Load and Overload Control Over N4/Sx Interface

Feature Summary and Revision History

Summary Data

Applicable Product(s) or Functional Area UPF
Applicable Platform(s)

VPC-SI

SMI
Feature Default Setting Disabled – Configuration Required
Related Changes in this Release Not Applicable
Related Documentation Not Applicable

Revision History

Table 1. Feature History
Feature Name Release Information

Description
Reporting of Load Control over N4 Interface 2020.02.0

This feature enables load control on UPF. This feature allows UPF to send its load information to the SMF in order to balance PFCP session load across the UPFs as per their effective load.

Default Setting: Disabled-Configuration required to enable.

Feature Description

Load control enables the UPF to send its load information to the CP-F in order to balance PFCP session load across the UPF according to their effective load. The load information reflects the operating status of the resources of the UPF. Load control allows for better balancing of the PFCP session load to prevent overload.

NOTE: Overload mitigation actions are not triggered even if the UPF reports a high load.

Supported IE and Messages

To report Load Control Information (LCI) to the SMF, 3GPP specification has defined the following IEs:

  • Load Control Information IE – The load control Information IE is as follows: It contains the sequence number IE and load metric IE. This IE is sent in Session Establishment Response, Session Modification Response, Session Deletion Response, and Session Report Request messages sent from UPF.

  • Sequence Number IE – The Sequence Number IE contains an Unsigned32 binary integer value. The Load Control Sequence Number increases whenever the load control information changes.

  • Load Metric IE – The Load Metric parameter indicates the current load level of the originating node. The computation of the Load Metric happens at the implementation basis. The node considers the various aspects, such as:

    • The used capacity of the UPF

    • The load in the node. For example, memory or CPU usage in relationship to the total memory or CPU available, and so on.

    The Metric IE encoding is as follows: It indicates a percentage and takes binary coded integer values from and including 0 up to and including 100. Considers the other values as 0.

How Load and Overload Control Works

CP-F sends the load and overload features as supported in the Sx Association Request and Response message to UPF. Based on this information, UPF decides whether to send the load or overload information toward CP-F peer.

Load and Overload Reporting to CP-F

UPF sends load or overload information to CP-F through N4 messages.

If the UPF is in...

It sends...

Load State

LCI IE

Overload State

OCI IE

CP-F parses all the incoming N4 messages and N4 responses for LCI and OCI IEs. Whenever CP-F receives LCI IE or OCI IE from a peer node, CP-F learns about the load and overload parameter of that UPF node.

For more details on how CP-F node handles the LCI and OCI IEs respectively, see the following guides:

  • UCC 5G Session Management Function Configuration and Administration Guide, Release 2024.04.

  • UCC 5G cnSGWc Configuration and Administration Guide, Release 2024.04.

Load Information Reporting to CP-F

The UPF sends its load control information to reflect the operating status of its resources at the node level. It allows the CP-F to use this information to augment the UPF selection procedures. The load control information is piggybacked in PFCP request or response messages such that the exchange of load control information does not trigger extra signaling.

Load Control Information IE can be present in the following messages from UPF to CP-F:

  • PFCP Session Establishment Response

  • PFCP Session Modification Response

  • PFCP Session Deletion Response

  • PFCP Session Report Request

Configuring the Max Sessions

Based on various deployment scenarios, if you do not want to load UPF to its maximum capacity in terms of the count of sessions, especially, given that in 5G a single user-session can go up to 5 Gbps. To alter the max session supported in UPF, a CLI command is available under the User Plane Service configuration. It allows the operator to configure the required number of max-sessions that are supported on the UPF so that the SMF can load balance the sessions across the UPF. The following is a sample configuration: 


configure 
   context context_name 
      user-plane-service user_plane_service 
         load-control capacity session_value 
         end

NOTES:

  • session_value must be an integer in the range of 1 through the maximum value that is allowed in the platform.

  • The use of this configuration is only for the LCI reporting to SMF, and not for any other purpose, such as congestion control, and so on.

The following is an example configuration:


configure 
   context ingress 
      user-plane-service ups1 
         load-control capacity 2500 
         end

Show Command Support

The output of the show command to display the User Plane Service includes the value of configured max sessions.

show user-plane-service all 


Service name                      : user-plane-service 
Service-Id                        : 4 
Context                           : ingress 
Status                            : STARTED 
UPF Ingress GTPU Service          : n3-gtpu-service 
SGW Ingress GTPU Service          : Not defined 
SGW Egress GTPU Service           : Not defined 
Control Plane Tunnel GTPU Service : n4-gtpu-service 
Sx Service                        : n4-sx 
Control Plane Group               : g1 
Load Control Parameters                       
                Capacity          : 1000