Message Overhead Budget
The proportion of each transmission spent on protocol rather than payload, which on a metered or battery-powered link determines the running cost.
A device sending a twenty-byte reading over HTTPS transmits headers many times the size of the data, plus a TLS handshake if the connection is not persistent. On a cellular plan billed by the megabyte, or a battery sized for years of operation, that ratio is the dominant cost — radio transmission is typically the largest consumer of power in the device.
Constrained protocols exist to change the ratio. MQTT keeps a persistent connection with a very small fixed header, so a publish costs a few bytes of overhead. CoAP uses compact binary encoding over UDP, avoiding connection setup entirely. Both trade features for size.
The budget is worth calculating explicitly, per message, at the design stage: payload bytes, protocol overhead, transport overhead, and the amortised cost of connection establishment at the expected reconnection rate. Multiplied by message frequency and fleet size, it produces the monthly data bill and the battery life estimate — two numbers that are extremely expensive to change after hardware has shipped.
The related decision is payload encoding. A binary format against a schema is frequently a third the size of the equivalent JSON, and on a constrained link that difference is the same order as the protocol choice itself.