LSR Working Group Y. Zhu
Internet-Draft China Telecom
Intended status: Standards Track J. Dong
Expires: September 10, 2020 Z. Hu
Huawei Technologies
March 9, 2020
Using Flex-Algo for Segment Routing based VTN
draft-zhu-lsr-isis-sr-vtn-flexalgo-00
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+ requries
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 toplogy 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 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
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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 September 10, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Advertisement of SR VTN Topology Attribute . . . . . . . . . 3
3. Extensions to IGP L2 Bundles . . . . . . . . . . . . . . . . 4
4. Scalability Considerations . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. 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. These
properties cannot be met with pure overlay networks, as they 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) need to be created, each with a subset of
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the underlay network topology and a set of network resources
allocated to meet the requirement of a specific VPN+ service or a
group of VPN+ services. Another existing approach is to build 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, it has some scalability issues in large networks.
[I-D.dong-spring-sr-for-enhanced-vpn] specifies how segment routing
(SR) [RFC8402] 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 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 has an independent topology and a set of dedicated network
resources. 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 provides the topological constraints for
path computation. When each VTN has an independent network topology,
the Flex-Algo Identifier could be used as the identifier of a VTN in
control plane. Thus the topology attribute of a VTN could be defined
using Flex-Algo.
According to the network nodes' participation of a Flex-Algo, and the
rules of including or excluding specific Admin Groups (colors) and
Shared Risk Link Groups (SRLGs), the topology attribute of a VTN can
be determined using the associated Flex-Algo.
With the mechanisms defined in[RFC8667] [I-D.ietf-lsr-flex-algo],
prefix-SID advertisement can be associated with a specific topology
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and a specific algorithm, which can be a Flex-Algo. This allows the
nodes to use the prefix-SID to steer traffic along distributed
computed 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 the nodes to used the SRv6
locator to steer traffic along distributed computed paths according
to the identified Flex-Algo in the associated topology. In addition,
topology/algorithm specific SRv6 End SID and End.X SID can be used to
enforce traffic over the LFA computed backup path.
3. Extensions to IGP L2 Bundles
In order to perform constraint based path computation for each VTN on
network controller and the ingress nodes, the resource attribute of
VTN also needs to be advertised.
[RFC8668] was defined to advertise 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 one
Layer 3 interface.
On a Layer 3 interface, it is assumed that different subset of the
link resources are allocated to different VTNs it participates in. A
subset of the link resource can be seen as a virtual layer-2 member
link of the Layer 3 interface. If the Layer 3 interface is a L2 link
bundle, it is also possible that the subset of link resource for a
specific VTN is provided by a physical Layer 2 member link.
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 member links. 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 such as
the Maximum Link Bandwidth and admin-groups can be advertised using
the mechanism as defined in [RFC8668].
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In order to associate different virtual or physical member links with
the corresponding VTNs, each member link SHOULD be assigned with a
dedicated admin-group or extended admin-group (color), which is
included in the definition of the corresponding Flex-Algo. Note that
in this case the admin-group or extended admin-group of the Layer 3
interface SHOULD be set to the union of all the admin-groups of the
virtual or physical member links. This is to ensure that the Layer 3
link will be included in the constraint-based computation of the
corresponding Flex-Algo.
4. Scalability Considerations
The mechanism described in this document requires that each VTN maps
to an independent Flex-Algo. Even if multiple VTNs share the same
topology constraints. While this brings the benefits of simplicity,
it also has some limitations. For example, it means that even if
multiple VTNs have the same topology constraints, they would still
need to be identified using different Flex-Algos in the control
plane. Then this requires that for each VTN, independent path
computation would be executed. 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 is that
at most 128 Flex-Algos can be defined in a network.
5. Security Considerations
This document introduces no additional security vulnerabilities to
IS-IS and OSPF.
The mechanism proposed in this document is subject to the same
vulnerabilities as any other protocol that relies on IGPs.
6. IANA Considerations
This document does not request any IANA actions.
7. Acknowledgments
The authors would like to thank Zhenbin Li for the review and
discussion of this document.
8. References
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8.1. Normative References
[I-D.dong-spring-sr-for-enhanced-vpn]
Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., and
Z. Li, "Segment Routing for Resource Partitioned Virtual
Networks", draft-dong-spring-sr-for-enhanced-vpn-06 (work
in progress), December 2019.
[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-06 (work in progress), February 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-06
(work in progress), March 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>.
[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>.
[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>.
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[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>.
8.2. Informative References
[I-D.dong-lsr-sr-enhanced-vpn]
Dong, J., Hu, Z., and S. Bryant, "IGP Extensions for
Segment Routing based Enhanced VPN", draft-dong-lsr-sr-
enhanced-vpn-02 (work in progress), November 2019.
[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-12 (work in
progress), March 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+)
Services", draft-ietf-teas-enhanced-vpn-05 (work in
progress), February 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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