It is important that the upper layer for a specific 802.11 client sees one interface that it can send data out on. That upper MAC interface was the Multi-Link Device (MLD) SAP interface. The below diagram describes the different aspects of what the MLD handles and what the lower link layers handle.
As can be seen from the diagram above, the MAC layer operations are now handled by the MLD and the lower link layers handle backoff/EDCA, NAV, MAC header formation etc. The “From address” field will contain the Link MAC address inserted. The MLD address will be used for deriving encryption keys and used in EAPOL-key frames. There are a few exceptions but the regular operation of AP to station operation follow the above rules.
The below table outlines which layer handles what:
The section 35.3 refers to the 802.11be 2024 standard section which describes the different aspects of the MLD operation. The next article defines the base Information elements for an MLO operation.