C.3 Message prioritisation over STa, SWm and SWa

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

This clause gives an analysis of possible behaviours of the trusted non 3GPP access network over STa, of the untrusted non 3GPP access network over SWa and of the ePDG over SWm regarding message prioritisation as guidance and for an informative purpose.

In the rest of the clause, a reacting node identifies:

– a trusted non 3GPP access network over STa;

– an untrusted non 3GPP access network over SWa;

– an ePDG over SWm.

As applying Diameter overload control over SWm and SWa may overlap, this is an operator policy to apply Diameter overload control either on SWm or on SWa or on both.

When the 3GPP AAA server is overloaded, the reacting node will receive overload reports from the 3GPP AAA server requesting a reduction of requests sent by the reacting node. This will apply to DER, STR (and also AAR for STa and SWm) commands.

The reacting node can consider some messages with a lower or a higher priority; lower priority messages will be candidates for throttling before higher priority messages. The reacting node can take into account the following considerations:

– 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 reacting node has to apply traffic reduction;

– if the user has no existing context in the reacting node and the user is not establishing a PDN connection for emergency services, the reacting node gives a lower priority to authentication and authorisation procedures, so to DER commands. This avoids adding new users on the 3GPP AAA server. There is nevertheless a consequence in limiting an inter RAT mobility between non 3GPP accesses when such a DER is throttled, this is considered as acceptable;

– if the initial request was not throttled, the subsequent requests have a higher priority, otherwise it would mean that the initial request processed by the (overloaded) 3GPP AAA server has been useless. An example is the call flow described in Annex A.2 for STa;

– If the user has an existing context in the reacting node, new requests have a higher priority so to maintain the service to the user;

– DER Requests (and also AAR for STa) resulting from the reception of a re-authentication re-authorisation procedure (or also re-authorisation procedure over STa) from the 3GPP AAA server have a higher priority, so to maintain the service to the user;

– The trusted non 3GPP access network has the possibility (see 5.1.2.3.1clause), at any time, to send a AAR command for check if there is any modification in the user authorization parameters previously provided by the 3GPP AAA Server. The trusted non 3GPP access network may defer such AAR requests to diminish the traffic without impacting the service to the user;

– Session termination procedures initiated by the reacting node may have a higher priority as releasing resources in the 3GPP AAA server.