Dear Osmocom Community,

I hope this message finds you well.

I am currently setting up a testing environment to evaluate the performance of my Packet Gateway (PGW) in terms of maximum simultaneous active sessions and bandwidth.

The architecture of my test setup includes: VM osmo-ttcn3: Hosts the TTCN-3 framework, which initiates GTP-C session creation and integrates with the osmo-uecups solution to simulate user sessions by sending ICMP echo requests (pings) to public IP addresses (e.g., 8.8.8.8).​ VM osmo-uecups: Manages network namespaces (netns), each representing a simulated user session.​ VM pgw: My Packet Gateway, handling the routing and forwarding of packets between the simulated users and the external network via the GTP-U protocol.​

Issue Encountered: While using the TTCN-3/PGW test suite scripts that execute ping tests inside each created GTP tunnel, I observed the following on the osmo-uecups VM:​ ICMP echo requests are successfully sent from the netns and are visible in tcpdump captures both inside each netns and on the physical interfaces of the osmo-uecups VM.​ ICMP echo replies are sent back by my PGW for each UE session, and I can see them when capturing traces on the physical interface of the osmo-uecups VM. However, these ICMP echo replies are not injected into each netns. When capturing tcpdump inside each netns, I can see the ICMP echo requests but no corresponding replies.​ This behavior results in ping test failures in the TTCN-3 test suite, as the echo replies do not reach the originating netns.​

Additional Context: Initially, when running the test suite, I encountered the following error in the osmo-uecups-daemon logs:​ DEP NOTICE X.X.X.X:2152: Unexpected GTP Flags: 0x32 (gtp_endpoint.c:59)​ Upon investigation, I found that this was due to my PGW including a sequence number in the GTP-U header, resulting in a flag value of 0x32.​ To address this, I manually modified the gtp_endpoint.c file to accept this flag. After this adjustment, the "Unexpected GTP Flags: 0x32" message no longer appears.​ However, despite resolving this issue, the ping tests continue to fail because the ICMP echo replies are not being injected back into the respective netns.​ Additionally, I now receive the following message in the osmo-uecups-daemon log:​ DEP NOTICE X.X.X.X:2152: Short GTP Message: 98 < len=96 (gtp_endpoint.c:74)​

I would greatly appreciate any guidance or suggestions you can provide to help resolve this issue.​

Please find below the configuration changes I made in the gtp_endpoint.c file:: ************************************************************************************** /* check GTP header contents */ if (gtph->flags != 0x30 && gtph->flags != 0x32) { LOGEP(ep, LOGL_NOTICE, "Unexpected GTP Flags: 0x%02x\n", gtph->flags); continue; } ////////////////////////////// unsigned int hdr_len = sizeof(*gtph); if (gtph->flags & 0x02) { hdr_len += 2; // Si le flag Sequence est activé }

/* Vérifie que la longueur totale attendue (header + payload) est présente */ if (hdr_len + ntohs(gtph->length) > nread) { LOGEP(ep, LOGL_NOTICE, "Shotr GTP Message: %u < len=%d\n", hdr_len + ntohs(gtph->length), nread); continue; }

/* Transmission du payload au TUN device à partir de l'offset hdr_len */ rc = write(outfd, buffer+hdr_len, ntohs(gtph->length)); ***********************************************************************************

Thank you for your time and assistance.

Best regards, Imen Boufares