IDR Working Group                                                 Y. Zhu
Internet-Draft                                             China Telecom
Intended status: Standards Track                                 J. Dong
Expires: September 10, 2020                                        Z. Hu
                                                     Huawei Technologies
                                                           March 9, 2020

   BGP-LS with Flex-Algo for Segment Routing based Virtual Transport


   Enhanced VPN (VPN+) as defined in I-D.ietf-teas-enhanced-vpn aims to
   provide enhanced VPN service to support applications's needs of
   enhanced isolation and stringent performance requirements.  VPN+
   requries integration between the overlay VPN and the underlay
   network.  A Virtual Transport Network (VTN) is a virtual network
   which consists of a subset of the network toplogy and network
   resources allocated from the underlay network.  A VTN could be used
   as the underlay for one or a group of VPN+ services.

   I-D.dong-idr-bgpls-sr-enhanced-vpn defines the BGP-LS extensions to
   distribute the information of Segment Routing (SR) based VTNs to
   external entities, such as the network controllers.  This document
   describes a simplified mechanism to distribute the information of SR
   based VTNs using BGP-LS with Flex-Algo.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   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 September 10, 2020.

Copyright Notice

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   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Advertisement of VTN Topology Attribute . . . . . . . . . . .   3
     2.1.  Intra-domain Topology Advertisement . . . . . . . . . . .   3
     2.2.  Inter-Domain Topology Advertisement . . . . . . . . . . .   4
   3.  Advertisement of VTN Resource Attribute . . . . . . . . . . .   5
   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.  These
   properties cannot be met with pure overlay networks, as they 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 layers.  An enhanced VPN can be used for 5G

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   transport network slicing, and will also be of use in other generic

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

   [I-D.dong-spring-sr-for-enhanced-vpn] specifies how segment routing
   (SR) [RFC8402] can be used to build virtual transport networks (VTNs)
   with the required network topology and network resources, which could
   be used as the underlay of enhanced VPN services.
   [I-D.dong-lsr-sr-enhanced-vpn] and [I-D.zhu-lsr-isis-sr-vtn-flexalgo]
   specifies the IGP mechanism and extensions to build a set of SR based
   VTNs.  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 inter-area or inter-
   AS SR VTNs.

   [I-D.dong-idr-bgpls-sr-enhanced-vpn] defines the BGP-LS extensions to
   distribute the information of Segment Routing (SR) based VTNs to
   external entities, such as the network controllers.  The mechanism
   described in [I-D.dong-idr-bgpls-sr-enhanced-vpn] allows flexible
   combination of the topology and resource attribute to build
   customized VTNs.  While in some network scenarios, it is assumed that
   each VTN has an independent topology and a set of dedicated network
   resources.  For those scenarios, this document describes a simplified
   mechanism to distribute the information of SR based VTNs using BGP-LS
   with Flex-Algo.

2.  Advertisement of VTN Topology Attribute

   [I-D.zhu-lsr-isis-sr-vtn-flexalgo] describes the mechanism of using
   ISIS Flex-Algo to distribute the topology constraints 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

   The Flex-Algo definition [I-D.ietf-lsr-flex-algo] can be used to
   describe the topological constraints for path computation on a
   network topology.  As specified in
   [I-D.zhu-lsr-isis-sr-vtn-flexalgo], the topology of a VTN can be
   determined by applying Flex-Algo on a default topology.

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   BGP-LS extensions for Flex-Algo [I-D.ietf-idr-bgp-ls-flex-algo]
   provide the mechanisms to advertise the Flex-Algo definition
   information.  BGP-LS extensions for SR-MPLS
   [I-D.ietf-idr-bgp-ls-segment-routing-ext] and SRv6
   [I-D.ietf-idr-bgpls-srv6-ext] provide the mechanism to advertise the
   algorithm-specific segment routing information.

   In[I-D.ietf-idr-bgp-ls-segment-routing-ext], algorithm-specific
   prefix-SIDs can be advertised in BGP-LS attribute associated with
   Prefix NLRI.

   In [I-D.ietf-idr-bgpls-srv6-ext], algorithm-specific SRv6 Locators
   can be advertised in the Prefix NLRI with the SRv6 Locator TLV
   carried in the associated BGP-LS Attribute, and algorithm-specific
   End.X SID can be advertised in BGP-LS Attribute associated with the
   corresponding Link NLRI.  Other types of SRv6 SIDs can also be
   algorithm-specific and are advertised using the SRv6 SID NLRI .

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 VTNs using segment routing, it is
   necessary to advertise the topology and resource attribute of VTN on
   the inter-domain links and the associated BGP Peering SIDs.

   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.  In both
      cases, different BGP Peer-Adj-SIDs SHOULD be allocated to each
      underlying physical or virtual member link, and ASBRs SHOULD

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      advertise the VTN identifier associated with each BGP Peer-Adj-

   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 ASBRs SHOULD
      advertise the VTN identifier associated with each BGP Peer-node-

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

   When Flex-Algo is used consistently in multiple ASes covered by a
   VTN, the topology-specific BGP peering SIDs can be advertised
   together with the admin-group (color) of the corresponding Flex-Algo
   in the BGP-LS attribute.

   In network scenarios where consistent usage of Flex-Algo among
   multiple ASes can not be expected, then the global-significant VTN-ID
   can be used to define the AS level topologies.  Within each domain,
   the Flex-Algo based mechanism could be used for intra-domain topology
   advertisement.  The detailed mechanism is specified in

3.  Advertisement of VTN Resource Attribute

   [I-D.zhu-lsr-isis-sr-vtn-flexalgo] specifies the mechanism to
   advertise the resource information associated with each VTN.  It is
   based on the extensions to the advertisement of L2 bundle member
   links information[RFC8668].  This section defines the corresponding
   BGP-LS extensions.

   A new TLVs is defined to specify the attribute flags of either a
   Layer 3 link or a L2 bundle member link.  It can be carried in BGP-LS
   attribute which is associated with a Link NLRI, or it could be
   carried as a sub-TLV in the L2 Bundle Member Attribute TLV.  The
   format of the sub-TLV is as below:

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |              Type             |             Length            |
      |             Flags             |


      Type: TBD

      Length: 4 octets.

      Flags: 16-bit flags.  This field is consistent with the Flag field
      in IS-IS Link Attribute sub-TLV in [RFC5029].  In addition to the
      flags defined in [RFC5029], A new Flag V is defined in this
      document.  When the V flag is set, it indicates this link is a
      virtual link.

   In order to associate different virtual or physical bundle member
   links with the corresponding VTNs, each member link SHOULD be
   assigned with a dedicated admin-group or extended admin-group
   (color), which is included in the definition of the corresponding
   Flex-Algo.  Note that in this case the admin-group or extended admin-
   group of the Layer 3 interface SHOULD be set to the union of all the
   admin-groups of the virtual or physical member links.  This is to
   ensure that the Layer 3 link will be included in the constraint-based
   computation of the corresponding Flex-Algo.

   The TE attributes of each Layer 3 link or Layer 2 bundle member link,
   such as the bandwidth, the adj-SIDs or the End.X SIDs, can be
   advertised using the mechanism as defined in [I-D.ietf-idr-bgp-ls-seg
   ment-routing-ext][I-D.ietf-idr-bgpls-segment-routing-epe] and

4.  Scalability Considerations

   The mechanism described in this document requires that each VTN maps
   to an independent Flex-Algo.  Even if multiple VTNs share the same
   topology constraints.  While this brings the benefits of simplicity,
   it also has some limitations.  For example, it means that even if
   multiple VTNs have the same topology constraints, they would still
   need to be identified using different Flex-Algos in the control
   plane.  Then this requires that for each VTN, independent path
   computation would be executed.  The number of VTNs supported in a
   network may be dependent on the number of Flex-Algos supported, which
   is related to the control plane computation overhead.

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   Another aspect which may impact the number of VTNs supported is that
   at most 128 Flex-Algos can be defined in a network.

5.  Security Considerations

   This document introduces no additional security vulnerabilities to

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

6.  IANA Considerations


7.  Acknowledgments

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

8.  References

8.1.  Normative References

              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., and
              Z. Li, "Segment Routing for Resource Partitioned Virtual
              Networks", draft-dong-spring-sr-for-enhanced-vpn-06 (work
              in progress), December 2019.

              Talaulikar, K., Psenak, P., Zandi, S., and G. Dawra,
              "Flexible Algorithm Definition Advertisement with BGP
              Link-State", draft-ietf-idr-bgp-ls-flex-algo-02 (work in
              progress), January 2020.

              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.

              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.

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              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
    , d., and B. Decraene, "BGP Link
              State Extensions for SRv6", draft-ietf-idr-bgpls-
              srv6-ext-02 (work in progress), January 2020.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5029]  Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
              Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
              September 2007, <>.

   [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,

   [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, <>.

8.2.  Informative References

              Dong, J. and Z. Hu, "BGP-LS Extensions for Segment Routing
              based Enhanced VPN", draft-dong-idr-bgpls-sr-enhanced-
              vpn-00 (work in progress), November 2019.

              Dong, J., Hu, Z., and S. Bryant, "IGP Extensions for
              Segment Routing based Enhanced VPN", draft-dong-lsr-sr-
              enhanced-vpn-02 (work in progress), November 2019.

              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-06 (work in progress), February 2020.

              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-06
              (work in progress), March 2020.

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              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Networks (VPN+)
              Services", draft-ietf-teas-enhanced-vpn-05 (work in
              progress), February 2020.

              Zhu, Y., Dong, J., and Z. Li, "Using Flex-Algo for Segment
              Routing based VTN", March 2020,

   [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, <>.

Authors' Addresses

   Yongqing Zhu
   China Telecom


   Jie Dong
   Huawei Technologies


   Zhibo Hu
   Huawei Technologies


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