LSR Working Group Y. Zhu
Internet-Draft China Telecom
Intended status: Standards Track J. Dong
Expires: 12 January 2023 Z. Hu
Huawei Technologies
11 July 2022
Using Flex-Algo for Segment Routing (SR) based Virtual Transport Network
(VTN)
draft-zhu-lsr-isis-sr-vtn-flexalgo-05
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 has a customized network topology and a set of
network resources allocated from the physical network. A VTN could
be used as the underlay for one or a group of VPN+ services.
The topological constraints of a VTN can be defined using Flex-Algo,
a mechanism to provide distributed constraint-path computation. In
some network scenarios, each VTN can be associated with a unique
Flex-Algo, and the set of network resources allocated to a VTN can be
instantiated as layer-2 sub-interfaces or member links of the layer-3
interfaces. This document describes the mechanisms to build the
Segment Routing (SR) based VTNs using SR Flex-Algo and IGP L2 bundles
with minor extensions. This document updates RFC 8668 by defining a
new flag in the Parent L3 Neighbor Descriptor in the L2 Bundle Member
Attributes TLV.
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 12 January 2023.
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Advertisement of SR VTN Topology Attributes . . . . . . . . . 4
3. Advertisement of SR VTN Resource Attributes . . . . . . . . . 4
4. Forwarding Plane Operations . . . . . . . . . . . . . . . . . 6
5. Scalability Considerations . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 could be provided with existing overlay VPN techniques.
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.
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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 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.ietf-spring-sr-for-enhanced-vpn], the resource-aware segment
identifiers (SIDs) can be used to build VTNs with the required
network topology and network resource attributes to support VPN+
services. With a segment routing based data plane, SIDs can be used
to represent both the topology and the set of network resources
allocated by network nodes to a VTN. The SIDs of each VTN together
with its associated topology and resource attributes need to be
distributed using the control plane.
[I-D.dong-lsr-sr-enhanced-vpn] defines the IGP mechanisms and
extensions to provide scalable SR based VTNs. The mechanism in
[I-D.dong-lsr-sr-enhanced-vpn] allows flexible combination of the
topology and resource attribute to provide a relatively large number
of VTNs. In some network scenarios, the number of required VTNs may
be small, thus a solution to provide a small number of VTNs may also
be desired.
This document describes a mechanism to build the SR based VTNs using
SR Flex-Algo [I-D.ietf-lsr-flex-algo] and IGP L2 bundle [RFC8668]
with minor extensions. With this mechanism, each VTN is associated
with a unique Flex-Algo, and the set of network resources allocated
to the VTN is instantiated using layer-2 sub-interfaces or layer-2
member links of the L3 interfaces. It can provide a relatively small
number of VTNs, and can be considered as a transitional solution for
the VTN deployment.
This document updates [RFC8668] by defining a new flag in the Parent
L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV.
[RFC8668] states that all bit fields not defined in that document
"MUST be set to zero on transmission and ignored on receipt".
Section 3 of this document defines a new flag and specifies both when
it is set and how it should be processed.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Advertisement of SR VTN Topology Attributes
[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 (FAD) is the combination of calculation-
type, metric-type and the topological constraints used 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), the topology of a VTN
can be described using the associated Flex-Algo. If each VTN is
associated with a unique Flex-Algo, the Flex-Algo identifier could be
reused as the identifier of the VTN in the control plane.
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 associated
Flex-Algo in a particular topology.
[I-D.ietf-lsr-isis-srv6-extensions] specifies the IS-IS extensions to
support SRv6 data plane, in which the SRv6 locators advertisement is
associated with a topology and a specific algorithm, which can be a
Flex-Algo. This allows the nodes to use the SRv6 locators to steer
traffic along distributed computed constraint paths according to the
associated Flex-Algo in a particular 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 Attributes
Each VTN can be allocated a set of dedicated network resources on
different network nodes and links.
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In order for a network controller or the ingress nodes to perform
constraint based path computation for each VTN, the resource
attributes of each VTN need to be advertised. This way, the network
controller or the ingress node can compute an SR TE path in a VTN by
taking both the Flex-Algo constraints and the resource attributes of
the VTN into consideration.
IS-IS L2 Bundle [RFC8668] was defined to advertise the link
attributes of the layer-2 bundle member links. In this section, it
is extended to advertise the set of network resource attributes
associated with different VTNs on a layer-3 link.
The layer-3 link must have the capability of partitioning the link
resources into different subsets for the different VTNs it
participates in. It may or may not be a bundle of layer-2 links to
achieve this. One partition of the link resources can be
instantiated as a layer-2 sub-interface, which can be seen as a
virtual layer-2 member link of the layer-3 link. If the layer-3 link
is a layer-2 link bundle, it is possible that the set of link
resources allocated to a specific VTN is provided by one or multiple
physical layer-2 member links.
A new flag "E" (Exclusive) 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|E| |
+-+-+-+-+-+-+-+-+
E flag: When the E flag is set, it indicates each member link under
the Parent L3 link is used exclusively for one VTN, and load sharing
among the member links is not allowed. When the E flag is clear, it
indicates load balancing and sharing among the member links are
allowed.
Note that legacy implementations of [RFC8668] will set the E flag to
zero (clear) meaning that load balancing among component links is the
default behavior. Further, when a legacy implementation receives an
E flag that is set, it will ignore the flag and so will assume that
load balancing among component links is allowed even when the sender
has requested it to not be used. The Flex-Algo associated with the
VTN can be defined that only nodes which support the E flag are
included in the constraint-based path computation and packet
forwarding of the VTN.
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For each virtual or physical layer-2 member link, the Admin Groups
(AG) or Extended Admin Group (EAG) attribute MUST be advertised using
the mechanisms as defined in [RFC8668]. This is for the correlation
between the Flex-Algo specific forwarding entries and the layer-2
member link. Other TE attributes as defined in [RFC5305] such as the
Maximum Link Bandwidth attribute MAY also be advertised for the
constraint-based path computation performed by the controller or the
ingress nodes. The SR-MPLS Adj-SIDs or SRv6 End.X SIDs associated
with each of the virtual or physical layer-2 member links MUST be
advertised according to [RFC8668] and [I-D.dong-lsr-l2bundle-srv6].
In order to correlate the virtual or physical layer-2 member links
with the Flex-Algo ID which is used to identify the VTN, each VTN is
assigned with a unique Admin Group (AG) or Extended Admin Group
(EAG), and the virtual or physical layer-2 member links associated
with this VTN are configured with the AG or EAG assigned to the VTN.
The AG or EAG of the parent layer-3 link is set to the union of all
the AGs or EAGs of its virtual or physical layer-2 member links. In
the definition of the Flex-Algo corresponding to the VTN, it MUST use
the Include-Any Admin Group rule with only the AG or EAG assigned to
the VTN as the link constraints, the Include-All Admin Goup rule or
the Exclude Admin Group rule MUST NOT be used. This is to ensure
that the layer-3 link is included in the Flex-Algo constraint based
path computation for each VTN it participates in.
4. Forwarding Plane Operations
For the SR-MPLS data plane, a prefix SID is associated with the paths
calculated using the Flex-Algo corresponding to 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
layer-2 member link which is associated with the VTN on the outgoing
layer-3 interface for packet forwarding. A forwarding entry MUST be
installed in the forwarding plane using the MPLS label that
corresponds to the Prefix-SID associated with the Flex-algorithm
corresponding to the VTN. The Adj-SIDs associated with the virtual
or physical member links of a VTN can be used together with the
prefix-SIDs of the same VTN to build SR-MPLS TE paths under the
topological and resource constraints of the VTN.
For the SRv6 data plane, an SRv6 Locator is a prefix which is
associated with the paths calculated using the Flex-Algo
corresponding to 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 layer-2 member link which is
associated with the VTN on the outgoing layer-3 interface for packet
forwarding. A forwarding entry for the SRv6 Locator prefix MUST be
installed in the forwarding plane for the Flex-algorithm
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corresponding to the VTN.The End.XU SIDs associated with the virtual
or physical member links of a VTN can be used together with other
types of SRv6 SIDs of the same VTN to build SRv6 paths under 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 Flex-Algo, so that the Flex-Algo 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
topological constraints, they still need to be identified using
different Flex-Algo IDs in the control plane, then independent path
computation needs to 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 number of Flex-Algos supported in
the protocol (which is 128) and the control plane overhead on network
nodes. 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 can be
found 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.
8. Acknowledgments
The authors would like to thank Zhenbin Li, Peter Psenak, Adrian
Farrel and Gyan Mishra for the review and discussion of this
document.
9. References
9.1. Normative References
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[I-D.dong-lsr-l2bundle-srv6]
Dong, J. and Z. Hu, "Advertising SRv6 SIDs for Layer 2
Bundle Member Links in IGP", Work in Progress, Internet-
Draft, draft-dong-lsr-l2bundle-srv6-01, 24 October 2021,
<https://www.ietf.org/archive/id/draft-dong-lsr-l2bundle-
srv6-01.txt>.
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", Work in Progress,
Internet-Draft, draft-ietf-lsr-flex-algo-20, 18 May 2022,
<https://www.ietf.org/archive/id/draft-ietf-lsr-flex-algo-
20.txt>.
[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>.
[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>.
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[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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 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>.
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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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