BFD Encapsulated in Large Packets

The Bidirectional Forwarding Detection (BFD) protocol is commonly used to verify connectivity between two systems. However, some applications may require that the Path MTU 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 of this is single-hop BFD . 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 ) is sufficient to permit verification of the Path MTU of such directly connected systems. Previous proposals () have been made for testing Path MTU for such directly connected systems. However, in the case of multi-hop BFD , this guarantee does not hold. The encapsulation of BFD in multi-hop sessions is a simple UDP packet. The BFD elements of procedure (Section 6.8.6 of ) covers 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.

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 when, and only when, they appear in all capitals, as shown here.

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 permitted by the element of BFD procedure; 24 or 26 - see Section 6.8.6 of .

The Don't Fragment bit (Section 2.3 of ) of the IP payload, when using IPv4 encapsulation, MUST be set.

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 warns against excessive pedantry, implementations may not work with this mechanism without additional support. , section 6.8.6, 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.

Since the consideration is path MTU, BFD sessions using this feature only need to use an appropriate value of bfd.PaddedPduSize appropriate 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.

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 bi-directional Path MTU, it is necessary to configure the bfd.PaddedPduSize parameter on each side of the BFD session. E.g., if the desire is to verify a 1500 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 1500. In the absence of such consistent configuration, BFD with Large Packets may correctly determine unidirectional connectivity at the tested MTU, but bi-directional 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.

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.

Various mechanisms are utilized to increase throughput between two endpoints at various network layers. Such features include Link Aggregate 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, , 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 the 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. The above text also applies to most, if not all, BFD techniques.

This mechanism also can be applied to other forms of BFD, including S-BFD .

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 Packet is supported by the implementation. Further, this YANG module augments the YANG modules for single-hop, multi-hop, 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 , 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.

This YANG module imports A YANG Data Model for Routing, and YANG Data Model for Bidirectional Forwading Detection (BFD).

file "ietf-bfd-large@2025-01-15.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: WG List: 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) 2024 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 XXXX (https://www.rfc-editor.org/info/rfcXXXX); 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 "2025-01-15" { description "Initial Version."; reference "RFC XXXX, 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 XXXX, BFD Encapsulated in Large Packets."; } grouping bfd-large-common { description "Common configuration and operational state for BFD Encapsulated in Large Packets."; reference "RFC XXXX, 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."; } }

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	    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.
            

            
                
                This section is modeled after the template described in Section 3.7
                of .
                

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

                
                The Network Configuration Access Control Model (NACM) 
                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. This module defines one such grouping,
                "bfd-large-common", which contains the "pdu-size" data node whose security
                considerations are documented above.
                
            
	

	
            
                This document registers one URIs in the "ns" subregistry of the
                "IETF XML" registry . Following the format in
                , the following registration is requested:

                    
                        
                        
                    
            

            
                This document registers one YANG modules in the "YANG Module Names"
                registry . Following the format in , the following registrations are requested:

                
                    
                
            
	

	
	    
	    The authors would like to thank Les Ginsberg, Mahesh Jethanandani, Robert Raszuk,
	    and Ketan Talaulikar, for their valuable feedback on this proposal.