Add details about D-Bus architecture (container to host).

doc/mgmt/Docker to Host communication.md

@@ -94,9 +94,13 @@ All deployments
 
 ## 2.2 Functional Description
 
-This feature enables applications such as image management, ZTP, etc. to issue
-requests to the host to perform actions such as image install, ZTP
-enable/disable, etc.
+This feature enables management applications to issue
+requests to the host to perform actions such as:
+* image install / upgrade
+* ZTP enable/disable
+* initiate reboot and warm reboot using existing scripts
+* create show-tech tar file using existing show-tech script
+* config save/reload using existing scripts
 
 # 3 Design
 ## 3.1 Overview
@@ -110,6 +114,20 @@ the host, and either wait for the response (if the request was synchronous), or
 receive a channel and wait for the request to return the response on the
 channel (asynchronous request).
 
+The architecture of a D-Bus host service in a SONiC environment is illustrated in the diagram below:
+![](images/docker-to-host-services-architecture.jpg)
+
+Note. The Linux D-Bus implementation uses Unix domain sockets for client to D-Bus service communications. 
+All containers that use D-Bus services will bind mount 
+(-v /var/run/dbus:/var/run/dbus:rw) the host directory where D-Bus service sockets are created.
+
+D-Bus provides a reliable communication channel between client (SONiC management container) and service (native host OS) – all actions are acknowledged and can provide return values. It should be noted that acknowledgements are important for operations such as “image upgrade” or “config-save”. In addition, D-Bus methods can return values of many types – not just ACKs. For instance, they can return strings, useful to return the output of a command.
+
+
+### 3.1.1 Security of D-Bus Communications
+In addition to standard Linux security mechanisms for file/Unix socket access rights (read/write), D-Bus provides a separate security layer, using the D-Bus service configuration files.
+This allows finer grain access control to D-Bus objects and methods.
+
 ## 3.2 DB Changes
 ### 3.2.1 CONFIG DB
 N/A