Low Level Reader Protocol (LLRP v1.0.1 / v1.1)

using System.Buffers.Binary;

public static class LlrpMessageDecoder
{
    public const ushort RO_ACCESS_REPORT_MSG_TYPE = 61;
    public const ushort PARAM_TAG_REPORT_DATA = 240;
    public const ushort PARAM_PEAK_RSSI = 218;
    public const ushort PARAM_ANTENNA_ID = 221;

    public static List<(string EPC, float RSSI, int Antenna)> DecodeRoAccessReport(byte[] buffer)
    {
        var tags = new List<(string EPC, float RSSI, int Antenna)>();
        if (buffer == null || buffer.Length < 10) return tags;

        ushort msgTypeHeader = BinaryPrimitives.ReadUInt16BigEndian(buffer.AsSpan(0, 2));
        ushort msgType = (ushort)(msgTypeHeader & 0x03FF);
        uint length = BinaryPrimitives.ReadUInt32BigEndian(buffer.AsSpan(2, 4));

        if (msgType != RO_ACCESS_REPORT_MSG_TYPE || buffer.Length < length) return tags;

        int index = 10;
        int maxIndex = (int)length;

        while (index + 4 <= maxIndex)
        {
            ushort paramHeader = BinaryPrimitives.ReadUInt16BigEndian(buffer.AsSpan(index, 2));
            ushort paramType = (ushort)(paramHeader & 0x03FF);
            ushort paramLength = BinaryPrimitives.ReadUInt16BigEndian(buffer.AsSpan(index + 2, 2));
            if (paramLength == 0 || index + paramLength > maxIndex) break;

            if (paramType == PARAM_TAG_REPORT_DATA)
            {
                // Parse EPC-96, RSSI, and Antenna Port
                var tag = ParseTagReportData(buffer, index + 4, index + paramLength);
                if (!string.IsNullOrEmpty(tag.EPC)) tags.Add(tag);
            }
            index += paramLength;
        }
        return tags;
    }
}