Learn in the textbookThe OSI Reference Model and Its 7 Layers
DefinitionThe transport layer (トランスポート層) is Layer 4; it manages data transfer between the sending and receiving devices and, when needed, makes up for lost or out-of-order data to ensure reliability.
ExplanationIts best-known protocol, TCP, keeps end-to-end transfer in order by resending data lost on the way and putting received data back in sequence. A port number (ポート番号) tells which application inside a device the communication belongs to.
AnalogyIt is like an inspector at the receiving end who checks that every box of a split shipment has arrived and asks for any missing box to be sent again.
Memory hookTransport means carrying goods. Link it to a shipping company that carries freight all the way from sender to recipient.