Segment Routing based Virtual Transport Network (VTN) for Enhanced VPN
draft-ietf-spring-sr-for-enhanced-vpn-01

Document Type Active Internet-Draft (spring WG)
Authors Jie Dong  , Stewart Bryant  , Takuya Miyasaka  , Yongqing Zhu  , Fengwei Qin  , Zhenqiang Li  , Francois Clad 
Last updated 2021-07-12
Replaces draft-dong-spring-sr-for-enhanced-vpn
Stream Internet Engineering Task Force (IETF)
Intended RFC status (None)
Formats plain text xml pdf htmlized bibtex
Stream WG state WG Document
Document shepherd No shepherd assigned
IESG IESG state I-D Exists
Consensus Boilerplate Unknown
Telechat date
Responsible AD (None)
Send notices to (None)
SPRING Working Group                                             J. Dong
Internet-Draft                                       Huawei Technologies
Intended status: Informational                                 S. Bryant
Expires: January 13, 2022                         Futurewei Technologies
                                                             T. Miyasaka
                                                        KDDI Corporation
                                                                  Y. Zhu
                                                           China Telecom
                                                                  F. Qin
                                                                   Z. Li
                                                            China Mobile
                                                                 F. Clad
                                                           Cisco Systems
                                                           July 12, 2021

 Segment Routing based Virtual Transport Network (VTN) for Enhanced VPN
                draft-ietf-spring-sr-for-enhanced-vpn-01

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 a set of
   network resources used for executing the instruction.  Such a segment
   is called resource-aware segment.

   Resource-aware Segment Identifiers (SIDs) may be used to build SR
   paths with a set of reserved network resources.  In addition, a group
   of resource-aware SIDs may be used to build SR based virtual underlay
   networks, which has customized network topology and resource
   attributes required by one or a group of customers and/or services.
   Such virtual networks are the SR instantiations of Virtual Transport
   Networks (VTNs).

   This document describes a suggested use of 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/.

Dong, et al.            Expires January 13, 2022                [Page 1]
Internet-Draft                 SR for VPN+                     July 2021

   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 13, 2022.

Copyright Notice

   Copyright (c) 2021 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.  Resource-Aware SIDs for VTN . . . . . . . . . . . . . . . . .   3
     2.1.  SR-MPLS based VTN . . . . . . . . . . . . . . . . . . . .   4
     2.2.  SRv6 based VTN  . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  VTN Identification  . . . . . . . . . . . . . . . . . . .   5
     2.4.  Scalability Considerations  . . . . . . . . . . . . . . .   5
   3.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  VTN Topology and Resource Planning  . . . . . . . . . . .   6
     3.2.  VTN Network Resource and SID Allocation . . . . . . . . .   7
     3.3.  Construction of SR based VTNs . . . . . . . . . . . . . .   9
     3.4.  Mapping Service to SR based VTN . . . . . . . . . . . . .  11
     3.5.  VTN Visibility to Customer  . . . . . . . . . . . . . . .  11
   4.  Characteristics of SR based VTN . . . . . . . . . . . . . . .  12
   5.  Service Assurance of VTN  . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

Dong, et al.            Expires January 13, 2022                [Page 2]
Internet-Draft                 SR for VPN+                     July 2021

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.
   [I-D.ietf-spring-resource-aware-segments] proposes to extend SR by
   associating SIDs with network resource attributes (e.g. bandwidth,
   processing or storage resources).  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 network segment, multiple resource-aware
   SIDs may be allocated, each of which is associated with a subset of
   network resources assigned to meet the requirements of one or a group
   of customers and/or services.

   Once allocated, Resource-aware SIDs can be used to build SR paths
   with a set of reserved network resources.  In addition, a group of
   resource-aware SIDs may be used to build SR based virtual networks,
   which has customized network topology and resource attributes
   required by one or a group of customers and/or services.  Such
   virtual networks are the SR instantiations of Virtual Transport
   Networks (VTNs) as defined in [I-D.ietf-teas-enhanced-vpn], and can
   be used to enable the enhanced VPN (VPN+) services.

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

2.  Resource-Aware SIDs for VTN

   A VTN is a virtual underlay network which has a specific network
   topology and a subset of network resources allocated from the
   physical network.

   When SR is used as the data plane to construct VTNs in the network,
   it is necessary to compute and instantiate the SR paths with the
   topology and/or algorithm constraints of the VTN, and steer the
   traffic to only use the set of network resources allocated to the
   VTN.

   Based on the resource-aware segments defined in
   [I-D.ietf-spring-resource-aware-segments], a group of resource-aware
   SIDs can be allocated to represent the network segments of one VTN.
   These resource-aware SIDs are associated with the group of network
   resources allocated to the VTN on network nodes and links which
   participate in the VTN.  These resource-aware SIDs can also identify

Dong, et al.            Expires January 13, 2022                [Page 3]
Internet-Draft                 SR for VPN+                     July 2021

   the network topological or functional instructions associated with
   the VTN.

   The resource-aware SIDs may be allocated either by a centralized
   network controller or by network nodes.  The control plane mechanisms
   for advertising the resource-aware SIDs for 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 a mechanism of allocating resource-aware SIDs
   to SR-MPLS based VTNs.

   For one IGP link, multiple Adj-SIDs are allocated, each of which is
   associated with a VTN that link participates in, and represents a
   subset of the link resources allocated to the VTN.  For one IGP node,
   multiple prefix-SIDs are allocated, each of which is associated with
   a VTN which the node participates in, and identifies the set of
   network resources allocated to the VTN on network nodes which
   participate in the VTN.  These set of resources will be used to
   process packets which have the resource-aware SIDs as the active
   segment.

   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.

2.2.  SRv6 based VTN

   This section describes a mechanism of allocating resource-aware SRv6
   Locators and SIDs to SRv6 based VTNs.

   For a network node, multiple SRv6 Locators are allocated, each of
   which is associated with a VTN the node participates in, and
   identifies the set of network resources allocated to the VTN on
   network nodes which participate in the VTN.  The SRv6 SIDs associated
   with a VTN are allocated from the SID space using the VTN-specific
   Locator as the prefix.  These SRv6 SIDs can be used to represent VTN-
   specific SRv6 functions, and can identify the set of resources used
   by network nodes to process packets.

Dong, et al.            Expires January 13, 2022                [Page 4]
Internet-Draft                 SR for VPN+                     July 2021

2.3.  VTN Identification

   In a simple case, each VTN can be mapped to a unique topology or
   algorithm.  Then the VTNs can be distinguished by the topology ID or
   algorithm ID in control plane, and the resource-aware SIDs associated
   with a VTN can be identified using the <topology, algorithm> tuple as
   described in [RFC8402].  The number of VTNs supported in a network
   relies on the number of topologies or algorithms supported.

   In a more complicated case, multiple VTNs may be mapped to the same
   <topology, algorithm> tuple, while each is allocated with a separate
   set of network resources.  Then a new VTN Identifier (VTN-ID) in the
   control plane is needed to identify the VTN.  The resource-aware SIDs
   associated with different VTNs can be distinguished using VTN-IDs.

   In the data plane, The resource-aware SIDs are used to identify the
   VTN, and are also used to determine the forwarding instructions and
   the set of network resources used for the packet processing action.

2.4.  Scalability Considerations

   Since multiple VTNs can be created in a network, and each VTN is
   allocated with a group of resource-aware SIDs, the mechanism of SR
   based VTNs increases the number of SIDs and SRv6 Locators needed in a
   network.  There may be some concern, especially about the SR-MPLS
   prefix-SIDs, which are allocated from the Segment Routing Global
   Block (SRGB).  The amount of network state will also increase
   accordingly.  However, based on the SR paradigm, resource-aware SIDs
   and the associated network state are allocated and maintained per
   VTN, thus per-path network state is avoided in the SR network.

3.  Procedures

   This section describes possible procedures for 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.

   Suppose a virtual network is requested by some customer or service.
   One of the basic requirement is that customer or service is allocated
   with some dedicated network resource, so that it does not experience
   unexpected interference from other services in the same network.
   Other possible requirements may include the required topology,
   bandwidth, latency, reliability, etc.

   According to the received requirement, a centralized network
   controller calculates a subset of the underlay network topology to
   support the service.  With this topology, the set of network

Dong, et al.            Expires January 13, 2022                [Page 5]
Internet-Draft                 SR for VPN+                     July 2021

   resources required on each network element is also determined.  The
   subset of network topology and network resources are the two major
   characteristics of a VTN.  Depending on the service requirement, the
   network topology and network resource of this VTN can be dedicated
   for an individual customer or service, or can be shared by a group of
   customers and/or services.

   Based on the mechanisms described in section 2, a group of resource-
   aware SIDs can be allocated for the VTN.  With SR-MPLS, it is a group
   of prefix-SIDs and adj-SIDs which are allocated to identify the
   network nodes and links in the VTN, and also identify the set of
   network resources allocated on these network nodes and links for the
   VTN.  As the resource-aware SIDs can be allocated either by a
   centralized network controller or by the network nodes, control plane
   protocols such as IGP (e.g.  IS-IS or OSPF) and BGP-LS can be used to
   distribute the SIDs and the associated resource and topology
   information of a VTN to other nodes in the same VTN and also to the
   controller, so that both the network nodes and the controller can
   generate the VTN-specific forwarding table or forwarding entries
   based on the resource-aware SIDs of the VTN.  The detailed control
   plane mechanisms and possible extensions are described in separate
   documents and are out of the scope of this document.

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 on network connectivity, network
   resources, network performance and any other relevant network states
   from the underlay network.  This can be done using either IGP TE
   extensions such as [RFC5305] [RFC3630] [RFC7471] [RFC8570], and/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, and the set of the resources needed on each network
   segment (e.g. links and nodes) in the sub-topology to meet the
   service requirements, whilst maintaining the needs of the existing
   services that are using the same network.  The subset of the network
   topology and network resources will be used to constitute a VTN, and
   will be used as the virtual underlay network of the requested
   service.

Dong, et al.            Expires January 13, 2022                [Page 6]
Internet-Draft                 SR for VPN+                     July 2021

3.2.  VTN Network Resource and SID Allocation

   According to the result of VTN planning, the network controller
   instructs the set of network nodes involved to join a specific VTN
   and allocate the required set of network resources for the VTN.  This
   may be done with Netconf/YANG [RFC6241] [RFC7950] or with any other
   control or management plane mechanism with necessary extensions.
   Thus, the controller not only allocates the resources to the newly
   computed VTN but also keeps track 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
   (e.g. link bandwidth) are allocated to the VTN.  Such set of network
   resources can be dedicated for the processing of traffic in that VTN,
   and cannot be used by traffic in other VTNs.  Note it is also
   possible that a group of VTNs may share a set of network resources on
   some network segments.  A group of resource-aware SIDs, such as
   prefix-SIDs and adj-SIDs are allocated to identify both the network
   segments and the set of resources allocated on the network segments
   for the VTN.  Such group of resource-aware SIDs, e.g. prefix-SIDs and
   adj-SIDs are used as the data plane identifiers of the nodes and
   links in the VTN.

   In the underlying forwarding plane, there can be multiple ways of
   allocating a subset of network resources to a VTN.  The candidate
   data plane technologies to support resource partitioning or
   reservation can be found in [I-D.ietf-teas-enhanced-vpn].  The
   resource-aware SIDs are considered as abstract data plane identifiers
   in the network layer, which can work with various network resource
   partitioning or reservation mechanisms in the underlying forwarding
   plane.

Dong, et al.            Expires January 13, 2022                [Page 7]
Internet-Draft                 SR for VPN+                     July 2021

    Prefix-SIDs:                         Prefix-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
            |                              |       \ +-----+Prefix-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 +-----+
  Prefix-SIDs:   g:2002:1G    g:2001:1G   Prefix-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 traffic of different customers or 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
   node, a resource-aware prefix-SID is allocated for each VTN it
   participates in.  And 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
   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.  Each resource-aware adj-

Dong, et al.            Expires January 13, 2022                [Page 8]
Internet-Draft                 SR for VPN+                     July 2021

   SID can be associated with a set of link resources (e.g.  bandwidth)
   allocated to different VTNs, so that different adj-SIDs can be used
   to steer service traffic into different set of link resources in
   packet forwarding.  A resource-aware prefix-SIDs in a VTN can be
   associated with the set of network resources allocated to this VTN on
   each involved network node and link.  Thus the prefix-SIDs can be
   used to build loose SR path within a VTN, and can be used by the
   transit nodes to steer traffic into the set of local network
   resources allocated to the VTN.

3.3.  Construction of SR based VTNs

   The network controller needs to obtain the information of all the
   VTNs in the network it oversees, including the resource-aware SIDs
   and their associated network resources and topology information.
   Based on this information, the controller can have a global view of
   the VTN topology, network resources and the associated SIDs, so as to
   perform VTN-specific explicit path computation, taking both the
   topology and resource constraints of the VTN into consideration, and
   use the resource-aware SIDs to build the SID list for the explicit
   path.  The controller may also compute the shortest paths in the VTN
   based on the resource-aware prefix-SIDs.

   The network nodes also need to obtain the information of the VTNs
   they participate in, including the resource-aware SIDs and their
   associated network resources and topology information.  Based on the
   collected information, the network nodes which are the headend of a
   path can perform VTN-specific path computation, and build the SID
   list using the collected resource-aware adj-SIDs and prefix-SIDs.
   The network nodes also need to generate the forwarding entries for
   the resource-aware prefix-SIDs in each VTN they participates in, and
   associate these forwarding entries with the set of local network
   resources (e.g. a set of bandwidth on the outgoing interface)
   allocated to the corresponding VTN.

   Thus after receiving the network controller's instruction of network
   resource and SID allocation, each network node needs to advertise the
   identifier of the VTNs it participates in, the group of resource-
   aware SIDs allocated to each VTN, and the resource attributes (e.g.
   bandwidth) associated with the resource-aware SIDs in the network.
   Each resource-aware adj-SID is advertised with the set of associated
   link resources, and each resource-aware prefix-SID is advertised with
   the identifier of the associated VTN, as all the prefix-SIDs in a VTN
   are associated with the same set of network resources allocated to
   the VTN.  Note that as described in section 2.3, the VTNs can be
   identified in the control plane either using existing identifiers,
   such as the MT-ID or Flex-Algo ID, or using a newly defined VTN ID.

Dong, et al.            Expires January 13, 2022                [Page 9]
Internet-Draft                 SR for VPN+                     July 2021

   The IGP mechanisms which reuse the existing IDs such as Multi-
   Topology [RFC5120] or Flex-Algo [I-D.ietf-lsr-flex-algo] as the
   identifier of VTNs, and distribute the resource-aware SIDs and the
   associated topology and resource information are described in
   [I-D.ietf-lsr-isis-sr-vtn-mt] and [I-D.zhu-lsr-isis-sr-vtn-flexalgo]
   respectively.  The corresponding BGP-LS mechanisms which can be used
   to distribute both the intra-domain VTN information and the inter-
   domain VTN-specfic link 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.  Note that with
   these mechanisms, the number of VTNs supported relies on the number
   of topologies or algorithms supported.

   The IGP mechanisms described in [I-D.dong-lsr-sr-enhanced-vpn]
   introduce a new VTN-ID in the control plane, so that multiple VTNs
   can be mapped to the same <topology, algorithm> tuple, while each VTN
   can have different resource attributes.  This allows flexible
   combination of network topology and network resources attributes to
   build a large number of VTNs with a relatively small number of
   topologies or algorithms.  The corresponding BGP-LS mechanisms which
   can be used to distribute the intra-domain VTN information and the
   inter-domain VTN-specific link information to the controller are
   described in [I-D.dong-idr-bgpls-sr-enhanced-vpn].

   Figure 2 shows the three 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                 2002  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 or SRv6 End SID
   allocated by a node for the VTN, the IGP path computation is also
   based on the topology and/or algorithm constraints of the VTN.

Dong, et al.            Expires January 13, 2022               [Page 10]
Internet-Draft                 SR for VPN+                     July 2021

   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 are steered to use different set of
   network resources even when they traverse the same 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 network resources allocated by these nodes 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 be 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.

3.4.  Mapping Service to SR based VTN

   Network services can be provisioned using SR based VTNs as the
   virtual underlay networks.  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 their data traffic
   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

   VTNs can be used by network operators to organize and split their
   network infrastructure into different virtual underlay networks for
   different customers or services.  Some customers may also request
   different granularity of visibility to the VTN which is used to
   deliver the service.  Depending on the requirement, VTN may be
   exposed to the customer either as a virtual network with both the
   edge nodes and the intermediate nodes, or a set of paths with some of
   the transit nodes, or simply a set of virtual connections between the
   endpoints without any transit node information.  The visibility may

Dong, et al.            Expires January 13, 2022               [Page 11]
Internet-Draft                 SR for VPN+                     July 2021

   be delivered through different possible mechanisms, such as IGPs
   (e.g.  IS-IS, OSPF), BGP-LS or Netconf/YANG.  On the other hand,
   network operators may want to restrict the visibility of the underlay
   network 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).  The information of VTNs
   which are not used by the customer should also be filtered.
   Mechanisms such as BGP-LS allow the flexibility of the advertisement
   of aggregated virtual network information and configurable filtering
   policies.

4.  Characteristics of SR based VTN

   The proposed mechanism provides several key characteristics:

   o  Customization: Different 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 shared physical network.  This provides an
      practical 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
      dedicated network resources, which can avoid resource competition
      and performance interference from other VTNs or 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 which are provisioned in the same VTN.  This
      gives the operators and the customers the flexibility in network
      planning and service provisioning.  In a VTN, the performance of
      critical services can be further ensured using other mechanisms,
      e.g. those as defined in [DetNet].

   o  Scalability: The introduction of resource aware SIDs for different
      VTNs would increase the amount of SIDs and state in the network.
      While the increased network state is considered an inevitable
      price in meeting the requirements of some customers or services,
      the SR based VTN 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 in a per VTN manner and represented as SIDs, this
      ensures that there is no per-path state introduced in the network.
      In addition, operators can choose the granularity of resource
      allocation on different network segments.  In network segments

Dong, et al.            Expires January 13, 2022               [Page 12]
Internet-Draft                 SR for VPN+                     July 2021

      where resource is scarce such that the service requirement may not
      always be met, the proposed approach can be used to allocate a set
      of resources to a VTN which contains such network segment to avoid
      possible competition.  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 of VTN

   In order to provide assurance for services provisioned in the SR
   based VTNs, it is necessary to instrument the network at multiple
   levels, e.g. in both the underlay network level and the VTN level.
   The operator or the customer may also monitor and measure the
   performance of the services carried by the VTN.  In principle these
   can be achieved using existing or in development techniques in IETF.
   The detailed mechanisms are out of the scope of this document.

   In case of failure or service performance degradation happens in a
   VTN, it is necessary that some recovery mechanisms, e.g. local
   protection or end-to-end protection mechanism is used to switch the
   traffic to another path in the same VTN which could meet the service
   performance requirement.  Care must be taken that the service or path
   recovery mechanism in one VTN does not impact other VTNs in the same
   network.

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

Dong, et al.            Expires January 13, 2022               [Page 13]
Internet-Draft                 SR for VPN+                     July 2021

   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, Joel
   Halpern, James Guichard, Adrian Farrel and Shunsuke Homma for the
   valuable discussion and suggestions to this document.

10.  References

10.1.  Normative References

   [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>.

   [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-03 (work in progress),
              February 2021.

Dong, et al.            Expires January 13, 2022               [Page 14]
Internet-Draft                 SR for VPN+                     July 2021

   [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-05 (work in
              progress), February 2021.

   [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-15 (work in progress), April 2021.

   [I-D.ietf-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-ietf-lsr-isis-sr-vtn-mt-00 (work in
              progress), March 2021.

   [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-02
              (work in progress), February 2021.

   [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-11 (work in progress),
              April 2021.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Garvia, P. C., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", draft-ietf-spring-srv6-
              network-programming-28 (work in progress), December 2020.

   [I-D.ietf-teas-enhanced-vpn]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Network (VPN+)
              Services", draft-ietf-teas-enhanced-vpn-07 (work in
              progress), February 2021.

Dong, et al.            Expires January 13, 2022               [Page 15]
Internet-Draft                 SR for VPN+                     July 2021

   [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-02 (work in
              progress), January 2021.

   [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-01 (work in progress),
              February 2021.

   [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-02
              (work in progress), February 2021.

   [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>.

   [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>.

Dong, et al.            Expires January 13, 2022               [Page 16]
Internet-Draft                 SR for VPN+                     July 2021

   [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>.

   [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

Dong, et al.            Expires January 13, 2022               [Page 17]
Internet-Draft                 SR for VPN+                     July 2021

   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

   Francois Clad
   Cisco Systems

   Email: fclad@cisco.com

Dong, et al.            Expires January 13, 2022               [Page 18]