IS-IS Working Group J. Tantsura
Internet-Draft U. Chunduri
Intended status: Standards Track Individual
Expires: March 30, 2017 September 26, 2016

Signaling MSD (Maximum SID Depth) using IS-IS


This document proposes a way to expose Maximum SID Depth (MSD) supported by a node at node and/or link level by an ISIS Router. In a Segment Routing (SR) enabled network a centralized controller that programs SR tunnels at the head-end node needs to know the MSD information at node level and/or link level to push the label stack of an appropriate depth.

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Table of Contents

1. Introduction

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 IS-IS to expose the MSD of the node or link to a centralized controller.

PCEP SR extensions [I-D.ietf-pce-segment-routing] 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 [RFC7752] 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 [I-D.tantsura-idr-bgp-ls-segment-routing-msd]. For this information to be advertised by BGP for the all nodes and links of the network, where this is provisioned, IS-IS module should have this information in the LSDB.

[I-D.ietf-isis-mpls-elc] 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.

1.1. Conventions used in this document

1.1.1. Terminology

BGP-LS: Distribution of Link-State and TE Information using Border Gateway Protocol

ISIS: 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

1.2. Requirements Language

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 [RFC2119].

2. Terminology

This memo makes use of the terms defined in [RFC4971].

3. Node MSD Advertisement

A new sub-TLV within the body of IS-IS Router Capability TLV [RFC4971], called Node MSD sub-TLV is defined to carry the provisioned SID depth of the router originating the Router Capability TLV. Node MSD is the lowest MSD supported by the node and can be provisioned in IS-IS instance.

The Type (1 byte) of this sub-TLV is TBD.

Length is 1 bytes, and

the Value field contains MSD of the router originating the Router Capability TLV. 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 optional. The scope of the advertisement is specific to the deployment.

4. LINK MSD Advertisement

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 (1 byte) of this TLV is TBD.

Length is 1 byte, and

the Value field contains Link MSD of the router originating the corresponding IS extended reachability TLV [RFC5305] or MT IS TLV [RFC5120]. 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.

5. Acknowledgements


6. IANA Considerations

This document includes a request to IANA to allocate sub-TLV type codes for the new TLV proposed in Section 3 of this document from IS-IS Router Capability TLV Registry as defined by [RFC4971]. Also for link MSD, we request IANA to allocate new sub-TLV codes as defined in Section 4 from IS extended reachability TLV (22) and MT IS TLV (222) registry.

7. Security Considerations

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

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4971] Vasseur, JP., Shen, N. and R. Aggarwal, "Intermediate System to Intermediate System (IS-IS) Extensions for Advertising Router Information", RFC 4971, DOI 10.17487/RFC4971, July 2007.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008.

8.2. Informative References

[I-D.ietf-isis-mpls-elc] Xu, X., Kini, S., Sivabalan, S., Filsfils, C. and S. Litkowski, "Signaling Entropy Label Capability Using IS-IS", Internet-Draft draft-ietf-isis-mpls-elc-01, November 2015.
[I-D.ietf-pce-segment-routing] Sivabalan, S., Medved, J., Filsfils, C., Crabbe, E., Lopez, V., Tantsura, J., Henderickx, W. and J. Hardwick, "PCEP Extensions for Segment Routing", Internet-Draft draft-ietf-pce-segment-routing-07, March 2016.
[I-D.tantsura-idr-bgp-ls-segment-routing-msd] Tantsura, J., Mirsky, G., Sivabalan, S. and U. Chunduri, "Signaling Maximum SID Depth using Border Gateway Protocol Link-State", Internet-Draft draft-tantsura-idr-bgp-ls-segment-routing-msd-01, July 2016.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, DOI 10.17487/RFC1195, December 1990.
[RFC5120] Przygienda, T., Shen, N. and N. Sheth, "M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)", RFC 5120, DOI 10.17487/RFC5120, February 2008.
[RFC7752] Gredler, H., Medved, J., Previdi, S., Farrel, A. and S. Ray, "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, DOI 10.17487/RFC7752, March 2016.

Authors' Addresses

Jeff Tantsura Individual EMail:
Uma Chunduri Individual EMail: