One of our customers is an MVNE and they reached out the other day with an issue.

They were turning up a new PGW and they’d see Create Session Request, everything looked OK, it’d get a response, but then in the GUI of the PGW-C they’d see the session drop.

The logs showed the newly setup session dropping shortly after being setup.

Have a look at the screenshot and see if you can work out why:

So what’s going on, and why is the PGW-C deleting sessions?

The initial reaction from the customer was there’s something up with the PGW, but the answer is bit more nuanced.

Per the specs, you can’t have two PDN sessions for the same subscriber (IMSI) on the same APN (DNN).

So if 50557000000001 is connected to the PGW-C on the internet APN, if I send another Create Session Request to the same PGW-C, it deletes the old session, before starting the new one.

In this case, the MVNE it was going through was dropping the Create Session Response, so it never made it back to the MNO, and then the MME in the MNO sent it again.

Joys of GTPv2-C being UDP based and connectionless!