C.2 S6a/S6d interfaces

29.2723GPPEvolved Packet System (EPS)Mobility Management Entity (MME) and Serving GPRS Support Node (SGSN) related interfaces based on Diameter protocolRelease 17TS

C.2.1 General

Diameter overload control mechanism is an optional feature.

It is recommended to make use of IETF RFC 7683 [50] on the S6a/S6d interfaces where, when applied, the MME or the SGSN shall behave as reacting nodes and the HSS as a reporting node.

C.2.2 HSS behaviour

The HSS requests traffic reduction from its clients when the HSS is in an overload situation, including OC-OLR AVP in answer commands as described in IETF RFC 7683 [50].

The HSS identifies that it is in an overload situation by implementation specific means. For example, the HSS may take into account the traffic over the S6a/d interfaces or other interfaces, the level of usage of internal resources (CPU, memory), the access to external resources, etc.

The HSS determines the specific contents of OC-OLR AVP in overload reports and the HSS decides when to send OC-OLR AVPs by implementation specific means.

C.2.3 MME/SGSN behaviour

The MME/SGSN applies required traffic reduction received in answer commands to subsequent applicable requests, as per RFC 7683 [50].

Requested traffic reduction is achieved by the MME/SGSN by implementation specific means. For example, it may implement message throttling with prioritization or a message retaining mechanism for operations that can be postponed.

Diameter requests related to priority traffic (e.g. MPS as identified by the MME/SGSN through access procedures) and emergency have the highest priority. Depending on regional/national regulatory and operator policies, these Diameter requests shall be the last to be throttled, when the MME/SGSN has to apply traffic reduction. Relative priority amongst various priority traffic (e.g. MPS) and emergency traffic is subject to regional/national regulatory and operator policies.

As a result of the need to throttle traffic, the MME or SGSN may reject Attach, Tracking Area Update or Service Requests initiated by UEs. The possible NAS causes are described in the Annex A and B.

Annex D (Informative):
Diameter overload control node behaviour