Signaling MSD (Maximum SID Depth) using IS-IS

When Segment Routing tunnels are computed by a centralized controller, it is critical that the controller learns the MSD "Maximum SID Depth" of the node or link SR tunnel exits over, so the SID stack depth of a path computed doesn't exceed the number of SIDs the node is capable of imposing. This document describes how to use IS-IS to signal the MSD of a node or link to a centralized controller. PCEP SR extensions draft signals MSD in SR PCE Capability TLV and METRIC Object. However, if PCEP is not supported/configured on the head-end of a SR tunnel or a Binding-SID anchor node and controller does not participate in IGP routing, it has no way to learn the MSD of nodes and links which has been configured. BGP-LS defines a way to expose topology and associated attributes and capabilities of the nodes in that topology to a centralized controller. MSD signaling by BGP-LS has been defined in . Typically, BGP-LS is configured on a small number of nodes, that do not necessarily act as head-ends. In order, for BGP-LS to signal MSD for the all nodes and links in the network MSD is relevant, MSD capabilites SHOULD be distributed to every IS-IS router in the network. defines Readable Label Depth Capability (RLDC) that is used by a head-end to insert Entropy Label (EL) at appropriate depth, so it could be read by transit nodes. MSD in contrary signals ability to impose SID's stack of a particular depth. MSD of type 1 (IANA Registry), called Base MSD, is used to signal the total number of SIDs a node is capable of imposing, to be used by a path computation element/controller. In case, there are additional SIDs (e.g. service) that are to be imposed to the stack - this would be signaled with an another MSD type (TBD), no adjustment to the Base MSD should be made. In the future, new MSD types could be defined to signal additional capabilities: entropy labels, SIDs that can be imposed thru recirculation, or another dataplane e.g IPv6.

BGP-LS: Distribution of Link-State and TE Information using Border Gateway Protocol IS-IS: Intermediate System to Intermediate System 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 sub-TLV within the body of IS-IS Router Capability TLV , Node MSD sub-TLV is defined to carry the provisioned MSD of the router originating the Router Capability TLV. Node MSD is the lowest MSD supported by the node of any interface and can be provisioned in IS-IS instance.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type and Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Type (1 byte) of this sub-TLV has value of 23. Length is variable (minimum of 2, multiple of 2 octets) and represents the total length of value field. Value field consists of a 1 octet sub-type (IANA Registry) and 1 octet value. Sub-Type 1 (IANA Section), MSD and the Value field contains maximum MSD of the router originating the Router Capability TLV. Node Maximum MSD is a number in the range of 0-254. 0 represents lack of the ability to impose MSD stack of any depth; any other value represents that of the node. This value SHOULD represent the lowest value supported by node. Other Sub-types other than defined above are reserved for future extensions. This sub-TLV is optional. The scope of the advertisement is specific to the deployment.

A new sub-TLV - Link MSD sub-TLV is defined for TLVs 22, 23, 141, 222, and 223 to carry the provisioned MSD of the interface associated with the link.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type and Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Type (1 byte) of this sub-TLV has value of 15. Length is variable and similar to what is defined in . Value field consists of a 1 octet sub-type (IANA Registry) and 1 octet value. Sub-Type 1 (IANA Section), MSD and the Value field contains Link MSD of the router originating the corresponding TLV's 22, 23, 141, 222, and 223. Link MSD is a number in the range of 0-254. 0 represents lack of the ability to impose MSD stack of any depth; any other value represents that of the particular link MSD value.

When both Node MSD and Link MSD are present, the value in the Link MSD MUST be used.

This document includes a request to IANA to allocate sub-TLV type codes for the new sub TLV proposed in of this document from IS-IS Router Capability TLV Registry as defined by . Following values have been allocated by IANA:

Value Description Reference ----- --------------- ------------- 23 Node MSD This document 15 Link MSD This document For the Link MSD, we request IANA to allocate new sub-TLV codes as defined in from Sub-TLVs for TLVs 22, 23, 141, 222 and 223 registry. Per TLV information where LINK MSD sub-TLV can be part of:

TLV 22 23 25 141 222 223 -------------------- y y y y y y This document requests the creation of a new IANA managed registry to identify MSD types as proposed in , . The registration procedure is "Expert Review" as defined in . Suggested registry name is "MSD Sub-types". Types are an unsigned 8 bit number. The following values are defined by this document

Value Name Reference ----- --------------------- ------------- 0 Reserved This document 1 Base MSD This document 2-250 Unassigned This document 251-254 Experimental This document 255 Reserved This document

This document describes a mechanism to signal Segment Routing MSD supported at node and/or link granularity through IS-IS LSPs and does not introduce any new security issues.

The following people contributed to this document: Peter Psenak Email: ppsenak@cisco.com

The authors would like to thank Stephane Litkowski and Bruno Decraene for their reviews and valuable comments.