datagram

UDP's unit: one send, with no handshake, no numbering, no retransmission and no state in the kernels. It is not a degraded TCP but the other answer to the question of what should happen to a lost packet — for a DNS query that fits in one packet, or an audio packet that will be worthless in 300 milliseconds, asking again or giving up costs less than waiting. ss -uan never prints LISTEN: every UDP socket is UNCONN, since there is no handshake and no connection to accept.