Applicability of BGP-LS with Multi-Topology (MT) for Segment Routing based Network Resource Partitions (NRP)
draft-ietf-idr-bgpls-sr-vtn-mt-07

IDR Working Group                                                 C. Xie
Internet-Draft                                                     C. Li
Intended status: Informational                             China Telecom
Expires: 7 May 2025                                              J. Dong
                                                                   Z. Li
                                                     Huawei Technologies
                                                         3 November 2024

  Applicability of BGP-LS with Multi-Topology (MT) for Segment Routing
                based Network Resource Partitions (NRP)
                   draft-ietf-idr-bgpls-sr-vtn-mt-07

Abstract

   Enhanced VPNs aim to deliver VPN services with enhanced
   characteristics to customers who have specific requirements on their
   connectivity, such as guaranteed resources, latency, or jitter.
   Enhanced VPNs require integration between the overlay VPN
   connectivity and the characteristics provided by the underlay
   network.  A Network Resource Partition (NRP) is a subset of the
   network resources and associated policies on each of a connected set
   of links in the underlay network.  An NRP could be used as the
   underlay to support one or a group of enhanced VPN services.

   When Segment Routing is used as the data plane of NRPs, each NRP can
   be allocated with a group of Segment Identifiers (SIDs) to identify
   the topology and resource attributes of network segments in the NRP.
   The association between the network topology, the network resource
   attributes and the SR SIDs may need to be distributed to a
   centralized network controller.  In some network scenarios, each NRP
   can be associated with a unique logical network topology.  This
   document describes a mechanism to distribute the information of SR
   based NRPs using BGP-Link State (BGP-LS) with Multi-Topology (MT).

Status of This Memo

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

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Advertisement of Topology Attribute for SR-based NRP  . . . .   4
     2.1.  Intra-domain Topology Advertisement . . . . . . . . . . .   4
     2.2.  Inter-Domain Topology Advertisement . . . . . . . . . . .   5
   3.  Advertisement of Resource Attribute for SR-based NRP  . . . .   6
   4.  Scalability Considerations  . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Enhanced VPNs aim to deliver VPN services with enhanced
   characteristics to customers who have specific requirements on their
   connectivity, such as guaranteed resources, latency, or jitter.
   Enhanced VPNs require integration between the overlay VPN
   connectivity and the characteristics provided by the underlay
   network.  [RFC9543] discusses the general framework, components, and
   interfaces for requesting and operating network slices using IETF
   technologies.  A network slice is considered as one target use case
   of enhanced VPNs.

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   [RFC9543] also introduces the concept of the Network Resource
   Partition (NRP), which is a subset of the buffer/queuing/scheduling
   resources and associated policies on each of a connected set of links
   in an underlay network.  An NRP can be associated with a logical
   network topology to select or specify the set of links and nodes
   involved.  [I-D.ietf-teas-enhanced-vpn] specifies the framework of
   NRP-based enhanced VPNs and describes the candidate component
   technologies in different network planes and network layers.  An NRP
   could be used as the underlay to meet the requirement of one or a
   group of enhanced VPN services.  To meet the requirement of enhanced
   VPN services, a number of NRPs can be created, each with a subset of
   network resources allocated on network nodes and links in a
   customized topology of the physical network.

   [I-D.ietf-spring-resource-aware-segments] introduces resource
   awareness to Segment Routing (SR) [RFC8402].  The resource-aware
   Segment Identifiers (SIDs) have additional semantics to identify the
   set of network resources available for the packet processing action
   associated with the SIDs.  As described in
   [I-D.ietf-spring-sr-for-enhanced-vpn], the resource- aware SIDs can
   be used to build SR-based NRPs with the required network topology and
   network resource attributes to support enhanced VPN services.  With
   SR-based data plane, SIDs can be used to represent both the
   topological instructions and a subset of network resources on the
   network nodes and links which are allocated to an NRP.

   To allow NRP-specific constraint-based path computation and/or NRP-
   specific shortest path computation to be performed by network
   controller and network nodes, the set of resource-aware SR SIDs and
   the associated topology and resource attributes of an NRP need to be
   distributed using a control plane.  When a centralized network
   controller is used for NRP-specific constraint-based path
   computation, especially when an NRP spans multiple IGP areas or
   multiple Autonomous Systems (ASes), BGP-Link State (BGP-LS) [RFC9552]
   is needed to advertise the NRP information in each IGP area or AS to
   the network controller, so that the controller could use the
   collected information to build the view of inter-area or inter-AS SR
   NRPs.

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   In some network scenarios, the required number of NRPs could be
   small, and it can be assumed that each NRP is associated with an
   independent topology and has a set of dedicated or shared network
   resources.  [I-D.ietf-lsr-isis-sr-vtn-mt] describes the IS-IS Multi-
   Topology (MT) [RFC5120] based mechanism to advertise an independent
   topology and the associated SR SIDs, together with the resource and
   Traffic Engineering (TE) attributes for each SR based NRP.  This
   document describes a mechanism to distribute the information of SR
   based NRPs to the network controller using BGP-LS with Multi-
   Topology.

2.  Advertisement of Topology Attribute for SR-based NRP

   [I-D.ietf-lsr-isis-sr-vtn-mt] describes the IS-IS Multi-Topology
   based mechanisms to distribute the topology and the SR SIDs
   associated with SR based NRPs.  This section describes the
   corresponding BGP-LS mechanism to distribute both the intra-domain
   and inter-domain topology attributes of SR based NRPs.

2.1.  Intra-domain Topology Advertisement

   Section 5.2.2.1 of [RFC9552] defines the Multi-Topology Identifier
   (MT-ID) TLV (Type 263), which can contain one or more IS-IS or OSPF
   Multi-Topology Identifiers for a link, node, or prefix.  The rules of
   the usage of MT-ID TLV is described in section 5.2.2.1 of [RFC9552]
   as follows:

   "The MT-ID TLV MAY be included as a Link Descriptor, as a Prefix
   Descriptor, or in the BGP-LS Attribute of a Node NLRI.  When included
   as a Link or Prefix Descriptor, only a single MT-ID TLV containing
   the MT-ID of the topology where the link or the prefix is reachable
   is allowed.  In case one wants to advertise multiple topologies for a
   given Link or Prefix Descriptor, multiple NLRIs MUST be generated
   where each NLRI contains a single unique MT-ID."

   [RFC9085] defines the BGP-LS extensions to carry the SR-MPLS
   information using TLVs of BGP-LS Attribute.  When Multi-Topology is
   used with the SR-MPLS data plane, topology-specific Prefix-SIDs and
   topology-specific Adjacency Segment Identifiers (Adj-SIDs) can be
   carried in the BGP-LS Attribute associated with the Prefix NLRI and
   Link NLRI respectively, the MT-ID TLV is carried in the prefix
   descriptor or link descriptor to identify the corresponding topology
   of the SIDs.

   [RFC9514] defines the BGP-LS extensions to advertise Segment Routing
   over IPv6 (SRv6) information along with their functions and
   attributes.  When Multi-Topology is used with the SRv6 data plane,
   the SRv6 Locator TLV is carried in the BGP-LS Attribute associated

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   with the Prefix NLRI, the MT-ID TLV can be carried as a Prefix
   Descriptor to identify the corresponding topology of the SRv6
   Locator.  The SRv6 End.X SIDs are carried in the BGP-LS Attribute
   associated with the Link NLRI, the MT-ID TLV can be carried in the
   link descriptor to identify the corresponding topology of the End.X
   SIDs.  The SRv6 SID NLRI is defined to advertise other types of SRv6
   SIDs, in which the SRv6 SID descriptors can include the MT-ID TLV so
   as to advertise topology-specific SRv6 SIDs.

2.2.  Inter-Domain Topology Advertisement

   [RFC9086] and [RFC9514] define the BGP-LS extensions for the
   advertisement of BGP inter-domain topology information and the BGP
   Egress Peering Segment Identifiers.  Such information could be used
   by a network controller for the computation and instantiation of
   inter-AS SR-TE paths.

   In some network scenarios, for instance, an operator's network
   consists of multiple parts such as metro area networks, backbone
   networks, or data center networks, each part being a different AS,
   there are needs to create NRPs which span multiple ASes.  The inter-
   domain NRPs could have different inter-domain connectivity, and may
   be associated with different sets of network resources in each domain
   and also on the inter-domain links.  In order to build the multi-
   domain SR based NRPs, it is necessary to advertise the topology and
   the associated BGP Peering SIDs of each NRP for inter-domain links.

   When MT-ID is used consistently in multiple domains covered by an
   NRP, the topology-specific BGP peering SIDs can be advertised with
   the MT-ID carried in the corresponding Link NLRI.  This can be
   achieved with the existing mechanisms as defined in
   [RFC9552][RFC9086] and [RFC9514].

   Depending on the requirement of inter-domain NRPs, different
   mechanisms can be used on the inter-domain connection:

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   *  One External BGP (EBGP) session between two ASes can be
      established over multiple underlying links.  In this case,
      different underlying links can be used for different inter-domain
      NRPs, which requires the links to be isolated from each other.  In
      another similar case, the EBGP session is established over a
      single link, while the network resource (e.g. bandwidth) on this
      link can be partitioned into several pieces, each of which can be
      considered as a virtual member link.  An NRP can be associated
      with one of the underlying physical or virtual member links.  In
      both cases, different BGP Peer-Adj-SIDs or SRv6 End.X SIDs need to
      be allocated to each underlying physical or virtual member link,
      and the association between the BGP Peer-Adj-SID/End.X SID and the
      MT-ID of the NRP needs to be advertised by the ASBR.

   *  For inter-domain connection between two ASes, multiple EBGP
      sessions can be established between different sets of peering
      ASBRs.  It is possible that some of these BGP sessions are used
      for one inter-domain NRP, while some other BGP sessions are used
      for another inter-domain NRP.  In this case, different BGP Peer
      Node SIDs need to be allocated to each BGP session and are
      advertised using the mechanism in [RFC9086] and [RFC9514].  The
      association between the BGP Peer Node SIDs and the MT-ID of the
      NRP also needs to be advertised by the ASBR.

   *  At the AS-level topology, different inter-domain NRPs may have
      different inter-AS connectivity.  In this case, different BGP Peer
      Set SIDs need to be allocated to represent the groups of BGP peers
      which can be used for load-balancing in each inter-domain NRP.
      The association between the BGP Peer Node SIDs and the MT-ID of
      the NRP needs to be advertised by the ASBR.

   In network scenarios where consistent allocation of MT-ID among
   multiple domains can not be achieved, the MT-ID advertised by the
   peering ASBRs of an inter-domain link could be different.  Some
   mapping mechanism may be used by the controller to match the MT-IDs
   of an inter-domain link in two directions, and concatenate the inter-
   domain topology of the NRP.  Alternatively, a globally-significant
   NRP identifier many be introduced to identify the inter-domain links
   of an NRP.  Within each domain, the MT based mechanism could be
   reused for intra-domain topology advertisement.  The detailed
   mechanism is out of the scope of this document.

3.  Advertisement of Resource Attribute for SR-based NRP

   [I-D.ietf-lsr-isis-sr-vtn-mt] specifies the mechanism to advertise
   the resource and TE attributes associated with each NRP.  This
   section describes the corresponding BGP-LS mechanisms for reporting
   NRP resource and TE attributes to network controllers.

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   The information of the network resources and TE attributes associated
   with a link of an NRP can be specified by carrying the TE Link
   attribute TLVs in BGP-LS Attribute [RFC9552], with the associated MT-
   ID carried in the corresponding Link NLRI.

   When the Maximum Link Bandwidth sub-TLV is carried in the BGP-LS
   attribute associated with the Link NLRI of an NRP, it indicates the
   amount of link bandwidth resource allocated to the corresponding NRP
   on the link.  The bandwidth allocated to an NRP can be exclusive for
   traffic in the corresponding NRP.  The advertisement of other TE
   attributes in BGP-LS for NRP is for further study.

4.  Scalability Considerations

   The mechanism described in this document assumes that each NRP is
   associated with an independent topology, and for the inter-domain
   NRPs, the MT-IDs used in the involved domains are consistent, so that
   the MT-IDs can be reused to identify the NRPs in the control plane.
   Reusing MT-ID can avoid introducing new mechanism with similar
   functionality in the control plane, while it also has some
   limitations.  For example, even if multiple NRPs share the same
   topology, each NRP still need to be identified using a unique MT-ID
   in the control plane.  Thus independent path computation needs be
   executed for each NRP.  The number of NRPs supported in a network may
   be dependent on the number of topologies supported, which is related
   to both the number of topologies supported in the protocol and the
   control plane overhead which the network could afford.  The mechanism
   described in this document is considered useful for network scenarios
   in which the required number of NRPs is small because no control
   protocol extension is required.  For network scenarios where the
   number of required NRPs is large, more scalable solution would be
   needed which may require further protocol extensions and
   enhancements.  A detailed analysis about the NRP scalability and the
   possible optimizations for supporting a large number of NRPs is
   described in [I-D.ietf-teas-nrp-scalability].

5.  Security Considerations

   This document introduces no additional security vulnerabilities to
   BGP-LS.

   The mechanism proposed in this document is subject to the same
   vulnerabilities as any other protocol that relies on BGP-LS.

6.  IANA Considerations

   This document does not request any IANA actions.

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7.  Acknowledgments

   The authors would like to thank Shunwan Zhuang, Adrian Farrel, Susan
   Hares and Jeffrey Haas for the review and discussion of this
   document.

8.  References

8.1.  Normative References

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Miyasaka, T., Zhu, Y., Qin, F., and Z. Li,
              "Introducing Resource Awareness to SR Segments", Work in
              Progress, Internet-Draft, draft-ietf-spring-resource-
              aware-segments-10, 12 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              resource-aware-segments-10>.

   [I-D.ietf-spring-sr-for-enhanced-vpn]
              Dong, J., Miyasaka, T., Zhu, Y., Qin, F., and Z. Li,
              "Segment Routing based Network Resource Partition (NRP)
              for Enhanced VPN", Work in Progress, Internet-Draft,
              draft-ietf-spring-sr-for-enhanced-vpn-08, 12 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              sr-for-enhanced-vpn-08>.

   [I-D.ietf-teas-enhanced-vpn]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Network Resource Partition (NRP) based
              Enhanced Virtual Private Networks", Work in Progress,
              Internet-Draft, draft-ietf-teas-enhanced-vpn-20, 14 June
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              teas-enhanced-vpn-20>.

   [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>.

   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

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   [RFC9086]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
              Ray, S., and J. Dong, "Border Gateway Protocol - Link
              State (BGP-LS) Extensions for Segment Routing BGP Egress
              Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
              2021, <https://www.rfc-editor.org/info/rfc9086>.

   [RFC9514]  Dawra, G., Filsfils, C., Talaulikar, K., Ed., Chen, M.,
              Bernier, D., and B. Decraene, "Border Gateway Protocol -
              Link State (BGP-LS) Extensions for Segment Routing over
              IPv6 (SRv6)", RFC 9514, DOI 10.17487/RFC9514, December
              2023, <https://www.rfc-editor.org/info/rfc9514>.

   [RFC9543]  Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S.,
              Makhijani, K., Contreras, L., and J. Tantsura, "A
              Framework for Network Slices in Networks Built from IETF
              Technologies", RFC 9543, DOI 10.17487/RFC9543, March 2024,
              <https://www.rfc-editor.org/info/rfc9543>.

   [RFC9552]  Talaulikar, K., Ed., "Distribution of Link-State and
              Traffic Engineering Information Using BGP", RFC 9552,
              DOI 10.17487/RFC9552, December 2023,
              <https://www.rfc-editor.org/info/rfc9552>.

8.2.  Informative References

   [I-D.ietf-lsr-isis-sr-vtn-mt]
              Xie, C., Ma, C., Dong, J., and Z. Li, "Applicability of
              IS-IS Multi-Topology (MT) for Segment Routing based
              Network Resource Partition (NRP)", Work in Progress,
              Internet-Draft, draft-ietf-lsr-isis-sr-vtn-mt-08, 18
              August 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lsr-isis-sr-vtn-mt-08>.

   [I-D.ietf-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., and G. S. Mishra,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-ietf-
              teas-nrp-scalability-06, 21 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              nrp-scalability-06>.

   [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>.

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Authors' Addresses

   Chongfeng Xie
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China
   Email: xiechf@chinatelecom.cn

   Cong Li
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China
   Email: licong@chinatelecom.cn

   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing
   100095
   China
   Email: jie.dong@huawei.com

   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing
   100095
   China
   Email: lizhenbin@huawei.com

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