C.2 Message prioritization over SWx

29.2733GPP3GPP EPS AAA interfacesEvolved Packet System (EPS)Release 18TS

This clause gives an analysis of possible behaviours of the 3GPP AAA server regarding message prioritisation as guidance and for an informative purpose.

When the HSS is overloaded, the 3GPP AAA server will receive overload reports from the HSS requesting a reduction of the requests sent by the 3GPP AAA server. This will apply to MAR and SAR requests.

The 3GPP AAA server can consider some messages with a lower or a higher priority; lower priority messages will be candidates for throttling before higher priority messages.

The 3GPP AAA server can take into account if it has already registered the user:

– Diameter requests related to PDN connections for emergency services have the highest priority. Depending on regional/national requirements and network operator policy, these Diameter requests are the last to be throttled, when the 3GPP AAA Server has to apply traffic reduction;

– if the user is not already registered in the 3GPP AAA server and the user is not establishing a PDN connection for emergency services, the 3GPP AAA server gives a lower priority to the MAR command to be sent to the HSS. This will correspond to the following cases:

– the user is not registered for non 3GPP access in the HSS and is doing an attach on a non 3GPP access;

– the user is not registered for non 3GPP access in the HSS and is doing an handover from a 3GPP access; if the MAR command is throttled, the PDN connection will be then maintained in the 3GPP access;

– the user is registered in the HSS but with another 3GPP AAA server, apart a restoration case (see further), this relates to a new session. There is nevertheless a limitation for an inter RAT mobility between non 3GPP accesses when such a MAR is throttled, this is considered as acceptable;

– after a not throttled and successful MAR, the subsequent requests (SARs) have a higher priority, otherwise it would mean that the initial MAR command processed by the (overloaded) HSS has been useless. An example is the call flow described in Annex C;

– if the user is registered in the 3GPP AAA server, the 3GPP AAA server gives a higher priority to the MAR or SAR commands to be sent to the HSS as to maintain the service to the user; nevertheless, if a SAR indicates a PGW_UPDATE and is related to an additional Diameter session over S6b for the user, so to establish an additional PDN connection, a lower priority may be given to this SAR;

– in a restoration procedure due to the failure of the old 3GPP AAA server, the new 3GPP AAA server is informed of such a restoration procedure by the presence of the AAA-Failure-Indication AVP in the request received from the access. As restoration procedures may be a source of a high signalling traffic and contribute to a HSS overload, the new 3GPP AAA server may give a lower priority to such MAR messages;

– the deregistration of the user by the 3GPP AAA server has a higher priority as releasing resources in the HSS.