Network Working Group X. de Foy
Internet-Draft A. Rahman
Intended status: Informational InterDigital Communications, LLC
Expires: September 7, 2017 March 6, 2017

Network Slicing – 3GPP Use Case


This document describes work conducted at the 3GPP standard organization on 5G Network Slicing. Its goal is to provide a detailed use case, and help better define requirements, for Internet Protocols supporting Network Slicing.

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Table of Contents

1. Introduction

This document describes work conducted at the 3GPP standard organization on 5G Network Slicing. Its goal is to provide a detailed use case, and help better define requirements, for Internet Protocols supporting Network Slicing.

The concept of Network Slicing (NS) is considered a key mechanism for 5G networks to serve vertical industries with widely different service needs, in term of latency, reliability, capacity, and domain specific extra functionalities. It does so by exposing isolated partitions of network resources and services. The IETF Bar-BoF NETSLICES activity studies the need for supporting protocols. In particular, [I-D.gdmb-netslices-intro-and-ps] defines Network Slicing in a broad context and suggests related problems and work areas. It also identifies the need to provide use cases such as, for example, ultra-low latency and massive-connectivity machine communication services. We propose to review ongoing NS work in 3GPP within the present document. This constitutes in our view another valid type of use case, since 3GPP architecture may ultimately use or integrate with NS solutions defined in IETF.

Sections 2 to 6 aim to represent the current state of network slicing and related aspects in 3GPP. We attempted to leave our own analysis out of these sections and present it into section 7. For simplicity, 3GPP-specific acronyms are defined in the section they are used, which is mostly section 3.

1.1. Background

The 3GPP standard organization is in the process of developing 5G system architecture, which includes network slicing. In the present document we will use information collected from 3GPP Release 15 specifications (as well as preliminary studies from Release 14 when specifications are not yet available). This release aims to address a more urgent subset of commercial needs in "5G Phase 1", with expected deployments in 2020. Work on network slicing is split between different working and study groups, reviewed here in separate sections.

1.2. NS Work in 3GPP Prior to 5G Phase 1

While the present document will only focus on Network Slicing in 5G Phase 1, an early form of network slicing was introduced in Release 13, with the Dedicated Core Network (DCN) or DECOR feature, where dedicated core network nodes are forming a DCN serving subscribers or devices with a certain "usage type" (e.g. machine-to-machine or enterprise). In the Release 14 eDECOR feature, the DCN selection mechanism was extended to be assisted by the device, which can now send its usage type to the RAN. This is specified in [_3GPP.23.401]. Deployments are expected in 2017 and 2018.

2. Network Slicing Requirements in 3GPP

Network slicing requirements are included in [_3GPP.22.261]. There are requirements related to:

3. Network Slicing in Logical 5G Architecture in 3GPP

5G network architecture is entering its normative stage in 2017 with a "5G System - Phase 1" Release 15 work item, which is in the process of producing two technical specifications: 23.501 "System Architecture for the 5G System" and 23.502 "Procedures for the 5G System". At this early stage, though, we will still need to refer to the earlier technical report [_3GPP.23.799]. The following text summarizes what has been agreed about network slicing (refer to section 8.1 of that report for more details).

A network slice is a complete logical network including Radio Access Network (RAN) and Core Network (CN). It provides telecommunication services and network capabilities, which may vary (or not) from slice to slice. Distinct RAN and core network slices will exist. A device may access multiple slices simultaneously through a single RAN.

The device may provide Network Slice Selection Assistance Information (NSSAI) parameters to the network to help it select a RAN and a core network part of a slice instance for the device. A single NSSAI may lead to the selection of several slices. The network may also use device capabilities, subscription information and local operator policies to do the selection.

A NSSAI is a collection of smaller components, Session Management NSSAIs (SM-NSSAI), which each include a Slice Service Type (SST) and possibly a Slice Differentiator (SD). Slice service type refers to an expected network behavior in terms of features and services (e.g. specialized for broadband or massive IoT), while the slice differentiator can help selecting among several network slice instances of the same type, e.g. to isolate traffic related to different services into different slices.

A PDU session is a 5G concept for an association between the device and a data network, which can be IP, Ethernet or Unstructured (i.e. transparent to the 5G system). The device will associate an application with one out of multiple parallel PDU sessions, each PDU session correspond to one core network slice and one RAN slice. Different PDU sessions may belong to different slices. More precisely, an application will be associated with a SM-NSSAI (as mentioned above, this includes a slice service type and may also include a service differentiator), and data for this application will be routed to a PDU session associated to this SM-NSSAI.

Part of the control plane, the Common Control Network Function (CCNF), is common to all or several slices. It includes the Access and mobility Management Function (AMF) as well as the Network Slice Selection Function (NSSF), which is in charge of selecting core network slice instances. Besides those shared functions, different slices may also have dedicated control plane functions such as the Session Management Function (SMF), which manages PDU sessions. User plane functions are dedicated to each slice. The RAN selects a CCNF for a new PDU session. CCNF may initiate the redirection of service for a device towards another CCNF, initially at session setup, or later on.

In figures 1 and 2 we attempt to represent the use of network slicing in 3GPP logical architecture (those figures are our interpretation and are not directly adapted from the report). Figure 1 represents the role of NSSAI in network selection. Figure 2 represents the major network functions and interfaces in the context of RAN and Core Network slicing. The terms used in these diagrams were introduced earlier. System description and diagrams in section 4 of [_3GPP.23.501] can provide additional context.

                         |       |
                         |Device |
                         |       |
         RAN uses NSSAI  +---+---+
         to select CCNF      |
                       \     |(NSSAI)
                        \    |
                         |       +-------------+
       CCNF uses NSSAI   |  RAN  +---------+   |
       to select slice   |       |         |   |
       or redirect to    +---+---+         |   |
       another CCNF          |             |   |
                   \         |(NSSAI)      |   |
                    \        |             |   |
                     +-------+--------+    |   |
                     | Common Control |    |   |
                     | Plane Network  |    |   |
                     |    Functions   |    |   |
                     |    (CCNF)      |    |   |
                     +-----+----+-----+    |   |
                           |    |          |   |
                           |    |          |   |
                           |    |          |   |
                 |         |               |           |
              +------------|---------------|-------+   |
              |  +---------++        +-----+----+  |   |
              |  |          |        |          |  |   |
              |  | +------+ |        | +------+ |  |   |
              |  | |      | |        | |      | |  |   |
              |  | |CP NF1| |        | |UP NF1| |  |   |
              |  | |      | |        | |      | |  |   |
              |  | +------+ |        | +------+ |  |   |
              |  |    ...   |        |    ...   |  |   |
              |  |          |        |          |  |   |
              |  | +------+ |        | +------+ |  |   |
              |  | |      | |        | |      | |  |   |
              |  | |CP NFn| |        | |UP NFn| |  |   |
              |  | |      | |        | |      | |  |   |
              |  | +------+ |        | +------+ |  |   |
              |  |          |        |          |  +---+
              |  +----------+        +----------+  |
                   Core Network Slice Instances

Figure 1: Network Slice Selection in 3GPP architecture

                 CCNF         Network Slice Instance
         |                 |                     |
         |                 |                     |
         |   +--------+    |     +--------+      |
         |   | Control|    |     | Control|      |
    +--------+ Plane  +----------+ Plane  |      |
    |    |   | AMF... |    |     | SMF... |      |
    |    |   +--------+    |     +----+---+      |
    |    |                 |          |          |
    |    |                 |          |          |
    |    |                 |          |          |
+---+--+ +-----------------+          |          |
|Device| |                 |          |          |
+---+--+ |                 |          |          |
    |    |                 |          |          |
    |    |   +--------+    |   +------+-----+    |
    |    |   |        |    |   | User Plane |    | +---------------+
    +--------+  RAN   +--------+ Functions  +------+Data Network or|
         |   |        |    |   |            |    | | The Internet  |
         |   +--------+    |   +------------+    | +---------------+
         |                 |                     |
         |                 |                     |
              RAN Slice

Figure 2: Network Slices in 3GPP architecture

3.1. Early Work on RAN Slicing

In line with the logical architecture described above, early work on RAN slicing is being conducted as part of the larger Release 14 "Study on New Radio Access Technology". Key principles are likely to include the following, extracted from [_3GPP.38.801]:

  1. RAN will support differentiated handling of traffic between pre-configured, isolated RAN slices. How to perform this is left to implementation.
  2. Selection of the RAN slice will be based on IDs (which should be the slice service type and slice differentiator defined above) provided by the device or core network.
  3. A RAN slice may or may not be available at a given location.
  4. RAN will select the core network slice.
  5. QoS differentiation within a RAN slice will be supported as well.

4. Management Aspect of Network Slicing in 3GPP

3GPP is developing a Release 14 "Study on management and orchestration of network slicing for next generation network" technical report [_3GPP.28.801], which defines an information model where the network slice as well as physical and virtualized network functions belong to the network operator domain, while the virtualized resources belong to another domain operated by a virtualization infrastructure service provider.

The concept of "sub-netslice" is used in the model, as defined originally in [NGMN_Network_Slicing] (we will use the term sub-netslices here, instead of the original subnet or sub-networks, which can be confusing). Sub-netslice instances are comprised of physical and virtual resources, have a life cycle independent from network slices they belong to, can be shared between several network slices and may be associated with other sub-netslice instances.

Multiple management use cases are described, ranging from creating and monitoring a slice instance to configuring its SLA policy, capacity and roaming support. It is also expected that some level of slice management will be exposed to customers, and that operators will have the possibility to create end-to-end network slices involving multiple operators' networks.

Key issues are identified, including creating a slice across multiple administrative domains, sharing a network slice between multiple services, moving towards a more autonomous management, as well as additional management specific key issues.

Finally, the life cycle of a slice is defined over 4 phases: preparation phase including design and pre-provisioning, an "instantiation, configuration and activation" phase, a run-time phase including supervision and reporting, as well as upgrade, reconfiguration and scaling, and a decommissioning phase.

5. 5G Virtual Infrastructure Management in 3GPP

To support the logical architecture defined earlier, some aspects of virtualization infrastructure management are also being standardized by 3GPP, through the activity "Management of mobile networks that include virtualized network functions". This includes 5 work tasks, the first of which deals with concept, architecture and requirements [_3GPP.28.500], and 4 additional specialized work tasks on configuration, fault, lifecycle and performance management, are in the process of creating more detailed technical specification documents.

The new 5G management system is tied to NFV-MANO, as defined by ETSI. Its system architecture is described in [_3GPP.28.500] and represented in Figure 3 (directly adapted from TS28.500). It defines interconnections between the 3GPP management system and the NFV-MANO system. NFV-MANO has the responsibility to manage NFV Infrastructure (NFVI) and VNF lifecycle, and to report performance data, fault and VNF instance information to 3GPP management system.

+--------------------------------------------+    +-------------+
| +----------------------------------+OSS/BSS|    |   NFV       |(3)
| |      NM                          |       | (1)|Orchestrator +--+
| +--+--------------+--------------+-+       +----+   (NFVO)    |  |
+----|--------------|--------------|---------+    +-------------+  |
     |              |              |                     |(2)      |
     | Itf-N        | Itf-N        | Itf-N               |         |
     |              |              |              +------+------+  |
     |              |              |              |             |  |
     |    +---------+------------+ |              |             |  |
     |    |DM +----------------+ | |          (5) |             |  |
     |    |   |       EM       +------------------+   VNF       |  |
     |    |   +-+--------+-----+ | |          (5) |  Manager    |  |
     |    +-----|--------|-------+ |   +----------+  (VNFM)     |  |
     |          |        |         |   |          |             |  |
     |          |        |         |   |          |             |  |
     |          |        |         |   |          |             |  |
     |          |        |         |   |          |             |  |
 +-+-+--+       |        |      +--+---++--+  (5) |             |  |
 | | EM |       |        |      | EM    |  +------+             |  |
 | +----+       |        |      +-------+  |      +------+------+  |
 |      |       |        |      |    VNF   |             |         |
 |      |  +----++  +----+----+ +----+-----+             |(4)      |
 |      |  |     |  |         |      |                   |         |
 |      |  |     |  |  VNF    |      |                   |         |
 |      |  |     |  |         |      |(7)                |         |
 |      |  |     |  +-----+---+      |            +------+------+  |
 |      |  |     |        |(7)       |            |             |  |
 |  NE  |  | NE  |  +-----+----------+----+       | Virtualized |  |
 | (PNF)|  |(PNF)|  |                     |       | Infrastruct.|  |
 |      |  |     |  | NFV Infrastructure  |  (6)  | Manager     +--+
 |      |  |     |  |     (NFVI)          +-------+  (VIM)      |
 |      |  |     |  |                     |       |             |
 +------+  +-----+  +---------------------+       +-------------+

Figure 3: 3GPP Network Management Architecture

The major building blocks on the left are from pre-existing 3GPP architecture and on the right are from ETSI NFV architecture. Itf-N is the traditional 3GPP management interface between the network manager and domain and element managers, some of which will now be collocated with VNFs. Other interfaces identified in the diagram are defined as part of the NFV architecture and described in published NFV specifications (which themselves do not make mention of network slicing).

  • (1) Os-Ma-nfvo enables managing Network Service Descriptors (NSD), network service lifecycle, performance, faults and VNF packages. The NSD information model is specified by ETSI NFV as well. It enables describing network connectivity topology graphs where VNFs are connected together through virtual links. An NSD also includes VNF descriptors, which include memory and CPU requirements, a link to a software image, initial setup scripts, etc.
  • (2) Or-Vnfm includes package management, VNF lifecycle/fault/configuration management, virtualized resources management (in indirect mode as seen below), and relays notifications from the VNF or EM.
  • (3) Or-Vi and (4) Vi-Vnfm are the northbound interface of the Virtual Infrastructure Manager (VIM). The orchestrator can basically use a direct interface to VIM, or indirectly go through the VNF manager over Or-Vnfm. The orchestrator can add software images, create/update/terminate virtualized compute/network/storage resources allocations, manage resource capacity, manage network virtual paths, run and query performance collection jobs, set quotas. Location/affinity constraints can be applied when creating resources.
  • (5) Ve-Vnfm is used both between VNFM and EM and between VNFM and VNFs. It includes lifecycle, performance, fault and configuration management, as well as notifications from the EM that VNFM can subscribe to.
  • (6) Nf-Vi is for assignment of virtualized resources, hardware resource configuration and state information exchange.
  • (7) Vn-Nf represents the execution environment provided to the VNF.

6. Security Considerations

The present section on network slicing security is based on the "Study on Architecture and Security for Next Generation System", a Release 14 study item. Its related technical report is [_3GPP.33.899], and covers, among other areas, a network slicing security area dealing with service access, network function sharing and isolation. Multiple key issues are summarized here (refer to the TR section 5.8 for more details):

  1. Isolation requirements, especially performance isolation, ask for data plane actions on one slice to have no influence on other slices and for control plane actions (e.g. creation/update/deletion) to have little influence on other slices. Lack of isolation can enable DoS attacks from one slice to another, especially since some functions can be shared between slices. Moreover, a device can access multiple slices, which increases the possibility for leakage/breach type of issues between slices, both from the device and the service side.
  2. Different slices may need to implement different security policies, e.g. in term of authentication requirements (IoT vs. mobile broadband user). Authentication may be centralized and/or per-slice.
  3. Security and privacy of devices’ access to slices is also a concern. It may be possible to forge slice selection information for a device. Slice selection information sent over the access network can also lead to confidentiality issues. The proposed key hierarchy supports having different keys for different slices. How to enable security isolation of a common control plane between different slices is not addressed at this point.
  4. Security of sensitive shared network elements such as the HSS (which holds customers’ profiles) is identified as a key issue as well.
  5. Slice management functions should be secured, since an attacker may use them, e.g. to delete a slice. Slice management capabilities exposed through APIs for 3rd parties are especially vulnerable.
  6. Security on inter-slice communications is an issue in several scenarios, for example when multiple slices share control plane functions, and when a RAN slice and a core network slice are interconnected to form a complete slice. Each slice is considered a different trusted domain.
  7. A range of potential issues related to virtualization are yet to be explored further, though it is not clear if they are in the scope of 3GPP. Issues include for example isolation between VNFs hosted by the same hypervisor, authentication between VNFs, performance isolation between VNFs.

7. Conclusion

This document summarized our understanding of 5G architecture and requirements for network slicing, as defined by 3GPP, in their current state. Our goal was to provide context to IETF NETSLICES, since a protocol or framework defined by IETF for network slicing may be used to implement or interoperate with 3GPP-compliant 5G systems. In reference to Figure 3, the scope of IETF involvement with network slicing could be within NFV Infrastructure, as well as some aspects of the control plane on the right-hand side of the figure.

Here is a list of discussion points gathered while preparing this document, in no particular order:

  1. The concept of "sub-netslice" in section 4 may be useful as a building block, e.g. a single-domain, indivisible and composable service chain that is used to assemble a network slice. Defining the interfaces of such a component could help focusing on sharing between slices and extending slices across domains.
  2. A large portion of the NS-related logical architecture agreements so far is focusing on network slice instance selection. 3GPP architecture demonstrate a requirement for common authentication and network slice selection functions. It is our understanding that the described mechanism enables a wide variation of cases associating "n" slices with "m" network services or applications involving "p" end devices. For example: a single slice instance could be associated with multiple IoT applications, each connected to multiple devices. In another example, an application may split its end users in 2 service categories with different SLAs, using different network slice instances.
  3. Interoperation between slices is a major risk factor on isolation and can occur in various scenarios:
    • "Interoperation for extension" when data and control plane are interconnected for extending a slice between RAN and CN, or between visitor and home networks in a roaming scenario.
    • "Interoperation through network function sharing" occurs in 3GPP when some control planes functions are performed by common functions.
    • "Interoperation through end points" can occur on user devices connected to multiple network slices, or on an application server side interacting with clients over different slices.

  4. Sharing and interoperation within slices is also a concern and occurs at multiple levels: QoS differentiation within a given slice, and sharing a single slice between multiple services. Deployment of a single slice over multiple data centers is not an explicitly described scenario but may possibly be implied by the use of virtualization of core network function.
  5. Managing and deploying 3rd party network services is a potential use case for network slicing. For example, network slice management will likely be partially exposed to customers. Additionally, an early requirement, that was finally not included in the 3GPP specifications for network slicing, mentioned "hosting multiple 3rd parties (e.g., enterprises) or MVNOs".
  6. There is a strict requirement for security and performance isolation from data plane and control plane actions between slices. Should network slices be allowed to tap into currently unused resource capacity? There is a possible tradeoff here between performance/network efficiency and isolation, since in this case, through its normal operation, a slice may influence another slice by denying it this extra capacity.
  7. Beyond performance isolation, there are multiple security concerns related to network slicing, including the need for separate per-netslice security policies.

We hope these points will contribute to the ongoing discussion taking place in IETF NETSLICES to define requirements and work areas related to network slicing.

8. IANA Considerations

This document requests no IANA actions.

9. Acknowledgements

The authors would like to thank Ulises Olvera-Hernandez for his contribution and comments.

10. Informative References

[_3GPP.22.261] 3GPP, "Service requirements for next generation new services and markets", 3GPP TS 22.261 1.1.0, February 2017.
[_3GPP.23.401] 3GPP, "General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access", 3GPP TS 23.401 14.2.0, 12 2016.
[_3GPP.23.501] 3GPP, "System Architecture for the 5G System", 3GPP TS 23.501 0.2.0, 2 2017.
[_3GPP.23.799] 3GPP, "Study on Architecture for Next Generation System", 3GPP TR 23.799 14.0.0, 12 2016.
[_3GPP.28.500] 3GPP, "Telecommunication management; Management concept, architecture and requirements for mobile networks that include virtualized network functions", 3GPP TS 28.500 1.3.0, 11 2016.
[_3GPP.28.801] 3GPP, "Study on management and orchestration of network slicing for next generation network", 3GPP TR 28.801 0.4.0, 1 2017.
[_3GPP.33.899] 3GPP, "Study on the security aspects of the next generation system", 3GPP TR 33.899 0.6.0, 11 2016.
[_3GPP.38.801] 3GPP, "Study on the security aspects of the next generation system", 3GPP TR 38.801 1.0.0, 12 2016.
[I-D.gdmb-netslices-intro-and-ps] Galis, A., Dong, J.,, k., Bryant, S., Boucadair, M. and P. Martinez-Julia, "Network Slicing - Introductory Document and Revised Problem Statement", Internet-Draft draft-gdmb-netslices-intro-and-ps-02, February 2017.
[NGMN_Network_Slicing] NGMN, "Description of Network Slicing Concept", 1 2016.

Authors' Addresses

Xavier de Foy InterDigital Communications, LLC Montreal, Canada EMail:
Akbar Rahman InterDigital Communications, LLC Montreal, Canada EMail: