RFC 9764: Bidirectional Forwarding Detection (BFD) Encapsulated in Large Packets

4.1. Implementations That Do Not Support Large BFD Packets

While this document proposes no change to the BFD protocol, implementations may not permit arbitrarily padded transport PDUs to carry BFD packets. While Section 6 of [RFC5880] warns against excessive pedantry, implementations may not work with this mechanism without additional support.¶

Section 6.8.6 of [RFC5880] discusses the procedures for receiving BFD Control packets. The length of the BFD Control packet is validated to be less than or equal to the payload of the encapsulating protocol. When a receiving implementation is incapable of processing large BFD packets, it could manifest in one of two possible ways:¶

In each of these cases, the BFD state machine would behave as if it were not receiving Control packets, and the receiving implementation would follow normal BFD procedures regarding not having received Control packets.¶

If large BFD packets is enabled on a session that is already in the Up state and the remote BFD system does not (or cannot) support receiving the padded BFD control packets, the session will go Down.¶

4.2. Selecting MTU Size To Be Detected

Since the consideration is Path MTU, BFD sessions using this feature only need to use an appropriate value of bfd.PaddedPduSize to exercise the Path MTU for the desired application. This may be significantly smaller than the system's link MTU, e.g., desired Path MTU is 1512 bytes, while the interface MTU that BFD with large packets is running on is 9000 bytes.¶

In the case multiple BFD clients desire to test the same BFD endpoints using different bfd.PaddedPduSize parameters, implementations SHOULD select the largest bfd.PaddedPduSize parameter from the configured sessions. This is similar to how implementations of BFD select the most aggressive timing parameters for multiple sessions to the same endpoint. Failure to select the largest size will result in BFD sessions going to the Up state and dependent applications not having their MTU requirements satisfied.¶

4.3. Detecting MTU Mismatches

The accepted MTU for an interface is impacted by packet encapsulation considerations at a given layer, e.g., Layer 2, Layer 3, tunnel, etc. A common misconfiguration of interface parameters is inconsistent MTU. In the presence of inconsistent MTU, it is possible for applications to have unidirectional connectivity.¶

When it is necessary for an application using BFD with Large Packets to test the bidirectional Path MTU, it is necessary to configure the bfd.PaddedPduSize parameter on each side of the BFD session. For example, if the desire is to verify a 1512-byte MTU in both directions on an Ethernet or point-to-point link, each side of the BFD session must have bfd.PaddedPduSize set to 1512. In the absence of such consistent configuration, BFD with Large Packets may correctly determine unidirectional connectivity at the tested MTU, but bidirectional MTU may not be properly validated.¶

It should be noted that some interfaces may intentionally have different MTUs. Setting the bfd.PaddedPduSize appropriately for each side of the BFD session supports such scenarios.¶

4.4. Detecting MTU Changes

Once BFD sessions using Large Packets has reached the Up state, connectivity at the tested MTU(s) for the session is being validated. If the Path MTU tested by the BFD with Large Packets session falls below the tested MTU, the BFD session will go Down.¶

In the opposite circumstance (where the Path MTU increases), the BFD session will continue without being impacted. BFD for Large Packets only ensures that the minimally acceptable MTU for the session can be used.¶

4.5. Equal-Cost Multipath (ECMP) or Other Load-Balancing Considerations

Various mechanisms are utilized to increase throughput between two endpoints at various network layers. Such features include Link Aggregation Groups (LAGs) or ECMP forwarding. Such mechanisms balance traffic across multiple physical links while hiding the details of that balancing from the higher networking layers. The details of that balancing are highly implementation specific.¶

In the presence of such load-balancing mechanisms, it is possible to have member links that are not properly forwarding traffic. In such circumstances, this will result in dropped traffic when traffic is chosen to be load balanced across those member links.¶

Such load-balancing mechanisms may not permit all link members to be properly tested by BFD. This is because the BFD Control packets may be forwarded only along links that are up. BFD on LAG interfaces, [RFC7130], was developed to help cover one such scenario. However, for testing forwarding over multiple hops, there is no such specified general-purpose BFD mechanism for exercising all links in an ECMP. This may result in a BFD session being in the Up state while some traffic may be dropped or otherwise negatively impacted along some component links.¶

Some BFD implementations utilize their internal understanding of the component links and their resultant forwarding to exercise BFD in such a way to better test the ECMP members and to tie the BFD session state to the health of that ECMP. Due to implementation-specific load balancing, it is not possible to standardize such additional mechanisms for BFD.¶

Misconfiguration of some member MTUs may lead to load balancing that may have an inconsistent Path MTU depending on how the traffic is balanced. While the intent of BFD with large packets is to verify Path MTU, it is subject to the same considerations above.¶

This section applies to most, if not all, BFD techniques.¶

4.6. S-BFD

This mechanism also can be applied to other forms of BFD, including Seamless BFD (S-BFD) [RFC7880].¶

6.1. YANG Security Considerations

This section is modeled after the template described in Section 3.7 of [YANG-GUIDELINES].¶

The "ietf-bfd-large" YANG module defines a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF [RFC6241] and RESTCONF [RFC8040]. These protocols have to use a secure transport layer (e.g., SSH [RFC4252], TLS [RFC8446], and QUIC [RFC9000]) and have to use mutual authentication.¶

The Network Configuration Access Control Model (NACM) [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.¶

There is one data node defined in this YANG module that is writable/creatable/deletable (i.e., "config true", which is the default). All writable data nodes are likely to be reasonably sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) and delete operations to these data nodes without proper protection or authentication can have a negative effect on network operations. The data node has particular sensitivities/vulnerabilities:¶

There are no particularly sensitive readable data nodes.¶

There are no particularly sensitive RPC or action operations.¶

Modules that use the groupings that are defined in this document should identify the corresponding security considerations. For example, reusing some of these groupings will expose privacy-related information (e.g., 'node-example'). This module defines one such grouping, "bfd-large-common", which contains the "pdu-size" data node whose security considerations are documented above.¶

7.1. The "IETF XML" Registry

IANA has registered the following URI in the "ns" subregistry of the "IETF XML Registry" [RFC3688].¶

URI:

urn:ietf:params:xml:ns:yang:ietf-bfd-large¶

Registrant Contact:

The IESG¶

XML:

N/A; the requested URI is an XML namespace.¶

7.2. The "YANG Module Names" Registry

IANA has registered the following YANG module in the "YANG Module Names" registry [RFC6020].¶

Name:

ietf-bfd-large¶

Maintained by IANA:

N¶

Namespace:

urn:ietf:params:xml:ns:yang:ietf-bfd-large¶

Prefix:

bfdl¶

Reference:

RFC 9764¶

8.1. Normative References

[RFC0791]

Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981, <https://www.rfc-editor.org/info/rfc791>.

[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>.

[RFC3688]

Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004, <https://www.rfc-editor.org/info/rfc3688>.

[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>.

[RFC5881]

Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI 10.17487/RFC5881, June 2010, <https://www.rfc-editor.org/info/rfc5881>.

[RFC5883]

Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, June 2010, <https://www.rfc-editor.org/info/rfc5883>.

[RFC6020]

Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010, <https://www.rfc-editor.org/info/rfc6020>.

[RFC7130]

Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed., Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional Forwarding Detection (BFD) on Link Aggregation Group (LAG) Interfaces", RFC 7130, DOI 10.17487/RFC7130, February 2014, <https://www.rfc-editor.org/info/rfc7130>.

[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>.

[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>.

[RFC8341]

Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, March 2018, <https://www.rfc-editor.org/info/rfc8341>.

[RFC8349]

Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for Routing Management (NMDA Version)", RFC 8349, DOI 10.17487/RFC8349, March 2018, <https://www.rfc-editor.org/info/rfc8349>.

[RFC9314]

Jethanandani, M., Ed., Rahman, R., Ed., Zheng, L., Ed., Pallagatti, S., and G. Mirsky, "YANG Data Model for Bidirectional Forwarding Detection (BFD)", RFC 9314, DOI 10.17487/RFC9314, September 2022, <https://www.rfc-editor.org/info/rfc9314>.

8.2. Informative References

[BFD-ECHO-PATH-MTU]

Min, X., Ed. and J. Haas, Ed., "Application of the BFD Echo function for Path MTU Verification or Detection", Work in Progress, Internet-Draft, draft-haas-xiao-bfd-echo-path-mtu-01, 11 July 2011, <https://datatracker.ietf.org/doc/html/draft-haas-xiao-bfd-echo-path-mtu-01>.

[RFC1191]

Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990, <https://www.rfc-editor.org/info/rfc1191>.

[RFC4252]

Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, January 2006, <https://www.rfc-editor.org/info/rfc4252>.

[RFC6241]

Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011, <https://www.rfc-editor.org/info/rfc6241>.

[RFC8040]

Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017, <https://www.rfc-editor.org/info/rfc8040>.

[RFC8446]

Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.

[RFC9000]

Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, <https://www.rfc-editor.org/info/rfc9000>.

[YANG-GUIDELINES]

Bierman, A., Boucadair, M., Ed., and Q. Wu, "Guidelines for Authors and Reviewers of Documents Containing YANG Data Models", Work in Progress, Internet-Draft, draft-ietf-netmod-rfc8407bis-22, 14 January 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-rfc8407bis-22>.