Which attribute is unique to User Datagram Protocol (UDP)?

User Datagram Protocol (UDP) is characterized by its approach to data transmission that does not establish a connection prior to sending data, which is a key identifier of its operation. The essence of UDP is that it provides an unreliable best-effort delivery service. This means that it does not provide the same reliability features as other protocols, such as Transmission Control Protocol (TCP), which ensures that messages are delivered in order, guarantees delivery, and manages retransmission of lost packets.

UDP, on the other hand, sends packets without establishing a connection and does not confirm whether packets reached their destination. This makes it faster and more efficient for certain applications, particularly those that can tolerate some data loss, such as video streaming or online gaming. The design of UDP prioritizes speed and low latency, making it suitable for real-time applications where the timely arrival of data is more critical than ensuring every single packet arrives intact.

This best-effort delivery model is distinctive. Other options in the question refer to features such as connection establishment, message ordering, and retransmission, which are not attributes of UDP. These features align more closely with the capabilities of TCP, illustrating why understanding the unique characteristics of UDP is essential for differentiating it from other transport layer protocols.