Line-Rate GTP-U Admission Control at the Edge of a Cloud-Native 5G Core: An XDP-Based Design for Kubernetes-Hosted User Plane Functions

Abstract

The User Plane Function (UPF) of a 5G Standalone core terminates every GPRS Tunnelling Protocol user-plane (GTP-U) packet arriving from the radio access network, which makes its N3 interface both the busiest and the most exposed point of the mobile data path. As operators migrate the core onto Kubernetes and public cloud, the UPF increasingly shares a general-purpose Linux kernel with other workloads, and kernel-bypass frameworks such as DPDK become harder to operate. This paper presents GTP-Guard, a design for line-rate GTP-U admission control built on the eXpress Data Path (XDP) hook of the Linux kernel. GTP-Guard drops illegitimate tunnel traffic in the network driver, before socket-buffer allocation, using six ordered stages: peer allow-listing, GTP-U header validation, Tunnel Endpoint Identifier (TEID) admission against session state derived from the Packet Forwarding Control Protocol (PFCP), bounded extension-header parsing, inner-packet anti-spoofing with GTP-in-GTP detection, and per-session policing. We formalize stage ordering as a cost-minimization problem and show that sorting stages by the ratio of per-packet cost to rejection probability minimizes expected per-packet work, which motivates run-time reordering through tail-call program arrays when traffic mix shifts under attack. We further describe a Kubernetes deployment model based on a node-level DaemonSet with pinned eBPF maps that survive agent upgrades, discuss constraints specific to public-cloud virtual NICs, and define a reproducible evaluation methodology, with explicit hypotheses, for throughput, latency, CPU efficiency and attack resilience. This paper presents the design and methodology; experimental results will be reported in a subsequent version.