read14.cs

// See https://aka.ms/new-console-template for more information
using System.Buffers.Binary;

var data = new byte[] {
    100,
    100,100,10,10,
    11,11,11,11,
    12,00,
    4,
    4,

    100,
    100,
    10,10,10,10,
    11,11,11,11,
    13,00,
    4,
    4
};
var packetTemplateData = new byte[] {
    100,
    100,
    0,0,0,0,
    0,0,0,0,
    0,0,
    4,
    4,
    0,0
};
var packetBitmask = new byte[] {
    255,
    255,
    0,0,0,0,
    0,0,0,0,
    0,0,
    255,
    255,
    0,0
};

var template = BinaryPrimitives.ReadInt128BigEndian(packetTemplateData);
var bitmask = BinaryPrimitives.ReadInt128BigEndian(packetBitmask);
using var ms = new MemoryStream(data);

var buffer = new byte[16].AsMemory();

ms.ReadExactly(buffer.Span[..14]);

var packet = BinaryPrimitives.ReadInt128BigEndian(buffer.Span);

while ((packet & bitmask) != template)
{
    packet <<= 8;
    packet |= ms.ReadByte() << 16;
}

//parse package

while ((packet & bitmask) != template)
{
    ms.ReadExactly(buffer.Span[..14]);
    packet = BinaryPrimitives.ReadInt128BigEndian(buffer.Span);
    // parse package
}
//error. reset or read end package

Console.ReadLine();