Hi Avatli,

can you please explain what exactly you are trying to do?

What is your overall goal? Which software do you want to use as GGSN/P-GW?

In case you assume that GTP is symmetric and that you can simply establish GTP tunnels with some command line tunnels between two linux machines: That's not the case.

GTP exists to dynamically manage [lots of] per-subscriber tunnels, all of which are represented within one GTP "link" by the kernel GTP module.

You always need a userspace application implementing the control plane instance (GTP-C v1 for GGSN, v2 for P-GW). And even with that, it always only implements the GGSN / P-GW side and not the SGSN / S-GW side.

Hi,

http://lists.osmocom.org/pipermail/osmocom-net-gprs/2020-July/001151.html (Sorry I could not respond to the original email to show this in thread)

I have confirmed that "ping: sendmsg: Network is unreachable" could be reproduced on Ubuntu 18.04.4 with Kernel 4.15.0-111-generic, while it does not (could ping through gtp tunnel) on 4.15.0-76-generic.

To be more specific, the issue is GTP packet would not be created even if I send packet with destination IP address matching <ms-addr> to gtp link using 4.15.0-111-generic. (and most likely on later versions) However, when I switch kernel version to 4.15.0-76-generic on the same host, it will generate GTP packet with exactly the same rule configured to the GTP link.

I have created GTP tunnel using command below.

./gtp-link add gtp1 ./gtp-tunnel add gtp1 v1 200 100 172.99.0.2 172.0.0.2

and sent packet with dst IP 172.99.0.2, which is same as the ms-addr in the above rule.

I have monitored veth1 using "tcpdump -n -i veth1" and GTP packet will be captured on 4.15.0-76-generic, but not on 4.15.0-111-generic.

Can anyone think of the difference between the two kernel or gtp module version?

### FYI: list of commands to create gtp link, tunnel rule and routing entry to route packet to gtp link.

# Create veth pair so GTP packet can flow within the host. ip link add veth1 type veth peer name veth2 ip addr add 172.0.0.1/24 dev veth1 ip link set veth1 up ip addr add 172.99.0.1/32 dev lo

# Create gtp link, gtp1 ./gtp-link add gtp1 WARNING: attaching dummy socket descriptors. Keep this process running for testing purposes.

-- open new terminal --

# Create tunnel rule from netns default to ns2 ./gtp-tunnel add gtp1 v1 200 100 172.99.0.2 172.0.0.2 ./gtp-tunnel list version 1 tei 200/100 ms_addr 172.99.0.2 sgsn_addr 172.0.0.2

# Add route to navigte packets to 172.99.0.2 (ms-addr) via gtp1 ip route add 172.99.0.2/32 dev gtp1

# Create netns ns2 to receive GTP packets. ip netns add ns2 ip link set veth2 netns ns2 ip netns exec ns2 ip addr add 172.0.0.2/24 dev veth2 ip netns exec ns2 ip link set veth2 up ip netns exec ns2 ip addr add 172.99.0.2/32 dev lo ip netns exec ns2 ip link set lo up

# Create gtp link, gtp2 ip netns exec ns2 ./gtp-link add gtp2 WARNING: attaching dummy socket descriptors. Keep this process running for testing purposes.

-- open new terminal --

# Create tunnel rule from netns ns2 to default ip netns exec ns2 ./gtp-tunnel add gtp2 v1 100 200 172.99.0.1 172.0.0.1

# Add route to navigte packets to 172.99.0.1 (ms-addr) via gtp2 ip netns exec ns2 ip route add 172.99.0.1/32 dev gtp2

# Send ping ... $ ping 172.99.0.2

# error below will shown on Kernel 4.15.0-111-generic. $ ping 172.99.0.2 PING 172.99.0.2 (172.99.0.2) 56(84) bytes of data. ping: sendmsg: Network is unreachable ping: sendmsg: Network is unreachable

# GTP packet will be generated and ping success on Kernel 4.15.0-76-generic $ ping 172.99.0.2 PING 172.99.0.2 (172.99.0.2) 56(84) bytes of data. 64 bytes from 172.99.0.2: icmp_seq=1 ttl=64 time=0.130 ms 64 bytes from 172.99.0.2: icmp_seq=2 ttl=64 time=0.035 ms

Thanks,