RFC 9764 PROPOSED STANDARD ROUTING · BFD · MTU
BFD large packets: Bidirectional Forwarding Detection (BFD) Encapsulated in Large Packets
Pad BFD packets with zeros so a BFD session also proves the path can carry packets of a chosen size.
RFC EDITOR
Haas, J. and A. Fu, "Bidirectional Forwarding Detection (BFD) Encapsulated in Large Packets", RFC 9764, DOI 10.17487/RFC9764, April 2025, <https://www.rfc-editor.org/info/rfc9764>.
BIBTEX
@misc{rfc9764,
series = {Request for Comments},
number = 9764,
howpublished = {RFC 9764},
publisher = {RFC Editor},
doi = {10.17487/RFC9764},
url = {https://www.rfc-editor.org/info/rfc9764},
author = {J. Haas and A. Fu},
title = {{Bidirectional Forwarding Detection (BFD) Encapsulated in Large Packets}},
pagetotal = 14,
year = 2025,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Haas
Request for Comments: 9764 Juniper Networks, Inc.
Category: Standards Track A. Fu
ISSN: 2070-1721 Bloomberg L.P.
April 2025
Bidirectional Forwarding Detection (BFD) Encapsulated in Large Packets
Abstract
The Bidirectional Forwarding Detection (BFD) protocol is commonly
used to verify connectivity between two systems. BFD packets are
typically very small. It is desirable in some circumstances to know
not only that the path between two systems is reachable, but also
that it is capable of carrying a payload of a particular size. This
document specifies how to implement such a mechanism using BFD in
Asynchronous mode.
YANG modules for managing this mechanism are also defined in this
document. These YANG modules augment the existing BFD YANG modules
defined in RFC 9314. The YANG modules in this document conform to
the Network Management Datastore Architecture (NMDA) (RFC 8342).
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/rfc9764.
Copyright Notice
Copyright (c) 2025 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.
Table of Contents
1. Introduction
2. Requirements Language
3. BFD Encapsulated in Large Packets
4. Implementation and Deployment Considerations
4.1. Implementations That Do Not Support Large BFD Packets
4.2. Selecting MTU Size To Be Detected
4.3. Detecting MTU Mismatches
4.4. Detecting MTU Changes
4.5. Equal-Cost Multipath (ECMP) or Other Load-Balancing
Considerations
4.6. S-BFD
5. BFD Encapsulated in Large Packets YANG Module
5.1. Data Model Overview
5.2. YANG Module
6. Security Considerations
6.1. YANG Security Considerations
7. IANA Considerations
7.1. The "IETF XML" Registry
7.2. The "YANG Module Names" Registry
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
The Bidirectional Forwarding Detection (BFD) [RFC5880] protocol is
commonly used to verify connectivity between two systems. However,
some applications may require that the Path MTU [RFC1191] between
those two systems meets a certain minimum criterion. When the Path
MTU decreases below the minimum threshold, those applications may
wish to consider the path unusable.
BFD may be encapsulated in a number of transport protocols. An
example is single-hop BFD [RFC5881]. In that case, the link MTU
configuration is typically enough to guarantee communication between
the two systems for that size MTU. BFD Echo mode (Section 6.4 of
[RFC5880]) is sufficient to permit verification of the Path MTU of
such directly connected systems. Previous proposals (e.g.,
[BFD-ECHO-PATH-MTU]) have been made for testing Path MTU for such
directly connected systems. However, in the case of multihop BFD
[RFC5883], this guarantee does not hold.
The encapsulation of BFD in multihop sessions is a simple UDP packet.
The BFD elements of procedure (Section 6.8.6 of [RFC5880]) cover
validating the BFD payload. However, the specification is silent on
the length of the encapsulation that is carrying the BFD PDU. While
it is most common that the transport protocol payload (i.e., UDP)
length is the exact size of the BFD PDU, this is not required by the
elements of procedure. This leads to the possibility that the
transport protocol length may be larger than the contained BFD PDU.
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.
3. BFD Encapsulated in Large Packets
Support for BFD between two systems is typically configured, even if
the actual session may be dynamically created by a client protocol.
A new BFD variable is defined in this document:
bfd.PaddedPduSize
The BFD transport protocol payload size (in bytes) is increased to
this value. The contents of this additional payload MUST be zero.
The contents of this additional payload SHOULD NOT be validated by
the receiver. The minimum size of this variable MUST NOT be
smaller than 24 or 26 bytes, as permitted by the element of BFD
procedure; see Section 6.8.6 of [RFC5880].
The Don't Fragment bit (Section 2.3 of [RFC0791]) of the IP payload,
when using IPv4 encapsulation, MUST be set.
4. Implementation and Deployment Considerations
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:
* A receiving BFD implementation is incapable of accepting large BFD
packets. This is identical to the packet being discarded.
* A receiving BFD implementation is capable of accepting large BFD
packets, but the Control packet is improperly rejected during
validation procedures. This is identical to the packet being
discarded.
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].
5. BFD Encapsulated in Large Packets YANG Module
5.1. Data Model Overview
This YANG module augments the "ietf-bfd" module to add a flag
'padding' to enable this feature. The feature statement 'padding'
needs to be enabled to indicate that BFD encapsulated in large
packets is supported by the implementation.
Further, this YANG module augments the YANG modules for single-hop,
multihop, LAG, and MPLS to add the "pdu-size" parameter to those
session types to configure large BFD packets.
Finally, similar to the grouping "client-cfg-parms" defined in
Section 2.1 of [RFC9314], this YANG module defines a grouping "bfd-
large-common" that may be utilized by BFD clients using "client-cfg-
params" to uniformly add support for the feature defined in this RFC.
module: ietf-bfd-large
augment /rt:routing/rt:control-plane-protocols
/rt:control-plane-protocol/bfd:bfd/bfd-ip-sh:ip-sh
/bfd-ip-sh:sessions/bfd-ip-sh:session:
+--rw pdu-size? padded-pdu-size {padding}?
augment /rt:routing/rt:control-plane-protocols
/rt:control-plane-protocol/bfd:bfd/bfd-ip-mh:ip-mh
/bfd-ip-mh:session-groups/bfd-ip-mh:session-group:
+--rw pdu-size? padded-pdu-size {padding}?
augment /rt:routing/rt:control-plane-protocols
/rt:control-plane-protocol/bfd:bfd/bfd-lag:lag
/bfd-lag:sessions/bfd-lag:session:
+--rw pdu-size? padded-pdu-size {padding}?
augment /rt:routing/rt:control-plane-protocols
/rt:control-plane-protocol/bfd:bfd/bfd-mpls:mpls
/bfd-mpls:session-groups/bfd-mpls:session-group:
+--rw pdu-size? padded-pdu-size {padding}?
Figure 1
5.2. YANG Module
This YANG module imports "A YANG Data Model for Routing Management
(NMDA Version)" [RFC8349] and "YANG Data Model for Bidirectional
Forwarding Detection (BFD)" [RFC9314].
<CODE BEGINS> file "ietf-bfd-large@2025-04-04.yang"
module ietf-bfd-large {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-large";
prefix bfdl;
import ietf-routing {
prefix rt;
reference
"RFC 8349: A YANG Data Model for Routing Management
(NMDA version)";
}
import ietf-bfd {
prefix bfd;
reference
"RFC 9314: YANG Data Model for Bidirectional
Forwarding Detection.";
}
import ietf-bfd-ip-sh {
prefix bfd-ip-sh;
reference
"RFC 9314: YANG Data Model for Bidirectional
Forwarding Detection.";
}
import ietf-bfd-ip-mh {
prefix bfd-ip-mh;
reference
"RFC 9314: YANG Data Model for Bidirectional
Forwarding Detection.";
}
import ietf-bfd-lag {
prefix bfd-lag;
reference
"RFC 9314: YANG Data Model for Bidirectional
Forwarding Detection.";
}
import ietf-bfd-mpls {
prefix bfd-mpls;
reference
"RFC 9314: YANG Data Model for Bidirectional
Forwarding Detection.";
}
organization
"IETF BFD Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/bfd>
WG List: <rtg-bfd@ietf.org>
Authors: Jeffrey Haas (jhaas@juniper.net)
Albert Fu (afu14@bloomberg.net).";
description
"This YANG module augments the base BFD YANG module to add
attributes related to support for BFD Encapsulated in Large
Packets. In particular, it adds a per-session parameter for the
BFD Padded PDU Size.
Copyright (c) 2025 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject to
the license terms contained in, the Revised BSD License set
forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 9764
(https://www.rfc-editor.org/info/rfc9764); see the RFC itself
for full legal notices.
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.";
revision 2025-04-04 {
description
"Initial Version.";
reference
"RFC 9764, Bidirectional Forwarding Detection (BFD)
Encapsulated in Large Packets.";
}
feature padding {
description
"If supported, the feature allows for BFD sessions to be
configured with padded PDUs in support of BFD Encapsulated in
Large Packets.";
}
typedef padded-pdu-size {
type uint16 {
range "24..65535";
}
units "bytes";
description
"The size of the padded and encapsulated BFD control packets
to be transmitted at Layer 3. The BFD minimum control packet
size is 24 or 26 octets; see Section 6.8.6 of RFC 5880.
If the configured padded PDU size is smaller than the minimum
sized packet of a given BFD session, then the minimum sized
packet for the session will be used.
The maximum padded PDU size may be limited by the supported
interface MTU of the system.";
reference
"RFC 9764, Bidirectional Forwarding Detection (BFD)
Encapsulated in Large Packets.";
}
grouping bfd-large-common {
description
"Common configuration and operational state for BFD
Encapsulated in Large Packets.";
reference
"RFC 9764, Bidirectional Forwarding Detection (BFD)
Encapsulated in Large Packets.";
leaf pdu-size {
if-feature "padding";
type padded-pdu-size;
description
"If set, this configures the padded PDU size for the
Asynchronous mode BFD session. By default, no additional
padding is added to such packets.";
}
}
augment "/rt:routing/rt:control-plane-protocols/"
+ "rt:control-plane-protocol/bfd:bfd/bfd-ip-sh:ip-sh/"
+ "bfd-ip-sh:sessions/bfd-ip-sh:session" {
uses bfd-large-common;
description
"Augment the 'bfd' container to add attributes related to BFD
Encapsulated in Large Packets.";
}
augment "/rt:routing/rt:control-plane-protocols/"
+ "rt:control-plane-protocol/bfd:bfd/bfd-ip-mh:ip-mh/"
+ "bfd-ip-mh:session-groups/bfd-ip-mh:session-group" {
uses bfd-large-common;
description
"Augment the 'bfd' container to add attributes related to BFD
Encapsulated in Large Packets.";
}
augment "/rt:routing/rt:control-plane-protocols/"
+ "rt:control-plane-protocol/bfd:bfd/bfd-lag:lag/"
+ "bfd-lag:sessions/bfd-lag:session" {
uses bfd-large-common;
description
"Augment the 'bfd' container to add attributes related to BFD
Encapsulated in Large Packets.";
}
augment "/rt:routing/rt:control-plane-protocols/"
+ "rt:control-plane-protocol/bfd:bfd/bfd-mpls:mpls/"
+ "bfd-mpls:session-groups/bfd-mpls:session-group" {
uses bfd-large-common;
description
"Augment the 'bfd' container to add attributes related to BFD
Encapsulated in Large Packets.";
}
}
<CODE ENDS>
Figure 2
6. Security Considerations
This document does not change the underlying security considerations
of the BFD protocol or its encapsulations.
On-path attackers that can selectively drop BFD packets, including
those with large MTUs, can cause BFD sessions to go Down.
The contents of the padding payload are set to zero. This avoids
implementation issues where the local uninitialized data may be
leaked.
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:
* 'pdu-size' specifies the targeted size of BFD control packets
encapsulated according to this proposal. Changing this value for
a session in the Up state may cause the session to go down,
perhaps intentionally, if the session cannot accommodate such BFD
control packets. Operators should be mindful that multiple BFD
clients may rely on the status of a given BFD session when
changing this value.
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. IANA Considerations
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. References
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>.
Acknowledgments
The authors would like to thank Les Ginsberg, Mahesh Jethanandani,
Robert Raszuk, and Ketan Talaulikar, for their valuable feedback on
this proposal.
Authors' Addresses
Jeffrey Haas
Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: jhaas@juniper.net
Albert Fu
Bloomberg L.P.
731 Lexington Avenue
New York, NY 10022
United States of America
Email: afu14@bloomberg.net
AI ELI5
On this page
PROBLEMBFD packets are tiny, so a BFD session going Up shows only that a path works for small packets. Some applications also need the Path MTU between two systems to meet a minimum.
MECHANISMA new BFD variable, bfd.PaddedPduSize, makes the sender pad the BFD transport payload with zero bytes up to that size. If the path cannot carry the bigger packets, they are lost and the session goes Down.
WHY IT MATTERSBFD sessions can now double as a continuous minimum-Path-MTU check, with YANG modules to configure it. It is a Proposed Standard and it does not obsolete or update any other RFC.
§1Why Plain BFD Cannot Test Packet Size
Bidirectional Forwarding Detection (BFD) is commonly used to verify connectivity between two systems. BFD packets are typically very small, so a working session says nothing about whether a bigger packet would get through.
Some applications require that the Path MTU between the two systems meets a minimum. When it drops below that threshold they may wish to treat the path as unusable.
For directly connected systems, the link MTU configuration is typically enough, and BFD Echo mode is sufficient to verify the Path MTU. With multihop BFD that guarantee does not hold. Multihop BFD is carried in a simple UDP packet, and the base BFD specification is silent on the length of the encapsulation carrying the BFD message. The UDP payload is usually exactly the size of the BFD message, but nothing requires that, so it may be larger.
A tiny test packet slips through almost anywhere. This RFC notices that nothing stops us from sending a bigger one, and a bigger one is a better test.
§3The Padded PDU Size Variable
The RFC defines one new BFD variable, bfd.PaddedPduSize. The BFD transport protocol payload size, in bytes, is increased to this value. The extra payload MUST be zero, and the receiver SHOULD NOT validate its contents.
The variable must not be smaller than 24 or 26 bytes, the BFD control packet sizes permitted by the BFD receive procedures. When IPv4 encapsulation is used, the Don't Fragment bit MUST be set. Together these mean a packet that is too big for the path is dropped instead of being fragmented, so BFD notices.
The mechanism applies to BFD in Asynchronous mode. The RFC says it proposes no change to the BFD protocol itself.
You pick a size and the sender stuffs the packet with empty padding until it is that big. The padding is just filler, and the receiver does not need to look at it.
§4.1Peers That Do Not Support Large Packets
Some implementations may not accept arbitrarily padded transport payloads. A receiver that cannot take large BFD packets, or that wrongly rejects them during validation, behaves exactly as if the packet had been discarded. The BFD state machine then acts as if no Control packets were arriving and follows the normal procedures for that.
The consequence is that if large BFD packets are enabled on a session already in the Up state and the remote system cannot receive the padded packets, the session goes Down.
If the other side cannot handle the big envelopes, it acts like they never arrived, and the connection is declared broken.
§4.2Choosing and Checking the Size
Only the Path MTU the application needs has to be exercised. The RFC gives an example where the desired Path MTU is 1512 bytes while the interface MTU is 9000 bytes, so the padded size may be much smaller than the link MTU.
If several BFD clients want to test the same endpoints with different sizes, implementations SHOULD select the largest bfd.PaddedPduSize among the configured sessions, like BFD picks the most aggressive timers for multiple sessions to one endpoint. Otherwise a session can be Up while some application's MTU needs are not met.
To test the Path MTU in both directions, the parameter must be configured on each side of the session. For example, verifying a 1512-byte MTU both ways needs 1512 on both sides. Without consistent configuration the mechanism may correctly find unidirectional connectivity at the tested MTU but not validate the bidirectional MTU. Interfaces that intentionally differ can be set appropriately per side.
Once a session is Up, it keeps validating the tested size. If the Path MTU falls below it, the session goes Down. If the Path MTU grows, the session is not affected, because the mechanism only ensures the minimally acceptable MTU can be used.
You only need to test for the size you care about, not the biggest the road allows. Both ends should test, or you only learn about one direction.
- Path MTU falls belowThe BFD session goes Down.
- Path MTU risesThe session continues unaffected.
- Peer cannot take big packetsPackets are treated as discarded; an Up session goes Down.
- Both directionsConfigure the size on each side of the session.
§4.5Load Balancing and Other Forms of BFD
Link aggregation groups and equal-cost multipath (ECMP) forwarding spread traffic across several links while hiding the details from higher layers. A member link may not forward properly, and BFD packets may only be sent along links that are up, so a BFD session can be Up while some traffic is dropped on some component links.
BFD on LAG interfaces (RFC 7130) covers one such scenario, but for multihop paths there is no general-purpose BFD mechanism to exercise all ECMP links. Some implementations use internal knowledge of the component links to test them, but because load balancing is implementation specific, this cannot be standardized. Misconfigured member MTUs can make the Path MTU inconsistent depending on how traffic is balanced, and large-packet BFD is subject to the same limits. The RFC says this applies to most, if not all, BFD techniques.
The mechanism can also be applied to other forms of BFD, including Seamless BFD (S-BFD, RFC 7880).
If a road has many lanes and one has a pothole, your test car might happen to drive on a good lane. This trick cannot promise every lane is checked.
§5The YANG Modules
The RFC defines the ietf-bfd-large YANG module, which augments the BFD YANG modules from RFC 9314 and conforms to the Network Management Datastore Architecture. A feature named padding must be enabled to indicate an implementation supports large BFD packets.
The module adds an optional pdu-size leaf, of type padded-pdu-size, to single-hop, multihop, LAG and MPLS session types. The type is a uint16 in bytes with a range of 24 to 65535. If the configured size is smaller than the minimum packet of a session, the session's minimum is used, and by default no extra padding is added. The module also defines a grouping, bfd-large-common, that BFD clients can reuse to add the feature uniformly.
This part is the settings form network tools use to turn the feature on and type in the packet size.
§6Security and IANA
The document does not change the underlying security considerations of BFD or its encapsulations. On-path attackers who can selectively drop BFD packets, including large ones, can make BFD sessions go Down. The padding is set to zero so that uninitialized local data is not leaked.
The one writable YANG node, pdu-size, is sensitive: changing it on an Up session may take the session down if it cannot accommodate the packets. Operators should remember that several BFD clients may depend on one session. IANA registered the XML namespace and the ietf-bfd-large module name.
Someone in the middle can throw away your packets and make the link look broken. The filler is all zeros so no secret scraps of memory are sent by accident.
§8References
[RFC5880]RFC 5880Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, June 2010.June 2010[RFC5881]RFC 5881Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June 2010.June 2010[RFC5883]RFC 5883Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for Multihop Paths", RFC 5883, June 2010.June 2010[RFC7130]RFC 7130Bhatia, M., Ed., et al., "Bidirectional Forwarding Detection (BFD) on Link Aggregation Group (LAG) Interfaces", RFC 7130, February 2014.February 2014[RFC7880]RFC 7880Pignataro, C., et al., "Seamless Bidirectional Forwarding Detection (S-BFD)", RFC 7880, July 2016.July 2016[RFC9314]RFC 9314Jethanandani, M., Ed., et al., "YANG Data Model for Bidirectional Forwarding Detection (BFD)", RFC 9314, September 2022.September 2022[RFC1191]RFC 1191Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990.November 1990