SPRING Working Group J. Dong Internet-Draft Huawei Technologies Intended status: Informational S. Bryant Expires: June 6, 2021 Futurewei Technologies T. Miyasaka KDDI Corporation Y. Zhu China Telecom F. Qin Z. Li China Mobile F. Clad Cisco Systems December 3, 2020 Segment Routing based Virtual Transport Network (VTN) for Enhanced VPN draft-dong-spring-sr-for-enhanced-vpn-12 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. Resource-aware SIDs may be used to build SR paths with a set of reserved network resources. In addition, resource-aware SIDs may be used to build SR based virtual underlay networks, which can provide the customized network topology and resource attributes required by different customers and/or services. Such virtual networks are called SR based Virtual Transport Networks (VTNs). The SR based VTNs can be used as the underlay network to enable services with required topology and resource characteristics. 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 June 6, 2021 [Page 1] Internet-Draft SR for VPN+ December 2020 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 June 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 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. Scalability Considerations . . . . . . . . . . . . . . . 4 3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. VTN Topology and Resource Planning . . . . . . . . . . . 5 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 . . . . . . . . . . . . . 9 3.5. VTN Visibility to Customer . . . . . . . . . . . . . . . 10 4. Characteristics of SR based VTN . . . . . . . . . . . . . . . 10 5. Service Assurance of VTN . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 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 Dong, et al. Expires June 6, 2021 [Page 2] Internet-Draft SR for VPN+ December 2020 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. When compared with RSVP-TE [RFC3209], SR currently does not have the capability to reserve network resources or identify different sets of network resources reserved for different customers and/or services. [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). On a network segment, multiple resource-aware SIDs may be allocated, each of which represents 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 using a set of reserved network resources. In addition, a group of resource-aware SIDs can be used to build SR based virtual networks with customized network topology and resource attributes. In this document, such virtual networks are called SR based Virtual Transport Networks (VTNs), and can be used to enable services with required topology and resource characteristics, such as the enhanced VPN (VPN+) services as described in [I-D.ietf-teas-enhanced-vpn]. 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 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 to compute and instantiate SR paths with the topology constraints of the VTN, and from 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, with each SID used to identify both the network topological instruction, and the set of network resources allocated for a VTN. The mechanisms to identify the network topology or path with a SID as defined in [RFC8402] are reused. The control plane mechanisms for advertising resource-aware SIDs for different VTNs may be based on [RFC4915], [RFC5120] and Dong, et al. Expires June 6, 2021 [Page 3] Internet-Draft SR for VPN+ December 2020 [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. Similarly, for one IGP node, multiple prefix-SIDs are allocated, each of which is associated with a VTN the node participates in, and represents 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. 2.2. SRv6 based VTN This section describes a mechanism of allocating resource-aware SIDs to VTN based on SRv6. For a network node, multiple SRv6 Locators are allocated, each of which is associated with a VTN that node 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 using the VTN-specific Locators as the prefix. These SRv6 SIDs can be used to represent VTN-specific SRv6 functions which are executed using the network resources allocated to the VTN. 2.3. Scalability Considerations Note that the introduction of SR based VTNs increases the number of SIDs and SRv6 Locators needed in a network, there may be some concern, especially about the 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, and per-path network state is avoided in the SR network. Dong, et al. Expires June 6, 2021 [Page 4] Internet-Draft SR for VPN+ December 2020 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 service requirement, a centralized network controller calculates a subset of the underlay 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 an individual customer or service, or can be shared by a group of customers and/or services. 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 will be used as the virtual underlay network for the requested service. 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 information of each VTN. The detailed control plane mechanisms and possible extensions 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 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], 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 Dong, et al. Expires June 6, 2021 [Page 5] Internet-Draft SR for VPN+ December 2020 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 for the VTN accordingly. This may be done with PCEP [RFC5440], Netconf/YANG [RFC6241] [RFC7950] or with any other control 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 a VTN, a set of network resources are allocated to that VTN. Such set of network resources can be dedicated for the processing of traffic in that VTN, and cannot be used for 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. 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 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 a unified abstraction in the network layer, which can work with various network resource partition or reservation mechanisms in the underlying forwarding plane. Dong, et al. Expires June 6, 2021 [Page 6] Internet-Draft SR for VPN+ December 2020 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 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 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. The adj-SIDs can be associated with different set of link resources (e.g. bandwidth) Dong, et al. Expires June 6, 2021 [Page 7] Internet-Draft SR for VPN+ December 2020 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 a 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. 3.3. Construction of SR based VTNs The network controller needs to obtain the information of all the VTNs in the network it oversees, 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 an IGP mechanism to advertise the customized topology and resource attributes of VTN, which allows flexible combination of the virtual network topology and the network resources attribute to provide a relatively large number of 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 flexibility or scalability, the simplified control plane mechanisms based on Multi-Topology [RFC5120] or Flex-Algo [I-D.ietf-lsr-flex-algo] are described in [I-D.xie-lsr-isis-sr-vtn-mt] and [I-D.zhu-lsr-isis-sr-vtn-flexalgo] respectively. 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 resources and the associated SIDs of VTNs, both the controller and network nodes can construct the SR based VTNs and generate the forwarding tables and entries for each VTN based on the SIDs and SRv6 Locators 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 isolation between different customers and/or services in the same network. Figure 2 shows the SR based VTNs created in the network in Figure 1. Dong, et al. Expires June 6, 2021 [Page 8] Internet-Draft SR for VPN+ December 2020 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 or SRv6 End 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 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 they will not Dong, et al. Expires June 6, 2021 [Page 9] Internet-Draft SR for VPN+ December 2020 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 VTN which deliver the service. Depending on the requirement, the network can 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 connectivity between endpoints without any transit node information. The visibility may 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 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). Mechanisms such as BGP-LS allow the flexibility of the advertisement of aggregated virtual network information. 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 Dong, et al. Expires June 6, 2021 [Page 10] Internet-Draft SR for VPN+ December 2020 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 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. Dong, et al. Expires June 6, 2021 [Page 11] Internet-Draft SR for VPN+ December 2020 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, Joel Halpern and James Guichard 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, . Dong, et al. Expires June 6, 2021 [Page 12] Internet-Draft SR for VPN+ December 2020 [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, . 10.2. Informative References [DetNet] "DetNet WG", 2016, . [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. [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-09 (work in progress), November 2020. Dong, et al. Expires June 6, 2021 [Page 13] Internet-Draft SR for VPN+ December 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-26 (work in progress), November 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, . [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2", RFC 3630, DOI 10.17487/RFC3630, September 2003, . Dong, et al. Expires June 6, 2021 [Page 14] Internet-Draft SR for VPN+ December 2020 [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, . [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, . [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, . [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, . [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, . [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, . [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, . [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016, . [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, . Dong, et al. Expires June 6, 2021 [Page 15] Internet-Draft SR for VPN+ December 2020 [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, . 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 Francois Clad Cisco Systems Email: fclad@cisco.com Dong, et al. Expires June 6, 2021 [Page 16]