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How the Arpanet Protocols Worked (2021)
An ascent through the four levels of the ARPANET protocol hierarchy.
The US government did not directly build the network; instead, it contracted the work out to a Boston-based consulting firm called Bolt, Beranek, and Newman, more commonly known as BBN. So level 0 was a layer without a real protocol, in the sense of a publicly known and agreed-upon set of rules, and its existence could be ignored by software running on the ARPANET hosts. It’s a simple approach, but it means that only one segment is ever in flight across the network, making for a very slow protocol—which is why Kurose and Ross present “stop-and-wait” as merely a stepping stone on the way to a fully featured transport layer protocol.
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