The Bidirectional Forwarding Detection (BFD) protocol (RFC 5880) is used to detect loss of connectivity between two forwarding engines, typically with low latency. BFD is leveraged by routing protocols, including the Border Gateway Protocol (BGP), to bring down routing protocol connections more quickly than the original protocol timers. This document defines a subcode for the BGP Cease NOTIFICATION message (Section 6.7 of RFC 4271) for use when a BGP connection is being closed due to a BFD session going down.

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The Bidirectional Forwarding Detection (BFD) protocol [RFC 5880] is used to detect loss of connectivity between two forwarding engines, typically with low latency. BFD is utilized as a service for various clients, including routing protocols, to provide an advisory mechanism for those clients to take appropriate actions when a BFD session goes down [RFC 5882]. This is typically used by the clients to trigger closure of their connections more quickly than the original protocol timers might allow. Border Gateway Protocol version 4 (BGP-4) [RFC 4271] terminates its connections upon Hold Timer expiration when the speaker does not receive a BGP message within the negotiated Hold Time interval. As per Sections 4.2 and 4.4 of [RFC 4271], the minimum Hold Time interval is at least three seconds, unless KEEPALIVE processing has been disabled by negotiating the distinguished Hold Time of zero. If a BGP speaker desires to have its connections terminate more quickly than the negotiated BGP Hold Timer can accommodate upon loss of connectivity with a neighbor, the BFD protocol can be relied upon by BGP speakers to supply that faster detection. When the BFD session state changes to Down, the BGP speaker terminates the connection with a Cease NOTIFICATION message sent to the neighbor, if possible, and then closes the TCP connection for the session. This document defines a subcode, "BFD Down", to be sent with the Cease NOTIFICATION message that indicates the reason for this type of connection termination.

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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

The value 10 has been allocated by IANA for the "BFD Down" Cease NOTIFICATION message subcode. When a BGP connection is terminated due to a BFD session going into the Down state, the BGP speaker SHOULD send a NOTIFICATION message with the error code "Cease" and the error subcode "BFD Down".

A BFD session may go into the Down state when there is only a partial loss of connectivity between two BGP speakers. Operators using BFD for their BGP connections make choices regarding what BFD timers are used based upon a variety of criteria -- for example, stability vs. fast failure. In the event of a BGP connection being terminated due to a "BFD Down" event from partial loss of connectivity as detected by BFD, the remote BGP speaker might be able to receive a BGP Cease NOTIFICATION message with the "BFD Down" subcode. The receiving BGP speaker will then have an understanding that the connection is being terminated because of a BFD-detected issue and not an issue with the BGP speaker. When there is a total loss of connectivity between two BGP speakers, it may not have been possible for the Cease NOTIFICATION message to have been sent. Even so, BGP speakers SHOULD provide this reason as part of their operational state. Examples include bgpPeerLastError per the BGP MIB [RFC 4273] and "last-error" per [BGP-YANG]. When the procedures in [RFC 8538] for sending a NOTIFICATION message with a "Cease" code and "Hard Reset" subcode are required, and the BGP connection is being terminated because BFD has gone into the Down state, the "BFD Down" subcode SHOULD be encapsulated in the Hard Reset's data portion of the NOTIFICATION message.

Similar to [RFC 4486], this document defines a subcode for the BGP Cease NOTIFICATION message that provides information to aid network operators in correlating network events and diagnosing BGP peering issues. This subcode is purely informational and has no impact on the BGP Finite State Machine beyond that already documented by [RFC 4271], Sections 6.6 and 6.7.

IANA has assigned the value 10 from the "BGP Cease NOTIFICATION message subcodes" registry , with the name "BFD Down" and a reference to this document.

[RFC2119]

S. Bradner, "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>.

[RFC4271]

Y. Rekhter, T. Li, and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC5880]

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

[RFC5882]

D. Katz, and D. Ward, "Generic Application of Bidirectional Forwarding Detection (BFD)", RFC 5882, DOI 10.17487/RFC5882, June 2010,
<https://www.rfc-editor.org/info/rfc5882>.

[RFC8174]

B. Leiba, "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>.

[RFC8538]

K. Patel, R. Fernando, J. Scudder, and J. Haas, "Notification Message Support for BGP Graceful Restart", RFC 8538, DOI 10.17487/RFC8538, March 2019,
<https://www.rfc-editor.org/info/rfc8538>.

[BGP-YANG]

Juniper Networks, M. Jethanandani, K. Patel, S. Hares, and J. Haas, "YANG Model for Border Gateway Protocol (BGP-4)", Internet-Draft draft-ietf-idr-bgp-model-16, March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-model-16>.

[RFC4273]

J. Haas, and S. Hares, "Definitions of Managed Objects for BGP-4", RFC 4273, DOI 10.17487/RFC4273, January 2006,
<https://www.rfc-editor.org/info/rfc4273>.

[RFC4486]

E. Chen, and V. Gillet, "Subcodes for BGP Cease Notification Message", RFC 4486, DOI 10.17487/RFC4486, April 2006,
<https://www.rfc-editor.org/info/rfc4486>.

Thanks to Jeff Tantsura and Dale Carder for their comments on this document. Mohamed Boucadair provided feedback as part of the Routing Directorate review of this document. In 2006, Bruno Rijsman had written a proposal that was substantively similar to this document: draft-rijsman-bfd-down-subcode. That draft did not progress in the Inter-Domain Routing (IDR) Working Group at that time. The author of this document was unaware of Bruno's prior work when creating this proposal.

Jeffrey Haas