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DMM Working Group                                       U. Chunduri, Ed.
Internet-Draft                                                     R. Li
Intended status: Informational                                 Futurewei
Expires: May 7, 2020                                        S. Bhaskaran
                                                               Altiostar
                                                       J. Kaippallimalil
                                                               Futurewei
                                                             J. Tantsura
                                                            Apstra, Inc.
                                                            L. Contreras
                                                              Telefonica
                                                                P. Muley
                                                                   Nokia
                                                        November 4, 2019


                Transport Network aware Mobility for 5G
                   draft-clt-dmm-tn-aware-mobility-05

Abstract

   This document specifies a framework and a mapping function for 5G
   mobile user plane with transport network slicing, integrated with
   Mobile Radio Access and a Virtualized Core Network.  The integrated
   approach is specified in a way to fit into the 5G core network
   architecture defined in [TS23.501].

   It focuses on an optimized mobile user plane functionality with
   various transport services needed for some of the 5G traffic needing
   low and deterministic latency, real-time, mission-critical services.
   This document describes, how this objective is achieved agnostic to
   the transport underlay used (IPv6, MPLS, IPv4) in various deployments
   and with a new transport network underlay routing, called Preferred
   Path Routing (PPR).

Requirements Language

   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 [RFC2119].

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



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   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 May 7, 2020.

Copyright Notice

   Copyright (c) 2019 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
     1.1.  Problem Statement . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Solution Approach . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Transport Network and Slice aware Mobility on N3/N9 . . . . .   6
     2.1.  XHaul Transport Network . . . . . . . . . . . . . . . . .   7
     2.2.  Mobile Transport Network Context (MTNC) and Scalability .   8
     2.3.  Transport Network Function (TNF)  . . . . . . . . . . . .   9
     2.4.  TNF Interfaces  . . . . . . . . . . . . . . . . . . . . .   9
     2.5.  Transport Provisioning  . . . . . . . . . . . . . . . . .  10
     2.6.  MTNC-ID in the Data Packet  . . . . . . . . . . . . . . .  11
     2.7.  Functionality for E2E Management  . . . . . . . . . . . .  12
   3.  Using PPR as TN Underlay  . . . . . . . . . . . . . . . . . .  13
     3.1.  PPR with Transport Awareness for 5GS on N3/N9 Interfaces   14
     3.2.  Path Steering Support to native IP user planes  . . . . .  16
     3.3.  Service Level Guarantee in Underlay . . . . . . . . . . .  16
   4.  Other TE Technologies Applicability . . . . . . . . . . . . .  16
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .  17



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   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Appendix A.  New Control Plane and User Planes  . . . . . . . . .  20
     A.1.  Slice aware Mobility: Discrete Approach . . . . . . . . .  20
   Appendix B.  PPR with various 5G Mobility procedures  . . . . . .  21
     B.1.  SSC Mode1 . . . . . . . . . . . . . . . . . . . . . . . .  22
     B.2.  SSC Mode2 . . . . . . . . . . . . . . . . . . . . . . . .  23
     B.3.  SSC Mode3 . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   The 3GPP architecture for 5GS is defined in [TS.23.501-3GPP],
   [TS.23.502-3GPP] and [TS.23.503-3GPP].  The architecture defines a
   comprehensive set of functions for access mobility, session handling
   and related functions for subscription management, authentication and
   policy among others.  These network functions (NF) are defined using
   a service-based architecture (SBA) that allows NFs to expose their
   functions via an API and common service framework.  The architecture
   also defines slicing aspects where the Network Slice Selection
   Function (NSSF) assists the Access Mobility Manager (AMF) and Session
   Management Function (SMF) to assist and select the right entities and
   resources corresponding to the slice requested by the user equipment
   (UE).  The User Equipment (UE) indicates slice information in the
   Network Slice Selection Assistance Information (NSSAI) field during
   session establishment (Attach).  The AMF selects the right SMF and
   the SMF in turn selects the User Plane Functions (UPF) so that the
   QoS and capabilities requested can be fulfilled.  UPFs are the data
   forwarding entities in the 5GC architecture.  The architecture allows
   the placement of Branching Point (BP) and Uplink Classifier (ULCL)
   UPFs closer to the access network (5G-AN).  The 5G-AN can be a radio
   access network or any non-3GPP access network, for example, WLAN.
   The IP address is anchored by a PDU session anchor UPF (PSA UPF).

   5GS allows more than one UPF on the path for a PDU (Protocol Data
   Unit) session that provides various functionality including session
   anchoring, uplink classification and branching point for a multihomed
   IPv6 PDU session.  The interface between the BP/ULCL UPF and the PSA
   UPF is called N9 [TS.23.501-3GPP]. 3GPP has adopted GTP-U for the N9
   and N3 interface between the various UPF instances and the (R)AN.
   3GPP has specified control and user plane aspects in [TS.23.501-3GPP]
   to provide slice and QoS support.  3GPP has defined three broad slice
   types to cover enhanced mobile broadband (eMBB) communications,
   ultra-reliable low latency communications(URLLC) and massive internet
   of things (mIoT).  ATIS [ATIS075] has defined an additional slice
   type for V2X services.  There may be multiple instances of a slice
   type to satisfy some characteristics like isolation.  The slice



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   details in 3GPP, ATIS or NGMN do not specify how slice
   characteristics for QoS, hard /soft isolation, protection and other
   aspects should be satisfied in IP transport networks.  This is
   explored further in this document.

1.1.  Problem Statement

   [TS.23.501-3GPP] and [TS.23.502-3GPP] define network slicing as one
   of the core capability of 5GC with slice awareness from Radio and 5G
   Core (5GC) network.  The 5G System (5GS) as defined, does not
   consider the resources and functionalities needed from transport
   network for the selection of UPF.  This is seen as independent
   functionality and currently not part of 5GS.

   However, the lack of underlying Transport Network (TN) awareness may
   lead to selection of sub-optimal UPF(s) and/or 5G-AN during 5GS
   various procedures (e.g., session establishment, mobility).  Meeting
   the specific slice characteristics on the N3 and N9 interfaces
   depends on the IP transport underlay providing these resources and
   capabilities.  This could also lead to the inability in meeting SLAs
   for real-time, mission-critical or latency sensitive services.  5GS
   procedures including but not limited to Service Request, PDU Session
   Establishment, or UE mobility need same service level characteristics
   from the TN for the Protocols Data Unit (PDU) session, similar to as
   provided in Radio and 5GC for the various Slice Service Types (SST)
   and 5QI's defined in [TS.23.501-3GPP].

   The 5GS provides slices to its clients (UEs).  The UE's PDU session
   spans the access network (radio) and N3 and N9 transport segments
   which have an IP transport underlay.  The 5G operator needs to obtain
   slice capability from the IP transport provider.  Several UE sessions
   that match a slice may be mapped to an IP transport segment.  Thus
   there needs to be a mapping between the slice capability offered to
   the UE (NSSAI) and what is provided by the IP transport.

1.2.  Solution Approach

   This document specifies an approach to fulfil the needs of 5GS to
   transport user plane traffic from 5G-AN to UPF for all service
   continuity modes [TS.23.501-3GPP] in an optimized fashion.  This is
   done by, keeping mobility procedures aware of underlying transport
   network along with slicing requirements.

   Section 2 describes in detail on how TN aware mobility can be built
   irrespective of underlying TN technology used.  Using Preferred Path
   Routing (PPR), applicable to any transport network underlay (IPv6,
   MPLS and IPv4) is detailed in Section 3.  How other IETF TE
   technologies applicable for this draft is specified in Section 4.  At



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   the end, Appendix B further describes the applicability and
   procedures of PPR with 5G SSC modes on N3 and N9 interfaces.

1.3.  Acronyms

   5QI      -  5G QoS Indicator

   5G-AN    -  5G Access Network

   AMF      -  Access and Mobility Management Function (5G)

   BP       -  Branch Point (5G)

   CSR      -  Cell Site Router

   DN       -  Data Network (5G)

   eMBB     -  enhanced Mobile Broadband (5G)

   FRR      -  Fast ReRoute

   gNB      -  5G NodeB

   GBR      -  Guaranteed Bit Rate (5G)

   GTP-U    -  GPRS Tunneling Protocol - Userplane (3GPP)

   IGP      -  Interior Gateway Protocols (e.g.  IS-IS, OSPFv2, OSPFv3)

   LFA      -  Loop Free Alternatives (IP FRR)

   mIOT     -  Massive IOT (5G)

   MPLS     -  Multi Protocol Label Switching

   QFI      -  QoS Flow ID (5G)

   PPR      -  Preferred Path Routing

   PDU      -  Protocol Data Unit (5G)

   PW       -  Pseudo Wire

   RQI      -  Reflective QoS Indicator (5G)

   SBI      -  Service Based Interface (5G)

   SID      -  Segment Identifier



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   SMF      -  Session Management Function (5G)

   SSC      -  Session and Service Continuity (5G)

   SST      -  Slice and Service Types (5G)

   SR       -  Segment Routing

   TE       -  Traffic Engineering

   ULCL     -  Uplink Classifier (5G)

   UPF      -  User Plane Function (5G)

   URLLC    -  Ultra reliable and low latency communications (5G)

2.  Transport Network and Slice aware Mobility on N3/N9

   Currently specified Control Plane (CP) functions - the AMF, the SMF
   and the User plane (UP) components 5G-AN (e.g. gNB), UPF with N2, N3,
   N4, N6 and N9 interfaces are relevant to this document.  Other
   Virtualized 5G control plane components NRF, AUSF, PCF, AUSF, UDM,
   NEF, and AF are not directly relevant for the discussion in this
   document and one can see the functionalities of these in
   [TS.23.501-3GPP].

   From encapsulation perspective, N3 interface is similar to S1U in 4G/
   LTE [TS.23.401-3GPP] network and uses GTP-U [TS.29.281-3GPP] to
   transport any UE PDUs (IPv4, IPv6, IPv4v6, Ethernet or Unstructured).
   Unlike S1U, N3 has some additional aspects as there is no bearer
   concept and no per bearer GTP-U tunnels.  Instead, QoS information is
   carried in the PDU Session Container GTP-U extension header.



















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                  +------+     +-----+    +-------+    +-----+
                  | NSSF |     | NRF |    | NWDAF |    | PCF |
                  +---+--+     +--+--+    +---+---+    +--+--+
                      |           |           |           |
              --+-----+--------+--+-----------+-----+-----+-----
                |              | Service Based Interfaces (SBI)
                |        +-----+--------+           |
            +--+--+      | +-----+ NSSMF|        +--+--+
      +-----+ AMF |      | | TNF |      |        | SMF |
      |     +--+--+      | +--+--+      |        +--+--+
      |                  +---+----------+           |
      |                      |ACTN                  |
      |                  +---+--+                   |
   N2 |         +--------|SDN-C |-------+           |
      |         |        +--+---+       |       +----------+
      |         |Ns       _______     Nn|       |    N4    |
      |         |     ___/       \______|_______|__________|___
      |         |    /                  |       |          |   \
 +------+    +--+---+       IP        +-++   +--+---+   +--+----+   +--+
 |(R)AN |====|CSR/PE|    Backhaul     |PE|+++|UP-NF2|+++|PE+UPF2|---|DN|
 +------+    +------+                 +--+   +------+   +-------+   +--+
                     \___         _____________________________/
         Front/          \       /  N3                N9          N6
         Midhaul          +-----+


      Figure 1: 5G Service Based Architecture with Transport Network

   TN Aware Mobility with optimized transport network functionality is
   explained below.  How PPR fits in this framework in detail along with
   other various TE technologies briefly are in Section 3 and Section 4
   respectively.

2.1.  XHaul Transport Network

   Figure 1 depicts IP Xhaul network with SDN-C and CSR/PE (Provider
   Edge) routers provide IP transport service to 5GS user plane entities
   5G-AN (e.g. gNB) and UPF.  5GS architecture with key control and user
   plane entities and its interaction with the IP transport plane is
   shown here.  When a UE initiates session establishment, it indicates
   the desired slice type in the S-NSSAI (Specific Network Slice
   Selection Assistance Information) field.  The AMF uses the S-NSSAI,
   other subscription information and configuration in the NSSF to
   select the appropriate SMF and the SMF in turn selects UPFs (User
   Plane Functions) that are able to provide the specified slice
   resources and capabilities.  Some of the slice capabilities along the
   user plane path between the (R)AM and UPFs (N3, N9 segments) such as




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   a low latency path, jitter, protection and priority needs these to be
   provided by the IP transport network.

   Interface between (R)AN/5G-AN and CSR includes fronthaul and midhaul
   part of the transport network.  In some cases midhaul can also a
   layer-2 network.  In deployments where virtualized 5G-AN is used, F1U
   interface with GTP encapsulation is considered between the
   Distributed Unit (DU) and Centralized Unit (CU).

   Slice capability required in IP transport networks is estimated and
   provisioned by the functionality as specified in Section 2.3 (TNF)
   with support from various other functions such as the Network Data
   Analytics Function (NWDAF), Network Resource Function (NRF) and
   Policy Control Function (PCF).  Figure 1 depicts the PE router, where
   transport paths are initiated/terminated can be deployed seperately
   with UPF or both functionalities can be in the same node.  The TNF
   provisions this in the SDN-C of the IP XHaul network using ACTN
   [RFC8453].  When a GTP encapsulated user packet from the (R)AN or UPF
   with the slice information traverses the F1U/N3/N9 segment, the PE
   router of the IP transport underlay can inspect the slice information
   and provide the provisioned capabilities.  This is elaborated further
   in Section 2.5.

2.2.  Mobile Transport Network Context (MTNC) and Scalability

   The MTNC represents a slice, QoS configuration for a transport path
   between two 3GPP user plane functions.  The Mobile-Transport Network
   Context Identifier (MTNC-ID) is generated by the TNF to be unique for
   each path and per traffic class (including QoS and slice aspects).
   Thus, there may be more than one MTNC-ID for the same QoS and path if
   there is a need to provide isolation (slice) of the traffic.  It
   should be noted that MTNC are per class/path and not per user session
   (nor is it per data path entity).  The MTNC-IDs are configured by the
   TNF to be unique within a provisioning domain.

   Since the MTNC-IDs are not generated per user flow or session, there
   is no need for unique MTNC-IDs per flow/session.  In addition, since
   the traffic estimation not performed at the time of session
   establishment, there is no provisioning delay experienced during
   session setup.  The MTNC-ID space scales as a square of the number
   sites between which 3GPP user plane functions require paths.  If
   there are T traffic classes across N sites, the number of MTNC-IDs in
   a fully meshed network is (N*(N-1)/2) * T.  For example, if there are
   3 traffic classes between 25 sites, there would be at most 900 MTNC-
   IDs required.  Multiple slices for the same QoS class that need to be
   fully isolated, will add to the MTNC provisioning.  An MTNC-ID space
   of 16 bits (65K+ identifiers) can be expected to be sufficient.




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2.3.  Transport Network Function (TNF)

   Figure 1 shows a view of the functions and interfaces for
   provisioning the MTNC-IDs.  The focus is on provisioning between the
   3GPP management plane (NSSMF), transport network (SDN-C) and carrying
   the MTNC-IDs in PDU packets for the transport network to grant the
   provisioned resources.

   In Figure 1, the TNF (logical functionality within the NSSMF)
   requests the SDN-C in the transport domain to program the TE path
   using ACTN [RFC 8453].  The SDN-C programs the Provider Edge (PE)
   routers and internal routers according to the underlay transport
   technology (e.g., PPR, MPLS, SRv6).  The PE router inspects incoming
   PDU data packets for the MTNC-ID, classifies and provides the VN
   service provisioned across the transport network.

   The detailed mechanisms by which the NSSMF provides the MTNC-IDs to
   the control plane and user plane functions are for 3GPP to specify.
   Two possible options are outlined below for completeness.  The NSSMF
   may provide the MTNC-IDs to the 3GPP control plane by either
   providing it to the Session Management Function (SMF), and the SMF in
   turn provisions the user plane functions (UP-NF1, UP-NF2) during PDU
   session setup.  Alternatively, the user plane functions may request
   the MTNC-IDs directly from the TNF/NSSMF.  Figure 1 shows the case
   where user plane entities request the TNF/NSSMF to translate the
   Request and get the MTNC-ID (over interface Ns/Nn).

   The TNF should be seen as a logical entity that can be part of NSSMF
   in the 3GPP management plane [TS.28.533-3GPP].  The NSSMF may use
   network configuration, policies, history, heuristics or some
   combination of these to derive traffic estimates that the TNF would
   use.  How these estimates are derived are not in the scope of this
   document.  The focus here is only in terms of how the TNF and SDN-C
   are programmed given that slice and QoS characteristics across a
   transport path can be represented by an MTNC-ID.  The TNF requests
   the SDN-C in the transport network to provision paths in the
   transport domain based on the MTNC-ID.  The TNF is capable of
   providing the MTNC-ID provisioned to control and user plane functions
   in the 3GPP domain.  Detailed mechanisms for programming the MTNC-ID
   should be part of the 3GPP specifications.

2.4.  TNF Interfaces

   A south bound interface Ns is shown which interacts with the 5G
   Access Network (e.g.  CSR).  'Ns' can use one or more mechanism
   available today (PCEP [RFC5440], NETCONF [RFC6241], RESTCONF
   [RFC8040] or gNMI) to provision the L2/L3 VPNs along with TE underlay
   paths from CSR to PE@UPF.  Ns and Nn interfaces can be part of the



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   integrated 3GPP architecture, but the specification/ownership of
   these interfaces SHOULD be left out of scope of 3GPP.

   A north bound interface 'Nn' is shown from one or more of the
   transport network nodes (or PE @ ULCL/BP UPF, Anchor Point UPF) to
   TNF as shown in Figure 1.  It would enable learning the TE
   characteristics of all links and nodes of the network continuously
   (through BGP-LS [RFC7752] or through a passive IGP adjacency and PCEP
   [RFC5440]).

   These VPNs and/or underlay TE paths MUST be similar on all 5G-AN/CSRs
   and UPFs concerned to allow mobility of UEs while associated with one
   of the Slice/Service Types (SSTs)as defined in [TS.23.501-3GPP].

2.5.  Transport Provisioning

   Functionality of transport provisioning for an engineered IP
   transport that supports 3GPP slicing and QoS requirements in
   [TS.23.501-3GPP] is described in this section.

   During a PDU session setup, the AMF using input from the NSSF selects
   a network slice and SMF.  The SMF with user policy from Policy
   Control Function (PCF) sets 5QI (QoS parameters) and the UPF on the
   path of the PDU session.  While QoS and slice selection for the PDU
   session can be applied across the 3GPP control and user plane
   functions as outlined in Section 2, the IP transport underlay across
   N3 and N9 segments do not have enough information to apply the
   resource constraints represented by the slicing and QoS
   classification.  Current guidelines for interconnection with
   transport networks [IR.34-GSMA] provide an application mapping into
   DSCP.  However, these recommendations do not take into consideration
   other aspects in slicing like isolation, protection and replication.

   IP transport networks have their own slice and QoS configuration
   based on domain policies and the underlying network capability.
   Transport networks can enter into an agreement for virtual network
   services (VNS) with client domains using the ACTN [RFC8453]
   framework.  An IP transport network may provide such slice instances
   to mobile network operators, CDN providers or enterprises for
   example.  The 3GPP mobile network, on the other hand, defines a slice
   instance for UEs as are the the mobile operator's 'clients'.  The
   Network Slice Selection Management Function (NSSMF) [TS 28.533] that
   interacts with a TN controller like an SDN-C (that is out of scope of
   3GPP).

   The ACTN VN service can be used across the IP transport networks to
   provision and map the slice instance and QoS of the 3GPP domain to
   the IP transport domain.  An abstraction that represents QoS and



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   slice instance in the mobile domain and mapped to ACTN VN service in
   the transport domain is represented here as MTNC-IDs.  Details of how
   the MTNC-IDs are derived are upto functions that can estimate the
   level of traffic demand.

   The 3GPP network/5GS provides slices instances to its clients (UE)
   that include resources for radio and mobile core segments.  The UE's
   PDU session spans the access network (radio) and N3 and N9 transport
   segments which have an IP transport underlay.  The 5G operator needs
   to obtain slice capability from the IP transport provider since these
   resources are not seen by the 5GS.  Several UE sessions that match a
   slice may be mapped to an IP transport segment.  Thus, there needs to
   be a mapping between the slice capability offered to the UE (NSSAI)
   and what is provided by the IP transport.

   When the 3GPP user plane function (5G-AN, UPF) does not terminate the
   transport underlay protocol (e.g., MPLS), it needs to be carried in
   the IP protocol header from end-to-end of the mobile transport
   connection (N3, N9).  [I-D.ietf-dmm-5g-uplane-analysis] discusses
   these scenarios in detail.

2.6.  MTNC-ID in the Data Packet

   When the 3GPP user plane function (5G-AN, UPF) and transport provider
   edge is on different nodes, the PE router needs to have the means by
   which to classify the PDU packet.  The mapping information is
   provisioned between the 5G provider and IP transport network and
   corresponding information should be carried in each IP packet on the
   N3, N9 interface.  To allow the IP transport edge nodes to inspect
   the transport context information efficiently, it should be carried
   in an IP header field that is easy to inspect.  It may be noted that
   the N3 and N9 interfaces in 5GS are IP interfaces.  Thus, Layer 2
   alternatives such as VLAN will fail if there are multiple L2 networks
   on the N3 or N9 path.  Another alternative is to use L3 VPNs.
   However, in the 5G architecture, there may be a large number of
   smaller UPFs that are dynamically inserted on path.  Adding this VPN
   information for dynamic UPF insertion is configuration intensive and
   error prone.  GTP (N3, N9 encapsulation header) field extensions
   offer a possibility, however these extensions are hard for a
   transport edge router to parse efficiently on a per packet basis.
   Other IP header fields like DSCP are not suitable as it only conveys
   the QoS aspects (but not other aspects like isolation, protection,
   etc.)

   IPv6 extension headers like FaST, or SRv6 may be options to carry the
   MTNC-ID when such mechanism is a viable (if complete transport
   network is IPv6 based).  To mininise the protocol changes are
   required and make this underlay tranport independed (IPv4/IPv6/MPLS/



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   L2), an option is to provision a mapping of MTNC-ID to a UDP port
   range of the GTP encapsulated user packet.  A simple mapping table
   between the MTNC-ID and the source UDP port number can be configured
   to ensure that ECMP /load balancing is not affected adversely by
   encoding the UDP source port with an MTNC-ID mapping.  This mapping
   is configured in 3GPP user plane functions (5G-AN, UPF) and Provider
   Edge (PE) Routers that process MTNC-IDs.  PE routers can thus
   provision a policy based on the source UDP port number (which
   reflects the mapped MTNC-ID) to underlying transport path and then
   deliver the QoS/slice resource provisioned in the transport network.
   The source UDP port that is encoded is the outer IP (corresponding to
   GTP header) while the inner IP packet (UE payload) is unaltered.

2.7.  Functionality for E2E Management

   With the TNF finctionality in 5GS Service Based Interface, the
   following additional functionalities are required for end-2-end slice
   management including the transport network:

   o  The Specific Network Slice Selection Assistance Information
      (S-NSSAI) of PDU session SHOULD be mapped to the assigned
      transport VPN and the TE path information for that slice.

   o  For transport slice assignment for various SSTs (eMBB, URLLC,
      MIoT) corresponding underlay paths need to be created and
      monitored from each transport end point (CSR and PE@UPF).

   o  During PDU session creation, apart from radio and 5GC resources,
      transport network resources needed to be verified matching the
      characteristics of the PDU session traffic type.

   o  The TNF MUST provide an API that takes as input the source and
      destination 3GPP user plane element address, required bandwidth,
      latency and jitter characteristics between those user plane
      elements and returns as output a particular TE path's identifier,
      that satisfies the requested requirements.

   o  Mapping of PDU session parameters to underlay SST paths need to be
      done.  One way to do this to let the SMF install a Forwarding
      Action Rule (FAR) in the UPF via N4 with the FAR pointing to a
      "Network Instance" in the UPF.  A "Network Instance" is a logical
      identifier for an underlying network.  The "Network Instance"
      pointed by the FAR can be mapped to a transport path (through L2/
      L3 VPN).  FARs are associated with Packet Detection Rule (PDR).
      PDRs are used to classify packets in the uplink (UL) and the
      downlink (DL) direction.  For UL procedures specified in
      Section 2.5, Section 2.6 can be used for classifying a packet
      belonging to a particular slice characteristics.  For DL, at a PSA



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      UPF, the UE IP address is used to identify the PDU session, and
      hence the slice a packet belongs to and the IP 5 tuple can be used
      for identifying the flow and QoS characteristics to be applied on
      the packet at UPF.  If a PE is not co-located at the UPF then
      mapping to the underlying TE paths at PE happens based on the
      encapsulated GTP-US packet as specified in Section 2.6.

   o  If any other form of encapsulation (other than GTP-U) either on N3
      or N9 corresponding QFI information MUST be there in the
      encapsulation header.

   o  In some SSC modes Appendix B, if segmented path (CSR to
      PE@staging/ULCL/BP-UPF to PE@anchor-point-UPF) is needed, then
      corresponding path characteristics MUST be used.  This includes a
      path from CSR to PE@UL-CL/BP UPF [TS.23.501-3GPP] and UL-CL/BP UPF
      to eventual UPF access to DN.

   o  Continuous monitoring of the underlying transport path
      characteristics should be enabled at the endpoints (technologies
      for monitoring depends traffic engineering technique used as
      described in Section 3 and Section 4).  If path characteristics
      are degraded, reassignment of the paths at the endpoints should be
      performed.  For all the affected PDU sessions, degraded transport
      paths need to be updated dynamically with similar alternate paths.

   o  During UE mobility event similar to 4G/LTE i.e., gNB mobility (Xn
      based or N2 based), for target gNB selection, apart from radio
      resources, transport resources MUST be factored.  This enables
      handling of all PDU sessions from the UE to target gNB and this
      require co-ordination of gNB, AMF, SMF with the TNF module.

   Integrating the TNF as part of the 5GS Service Based Interfaces,
   provides the flexibility to control the allocation of required
   characteristics from the TN during a 5GS signaling procedure (e.g.
   PDU Session Establishment).  If TNF is seen as part of management
   plane, this real time flexibility is lost.  Changes to detailed
   signaling to integrate the above for various 5GS procedures as
   defined in [TS.23.502-3GPP] is beyond the scope of this document.

3.  Using PPR as TN Underlay

   In a network implementing source routing, packets may be transported
   through the use of Segment Identifiers (SIDs), where a SID uniquely
   identifies a segment as defined in [I-D.ietf-spring-segment-routing].
   Section 5.3 [I-D.bogineni-dmm-optimized-mobile-user-plane] lays out
   all SRv6 features along with a few concerns in Section 5.3.7 of the
   same document.  Those concerns as well as need for underlay agnostic
   (L2/Ipv4/IPv6/MPLS) TE requirements are addressed by a new XHaul



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   routing mechanism called Preferred Path Routing (PPR), of which this
   section provides an overview.

   With PPR, the label/PPR-ID refer not to individual segments of which
   the path is composed, but to the identifier of a path that is
   deployed on network nodes.  The fact that paths and path identifiers
   can be computed and controlled by a controller, not a routing
   protocol, allows the deployment of any path that network operators
   prefer, not just shortest paths.  As packets refer to a path towards
   a given destination and nodes make their forwarding decision based on
   the identifier of a path, not the identifier of a next segment node,
   it is no longer necessary to carry a sequence of labels.  This
   results in multiple benefits including significant reduction in
   network layer overhead, increased performance and hardware
   compatibility for carrying both path and services along the path.

   Details of the IGP extensions for PPR are provided here:

   o  IS-IS - [I-D.chunduri-lsr-isis-preferred-path-routing]

   o  OSPF - [I-D.chunduri-lsr-ospf-preferred-path-routing]

3.1.  PPR with Transport Awareness for 5GS on N3/N9 Interfaces

   PPR does not remove GTP-U, unlike some other proposals laid out in
   [I-D.bogineni-dmm-optimized-mobile-user-plane].  Instead, PPR works
   with the existing cellular user plane (GTP-U) for both N3 and any
   approach selected for N9 (encapsulation or no-encapsulation).  In
   this scenario, PPR will only help providing TE benefits needed for 5G
   slices from transport domain perspective.  It does so for any
   underlying data plane used in the transport network (L2/IPv4/IPv6/
   MPLS).  This is achieved by:

   o  For 3 different SSTs, 3 PPR-IDs can be signaled from any node in
      the transport network.  For Uplink traffic, the 5G-AN will choose
      the right PPR-ID of the UPF based on the S-NSSAI the PDU Session
      belongs to and/or the UDP Source port (corresponds to the MTNC-ID
      Section 2.5) of the GTP-U encapsulation header.  Similarly in the
      Downlink direction matching PPR-ID of the 5G-AN is chosen based on
      the S-NSSAI the PDU Session belongs to.  The table below shows a
      typical mapping:










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      +----------------+------------+------------------+-----------------+
      |GTP/UDP SRC PORT|   SST      |   Transport Path | Transport Path  |
      |                | in S-NSSAI |   Info           | Characteristics |
      +----------------+------------+------------------+-----------------+
      | Range Xx - Xy  |            |                  |                 |
      | X1, X2(discrete|  MIOT      | PW ID/VPN info,  | GBR (Guaranteed |
      | values)        |  (massive  |   PPR-ID-A       |       Bit Rate) |
      |                |   IOT)     |                  |   Bandwidth: Bx |
      |                |            |                  |   Delay:     Dx |
      |                |            |                  |   Jitter:    Jx |
      +----------------+------------+------------------+-----------------+
      | Range Yx - Yy  |            |                  |                 |
      | Y1, Y2(discrete|  URLLC     | PW ID/VPN info,  | GBR with Delay  |
      | values)        | (ultra-low | PPR-ID-B         |     Req.        |
      |                |  latency)  |                  |   Bandwidth: By |
      |                |            |                  |   Delay:     Dy |
      |                |            |                  |   Jitter:    Jy |
      +----------------+------------+------------------+-----------------+
      | Range Zx - Zy  |            |                  |                 |
      | Z1, Z2(discrete|  EMBB      | PW ID/VPN info,  |   Non-GBR       |
      | values)        | (broadband)| PPR-ID-C         |   Bandwidth: Bx |
      +----------------+------------+------------------+-----------------+



                   Figure 2: Mapping of PPR-IDs on N3/N9

   o  It is possible to have a single PPR-ID for multiple input points
      through a PPR tree structure separate in UL and DL direction.

   o  Same set of PPRs are created uniformly across all needed 5G-ANs
      and UPFs to allow various mobility scenarios.

   o  Any modification of TE parameters of the path, replacement path
      and deleted path needed to be updated from TNF to the relevant
      ingress points.  Same information can be pushed to the NSSF, and/
      or SMF as needed.

   o  PPR can be supported with any native IPv4 and IPv6 data/user
      planes (Section 3.2) with optional TE features (Section 3.3) . As
      this is an underlay mechanism it can work with any overlay
      encapsulation approach including GTP-U as defined currently for N3
      interface.








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3.2.  Path Steering Support to native IP user planes

   PPR works in fully compatible way with SR defined user planes (SR-
   MPLS and SRv6) by reducing the path overhead and other challenges as
   listed in Section 5.3.7 of
   [I-D.bogineni-dmm-optimized-mobile-user-plane].  PPR also expands the
   source routing to user planes beyond SR-MPLS and SRv6 i.e., L2,
   native IPv6 and IPv4 user planes.

   This helps legacy transport networks to get the immediate path
   steering benefits and helps in overall migration strategy of the
   network to the desired user plane.  Some of these benefits with PPR
   can be realized with no hardware upgrade except control plane
   software for native IPv6 and IPv4 user planes.

3.3.  Service Level Guarantee in Underlay

   PPR also optionally allows to allocate resources that are to be
   reserved along the preferred path.  These resources are required in
   some cases (for some 5G SSTs with stringent GBR and latency
   requirements) not only for providing committed bandwidth or
   deterministic latency, but also for assuring overall service level
   guarantee in the network.  This approach does not require per-hop
   provisioning and reduces the OPEX by minimizing the number of
   protocols needed and allows dynamism with Fast-ReRoute (FRR)
   capabilities.

4.  Other TE Technologies Applicability

   RSVP-TE [RFC3209] provides a lean transport overhead for the TE path
   for MPLS user plane.  However, it is perceived as less dynamic in
   some cases and has some provisioning overhead across all the nodes in
   N3 and N9 interface nodes.  Also it has another drawback with
   excessive state refresh overhead across adjacent nodes and this can
   be mitigated with [RFC8370].

   SR-TE [I-D.ietf-spring-segment-routing] does not explicitly signal
   bandwidth reservation or mechanism to guarantee latency on the nodes/
   links on SR path.  But, SR allows path steering for any flow at the
   ingress and particular path for a flow can be chosen.  Some of the
   issues around path overhead/tax, MTU issues are documented at
   Section 5.3 of [I-D.bogineni-dmm-optimized-mobile-user-plane].  SR-
   MPLS allows reduction of the control protocols to one IGP (with out
   needing for LDP and RSVP-TE).

   However, as specified above with PPR (Section 3), in the integrated
   transport network function (TNF) a particular RSVP-TE path for MPLS




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   or SR path for MPLS and IPv6 with SRH user plane, can be supplied to
   SMF for mapping a particular PDU session to the transport path.

5.  Acknowledgements

   Thanks to Young Lee for discussions on this document including ACTN
   applicability for the proposed TNF.  Thanks to Sri Gundavelli and
   3GPP delegates who provided detailed feedback on this document.

6.  IANA Considerations

   This document has no requests for any IANA code point allocations.

7.  Security Considerations

   This document does not introduce any new security issues.

8.  Contributing Authors

   The following people contributed substantially to the content of this
   document and should be considered co-authors.

   Xavier De Foy
      InterDigital Communications, LLC
      1000 Sherbrooke West
      Montreal
      Canada

      Email: Xavier.Defoy@InterDigital.com

9.  References

9.1.  Normative References

   [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>.

9.2.  Informative References

   [ATIS075]  Alliance for Telecommunications Industry Solutions (ATIS),
              "IOT Categorization: Exploring the Need for Standardizing
              Additional Network Slices ATIS-I-0000075", September 2019.







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   [I-D.bashandy-rtgwg-segment-routing-ti-lfa]
              Bashandy, A., Filsfils, C., Decraene, B., Litkowski, S.,
              Francois, P., daniel.voyer@bell.ca, d., Clad, F., and P.
              Camarillo, "Topology Independent Fast Reroute using
              Segment Routing", draft-bashandy-rtgwg-segment-routing-ti-
              lfa-05 (work in progress), October 2018.

   [I-D.bogineni-dmm-optimized-mobile-user-plane]
              Bogineni, K., Akhavain, A., Herbert, T., Farinacci, D.,
              Rodriguez-Natal, A., Carofiglio, G., Auge, J.,
              Muscariello, L., Camarillo, P., and S. Homma, "Optimized
              Mobile User Plane Solutions for 5G", draft-bogineni-dmm-
              optimized-mobile-user-plane-01 (work in progress), June
              2018.

   [I-D.chunduri-lsr-isis-preferred-path-routing]
              Chunduri, U., Li, R., White, R., Tantsura, J., Contreras,
              L., and Y. Qu, "Preferred Path Routing (PPR) in IS-IS",
              draft-chunduri-lsr-isis-preferred-path-routing-04 (work in
              progress), July 2019.

   [I-D.chunduri-lsr-ospf-preferred-path-routing]
              Chunduri, U., Qu, Y., White, R., Tantsura, J., and L.
              Contreras, "Preferred Path Routing (PPR) in OSPF", draft-
              chunduri-lsr-ospf-preferred-path-routing-03 (work in
              progress), May 2019.

   [I-D.farinacci-lisp-mobile-network]
              Farinacci, D., Pillay-Esnault, P., and U. Chunduri, "LISP
              for the Mobile Network", draft-farinacci-lisp-mobile-
              network-06 (work in progress), September 2019.

   [I-D.ietf-dmm-5g-uplane-analysis]
              Homma, S., Miyasaka, T., Matsushima, S., and D. Voyer,
              "User Plane Protocol and Architectural Analysis on 3GPP 5G
              System", draft-ietf-dmm-5g-uplane-analysis-02 (work in
              progress), July 2019.

   [I-D.ietf-dmm-srv6-mobile-uplane]
              Matsushima, S., Filsfils, C., Kohno, M., Camarillo, P.,
              Voyer, D., and C. Perkins, "Segment Routing IPv6 for
              Mobile User Plane", draft-ietf-dmm-srv6-mobile-uplane-06
              (work in progress), September 2019.

   [I-D.ietf-intarea-gue-extensions]
              Herbert, T., Yong, L., and F. Templin, "Extensions for
              Generic UDP Encapsulation", draft-ietf-intarea-gue-
              extensions-06 (work in progress), March 2019.



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   [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.

   [IR.34-GSMA]
              GSM Association (GSMA), "Guidelines for IPX Provider
              Networks (Previously Inter-Service Provider IP Backbone
              Guidelines, Version 14.0", August 2018.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/info/rfc6830>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <https://www.rfc-editor.org/info/rfc7490>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.






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   [RFC8370]  Beeram, V., Ed., Minei, I., Shakir, R., Pacella, D., and
              T. Saad, "Techniques to Improve the Scalability of RSVP-TE
              Deployments", RFC 8370, DOI 10.17487/RFC8370, May 2018,
              <https://www.rfc-editor.org/info/rfc8370>.

   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,
              <https://www.rfc-editor.org/info/rfc8453>.

   [TS.23.401-3GPP]
              3rd Generation Partnership Project (3GPP), "Procedures for
              4G/LTE System; 3GPP TS 23.401, v15.4.0", June 2018.

   [TS.23.501-3GPP]
              3rd Generation Partnership Project (3GPP), "System
              Architecture for 5G System; Stage 2, 3GPP TS 23.501
              v2.0.1", December 2017.

   [TS.23.502-3GPP]
              3rd Generation Partnership Project (3GPP), "Procedures for
              5G System; Stage 2, 3GPP TS 23.502, v2.0.0", December
              2017.

   [TS.23.503-3GPP]
              3rd Generation Partnership Project (3GPP), "Policy and
              Charging Control System for 5G Framework; Stage 2, 3GPP TS
              23.503 v1.0.0", December 2017.

   [TS.28.533-3GPP]
              3rd Generation Partnership Project (3GPP), "Management and
              Orchestration Architecture Framework (Release 15)", June
              2018.

   [TS.29.281-3GPP]
              3rd Generation Partnership Project (3GPP), "GPRS Tunneling
              Protocol User Plane (GTPv1-U), 3GPP TS 29.281 v15.1.0",
              December 2018.

Appendix A.  New Control Plane and User Planes

A.1.  Slice aware Mobility: Discrete Approach

   In this approach transport network functionality from the 5G-AN to
   UPF is discrete and 5GS is not aware of the underlying transport
   network and the resources available.  Deployment specific mapping
   function is used to map the GTP-U encapsulated traffic at the 5G-AN
   (e.g. gNB) in UL and UPF in DL direction to the appropriate transport



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   slice or transport Traffic Engineered (TE) paths.  These TE paths can
   be established using RSVP-TE [RFC3209] for MPLS underlay, SR
   [I-D.ietf-spring-segment-routing] for both MPLS and IPv6 underlay or
   PPR [I-D.chunduri-lsr-isis-preferred-path-routing] with MPLS, IPv6
   with SRH, native IPv6 and native IPv4 underlays.

   As per [TS.23.501-3GPP] and [TS.23.502-3GPP] the SMF controls the
   user plane traffic forwarding rules in the UPF.  The UPFs have a
   concept of a "Network Instance" which logically abstracts the
   underlying transport path.  When the SMF creates the packet detection
   rules (PDR) and forwarding action rules (FAR) for a PDU session at
   the UPF, the SMF identifies the network instance through which the
   packet matching the PDR has to be forwarded.  A network instance can
   be mapped to a TE path at the UPF.  In this approach, TNF as shown in
   Figure 1 need not be part of the 5G Service Based Interface (SBI).
   Only management plane functionality is needed to create, monitor,
   manage and delete (life cycle management) the transport TE paths/
   transport slices from the 5G-AN to the UPF (on N3/N9 interfaces).
   The management plane functionality also provides the mapping of such
   TE paths to a network instance identifier to the SMF.  The SMF uses
   this mapping to install appropriate FARs in the UPF.  This approach
   provide partial integration of the transport network into 5GS with
   some benefits.

   One of the limitations of this approach is the inability of the 5GS
   procedures to know, if underlying transport resources are available
   for the traffic type being carried in PDU session before making
   certain decisions in the 5G CP.  One example scenario/decision could
   be, a target 5G-AN selection during a N2 mobility event, without
   knowing if the target 5G-AN is having a underlay transport slice
   resource for the S-NSSAI and 5QI of the PDU session.  The Integrated
   approach specified below can mitigate this.

Appendix B.  PPR with various 5G Mobility procedures

   PPR fulfills the needs of 5GS to transport the user plane traffic
   from 5G-AN to UPF in all 3 SSC modes defined [TS.23.501-3GPP].  This
   is done in keeping the backhaul network at par with 5G slicing
   requirements that are applicable to Radio and virtualized core
   network to create a truly end-to-end slice path for 5G traffic.  When
   UE moves across the 5G-AN (e.g. from one gNB to another gNB), there
   is no transport network reconfiguration required with the approach
   above.

   SSC mode would be specified/defaulted by SMF.  No change in the mode
   once connection is initiated and this property is not altered here.





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B.1.  SSC Mode1


                           +--------------+
             +---+----+    |NSSMF +-----+ | +----------------+
             |  AMF   |    |      | TNF | | |      SMF       |
             +---+--+-+    |      +-+-+-+ | +-+--------------+
                N1  |      +--------+-+---+   |
                 |  |               | |       |
        +--------+  N2    +----Ns---+ +-Nn-+  N4
        |           |     |                |  |
        +   +---+---+  +--++             +-+--+---+     +----+
       UE1  |gNB|======|CSR|------N3---- | PE+UPF |-N6--| DN |
       ==   +---+      +---+             +--------+     +----+


       Figure 3: SSC Mode1 with integrated Transport Slice Function

   After UE1 moved to another gNB in the same UPF serving area



                           +--------------+
             +---+----+    |NSSMF +-----+ | +----------------+
             |  AMF   |    |      | TNF | | |      SMF       |
             +---+--+-+    |      +-+-+-+ | +-+--------------+
                    |      +--------+-+---+   |
                    |               | |       |
                    N2    +----Ns---+ +-Nn-+  N4
                    |     |                |  |
            +----+--+   +-+-+             ++--+----+     +----+
            |gNB1|======|CSR|------N3-----| PE+UPF |-N6--| DN |
            +----+      +---+             +---+----+     +----+
                                             |
                                             |
                                             |
                                             |
            +----+      +--++                |
       UE1  |gNB2|======|CSR|------N3--------+
       ==   +----+      +---+


       Figure 4: SSC Mode1 with integrated Transport Slice Function

   In this mode, IP address at the UE is preserved during mobility
   events.  This is similar to 4G/LTE mechanism and for respective
   slices, corresponding PPR-ID (TE Path) has to be assigned to the
   packet at UL and DL direction.  During Xn mobility as shown above,



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   source gNB has to additionally ensure transport path's resources from
   TNF are available at the target gNB apart from radio resources check
   (at decision and request phase of Xn/N2 mobility scenario).

B.2.  SSC Mode2

   In this case, if IP Address is changed during mobility (different UPF
   area), then corresponding PDU session is released.  No session
   continuity from the network is provided and this is designed as an
   application offload and application manages the session continuity,
   if needed.  For PDU Session, Service Request and Mobility cases
   mechanism to select the transport resource and the PPR-ID (TE Path)
   is similar to SSC Mode1.

B.3.  SSC Mode3

   In this mode, new IP address may be assigned because of UE moved to
   another UPF coverage area.  Network ensures UE suffers no loss of
   'connectivity'.  A connection through new PDU session anchor point is
   established before the connection is terminated for better service
   continuity.  There are two ways in which this happens.

   o  Change of SSC Mode 3 PDU Session Anchor with multiple PDU
      Sessions.

   o  Change of SSC Mode 3 PDU Session Anchor with IPv6 multi-homed PDU
      Session.

   In the first mode, from user plane perspective, the two PDU sessions
   are independent and the use of PPR-ID by gNB and UPFs is exactly
   similar to SSC Mode 1 described above.  The following paragraphs
   describe the IPv6 multi-homed PDU session case for SSC Mode 3.



















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                           +--------------+
             +---+----+    |NSSMF +-----+ | +----------------+
             |  AMF   |    |      | TNF | | |      SMF       |
             +---+--+-+    |      +-+-+-+ | +-+-----------+--+
                 |  |      +--------+-+---+   |           |
                N1  |               | |       |           |
       to-UE+----+  N2  +------Ns---+ +-Nn-+  N4        N4|
                +---+   |                  |  |           |
                |       |                  |  |           |
            +---+   +---++        +--+-------+--+     +---+---+
            |gNB|===|CSR |---N3---| PE+BP UPF   |-N9--| PE+UPF|-N6->
            +---+   +----+        +----------+--+     +-------+ to DN
                                           | +----+
                                           +-| DN |
                                         N6  +----+



                Figure 5: SSC Mode3 and Service Continuity

   In the uplink direction for the traffic offloading from the Branching
   Point UPF, packet has to reach to the right exit UPF.  In this case
   packet gets re-encapsulated by the BP UPF (with either GTP-U or the
   chosen encapsulation) after bit rate enforcement and LI, towards the
   anchor UPF.  At this point packet has to be on the appropriate VPN/PW
   to the anchor UPF.  This mapping is done based on the S-NSSAI the PDU
   session belongs to and/or with the UDP source port (corresponds to
   the MTNC-ID Section 2.5) of the GTP-U encapsulation header to the
   PPR-ID of the exit node by selecting the respective TE PPR-ID (PPR
   path) of the UPF.  If it's a non-MPLS underlay, destination IP
   address of the encapsulation header would be the mapped PPR-ID (TE
   path).

   In the downlink direction for the incoming packet, UPF has to
   encapsulate the packet (with either GTP-U or the chosen
   encapsulation) to reach the BP UPF.  Here mapping is done based on
   the S-NSSAI the PDU session belongs, to the PPR-ID (TE Path) of the
   BP UPF.  If it's a non-MPLS underlay, destination IP address of the
   encapsulation header would be the mapped PPR-ID (TE path).  In
   summary:

   o  Respective PPR-ID on N3 and N9 has to be selected with correct
      transport characteristics from TNF.

   o  For N2 based mobility SMF has to ensure transport resources are
      available for N3 Interface to new BP UPF and from there the
      original anchor point UPF.




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   o  For Service continuity with multi-homed PDU session same transport
      network characteristics of the original PDU session (both on N3
      and N9) need to be observed for the newly configured IPv6
      prefixes.

Authors' Addresses

   Uma Chunduri (editor)
   Futurewei
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Email: umac.ietf@gmail.com


   Richard Li
   Futurewei
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Email: richard.li@futurewei.com


   Sridhar Bhaskaran
   Altiostar

   Email: sridharb@altiostar.com


   John Kaippallimalil
   Futurewei

   Email: john.kaippallimalil@futurewei.com


   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com










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   Luis M. Contreras
   Telefonica
   Sur-3 building, 3rd floor
   Madrid  28050
   Spain

   Email: luismiguel.contrerasmurillo@telefonica.com


   Praveen Muley
   Nokia
   440 North Bernardo Ave
   Mountain View, CA  94043
   USA

   Email: praveen.muley@nokia.com



































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