Fully Qualified Session Endpoint Identifier

Description

The Fully Qualified Session Endpoint Identifier (F-SEID) is a critical data structure defined within the 5G Core network's Service-Based Architecture (SBA). It serves as a globally unambiguous address for a specific session endpoint, such as a Protocol Data Unit (PDU) session anchored at a Session Management Function (SMF) or a User Plane Function (UPF). The F-SEID is constructed by concatenating two primary components: the Session Endpoint Identifier (SEID) and the IP address of the network function hosting that endpoint. The SEID itself is a locally significant identifier assigned by the network function, but when combined with the node's IP address, it becomes globally unique across the entire network. This composite structure is fundamental for the HTTP/2-based service-based interfaces (e.g., N4, N7) used between Control Plane functions like the SMF and the UPF.

Architecturally, the F-SEID is used within the Packet Forwarding Control Protocol (PFCP) session context, particularly in the N4 interface between the SMF (control plane) and the UPF (user plane). When an SMF establishes a PDU session, it creates a PFCP session and assigns an F-SEID for the session endpoint on the UPF. This F-SEID is then used in all subsequent PFCP messages (e.g., Session Establishment, Modification, Deletion) to uniquely identify the session and the specific UPF instance handling it. The IP address component ensures that messages are correctly routed to the physical or virtualized network function instance, even in a cloud-native, distributed deployment where multiple UPF instances may exist.

The role of the F-SEID extends beyond simple identification; it is integral to session management, mobility, and network slicing. For mobility events like handovers, the SMF may need to redirect traffic by updating the F-SEID to point to a different UPF. In network slicing, the F-SEID helps associate a session with a specific slice instance, as the UPF may be slice-specific. The identifier also supports advanced features like session and service continuity (SSC) modes, where the PDU session anchor (and thus the F-SEID) may change. Its design aligns with the 5G Core's principle of separation of control and user plane, enabling flexible scaling and deployment of user plane resources.

Purpose & Motivation

The F-SEID was introduced to address the need for unambiguous session endpoint identification in the 5G Core's distributed, cloud-native architecture. Previous 4G EPC architectures used identifiers like the Fully Qualified TEID (F-TEID) in GTP-based interfaces, but these were tied to the GPRS Tunnelling Protocol. With 5G's shift to a Service-Based Architecture (SBA) and the introduction of new protocols like PFCP for the N4 interface, a new identifier mechanism was required. The F-SEID provides a standardized way to uniquely identify a session endpoint across potentially numerous, geographically dispersed network function instances, solving the problem of session routing and management in a decomposed network.

Its creation was motivated by the limitations of earlier identifiers that were not fully qualified or were protocol-specific. In a monolithic network, locally significant identifiers might suffice, but in a microservices-based 5G Core, control plane functions (like SMF) need to communicate with specific user plane instances (UPFs) without ambiguity. The F-SEID ensures that even if multiple UPFs use the same local SEID value, the combination with the IP address prevents collisions. This is crucial for automation, orchestration, and fault isolation, as it allows network operators to pinpoint exactly where a session is anchored. The F-SEID thus supports the scalability, resilience, and service differentiation goals of 5G, including network slicing and edge computing.