Using IS-IS Multi-Topology (MT) for
Segment Routing based Virtual Transport Network
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
machh@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
LSR Working Group
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
consists of a subset of network resources allocated on network nodes and
links in a customized topology of the physical network. A VTN could be
used as the underlay to support one or a group of VPN+ services.
In some network scenarios, each VTN can be associated with a unique
logical network topology. This document describes a mechanism to build
the SR based VTNs using IS-IS Multi-Topology together with other
well-defined IS-IS extensions.
Enhanced VPN (VPN+) is an enhancement to VPN services to support the
needs of new applications, particularly 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. specifies the framework of
enhanced VPN and describes the candidate component technologies in
different network planes and layers. VPN+ can be used to underpin
network slicing, but could also be of use in its own right providing
enhanced connectivity services between customer sites.
To meet the requirement of VPN+ services, a number of virtual
transport networks (VTN) can be created, each with a subset of network
resources allocated on network nodes and links in a customized topology
of the physical network. A VTN could be used as the underlay to meet the
requirement of one 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 resources reserved along the path, such paths are called Virtual
Transport Path (VTP). Although using a set of dedicated VTPs can provide
similar characteristics as a VTN, it has some scalability issues due to
the per-path state in the network.
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 SIDs
can be used to build SR based 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 topological instructions and the set of network
resources allocated by network nodes to a VTN. The SR SIDs and the
associated topology and resource attributes of a VTN need to be
distributed using control plane.
defines the IGP
mechanisms with necessary extensions to provide scalable Segment Routing
(SR) based VTNs. The VTNs could be used as the underlay of the VPN+
service. The mechanism described in allows flexible combination of
the topology and resource attribute to build a relatively large number
of VTNs. In some network scenarios, the required number of VTNs could be
small, and it is assumed that each VTN is associated with an independent
topology and has a set of dedicated or shared network resources. This
document describes a simplified mechanism to build SR based VTNs in
those scenarios. The resource-aware segments can be used with this
approach to provide resource guaranteed SR VTNs, while the normal SR
segments may also be used to provide SR VTNs with shared network
resources in the forwarding plane.
The proposed approach is to use IS-IS Multi-Topology with segment routing to
define the independent network topology of each VTN. The network
resources and other TE attributes of a VTN can be advertised using IS-IS
MT with the Traffic Engineering (TE) extensions defined in and .
IS-IS Multi-Topology Routing (MTR) has been
defined to create independent topologies in one network. In , MT-based TLVs are introduced to carry
topology-specific link-state information. The MT-specific Link or Prefix
TLVs are defined by adding additional two bytes, which includes 12-bit
MT-ID field in front of the ISN TLV and IP or IPv6 Reachability TLVs.
This provides the capability of specifying the customized attributes of
each topology. When each VTN is associated with an independent network
topology, MT-ID could be used as the identifier of VTN in control
plane.
MTR can be used with segment routing based data plane. Thus the
topology attribute of an SR based VTN could be advertised using MTR with
segment routing. The IS-IS extensions to support the advertisement of
topology-specific MPLS SIDs are specified in .
Topology-specific Prefix-SIDs can be advertised by carrying the
Prefix-SID sub-TLVs in the IS-IS TLV 235 (MT IP Reachability) and TLV
237 (MT IPv6 IP Reachability). Topology-specific Adj-SIDs can be
advertised by carrying the Adj-SID sub-TLVs in IS-IS TLV 222 (MT-ISN)
and TLV 223 (MT IS Neighbor Attribute). The topology-specific
Prefix-SIDs and Adj-SIDs can be resource-aware segments or normal SR
segments.
The IS-IS extensions to support the advertisement of
topology-specific SRv6 Locators and SIDs are specified in . The topology-specific SRv6
locators are advertised using SRv6 Locator TLV, and SRv6 End SIDs
inherit the MT-ID from the parent locator. The topology-specific End.X
SID are advertised by carrying SRv6 End.X SID sub-TLVs in the IS-IS TLV
222 (MT-ISN) and TLV 223 (MT IS Neighbor Attribute). The
topology-specific SRv6 locators can be resource-aware locator or normal
SRv6 locator, and accordingly the topology-specific SRv6 SIDs can be
resource-aware SRv6 segments or normal SRv6 segments.
In order to perform constraint based path computation for each VTN on
the network controller or on the ingress nodes, the network resource
attributes and other attributes associated with each VTN need to be
advertised.
On each network link, the information of the network resources and
other attributes associated with a VTN can be specified by carrying
the TE attributes sub-TLVs and in the IS-IS TLV 222 (MT-ISN) and TLV 223 (MT IS
Neighbor Attribute) of the corresponding topology.
When Maximum Link Bandwidth sub-TLV is carried in the MT-ISN TLV of
a topology, it indicates the amount of link bandwidth allocated to the
corresponding VTN. The bandwidth allocated to a VTN can be exclusive
for services carried in the corresponding VTN. The usage of other TE
attributes in topology-specific TLVs is for further study.
Editor's note: It is noted that carrying per-topology TE attributes
was considered as a possible feature in future when the encoding of
IS-IS multi-topology was defined in .
For SR-MPLS data plane, the Adj-SIDs and Prefix-SIDs associated with
the same VTN can be used together to build SR-MPLS paths with the
topological and resource constraints of the VTN taken into
consideration. A Prefix-SID is associated with the paths calculated in
the corresponding topology of the VTN. An outgoing interface is
determined for each path. In addition, the resource-aware prefix-SID can
steer the traffic to use the subset of network resources allocated to
the VTN on the outgoing interface for packet forwarding. A forwarding
entry is installed in the forwarding plane using the MPLS label that
corresponds to the Prefix-SID associated with the topology corresponding
to the VTN. A resource-aware Adj-SID is associated with a subset of
network resources allocated to the VTN on the link it identifies, and
can be used together with the prefix-SIDs of the same VTN to build
SR-MPLS TE paths of the VTN.
For SRv6 data plane, the SRv6 SIDs associated with the same VTN can
be used together to build SRv6 paths with the topological and resource
constraints of the VTN taken into consideration. An SRv6 Locator is a
prefix which is associated with the paths calculated in the
corresponding topology of the VTN. An outgoing interface is determined
for each path. In addition, the resource-aware SRv6 Locator prefix also
steers the traffic to use the subset of network resources which are
allocated to the VTN on the outgoing interface for packet forwarding. A
forwarding entry for the SRv6 Locator prefix is installed in the
forwarding plane for the topology corresponding to the VTN. A
resource-aware End.X SID is associated with a subset of network
resources allocated to the VTN on the link it identifies, and can be
used together with other types of SRv6 SIDs of the same VTN to build
SRv6 TE paths of the VTN.
The mechanism described in this document assumes that each VTN is
associated with a unique multi-topology, so that the MT-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 topology, they would
still need to be identified using different MT-IDs in the control plane,
then independent path computation needs to be executed for each VTN.
Thus the number of VTNs supported in a network may be dependent on the
number of topologies supported, which is related to both the number of
topologies supported in the protocol and the control plane overhead
which the network nodes could afford. 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
IS-IS.
The mechanism proposed in this document is subject to the same
vulnerabilities as any other protocol that relies on IGPs.
This document does not request any IANA actions.
The authors would like to thank Zhibo Hu, Dean Cheng, Les Ginsberg
and Peter Psenak for the review and discussion of this document.