Computing-Aware Traffic Steering (CATS) Using Segment Routing

Solution Overview This section describes the details of realizing CATS identifiers, CATS components, and workflow.

Realization of CATS Framework Components

CATS Identifiers A CATS Service ID (CS-ID) is an anycast IPv4 or IPv6 address. Such an IP address is associated with a specific service that is reachable via one or multiple service contact instances. The CATS overlay encapsulation is established from an Ingress CATS-Router to an Egress CATS-Router connected to a service contact instance. The service contact instance is typically hosted in a service site. Depending on the deployment requirements, CIS-IDs may be needed to indicate where to forward the packet to a specific interface pointing to a specific site in the case that multiple sites connect to the same Egress CATS-Router.

CATS Components In the context of this document, CATS-Routers are required to support SR encapsulation, including SR-MPLS and SRv6 . The CATS Traffic Classifier (C-TC) is assumed to be running on Ingress CATS-Routers. For each service site, one or multiple C-SMAs and C-NMAs can be implemented within the site to collect the metrics of the service instances.

Realization of the CATS Framework Workflow

Service Announcement The service anycast IP address may announced using a rendezvous service (DNS, for example). Clients can obtain the CS-ID of the service from the rendezvous service used by the application (e.g., DNS). It is out of scope of this document to provide a comprehensive list of all candidate rendezvous services.

Metrics Distribution As per the CATS framework, CS-ID routes with metrics are distributed among the overlay CATS Routers. The detailed control plane solutions of metrics distribution are out of the scope of this document. However, a sample procedure is provided for the readers convenience. For example, BGP can be used to distribute CS-ID routes with metrics. In the case of the C-SMA running as stand alone outside an Egress CATS-Router, the C-SMA collects the metrics of computing resource within a service site and distributes the CS-ID routes with the collected metrics to the Egress CATS-Router. Egress CATS-Routers will generate the new metrics combined with network metrics and computing-related metrics, and redistribute the CS-ID route to Ingress CATS-Routers. In the case of the C-SMA running as a logic entity on an Egress CATS-Router, the same process will be performed inside the Egress CATS-Router. As described in , CATS can be deployed in a distributed model, centralized model, or a hybrid model. In a centralized model or hybrid model, the routes with metrics may be collected by centralized controllers. BGP-LS may be a candidate solution to collect the route with metrics from CATS-Routers to controllers; the use of BGP-LS is however out of the scope of this document. A centralized controller may also install the forwarding policy on Ingress CATS-Routers to steer the traffic; how these policies are communicated to the routers is out of the scope of this document.

Service Demand Processing Two SR data plane approaches are supported: SRv6 and SR-MPLS . This section introduces a solution based upon SRv6 and SR-MPLS as data planes for CATS purposes. An Ingress CATS-Router generates SRv6/SR-MPLS encapsulations from itself to Egress CATS-Routers according to the SR policy received from a controller. An Ingress CATS-Router receives service routes with network and computing-related metrics from Egress CATS-Routers. An C-PS will select the best service site according to the received service routes and routing policies. Once the best service site is selected, the associated Egress CATS-Router can be determined and the appropriate SR encapsulation from an Ingress CATS-Router to the C-PS-computed Egress CATS-Router can be selected. When a service demand is received by an Ingress CATS-Router, it is classified by the C-TC component. When a matching classification entry is found for this demand, the Ingress CATS-Router encapsulates and forwards it to the C-PS selected Egress CATS-Router via the matching SR tunnel.

SRv6 As shown in , SRv6 tunnels are established from Ingress CATS-Routers to Egress CATS-Routers. There may be multiple encapsulations from a single Ingress CATS-Router to different Egress CATS-Routers so that the ingress can choose the best Egress CATS-Router connected to the target site. Furthermore, there may be multiple tunnels from a single Ingress CATS-Router to a single Egress CATS-Router, e.g., to provide different connectivity performance guarantees.

Using SRv6 in CATS In some cases, multiple service sites may be connected to a single Egress CATS-Router. To demux these sites, a specific attachment circuit must be provided to indicate the specific target service. In order to explicitly indicate the interface towards a site, an END.DX is encoded as the last segment in the SRv6 encapsulation. The associated END.DX is learned from the control plane. When the traffic reaches the Egress CATS-Router, the SRv6 packet is decapsulated and the traffic is forwarded to the service contact instance. How the packet is handled beyond that point is out of the scope.

SR-MPLS Similarly, SR-MPLS can be used as the overlay CATS encapsulation. The forwarding path is encoded as an MPLS label stack, and a potential VPN label can be included as the last label to indicate to steer the traffic through a specific interface to a target service contact instance in the case multiple service sites connect to the same Egress CATS-Router.

Service Instance Affinity As per , different services may have different notions of what constitutes a 'flow' and may thus identify a flow differently. Typically, a flow is identified by the 5-tuple transport coordinates (source and destination addresses, source and destination port numbers, and protocol).

Note: This section will be updated to reflect the discussion in the WG about affinity.