Using IS-IS Multi-Topology (MT) for Segment Routing based Virtual Transport Network
draft-ietf-lsr-isis-sr-vtn-mt-02

LSR Working Group                                                 C. Xie
Internet-Draft                                                     C. Ma
Intended status: Informational                             China Telecom
Expires: 17 July 2022                                            J. Dong
                                                                   Z. Li
                                                     Huawei Technologies
                                                         13 January 2022

   Using IS-IS Multi-Topology (MT) for Segment Routing based Virtual
                           Transport Network
                    draft-ietf-lsr-isis-sr-vtn-mt-02

Abstract

   Enhanced VPN (VPN+) aims to provide enhanced VPN service to support
   some application's needs of enhanced isolation and stringent
   performance requirements.  VPN+ requires integration between the
   overlay VPN connectivity and the characteristics provided by the
   underlay network.  A Virtual Transport Network (VTN) is a virtual
   underlay network which consists of a subset of network resources
   allocated on network nodes and links in a customized topology of the
   physical network.  A VTN could be used as the underlay to support one
   or a group of VPN+ services.

   In some network scenarios, each VTN can be associated with a unique
   logical network topology.  This document describes a mechanism to
   build the SR based VTNs using IS-IS Multi-Topology together with
   other well-defined IS-IS extensions.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on 17 July 2022.

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Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Advertisement of SR VTN Topology Attribute  . . . . . . . . .   4
   3.  Advertisement of SR VTN Resource Attribute  . . . . . . . . .   4
     3.1.  Advertising Topology-specific TE attributes . . . . . . .   5
   4.  Forwarding Plane Operations . . . . . . . . . . . . . . . . .   5
   5.  Scalability Considerations  . . . . . . . . . . . . . . . . .   6
   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 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.  VPN+ can be used to underpin network
   slicing, but could also be of use in its own right providing enhanced
   connectivity services between customer sites.

   To meet the requirement of VPN+ services, a number of virtual
   transport networks (VTN) can be created, each with a subset of
   network resources allocated on network nodes and links in a
   customized topology of the physical network.  A VTN could be used as

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   the underlay to meet the requirement of one 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 Path (VTP).  Although
   using a set of dedicated VTPs can provide similar characteristics as
   a 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].  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 SIDs can be used to build SR based VTNs with the required
   network topology and network resource attributes to support VPN+
   services.  With segment routing based data plane, Segment Identifiers
   (SIDs) can be used to represent both the topological instructions and
   the set of network resources allocated by network nodes to a VTN.
   The SR SIDs and the associated topology and resource attributes of a
   VTN need to be distributed using control plane.

   [I-D.dong-lsr-sr-enhanced-vpn] defines the IGP mechanisms with
   necessary extensions to provide scalable Segment Routing (SR) based
   VTNs.  The VTNs could be used as the underlay of the VPN+ service.
   The mechanism described in [I-D.dong-lsr-sr-enhanced-vpn] allows
   flexible combination of the topology and resource attribute to build
   a relatively large number of VTNs.  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.  This document
   describes a simplified mechanism to build SR based VTNs in those
   scenarios.  The resource-aware segments can be used with this
   approach to provide resource guaranteed SR VTNs, while the normal SR
   segments may also be used to provide SR VTNs with shared network
   resources in the forwarding plane.

   The proposed approach is to use IS-IS Multi-Topology [RFC5120] with
   segment routing [RFC8667] to define the independent network topology
   of each VTN.  The attribute of network resources allocated to a VTN
   can be advertised using IS-IS MT with the Traffic Engineering (TE)
   extensions defined in [RFC5305] and [RFC8570].

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2.  Advertisement of SR VTN Topology Attribute

   IS-IS Multi-Topology Routing (MTR) [RFC5120] has been defined to
   create independent topologies in one network.  In [RFC5120], MT-based
   TLVs are introduced to carry topology-specific link-state
   information.  The MT-specific Link or Prefix TLVs are defined by
   adding additional two bytes, which includes 12-bit MT-ID field in
   front of the ISN TLV and IP or IPv6 Reachability TLVs.  This provides
   the capability of specifying the customized attributes of each
   topology.  When each VTN is associated with an independent network
   topology, MT-ID could be used as the identifier of VTN in control
   plane.

   MTR can be used with segment routing based data plane.  Thus the
   topology attribute of an SR based VTN could be advertised using MTR
   with segment routing.  The IS-IS extensions to support the
   advertisement of topology-specific MPLS SIDs are specified in
   [RFC8667].  Topology-specific Prefix-SIDs can be advertised by
   carrying the Prefix-SID sub-TLVs in the IS-IS TLV 235 (MT IP
   Reachability) and TLV 237 (MT IPv6 IP Reachability).  Topology-
   specific Adj-SIDs can be advertised by carrying the Adj-SID sub-TLVs
   in IS-IS TLV 222 (MT-ISN) and TLV 223 (MT IS Neighbor Attribute).
   The topology-specific Prefix-SIDs and Adj-SIDs can be resource-aware
   segments or normal SR segments.

   The IS-IS extensions to support the advertisement of topology-
   specific SRv6 Locators and SIDs are specified in
   [I-D.ietf-lsr-isis-srv6-extensions].  The topology-specific SRv6
   locators are advertised using SRv6 Locator TLV, and SRv6 End SIDs
   inherit the MT-ID from the parent locator.  The topology-specific
   End.X SID are advertised by carrying SRv6 End.X SID sub-TLVs in the
   IS-IS TLV 222 (MT-ISN) and TLV 223 (MT IS Neighbor Attribute).  The
   topology-specific SRv6 locators can be resource-aware locator or
   normal SRv6 locator, and accordingly the topology-specific SRv6 SIDs
   can be resource-aware SRv6 segments or normal SRv6 segments.

3.  Advertisement of SR VTN Resource Attribute

   In order to perform constraint based path computation for each VTN on
   the network controller or on the ingress nodes, the network resource
   attributes and other attributes associated with each VTN need to be
   advertised.

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3.1.  Advertising Topology-specific TE attributes

   On each network link, the information of the network resources and
   other attributes associated with a VTN can be specified by carrying
   the TE attributes sub-TLVs [RFC5305] and [RFC8570] in the IS-IS TLV
   222 (MT-ISN) and TLV 223 (MT IS Neighbor Attribute) of the
   corresponding topology.

   When Maximum Link Bandwidth sub-TLV is carried in the MT-ISN TLV of a
   topology, it indicates the amount of link bandwidth allocated to the
   corresponding VTN.  The bandwidth allocated to a VTN can be exclusive
   for services carried in the corresponding VTN.  The usage of other TE
   attributes in topology-specific TLVs is for further study.

   Editor's note1: It is noted that carrying per-topology TE attributes
   was considered as a possible feature in future when the encoding of
   IS-IS multi-topology was defined in [RFC5120].

4.  Forwarding Plane Operations

   For SR-MPLS data plane, the Adj-SIDs and Prefix-SIDs associated with
   the same VTN can be used together to build SR-MPLS paths with the
   topological and resource constraints of the VTN taken into
   consideration.  A Prefix-SID is associated with the paths calculated
   in the corresponding topology of a VTN.  An outgoing interface is
   determined for each path.  In addition, the resource-aware prefix-SID
   can steer the traffic to use the subset of network resources
   allocated to the VTN on the outgoing interface for packet forwarding.
   Similarly, the resource-aware Adj-SID is associated with a subset of
   network resources allocated to a VTN on the link it identifies.

   For SRv6 data plane, the End.X SIDs and the SRv6 Locator prefixes
   associated with the same VTN can be used together to build SRv6 paths
   with the topological and resource constraints of the VTN taken into
   consideration.  An SRv6 Locator is a prefix which is associated with
   the paths calculated in the corresponding topology of a VTN.  An
   outgoing interface is determined for each path.  In addition, the
   resource-aware SRv6 Locator prefix also steers the traffic to use the
   subset of network resources which are allocated to the VTN on the
   outgoing interface for packet forwarding.  Similarly, an End.X SID is
   associated with a subset of network resources allocated to a VTN on
   the link it identifies.

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5.  Scalability Considerations

   The mechanism described in this document assumes that each VTN is
   associated with a unique topology, so that the MT-IDs can be reused
   to identify the VTNs in the control plane.  While this brings the
   benefit of simplicity, it also has some limitations.  For example, it
   means that even if multiple VTNs have the same topology, they would
   still need to be identified using different MT-IDs in the control
   plane, then independent path computation needs to be executed for
   each VTN.  Thus the number of VTNs supported in a network may be
   dependent on the number of topologies supported, which is related to
   the number of topologies supported in the protocol and the control
   plane overhead on network nodes.  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.dong-teas-enhanced-vpn-vtn-scalability].

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 Zhibo Hu, Dean Cheng, Les Ginsberg
   and Peter Psenak for the review and discussion of this document.

9.  References

9.1.  Normative References

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extensions to Support Segment Routing over
              IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
              ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
              extensions-18.txt>.

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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", Work in Progress, Internet-Draft, draft-ietf-
              spring-resource-aware-segments-03, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              resource-aware-segments-03.txt>.

   [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-01,
              12 July 2021, <https://www.ietf.org/archive/id/draft-ietf-
              spring-sr-for-enhanced-vpn-01.txt>.

   [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>.

   [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>.

   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

   [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>.

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 Scalable Segment
              Routing based Enhanced VPN", Work in Progress, Internet-

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              Draft, draft-dong-lsr-sr-enhanced-vpn-06, 11 July 2021,
              <https://www.ietf.org/archive/id/draft-dong-lsr-sr-
              enhanced-vpn-06.txt>.

   [I-D.dong-teas-enhanced-vpn-vtn-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J. N.,
              Mishra, G., and F. Qin, "Scalability Considerations for
              Enhanced VPN (VPN+)", Work in Progress, Internet-Draft,
              draft-dong-teas-enhanced-vpn-vtn-scalability-04, 25
              October 2021, <https://www.ietf.org/archive/id/draft-dong-
              teas-enhanced-vpn-vtn-scalability-04.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-09, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-teas-enhanced-
              vpn-09.txt>.

Authors' Addresses

   Chongfeng Xie
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China

   Email: xiechf@chinatelecom.cn

   Chenhao Ma
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China

   Email: machh@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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