Using IS-IS Multi-Topology (MT) for Segment Routing based Virtual Transport Network
draft-ietf-lsr-isis-sr-vtn-mt-00

Document Type Active Internet-Draft (lsr WG)
Authors Chongfeng Xie  , Chenhao Ma  , Jie Dong  , Zhenbin Li 
Last updated 2021-03-26
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LSR Working Group                                                 C. Xie
Internet-Draft                                                     C. Ma
Intended status: Informational                             China Telecom
Expires: September 27, 2021                                      J. Dong
                                                                   Z. Li
                                                     Huawei Technologies
                                                          March 26, 2021

   Using IS-IS Multi-Topology (MT) for Segment Routing based Virtual
                           Transport Network
                    draft-ietf-lsr-isis-sr-vtn-mt-00

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 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.

   In some network scenarios, each VTN can be associated with a unique
   logicial 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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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 27, 2021.

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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
   3.  Advertisement of SR VTN Resource Attribute  . . . . . . . . .   4
     3.1.  Advertising Topology-specific TE attributes . . . . . . .   4
   4.  Forwarding Plane Operations . . . . . . . . . . . . . . . . .   5
   5.  Scalability Considerations  . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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
   underlay network topology and a subset of network resources allocated

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   from the underlay network 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 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.  This document describes a simplified mechanism to
   build SR based VTNs in those scenarios.

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

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

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   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 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).

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
   and other attributes associated with each VTN need to be advertised.

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].

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4.  Forwarding Plane Operations

   For SR-MPLS data plane, 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 prefix-SID
   also steers the traffic to use the subset of network resources
   allocated to the VTN on the outgoing interface for packet forwarding.
   An Adj-SID is associated with a subset of network resources allocated
   to a VTN on the link.  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.

   For SRv6 data plane, 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
   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.  An End.X SID is associated with a
   subset of network resources allocated to a VTN on the link.  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.

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 control plane computation overhead.

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.

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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 Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-11
              (work in progress), October 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.

   [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", February 2021,
              <https://tools.ietf.org/html/draft-ietf-spring-sr-for-
              enhanced-vpn>.

   [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>.

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   [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 Segment Routing based
              Enhanced VPN", draft-dong-lsr-sr-enhanced-vpn-04 (work in
              progress), June 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.

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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