UDP

What UDP is

UDP (User Datagram Protocol) sends independent messages called datagrams, with no handshake, no retransmission and no ordering. Its header is just 8 bytes: source port, destination port, length and checksum.

When UDP wins

• Small request/response exchanges such as DNS lookups: one packet each way, no handshake.
• Real-time media: a late video frame is useless, so retransmitting it only adds delay.
• Custom reliability: QUIC, the transport under HTTP/3, builds its own reliability and encryption on top of UDP so it can evolve faster than operating-system TCP stacks.

import random

random.seed(3)
LOSS = 0.2
sent = [f"frame-{i}" for i in range(1, 11)]
received = [f for f in sent if random.random() > LOSS]
print("received:", received)
print(f"{len(sent) - len(received)} of {len(sent)} datagrams lost; UDP doesn't resend them")
print("a video call just shows the next frame instead of waiting")

received: ['frame-1', 'frame-2', 'frame-3', 'frame-4', 'frame-5', 'frame-8', 'frame-9', 'frame-10']
2 of 10 datagrams lost; UDP doesn't resend them
a video call just shows the next frame instead of waiting

Firewalls and UDP

Because UDP has no connection, stateful firewalls and NAT devices track UDP "flows" by timeouts instead. Many corporate networks block most UDP, which is why HTTP/3 clients always fall back to HTTP/2 over TCP.