RE: Sequence Number in BLE Mesh - nRF Connect SDK 1.9.1

Joakim Jakobsen — Fri, 30 Jun 2023 01:09:30 GMT

From the Mesh Profile Specification - Section 3.8.3:

The sequence number, a 24-bit value contained in the SEQ field of the Network PDU, is primarily designed to protect against replay attacks. Elements within the same node may or may not share the sequence number space with each other. Having a different sequence number in each new Network PDU for every message source (identified by the unicast address contained in the SRC field) is critical for the security of the mesh network.

With a 24-bit sequence number, an element can transmit 16,777,216 messages before repeating a nonce. If an element transmits a message on average once every five seconds (representing a fairly high frequency message for known use cases), the element can transmit for 2.6 years before the nonce repeats.

Each element shall use strictly increasing sequence numbers for the Network PDUs it generates. Before the sequence number approaches the maximum value (0xFFFFFF), the element shall update the IV Index using the IV Update procedure (see Section 3.10.5). This is done to ensure that the sequence number will never wrap around.

Br,
Joakim