5.21 UE IP address/prefix Allocation and Release

29.2443GPPInterface between the Control Plane and the User Plane nodesRelease 17TS

5.21.1 General

Stage 2 requirements for UE IP address allocation in the EPC and 5GC are specified in clause 5.3.1 of 3GPP TS 23.401 [14], clause 5.8.2.2 of 3GPP TS 23.501 [28] and clause 4.6.2 of 3GPP TS 23.316 [57]. The following types of UE IP addresses may be assigned over N4 or Sxb:

– IPv4 address;

– /64 IPv6 Prefix;

– IPv6 prefix other than default /64, including individual /128 IPv6 address, if the UPF indicates supports of the IP6PL feature (see clause 8.2.25), for IPv6 address allocation using DHCPv6 specified in clause 4.6.2.2 of 3GPP TS 23.316 [57];

– IPv6 prefix shorter than the default /64 prefix for support of IPv6 prefix delegation as specified in clause 4.6.2.3 of 3GPP TS 23.316 [57] and clause 5.14.

More than one UE IP address may be assigned to a PDU session, if the UPF indicates supports of the IP6PL feature (see clause 8.2.25), by provisioning multiple instances of the UE IP Address IE in a same PDI or Traffic Endpoint, or by provisioning multiple PDIs or Traffic Endpoints with a different UE IP Address, where the UE IP addresses may correspond e.g. to:

– multiple /128 IPv6 addresses; or

– an /64 default prefix used for IPv6 stateless autoconfiguration and an IPv6 prefix shorter than the default /64 prefix for IPv6 Prefix Delegation not including the /64 IPv6 Prefix (i.e. when the total IPv6 address space available for the PDU Session cannot be aggregated into one single IPv6 prefix); the IPv6 prefix shorter than the default /64 prefix may be assigned by setting either the IPv6D flag or the IP6PL flag as specified in clause 5.14.

A UE IPv4 address or IPv6 prefix may be allocated by the CP function or the UP function. A given IP address pool shall be controlled by a unique entity (either the SMF/PGW-C or the UPF/PGW-U or an external server).

The support of UE IP address/prefix allocation by the CP function is mandatory. The support of UE IP address/prefix allocation by the UP function is optional. See clause 5.8.2.2 of 3GPP TS 23.501 [28]. A UPF supporting the SSET feature (see clause 5.22.2) or the MPAS feature (see clause 5.22.3) shall support UE IP address/prefix allocation in the UP function.

The UP function shall set the UEIP feature flag in the UP Function Features IE if it supports UE IP address/prefix allocation in the UP function (see clause 8.2.25). If so, the CP function shall determine whether UE IP address or prefix is allocated by the CP function or the UP function based on network configuration.

If the SSET feature (see clause 5.22.2) or the MPAS feature (see clause 5.22.3) is supported, the SMFs in an SMF set may decide that the CP function should allocate the UE IP address or prefix. In such a case, the SMFs in the SMF set may query a DHCP or AAA server for UE IP address or prefix allocation as specified in clause 5.21.2.2.

5.21.2 UE IP address/prefix allocation in the CP function

5.21.2.1 General

When performing UE IP address allocation in the CP function, the CP function shall assign the UE IP address/prefix and provide the assigned address/prefix to the UP function in the UE IP Address IE of the PDR IE (see Table 7.5.2.2-1) or of the Traffic Endpoint (see Table 7.5.2.7-1). The CP function shall always provide a full list of assigned address(es)/prefix(es) to the UP function in the PDI or Create/Update Traffic Endpoint IE.

5.21.2.2 UE IP Address/prefix allocation using DHCP or AAA server

When performing the UE IP address allocation by an SMF set, the SMFs in the SMF set may be configured to use DHCP or AAA server to allocate the UE IP address/prefix as specified in clause 10 of 3GPP TS 29.561 [49] and clauses 11 and clauses 12 of 3GPP TS 29.561 [49], with the following additions:

– one (or more) DHCPv4 or DHCPv6 server or one (or more) AAA server may be deployed within the 5GC for resiliency;

– different SMFs of an SMF set may request, renew and release the IP address assigned to a given PDU session, e.g. when the PDU session is taken over by a different SMF during the lifetime of the PDU session.

The lease time assigned by the DHCP or AAA server for IP addresses may be set by the operator to a long time to minimize the need for IP address renewal. When the lease time is approaching expiry, the SMF should request the DHCP server to renew the IP address (keeping the IP address unchanged) or the SMF should initiate a AAA re-authorization to renew the IP address (keeping the IP address unchanged).

NOTE: The lease time is stored in the PDU session context (which is shared by the SMFs of the SMF set). So any SMF in the SMF set taking over the control of the PDU session can determine whether the lease time is approaching expiry.

5.21.3 UE IP address/prefix allocation in the UP function

5.21.3.1 General

When performing UE IP address/prefix allocation in the UP function, the CP function shall request the UP function to allocate the UE IP address/prefix by:

– setting the CHOOSE flags (CHOOSE IPV4 and/or CHOOSE IPV6) in the UE IP Address IE of the PDR IE (see Table 7.5.2.2-1) or of the Traffic Endpoint (see Table 7.5.2.7-1); the IPv6 prefix length shall be indicated in the UE IP Address if an IPv6 prefix other than default /64 and other than for IPv6 prefix delegation (see clause 5.14) is to be assigned and the UPF indicated support of the IP6PL feature (see clause 8.2.25); and

– including the Network Instance IE to indicate the IP address pool from which the UE IP address/prefix is to be assigned.

– optionally including the UE IP address Pool Identity from which the UE IP address shall be allocated by the UP function.

The CP function may request the UP function to allocate the same UE IP address/prefix to several PDRs to be created (i.e. using Create PDR) within one single PFCP Session Establishment Request or PFCP Session Modification Request, or to several PDRs to be modified (i.e. using Update PDR) within one single PFCP Session Modification Request by:

– setting the CHOOSE flags (CHOOSE IPV4 and/or CHOOSE IPV6) in the UE IP Address IE of each PDR to be created with a new UE IP address/prefix or each PDR to be modified;

or, if the UP function indicated support of the PDI optimization (see clause 8.2.25), by:

– including the UE IP Address IE in the Create Traffic Endpoint IE or Update Traffic Endpoint IE, and by setting the CHOOSE flags (CHOOSE IPV4 and/or CHOOSE IPV6) in that UE IP Address IE; and

– including the Traffic Endpoint ID in all the PDRs to be created with the same UE IP address or all PDRs to be modified with additional UE IP address(es).

If the PDR(s) is created or modified successfully or the Traffic Endpoint(s) is created or modified successfully, the UP function shall always return the full list of UE IP address/prefix in the UE IP Address IE(s) it has assigned to the PDR(s) or to the Traffic Endpoint(s) in the PFCP Session Establishment Response or PFCP Session Modification Response.

Upon receiving a request to delete a PFCP session, to remove a Traffic Endpoint associated with the UE IP address/prefix, or to remove the last PDR associated with the UE IP address/prefix, the UP function shall release the UE IP address/prefix that was assigned to the PFCP session, to the Traffic Endpoint, or to the PDR.

NOTE 1: When the CP function requests additional UE IP Address in the Update PDR or Update Traffic Endpoint IE, it needs not include any existing UE IP Address(es).

If the UP Function supports the "Per Slice UP Resource management" feature and if the feature is enabled in the UP Function, the UP Function shall take the S-NSSAI associated with the PFCP session into account when allocating UE IP address, together with other information present (e.g. Network Instance, UE IP address Pool Identity).

5.21.3.2 Reporting UE IP Address Usage to the CP function

The UE IP Address Usage Reporting feature (see clause 8.2.58) is an optional feature. The following requirements shall apply if UE IP addresses are allocated by the UP function and both the CP function and UP function support the UE IP Address Usage Reporting feature.

The UP Function should report UE IP address usage information to the CP function for network instances and/or IP address pools whose ratio of occupied (i.e. already assigned) IP addresses to the configured IP addresses in the UP Function exceeds a configurable threshold. If the UP Function supports the "Per Slice UP Resource management" feature and if the feature is enabled in the UP Function, the UP Function shall additionally indicate the S-NSSAI associated with the UE IP address usage information. The UP function shall do so by sending one or more PFCP Association Update Request messages to the CP function, including a UE IP Address Usage Information IE per network instance and/or IP address pool. Each UE IP Address Usage Information IE shall include a validity timer that informs the CP function for how long the UE IP address Usage Information shall be considered as valid. Each UE IP Address Usage Information shall also contain a UE IP Address Usage Sequence Number, which enables the CP function to determine the latest UE IP Address Usage Information generated by the UP function for a given network instance and/or IP address pool.

The UP function may update the UE IP address usage information reported to the CP function if needed, by sending subsequent PFCP Association Update Request/Response messages, including updated UE IP address Usage Information IEs with a (new) validity timer. The UP function shall increment the UE IP Address Usage Sequence Number when updating UE IP address usage information. The UP function shall also increment the UE IP Address Usage Sequence Number when the IP address usage has not changed but the validity timer needs to be renewed.

NOTE 1: The threshold value in the UP function needs to be selected in such way that it avoids frequent UE IP address usage reporting for network instances and/or UE IP address pools with low usage.

The CP function shall use the latest updated UE IP address Usage Information received for a given network instance and/or UE IP Address Pool. The CP function shall ignore a UE IP Address Usage Information IE received for a given network instance and/or IP address pool with a UE IP Address Usage Sequence Number smaller or equal to the UE IP Address Usage Sequence Number of an already received and stored information.

If the validity timer has not expired, the CP function shall keep the latest updated UE IP Address Usage Information received for a given Network Instance and/or UE IP Address Pool if it receives a PFCP Association Update Request/Response not including a UE IP Address Usage Information IE for this network instance and/or UE IP Address Pool.

NOTE 2: The UE IP Address Usage Information IE may be absent e.g. if the PFCP Association Update Request/Response is sent for other regular purposes, or if the ratio remains the same, or if the ratio has not changed enough to justify being reported again.

The CP function shall delete UE IP Address Usage Information if its validity period has expired.