Service Segmentation Considerations for CATS-MUP
draft-dcn-dmm-cats-mup-service-segmentation-00

Distributed Mobility Management                          K. Nguyen-Trung
Internet-Draft                                                   N. Tran
Intended status: Informational                                    Y. Kim
Expires: 2 September 2026                            Soongsil University
                                                            1 March 2026

            Service Segmentation Considerations for CATS-MUP
             draft-dcn-dmm-cats-mup-service-segmentation-00

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 a MUP Sequence Session Transform to support
   ordered traversal across multiple subtask instances and to maintain
   service continuity during pipeline updates, particularly when
   stateful subtasks are involved.

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   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Architecture Overview . . . . . . . . . . . . . . . . . . . .   3
   4.  MUP Sequence Session Transform Mechanism  . . . . . . . . . .   5
   5.  Service Continuity for Stateful Service Segmentation  . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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

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   traverse service instances in a predefined sequence, or as a parallel
   service pipeline, where traffic is distributed to multiple service
   instances and later merged.  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].

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.

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                             +----------------+
                             |    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 or parallel branches,
   requiring traffic steering to evolve from single-instance selection
   toward pipeline-aware steering across distributed subtasks.

   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

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   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.

   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.

   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.

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                              +-----------+                +-----------+
                              | 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

   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:

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      -  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.

5.  Service Continuity for Stateful Service Segmentation

   In practical deployments, service segmentation often involves
   stateful service instances, where execution state must be preserved
   across service instance relocation or pipeline reconfiguration.  In
   such scenarios, immediate redirection of traffic to a newly selected
   service pipeline may result in service disruption if the target
   instances are not yet ready to process incoming requests or if state
   migration is still in progress.

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   For example, consider a sequential service pipeline composed of five
   subtasks executed in order.  The first four subtasks are stateless,
   while the fifth subtask is a stateful application.  When the stateful
   subtask is migrated due to mobility events or computing resource
   degradation, packets processed by the upstream stateless subtasks
   (i.e., Subtasks 1 through 4) may reach at the stateful subtask before
   migration is completed.  As a result, these packets may be dropped or
   fail to be processed correctly, leading to wasted computing resources
   and unnecessary consumption of network bandwidth.

   To preserve service continuity during pipeline transition, this
   document considers new use behaviors
   [future-SRv6-service-continuity], such as _End.M.GTP6.D.Buffer_, an
   extension of the End.M.GTP6.D function, integrated into all MUP
   Session Transform Routes.  These behaviors enable traffic to be
   temporarily held while stateful service instances are being migrated.

   *  The MUP-C installs MUP Session Transform Routes with buffer
      [ieee-access-service-continuity-stateful-service-segmentation] at
      the relevant MUP-PEs, inserting a buffer SID as the first segment
      in the SRv6 segment list.  As a result, packets matching the
      session policy are first steered toward the buffer.  The buffer
      MAY be realized as a dedicated virtual network function (VNF).

   *  Furthermore, the MUP-C updates the MUP-PE of all subtasks (current
      pipeline) so that packets returning from upstream subtask are re-
      encapsulated using _SR Policy headend behaviors_ with a new SRH.
      The new segment list places the buffer SID as the first segment,
      followed by the MUP-PE of the next subtask in the newly selected
      service pipeline.

   *  Once a new optimal service pipeline is determined, incoming
      traffic is redirected toward buffer, allowing migration of
      multiple stateful subtasks to be performed *in parallel* while
      maintaining service continuity.  Parallel migration reduces
      service disruption time and increases efficiency.

   *  After migration is completed, the MUP-C triggers buffer release
      procedures.  Buffered packets are flushed toward the new pipeline,
      and the buffer removes its SID from the SRH.

   *  Following buffer release, MUP-PE are updated so that subsequent
      packets bypass the buffer and are steered directly along the new
      service pipeline.

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6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   TBD

8.  References

8.1.  Normative References

   [I-D.draft-dcn-dmm-cats-mup]
              Tran, N., "Computing Aware Traffic Steering Consideration
              for Mobile User Plane Architecture", 2026,
              <https://datatracker.ietf.org/doc/draft-dcn-dmm-cats-
              mup/>.

   [I-D.draft-dcn-cats-req-service-segmentation]
              Tran, N., "Additional CATS Requirements Consideration for
              Service Segmentation-related Use Cases", 2026,
              <https://datatracker.ietf.org/doc/draft-dcn-cats-req-
              service-segmentation/>.

   [RFC9491]  Guichard, J. and J. Tantsura, "Integration of the Network
              Service Header (NSH) and Segment Routing for Service
              Function Chaining (SFC)", November 2023,
              <https://datatracker.ietf.org/doc/rfc9491/>.

8.2.  Informative References

   [ieee-access-service-segmentation]
              Tran, M-N., "Design of 5G Architecture Enhancements for
              Supporting Edge Split Computing Service Pipeline", 2025,
              <https://doi.org/10.1109/ACCESS.2025.3630182>.

   [ieee-access-service-continuity-stateful-service-segmentation]
              Nguyen Trung, K., "Enabling Service Continuity for
              Stateful Service Segmentation in Mobile Edge Computing
              Toward 6G", 2026,
              <https://doi.org/10.1109/ACCESS.2026.3661972>.

   [ieee-sfc-over-srv6]
              Nguyen Trung, K., "A Design and Implementation of Service
              Function Chaining Over Segment Routing IPv6 Network",
              2024, <https://doi.org/10.1109/ICTC62082.2024.10827193>.

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   [future-SRv6-service-continuity]
              Lemmi, J., "SRv6-Based Edge Service Continuity in 5G
              Mobile Networks", 2023,
              <https://doi.org/10.3390/fi16040138>.

Authors' Addresses

   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
   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

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