into layers, each solving one problem and relying on the layer below. HTTP doesn't care whether packets travel over Wi-Fi or fibre; TCP doesn't care what's inside the bytes it delivers.

The TCP/IP model (what the internet actually uses)

The OSI model (the vocabulary)

The seven-layer OSI model is mostly used for naming things:

Encapsulation

Each layer wraps the data from the layer above in its own header: an HTTP request goes inside a TCP segment, inside an IP packet, inside an Ethernet frame. The receiver unwraps them in reverse.

http = b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n"
layers = [("TCP header", 20), ("IPv4 header", 20), ("Ethernet header + trailer", 18)]

size = len(http)
print(f"{'HTTP request':28} {size:4} bytes")
for name, overhead in layers:
    size += overhead
    print(f"+ {name:26} {size:4} bytes on the wire")
print(f"headers are {1 - len(http) / size:.0%} of this small packet")

HTTP request                   37 bytes
+ TCP header                   57 bytes on the wire
+ IPv4 header                  77 bytes on the wire
+ Ethernet header + trailer    95 bytes on the wire
headers are 61% of this small packet