LSR Working Group C. Xie
Internet-Draft C. Ma
Intended status: Informational China Telecom
Expires: January 13, 2022 J. Dong
Z. Li
Huawei Technologies
July 12, 2021
Using IS-IS Multi-Topology (MT) for Segment Routing based Virtual
Transport Network
draft-ietf-lsr-isis-sr-vtn-mt-01
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
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
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This Internet-Draft will expire on January 13, 2022.
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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 . . . . . . . . . . . . . . . . . . . 6
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 SR based 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 provide scalable 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 a relatively large number of VTNs. In some network scenarios,
it is assumed that each VTN can have an independent topology and 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, the normal SR
segments may also be used to provide SR VTNs with shared network
resources in the forwarding plane.
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
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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
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.
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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, 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 overhead. 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].
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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 Extension to Support Segment Routing over
IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-14
(work in progress), April 2021.
[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-02
(work in progress), February 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", draft-ietf-spring-sr-for-
enhanced-vpn-00 (work in progress), February 2021.
[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>.
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[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 Segment Routing based
Enhanced VPN", draft-dong-lsr-sr-enhanced-vpn-05 (work in
progress), February 2021.
[I-D.dong-teas-enhanced-vpn-vtn-scalability]
Dong, J., Li, Z., Qin, F., Yang, G., and J. N. Guichard,
"Scalability Considerations for Enhanced VPN (VPN+)",
draft-dong-teas-enhanced-vpn-vtn-scalability-02 (work in
progress), February 2021.
[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", draft-ietf-teas-enhanced-vpn-07 (work in
progress), February 2021.
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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