Learning UDP message

[rogier]

Can anyone te me how this works / use it in Node Red

in the 2 picture below you see a snap of wire shark.
in picture 1 i ask (with company software) 010101 from a DDC device (red understripe).
in picture 2 i ask (with company software) 010102 from de DDC device (red understripe)

the only thing that change in the 2 requests is de last part of the data (green understripe)
my question: how does the company software calculate the last part.. and the big question: can node red do the calculation?

[TotallyInformation]

There is absolutely no way we can help with this. We have no idea what is generating the data nor what it means.

[rogier]

thank you. I thought it was a reducible (calculated) value. it probably is but not standard in a UDP message ... unfortunate

[TotallyInformation]

UDP is just a protocol. It does not define the data.

[dceejay]

indeed - you can see the UDP checksum a few lines earlier in your dump - nothing to do with the data. But if you use something like https://crccalc.com/ in CRC-16 mode and stick in the data part I'm sure you'll find it...

Hint - try CRC-16/XMODEM

[rogier]

thats it. the CRC-16/XMODEM . hit de string in https://crccalc.com/ and the correct value came up! thank you.. now trying in node red...

[rogier]

ok, now getting it done in node red is my next issue...
i found a funtion wat does the math. now i neet to paste the crc at the ens of the buffer (10 and 11).

[{"id":"95de803e.d9674","type":"tab","label":"Flow 3","disabled":false,"info":""},{"id":"272ea949.947526","type":"inject","z":"95de803e.d9674","name":"as hex string 745207020315d2","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"0251FA019A00000A6209","payload":"0251FA019A00000A6209","payloadType":"str","x":1190,"y":360,"wires":[["c493e18d.d3b86"]]},{"id":"70ac250.402efdc","type":"debug","z":"95de803e.d9674","name":"","active":true,"tosidebar":true,"console":true,"tostatus":true,"complete":"true","targetType":"full","statusVal":"payload","statusType":"auto","x":1730,"y":360,"wires":[]},{"id":"c493e18d.d3b86","type":"function","z":"95de803e.d9674","name":"calculate CRC","func":"msg.payload = Buffer.from(msg.payload,\"hex\")\n\nconst ccitt_16 = [\n    0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,\n    0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,\n    0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,\n    0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,\n    0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,\n    0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,\n    0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,\n    0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,\n    0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,\n    0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,\n    0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12,\n    0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,\n    0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,\n    0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,\n    0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,\n    0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,\n    0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,\n    0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,\n    0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,\n    0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,\n    0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,\n    0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,\n    0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,\n    0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,\n    0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,\n    0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,\n    0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,\n    0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,\n    0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,\n    0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,\n    0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,\n    0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0\n];\nfunction Hi(word){\n    return (word >> 8) \n}\nfunction Lo(word){\n    return (word & 0x00ff) \n}\nfunction CalculateCharacterCRC16(crc, byte){\n    let shifted8_int16 = ((crc << 8) & 0xffff);\n    let b = (Hi(crc) ^ byte);\n    let newCRC = (shifted8_int16 ^ ccitt_16[b]);\n    return newCRC;\n}\n\nvar data = msg.payload;\nvar crc = 0;\nfor (let i=0; i < data.length; i++) {\n    crc = CalculateCharacterCRC16(crc, data[i]);\n} \nmsg.crc = crc ;\n\nreturn msg;\n\n\n","outputs":1,"noerr":0,"initialize":"","finalize":"","x":1520,"y":360,"wires":[["70ac250.402efdc"]]}]

does anyone know how to do this in one function code??