I am currently working in a project where I was asked to implement a GTP tunnel from client to company servers. 

I have read through osmocom pages and tried to implement the simple network structure. I have seen from Basic testing of openGGSN page:

"     test GTP-U interoperability of the kernel with at least one other implementation, not just kernel-to-kernel (which currently is not supported in the kernel, as it only implements the GGSN/P-GW role)   "

I would not like to implement openGGSN but just linux-kernel-gtp-u implementation between two kernels.

To achieve kernel-to-kernel implementation, I found sgsn patch on the internet:

             

I would like make a tunnel between sgsn and ggsn. I have used libgtpnl tools to create link and then tunnel. I have added a default gateway GTP interface to route table on sgsn side, but the packets cannot reach ggsn side. 

Is there any detailed explanation for a simple bidirectional tunnel setup. If so, it would be so nice to know about.

Thank you for all your efforts,

Fırat