The Header

Length Covers Payload

The UDP length field counts the header and the tiny payload together.

highlighted = computed this step

Length covers header and payload

The UDP length field counts the UDP header plus the UDP payload. Here the header is 8 bytes and the payload is 2 bytes.

8+2=108+2=10
UDP lengthThe UDP header and tiny payload are decoded from exact bytes.UDP header64 bits / 8 bytes0x9c400035000a779fbyte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100111011110011111source-port40000destination-port53length10checksum0x779fPayload16 bits / 2 bytes0x6869byte 0byte 10110100001101001payload0x6869

Tiny payload

The payload bytes are 0x6869. They are the two ASCII bytes for hi.

payload bytes=2\text{payload bytes}=2
UDP lengthThe UDP header and tiny payload are decoded from exact bytes.UDP header64 bits / 8 bytes0x9c400035000a779fbyte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100111011110011111source-port40000destination-port53length10checksum0x779fPayload16 bits / 2 bytes0x6869byte 0byte 10110100001101001payload0x6869

Length field

The length field is 10, matching header plus payload.

UDP length=10\text{UDP length}=10
UDP lengthThe UDP header and tiny payload are decoded from exact bytes.UDP header64 bits / 8 bytes0x9c400035000a779fbyte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100111011110011111source-port40000destination-port53length10checksum0x779fPayload16 bits / 2 bytes0x6869byte 0byte 10110100001101001payload0x6869

Summary

The receiver does not guess where this UDP datagram ends. The length field is checked against the exact header and payload byte count.

length=10\text{length}=10
UDP lengthThe UDP header and tiny payload are decoded from exact bytes.UDP header64 bits / 8 bytes0x9c400035000a779fbyte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100111011110011111source-port40000destination-port53length10checksum0x779fPayload16 bits / 2 bytes0x6869byte 0byte 10110100001101001payload0x6869