USB4 allows tunneling of USB3, PCIe and DisplayPort and all that is managed by a Host Router. On Apple Silicon that thing is called acio. The host router contains various protocol adapters which convert the incoming data from e.g. PCIe to the tunneled package format and combines that stream with other tunneled protocols to send it out to the Type-C PHY. The PHY then converts the packets to the on-wire protocol sent over the Type-C port. ACIO also contains an IOMMU and the Native Host Interface (NHI) which controls setting up all these tunnels. This is reason why supporting that entire thing is so complicated: All the various components have to interact with each other and on x86 machines that's mostly abstracted away by firmware running on the router. Here, we don't get that luxury and have to implement all the management ourselves. That also means the first Thunderbolt series will be very boring for end-user: It'll just be the Host Router and the NHI such that tunnels can be established. They will be useless though before we then later implement the protocol adapters. (2/2)