Transport protocols
The layer above IP that picks the program by port number and decides what delivery means: a reliable stream (TCP), single datagrams (UDP) or encrypted, multiplexed streams (QUIC).
How they work
IP gets a datagram to a host; a transport protocol gets it to the right program and decides what happens when packets are lost. TCP opens a connection with a three-way handshake, then delivers a reliable, in-order byte stream, resending lost segments and backing off when the network is congested. UDP adds only ports, a length and a checksum in an 8-byte header: a datagram may arrive once, twice or not at all, in any order, and the application deals with it.
QUIC, standardised in 2021, runs over UDP but gives applications what TCP does and more: many independent streams on one connection, so a lost packet stalls only the streams it carried; a handshake that sets up TLS 1.3 encryption at the same time; and connection IDs that let a client keep its connection when its network path changes.
Pick TCP for request and response traffic and anything that must arrive complete; UDP for small queries and real-time media, where a resent packet would arrive too late; QUIC when you speak HTTP/3 or control both ends. Keep a TCP fallback for QUIC: measurements cited by the IETF found 3% to 5% of networks block UDP entirely.