RFC 9247 | BGP-LS Extensions for S-BFD | June 2022
Li, et al. | Standards Track | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  9247
Category:
  Standards Track
Published:
  June 2022
ISSN:
  2070-1721
Authors:
  Z. Li
  Huawei
  S. Zhuang
  Huawei
  K. Talaulikar, Ed.
  Arrcus, Inc.
  S. Aldrin
  Google, Inc.
  J. Tantsura
  Microsoft
  G. Mirsky
  Ericsson

# RFC 9247

# BGP - Link State (BGP-LS) Extensions for Seamless Bidirectional Forwarding Detection (S-BFD)

## Abstract

Seamless Bidirectional Forwarding Detection (S-BFD) defines a simplified mechanism to use Bidirectional Forwarding Detection (BFD) with large portions of negotiation aspects eliminated, thus providing benefits such as quick provisioning as well as improved control and flexibility to network nodes initiating the path monitoring. The link-state routing protocols (IS-IS and OSPF) have been extended to advertise the S-BFD Discriminators.¶

This document defines extensions to the BGP - Link State (BGP-LS) address family to carry the S-BFD Discriminators' information via BGP.¶

## Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9247.¶

## Copyright Notice

Copyright (c) 2022 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 (https://trustee.ietf.org/license-info) 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

## 1. Introduction

Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880] defines a simplified mechanism to use Bidirectional Forwarding Detection (BFD) [RFC5880] with large portions of negotiation aspects eliminated, thus providing benefits such as quick provisioning as well as improved control and flexibility to network nodes initiating the path monitoring.¶

For the monitoring of a service path end to end via S-BFD, the headend node (i.e., Initiator) needs to know the S-BFD Discriminator of the destination/tail-end node (i.e., Responder) of that service. The link-state routing protocols (IS-IS [RFC7883] and OSPF [RFC7884]) have been extended to advertise the S-BFD Discriminators. With this, an Initiator can learn the S-BFD Discriminator for all Responders within its IGP area/level or optionally within the domain. With networks being divided into multiple IGP domains for scaling and operational considerations, the service endpoints that require end-to-end S-BFD monitoring often span across IGP domains.¶

BGP - Link State (BGP-LS) [RFC7752] enables the collection and distribution of IGP link-state topology information via BGP sessions across IGP areas/levels and domains. The S-BFD Discriminator(s) of a node can thus be distributed along with the topology information via BGP-LS across IGP domains and even across multiple Autonomous Systems (ASes) within an administrative domain.¶

This document defines extensions to BGP-LS for carrying the S-BFD Discriminators' information.¶

## 2. Terminology

This memo makes use of the terms defined in [RFC7880].¶

### 2.1. 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.¶

## 3. BGP-LS Extensions for S-BFD Discriminators

BGP-LS [RFC7752] specifies the Node Network Layer Reachability Information (NLRI) for the advertisement of nodes and their attributes using the BGP-LS Attribute. The S-BFD Discriminators of a node are considered a node-level attribute and are advertised as such.¶

This document defines a new BGP-LS Attribute TLV called "S-BFD Discriminators TLV", and its format is as follows:¶

     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            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Discriminator 1                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Discriminator 2 (Optional)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               ...                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Discriminator n (Optional)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: S-BFD Discriminators TLV

where:¶

Type:
  1032¶
Length:
  variable. It MUST be a minimum of 4 octets, and it increments by 4 octets for each additional discriminator.¶
Discriminator n:
  4 octets each, carrying an S-BFD local discriminator value of the node. At least one discriminator MUST be included in the TLV.¶

The S-BFD Discriminators TLV can be added to the BGP-LS Attribute associated with the Node NLRI that originates the corresponding underlying IGP TLV/sub-TLV as described below. This information is derived from the protocol-specific advertisements as follows:¶

- IS-IS, as defined by the S-BFD Discriminators sub-TLV in [RFC7883].¶
- OSPFv2/OSPFv3, as defined by the S-BFD Discriminator TLV in [RFC7884].¶

## 4. IANA Considerations

IANA has permanently allocated the following code point in the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry. The column "IS-IS TLV/Sub-TLV" defined in the registry does not require any value and should be left empty.¶

Table 1: S-BFD Discriminators TLV Code Point Allocation
TLV Code Point | Description | Reference
--- | --- | ---
1032 | S-BFD Discriminators | This document

## 5. Manageability Considerations

The new protocol extensions introduced in this document augment the existing IGP topology information that was distributed via BGP-LS [RFC7752]. Procedures and protocol extensions defined in this document do not affect BGP protocol operations and management other than as discussed in "Manageability Considerations" (Section 6) of [RFC7752]. Specifically, the malformed NLRIs attribute tests in "Fault Management" (Section 6.2.2) of [RFC7752] now encompass the new TLV for the BGP-LS NLRI in this document.¶

## 6. Security Considerations

The new protocol extensions introduced in this document augment the existing IGP topology information that can be distributed via BGP-LS [RFC7752]. Procedures and protocol extensions defined in this document do not affect the BGP security model other than as discussed in "Security Considerations" (Section 8) of [RFC7752], i.e., the aspects related to limiting the nodes and consumers with which the topology information is shared via BGP-LS to trusted entities within an administrative domain.¶

The TLV introduced in this document is used to propagate IGP-defined information (see [RFC7883] and [RFC7884]). The TLV represents information used to set up S-BFD sessions. The IGP instances originating this information are assumed to support any required security and authentication mechanisms (as described in [RFC7883] and [RFC7884]).¶

Advertising the S-BFD Discriminators via BGP-LS makes it possible for attackers to initiate S-BFD sessions using the advertised information. The vulnerabilities this poses and how to mitigate them are discussed in [RFC7880].¶

## 7. References

### 7.1. 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, <https://www.rfc-editor.org/info/rfc2119>.
[RFC7752]
  Gredler, H., Ed., 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, <https://www.rfc-editor.org/info/rfc7752>.
[RFC7880]
  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S. Pallagatti, "Seamless Bidirectional Forwarding Detection (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016, <https://www.rfc-editor.org/info/rfc7880>.
[RFC7883]
  Ginsberg, L., Akiya, N., and M. Chen, "Advertising Seamless Bidirectional Forwarding Detection (S-BFD) Discriminators in IS-IS", RFC 7883, DOI 10.17487/RFC7883, July 2016, <https://www.rfc-editor.org/info/rfc7883>.
[RFC7884]
  Pignataro, C., Bhatia, M., Aldrin, S., and T. Ranganath, "OSPF Extensions to Advertise Seamless Bidirectional Forwarding Detection (S-BFD) Target Discriminators", RFC 7884, DOI 10.17487/RFC7884, July 2016, <https://www.rfc-editor.org/info/rfc7884>.
[RFC8174]
  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

### 7.2. Informative References

[RFC5880]
  Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, <https://www.rfc-editor.org/info/rfc5880>.

## Acknowledgements

The authors would like to thank Nan Wu for his contributions to this work. The authors would also like to thank Gunter Van de Velde and Thomas Fossati for their reviews as well as Jeff Haas for his shepherd review and Alvaro Retana for his AD review of this document.¶

## Authors' Addresses