Distributed Mobility Management D. K. Bui Internet-Draft K. Nguyen-Trung Intended status: Informational N. Tran Expires: 16 April 2027 Y. Kim Soongsil University 13 October 2026 Service Segmentation Considerations for CATS-MUP draft-dcn-dmm-cats-mup-service-segmentation-01 Abstract Service segmentation introduces an emerging deployment paradigm in which a service is composed of multiple distributed subtasks forming a service pipeline. This document discusses architectural considerations for steering sessions across multiple subtask instances in sequential, parallel, and iterative service pipelines, including a MUP Sequence Session Transform to support ordered traversal, and for maintaining service continuity during pipeline updates, particularly when stateful subtasks are involved. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 16 April 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights Bui, et al. Expires 16 April 2027 [Page 1] Internet-Draft CATS-MUP Service Segmentation October 2026 and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Architecture Overview . . . . . . . . . . . . . . . . . . . . 4 4. MUP Sequence Session Transform Mechanism . . . . . . . . . . 6 5. Iterative Execution with a Remote Service Subtask . . . . . . 8 6. Service Continuity for Stateful Service Segmentation . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 9.1. Normative References . . . . . . . . . . . . . . . . . . 10 9.2. Informative References . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction The document [I-D.draft-dcn-dmm-cats-mup] describes how to integrate Computing-Aware Traffic Steering (CATS) capabilities into the Mobile User Plane (MUP) architecture. In that design, when multiple candidate service instances are deployed at different locations, the MUP Controller (MUP-C), as the core component of the architecture, selects the optimal service instance by considering computing and network information. The MUP-C receives user mobility session information from the control plane entity and converts such session information into IPv6 dataplane routing information. Instead of relying on traditional anchoring mechanisms or intermediate user plane forwarding nodes, the session is directly mapped into SRv6-based routing instructions in the underlay network. The Type 2 Session Transformed Route (T2ST) and Type 1 Session Transformed Route (T1ST) are used to convert session information into SRv6-based routing paths toward the selected service endpoint. As a result, traffic steering is realized at the IP routing level without requiring dedicated anchor or intermediate nodes in the mobile user plane. However, emerging 6G applications introduce a new deployment paradigm, where a service is decomposed into multiple subtasks that are distributed across different edge locations. These subtasks can be organized as a sequential service pipeline, where traffic must traverse service instances in a predefined sequence, as a parallel service pipeline, where traffic is distributed to multiple service instances and later merged, or as an iterative service pipeline, Bui, et al. Expires 16 April 2027 [Page 2] Internet-Draft CATS-MUP Service Segmentation October 2026 where two subtasks exchange intermediate results repeatedly before a result is returned. Distributed AI Inference gives examples of all three: an ML model inference service can be segmented by model structure (parallel), by inference phase as in prefill-decode disaggregation (sequential), or by model role as in speculative decoding (iterative), in which one of the subtasks may be located at a remote site outside the CATS domain. Such service segmentation scenarios are discussed in [I-D.draft-dcn-cats-req-service-segmentation]. The existing T1ST and T2ST mechanisms are defined for mapping a session to a selected service instance, where traffic is steered toward a specific execution endpoint. While this model is suitable when a session is directed to one optimal service instance, service segmentation introduces scenarios in which a sequential service pipeline spans multiple service instances as part of a single service execution. In such cases, the session-to-route transformation must not only select appropriate service instances, but also support ordered traversal across them and preserve overall service continuity, particularly when the pipeline includes stateful subtasks. The existing CATS-MUP architecture focuses on mapping a session to a selected service instance. This document discusses scenarios where a service execution consists of multiple distributed subtasks forming a pipeline. It discusses extensions to session transformation behavior to enable ordered traversal across multiple subtask instances and to support service continuity during pipeline updates, particularly when stateful subtasks are involved. 2. Terminology This document uses the terminology defined in [I-D.draft-dcn-dmm-cats-mup] and [I-D.draft-dcn-cats-req-service-segmentation]. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174]. Bui, et al. Expires 16 April 2027 [Page 3] Internet-Draft CATS-MUP Service Segmentation October 2026 3. Architecture Overview This document does not define mechanisms for constructing or orchestrating service pipelines. Pipeline composition, subtask placement, and lifecycle management are considered out of scope. Furthermore, it does not modify or redefine existing SRv6 behavior definitions and relies on procedures specified in existing SRv6 and SFC specifications. +----------------+ | Mobility | | Management | | System | +----------------+ | Session Information | STR +--------v-------+ +------| CATS-MUP-C | +--|------| +------+ |----------+ | | | | C-PS | | | | | +----------------+ | +-----------------+ UE- | v | | C-SMA | \+---+ +------+ DSD +------+ |-----------------| UE--|RAN|---| PE |<----------------------| PE |----| Service Site A | +---+ +------+<-----------\ +------+ | Subtask 1 | UE-/ |Buffer| \ |Buffer| | Subtask 3 | +------+ \ +------+ +-----------------+ | \ | | MUP network \ +------+ +-----------------+ | +-------+ \-----| PE |----| C-SMA | | | C-NMA | +------+ |-----------------| | +-------+ |Buffer| | Service Site B | +------------------------------+------+ | Subtask 1 | | Subtask 2 | +-----------------+ Figure 1: Service Segmentation Extensions for CATS-MUP Architecture Figure 1 illustrates the high-level architecture for supporting service segmentation in the CATS-MUP environment, where an application is decomposed into multiple subtasks deployed across different edge sites. Depending on deployment and execution logic, these subtasks may form sequential chains, parallel branches, or iterative exchanges between two subtasks, requiring traffic steering to evolve from single-instance selection toward pipeline-aware steering across distributed subtasks. Bui, et al. Expires 16 April 2027 [Page 4] Internet-Draft CATS-MUP Service Segmentation October 2026 For sequential pipelines, existing Session Transform Routes, including T1ST and T2ST, require the C-PS to configure traffic steering independently for each MUP-PE along the execution chain. As a result, the completion time of the CATS-MUP-C operation increases proportionally with the number of subtasks composing the pipeline. The situation becomes more complex when the active pipeline must be updated or replaced due to mobility events or changing resource conditions, since multiple steering configurations must be recomputed and consistently applied across all participating nodes. To address this limitation, this document proposes a new Session Transform Route referred to as the _MUP Sequence Session Transform_, enabling pipeline-level traffic steering for sequential service execution. The detailed design is presented in Section 4. For parallel pipelines, existing T1ST and T2ST mechanisms remain applicable because traffic distribution and result aggregation can be handled by split and merge functions. User requests may be distributed across parallel subtasks, and intermediate results are combined before delivering the final outcome to the UE. However, each parallel branch may itself consist of a sequential chain of subtasks, in which case the same scalability and steering reconfiguration issues as observed in sequential pipelines arise. Therefore, the proposed _MUP Sequence Session Transform_ can also be applied within such branches to enable pipeline-level traffic steering. For iterative pipelines, a segment list installed when the session is established encodes a fixed, ordered traversal, whereas the number of exchanges between the two subtasks of an iterative pipeline depends on the processed data and is not known in advance. Moreover, these exchanges are generated by the subtask instances themselves rather than carried in the uplink traffic of the session, and, in the case considered in this document, one of the two subtasks is a remote service subtask outside the CATS domain. The session is therefore steered to the selected subtask instance in the CATS domain by a T2ST, as for a single service instance, and the iterative exchange with the remote service subtask takes place outside the MUP, as described in Section 5. Service segmentation may involve stateful subtasks, making subtask migration necessary when an existing pipeline no longer satisfies QoS requirements. Maintaining service continuity during pipeline transitions requires preventing packet loss while new subtask instances are activated and traffic steering is updated. One approach to enabling such transitions is the use of SRv6-based buffering, which temporarily stores packets during path updates and releases them once the new execution pipeline becomes operational. Bui, et al. Expires 16 April 2027 [Page 5] Internet-Draft CATS-MUP Service Segmentation October 2026 Furthermore, a single subtask type may have multiple instances deployed across different MEC sites. Selecting an optimal pipeline based solely on per-instance evaluation may introduce significant decision overhead as deployment scale increases. To address this challenge, Service Pipeline Metrics are introduced to evaluate pipelines as unified entities rather than independent instances, enabling efficient pipeline selection as described in [I-D.draft-dcn-cats-req-service-segmentation]. 4. MUP Sequence Session Transform Mechanism The MUP Sequence Session Transform defines how the Mobile User Plane steers packets across an ordered set of service stages using a single session transformation operation. Under this mechanism, the execution sequence of a service pipeline is encoded directly into packet forwarding behavior at MUP-PE nodes. +-----------+ +-----------+ | Subtask 1 | | Subtask 2 | +-----------+ +-----------+ | | +-----------+ +-----------+ | SFC Proxy | | SFC Proxy | +-----------+ +-----------+ ^ ^ +----------+ | +----------+ +----------+ | +----------+ |NSH(X,255)| | |Inner Pkt | |NSH(X,254)| | |Inner Pkt | +----------+ | +----------+ +----------+ | +----------+ |Inner Pkt | | |NSH(X,254)| |Inner Pkt | | |NSH(X,253)| +----------+ v +----------+ +----------+ v +----------+ +--+ +------+ +--------+ +--------+ |UE|->|RAN PE|---------->|MUP PE 1|------------------>|MUP PE 2| +--+ +------+ +--------+ +--------+ +-------------+ +-------------+ | SRH | | SRH | |[0]: MUP PE 2| |[0]: MUP PE 2| |[1]: MUP PE 1| |[1]: MUP PE 1| | SL: 2 | | SL: 1 | +-------------+ +-------------+ | NSH(X,255) | | NSH(X,254) | +-------------+ +-------------+ | Inner Pkt | | Inner Pkt | +-------------+ +-------------+ Figure 2: MUP Sequence Session Transform Example Bui, et al. Expires 16 April 2027 [Page 6] Internet-Draft CATS-MUP Service Segmentation October 2026 The execution order of a service pipeline is encoded directly into packet forwarding state. Specifically, the SRv6 segment list carried in the SRH defines the traversal sequence across MUP-PE nodes, while service processing context is maintained within the packet using SRv6 Service Function Chaining (SFC) mechanisms, as specified in [RFC9491]. As illustrated in Figure 2, packets advance along the pipeline as each MUP-PE processes its local segment and updates the associated service context. * The MUP Sequence Session Transform is installed by the CATS-MUP-C at the ingress MUP-PE located at the RAN side. For UE traffic matching the corresponding session policy, the ingress MUP-PE decapsulates the GTP header and re-encapsulates the packet using SRv6 Service Function Chaining (SFC). * When traffic is steered according to the SRv6 segment list, each MUP-PE performs the following processing steps: - The MUP-PE processes its local SRv6 SID and removes the outer SRv6 encapsulation, leaving the NSH and inner packet intact. - The packet is delivered toward the corresponding subtask instance. Depending on deployment, the MUP-PE MAY apply a site-specific encapsulation (such as VXLAN-gre, GRE [ieee-sfc-over-srv6]) before forwarding the packet to the service instance. - If the subtask instance does not support NSH processing, an SFC proxy MAY be used to terminate or translate the NSH on behalf of the service function. - After processing, the packet is returned to the MUP-PE together with updated service context. - The MUP-PE re-applies the SRv6 encapsulation, updates SRH processing state, and forwards the packet toward the next segment indicated in the remaining SRv6 segment list without additional control-plane interaction. * The NSH context header may include service pipeline identifiers, UE-related attributes, or intermediate processing state, enabling distributed service functions to coordinate execution and maintain continuity across the pipeline. By embedding pipeline progression into dataplane processing, the MUP Sequence Session Transform enables ordered service execution across multiple edge locations. Bui, et al. Expires 16 April 2027 [Page 7] Internet-Draft CATS-MUP Service Segmentation October 2026 5. Iterative Execution with a Remote Service Subtask +----------------------------------------+ | MUP network +-----------+ +-----------+ | +---------+ | Subtask A |=L1=>| | | |MUP PE 1 |<-->| (draft) || RAN PE |-----------+ | | (target) | | | | |(ingress)| +-----------+ |Remote site| +--+ | +---------+ +---------+ | Subtask A |=L1=>| | | |MUP PE 2 |<-->| (draft) |. [I-D.draft-dcn-cats-req-service-segmentation] Bui, D. K., "Additional CATS Requirements Consideration for Service Segmentation-related Use Cases", 2026, . [RFC9491] Guichard, J. and J. Tantsura, "Integration of the Network Service Header (NSH) and Segment Routing for Service Function Chaining (SFC)", November 2023, . 9.2. Informative References Bui, et al. Expires 16 April 2027 [Page 10] Internet-Draft CATS-MUP Service Segmentation October 2026 [ieee-access-service-segmentation] Tran, M-N., "Design of 5G Architecture Enhancements for Supporting Edge Split Computing Service Pipeline", 2025, . [ieee-access-service-continuity-stateful-service-segmentation] Nguyen Trung, K., "Enabling Service Continuity for Stateful Service Segmentation in Mobile Edge Computing Toward 6G", 2026, . [ieee-sfc-over-srv6] Nguyen Trung, K., "A Design and Implementation of Service Function Chaining Over Segment Routing IPv6 Network", 2024, . [future-SRv6-service-continuity] Lemmi, J., "SRv6-Based Edge Service Continuity in 5G Mobile Networks", 2023, . Authors' Addresses Duong Kham Bui Soongsil University 369, Sangdo-ro, Dongjak-gu Seoul 06978 Republic of Korea Phone: +82 10 5812 2230 Email: bdkham@dcn.ssu.ac.kr Kiem Nguyen Trung Soongsil University 369, Sangdo-ro, Dongjak-gu Seoul 06978 Republic of Korea Email: kiemnt@dcn.ssu.ac.kr Minh-Ngoc Tran Soongsil University 369, Sangdo-ro, Dongjak-gu Seoul 06978 Republic of Korea Bui, et al. Expires 16 April 2027 [Page 11] Internet-Draft CATS-MUP Service Segmentation October 2026 Email: mipearlska1307@dcn.ssu.ac.kr Younghan Kim Soongsil University 369, Sangdo-ro, Dongjak-gu Seoul 06978 Republic of Korea Phone: +82 10 2691 0904 Email: younghak@ssu.ac.kr Bui, et al. Expires 16 April 2027 [Page 12]