Moving to 4.0.0-rc1/rc2/rc3 from 3.x.x:
--------------------------------

Version 4.0.0-rc1/rc2/rc3 adds new files to support IPv6 functionality, breaking each file into logically separated IPv4 and IPv6 files. The folder structure of FreeRTOS-Plus-TCP remains unchanged. Build separation is added to make the library modular, enabling users to compile and create a low-footprint binary with only the required functionalities. For more details on supported build combinations, see [History.txt](https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/History.txt) .

Some of the APIs have changed which is illustrated in the section below. However, there is a backward compatibility mode provided as well.

Backward Compatibility Mode:
---------------------------
   If you are moving your code from V3.x.x to V4.0.0-rc1/rc2/rc3, then set the "ipconfigIPv4_BACKWARD_COMPATIBLE" macro to 1 in “FreeRTOSIPConfigDefaults.h” to run the code in backward compatible mode.
   The "Existing API's" defined in all the API changes below work only when the backward compatibility mode is enabled.

API changes in 4.0.0-rc1/rc2/rc3:
----------------------

The changes mentioned below uses ipconfigIPv4_BACKWARD_COMPATIBLE flag to differentiate between old API and new API.

Change 1:

   - Existing API: FreeRTOS_IPInit
      - Backward compatibility with the IPv4 FreeRTOS+TCP V3.x.x which only supports single network interface. This can be used for single  interface IPv4 systems.
   - New API: FreeRTOS_IPInit_Multi
      - Supports multiple interfaces. Before calling this function, at least 1 interface and 1 end-point must have been set-up. FreeRTOS_IPInit_Multi() replaces the earlier FreeRTOS_IPInit().

Change 2:

   - Existing API: FreeRTOS_GetAddressConfiguration/FreeRTOS_SetAddressConfiguration
      - Get/Set the address configuration from the global variables initialised during FreeRTOS_IPInit
   - New API: FreeRTOS_GetEndPointConfiguration/FreeRTOS_SetEndPointConfiguration
      - Get/Set the same address configuration from/to the end point

Change 3:

   - Existing API:  FreeRTOS_GetUDPPayloadBuffer
      - Backward compatibility with the IPv4 FreeRTOS+TCP V3.x.x. This can still be used for IPv4 use cases.
   - New API:FreeRTOS_GetUDPPayloadBuffer_Multi
      - A new argument (uint8_t ucIPType) to specify IP type to support both IPv4 and IPv6

Change 4:

   - Existing API: pxFillInterfaceDescriptor
      - It is there for backward compatibility. The function FreeRTOS_IPInit() will call it to initialise the interface and end-point objects
   - New API: prefix_pxFillInterfaceDescriptor
      - where prefix = Network Interface Name
      - E.g pxWinPcap_FillInterfaceDescriptor
      - New function with the same functionality

Change 5:

   - Existing API: vApplicationIPNetworkEventHook
   - New API: vApplicationIPNetworkEventHook_Multi
      - New argument “struct xNetworkInterface * pxNetworkInterface” added.

Change 6:

   - Existing API: xApplicationDHCPHook
   - New API: xApplicationDHCPHook_Multi
      - New argument "struct xNetworkEndPoint * pxEndPoint" added.

Change 7:

   - Existing API: xApplicationDNSQueryHook
   - New API: xApplicationDNSQueryHook_Multi
      - New argument "struct xNetworkEndPoint * pxEndPoint" added.

**NOTE** : We are NOT considering the APIs changes in FreeRTOS_IP_Private.h for backward compatibility as those are not part of published interface.

Running Demos:
-------------
The demos can be found on [this page](https://github.com/FreeRTOS/FreeRTOS/tree/main/FreeRTOS-Plus/Demo).

In all the demos, there is a backward compatibility mode which can be enabled by setting the flag “ipconfigIPv4_BACKWARD_COMPATIBLE” to 1 in the header file “FreeRTOSIPConfigDefaults.h”.
This flag is by default set to zero.

New IPv6 WinSim Demo can be found on [this page](https://github.com/FreeRTOS/FreeRTOS/tree/main/FreeRTOS-Plus/Demo/FreeRTOS_Plus_TCP_IPv6_Demo/IPv6_Multi_WinSim_demo).