UDP carries Ethereum's discovery traffic and the QUIC transport underneath the consensus stack.
Discovery datagrams
Node discovery v4 messages are sent as UDP datagrams, with a maximum packet size of 1280 bytes. Discovery v5 sends its messages as UDP datagrams as well and responds to the UDP envelope address of a request.
QUIC substrate
The consensus libp2p QUIC transport runs over UDP. Consensus clients MUST
support the QUIC (UDP) transport for dialing and listening; consensus ENRs
announce the QUIC listener under the quic and quic6 keys.
UDP provides no reliability or ordering; each protocol above it supplies its own retransmission or stream semantics.