BGP-LS with Multi-topology for Segment Routing based Virtual Transport Networks
draft-xie-idr-bgpls-sr-vtn-mt-02

Document Type Active Internet-Draft (individual)
Authors Chongfeng Xie  , Cong Li  , Jie Dong  , Zhenbin Li 
Last updated 2021-01-25
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IDR Working Group                                                 C. Xie
Internet-Draft                                                     C. Li
Intended status: Standards Track                           China Telecom
Expires: July 30, 2021                                           J. Dong
                                                                   Z. Li
                                                     Huawei Technologies
                                                        January 26, 2021

 BGP-LS with Multi-topology for Segment Routing based Virtual Transport
                                Networks
                    draft-xie-idr-bgpls-sr-vtn-mt-02

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 customized
   network topology and a set of network resource allocated from the
   physical network.  A VTN could be used as the underlay to support 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 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.  For network scenarios where each VTN can be identified
   by a unique topology ID, 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute

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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 30, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Advertisement of SR VTN Topology Attribute  . . . . . . . . .   3
     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  . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 can be provided with traditional overlay VPNs.  Thus these

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   properties require integration between the overlay connectivity and
   the characteristics provided by the underlay networks.
   [I-D.ietf-teas-enhanced-vpn] specifies the framework of enhanced VPN
   and describes the candidate component technologies in different
   network planes and layers.  An enhanced VPN can be used for 5G
   network slicing, and will also be of use in more generic scenarios.

   To meet the requirement of enhanced VPN services, a number of Virtual
   Transport Networks (VTNs) need to be created, each consists of a
   subset of the underlay network topology and a set of network
   resources allocated from the underlay network to meet the requirement
   of one or a group of VPN+ services.

   [I-D.ietf-spring-resource-aware-segments] introduces resource
   awareness to Segment Routing (SR) [RFC8402], by associating existing
   type of SIDs with network resource attributes (e.g.  bandwidth,
   processing or storage resources).  These resource-aware SIDs retain
   their original functionality, with the additional semantics of
   identifying the set of network resources available for the packet
   processing action.[I-D.dong-spring-sr-for-enhanced-vpn] describes the
   use of resource-aware segments to build SR based VTNs.  To allow the
   network controller and network nodes to perform VTN-specific explicit
   path computation and/or shortest path computation, the group of
   resource-aware SIDs allocated by network nodes to each VTN and the
   associated topology and resource attributes need to be distributed in
   the control plane.  When a centralized network controller is used for
   VTN-specific 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.

   In some network scenarios, each VTN can be identified by a unique
   topology ID [RFC5120], [I-D.xie-lsr-isis-sr-vtn-mt] describes an IGP
   mechanism to advertise the association between the topology, resource
   attributes and the SR SIDs for each VTN.  This document describes a
   mechanism to distribute the information of SR based VTNs to the
   network controller using BGP-LS with Multi-Topology.

2.  Advertisement of SR VTN Topology Attribute

   [I-D.xie-lsr-isis-sr-vtn-mt] describes the IS-IS Multi-topology based
   mechanisms to distribute the topology attributes 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.

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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.

   [I-D.ietf-idr-bgp-ls-segment-routing-ext] defines the BGP-LS
   extensions to carry the segment routing information using TLVs of
   BGP-LS Attribute.  When MTR 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 MTR 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.

   [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

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   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 will be elaborated in a future version.

2.2.  Inter-Domain Topology Advertisement

   [I-D.ietf-idr-bgpls-segment-routing-epe] and
   [I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS extensions for
   advertisement of BGP topology information between ASes and the BGP
   Peering Segment Identifiers.  Such information could be used by a
   network controller for the computation and instantiation of inter-AS
   traffic engineering SR 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 resource attribute of each VTN and the
   associated BGP Peering SIDs on the inter-domain links.

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

   o  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
      is associated with one of the 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 identifier of the VTN SHOULD be advertised by the ASBR.

   o  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 multi-domain VTN, while some other BGP sessions are used
      for another multi-domain VTN.  In this case, different BGP Peer
      Node SIDs are allocated to each BGP session and are advertised
      using the mechanism in [I-D.ietf-idr-bgpls-segment-routing-epe]
      and [I-D.ietf-idr-bgpls-srv6-ext], the association between the BGP
      Peer Node SIDs and the identifier of the VTN SHOULD be advertised
      by the ASBR.

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   o  At the AS-level topology, different multi-domain VTNs may have
      different inter-domain connectivity.  Different BGP Peer Set SIDs
      MAY be allocated to represent the groups of BGP peers which can be
      used for load-balancing in each multi-domain VTN.

   When MT-ID is used consistently in multiple ASes 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
   [RFC7752][I-D.ietf-idr-bgpls-segment-routing-epe] and
   [I-D.ietf-idr-bgpls-srv6-ext].

   In network scenarios where consistent usage of MT-ID among multiple
   domains can not be expected, a global-significant VTN-ID needs to be
   introduced to define the inter-domain topologies.  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.xie-lsr-isis-sr-vtn-mt] specifies the mechanism to advertise the
   resource information associated with each VTN.  This section
   describes the corresponding BGP-LS mechanisms.

   The information of the network resources associated with a VTN can be
   specified by carrying the TE Link attribute TLVs in BGP-LS Attribute
   [RFC7752], with the associated MT-ID carried in the corresponding
   Link NLRI.

   When 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 each VTN is for further study.

4.  Scalability Considerations

   The mechanism described in this document requires that each VTN
   mapped to an independent topology, and for the inter-domain VTNs, the
   MT-IDs used in each involved domain need to be consistent.  Reusing
   MT-IDs as the identifier of VTN can avoid introducing new identifiers
   in the control plane, while it also has some limitations.  For
   example, when multiple VTNs shares the same topology, each VTN still
   need to be identified using different MT-IDs in the control plane,
   thus independent path computation needs be executed for each VTN.
   The number of VTNs supported in a network may be dependent on the

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   number of topologies supported, which is related to the control plane
   overhead.

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.

8.  References

8.1.  Normative References

   [I-D.dong-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", draft-dong-spring-sr-for-
              enhanced-vpn-13 (work in progress), January 2021.

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
              (work in progress), June 2019.

   [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-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              daniel.bernier@bell.ca, d., and B. Decraene, "BGP Link
              State Extensions for SRv6", draft-ietf-idr-bgpls-
              srv6-ext-05 (work in progress), November 2020.

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   [I-D.ietf-idr-rfc7752bis]
              Talaulikar, K., "Distribution of Link-State and Traffic
              Engineering Information Using BGP", draft-ietf-idr-
              rfc7752bis-05 (work in progress), November 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-01
              (work in progress), January 2021.

   [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>.

   [RFC5029]  Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
              Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
              September 2007, <https://www.rfc-editor.org/info/rfc5029>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

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 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.

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   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-11
              (work in progress), October 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-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.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc8667>.

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

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   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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