5.8 PFCP Association

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

5.8.1 General

A PFCP Association shall be set up between the CP function and the UP function prior to establishing PFCP sessions on that UP function. Only one PFCP association shall be setup between a given pair of CP and UP functions, even if the CP and/or UP function exposes multiple IP addresses. A single PFCP association may also be setup between a SMF set and a UPF (see clause 5.22.2).

The CP function and the UP function shall support the PFCP Association Setup procedure initiated by the CP function (see clause 6.2.6.2). The CP function and the UP function may additionally support the PFCP Association Setup procedure initiated by the UP function (see clause 6.2.6.3).

A CP function may have PFCP Associations set up with multiple UP functions. A UP function may have PFCP Associations set up with multiple CP functions.

In PFCP signaling, a CP function or a UP function shall be identified by a unique Node ID. A Node ID may be set to an FQDN or to an IP address (either IPv4 or IPv6, see clause 8.2.38). When set to an IP address, it indicates that the CP/UP function only exposes one IP address for the PFCP Association signalling. Once a PFCP association is set up between a CP function and a UP function, the same Node ID as signaled respectively by the CP function and the UP function during the PFCP association setup procedure shall be used in all subsequent PFCP node related messages (other than Heartbeat messages) until the PFCP association is released.

NOTE 1: The control of a PFCP session can be taken over by a new SMF in an SMF set as specified in clause 5.22. When using the MPAS feature (see clause 5.22.3), when a different SMF takes over the control of the PFCP session, the UP function receives the NODE ID of the new SMF that is different from the NODE ID of the SMF that was handling the PFCP session earlier.

The PFCP entities shall accept any new IP address allocated as part of F-SEID other than the one(s) communicated in the Node Id.

NOTE 2: The source IP address to send PFCP Association Setup request can not be used as the destination IP address when the peer sends a PFCP Association Update Request message, e.g. for a scenario when a NAT is deployed in the network.

Prior to establishing a PFCP Association, the function responsible for establishing the PFCP Association (e.g. CP function) shall look up a peer function (e.g. UP function), e.g. using DNS procedures (see 3GPP TS 29.303 [25]), NRF procedures (see 3GPP TS 29.510 [43]) or local configuration. If the peer function is found to support multiple IP addresses (in the look up information), one of these addresses (any one) shall be used as destination IP address to send the PFCP Association Setup Request. Once the PFCP Association is established, any of the IP addresses of the peer function (found during the look-up) may then be used to send subsequent PFCP node related messages and PFCP session establishment requests for that PFCP Association.

NOTE 3: The look up information (e.g. in DNS, NRF or local configuration of the function responsible for establishing the PFCP association) needs to be configured consistently with the addressing information of the peer function. If a FQDN is configured to identify a function in DNS or NRF, then the Node ID of that function included in PFCP messages need to be set to the same FQDN. For instance, if the CP function is responsible for establishing the PFCP association, a UP function that exposes multiple IP addresses (for PFCP node related messages and PFCP session establishment requests) needs to be configured in the look up information as one (single) UP function that is associated to multiple IP addresses. The Node ID needs to be set to an SMF set FQDN when a single association is setup between an SMF set and UPF (see clause 5.22.2).

NOTE 4: PFCP session related messages for sessions that are already established are sent to the IP address received in the F-SEID allocated by the peer function or to the IP address of an alternative SMF in the SMF set (see clause 5.22). The former IP address needs not be configured in the look up information. See clauses 4.3.2 and 4.3.3.

5.8.2 Behaviour with an Established PFCP Association

When a PFCP Association is established with a UP function, the CP function:

– shall provision node related parameters (i.e. parameters that apply to all PFCP sessions) in the UP function, if any, e.g. PFDs;

– shall provision the UP function with the list of features (affecting the UP function behaviour) the CP function supports, if any, e.g. support of load and/or overload control;

– shall check the responsiveness of the UP function using the Heartbeat procedure as specified in clause 6.2.2;

– may establish PFCP sessions on that UP function;

– shall refrain from attempting to establish new PFCP sessions on the UP function, if the UP function has indicated it will shut down gracefully.

When a PFCP Association is established with a CP function, the UP function:

– shall update the CP function with the list of features it supports;

– shall update the CP function with its load and/or overload control information, if load and/or overload control is supported by the CP and UP functions;

– shall accept PFCP Session related messages from that CP function (unless prevented by other reasons, e.g. overload);

– shall check the responsiveness of the CP function using the Heartbeat procedure as specified in clause 6.2.2;

– shall indicate to the CP function if it will shut down within a graceful period and, when possible, if it fails and becomes out of service;

– may report UE IP address usage information to the CP function, if UE IP addresses are allocated by the UP function and the UE IP Address Usage Reporting feature is supported by the CP function (see clause 5.21.3.2).

5.8.3 Behaviour without an Established PFCP Association

When a PFCP Association is not established with a UP function, the CP function:

– shall reject any incoming PFCP Session related messages from that UP function, with a cause indicating that no PFCP association exists with the peer entity.

When a PFCP Association is not yet established with a CP function, the UP function:

– shall reject any incoming PFCP Session related messages from that CP function, with a cause indicating that no PFCP association exists with the peer entity.