UPF not forwarding the GTP packets sent using tcpreplay


I am trying to send GTP packets using tcpreplay from UERANSIM gNB (VM) to the Data Network (VM) through the UPF (VM). However, the UPF is not forwarding the traffic to the DN. The gNB, DN and the UPF are all running on different VMs.

The pcap file I am trying to replay is captured on the N3 interface of the gNB VM during the ping traffic sent from the UE to the DN. The UPF successfully forwards the normal ping traffic sent from the UE using the commad ping -I uesimtun0 However, replaying the exact same captured pcap file using tcpreplay on N3 of the gNB are not forwarded to the DN by the UPF.

I did some troubleshooting on the UPF using tcpdump and I can see the packets reach the N3 interface of the UPF conntected to the gNB, however, using tcpdump on “upfgtp” interface do not show any packets forwarded to the N6 interface connected to the DN.

A similar unanswered topic is here: GTP data not forwarded by UPF, so I am asking it again.

My main goal is to replay pcap traces from the UE to the DN for my experiments, however, tcpreplay is not working on UERANSIM UE interface due to “Unsupported physical layer type 0xfffe on uesimtun0”. This is why, as a workaround, I am trying to replay a GTP traffic from the N3 interface of the gNB.

Please let me know if this is a limitation of Free5GC UPF or is there another way to replay the traffic. Or if there is any specific configuration I should do on the UPF to allow the forwarding of pcap files or on the UE to allow tcpreplay without “Unsupported physical layer” warning.


Could you please provide

  1. the versions of free5gc, gtp5g, and UERANSIM that you are using,
  2. free5gc & UERANSIM configuration
  3. free5gc & UERANSIM log files about all error and warning,
    so that we can track down the cause of this error? Thank you.

Starting with version 3.4.0, free5gc supports charging. When the charging cost for a UE is depleted, all traffic for that UE will be blocked. Please check if this is the cause of the issue.