UDP (User Datagram Protocol)

核心 · Key Idea

In one line: UDP sets up no connection, doesn't guarantee delivery or order, but has only an 8-byte header and fires-and-forgets. Real-time use cases (DNS / video / voice / gaming / QUIC) almost all pick it.

What it is#

UDP datagram format:

+-----------------+----------------+
| src port (16)   | dst port (16)  |
+-----------------+----------------+
| length (16)     | checksum (16)  |
+-----------------+----------------+
|              data                |
+----------------------------------+

8-byte header + data — no sequence, no ACK, no retransmit.

Analogy#

打个比方 · Analogy

UDP is a walkie-talkie: hold the button and shout. Heard it? lucky. Missed it? not retried. Realtime beats complete.

Key concepts#

ConnectionlessConnectionless

No handshake before sending — any host just sends.

DatagramDatagram

UDP data has boundaries — one send = one recv.

UnreliableUnreliable

No retransmit on loss; the application must handle it.

Out-of-orderOut-of-order

Packets may arrive in any order; the application must sort.

Broadcast / MulticastBroadcast / Multicast

UDP supports both natively; TCP doesn't (point-to-point only).

How it works#

The OS hands the UDP packet to the listening process — no connection, no state.

Practical notes#

Typical protocols: DNS (simple Q&A), DHCP, SNMP, NTP, VoIP (RTP), QUIC (HTTP/3 underlying).
UDP doesn't natively give half-duplex / order / reliability — build them in the app (QUIC does this).
Less NAT-friendly: routers maintain UDP entries by timeout (TCP has SYN/FIN); timeouts vary widely — pain point for P2P.
MTU sensitive: large UDP fragments easily; keep payload within MTU (typically 1472 IPv4, 1452 IPv6).
Capture: tcpdump 'udp port 53' to see DNS.

Easy confusions#

UDP

The barebones transport itself.
Reliability is the app's call.

QUIC

Implements reliability / encryption / multistream **on top of** UDP.
HTTP/3 uses it instead of TCP.