What is OSI Layer 4?

Understanding OSI Layer 4

Transport layer. Layer 4 of the OSI model ensures reliable end-to-end data delivery between applications, providing services such as connection establishment, flow control, error recovery, and segmentation/reassembly. It uses protocols like TCP (connection-oriented) and UDP (connectionless). Security at this layer addresses issues like port scanning and denial-of-service attacks. Transport layer security is addressed in standards like NIST SP 800-53, ISO 27001, and various IETF RFCs. Organizations implement Layer 4 security through stateful firewalls, port filtering, TCP wrapper controls, rate limiting, and SYN flood protection. For example, a security team might configure stateful firewalls to track connection states, implement rate limiting to prevent SYN flood attacks, and use port-based access controls to restrict access to services based on source/destination port numbers. Related terms: TCP, UDP, Ports, Stateful inspection, Connection tracking, Flow control, SYN flood, Session establishment.