Which protocol is used when guaranteed delivery of each packet is not necessary?

When guaranteed delivery of each packet is not necessary, the User Datagram Protocol (UDP) is the appropriate choice. UDP is a connectionless protocol that provides a method for sending messages in the form of datagrams without establishing a connection between the sender and the receiver. This results in lower overhead and reduced latency, making it suitable for applications where speed is preferred over reliability.

UDP does not ensure that packets are delivered in order, nor does it guarantee delivery at all. This is beneficial for applications such as video streaming, online gaming, or voice over IP (VoIP), where occasional packet loss or out-of-order delivery is acceptable as it does not significantly impact the experience.

In contrast, protocols such as TCP (Transmission Control Protocol) establish a connection and provide reliable delivery of packets, making them more suited for applications where data integrity is critical. Other options like HTTP (Hypertext Transfer Protocol) and FTP (File Transfer Protocol) rely on TCP for their operations, thus inheriting its reliability features. This distinction highlights why UDP is chosen when timeliness is more critical than the guaranteed delivery of every single packet.