https://github.com/open5gs/nextepc/issues/238

Recently we saw Open5Gs’s Update Location Answer response putting the Subscribed-Periodic-RAU-TAU-Timer AVP in the top level and not in the AVP Code 1400 (APN Configuration) Diameter payload from the HSS to the MME.

But what exactly does the Subscribed-Periodic-RAU-TAU-Timer AVP in the Update Location Answer response do?

Folks familiar with EUTRAN might recognise TAU as Tracking Area Update, while RAU is Routing Area Update in GERAN/UTRAN (UMTS).

Periodic tracking area updating is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). The value of timer T3412 is sent by the network to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking areas of the list of tracking areas assigned to the UE, until a new value is received.

Section 5.3.5 of 24301-9b0 (3GPP TS 24.301 V9.11.0)

So the Periodic Tracking Area Update timer simply defines how often the UE should send a Tracking Area Update when stationary (not moving between cells / tracking area lists).