Interconnecting (or Stitching) Network Slice Subnets
draft-defoy-coms-subnet-interconnection-04

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none                                                           X. de Foy
Internet-Draft                                                 A. Rahman
Intended status: Informational                         InterDigital Inc.
Expires: September 9, 2020                                      A. Galis
                                               University College London
                                                            K. Makhijani
                                                                L. Qiang
                                                     Huawei Technologies
                                                                S. Homma
                                                                     NTT
                                                       P. Martinez-Julia
                                                                    NICT
                                                           March 8, 2020

          Interconnecting (or Stitching) Network Slice Subnets
               draft-defoy-coms-subnet-interconnection-04

Abstract

   This document defines the network slice (NS) subnet as a general
   management plane concept that augments a baseline YANG network slice
   model with management attributes and operations enabling
   interconnections (or stitching) between network slices.  The
   description of NS subnet interconnections is technology agnostic, and
   is not tied to a particular implementation of the interconnection in
   data plane.

Status of This Memo

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   This Internet-Draft will expire on September 9, 2020.

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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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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation and Roles of NS Subnet . . . . . . . . . . . .   3
     1.2.  Usage of NS Subnets . . . . . . . . . . . . . . . . . . .   3
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Information Model . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Base Information Model  . . . . . . . . . . . . . . . . .   5
     2.2.  Interconnection Anchors . . . . . . . . . . . . . . . . .   6
     2.3.  Interconnection Instances . . . . . . . . . . . . . . . .   8
     2.4.  Stitching Operation . . . . . . . . . . . . . . . . . . .   9
       2.4.1.  Operation Overview  . . . . . . . . . . . . . . . . .   9
       2.4.2.  Stitching Scenarios . . . . . . . . . . . . . . . . .  10
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   5.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Network Slicing enables deployment and management of services with
   diverse requirements on end-to-end partitioned virtual networks over
   the same infrastructure, including networking, compute and storage
   resources.  There were recent efforts in the IETF to define a
   transport slice ([I-D.nsdt-teas-transport-slice-definition]) and to
   define a north-bound interface for such a transport slice
   ([I-D.contreras-teas-slice-nbi]).  The mapping of transport slices in
   5G mobile systems is also studied in [I-D.clt-dmm-tn-aware-mobility]
   and [I-D.geng-teas-network-slice-mapping].

   Network slices may be managed through usage of YANG data models.  For
   example, [I-D.liu-teas-transport-network-slice-yang] describes how
   existing YANG models can be augmented with network slice attributes.

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   Nevertheless, defining and managing a network slice (NS) end-to-end
   does not always have to be done directly.  It may be convenient to
   define and manage separately subsets of an end-to-end slice.  The
   concept of network slice subnet is defined originally in
   [NGMN_Network_Slicing], though we only need to retain its definition
   in the most universal form: network slice subnets are similar to
   network slices in most ways but cannot be operated in isolation as a
   complete network slice (e.g., a NS subnet can be seen as a network
   slice with unconnected links).  NS subnets are interconnected with
   other NS subnets to form a complete, end-to-end network slice (i.e.
   interconnection and/or stitching of NS subnets).  In the present
   draft, we describe a data model for describing interconnections
   between NS subnets, that enables assembling them in a hierarchical
   fashion.

1.1.  Motivation and Roles of NS Subnet

   NS subnet is a management plane concept that facilitates
   interconnections (also known as stitching) of network slices.  It
   augments the base slice information model, that can be used to
   represent an end-to-end network slice.  The extensions described in
   this document can be used to represent a slice subnet instead, and
   can also be used to represent an interconnection inside an end-to-end
   slice, i.e.  they aim to represent interconnection points both
   "before" and "after" the interconnection takes place.  Operations
   such as stitching subnets are also described.

   The description of NS subnet interconnections is technology agnostic
   following the approach of the slice information model.  Some
   interconnections may be implemented using the interplay between
   management plane and gateways in the data plane.
   [I-D.homma-rtgwg-slice-gateway] describes the requirements on such
   data plane network elements, and will provide input for the
   management plane mechanisms described in the present document.

1.2.  Usage of NS Subnets

   Using NS subnets can help:

   o  Isolate management and maintenance of different portions of a
      network slice, over multiple infrastructure domains, or even
      within a single domain.  For example, in Figure 1, NS orchestrator
      (NSO) 2 manages subnet A, in isolation from subnets B and C
      managed by NSO 3.  NSO 1 can still manage the end-to-end slice as
      a whole, but it does not need to deal in detail with each subnet.

   o  Isolate mapping towards different infrastructure technologies,
      even within the same domain.  This can simplify NS orchestrator

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      implementation, since each NSO can specialize in managing a
      smaller set of technologies.

   o  Enable advanced functions such as sharing a slice subnet between
      several slices, or substituting one slice subnet for another, e.g.
      for coping with load.

                      +-----------+
                ******| NS Orch. 1|********
                *     +-----------+       *
                *                         *
                *                         *
           +-----------+              +-----------+
           | NS Orch. 2|              | NS Orch. 3|*****
           +-----------+              +-----------+    *
                *                         *            *
                *                         *            *
                *   A-B Inter-            * B-C Inter- *
                *   connection            * connection *
   +-----------------+   .  +-----------------+  .  +-----------------+
   |      +--+       |   .  |      +--+       |  .  |      +--+       |
   |      |  +---------------------+  +--------------------+  |       |
   |      ++-+       |   .  |      ++-+       |  .  |      ++-+       |
   |       |         |   .  |       |         |  .  |       |         |
   | +---+ |  +---+  |   .  | +---+ |  +---+  |  .  | +---+ |  +---+  |
   | |   +-+--+   +-----------+   +-+--+   +----------+   +-+--+   |  |
   | +---+    +---+  |   .  | +---+    +---+  |  .  | +---+    +---+  |
   +-----------------+   .  +-----------------+  .  +-----------------+

   <.. NS subnet A ..>      <.. NS subnet B ..>     <.. NS subnet C ..>

   <....................... end-to-end slice .........................>

        Figure 1: Overview of Network Slice Subnets Interconnection

   Figure 1 illustrates how an end-to-end network slice may be composed
   of multiple slice subnets, each managed independently by a same or
   different NSO.  In multi-administrative domain scenarios, using NS
   subnets can help limiting the information that needs to be shared
   between domains.  At the infrastructure layer (i.e. in the data
   plane), the interconnection between NS subnets may involve:

   o  a gateway, that performs protocol and/or identifier/label
      translation as needed,

   o  two gateways, especially in cases where interconnected NS subnets
      are in different administrative domains,

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   o  nothing at all, in cases where the interconnection point can be
      abstracted away, e.g.  when the NS subnets share a common
      infrastructure.  In this case nodes from both NS subnets end up
      being directly interconnected between each other.

   More detailed usage scenarios are described in Section 2.4.2.

1.3.  Terminology

   Network slicing terminology, especially focusing on transport slices,
   is defined in [I-D.nsdt-teas-transport-slice-definition].

   Network Slice Subnet (NS subnet): a network slice designed to be
   interconnected with other network slices.

   NS Stitching: a management operation consisting in creating an end-
   to-end NS or a larger NS subnet, by interconnecting a set of NS
   subnets together.

   Interconnection Anchor: a management plane entity, part of a NS
   subnet model, representing an end point for use in future stitching
   operation.

   Interconnection Instance (or Interconnect): a management plane
   entity, part of a NS subnet model, representing an interconnection
   realized by a stitching operation.  It is distinct from a (data
   plane) gateway: an interconnect may be realized with or without using
   a gateway in the data plane.

2.  Information Model

2.1.  Base Information Model

   The information model we use as base for network slicing is the
   network topology model ietf-network defined in [RFC8345], in which
   networks are composed of nodes and links, and in which termination
   points (TP), defined in nodes, are used to define source and
   destination of links.

   A network slice data model instance, i.e. a YANG data model augmented
   using [I-D.liu-teas-transport-network-slice-yang]), represents a
   network slice.  When such a data model instance includes at least an
   "interconnection anchor", as defined below, it represents a network
   slice subnet instance.

   At high level, the extensions defined in this document will augment
   nodes and termination points:

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   module: ietf-network
   +--rw networks
      +--rw network* [network-id]
         +--rw network-id
         +--rw network-types
         +--rw supporting-network* [network-ref]
         |  +--rw network-ref
         +--rw node* [node-id]
         |  +--... (augmented with attributes for
         |  |       anchor/interconnection nodes)
         |  +--rw nt:termination-point* [tp-id]
         |  |  ... (augmented with attributes for
         |  |       anchor/interconnection TP)

2.2.  Interconnection Anchors

   To represent an anchor point for future interconnections (i.e. an
   unconnected end of a link), a simple solution is to use an
   "interconnection anchor" termination point (or anchor TP).  Within
   the data model describing a subnet, any link not entirely contained
   within the NS subnet must be terminated with such an anchor TP as
   source or destination.  An anchor TP belongs to a "node" attribute,
   which we refer to as interconnection anchor node (or anchor node).
   Several anchor TPs can be grouped together in an anchor node, and
   such grouping may be used as a hint during a stitching operation
   (e.g. to place all interconnection points at a same location).

   Figure 2 represents 2 interconnected network slice subnets.

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                               Slice Provider
                                     |
   +---------------------------------v---------------------------------+
   |  Network Slice Orchestrator                                       |
   |                                                                   |
   | +---------------------------------------------------------------+ |
   | |   Data model: network slice composed of NS subnet 1 and 2     | |
   | |                                                               | |
   | |      Network Slice Subnet 1            Network Slice Subnet 2 | |
   | | +---------------------------+  +----------------------------+ | |
   | | |     cross-subnet link     |  |   cross-subnet             | | |
   | | |    +----------------+     |  |       link    +------+     | | |
   | | |    |                |     |  |      +--------o node |     | | |
   | | |    |                |Interconnection|        +---o--+     | | |
   | | |+---o--+     +-------|-----+--+------|------+     |        | | |
   | | || node |     |       |     |  |      |      |     |        | | |
   | | |+---o--+     | +-----|---+ |  | +----|----+ |     |        | | |
   | | |    |        | |     |   | |  | |    |    | |     |        | | |
   | | |    |        | |     O - - - - - - - O    | |     |        | | |
   | | |    |        | |         | |  | |         | |     |        | | |
   | | |    |        | | anchor  | |  | | anchor  | |     |        | | |
   | | |    |        | |  node   | |  | |  node   | |     |        | | |
   | | |    |        | |         | |  | |         | |     +---+    | | |
   | | |    |        | |     O - - - - - - - O    | |         |    | | |
   | | |    |        | |     |   | |  | |    |    | |         |    | | |
   | | |    |        | +-----|---+ |  | +----|----+ |     +---o--+ | | |
   | | |    |        |       |     |  |      |      |     | node | | | |
   | | |    |        +-------|-----+--+------|------+     +---o--+ | | |
   | | |    | +------+       |     |  |      |                |    | | |
   | | |    +-o node o-------+     |  |      +----------------+    | | |
   | | |      +------+ cross-subnet|  |         cross-subnet       | | |
   | | |                link       |  |           link             | | |
   | | +---------------------------+  +----------------------------+ | |
   | +---------------------------------------------------------------+ |
   +--------------------------------+----------------------------------+
                                    |
                                    v
                            Network Infrastructure

        Legend: o = termination point, O = anchor termination point

              Figure 2: Network Slice Subnets Interconnection

   Attributes of interconnection anchor nodes and termination points
   include:

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   o  Information enabling NS orchestrators to match anchor nodes and
      TPs from both NS during a stitching operation.  A label may be a
      simple way to enable this.

   o  Information to help locate the interconnection.  For example, it
      could be a (sub-)domain name or geo-location information, that
      indicates where the interconnection point should be located.  This
      can help for example in cases where the subnet is instantiated
      before stitching.

   o  Information to help select the type of interconnection
      establishment: for example, this can indicate a preference for
      using interconnection over a gateway, or for abstracting away the
      interconnection point in the infrastructure plane.

         +--rw node* [node-id]
            +-- (...)
            +-- anchor_node_config
            |   +-- label (and/or other auto stitching help)
            |   +-- hint for location (domain, geolocation, etc.)
            |   +-- hint for type (1 gateway, 2 gateways, ...)
            +--rw nt:termination-point* [tp-id]
                +-- (...)
                +-- anchor_tp_config
                    +-- label (and/or other auto stitching help)
                    +-- location (domain, geolocation, etc.)
                    +-- type (1 gateway, 2 gateways, ...)

2.3.  Interconnection Instances

   There are two options for representing post-stitching network slices
   (or subnets).  They are not mutually exclusive:

   o  Option 1: subnet data models are updated with information
      describing the interconnection (e.g. anchor TPs and nodes are
      updated with new attributes representing the existing connection,
      if necessary).

   o  Option 2: a new data model is generated to represent the resulting
      network slice (or subnet).  In this composite data model, the
      interconnection may or may not be represented, this can be a
      choice made by the operator.

   Option 1 and 2 can be used concurrently in a network.  For example, a
   parent NS orchestrator may manage stitched NS subnets through
   underlying NS orchestrators, and at the same time expose to the NS
   operator a composite data model representing the resulting end-to-end
   slice.

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   To represent an existing interconnection in option 1, a simple
   solution is to add attributes to existing anchor nodes and anchor
   TPs.  Those attributes will be described below.  They aim to describe
   state and configuration associated with an active interconnection.

   To represent an existing interconnection in option 2, a simple
   solution is to create new interconnection instance nodes and
   termination point.  The same attributes as in option 1 may be
   associated with these nodes and TPs.

   Attributes of interconnection instance nodes and termination points
   include:

   o  State information (interconnection type, status, location...).

   o  Service assurance related information: besides measurements (on
      throughput, loss rate, etc.), triggers depending on throughput,
      latency, etc. can be linked with a management action or event.  A
      NS operator can use such events to take the decision to disable a
      NS subnet, replace a NS subnet with another, etc. to maintain
      overall service performance.

         +--rw node* [node-id]
            +-- (...)
            +-- interconnection_instance_node_state
            |   +-- status
            |   +-- location (domain, geolocation, etc.)
            |   +-- type (1 gateway, 2 gateways, ...)
            +-- interconnection_instance_node_service_assurance
            |   +-- events (including triggers and event IDs)
            |   +-- measurements
            +--rw nt:termination-point* [tp-id]
                +-- (...)
                +-- interconnection_instance_tp_state
                |   +-- status
                |   +-- location (domain, geolocation, etc.)
                |   +-- type (1 gateway, 2 gateways, ...)
                +-- interconnection_instance_node_service_assurance
                    +-- events (including triggers and event IDs)
                    +-- measurements

2.4.  Stitching Operation

2.4.1.  Operation Overview

   Stitching is an operation that takes two or more NS subnets as input,
   and produces a single composite NS subnet or end-to-end slice.  It
   may occur when the slice subnets are being instantiated, or later.

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   The first step in this operation is to identify the anchors that will
   be used in the interconnection.  This may be done by an automated
   algorithm that matches the possible interconnection points and
   decides which one will be used, according to the policies established
   by the NS operator.  The operation in this case will require the
   presence of semantically-rich attributes in the candidate anchors to
   enable automatic matching without human intervention.

   Other attributes of slices and anchors will also influence the
   operation and the resulting stitched (composite) object.  For
   instance, network links that are interconnected must have compatible
   QoS attributes.  Moreover, available networking protocols must also
   match among the underlying network elements that are being stitched.
   Otherwise, the operation will fail unless the NS operator (based on
   policy and/or NS subnet attributes) enables it to search for, and
   use, some "bridge" element in the underlying infrastructure.

2.4.2.  Stitching Scenarios

   This section briefly describes examples of usage for subnet
   stitching.

   Traversal through a transport network.

      Let's consider a network slice composed of (NS) subnet-A, and
      subnet-C (Figure 3).  Subnet-A and subnet-C are deployed in
      independent domains and are mapped into a slice information model;
      in order to stitch these two together a transport segment is
      needed.  N1 and N2 are anchor nodes within NS subnets A and C.
      Segment-B could be a simple link between the two NS subnets but it
      may also be a TE-link made available by a transport network
      provider.  Segment-B may be involved in the stitching operation in
      one of several ways:

         Segment-B may be set up as part of the stitching operation
         between NS subnets A and C, as a form of "bridge" mentioned in
         Section 2.4.  Segment-B will need to comply with service
         specific traffic constraints that are determined during the
         stitching operation, possibly using attributes from NS subnets
         A and C.  In this case, the data plane implementation of N1 and
         N2 in the composite slice may be, for example, 2 distinct
         gateway functions terminating segment-B.

         Segment-B may alternatively be represented as a distinct NS
         subnet, e.g. in cases where segment-B is complex and/or
         involves multiple network functions.  In this case, the
         stitching operation may therefore involve 3 NS subnets A-B-C.

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                   +-----------+                     +----------+
                   |   +--+    |      ______         |   +--+   |
                   |   |N1+==========(______)============|N2|   |
                   |   +--+    |   --transport--     |   +--+   |
                   +-----------+                     +----------+
                   --subnet-A---  --segment-B------  --subnet-C--
                   <---------------end to end slice ------------>

     Figure 3: Example of NS subnets interconnection through transport
                                  network

   Subnets in a single domain.

      In this scenario multiple network slice subnets are defined as
      basic building blocks with specific service functions (or chains),
      topologies and traffic handling characteristics.  These building
      blocks can be assembled through stitching to build end-to-end
      customized slices, but also to dynamically extend slices to adapt
      to traffic load.  Additionally, stitching can also be used to
      share building blocks between multiple slices, e.g. to
      interconnect multiple slices with a shared function.  In all these
      cases, interconnection instances may be entirely abstracted away,
      although they may also be implemented through one or multiple
      gateways, e.g. when stitched subnets belong to different sub-
      domains.

3.  Security Considerations

   Security aspects relative to network slices (e.g., for transport
   slices, in [I-D.liu-teas-transport-network-slice-yang]) are
   applicable to slice subnets, including transport security aspects,
   access control and protection of write operation on newly introduced
   nodes (e.g., termination-point).

4.  IANA Considerations

   This document has no actions for IANA.

5.  Informative References

   [I-D.clt-dmm-tn-aware-mobility]
              Chunduri, U., Li, R., Bhaskaran, S., Kaippallimalil, J.,
              Tantsura, J., Contreras, L., and P. Muley, "Transport
              Network aware Mobility for 5G", draft-clt-dmm-tn-aware-
              mobility-05 (work in progress), November 2019.

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   [I-D.contreras-teas-slice-nbi]
              Contreras, L., Homma, S., and J. Ordonez-Lucena,
              "Considerations for defining a Transport Slice NBI",
              draft-contreras-teas-slice-nbi-00 (work in progress),
              November 2019.

   [I-D.geng-teas-network-slice-mapping]
              Geng, X., Dong, J., Niwa, T., and J. Jin, "5G End-to-end
              Network Slice Mapping from the view of Transport Network",
              draft-geng-teas-network-slice-mapping-00 (work in
              progress), February 2020.

   [I-D.homma-rtgwg-slice-gateway]
              Homma, S., Foy, X., Galis, A., and L. Contreras, "Gateway
              Function for Network Slicing", draft-homma-rtgwg-slice-
              gateway-01 (work in progress), November 2019.

   [I-D.liu-teas-transport-network-slice-yang]
              Liu, X., Tantsura, J., Bryskin, I., Contreras, L., and Q.
              WU, "Transport Network Slice YANG Data Model", draft-liu-
              teas-transport-network-slice-yang-00 (work in progress),
              November 2019.

   [I-D.nsdt-teas-transport-slice-definition]
              Rokui, R., Homma, S., and K. Makhijani, "IETF Definition
              of Transport Slice", draft-nsdt-teas-transport-slice-
              definition-00 (work in progress), November 2019.

   [NGMN_Network_Slicing]
              NGMN, "Description of Network Slicing Concept", 10 2016,
              <https://www.ngmn.org/uploads/
              media/161010_NGMN_Network_Slicing_framework_v1.0.8.pdf>.

   [RFC8345]  Clemm, A., Medved, J., Varga, R., Bahadur, N.,
              Ananthakrishnan, H., and X. Liu, "A YANG Data Model for
              Network Topologies", RFC 8345, DOI 10.17487/RFC8345, March
              2018, <https://www.rfc-editor.org/info/rfc8345>.

Authors' Addresses

   Xavier de Foy
   InterDigital Inc.
   1000 Sherbrooke West
   Montreal
   Canada

   Email: Xavier.Defoy@InterDigital.com

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   Akbar Rahman
   InterDigital Inc.
   1000 Sherbrooke West
   Montreal
   Canada

   Email: Akbar.Rahman@InterDigital.com

   Alex Galis
   University College London
   Torrington Place
   London  WC1E 7JE
   United Kingdom

   Email: a.galis@ucl.ac.uk

   Kiran Makhijani
   Huawei Technologies
   2890 Central Expressway
   Santa Clara  CA 95050
   USA

   Email: kiran.makhijani@huawei.com

   Li Qiang
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: qiangli3@huawei.com

   Shunsuke Homma
   NTT, Corp.
   3-9-11, Midori-cho
   Musashino-shi, Tokyo  180-8585
   Japan

   Email: homma.shunsuke@lab.ntt.co.jp

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Internet-Draft               Network slicing                  March 2020

   Pedro Martinez-Julia
   National Institute of Information and Communications Technology
   Japan

   Email: pedro@nict.go.jp

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