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Versions: (draft-matsushima-spring-dmm-srv6-mobile-uplane) 00 01 02

DMM Working Group                                          S. Matsushima
Internet-Draft                                                  SoftBank
Intended status: Standards Track                             C. Filsfils
Expires: January 3, 2019                                        M. Kohno
                                                            P. Camarillo
                                                     Cisco Systems, Inc.
                                                                D. Voyer
                                                             Bell Canada
                                                              C. Perkins
                                                               Futurewei
                                                            July 2, 2018


               Segment Routing IPv6 for Mobile User Plane
                  draft-ietf-dmm-srv6-mobile-uplane-02

Abstract

   This document discusses the applicability of SRv6 (Segment Routing
   IPv6) to user-plane of mobile networks (N3 and N9 interfaces).  The
   source routing capability and the network programming nature of SRv6,
   accomplish mobile user-plane functions in a simple manner.  The
   statelessness and the ability to control underlying layer will be
   even more beneficial to the mobile user-plane, in terms of providing
   flexibility and SLA control for various applications.  It also
   simplifies the network architecture by eliminating the necessity of
   tunnels, such as GTP-U [TS.29281], PMIP [RFC5213], Mac-in-Mac, MPLS,
   and so on.  In addition, Segment Routing provides an enhanced method
   for network slicing, which is briefly introduced by this document.

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 January 3, 2019.





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Copyright Notice

   Copyright (c) 2018 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 and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Reference Architecture  . . . . . . . . . . . . . . . . . . .   5
   5.  User-plane behaviors  . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Traditional mode (formerly Basic mode)  . . . . . . . . .   7
       5.1.1.  Packet flow - Uplink  . . . . . . . . . . . . . . . .   7
       5.1.2.  Packet flow - Downlink  . . . . . . . . . . . . . . .   8
       5.1.3.  IPv6 user-traffic . . . . . . . . . . . . . . . . . .   8
     5.2.  Enhanced Mode (formerly Aggregate mode) . . . . . . . . .   8
       5.2.1.  Packet flow - Uplink  . . . . . . . . . . . . . . . .   9
       5.2.2.  Packet flow - Downlink  . . . . . . . . . . . . . . .  10
       5.2.3.  IPv6 user-traffic . . . . . . . . . . . . . . . . . .  10
     5.3.  Enhanced mode with unchanged gNB GTP behavior . . . . . .  11
       5.3.1.  Interworking with IPv6 GTP  . . . . . . . . . . . . .  11
       5.3.2.  Interworking with IPv4 GTP  . . . . . . . . . . . . .  14
       5.3.3.  Extensions to the interworking mechanisms . . . . . .  17
   6.  SRv6 SID Mobility Functions . . . . . . . . . . . . . . . . .  17
     6.1.  End.MAP: Endpoint function with SID mapping . . . . . . .  17
     6.2.  End.M.GTP6.D: Endpoint function with decapsulation from
           IPv6/GTP tunnel . . . . . . . . . . . . . . . . . . . . .  17
     6.3.  End.M.GTP6.E: Endpoint function with encapsulation for
           IPv6/GTP tunnel . . . . . . . . . . . . . . . . . . . . .  18
     6.4.  End.M.GTP4.E: Endpoint function with encapsulation for
           IPv4/GTP tunnel . . . . . . . . . . . . . . . . . . . . .  18
     6.5.  T.M.Tmap: Transit behavior with IPv4/GTP decapsulation
           and mapping into an SRv6 Policy . . . . . . . . . . . . .  19
     6.6.  End.Limit: Rate Limiting function . . . . . . . . . . . .  20
   7.  SRv6 supported PDU session types  . . . . . . . . . . . . . .  20
   8.  Network Slicing Considerations  . . . . . . . . . . . . . . .  21
   9.  Control Plane Considerations  . . . . . . . . . . . . . . . .  21



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   10. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     14.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Implementations  . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   In mobile networks, mobility management systems provide connectivity
   while mobile nodes move around.  While the control-plane of the
   system signals movements of a mobile node, user-plane establishes
   tunnel between the mobile node and anchor node over IP based backhaul
   and core networks.

   This document discusses the applicability of SRv6 (Segment Routing
   IPv6) to those mobile networks.  SRv6 provides source routing to
   networks where operators can explicitly indicate a route for the
   packets from and to the mobile node.  SRv6 endpoint nodes perform the
   roles of anchor of mobile user-plane.

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   SRH is the abbreviation for the Segment Routing Header.  We assume
   that the SRH may be present multiple times inside each packet.

   NH is the abbreviation of the IPv6 next-header field.

   NH=SRH means that the next-header field is 43 with routing type 4.

   When there are multiple SRHs, they must follow each other: the next-
   header field of all SRH, except the last one, must be SRH.

   The effective next-header (ENH) is the next-header field of the IP
   header when no SRH is present, or is the next-header field of the
   last SRH.

   In this version of the document, we assume that there is no other
   extension header than the SRH.  This will be lifted in future
   versions of the document.




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   SID: A Segment Identifier which represents a specific segment in
   segment routing domain.  The SID type used in this document is IPv6
   address (also referenced as SRv6 Segment or SRv6 SID).

   A SID list is represented as <S1, S2, S3> where S1 is the first SID
   to visit, S2 is the second SID to visit and S3 is the last SID to
   visit along the SR path.

   (SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:

   o  IPv6 header with source and destination addresses respectively SA
      and DA and next-header is SRH
   o  SRH with SID list <S1, S2, S3> with SegmentsLeft = SL
   o  Note the difference between the <> and () symbols: <S1, S2, S3>
      represents a SID list where S1 is the first SID and S3 is the last
      SID.  (S3, S2, S1; SL) represents the same SID list but encoded in
      the SRH format where the rightmost SID in the SRH is the first SID
      and the leftmost SID in the SRH is the last SID.  When referring
      to an SR policy in a high-level use-case, it is simpler to use the
      <S1, S2, S3> notation.  When referring to an illustration of the
      detailed behavior, the (S3, S2, S1; SL) notation is more
      convenient.
   o  The payload of the packet is omitted.

   SRH[SL] represents the SID pointed by the SL field in the first SRH.
   In our example, SRH[2] represents S1, SRH[1] represents S2 and SRH[0]
   represents S3.

   FIB is the abbreviation for the forwarding table.  A FIB lookup is a
   lookup in the forwarding table.  When a packet is intercepted on a
   wire, it is possible that SRH[SL] is different from the DA.

3.  Motivation

   Every day mobility networks are getting more challenging to operate:
   on one hand, traffic is constantly growing, and latency requirements
   are more strict; on the other-hand, there are new use-cases like NFV
   that are also challenging network management.

   Problem comes from the fact that the current architecture of mobile
   networks is agnostic to the underlying transport.  Indeed, it rigidly
   fragments the user-plane into radio access, core and service networks
   and connects them by tunneling techniques through the user-plane
   roles such as access and anchor nodes.  Such agnosticism and
   rigidness make it difficult for the operator to optimize and operate
   the data-path.





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   While the mobile network industry has been trying to solve those
   problems, applications have shifted to use IPv6, and network
   operators have started adopting IPv6 as their IP transport as well.
   SRv6, the IPv6 instantiation of Segment Routing
   [I-D.ietf-spring-segment-routing], integrates both the application
   data-path and the underlying transport layer into one single
   protocol, allowing operators to optimize the network in a simplified
   manner and removing forwarding state from the network.

   Further on, SRv6 introduces the notion of network-programming
   [I-D.filsfils-spring-srv6-network-programming], that applied to
   mobility fulfils the user-plane functions of mobility management.
   SRv6 takes advantage of underlying transport awareness and
   flexibility to deploy mobility user-plane functions in an optimized
   manner.  Those are the motivations to adopt SRv6 for mobile user-
   plane.

4.  Reference Architecture

   This section describes a reference architecture and possible
   deployment scenarios.

   Figure 1 shows a reference architecture, based on 5G packet core
   architecture [TS.23501].

   Please note that all the user-plane described in this document does
   not depend on any specific architecture.  This architecture is just
   used as a reference based on the latest 3GPP standards at the time of
   writing this draft.  Other type of architectures can be seen in
   [I-D.gundavelli-dmm-mfa] and [WHITEPAPER-5G-UP].

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

                     Figure 1: Reference Architecture




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   o  UE : User Equipment
   o  gNB : gNodeB
   o  UPF : User Plane Function

      *  UPF1: Interfaces N3 and N9
      *  UPF2: Interfaces N9 and N6
      *  Note: For simplicity we don't depict a UPF that is only
         connected to N9 interfaces, although the techniques described
         in this document are also valid in such case.
   o  SMF : Session Management Function
   o  AMF : Access and Mobility Management Function
   o  DN : Data Network e.g. operator services, Internet access

   A session from an UE gets assigned to an UPF.  Sometimes more than
   one UPF may be used for providing a certain kind of richer service
   functions.  UE gets its IP address from the DHCP block of its UPF.
   The UPF advertises the IP address block towards the Internet ensuring
   that return traffic is routed to the right UPF.

5.  User-plane behaviors

   This section describes the mobile user-plane behaviors using SRv6.

   In order to simplify the SRv6 adoption, we present two different
   "modes" that vary with respect the SRv6 SID allocation.  The first
   one is the "Traditional mode", which inherits the traditional mobile
   user-plane.  In this mode there is no change to mobility networks
   architecture, except for the pure replacement of GTP-U [TS.29281] for
   SRv6.

   The second mode is the "Enhanced mode", which aggregates the mobile
   sessions and allocates SID on a per policy basis.  The benefit of the
   latter is that the SR policy contains SIDs for Traffic Engineering
   and VNFs.  Both of these modes assume both the gNB and UPFs are SR-
   aware (N3 and N9 interfaces are SRv6).

   Additionally, we introduce a new "Enhanced mode with unchanged gNB
   GTP behavior".  This mode consists of two mechanisms for interworking
   with legacy access networks -interface N3 unmodified-. One of these
   mechanism is designed to interwork with legacy gNBs using GTP/IPv4.
   The second method is designed to interwork with legacy gNBs using
   GTP/IPv6.

   This section makes reference to already existing SRv6 functions
   defined in [I-D.filsfils-spring-srv6-network-programming] as well as
   new SRv6 functions designed for the mobile userplane.  The new SRv6
   functions are detailed in the Section 6.




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5.1.  Traditional mode (formerly Basic mode)

   In the traditional mode, we assume that mobile user-plane functions
   are the same as existing ones except the use of SRv6 as the data
   plane instead of GTP-U.  No impact to the rest of mobile system
   should be expected.

   In the traditional mobile network, an UE session is mapped 1-for-1
   with a specific GTP tunnel (TEID).  This 1-for-1 mapping is
   replicated here to replace the GTP encaps with the SRv6 encaps, while
   not changing anything else.

   This mode minimizes the changes required to the entire system and it
   is a good starting point for forming the common basis.  Note that in
   this mode the TEID is embedded in each SID.

   Our reference topology is shown in Figure 2.  In this mode we assume
   that the gNB and the UPFs are SR-aware.

                                                              ________
                     SRv6           SRv6                     /        \
   +--+      +-----+ [N3] +------+  [N9]  +------+  [N6]    /          \
   |UE|------| gNB |------| UPF1 |--------| UPF2 |--------- \    DN    /
   +--+      +-----+      +------+        +------+           \________/
            SRv6 node     SRv6 node       SRv6 node

              Figure 2: Traditional mode - Reference topology

5.1.1.  Packet flow - Uplink

   The uplink packet flow is the following:

       UE_out  : (A,Z)
       gNB_out : (gNB, U1::1) (A,Z)     -> T.Encaps.Reduced <U1::1>
       UPF1_out: (gNB, U2::1) (A,Z)     -> End.MAP
       UPF2_out: (A,Z)                  -> End.DT4 or End.DT6

   The UE packet arrives to the gNB.  The gNB performs a
   T.Encaps.Reduced operations.  Since there is only one SID, there is
   no need to push an SRH. gNB only adds an outer IPv6 header with IPv6
   DA U1::1.  U1::1 represents an anchoring SID specific for that
   session at UPF1.  The SID U1::1 is retrieved through the existing
   control plane (N2 interface).

   Upon packet arrival on UPF1, the SID U1::1 is a local End.MAP
   function.  This function maps the SID with the next anchoring point
   and replaces U1::1 by U2::1, that belongs to the next anchoring
   point.



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   Upon packet arrival on UPF2, the SID U2::1 corresponds to an End.DT
   function.  UPF2 decapsulates the packet, performs a lookup in a
   specific table and forwards the packet towards the data network.

5.1.2.  Packet flow - Downlink

   The downlink packet flow is the following:

       UPF2_in : (Z,A)
       UPF2_out: (U2::, U1::1) (Z,A)    -> T.Encaps.Reduced <U1::1>
       UPF1_out: (U2::, gNB::1) (Z,A)   -> End.MAP
       gNB_out : (Z,A)                  -> End.DX4 or End.DX6

   When the packet arrives to the UPF2, the UPF2 will map that
   particular flow into a UE session.  This UE session is associated
   with the policy <U1::1>.  The UPF2 performs a T.Encaps.Reduced
   operation, encapsulating the packet into a new IPv6 header with no
   SRH since there is only one SID.

   Upon packet arrival on UPF1, the SID U1::1 is a local End.MAP
   function.  This function maps the SID with the next anchoring point
   and replaces U1::1 by gNB::1, that belongs to the next anchoring
   point.

   Upon packet arrival on gNB, the SID gNB::1 corresponds to an End.DX4/
   End.DX6 function.  The gNB will decapsulates the packet, removing the
   IPv6 header and all it's extensions headers and will forward the
   traffic towards the UE.

5.1.3.  IPv6 user-traffic

   For IPv6 user-traffic it is RECOMMENDED to perform encapsulation.
   However based on local policy, a service provider MAY choose to do
   SRH insertion.  The main benefit is a lower overhead.  In such case,
   the functions used are T.Insert.Red at gNB, End.MAP at UPF1 and End.T
   at UPF2 on Uplink, T.Insert.Red at UPF2, End.MAP at UPF1 and End.X at
   gNB on Downlink.

5.2.  Enhanced Mode (formerly Aggregate mode)

   This mode improves the scalability.  In addition, it provides key
   improvements in terms of traffic steering and service programming
   [I-D.xuclad-spring-sr-service-programming] , thanks to the use of an
   SR policy of multiple SIDs, instead of single one in the Traditional
   mode.

   Key points:




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   o  Several UE share the same SR Policy (and it's composing SID)
   o  The SR policy MAY include SIDs for traffic engineering and service
      programming on top of the UPF anchor.

   The gNB control-plane (N2 interface) is unchanged, specifically a
   single IPv6 address is given to the gNB.

   o  The gNB MAY resolve the IP address into a SID list through a
      mechanism like PCEP, DNS-lookup, small augment for LISP control-
      plane, etc.

   Our reference topology is shown in Figure 3.  In this mode we assume
   that the gNB and the UPF are SR-aware.  We also assume that we have
   two services segments, S1 and C1.  S1 represents a VNF in the
   network, and C1 represents a constraint path on a router over which
   we are going to perform Traffic Engineering.  Note that S1 and C1
   belong to the underlay and don't have an N4 interface.  For this
   reason we don't consider them UPFs.

                                    +----+  SRv6               _______
                    SRv6          --| C1 |--[N3]              /       \
   +--+    +-----+  [N3]         /  +----+  \  +------+ [N6] /         \
   |UE|----| gNB |--       SRv6 /    SRv6    --| UPF2 |------\   DN    /
   +--+    +-----+  \      [N3]/      TE       +------+       \_______/
          SRv6 node  \ +----+ /               SRv6 node
                      -| S1 |-
                       +----+
                      SRv6 node
                        VNF

               Figure 3: Enhanced mode - Reference topology

5.2.1.  Packet flow - Uplink

   The uplink packet flow is the following:

   UE_out  : (A,Z)
   gNB_out : (gNB, S1)(U2::1, C1; SL=2)(A,Z)-> T.Encaps.Red<S1,C1,U2::1>
   S1_out  : (gNB, C1)(U2::1, C1; SL=1 (A,Z)
   C1_out  : (gNB, U2::1)(A,Z)              -> PSP
   UPF2_out: (A,Z)                          -> End.DT4 or End.DT6

   UE sends its packet (A,Z) on a specific bearer session to its gNB.
   gNB's CP associates that session from the UE(A) with the IPv6 address
   B and GTP TEID T. gNB's CP does a lookup on B (by reverseDNS, LISP,
   etc.) to find the related SID list <S1, C1, U2::1>.





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   Once the packet leaves the gNB, it already contains all the segments
   of the SR policy.  This SR policy contains segments for traffic
   engineering (C1) and for service programming (S1).

   The nodes S1 and C1 perform their related Endpoint functionality and
   forward.

   When the packet arrives to UPF2, the active segment (U2::1) is an
   End.DT4/6 which performs the decapsulation (removing the IPv6 header
   with all it's extension headers) and forward towards the data
   network.

   Note that in case several APNs are using duplicated IPv4 private
   address spaces, then the aggregated SR policies are unique per APNs.

5.2.2.  Packet flow - Downlink

   The downlink packet flow is the following:

   UPF2_in : (Z,A)                              -> UPF2 maps the flow w/
                                                   SID list <C1,S1, gNB>
   UPF2_out: (U2::1, C1)(gNB, S1; SL=2)(Z,A)    -> T.Encaps.Red
   C1_out  : (U2::1, S1)(gNB, S1; SL=1)(Z,A)
   S1_out  : (U2::1, gNB)(Z,A)                  -> PSP
   gNB_out : (Z,A)                              -> End.DX4 or End.DX6

   When the packet arrives to the UPF2, the UPF2 will map that
   particular flow into a UE session.  This UE session is associated
   with the policy <C1, S1, gNB>.  The UPF2 performs a T.Encaps.Reduced
   operation, encapsulating the packet into a new IPv6 header with its
   corresponding SRH.

   The nodes C1 and S1 perform their related Endpoint processing.

   Once the packet arrives to the gNB, the IPv6 DA corresponds to an
   End.DX4 or End.DX6 (depending on the underlying traffic).  The gNB
   will decapsulate the packet, removing the IPv6 header and all it's
   extensions headers and will forward the traffic towards the UE.

5.2.3.  IPv6 user-traffic

   For IPv6 user-traffic it is RECOMMENDED to perform encapsulation.
   However based on local policy, a service provider MAY choose to do
   SRH insertion.  The main benefit is a lower overhead.  In such case,
   the functions used are T.Insert.Red at gNB and End.T at UPF2 on
   Uplink, T.Insert.Red at UPF2 and End.X at gNB on Downlink.





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5.3.  Enhanced mode with unchanged gNB GTP behavior

   In this section we introduce two mechanisms for interworking with
   legacy gNBs that still use GTP.  One of the mechanisms is valid for
   IPv4 while the other for IPv6.

   In this scenario, it is assumed that gNB does not support SRv6.  It
   just supports GTP encapsulation over IPv4 or IPv6.  Hence in order to
   achieve interworking we are going to add a new SR Gateway (SRGW-UPF1)
   entity.  This SRGW is going to map the GTP traffic into SRv6.  Note
   that the SR GW is not an anchor point.

   The SRGW maintains very little state on it.  For this reason, both of
   these methods (IPv4 and IPv6) scale to millions of UEs.

                                                              _______
                     IP GTP          SRv6                    /       \
    +--+      +-----+ [N3] +------+  [N9]  +------+  [N6]   /         \
    |UE|------| gNB |------| UPF1 |--------| UPF2 |---------\   DN    /
    +--+      +-----+      +------+        +------+          \_______/
                          SR Gateway       SRv6 node

               Figure 4: Reference topology for interworking

5.3.1.  Interworking with IPv6 GTP

   In this interworking mode we assume that the gNB is using GTP over
   IPv6 in the N3 interface

   Key points:

   o  gNB is unchanged (control-plane or user-plane) and encaps into GTP
      (N3 interface is not modified).
   o  5G Control-Plane (N2 interface) is unmodified: 1 IPv6 address
      (i.e. a BSID at the SRGW)
   o  SRGW removes GTP, finds SID list related to DA, add SRH with the
      SID list.
   o  There is NO state for the downlink at the SRGW.
   o  There is simple state in the uplink at the SRGW (leveraging the
      enhanced mode results in few SR policies on this node.  A SR
      policy can be shared across UEs).
   o  As soon as the packet leaves the gNB (uplink), the traffic is SR-
      routed.  This simplifies considerably network slicing
      [I-D.hegdeppsenak-isis-sr-flex-algo].
   o  In the uplink, we use the IPv6 DA BSID to steer the traffic into
      an SR policy when it arrives at the SRGW-UPF1-.





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   Our reference topology is shown in Figure 5.  In this mode we assume
   that the gNB is an unmodified gNB using IPv6/GTP.  The UPFs are SR-
   aware.  Also, as explained before, we introduce a new SRGW entity
   that is going to map the IPv6/GTP traffic to SRv6.

   We also assume that we have two service segment, S1 and C1.  S1
   represents a VNF in the network, and C1 represents a router over
   which we are going to perform Traffic Engineering.

                                  +----+
                IPv6/GTP         -| S1 |-                            ___
   +--+  +-----+ [N3]           / +----+ \                          /
   |UE|--| gNB |-         SRv6 /   SRv6   \ +----+   +------+ [N6] /
   +--+  +-----+ \        [N9]/     VNF    -| C1 |---| UPF2 |------\  DN
           GTP    \ +------+ /              +----+   +------+       \___
                   -| UPF1 |-                SRv6      SRv6
                    +------+                  TE
                   SR Gateway

       Figure 5: Enhanced mode with unchanged gNB IPv6/GTP behavior

5.3.1.1.  Packet flow - Uplink

   The uplink packet flow is the following:

   UE_out  : (A,Z)
   gNB_out : (gNB, B)(GTP: TEID T)(A,Z)       -> Interface N3 unmodified
                                                 (IPv6/GTP)
   SRGW_out: (SRGW, S1)(U2::1, C1; SL=2)(A,Z) -> B is an End.M.GTP6.D
                                                 SID at the SRGW
   S1_out  : (SRGW, C1)(U2::1, C1; SL=1)(A,Z)
   C1_out  : (SRGW, U2::1)(A,Z)               -> PSP
   UPF2_out: (A,Z)                            -> End.DT4 or End.DT6

   The UE sends a packet destined to Z towards the gNB on a specific
   bearer for that session.  The gNB, which is unmodified, encapsulates
   the packet into a new IPv6, UDP and GTP headers.  The IPv6 DA B, and
   the GTP TEID T are the ones received in the N2 interface.

   The IPv6 address that was signalled over the N2 interface for that UE
   session, B, is now the IPv6 DA.  B is an SRv6 Binding SID
   instantiated at the SRGW.  Hence the packet, will be routed up to the
   SRGW.

   When the packet arrives at the SRGW, the SRGW realises that B is an
   End.M.GTP6.D BindingSID.  Hence, the SRGW will remove the IPv6, UDP
   and GTP headers, and will push a new IPv6 header with its own SRH
   containing the SIDs bound to the SR policy associated with this



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   BindingSID.  Note that there will be one instance of the End.M.GTP6.D
   SID per PDU type.

   The nodes S1 and C1 perform their related Endpoint functionality and
   forward.

   When the packet arrives to UPF2, the active segment is (U2::1) which
   bound to End.DT4/6 which is going to perform the decapsulation
   (removing the outer IPv6 header with all it's extension headers) and
   forward towards the data network.

5.3.1.2.  Packet flow - Downlink

   The downlink packet flow is the following:

   UPF2_in : (Z,A)                           -> UPF2 maps the flow with
                                                <C1, S1, SRGW::TEID,gNB>
   UPF2_out: (U2::1, C1)(gNB, SRGW::TEID, S1; SL=3)(Z,A) -> T.Encaps.Red
   C1_out  : (U2::1, S1)(gNB, S1; SL=2)(Z,A)
   S1_out  : (U2::1, SRGW::TEID)(gNB, SRGW::TEID, S1, SL=1)(Z,A)
   SRGW_out: (SRGW, gNB)(GTP: TEID=T)(Z,A)   -> SRGW/96 is End.M.GTP6.E
   gNB_out : (Z,A)

   When a packet destined to A arrives at the UPF2, the UPF2 performs a
   lookup in the associated table to A and finds the SID list <C1, S1,
   SRGW::TEID, gNB>.  The UPF2 performs a T.Encaps.Reduced operation,
   encapsulating the packet into a new IPv6 header with its
   corresponding SRH.

   The nodes C1 and S1 perform their related Endpoint processing.

   Once the packet arrives to the SRGW, the SRGW realizes the active SID
   is an End.M.GTP6.E function.  The SRGW removes the IPv6 header and
   all it's extensions headers.  The SRGW generates an IPv6, UDP and GTP
   headers.  The new IPv6 DA is the gNB which is the last SID in the
   received SRH.  The TEID in the generated GTP header is the arguments
   of the received End.M.GTP6.E SID.  The SRGW pushes the headers to the
   packet and forwards the packet towards the gNB.  Note that there will
   be one instance of the End.M.GTP6.E SID per PDU type.

   Once the packet arrives to the gNB, the packet is a regular IPv6/GTP
   packet.  The gNB looks for the specific radio bearer for that TEID
   and forward it on the bearer.  This gNB behavior is not modified from
   current and previous generations.







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5.3.1.3.  Scalability

   For the downlink traffic, the SRGW is stateless.  All the state is in
   the SRH imposed by the UPF2.  The UPF2 must have the UE states as the
   session anchor point.

   For the uplink traffic, the state at the SRGW does not necessarily
   need to be per UE session basis.  A state of SR policy of which state
   can be shared among UE's.  Hence it is possible to deploy SRGW in
   very scalable way compared to hold millions of states per UE session
   basis.

5.3.1.4.  IPv6 user-traffic

   For IPv6 user-traffic it is RECOMMENDED to perform encapsulation.
   However based on local policy, a service provider MAY choose to do
   SRH insertion.  The main benefit is a lower overhead.

5.3.2.  Interworking with IPv4 GTP

   In this interworking mode we assume that the gNB is using GTP over
   IPv4 in the N3 interface

   Key points:

   o  gNB is unchanged and encaps into GTP (N3 interface is not
      modified).
   o  In the uplink, traffic is classified at SRGW by UL CL(Uplink
      Classifier) and steered into an SR policy.  The SRGW is a UPF1
      functionality, hence it can coexist with UPF UL CL functionality.
   o  SRGW removes GTP, finds SID list related to DA, add SRH with SID
      list.

   Our reference topology is shown in Figure 6.  In this mode we assume
   that the gNB is an unmodified gNB using IPv4/GTP.  The UPFs are SR-
   aware.  Also, as explained before, we introduce a new SRGW entity
   that is going to map the IPv4/GTP traffic to SRv6.

   We also assume that we have two service segment, S1 and C1.  S1
   represents a VNF in the network, and C1 represents a router over
   which we are going to perform Traffic Engineering.










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                                  +----+
                IPv4/GTP         -| S1 |-                            ___
   +--+  +-----+ [N3]           / +----+ \                          /
   |UE|--| gNB |-         SRv6 /   SRv6   \ +----+   +------+ [N6] /
   +--+  +-----+ \        [N9]/     VNF    -| C1 |---| UPF2 |------\  DN
           GTP    \ +------+ /              +----+   +------+       \___
                   -| UPF1 |-                SRv6      SRv6
                    +------+                  TE
                   SR Gateway

       Figure 6: Enhanced mode with unchanged gNB IPv4/GTP behavior

5.3.2.1.  Packet flow - Uplink

   The uplink packet flow is the following:

    gNB_out : (gNB, B)(GTP: TEID T)(A,Z)          -> Interface N3
                                                     unchanged IPv4/GTP
    SRGW_out: (SRGW, S1)(U2::1, C1; SL=2)(A,Z)    -> T.M.Tmap function
    S1_out  : (SRGW, C1)(U2::1, C1; SL=1)(A,Z)
    C1_out  : (SRGW, U2::1) (A,Z)                 -> PSP
    UPF2_out: (A,Z)                               -> End.DT4 or End.DT6

   The UE sends a packet destined to Z towards the gNB on a specific
   bearer for that session.  The gNB, which is unmodified, encapsulates
   the packet into a new IPv4, UDP and GTP headers.  The IPv4 DA, B, and
   the GTP TEID are the ones received at the N2 interface.

   When the packet arrives to the SRGW -UPF1-, the SRGW has an UL CL
   (uplink classifier) rule for incoming traffic from the gNB that
   steers the traffic into an SR policy by using the function T.M.TMap.
   The SRGW removes the IPv4, UDP and GTP headers and pushes an IPv6
   header with its own SRH containing the SIDs related to the SR policy
   associated with this traffic.  The SRGW forwards according to the new
   IPv6 DA.

   The nodes S1 and C1 perform their related Endpoint functionality and
   forward.

   When the packet arrives at UPF2, the active segment is (U2::1) which
   is bound to End.DT4/6 which performs the decapsulation (removing the
   outer IPv6 header with all it's extension headers) and forwards
   towards the data network.








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5.3.2.2.  Packet flow - Downlink

   The downlink packet flow is the following:

   UPF2_in : (Z,A)                            -> UPF2 maps flow with SID
                                               <C1, S1,SRGW::SA:DA:TEID>
   UPF2_out: (U2::1, C1)(SRGW::SA:DA:TEID, S1; SL=2)(Z,A) ->T.Encaps.Red
   C1_out  : (U2::1, S1)(SRGW::SA:DA:TEID, S1; SL=1)(Z,A)
   S1_out  : (U2::1, SRGW::SA:DA:TEID)(Z,A)
   SRGW_out: (SA, DA)(GTP: TEID=T)(Z,A)       -> End.M.GTP4.E
   gNB_out : (Z,A)

   When a packet destined to A arrives to the UPF2, the UPF2 performs a
   lookup in the associated table to A and finds the SID list <C1, S1,
   SRGW::SA:DA:TEID>.  The UPF2 performs a T.Encaps.Reduced operation,
   encapsulating the packet into a new IPv6 header with its
   corresponding SRH.

   The nodes C1 and S1 perform their related Endpoint processing.

   Once the packet arrives to the SRGW, the SRGW realizes the active SID
   is an End.M.GTP4.E function.  The SRGW removes the IPv6 header and
   all it's extensions headers.  The SRGW generates an IPv4, UDP and GTP
   headers.  The IPv4 SA and DA will the ones received as part of the
   SID arguments.  The TEID in the generated GTP header is also the
   arguments of the received End.M.GTP4.E SID The SRGW pushes the
   headers to the packet and forwards the packet towards the gNB.

   Once the packet arrives to the gNB, the packet is a regular IPv4/GTP
   packet.  The gNB looks for the specific radio bearer for that TEID
   and forward it on the bearer.  This gNB behavior is not modified from
   current and previous generations.

5.3.2.3.  Scalability

   For the downlink traffic, the SRGW is stateless.  All the state is in
   the SRH imposed by the UPF.  The UPF must have this UE-base state
   anyway (it is its anchor point).

   For the uplink traffic, the state at the SRGW is dedicated on a per
   UE/session basis.  This is an UL CL (uplink classifier).  There is
   state for steering the different sessions on a SR policies.  Notice
   however that the SR policies are shared among several UE/sessions.








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5.3.2.4.  IPv6 user-traffic

   For IPv6 user-traffic it is RECOMMENDED to perform encapsulation.
   However based on local policy, a service provider MAY choose to do
   SRH insertion.  The main benefit is a lower overhead.

5.3.3.  Extensions to the interworking mechanisms

   In this section we presented two mechanisms for interworking with
   gNBs that do not support SRv6.  These mechanism are done to support
   GTP over IPv4 and GTP over IPv6.

   Even though we have presented these methods as an extension to the
   "Enhanced mode", it is straightforward in its applicability to the
   "Traditional mode".

   Furthermore, although these mechanisms are designed for interworking
   with legacy RAN at the N3 interface, these methods could also be
   applied for interworking with a non-SRv6 capable UPF at the N9
   interface (e.g.  L3-anchor is SRv6 capable but L2-anchor is not).

6.  SRv6 SID Mobility Functions

6.1.  End.MAP: Endpoint function with SID mapping

   The "Endpoint function with SID mapping" function (End.MAP for short)
   is used in several scenarios.  Particularly in mobility, it is used
   in the UPFs for the anchor functionality in some of the use-cases.

   When a SR node N receives a packet destined to S and S is a local
   End.MAP SID, N does:

   1.    look up the IPv6 DA in the mapping table
   2.    update the IPv6 DA with the new mapped SID              ;; Ref1
   3.    forward according to the new mapped SID
   4. ELSE
   5.    Drop the packet

   Ref1: Note that the SID in the SRH is NOT modified.

6.2.  End.M.GTP6.D: Endpoint function with decapsulation from IPv6/GTP
      tunnel

   The "Endpoint function with IPv6/GTP decapsulation into SR policy"
   function (End.M.GTP6.D for short) is used in interworking scenario
   for the uplink towards from the legacy gNB using IPv6/GTP.  This SID
   is associated with an SR policy <S1, S2, S3> and an IPv6 Source
   Address A.



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   When the SR Gateway node N receives a packet destined to S and S is a
   local End.M.GTP6.D SID, N does:

   1. IF NH=UDP & UDP_PORT = GTP THEN
   2.    pop the IP, UDP and GTP headers
   3.    push a new IPv6 header with its own SRH <S2, S3>
   4.    set the outer IPv6 SA to A
   5.    set the outer IPv6 DA to S1
   6.    forward according to the first segment of the SRv6 Policy
   7. ELSE
   8.    Drop the packet

6.3.  End.M.GTP6.E: Endpoint function with encapsulation for IPv6/GTP
      tunnel

   The "Endpoint function with encapsulation for IPv6/GTP tunnel"
   function (End.M.GTP6.E for short) is used in interworking scenario
   for the downlink towards the legacy gNB using IPv6/GTP.

   The End.M.GTP6.E function has a 32-bit argument space.  This argument
   corresponds to the GTP TEID.

   When the SR Gateway node N receives a packet destined to S and S is a
   local End.M.GTP6.E SID, N does:

    1. IF NH=SRH & SL = 1  THEN                                  ;; Ref1
    2.    decrement SL
    3.    store SRH[SL] in variable new_DA
    4.    store TEID in variable new_TEID                        ;; Ref2
    5.    pop IP header and all it's extension headers
    6.    push new IPv6 header and GTP-U header
    7.    set IPv6 DA to new_DA
    8.    set GTP_TEID to new_TEID
    9.    lookup the new_DA and forward the packet accordingly
   10. ELSE
   11.    Drop the packet

   Ref1: An End.M.GTP6.E SID MUST always be the penultimate SID.

   Ref2: TEID is extracted from the argument space of the current SID.

6.4.  End.M.GTP4.E: Endpoint function with encapsulation for IPv4/GTP
      tunnel

   The "Endpoint function with encapsulation for IPv4/GTP tunnel"
   function (End.M.GTP4.UP for short) is used in the downlink when doing
   interworking with legacy gNB using IPv4/GTP.




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   When the SR Gateway node N receives a packet destined to S and S is a
   local End.M.GTP4.E SID, N does:

   1. IF NH=SRH & SL > 0 THEN
   2.    decrement SL
   3.    update the IPv6 DA with SRH[SL]
   4.    pop the SRH
   5.    push header of TUN-PROTO with tunnel ID from S          ;; Ref1
   6.    push outer IPv4 header with SA, DA from S
   7. ELSE
   8.    Drop the packet

   Ref1: TUN-PROTO indicates target tunnel type.

   Note that S has the following format:

             +----------------------+-------+-------+-------+
             |  SRGW-IPv6-LOC-FUNC  |IPv4DA |IPv4SA |TUN-ID |
             +----------------------+-------+-------+-------+
                     128-a-b-c          a      b       c

                         End.M.GTP4.E SID Encoding

6.5.  T.M.Tmap: Transit behavior with IPv4/GTP decapsulation and mapping
      into an SRv6 Policy

   The "Transit with tunnel decapsulation and map to an SRv6 policy"
   function (T.Tmap for short) is used in the direction from legacy
   user-plane to SRv6 user-plane network.

   When the SR Gateway node N receives a packet destined to a IW-
   IPv4-Prefix, N does:

   1. IF P.PLOAD == TUN-PROTO THEN    ;; Ref1
   2.    pop the outer IPv4 header and tunnel headers
   3.    copy IPv4 DA, SA, TUN-ID to form SID B with SRGW-IPv6-Prefix
   4.    encapsulate the packet into a new IPv6 header  ;; Ref2, Ref2bis
   5.    set the IPv6 DA = B
   6.    forward along the shortest path to B
   7. ELSE
   8.    Drop the packet

   Ref1: TUN-PROTO indicates target tunnel type.

   Note that B has the following format:






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             +----------------------+-------+-------+-------+
             |  SRGW-IPv6-LOC-FUNC  |IPv4DA |IPv4SA |TUN-ID |
             +----------------------+-------+-------+-------+
                     128-a-b-c          a      b       c

                         End.M.GTP4.E SID Encoding

   Note that the B SID, is going to be an SRv6 BindingSID instantiated
   at the first UPF (anchor point).  A static format is leveraged to
   instantiate this Binding SIDs in order to remove state from the SRGW.

6.6.  End.Limit: Rate Limiting function

   Mobile user-plane requires a rate-limit feature.  SID is able to
   encode limiting rate as an argument in SID.  Multiple flows of
   packets should have same group identifier in SID when those flows are
   in an same AMBR group.  This helps to keep user-plane stateless.
   That enables SRv6 endpoint nodes which are unaware from the mobile
   control-plane information.  Encoding format of rate limit segment SID
   is following:

              +----------------------+----------+-----------+
              | LOC+FUNC rate-limit  | group-id | limit-rate|
              +----------------------+----------+-----------+
                    128-i-j                i          j

             End.Limit: Rate limiting function argument format

   In case of j bit length is zero in SID, the node should not do rate
   limiting unless static configuration or control-plane sets the limit
   rate associated to the SID.

7.  SRv6 supported PDU session types

   The 3GPP [TS.23501] defines the following PDU session types:

   o  IPv4
   o  IPv6
   o  IPv4v6
   o  Ethernet
   o  Unstructured

   SRv6 supports all the PDU session types without any protocol overhead
   by using the corresponding SRv6 functions (End.DX4, End.DT4 for IPv4
   PDU sessions; End.DX6, End.DT6, End.T for IPv6 PDU sessions; End.DT46
   for IPv4v6 PDU sessions; End.DX2, End.DT2M for L2 PDU sessions;
   End.DX2 for Unstructured PDU sessions).




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8.  Network Slicing Considerations

   A mobile network may be required to implement "network slices", which
   logically separate network resources.  User-plane functions
   represented as SRv6 segments would be part of a slice.

   [I-D.filsfils-spring-segment-routing-policy] describes a solution to
   build basic network slices with SR.  Depending on the requirements,
   these slices can be further refined by leveraging the mechanisms
   from:

   o  IGP Flex-Algo [I-D.hegdeppsenak-isis-sr-flex-algo]
   o  Inter-Domain policies
      [I-D.ietf-spring-segment-routing-central-epe]

   Furthermore, these can be combined with ODN/AS
   [I-D.filsfils-spring-segment-routing-policy] for automated slice
   provisioning and traffic steering.

   A separate document will explain in detail how each one of these
   tools is leveraged to build a network slice.

9.  Control Plane Considerations

   This documents focuses on the user-plane behavior and it's
   independent from the control plane.

   The control plane could be the current 3GPP-defined control plane
   with slight modifications to the N4 interface [TS.29244].

   Alternatively, SRv6 could be used in conjunction with a new mobility
   control plane as described in LISP [I-D.rodrigueznatal-lisp-srv6],
   hICN [I-D.auge-dmm-hicn-mobility-deployment-options], MFA
   [I-D.gundavelli-dmm-mfa] or in cunjunction with FPC
   [I-D.ietf-dmm-fpc-cpdp].  The analysis of new mobility control-planes
   and it's applicability to SRv6 is is out of the scope of this
   document.

   Note that the IANA section of this document allocates the SRv6
   endpoint function types for the new functions defined in this
   document.  All control-plane protocols are expected to leverage these
   function type-codes to signal each function.

   It's notable that SRv6's network programming nature allows a flexible
   and dynamic anchor placement.






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10.  Security Considerations

   TBD

11.  IANA Considerations

   This I-D requests to IANA to allocate, within the "SRv6 Endpoint
   Types" sub-registry belonging to the top-level "Segment-routing with
   IPv6 dataplane (SRv6) Parameters" registry
   [I-D.filsfils-spring-srv6-network-programming], the following
   allocations:

           +-------------+-----+-------------------+-----------+
           | Value/Range | Hex | Endpoint function | Reference |
           +-------------+-----+-------------------+-----------+
           | TBA         | TBA |      End.MAP      | [This.ID] |
           | TBA         | TBA |    End.M.GTP6.D   | [This.ID] |
           | TBA         | TBA |    End.M.GTP6.E   | [This.ID] |
           | TBA         | TBA |    End.M.GTP4.E   | [This.ID] |
           | TBA         | TBA |     End.Limit     | [This.ID] |
           +-------------+-----+-------------------+-----------+

              Table 1: SRv6 Mobile User-plane Endpoint Types

12.  Acknowledgements

   The authors would like to thank Daisuke Yokota, Bart Peirens,
   Ryokichi Onishi, Kentaro Ebisawa, Peter Bosch, Darren Dukes, Francois
   Clad, Sridhar Bhaskaran and Arashmid Akhavain for their useful
   comments of this work.

13.  Contributors

   Kentaro Ebisawa
   Ponto Networks
   Japan

   Email: ebiken@pontonetworks.com

14.  References

14.1.  Normative References









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   [I-D.filsfils-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Hegde, S.,
              daniel.voyer@bell.ca, d., Lin, S., bogdanov@google.com,
              b., Krol, P., Horneffer, M., Steinberg, D., Decraene, B.,
              Litkowski, S., Mattes, P., Ali, Z., Talaulikar, K., Liste,
              J., Clad, F., and K. Raza, "Segment Routing Policy
              Architecture", draft-filsfils-spring-segment-routing-
              policy-06 (work in progress), May 2018.

   [I-D.filsfils-spring-srv6-network-programming]
              Filsfils, C., Li, Z., Leddy, J., daniel.voyer@bell.ca, d.,
              daniel.bernier@bell.ca, d., Steinberg, D., Raszuk, R.,
              Matsushima, S., Lebrun, D., Decraene, B., Peirens, B.,
              Salsano, S., Naik, G., Elmalky, H., Jonnalagadda, P., and
              M. Sharif, "SRv6 Network Programming", draft-filsfils-
              spring-srv6-network-programming-04 (work in progress),
              March 2018.

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Previdi, S., Leddy, J., Matsushima, S., and
              d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-segment-routing-header-14 (work in
              progress), June 2018.

   [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.xuclad-spring-sr-service-programming]
              Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C.,
              Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and
              S. Salsano, "Service Programming with Segment Routing",
              draft-xuclad-spring-sr-service-programming-00 (work in
              progress), July 2018.

   [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>.

14.2.  Informative References








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   [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-00 (work in progress), June
              2018.

   [I-D.camarillo-dmm-srv6-mobile-pocs]
              Camarillo Garvia, P., Filsfils, C., Bertz, L., Akhavain,
              A., Matsushima, S., and D. Voyer, "Segment Routing IPv6
              for mobile user-plane PoCs", draft-xuclad-spring-sr-
              service-programming-00 (work in progress), July 2018.

   [I-D.gundavelli-dmm-mfa]
              Gundavelli, S., Liebsch, M., and S. Matsushima, "Mobility-
              aware Floating Anchor (MFA)", draft-gundavelli-dmm-mfa-00
              (work in progress), February 2018.

   [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-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-12
              (work in progress), June 2018.

   [I-D.ietf-spring-segment-routing-central-epe]
              Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
              Afanasiev, "Segment Routing Centralized BGP Egress Peer
              Engineering", draft-ietf-spring-segment-routing-central-
              epe-10 (work in progress), December 2017.

   [I-D.rodrigueznatal-lisp-srv6]
              Rodriguez-Natal, A., Ermagan, V., Maino, F., Dukes, D.,
              Camarillo, P., and C. Filsfils, "LISP Control Plane for
              SRv6 Endpoint Mobility", draft-rodrigueznatal-lisp-srv6-00
              (work in progress), July 2018.

   [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>.






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   [TR.29891]
              3GPP, "5G System - Phase 1 CT WG4 Aspects", 3GPP TR
              29.891 15.0.0, December 2017.

   [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.

   [TS.29281]
              3GPP, "General Packet Radio System (GPRS) Tunnelling
              Protocol User Plane (GTPv1-U)", 3GPP TS 29.281 15.1.0,
              December 2017.

Appendix A.  Implementations

   This I-D introduces new SRv6 functions.  These functions have an
   open-source P4 implementation available in
   <https://github.com/ebiken/p4srv6>.

   Additionally, there are ongoing PoC efforts in M-CORD NGIC and Open
   Air Interface (OAI).  Progress and results can be found in
   [I-D.camarillo-dmm-srv6-mobile-pocs].

Authors' Addresses

   Satoru Matsushima
   SoftBank
   Tokyo
   Japan

   Email: satoru.matsushima@g.softbank.co.jp


   Clarence Filsfils
   Cisco Systems, Inc.
   Belgium

   Email: cf@cisco.com









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   Miya Kohno
   Cisco Systems, Inc.
   Japan

   Email: mkohno@cisco.com


   Pablo Camarillo Garvia
   Cisco Systems, Inc.
   Spain

   Email: pcamaril@cisco.com


   Daniel Voyer
   Bell Canada
   Canada

   Email: daniel.voyer@bell.ca


   Charles E. Perkins
   Futurewei Inc.
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Phone: +1-408-330-4586
   Email: charliep@computer.org






















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