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DMM Working Group                                               M. Kohno
Internet-Draft                                                   F. Clad
Intended status: Informational                              P. Camarillo
Expires: May 6, 2021                                              Z. Ali
                                                     Cisco Systems, Inc.
                                                        November 2, 2020


           Architecture Discussion on SRv6 Mobile User plane
                    draft-kohno-dmm-srv6mob-arch-03

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on May 6, 2021.

Copyright Notice

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   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Definition  . . . . . . . . . . . . . . . . . . . . .   2
   3.  Common data plane across domains and across overlay/underlay    3
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  SRv6 mobile user plane and the 5G use cases . . . . . . . . .   4
     5.1.  Network Slicing . . . . . . . . . . . . . . . . . . . . .   4
     5.2.  Edge Computing  . . . . . . . . . . . . . . . . . . . . .   5
     5.3.  URLLC (Ultra-Reliable Low-Latency Communication) support    6
   6.  Control Plane Considerations  . . . . . . . . . . . . . . . .   7
   7.  Incremental Deployment  . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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, it will not be able to efficiently meet the diverse SLA
   requirements of the 5G era.  Also, it will not be able to meet the
   demands of new mobile first and/or data intensive applications.

   This document discusses a solution approach and its architectural
   benefits of common data plane across domains (e.g., mobile including
   UE, IP transport, data center, applications) and across overlay/
   underlay.

   This approach is in a sense contrary to proposals that the underlying
   transport can be anything (L2, IPv4, IPv6, MPLS, SR, SRv6).  But it
   is an approach to make the network as flat as possible, making it
   suitable for the distributed mobile deployment model.

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  lack a way to map to the underlay



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   o  Non-optimal for edge/distributed computing
   o  Lack a way for application developers to manipulate

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

   For IP and data center networking, tunnel sessions can be eliminated
   when necessary and if possible (e.g.  PPPoE -> IPoE, VXLAN/GENEVE/NSH
   -> SRv6), but such an architectural change used to be difficult for
   mobile domain.

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 [I-D.ietf-spring-srv6-network-programming]
   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.

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

   Note that the interaction with underlay infrastructure is not a
   mandatory in the data plane commonality.  It just gives a design
   option to interact with the underlay and optimize it, and it is
   totally fine to keep ovelray underlay-agnostic.

4.  Terminology

   The terminology used in this document leverages and conforms to
   [I-D.ietf-dmm-srv6-mobile-uplane].













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                                  +-----+
                                  | AMF |
                                  +-----+
                                 /    | [N11]
                          [N2]  /  +-----+
                        +------/   | SMF |
                       /           +-----+
                      /              / \
                     /              /   \  [N4]
                    /              /     \                    ________
                   /              /       \                  /        \
   +--+      +-----+ [N3] +------+  [N9]  +------+  [N6]    /          \
   |UE|------| gNB |------| UPF1 |--------| UPF2 |--------- \    DN    /
   +--+      +-----+  TN  +------+   TN   +------+           \________/
                              |
                           _______
                          /       \
                         /  Local  \
                         \   DN    /
                          \_______/


                     Figure 1: Reference Architecture

   - UE : User Equipment
   - gNB : gNodeB
   - UPF : User Plane Function
   - SMF : Session Management Function
   - AMF : Access and Mobility Management Function
   - 3GPP data plane entities : 3GPP entities responsible for data plane
     forwarding, i.e.  gNB and UPF
   - TN : Transport Network - IP network where 3GPP data plane entities
     connected
   - DN : Data Network e.g. operator services, Internet access
   - CUPS : Control Plane and User Plane Separation
   - VNF : Virtual Network Function
   - CNF : Cloud native Network Function

5.  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.

5.1.  Network Slicing

   Network slicing enables network segmentation, isolation, and SLA
   differentiation in terms of latency and availability.  End-to-end




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   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
   [I-D.ali-spring-network-slicing-building-blocks].

   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 efficiently 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
      required.

   And In some deployment scenarios, it may be better to have a
   transport network PE present.  For such a case, 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.

5.2.  Edge Computing

   Edge computing, where the computing workload is placed closer to
   users, is recognized as one of the key pillars to meet 5G's demanding
   key performance indicators (KPIs), especially with regard to low
   latency and bandwidth efficiency.  The computing workload includes
   network services, security, analytics, content cache and various
   applications.  (UPF 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-
   end throughput because it's largely bound to round trip delay.

   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.





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   SRv6 has an advantage in enabling edge computing for the following
   reasons.

   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.

5.3.  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.

   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



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   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.

   SRv6 mobile user plane has some advantages for URLLC traffic.  First,
   it can be used to enforce a low-latency path in the network by means
   of 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.

6.  Control Plane Considerations

   This draft focuses on commonalization of data plane, and control
   plane is out of scope for now.  Having said that, IGP and BGP
   extension for SRv6 can be used as the control plane as they are.

   As for the mobility management, BGP based Loc/ID mapping would be
   straightforward to implement.  Or even pure ID based anchorless
   protocol such as hICN [I-D.auge-dmm-hicn-mobility] can be used.

   The co-existence with the 3GPP control plane is for further study.

7.  Incremental Deployment

   Although it may seem difficult to migrate from the current mobile
   architecture, the conversion between GTP-U and SRv6 has been defined
   and can co-exist with the current mobile architecture as needed.
   Since the conversion is done completely statelessly (i.e., all
   necessary information is retained in the packet), there will not be a
   scaling bottleneck or a single point of failure.





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   With regard to the architectural migration, the insertion with no
   modification to the existing 3GPP architecture is considered first,
   and then the tighter integration of data plane is to be achieved. as
   described in [I-D.auge-dmm-hicn-mobility-deployment-options].

8.  Security Considerations

   TBD

9.  IANA Considerations

   NA

10.  Acknowledgements

   Authors would like to thank Satoru Matsushima and Shunsuke Homma for
   their insights and comments.

11.  References

11.1.  Normative References

   [I-D.hegdeppsenak-isis-sr-flex-algo]
              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.

   [I-D.ietf-dmm-srv6-mobile-uplane]
              Matsushima, S., Filsfils, C., Kohno, M., Camarillo, P.,
              Voyer, D., and C. Perkins, "Segment Routing IPv6 for
              Mobile User Plane", draft-ietf-dmm-srv6-mobile-uplane-09
              (work in progress), July 2020.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-24 (work in
              progress), October 2020.







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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

11.2.  Informative References

   [ETSI-MEC]
              ETSI, "MEC in 5G Networks", ETSI White Paper No.28, June
              2018.

   [I-D.ali-spring-network-slicing-building-blocks]
              Ali, Z., Filsfils, C., Camarillo, P., and D. Voyer,
              "Building blocks for Slicing in Segment Routing Network",
              draft-ali-spring-network-slicing-building-blocks-02 (work
              in progress), November 2019.

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

   [I-D.auge-dmm-hicn-mobility-deployment-options]
              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
              2020.

   [I-D.clt-dmm-tn-aware-mobility]
              Chunduri, U., Li, R., Bhaskaran, S., Kaippallimalil, J.,
              Tantsura, J., Contreras, L., and P. Muley, "Transport
              Network aware Mobility for 5G", draft-clt-dmm-tn-aware-
              mobility-07 (work in progress), September 2020.

   [I-D.filsfils-spring-srv6-interop]
              Filsfils, C., Clad, F., Camarillo, P., Abdelsalam, A.,
              Salsano, S., Bonaventure, O., Horn, J., and J. Liste,
              "SRv6 interoperability report", draft-filsfils-spring-
              srv6-interop-02 (work in progress), March 2019.






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   [I-D.filsfils-spring-srv6-stateless-slice-id]
              Filsfils, C., Clad, F., Camarillo, P., and K. Raza,
              "Stateless and Scalable Network Slice Identification for
              SRv6", draft-filsfils-spring-srv6-stateless-slice-id-01
              (work in progress), July 2020.

   [I-D.guichard-spring-srv6-simplified-firewall]
              Guichard, J., Filsfils, C., daniel.bernier@bell.ca, d.,
              Li, Z., Clad, F., Camarillo, P., and A. Abdelsalam,
              "Simplifying Firewall Rules with Network Programming and
              SRH Metadata", draft-guichard-spring-srv6-simplified-
              firewall-02 (work in progress), April 2020.

   [I-D.ietf-dmm-fpc-cpdp]
              Matsushima, S., Bertz, L., Liebsch, M., Gundavelli, S.,
              Moses, D., and C. Perkins, "Protocol for Forwarding Policy
              Configuration (FPC) in DMM", draft-ietf-dmm-fpc-cpdp-14
              (work in progress), September 2020.

   [I-D.rokui-5g-transport-slice]
              Rokui, R., Homma, S., Lopez, D., Foy, X., Contreras, L.,
              Ordonez-Lucena, J., Martinez-Julia, P., Boucadair, M.,
              Eardley, P., Makhijani, K., and H. Flinck, "5G Transport
              Slice Connectivity Interface", draft-rokui-5g-transport-
              slice-00 (work in progress), July 2019.

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,
              <https://www.rfc-editor.org/info/rfc5213>.

   [TR.23725]
              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.

   [TR.29892]
              3GPP, "Study on User Plane Protocol in 5GC", 3GPP TR
              29.892 16.1.0, April 2019.

   [TS.23501]
              3GPP, "System Architecture for the 5G System", 3GPP TS
              23.501 15.0.0, November 2017.

   [TS.29244]
              3GPP, "Interface between the Control Plane and the User
              Plane Nodes", 3GPP TS 29.244 15.0.0, December 2017.




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   [TS.29281]
              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.
   Japan

   Email: mkohno@cisco.com


   Francois Clad
   Cisco Systems, Inc.
   France

   Email: fclad@cisco.com


   Pablo Camarillo Garvia
   Cisco Systems, Inc.
   Spain

   Email: pcamaril@cisco.com


   Zafar Ali
   Cisco Systems, Inc.
   Canada

   Email: zali@cisco.com


















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