Internet DRAFT - draft-ietf-idr-bgpls-sr-vtn-mt
draft-ietf-idr-bgpls-sr-vtn-mt
IDR Working Group C. Xie
Internet-Draft C. Li
Intended status: Informational China Telecom
Expires: 14 March 2024 J. Dong
Z. Li
Huawei Technologies
11 September 2023
BGP-LS with Multi-topology for Segment Routing based Virtual Transport
Networks
draft-ietf-idr-bgpls-sr-vtn-mt-03
Abstract
Enhanced VPN (VPN+) aims to provide enhanced VPN service to support
some applications' 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.
When Segment Routing is used as the data plane of VTNs, each VTN can
be allocated with a group of Segment Identifiers (SIDs) to identify
the topology and resource attributes of network segments in the VTN.
The association between the network topology, the network resource
attributes and the SR SIDs may need to be distributed to a
centralized network controller. In network scenarios where each VTN
can be associated with a unique logical network topology, this
document describes a mechanism to distribute the information of SR
based VTNs using BGP-LS with Multi-Topology.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 14 March 2024.
Copyright Notice
Copyright (c) 2023 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Advertisement of SR VTN Topology Attribute . . . . . . . . . 3
2.1. Intra-domain Topology Advertisement . . . . . . . . . . . 4
2.2. Inter-Domain Topology Advertisement . . . . . . . . . . . 5
3. Advertisement of SR VTN Resource Attribute . . . . . . . . . 6
4. Scalability Considerations . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Enhanced VPN (VPN+) provides 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 stringent performance
requirements. VPN+ requires integration between the overlay
connectivity and the characteristics provided by the underlay
networks. [I-D.ietf-teas-enhanced-vpn] specifies the framework of
VPN+ and describes the candidate component technologies in different
network planes and layers. VPN+ can be used to underpin network
slicing, and will also be of use in more generic scenarios.
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To meet the requirement of VPN+ services, a number of Virtual
Transport Networks (VTNs) need to be created, each of which consists
of a subset of network resources allocated from the underlay network,
and is associated with a customized logical topology. A VTN can be
used to support one or a group of VPN+ services.
[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 segments can be used to build SR based VTNs with the required
network topology and network resource attributes to support VPN+
services.
To allow the VTN-specific constraint-based path computation and/or
VTN-specific shortest path computation to be performed by network
controller and network nodes, the group of resource-aware SIDs
allocated by the network nodes for the VTN, together with the
associated topology and resource attributes of the VTN need to be
distributed in the control plane. When a centralized network
controller is used for VTN-specific constraint-based path
computation, especially when a VTN spans multiple IGP areas or
multiple Autonomous Systems (ASes), BGP-LS is needed to advertise the
VTN information in each IGP area or AS to the network controller, so
that the controller could use the collected information to build the
view of inter-area or inter-AS SR VTNs.
In some network scenarios, it is assumed that each VTN is associated
with an independent topology and has a set of dedicated or shared
network resources. [I-D.ietf-lsr-isis-sr-vtn-mt] describes the IGP
Multi-Topology (MT) [RFC5120] based mechanism to advertise the
topology and the associated SR SIDs, together with the resource and
TE attributes for each SR based VTN. This document describes a
mechanism to distribute the information of SR based VTNs to the
network controller using BGP-LS [I-D.ietf-idr-rfc7752bis] with Multi-
Topology.
2. Advertisement of SR VTN Topology Attribute
[I-D.ietf-lsr-isis-sr-vtn-mt] describes the IS-IS Multi-topology
based mechanisms to distribute the topology and the associated SR
SIDs of SR based VTNs. This section describes the corresponding BGP-
LS mechanism to distribute both the intra-domain and inter-domain
topology attributes of SR based VTNs.
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2.1. Intra-domain Topology Advertisement
In section 4.2.2.1 of [I-D.ietf-idr-rfc7752bis], Multi-Topology
Identifier (MT-ID) TLV is defined, which can contain one or more IS-
IS or OSPF Multi-Topology IDs. The MT-ID TLV MAY be present in a
Link Descriptor, a Prefix Descriptor, or the BGP-LS Attribute of a
Node NLRI.
[RFC9085] defines the BGP-LS extensions to carry the segment routing
information using TLVs of BGP-LS Attribute. When Multi-Topology is
used with SR-MPLS data plane, topology-specific prefix-SIDs and
topology-specific Adj-SIDs can be carried in the BGP-LS Attribute
associated with the prefix NLRI and link NLRI respectively, the MT-ID
TLV is carried in the prefix descriptor or link descriptor to
identify the corresponding topology of the SIDs.
[I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS extensions to
advertise SRv6 segments along with their functions and attributes.
When Multi-Topology is used with SRv6 data plane, the SRv6 Locator
TLV is carried in the BGP-LS Attribute associated with the prefix-
NLRI, the MT-ID TLV can be carried in the prefix descriptor to
identify the corresponding topology of the SRv6 Locator. The SRv6
End.X SIDs are carried in the BGP-LS Attribute associated with the
link NLRI, the MT-ID TLV can be carried in the link descriptor to
identify the corresponding topology of the End.X SIDs. The SRv6 SID
NLRI is defined to advertise other types of SRv6 SIDs, in which the
SRv6 SID descriptors can include the MT-ID TLV so as to advertise
topology-specific SRv6 SIDs.
[I-D.ietf-idr-rfc7752bis] also defines the rules of the usage of MT-
ID TLV:
"In a Link or Prefix Descriptor, only a single MT-ID TLV containing
the MT-ID of the topology where the link or the prefix is reachable
is allowed. In case one wants to advertise multiple topologies for a
given Link Descriptor or Prefix Descriptor, multiple NLRIs MUST be
generated where each NLRI contains a single unique MT-ID."
Editor's note: the above rules indicates that only one MT-ID is
allowed to be carried the Link or Prefix descriptors. When a link or
prefix needs to be advertised in multiple topologies, multiple NLRIs
needs to be generated to report all the topologies the link or prefix
participates in, together with the topology-specific segment routing
information and link attributes. This may increase the number of BGP
Updates needed for advertising MT-specific topology attributes, and
may introduce additional processing burden to both the sending BGP
speaker and the receiving network controller. When the number of
topologies in a network is not a small number, some optimization may
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be needed for the reporting of multi-topology information and the
associated segment routing information in BGP-LS. Based on the WG's
opinion, this may be elaborated in a future version.
2.2. Inter-Domain Topology Advertisement
[RFC9086] and [I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS
extensions for advertisement of BGP inter-domain topology information
and the BGP Egress Peering Segment Identifiers. Such information
could be used by a network controller for the computation and
instantiation of inter-AS SR TE paths.
In some network scenarios, there are needs to create VTNs which span
multiple ASes. The inter-domain VTNs could have different inter-
domain connectivity, and may be associated with different set of
network resources in each domain and also on the inter-domain links.
In order to build the multi-domain SR based VTNs, it is necessary to
advertise the topology and the associated BGP Peering SIDs of each
VTN for inter-domain links.
When MT-ID is used consistently in multiple domains covered by a VTN,
the topology-specific BGP peering SIDs can be advertised with the MT-
ID carried in the corresponding Link NLRI. This can be achieved with
the existing mechanisms as defined in
[I-D.ietf-idr-rfc7752bis][RFC9086] and [I-D.ietf-idr-bgpls-srv6-ext].
Depending on the requirement of inter-domain VTNs, different
mechanisms can be used on the inter-domain connection:
* One EBGP session between two ASes can be established over multiple
underlying links. In this case, different underlying links can be
used for different inter-domain VTNs which requires link isolation
between each other. In another similar case, the EBGP session is
established over a single link, while the network resource (e.g.
bandwidth) on this link can be partitioned into several pieces,
each of which can be considered as a virtual member link. A VTN
can be associated with one of the underlying physical or virtual
member links. In both cases, different BGP Peer-Adj-SIDs or SRv6
End.X SID SHOULD be allocated to each underlying physical or
virtual member link, the association between the BGP Peer Adj-SID/
End.X SID and the MT-ID of the VTN SHOULD be advertised by the
ASBR.
* For inter-domain connection between two ASes, multiple EBGP
sessions can be established between different set of peering
ASBRs. It is possible that some of these BGP sessions are used
for one inter-domain VTN, while some other BGP sessions are used
for another inter-domain VTN. In this case, different BGP Peer
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Node SIDs SHOULD be allocated to each BGP session and are
advertised using the mechanism in [RFC9086] and
[I-D.ietf-idr-bgpls-srv6-ext], the association between the BGP
Peer Node SIDs and the MT-ID of the VTN SHOULD be advertised by
the ASBR.
* At the AS-level topology, different inter-domain VTNs may have
different inter-AS connectivity. Then different BGP Peer Set SIDs
MAY be allocated to represent the groups of BGP peers which can be
used for load-balancing in each inter-domain VTN. The association
between the BGP Peer Node SIDs and the MT-ID of the VTN SHOULD be
advertised by the ASBR.
In network scenarios where consistent usage of MT-ID among multiple
domains can not be achieved, a global-significant identifier MAY be
introduced to identify the inter-domain topology of a VTN. Within
each domain, the MT based mechanism could be reused for intra-domain
topology advertisement. The detailed mechanism is specified in
[I-D.dong-idr-bgpls-sr-enhanced-vpn].
3. Advertisement of SR VTN Resource Attribute
[I-D.ietf-lsr-isis-sr-vtn-mt] specifies the mechanism to advertise
the resource and TE attributes associated with each VTN. This
section describes the corresponding BGP-LS mechanisms for reporting
VTN resource and TE attributes to network controllers.
The information of the network resources and TE attributes associated
with a link of a VTN can be specified by carrying the TE Link
attribute TLVs in BGP-LS Attribute [I-D.ietf-idr-rfc7752bis], with
the associated MT-ID carried in the corresponding Link NLRI.
When the Maximum Link Bandwidth sub-TLV is carried in the BGP-LS
attribute associated with the Link NLRI of a VTN, it indicates the
amount of link bandwidth resource allocated to the corresponding VTN
on the link. The bandwidth allocated to a VTN can be exclusive for
traffic in the corresponding VTN. The advertisement of other TE
attributes in BGP-LS for VTN is for further study.
4. Scalability Considerations
The mechanism described in this document requires that each VTN is
associated with an independent topology, and for the inter-domain
VTNs, the MT-IDs used in all the involved domains need to be
consistent. Reusing MT-ID as the identifier of VTN can avoid
introducing new mechanism with similar functionality in the control
plane, while it also has some limitations. For example, when
multiple VTNs have the same topology, each VTN still need to be
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identified using a unique MT-ID in the control plane, thus
independent path computation needs be executed for each VTN, although
the result of computation for these VTNs would be the same. 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.ietf-teas-nrp-scalability].
5. Security Considerations
This document introduces no additional security vulnerabilities to
BGP-LS.
The mechanism proposed in this document is subject to the same
vulnerabilities as any other protocol that relies on BGP-LS.
6. IANA Considerations
This document does not request any IANA actions.
7. Acknowledgments
The authors would like to thank Shunwan Zhuang for the review and
discussion of this document.
8. References
8.1. Normative References
[I-D.ietf-idr-bgpls-srv6-ext]
Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
Bernier, D., and B. Decraene, "BGP Link State Extensions
for SRv6", Work in Progress, Internet-Draft, draft-ietf-
idr-bgpls-srv6-ext-14, 17 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
bgpls-srv6-ext-14>.
[I-D.ietf-idr-rfc7752bis]
Talaulikar, K., "Distribution of Link-State and Traffic
Engineering Information Using BGP", Work in Progress,
Internet-Draft, draft-ietf-idr-rfc7752bis-17, 25 August
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
idr-rfc7752bis-17>.
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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", Work in Progress, Internet-Draft, draft-ietf-
spring-resource-aware-segments-07, 31 May 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
resource-aware-segments-07>.
[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-05,
31 May 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-spring-sr-for-enhanced-vpn-05>.
[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>.
[RFC9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., and M. Chen, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing", RFC 9085,
DOI 10.17487/RFC9085, August 2021,
<https://www.rfc-editor.org/info/rfc9085>.
[RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
Ray, S., and J. Dong, "Border Gateway Protocol - Link
State (BGP-LS) Extensions for Segment Routing BGP Egress
Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
2021, <https://www.rfc-editor.org/info/rfc9086>.
8.2. Informative References
[I-D.dong-idr-bgpls-sr-enhanced-vpn]
Dong, J., Hu, Z., Li, Z., Tang, X., and R. Pang, "BGP-LS
Extensions for Scalable Segment Routing based Enhanced
VPN", Work in Progress, Internet-Draft, draft-dong-idr-
bgpls-sr-enhanced-vpn-04, 4 March 2022,
<https://datatracker.ietf.org/doc/html/draft-dong-idr-
bgpls-sr-enhanced-vpn-04>.
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[I-D.ietf-lsr-isis-sr-vtn-mt]
Xie, C., Ma, C., Dong, J., and Z. Li, "Using IS-IS Multi-
Topology (MT) for Segment Routing based Virtual Transport
Network", Work in Progress, Internet-Draft, draft-ietf-
lsr-isis-sr-vtn-mt-05, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-lsr-
isis-sr-vtn-mt-05>.
[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+)",
Work in Progress, Internet-Draft, draft-ietf-teas-
enhanced-vpn-14, 28 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
enhanced-vpn-14>.
[I-D.ietf-teas-nrp-scalability]
Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J.,
Mishra, G. S., Qin, F., Saad, T., and V. P. Beeram,
"Scalability Considerations for Network Resource
Partition", Work in Progress, Internet-Draft, draft-ietf-
teas-nrp-scalability-02, 2 June 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
nrp-scalability-02>.
[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>.
Authors' Addresses
Chongfeng Xie
China Telecom
China Telecom Beijing Information Science & Technology, Beiqijia
Beijing
102209
China
Email: xiechf@chinatelecom.cn
Cong Li
China Telecom
China Telecom Beijing Information Science & Technology, Beiqijia
Beijing
102209
China
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Email: licong@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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