Five Bytes Shipped for Every One That Matters

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

Five Bytes Shipped for Every One That Matters

A 12-byte sensor reading picks up 28 bytes of IPv4-plus-UDP header, or 48 bytes over IPv6 — which is why the chapter notes the packet carries “more header than payload”. But “more” is vague, and that ratio is the whole reason a tiny reading can still be expensive to send. This audit puts a number on it, asking how many bytes ship for every one that matters.

Companion to the chapter Devices, Packets, and Layers — every number here comes from that chapter.

Ada: The worked example says a 12-byte reading picks up 28 bytes of IPv4-plus-UDP header, or 48 bytes with IPv6-plus-UDP, and concludes the packet “carries more header than payload.” That is true, but “more” is vague — let me put a number on how much, because the ratio is the whole reason a tiny reading can still be expensive.

The header sums first: 20 + 8 = 28 bytes for IPv4+UDP, and 40 + 8 = 48 bytes for IPv6+UDP. Now the useful fraction of each packet, before any link framing or security is added:

• IPv4 + UDP: total 12 + 28 = 40 bytes, payload efficiency 12 / 40 = 0.30, that is 30%
• IPv6 + UDP: total 12 + 48 = 60 bytes, payload efficiency 12 / 60 = 0.20, that is 20%

Flip that around and it is starker: 60 / 12 = 5.000 on the IPv6 path, so the device transmits five bytes for every one byte of reading; the IPv4 path is 40 / 12 = 3.333, about three and a third. And this is the optimistic figure — it counts only network and transport headers, before the data-link frame, addressing, and any encryption the chapter says still have to be added on top, all of which push the useful fraction lower.

That is why the chapter starts a network review from the message, not the protocol: at 12 bytes of payload the overhead dominates, so how often the reading repeats — and how much header each repeat drags along — decides the airtime and energy bill, a cost that is invisible if you look only at the reading itself.

Every number above is taken from the chapter’s own material and re-derived step by step.