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LSR Working Group Y. Zhu
Internet-Draft China Telecom
Intended status: Standards Track J. Dong
Expires: March 15, 2021 Z. Hu
Huawei Technologies
September 11, 2020
Using Flex-Algo for Segment Routing based VTN
draft-zhu-lsr-isis-sr-vtn-flexalgo-01
Abstract
As defined in I-D.ietf-teas-enhanced-vpn, enhanced VPN (VPN+) aims to
provide enhanced VPN service to support the 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 network which consists
of a subset of network topology and network resources allocated from
the underlay network. A VTN could be used as the underlay for one or
a group of VPN+ services.
I-D.dong-lsr-sr-enhanced-vpn defines the IGP mechanisms with
necessary extensions to build a set of Segment Routing (SR) based
VTNs. This document describes a simplified mechanism to build the SR
based VTNs using SR Flex-Algo together with minor extensions to IGP
L2 bundle.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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."
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This Internet-Draft will expire on March 15, 2021.
Copyright Notice
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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
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 . . . . . . . . . . . . . . . . . . . . . . . 8
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
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underlay network topology and a set of network resources allocated
from the underlay network to meet the requirement of a specific VPN+
service 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 Paths (VTPs). Although using a set of dedicated VTPs can
provide similar characteristics as 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]. As described in
[I-D.dong-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, and the number of the VTNs required is limited.
This document describes a simplified mechanism to build the SR based
VTNs in those scenarios.
2. Advertisement of SR VTN Topology Attribute
[I-D.ietf-lsr-flex-algo] specifies the mechanism to provide
distributed constraint-path computation, and the usage of SR-MPLS
prefix-SIDs and SRv6 locators for steering traffic along the
constrained paths.
The Flex-Algo definition consists of the topological constraints for
path computation. According to the network nodes' participation of a
Flex-Algo, and the rules of including or excluding Admin Groups (i.e.
colors) and Shared Risk Link Groups (SRLGs), thus the topology
attribute of a VTN can be described using the associated Flex-Algo.
And when each VTN has an independent set of network topology
constraints, the Flex-Algo identifier could be reused as the
identifier of the VTN in control plane.
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With the mechanisms defined in[RFC8667] [I-D.ietf-lsr-flex-algo], SR-
MPLS prefix-SID advertisement can be associated with a specific
topology and a specific algorithm, which can be a Flex-Algo. This
allows the nodes to use the prefix-SIDs to steer traffic along
distributed computed constraint paths according to the identified
Flex-Algo in the associated topology.
[I-D.ietf-lsr-isis-srv6-extensions] specifies the IS-IS extensions to
support SRv6 data plane, in which the SRv6 locators advertisement can
be associated with a specific topology and a specific algorithm,
which can be a Flex-Algo. This allows the nodes to used the SRv6
locators to steer traffic along distributed computed constraint paths
according to the identified Flex-Algo in the associated topology. In
addition, topology/algorithm specific SRv6 End SIDs and End.X SIDs
can be used to enforce traffic over the Loop-Free Alternatives (LFA)
computed backup paths.
3. Advertisement of SR VTN Resource Attribute
Each VTN may have customized network resource attributes. In order
to perform constraint based path computation for each VTN on network
controller and the ingress nodes, the resource attribute of each VTN
also needs to be advertised.
[RFC8668] was defined to advertise the link attributes of the Layer 2
bundle member links. In this section, it is extended to advertise
the network resource attributes associated with different VTNs on a
Layer 3 link.
The Layer 3 link may or may not be a Layer 2 link bundle, as long as
it has the capability of allocating different subsets of link
resources to different VTNs it participates in. A subset of the link
resource can be considered as a virtual Layer 2 member link (or sub-
interface) of the Layer 3 link. If the Layer 3 interface is a Layer
2 link bundle, it is possible that the subset of link resource
allocated to a specific VTN is provided by one of the physical Layer
2 member links.
A new flag "V" (Virtual) is defined in the flag field of the Parent
L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV (25).
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|P|V| |
+-+-+-+-+-+-+-+-+
V flag: When the V flag is set, it indicates the advertised member
links under the Parent L3 link are virtual Layer 2 member links.
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When the V flag is clear, it indicates the member links are physical
member links.
For each virtual or physical member link, the TE attributes defined
in [RFC5305] such as the Maximum Link Bandwidth and Admin Groups
SHOULD be advertised using the mechanism as defined in [RFC8668].
The Adj-SIDs or SRv6 End.X SIDs associated with each of the virtual
or physical member links of an L2 bundle SHOULD also be advertised.
In order to correlate the virtual or physical member links with the
corresponding VTNs, each member link SHOULD be assigned with a
dedicated Admin Group or Extended Admin Group, which is included in
the definition of the Flex-Algo of the corresponding VTN. Note that
in this case the Admin Group or Extended Admin Group of the Layer 3
link SHOULD be set to the union of all the Admin Groups or Extended
Admin Groups of its virtual or physical member links. This is to
ensure that the Layer 3 link is always included in the Flex-Algo
specific constraint path computation of the VTNs it participates in.
4. Forwarding Plane Operations
For SR-MPLS data plane, a prefix SID is associated with the paths
calculated using the corresponding Flex-Algo of a VTN. An outgoing
Layer 3 interface is determined for each path. In addition, the
prefix-SID also steers the traffic to use the virtual or physical
member link which is associated with the VTN on the outgoing Layer 3
interface for packet forwarding. The Adj-SIDs associated with the
virtual or physical member links of a VTN MAY be used with the
prefix-SIDs of the same VTN 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 using the corresponding Flex-Algo of a VTN.
An outgoing Layer 3 interface is determined for each path. In
addition, the SRv6 Locator prefix also steers the traffic to use the
virtual or physical member link which is associated with the VTN on
the outgoing Layer 3 interface for packet forwarding. The End.X SIDs
associated with the virtual or physical member links of a VTN MAY be
used with the SRv6 Locator prefix of the same VTN together to build
SRv6 paths with the topological and resource constraints of the VTN.
5. Scalability Considerations
The mechanism described in this document requires that each VTN
associated with an independent Flex-Algo, so that the VTNs can be
identified using the corresponding Flex-Algo ID. While this brings
the benefits of simplicity, it also has some limitations. For
example, it means that even if multiple VTNs share the same topology
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constraints, they would still need to be identified using different
Flex-Algos in the control plane. This means independent path
computation would be executed for each VTN. The number of VTNs
supported in a network may be dependent on the number of Flex-Algos
supported, which is related to the control plane computation
overhead.
Another aspect which may impact the number of VTNs supported with
this mechanism is that at most 128 Flex-Algos can be used in a
network.
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 Zhenbin Li for the review and
discussion of this document.
9. References
9.1. Normative References
[I-D.dong-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 for Enhanced VPN", draft-dong-spring-sr-for-
enhanced-vpn-10 (work in progress), August 2020.
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-11 (work in progress), September 2020.
[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-09
(work in progress), September 2020.
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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", draft-ietf-spring-resource-aware-segments-00
(work in progress), July 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[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>.
[RFC8668] Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
M., and E. Aries, "Advertising Layer 2 Bundle Member Link
Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
December 2019, <https://www.rfc-editor.org/info/rfc8668>.
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-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
draft-ietf-spring-srv6-network-programming-18 (work in
progress), August 2020.
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[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
Yongqing Zhu
China Telecom
Email: zhuyq8@chinatelecom.cn
Jie Dong
Huawei Technologies
Email: jie.dong@huawei.com
Zhibo Hu
Huawei Technologies
Email: huzhibo@huawei.com
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