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UDP

The smallest transport: ports and a checksum, nothing else.

A UDP datagram

The whole header fits in one row. Everything after it is somebody else's business.

UDP datagram · 8-byte header + 29 bytes of data08162432D411010100310011000100000000003500110101000000000025001001012C001011009C10011100AB10101011CD11001101010000000100000000000000000000010000000100000000000000000000000000000000000000000000000000000000000007000001116501100101780111100061011000016D0110110170011100006C011011006501100101030000001163011000116F011011116D0110110100000000000000000000010000000100000000000100000001Source portDestination portLengthChecksumData (a DNS query)Click a field to inspect it

Whole frame

Eight bytes of header: two ports, a length and a checksum. No connection, no sequence numbers, no retransmission. D4 31 00 35 00 25 2C 9C

FieldOffsetExample valueMeaning
Source portByte 0–10xD431 = 54321The sender's port, so the reply knows where to go. The OS picks a random 'ephemeral' port (49152–65535 by convention).
Destination portByte 2–30x0035 = 53 (DNS)Which service on the destination host should receive this: 53 is DNS. The IP address picks the machine, the port picks the program.
LengthByte 4–537Header plus data in bytes: 8 + 29 = 37.
ChecksumByte 6–70x2C9C (✓)A 16-bit checksum over a pseudo-header (source IP, destination IP, protocol 17, length), the UDP header and the data. Computed for real here. In IPv4 it is optional (0 = none); in IPv6 it is mandatory.
Data (a DNS query)Byte 8–36DNS queryUDP does not care what is inside. Here it is a 29-byte DNS question about example.com (see the DNS page). Whatever the application hands over is sent as one datagram.

Overview

UDP (User Datagram Protocol) adds just two things to IP: port numbers, so many programs can share one machine, and a checksum. It is connectionless: each datagram stands alone, may arrive out of order, twice, or not at all, and the sender never finds out.

That minimalism is the feature: no handshake means no setup delay, and applications that tolerate loss (voice, games, DNS) or implement their own recovery (QUIC) choose it.

Key facts

Header
8 bytes
Ports
16 bits (0 – 65535)
IP protocol number
17
Delivery
Unreliable, unordered
Max datagram
65,507 bytes (IPv4)
Standard
RFC 768

Where you meet it

  • DNS queries, DHCP, NTP
  • Voice and video calls (RTP), online games
  • QUIC and HTTP/3 run on top of UDP

Watch out for

  • A datagram bigger than the path MTU (about 1472 bytes of data) gets fragmented by IP; lose one fragment and the whole datagram is lost.
  • No congestion control: a UDP sender can flood a network. Protocols built on UDP must add their own limits.
  • UDP amplification attacks abuse small spoofed queries that trigger large replies (DNS, NTP, memcached).

Standards

  • RFC 768: User Datagram Protocol

Related protocols

DNSIPv4TCP