What guarantees packet delivery?

Packet delivery is primarily guaranteed by protocols that implement error detection, acknowledgment, and retransmission mechanisms. In the TCP (Transmission Control Protocol), for instance, packets are tracked and acknowledged, ensuring that lost packets are retransmitted. Additionally, techniques like checksums and sequence numbers help verify that packets arrive intact and in the correct order. In contrast, protocols like UDP (User Datagram Protocol) do not guarantee delivery, making it suitable for applications where speed is prioritized over reliability.