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Versions: 00 01 02 03 04 05                                             
DMM Working Group                                               M. Kohno
Internet-Draft                                                   F. Clad
Intended status: Informational                              P. Camarillo
Expires: May 12, 2022                                             Z. Ali
                                                     Cisco Systems, Inc.
                                                        November 8, 2021

           Architecture Discussion on SRv6 Mobile User plane


   SRv6 mobile user plane is standardized in IETF.  It accomplishes the
   mobile user-plane functions in a simple, flexible and scalable
   manner, by utilizing the network programming nature of SRv6.  It
   leverages common native IPv6 data plane and creates interoperable
   overlays with underlay optimization.

   This document discusses the solution approach and its architectural
   benefits of common data plane across domains and across overlay/

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 12, 2022.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (https://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Definition  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Common data plane across domains and across overlay/underlay    3
   4.  Control Plane Considerations  . . . . . . . . . . . . . . . .   4
   5.  Incremental Deployability . . . . . . . . . . . . . . . . . .   4
   6.  SRv6 mobile user plane and the 5G use cases . . . . . . . . .   5
     6.1.  Network Slicing . . . . . . . . . . . . . . . . . . . . .   5
     6.2.  Edge Computing  . . . . . . . . . . . . . . . . . . . . .   5
     6.3.  URLLC (Ultra-Reliable Low-Latency Communication) support    6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Mobile architectures have evolved individually, and the user plane,
   GTP-U, has been defined as an overlay tunnel that is agnostic to the
   IP infrastructure.

   However, the system requirements are changing as digitalization goes
   into full swing.  The continued use of GTP-U as a user plane protocol
   will lock-in to the existing architectural structure and hinder the
   innovation.  GTP-U will not be able to meet the diverse SLA
   requirements of the 5G era and beyond with efficiency and
   scalability.  Also it will not be able to meet the demands of new
   mobile-first data intensive applications, which will be more
   dynamically distributed.

   SRv6 mobile user plane [I-D.ietf-dmm-srv6-mobile-uplane] is
   standardized in IETF.  It accomplishes the mobile user-plane
   functions in a simple, flexible and scalable manner, by utilizing the
   network programming nature of SRv6.  It leverages common native IPv6
   data plane and creates interoperable overlays with underlay

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   This document discusses the solution approach and its architectural
   benefits of common data plane across domains (e.g., mobile domain, IP
   infrastructure, data center, applications) and across overlay/

2.  Problem Definition

   The current mobile user plane, GTP-U, defined as an overlay tunnel
   that is agnostic to the IP infrastructure, has the following
   limitations that prevent it from supporting new application demands.

   o  Non-optimal for any-to-any communication
   o  No control of the underlay path
   o  Non-optimal for edge/distributed computing
   o  Non-optimal for fixed and mobile path convergence
   o  Lack a way for application/service developers to manipulate and

   In addition, the centralized tunnel terminating gateway becomes a
   scaling bottleneck and a single point of failure

   For residential broadband IP and data center networking, tunnel
   sessions could be eliminated (e.g.  PPPoE -> IPoE, VXLAN/NSH ->
   SRv6).  This indicates that a tunnel session is not necessarily
   absolute.  But such a thing was unlikely to happen in the mobile

   As for FMC, there is currently a coordinated standardization effort
   between 3GPP WWC [TS.23316] and BBF [BBF407].  However, the idea is
   to anchor even wireline traffic in the mobile packet core, which
   compromises simplicity and scalability.

3.  Common data plane across domains and across overlay/underlay

   [I-D.ietf-dmm-srv6-mobile-uplane] defines SRv6 mobile user plane as
   an alternative or co-existing solution to GTP-U.

   Since SRv6 is a native IPv6 data plane, it can be a common data plane
   regardless of the domain.

   SRv6 Network Programming [RFC8986] enables the creation of overlays
   with underlay optimization.  In addition, SRv6 can be operated by
   application developers because of its implementation in the computing
   stack, e.g.  VPP, Linux Kernel, smart NIC, and cloud native platform
   such as Network Service Mesh.

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   Data plane commonality offers significant advantage regarding
   function, scaling, and cost.  In particular, the benefits of the 5G
   era are shown in Section 6.

   Note that the interaction with underlay infrastructure is not a
   mandatory in the data plane commonality.  It just gives a design
   choice to interact with the underlay and optimize it, and it is
   totally fine to keep ovelray underlay-agnostic, which will allow the
   coexistence of different capability of nodes.

4.  Control Plane Considerations

   This document focuses on the commonalization of data plane, and the
   control plane is out of scope.  The actual system characteristics
   such as scaling and functionality depend heavily on the control
   plane, though.

   The potential of the SRv6 mobile user plane is huge, in the sense
   that it can realize various functions of mobile management using SRv6
   Network Programming.  Protocols such as GTP-C, PMIPv6, BGP, LISP,
   ILNP, hICN, or even others can be applied as a control plane to
   control mobility.

   For example, if hICN [I-D.auge-dmm-hicn-mobility] was used,
   anchorless mobility can be realised.

5.  Incremental Deployability

   The mobile domain is a compound domain that includes Radio Access,
   and it is difficult to implement a completely new architecture, and
   incremental deployability is required.

   [I-D.ietf-dmm-srv6-mobile-uplane] defines the conversion between
   GTP-U and SRv6, so that it can co-exist with the current mobile
   architecture as needed.  Since the conversion is done statelessly
   (i.e., all necessary information is retained in the packet), there
   will not be a scaling bottleneck or a single point of failure.

   Further, [I-D.mhkk-dmm-srv6mup-architecture] defines the SRv6 MUP
   architecture for Distributed Mobility Management, which can be
   plugged to the existing mobile service architecture.

   In this way, SRv6 Network Programmability allows for proper

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6.  SRv6 mobile user plane and the 5G use cases

   This section describes the advantages of the common data plane and of
   applying SRv6 mobile user plane for 5G use cases.

6.1.  Network Slicing

   Network slicing enables network segmentation, isolation, and SLA
   differentiation in terms of latency and availability.  End-to-end
   slicing will be achieved by mapping and coordinating IP network
   slicing, RAN and mobile packet core slicing.

   However, as pointed out in [I-D.clt-dmm-tn-aware-mobility], the 5G
   System as defined, does not have underlying IP network awareness,
   which could lead to the inability in meeting SLAs.

   Segment Routing has a comprehensive set of slice engineering
   technologies.  How to build network slicing using the Segment Routing
   based technology is described in

   In the typical GTP-U over IP/MPLS/SR configuration, 3GPP data plane
   entity such as UPF is a CE to the transport networks PE.  But if 3GPP
   they support SRv6 mobile user plane, they can directly participate in
   network slicing, and solves the following issues.

   o  A certain Extra ID such as VLAN-ID is needed for segregating
      traffic and mapping it onto a designated slice.
   o  PE and the PE-CE connection is a single point of failure, so some
      form of PE redundancy (using routing protocols, MC-LAG, etc.) is

   Moreover, the stateless slice identifier encoding
   [I-D.filsfils-spring-srv6-stateless-slice-id] can be applicable to
   enable per-slice forwarding policy using the IPv6 header.

6.2.  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 5G's demanding requirements, with regard to low latency,
   bandwidth efficiency, and data privacy.  The computing workload
   includes network services, security, data analytics, content cache
   and various applications.  (UPF itself can also be viewed as a
   distributed network service function.)

   Edge computing is more important than ever.  This is because no
   matter how much 5G improves access speeds, it won't improve end-to-

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   end throughput because it's largely bound to round trip delay.  It is
   also important from the viewpoint of "local production for local
   consumption" and privacy protection.

   However, the current MEC discussion [ETSI-MEC] focuses on how to
   properly select the UPF of adequate proximity, and not on how to
   interact with applications.

   SRv6 has an advantage in enabling edge computing for the following

   o  Programmable and Flexible Traffic Steering : SRv6's flexible
      traffic steering capabilities and the network programming concept
      is suitable for flexible placement of computing workload.
   o  Common data plane across domains : SRv6/IPv6 can be a common data
      plane regardless of the domains such as mobile including UE, IP
      transport, data center, applications.
   o  Stateless Service Chaining : It does not require any per-flow
      state in network fabric.
   o  Interaction with Applications : SRv6 can be implemented in the
      compute stack and can be manipulated by applications using socket
      API.  Also, SRv6 can carry meta data, which can be used for
      interacting with applications.
   o  Functionality without performance degradation : Various
      information can be exposed in IP header, but it does not degrade
      performance thanks to the longest match mechanism in the IP
      routing.  Only who needs the information for granular processing
      are to lookup.

   It is even more beneficial if service functions/applications directly
   support SRv6.

6.3.  URLLC (Ultra-Reliable Low-Latency Communication) support

   3GPP [TR.23725] 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.

   Some of the issues can be solved more simply without GTP-U tunnel.
   SRv6 mobile user plane can exposes session and QoS flow information
   in IP header as discussed in the previous section.  This would make
   routing and forwarding path optimized for URLLC, much simpler than
   the case with GTP-U tunnel.

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   Another issue that deserves special mention is the ultra-reliability
   issue.  In 3GPP, in order to support ultra-reliability, redundant
   user planes paths based on dual connectivity has been proposed.  The
   proposal has two main options.

   o  Dual Connectivity based end-to-end Redundant User Plane Paths
   o  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, the 3GPP data plane entities are to
   replicate and merge the packets 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.

   SRv6 mobile user plane has some advantages for URLLC traffic.  First,
   with SRv6, Traffic can be mapped to a disjoint path or low latency
   path as needed, by means of the scalable Traffic Engineering.

   Additionally, SRv6 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.

   With the case that dual live-live path is required, the problem is
   not only the complexity but that the replication point and the
   merging point would be the single point of failure.  The SRv6 mobile
   user plane also has an advantage in this respect, because any
   endpoints or 3GPP data plane nodes themselves can be the replication/
   merging point when they are SRv6 aware.

   Furthermore, SRv6 supports inband telemetry/time stamping for latency
   monitoring and control.

7.  Security Considerations


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8.  IANA Considerations


9.  Acknowledgements

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

10.  References

10.1.  Normative References

              Psenak, P., Hegde, S., Filsfils, C., and A. Gulko, "ISIS
              Segment Routing Flexible Algorithm", draft-hegdeppsenak-
              isis-sr-flex-algo-02 (work in progress), February 2018.

              Matsushima, S., Filsfils, C., Kohno, M., Garvia, P. C.,
              Voyer, D., and C. E. Perkins, "Segment Routing IPv6 for
              Mobile User Plane", draft-ietf-dmm-srv6-mobile-uplane-17
              (work in progress), October 2021.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,

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10.2.  Informative References

   [BBF407]   BBF, "5G Wireless Wireline Convergence Architecture", BBF
              TR-407 Issue:1, August 2020.

              ETSI, "MEC in 5G Networks", ETSI White Paper No.28, June

              Ali, Z., Filsfils, C., Camarillo, P., and D. Voyer,
              "Building blocks for Slicing in Segment Routing Network",
              draft-ali-spring-network-slicing-building-blocks-04 (work
              in progress), February 2021.

              Auge, J., Carofiglio, G., Muscariello, L., and M.
              Papalini, "Anchorless mobility through hICN", draft-auge-
              dmm-hicn-mobility-04 (work in progress), July 2020.

              Auge, J., Carofiglio, G., Muscariello, L., and M.
              Papalini, "Anchorless mobility management through hICN
              (hICN-AMM): Deployment options", draft-auge-dmm-hicn-
              mobility-deployment-options-04 (work in progress), July

              Chunduri, U., Li, R., Bhaskaran, S., Kaippallimalil, J.,
              Tantsura, J., Contreras, L. M., and P. Muley, "Transport
              Network aware Mobility for 5G", draft-clt-dmm-tn-aware-
              mobility-09 (work in progress), February 2021.

              Filsfils, C., Clad, F., Camarillo, P., Raza, K., Voyer,
              D., and R. Rokui, "Stateless and Scalable Network Slice
              Identification for SRv6", draft-filsfils-spring-srv6-
              stateless-slice-id-04 (work in progress), July 2021.

              Matsushima, S., Horiba, K., Khan, A., Kawakami, Y.,
              Murakami, T., Patel, K., Kohno, M., Kamata, T., and P.
              Camarillo, "Segment Routing IPv6 Mobile User Plane
              Architecture for Distributed Mobility Management", draft-
              mhkk-dmm-srv6mup-architecture-00 (work in progress),
              October 2021.

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   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,

              3GPP, "Study on enhancement of Ultra-Reliable Low-Latency
              Communication (URLLC) support in the 5G Core network
              (5GC)", 3GPP TR 23.725 16.2.0, June 2019.

              3GPP, "Study on User Plane Protocol in 5GC", 3GPP TR
              29.892 16.1.0, April 2019.

              3GPP, "Wireless and wireline convergence access support
              for the 5G System (5GS)", 3GPP TS 23.316 16.7.0, September

              3GPP, "System Architecture for the 5G System", 3GPP TS
              23.501 15.0.0, November 2017.

              3GPP, "Interface between the Control Plane and the User
              Plane Nodes", 3GPP TS 29.244 15.0.0, December 2017.

              3GPP, "General Packet Radio System (GPRS) Tunnelling
              Protocol User Plane (GTPv1-U)", 3GPP TS 29.281 15.1.0,
              December 2017.

Authors' Addresses

   Miya Kohno
   Cisco Systems, Inc.

   Email: mkohno@cisco.com

   Francois Clad
   Cisco Systems, Inc.

   Email: fclad@cisco.com

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   Pablo Camarillo Garvia
   Cisco Systems, Inc.

   Email: pcamaril@cisco.com

   Zafar Ali
   Cisco Systems, Inc.

   Email: zali@cisco.com

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