Using Flex-Algo for Segment Routing (SR) based Network Resource Partition (NRP)

Abstract

Enhanced VPNs aim to deliver VPN services with enhanced characteristics, such as guaranteed resources, latency, jitter, etc., so as to support customers requirements for connectivity services with these enhanced characteristics. Enhanced VPN requires integration between the overlay VPN connectivity and the characteristics provided by the underlay network. A Network Resource Partition (NRP) is a subset of the network resources and associated policies on each of a connected set of links in the underlay network. An NRP could be used as the underlay to support one or a group of enhanced VPN services.¶

In some network scenarios, each NRP can be associated with a unique Flexible Algorithm (Flex-Algo), which can provide constraint-path computation based on the customized topological constraints. This document specifies a mechanism to build Segment Routing (SR) based NRPs by combining SR Flex-Algo and IGP L2 bundles with minor extensions.¶

2. Advertisement of NRP Topology Attributes

[RFC9350] specifies the mechanism to provide distributed constraint-path computation, and the use of SR-MPLS prefix-SIDs and SRv6 locators in data plane 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 Flex-Algo, and the rules of including or excluding Admin Groups (i.e. colors) and Shared Risk Link Groups (SRLGs), the topology of an NRP can be described using the associated Flex-Algo. If each NRP is associated with a unique Flex-Algo, the Flex-Algo identifier could be used as the identifier of the NRP in the control plane.¶

With the mechanisms defined in[RFC8667] [RFC9350], SR-MPLS prefix-SID advertisement can be associated with a constrained topology, which can be defined by 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.¶

[RFC9352] specifies the IS-IS extensions to support SRv6 data plane, in which the SRv6 locators advertisement is associated with a constrained topology, which can be defined by a Flex-Algo. This allows the nodes to use the SRv6 locators to steer traffic along constraint paths computed using the Flex-Algo in the associated topology. In addition, SRv6 End SIDs and End.X SIDs which are associated with a Flex-Algo can be used to enforce traffic over the Loop-Free Alternatives (LFA) computed backup paths.¶

3. Advertisement of NRP Resource Attributes

Each NRP can be allocated with a set of dedicated or shared network resources on a connected set of links in the underlay network.¶

In order for a network controller or the ingress nodes to perform constraint based path computation for each NRP, the resource attributes of each NRP need to be advertised in the control plane, and distributed to the network controller. This way, the network controller or the ingress node can compute an SR-TE path in the NRP by taking both the Flex-Algo constraints and the resource attributes of the NRP 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 different subset of link resources and attributes associated with different NRPs on a layer-3 link.¶

The layer-3 link must have the capability of partitioning the link resources into different subsets and allocate them to different NRPs it participates in. Each partition of the link resources can be instantiated as a virtual sub-interface, which can be seen as a virtual layer-2 member link of the layer-3 link. When the layer-3 link is a layer-2 link bundle, the subset of link resources allocated to a specific NRP may be provided by one or multiple physical layer-2 member links in the link bundle.¶

Normally the member links of a L2 link bundle are used in load-balancing mode, some extension is needed to indicate that each member link is used exclusively for traffic of a specific NRP. 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 is used exclusively for one NRP, and load balancing 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.¶

Note that legacy implementations of [RFC8668] will set the E flag to zero (clear) meaning that load balancing among the component links is the default behavior. Further, when a legacy implementation receives an E flag that is set, it will ignore the flag and so will assume that load balancing among component links is allowed even when the sender has requested it to not be used. The Flex-Algo associated with the NRP can be defined that only nodes which support the E flag and the mechanisms defined in this document are included in the constraint-based path computation and packet forwarding of the NRP.¶

For each virtual or physical layer-2 member link under the layer-3 interface, the Admin Groups (AG) or Extended Admin Group (EAG) attribute MUST be advertised using the mechanisms as defined in [RFC8668]. Other TE attributes as defined in [RFC5305] such as the Maximum Link Bandwidth attribute MAY be advertised for the constraint-based path computation. The SR-MPLS Adj-SIDs or SRv6 End.X SIDs associated with each of the virtual or physical layer-2 member links SHOULD 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 associated Flex-Algo ID of the NRP, each NRP 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 NRP SHOULD be configured with the AG or EAG of the NRP. 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 which is associated with the NRP, the Include-Any Admin Group with only the AG or EAG assigned to the NRP SHOULD be used as the link constraints, the Include-All Admin Goup or the Exclude Admin Group rules MUST NOT be used for a Flex-Algo associated with an NRP. This is to ensure that the layer-3 link is included in the Flex-Algo constraint path computation for each NRP it participates in.¶

4. Forwarding Plane Operations

For the SR-MPLS data plane, a prefix SID is associated with the paths calculated using the Flex-Algo corresponding to the NRP. 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 NRP on the outgoing layer-3 interface for packet forwarding. A forwarding entry MUST be installed in the forwarding plane using the MPLS label that corresponds to the Prefix-SID associated with the Flex-algorithm corresponding to the NRP. The Adj-SIDs associated with the virtual or physical member links of an NRP MAY be used together with the prefix-SIDs of the same NRP to build SR-MPLS TE paths under the topological and resource constraints of the NRP.¶

For the SRv6 data plane, an SRv6 Locator is a prefix which is associated with the paths calculated using the Flex-Algo corresponding to the NRP. 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 NRP on the outgoing layer-3 interface for packet forwarding. A forwarding entry for the SRv6 Locator prefix MUST be installed in the forwarding plane for the Flex-algorithm corresponding to the NRP.The End.XU SIDs associated with the virtual or physical member links of an NRP MAY be used together with other types of SRv6 SIDs of the same NRP to build SRv6 paths under the topological and resource constraints of the NRP.¶

5. Scalability Considerations

The mechanism described in this document assumes that each NRP is associated with a unique Flex-Algo, so that the Flex-Algo IDs can be reused to identify the NRPs in the control plane. While this brings the benefit of simplicity, it also has some limitations. For example, it means that even if multiple NRPs share the same constrained topology, they would still need to be identified using different Flex-Algos in the control plane, then independent path computation needs to be executed for each NRP. Thus the number of NRPs 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 accomodate. The mechanism described in this document is considered useful for network scenarios in which the required number of NRPs is small, as minor control protocol extension is required. For network scenarios where the number of required NRPs is large, more scalable solution would be needed, which may require further protocol extensions and enhancements. A detailed analysis about the NRP scalability and the possible optimizations for supporting a large number of NRPs is described in [I-D.ietf-teas-nrp-scalability].¶

9. References