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DMM Working Group                                       U. Chunduri, Ed.
Internet-Draft                                                     R. Li
Intended status: Informational                                 Futurewei
Expires: September 10, 2020                                 S. Bhaskaran
                                                               Altiostar
                                                  J. Kaippallimalil, Ed.
                                                               Futurewei
                                                             J. Tantsura
                                                            Apstra, Inc.
                                                            L. Contreras
                                                              Telefonica
                                                                P. Muley
                                                                   Nokia
                                                           March 9, 2020


                Transport Network aware Mobility for 5G
                   draft-clt-dmm-tn-aware-mobility-06

Abstract

   This document specifies a framework and mapping from slices in 5G
   mobile systems to transport slices in IP and Layer 2 transport
   networks.  Slices in 5G systems are characterized by latency bounds,
   reservation guarantees, jitter, data rates, availability, mobility
   speed, usage density, criticality and priority should be mapped to
   transport slice characteristics that include bandwidth, latency and
   criteria such as isolation, directionality and disjoint routes.
   Mobile slice criteria need to be mapped to the appropriate transport
   slice and capabilities offered in backhaul, midhaul and fronthaul
   connectivity segments between radio side network functions and user
   plane function (gateway).

   This document describes how mobile network functions map its slice
   criteria to identifiers in IP packets that transport segments use to
   grant transport layer services.  This is based on mapping between
   mobile and IP transport underlays (IPv6, MPLS, IPv4, Segment
   Routing).  Applicability of this framework and a new transport
   network underlay routing mechanism, Preferred Path Routing (PPR),
   which brings slice properties and works with any underlying transport
   (L2, IPv4, SR and MPLS) is also discussed.

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].




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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 September 10, 2020.

Copyright Notice

   Copyright (c) 2020 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
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   publication of this document.  Please review these documents
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   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 and Slice aware Mobility in 5G Networks . . . . . .   6
     2.1.  Backhaul and Mid-Haul Transport Network . . . . . . . . .   7
     2.2.  Front Haul Transport Network  . . . . . . . . . . . . . .   9
     2.3.  Mobile Transport Network Context (MTNC) and Scalability .   9
     2.4.  Transport Network Function (TNF)  . . . . . . . . . . . .  10
     2.5.  Transport Provisioning  . . . . . . . . . . . . . . . . .  11
     2.6.  MTNC-ID in the Data Packet  . . . . . . . . . . . . . . .  12
     2.7.  Functionality for E2E Management  . . . . . . . . . . . .  13
   3.  Transport Network Underlays . . . . . . . . . . . . . . . . .  15
     3.1.  Using PPR as TN Underlay  . . . . . . . . . . . . . . . .  15



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       3.1.1.  PPR on F1-U/N3/N9 Interfaces  . . . . . . . . . . . .  16
       3.1.2.  Path Steering Support to native IP user planes  . . .  17
       3.1.3.  Service Level Guarantee in Underlay . . . . . . . . .  17
     3.2.  Other TE Technologies Applicability . . . . . . . . . . .  18
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .  18
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  New Control Plane and User Planes  . . . . . . . . .  22
     A.1.  Slicing Framework and RAN Aspects . . . . . . . . . . . .  22
     A.2.  Slice aware Mobility: Discrete Approach . . . . . . . . .  23
   Appendix B.  PPR with various 5G Mobility procedures  . . . . . .  24
     B.1.  SSC Mode1 . . . . . . . . . . . . . . . . . . . . . . . .  24
     B.2.  SSC Mode2 . . . . . . . . . . . . . . . . . . . . . . . .  25
     B.3.  SSC Mode3 . . . . . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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.

   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). 3GPP slicing and RAN aspects are further described in
   (Appendix A.1).

   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 and
   also for the F1-U interface between the DU and the CU in the RAN.
   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,



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   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
   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 F1-U, N3, 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 Transport Network (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 network including the F1-U) 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 (S-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 establishment and mobility procedures aware of
   underlying transport network along with slicing requirements.




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   (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 3.1).  How other IETF TE
   technologies applicable for this draft is specified in (Section 3.2).
   At the end, (Appendix B) further describes the applicability and
   procedures of PPR with 5G SSC modes on F1-U, 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

   CP       -  Control Plane (5G)

   CU       -  Centralized Unit (5G, gNB)

   DN       -  Data Network (5G)

   DU       -  Distributed Unit (5G, gNB)

   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

   NSSMF    -  Network Slice Selection Management Function




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   QFI      -  QoS Flow ID (5G)

   PPR      -  Preferred Path Routing

   PDU      -  Protocol Data Unit (5G)

   PW       -  Pseudo Wire

   RAN      -  Radio Access Network

   RQI      -  Reflective QoS Indicator (5G)

   SBI      -  Service Based Interface (5G)

   SID      -  Segment Identifier

   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)

   UP       -  User Plane(5G)

   UPF      -  User Plane Function (5G)

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

2.  Transport and Slice aware Mobility in 5G Networks

   3GPP architecture [TS.23.501-3GPP], [TS.23.502-3GPP] describe slicing
   in 5GS.  However, the application of 5GS slices in transport network
   for backhaul, mid-haul and front haul are not explicitly covered.  To
   support specific characteristics in backhaul (N3, N9), mid-haul (F1)
   and front haul, it is necessary to map and provision corresponding
   resources in the transport domain.  This section describes how to
   provision the mapping information in transport network and apply it
   so that user plane packets can be provided the transport resources
   (QoS, isolation, protection, etc.) expected by the 5GS slices.

   TN Aware Mobility with optimized transport network functionality is
   explained below.  How an underlay agnostic routing technology fits in



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   this framework in detail along with other various TE technologies
   briefly are in (Section 3.1) and (Section 3.2) respectively.



                5G Control and Management Planes
    +------------------------------------------------------------------------+
    | +--------------------------------------------------------------------+ |
    | |                    (TNF)   5G Management Plane   (TNF)             | |
    | +----+-----------------+-------------+---------------+-----------+---+ |
    |      |                 |             |               |           |     |
    | +----+-----+           |   F1-C +----+-----+         |   N2 +----+---+ |
    | |          |----------(---------|gNB-CU(CP)|--------(-------| 5GC CP | |
    | |          |           |        +----+-----+         |      +----+---+ |
    +-|          |-----------|-------------|---------------|-----------|-----+
      |          |           |             |               |           |
      |          |           |             |               |           |
      |          |           | ACTN        |               | ACTN      |
      |          |       +---+---+         |           +---+---+       |
      |          |       |       |         |           |       |       |
      | gNB-DU   |       | SDN-C |         E1          | SDN-C |       |
      |          |       |       |         |           |       |       |
      |          |       +---+---+         |           +---+---+       |
      |          |           |             |               |           |
      |          |           |             |               |           |
      |          |        __ +__           |            ___+__         |
      |          |     __/      \__     +--+---+     __/      \__    +-+-+
      |          |    /     IP     \    | gNB  |    /     IP     \   |   |
 UE---|          |-(PE) Mid-haul (PE)---+CU(UP)+--(PE) Backhaul(PE)--+UPF|--DN
      +----------|    \__        __/    +------+    \__        __/   +---+
                         \______/                      \______/

                   |------ F1-U -------|        |------ N3 OR N9 ------|


         Figure 1: Backhaul and Mid-haul Transport Network for 5G

2.1.  Backhaul and Mid-Haul Transport Network

   Figure 1 depicts IP Xhaul network with SDN-C and PE (Provider Edge)
   routers provide IP transport service to 5GS user plane entities 5G-AN
   (e.g. gNB) and UPF.  5GS architecture with high level management,
   control and user plane entities and its interaction with the IP
   transport plane is shown here.  The slice capability required in IP
   transport networks is estimated and provisioned by the functionality
   as specified in Section 2.4 (TNF) with support from various other
   control plane functions such as the Network Data Analytics Function
   (NWDAF), Network Function Repository Function (NRF) and Policy



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   Control Function (PCF).  The TNF is only a logical function that
   maybe realized in a 3GPP management function such as Network Slice
   Selection Management Function (NSSMF) defined in [TS.28.533-3GPP].
   The TNF requests the SDN-C to provision the IP XHaul network using
   ACTN [RFC8453].

   The 5G management plane in Figure 1 interacts with the 5G control
   plane - the 5GC (5G Core), gNB-CU (5G NodeB Centralized Unit) and
   gNB-DU (5G Node B Distributed Unit).  Non-access stratum (NAS)
   signaling from the UE for session management, mobility is handled by
   the 5GC.  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.

   The AMF, SMF, NSSF, PCF, NRF, NWDAF and other control functions in
   5GC are described in [TS.23.501-3GPP] Some of the slice capabilities
   along the user plane path between the (R)AN and UPFs (F1-U, N3, N9
   segments) such as a low latency path, jitter, protection and priority
   needs these to be provided by the IP transport network.

   The 5G user plane from UE to DN (Data Network) includes a mid-haul
   segment (F1-U between gNB DU(UP), gNB CU(UP)) and backhaul (N3
   between gNB - UPF; N9 between UPFs).  If the RAN uses lower layer
   split architecture as specified by O-RAN alliance, then the user
   plane path from UE to DN also includes the fronthaul interface.  The
   fronthaul interface carries the radio frames in the form of In-phase
   (I) and Quadrature (Q) samples using eCPRI encapsulation over
   Ethernet or UDP over IP.

   The N3, N9 and F1 user planes use GTP-U [TS.29.281-3GPP] to transport
   UE PDUs (IPv4, IPv6, IPv4v6, Ethernet or Unstructured).  For the
   front haul described further in Section 2.2, an Ethernet transport
   with VLANs can be expected to be the case in many deployments.

   Figure 1 also depicts the PE router, where transport paths are
   initiated/terminated can be deployed separately 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 (gNB) 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).




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2.2.  Front Haul Transport Network

   The O-RAN Alliance has specified the fronthaul interface between the
   O-RU and the O-DU in [ORAN-WG4.CUS-O-RAN].  The radio layer
   information, in the form of In-phase (I) and Quadrature (Q) samples
   are transported using Enhanced Common Public Radio Interface (eCPRI)
   framing over Ethernet or UDP.  On the Ethernet based fronthaul
   interface, the slice information is carried in the Ethernet header
   through the VLAN tags.  The Ethernet switches in the fronthaul
   transport network can inspect the slice information (VLAN tag) in the
   Ethernet header and provide the provisioned capabilities.  The
   mapping of I and Q samples of different radio resources (radio
   resource blocks or carriers etc.,.) to different slices and to their
   respective VLAN tags on the fronthaul interface is controlled by the
   O-RAN fronthaul C-Plane and M-Plane interfaces.  On UDP based
   fronthaul interface, the slice information is carried in the IP or
   UDP header.  The PE routers of the fronthaul transport network can
   inspect the slice information in the IP or UDP header and provide the
   provisioned capabilities.  The fronthaul transport network is latency
   and jitter sensitive.  The provisioned slice capabilities in the
   fronthaul transport network MUST take care of the latency and jitter
   budgets of the specific slice for the fronthaul interface.  The
   provisioning of the fronthaul transport network is handled by the
   SDN-C pertaining to the fronthaul transport.

2.3.  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




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   fully isolated, will add to the MTNC provisioning.  An MTNC-ID space
   of 16 bits (65K+ identifiers) can be expected to be sufficient.

2.4.  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.  Another alternative is for the TNF to
   provide a mapping of the 3GPP Network Instance Identifier, described
   in (Section 2.7) and the MTNC-ID to the user plane entities via
   configuration.

   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.





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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 F1-U, 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 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
   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 up to 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 F1-U/N3/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.



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   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
   F1-U, 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 F1-U, 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 F1-U or N3 or N9 path.  GTP (F1-U, 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 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 independent (IPv4/IPv6/MPLS/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.  The source UDP port is encoded by the node
   that creates the GTP-U encapsulation and therefore, this mechanism
   has no impact to UDP checksum calculations.

   3GPP network operators may use IPSec gateways (SEG) to secure packets
   between two sites - for example over an F1-U, N3 or N9 segment.  The



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   MTNC identifier in the GTP-U packet should be in the outer IP source
   port even after IPSec encryption for PE transport routers to inspect
   and provide the level of service provisioned.  Tunnel mode - which is
   the case for SEG/IPSec gateways - adds an outer IP header in both AH
   (Authenticated Header) and ESP (Encapsulated Security Payload) modes.
   The GTP-U / UDP source port with encoded MTNC identifier should be
   copied to the IPSec tunnel ESP header.  One option is to use 16 bits
   from the SPI field of the ESP header to encode the MTNC identifier
   and use the remaining 16 bits in SPI field to identify an SA.  Load
   balancing entropy for ECMP will not be affected as the MTNC encoding
   mechanism already accounts for this.

   If the RAN uses O-RAN lower layer split architecture, then a
   fronthaul network is involved.  On an Ethernet based fronthaul
   transport network, VLAN tag may be an option to carry the MTNC-ID.
   The VLAN ID provides a 12 bit space and is sufficient to support up
   to 4096 slices on the fronthaul transport network.  The mapping of
   fronthaul traffic to corresponding network slice is based on the
   radio resource for which the fronthaul carries the I and Q samples.
   The mapping of fronthaul traffic to the VLAN tag corresponding to the
   network slice is specified in Section 2.2.  On UDP based fronthaul
   transport network, the UDP source port can be used to carry the MTNC-
   ID.

2.7.  Functionality for E2E Management

   With the TNF functionality 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.




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   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
      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 3.1) and (Section 3.2)).  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



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   signaling to integrate the above for various 5GS procedures as
   defined in [TS.23.502-3GPP] is beyond the scope of this document.

3.  Transport Network Underlays

   Apart from the various flavors of IETF VPN technologies to share the
   transport network resources and capacity, TE capabilities in the
   underlay network is an essential component to realize the 5G TN
   requirements.  This section focuses on various transport underlay
   technologies (not exhaustive) and their applicability to realize
   Midhaul/Backhaul transport networks.  Focus is on the user/data plane
   i.e., F1-U/N3/N9 interfaces as laid out in the framework Figure 1.

3.1.  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
   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]








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3.1.1.  PPR on F1-U/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 F1-U/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 user/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:


      +----------------+------------+------------------+-----------------+
      |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




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   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 3.1.2)) with optional TE features (
      (Section 3.1.3)) . As this is an underlay mechanism it can work
      with any overlay encapsulation approach including GTP-U as defined
      currently for N3 interface.

3.1.2.  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.

3.1.3.  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.








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3.2.  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 3.1)), in the
   integrated transport network function (TNF) a particular RSVP-TE path
   for MPLS 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.

4.  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.

5.  IANA Considerations

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

6.  Security Considerations

   This document does not introduce any new security issues.

7.  Contributing Authors

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









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

      Email: Xavier.Defoy@InterDigital.com

8.  References

8.1.  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>.

8.2.  Informative References

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

   [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-05 (work in
              progress), March 2020.








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   [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-04 (work in
              progress), March 2020.

   [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-03 (work in
              progress), November 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-07
              (work in progress), November 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.

   [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.

   [ORAN-WG4.CUS-O-RAN]
              O-RAN Alliance (O-RAN), "O-RAN Fronthaul Working Group;
              Control, User and Synchronization Plane Specification;
              v2.0.0", August 2019.

   [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>.



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   [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>.

   [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.



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   [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.

   [TS.38.300-3GPP]
              3rd Generation Partnership Project (3GPP), "NR; NR and NG-
              RAN Overall Description; Stage 2; v15.7.0", September
              2019.

   [TS.38.401-3GPP]
              3rd Generation Partnership Project (3GPP), "NG-RAN;
              Architecture description; v15.7.0", September 2019.

Appendix A.  New Control Plane and User Planes

A.1.  Slicing Framework and RAN Aspects

   The 3GPP architecture 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 information
   regarding the set of slices it wishes to connect, in the Network
   Slice Selection Assistance Information (NSSAI) field during network
   registration procedure (Attach) and the specific slice the UE wants
   to establish an IP session, in the Specific NSSAI (S-NSSAI) field
   during the session establishment procedure (PDU Session
   Establishment).  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.





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   The architecture for the Radio Access Network (RAN) is defined in
   [TS.38.300-3GPP] and [TS.38.401-3GPP].  The 5G RAN architecture
   allows disaggregation of the RAN into a Distributed Unit (DU) and a
   Centralized Unit (CU).  The CU is further split into control plane
   (CU-CP) and user plane (CU-UP).  The interface between CU-UP and the
   DU for the user plane traffic is called the F1-U and between the CU-
   CP and DU for the control plane traffic is called the F1-C.  The F1-C
   and the F1-U together are called the mid-haul interfaces.  The DU
   does not have a CP/UP split.  Apart from 3GPP, O-RAN Alliance has
   specified further disaggregation of the RAN at the lower layer
   (physical layer).  The DU is disaggregated into a ORAN DU (O-DU)
   which runs the upper part of the physical layer, MAC and RLC and the
   ORAN Radio Unit (O-RU) which runs the lower part of the physical
   layer.  The interface between the O-DU and the O-RU is called the
   Fronthaul interface and is specified in [ORAN-WG4.CUS-O-RAN].

A.2.  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
   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.





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   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.

B.1.  SSC Mode1


                           +--------------+
             +---+----+    |NSSMF +-----+ | +----------------+
             |  AMF   |    |      | TNF | | |      SMF       |
             +---+--+-+    |      +-+-+-+ | +-----+----------+
                N1  |      +--------+-+---+       |
                 |  |               | |           |
                 |  |           +---+-+--+        |
                 |  |           | SDN-C  |        |
                 |  |           +---+-+--+        |
                 |  |               | |           |
        +--------+  N2    +---------+ + ---+      |
        |           |     |                |      |
        +    +---+--+   +--++             +---+ +-+--+     +----+
        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





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                           +--------------+
             +---+----+    |NSSMF +-----+ | +----------------+
             |  AMF   |    |      | TNF | | |      SMF       |
             +------+-+    |      +-+-+-+ | +-----+----------+
                    |      +--------+-+---+       |
                    |               | |           |
                    |           +---+-+--+        |
                    |           | SDN-C  |        |
                    |           +---+-+--+        |
                    |               | |           |
                    N2    +---------+ + ---+      |
                    |     |                |      |
             +---+--+   +--++             +---+ +-+--+     +----+
             |gNB|======|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,
   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.








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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.



                          +--------------+
            +---+----+    |NSSMF +-----+ | +----------------+
            |  AMF   |    |      | TNF | | |      SMF       |
            +---+--+-+    |      +-+-+-+ | +-+-----------+--+
                |  |      +--------+-+---+   |           |
               N1  |               | |       |           |
                |  |           +---+-+--+    |           |
                |  |           | SDN-C  |    |           |
                |  |           +---+-+--+    |           |
                |  |               | |       |           |
      to-UE+----+  N2  +-----------+ |       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



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   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.

   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



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   Sridhar Bhaskaran
   Altiostar

   Email: sridharb@altiostar.com


   John Kaippallimalil (editor)
   Futurewei

   Email: john.kaippallimalil@futurewei.com


   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com


   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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