BGP-LS with Multi-topology for
Segment Routing based Virtual Transport Networks
China Telecom
China Telecom Beijing Information Science & Technology,
Beiqijia
Beijing
102209
China
xiechf@chinatelecom.cn
China Telecom
China Telecom Beijing Information Science & Technology,
Beiqijia
Beijing
102209
China
licong@chinatelecom.cn
Huawei Technologies
Huawei Campus, No. 156 Beiqing Road
Beijing
100095
China
jie.dong@huawei.com
Huawei Technologies
Huawei Campus, No. 156 Beiqing Road
Beijing
100095
China
lizhenbin@huawei.com
Routing Area
IDR Working Group
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.
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.
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. 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.
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.
introduces
resource awareness to Segment Routing (SR) . 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 , 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.
describes the IGP Multi-Topology (MT) 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 with Multi-Topology.
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.
In section 4.2.2.1 of ,
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.
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.
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.
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
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.
and 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 and .
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 Node
SIDs SHOULD be allocated to each BGP session and are advertised
using the mechanism in and , 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 .
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 , 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.
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 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
.
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.
This document does not request any IANA actions.
The authors would like to thank Shunwan Zhuang for the review and
discussion of this document.
Key words for use in RFCs to Indicate Requirement
Levels
In many standards track documents several words are used to
signify the requirements in the specification. These words are
often capitalized. This document defines these words as they
should be interpreted in IETF documents. This document specifies
an Internet Best Current Practices for the Internet Community, and
requests discussion and suggestions for improvements.