



LSR Working Group                                                 C. Xie
Internet-Draft                                                     C. Ma
Intended status: Informational                             China Telecom
Expires: 16 April 2026                                           J. Dong
                                                                   Z. Li
                                                     Huawei Technologies
                                                         13 October 2025


  Applicability of IS-IS Multi-Topology (MT) for Segment Routing based
                    Network Resource Partition (NRP)
                    draft-ietf-lsr-isis-sr-vtn-mt-11

Abstract

   Enhanced VPNs aim to deliver VPN services with enhanced
   characteristics, such as guaranteed resources, latency, jitter, etc.,
   so as to support customers requirements for connectivity services
   with these enhanced characteristics.  Enhanced VPN requires
   integration between the overlay VPN connectivity and the
   characteristics provided by the underlay network.  A Network Resource
   Partition (NRP) is a subset of the network resources and associated
   policies on each of a connected set of links in the underlay network.
   An NRP could be used as the underlay to support one or a group of
   enhanced VPN services.

   In some network scenarios, each NRP can be associated with a unique
   logical network topology.  This document describes a mechanism to
   build the SR-based NRPs 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
   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."

   This Internet-Draft will expire on 16 April 2026.




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Copyright Notice

   Copyright (c) 2025 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Advertisement of Topology Attribute for SR-based NRP  . . . .   3
   3.  Advertisement of Resource Attribute for SR-based NRP  . . . .   4
   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  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Enhanced VPNs aim to deliver VPN services with enhanced
   characteristics, such as guaranteed resources, latency, jitter, etc.,
   so as to support customers requirements for connectivity services
   with these enhanced characteristics.  Enhanced VPN requires
   integration between the overlay VPN connectivity and the
   characteristics provided by the underlay network.  [RFC9543]
   discusses the general framework, components, and interfaces for
   requesting and operating network slices using IETF technologies.
   Network slice is considered as one target use case of enhanced VPNs.

   [RFC9543] also introduces the concept of the Network Resource
   Partition (NRP), which is a subset of the buffer/queuing/scheduling
   resources and associated policies on each of a connected set of links
   in an underlay network.  An NRP can be associated with a logical
   network topology to select or specify the set of links and nodes
   involved.  [RFC9732] specifies the framework of NRP-based enhanced
   VPNs and describes the candidate component technologies in different



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   network planes and network layers.  An NRP could be used as the
   underlay to meet the requirement of one or a group of enhanced VPN
   services.  To meet the requirement of enhanced VPN services, a number
   of NRPs can be created, each with a subset of network resources
   allocated on network nodes and links in a customized topology of the
   physical 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 NRPs with the required
   network topology and network resource attributes to support enhanced
   VPN services.  In an SR-based data plane, Segment Identifiers (SIDs)
   can be used to represent both the topological instructions and a
   subset of network resources on the network nodes and links which are
   allocated to an NRP.  The SR SIDs and the associated topology and
   resource attributes of an NRP need to be distributed using a control
   plane.

   In some network scenarios, the required number of NRPs could be
   small, and it can be assumed that each NRP is associated with an
   independent topology and has a set of dedicated or shared network
   resources.  For such scenarios, this document describes a simplified
   mechanism to build SR-based NRPs.  It proposes to use IS-IS Multi-
   Topology [RFC5120] with segment routing [RFC8667] to define the
   independent network topology of each NRP.  The network resources and
   other TE attributes of an NRP can be advertised using IS-IS MT with
   the Traffic Engineering (TE) extensions defined in [RFC5305] and
   [RFC8570].  The resource-aware segments can be used with this
   approach to provide resource-guaranteed SR-based NRPs, while the
   normal SR segments may also be used to provide SR-based NRPs with
   shared network resources in the forwarding plane.

   Alternate enhancements will be proposed to provide a flexible
   combination of the topology and resource attribute to build a
   relatively large number of NRPs.  The detailed mechanism is out of
   the scope of this document.

2.  Advertisement of Topology Attribute for SR-based NRP

   As each SR-based NRP is associated with a network topology, the
   topology attribute and SR SIDs of NRPs need to be advertised, so that
   the SR shortest path could be calculated using the topology of the
   corresponding NRP.  In this document, IS-IS MT and IS-IS SR are
   reused for advertising the topology and SR SIDs of NRPs.




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   IS-IS Multi-Topology (MT) [RFC5120] has been defined to create
   independent topologies in one network.  In [RFC5120], MT-based TLVs
   are introduced to advertise 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 NRP
   is associated with an independent network topology, MT-ID could be
   used as the identifier of NRP in the control plane.

   IS-IS MT can be used with segment routing based data plane.  Thus the
   topology attribute of an SR based NRP could be advertised using MT
   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)
   [RFC5311].  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 [RFC9352].  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 Resource Attribute for SR-based NRP

   In order to perform constraint based path computation for each NRP on
   the network controller or on the ingress nodes, the network resource
   attributes and other attributes associated with each NRP need to be
   advertised.  In this document, IS-IS MT is reused to advertise
   topology-specific TE attributes for different NRPs.

   On each network link, the information of the network resources and
   other attributes associated with an NRP can be specified by
   advertising the TE attributes sub-TLVs [RFC5305] and [RFC8570] in the
   IS-IS TLV 222 (MT-ISN) and TLV 223 (MT IS Neighbor Attribute)
   [RFC5311] of the corresponding topology.





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   When Maximum Link Bandwidth sub-TLV is advertised in the MT-ISN TLV
   of a topology, it indicates the amount of link bandwidth allocated to
   the corresponding NRP.  The bandwidth allocated to an NRP can be
   exclusive for services utilizing the corresponding NRP.  The usage of
   other TE attributes in topology-specific TLVs is out of the scope of
   this document.

   Editor's note: It is noted that advertising 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 planes, the Adj-SIDs and Prefix-SIDs associated with
   the same NRP can be used together to build SR-MPLS paths with the
   topological and resource constraints of the NRP taken into
   consideration.  A Prefix-SID is associated with the paths calculated
   in the topology corresponding to the NRP.  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 NRP on the outgoing interface for packet forwarding.
   A forwarding entry is installed in the forwarding plane using the
   MPLS label that corresponds to the Prefix-SID associated with the
   topology corresponding to the NRP.  A resource-aware Adj-SID is
   associated with a subset of network resources allocated to the NRP on
   the link it identifies, and can be used together with the prefix-SIDs
   of the same NRP to build SR-MPLS TE paths using the NRP.

   For SRv6 data planes, the SRv6 SIDs associated with the same NRP can
   be used together to build SRv6 paths with the topological and
   resource constraints of the NRP taken into consideration.  An SRv6
   Locator is a prefix which is associated with the paths calculated in
   the topology corresponding to the NRP.  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 NRP on the outgoing interface
   for packet forwarding.  A forwarding entry for the SRv6 Locator
   prefix is installed in the forwarding plane for the topology
   corresponding to the NRP.  A resource-aware End.X SID is associated
   with a subset of network resources allocated to the NRP on the link
   it identifies, and can be used together with other types of SRv6 SIDs
   of the same NRP to build SRv6 TE paths using the NRP.









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5.  Scalability Considerations

   The mechanism described in this document assumes that each NRP is
   associated with a unique multi-topology, so that the MT-IDs can be
   reused to identify the NRPs in the control plane.  While this brings
   the benefit of simplicity, it also has some limitations.  For
   example, it means that even if multiple NRPs share 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 NRP.  Thus the number of NRPs supported in a network may be
   dependent on the number of topologies supported, which is related to
   both the number of topologies supported in the protocol and the
   control plane overhead which the network nodes could accomodate.  The
   mechanism described in this document is considered useful for network
   scenarios in which the required number of NRPs is small, as no
   control protocol extension is required.  For network scenarios where
   the number of required NRPs is large, more scalable solution would be
   needed, which may require further protocol extensions and
   enhancements.  A detailed analysis about the NRP scalability and the
   possible optimizations for supporting a large number of NRPs is
   described in [I-D.ietf-teas-nrp-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,
   Peter Psenak, Daniele Ceccarelli, Jia He, Xuesong Geng and Acee
   Lindem for the review and discussion of this document.

9.  References

9.1.  Normative References

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Miyasaka, T., Zhu, Y., Qin, F., and Z. Li,
              "Introducing Resource Awareness to SR Segments", Work in
              Progress, Internet-Draft, draft-ietf-spring-resource-



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              aware-segments-15, 3 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              resource-aware-segments-15>.

   [I-D.ietf-spring-sr-for-enhanced-vpn]
              Dong, J., Miyasaka, T., Zhu, Y., Qin, F., and Z. Li,
              "Segment Routing based Network Resource Partition (NRP)
              for Enhanced VPN", Work in Progress, Internet-Draft,
              draft-ietf-spring-sr-for-enhanced-vpn-09, 10 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              sr-for-enhanced-vpn-09>.

   [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>.

   [RFC5311]  McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
              Shand, "Simplified Extension of Link State PDU (LSP) Space
              for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
              <https://www.rfc-editor.org/info/rfc5311>.

   [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>.

   [RFC9352]  Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
              and Z. Hu, "IS-IS Extensions to Support Segment Routing
              over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
              February 2023, <https://www.rfc-editor.org/info/rfc9352>.




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   [RFC9543]  Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S.,
              Makhijani, K., Contreras, L., and J. Tantsura, "A
              Framework for Network Slices in Networks Built from IETF
              Technologies", RFC 9543, DOI 10.17487/RFC9543, March 2024,
              <https://www.rfc-editor.org/info/rfc9543>.

   [RFC9732]  Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for NRP-Based Enhanced Virtual Private
              Networks", RFC 9732, DOI 10.17487/RFC9732, March 2025,
              <https://www.rfc-editor.org/info/rfc9732>.

9.2.  Informative References

   [I-D.ietf-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., and G. S. Mishra,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-ietf-
              teas-nrp-scalability-07, 2 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              nrp-scalability-07>.

Authors' Addresses

   Chongfeng Xie
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China
   Email: chongfeng.xie@foxmail.com


   Chenhao Ma
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China
   Email: chenhao.m@outlook.com


   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing
   100095
   China
   Email: jie.dong@huawei.com



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   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing
   100095
   China
   Email: lizhenbin@huawei.com












































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