C.4 Message prioritization over S6b

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

This clause gives an analysis of possible behaviours of the PDN-GW regarding message prioritisation as guidance and for an informative purpose.

When the 3GPP AAA server is overloaded, the PDN-GW will receive overload reports from the 3GPP AAA server requesting a reduction of requests sent by the PDN-GW. This will apply to AAR and STR commands.

The PDN-GW can consider some messages with a lower or a higher priority; lower priority messages will be candidates for throttling before higher priority messages.

Following considerations can be taken into account:

– An important point to consider is that UEs accessing the PDN-GW via S2a or S2b have already been previously authorized to use the non 3GPP access (i.e. via STa or SWm or SWa). The AAR command over S6b following this initial authorisation has a higher priority, otherwise, if the AAR is throttled, it would mean that the initial authorisation procedure (and its process by the 3GPP AAA server) has been useless;

– An exception is in the Multi Connection Mode (MCM) (described in 3GPP TS 23.402 [3]), where after a first PDN connection being established, the UE requests the establishment of additional PDN connections. As the priority, when overload in the 3GPP AAA server, is to maintain the existing service in preference to setting up new services for the user, such AAR commands over S6b may have a lower priority;

– Traffic reduction over the STa or SWm or SWa interfaces with the throttling of new authentication authorisation procedures, results in diminishing requests for new PDN connections to the PDN-GW and diminishing the traffic over S6b;

– regarding the 3GGP AAA Server behaviour, it is better to request traffic reduction over the STa or SWm or SWa interfaces than over the S6b interface, so following the principle to request the traffic reduction as early and as close as possible to the traffic source (i.e. the UE);

– AAR Requests resulting from the reception of a re-authorisation procedure from the 3GPP AAA server have a higher priority, so to maintain the service to the user;

– the session termination procedures over S6b initiated by the PDN-GW resulting from a UE request may have a higher priority as releasing resources in the 3GPP AAA server.

Annex D (normative):
Diameter message priority mechanism