Answer by Keith Winstein on Quora, A lot of people ask this question and think in these terms, but here's a different point of view. TCP and UDP are not at the same level of abstraction and they're ...

The two most common transport layer protocols are the transmission control protocol (TCP) and the user datagram protocol (UDP). One of the main differences between the two protocols is reliability.

Security practitioners for decades have advised people to limit DNS queries against their DNS servers to only use UDP port 53. The reality is that DNS queries can also use TCP port 53 if UDP port 53 ...