\subsection{The update mechanism}

The implementation of the update mechanism consists of four parts which interact closely: checking for updates, reprogramming the device, calculating and verifying the firmware signature and reconfiguring the boot process.
This sections describes all four of these parts in detail.

The build-time configuration was extended to include a new option called \texttt{UPDATER\_URL}, which is the base URL used to query the update server.
Each device requires to have this option set to make the update work.
Skipping the option results in the exclusion of the code for update management during the build.


\subsubsection{Checking for updates}

Each device queries the update server regularly for the current firmware version and initializes the update process if remote and local versions differ.
To do so, the update server provides a file for each device type containing meta-information about the available firmware which consists of the version identifier and the signatures of both of the firmware binaries.
These meta-information files are provided by the update server using \textit{HTTP 1.1}\cite{HTTP_1.1} under the following path pattern: \texttt{\$\{DEVICE\}.version} (whereas \texttt{\$\{DEVICE\}} is the device type name).

The meta-information file has a very simple line oriented, ASCII based format which is easy to generate and efficient to pars in the limited constrains of the embedded device.
The first line is the version identifier which can be an arbitrary string as the content is not interpreted semantically but only compared to the version identifier used during build time.
The other two remaining lines is the hexadecimal representation of the firmware signatures, one line for each firmware.

Each device tries to fetch the meta-information file once every hour.
After the file has been downloaded successfully, the version identifier is extracted and compared to the version identifier provided during build time.
If the version identifiers differ, the update process is initialized; in cases where the download has failed, the update server or the network connection was not available or any other error occurred, another attempt will be made at the next interval.

In addition to the interval, a special \textit{MQTT} topic shared by all devices is subscribed on device startup: \texttt{\$\{MQTT\_REALM\}/update}.
Every time a message is received on this topic, a fetch attempt for the meta-information file is triggered and the process restarts.
This allows faster roll-outs of updates and finer control for manual maintenance.


\subsubsection{Reprogramming the device}

As the \textit{ESP-01s} is only equipped with 1 MB of flash, this means that the whole memory is mapped to a contiguous address space.
Therefore, the second ROM can not be re-mapped to have the same start address as the first ROM.
While the firmware is executed without any dynamic linking mechanism and the chip does not support position independent code, the addresses used in the ROMs are dependent to the offset at which the firmware is stored.
This arises the need for building two firmware binaries, one for each target location.
To do so, a linker script for each of the two ROM slots was created, which is used to create two variations of the same firmware, only differing in ROM placement.
The two resulting firmware binary files are both provided for download via \textit{HTTP 1.1} - which one to download depends on the target ROM slot and is selected by the device during the update process.
Listing~\ref{lst:linker_script} shows the only difference between the two linker scripts, where \texttt{\$\{SLOT\}} must be replaced with the slot number according to the current build.

\begin{lstlisting}[language=,
                   caption={Linker script to build firmware for two ROM slots.},
                   label=lst:linker_script, basicstyle=\ttfamily\scriptsize]
irom0_0_seg : 
  org = ( 0x40200000       // The memory mapping address
        + 0x2000           // Bootloader code and config
        + 0x10             // Data offset after header
        + 1M / 2 * ${SLOT} // Offset for the ROM slot
        ),
  len = ( 1M / 2 - 0x2010 )// Half ROM size excl. bootloader
\end{lstlisting}


\textit{rBoot}\cite{rBoot} has been choosen as it is integrated within the \textit{Sming} framework and allows to boot to multiple ROMs.
For configuration, an \textit{rBoot} specific structure is placed in the flash at a well-known location directly after the space reserved for the bootloader code.
This structure contains, among other things, the target offsets for all known ROMs and the number of the ROM to boot from on next reboot.

%The full memory layout of this approach is shown in Figure~\ref{fig:memory_layout}.
To calculate the origin of application data for each slot, the available memory of 1 MB is split in half and an offset of the size of the bootloader code and its configuration (0x2000 bytes) is added.
For alignment and easy debugging, the second block is also shifted by the same amount ob bytes as the first block.
The unused gap of 8192 bytes is used by some applications to store data which can persist over application updates.

\begin{lstlisting}[caption={The flash layout used for two ROMs.},
                   label=lst:choosing_rom, basicstyle=\ttfamily\scriptsize]
#define URL_ROM(slot) (( URL "/" DEVICE ".rom" slot ))

// Select rom slot to flash
const auto& bootconf = rboot_get_config();

// Add items to flash
if (bootconf.current_rom == 0) {
  updater.addItem(bootconf.roms[1], URL_ROM("1"));
  updater.switchToRom(1);
} else {
  updater.addItem(bootconf.roms[0], URL_ROM("0"));
  updater.switchToRom(0);
}
\end{lstlisting}

The firmware files provided on the update server are the exact same ones as used to initially flash the chip for the according version.
Using the same files for flashing and updating allows better debugging by eliminating errors related to the update process itself and makes development and initial installation very easy.
Listing~\ref{lst:choosing_rom} shows the algorithm used to determine the download address and reconfigure the bootloader.
The update server provides these files in the exact same way as it provides the meta-information files, but the path pattern differs: the suffixes \texttt{.rom0} and \texttt{.rom1} are used to provide the firmware binary files for the first and second slot respectively.

For installing a firmware update, the new firmware binary file is downloaded using an HTTP GET request.
While the image is being downloaded each chunk received in the download stream is used to update the \textit{SHA256} hash before it is written to the flash.
When the write has been finished, the next chunk is received and the process continues until all chunks have been received.

After the download of a new ROM has been finished successfully, the calculated hash is checked against the signature provided in the meta-information file triggering the update and the public key baked into the running firmware. Only if the firmware is considered valid, the bootloader configuration is altered to boot into the new ROM slot and the device is rebooted.
