Mobility aware Transport Network Slicing for 5G

3.1. Overview of 5G End-to-End Network Slicing

3GPP architecture in [TS.23.501-3GPP], [TS.23.502-3GPP] specify slicing in 5GS and an overview is provided here. 5GS comprises of control plane network functions and user plane network functions. Communication services offered to 3GPP clients (UE) are associated to one or more slices represented by NSSAI (Network Slice Selection Assistance Information) both on the control plane and the user plane. The NSSAI is realized through the 5G management plane using network slice subnet (NSS), for example, a network slice subnet that contains network function instances of the core network control plane functions (e.g., SMF, NRF), user plane network functions (e.g., UPF), radio network slice common functions (e.g., gNB-DU, gNB-CU-CP) and and radio network (e.g., gNB-CU-UP). Within the 3GPP domain, the control plane slicing is end-to-end while the user plane slicing ends at the UPF. User plane slicing outside of the UPF towards IP services is outside the scope of 3GPP and is addressed in [I-D.mcd-rtgwg-extension-tn-aware-mobility]. 3GPP documents mention transport network in the context of network slice subnets, but do not provide any details. The application of 5GS slices in transport network for backhaul, mid-haul and front haul are not explicitly covered in 3GPP and is the topic here. To support specific characteristics in backhaul (N3, N9), mid-haul (F1) and front haul, it is necessary to provision corresponding resources in the transport domain and carry a slice identifier that is understood by both the customer (3GPP network) and the provider (transport network). This section provides an overview and subsequent sections describe how to provision the mapping information in the 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.¶

                5G Control and Management Planes
  +--------------------------------------------------------------------+
  | +----------------------------------------------------------------+ |
  | |                   5G Management Plane  (NSSMF)                 | |
  | +---+--------------+-------------+---------------+-----------+---+ |
  |     |              |             |               |           |     |
  | +---+--+           |   F1-C +----+-----+         |   N2 +----+---+ |
  | |      |----------(---------|gNB-CU(CP)|--------(-------| 5GC CP | |
  | |      |           |        +----+-----+         |      +----+---+ |
  +-|      |-----------|-------------|---------------|-----------|-----+
    |      |           |             |               |           |
    |      |           | e.g.,        |              | e.g.,     |
    |      |           | ACTN        |               | ACTN      |
    |      |       +---+---+         |           +---+---+       |
    |      |       |       |         |           |       |       |
    |gNB-DU|       |  NSC  |         E1          |  NSC  |       |
    |      |       |       |         |           |       |       |
    |      |       +---+---+         |           +---+---+       |
    |      |           |             |               |           |
    |      |           |             |               |           |
    |      |        __ +__           |            ___+__         |
    |      |     __/      \__     +--+---+     __/      \__    +-+-+
    |      |    /   IP/L2    \    | gNB  |    /     IP     \   |   |
UE--+      +--(PE) Mid-haul (PE)--+CU(UP)+--(PE) Backhaul(PE)--+UPF+--DN
    +------+    \__        __/    +------+    \__        __/   +---+
                   \______/                      \______/

            |------ F1-U -------|           |----- N3 or N9 ------|

Figure 1 depicts a 5G System (5GS) expanded to show the IP transport and PE (Provider Edge) routers providing IP transport service to 5GS user plane entities 5G-AN (e.g., gNB) and UPF. The Provider Edge (PE) represents the Service Demarcation Point (SDP) to the transport network that provides the slice capabilities. The IETF Network Slice Controller (NSC) interfaces with the 3GPP network (customer network) that requests for transport network slices (IETF network slice). The 5G management plane in turn requests the Network Slice Controller (NSC) to setup resources and connectivity in the transport network to realize the particular network slice. 5G network slice orchestration is defined in [TS.28.533-3GPP] and is represented in Figure 1 as Network Slice Selection Management Function (NSSMF)) which is a part of 5G Management system responsible for managing network slice subnets. The 3GPP network (customer network) requests for IP transport network slice (slice provider network) based on estimated demand. The 5G management plane slice orchestration functionality in 3GPP requests for transport slices via the NSC and may use ACTN [RFC8453]. The Network Data Analytics Function (NWDAF), Network Slice Selection Management Function (NSSMF) and other 3GPP functions in the control and management planes provide data and functionality to estimate slice capabilities required from the transport network. The slices provisioned in the IP transport network correspond to 3GPP slices represented by NSSAI (set of 3GPP slices) available at 3GPP access/data networks and locations. During 3GPP procedures for session initialization, the network grants an S-NSSAI (single one out of the NSSAI) based on the user's session request. The S-NSSAI for the UE's session is signaled to the user plane nodes (gNB, UPF) during the session setup and used to associate to the corresponding IP transport slice. An overview of the sequence of operations from when a user (UE) requests during session setup to how it relates to the fronthaul, mid-haul and backhaul transport network slices is provided below.¶

Prior to 3GPP user (UE) signaling to setup a session, the UE attaches to the radio access network and has the parameters for operation configured. During this sequence of operation, the signaling is between the UE and the gNB. When the gNB functionality is split between a central unit (CU) and a distributed unit (DU), a mid-haul transport segment provides the connectivity as shown in Figure 1. Further, the gNB central unit can be split into the control plane (CP) and user plane (UP) logical functions as shown in Figure 1. 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. An important point to note is that signaling and data transport over the fronthaul transport has no notion of an end-user/UE session, but is rather defined by low latency and other requirements required for processing radio signaling and data transport between the network entities that compose gNB. For the front-haul described further in Section 3.2, an Ethernet transport with VLANs can be expected to be the case in many deployments.¶

Following the radio access setup and attach, the 3GPP user (UE) signals to setup a session. 5G core network (5GC) functionality to handle access mobility (AMF), UE session management (SMF), policy (PCF) and various other assisting functionality including 3GPP slice selection (NSSF) is used to authenticate the UE and setup the data plane to transport the UE PDU (Protocol Data Units). The N3, N9 and F1-U user planes use GTP-U [TS.29.281-3GPP] to transport UE PDUs (IPv4, IPv6, IPv4v6, Ethernet or Unstructured). From an IP transport network perspective, these GTP-U connections can be viewed as multiple overlay connection segments between each of the 3GPP data plane entities (gNB, UPF) on a per UE basis. The GTP-U/overlay transport capabilities required are signaled between the RAN and 5GC during UE session setup. Note that unlike the slice requirements for mid-haul segment (F1-U), the slice requirements for the backhaul (N3, N9) are setup in the 3GPP network on a per UE basis. Thus, in the backhaul, an IP transport slice with capabilities corresponding to the S-NSSAI negotiated between UE 5GC is provided. For example, a UE that sets up an eMBB session may require high bandwidth but can tolerate delay whereas a URLLC session requires low latency, low jitter and low error rates. Slice capabilities along the user plane path between the (R)AN and UPFs such as a low latency path, jitter, protection and priority need to be provided by the IP transport network. 3GPP core network entities may be deployed across multiple data centers and in such cases require the IP transport network to provide the resources and connectivity for each of the slice segments.¶

3.2. Fronthaul and Mid-Haul Transport Network

The O-RAN Alliance defined a lower layer split at the physical layer of the radio access network whereby the gNB-DU is split into two entities (O-RU and O-DU) and 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 an Ethernet based fronthaul interface, the network slice instance (NSI) information is carried in the Ethernet header through the VLAN tags and/or PCP marking. The Ethernet switches in the fronthaul transport network inspects the slice information (VLAN tag and/or PCP marking) 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) to different slices and to their respective VLAN tags and or PCP marking on the fronthaul interface is controlled by the O-RAN fronthaul C-Plane and M-Plane interfaces. 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 NC pertaining to the fronthaul transport. 3GPP functions gNB-CU (user plane) and gNB-DU may also be distributed and have a mid-haul transport between the two 3GPP network functions. If an IP based mid-haul interface is used, the network slice instance (NSI) information can be MPLS, SRv6 based as defined in [TS.28.541-3GPP]. However if the 3GPP network function (slice customer) is physically separated from the slice provider network (e.g., a gNB-CU (user plane) with baseband units deployed in a data center), the MPLS, SRv6 information may not be practical to carry across to the separated IP transport network (slice provider). In this case, the source UDP port of the GTP-U can be used to indicate the slice in the IP transport network (slice provider).¶

3.3. Backhaul Transport Network

The backhaul transport over which the protocols for N3 and N9 interfaces run are described in [TS.23.501-3GPP] and [TS.23.502-3GPP]. The end-user (UE) sessions (IP, Ethernet, unstructured) are carried over GTP-U transport protocol over the N3 and N9 interfaces. GTP-U between the 3GPP network functions (gnB, UPF) serves as an overlay protocol across one or more MPLS, SRv6 or Ethernet transport networks in between. During UE session setup, a number of parameters for context management are configured in the gNB, UPF and that includes allowed network slice (S-NSSAI). On an Ethernet based backhaul interface, the slice information is carried in the Ethernet header through the VLAN tags. If an IP based backhaul interface is used, the network slice instance (NSI) information can be MPLS, SRv6 based as defined in [TS.28.541-3GPP]. However, if the 3GPP network function (slice customer) is physically separated from the slice provider network (e.g., a gNB-CU (user plane) or UPF, or both are deployed in a data center), the MPLS, SRv6 information may not be practical to carry across to the separated IP transport network (slice provider). In this case, the source UDP port of the GTP-U can be used to indicate the slice in the IP transport network (slice provider).¶

3.4. Slice Mapping using UDP Source Port

Communication services offered by 3GPP and the concepts used to provision and manage it are described in [TS.28.530-3GPP]. A brief overview is given here with the intent to describe how it is related to an IP transport slice and the mapping between it and the 3GPP slice. Communication services (e.g., an eMBB service) may be realized in a 3GPP network using one or more slices identified by NSSAI (Network Slice Selection Assistance Information) in the 3GPP control plane signaling. In the 3GPP management plane, the network slice identified by NSSAI is realized in as a Network Slice Subnet (NSS). For example, a slice S-NSSAI is available to a user at different locations (and even PLMNs) and maybe realized in an NSS at that a location. The NSS consists of sets of functions from 5GC and RAN that belong to the NSS. Network interfaces of functions in an NSS may be associated to one or more slice subnets. These relationships are illustrated in Figure 2. From the viewpoint of IP transport slicing and mapping to 3GPP slices, an IP transport network slice is associated to 3GPP core or RAN network functions in a 3GPP Network Slice Subnet (NSS). Thus, it can represent a slice of a transport path for a tenant between two 3GPP user plane functions.¶

+-------------------+   +-------------------+    +-------------------+
|  Comm. Service A  |   |  Comm. Service B  |    |  Comm. Service C  |
+-----+-------------+   +--+-----+----------+    +--------+----------+
      |     ______________/      |                         \
      |    /                     |                          \
+-----+---+---+          +-------+-----+              +------+------+
|NSSAI = 000A |          |NSSAI = 000B |              |NSSAI = 000C |
+-------^-----+          +------^------+              +------^------+
       /                       /                             |
______/______            _____/_______                 ______|_______
\  Net.Slice \           \  Net.Slice \               | Net.Slice    |
 \  Subnet-A  \           \  Subnet-B  \              | Subnet-C     |
  \ (NSS-A)    \           \   (NSS-B)  \             |   (NSS-C)    |
   \            \           \            \            |              |
    \  +--------+\           \  +--------+\           |  +--------+  |
     \ |NSSI=CN1| \           \ |NSSI=CN1| \          |  |NSSI=CN3|  |
      \+-----\--+  \           \+---\----+  \         |  +---|----+  |
       \      \     \           \    \       \        |      |       |
        \  +===\====+\           \ +==\=====+ \       |  +===|====+  |
         \ |NS = IP1| \           \|NS = IP2|  \      |  |NS = IP3|  |
          \+====\===+  \           +====\===+   \     |  +===|====+  |
           \     \      \           \    \       \    |      |       |
            \  +--\-----+\       +--------\-----------+      |       |
             \ |NSSI=AN1| \      \    \ +--\-----+ \         |       |
              \+--------+  \      \    \|NSSI=AN2+-----------+       |
               \____________\      \    +--------+   \               |
                                    +----\------------\--------------+
                                          +------------+

Figure 2 shows the slice hierarchy described in [TS.28.530-3GPP] with 3 communication services enhanced to show the IP transport slice instances (IP1, IP2, IP3). As an example, when a UE registers with 5GC with NSSAI 000A, OOOB and others, the AMF may select NSSAI 000B in list of NSSAI allowed for the UE. One of the factors in selecting the NSSAI is whether the UE may move to another location during the lifetime of the session. In this case, the NSSAI should be such that it has a mapping to IP transport network slice during initial attach, and following handover. For example, a UE that attaches to 5GC with S-NSSAI = 000B and served by user plane instances CN1 and AN2 uses IP transport network slice NS = IP2 to provide the resources in the IP network that corresponds to the UE session. Following handover with S-NSSAI = 000B, the UE may be served by user plane instances CN1' and AN2' over an IP slice IP2' in the new location.¶

When the 3GPP user plane function (5G-AN, UPF) and IP transport provider edge are on different nodes or separated across a network, the PE router needs to have the means by which to classify the IP packet from 3GPP entity based on some header information. In [I-D.ietf-teas-ietf-network-slices] terminology, this is a scenario where there is an Attachment Circuit (AC) between the 3GPP entity (customer edge) and the SDP (Service Demarcation Point) in the IP transport network (provider edge). The Attachment Circuit(AC) may for example be between a UPF in a data center to a (provider edge) router that serves as the service demarcation point for the transport network slice. The identification 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. For 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. This is illustrated in Figure 3.¶

                          upstream GTP-U packet
                    =====================================>

      customer edge     attachment       slice provider    customer edge
                         circuit             ______
     +-------------+      ______          __/      \__     +-----------+
     |   gNB-CU    |   __/      \__      /     IP     \    |   UPF     |
     |N3 IP i/f =  +--/ Data Center\---(PE) Backhaul (PE)--+N3 IP i/f =|
     |  gNB-AN2-if |  \__ Network__/     \__        __/    |UPF-CN1-if |
     +-------\-----+     \______/           \___\__/       +-----------+
              \                                  \
               \                                  \
                \                                  \
                 \                       +----------\---------------+
 +----------------\---------------+      |   Slice Mapping:         |
 |+-------------------------+     |      |Match:                    |
 ||3GPP CP Config:          |     |      | src-IP-addr = gNB-AN2-if |
 ||NSSAI = {000B, 000C, ..} |     |      |    src-port = 5678       |
 ||NSSI  = AN2              |     |      |Action:                   |
 |+-------------------------+     |      |   select NS = IP2        |
 |                                |      +--------------------------+
 |+------------------------------+|
 ||Slice Mapping to UPF-CN1-if:  ||
 ||EP_Transport S-NSSAI=000B     ||
 ||logicInterfaceType = UDPSrcPrt||
 ||logicInterfaceId = 5678       ||
 ||ipAddress = UPF-CN1-if        ||
 |+------------------------------+|
 +--------------------------------+

Figure 3 shows the configuration and mapping applied to network instances in a 3GPP network slice subnet and corresponding IP transport network instances for sending an upstream GTP packet from gNB-CU (user plane) to UPF. The gNB-CU (user plane) function is in a data center and separated from the IP transport slice provider by an attachment circuit (AC - i.e., the data center network). In this example, a GTP-U packet at gNB-CU (user plane) belonging a UE session with S-NSSAI = 000B (and thus associated to 3GPP and IP transport network instances in the figure for providing slice resources). Since the GTP-U packet belongs to a session with S-NSSAI = 000B, the gNB-CU (UP) maps it to UDP port 5678 in the GTP-U header (outer encapsulation source port). The GTP-U packet is forwarded by the data center network to the PE router at IP backhaul network. The PE matches on GTP-U source IP address and port to select the provisioned slice (NS = IP2). The UPF customer edge may be attached to the PE as shown in the figure or alternatively via a data center network. A similar set of mappings exist for downlink GTP-U, but they do not necessarily use the same resources.¶

PE routers can thus provision a policy based on the source UDP port number to the 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-U 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 on 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 IP network slice 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 slice 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 IP network slice 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 slice encoding mechanism already accounts for this.¶