LSR Working Group C. Xie
Internet-Draft C. Ma
Intended status: Informational China Telecom
Expires: 11 January 2023 J. Dong
Z. Li
Huawei Technologies
10 July 2022
Using IS-IS Multi-Topology (MT) for Segment Routing based Virtual
Transport Network
draft-ietf-lsr-isis-sr-vtn-mt-03
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
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 11 January 2023.
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Copyright Notice
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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 . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
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 resources 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, the
required number of VTNs could be small, and 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 network resources and other TE attributes of 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 note: 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 the 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.
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 VTN. A resource-aware Adj-SID is
associated with a subset of network resources allocated to the VTN on
the link it identifies, and can be used together with the prefix-SIDs
of the same VTN to build SR-MPLS TE paths of the VTN.
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For SRv6 data plane, the SRv6 SIDs 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 the 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. A forwarding entry for the SRv6 Locator
prefix is installed in the forwarding plane for the topology
corresponding to the VTN. A resource-aware End.X SID is associated
with a subset of network resources allocated to the VTN on the link
it identifies, and can be used together with other types of SRv6 SIDs
of the same VTN to build SRv6 TE paths of the VTN.
5. Scalability Considerations
The mechanism described in this document assumes that each VTN is
associated with a unique multi-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 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 VTN. Thus the number of VTNs 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 afford. 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-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.
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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>.
[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-04, 5 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-spring-
resource-aware-segments-04.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-02,
5 March 2022, <https://www.ietf.org/archive/id/draft-ietf-
spring-sr-for-enhanced-vpn-02.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>.
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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 Scalable Segment
Routing based Enhanced VPN", Work in Progress, Internet-
Draft, draft-dong-lsr-sr-enhanced-vpn-07, 29 January 2022,
<https://www.ietf.org/archive/id/draft-dong-lsr-sr-
enhanced-vpn-07.txt>.
[I-D.dong-teas-nrp-scalability]
Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J. N.,
Mishra, G., Qin, F., Saad, T., and V. P. Beeram,
"Scalability Considerations for Network Resource
Partition", Work in Progress, Internet-Draft, draft-dong-
teas-nrp-scalability-02, 16 May 2022,
<https://www.ietf.org/archive/id/draft-dong-teas-nrp-
scalability-02.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-10, 6 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-teas-enhanced-
vpn-10.txt>.
Authors' Addresses
Chongfeng Xie
China Telecom
China Telecom Beijing Information Science & Technology, Beiqijia
Beijing
102209
China
Email: xiechf@chinatelecom.cn
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Chenhao Ma
China Telecom
China Telecom Beijing Information Science & Technology, Beiqijia
Beijing
102209
China
Email: machh@chinatelecom.cn
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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