sub pkt_escape {
    my $pkt = shift;
    my $esc = pack('C', 0x10);
    my $buf;

    for (my $i = 0; $i < length $pkt; $i++) {
        my $byte = unpack("x[$i]C", $pkt);

        if ($i > 4 && $i < length($pkt) - 4 && ($byte == 0x10 || $byte == 0x01 || $byte == 0x04 || $byte == 0x17)) {
            $buf .= $esc;
        }

        $buf .= pack('C', $byte);
    }

    return $buf;
}