{
  "id": "wire-format/udp-header",
  "family": "wire-format",
  "slug": "udp-header",
  "title": "UDP header",
  "summary": "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.",
  "kind": "wire-format",
  "aliases": [
    "UDP datagram header",
    "User Datagram Protocol header"
  ],
  "status": "standard",
  "verification": "verified",
  "tier": "B",
  "source_url": "https://www.rfc-editor.org/rfc/rfc768",
  "source_version": "RFC 768 (User Datagram Protocol)",
  "retrieved_date": "2026-05-29",
  "see_also": [
    "wire-format/tcp-header",
    "wire-format/ipv4-header",
    "wire-format/dns-message"
  ],
  "ext_type": "wire-format@1",
  "ext": {
    "spec": "RFC 768",
    "summary": "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.",
    "structure": [
      {
        "field": "Source Port",
        "size": "16 bits",
        "meaning": "Sending port; may be zero if no reply is expected."
      },
      {
        "field": "Destination Port",
        "size": "16 bits",
        "meaning": "Receiving port."
      },
      {
        "field": "Length",
        "size": "16 bits",
        "meaning": "Length in bytes of the UDP header plus data (minimum 8)."
      },
      {
        "field": "Checksum",
        "size": "16 bits",
        "meaning": "Ones-complement checksum over a pseudo-header, the UDP header, and data. Optional in IPv4 (0 = unused); required in IPv6."
      }
    ],
    "example_hex": "00 35 00 35 00 1C 00 00",
    "example_decoded": "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"
    ],
    "notes": [
      "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."
    ]
  },
  "updated": "2026-05-29T00:00:00Z"
}