[bhk]

TCP does have retransmission, but it was designed for networks with very low packet loss and where most packet loss is due to congestion, so it does not react quickly to packet loss and when multiple packets are lost it slows down dramatically. TCP typically uses an exponential backoff in re-transmission times to avoid exacerbating congestion.

When dealing directly with UDP, one can implement different application-level logic for dealing with packet loss: "NAK" packets, different backoff strategies, and even forward error correction.