DMM Working Group U. Chunduri, Ed. Internet-Draft R. Li Intended status: Standards Track Huawei USA Expires: August 18, 2019 S. Bhaskaran Huawei Technologies India Pvt Ltd J. Tantsura Apstra, Inc. L. Contreras Telefonica P. Muley Nokia February 14, 2019 Transport Network aware Mobility for 5G draft-clt-dmm-tn-aware-mobility-03 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 (IPv4, IPv6, MPLS) 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 working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Chunduri, et al. Expires August 18, 2019 [Page 1] Internet-Draft Transport Network aware Mobility for 5G February 2019 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 August 18, 2019. 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 and Problem Statement . . . . . . . . . . . . . 3 1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Solution Approach . . . . . . . . . . . . . . . . . . . . 5 2. Transport Network (TN) and Slice aware Mobility on N3/N9 . . 5 2.1. Discrete Approach . . . . . . . . . . . . . . . . . . . . 7 2.2. Integrated Approach . . . . . . . . . . . . . . . . . . . 8 2.3. Transport Network Function . . . . . . . . . . . . . . . 10 3. Using PPR as TN Underlay . . . . . . . . . . . . . . . . . . 10 3.1. PPR with Transport Awareness for 5GS on N3/N9 Interfaces 11 3.2. Path Steering Support to native IP user planes . . . . . 13 3.3. Service Level Guarantee in Underlay . . . . . . . . . . . 13 3.4. PPR with various 5G Mobility procedures . . . . . . . . . 13 3.4.1. SSC Mode1 . . . . . . . . . . . . . . . . . . . . . . 13 3.4.2. SSC Mode2 . . . . . . . . . . . . . . . . . . . . . . 14 3.4.3. SSC Mode3 . . . . . . . . . . . . . . . . . . . . . . 15 4. Other TE Technologies Applicability . . . . . . . . . . . . . 16 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 17 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 9.1. Normative References . . . . . . . . . . . . . . . . . . 17 9.2. Informative References . . . . . . . . . . . . . . . . . 17 Appendix A. Appendix: New Control Plane and User Planes . . . . 20 Chunduri, et al. Expires August 18, 2019 [Page 2] Internet-Draft Transport Network aware Mobility for 5G February 2019 A.1. LISP and PPR . . . . . . . . . . . . . . . . . . . . . . 20 A.2. ILA and PPR . . . . . . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction and Problem Statement 3GPP Release 15 for 5GC is defined in [TS.23.501-3GPP], [TS.23.502-3GPP] and [TS.23.503-3GPP]. User Plane Functions (UPF) 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). The interface between the BP/ULCL UPF and the PSA UPF is called N9. While in REL15, 3GPP has adopted GTP-U for the N9 interface, new user plane protocols along with GTP-U are being investigated for N9 interface in REL16, as part of [CT4SID]. Concerning to this document another relevant interface is N3, which is between the 5G-AN and the UPF. N3 interface is similar to the user plane interface S1U in LTE [TS.23.401-3GPP]. This document: o does not propose any change to existing N3 user plane encapsulations to realize the benefits with the approach specified here o and can work with any encapsulation (including GTP-U) for the N9 interface. [TS.23.501-3GPP] defines network slicing as one of the core capability of 5GC with slice awareness from Radio and 5G Core (5GC) network. It also defines various Session and Service Continuity (SSC) modes and mobility scenarios for 5G. The 5G System (5GS) as defined, allows transport network between N3 and N9 interfaces work independently with various IETF Traffic Engineering (TE) technologies. However, lack of underlying Transport Network (TN) awareness may lead to selection of sub-optimal UPF(s) and/or 5G-AN during 5GS procedures. This could lead to inability to meet SLAs for real-time, mission-critical or latency sensitive services. These 5GS procedures including but not limited to Service Request, PDU Session Establishment, or User Equipment (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] . Chunduri, et al. Expires August 18, 2019 [Page 3] Internet-Draft Transport Network aware Mobility for 5G February 2019 1.1. 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) 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 SMF - Session Management Function (5G) SSC - Session and Service Continuity (5G) Chunduri, et al. Expires August 18, 2019 [Page 4] Internet-Draft Transport Network aware Mobility for 5G February 2019 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) 1.2. Solution Approach This document specifies a mechanism 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 two methods, with which Transport Network (TN) aware mobility can be built irrespective of underlying TN technology used. Using Preferred Path Routing (PPR) as TN Underlay is detailed in Section 3. Section 3.4 further describes the applicability and procedures of the same with 5G SSC modes on N3 and N9 interfaces. 2. Transport Network (TN) and Slice aware Mobility on N3/N9 Chunduri, et al. Expires August 18, 2019 [Page 5] Internet-Draft Transport Network aware Mobility for 5G February 2019 Service Based Interfaces (SBI) ----+-----+-----+----+----+-----+----+--------+-----+----+------ | | | | | | | | | | +---+---+ | +--+--+ | +--+---+ | +--+--+ +--+--+ | +-+--+ | NSSF | | | NRF | | | AUSF | | | UDM | | NEF | | | AF | +-------+ | +-----+ | +------+ | +-----+ +-----+ | +----+ +---+----+ +--+--+ +---=++ +--------------+-+ | AMF | | PCF | | TNF | | SMF | +---+--+-+ +-----+ +-+-+-+ +-+-----------+--+ N1 | | | | To | to-UE+----+ N2 +----Ns---+ +-Nn-+ N4 +--Nn-+ N4 | | | | | | +---+---+ +--++ +-+--+---+ +-+-----+ +----+ |gNB+======+CSR+------N3-----+ UPF +-N9--+ UPF +--N6--+ DN | +---+ +---+ +-+------+ +-------+ +----+ | +----+ +-| DN | N6 +----+ Figure 1: 5G Service Based Architecture The above diagrams depicts one of the scenarios of the 5G network specified in [TS.23.501-3GPP] and with a new and virtualized control component Transport Network Function (TNF). A Cell Site Router (CSR) is shown connecting to gNB. gNB is an entity in 5G-AN. Though it is shown as a separate block from gNB, in some cases both of these can be co-located. This document concerns with backhaul TN, from CSR to UPF on N3 interface or from Staging UPF to Anchor UPF on N9 interface. Currently specified Control Plane (CP) functions - the Access and Mobility Management Function (AMF), the Session Management Function (SMF) and the User plane (UP) components gNodeB (gNB), User Plane Function (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. N9 interface is a new interface to connect UPFs and the right user plane Chunduri, et al. Expires August 18, 2019 [Page 6] Internet-Draft Transport Network aware Mobility for 5G February 2019 protocols for N9, including GTP-U, are being studied through 3GPP CT4 WG approved study item [CT4SID] for REL-16. 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. 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. 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 gNB selection during a N2 mobility event, without knowing if the target gNB 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. Chunduri, et al. Expires August 18, 2019 [Page 7] Internet-Draft Transport Network aware Mobility for 5G February 2019 2.2. Integrated Approach Network Slice Selection Function (NSSF) as defined in [TS.23.501-3GPP] concerns with multiple aspects including selecting a network slice instance when requested by AMF based on the requested SNSSAI, current location of UE, roaming indication etc. It also notifies NF service consumers (e.g AMF) whenever the status about the slice availability changes. However, the scope is only in 5GC (both control and user plane) and NG Radio Access network including the N3IWF for the non-3GPP access. The network slice instance(s) selected by the NSSF are applicable at a per PDU session granularity. An SMF and UPF are allocated from the selected slice instance during the PDU session establishment procedure. [TS.23.501-3GPP] and [TS.23.502-3GPP] do 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. If transport network is not factored in an integrated fashion w.r.t available resources (with network characteristics from desired bandwidth, latency, burst size handling and optionally jitter) some of the gains made with optimizations through 5GNR and 5GC can be degraded. To assuage the above situation, TNF is described (Figure 1) as part of control plane. This has the view of the underlying transport network with all links and nodes as well as various possible underlay paths with different characteristics. TNF can be seen as supporting PCE functionality [RFC5440] and optionally BGP-LS [RFC7752] to get the TE and topology information of the underlying IGP network. A south bound interface Ns is shown which interacts with the 5G Access Network (e.g. gNB/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 gNB to UPF. Ns and Nn interfaces can be part of the integrated 3GPP architecture, but the specification/ownership of these interfaces SHOULD be left out of scope of 3GPP. 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]. A north bound interface 'Nn' is shown from one or more of the transport network nodes (or 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]). Chunduri, et al. Expires August 18, 2019 [Page 8] Internet-Draft Transport Network aware Mobility for 5G February 2019 With the TNF 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 (SNSSAI) of PDU session's 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 (gNB/CSR and 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 GTP-U TEID and/or the QFI marked in the GTPU packet 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. 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 Section 3.4, if segmented path (gNB to staging/ULCL/BP-UPF to anchor-point-UPF) is needed, then corresponding path characteristics MUST be used. This includes a path from gNB/CSR to UL-CL/BP UPF [TS.23.501-3GPP] and UL-CL/BP UPF to eventual UPF access to DN. Chunduri, et al. Expires August 18, 2019 [Page 9] Internet-Draft Transport Network aware Mobility for 5G February 2019 o Continuous monitoring of transport path characteristics and reassignment at the endpoints MUST 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 allcoation of required characteristics from the TN during a 5GS signalling 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. 2.3. Transport Network Function Proposed TNF as part of the 5GC shown in Figure 1 can be realized using Abstraction and Control of TE Networks (ACTN). ACTN architecture, underlying topology abstraction methods and manageability considerations of the same are detailed in [RFC8453]. 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 are addressed by a new backhaul routing mechanism called Preferred Path Routing (PPR), of which this section provides an overview. 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 Chunduri, et al. Expires August 18, 2019 [Page 10] Internet-Draft Transport Network aware Mobility for 5G February 2019 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 (encap or no-encap). In this scenario, PPR will only help providing TE benefits needed for 5G slices from transport domain perspective. It does so without adding any additional overhead to the user plane, unlike SR-MPLS or SRv6. 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 QFI (e.g. 5QI) marking on 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: Chunduri, et al. Expires August 18, 2019 [Page 11] Internet-Draft Transport Network aware Mobility for 5G February 2019 +----------------+------------+------------------+-----------------+ | QFI (Ranges) | 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: QFI Mapping with 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. Chunduri, et al. Expires August 18, 2019 [Page 12] Internet-Draft Transport Network aware Mobility for 5G February 2019 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 [I-D.chunduri-lsr-isis-preferred-path-routing] or 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., 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. It is important to note, these benefits 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. 3.4. 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. 3.4.1. SSC Mode1 Chunduri, et al. Expires August 18, 2019 [Page 13] Internet-Draft Transport Network aware Mobility for 5G February 2019 +---+----+ +-----+ +----------------+ | AMF | | TNF | | SMF | +---+--+-+ +-+-+-+ +-+--------------+ N1 | | | | +--------+ N2 +----Ns---+ +-Nn-+ N4 | | | | | + +---+---+ +--++ +-+--+---+ +----+ UE1 |gNB+======+CSR+------N3-----+ UPF +-N6--+ DN | == +---+ +---+ +--------+ +----+ Figure 3: SSC Mode1 with integrated Transport Slice Function After UE1 moved to another gNB in the same UPF serving area +---+----+ +-----+ +----------------+ | AMF | | TNF | | SMF | +---_--+-+ +-+-+-+ +-+--------------+ | | | | N2 +----Ns---+ +-Nn-+ N4 | | | | +----+--+ +-+-+ ++--+----+ +----+ |gNB1+======+CSR+------N3-----+ 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). 3.4.2. 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 Chunduri, et al. Expires August 18, 2019 [Page 14] Internet-Draft Transport Network aware Mobility for 5G February 2019 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. 3.4.3. 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. +---+----+ +-----+ +----------------+ | AMF | | TNF | | SMF | +---+--+-+ +-+-+-+ +-+-----------+--+ N1 | | | | | to-UE+----+ N2 +-------Ns---+ +-Nn-+ N4 N4| | | | | | +-------+--+ +--+-------+--+ +-----+-+ |gNB/CSR +---N3---+ BP UPF +-N9--+ 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 Chunduri, et al. Expires August 18, 2019 [Page 15] Internet-Draft Transport Network aware Mobility for 5G February 2019 to the anchor UPF. This mapping is done based on the S-NSSAI the PDU session belongs to and/or the QFI marking in the GTPU encapsulation header (e.g. 5QI value) 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. 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 Chunduri, et al. Expires August 18, 2019 [Page 16] Internet-Draft Transport Network aware Mobility for 5G February 2019 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 and John Kaippallimalil 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, . 9.2. Informative References [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. Chunduri, et al. Expires August 18, 2019 [Page 17] Internet-Draft Transport Network aware Mobility for 5G February 2019 [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-02 (work in progress), February 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-02 (work in progress), February 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-04 (work in progress), September 2018. [I-D.ietf-dmm-srv6-mobile-uplane] Matsushima, S., Filsfils, C., Kohno, M., Camarillo, P., daniel.voyer@bell.ca, d., and C. Perkins, "Segment Routing IPv6 for Mobile User Plane", draft-ietf-dmm-srv6-mobile- uplane-03 (work in progress), October 2018. [I-D.ietf-spring-segment-routing] Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", draft-ietf-spring-segment-routing-15 (work in progress), January 2018. [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, . [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, . Chunduri, et al. Expires August 18, 2019 [Page 18] Internet-Draft Transport Network aware Mobility for 5G February 2019 [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, . [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The Locator/ID Separation Protocol (LISP)", RFC 6830, DOI 10.17487/RFC6830, January 2013, . [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, . [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, . [RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017, . [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, . [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, . [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. Chunduri, et al. Expires August 18, 2019 [Page 19] Internet-Draft Transport Network aware Mobility for 5G February 2019 [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.29.281-3GPP] 3rd Generation Partnership Project (3GPP), "GPRS Tunneling Protocol User Plane (GTPv1-U), 3GPP TS 29.281 v15.1.0", December 2017. Appendix A. Appendix: New Control Plane and User Planes A.1. LISP and PPR PPR can also be used with LISP control plane for 3GPP as described in [I-D.farinacci-lisp-mobile-network]. This can be achieved by mapping the UE IP address (EID) to PPR-ID, which acts as Routing Locator (RLOC). Any encapsulation supported by LISP can work well with PPR. If the RLOC refers to an IPv4 or IPv6 destination address in the LISP encapsulated header, packets are transported on the preferred path in the network as opposed to traditional shortest path routing with no additional user plane overhead related to TE path in the network layer. Some of the distinct advantages of the LISP approach is, its scalability, support for service continuity in SSC Mode3 as well as native support for session continuity (session survivable mobility). Various other advantages are documented at [I-D.farinacci-lisp-mobile-network]. A.2. ILA and PPR If an ILA-prefix is allowed to refer to a PPR-ID, ILA can be leveraged with all the benefits (including mobility) that it provides. This works fine in the DL direction as packet is destined to UE IP address at UPF. However, in the UL direction, packet is destined to an external internet address (SIR Prefix to ILA Prefix transformation happens on the Source address of the original UE packet). One way to route the packet with out bringing the complete DFZ BGP routing table is by doing a default route to the UPF (ILA-R). In this case, how TE can be achieved is TBD (to be expanded further with details). Authors' Addresses Chunduri, et al. Expires August 18, 2019 [Page 20] Internet-Draft Transport Network aware Mobility for 5G February 2019 Uma Chunduri (editor) Huawei USA 2330 Central Expressway Santa Clara, CA 95050 USA Email: uma.chunduri@huawei.com Richard Li Huawei USA 2330 Central Expressway Santa Clara, CA 95050 USA Email: renwei.li@huawei.com Sridhar Bhaskaran Huawei Technologies India Pvt Ltd Survey No.37, Whitefield Road, Kundalahalli Bengaluru, Karnataka India Email: sridhar.bhaskaran@huawei.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 Chunduri, et al. Expires August 18, 2019 [Page 21] Internet-Draft Transport Network aware Mobility for 5G February 2019 Praveen Muley Nokia 440 North Bernardo Ave Mountain View, CA 94043 USA Email: praveen.muley@nokia.com Chunduri, et al. Expires August 18, 2019 [Page 22]