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SPRING Working Group                                             J. Dong
Internet-Draft                                       Huawei Technologies
Intended status: Informational                                 S. Bryant
Expires: May 6, 2021                              Futurewei Technologies
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                                  Y. Zhu
                                                           China Telecom
                                                                  F. Qin
                                                                   Z. Li
                                                            China Mobile
                                                                 F. Clad
                                                           Cisco Systems
                                                        November 2, 2020


 Segment Routing based Virtual Transport Network (VTN) for Enhanced VPN
                draft-dong-spring-sr-for-enhanced-vpn-11

Abstract

   Segment Routing (SR) leverages the source routing paradigm.  A node
   steers a packet through an ordered list of instructions, called
   "segments".  A segment can represent topological or service based
   instructions.  A segment can further be associated with network
   resources allocated for executing the instruction.  Such a segment is
   called resource-aware SID.

   The resource-aware SIDs can be used to build SR paths with a set of
   reserved network resources.  In addition, the resource-aware SIDs can
   be used to build SR based virtual networks, which can provide the
   customized network topology and resource attributes required by
   different customers or services.  Such virtual networks are called SR
   based Virtual Transport Networks (VTNs).  The SR based VTNs can be
   used to enable the enhanced VPN (VPN+) services.  This document
   describes the mechanisms of using resource-aware SIDs to build SR
   based VTNs.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.




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   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
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   This Internet-Draft will expire on May 6, 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Resource-Aware SIDs for VTN . . . . . . . . . . . . . . . . .   3
     2.1.  SR-MPLS based VTN . . . . . . . . . . . . . . . . . . . .   4
     2.2.  SRv6 based VTN  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  VTN Topology and Resource Planning  . . . . . . . . . . .   6
     3.2.  VTN Network Resource and SID Allocation . . . . . . . . .   6
     3.3.  Construction of SR based VTNs . . . . . . . . . . . . . .   8
     3.4.  Mapping Service to SR based VTN . . . . . . . . . . . . .  10
     3.5.  VTN Visibility to Customer  . . . . . . . . . . . . . . .  10
   4.  Benefits of the Proposed Mechanism  . . . . . . . . . . . . .  10
   5.  Service Assurance . . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16







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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.  While
   compared with RSVP-TE [RFC3209], currently SR does not have the
   capability of reserving network resources or identifying different
   set of network resources reserved for different services or
   customers.  [I-D.ietf-spring-resource-aware-segments] extends the SR
   paradigm by associating SIDs with network resource attributes.  On a
   network segment, multiple resource-aware SIDs can be allocated, each
   of which represents a subset of network resources allocated to meet
   the requirement of some customers or services.

   The resource-aware SIDs can be used to build SR paths with a set of
   reserved network resources.  In addition, the resource-aware SIDs can
   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.  Such virtual networks are called virtual transport networks
   (VTNs), which can be used to enable enhanced VPN (VPN+) services as
   described in [I-D.ietf-teas-enhanced-vpn].

   This document describes the mechanism of using resource-aware SIDs to
   build SR based VTNs.  Although the procedure is illustrated using SR-
   MPLS, the proposed mechanism is applicable to both segment routing
   over MPLS data plane (SR-MPLS) and segment routing over IPv6 data
   plane (SRv6).

2.  Resource-Aware SIDs for VTN

   When SR is used as the data plane to provide multiple VTNs in one
   network, it is necessary that SR paths in a VTN are computed with the
   topology constraints, and are instantiated with the set of network
   resources allocated to the VTN.

   With the mechanism defined in
   [I-D.ietf-spring-resource-aware-segments], multiple SR SIDs can be
   allocated for each network segment, each SID is used to identify both
   the network topological instruction, and the set of network resources
   allocated on the network segment for a VTN.  The mechanisms to
   identify the network topology or path with a SID as defined in
   [RFC8402] are reused.





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   The control plane mechanisms for advertising the resource-aware SIDs
   for different VTNs can be based on [RFC4915], [RFC5120] and
   [I-D.ietf-lsr-flex-algo] with necessary extensions.  This is further
   described in section 3.3.

2.1.  SR-MPLS based VTN

   This section describes the mechanism of allocating resource-aware
   SIDs for VTNs based on SR-MPLS.

   For one IGP link, multiple Adj-SIDs are allocated, each of which is
   associated with a VTN the link participates in, and represents a
   subset of the link resources allocated to the VTN.  Similarly, for
   one IGP node, multiple prefix-SIDs are allocated, each of which is
   associated with a VTN the node participates in, and represent a
   subset of the node level processing resources allocated to the VTN.

   In the case of multi-domain VTNs, on an inter-domain link, multiple
   BGP peering SIDs [I-D.ietf-idr-bgpls-segment-routing-epe] are
   allocated, each of which is associated with a VTN which spans
   multiple domains, and represents a subset of resources allocated on
   the inter-domain link.

   This way, a group of resource-aware SIDs associated with the same VTN
   can be used to represent the VTN topology and the network resources
   allocated in data plane.  An SR SID-list built with SIDs in this
   group can be used to steer service traffic to follow a path within
   the VTN topology, and each SID in the SID-list can be used to steer
   the service traffic to be processed with the set of network resources
   allocated to the VTN.

   Note that the introduction of SR-MPLS based VTN increases the number
   of SIDs needed, there may be some concern especially for the prefix-
   SIDs, which are allocated from the Segment Routing Global Block
   (SRGB).  The amount of network state will also increase accordingly.
   While thanks to the SR paradigm, the resource-aware SIDs are
   associated with VTNs rather than paths, thus per-path state is still
   avoided in the SR network.

2.2.  SRv6 based VTN

   This section describes the mechanism of allocating resource-aware
   SIDs for VTN based on SRv6.

   For a network node, multiple SRv6 LOCs are allocated, each of which
   is associated with a VTN it participates in, and represents a subset
   of the network resources allocated by the network node to the VTN.
   The SRv6 SIDs associated with a VTN are allocated from the SID space



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   using the VTN-specific LOC as the prefix.  These SRv6 SIDs can be
   used to represent VTN-specific functions.

   A group of SRv6 SIDs associated with the same VTN can be used to
   represent the VTN topology and the network resources allocated in
   data plane.  An SRv6 SID-list built with SIDs in this group can be
   used to steer service traffic to follow a path within the VTN
   topology, and each SID in the SID-list can be used to steer the
   service traffic to be processed with the set of network resources
   allocated to the VTN.

   Note that the introduction of SRv6 based VTN increases the number of
   SRv6 Locators and SIDs needed, and the amount of network state is
   also increased.  While thanks to the SR paradigm, the resource-aware
   SIDs are associated with VTNs rather than paths, thus per-path state
   is still avoided in the SR network.

3.  Procedures

   This section describes the procedures of creating SR based VTNs and
   the corresponding forwarding tables and entries.  Although it is
   illustrated using SR-MPLS, the proposed mechanism is applicable to
   both SR-MPLS and SRv6.

   According to the received service requirement, a centralized network
   controller calculates a subset of the 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 VTN.  Depending
   on the service requirement, the network topology and resource can be
   dedicated for a individual service or customer, or can be shared by a
   group of services or customers.

   Based on the mechanisms defined in
   [I-D.ietf-spring-resource-aware-segments], the network topology and
   resources of a VTN can be represented by a group of resource-aware
   SIDs.  With SR-MPLS, a group of prefix-SIDs and adj-SIDs will be used
   by network nodes and the network controller to construct an SR based
   VTN, which is considered as the virtual underlay network for the
   service.  Control plane protocols such as IGP and BGP-LS needs to be
   extended to distribute the SIDs and the associated resource
   information of each VTN.  The detailed control plane extensions are
   out of the scope of this document.

   Suppose a virtual network is requested by some customer or service.
   The basic requirement is that customer or service is allocated with
   some dedicated network resource and does not experience unexpected
   interference from other services in the same network.  Other possible



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   requirements may include the required topology, bandwidth, latency,
   reliability, etc.

3.1.  VTN Topology and Resource Planning

   A centralized network controller can be responsible for the planning
   of a VTN to meet the received service request.  The controller needs
   to collect 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] [RFC8570] or BGP-LS [RFC7752] [RFC8571] or any
   other form of control plane signaling.

   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, the controller determines 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
   service requirements, whilst maintaining the needs of the existing
   services that are using the same network.  The subset of network
   topology and network resources constitute a VTN, which will be used
   as the virtual underlay network of the requested service.

3.2.  VTN Network Resource and SID Allocation

   According to the result of VTN planning, the network controller
   instructs the network nodes with the information of the VTN
   identifier and the required network resources to be allocated to the
   VTN, so that the involved network nodes could join the VTN and
   allocate the network resources accordingly.  This can be done with
   PCEP [RFC5440], Netconf/YANG [RFC6241] [RFC7950] or with any other
   contraol plane mechanism with necessary extensions.  Thus, the
   controller not only allocates the resources to the newly computed VTN
   but also keep trace of the remaining available resources in order to
   cope with subsequent VTN requests.

   On each network node involved in the VTN, a set of network resources
   are allocated on a per virtual network basis, and a group of
   resource-aware SIDs are allocated to represent the set of resources
   allocated on the network node and the attached links.  Such group of
   resource-aware SIDs, e.g. prefix-SIDs and adj-SIDs are used as the
   data plane identifiers of the node and links in the VTN.

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



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   other.  The candidate data plane technologies to support resource
   partitioning can be found in [I-D.ietf-teas-enhanced-vpn].  The SR
   SIDs are considered as a unified abstraction in network layer , which
   can work with 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 VTNs

   Figure 1 shows an example of providing multiple VTNs in an SR based
   network.  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 VTNs: red (r) , green (g) and blue (b) are
   created to carry different services.  Both the red and green VTNs
   consist of nodes A, B, C, D, and E with all their interconnecting
   links, whilst the blue VTN only consists of nodes A, B, C and D with
   all their interconnecting links.  Note that different VTNs may have a
   set of shared nodes and links.  On each link, a resource-aware adj-
   SID is allocated for each VTN it participates in.

   In Figure 1, the notation x:nnnn:y means that in VTN x, the adj-SID
   nnnn will steer the packet over a link which has bandwidth y reserved
   for that VTN.  For example, r:1002:1G in link C->D says that the VTN



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   red has a reserved bandwidth of 1Gb/s on link C->D, 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 resource-aware prefix-SID is
   allocated for each VTN it participates in.  The adj-SIDs can be
   associated with different set of link resources (e.g. bandwidth)
   allocated to different VTNs, 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 VTNs.  In addition, the prefix-SIDs
   can be used to build loose SR path within each VTN, in this case it
   can be used by the transit nodes to steer service traffic into the
   set of local network resources allocated to the VTN in forwarding
   plane.

3.3.  Construction of SR based VTNs

   The network controller needs to obtain the information of all the
   VTNs in the network it is in charge of, and the network nodes need to
   obtain the information of the VTNs they participate in.  To achieve
   this, each network node needs to advertise the identifiers of the
   VTNs it participates in, together with the group of SIDs and the
   associated resource attributes both to other network nodes and to the
   controller.

   [I-D.dong-lsr-sr-enhanced-vpn] defines the IGP extensions to
   advertise the customized topology and resource attributes of
   different VTNs.  The corresponding BGP-LS mechanism used to
   distribute the VTN information to the controller is described in
   [I-D.dong-idr-bgpls-sr-enhanced-vpn].

   For network scenarios which require less customization or
   scalability, the simplified mechanisms based on Multi-Topology or
   Flex-Algo are described in [I-D.xie-lsr-isis-sr-vtn-mt] and
   [I-D.zhu-lsr-isis-sr-vtn-flexalgo].  The corresponding BGP-LS
   mechanisms used to distribute the VTN information to the controller
   are described in [I-D.xie-idr-bgpls-sr-vtn-mt] and
   [I-D.zhu-idr-bgpls-sr-vtn-flexalgo] respectively.

   Based on the collected information of the topology, the allocated
   network resource information and the associated SIDs of VTN, the
   controller and network nodes are able to construct the SR based VTNs
   and generate the forwarding tables and entries for each VTN based on
   the prefix-SIDs and adj-SIDs of each VTN.  Unlike classic segment
   routing in which network resources on a network segment are shared by
   all the SR traffic, different SR VTNs can be associated with
   different set of resources allocated in the underlay forwarding
   plane, so that they can be used to provide the required resource




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   isolation between different services or customers in the same
   network.

   Figure 2 shows the SR based VTNs created in the network in Figure 1.

      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
       VTN Red                   VTN Green                   VTN Blue

         Figure 2.  SR based VTNs with different groups of SIDs

   For each SR based VTN, SR paths are computed within the VTN, taking
   the VTN 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 VTN.  The SR path can also
   be an IGP computed path associated with a prefix-SID allocated by a
   node for the VTN, the IGP computation is also based on the VTN
   constraints.  Different SR paths in the same VTN may use shared
   network resources when they use the same resource-aware SIDs
   allocated to the VTN, while SR paths in different VTNs can be steered
   to use different set of network resources over the shared network
   links or nodes.  These VTN-specific SR paths need to be installed in
   the corresponding forwarding tables.

   For example, to create an explicit path A-B-D-E in VTN 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 VTN 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 VTN 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 VTN green.  At node D
   the packet is forwarded to E using the link and node resource
   allocated for VTN green.  Similarly, a packet to sent via loose path
   A-D-E in VTN 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 prefix-
   SID of egress node E in the corresponding VTN.





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3.4.  Mapping Service to SR based VTN

   Network services can be provisioned using SR based VTN as the
   underlay network.  For example, different services may be provisioned
   in different SR based VTNs, each of which would use the network
   resources allocated to the VTN, so that they will not interfere with
   each other.  In another case, a group of services which have similar
   characteristics and requirements may be provisioned in the same VTN,
   in this case the network resources allocated to the VTN are only
   shared among this group of services, but will not be shared with
   other services in the network.  The steering of service traffic to SR
   based VTNs can be based on either local policy or the mechanisms as
   defined in [I-D.ietf-spring-segment-routing-policy].

3.5.  VTN Visibility to Customer

   The customers may request different granularity of visibility to the
   network which deliver the service.  Depending on the requirement, the
   network can be exposed to the customer 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 customer 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.

4.  Benefits of the Proposed Mechanism

   The proposed mechanism provides several key characteristics:

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

   o  Resource Isolation: The computation and instantiation of SR paths
      in one VTN can be independent from other VTNs or other services in
      the network.  In addition, a VTN 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
      service flows of the same customer, or between a group of services



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      which are provisioned in the same VTN.  This gives the operator
      and the customers 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 introduction of resource guaranteed paths or
      virtual networks would increase the amount of state to the
      network.  The proposed mechanism seeks to achieve a balance
      between the state limitations of traditional end-to-end TE
      mechanism and the lack of resource awareness in classic segment
      routing.  Following the segment routing paradigm, network
      resources are allocated on network segments per VTN 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 VTN 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 VTNs.  The decision to do this is in the hands of the operator.
      Because of the segmented nature of the SR based VTN, resource
      aggregation is easier and more flexible than RSVP-TE based
      approach.

5.  Service Assurance

   In order to provide service assurance for services provisioned in the
   SR based VTNs, 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 VTNs 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
   VTN, it is necessary that either local protection or end-to-end
   protection mechanism is used to switch to another path in the same
   VTN which could meet the service performance requirement and does not
   impact other services in the network.





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6.  IANA Considerations

   This document makes no request of IANA.

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

7.  Security Considerations

   The security considerations of segment routing and resource-aware
   SIDs are applicable to this document.

   The SR VTNs may be used carry services with specific SLA parameters.
   An attack can be directly targeted at the customer application by
   disrupting the SLA, and can be targeted at the network operator by
   causing them to violate their SLA, triggering commercial
   consequences.  By rigorously policing ingress traffic and carefully
   provisioning the resources provided to the VTN, this type of attack
   can be prevented.  However care needs to be taken when shared
   resources are provided between VTNs at some point in the network, and
   when the network needs to be reconfigured as part of ongoing
   maintenance or in response to a failure.

   The details of the underlying network should not be exposed to third
   parties, some abstraction would be needed, this is also to prevent
   attacks aimed at exploiting a shared resource between VTNs.

8.  Contributors

   Zhenbin Li
   Email: lizhenbin@huawei.com

   Zhibo Hu
   Email: huzhibo@huawei.com

9.  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.

10.  References

10.1.  Normative References







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   [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>.

10.2.  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-13 (work in progress), October 2020.

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Introducing Resource Awareness to SR
              Segments", draft-ietf-spring-resource-aware-segments-00
              (work in progress), July 2020.



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   [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-24 (work in
              progress), October 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+)
              Service", draft-ietf-teas-enhanced-vpn-06 (work in
              progress), July 2020.

   [I-D.xie-idr-bgpls-sr-vtn-mt]
              Xie, C., Li, C., Dong, J., and Z. Li, "BGP-LS with Multi-
              topology for Segment Routing based Virtual Transport
              Networks", draft-xie-idr-bgpls-sr-vtn-mt-01 (work in
              progress), July 2020.

   [I-D.xie-lsr-isis-sr-vtn-mt]
              Xie, C., Ma, C., Dong, J., and Z. Li, "Using IS-IS Multi-
              Topology (MT) for Segment Routing based Virtual Transport
              Network", draft-xie-lsr-isis-sr-vtn-mt-02 (work in
              progress), October 2020.

   [I-D.zhu-idr-bgpls-sr-vtn-flexalgo]
              Zhu, Y., Dong, J., and Z. Hu, "BGP-LS with Flex-Algo for
              Segment Routing based Virtual Transport Networks", draft-
              zhu-idr-bgpls-sr-vtn-flexalgo-00 (work in progress), March
              2020.

   [I-D.zhu-lsr-isis-sr-vtn-flexalgo]
              Zhu, Y., Dong, J., and Z. Hu, "Using Flex-Algo for Segment
              Routing based VTN", draft-zhu-lsr-isis-sr-vtn-flexalgo-01
              (work in progress), September 2020.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.







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   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

   [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


   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




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   Francois Clad
   Cisco Systems

   Email: fclad@cisco.com















































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