TCP is the protocol that builds a session before application data is sent; UDP does not. In the CCNA v1.1 network fundamentals scope, this is the practical distinction between connection-oriented and connectionless transport. TCP uses a three-way handshake: the client sends SYN, the server replies with SYN-ACK, and the client answers with ACK. After that, TCP can number segments, acknowledge received data, retransmit missing data, and apply flow control through windowing. UDP skips that establishment process. It sends datagrams without negotiating a session and without guaranteeing that the receiver gets every datagram in order. That makes UDP faster and simpler for voice, video, DNS, DHCP, and other traffic that can tolerate loss or implement recovery at the application layer. Options A, B, and C distort the protocol behavior. UDP does not use SYN/SYN-ACK/ACK, and UDP is not the reliable transport. Cisco CCNA v1.1 expects this exact operational contrast: TCP establishes and protects delivery; UDP sends without connection setup.
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