MARGINALIA · ONE NUMBER
The physical frame, before cryptography takes its share.
127 octets
That is the maximum physical-layer packet size used in the IEEE 802.15.4 calculation set out by RFC 4944.
The number is only the beginning. The RFC subtracts a maximum frame overhead of 25 octets, leaving 102 at the media-access layer. In its largest AES-CCM-128 security case, another 21 octets are consumed. What remains is 81 octets, before an adaptation layer and the application have expressed anything.
At this scale, a nonce, authentication tag, key identifier, counter and algorithm indication are not negligible metadata. They compete directly with the message. A construction that is compact in an API can require fragmentation, new state and new failure paths on the wire.
The number is useful because it moves the security discussion from algorithm names to complete encodings.
Source: RFC 4944, Section 4.
Related research: The handshake has a budget of fourteen kilobytes.
