First of all, the official location of the FreeRTOS+TCP library is here, and the IPv6/multi branch can be found here.
You will find several clones on Github like mine, but those are all running more or less behind.
Beside these 2 locations there are LTS releases ( Long Term Support ).
When I google the missing function xApplicationGetRandomNumber(), it brings to Working Example for FreeRTOS+TCP and STM32F7. It describes well what the function should do.
The following function ( “application hook” ) probably also lacks some description:
/* Create an initial sequence number (ISN), to be used for a TCP connection
* between two IP-addresses / port numbers.
* In most cases it will be sufficient to call xApplicationGetRandomNumber.
*/
uint32_t ulApplicationGetNextSequenceNumber( uint32_t ulSourceAddress,
uint16_t usSourcePort,
uint32_t ulDestinationAddress,
uint16_t usDestinationPort );
It is asking for a random an initial sequence number. In the simplest case you just return a random number that you obtained by calling xApplicationGetRandomNumber().
I have written documentation about these functions but apparently it is still “in the pipeline”.