Layer 4 of the seven-layer Open Systems Interconnection (OSI) Reference Model. Software at the Transport Layer is responsible for maintaining the end-to-end integrity and control of the session. Data is accepted from the Session Layer (Layer 5) and passed through to the Network Layer (Layer 3). Software at this layer fragments long message blocks into shorter packets for transmission and reassembles them at the receiver, manages flow control, adds sequence numbers, calculates and appends checksums, generates retransmissions in the event of errored message blocks or timeouts, and adds security measures. Example protocols that can be used at this layer include Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and the five classes of the OSI Transport Protocol (TP).The X.25 packet-switching protocol operates at the Transport, Network, Data Link, and Physical Layers. See also block, code, compression, flow control, layer, network architecture, OSI Reference Model, packet, protocol, and software.