Flow control regulates the amount of data a source can send before receiving an acknowledgment from the destination.

In communication at the transport layer, we are dealing with four entities: sender process, sender transport layer, receiver transport layer, and receiver process.

The sending process at the application layer is only a producer. It produces message chunks and pushes them to the transport layer. The sending transport layer has a double role: it is both a consumer and the producer. It consumes the messages pushed by the producer. It encapsulates the messages in packets and pushes them to the receiving transport layer.

The receiving transport layer has also a double role: it is the consumer for the packets received from the sender. It is also a producer; it needs to decapsulate the messages and deliver them to the application layer. The last delivery, however, is normally a pulling delivery; the transport layer waits until the application-layer process asks for messages.