GTP tunnel seems to exist, but No encapsulation when using libgtpnl

[Papa Tana]

Hi All,

This is my first post, I have a similar problem to the topic "Network
is unreachable error for GTP interface".

I followed all instructions, installed all needed dependencies,
upgrade my kernel version to 4.9.0-6 as stated in
https://osmocom.org/projects/openggsn/wiki/Kernel_GTP

Unfortunately, no GTP T-PDU encapsulation for my packets.

## Tunnel listing is OK
root at routeurA:/home/bob/libgtpnl/tools# ./gtp-tunnel list
version 1 tei 200/100 ms_addr 172.23.10.163 sgsn_addr 10.11.12.14

## I have upgraded my Kernel version to 4.9.0-6 as stated in
https://osmocom.org/projects/openggsn/wiki/Kernel_GTP
At the time of writing (2018-04-26) of this wiki, below listed
distributions have support of GTP kernels :

    Debian
        Debian 9 "stretch" (kernel 4.9.0-6)

root at routeurA:/home/bob/libgtpnl/tools# uname -r
4.9.0-6-amd64

## GTP module
root at routeurA:/home/bob/libgtpnl/tools# lsmod | grep gtp
gtp                    28672  0
udp_tunnel             16384  1 gtp

## ping remote ms_addr is not ok
root at routeurA:/home/bob/libgtpnl/tools# ping 172.23.10.163
PING 172.23.10.163 (172.23.10.163) 56(84) bytes of data.
^C
--- 172.23.10.163 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms

## remove the route using "gtpa" device
root at routeurA:/home/bob/libgtpnl/tools# ip route del 172.23.10.163/32 dev gtpa

## add new route using normal interface
root at routeurA:/home/bob/libgtpnl/tools# ip route add 172.23.10.163/32
via 10.11.12.14

## ping is OK
root at routeurA:/home/bob/libgtpnl/tools# ping 172.23.10.163
PING 172.23.10.163 (172.23.10.163) 56(84) bytes of data.
64 bytes from 172.23.10.163: icmp_seq=1 ttl=64 time=0.592 ms
64 bytes from 172.23.10.163: icmp_seq=2 ttl=64 time=0.713 ms

## remove again the route
root at routeurA:/home/bob/libgtpnl/tools# ip route del 172.23.10.163/32

## switch it to "gtpa" device
root at routeurA:/home/bob/libgtpnl/tools# ip route add 172.23.10.163/32 dev gtpa
root at routeurA:/home/bob/libgtpnl/tools# ping 172.23.10.163
PING 172.23.10.163 (172.23.10.163) 56(84) bytes of data.
^C

## tcpdump shows ICMP between the 2 ms_addr, no encapsulation at all

Am I missing something somewhere?

FYI, I'm not using openggsn or ergw, I have developped my small
userspace GTP-C ready, but I'm stuck at GTP-U side.

Thanks in advance,

Best Regards,