Signaling MSD (Maximum SID Depth) using OSPF

When Segment Routing tunnels are computed by a centralized controller, it is crucial that the controller knows the MSD "Maximum SID Depth" of the node or link SR tunnel exits over, so it doesn't download a path with SID (label stack) of a depth more than the node or link used is capable of imposing. This document describes how to use OSPF to expose the MSD of the node or link to a centralized controller. PCEP SR extensions has defined MSD, to signal in SR PCE Capability TLV, METRIC Object. However, If PCEP is not supported by a node (head-end of the SR tunnel) and controller does not participate in IGP routing it has no way to learn the MSD of the node or link configured. BGP-LS defines a way to expose topology and associated different attributes, capabilities of the nodes in that topology to a centralized controller and MSD has been defined in . For this information to be advertised by BGP for the all nodes and links of the network, where this is provisioned, OSPF module should have this information in the LSDB. defines, RLSDC which indicates how many labels a node can read to take a decision to insert an Entropy Label (EL) and is different than how many labels a node can push as defined by MSD in this draft.

BGP-LS: Distribution of Link-State and TE Information using Border Gateway Protocol OSPF: Open Shortest Path First MSD: Maximum SID Depth PCC: Path Computation Client PCE: Path Computation Element PCEP: Path Computation Element Protocol SID: Segment Identifier SR: Segment routing

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.

This memo makes use of the terms defined in .

A new TLV within the body of the OSPF RI Opaque LSA, called Node MSD TLV is defined to carry the provisioned SID depth of the router originating the RI LSA. Node MSD is the lowest MSD supported by the node. The Type (2 bytes) of this TLV is TBD. Length is 2 bytes, and the Value field contains MSD of the router originating the RI LSA. Node MSD is a number in the range of 0-254. 0 represents lack of the ability to push MSD of any depth; any other value represents that of the node. This value SHOULD represent the lowest value supported by node. This TLV is applicable to OSPFv2 and to OSPFv3 and is optional. The scope of the advertisement is specific to the deployment.

A new sub-TLV called Link MSD sub-TLV is defined to carry the provisioned SID depth of the interface associated with the link. The Type (2 bytes) of this TLV is TBD. Length is 2 bytes, and the Value field contains Link MSD of the router originating the corresponding LSA as specified for OSPFv3 and OSPFv3. Link MSD is a number in the range of 0-254. 0 represents lack of the ability to push MSD of any depth; any other value represents that of the particular link MSD value. For OSPFv2, the Link level MSD value is advertised as an optional Sub-TLV of OSPFv2 Extended Link TLV as defined in . For OSPFv3, the Link level MSD value is advertised as an optional Sub-TLV of the Router-Link TLV as defined in .

TBD

This document includes a request to IANA to allocate TLV type codes for the new TLV proposed in of this document from OSPF Router Information (RI) TLVs Registry as defined by . Also for link MSD, we request IANA to allocate new sub-TLV codes as proposed in from OSPFv2 Extended Link Opaque LSAs Extended Link TLV registry and from Router-Link TLV defined in OSPFv3 Extend-LSA Sub-TLV registry.

This document describes a mechanism for advertising Segment Routing SID depth supported at node and link level information through OSPF LSAs and does not introduce any new security issues.