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Layer 4 refers to the fourth layer of the Open Systems Interconnection (OSI) Model, known as the transport layer. It provides the transparent transmission or transfer of data between end systems or hosts and is responsible for end-to-end error recovery, as well as flow control.
As the name suggests, the transport layer ensures complete data transfers.
Layer 4 provides the host-to-host or end-to-end transfer of data and communication services for applications that use the layered structure of the OSI model. Layer 4 provides such services as connection-oriented data stream support, flow control, multiplexing and reliability.
The TCP/IP model is the foundation of the Internet, and transport layer implementations are contained within it. Meanwhile, the OSI model for general networking also contains transport layer implementations, which are different than that of the TCP/IP model, and here it is generally referred to as Layer 4.
Layer 4 is responsible for the delivery of data to appropriate application processes running on host computers, which involves statistical multiplexing of various data from different application processes. This involves the creation of data packets from raw data and the addition of source and destination specifics like port numbers.
Working together with destination IP addresses, these ports form a network socket or simply the identification address of the process-to-process communication. The session layer, Layer 5, supports this process in the OSI model. Some common protocols used in OSI Layer 4 are: