The Header

Source and Destination Ports

UDP starts by naming the local endpoint numbers on each side.

highlighted = computed this step

Ports name local endpoints

A UDP header begins with two port fields. In this toy datagram, those fields name the sending endpoint and the receiving endpoint.

source port=40000,destination port=53\text{source port}=40000,\quad \text{destination port}=53
Ports in the headerThe 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

Source port

The source port is 40000. It is a field in the UDP header, not a process lookup.

source port=40000\text{source port}=40000
Ports in the headerThe 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

Destination port

The destination port is 53. At this toy level, that is the socket-side name the receiver checks.

destination port=53\text{destination port}=53
Ports in the headerThe 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

UDP ports are exact header fields. This book does not model process discovery, firewall behavior, or NAT behavior.

40000→5340000\to53
Ports in the headerThe 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