BGP-LS with Multi-topology for Segment Routing based Virtual Transport Networks
draft-ietf-idr-bgpls-sr-vtn-mt-01

IDR Working Group                                                 C. Xie
Internet-Draft                                                     C. Li
Intended status: Informational                             China Telecom
Expires: 17 March 2023                                           J. Dong
                                                                   Z. Li
                                                     Huawei Technologies
                                                       13 September 2022

 BGP-LS with Multi-topology for Segment Routing based Virtual Transport
                                Networks
                   draft-ietf-idr-bgpls-sr-vtn-mt-01

Abstract

   Enhanced VPN (VPN+) aims to provide enhanced VPN service to support
   some applications' needs of enhanced isolation and stringent
   performance requirements.  VPN+ requires integration between the
   overlay VPN and the underlay network.  A Virtual Transport Network
   (VTN) is a virtual underlay network which consists of a subset of the
   network topology and network resources allocated from the physical
   network.  A VTN could be used as the underlay for one or a group of
   VPN+ services.

   When Segment Routing is used as the data plane of VTNs, each VTN can
   be allocated with a group of Segment Identifiers (SIDs) to identify
   the topology and resource attributes of network segments in the VTN.
   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 network scenarios where each VTN
   can be associated with a unique logical network topology, this
   document describes a mechanism to distribute the information of SR
   based VTNs using BGP-LS with Multi-Topology.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

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

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Table of Contents

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

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1.  Introduction

   Enhanced VPN (VPN+) provides 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 stringent performance
   requirements.  VPN+ requires integration between the overlay
   connectivity and the characteristics provided by the underlay
   networks.  [I-D.ietf-teas-enhanced-vpn] specifies the framework of
   VPN+ and describes the candidate component technologies in different
   network planes and layers.  VPN+ can be used to underpin network
   slicing, and will also be of use in more generic scenarios.

   To meet the requirement of VPN+ services, a number of Virtual
   Transport Networks (VTNs) need to be created, each of which consists
   of a subset of network resources allocated from the underlay network,
   and is associated with a customized logical topology.  A VTN can be
   used to support one or a group of VPN+ services.

   [I-D.ietf-spring-resource-aware-segments] introduces resource
   awareness to Segment Routing (SR) [RFC8402].  The resource-aware 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 segments can be used to build SR based VTNs with the required
   network topology and network resource attributes to support VPN+
   services.

   To allow the VTN-specific constraint-based path computation and/or
   VTN-specific shortest path computation to be performed by network
   controller and network nodes, the group of resource-aware SIDs
   allocated by the network nodes for the VTN, together with the
   associated topology and resource attributes of the VTN need to be
   distributed in the control plane.  When a centralized network
   controller is used for VTN-specific constraint-based path
   computation, especially when a VTN spans multiple IGP areas or
   multiple Autonomous Systems (ASes), BGP-LS is needed to advertise the
   VTN 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 VTNs.

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   In some network scenarios, it is assumed that each VTN 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 IGP
   Multi-Topology (MT) [RFC5120] based mechanism to advertise the
   topology and the associated SR SIDs, together with the resource and
   TE attributes for each SR based VTN.  This document describes a
   mechanism to distribute the information of SR based VTNs to the
   network controller using BGP-LS [I-D.ietf-idr-rfc7752bis] with Multi-
   Topology.

2.  Advertisement of SR VTN Topology Attribute

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

2.1.  Intra-domain Topology Advertisement

   In section 4.2.2.1 of [I-D.ietf-idr-rfc7752bis], Multi-Topology
   Identifier (MT-ID) TLV is defined, which can contain one or more IS-
   IS or OSPF Multi-Topology IDs.  The MT-ID TLV MAY be present in a
   Link Descriptor, a Prefix Descriptor, or the BGP-LS Attribute of a
   Node NLRI.

   [RFC9085] defines the BGP-LS extensions to carry the segment routing
   information using TLVs of BGP-LS Attribute.  When Multi-Topology is
   used with SR-MPLS data plane, topology-specific prefix-SIDs and
   topology-specific 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.

   [I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS extensions to
   advertise SRv6 segments along with their functions and attributes.
   When Multi-Topology is used with SRv6 data plane, the SRv6 Locator
   TLV is carried in the BGP-LS Attribute associated with the prefix-
   NLRI, the MT-ID TLV can be carried in the 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.

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   [I-D.ietf-idr-rfc7752bis] also defines the rules of the usage of MT-
   ID TLV:

   "In 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 Descriptor or Prefix Descriptor, multiple NLRIs MUST be
   generated where each NLRI contains a single unique MT-ID."

   Editor's note: the above rules indicates that only one MT-ID is
   allowed to be carried the Link or Prefix descriptors.  When a link or
   prefix needs to be advertised in multiple topologies, multiple NLRIs
   needs to be generated to report all the topologies the link or prefix
   participates in, together with the topology-specific segment routing
   information and link attributes.  This may increase the number of BGP
   Updates needed for advertising MT-specific topology attributes, and
   may introduce additional processing burden to both the sending BGP
   speaker and the receiving network controller.  When the number of
   topologies in a network is not a small number, some optimization may
   be needed for the reporting of multi-topology information and the
   associated segment routing information in BGP-LS.  Based on the WG's
   opinion, this may be elaborated in a future version.

2.2.  Inter-Domain Topology Advertisement

   [RFC9086] and [I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS
   extensions for 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, there are needs to create VTNs which span
   multiple ASes.  The inter-domain VTNs could have different inter-
   domain connectivity, and may be associated with different set of
   network resources in each domain and also on the inter-domain links.
   In order to build the multi-domain SR based VTNs, it is necessary to
   advertise the topology and the associated BGP Peering SIDs of each
   VTN for inter-domain links.

   When MT-ID is used consistently in multiple domains covered by a VTN,
   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
   [I-D.ietf-idr-rfc7752bis][RFC9086] and [I-D.ietf-idr-bgpls-srv6-ext].

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

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   *  One 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 VTNs which requires link isolation
      between 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.  A VTN
      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 SID SHOULD be allocated to each underlying physical or
      virtual member link, the association between the BGP Peer Adj-SID/
      End.X SID and the MT-ID of the VTN SHOULD be advertised by the
      ASBR.

   *  For inter-domain connection between two ASes, multiple EBGP
      sessions can be established between different set of peering
      ASBRs.  It is possible that some of these BGP sessions are used
      for one inter-domain VTN, while some other BGP sessions are used
      for another inter-domain VTN.  In this case, different BGP Peer
      Node SIDs SHOULD be allocated to each BGP session and are
      advertised using the mechanism in [RFC9086] and
      [I-D.ietf-idr-bgpls-srv6-ext], the association between the BGP
      Peer Node SIDs and the MT-ID of the VTN SHOULD be advertised by
      the ASBR.

   *  At the AS-level topology, different inter-domain VTNs may have
      different inter-AS connectivity.  Then different BGP Peer Set SIDs
      MAY be allocated to represent the groups of BGP peers which can be
      used for load-balancing in each inter-domain VTN.  The association
      between the BGP Peer Node SIDs and the MT-ID of the VTN SHOULD be
      advertised by the ASBR.

   In network scenarios where consistent usage of MT-ID among multiple
   domains can not be achieved, a global-significant identifier MAY be
   introduced to identify the inter-domain topology of a VTN.  Within
   each domain, the MT based mechanism could be reused for intra-domain
   topology advertisement.  The detailed mechanism is specified in
   [I-D.dong-idr-bgpls-sr-enhanced-vpn].

3.  Advertisement of SR VTN Resource Attribute

   [I-D.ietf-lsr-isis-sr-vtn-mt] specifies the mechanism to advertise
   the resource and TE attributes associated with each VTN.  This
   section describes the corresponding BGP-LS mechanisms for reporting
   VTN 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 a VTN can be specified by carrying the TE Link
   attribute TLVs in BGP-LS Attribute [I-D.ietf-idr-rfc7752bis], 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 a VTN, it indicates the
   amount of link bandwidth resource allocated to the corresponding VTN
   on the link.  The bandwidth allocated to a VTN can be exclusive for
   traffic in the corresponding VTN.  The advertisement of other TE
   attributes in BGP-LS for VTN is for further study.

4.  Scalability Considerations

   The mechanism described in this document requires that each VTN is
   associated with an independent topology, and for the inter-domain
   VTNs, the MT-IDs used in all the involved domains need to be
   consistent.  Reusing MT-ID as the identifier of VTN can avoid
   introducing new mechanism with similar functionality in the control
   plane, while it also has some limitations.  For example, when
   multiple VTNs have the same topology, each VTN still need to be
   identified using a unique MT-ID in the control plane, thus
   independent path computation needs be executed for each VTN, although
   the result of computation for these VTNs would be the same.  The
   number of VTNs supported in a network may be dependent on the number
   of topologies supported, which is related to the control plane
   overhead.  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 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.

7.  Acknowledgments

   The authors would like to thank Shunwan Zhuang for the review and
   discussion of this document.

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8.  References

8.1.  Normative References

   [I-D.ietf-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              Bernier, D., and B. Decraene, "BGP Link State Extensions
              for SRv6", Work in Progress, Internet-Draft, draft-ietf-
              idr-bgpls-srv6-ext-09, 10 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-idr-bgpls-
              srv6-ext-09.txt>.

   [I-D.ietf-idr-rfc7752bis]
              Talaulikar, K., "Distribution of Link-State and Traffic
              Engineering Information Using BGP", Work in Progress,
              Internet-Draft, draft-ietf-idr-rfc7752bis-10, 10 November
              2021, <https://www.ietf.org/archive/id/draft-ietf-idr-
              rfc7752bis-10.txt>.

   [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-05, 8 September 2022,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              ietf-spring-resource-aware-segments/>.

   [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-03,
              8 September 2022, <https://www.ietf.org/archive/id/draft-
              ietf-spring-sr-for-enhanced-vpn-03.txt>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

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   [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>.

   [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>.

8.2.  Informative References

   [I-D.dong-idr-bgpls-sr-enhanced-vpn]
              Dong, J., Hu, Z., Li, Z., Tang, X., and R. Pang, "BGP-LS
              Extensions for Scalable Segment Routing based Enhanced
              VPN", Work in Progress, Internet-Draft, draft-dong-idr-
              bgpls-sr-enhanced-vpn-04, 4 March 2022,
              <https://www.ietf.org/archive/id/draft-dong-idr-bgpls-sr-
              enhanced-vpn-04.txt>.

   [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", Work in Progress, Internet-Draft, draft-ietf-
              lsr-isis-sr-vtn-mt-03, 10 July 2022,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-isis-sr-
              vtn-mt-03.txt>.

   [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", Work in Progress, Internet-Draft, draft-ietf-
              teas-enhanced-vpn-10, 6 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-teas-enhanced-
              vpn-10.txt>.

   [I-D.ietf-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J. N.,
              Mishra, G., Qin, F., Saad, T., and V. P. Beeram,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-ietf-
              teas-nrp-scalability-00, 11 July 2022,
              <https://www.ietf.org/archive/id/draft-ietf-teas-nrp-
              scalability-00.txt>.

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   [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>.

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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