TCP, the Transmission Control Protocol, is a connection-oriented transport protocol used to deliver data reliably between hosts. Before data is exchanged, TCP establishes a session and tracks sequence numbers, acknowledgments, retransmissions, and flow control. That state lets each side know which bytes were received and which need to be sent again.
In cyber security, TCP matters because many services depend on stable session state. Attackers may try to disrupt that state, reset connections, inject traffic, or hijack an established session if a network device or application accepts the wrong packets. Stateful middleboxes such as NATs and firewalls also rely on TCP flow tracking, so weak handling of connection tables can create security risks. Defenders monitor for abnormal resets, unexpected sequence behavior, and suspicious state changes, and they use strong validation and encryption to reduce trust in the network path.