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Versions: 00 01

LSR Working Group                                                 Y. Zhu
Internet-Draft                                             China Telecom
Intended status: Standards Track                                 J. Dong
Expires: March 15, 2021                                            Z. Hu
                                                     Huawei Technologies
                                                      September 11, 2020


             Using Flex-Algo for Segment Routing based VTN
                 draft-zhu-lsr-isis-sr-vtn-flexalgo-01

Abstract

   As defined in I-D.ietf-teas-enhanced-vpn, enhanced VPN (VPN+) aims to
   provide enhanced VPN service to support the 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 network which consists
   of a subset of network topology 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-lsr-sr-enhanced-vpn defines the IGP mechanisms with
   necessary extensions to build a set of Segment Routing (SR) based
   VTNs.  This document describes a simplified mechanism to build the SR
   based VTNs using SR Flex-Algo together with minor extensions to IGP
   L2 bundle.

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
   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
   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."



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   This Internet-Draft will expire on March 15, 2021.

Copyright Notice

   Copyright (c) 2020 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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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Advertisement of SR VTN Topology Attribute  . . . . . . . . .   3
   3.  Advertisement of SR VTN Resource Attribute  . . . . . . . . .   4
   4.  Forwarding Plane Operations . . . . . . . . . . . . . . . . .   5
   5.  Scalability Considerations  . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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
   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 layers.  An enhanced VPN may be used for
   5G transport network slicing, and will also be of use in other
   generic scenarios.

   To meet the requirement of enhanced VPN services, a number of virtual
   transport networks (VTN) can be created, each with a subset of the



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   underlay network topology and a set of network resources allocated
   from the underlay network to meet the requirement of a specific VPN+
   service 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.dong-spring-sr-for-enhanced-vpn], the resource-aware SIDs can be
   used to build virtual transport networks (VTNs) with the required
   network topology and network resource attributes to support enhanced
   VPN services.  With segment routing based data plane, Segment
   Identifiers (SIDs) can be used to represent both the topology and the
   set of network resources allocated by network nodes to a virtual
   network.  The SIDs of each VTN and the associated topology and
   resource attributes need to be distributed using control plane.

   [I-D.dong-lsr-sr-enhanced-vpn] defines the IGP mechanisms with
   necessary extensions to build a set of Segment Routing (SR) based
   VTNs.  The VTNs could be used as the underlay of the enhanced VPN
   service.  The mechanism described in [I-D.dong-lsr-sr-enhanced-vpn]
   allows flexible combination of the topology and resource attribute to
   build customized VTNs.  In some network scenarios, it is assumed that
   each VTN can have an independent topology and a set of dedicated
   network resources, and the number of the VTNs required is limited.
   This document describes a simplified mechanism to build the SR based
   VTNs in those scenarios.

2.  Advertisement of SR VTN Topology Attribute

   [I-D.ietf-lsr-flex-algo] 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 consists of the topological constraints 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), thus the topology
   attribute of a VTN can be described using the associated Flex-Algo.
   And when each VTN has an independent set of network topology
   constraints, the Flex-Algo identifier could be reused as the
   identifier of the VTN in control plane.





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   With the mechanisms defined in[RFC8667] [I-D.ietf-lsr-flex-algo], 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 identified
   Flex-Algo in the associated topology.

   [I-D.ietf-lsr-isis-srv6-extensions] specifies the IS-IS extensions to
   support SRv6 data plane, in which the SRv6 locators advertisement can
   be associated with a specific topology and a specific algorithm,
   which can be a Flex-Algo.  This allows the nodes to used the SRv6
   locators to steer traffic along distributed computed constraint paths
   according to the identified Flex-Algo in the associated 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.

3.  Advertisement of SR VTN Resource Attribute

   Each VTN may have customized network resource attributes.  In order
   to perform constraint based path computation for each VTN on network
   controller and the ingress nodes, the resource attribute of each VTN
   also needs to be advertised.

   [RFC8668] was defined to advertise the link attributes of the Layer 2
   bundle member links.  In this section, it is extended to advertise
   the network resource attributes associated with different VTNs on a
   Layer 3 link.

   The Layer 3 link may or may not be a Layer 2 link bundle, as long as
   it has the capability of allocating different subsets of link
   resources to different VTNs it participates in.  A subset of the link
   resource can be considered as a virtual Layer 2 member link (or sub-
   interface) of the Layer 3 link.  If the Layer 3 interface is a Layer
   2 link bundle, it is possible that the subset of link resource
   allocated to a specific VTN is provided by one of the physical Layer
   2 member links.

   A new flag "V" (Virtual) 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|V|           |
               +-+-+-+-+-+-+-+-+

   V flag: When the V flag is set, it indicates the advertised member
   links under the Parent L3 link are virtual Layer 2 member links.



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   When the V flag is clear, it indicates the member links are physical
   member links.

   For each virtual or physical member link, the TE attributes defined
   in [RFC5305] such as the Maximum Link Bandwidth and Admin Groups
   SHOULD be advertised using the mechanism as defined in [RFC8668].
   The Adj-SIDs or SRv6 End.X SIDs associated with each of the virtual
   or physical member links of an L2 bundle SHOULD also be advertised.

   In order to correlate the virtual or physical member links with the
   corresponding VTNs, each member link SHOULD be assigned with a
   dedicated Admin Group or Extended Admin Group, which is included in
   the definition of the Flex-Algo of the corresponding VTN.  Note that
   in this case the Admin Group or Extended Admin Group of the Layer 3
   link SHOULD be set to the union of all the Admin Groups or Extended
   Admin Groups of its virtual or physical member links.  This is to
   ensure that the Layer 3 link is always included in the Flex-Algo
   specific constraint path computation of the VTNs it participates in.

4.  Forwarding Plane Operations

   For SR-MPLS data plane, a prefix SID is associated with the paths
   calculated using the corresponding Flex-Algo of 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
   member link which is associated with the VTN on the outgoing Layer 3
   interface for packet forwarding.  The Adj-SIDs associated with the
   virtual or physical member links of a VTN MAY be used with the
   prefix-SIDs of the same VTN together to build SR-MPLS paths with the
   topological and resource constraints of the VTN.

   For SRv6 data plane, an SRv6 Locator is a prefix which is associated
   with the paths calculated using the corresponding Flex-Algo of 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 member link which is associated with the VTN on
   the outgoing Layer 3 interface for packet forwarding.  The End.X SIDs
   associated with the virtual or physical member links of a VTN MAY be
   used with the SRv6 Locator prefix of the same VTN together to build
   SRv6 paths with the topological and resource constraints of the VTN.

5.  Scalability Considerations

   The mechanism described in this document requires that each VTN
   associated with an independent Flex-Algo, so that the VTNs can be
   identified using the corresponding Flex-Algo ID.  While this brings
   the benefits of simplicity, it also has some limitations.  For
   example, it means that even if multiple VTNs share the same topology



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   constraints, they would still need to be identified using different
   Flex-Algos in the control plane.  This means independent path
   computation would be executed for each VTN.  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.

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

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.

7.  IANA Considerations

   This document does not request any IANA actions.

8.  Acknowledgments

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

9.  References

9.1.  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 for Enhanced VPN", draft-dong-spring-sr-for-
              enhanced-vpn-10 (work in progress), August 2020.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-11 (work in progress), September 2020.

   [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-09
              (work in progress), September 2020.



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   [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-00
              (work in progress), July 2020.

   [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>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [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>.

   [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>.

   [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, <https://www.rfc-editor.org/info/rfc8668>.

9.2.  Informative References

   [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.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-18 (work in
              progress), August 2020.







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   [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.

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



























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