Section: 4.3.7.1a.2 GTP-C Overload Control
Content: GTP-C Overload Control feature is an optional feature. Nodes using GTP control plane signalling may support communication of Overload Control Information in order to mitigate overload situation for the overloaded node through actions taken by the peer node(s). This feature is supported over S4, S11, S5 and S8 interfaces via GTPv2 control plane protocol. A GTP-C node is considered to be in overload when it is operating over its nominal capacity resulting in diminished performance (including impacts to handling of incoming and outgoing traffic). Overload Control Information reflects an indication of when the originating node has reached such situation. This information, when transmitted between GTP-C nodes may be used to reduce and/or throttle the amount of GTP-C signalling traffic between these nodes. As such, the Overload Control Information provides guidance to the receiving node to decide actions which leads to mitigation towards the sender of the information. NOTE 1: How a node determines its Overload Control Information is implementation dependent. The Overload Control Information may convey information regarding the node itself and/or regarding specific APN(s) status. In order to mitigate overload, - it shall be possible to signal control information about the overload of a GTP-C node (e.g. S-GW, P-GW); - the PDN GW may detect overload for certain APNs, e.g. based on Diameter overload indication received from a PCRF or from an external AAA server, or e.g. based on shortage of resources for an APN (IP address pool). It shall be possible to signal appropriate control information about the APN status in addition to the mechanism described in clause 4.3.7.5. For a given APN, the PDN GW shall either activate the congestion control by conveying the Overload Control Information at APN level or by conveying the "PDN GW back-off time" (as specified in clause 4.3.7.5), but not both at the same time, as specified in more detail in clause 12.3.8 of TS 29.274[ 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 ]  [43]. GTP-C Overload Control feature allows the MME/SGSN to send its Overload Control Information to the PDN GW via Serving GW. GTP-C Overload Control feature allows the Serving GW to send its Overload Control Information to the MME/SGSN and P-GW. GTP-C Overload Control feature also allows the PDN GW to send its Overload Control Information to the MME/SGSN via a Serving GW. GTP-C overload Control feature should continue to allow for preferential treatment of priority users (eMPS) and emergency services as per existing specifications. An MME/SGSN may during ESM and EMM procedures apply certain restrictions towards GWs (Serving GW and/or PDN GW as applicable) that have indicated overload, e.g.: - reject EPS Session Management requests from the UE (e.g. PDN Connectivity, Bearer Resource Allocation or Bearer Resource Modification Requests) with a Session Management back-off timer as described in clause 4.3.7.4.2; - reject Mobility Management signalling requests from UEs (such as Attach, Detach, Service Request, Tracking Area Update) with a Mobility Management back-off timer (e.g. reject Service Request requiring to activate user plane bearers in an overloaded SGW) as described in clause 4.3.7.4.2; - reject or accept requests for data transmission via Control Plane CIoT EPS Optimisation from UEs (e.g. Control Plane Service Request and ESM Data Transport) with a Control Plane data back-off timer as described in clause 4.3.7.4.2.7; - may reduce/throttle messages towards the GWs indicating overload status; - other implementation specific mechanisms, which are outside the scope of 3GPP specifications. A PDN GW may take the following actions for MME/SGSN which have indicated overload: - Limit or completely block non-GBR dedicated bearer establishment; - Limit or completely block all Dedicated Bearer establishments or modification, except QCI=1 bearers; - Limit or completely block all Dedicated Bearer establishments, including the QCI=1 bearers; - other implementation specific mechanisms, which are outside the scope of 3GPP specifications. A node supporting GTP-C Overload Control feature sends Overload Control Information in any GTP control plane request or response message such that exchange of Overload Control Information does not trigger extra signalling. The computation and transfer of the Overload Control Information shall not add significant additional load to the node itself and to its corresponding peer nodes. The calculation of Overload Control Information should not severely impact the resource utilization of the node. Based on local policies/configuration, a GTP-C node may support Overload Control feature and act upon or ignore Overload Control Information in the VPLMN when received from HPLMN and in the HPLMN when received from VPLMN. When this feature is supported, a GTP-C node may decide to send different values of Overload Control Information on inter-network (roaming) and on intra-network (non-roaming) interfaces based on local policies/configuration. NOTE 2: Refer to clause 12 of TS 29.274[ 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 ]  [43] for the details, such as exact format of the Overload Control Information, mechanisms to discover the support of the feature by the peer node, interfaces for which this feature is applicable, APN level overload control, etc.