What is the primary function of the OSI model's transport layer?

The transport layer (Layer 4) of the OSI model is responsible for managing end-to-end communication between devices. This layer ensures that data is transferred reliably and efficiently, managing flow control, error detection, and retransmission. It handles segmentation of data for transmission, reassembly upon arrival, and keeps track of data integrity. Protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) operate at this layer. Option A is incorrect because encryption/decryption happens at the presentation layer (Layer 6). Option B is incorrect as the physical transmission of data occurs at the physical layer (Layer 1). Option D is incorrect; defining physical topologies is related to the data link and physical layers. Option E is incorrect because packet routing occurs at the network layer (Layer 3).