IDR Working Group J. Tantsura
Internet-Draft Apstra, Inc.
Intended status: Standards Track U. Chunduri
Expires: December 3, 2019 Futurewei Technologies
K. Talaulikar
Cisco Systems
G. Mirsky
ZTE Corp.
N. Triantafillis
Apstra, Inc.
June 1, 2019

Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link-State


This document defines a way for a Border Gateway Protocol Link-State (BGP-LS) speaker to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity.

Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment Identifier (SID) stack can be supported in a given network.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on December 3, 2019.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

When Segment Routing (SR) [RFC8402] paths are computed by a centralized controller, it is critical that the controller learns the Maximum SID Depth (MSD) that can be imposed at each node/link on a given SR path. This ensures that the Segment Identifier (SID) stack depth of a computed path doesn't exceed the number of SIDs the node is capable of imposing.

[I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path Computation Element Protocol (PCEP). The OSPF and IS-IS extensions for signaling of MSD are defined in [RFC8476] and [RFC8491] respectively.

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 of learning the MSD of nodes and links. BGP-LS [RFC7752] defines a way to advertise topology and associated attributes and capabilities of the nodes in that topology to a centralized controller. This document defines extensions to BGP-LS to advertise one or more types of MSDs at node and/or link granularity.

Other types of MSD are known to be useful. For example, [I-D.ietf-ospf-mpls-elc] and [I-D.ietf-isis-mpls-elc] define Readable Label Depth Capability (RLDC) that is used by a head-end to insert an Entropy Label (EL) at a depth that can be read by transit nodes.

In the future, it is expected that new MSD-Types will be defined to signal additional capabilities, e.g., ELs, SIDs that can be imposed through recirculation, or SIDs associated with another data plane such as IPv6. MSD advertisements MAY be useful even if SR itself is not enabled. For example, in a non-SR MPLS network, MSD defines the maximum label depth.

1.1. Conventions used in this document

1.1.1. Terminology

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

MSD: Maximum SID Depth

PCC: Path Computation Client

PCE: Path Computation Element

PCEP: Path Computation Element Protocol

SID: Segment Identifier

SR: Segment routing

Label Imposition: Imposition is the act of modifying and/or adding labels to the outgoing label stack associated with a packet. This includes:

1.1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here .

2. Advertisement of MSD via BGP-LS

This document describes extensions that enable BGP-LS speakers to signal the MSD capabilities of nodes and their links in a network to a BGP-LS consumer of network topology such as a centralized controller. The centralized controller can leverage this information in computation of SR paths and their instantiation on network nodes based on their MSD capabilities. When a BGP-LS speaker is originating the topology learnt via link-state routing protocols like OSPF or IS-IS, the MSD information for the nodes and their links is sourced from the underlying extensions as defined in [RFC8476] and [RFC8491] respectively. The BGP-LS speaker may also advertise the MSD information for the local node and its links when not running any link-state IGP protocol e.g. when running BGP as the only routing protocol.

The extensions introduced in this document allow for advertisement of different MSD-Types. This document does not define these MSD-Types but leverages the definition, guidelines and the code-point registry specified in [RFC8491]. This enables sharing of MSD-Types that may be defined in the future by the IGPs in BGP-LS.

3. Node MSD TLV

Node MSD is encoded in a new Node Attribute TLV [RFC7752] using the following format:

   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            | 
  |    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |

Figure 1: Node MSD TLV Format


4. Link MSD TLV

Link MSD is encoded in a new Link Attribute TLV [RFC7752] using the following format:

   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            | 
  |    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |

Figure 2: Link MSD TLV Format


5. IANA Considerations

This document requests assigning code-points from the registry "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" based on table below. Early allocation for these code-points have been done by IANA.

| Code Point |   Description   |     IS-IS TLV/Sub-TLV     |
|    266     | Node MSD        | 242/23                    |
|    267     | Link MSD        | (22,23,25,141,222,223)/15 |

6. Security Considerations

The advertisement of an incorrect MSD value may have negative consequences. If the value is smaller than supported, path computation may fail to compute a viable path. If the value is larger than supported, an attempt to instantiate a path that can't be supported by the head-end (the node performing the SID imposition) may occur. The presence of this information may also inform an attacker of how to induce any of the aforementioned conditions.

This document does not introduce security issues beyond those discussed in [RFC7752], [RFC8476] and [RFC8491]

7. Contributors

Siva Sivabalan
Cisco Systems Inc.


8. Acknowledgements

We like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene for their reviews and valuable comments.

9. References

9.1. 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.
[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.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S. and P. Psenak, "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, DOI 10.17487/RFC8476, December 2018.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S. and L. Ginsberg, "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, DOI 10.17487/RFC8491, November 2018.

9.2. Informative References

[I-D.ietf-isis-mpls-elc] Xu, X., Kini, S., Psenak, P., Filsfils, C. and S. Litkowski, "Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS", Internet-Draft draft-ietf-isis-mpls-elc-07, May 2019.
[I-D.ietf-ospf-mpls-elc] Xu, X., Kini, S., Psenak, P., Filsfils, C. and S. Litkowski, "Signaling Entropy Label Capability and Entropy Readable Label-stack Depth Using OSPF", Internet-Draft draft-ietf-ospf-mpls-elc-08, May 2019.
[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-16, March 2019.
[RFC3031] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, DOI 10.17487/RFC3031, January 2001.
[RFC8402] Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., Litkowski, S. and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018.

Authors' Addresses

Jeff Tantsura Apstra, Inc. EMail:
Uma Chunduri Futurewei Technologies EMail:
Ketan Talaulikar Cisco Systems EMail:
Greg Mirsky ZTE Corp. EMail:
Nikos Triantafillis Apstra, Inc. EMail: