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8. Control Plane

8. Control Plane​

In an SDN environment, one expects the controller to explicitly provision the SIDs and/or discover them as part of a service discovery function. Applications residing on top of the controller could then discover the required SIDs and combine them to form a distributed network program.

The concept of "SRv6 Network Programming" refers to the capability of an application to encode any complex program as a set of individual functions distributed through the network. Some functions relate to underlay SLA, others to overlay/tenant, and others to complex applications residing in VMs and containers.

While not necessary for an SDN control plane, the remainder of this section provides a high-level illustrative overview of how control-plane protocols may be involved with SRv6. Their specification is outside the scope of this document.

8.1. IGP​

The End, End.T, and End.X SIDs express topological behaviors and hence are expected to be signaled in the IGP together with the flavors PSP, USP, and USD. The IGP should also advertise the Maximum SID Depth (MSD) capability of the node for each type of SRv6 operation -- in particular, the SR source (e.g., H.Encaps), intermediate endpoint (e.g., End and End.X), and final endpoint (e.g., End.DX4 and End.DT6) behaviors. These capabilities are factored in by an SR source node (or a controller) during the SR Policy computation.

The presence of SIDs in the IGP does not imply any routing semantics to the addresses represented by these SIDs. The routing reachability to an IPv6 address is solely governed by the non-SID-related IGP prefix reachability information that includes locators. Routing is neither governed nor influenced in any way by a SID advertisement in the IGP.

These SIDs provide important topological behaviors for the IGP to build Fast Reroute (FRR) solutions based on TI-LFA [SR-TI-LFA] and for TE processes relying on an IGP topology database to build SR Policies.

8.2. BGP-LS​

BGP-LS provides the functionality for topology discovery that includes the SRv6 capabilities of the nodes, their locators, and locally instantiated SIDs. This enables controllers or applications to build an inter-domain topology that can be used for computation of SR Policies using the SRv6 SIDs.

8.3. BGP IP/VPN/EVPN​

The End.DX4, End.DX6, End.DT4, End.DT6, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M SIDs can be signaled in BGP.

In some scenarios, an egress PE advertising a VPN route might wish to abstract the specific behavior bound to the SID from the ingress PE and other routers in the network. In such case, the SID may be advertised using the Opaque SRv6 Endpoint Behavior codepoint defined in Table 6. The details of such control-plane signaling mechanisms are out of the scope of this document.

8.4. Summary​

The following table summarizes which SID behaviors may be signaled in which control-plane protocol.

BehaviorIGPBGP-LSBGP IP/VPN/EVPN
End (PSP, USP, USD)XX
End.X (PSP, USP, USD)XX
End.T (PSP, USP, USD)XX
End.DX6XXX
End.DX4XXX
End.DT6XXX
End.DT4XXX
End.DT46XXX
End.DX2XX
End.DX2VXX
End.DT2UXX
End.DT2MXX
End.B6.EncapsX
End.B6.Encaps.RedX
End.B6.BMX

Table 3: SRv6 Locally Instantiated SIDs Signaling

The following table summarizes which SR Policy Headend capabilities may be signaled in which control-plane protocol.

BehaviorIGPBGP-LSBGP IP/VPN/EVPN
H.EncapsXX
H.Encaps.RedXX
H.Encaps.L2X
H.Encaps.L2.RedX

Table 4: SRv6 Policy Headend Behaviors Signaling

The previous table describes generic capabilities. It does not describe specific instantiated SR Policies.

For example, a BGP-LS advertisement of H.Encaps behavior would describe the capability of node N to perform H.Encaps behavior. Specifically, it would describe how many SIDs could be pushed by N without significant performance degradation.

As a reminder, an SR Policy is always assigned a Binding SID [RFC8402]. Binding SIDs are also advertised in BGP-LS as shown in Table 3. Hence, Table 4 only focuses on the generic capabilities related to H.Encaps.

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