DNS is a great example of an application layer service that uses UDP for the transport layer instead of TCP. UDP is a connectionless this means there’s no set up or tear down of connections. So much less traffic needs to be transmitted overall
A single DNS request and its response can usually fit inside of a single UDP datagram, making it an ideal candidate for a connectionless protocol. It’s also worth calling out that DNS can generate a lot of traffic. If the full resolution needs to be processed, we’re talking about a lot more traffic.
As number of server visited increases client to server has to establish connection for each transfer causing the amount of traffic to increase exponentially.
It will cost 44 packets at the minimum in order for a fully recursive DNS requests to be fulfilled via TCP in contrast to UDP which would only cost 8 packets.
DNS over TCP does in fact exist and is also in use all over. As the web has gotten more complex, it’s no longer the case that all DNS lookup responses can fit in a single UDP datagram. In these situations, a DNS name server would respond with a packet explaining that the response is too large. The DNS client would then establish a TCP connection in order to perform the lookup.