Feature Description

Important

The PGW-C term used in this chapter denotes the EPS interworking functionality supported by SMF and must not be assumed as a standalone P-GW that is used in the LTE network.

In the 5G System Architecture, the SMF performs the session management functions that the 4G Mobility Management Entity (MME), Serving Gateway Control plane function (SGW-C), and PDN Gateway Control plane function (PGW-C) handle. The SMF is one of the elements of the Service-Based Architecture (SBA). SMF is responsible for communicating with the decoupled data plane, creating, updating, and removing Protocol Data Unit (PDU) sessions. SMF also manages the session context with the User Plane Function (UPF). For the session management-related functions, SMF communicates with various interfaces, such as N1, N4, and N10.

At a given time, the SBI interfaces (N7, N10, N11, and N40) support only an IPv4 or IPv6 address. However, the N3, N4 and GTPC interfaces support either IPv4 or IPv6 address or both. For the IP address support, both the endpoint and interfaces configuration must include unique VIP IP and port. For configuration details, see the Configuring Interfaces section.

SMF prioritizes IPv6 over IPv4 addresses while initiating a message on the N4 interface. If the peer GTPC uses both IPv4 and IPv6 addresses, SMF uses the same IP address type on which it has received the last message from the GTPC peer for that particular session, while initiating any new message.

If SMF receives both the IPv4 and IPv6 address as part of a CSR or MBR message, SMF sends an echo using an IPv4 and IPv6 address on the GTPC interface. The peer is considered to be down, only if echo fails on both the interfaces. SMF determines it as path failure and clears the session.

For SBI interfaces, if the discovered NF profile contains both IPv4 and IPv6 addresses, then SMF selects the IP to communicate with the peer NF based on the IP type configuration at SBI endpoint level or interface level for that particular interface.

SMF negotiates between UPF tunnel and RAN by exchanging the IPv6 endpoint identifier information and tunnel information for both.

During HO, SMF creates the tunnel based on the tunnel information received from the target peer and exchanges the tunnel information between UPF and the target peer.

Each interface and endpoint can be independently configured for IPv4 or IPv6 or both based on the current support.

During UPF association setup, the SMF checks if the transport type in the setup request is the same as the configured address. The SMF proceeds with the association request or rejects the request based on the validation result.

Similarly, during NRF discovery, the transport type must match the statically configured transport type either at the endpoint level or interface level. The SMF performs NF selection based on the IP address-matching criteria.

For 4G calls with legacy interfaces, peer SGW IPv4, IPv6, or IPv4v6 data address is supported.

Note

DNS, RADIUS, and roaming interfaces currently don’t support the IPv6 address.