IS-IS Working Group J. Tantsura
Internet-Draft Individual
Intended status: Standards Track U. Chunduri
Expires: December 6, 2017 Huawei Technologies
S. Aldrin
Google, Inc
L. Ginsberg
Cisco Systems
June 04, 2017

Signaling MSD (Maximum SID Depth) using IS-IS


This document proposes a way to signal Maximum SID Depth (MSD) supported by a node at node and/or link granularity by an IS-IS Router. In a Segment Routing (SR) enabled network a centralized controller that programs SR tunnels needs to know the MSD supported by the head-end at node and/or link granularity to push the SID stack of an appropriate depth. MSD is relevant to the head-end of a SR tunnel or Binding-SID anchor node where Binding-SID expansions might result in creation of a new SID stack.

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Table of Contents

1. Introduction

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 [I-D.ietf-pce-segment-routing] 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 [RFC7752] 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 [I-D.tantsura-idr-bgp-ls-segment-routing-msd]. 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.

[I-D.ietf-isis-mpls-elc] 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 push 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 labels (e.g. service) that are to be pushed 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, labels that can be pushed thru recirculation, or another dataplane e.g IPv6.

1.1. Conventions used in this document

1.1.1. Terminology

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

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.

2. Terminology

This memo makes use of the terms defined in [RFC7981].

3. Node MSD Advertisement

A new sub-TLV within the body of IS-IS Router Capability TLV [RFC7981], 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          |   


Figure 1: Node MSD Sub-TLV

The Type (1 byte) of this sub-TLV is 23 (Suggested value - to be assigned by IANA).

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 push MSD 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.

4. LINK MSD Advertisement

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       |   


Figure 2: Link MSD Sub-TLV

The Type (1 byte) of this sub-TLV is 15 (Suggested value - to be assigned by IANA).

Length is variable and similar to what is defined in Section 3.

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 push MSD of any depth; any other value represents that of the particular link MSD value.

5. Node MSD vs Link MSD conflict resolution

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

6. IANA Considerations

This document includes a request to IANA to allocate sub-TLV type codes for the new sub TLV proposed in Section 3 of this document from IS-IS Router Capability TLV Registry as defined by [RFC7981].

Type: 23 (suggested - to be assigned by IANA)

Description: Node MSD

Also for link MSD, we request IANA to allocate new sub-TLV codes as defined in Section 4 from Sub-TLVs for TLVs 22, 23, 141, 222 and 223 registry.

Type: 15 (suggested - to be assigned by IANA)

Description: Link MSD

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

Figure 3: TLVs where LINK MSD Sub-TLV can be present

This document requests the creation of a new IANA managed registry to identify MSD types as proposed in Section 3, Section 4. The registration procedure is "Expert Review" as defined in [RFC5226]. 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

Figure 4: MSD Sub-type Codepoints Registry

7. Security Considerations

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.

8. Contributors

The following people contributed to this document:

Peter Psenak


9. Acknowledgements

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

10. References

10.1. 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.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008.
[RFC7981] Ginsberg, L., Previdi, S. and M. Chen, "IS-IS Extensions for Advertising Router Information", RFC 7981, DOI 10.17487/RFC7981, October 2016.

10.2. 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-02, October 2016.
[I-D.ietf-pce-segment-routing] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W. and J. Hardwick, "PCEP Extensions for Segment Routing", Internet-Draft draft-ietf-pce-segment-routing-09, April 2017.
[I-D.tantsura-idr-bgp-ls-segment-routing-msd] Tantsura, J., Chunduri, U., Mirsky, G. and S. Sivabalan, "Signaling Maximum SID Depth using Border Gateway Protocol Link-State", Internet-Draft draft-tantsura-idr-bgp-ls-segment-routing-msd-04, March 2017.
[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 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 Huawei Technologies EMail:
Sam Aldrin Google, Inc EMail:
Les Ginsberg Cisco Systems EMail: