Interfaces X1, X2 et X3 dans la 5G : architecture LI du 3GPP

The X1 X2 X3 5G interface architecture represents a fundamental evolution in lawful interception design. The arrival of 5G has brought fundamental changes to how telecommunications networks are designed, deployed, and operated. For lawful interception, these changes are equally profound. The 3GPP LI architecture for 5G builds on the X1, X2 and X3 interfaces already known from 3G and 4G, but now with standardised protocols (ETSI TS 103 221-1 and TS 103 221-2) instead of the vendor-specific implementations used in previous network generations. For operators deploying 5G networks, understanding the X1/X2/X3 architecture is essential to building LI capabilities that meet regulatory requirements and function effectively in the new network environment.

Cet article présente une explication détaillée de l'architecture LI du 3GPP pour la 5G, en mettant l'accent sur les interfaces X1, X2 et X3, leurs relations avec les interfaces HI de l'ETSI, ainsi que les implications pratiques pour les opérateurs et Solution LI prestataires.

L'architecture X1, X2, X3 5G

Le réseau central 5G, tel que défini par le 3GPP, utilise une architecture basée sur les services (SBA) qui diffère fondamentalement des architectures basées sur les nœuds des réseaux 3G et 4G. Au lieu d’éléments de réseau dédiés remplissant des fonctions spécifiques, le cœur de réseau 5G se compose d’un ensemble de fonctions réseau (NF) qui communiquent entre elles via des interfaces basées sur les services. Parmi les principales fonctions réseau, on trouve la fonction de gestion de l'accès et de la mobilité (AMF), la fonction de gestion de session (SMF), la fonction de plan utilisateur (UPF) et bien d'autres encore.

Cette évolution architecturale a des implications importantes pour l’interception légale (LI). Dans les générations précédentes, l’interception était généralement mise en œuvre au niveau de nœuds réseau spécifiques — le MSC pour la voix en 3G, le P-GW pour les données en 4G — qui constituaient des points bien définis où convergeaient le trafic de signalisation et le trafic du plan utilisateur de la cible. En 5G, la nature distribuée et virtualisée des fonctions réseau implique que l’interception doit être abordée différemment. L’architecture LI du 3GPP répond à ce défi en définissant des interfaces normalisées entre le système LI et les fonctions réseau, plutôt que de s’appuyer sur des mécanismes d’interception spécifiques à chaque nœud.

L'architecture LI dans les spécifications 3GPP TS 33.127 et TS 33.128

The 3GPP LI architecture for 5G is defined primarily in three specifications: TS 33.126 (Lawful Interception requirements), TS 33.127 (Lawful Interception architecture and functions) and TS 33.128 (Lawful Interception protocols and procedures, stage 3). These specifications define the functional components of the LI system within the 5G network, the interfaces between them, and the interfaces to the law enforcement domain.

The key functional components defined in TS 33.127 are the Administration Function (ADMF), which receives warrants (in its LI Control Function, LICF) and provisions the LI system (through its LI Provisioning Function, LIPF); the Points of Interception (POIs) inside or alongside the network functions, split into IRI-POIs and CC-POIs; Triggering Functions (TFs), which activate interception at POIs such as the UPF on demand; and the Mediation and Delivery Function (MDF), split into MDF2 for IRI and MDF3 for content, which delivers the result to law enforcement.

The interfaces between these components are designated X1, X2, and X3. These interfaces are internal to the operator’s domain — they connect the LI management and delivery functions to the network functions — and are distinct from the ETSI HI interfaces, which connect the operator to the LEMF. In practice, X2 and X3 feed the handover interfaces: the MDF2 turns xIRI received over X2 into IRI for HI2, and the MDF3 turns xCC received over X3 into CC for HI3. X1 has no HI counterpart: it is the operator’s internal provisioning channel, while HI1 carries the warrant from the authority to the operator.

X1 : L'interface d'administration

X1 is the interface over which the ADMF provisions and manages the POIs, the triggering functions and the MDF. Through X1, the ADMF sends instructions to the POIs to activate, modify, or deactivate interceptions. The X1 interface carries the target identification parameters, the scope of the interception, and any technology-specific configuration information needed by the POI.

X1 uses ETSI TS 103 221-1: XML messages sent as HTTP POST requests over HTTPS (HTTP/1.1 or HTTP/2), with mutual TLS authentication using X.509 certificates. X1, X2 and X3 are not new as concepts (3GPP TS 33.107 already used them for 3G and 4G), but in previous generations their protocols were left to vendors. For 5G, TS 33.128 mandates the standardised ETSI protocols TS 103 221-1 (X1) and TS 103 221-2 (X2/X3), which can also be used for 4G and IMS.

The X1 interface supports various target identification methods, including SUPI (Subscription Permanent Identifier), GPSI (Generic Public Subscription Identifier), PEI (Permanent Equipment Identifier), and IP addresses. Because 5G conceals the permanent identifier on the radio interface (SUCI, 5G-GUTI), TS 33.127 adds identifier-association functions (IEF, ICF and IQF) that let the operator and the LEA map temporary identifiers to the target’s SUPI.

X2 : L'interface de livraison IRI

X2 is the interface for delivering intercept-related information from the Points of Interception to the LI Mediation and Delivery Function. When a target’s communications trigger events at the POI — such as registration, session establishment, mobility events, or session termination — the IRI-POI generates xIRI records and sends them to the MDF2 over X2.

The X2 IRI records in the 5G architecture are significantly richer than those in previous generations, reflecting the more complex signalling and session management in 5G networks. X2 records may include information about PDU (Protocol Data Unit) session establishment, QoS (Quality of Service) flow creation and modification, handover events, network slice selection, and service-specific parameters. The X2 records (xIRI) are defined in TS 33.128 as BER-encoded ASN.1 and carried in the binary X2 PDU format of ETSI TS 103 221-2.

The X2 interface operates in a push model — the POI generates and sends xIRI records to the MDF2 as events occur, without waiting for a request. The delivery must be timely and reliable, as the IRI forms a critical part of the intercepted material delivered to law enforcement. The MDF2 receives X2 records from potentially multiple POIs across the network, correlates them, and formats them for delivery to the LEMF over the HI2 interface.

X3 : L'interface de diffusion CC

X3 carries content (xCC) from the CC-POIs to the MDF3. For 5G data sessions the CC-POI is in the UPF: when the target’s PDU session is set up, the triggering function in the SMF instructs the UPF over LI_T3 to duplicate the session’s user-plane packets — IP packets carrying voice, data, messaging, and other services — and send them over X3.

The X3 interface handles potentially large volumes of data, particularly for data-heavy targets. The interface must be dimensioned to handle the throughput requirements without packet loss, and the delivery mechanism must maintain the integrity and ordering of the intercepted packets. The MDF3 receives the X3 traffic, processes it as needed, and delivers it to the LEMF over the HI3 interface.

In the 5G architecture, the UPF is the primary POI for user-plane interception. However, the UPF may be deployed in different locations within the network — at the edge, in regional data centres, or in central facilities — depending on the network architecture and the services being provided. The LI system must be capable of interacting with UPFs wherever they are deployed, which may require distributed X3 collection points and centralised processing at the MDF3.

Le « network slicing » ajoute une dimension supplémentaire à l'interception X3. Une même cible peut voir son trafic transiter par plusieurs tranches de réseau, chacune disposant de sa propre instance UPF. Le système LI doit identifier et intercepter le trafic de la cible sur l'ensemble des tranches concernées, garantissant ainsi une couverture complète des communications de la cible.

Relation entre X1/X2/X3 et HI1/HI2/HI3

The X interfaces and the HI interfaces serve different but complementary roles in the overall LI architecture. The X interfaces operate within the operator’s domain, connecting the LI management and delivery functions to the network infrastructure. The HI interfaces operate at the boundary between the operator and law enforcement: HI1 carries the warrant to the operator, and HI2 and HI3 deliver the intercepted IRI and content to the LEMF.

The LI Mediation and Delivery Function serves as the bridge between these two interface sets. It receives X2 IRI records from the POIs, processes and formats them, and delivers the resulting ETSI-compliant IRI over the HI2 interface to the LEMF. Similarly, it receives X3 content (xCC) from the CC-POIs, such as the UPF, processes it, and delivers it over the HI3 interface. On the administration side, the ADMF receives the warrant over HI1 (typically ETSI TS 103 120), and its provisioning function sends X1 messages (ETSI TS 103 221-1) to the POIs, the triggering functions and the MDF itself.

This layered architecture provides a clean separation between the internal network interception mechanisms and the external handover to law enforcement. It allows the network functions to implement interception using standardised X interfaces without needing to understand the specific requirements of the national HI interface implementation, and it allows the MDF to adapt between different internal and external interface versions as standards evolve.

Implications pratiques pour les opérateurs

For operators deploying 5G networks, the X1/X2/X3 architecture has several practical implications. First, LI must be considered from the earliest stages of 5G network design. The POI interfaces within the network functions must be planned and provisioned, the MDF must be dimensioned for the expected interception volumes, and the connectivity between the POIs and the MDF must be established.

Second, operators must ensure that their 5G network function vendors support the X1, X2, and X3 interfaces as defined in TS 33.127 and TS 33.128 (ETSI TS 103 221-1/-2 as profiled by TS 33.128). TS 33.128 also makes ETSI TS 103 120 the HI1 protocol and ETSI TS 102 232-1/-7 the HI2/HI3 transport for 5G. Vendor support for the LI interfaces has historically been uneven, and operators should include LI interface compliance as a requirement in their procurement specifications.

Third, the virtualised and cloud-native nature of 5G networks introduces new challenges for LI deployment. The MDF and POIs may need to operate in containerised environments, scale dynamically, and maintain performance and reliability across distributed deployments. Operators must ensure that their LI solutions are designed for cloud-native operation and can keep pace with the dynamic nature of the 5G network.

Conclusion

Les interfaces X1/X2/X3 constituent une évolution majeure de l’architecture LI du 3GPP, reflétant les changements fondamentaux que la 5G apporte à la conception et à l’exploitation des réseaux. L’interface X1 fournit le canal d’administration permettant de gérer les interceptions, l’interface X2 transmet les métadonnées détaillées dont les forces de l’ordre ont besoin pour mener leurs enquêtes, et l’interface X3 transmet le contenu des communications interceptées. Associées aux interfaces HI de l’ETSI, elles forment une architecture complète et stratifiée qui permet l’interception légale au sein des réseaux de télécommunications les plus avancés. Pour les opérateurs, il est essentiel de comprendre et de mettre en œuvre ces interfaces afin de respecter leurs obligations légales et de garantir que leurs réseaux 5G prennent en charge des capacités d’interception légale efficaces et conformes.

À mesure que les déploiements 5G en mode autonome s'accélèrent, les interfaces 5G X1, X2 et X3 revêtent une importance croissante. Les opérateurs doivent s'assurer que leurs implémentations 5G X1, X2 et X3 sont conformes aux dernières spécifications du 3GPP.

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