Using Flex-Algo for Segment Routing based VTN
draft-zhu-lsr-isis-sr-vtn-flexalgo-04
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| Authors | Yongqing Zhu , Jie Dong , Zhibo Hu | ||
| Last updated | 2022-03-06 | ||
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draft-zhu-lsr-isis-sr-vtn-flexalgo-04
LSR Working Group Y. Zhu
Internet-Draft China Telecom
Intended status: Standards Track J. Dong
Expires: 8 September 2022 Z. Hu
Huawei Technologies
7 March 2022
Using Flex-Algo for Segment Routing based VTN
draft-zhu-lsr-isis-sr-vtn-flexalgo-04
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.
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 SR
based VTNs using SR Flex-Algo and IGP L2 bundle with minor
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
Task Force (IETF). Note that other groups may also distribute
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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."
This Internet-Draft will expire on 8 September 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Advertisement of SR VTN Topology Attributes . . . . . . . . . 3
3. Advertisement of SR VTN Resource Attributes . . . . . . . . . 4
4. Forwarding Plane Operations . . . . . . . . . . . . . . . . . 5
5. Scalability Considerations . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 set of network resources allocated
from the underlay network to meet the requirement of a specific VPN+
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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 SIDs can be
used to build 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 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 control plane.
[I-D.dong-lsr-sr-enhanced-vpn] defines the IGP mechanisms and
extensions to provide scalable Segment Routing (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, each VTN
can be associated with a unique Flex-Algo, and the set of network
resources allocated to the VTN can be instantiated using layer-2 sub-
interfaces or member links of the L3 interfaces. This document
describes a mechanism to build the SR based VTNs using SR Flex-Algo
and IGP L2 bundle with minor extensions.
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
BCP14 RFC 2119 [RFC2119] RFC 8174 [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
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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 with a set of dedicated network resources
on different network nodes and links. 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. 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 attribute 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 may or may not be a bundle of layer-2 links, as long
as it has the capability of partitioning the link resources into
different subsets for different VTNs it participates in. 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 resource allocated to a specific VTN is
provided by one or multiple physical layer-2 member links.
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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 are 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.
For each virtual or physical layer-2 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 SR-MPLS Adj-SIDs or SRv6 End.X SIDs associated with
each of the virtual or physical Layer-2 member links SHOULD also 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
SHOULD be 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 SHOULD be configured with the AG or EAG
assigned to the VTN. The AG or EAG of the parent layer-3 link SHOULD
be 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 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. 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 TE paths
with the topological and resource constraints of the VTN.
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For 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. The
End.XU 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 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 defined 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 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.
8. Acknowledgments
The authors would like to thank Zhenbin Li and Peter Psenak for the
review and discussion of this document.
9. References
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9.1. Normative References
[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-18, 25 October
2021, <https://www.ietf.org/archive/id/draft-ietf-lsr-
flex-algo-18.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-03, 12 July 2021,
<https://www.ietf.org/archive/id/draft-ietf-spring-
resource-aware-segments-03.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-01,
12 July 2021, <https://www.ietf.org/archive/id/draft-ietf-
spring-sr-for-enhanced-vpn-01.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-09, 25 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-teas-enhanced-
vpn-09.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-01, 7 February 2022,
<https://www.ietf.org/archive/id/draft-dong-teas-nrp-
scalability-01.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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