How networks work: layers and packets

When you open a web page, your request does not travel as one stream. It is cut into packets: small pieces, each carrying the address it is going to. Routers pass each packet along on its own, and links are shared by everyone's packets at once. This is packet switching, and it is why one busy line does not block the whole internet. (The old telephone network instead reserved a whole circuit for each call: circuit switching.)

Five layers

Networking is split into layers. Each layer solves one problem and uses the layer below it, so a web browser never needs to know whether the bits travel over Wi-Fi or fibre.

The OSI model used in many textbooks has seven layers: it adds Presentation and Session between Transport and Application. The internet itself follows the five above (often drawn as four, with Link and Physical together).

Encapsulation

On the way down, each layer wraps the data from the layer above with its own header: who it is from, who it is for, and what it needs to do its job. On the way up, the receiver removes the headers in the opposite order.

message = "GET /index.html"
segment = {"src_port": 51000, "dst_port": 80, "data": message}
packet = {"src_ip": "192.168.1.10", "dst_ip": "93.184.216.34", "data": segment}
frame = {"src_mac": "aa:bb:cc:00:00:01", "dst_mac": "aa:bb:cc:00:00:fe", "data": packet}

# The receiver removes the layers in the opposite order.
inside = frame
for layer in ["frame", "packet", "segment"]:
    header = [key for key in inside if key != "data"]
    print(layer, "header:", header)
    inside = inside["data"]
print("message:", inside)

Headers cost space. A full Ethernet frame carries 1460 bytes of data under 58 bytes of headers, but a tiny message is mostly headers:

headers = {"TCP": 20, "IP": 20, "Ethernet": 18}
total_headers = sum(headers.values())

for data in [1460, 10]:
    frame = data + total_headers
    useful = round(100 * data / frame, 1)
    print(data, "bytes of data in a", frame, "byte frame:", useful, "% useful")

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