Planned for version 4.6 -- comments/suggestions about this are more than welcome
Example of multicast packet as of 2023-10-20
===================================================================
{
  "app": "Hamlib",
  "version": "4.6~git 2023-10-20T16:52:59Z SHA=d87671",
  "seq": 183,
  "time": "2023-10-20T20:13:53.139869-0000",
  "crc": 0,
  "rig": {
    "id": "FLRig:127.0.0.1:12345:30508",
    "status": "OK",
    "errorMsg": "",
    "name": "FLRig",
    "split": false,
    "splitVfo": "VFOA",
    "satMode": false,
    "modelist": "AM CW USB LSB FM CWR PKTLSB PKTUSB "
  },
  "vfos": [
    {
      "name": "VFOA",
      "freq": 14074270,
      "mode": "PKTUSB",
      "width": 3,
      "ptt": false,
      "rx": true,
      "tx": true
    },
    {
      "name": "VFOB",
      "freq": 14074500,
      "mode": "",
      "width": 0,
      "ptt": false,
      "rx": false,
      "tx": false
    }
  ],
  "spectra": [
    {
      "id": 0,
      "name": "?",
      "type": "FIXED",
      "minLevel": 0,
      "maxLevel": 0,
      "minStrength": 0,
      "maxStrength": 0,
      "centerFreq": 0,
      "span": 0,
      "lowFreq": 0,
      "highFreq": 0,
      "length": 0,
      "data": ""
    }
  ]
}
===========================================================
Multicast UDP broadcast containing rig snapshot data
Bidirectional rig control and status
Choice of token pairs or JSON
All packets will be tagged with ID=[unique name] -- so multiple rigs can broadcast/rx on the same port
Will eventually be able to set freq, mode, width, ptt, satmode, and split to start since those are common to many apps.
More functions will be added as time goes on.

Broadcast packet contents to be based on get_rig_info output
Can be multiple VFO lines
Parsers should allow for unknown tags that may be added in the future

Rig command: \get_rig_info
VFO=Main Freq=145000000 Mode=None Width=0 RX=1 TX=1
VFO=VFOB Freq=145000000 Mode=FM Width=15000 RX=0 TX=0
Split=0 SatMode=0
PTT=1   (1=On, 0=Off)
Rig=Dummy (name of connected rig)
App=Hamlib (name of application providing the packet)
Version=20210429 1.0.0 (version YYYYMMDD)
Status=OK   (possible values OK, Offline, Error)
ErrorMsg=msg text (pipe delimited multi-line error message)
CRC=0xf49f4708   (this is just an example CRC and not accurate for this example)

JSON data snapshot format for UDP packets:
{
  "app": "Hamlib",
  "__comment_version__": "protocol version YYYYMMDD x.x.x, 1.0.0 will be used when this is implemented",
  "version": "20210521 0.0.0",
  "__comment_seq__": "Seq is 1-up sequence number 32-bit -- wraps around to 1 from 2^32-1",
  "seq": 1,
  "__comment_crc__": "32-bit CRC of the entire JSON record replacing the CRC value with 0",
  "crc": 0,
  "rig": {
    "__comment1__": "customizable rig identification -- will allow multiple rigs to be on the multicast",
    "id": {
        "model":"IC-7300",
        "endpoint":"com1",
        "process":"53535",
    }
    "name": "IC-7300",
    "ptt" : false,
    "split": true,
    "splitVfo": "VFOB",
    "satMode": false,
    "status": "OK";
    "errorMsg": "OK",
    "modes":[
        "AM",
        "CW",
        "USB"
    ]
  },
  "vfos": [
  {
    "name": "VFOA",
    "freq": 14074000,
    "mode": "USB",
    "width": 5000,
    "rx": true,
    "tx": false
  },
  {
    "name": "VFOB",
    "freq": 14076000,
    "mode": "USB",
    "width": 5000,
    "rx": false,
    "tx": true
  }],

  "__comment_spectra__": "Rigs that have spectrum output may include this array data",
  "spectra": [
  {
    "__comment_id__": "A numeric ID for the spectrum data stream. These IDs are exposed in rig caps.",
    "id": 0,
    "__comment_name__": "Name identifying the spectrum data stream and matching the ID. The name corresponds to VFOs.",
    "name": "Main",
    "__comment_spectrum_length__": "Length of spectrum FFT data in bytes",
    "length": 475,
    "__comment_spectrum_data__": "Spectrum FFT data in 2-char hexadecimal byte format, so that length of the string is 2 * length",
    "data": "00AAFF75BD2AAA...",

    "type": "FIXED|CENTER",
    "minLevel": 0,
    "maxLevel": 140,
    "minStrength": -100,
    "maxStrength": 0,

    "__comment_spectrum_frequencies__": "The following fields will be calculated automatically by Hamlib based on the spectrum information exposed by the rig",
    "centerFreq": 14267000,
    "span": 25000,
    "lowFreq": 14000000,
    "highFreq": 14250000
  },
  {
    "__comment_id__": "A numeric ID for the spectrum data stream. These IDs are exposed in rig caps.",
    "id": 1,
    "__comment_name__": "Name identifying the spectrum data stream and matching the ID. The name corresponds to VFOs.",
    "name": "Sub",
    "__comment_spectrum_length__": "Length of spectrum FFT data in bytes",
    "length": 475,
    "__comment_spectrum_data__": "Spectrum FFT data in 2-char hexadecimal byte format, so that length of the string is 2 * length",
    "data": "00AAFF75BD2AAA...",

    "type": "FIXED|CENTER",
    "minLevel": 0,
    "maxLevel": 140,
    "minStrength": -100,
    "maxStrength": 0,

    "__comment_spectrum_frequencies__": "The following fields will be calculated automatically by Hamlib based on the spectrum information exposed by the rig",
    "centerFreq": 14267000,
    "span": 25000,
    "lowFreq": 14000000,
    "highFreq": 14250000
  }],
  "lastCommand": {
      "id": "MyApp 123",
      "command": "set_freq VFOA 14074000",
      "status": "OK"
  },
}

An example UDP packet containing spectrum data from IC-7300 (crc, id and lastCommand not implemented yet):

{
  "app": "Hamlib",
  "version": "4.5~git Sun Dec 19 20:56:24 2021 +0000 SHA=0fe723",
  "seq": 109,
  "crc": 0,
  "rig": {
    "id": "rig_id",
    "status": "OK",
    "errorMsg": "",
    "name": "IC-7300",
    "split": false,
    "splitVfo": "VFOA",
    "satMode": false
  },
  "vfos": [
    {
      "name": "VFOA",
      "freq": 3718000,
      "mode": "LSB",
      "width": 3000,
      "ptt": false,
      "rx": true,
      "tx": true
    },
    {
      "name": "VFOB",
      "freq": 3698750,
      "mode": "",
      "width": 0,
      "ptt": false,
      "rx": false,
      "tx": false
    }
  ],
  "spectra": [
    {
      "id": 0,
      "name": "Main",
      "type": "CENTER",
      "minLevel": 0,
      "maxLevel": 160,
      "minStrength": -80,
      "maxStrength": 0,
      "centerFreq": 3718000,
      "span": 50000,
      "lowFreq": 3693000,
      "highFreq": 3743000,
      "length": 475,
      "data": " "
    }
  ]
}
