Message-Priority Header for PFCP
When the message throttling is performed:
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PFCP requests related to priority traffic (that is, eMPS as described in 3GPP TS 22.153) and emergency have the highest priority. Depending on regional/national requirements and network operator policy, these PFCP requests are the last to be throttled when applying traffic reduction. Throttling exempts the priority traffic due to PFCP overload control up to the point where the requested traffic reduction cannot be achieved without throttling the priority traffic.
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For other types of sessions, the message throttling considers the relative priority of the messages so that the messages with low priority are first considered for the throttling. The relative priority of the messages is derived from the relative priority of the procedure for which the message is being sent or derived from the session parameters such as APN and ARP.
An overloaded node (UPF, SMF) may apply these message prioritization schemes when handling incoming initial messages during an overloaded condition, as part of a self-protection mechanism. Incoming messages are handled during an overloaded condition based on the relative PFCP message priority signaled in the PFCP header.
A PFCP entity determines whether to set and use the message priority in PFCP signalling, based on operator policy. A sending PFCP entity determines the relative message priority to signal in the message which are derived from the session parameters such as APN and ARP. If the message affects multiple bearers, the relative message priority is determined considering the highest priority ARP among all the bearers. A PFCP entity must set the same message priority in a Response message as received in the corresponding Request message.
For incoming PFCP messages that do not have a message priority in the PFCP header, the receiving PFCP entity:
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Applies a default priority if the incoming message is a Request message.
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Applies the message priority sent in the Request message if the incoming message is a Response message.
The SMF and UPF functions in the network homogenously support this feature; otherwise an overloaded node will process the Request messages received from the non-supporting nodes according to the default priority and Request messages received from supporting nodes will be processed according to the message priority signalled in the PFCP message.