Session Endpoint Identifier

Description

The Session Endpoint Identifier (SEID) is a fundamental parameter in the Packet Forwarding Control Protocol (PFCP), which is the protocol used for communication between the control plane and user plane in the 3GPP 5G Core network (5GC) and evolved EPC. Specifically, PFCP operates between the Session Management Function (SMF) and the User Plane Function (UPF) in 5GC, and between the Control Plane PGW (PGW-C) and User Plane PGW (PGW-U) in EPC. The SEID is a 64-bit unsigned integer that uniquely identifies a PFCP session on a specific node (either the control plane node or the user plane node).

A PFCP session is established to manage the user plane forwarding rules for a user's data session, known as a PDU Session in 5G. When an SMF needs to instruct a UPF on how to handle a user's traffic (e.g., which QoS flows to map to which tunnels, which policies to apply), it initiates a PFCP Session Establishment Request. This request includes the SMF's local SEID for this session. The UPF, upon accepting, creates its own local session context and responds with its own SEID. Subsequently, all PFCP messages related to this session (modification, deletion, reporting) must include both the SEID of the sender and, where relevant, the SEID of the peer. This creates a bidirectional association.

The SEID works in conjunction with the F-SEID (Fully Qualified SEID), which includes both the SEID value and the IP address of the node that allocated it. This allows for unambiguous identification in networks where multiple nodes exist. The SEID is locally significant to the node that assigns it, meaning the same numerical value could be used by different SMFs or UPFs for different sessions without conflict. The protocol uses these identifiers to correlate requests and responses and to maintain the state of potentially millions of simultaneous user sessions efficiently.

Architecturally, the SEID is a key enabler of the control and user plane separation (CUPS) and the service-based architecture of 5GC. It allows the stateless control plane functions (like SMF) to manage stateful user plane functions (UPF) without having to store the detailed packet forwarding state themselves. The SEID provides the handle by which the SMF can remotely program the UPF's forwarding paths, QoS enforcement, charging, and traffic reporting functions for each individual user session.

Purpose & Motivation

The SEID was created to address the need for a scalable and efficient session identifier in the protocol governing the separated control and user plane architecture (CUPS). Prior to CUPS, in monolithic gateways like the SGSN or GGSN, session state was managed internally within a single node. With the separation introduced in EPC and fully realized in 5GC, a standardized protocol (PFCP) was needed for the control entity to manage the user plane entity remotely. This protocol required a way to uniquely reference each session context on both ends.

The problem it solves is the unambiguous correlation of control messages with the correct user session state on the user plane node. Without a unique session identifier like the SEID, the UPF would have no efficient way to distinguish which set of packet detection rules (PDRs), forwarding action rules (FARs), and QoS enforcement rules (QERs) a given PFCP message refers to, especially when handling traffic for millions of concurrent users. The SEID provides this direct lookup key.

Historically, the SEID was introduced with the PFCP protocol in 3GPP Release 14 as part of the CUPS architecture for the EPC. It became even more critical in Release 15 with the 5G Core, where the service-based architecture and the decomposition of network functions made dynamic, session-specific control of the user plane a central design principle. The SEID is a foundational element that enables the flexibility, scalability, and network slicing capabilities of modern 5G networks by providing a simple yet powerful handle for session management across distributed network functions.