| bits | 0 - 15 | 16 - 31 | ||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | Source Port | Destination Port | ||||||||||||||||||||||||||||||
| 32 | Length | Checksum | ||||||||||||||||||||||||||||||
| 64 | Data | |||||||||||||||||||||||||||||||
The UDP header consists of only 4 fields.
- Source port -- This field identifies the sending port when meaningful and should be assumed to be the port to reply to if needed. If not used, then it should be zero.
- Destination port -- This field identifies the destination port and is required.
- Length -- A 16-bit field that specifies the length in bytes of the entire datagram: header and data. The minimum length is 8 bytes since that's the length of the header. The field size sets a theoretical limit of 65,535 bytes (8 byte header + 65527 bytes of data) for a UDP datagram. The practical limit for the data length which is imposed by the underlying IPv4 protocol is 65,507 bytes.
- Checksum -- The 16-bit checksum field is used for error-checking of the header and data. The algorithm for computing the checksum is different for transport over IPv4 and IPv6.
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