Feature/bacnet secure connect hub (#818)

* Added BACnet Secure Connect datalink.

* Added BACnet/SC hub application
---------

Co-authored-by: Kirill Neznamov <kirill.neznamov@dsr-corporation.com>
Co-authored-by: Mikhail Antropov <michail.antropov@dsr-corporation.com>
Co-authored-by: Ondřej Hruška <ondra@ondrovo.com>
Co-authored-by: Patrick Grimm <patrick@lunatiki.de>

src/bacnet/datalink/bsc/README.md

@@ -0,0 +1,54 @@
+This readme explains how to enable support of BACNet Secure Connect datalink
+(ANNEX AB – BACnet Secure Connect in standard), how to set up the building
+environment and clarifies some important moments regarding implementation
+of that functionality.
+
+By default the support of that functionality is turned off, to enable it
+you need to set option BACDL_BSC=ON in CMakeLists.txt if cmake build system
+is used or set BACDL_BSC=1 define in a case if using makefiles.
+
+BACNet/SC standard uses websockets as a transport layer, so Windows/Linux/BSD
+implementations use libwebsocket library. Implementation for Zephyr uses
+native websocket API for the client side and mongoose library for the
+server side. Websocket layer which is built on the top of libwebsockets
+uses 1 service thread per websocket server instance and 1 service thread
+per 1 websocket client instance. As a result, libwebsocket must be built
+with LWS_MAX_SMP > 1, otherwise rarely crashes may ocure in the application
+which uses bacnet stack library. Recommended value for that define is 32.
+You should note, that by default libwebsocket is built with LWS_MAX_SMP=1,
+so a packet manager like vcpkg and apt can have that lib built with
+LWS_MAX_SMP=1, which can lead to unstable work. It's better to build
+libwebsockets from recent stable sources for your platform, corresponded
+examples of how to do that you can find in .github/workflows/bsc-tests-platform.yml,
+(for example bsc-tests-linux.yml).
+
+In order to build bacnet stack library for linux, user needs to install
+libconfig-dev, libcap-dev and libssl-dev to the system. Most easiest way to do
+that is to use Advanced Packaging Tool (APT), check bsc-test-linux.yml.
+
+For MacOSX build user must install brew packet manager (https://brew.sh),
+then install openssl using brew then build libwebsocket, check
+.github/workflows/bsc-tests-macosx.yml.
+
+Windows build may be a challenge because libwebsocket depends on libpthreads
+which not so easy to find and build on windows, refer to the libwebsocket
+build instruction https://libwebsockets.org/lws-api-doc-master/html/md_README_8build.html.
+Using of vcpkg packet manager can save a lot of time, but you must ensure that
+installed libwebsocket library is compiled with LWS_MAX_SMP > 1.
+A good example how to use vcpkg packet manager with custom build of
+libwebsocket can be found in .github/workflows/bsc-tests-windows.yml.
+
+One moment is still needed to be improved. The current implementation processes
+dataflow between BACNET/SC nodes ignoring max_bvlc_size and max_npdu_size
+params in connect request and connect accept packets as that is not
+explicitly explained in standard. As a result if a node which has max_npdu_size
+less than remote peer can drop received packet if it does not fit into it's internal
+buffer which depends on BVLC_SC_NPDU_SIZE_CONF parameter. In opposite, a node
+can send a PDU more than max_npdu_size which may also lead to the drop of PDU
+on remote peer side.
+
+The current implementation does not support Certificate_Signing_Request_File
+property of BACNET/SC netport and properties Operational_Certificate_File
+and Issuer_Certificate_Files related to certificates are readonly and can't
+be changed remotely. So, management of device certificates is out of the scope
+of current BACNet/SC implementation.