Error in achieving Communication between two esp32 using ipv6

[joshity]

configuration : The two esp32 devices are connect via UART and we are using a SLIP protocol attached to netif for the communiction.
we are running a UDP echo client server and are trying to test the setup.
For ipv6 we are not able to extablish communication bewteen devices but for ipv4 setting its working fine.

we have enabled ipv6 and as well SLAAC but still no result.
only packet that goes out via SLIP to UART is Router solicitaion message which has no use to use as this is a point to point communication.
As per my understanding the neighbour solicitaion/discovery message should be sent out. but there are no trace of them.

when the client tries to send a message to server it shows the host is unreachable.

Please can anyone help me work out what excatly is happaning over here. I am stuct here for almost a month now.
Thank You!!