notes from Brookshear’s book 3.6 “Network Protocols”

Four layers of in Internet software: application, transport, network, and link

Open System Interconnection (OSI) is based on a seven-level hierarchy as opposed to the four-level system used by the Internet. … it has not been readily implemented, mainly because before it was established the TCP/IP protocol suite had already been developed, implemented, widely publicized, and proven reliable as the protocol system for the Internet.

The TCP/IP protocol suite is a collection of protocols defining the four-level hierarchy used by the Internet. At the transport layer, TCP (“reliable”) and UDP (connectionless) are two versions. At the network layer, it is IP.