Re: [Secdispatch] EDHOC Summary

[Tero Kivinen <[e-mail]>]

John Mattsson writes:
> constrained some of them are. For the target network technologies
> LPWAN over LoRaWAN and 6TiSCH over IEEE 802.15.4, the requirements
> for message size are clear (under 50 bytes).

IEEE 802.15.9 specifies how to use KMPs in IEEE 802.15.4, and it
provides fragmentation of larger messages, so message size limits are
not absolute in IEEE 802.15.4 environment. Of course the fewer
fragments are needed the more efficient the protocol will be, but
there is no absolute limit that is required.

Some of the PHYs in IEEE 802.15.4 support larger frame sizes (up to
2048 bytes) and some of them support smaller (smallest are 20-30 bytes
or so, but that PHY also includes another layer of fragmentation).

The most common maximum frame size is 127 bytes, including header, and
the header is usually less than 20 octets if no security header is
used (which normally is not used for key management protocols as there
is no keys yet). Even with security the overhead is usually about 12
bytes more, thus the total overhead is around 20-40 bytes. This means
there is space for around 80-100 bytes of actual frame payload for
IEEE 802.15.4 in normal cases.

Targetting for 50 bytes is quite pessimistic for IEEE 802.15.4.
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