UDP header

The UDP header is a fixed 8 bytes: 16-bit source port, 16-bit destination port, 16-bit length (header + data), and 16-bit checksum. There is no sequencing, acknowledgement, or connection state — UDP is a thin, connectionless datagram wrapper over IP.

wire format

spec: RFC 768

An 8-byte connectionless header. Length covers the header plus data; the checksum is optional in IPv4 (zero means not computed) but mandatory in IPv6.

field	size	meaning
Source Port	16 bits	Sending port; may be zero if no reply is expected.
Destination Port	16 bits	Receiving port.
Length	16 bits	Length in bytes of the UDP header plus data (minimum 8).
Checksum	16 bits	Ones-complement checksum over a pseudo-header, the UDP header, and data. Optional in IPv4 (0 = unused); required in IPv6.

example:

00 35 00 35 00 1C 00 00

src port 0x0035 = 53 (DNS), dst port 53, length 0x001C = 28 bytes (8 header + 20 data), checksum 0x0000 (not computed).

see: wire-format/dns-message

Only 8 bytes of overhead, versus TCP's 20+, which is why DNS, QUIC, DHCP, and real-time media use UDP.
No retransmission, ordering, or flow control — those are left to the application or a higher protocol (QUIC, RTP).
Length field max 65535 caps a single datagram, though the IP layer may fragment it.

wire format

provenance

see also