Segment Routing IPv6 for Mobile User Plane
draft-ietf-dmm-srv6-mobile-uplane-16

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Document Type Active Internet-Draft (dmm WG)
Authors Satoru Matsushima  , Clarence Filsfils  , Miya Kohno  , Pablo Camarillo  , Dan Voyer  , Charles Perkins 
Last updated 2021-08-11 (latest revision 2021-07-27)
Replaces draft-matsushima-spring-dmm-srv6-mobile-uplane
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DMM Working Group                                     S. Matsushima, Ed.
Internet-Draft                                                  SoftBank
Intended status: Standards Track                             C. Filsfils
Expires: 12 February 2022                                       M. Kohno
                                                       P. Camarillo, Ed.
                                                     Cisco Systems, Inc.
                                                                D. Voyer
                                                             Bell Canada
                                                            C.E. Perkins
                                                             Lupin Lodge
                                                          11 August 2021

               Segment Routing IPv6 for Mobile User Plane
                  draft-ietf-dmm-srv6-mobile-uplane-16

Abstract

   This document shows the applicability of SRv6 (Segment Routing IPv6)
   to the user-plane of mobile networks.  The network programming nature
   of SRv6 accomplishes mobile user-plane functions in a simple manner.
   The statelessness of SRv6 and its ability to control both service
   layer path and underlying transport can be beneficial to the mobile
   user-plane, providing flexibility, end-to-end network slicing, and
   SLA control for various applications.  This document describes the
   SRv6 mobile user plane.

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
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   This Internet-Draft will expire on 12 February 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Predefined SRv6 Endpoint Behaviors  . . . . . . . . . . .   4
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  3GPP Reference Architecture . . . . . . . . . . . . . . . . .   5
   5.  User-plane behaviors  . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Traditional mode  . . . . . . . . . . . . . . . . . . . .   7
       5.1.1.  Packet flow - Uplink  . . . . . . . . . . . . . . . .   8
       5.1.2.  Packet flow - Downlink  . . . . . . . . . . . . . . .   9
     5.2.  Enhanced Mode . . . . . . . . . . . . . . . . . . . . . .   9
       5.2.1.  Packet flow - Uplink  . . . . . . . . . . . . . . . .  10
       5.2.2.  Packet flow - Downlink  . . . . . . . . . . . . . . .  11
       5.2.3.  Scalability . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Enhanced mode with unchanged gNB GTP behavior . . . . . .  12
       5.3.1.  Interworking with IPv6 GTP  . . . . . . . . . . . . .  12
       5.3.2.  Interworking with IPv4 GTP  . . . . . . . . . . . . .  15
       5.3.3.  Extensions to the interworking mechanisms . . . . . .  17
     5.4.  SRv6 Drop-in Interworking . . . . . . . . . . . . . . . .  17
   6.  SRv6 Segment Endpoint Mobility Behaviors  . . . . . . . . . .  19
     6.1.  Args.Mob.Session  . . . . . . . . . . . . . . . . . . . .  19
     6.2.  End.MAP . . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  End.M.GTP6.D  . . . . . . . . . . . . . . . . . . . . . .  20
     6.4.  End.M.GTP6.D.Di . . . . . . . . . . . . . . . . . . . . .  22
     6.5.  End.M.GTP6.E  . . . . . . . . . . . . . . . . . . . . . .  23
     6.6.  End.M.GTP4.E  . . . . . . . . . . . . . . . . . . . . . .  24
     6.7.  H.M.GTP4.D  . . . . . . . . . . . . . . . . . . . . . . .  25
     6.8.  End.Limit: Rate Limiting behavior . . . . . . . . . . . .  26
   7.  SRv6 supported 3GPP PDU session types . . . . . . . . . . . .  26
   8.  Network Slicing Considerations  . . . . . . . . . . . . . . .  26
   9.  Control Plane Considerations  . . . . . . . . . . . . . . . .  27
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  28
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28

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     14.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     14.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Appendix A.  Implementations  . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   In mobile networks, mobility management systems provide connectivity
   over a wireless link to stationary and non-stationary nodes.  The
   user-plane establishes a tunnel between the mobile node and its
   anchor node over IP-based backhaul and core networks.

   This document shows the applicability of SRv6 (Segment Routing IPv6)
   to mobile networks.

   Segment Routing [RFC8402] is a source routing architecture: a node
   steers a packet through an ordered list of instructions called
   "segments".  A segment can represent any instruction, topological or
   service based.

   SRv6 applied to mobile networks enables a source-routing based mobile
   architecture, where operators can explicitly indicate a route for the
   packets to and from the mobile node.  The SRv6 Endpoint nodes serve
   as mobile user-plane anchors.

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].

2.1.  Terminology

   *  CNF: Cloud-native Network Function
   *  NFV: Network Function Virtualization
   *  PDU: Packet Data Unit
   *  PDU Session: Context of a UE connects to a mobile network.
   *  UE: User Equipment
   *  UPF: User Plane Function
   *  VNF: Virtual Network Function (including CNFs)

   The following terms used within this document are defined in
   [RFC8402]: Segment Routing, SR Domain, Segment ID (SID), SRv6, SRv6
   SID, Active Segment, SR Policy, Prefix SID, Adjacency SID and Binding
   SID.

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   The following terms used within this document are defined in
   [RFC8754]: SRH, SR Source Node, Transit Node, SR Segment Endpoint
   Node and Reduced SRH.

   The following terms used within this document are defined in
   [RFC8986]: NH, SL, FIB, SA, DA, SRv6 SID behavior, SRv6 Segment
   Endpoint Behavior.

2.2.  Conventions

   An SR Policy is resolved to a SID list.  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:

   *  Source Address is SA, Destination Address is DA, and next-header
      is SRH
   *  SRH with SID list <S1, S2, S3> with Segments Left = SL
   *  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 to traverse.  (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 packet behavior, the (S3, S2, S1; SL)
      notation is more convenient.
   *  The payload of the packet is omitted.

   SRH[n]: A shorter representation of Segment List[n], as defined in
   [RFC8754].  SRH[SL] can be different from the DA of the IPv6 header.

   *  gNB::1 is an IPv6 address (SID) assigned to the gNB.
   *  U1::1 is an IPv6 address (SID) assigned to UPF1.
   *  U2::1 is an IPv6 address (SID) assigned to UPF2.
   *  U2:: is some other IPv6 address (SID) assigned to UPF2.

2.3.  Predefined SRv6 Endpoint Behaviors

   The following SRv6 Endpoint Behaviors are defined in [RFC8986].

   *  End.DT4: Decapsulation and Specific IPv4 Table Lookup
   *  End.DT6: Decapsulation and Specific IPv6 Table Lookup
   *  End.DT46: Decapsulation and Specific IP Table Lookup
   *  End.DX4: Decapsulation and IPv4 Cross-Connect
   *  End.DX6: Decapsulation and IPv6 Cross-Connect
   *  End.DX2: Decapsulation and L2 Cross-Connect

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   *  End.T: Endpoint with specific IPv6 Table Lookup

   This document defines new SRv6 Segment Endpoint Behaviors in
   Section 6.

3.  Motivation

   Mobile networks are becoming more challenging to operate.  On one
   hand, traffic is constantly growing, and latency requirements are
   tighter; on the other-hand, there are new use-cases like distributed
   NFVi that are also challenging network operations.

   The current architecture of mobile networks does not take into
   account the underlying transport.  The user-plane is rigidly
   fragmented into radio access, core and service networks, connected by
   tunneling according to user-plane roles such as access and anchor
   nodes.  These factors have made it difficult for the operator to
   optimize and operate the data-path.

   In the meantime, applications have shifted to use IPv6, and network
   operators have started adopting IPv6 as their IP transport.  SRv6,
   the IPv6 dataplane instantiation of Segment Routing [RFC8402],
   integrates both the application data-path and the underlying
   transport layer into a single protocol, allowing operators to
   optimize the network in a simplified manner and removing forwarding
   state from the network.  It is also suitable for virtualized
   environments, like VNF/CNF to VNF/CNF networking.  SRv6 has been
   deployed in dozens of networks
   [I-D.matsushima-spring-srv6-deployment-status].

   SRv6 defines the network-programming concept [RFC8986].  Applied to
   mobility, SRv6 can provide the user-plane behaviors needed for
   mobility management.  SRv6 takes advantage of the underlying
   transport awareness and flexibility together with the ability to also
   include services to optimize the end-to-end mobile dataplane.

   The use-cases for SRv6 mobility are discussed in
   [I-D.camarilloelmalky-springdmm-srv6-mob-usecases], and the
   architetural benefits are discussed in [I-D.kohno-dmm-srv6mob-arch].

4.  3GPP Reference Architecture

   This section presents a reference architecture and possible
   deployment scenarios.

   Figure 1 shows a reference diagram from the 5G packet core
   architecture [TS.23501].

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   The user plane described in this document does not depend on any
   specific architecture.  The 5G packet core architecture as shown is
   based on the latest 3GPP standards at the time of writing this draft.

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

                  Figure 1: 3GPP 5G Reference Architecture

   *  UE: User Endpoint
   *  gNB: gNodeB with N3 interface towards packet core (and N2 for
      control plane)
   *  UPF1: UPF with Interfaces N3 and N9 (and N4 for control plane)
   *  UPF2: UPF with Interfaces N9 and N6 (and N4 for control plane)
   *  SMF: Session Management Function
   *  AMF: Access and Mobility Management Function
   *  DN: Data Network e.g. operator services, Internet access

   This reference diagram does not depict a UPF that is only connected
   to N9 interfaces, although the mechanisms defined in this document
   also work in such case.

   Each session from a UE gets assigned to a UPF.  Sometimes multiple
   UPFs may be used, providing richer service functions.  A UE gets its
   IP address from the DHCP block of its UPF.  The UPF advertises that
   IP address block toward the Internet, ensuring that return traffic is
   routed to the right UPF.

5.  User-plane behaviors

   This section introduces an SRv6 based mobile user-plane.

   In order to simplify the adoption of SRv6, we present two different
   "modes" that vary with respect to the use of SRv6.  The first one is
   the "Traditional mode", which inherits the current 3GPP mobile
   architecture.  In this mode GTP-U protocol [TS.29281] is replaced by

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   SRv6, however the N3, N9 and N6 interfaces are still point-to-point
   interfaces with no intermediate waypoints as in the current mobile
   network architecture.

   The second mode is the "Enhanced mode".  This is an evolution from
   the "Traditional mode".  In this mode the N3, N9 or N6 interfaces
   have intermediate waypoints -SIDs- that are used for Traffic
   Engineering or VNF purposes.  This results in optimal end-to-end
   policies across the mobile network with transport and services
   awareness.

   In both, the Traditional and the Enhanced modes, we assume that the
   gNB as well as the UPFs are SR-aware (N3, N9 and -potentially- N6
   interfaces are SRv6).

   In addition to those two modes, we introduce two mechanisms for
   interworking with legacy access networks (those where the N3
   interface is unmodified).  In this document we introduce them as a
   variant to the Enhanced mode, however they are equally applicable to
   the Traditional mode.

   One of these mechanisms is designed to interwork with legacy gNBs
   using GTP/IPv4.  The second mechanism is designed to interwork with
   legacy gNBs using GTP/IPv6.

   This document uses SRv6 Segment Endpoint Behaviors defined in
   [RFC8986] as well as new SRv6 Segment Endpoint Behaviors designed for
   the mobile user plane that are defined in this document in Section 6.

5.1.  Traditional mode

   In the traditional mode, the existing mobile UPFs remain unchanged
   with the sole exception of the use of SRv6 as the data plane instead
   of GTP-U.  There is no impact to the rest of the mobile system.

   In existing 3GPP mobile networks, a PDU Session is mapped 1-for-1
   with a specific GTP tunnel (TEID).  This 1-for-1 mapping is mirrored
   here to replace GTP encapsulation with the SRv6 encapsulation, while
   not changing anything else.  There will be a unique SRv6 SID
   associated with each PDU Session, and the SID list only contains a
   single SID.

   The traditional mode minimizes the changes required to the mobile
   system; hence it is a good starting point for forming a common
   ground.

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   The gNB/UPF control-plane (N2/N4 interface) is unchanged,
   specifically a single IPv6 address is provided to the gNB.  The same
   control plane signalling is used, and the gNB/UPF decides to use SRv6
   based on signaled GTP-U parameters per local policy.  The only
   information from the GTP-U parameters used for the SRv6 policy is the
   TEID and the IPv6 Destination Address.

   Our example topology is shown in Figure 2.  In traditional mode the
   gNB and the UPFs are SR-aware.  In the descriptions of the uplink and
   downlink packet flow, A is an IPv6 address of the UE, and Z is an
   IPv6 address reachable within the Data Network DN.  A new SRv6
   Endpoint Behavior, End.MAP, defined in Section 6.2, is used.

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

               Figure 2: Traditional mode - example topology

5.1.1.  Packet flow - Uplink

   The uplink packet flow is as follows:

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

   When the UE packet arrives at the gNB, the gNB performs a
   H.Encaps.Red operation.  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. gNB obtains the SID U1::1 from the existing control plane
   (N2 interface).

   When the packet arrives at UPF1, the SID U1::1 is associated with the
   End.MAP SRv6 Endpoint Behavior.  End.MAP replaces U1::1 by U2::1,
   that belongs to the next UPF (U2).

   When the packet arrives at UPF2, the SID U2::1 corresponds to an
   End.DT4/End.DT6/End.DT46 SRv6 Endpoint Behavior.  UPF2 decapsulates
   the packet, performs a lookup in a specific table associated with
   that mobile network and forwards the packet toward the data network
   (DN).

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5.1.2.  Packet flow - Downlink

   The downlink packet flow is as follows:

       UPF2_in : (Z,A)
       UPF2_out: (U2::, U1::2) (Z,A)    -> H.Encaps.Red <U1::2>
       UPF1_out: (U2::, gNB::1) (Z,A)   -> End.MAP
       gNB_out : (Z,A)                  -> End.DX4, End.DX6, End.DX2

   When the packet arrives at the UPF2, the UPF2 maps that flow into a
   PDU Session.  This PDU Session is associated with the segment
   endpoint <U1::2>.  UPF2 performs a H.Encaps.Red 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::2 is a local SID associated
   with the End.MAP SRv6 Endpoint Behavior.  It maps the SID to the next
   anchoring point and replaces U1::2 by gNB::1, that belongs to the
   next hop.

   Upon packet arrival on gNB, the SID gNB::1 corresponds to an End.DX4,
   End.DX6 or End.DX2 behavior (depending on the PDU Session Type).  The
   gNB decapsulates the packet, removing the IPv6 header and all its
   extensions headers, and forwards the traffic toward the UE.

5.2.  Enhanced Mode

   Enhanced mode improves scalability, provides traffic engineering
   capabilities, and allows service programming
   [I-D.ietf-spring-sr-service-programming], thanks to the use of
   multiple SIDs in the SID list (instead of a direct connectivity in
   between UPFs with no intermediate waypoints as in Traditional Mode).

   Thus, the main difference is that the SR policy MAY include SIDs for
   traffic engineering and service programming in addition to the
   anchoring SIDs at UPFs.

   Additionally in this mode the operator may choose to aggregate
   several devices under the same SID list (e.g., stationary residential
   meters connected to the same cell) to improve scalability.

   The gNB/UPF control-plane (N2/N4 interface) is unchanged,
   specifically a single IPv6 address is provided to the gNB.  A local
   policy instructs the gNB to use SRv6.

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   The gNB MAY resolve the IP address received via the control plane
   into a SID list using a mechanism like PCEP, DNS-lookup, LISP
   control-plane or others.  The resolution mechanism is out of the
   scope of this document.

   Note that the SIDs MAY use the arguments Args.Mob.Session if required
   by the UPFs.

   Figure 3 shows an Enhanced mode topology.  In the Enhanced mode, the
   gNB and the UPF are SR-aware.  The Figure shows two service segments,
   S1 and C1.  S1 represents a VNF in the network, and C1 represents an
   intermediate router used for Traffic Engineering purposes to enforce
   a low-latency path in the network.  Note that neither S1 nor C1 are
   required to have an N4 interface.

                                    +----+  SRv6               _______
                    SRv6          --| C1 |--[N3]              /       \
   +--+    +-----+  [N3]         /  +----+  \  +------+ [N6] /         \
   |UE|----| gNB |--       SRv6 /    SRv6    --| UPF1 |------\   DN    /
   +--+    +-----+  \      [N3]/      TE       +------+       \_______/
          SRv6 node  \ +----+ /               SRv6 node
                      -| S1 |-
                       +----+
                      SRv6 node
                        VNF

                 Figure 3: Enhanced mode - Example topology

5.2.1.  Packet flow - Uplink

   The uplink packet flow is as follows:

   UE_out  : (A,Z)
   gNB_out : (gNB, S1)(U1::1, C1; SL=2)(A,Z)->H.Encaps.Red<S1,C1,U1::1>
   S1_out  : (gNB, C1)(U1::1, C1; SL=1)(A,Z)
   C1_out  : (gNB, U1::1)(A,Z)              ->End with PSP
   UPF1_out: (A,Z)                          ->End.DT4,End.DT6,End.DT2U

   UE sends its packet (A,Z) on a specific bearer to its gNB.  gNB's
   control plane associates that session from the UE(A) with the IPv6
   address B.  gNB's control plane does a lookup on B to find the
   related SID list <S1, C1, U1::1>.

   When gNB transmits the packet, it contains all the segments of the SR
   policy.  The SR policy includes segments for traffic engineering (C1)
   and for service programming (S1).

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   Nodes S1 and C1 perform their related Endpoint functionality and
   forward the packet.

   When the packet arrives at UPF1, the active segment (U1::1) is an
   End.DT4/End.DT6/End.DT2U which performs the decapsulation (removing
   the IPv6 header with all its extension headers) and forwards toward
   the data network.

5.2.2.  Packet flow - Downlink

   The downlink packet flow is as follows:

   UPF1_in : (Z,A)                             ->UPF1 maps the flow w/
                                                 SID list <C1,S1, gNB>
   UPF1_out: (U1::1, C1)(gNB::1, S1; SL=2)(Z,A)->H.Encaps.Red
   C1_out  : (U1::1, S1)(gNB::1, S1; SL=1)(Z,A)
   S1_out  : (U1::1, gNB::1)(Z,A)              ->End with PSP
   gNB_out : (Z,A)                             ->End.DX4/End.DX6/End.DX2

   When the packet arrives at the UPF1, the UPF1 maps that particular
   flow into a UE PDU Session.  This UE PDU Session is associated with
   the policy <C1, S1, gNB>.  The UPF1 performs a H.Encaps.Red
   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 at the gNB, the IPv6 DA corresponds to an
   End.DX4, End.DX6 or End.DX2 behavior at the gNB (depending on the
   underlying traffic).  The gNB decapsulates the packet, removing the
   IPv6 header, and forwards the traffic towards the UE.  The SID gNB::1
   is one example of a SID associated to this service.

   Note that there are several means to provide the UE session
   aggregation.  The decision on which one to use is a local decision
   made by the operator.  One option is to use the Args.Mob.Session
   (Section 6.1).  Another option comprises the gNB performing an IP
   lookup on the inner packet by using the End.DT4, End.DT6, and End.DT2
   behaviors.

5.2.3.  Scalability

   The Enhanced Mode improves since it allows the aggregation of several
   UEs under the same SID list.  For example, in the case of stationary
   residential meters that are connected to the same cell, all such
   devices can share the same SID list.  This improves scalability
   compared to Traditional Mode (unique SID per UE) and compared to
   GTP-U (dedicated TEID per UE).

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5.3.  Enhanced mode with unchanged gNB GTP behavior

   This section describes two mechanisms for interworking with legacy
   gNBs that still use GTP: one for IPv4, and another for IPv6.

   In the interworking scenarios as illustrated in Figure 4, the gNB
   does not support SRv6.  The gNB supports GTP encapsulation over IPv4
   or IPv6.  To achieve interworking, an SR Gateway (SRGW) entity is
   added.  The SRGW maps the GTP traffic into SRv6.

   The SRGW is not an anchor point and maintains very little state.  For
   this reason, both IPv4 and IPv6 methods scale to millions of UEs.

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

                Figure 4: Example topology for interworking

   Both of the mechanisms described in this section are applicable to
   either the Traditional Mode or the Enhanced Mode.

5.3.1.  Interworking with IPv6 GTP

   In this interworking mode the gNB at the N3 interface uses GTP over
   IPv6.

   Key points:

   *  The gNB is unchanged (control-plane or user-plane) and
      encapsulates into GTP (N3 interface is not modified).
   *  The 5G Control-Plane towards the gNB (N2 interface) is unmodified;
      one IPv6 address is needed per SLA(i.e. a BSID at the SRGW).  The
      SRv6 SID is different depending on the required SLA.
   *  In the uplink, the SRGW removes GTP, finds the SID list related to
      the IPv6 DA, and adds SRH with the SID list.
   *  There is no state for the downlink at the SRGW.
   *  There is simple state in the uplink at the SRGW; using Enhanced
      mode results in fewer SR policies on this node.  An SR policy is
      shared across UEs as long as they belong to the same context
      (i.e., tenant).  A set of many different policies (i.e., different
      SLAs) increases the amount of state required.
   *  When a packet from the UE leaves the gNB, it is SR-routed.  This
      simplifies network slicing [I-D.ietf-lsr-flex-algo].

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   *  In the uplink, the SRv6 BSID located in the IPv6 DA steers traffic
      into an SR policy when it arrives at the SRGW.

   An example topology is shown in Figure 5.

   S1 and C1 are two service segments.  S1 represents a VNF in the
   network, and C1 represents a router configured for Traffic
   Engineering.

                                  +----+
                IPv6/GTP         -| S1 |-                            ___
   +--+  +-----+ [N3]           / +----+ \                          /
   |UE|--| gNB |-         SRv6 /   SRv6   \ +----+   +------+ [N6] /
   +--+  +-----+ \        [N9]/     VNF    -| C1 |---| UPF2 |------\  DN
           GTP    \ +------+ /              +----+   +------+       \___
                   -| SRGW |-                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 as follows:

   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)               -> End with PSP
   UPF2_out: (A,Z)                            -> End.DT4 or End.DT6

   The UE sends a packet destined to Z toward the gNB on a specific
   bearer for that session.  The gNB, which is unmodified, encapsulates
   the packet into 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 signaled over the N2 interface for that UE
   PDU Session, B, is now the IPv6 DA.  B is an SRv6 Binding SID at the
   SRGW.  Hence the packet is routed to the SRGW.

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   When the packet arrives at the SRGW, the SRGW identifies B as an
   End.M.GTP6.D Binding SID (see Section 6.3).  Hence, the SRGW removes
   the IPv6, UDP, and GTP headers, and pushes an IPv6 header with its
   own SRH containing the SIDs bound to the SR policy associated with
   this BindingSID.  There at least one instance of the End.M.GTP6.D SID
   per PDU type.

   S1 and C1 perform their related Endpoint functionality and forward
   the packet.

   When the packet arrives at UPF2, the active segment is (U2::1) which
   is bound to End.DT4/6.  UPF2 then decapsulates (removing the outer
   IPv6 header with all its extension headers) and forwards the packet
   toward the data network.

5.3.1.2.  Packet flow - Downlink

   The downlink packet flow is as follows:

   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) -> H.Encaps.Red
   C1_out  : (U2::1, S1)(gNB, SRGW::TEID, 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 table associated to A and finds the SID list <C1, S1,
   SRGW::TEID, gNB>.  The UPF2 performs an H.Encaps.Red operation,
   encapsulating the packet into a new IPv6 header with its
   corresponding SRH.

   C1 and S1 perform their related Endpoint processing.

   Once the packet arrives at the SRGW, the SRGW identifies the active
   SID as an End.M.GTP6.E function.  The SRGW removes the IPv6 header
   and all its extensions headers.  The SRGW generates new 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 an
   argument of the received End.M.GTP6.E SID.  The SRGW pushes the
   headers to the packet and forwards the packet toward the gNB.  There
   is one instance of the End.M.GTP6.E SID per PDU type.

   Once the packet arrives at 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 pushed by the UPF2.  The UPF2 must have the UE states since
   it is the UE's session anchor point.

   For the uplink traffic, the state at the SRGW does not necessarily
   need to be unique per PDU Session; the SR policy can be shared among
   UEs.  This enables more scalable SRGW deployments compared to a
   solution holding millions of states, one or more per UE.

5.3.2.  Interworking with IPv4 GTP

   In this interworking mode the gNB uses GTP over IPv4 in the N3
   interface

   Key points:

   *  The gNB is unchanged and encapsulates packets into GTP (the N3
      interface is not modified).
   *  In the uplink, traffic is classified by SRGW's classification
      engine and steered into an SR policy.  The SRGW may be implemented
      in a UPF or as a separate entity.
   *  SRGW removes GTP, finds the SID list related to DA, and adds an
      SRH with the SID list.

   An example topology is shown in Figure 6.  In this mode the gNB is an
   unmodified gNB using IPv4/GTP.  The UPFs are SR-aware.  As before,
   the SRGW maps the IPv4/GTP traffic to SRv6.

   S1 and C1 are two service segment endpoints.  S1 represents a VNF in
   the network, and C1 represents a router configured for Traffic
   Engineering.

                                  +----+
                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

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5.3.2.1.  Packet flow - Uplink

   The uplink packet flow is as follows:

   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)    -> H.M.GTP4.D 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 toward 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 at the SRGW for UPF1, the SRGW has an
   classification engine rule for incoming traffic from the gNB, that
   steers the traffic into an SR policy by using the function
   H.M.GTP4.D.  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.

   S1 and C1 perform their related Endpoint functionality and forward
   the packet.

   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 its extension headers) and forwards toward
   the data network.

   Note that the interworking mechanisms for IPv4/GTP and IPv6/GTP
   differs.  This is due to the fact that in IPv6/GTP we can leverage
   the remote steering capabilities provided by SRv6.  In IPv4 this is
   not the case, and it would require a significant address consumption.

5.3.2.2.  Packet flow - Downlink

   The downlink packet flow is as follows:

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

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   When a packet destined to A arrives at the UPF2, the UPF2 performs a
   lookup in the table associated to A and finds the SID list <C1, S1,
   SRGW::SA:DA:TEID>.  The UPF2 performs a H.Encaps.Red 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 at the SRGW, the SRGW identifies the active
   SID as an End.M.GTP4.E function.  The SRGW removes the IPv6 header
   and all its extensions headers.  The SRGW generates an IPv4, UDP, and
   GTP headers.  The IPv4 SA and DA are received as 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 toward the gNB.

   When the packet arrives at the gNB, the packet is a regular IPv4/GTP
   packet.  The gNB looks for the specific radio bearer for that TEID
   and forwards 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 pushed by the UPF2.  The UPF must have this UE-base state
   anyway (since it is its anchor point).

   For the uplink traffic, the state at the SRGW is dedicated on a per
   UE/session basis according to a classification engine.  There is
   state for steering the different sessions in the form of an SR
   Policy.  However, SR policies are shared among several UE/sessions.

5.3.3.  Extensions to the interworking mechanisms

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

5.4.  SRv6 Drop-in Interworking

   In this section we introduce another mode useful for legacy gNB and
   UPFs that still operate with GTP-U.  This mode provides an
   SRv6-enabled user plane in between two GTP-U tunnel endpoints.

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   In this mode we employ two SRGWs that map GTP-U traffic to SRv6 and
   vice-versa.

   Unlike other interworking modes, in this mode both of the mobility
   overlay endpoints use GTP-U.  Two SRGWs are deployed in either N3 or
   N9 interface to realize an intermediate SR policy.

                               +----+
                              -| S1 |-
   +-----+                   / +----+ \
   | gNB |-            SRv6 /   SRv6   \ +----+   +--------+    +-----+
   +-----+  \              /     VNF    -| C1 |---| SRGW-B |----| UPF |
      GTP[N3]\ +--------+ /              +----+   +--------+    +-----+
              -| SRGW-A |-                SRv6   SR Gateway-B     GTP
               +--------+                  TE
              SR Gateway-A

              Figure 7: Example topology for SRv6 Drop-in mode

   The packet flow of Figure 7 is as follows:

   gNB_out : (gNB, U::1)(GTP: TEID T)(A,Z)
   GW-A_out: (GW-A, S1)(U::1, SGB::TEID, C1; SL=3)(A,Z)->U::1 is an
                                                         End.M.GTP6.D.Di
                                                         SID at SRGW-A
   S1_out  : (GW-A, C1)(U::1, SGB::TEID, C1; SL=2)(A,Z)
   C1_out  : (GW-A, SGB::TEID)(U::1, SGB::TEID, C1; SL=1)(A,Z)
   GW-B_out: (GW-B, U::1)(GTP: TEID T)(A,Z)            ->SGB::TEID is an
                                                         End.M.GTP6.E
                                                         SID at SRGW-B
   UPF_out : (A,Z)

   When a packet destined to Z is sent to the gNB, which is unmodified
   (control-plane and user-plane remain GTP-U), gNB performs
   encapsulation into a new IP, UDP, and GTP headers.  The IPv6 DA,
   U::1, and the GTP TEID are the ones received at the N2 interface.

   The IPv6 address that was signaled over the N2 interface for that PDU
   Session, U::1, is now the IPv6 DA.  U::1 is an SRv6 Binding SID at
   SRGW-A.  Hence the packet is routed to the SRGW.

   When the packet arrives at SRGW-A, the SRGW identifies U::1 as an
   End.M.GTP6.D.Di Binding SID (see Section 6.4).  Hence, the SRGW
   removes the IPv6, UDP, and GTP headers, and pushes an IPv6 header
   with its own SRH containing the SIDs bound to the SR policy
   associated with this Binding SID.  There is one instance of the
   End.M.GTP6.D.Di SID per PDU type.

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   S1 and C1 perform their related Endpoint functionality and forward
   the packet.

   Once the packet arrives at SRGW-B, the SRGW identifies the active SID
   as an End.M.GTP6.E function.  The SRGW removes the IPv6 header and
   all its extensions headers.  The SRGW generates new IPv6, UDP, and
   GTP headers.  The new IPv6 DA is U::1 which is the last SID in the
   received SRH.  The TEID in the generated GTP header is an argument of
   the received End.M.GTP6.E SID.  The SRGW pushes the headers to the
   packet and forwards the packet toward UPF.  There is one instance of
   the End.M.GTP6.E SID per PDU type.

   Once the packet arrives at UPF, the packet is a regular IPv6/GTP
   packet.  The UPF looks for the specific rule for that TEID to forward
   the packet.  This UPF behavior is not modified from current and
   previous generations.

6.  SRv6 Segment Endpoint Mobility Behaviors

6.1.  Args.Mob.Session

   Args.Mob.Session provide per-session information for charging,
   buffering and lawful intercept (among others) required by some mobile
   nodes.  The Args.Mob.Session argument format is used in combination
   with End.Map, End.DT4/End.DT6/End.DT46 and End.DX4/End.DX6/End.DX2
   behaviors.  Note that proposed format is applicable for 5G networks,
   while similar formats could be used for legacy networks.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   QFI     |R|U|                PDU Session ID                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |PDU Sess(cont')|
     +-+-+-+-+-+-+-+-+

                     Figure 8: Args.Mob.Session format

   *  QFI: QoS Flow Identifier [TS.38415]
   *  R: Reflective QoS Indication [TS.23501].  This parameter indicates
      the activation of reflective QoS towards the UE for the
      transferred packet.  Reflective QoS enables the UE to map UL User
      Plane traffic to QoS Flows without SMF provided QoS rules.
   *  U: Unused and for future use.  MUST be 0 on transmission and
      ignored on receipt.
   *  PDU Session ID: Identifier of PDU Session.  The GTP-U equivalent
      is TEID.

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   Arg.Mob.Session is required in case that one SID aggregates multiple
   PDU Sessions.  Since the SRv6 SID is likely NOT to be instantiated
   per PDU session, Args.Mob.Session helps the UPF to perform the
   behaviors which require per QFI and/or per PDU Session granularity.

   Note that the encoding of user-plane messages (e.g., Echo Request,
   Echo Reply, Error Indication and End Marker) is out of the scope of
   this draft.  [I-D.murakami-dmm-user-plane-message-encoding] defines
   one possible encoding.

6.2.  End.MAP

   The "Endpoint behavior with SID mapping" behavior (End.MAP for short)
   is used in several scenarios.  Particularly in mobility, End.MAP is
   used in the UPFs for the PDU Session anchor functionality.

   When node N receives a packet whose IPv6 DA is S and S is a local
   End.MAP SID, N does:

   S01. If (IPv6 Hop Limit <= 1) {
   S02.    Send an ICMP Time Exceeded message to the Source Address,
              Code 0 (Hop limit exceeded in transit),
              interrupt packet processing, and discard the packet.
   S03. }
   S04. Decrement IPv6 Hop Limit by 1
   S05. Lookup the IPv6 DA in the mapping table
   S06. Update the IPv6 DA with the new mapped SID
   S07. Submit the packet to the egress IPv6 FIB lookup for
           transmission to the new destination

   Notes: The SIDs in the SRH are not modified.

6.3.  End.M.GTP6.D

   The "Endpoint behavior with IPv6/GTP decapsulation into SR policy"
   behavior (End.M.GTP6.D for short) is used in interworking scenario
   for the uplink towards SRGW from the legacy gNB using IPv6/GTP.  Any
   SID instance of this behavior is associated with an SR Policy B and
   an IPv6 Source Address A.

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

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   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-layer header of a packet matching a FIB
   entry locally instantiated as an End.M.GTP6.D SID, N does:

   S01. If (Next Header = UDP & UDP_Dest_port = GTP) {
   S02.    Copy the GTP TEID to buffer memory
   S03.    Pop the IPv6, UDP, and GTP Headers
   S04.    Push a new IPv6 header with its own SRH containing B
   S05.    Set the outer IPv6 SA to A
   S06.    Set the outer IPv6 DA to the first SID of B
   S07.    Set the outer Payload Length, Traffic Class, Flow Label,
              Hop Limit, and Next-Header fields
   S08.    Write in the last SID of the SRH the Args.Mob.Session
              based on the information of buffer memory
   S09.    Submit the packet to the egress IPv6 FIB lookup and
              transmission to the new destination
   S10. } Else {
   S11.    Process as per [RFC8986] Section 4.1.1
   S12. }

   Notes: The NH is set based on the SID parameter.  There is one
   instantiation of the End.M.GTP6.D SID per PDU Session Type, hence the
   NH is already known in advance.  For the IPv4v6 PDU Session Type, in
   addition we inspect the first nibble of the PDU to know the NH value.

   The prefix of last segment (S3 in above example) SHOULD be followed
   by an Arg.Mob.Session argument space which is used to provide the
   session identifiers.

   The prefix of A SHOULD be an End.M.GTP6.E SID instantiated at an SR
   gateway.

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6.4.  End.M.GTP6.D.Di

   The "Endpoint behavior with IPv6/GTP decapsulation into SR policy for
   Drop-in Mode" behavior (End.M.GTP6.D.Di for short) is used in SRv6
   drop-in interworking scenario described in Section 5.4.  The
   difference between End.M.GTP6.D as another variant of IPv6/GTP
   decapsulation function is that the original IPv6 DA of GTP packet is
   preserved as the last SID in SRH.

   Any SID instance of this behavior is associated with an SR Policy B
   and an IPv6 Source Address A.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-layer header of a packet matching a FIB
   entry locally instantiated as an End.M.GTP6.Di SID, N does:

   S01. If (Next Header = UDP & UDP_Dest_port = GTP) {
   S02.    Copy S to buffer memory
   S03.    Pop the IPv6, UDP, and GTP Headers
   S04.    Push a new IPv6 header with its own SRH containing B
   S05.    Set the outer IPv6 SA to A
   S06.    Set the outer IPv6 DA to the first SID of B
   S07.    Set the outer Payload Length, Traffic Class, Flow Label,
              Hop Limit, and Next-Header fields
   S08.    Write S to the SRH
   S09.    Submit the packet to the egress IPv6 FIB lookup and
              transmission to the new destination
   S10. } Else {
   S11.    Process as per [RFC8986] Section 4.1.1
   S12. }

   Notes: The NH is set based on the SID parameter.  There is one
   instantiation of the End.M.GTP6.D SID per PDU Session Type, hence the
   NH is already known in advance.  For the IPv4v6 PDU Session Type, in
   addition we inspect the first nibble of the PDU to know the NH value.

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   The prefix of A SHOULD be an End.M.GTP6.E SID instantiated at an SR
   gateway.

6.5.  End.M.GTP6.E

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

   The prefix of End.M.GTP6.E SID MUST be followed by the
   Arg.Mob.Session argument space which is used to provide the session
   identifiers.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 1) {
   S03.      Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-layer header of a packet matching a FIB
   entry locally instantiated as an End.M.GTP6.E SID, N does:

   S01.    Copy SRH[0] and S to buffer memory
   S02.    Pop the IPv6 header and all its extension headers
   S03.    Push a new IPv6 header with a UDP/GTP Header
   S04.    Set the outer IPv6 SA to A
   S05.    Set the outer IPv6 DA from buffer memory
   S06.    Set the outer Payload Length, Traffic Class, Flow Label,
              Hop Limit, and Next-Header fields
   S07.    Set the GTP TEID (from buffer memory)
   S08.    Submit the packet to the egress IPv6 FIB lookup and
              transmission to the new destination
   S09. }

   Notes: An End.M.GTP6.E SID MUST always be the penultimate SID.  The
   TEID is extracted from the argument space of the current SID.

   The source address A SHOULD be an End.M.GTP6.D SID instantiated at an
   SR gateway.

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6.6.  End.M.GTP4.E

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

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address,
                Code 0 (Erroneous header field encountered),
                Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-layer header of a packet matching a FIB
   entry locally instantiated as an End.M.GTP4.E SID, N does:

   S01. If (Next Header = UDP & UDP_Dest_port = GTP) {
   S02.    Sotre the IPv6 DA and SA in buffer memory
   S03.    Pop the IPv6 header and all its extension headers
   S04.    Push a new IPv4 header with a UDP/GTP Header
   S05.    Set the outer IPv4 SA and DA (from buffer memory)
   S06.    Set the outer Total Length, DSCP, Time To Live, and
              Next-Header fields
   S07.    Set the GTP TEID (from buffer memory)
   S08.    Submit the packet to the egress IPv6 FIB lookup and
              transmission to the new destination
   S09. } Else {
   S10.    Process as per [NET-PGM] Section 4.1.1
   S11. }

   Notes: The End.M.GTP4.E SID in S has the following format:

       0                                                         127
       +-----------------------+-------+----------------+---------+
       |  SRGW-IPv6-LOC-FUNC   |IPv4DA |Args.Mob.Session|0 Padded |
       +-----------------------+-------+----------------+---------+
              128-a-b-c            a            b           c

                    Figure 9: End.M.GTP4.E SID Encoding

   The IPv6 Source Address has the following format:

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       0                                                         127
       +----------------------+--------+--------------------------+
       |  Source UPF Prefix   |IPv4 SA | any bit pattern(ignored) |
       +----------------------+--------+--------------------------+
                128-a-b            a                  b

                Figure 10: IPv6 SA Encoding for End.M.GTP4.E

6.7.  H.M.GTP4.D

   The "SR Policy Headend with tunnel decapsulation and map to an SRv6
   policy" behavior (H.M.GTP4.D for short) is used in the direction from
   legacy IPv4 user-plane to SRv6 user-plane network.

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

   S01. IF Payload == UDP/GTP THEN
   S02.    Pop the outer IPv4 header and UDP/GTP headers
   S03.    Copy IPv4 DA, TEID to form SID B
   S04.    Copy IPv4 SA to form IPv6 SA B'
   S05.    Encapsulate the packet into a new IPv6 header   ;;Ref1
   S06.    Set the IPv6 DA = B
   S07.    Forward along the shortest path to B
   S08. ELSE
   S09.    Drop the packet

   Ref1: The NH value is identified by inspecting the first nibble of
   the inner payload.

   The SID B has the following format:

       0                                                         127
       +-----------------------+-------+----------------+---------+
       |Destination UPF Prefix |IPv4DA |Args.Mob.Session|0 Padded |
       +-----------------------+-------+----------------+---------+
              128-a-b-c            a            b           c

                     Figure 11: H.M.GTP4.D SID Encoding

   The SID B MAY be an SRv6 Binding SID instantiated at the first UPF
   (U1) to bind an SR policy [I-D.ietf-spring-segment-routing-policy].

   The prefix of B' SHOULD be an End.M.GTP4.E SID with its format
   instantiated at an SR gateway with the IPv4 SA of the receiving
   packet.

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6.8.  End.Limit: Rate Limiting behavior

   The mobile user-plane requires a rate-limit feature.  For this
   purpose, we define a new behavior "End.Limit".  The "End.Limit"
   behavior encodes in its arguments the rate limiting parameter that
   should be applied to this packet.  Multiple flows of packets should
   have the same group identifier in the SID when those flows are in the
   same AMBR (Aggregate Maximum Bit Rate) group.  The encoding format of
   the rate limit segment SID is as follows:

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

        Figure 12: End.Limit: Rate limiting behavior argument format

   If the limit-rate bits are set to zero, the node should not do rate
   limiting unless static configuration or control-plane sets the limit
   rate associated to the SID.

7.  SRv6 supported 3GPP PDU session types

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

   *  IPv4
   *  IPv6
   *  IPv4v6
   *  Ethernet
   *  Unstructured

   SRv6 supports the 3GPP PDU session types without any protocol
   overhead by using the corresponding SRv6 behaviors (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 for L2 and Unstructured PDU
   sessions).

8.  Network Slicing Considerations

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

   [I-D.ietf-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 adopting the mechanisms from:

   *  IGP Flex-Algo [I-D.ietf-lsr-flex-algo]

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   *  Inter-Domain policies
      [I-D.ietf-spring-segment-routing-central-epe]

   Furthermore, these can be combined with ODN/AS (On Demand Nexthop/
   Automated Steering) [I-D.ietf-spring-segment-routing-policy] for
   automated slice provisioning and traffic steering.

   Further details on how these tools can be used to create end to end
   network slices are documented in
   [I-D.ali-spring-network-slicing-building-blocks].

9.  Control Plane Considerations

   This document focuses on user-plane behavior and its independence
   from the control plane.  While there are benefits in an enhanced
   control plane (e.g., to dynamically configure SR policies from a
   controller), this document does not impose any change to the existant
   mobility control plane.

   Section 11 allocates SRv6 Segment Endpoint Behavior codepoints for
   the new behaviors defined in this document.

10.  Security Considerations

   The security considerations for Segment Routing are discussed in
   [RFC8402].  More specifically for SRv6 the security considerations
   and the mechanisms for securing an SR domain are discussed in
   [RFC8754].  Together, they describe the required security mechanisms
   that allow establishment of an SR domain of trust to operate
   SRv6-based services for internal traffic while preventing any
   external traffic from accessing or exploiting the SRv6-based
   services.

   The technology described in this document is applied to a mobile
   network that is within the SR Domain.

   This document introduces new SRv6 Endpoint Behaviors.  Those
   behaviors do not need any special security consideration given that
   it is deployed within that SR Domain.

11.  IANA Considerations

   The following values have been allocated within the "SRv6 Endpoint
   Behaviors" [RFC8986] sub-registry belonging to the top-level "Segment
   Routing Parameters" registry:

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            +=======+========+===================+===========+
            | Value |  Hex   | Endpoint behavior | Reference |
            +=======+========+===================+===========+
            | 40    | 0x0028 |      End.MAP      | [This.ID] |
            +-------+--------+-------------------+-----------+
            | 41    | 0x0029 |     End.Limit     | [This.ID] |
            +-------+--------+-------------------+-----------+
            | 69    | 0x0045 |    End.M.GTP6.D   | [This.ID] |
            +-------+--------+-------------------+-----------+
            | 70    | 0x0046 |   End.M.GTP6.Di   | [This.ID] |
            +-------+--------+-------------------+-----------+
            | 71    | 0x0047 |    End.M.GTP6.E   | [This.ID] |
            +-------+--------+-------------------+-----------+
            | 72    | 0x0048 |    End.M.GTP4.E   | [This.ID] |
            +-------+--------+-------------------+-----------+

                 Table 1: SRv6 Mobile User-plane Endpoint
                              Behavior Types

12.  Acknowledgements

   The authors would like to thank Daisuke Yokota, Bart Peirens,
   Ryokichi Onishi, Kentaro Ebisawa, Peter Bosch, Darren Dukes, Francois
   Clad, Sri Gundavelli, Sridhar Bhaskaran, Arashmid Akhavain, Ravi
   Shekhar, Aeneas Dodd-Noble, Carlos Jesus Bernardos, Dirk v.  Hugo and
   Jeffrey Zhang for their useful comments of this work.

13.  Contributors

   Kentaro Ebisawa Toyota Motor Corporation Japan

   Email: ebisawa@toyota-tokyo.tech

   Tetsuya Murakami Arrcus, Inc.  United States of America

   Email: tetsuya.ietf@gmail.com

14.  References

14.1.  Normative References

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-13, 28 May 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              segment-routing-policy-13>.

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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>.

   [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,
              <https://www.rfc-editor.org/info/rfc8754>.

   [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,
              <https://www.rfc-editor.org/info/rfc8986>.

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

14.2.  Informative References

   [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",
              Work in Progress, Internet-Draft, draft-ali-spring-
              network-slicing-building-blocks-04, 21 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ali-spring-
              network-slicing-building-blocks-04>.

   [I-D.camarilloelmalky-springdmm-srv6-mob-usecases]
              Garvia, P. C., Filsfils, C., Elmalky, H., Matsushima, S.,
              Voyer, D., Cui, A., and B. Peirens, "SRv6 Mobility Use-
              Cases", Work in Progress, Internet-Draft, draft-
              camarilloelmalky-springdmm-srv6-mob-usecases-02, 15 August
              2019, <https://datatracker.ietf.org/doc/html/draft-
              camarilloelmalky-springdmm-srv6-mob-usecases-02>.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", Work in Progress,
              Internet-Draft, draft-ietf-lsr-flex-algo-17, 6 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lsr-
              flex-algo-17>.

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   [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", Work in Progress, Internet-Draft, draft-
              ietf-spring-segment-routing-central-epe-10, 21 December
              2017, <https://datatracker.ietf.org/doc/html/draft-ietf-
              spring-segment-routing-central-epe-10>.

   [I-D.ietf-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",
              Work in Progress, Internet-Draft, draft-ietf-spring-sr-
              service-programming-04, 10 March 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              sr-service-programming-04>.

   [I-D.kohno-dmm-srv6mob-arch]
              Kohno, M., Clad, F., Camarillo, P., and Z. Ali,
              "Architecture Discussion on SRv6 Mobile User plane", Work
              in Progress, Internet-Draft, draft-kohno-dmm-srv6mob-arch-
              04, 6 May 2021, <https://datatracker.ietf.org/doc/html/
              draft-kohno-dmm-srv6mob-arch-04>.

   [I-D.matsushima-spring-srv6-deployment-status]
              Matsushima, S., Filsfils, C., Ali, Z., Li, Z., and K.
              Rajaraman, "SRv6 Implementation and Deployment Status",
              Work in Progress, Internet-Draft, draft-matsushima-spring-
              srv6-deployment-status-11, 17 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-matsushima-
              spring-srv6-deployment-status-11>.

   [I-D.murakami-dmm-user-plane-message-encoding]
              Murakami, T., Matsushima, S., Ebisawa, K., Camarillo, P.,
              and R. Shekhar, "User Plane Message Encoding", Work in
              Progress, Internet-Draft, draft-murakami-dmm-user-plane-
              message-encoding-03, 7 March 2021,
              <https://datatracker.ietf.org/doc/html/draft-murakami-dmm-
              user-plane-message-encoding-03>.

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

   [TS.38415] 3GPP, "Draft Specification for 5GS container (TS 38.415)",
              3GPP R3-174510 0.0.0, August 2017.

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Appendix A.  Implementations

   This document introduces new SRv6 Endpoint Behaviors.  These
   behaviors have an open-source P4 implementation available in
   https://github.com/ebiken/p4srv6.

   Additionally, a full implementation of this document is available in
   Linux Foundation FD.io VPP project since release 20.05.  More
   information available here: https://docs.fd.io/vpp/20.05/d7/d3c/
   srv6_mobile_plugin_doc.html.

   There are also experimental implementations in M-CORD NGIC and Open
   Air Interface (OAI).

Authors' Addresses

   Satoru Matsushima (editor)
   SoftBank
   Japan

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

   Clarence Filsfils
   Cisco Systems, Inc.
   Belgium

   Email: cf@cisco.com

   Miya Kohno
   Cisco Systems, Inc.
   Japan

   Email: mkohno@cisco.com

   Pablo Camarillo Garvia (editor)
   Cisco Systems, Inc.
   Spain

   Email: pcamaril@cisco.com

   Daniel Voyer
   Bell Canada
   Canada

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   Email: daniel.voyer@bell.ca

   Charles E. Perkins
   Lupin Lodge
   20600 Aldercroft Heights Rd.
   Los Gatos, CA 95033
   United States of America

   Email: charliep@computer.org

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