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SPRING Working Group                                             J. Dong
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                               S. Bryant
Expires: January 14, 2021                         Futurewei Technologies
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                                  Y. Zhu
                                                           China Telecom
                                                                  F. Qin
                                                                   Z. Li
                                                            China Mobile
                                                                 F. Clad
                                                           Cisco Systems
                                                           July 13, 2020


        Segment Routing for Resource Guaranteed Virtual Networks
                draft-dong-spring-sr-for-enhanced-vpn-09

Abstract

   This document describes the mechanism to associate network resource
   attributes to Segment Routing Identifiers (SIDs).  Such SIDs are
   referred to as resource-aware SIDs in this document.  The resource-
   aware SIDs retain their original forwarding semantics, but with the
   additional semantics to identify the set of network resources
   available for the packet processing action.  The resource-aware SIDs
   can therefore be used to build SR paths with a set of reserved
   network resources.  In addition, the resource-aware SIDs can also be
   used to build SR based virtual networks, which provide the network
   topology and resource attributes required by customers or services.
   The proposed mechanism is applicable to both segment routing with
   MPLS data plane (SR-MPLS) and segment routing with IPv6 data plane
   (SRv6).

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 RFC 2119 [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



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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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 January 14, 2021.

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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Segment Routing with Topology and Resource Awareness  . . . .   4
     2.1.  SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  SRv6  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Control Plane Considerations  . . . . . . . . . . . . . . . .   6
   4.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Virtual Network Topology and Resource Computation . . . .   8
     4.2.  Network Resource and SID Allocation . . . . . . . . . . .   8
     4.3.  Construction of SR based Virtual Networks . . . . . . . .  10
     4.4.  Service to SR Virtual Network Mapping . . . . . . . . . .  11
     4.5.  Virtual Network Visibility to Customer  . . . . . . . . .  12
   5.  Benefits of the Proposed Mechanism  . . . . . . . . . . . . .  12
   6.  Service Assurance . . . . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18



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1.  Introduction

   Segment Routing (SR) [RFC8402] specifies a mechanism to steer packets
   through an ordered list of segments.  A segment is referred to by its
   Segment Identifier (SID).  With SR, explicit source routing can be
   achieved without introducing per-path state into the network.
   Compared with RSVP-TE [RFC3209], currently SR does not have the
   capability of reserving network resources or identifying a set of
   network resources reserved for individual services or customers.
   Although a centralized controller can have a global view of network
   state and can provision different services using different SR paths,
   in data packet forwarding it still relies on traditional DiffServ QoS
   mechanism [RFC2474] [RFC2475] to provide coarse-grained traffic
   differentiation in the network.  While such kind of mechanism may be
   sufficient for some types of services, some customers or services may
   require a set of dedicated network resources to be allocated in the
   network to achieve resource isolation from other customers/services
   in the same network.  Also note the number of such customers or
   services can be larger than the number of traffic classes available
   with DiffServ QoS.

   This document extends the SR paradigm without the need of defining
   new SID types by associating SIDs with network resource attributes.
   These resource-aware SIDs retain their original functionality, with
   the additional semantics of identifying the set of network resources
   available for the packet processing action.  On a particular network
   segment, multiple resource-aware SIDs can be allocated, each of which
   represents a subset of network resources allocated to meet the
   requirement of individual customers or services.  This mechanism is
   applicable to SR with both MPLS (SR-MPLS) and IPv6 data planes
   (SRv6).

   The proposed resource-aware SIDs can be used to build SR paths with a
   set of reserved network resources, which can be used in network
   scenarios which require to allocate a set of network resources for
   the processing of groups of service traffic.  In addition, the
   resource-aware SIDs can also be used to build SR based virtual
   networks with the required network topology and resource attributes.
   A group of resource-aware SIDs together can be used to specify the
   customized topology of a virtual network, and can further be used to
   steer the service traffic to be processed with the set of network
   resources allocated to the virtual network.  The SR based virtual
   network can be used as the underlay of enhanced VPN services as
   described in [I-D.ietf-teas-enhanced-vpn].







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2.  Segment Routing with Topology and Resource Awareness

   When SR is used as the data plane to provide different virtual
   networks in the same network, it is necessary that the SR paths are
   computed within the virtual network topology, and are instantiated
   with the set of network resources allocated to the virtual network.

   In segment routing architecture [RFC8402], several types of segments
   are defined to represent either topological or service instructions.
   A topological segment can be a node segment or an adjacency segment.
   A service segment may be associated with specific service functions
   for service chaining purposes.  This document introduces additional
   resource semantics to these existing types of SIDs, so that the SIDs
   can be used to identify both the network topology and the set of
   network resources allocated on the network segments of a virtual
   network.

   This section describes the mechanisms of using SR SIDs to identify
   the additional resource information of virtual networks or paths with
   the two SR data plane instantiations: SR-MPLS and SRv6.  The
   mechanisms to identify the network topology or forwarding path with a
   SID as defined in [RFC8402] are unchanged, and the control plane can
   be based on [RFC4915], [RFC5120] and [I-D.ietf-lsr-flex-algo].

2.1.  SR-MPLS

   As specified in [RFC8402], an IGP Adjacency Segment (Adj-SID) is an
   IGP-segment attached to a unidirectional adjacency or a set of
   unidirectional adjacencies.  An IGP Prefix segment is an IGP segment
   representing an IGP prefix, and IGP node segment is an IGP-Prefix
   segment that identifies a specific router (e.g., a loopback).  As
   described in [I-D.ietf-spring-segment-routing-central-epe] and
   [I-D.ietf-idr-bgpls-segment-routing-epe], BGP PeerAdj SID is used as
   an instruction to steer over a specific local interface towards a
   specific peer node in a peering Autonomous System (AS).  These types
   of SIDs can be extended to represent both topological elements and
   the resources allocated on a network segment.  The MPLS instantiation
   of Segment Routing is specified in [RFC8660].

   For one IGP link, multiple Adj-SIDs SHOULD be allocated, each of
   which is associated with a virtual network it participates, and MAY
   represent a subset of link resources.  Several approaches can be used
   to partition the link resource, such as [FLEXE], Layer-2 logical sub-
   interfaces, dedicated queues, etc.  The detailed mechanism of
   resource partitioning is out of scope of this document.

   Similarly, for one IGP node, multiple prefix-SIDs SHOULD be
   allocated, each of which is associated with a virtual network, and



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   MAY represent a subset of the node resource (e.g. the processing
   resources).  For one inter-domain link, multiple BGP PeerAdj SIDs
   SHOULD be allocated, each of which is associated with a specific
   virtual network, which spans multiple domains, and MAY represent a
   subset of link resource allocated on the inter-domain link.  Note
   that this per-segment resource allocation complies to the SR
   paradigm, which avoids introducing per-path state into the network.

   A group of SIDs associated with the same virtual network can be used
   to construct the SR paths (either strict or loose) to steer traffic
   of a particular service within the topology of the virtual network.
   Each SID in the SID-list MAY also represent the set of network
   resources reserved on the corresponding network segment for that
   virtual network.

   In data packet forwarding, the SIDs are used to identify the topology
   the packet belongs to, so that a topology specific next-hop can be
   determined.  In addition, the adj-SIDs MAY also be used to steer
   traffic of different services into different set of link resources.
   The prefix-SIDs MAY be used to steer traffic of different services
   into different set of node resources.  When a prefix-SID is used in
   the SID-list to build an SR loose path, the transit nodes can use the
   prefix-SID to identify the virtual network topology, and MAY process
   the packet using the local resources allocated to the virtual
   network.  Note in this case, it is RECOMMENDED that Penultimate Hop
   Popping (PHP) [RFC3031] be disabled, otherwise the inner service
   label SHOULD be used to infer the set of resources to be used on the
   egress node of the SR path.

   This mechanism requires to allocate additional prefix-SIDs and adj-
   SIDs for different virtual networks.  As the number of virtual
   network increases, the number of SIDs would increase accordingly.  It
   is expected that this mechanism is applicable to networks with a
   limited number of virtual networks.

2.2.  SRv6

   As specified in [I-D.ietf-spring-srv6-network-programming], an SRv6
   Segment Identifier (SID) is a 128-bit value which consists of a
   locator (LOC) and a function (FUNCT), optionally it may also contain
   additional arguments (ARG) immediately after the FUNCT.  The LOC of
   the SID is routable and leads to the node which instantiates that
   SID, which means the LOC can be parsed by all nodes in the network.
   The FUNCT part of the SID is an opaque identification of a local
   function bound to the SID, which means the FUNCT and ARG parts can
   only be parsed by the node which instantiates that SID.





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   In order to support multiple virtual networks in a SRv6 network, all
   the nodes (including the edge nodes and transit nodes) belonging to
   the same virtual network need to have a consistent view of the
   virtual network, and performs consistent computation and forwarding
   behavior to comply to the network topology and resource constraints.
   The mechanisms to ensure the consistency of such view can be based on
   [RFC4915], [RFC5120] and [I-D.ietf-lsr-flex-algo].  In data plane, a
   node which participates in multiple virtual networks must be able to
   distinguish packets which belong to different virtual networks.

   Taking the above into consideration, for a network node, multiple
   SRv6 LOCs SHOULD be allocated, each of which is associated with a
   virtual network topology, and MAY represent a subset of the network
   resources associated with the virtual network.  The SRv6 SIDs of a
   particular virtual network SHOULD be allocated from the SID space
   using the virtual network specific LOC as the prefix.  These SRv6
   SIDs can be used to represent virtual network specific local
   functions.

   A group of SRv6 SIDs associated with the same virtual network can be
   used to construct the SR SID-lists (either strict or loose) to steer
   the traffic of particular service within the virtual network.  Each
   SID in the SID-list MAY also represent the set of network resources
   which are reserved on a network segment.

   In data packet forwarding, the LOC part of SRv6 SID is used by
   transit nodes to identify the virtual network the packet belongs to,
   so that a virtual network specific next-hop can be determined.  The
   LOC MAY also be used to indicate the set of local network resources
   on the transit nodes to be used for the forwarding of the received
   packet.  The SRv6 segment endpoint nodes use the virtual network
   specific SRv6 SID to identify the virtual network the packet belongs
   to, and the particular local function to perform on the received
   packet.  The local SRv6 SID MAY also be used to identify the set of
   network resource to be used for executing the local function.

   This mechanism requires to allocate additional SRv6 Locators and SIDs
   for each virtual network.  As the number of virtual networks
   increases, the number of Locators and SIDs would increase
   accordingly.  It is expected that this mechanism is applicable to
   networks with a limited number of virtual networks.

3.  Control Plane Considerations

   The mechanism described in this document makes use of a centralized
   controller to collect the information about the network
   (configuration, state, routing databases, etc.) as well as the
   service information (traffic matrix, performance statistics, etc.)



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   for the planning of virtual networks.  The controller is also
   responsible for the centralized computation and optimization of the
   SR paths within different virtual networks.  The SR SIDs can be
   either explicitly provisioned by the controller, or dynamically
   allocated by network nodes then reported to the controller.  The
   interaction between the controller and the network nodes can be based
   on PCEP [RFC5440], Netconf/YANG [RFC6241] [RFC7950] and BGP-LS
   [RFC7752].  In some scenarios, extensions to some of these protocols
   is needed, which are out of the scope of this document and will be
   specified in separate documents.  In some cases, a centralized
   controller may not be used, but this would complicate the operations
   and planning therefore not suggested.

   The distributed control plane is complementary to the centralized
   controller.  A distributed control plane can be used for the
   collection and distribution of the network topology and resource
   information of the virtual networks among network nodes.  Distributed
   route computation for services within a virtual network is also
   needed.  The distributed control plane may be based on [RFC4915],
   [RFC5120], [I-D.ietf-lsr-flex-algo] or the combination of some of
   them with necessary extensions.  The details are out of the scope of
   this document.

4.  Procedures

   This section describes the procedures of creating SR based virtual
   networks and the corresponding forwarding tables and entries.

   According to the received service requirement, a centralized network
   controller calculates a subset of the physical network topology to
   support the service.  Within this topology, the set of network
   resources required on each network element is also determined.  The
   subset of network topology and network resources together constitute
   a virtual network.  Depending on the service requirement, the network
   topology and resource can be dedicated for a particular service or
   customer, or can be shared by a group of services or customers.

   Following the segment routing paradigm, the network topology and
   resource is represented using a group of dedicated SIDs.  The group
   of prefix-SIDs and adj-SIDs allocated for a virtual network will be
   used by network nodes and the network controller to construct an SR
   based virtual network, which is considered as the underlay network
   for the service.  IGP and BGP-LS needs to be extended to distribute
   the SIDs and the associated resource information of each virtual
   network.  The detailed control plane extensions are out of the scope
   of this document.





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   Suppose tenant A requests for a virtual network from the network
   operator.  The requirement is that services of tenant A has dedicated
   network resource allocated and does not experience unexpected
   interference from other services in the same network, such as other
   tenants' services in the network.  Other requirements may include the
   service topology, the required bandwidth, latency, reliability, etc.

4.1.  Virtual Network Topology and Resource Computation

   As described in section 4, a centralized network controller is
   responsible for the planning of a virtual network to meet the
   received service request.  The controller collects the information of
   network connectivity, network resources, network performance and
   other relevant network states of the underlay network.  This can be
   done using either IGP [RFC5305] [RFC3630] [RFC7471] [RFC7810] or BGP-
   LS [RFC7752] [RFC8571].

   Based on the information collected from the underlay network, the
   controller obtains the underlay network topology and the information
   about the allocated and available network resources.  When a service
   request is received from a tenant, the controller computes the subset
   of the network topology, along with the set of the resources needed
   on each network segment (e.g. links and nodes) in the topology to
   meet the tenant's service requirements, whilst maintaining the needs
   of the existing tenants that are using the same network.  The subset
   of network topology and network resource constitute a virtual
   network, which will be used as the underlay of the requested service.

4.2.  Network Resource and SID Allocation

   According to the result of virtual network planning, the network
   controller instructs the network nodes with the information of the
   virtual network identifier and the required network resources to be
   allocated to the virtual network, so that the involved network nodes
   could join the virtual network and allocate the network resource
   accordingly.  This can be done with either PCEP [RFC5440] or Netconf/
   YANG [RFC6241] [RFC7950] with necessary extensions.  The network
   resources are allocated on a per virtual network basis, and
   represented by a group of SIDs.  Such group of dedicated SIDs, e.g.
   prefix-SIDs and adj-SIDs are used to represent the virtual network
   and the network resources allocated on each network segment for this
   virtual network.

   In the underlying forwarding plane, there can be multiple ways of
   partitioning and allocating a set of network resource to a virtual
   network.  For example, [FLEXE] may be used to partition the link
   resource into different sub-channels to achieve resource isolation
   between each other.  The candidate data plane technologies to support



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   resource partitioning can be found in [I-D.ietf-teas-enhanced-vpn].
   The SR SIDs are used as a unified abstraction in network layer for
   various network resource partition and allocation mechanisms in the
   underlying forwarding plane.

     Node-SIDs:                          Node-SIDs:
       r:101                               r:102
       g:201   Adj-SIDs:                   g:202
       b:301      r:1001:1G    r:1001:1G   b:302
          +-----+ g:2001:2G    g:2001:2G +-----+
          |  A  | b:3001:1G    b:3001:1G |  B  |Adj-SIDs:
          |     +------------------------+     + r:1003:1G
 Adj-SIDs +--+--+                        +--+--+\g:2003:2G
    r:1002:1G|                     r:1002:1G|    \
    g:2002:2G|                     g:2002:2G|     \ r:1001:1G
    b:3002:3G|                     b:3002:2G|      \g:2001:2G
             |                              |       \ +-----+ Node-SIDs:
             |                              |        \+  E  |   r:105
             |                              |        /+     |   g:205
    r:1001:1G|                     r:1002:1G|       / +-----+
    g:2001:2G|                     g:2002:2G|      /r:1002:1G
    b:3001:3G|                     b:3002:2G|     / g:2002:2G
          +--+--+                        +--+--+ /
          |     |                        |     |/r:1003:1G
          |  C  +------------------------+  D  + g:2003:2G
          +-----+ r:1002:1G    r:1001:1G +-----+
     Node-SIDs:   g:2002:1G    g:2001:1G   Node-SIDs:
       r:103      b:3002:2G    b:3001:2G     r:104
       g:203                                 g:204
       b:303                                 b:304

 Figure 1. SID and resource allocation for multiple virtual networks

   Figure 1 shows an exmple of SR network to support multiple virtual
   networks.  Note that the format of the SIDs in this figure is for
   illustration, both SR-MPLS and SRv6 can be used as the data plane.
   In this example, three virtual networks: red (r) , green (g) and blue
   (b) are created to carry different services.  Both the red and green
   virtual networks consist of nodes A, B, C, D, and E with all their
   interconnecting links, whilst the blue virtual network only consists
   of nodes A, B, C and D with all their interconnecting links.  Note
   that different virtual networks may have a set of shared nodes and
   links.  On each link, a dedicated adj-SID is allocated for each
   virtual network it participates in.  In Figure 1, the notation
   x:nnnn:y means that in virtual network x, the adj-SID nnnn will steer
   the packet over a link which has bandwidth y reserved for that
   virtual network.  For example, r:1002:1G in link C->D says that the
   virtual network red has a reserved bandwidth of 1Gb/s on link C->D,



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   and will be used by packets arriving at node C with an adj-SID 1002
   at the top of the label stack.  Similarly, on each node, a dedicated
   prefix-SID is allocated for each virtual network it participates in.
   The adj-SIDs can be associated with different set of link resources
   (e.g.  bandwidth) allocated to different virtual networks, so that
   the adj-SIDs can be used to steer service traffic into different set
   of link resources in packet forwarding.  The prefix-SIDs can be
   associated with the nodal resources allocated to different virtual
   network.  In addition, the prefix-SIDs can be used to build loose SR
   path within each virtual network, in this case it can be used by the
   transit nodes to steer different service traffic into different set
   of local network resources in the forwarding plane.

4.3.  Construction of SR based Virtual Networks

   In order to make both the network controller and network nodes aware
   of the information of the virtual networks in the network, each
   network node SHOULD advertise the identifiers of the virtual networks
   it participates in, together with the group of SIDs and the
   associated resource attributes both to other nodes in the network and
   to the controller.  This can be achieved by IGP extensions in
   [I-D.dong-lsr-sr-enhanced-vpn] and BGP-LS extensions
   in[I-D.dong-idr-bgpls-sr-enhanced-vpn].

   Based on the collected information of the virtual network topology,
   the associated network resource and SIDs information, the controller
   and network nodes are able to construct the SR virtual network and
   generate the forwarding tables and entries of each virtual network
   based on the prefix-SIDs and adj-SIDs allocated for each virtual
   network.  Unlike classic segment routing in which network resources
   are shared by all the services, different SR virtual networks can be
   associated with different set of resource allocated in the underlay
   forwarding plane, so that they can be used to meet the enhanced
   service requirement and provide the required resource isolation from
   other services in the same network.

   Figure 2 shows the SR based virtual networks created in the network
   in Figure 1.













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      1001  1001                 2001  2001                 3001  3001
   101---------102            201---------202            301---------302
    |           | \1003        |           | \2003        |           |
1002|       1002|  \ 1001  2002|       2002|  \ 2001  3002|       3002|
    |           |  105         |           |  205         |           |
1001|       1002|  / 1002  2001|       2002|  / 2002  3001|       3002|
    |           | / 1003       |           | / 2003       |           |
   103---------104            203---------204            303---------304
      1002  1001                 1002  2001                 3002  3001
    Topology Red              Topology Green              Topology Blue

Figure 2.  SR based virtual networks with different groups of SIDs

   For each SR virtual network, SR paths are computed within the virtual
   network, taking its network topology and resources as constraints.
   The SR path can be an explicit path instantiated using SR policy
   [I-D.ietf-spring-segment-routing-policy], in which the SID-list is
   built only with the SIDs allocated to the virtual network.  The SR
   path can also be an IGP computed path associated with a particular
   prefix-SID of the virtual network.  Different SR paths in the same
   virtual network would share the network resources allocated to the
   virtual network, while SR paths in different virtual networks can be
   steered to use different set of network resources on the shared
   network links or nodes.  These virtual network specific SR paths
   needs to be installed in the corresponding forwarding tables.

   For example, to create an explicit path A-B-D-E in virtual network
   red in Figure 2, the SR SID list encapsulated in the service packet
   would be (1001, 1002, 1003).  For the same explicit path A-B-D-E in
   virtual network green, the SR segment list would be (2001, 2002,
   2003).  In the case where we wish to construct a loose path A-D-E in
   virtual network green, the service packet SHOULD be encapsulated with
   the SR SID list (201, 204, 205).  At node A, the packet can be sent
   towards D via either node B or C using the link and node resources
   allocated for virtual network green.  At node D the packet is
   forwarded to E using the link and node resource allocated for virtual
   network green.  Similarly, a packet to sent via loose path A-D-E in
   virtual network red would be encapsulated with segment list (101,
   104, 105).  In the case where an IGP computed path can meet the
   service requirement, the packet can be simply encapsulated with the
   node SID of egress node E in the corresponding virtual network.

4.4.  Service to SR Virtual Network Mapping

   Network services can be provisioned using customized SR virtual
   networks as the underlay network.  For example, different services
   may be provisioned in different SR virtual networks, each of which
   would use the network resources allocated to a particular virtual



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   network, so that they will not interfere with each other.  In another
   case, a group of services which have similar characteristics and
   requirement can be provisioned in the same SR virtual network, in
   this case the network resources allocated to the virtual network are
   only shared among this group of services, but will not be shared with
   other services in the network.

4.5.  Virtual Network Visibility to Customer

   The tenants of service may request different granularity of
   visibility to the network which deliver the service.  Depending on
   the requirement, the network can be exposed to the tenant either as a
   virtual network, or a set of computed paths with transit nodes, or
   simply the abstract connectivity between endpoints without any path
   information.  The visibility can be delivered through different
   possible mechanisms, such as IGPs (e.g.  IS-IS, OSPF) or BGP-LS.  In
   addition, network operator may want to restrict the visibility of the
   information it delivers to the tenant by either hiding the transit
   nodes between sites (and only delivering the endpoints connectivity),
   or by hiding portions of the transit nodes (summarizing the path into
   fewer nodes).  Mechanisms such as BGP-LS allow the flexibility of the
   advertisement of aggregated virtual network information.

5.  Benefits of the Proposed Mechanism

   The proposed mechanism provides several key characteristics:

   o  Flexibility: Multiple customized virtual networks can be created
      in a shared network to meet different tenants' connectivity and
      service requirement.  Each tenant is only aware of the topology
      and attributes of his own virtual network, and provision services
      on the virtual network instead of the physical network.  This
      provides an efficient mechanism to support network slicing.

   o  Resource Isolation: Each virtual network can have independent SR
      path computation and instantiation.  In addition, a virtual
      network can be associated with a set of network resources, which
      can avoid resource competition and performance interference from
      other services in the network.  The proposed mechanism also allows
      resource sharing between different services in the same virtual
      network, or between a group of services which are provisioned in
      different virtual networks.  This gives the operator and the
      tenants the flexibility in network planning and service
      provisioning.  The performance of critical services can be further
      ensured using the mechanisms defined in [DetNet].

   o  Scalability: The proposed mechanism seeks to achieve a balance
      between the state limitations of traditional end-to-end TE



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      mechanism and the lack of resource awareness in basic segment
      routing.  Following the segment routing paradigm, network
      resources are allocated on network segments and represented as
      SIDs, thus there is no per-flow state introduced in the network.
      Operator can choose the granularity of resource allocation to
      network segments.  In network segments where resource is scarce
      such that the service requirement may not always be met, the
      proposed approach can be used to allocate specific resources to a
      virtual network which contains such network segment.  By contrast,
      in other segment of the network where resource is considered
      plentiful, the resource may be shared between a number of virtual
      networks.  The decision to do this is in the hands of the
      operator.  Because of the segmented nature of the virtual network,
      resource aggregation is easier and more flexible than RSVP-TE
      based approach.

6.  Service Assurance

   In order to provide service assurance for services provisioned in the
   SR virtual networks, it is necessary to instrument the network at
   multiple levels.  The network operator needs to ascertain that the
   underlay network is operating correctly.  A tenant needs to ascertain
   that their services are operating correctly.  In principle these can
   use existing techniques.  These are well known problems and solutions
   either exist or are in development to address them.

   New work may be needed to instrument the virtual networks that are
   created for particular services.  Such instrumentation needs to
   operate without causing disruption to other services using the
   network.  Given the sensitivity of some applications, care needs to
   be taken to ensure that the instrumentation itself does not cause
   disruption either to the service being instrumented or to other
   services.  In case of failure or performance degradation of a service
   path in a particular virtual network, it is necessary that either
   local protection or end-to-end protection mechanism is used to switch
   to another path in the same virtual network which could meet the
   service performance requirement and does not impact other services in
   the network.

7.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.






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8.  Security Considerations

   The security considerations of segment routing are applicable to this
   document.

   The Resource-aware SIDs may be used for provisioning of SR paths or
   virtual networks to carry traffic with latency as one of the SLA
   parameters.  By disrupting the latency of such traffic an attack can
   be directly targeted at the customer application, or can be targeted
   at the network operator by causing them to violate their SLA,
   triggering commercial consequences.  Dynamic attacks of this sort are
   not something that networks have traditionally guarded against, and
   networking techniques need to be developed to defend against this
   type of attack.  By rigorously policing ingress traffic and carefully
   provisioning the resources provided to such services, this type of
   attack can be prevented.  However care needs to be taken when
   providing shared resources, and when the network needs to be
   reconfigured as part of ongoing maintenance or in response to a
   failure.

   The details of the underlay network MUST NOT be exposed to third
   parties, to prevent attacks aimed at exploiting a shared resource.

9.  Contributors

   Zhenbin Li
   Email: lizhenbin@huawei.com

   Zhibo Hu
   Email: huzhibo@huawei.com

10.  Acknowledgements

   The authors would like to thank Mach Chen, Stefano Previdi, Charlie
   Perkins, Bruno Decraene, Loa Andersson, Alexander Vainshtein and Joel
   Halpern for the valuable discussion and suggestions to this document.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.






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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

11.2.  Informative References

   [DetNet]   "DetNet WG", 2016,
              <https://datatracker.ietf.org/wg/detnet>.

   [FLEXE]    "Flex Ethernet Implementation Agreement", March 2016,
              <http://www.oiforum.com/wp-content/uploads/OIF-FLEXE-
              01.0.pdf>.

   [I-D.dong-idr-bgpls-sr-enhanced-vpn]
              Dong, J., Hu, Z., Li, Z., Tang, X., and R. Pang, "BGP-LS
              Extensions for Segment Routing based Enhanced VPN", draft-
              dong-idr-bgpls-sr-enhanced-vpn-02 (work in progress), June
              2020.

   [I-D.dong-lsr-sr-enhanced-vpn]
              Dong, J., Hu, Z., Li, Z., Tang, X., Pang, R., JooHeon, L.,
              and S. Bryant, "IGP Extensions for Segment Routing based
              Enhanced VPN", draft-dong-lsr-sr-enhanced-vpn-04 (work in
              progress), June 2020.

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
              S., and J. Dong, "BGP-LS extensions for Segment Routing
              BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
              segment-routing-epe-19 (work in progress), May 2019.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-08 (work in progress), July 2020.





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   [I-D.ietf-spring-segment-routing-central-epe]
              Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
              Afanasiev, "Segment Routing Centralized BGP Egress Peer
              Engineering", draft-ietf-spring-segment-routing-central-
              epe-10 (work in progress), December 2017.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-08 (work in progress),
              July 2020.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-16 (work in
              progress), June 2020.

   [I-D.ietf-teas-enhanced-vpn]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Networks (VPN+)
              Services", draft-ietf-teas-enhanced-vpn-05 (work in
              progress), February 2020.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.







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   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5439]  Yasukawa, S., Farrel, A., and O. Komolafe, "An Analysis of
              Scaling Issues in MPLS-TE Core Networks", RFC 5439,
              DOI 10.17487/RFC5439, February 2009,
              <https://www.rfc-editor.org/info/rfc5439>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
              <https://www.rfc-editor.org/info/rfc7471>.







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   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC7810]  Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
              Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
              RFC 7810, DOI 10.17487/RFC7810, May 2016,
              <https://www.rfc-editor.org/info/rfc7810>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8571]  Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
              C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
              IGP Traffic Engineering Performance Metric Extensions",
              RFC 8571, DOI 10.17487/RFC8571, March 2019,
              <https://www.rfc-editor.org/info/rfc8571>.

Authors' Addresses

   Jie Dong
   Huawei Technologies

   Email: jie.dong@huawei.com


   Stewart Bryant
   Futurewei Technologies

   Email: stewart.bryant@gmail.com


   Takuya Miyasaka
   KDDI Corporation

   Email: ta-miyasaka@kddi.com







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   Yongqing Zhu
   China Telecom

   Email: zhuyq8@chinatelecom.cn


   Fengwei Qin
   China Mobile

   Email: qinfengwei@chinamobile.com


   Zhenqiang Li
   China Mobile

   Email: li_zhenqiang@hotmail.com


   Francois Clad
   Cisco Systems

   Email: fclad@cisco.com





























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