Transport layer

In computer networking, the transport layer is a conceptual layer in a network protocol stack that provides communication services between applications. Depending on the protocol, these services may include connection-oriented communication, reliable and ordered delivery, flow control, congestion control, and the multiplexing of communication between applications.

The transport layer is defined in both the Internet protocol suite and the OSI model, although the functions assigned to it differ between the two architectures. In the OSI model, the transport layer is Layer 4 (L4). The layers of the Internet protocol suite are not formally numbered.

The principal transport protocols of the Internet protocol suite are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). TCP provides a connection-oriented, reliable byte stream service, while UDP provides a connectionless datagram service with fewer protocol mechanisms. Other transport protocols include the Stream Control Transmission Protocol (SCTP) and the Datagram Congestion Control Protocol (DCCP). Newer transport protocols, such as QUIC, may be implemented over UDP.