LSR Working Group Y. Zhu Internet-Draft China Telecom Intended status: Standards Track J. Dong Expires: 11 January 2024 Z. Hu Huawei Technologies 10 July 2023 Using Flex-Algo for Segment Routing (SR) based Virtual Transport Network (VTN) draft-zhu-lsr-isis-sr-vtn-flexalgo-06 Abstract Enhanced VPN (VPN+) aims to provide enhanced VPN services to support some existing or emerging application's needs of enhanced isolation and stringent performance requirements. VPN+ requires integration between the overlay VPN connectivity and the characteristics provided by the underlay network. A VTN is a virtual underlay network that is associated with a network topology, and is allocated with a set of dedicated or shared resources from the underlay physical network. A VTN could be used as the underlay for one or a group of VPN+ services. The topological constraints of a VTN can be defined using Flex-Algo, a mechanism to provide distributed constraint-path computation. In some network scenarios, each VTN can be associated with a unique Flex-Algo, and the set of network resources allocated to different VTNs can be instantiated as layer-2 sub-interfaces or member links of the layer-3 interfaces. This document describes the mechanisms to build the Segment Routing (SR) based VTNs using SR Flex-Algo and IGP L2 bundles with minor extensions. This document updates RFC 8668 by defining a new flag in the Parent L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV. 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/. Zhu, et al. Expires 11 January 2024 [Page 1] Internet-Draft Flex-Algo for SR VTN July 2023 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 11 January 2024. Copyright Notice Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 2. Advertisement of SR VTN Topology Attributes . . . . . . . . . 4 3. Advertisement of SR VTN Resource Attributes . . . . . . . . . 5 4. Forwarding Plane Operations . . . . . . . . . . . . . . . . . 6 5. Scalability Considerations . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 9.1. Normative References . . . . . . . . . . . . . . . . . . 8 9.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction Enhanced VPN (VPN+) is an enhancement to VPN services to support the needs of new applications, particularly including the applications that are associated with 5G services. These applications require enhanced isolation and have more stringent performance requirements than that could be provided with conventional overlay VPN techniques. Thus these properties require integration between the underlay and the overlay networks. [I-D.ietf-teas-enhanced-vpn] specifies the framework of enhanced VPN and describes the candidate component technologies in different network planes and network layers to Zhu, et al. Expires 11 January 2024 [Page 2] Internet-Draft Flex-Algo for SR VTN July 2023 realize VPN+. VPN+ may be used to deliver IETF network slice services, and will also be of use in other generic scenarios. To meet the requirement of VPN+ services, a number of virtual transport networks (VTN) can be created, each is associated with a network topology, and is allocated with a set of dedicated or shared resources from the underlay physical network, so as to meet the requirements of one or a group of VPN+ services. Another possible approach is to create a set of point-to-point paths, each with a set of network resource reserved along the path, such paths are called Virtual Transport Paths (VTPs). Although using a set of dedicated VTPs can provide similar characteristics as VTN, it has some scalability issues due to the per-path state in the network. [I-D.ietf-spring-resource-aware-segments] introduces resource awareness to Segment Routing (SR) [RFC8402]. As described in [I-D.ietf-spring-sr-for-enhanced-vpn], the resource-aware segment identifiers (SIDs) can be used to build VTNs with the required network topology and network resource attributes to support VPN+ services. With a segment routing based data plane, SIDs can be used to represent both the topology and the set of network resources allocated by network nodes to a VTN. For VTN-specific path compuation and instantiation, the SIDs of each VTN together with its associated topology and resource attributes need to be distributed in control plane. [I-D.dong-lsr-sr-enhanced-vpn] defines the IGP mechanisms and extensions to provide scalable SR based VTNs. The mechanism in [I-D.dong-lsr-sr-enhanced-vpn] allows flexible combination of the topology and resource attributes to provide a relatively large number of VTNs with relatively small number of topologies. In some network scenarios, the number of required VTNs may be small, thus a simplified solution to provide a small number of VTNs may also be desired. This document describes a mechanism to build the SR based VTNs using SR Flex-Algo [RFC9350] and IGP L2 bundle [RFC8668] with minor extensions. With this mechanism, each VTN is associated with a unique Flex-Algo, and the set of network resources allocated to different VTNs are instantiated using layer-2 sub-interfaces or layer-2 member links of the L3 interfaces. It can provide a relatively small number of VTNs, and can be considered as a transitional solution for the VTN deployment. Zhu, et al. Expires 11 January 2024 [Page 3] Internet-Draft Flex-Algo for SR VTN July 2023 This document updates [RFC8668] by defining a new flag in the Parent L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV. [RFC8668] states that all bit fields not defined in that document "MUST be set to zero on transmission and ignored on receipt". Section 3 of this document defines a new flag and specifies when it should be set and how it should be processed. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. Advertisement of SR VTN Topology Attributes [RFC9350] specifies the mechanism to provide distributed constraint- path computation, and the usage of SR-MPLS prefix-SIDs and SRv6 locators for steering traffic along the constrained paths. The Flex-Algo Definition (FAD) is the combination of calculation- type, metric-type and the topological constraints used for path computation. According to the network nodes' participation of a Flex-Algo, and the rules of including or excluding Admin Groups (i.e. colors) and Shared Risk Link Groups (SRLGs), the topology of a VTN can be described using the associated Flex-Algo. If each VTN is associated with a unique Flex-Algo, the Flex-Algo identifier could be reused as the identifier of the VTN in the control plane. With the mechanisms defined in[RFC8667] [RFC9350], SR-MPLS prefix-SID advertisement can be associated with a specific topology and a specific algorithm, which can be a Flex-Algo. This allows the nodes to use the prefix-SIDs to steer traffic along distributed computed constraint paths according to the associated Flex-Algo in a particular topology. [RFC9352] specifies the IS-IS extensions to support SRv6 data plane, in which the SRv6 locators advertisement is associated with a topology and a specific algorithm, which can be a Flex-Algo. This allows the nodes to use the SRv6 locators to steer traffic along distributed computed constraint paths according to the associated Flex-Algo in a particular topology. In addition, topology/algorithm specific SRv6 End SIDs and End.X SIDs can be used to enforce traffic over the Loop-Free Alternatives (LFA) computed backup paths. Zhu, et al. Expires 11 January 2024 [Page 4] Internet-Draft Flex-Algo for SR VTN July 2023 3. Advertisement of SR VTN Resource Attributes Each VTN can be allocated with a set of dedicated or shared network resources on different network nodes and links. In order for a network controller or the ingress nodes to perform constraint based path computation for each VTN, the resource attributes of each VTN need to be advertised. This way, the network controller or the ingress node can compute an SR-TE path in a VTN by taking both the Flex-Algo constraints and the resource attributes of the VTN into consideration. IS-IS L2 Bundle [RFC8668] was defined to advertise the link attributes of the layer-2 bundle member links. In this section, it is extended to advertise the set of network resource attributes associated with different VTNs on a layer-3 link. The layer-3 link must have the capability of partitioning the link resources into different subsets and allocate them to different VTNs it participates in. Each partition of the link resources can be instantiated as a layer-2 sub-interface, which can be seen as a virtual layer-2 member link of the layer-3 link. If the layer-3 link itself is a layer-2 link bundle, the set of link resources allocated to a specific VTN may be provided by one or multiple physical layer-2 member links. A new flag "E" (Exclusive) is defined in the flag field of the Parent L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV (25). 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |P|E| | +-+-+-+-+-+-+-+-+ E flag: When the E flag is set, it indicates each member link under the Parent L3 link is used exclusively for one VTN, and load sharing among the member links in different VTNs is not allowed. When the E flag is clear, it indicates load balancing and sharing among the member links are allowed. Zhu, et al. Expires 11 January 2024 [Page 5] Internet-Draft Flex-Algo for SR VTN July 2023 Note that legacy implementations of [RFC8668] will set the E flag to zero (clear) meaning that load balancing among component links is the default behavior. Further, when a legacy implementation receives an E flag that is set, it will ignore the flag and so will assume that load balancing among component links is allowed even when the sender has requested it to not be used. The Flex-Algo associated with the VTN can be defined that only nodes which support the E flag and the mechanisms defined in this document are included in the constraint- based path computation and packet forwarding of the VTN. For each virtual or physical layer-2 member link, the Admin Groups (AG) or Extended Admin Group (EAG) attribute MUST be advertised using the mechanisms as defined in [RFC8668]. This is for the correlation between the Flex-Algo specific forwarding entries and the layer-2 member links of the VTN. Other TE attributes as defined in [RFC5305] such as the Maximum Link Bandwidth attribute MAY also be advertised for the constraint-based path computation performed by the controller or the ingress nodes. The SR-MPLS Adj-SIDs or SRv6 End.X SIDs associated with each of the virtual or physical layer-2 member links SHOULD be advertised according to [RFC8668] and [I-D.dong-lsr-l2bundle-srv6]. In order to correlate the virtual or physical layer-2 member links with the Flex-Algo ID which is used to identify the VTN, each VTN is assigned with a unique Admin Group (AG) or Extended Admin Group (EAG), and the virtual or physical layer-2 member links associated with this VTN are configured with the AG or EAG assigned to the VTN. The AG or EAG of the parent layer-3 link SHOULD be set to the union of all the AGs or EAGs of its virtual or physical layer-2 member links. In the definition of the Flex-Algo corresponding to the VTN, it MUST use the Include-Any Admin Group rule with only the AG or EAG assigned to the VTN as the link constraints, the Include-All Admin Goup rule or the Exclude Admin Group rule MUST NOT be used for a Flex-Algo associated with VTN. This is to ensure that the layer-3 link is included in the Flex-Algo constraint based path computation for each VTN it participates in. 4. Forwarding Plane Operations For the SR-MPLS data plane, a prefix SID is associated with the paths calculated using the Flex-Algo corresponding to a VTN. An outgoing layer-3 interface is determined for each path. In addition, the prefix-SID also steers the traffic to use the virtual or physical layer-2 member link which is associated with the VTN on the outgoing layer-3 interface for packet forwarding. A forwarding entry MUST be installed in the forwarding plane using the MPLS label that corresponds to the Prefix-SID associated with the Flex-algorithm corresponding to the VTN. The Adj-SIDs associated with the virtual Zhu, et al. Expires 11 January 2024 [Page 6] Internet-Draft Flex-Algo for SR VTN July 2023 or physical member links of a VTN MAY be used together with the prefix-SIDs of the same VTN to build SR-MPLS TE paths under the topological and resource constraints of the VTN. For the SRv6 data plane, an SRv6 Locator is a prefix which is associated with the paths calculated using the Flex-Algo corresponding to a VTN. An outgoing Layer-3 interface is determined for each path. In addition, the SRv6 Locator prefix also steers the traffic to use the virtual or physical layer-2 member link which is associated with the VTN on the outgoing layer-3 interface for packet forwarding. A forwarding entry for the SRv6 Locator prefix MUST be installed in the forwarding plane for the Flex-algorithm corresponding to the VTN.The End.XU SIDs associated with the virtual or physical member links of a VTN MAY be used together with other types of SRv6 SIDs of the same VTN to build SRv6 paths under the topological and resource constraints of the VTN. 5. Scalability Considerations The mechanism described in this document assumes that each VTN is associated with a unique Flex-Algo, so that the Flex-Algo IDs can be reused to identify the VTNs in the control plane. Although this has the benefit of simplified control plane, it also has some limitations. Firstly, it means that even if multiple VTNs share the same topological constraints, they still need to be identified using different Flex-Algo IDs in the control plane, then independent path computation needs to be executed for each VTN. Secondly, the number of VTNs supported in a network may be dependent on the number of Flex-Algos supported, which is related to the number of Flex-Algos supported both in the protocol specification (which is 128) and the control plane overhead on network nodes under a spedific network scale. Thus the mechanism described in this document is applicable to network scenarios where the number of required VTN is relatively small. A detailed analysis about the VTN scalability and the possible optimizations for supporting a large number of VTNs can be found in [I-D.ietf-teas-nrp-scalability]. 6. Security Considerations This document introduces no additional security vulnerabilities to IS-IS. The mechanism proposed in this document is subject to the same vulnerabilities as any other protocol that relies on IGPs. Zhu, et al. Expires 11 January 2024 [Page 7] Internet-Draft Flex-Algo for SR VTN July 2023 7. IANA Considerations This document does not request any IANA actions. 8. Acknowledgments The authors would like to thank Zhenbin Li, Peter Psenak, Adrian Farrel and Gyan Mishra for the review and discussion of this document. 9. References 9.1. Normative References [I-D.dong-lsr-l2bundle-srv6] Dong, J. and Z. Hu, "Advertising SRv6 SIDs for Layer 2 Bundle Member Links in IGP", Work in Progress, Internet- Draft, draft-dong-lsr-l2bundle-srv6-01, 24 October 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", Work in Progress, Internet-Draft, draft-ietf- spring-resource-aware-segments-07, 31 May 2023, . [I-D.ietf-spring-sr-for-enhanced-vpn] Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li, Z., and F. Clad, "Segment Routing based Virtual Transport Network (VTN) for Enhanced VPN", Work in Progress, Internet-Draft, draft-ietf-spring-sr-for-enhanced-vpn-05, 31 May 2023, . [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+)", Work in Progress, Internet-Draft, draft-ietf-teas- enhanced-vpn-13, 7 July 2023, . Zhu, et al. Expires 11 January 2024 [Page 8] Internet-Draft Flex-Algo for SR VTN July 2023 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . [RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., Bashandy, A., Gredler, H., and B. Decraene, "IS-IS Extensions for Segment Routing", RFC 8667, DOI 10.17487/RFC8667, December 2019, . [RFC8668] Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri, M., and E. Aries, "Advertising Layer 2 Bundle Member Link Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668, December 2019, . [RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, February 2023, . [RFC9352] Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B., and Z. Hu, "IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352, February 2023, . 9.2. Informative References [I-D.dong-lsr-sr-enhanced-vpn] Dong, J., Hu, Z., Li, Z., Tang, X., Pang, R., and S. Bryant, "IGP Extensions for Scalable Segment Routing based Enhanced VPN", Work in Progress, Internet-Draft, draft- dong-lsr-sr-enhanced-vpn-08, 11 July 2022, . Zhu, et al. Expires 11 January 2024 [Page 9] Internet-Draft Flex-Algo for SR VTN July 2023 [I-D.ietf-teas-nrp-scalability] Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J., Mishra, G. S., Qin, F., Saad, T., and V. P. Beeram, "Scalability Considerations for Network Resource Partition", Work in Progress, Internet-Draft, draft-ietf- teas-nrp-scalability-02, 2 June 2023, . Authors' Addresses Yongqing Zhu China Telecom Email: zhuyq8@chinatelecom.cn Jie Dong Huawei Technologies Email: jie.dong@huawei.com Zhibo Hu Huawei Technologies Email: huzhibo@huawei.com Zhu, et al. Expires 11 January 2024 [Page 10]