Section: 12.3.13 Implicit overload control mechanisms
Content: Implicit overload control mechanisms are mechanisms used between GTP-C entities when GTP-C overload control is not supported or not enabled between them, e.g. across PLMN boundary based on operator's policy, to help reducing the overload at the overloaded node: - a GTP-C entity which decides to not process an incoming request message due to overload should still send a reject response message, if possible, indicating the temporary unavailability of the resources, e.g. No resources available; otherwise the GTP-C entity may drop the incoming request message. NOTE: Without a response message, the source GTP-C entity cannot determine whether the request did not reach the target GTP-C entity due to a network error or whether the target GTP-C entity was in overload and not able to process the request and send a response message. This will cause the source GTP-C entity to retransmit the request message and hence will increase further the overload at the target node. - a GTP-C entity in overload may support messages throttling as a self protection mechanism and may apply message prioritization as described in clause 12.3.9.3 when selecting the incoming request messages to be throttled; - based on the number and rate of reject responses indicating temporary unavailability of resources, e.g. No resources available, a source GTP-C entity should try to assess the overload level of the target GTP-C entity and apply correspondingly message throttling as described in clause 12.3.9 to reduce the amount of traffic sent towards the overloaded GTP-C entity.