]> Cisco Systems, Inc.

Japan tkamata@cisco.com

Cisco Systems, Inc.

Czech Republic jakuhorn@cisco.com

Verizon

USA luay.jalil@verizon.com

China Mobile

China chengweiqiang@chinamobile.com

Keio University

Japan miya_kohno@keio.jp

Internet DMM Working Group This document describes the solution approach and its architectural benefits of transforming mobile session information into routing information, leveraging segment routing capabilities, and operating within the IP routing paradigm.

Introduction The existing mobile user plane is currently defined as an overlay tunnel session to a mobile anchor point (UPF: User Plane Function in 5G context). While this approach may be suited for use cases requiring frequent mobile handover and functionality tied to session initiation/ termination, it proves challenging to cost-effectively and scalably address the high traffic volumes of the 5G/Beyond 5G era and the increasingly distributed data and computing demands in the future. The requirements for wireless systems are becoming more diverse, and there are cases , such as some IoT and FWA (Fixed Wireless Access) systems, where the frequent mobile handover is not necessarily mandatory. This document describes the solution approach and its architectural benefits of transforming mobile session information into routing information, leveraging segment routing capabilities, and operating within the IP routing paradigm. And this document clarifies the motivation for the MUP initiatives :

Problem Definition The current tunnel session based mobile user plane has the following limitations and is getting hard to support new application requirements.

• Less suited for any-to-any communication
• Less suited for edge/distributed computing
• Less suited for fixed and mobile convergence (FMC) / wireless-wireline convergence (WWC)
• Limited control of the underlay path

The scalability comparison in this document refers to the amount of connectivity state needed to represent optimized any-to-any communication, rather than to the computational complexity of all mobile control-plane procedures. In the existing 3GPP architecture, the mobile control plane remains responsible for establishing and maintaining PDU sessions, including the selection of the relevant user-plane functions and GTP-U tunnel endpoints. The conventional anchor-based architecture can maintain one session per endpoint, but traffic between endpoints is generally carried through the selected anchor. If optimized any-to-any connectivity among N endpoints is instead represented using a separate tunnel or forwarding state for each communicating endpoint pair, the amount of such state can grow as N(N-1), or O(N^2). In the MUP architecture, the 3GPP session-management functions continue to manage the lifecycle and policy of mobile sessions. Relevant session information is learned by the MUP control plane and transformed into destination-based routing information. A route describing reachability to a destination can be shared by traffic from multiple sources, and transit nodes do not need to maintain GTP-U tunnel state for every communicating endpoint pair. Consequently, optimized any-to-any reachability can be represented by O(N) destination-based entries at a forwarding node, before route aggregation, rather than by O(N^2) pairwise entries. This comparison does not imply that all per-session state is eliminated. The 3GPP session-management functions continue to maintain the session lifecycle and policy information required by the mobile service. Rather, the scalability benefit described here comes from representing optimized any-to-any reachability using destination-based routing information that can be shared by multiple sources, instead of maintaining separate GTP-U tunnel or forwarding state for every communicating endpoint pair. Edge/distributed computing can be seen as a subset of any-to-any communication. IP Routing paradigm naturally supports ubiquitous computing. As for FMC/WWC, there is currently a coordinated standardization effort between 3GPP WWC and BBF . However, the idea is to anchor even wireline traffic in the mobile packet core, which compromises simplicity and scalability. In addition, the anchor point that terminates tunnel sessions becomes a scaling bottleneck. The IP routing paradigm naturally removes these tunnel session based restrictions. Segment Routing enables fast protection, policy, multi-tenancy, and provide reliability and SLA differentiation.

SRv6 MUP and the 5G/Beyond 5G use cases This section describes the advantages of applying the SRv6 Mobile User Plane approach for 5G/Beyond 5G use cases. These advantage comes from the fact that it transforms mobile session information into routing information, leverages Segment Routing, and operates within the IP Routing Paradigm. Another advantage, not mentioned here, is the ability to minimize overhead through SRv6 SID Compression. In particular, the adoption of the SRv6 uSID (NEXT-C-SID) flavor significantly reduces encapsulation overhead and enhances packet efficiency. Since uSID allows multiple segments to be compressed into a single IPv6 Destination Address (DA), it optimizes forwarding performance and minimizes the MTU impact even when long Segment Lists are required. This efficiency is a critical enabler for the use cases described below.

Network Slicing Network slicing enables network segmentation, isolation, and SLA differentiation such as latency and availability. End-to-end slicing will be achieved by mapping and coordinating IP network slicing, RAN and mobile packet core slicing. But existing mobile user plane which is overlay tunnel does not have underlying IP network awareness, which could lead to the inability in meeting SLAs. Removing the tunnel and treating it with a IP routing paradigm simplifies the problem. Segment Routing has a comprehensive set of slice engineering technologies. How to build network slicing using the Segment Routing technology is described in . Moreover, the stateless slice identifier encoding can be applicable to enable per-slice forwarding policy using the IPv6 header. The use of uSID facilitates efficient path steering for slice-specific policies. By compressing multiple service and topological SIDs into a compact header, it reduces the risk of fragmentation and MTU-related issues associated with deep SID stacks. This supports scalable deployment of strict SLAs—such as guaranteed latency and path isolation—across the transport network without compromising packet throughput.

Edge Computing Edge computing, where the computing workloads and datastores are placed closer to users, is recognized as one of the key pillars to meet demanding requirements of 5G/Beyond 5G era, with regard to low latency, bandwidth efficiency, data locality and privacy. Edge computing is more important than ever. This is because no matter how much New Radio improves access speeds, it won't improve throughput, latency and user experiences because they are largely bound to end-to-end round trip delay. Even with existing mobile architectures, it is possible to place UPFs in a multi-tier, or to distribute UPFs, to achieve Edge Computing. and describes how to properly select the UPF of adequate proximity. However, complicated and signaling-heavy mechanisms are required to branch traffic or properly use different UPFs. Also, if the UPF is distributed, seamless handover has to be compromised. When it comes to IP routing paradigum, ubiquitous computing is innately supported. uSID is particularly beneficial here, as edge computing frequently involves a high volume of small packets, such as those generated by IoT devices. Traditional SRv6 headers with long Segment Lists can impose a disproportionately high overhead on these small payloads. uSID minimizes this overhead, thereby maximizing effective throughput and ensuring efficient utilization of bandwidth-constrained links in distributed edge environments.

URLLC (Ultra-Reliable Low-Latency Communication) support 3GPP investigates the key issues for meeting the URLLC requirements on latency, jitter and reliability in the 5G System. The solutions provided in the document are focused at improving the overlay protocol (GTP-U) and limits to provide a few hints into how to map such tight-SLA into the transport network. These hints are based on static configuration or static mapping for steering the overlay packet into the right transport SLA. Such solutions do not scale and hinder network economics. Another issue that deserves special mention is the ultra-reliability issue. In order to support ultra-reliability with the tunnel session paradigm, redundant user planes paths based on dual connectivity has been proposed. The proposal has two main options.

• Dual Connectivity based end-to-end Redundant User Plane Path
• Support of redundant transmission on N3/N9 interfaces

In the case of the former, UE and hosts have RHF(Redundancy Handling Function). In sending, RFH is to replicate the traffic onto two GTP-U tunnels, and in receiving, RHF is to merge the traffic. In the case of the latter, traffic are to be replicated and merged with the same sequence for specific QoS flow, which requires further enhancements. And in either cases, the bigger problem is the lack of a reliable way for the redundant sessions to get through the disjoint path: even with the redundant sessions, if it ends up using the same infrastructure at some points, the redundancy is meaningless. These issues can be solved more simply without GTP-U tunnel. Segment routing has some advantages for URLLC traffic. First, traffic can be mapped to a disjoint path or low latency path as needed. Second, Segment routing provides an automated reliability protection mechanism known as TI-LFA, which is a sub-50ms FRR mechanism that provides protection regardless of the topology through the optimal backup path. It can be provisioned slice-aware. uSID further strengthens URLLC support by minimizing serialization delays and optimizing hardware processing at intermediate nodes. By shortening the overall packet length and reducing the number of required hardware lookups, uSID helps minimize jitter and provides more predictable end-to-end latency.

Co-existence and Incremental Deployability Mobile networks are composed of radio, mobile packet core, and IP networks (access and backbone), with separate standard organizations and communities. Therefore, in the steady state, it is difficult to innovate to a new architecture and requires coexistence and incremental deployment. defines the user plane convergence between GTP-U and SRv6, so that it can co-exist with the existing user plane as needed. defines the MUP architecture for Distributed Mobility Management, which can be plugged into the existing mobile service architecture. In the architecture, mobile session information is transformed to routing information, and operated in L3VPN scheme.

Security Considerations The deployment of this architecture is targeted in an administrative domain, and the functionality is domain specific.

IANA Considerations This memo includes no request to IANA.

References Normative References This document discusses the applicability of Segment Routing over IPv6 (SRv6) to the user plane of mobile networks. The network programming nature of SRv6 accomplishes mobile user-plane functions in a simple manner. The statelessness of SRv6 and its ability to control both service layer path and underlying transport can be beneficial to the mobile user plane, providing flexibility, end-to-end network slicing, and Service Level Agreement (SLA) control for various applications. This document discusses how SRv6 could be used as the user plane of mobile networks. This document also specifies the SRv6 Endpoint Behaviors required for mobility use cases. Segment Routing over IPv6 (SRv6) is the instantiation of Segment Routing (SR) on the IPv6 data plane. This document specifies new flavors for the SRv6 endpoint behaviors defined in RFC 8986, which enable the compression of an SRv6 segment list. Such compression significantly reduces the size of the SRv6 encapsulation needed to steer packets over long segment lists. This document updates RFC 8754 by allowing a Segment List entry in the Segment Routing Header (SRH) to be either an IPv6 address, as specified in RFC 8754, or a REPLACE-CSID container in packed format, as specified in this document. Informative References Cisco Systems Cisco Systems Cisco Systems Bell Canada Softbank Ciena Rakuten Airtel This document describes how to realize the IETF network slice using the Segment Routing based technology. It explains how the building blocks specified for the Segment Routing can be used for this purpose. Cisco Systems, Inc. Cisco Systems, Inc. Cisco Systems, Inc. Cisco Systems, Inc. Bell Canada Ciena This document defines a stateless and scalable solution to achieve network slicing with SRv6. SoftBank SoftBank Arrcus, Inc Arrcus, Inc Cisco Systems This document defines the Mobile User Plane (MUP) architecture for Distributed Mobility Management. The requirements for Distributed Mobility Management described in [RFC7333] can be satisfied by routing fashion. In MUP Architecture, session information between the entities of the mobile user plane is turned to routing information so that mobile user plane can be integrated into dataplane. MUP architecture is designed to be pluggable user plane part of existing mobile service architectures, enabled by auto-discovery for the use plane. Segment Routing provides network programmability for a scalable option with it. While MUP architecture itself is independent from a specific dataplane protocol, several dataplane options are available for the architecture. This document describes IPv6 dataplane in Segment Routing case (SRv6 MUP) due to the DMM requirement, and is suitable for mobile services which require a large IP address space. BBF ETSI 3GPP 3GPP 3GPP

Acknowledgements Authors would like to thank Satoru Matsushima, Shunsuke Homma, Yuji Tochio and Jeffrey Zhang, for their insights and comments.

Contributors Cisco Systems, Inc.

France fclad.ietf@gmail.com

Cisco Systems, Inc.

Spain pcamaril@cisco.com

Cisco Systems, Inc.

USA zali@cisco.com