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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (85 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-14, normative work from Rel-15.
In Release 15, the SEID (Session Endpoint Identifier) function was newly introduced as part of the Packet Forwarding Control Protocol (PFCP) for the N4 interface between the SMF and UPF. This enabled the core procedures for a PFCP session, including session establishment, modification, deletion, and reporting on N4. The release also introduced specific capabilities for these sessions, such as handling Ethernet PDU session types and managing an inactivity timer for Always-on PDU sessions.
- Session establishment on N4 TS 29.244CR0049
- Session Modification on N4 TS 29.244CR0050
- Session deletion on N4 TS 29.244CR0051
- Session Reporting on N4 TS 29.244CR0052
- PDR for Ethernet PDU session TS 29.244CR0112
- Adding 5G Session-AMBR TS 29.244CR0196
+ 3 more changes
In Release 16, the SEID function's context was enhanced to support PFCP sessions being successively or concurrently controlled by different SMFs within the same SMF set, improving control plane redundancy and session continuity. This release also introduced the ability to bundle multiple PFCP messages for efficiency and formally aligned terminology by correcting "Sx Session" to "PFCP Session" across the specification. Furthermore, new procedures were defined for UP function-initiated PFCP association and session release, and for session reestablishment after a UP function restart.
- Enhancement to the PFCP Association Release Procedure TS 29.244CR0240
- Update the PFCP association setup to support UE IP address Allocation by AAA/DHCP TS 29.244CR0252
- PFCP sessions successively controlled by different SMFs of a same SMF set TS 29.244CR0261
- PFCP messages bundling TS 29.244CR0285
- PFCP sessions controlled by different SMFs in a set TS 29.244CR0270
- PFCP usage over N16a for the support of traffic offload by UPF controlled by I-SMF TS 29.244CR0266
+ 27 more changes
In Release 17, the SEID function was extended to support the new N4mb interface for multicast and broadcast services (MBS), enabling PFCP session and node-related procedures for MBS sessions. This included new capabilities for MBS session deactivation and reactivation over N4, as well as the transport of MBS Session Identifier and related QoS information within PFCP messages. These enhancements integrated multicast session management into the existing PFCP framework used for individual PDU sessions.
- PFCP Node related messages supported over N4mb TS 29.244CR0606
- Transport Level Marking information for PFCP sessions over N4mb TS 29.244CR0622
- MBS Session Identifier TS 29.244CR0621
- DL MBS QFI Sequence Number in PDU Session Container TS 29.244CR0615
- Clarification to the MBS Session N4 Information TS 29.244CR0592
- Incorrect IE Type Value - MBS Session N4 Information IE TS 29.244CR0603
+ 15 more changes
In Release 18, the SEID function's scope was expanded through PFCP extensions to support new procedures and information elements. Key enhancements included support for High Reliability & Service-Based Operations (HR-SBO) PDU sessions, the introduction of Traffic Load (TL) Containers in session modification and deletion procedures, and the addition of S-NSSAI and DNN information to the session establishment procedure. Furthermore, specific mechanisms were introduced for handling overlapping UE IP addresses in HR-SBO sessions and for N6 tunneling between the V-UPF and V-EASDF.
- Update N4 Session modification procedure to support network slice replacement feature TS 29.244CR0730
- PFCP extensions for HR-SBO PDU sessions TS 29.244CR0750
- TL-Containers in PFCP Session Modification/Deletion Request/Response TS 29.244CR0767
- ECN marking during PDU session establishment procedure TS 29.244CR0772
- Support of DL Session AMBR in HR-SBO session TS 29.244CR0824
- Add S-NSSAI and DNN information in Session Establishment procedure TS 29.244CR0832
+ 10 more changes
In Release 19, the SEID function was enhanced to enable PFCP session restoration at an alternative SMF for improved session resilience. This allows a PFCP session to be excluded from restoration upon its original SMF's failure and subsequently restored at a different, designated SMF within the SMF set. These changes provide session continuity by allowing the control of a PFCP session to be seamlessly transferred between CP functions (SMFs) during failures.
- PFCP sessions excluded from the restoration upon a SMF failure with SMF set being deployed TS 29.244CR0895
- Providing alternative SMF(s) per PFCP Session TS 29.244CR0911
- Restore PFCP Sessions at an alternative SMF TS 29.244CR0914
- Link-specific and MPQUIC Proxy IP addresses for IPv4v6 PDU sessions TS 29.244CR1000
- Transport Level Marking for HR PDU sessions TS 29.244CR1008
- PFCP corrections for UE level measurements TS 29.244CR0957
Explore further
Broader topics and technologies where SEID plays a role.
Defining Specifications
3GPP specifications that define or reference SEID, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |