Which protocol is characterized as connectionless and unreliable at the Transport Layer?

• A. User Datagram Protocol (UDP)
• B. Transmission Control Protocol (TCP)
• C. Stream Control Transmission Protocol (SCTP)
• D. Internet Protocol (IP)

Answer: (A)

At the Transport Layer, the key distinction is whether a protocol establishes a session and guarantees delivery. UDP provides a connectionless, best-effort service: no handshake, no acknowledgments, no retransmission, and no guarantees about ordering. Data may arrive out of order or be lost, but the overhead is minimal and latency remains low. This makes UDP ideal for time-sensitive or simple messages where occasional loss is acceptable, such as DNS queries or certain streaming and real-time applications. In contrast, TCP requires a connection first and ensures reliable delivery through acknowledgments and retransmissions, so data arrives intact and in order. SCTP is also connection-oriented and reliable, with extra features like multi-streaming, while IP is a network-layer protocol and isn’t a transport-layer option.