IDR Working Group C. Xie
Internet-Draft C. Li
Intended status: Standards Track China Telecom
Expires: July 30, 2021 J. Dong
Z. Li
Huawei Technologies
January 26, 2021
BGP-LS with Multi-topology for Segment Routing based Virtual Transport
Networks
draft-xie-idr-bgpls-sr-vtn-mt-02
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 customized
network topology and a set of network resource allocated from the
physical network. A VTN could be used as the underlay to support 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 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. For network scenarios where each VTN can be identified
by a unique topology ID, this document describes a mechanism to
distribute the information of SR based VTNs using BGP-LS with Multi-
Topology.
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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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."
This Internet-Draft will expire on July 30, 2021.
Copyright Notice
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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+) 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
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properties require integration between the overlay connectivity and
the characteristics provided by the underlay 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 can be used for 5G
network slicing, and will also be of use in more generic scenarios.
To meet the requirement of enhanced VPN services, a number of Virtual
Transport Networks (VTNs) need to be created, each consists of a
subset of the underlay network topology and a set of network
resources allocated from the underlay network to meet the requirement
of one or a group of VPN+ services.
[I-D.ietf-spring-resource-aware-segments] introduces resource
awareness to Segment Routing (SR) [RFC8402], by associating existing
type of SIDs with network resource attributes (e.g. bandwidth,
processing or storage resources). These resource-aware SIDs retain
their original functionality, with the additional semantics of
identifying the set of network resources available for the packet
processing action.[I-D.dong-spring-sr-for-enhanced-vpn] describes the
use of resource-aware segments to build SR based VTNs. To allow the
network controller and network nodes to perform VTN-specific explicit
path computation and/or shortest path computation, the group of
resource-aware SIDs allocated by network nodes to each VTN and the
associated topology and resource attributes need to be distributed in
the control plane. When a centralized network controller is used for
VTN-specific 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, each VTN can be identified by a unique
topology ID [RFC5120], [I-D.xie-lsr-isis-sr-vtn-mt] describes an IGP
mechanism to advertise the association between the topology, resource
attributes and the SR SIDs for each VTN. This document describes a
mechanism to distribute the information of SR based VTNs to the
network controller using BGP-LS with Multi-Topology.
2. Advertisement of SR VTN Topology Attribute
[I-D.xie-lsr-isis-sr-vtn-mt] describes the IS-IS Multi-topology based
mechanisms to distribute the topology attributes 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.
[I-D.ietf-idr-bgp-ls-segment-routing-ext] defines the BGP-LS
extensions to carry the segment routing information using TLVs of
BGP-LS Attribute. When MTR 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 MTR 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
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topologies in a network is not a small number, some optimization may
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 will be elaborated in a future version.
2.2. Inter-Domain Topology Advertisement
[I-D.ietf-idr-bgpls-segment-routing-epe] and
[I-D.ietf-idr-bgpls-srv6-ext] defines the BGP-LS extensions for
advertisement of BGP topology information between ASes and the BGP
Peering Segment Identifiers. Such information could be used by a
network controller for the computation and instantiation of inter-AS
traffic engineering SR 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 resource attribute of each VTN and the
associated BGP Peering SIDs on the inter-domain links.
Depending on the requirement of inter-domain VTNs, different
mechanism can be used on the inter-domain connection:
o 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
is associated with one of the 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 identifier of the VTN SHOULD be advertised by the ASBR.
o 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 multi-domain VTN, while some other BGP sessions are used
for another multi-domain VTN. In this case, different BGP Peer
Node SIDs are allocated to each BGP session and are advertised
using the mechanism in [I-D.ietf-idr-bgpls-segment-routing-epe]
and [I-D.ietf-idr-bgpls-srv6-ext], the association between the BGP
Peer Node SIDs and the identifier of the VTN SHOULD be advertised
by the ASBR.
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o At the AS-level topology, different multi-domain VTNs may have
different inter-domain connectivity. Different BGP Peer Set SIDs
MAY be allocated to represent the groups of BGP peers which can be
used for load-balancing in each multi-domain VTN.
When MT-ID is used consistently in multiple ASes 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
[RFC7752][I-D.ietf-idr-bgpls-segment-routing-epe] and
[I-D.ietf-idr-bgpls-srv6-ext].
In network scenarios where consistent usage of MT-ID among multiple
domains can not be expected, a global-significant VTN-ID needs to be
introduced to define the inter-domain topologies. 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.xie-lsr-isis-sr-vtn-mt] specifies the mechanism to advertise the
resource information associated with each VTN. This section
describes the corresponding BGP-LS mechanisms.
The information of the network resources associated with a VTN can be
specified by carrying the TE Link attribute TLVs in BGP-LS Attribute
[RFC7752], with the associated MT-ID carried in the corresponding
Link NLRI.
When 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 each VTN is for further study.
4. Scalability Considerations
The mechanism described in this document requires that each VTN
mapped to an independent topology, and for the inter-domain VTNs, the
MT-IDs used in each involved domain need to be consistent. Reusing
MT-IDs as the identifier of VTN can avoid introducing new identifiers
in the control plane, while it also has some limitations. For
example, when multiple VTNs shares the same topology, each VTN still
need to be identified using different MT-IDs in the control plane,
thus independent path computation needs be executed for each VTN.
The number of VTNs supported in a network may be dependent on the
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number of topologies supported, which is related to the control plane
overhead.
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.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 (VTN) for Enhanced VPN", draft-dong-spring-sr-for-
enhanced-vpn-13 (work in progress), January 2021.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
(work in progress), June 2019.
[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
S., and J. Dong, "BGP-LS extensions for Segment Routing
BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
segment-routing-epe-19 (work in progress), May 2019.
[I-D.ietf-idr-bgpls-srv6-ext]
Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
daniel.bernier@bell.ca, d., and B. Decraene, "BGP Link
State Extensions for SRv6", draft-ietf-idr-bgpls-
srv6-ext-05 (work in progress), November 2020.
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[I-D.ietf-idr-rfc7752bis]
Talaulikar, K., "Distribution of Link-State and Traffic
Engineering Information Using BGP", draft-ietf-idr-
rfc7752bis-05 (work in progress), November 2020.
[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-01
(work in progress), January 2021.
[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>.
[RFC5029] Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
September 2007, <https://www.rfc-editor.org/info/rfc5029>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[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>.
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 Segment Routing based Enhanced VPN", draft-
dong-idr-bgpls-sr-enhanced-vpn-02 (work in progress), June
2020.
[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.
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[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-11
(work in progress), October 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+)
Service", draft-ietf-teas-enhanced-vpn-06 (work in
progress), July 2020.
[I-D.xie-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", draft-xie-lsr-isis-sr-vtn-mt-02 (work in
progress), October 2020.
[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>.
[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>.
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
Email: licong@chinatelecom.cn
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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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