653c0b1d_host_protocols_of_ARPANET

The responsibility for enabling communication between nodes fell to the hosts, not the IMPs. At the lowest level (equivalent to the link layer in the subsequent internet?), this consisted in moving raw bits between hosts regardless of what specific type of data they encoded.

In the book (Where Wizards Stay Up Late), there is a great description by analogy:

Some water out of the tap is used for making coffee, some for washing dishes, some for bathing. The pipe and the faucet do not care about this; they convey the water regardless.