



Internet Engineering Task Force (IETF)                    R. Rahman, Ed.
Request for Comments: 9127                                              
Category: Standards Track                                  L. Zheng, Ed.
ISSN: 2070-1721                                      Huawei Technologies
                                                    M. Jethanandani, Ed.
                                                     Xoriant Corporation
                                                           S. Pallagatti
                                                                  VMware
                                                               G. Mirsky
                                                                Ericsson
                                                            October 2021


      YANG Data Model for Bidirectional Forwarding Detection (BFD)

Abstract

   This document defines a YANG data model that can be used to configure
   and manage Bidirectional Forwarding Detection (BFD).

   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/rfc9127.

Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Tree Diagrams
   2.  Design of the Data Model
     2.1.  Design of the Configuration Model
       2.1.1.  Common BFD Configuration Parameters
       2.1.2.  Single-Hop IP
       2.1.3.  Multihop IP
       2.1.4.  MPLS Label Switched Paths
       2.1.5.  Link Aggregation Groups
     2.2.  Design of the Operational State Model
     2.3.  Notifications
     2.4.  RPC Operations
     2.5.  BFD Top-Level Hierarchy
     2.6.  BFD IP Single-Hop Hierarchy
     2.7.  BFD IP Multihop Hierarchy
     2.8.  BFD-over-LAG Hierarchy
     2.9.  BFD-over-MPLS-LSPs Hierarchy
     2.10. Interaction with Other YANG Modules
       2.10.1.  "ietf-interfaces" Module
       2.10.2.  "ietf-ip" Module
       2.10.3.  "ietf-mpls" Module
     2.11. IANA BFD YANG Module
     2.12. BFD Types YANG Module
     2.13. BFD Top-Level YANG Module
     2.14. BFD IP Single-Hop YANG Module
     2.15. BFD IP Multihop YANG Module
     2.16. BFD-over-LAG YANG Module
     2.17. BFD-over-MPLS YANG Module
   3.  Data Model Examples
     3.1.  IP Single-Hop
     3.2.  IP Multihop
     3.3.  LAG
     3.4.  MPLS
   4.  Security Considerations
   5.  IANA Considerations
     5.1.  IANA-Maintained "iana-bfd-types" Module
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Echo Function Configuration Example
     A.1.  Example YANG Module for BFD Echo Function Configuration
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document defines a YANG data model that can be used to configure
   and manage Bidirectional Forwarding Detection (BFD) [RFC5880].  BFD
   is a network protocol that is used for liveness detection of
   arbitrary paths between systems.  Some examples of different types of
   paths over which we have BFD are as follows:

   1.  Two systems directly connected via IP.  This is known as BFD over
       single-hop IP, a.k.a. BFD for IPv4 and IPv6 [RFC5881].

   2.  Two systems connected via multiple hops as described in
       "Bidirectional Forwarding Detection (BFD) for Multihop Paths"
       [RFC5883].

   3.  Two systems connected via MPLS Label Switched Paths (LSPs) as
       described in "Bidirectional Forwarding Detection (BFD) for MPLS
       Label Switched Paths (LSPs)" [RFC5884].

   4.  Two systems connected via a Link Aggregation Group (LAG)
       interface as described in "Bidirectional Forwarding Detection
       (BFD) on Link Aggregation Group (LAG) Interfaces" [RFC7130].

   5.  Two systems connected via pseudowires (PWs).  This is known as
       Virtual Circuit Connectivity Verification (VCCV), as described in
       "Bidirectional Forwarding Detection (BFD) for the Pseudowire
       Virtual Circuit Connectivity Verification (VCCV)" [RFC5885].
       This scenario is not addressed in this document.

   BFD typically does not operate on its own.  Various control
   protocols, also known as BFD clients, use the services provided by
   BFD for their own operation, as described in "Generic Application of
   Bidirectional Forwarding Detection (BFD)" [RFC5882].  The obvious
   candidates that use BFD are those that do not have "hellos" to detect
   failures, e.g., static routes, and routing protocols whose "hellos"
   do not support sub-second failure detection, e.g., OSPF and IS-IS.

   The YANG modules in this document conform to the Network Management
   Datastore Architecture (NMDA) [RFC8342].  This means that the data
   models do not have separate top-level or sibling containers for
   configuration data and operational state data.

1.1.  Tree Diagrams

   This document uses the graphical representation of data models, as
   defined in [RFC8340].

2.  Design of the Data Model

   Since BFD is used for liveness detection of various forwarding paths,
   there is no uniform key to identify a BFD session, and so the BFD
   data model is split into multiple YANG modules where each module
   corresponds to one type of forwarding path.  For example, BFD for IP
   single-hop is in one YANG module, and BFD for MPLS is in another YANG
   module.  The main difference between these modules is how a BFD
   session is uniquely identified, i.e., the key for the list containing
   the BFD sessions for that forwarding path.  To avoid duplication of
   BFD definitions, we have common types and groupings that are used by
   all the modules.

   A new control-plane protocol, "bfdv1", is defined, and a "bfd"
   container is created under "control-plane-protocol" as specified in
   "A YANG Data Model for Routing Management (NMDA Version)" [RFC8349].
   This new "bfd" container is augmented by the following YANG modules
   for their respective specific information:

   1.  The "ietf-bfd-ip-sh" module (Section 2.14) augments "/routing/
       control-plane-protocols/control-plane-protocol/bfd/" with the
       "ip-sh" container for BFD sessions over IP single-hop.

   2.  The "ietf-bfd-ip-mh" module (Section 2.15) augments "/routing/
       control-plane-protocols/control-plane-protocol/bfd/" with the
       "ip-mh" container for BFD sessions over IP multihop.

   3.  The "ietf-bfd-lag" module (Section 2.16) augments "/routing/
       control-plane-protocols/control-plane-protocol/bfd/" with the
       "lag" container for BFD sessions over a LAG.

   4.  The "ietf-bfd-mpls" module (Section 2.17) augments "/routing/
       control-plane-protocols/control-plane-protocol/bfd/" with the
       "mpls" container for BFD-over-MPLS LSPs.

   BFD can operate in the following contexts:

   1.  At the network device level.

   2.  In logical network elements (LNEs) as described in "YANG Model
       for Logical Network Elements" [RFC8530].

   3.  In network instances as described in "YANG Data Model for Network
       Instances" [RFC8529].

   When used at the network device level, the BFD YANG data model is
   used "as is".  When the BFD YANG data model is used in an LNE or
   network instance, the BFD YANG data model augments the mounted
   routing model for the LNE or network instance.

2.1.  Design of the Configuration Model

   The configuration model consists mainly of the parameters specified
   in BFD [RFC5880] -- for example, desired minimum transmit interval,
   required minimum receive interval, and detection multiplier.

   BFD clients are applications that use BFD for fast detection of
   failures.  Some implementations have BFD session configuration under
   the BFD clients -- for example, BFD session configuration under
   routing applications such as OSPF, IS-IS, or BGP.  Other
   implementations have BFD session configuration centralized under BFD,
   i.e., outside the multiple BFD clients.

   The main BFD parameters of interest to a BFD client are those related
   to the multiplier and interval(s), since those parameters impact the
   convergence time of the BFD clients when a failure occurs.  Other
   parameters, such as BFD authentication, are not specific to the
   requirements of the BFD client.  Configuration of BFD for all clients
   should be centralized.  However, this is a problem for BFD clients
   that auto-discover their peers.  For example, IGPs do not have the
   peer address configured; instead, the IGP is enabled on an interface,
   and the IGP peers are auto-discovered.  So, for an operator to
   configure BFD to an IGP peer, the operator would first have to
   determine the peer addresses.  And when a new peer is discovered, BFD
   configuration would need to be added.  To avoid this issue, we define
   the grouping "client-cfg-parms" in Section 2.12 for BFD clients to
   configure BFD: this allows BFD clients, such as the IGPs, to have
   configuration (multiplier and intervals) for the BFD sessions they
   need.  For example, when a new IGP peer is discovered, the IGP would
   create a BFD session to the newly discovered peer; similarly, when an
   IGP peer goes away, the IGP would remove the BFD session to that
   peer.  The mechanism for how the BFD sessions are created and removed
   by the BFD clients is outside the scope of this document, but this
   would typically be done by using an API implemented by the BFD module
   on the system.  In the case of BFD clients that create BFD sessions
   via their own configuration, authentication parameters (if required)
   are still specified in BFD.

2.1.1.  Common BFD Configuration Parameters

   The basic BFD configuration parameters are as follows:

   local-multiplier
      This is the detection time multiplier as defined in BFD [RFC5880].

   desired-min-tx-interval
      This is the Desired Min TX Interval as defined in BFD [RFC5880].

   required-min-rx-interval
      This is the Required Min RX Interval as defined in BFD [RFC5880].

   Although BFD [RFC5880] allows for different values for transmit and
   receive intervals, some implementations allow users to specify just
   one interval that is used for both transmit and receive intervals, or
   separate values for transmit and receive intervals.  The BFD YANG
   data model supports this: there is a choice between "min-interval",
   used for both transmit and receive intervals, and "desired-min-tx-
   interval" and "required-min-rx-interval".  This is supported via the
   "base-cfg-parms" grouping (Section 2.12), which is used by the YANG
   modules for the various forwarding paths.

   For BFD authentication, we have the following:

   key-chain
      This is a reference to "key-chain" as defined in "YANG Data Model
      for Key Chains" [RFC8177].  The keys, cryptographic algorithms,
      key lifetime, etc. are all defined in the "key-chain" model.

   meticulous
      This enables a meticulous mode as per BFD [RFC5880].

2.1.2.  Single-Hop IP

   For single-hop IP, there is an augment of the "bfd" data node, as
   described in Section 2.  The "ip-sh" node contains a list of IP
   single-hop sessions where each session is uniquely identified by the
   interface and destination address pair.  We use the configuration
   parameters defined in Section 2.1.1.  The "ip-sh" node also contains
   a list of interfaces and is used to specify authentication parameters
   for BFD sessions that are created by BFD clients.  See Section 2.1.

   [RFC5880] and [RFC5881] do not specify whether the Echo function
   operates continuously or on demand.  Therefore, the mechanism used to
   start and stop the Echo function is implementation specific and
   should be done by augmentation:

   1.  Configuration.  This is suitable for an Echo function that
       operates continuously.  An example is provided in Appendix A.

   2.  RPC.  This is suitable for an Echo function that operates on
       demand.

2.1.3.  Multihop IP

   For multihop IP, there is an augment of the "bfd" data node, as
   described in Section 2.

   Because of multiple paths, there could be multiple multihop IP
   sessions between a source and a destination address.  We identify
   this set of sessions as a "session-group".  The key for each
   "session-group" consists of the following:

   Source address
      Address belonging to the local system as per "Bidirectional
      Forwarding Detection (BFD) for Multihop Paths" [RFC5883].

   Destination address
      Address belonging to the remote system as per [RFC5883].

   We use the configuration parameters defined in Section 2.1.1.

   This document also provides the following parameters:

   tx-ttl
      TTL of outgoing BFD control packets.

   rx-ttl
      Minimum TTL of incoming BFD control packets.

2.1.4.  MPLS Label Switched Paths

   Here, we address MPLS LSPs whose Forwarding Equivalence Class (FEC)
   [RFC3031] is an IP address.  The "bfd" node (Section 2) is augmented
   with "mpls", which contains a list of sessions uniquely identified by
   an IP prefix.  Because of multiple paths, there could be multiple
   MPLS sessions to an MPLS FEC.  We identify this set of sessions as a
   "session-group".

   Since these LSPs are unidirectional, there is no LSP configuration on
   the egress node.

   The BFD parameters for the egress node are added under "mpls".

2.1.5.  Link Aggregation Groups

   Per "Bidirectional Forwarding Detection (BFD) on Link Aggregation
   Group (LAG) Interfaces" [RFC7130], configuring BFD on a LAG consists
   of having micro-BFD sessions on each LAG member link.  Since the BFD
   parameters are an attribute of the LAG, they should be under the LAG.
   However, there is no LAG YANG data model that we can augment.  So, a
   "lag" data node is added to the "bfd" node; see Section 2.  The
   configuration is per LAG: we have a list of LAGs.  The destination IP
   address of the micro-BFD sessions is configured per LAG and per
   address family (IPv4 and IPv6).

2.2.  Design of the Operational State Model

   The operational state model contains both the overall statistics for
   the BFD sessions running on the device and the per-session
   operational information.

   The overall statistics for the BFD sessions consist of the number of
   BFD sessions, the number of BFD sessions that are up, etc.  This
   information is available globally (i.e., for all BFD sessions) under
   the "bfd" node (Section 2) and also per type of forwarding path.

   For each BFD session, three main categories of operational state data
   are shown.

   1.  The first category includes fundamental information regarding a
       BFD session, such as the local discriminator, the remote
       discriminator, and the ability to support Demand mode.

   2.  The second category includes BFD "session-running" information,
       e.g., the remote BFD state and the diagnostic code received.
       Another example is the actual transmit interval between the
       control packets, which may be different from the configured
       desired minimum transmit interval.  Similar examples include the
       actual receive interval between the control packets and the
       actual transmit interval between the Echo packets.

   3.  The third category contains the detailed statistics for the
       session, e.g., when the session transitioned up/down and how long
       it has been in that state.

   For some path types, there may be more than one session on the
   virtual path to the destination.  For example, with IP multihop and
   MPLS LSPs, there could be multiple BFD sessions from the source to
   the same destination to test the various paths (ECMP) to the
   destination.  This is represented by having multiple "sessions" under
   each "session-group".

2.3.  Notifications

   This YANG data model defines notifications to inform end users of
   important events detected during the protocol operation.  The local
   discriminator identifies the corresponding BFD session on the local
   system, and the remote discriminator identifies the BFD session on
   the remote system.  Notifications also give more important details
   about BFD sessions, e.g., new state, time in previous state, network
   instance, and the reason that the BFD session state changed.  The
   notifications are defined for each type of forwarding path but use
   groupings for common information.

2.4.  RPC Operations

   None.

2.5.  BFD Top-Level Hierarchy

   At the "bfd" node under "control-plane-protocol", there is no
   configuration data -- only operational state data.  The operational
   state data consists of overall BFD session statistics, i.e., for BFD
   on all types of forwarding paths.

   module: ietf-bfd
     augment /rt:routing/rt:control-plane-protocols
               /rt:control-plane-protocol:
       +--rw bfd
          +--ro summary
             +--ro number-of-sessions?              yang:gauge32
             +--ro number-of-sessions-up?           yang:gauge32
             +--ro number-of-sessions-down?         yang:gauge32
             +--ro number-of-sessions-admin-down?   yang:gauge32

2.6.  BFD IP Single-Hop Hierarchy

   An "ip-sh" node is added under the "bfd" node in "control-plane-
   protocol".  The configuration data and operational state data for
   each BFD IP single-hop session are under this "ip-sh" node.

   module: ietf-bfd-ip-sh
     augment /rt:routing/rt:control-plane-protocols
               /rt:control-plane-protocol/bfd:bfd:
       +--rw ip-sh
          +--ro summary
          |  +--ro number-of-sessions?              yang:gauge32
          |  +--ro number-of-sessions-up?           yang:gauge32
          |  +--ro number-of-sessions-down?         yang:gauge32
          |  +--ro number-of-sessions-admin-down?   yang:gauge32
          +--rw sessions
          |  +--rw session* [interface dest-addr]
          |     +--rw interface                         if:interface-ref
          |     +--rw dest-addr                         inet:ip-address
          |     +--rw source-addr?                      inet:ip-address
          |     +--rw local-multiplier?                 multiplier
          |     +--rw (interval-config-type)?
          |     |  +--:(tx-rx-intervals)
          |     |  |  +--rw desired-min-tx-interval?    uint32
          |     |  |  +--rw required-min-rx-interval?   uint32
          |     |  +--:(single-interval) {single-minimum-interval}?
          |     |     +--rw min-interval?               uint32
          |     +--rw demand-enabled?                   boolean
          |     |       {demand-mode}?
          |     +--rw admin-down?                       boolean
          |     +--rw authentication! {authentication}?
          |     |  +--rw key-chain?    key-chain:key-chain-ref
          |     |  +--rw meticulous?   boolean
          |     +--ro path-type?                        identityref
          |     +--ro ip-encapsulation?                 boolean
          |     +--ro local-discriminator?              discriminator
          |     +--ro remote-discriminator?             discriminator
          |     +--ro remote-multiplier?                multiplier
          |     +--ro demand-capability?                boolean
          |     |       {demand-mode}?
          |     +--ro source-port?                      inet:port-number
          |     +--ro dest-port?                        inet:port-number
          |     +--ro session-running
          |     |  +--ro session-index?                uint32
          |     |  +--ro local-state?                  state
          |     |  +--ro remote-state?                 state
          |     |  +--ro local-diagnostic?
          |     |  |       iana-bfd-types:diagnostic
          |     |  +--ro remote-diagnostic?
          |     |  |       iana-bfd-types:diagnostic
          |     |  +--ro remote-authenticated?         boolean
          |     |  +--ro remote-authentication-type?
          |     |  |       iana-bfd-types:auth-type {authentication}?
          |     |  +--ro detection-mode?               enumeration
          |     |  +--ro negotiated-tx-interval?       uint32
          |     |  +--ro negotiated-rx-interval?       uint32
          |     |  +--ro detection-time?               uint32
          |     |  +--ro echo-tx-interval-in-use?      uint32
          |     |          {echo-mode}?
          |     +--ro session-statistics
          |        +--ro create-time?
          |        |       yang:date-and-time
          |        +--ro last-down-time?
          |        |       yang:date-and-time
          |        +--ro last-up-time?
          |        |       yang:date-and-time
          |        +--ro down-count?                     yang:counter32
          |        +--ro admin-down-count?               yang:counter32
          |        +--ro receive-packet-count?           yang:counter64
          |        +--ro send-packet-count?              yang:counter64
          |        +--ro receive-invalid-packet-count?   yang:counter64
          |        +--ro send-failed-packet-count?       yang:counter64
          +--rw interfaces* [interface]
             +--rw interface         if:interface-ref
             +--rw authentication! {authentication}?
                +--rw key-chain?    key-chain:key-chain-ref
                +--rw meticulous?   boolean

     notifications:
       +---n singlehop-notification
          +--ro local-discr?                 discriminator
          +--ro remote-discr?                discriminator
          +--ro new-state?                   state
          +--ro state-change-reason?         iana-bfd-types:diagnostic
          +--ro time-of-last-state-change?   yang:date-and-time
          +--ro dest-addr?                   inet:ip-address
          +--ro source-addr?                 inet:ip-address
          +--ro session-index?               uint32
          +--ro path-type?                   identityref
          +--ro interface?                   if:interface-ref
          +--ro echo-enabled?                boolean

2.7.  BFD IP Multihop Hierarchy

   An "ip-mh" node is added under the "bfd" node in "control-plane-
   protocol".  The configuration data and operational state data for
   each BFD IP multihop session are under this "ip-mh" node.  In the
   operational state model, we support multiple BFD multihop sessions
   per remote address (ECMP); the local discriminator is used as the
   key.

   module: ietf-bfd-ip-mh
     augment /rt:routing/rt:control-plane-protocols
               /rt:control-plane-protocol/bfd:bfd:
       +--rw ip-mh
          +--ro summary
          |  +--ro number-of-sessions?              yang:gauge32
          |  +--ro number-of-sessions-up?           yang:gauge32
          |  +--ro number-of-sessions-down?         yang:gauge32
          |  +--ro number-of-sessions-admin-down?   yang:gauge32
          +--rw session-groups
             +--rw session-group* [source-addr dest-addr]
                +--rw source-addr                       inet:ip-address
                +--rw dest-addr                         inet:ip-address
                +--rw local-multiplier?                 multiplier
                +--rw (interval-config-type)?
                |  +--:(tx-rx-intervals)
                |  |  +--rw desired-min-tx-interval?    uint32
                |  |  +--rw required-min-rx-interval?   uint32
                |  +--:(single-interval) {single-minimum-interval}?
                |     +--rw min-interval?               uint32
                +--rw demand-enabled?                   boolean
                |       {demand-mode}?
                +--rw admin-down?                       boolean
                +--rw authentication! {authentication}?
                |  +--rw key-chain?    key-chain:key-chain-ref
                |  +--rw meticulous?   boolean
                +--rw tx-ttl?                           bfd-types:hops
                +--rw rx-ttl                            bfd-types:hops
                +--ro sessions* []
                   +--ro path-type?              identityref
                   +--ro ip-encapsulation?       boolean
                   +--ro local-discriminator?    discriminator
                   +--ro remote-discriminator?   discriminator
                   +--ro remote-multiplier?      multiplier
                   +--ro demand-capability?      boolean {demand-mode}?
                   +--ro source-port?            inet:port-number
                   +--ro dest-port?              inet:port-number
                   +--ro session-running
                   |  +--ro session-index?                uint32
                   |  +--ro local-state?                  state
                   |  +--ro remote-state?                 state
                   |  +--ro local-diagnostic?
                   |  |       iana-bfd-types:diagnostic
                   |  +--ro remote-diagnostic?
                   |  |       iana-bfd-types:diagnostic
                   |  +--ro remote-authenticated?         boolean
                   |  +--ro remote-authentication-type?
                   |  |       iana-bfd-types:auth-type {authentication}?
                   |  +--ro detection-mode?               enumeration
                   |  +--ro negotiated-tx-interval?       uint32
                   |  +--ro negotiated-rx-interval?       uint32
                   |  +--ro detection-time?               uint32
                   |  +--ro echo-tx-interval-in-use?      uint32
                   |          {echo-mode}?
                   +--ro session-statistics
                      +--ro create-time?
                      |       yang:date-and-time
                      +--ro last-down-time?
                      |       yang:date-and-time
                      +--ro last-up-time?
                      |       yang:date-and-time
                      +--ro down-count?
                      |       yang:counter32
                      +--ro admin-down-count?
                      |       yang:counter32
                      +--ro receive-packet-count?
                      |       yang:counter64
                      +--ro send-packet-count?
                      |       yang:counter64
                      +--ro receive-invalid-packet-count?
                      |       yang:counter64
                      +--ro send-failed-packet-count?
                              yang:counter64

     notifications:
       +---n multihop-notification
          +--ro local-discr?                 discriminator
          +--ro remote-discr?                discriminator
          +--ro new-state?                   state
          +--ro state-change-reason?         iana-bfd-types:diagnostic
          +--ro time-of-last-state-change?   yang:date-and-time
          +--ro dest-addr?                   inet:ip-address
          +--ro source-addr?                 inet:ip-address
          +--ro session-index?               uint32
          +--ro path-type?                   identityref

2.8.  BFD-over-LAG Hierarchy

   A "lag" node is added under the "bfd" node in "control-plane-
   protocol".  The configuration data and operational state data for
   each BFD LAG session are under this "lag" node.

   module: ietf-bfd-lag
     augment /rt:routing/rt:control-plane-protocols
               /rt:control-plane-protocol/bfd:bfd:
       +--rw lag
          +--rw micro-bfd-ipv4-session-statistics
          |  +--ro summary
          |     +--ro number-of-sessions?              yang:gauge32
          |     +--ro number-of-sessions-up?           yang:gauge32
          |     +--ro number-of-sessions-down?         yang:gauge32
          |     +--ro number-of-sessions-admin-down?   yang:gauge32
          +--rw micro-bfd-ipv6-session-statistics
          |  +--ro summary
          |     +--ro number-of-sessions?              yang:gauge32
          |     +--ro number-of-sessions-up?           yang:gauge32
          |     +--ro number-of-sessions-down?         yang:gauge32
          |     +--ro number-of-sessions-admin-down?   yang:gauge32
          +--rw sessions
             +--rw session* [lag-name]
                +--rw lag-name                          if:interface-ref
                +--rw ipv4-dest-addr?
                |       inet:ipv4-address
                +--rw ipv6-dest-addr?
                |       inet:ipv6-address
                +--rw local-multiplier?                 multiplier
                +--rw (interval-config-type)?
                |  +--:(tx-rx-intervals)
                |  |  +--rw desired-min-tx-interval?    uint32
                |  |  +--rw required-min-rx-interval?   uint32
                |  +--:(single-interval) {single-minimum-interval}?
                |     +--rw min-interval?               uint32
                +--rw demand-enabled?                   boolean
                |       {demand-mode}?
                +--rw admin-down?                       boolean
                +--rw authentication! {authentication}?
                |  +--rw key-chain?    key-chain:key-chain-ref
                |  +--rw meticulous?   boolean
                +--rw use-ipv4?                         boolean
                +--rw use-ipv6?                         boolean
                +--ro member-links* [member-link]
                   +--ro member-link       if:interface-ref
                   +--ro micro-bfd-ipv4
                   |  +--ro path-type?              identityref
                   |  +--ro ip-encapsulation?       boolean
                   |  +--ro local-discriminator?    discriminator
                   |  +--ro remote-discriminator?   discriminator
                   |  +--ro remote-multiplier?      multiplier
                   |  +--ro demand-capability?      boolean
                   |  |       {demand-mode}?
                   |  +--ro source-port?            inet:port-number
                   |  +--ro dest-port?              inet:port-number
                   |  +--ro session-running
                   |  |  +--ro session-index?                uint32
                   |  |  +--ro local-state?                  state
                   |  |  +--ro remote-state?                 state
                   |  |  +--ro local-diagnostic?
                   |  |  |       iana-bfd-types:diagnostic
                   |  |  +--ro remote-diagnostic?
                   |  |  |       iana-bfd-types:diagnostic
                   |  |  +--ro remote-authenticated?         boolean
                   |  |  +--ro remote-authentication-type?
                   |  |  |       iana-bfd-types:auth-type
                   |  |  |       {authentication}?
                   |  |  +--ro detection-mode?               enumeration
                   |  |  +--ro negotiated-tx-interval?       uint32
                   |  |  +--ro negotiated-rx-interval?       uint32
                   |  |  +--ro detection-time?               uint32
                   |  |  +--ro echo-tx-interval-in-use?      uint32
                   |  |          {echo-mode}?
                   |  +--ro session-statistics
                   |     +--ro create-time?
                   |     |       yang:date-and-time
                   |     +--ro last-down-time?
                   |     |       yang:date-and-time
                   |     +--ro last-up-time?
                   |     |       yang:date-and-time
                   |     +--ro down-count?
                   |     |       yang:counter32
                   |     +--ro admin-down-count?
                   |     |       yang:counter32
                   |     +--ro receive-packet-count?
                   |     |       yang:counter64
                   |     +--ro send-packet-count?
                   |     |       yang:counter64
                   |     +--ro receive-invalid-packet-count?
                   |     |       yang:counter64
                   |     +--ro send-failed-packet-count?
                   |             yang:counter64
                   +--ro micro-bfd-ipv6
                      +--ro path-type?              identityref
                      +--ro ip-encapsulation?       boolean
                      +--ro local-discriminator?    discriminator
                      +--ro remote-discriminator?   discriminator
                      +--ro remote-multiplier?      multiplier
                      +--ro demand-capability?      boolean
                      |       {demand-mode}?
                      +--ro source-port?            inet:port-number
                      +--ro dest-port?              inet:port-number
                      +--ro session-running
                      |  +--ro session-index?                uint32
                      |  +--ro local-state?                  state
                      |  +--ro remote-state?                 state
                      |  +--ro local-diagnostic?
                      |  |       iana-bfd-types:diagnostic
                      |  +--ro remote-diagnostic?
                      |  |       iana-bfd-types:diagnostic
                      |  +--ro remote-authenticated?         boolean
                      |  +--ro remote-authentication-type?
                      |  |       iana-bfd-types:auth-type
                      |  |       {authentication}?
                      |  +--ro detection-mode?               enumeration
                      |  +--ro negotiated-tx-interval?       uint32
                      |  +--ro negotiated-rx-interval?       uint32
                      |  +--ro detection-time?               uint32
                      |  +--ro echo-tx-interval-in-use?      uint32
                      |          {echo-mode}?
                      +--ro session-statistics
                         +--ro create-time?
                         |       yang:date-and-time
                         +--ro last-down-time?
                         |       yang:date-and-time
                         +--ro last-up-time?
                         |       yang:date-and-time
                         +--ro down-count?
                         |       yang:counter32
                         +--ro admin-down-count?
                         |       yang:counter32
                         +--ro receive-packet-count?
                         |       yang:counter64
                         +--ro send-packet-count?
                         |       yang:counter64
                         +--ro receive-invalid-packet-count?
                         |       yang:counter64
                         +--ro send-failed-packet-count?
                                 yang:counter64

     notifications:
       +---n lag-notification
          +--ro local-discr?                 discriminator
          +--ro remote-discr?                discriminator
          +--ro new-state?                   state
          +--ro state-change-reason?         iana-bfd-types:diagnostic
          +--ro time-of-last-state-change?   yang:date-and-time
          +--ro dest-addr?                   inet:ip-address
          +--ro source-addr?                 inet:ip-address
          +--ro session-index?               uint32
          +--ro path-type?                   identityref
          +--ro lag-name?                    if:interface-ref
          +--ro member-link?                 if:interface-ref

2.9.  BFD-over-MPLS-LSPs Hierarchy

   An "mpls" node is added under the "bfd" node in "control-plane-
   protocol".  The configuration is per MPLS FEC under this "mpls" node.
   In the operational state model, we support multiple BFD sessions per
   MPLS FEC (ECMP); the local discriminator is used as the key.  The
   "mpls" node can be used in a network device (top level) or can be
   mounted in an LNE or network instance.

   module: ietf-bfd-mpls
     augment /rt:routing/rt:control-plane-protocols
               /rt:control-plane-protocol/bfd:bfd:
       +--rw mpls
          +--ro summary
          |  +--ro number-of-sessions?              yang:gauge32
          |  +--ro number-of-sessions-up?           yang:gauge32
          |  +--ro number-of-sessions-down?         yang:gauge32
          |  +--ro number-of-sessions-admin-down?   yang:gauge32
          +--rw egress
          |  +--rw enabled?                          boolean
          |  +--rw local-multiplier?                 multiplier
          |  +--rw (interval-config-type)?
          |  |  +--:(tx-rx-intervals)
          |  |  |  +--rw desired-min-tx-interval?    uint32
          |  |  |  +--rw required-min-rx-interval?   uint32
          |  |  +--:(single-interval) {single-minimum-interval}?
          |  |     +--rw min-interval?               uint32
          |  +--rw authentication! {authentication}?
          |     +--rw key-chain?    key-chain:key-chain-ref
          |     +--rw meticulous?   boolean
          +--rw session-groups
             +--rw session-group* [mpls-fec]
                +--rw mpls-fec                          inet:ip-prefix
                +--rw local-multiplier?                 multiplier
                +--rw (interval-config-type)?
                |  +--:(tx-rx-intervals)
                |  |  +--rw desired-min-tx-interval?    uint32
                |  |  +--rw required-min-rx-interval?   uint32
                |  +--:(single-interval) {single-minimum-interval}?
                |     +--rw min-interval?               uint32
                +--rw demand-enabled?                   boolean
                |       {demand-mode}?
                +--rw admin-down?                       boolean
                +--rw authentication! {authentication}?
                |  +--rw key-chain?    key-chain:key-chain-ref
                |  +--rw meticulous?   boolean
                +--ro sessions* []
                   +--ro path-type?              identityref
                   +--ro ip-encapsulation?       boolean
                   +--ro local-discriminator?    discriminator
                   +--ro remote-discriminator?   discriminator
                   +--ro remote-multiplier?      multiplier
                   +--ro demand-capability?      boolean {demand-mode}?
                   +--ro source-port?            inet:port-number
                   +--ro dest-port?              inet:port-number
                   +--ro session-running
                   |  +--ro session-index?                uint32
                   |  +--ro local-state?                  state
                   |  +--ro remote-state?                 state
                   |  +--ro local-diagnostic?
                   |  |       iana-bfd-types:diagnostic
                   |  +--ro remote-diagnostic?
                   |  |       iana-bfd-types:diagnostic
                   |  +--ro remote-authenticated?         boolean
                   |  +--ro remote-authentication-type?
                   |  |       iana-bfd-types:auth-type {authentication}?
                   |  +--ro detection-mode?               enumeration
                   |  +--ro negotiated-tx-interval?       uint32
                   |  +--ro negotiated-rx-interval?       uint32
                   |  +--ro detection-time?               uint32
                   |  +--ro echo-tx-interval-in-use?      uint32
                   |          {echo-mode}?
                   +--ro session-statistics
                   |  +--ro create-time?
                   |  |       yang:date-and-time
                   |  +--ro last-down-time?
                   |  |       yang:date-and-time
                   |  +--ro last-up-time?
                   |  |       yang:date-and-time
                   |  +--ro down-count?
                   |  |       yang:counter32
                   |  +--ro admin-down-count?
                   |  |       yang:counter32
                   |  +--ro receive-packet-count?
                   |  |       yang:counter64
                   |  +--ro send-packet-count?
                   |  |       yang:counter64
                   |  +--ro receive-invalid-packet-count?
                   |  |       yang:counter64
                   |  +--ro send-failed-packet-count?
                   |          yang:counter64
                   +--ro mpls-dest-address?      inet:ip-address

     notifications:
       +---n mpls-notification
          +--ro local-discr?                 discriminator
          +--ro remote-discr?                discriminator
          +--ro new-state?                   state
          +--ro state-change-reason?         iana-bfd-types:diagnostic
          +--ro time-of-last-state-change?   yang:date-and-time
          +--ro dest-addr?                   inet:ip-address
          +--ro source-addr?                 inet:ip-address
          +--ro session-index?               uint32
          +--ro path-type?                   identityref
          +--ro mpls-dest-address?           inet:ip-address

2.10.  Interaction with Other YANG Modules

   "Generic YANG Data Model for the Management of Operations,
   Administration, and Maintenance (OAM) Protocols That Use
   Connectionless Communications" [RFC8532] describes how the Layer-
   Independent OAM Management in the Multi-Layer Environment (LIME)
   connectionless OAM model could be extended to support BFD.

   Also, the operation of the BFD data model depends on configuration
   parameters that are defined in other YANG modules.

2.10.1.  "ietf-interfaces" Module

   The following boolean configuration is defined in "A YANG Data Model
   for Interface Management" [RFC8343]:

   /if:interfaces/if:interface/if:enabled
      If this configuration is set to "false", no BFD packets can be
      transmitted or received on that interface.

2.10.2.  "ietf-ip" Module

   The following boolean configuration is defined in "A YANG Data Model
   for IP Management" [RFC8344]:

   /if:interfaces/if:interface/ip:ipv4/ip:enabled
      If this configuration is set to "false", no BFD IPv4 packets can
      be transmitted or received on that interface.

   /if:interfaces/if:interface/ip:ipv4/ip:forwarding
      If this configuration is set to "false", no BFD IPv4 packets can
      be transmitted or received on that interface.

   /if:interfaces/if:interface/ip:ipv6/ip:enabled
      If this configuration is set to "false", no BFD IPv6 packets can
      be transmitted or received on that interface.

   /if:interfaces/if:interface/ip:ipv6/ip:forwarding
      If this configuration is set to "false", no BFD IPv6 packets can
      be transmitted or received on that interface.

2.10.3.  "ietf-mpls" Module

   The following boolean configuration is defined in "A YANG Data Model
   for MPLS Base" [RFC8960]:

   /rt:routing/mpls:mpls/mpls:interfaces/mpls:interface/
   mpls:mpls-enabled
      If this configuration is set to "false", no BFD MPLS packets can
      be transmitted or received on that interface.

2.11.  IANA BFD YANG Module

   This YANG module imports definitions from [RFC5880].  It references
   [RFC5880] and [RFC6428].

   <CODE BEGINS> file "iana-bfd-types@2021-10-21.yang"
   module iana-bfd-types {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:iana-bfd-types";
     prefix iana-bfd-types;

     organization
       "IANA";
     contact
       "Internet Assigned Numbers Authority

        Postal: ICANN
                12025 Waterfront Drive, Suite 300
                Los Angeles, CA 90094-2536
                United States of America
        Tel:    +1 310 301 5800
        <mailto:iana@iana.org>";
     description
       "This module defines YANG data types for IANA-registered
        BFD parameters.

        This YANG module is maintained by IANA and reflects the
        'BFD Diagnostic Codes' and 'BFD Authentication Types'
        registries.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2021-10-21 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Type definitions
      */

     typedef diagnostic {
       type enumeration {
         enum none {
           value 0;
           description
             "No Diagnostic.";
         }
         enum control-expiry {
           value 1;
           description
             "Control Detection Time Expired.";
         }
         enum echo-failed {
           value 2;
           description
             "Echo Function Failed.";
         }
         enum neighbor-down {
           value 3;
           description
             "Neighbor Signaled Session Down.";
         }
         enum forwarding-reset {
           value 4;
           description
             "Forwarding Plane Reset.";
         }
         enum path-down {
           value 5;
           description
             "Path Down.";
         }
         enum concatenated-path-down {
           value 6;
           description
             "Concatenated Path Down.";
         }
         enum admin-down {
           value 7;
           description
             "Administratively Down.";
         }
         enum reverse-concatenated-path-down {
           value 8;
           description
             "Reverse Concatenated Path Down.";
         }
         enum mis-connectivity-defect {
           value 9;
           description
             "Mis-connectivity defect.";
           reference
             "RFC 5880: Bidirectional Forwarding Detection (BFD)
              RFC 6428: Proactive Connectivity Verification, Continuity
              Check, and Remote Defect Indication for the MPLS Transport
              Profile";
         }
       }
       description
         "BFD diagnostic codes as defined in RFC 5880.  Values are
          maintained in the 'BFD Diagnostic Codes' IANA registry.
          Range is 0 to 31.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
     }

     typedef auth-type {
       type enumeration {
         enum reserved {
           value 0;
           description
             "Reserved.";
         }
         enum simple-password {
           value 1;
           description
             "Simple Password.";
         }
         enum keyed-md5 {
           value 2;
           description
             "Keyed MD5.";
         }
         enum meticulous-keyed-md5 {
           value 3;
           description
             "Meticulous Keyed MD5.";
         }
         enum keyed-sha1 {
           value 4;
           description
             "Keyed SHA1.";
         }
         enum meticulous-keyed-sha1 {
           value 5;
           description
             "Meticulous Keyed SHA1.";
         }
       }
       description
         "BFD authentication type as defined in RFC 5880.  Values are
          maintained in the 'BFD Authentication Types' IANA registry.
          Range is 0 to 255.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
     }
   }
   <CODE ENDS>

2.12.  BFD Types YANG Module

   This YANG module imports typedefs from [RFC6991] and [RFC8177].  It
   also imports definitions from [RFC5880], [RFC5881], [RFC5883],
   [RFC5884], and [RFC7130], as well as the "control-plane-protocol"
   identity from [RFC8349].

   <CODE BEGINS> file "ietf-bfd-types@2021-10-21.yang"
   module ietf-bfd-types {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-types";
     prefix bfd-types;

     import iana-bfd-types {
       prefix iana-bfd-types;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-inet-types {
       prefix inet;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-yang-types {
       prefix yang;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-routing {
       prefix rt;
       reference
         "RFC 8349: A YANG Data Model for Routing Management
          (NMDA Version)";
     }
     import ietf-key-chain {
       prefix key-chain;
       reference
         "RFC 8177: YANG Data Model for Key Chains";
     }

     organization
       "IETF BFD Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
        WG List:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains a collection of BFD-specific YANG data type
        definitions, as per RFC 5880, and also groupings that are common
        to other BFD YANG modules.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC 5880: Bidirectional Forwarding Detection (BFD)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2021-10-21 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Feature definitions
      */

     feature single-minimum-interval {
       description
         "This feature indicates that the server supports configuration
          of one minimum interval value that is used for both transmit
          and receive minimum intervals.";
     }

     feature authentication {
       description
         "This feature indicates that the server supports BFD
          authentication.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD),
          Section 6.7";
     }

     feature demand-mode {
       description
         "This feature indicates that the server supports BFD Demand
          mode.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD),
          Section 6.6";
     }

     feature echo-mode {
       description
         "This feature indicates that the server supports BFD Echo
          mode.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD),
          Section 6.4";
     }

     /*
      * Identity definitions
      */

     identity bfdv1 {
       base rt:control-plane-protocol;
       description
         "BFD protocol version 1.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
     }

     identity path-type {
       description
         "Base identity for the BFD path type.  The path type indicates
          the type of path on which BFD is running.";
     }

     identity path-ip-sh {
       base path-type;
       description
         "BFD on IP single-hop.";
       reference
         "RFC 5881: Bidirectional Forwarding Detection (BFD)
          for IPv4 and IPv6 (Single Hop)";
     }

     identity path-ip-mh {
       base path-type;
       description
         "BFD on IP multihop paths.";
       reference
         "RFC 5883: Bidirectional Forwarding Detection (BFD) for
          Multihop Paths";
     }

     identity path-mpls-te {
       base path-type;
       description
         "BFD on MPLS Traffic Engineering.";
       reference
         "RFC 5884: Bidirectional Forwarding Detection (BFD)
          for MPLS Label Switched Paths (LSPs)";
     }

     identity path-mpls-lsp {
       base path-type;
       description
         "BFD on an MPLS Label Switched Path.";
       reference
         "RFC 5884: Bidirectional Forwarding Detection (BFD)
          for MPLS Label Switched Paths (LSPs)";
     }

     identity path-lag {
       base path-type;
       description
         "Micro-BFD on LAG member links.";
       reference
         "RFC 7130: Bidirectional Forwarding Detection (BFD) on
          Link Aggregation Group (LAG) Interfaces";
     }

     identity encap-type {
       description
         "Base identity for BFD encapsulation type.";
     }

     identity encap-ip {
       base encap-type;
       description
         "BFD with IP encapsulation.";
     }

     /*
      * Type definitions
      */

     typedef discriminator {
       type uint32;
       description
         "BFD Discriminator as described in RFC 5880.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD)";
     }

     typedef state {
       type enumeration {
         enum adminDown {
           value 0;
           description
             "'adminDown' state.";
         }
         enum down {
           value 1;
           description
             "'Down' state.";
         }
         enum init {
           value 2;
           description
             "'Init' state.";
         }
         enum up {
           value 3;
           description
             "'Up' state.";
         }
       }
       description
         "BFD states as defined in RFC 5880.";
     }

     typedef multiplier {
       type uint8 {
         range "1..255";
       }
       description
         "BFD multiplier as described in RFC 5880.";
     }

     typedef hops {
       type uint8 {
         range "1..255";
       }
       description
         "This corresponds to Time To Live for IPv4 and corresponds to
          the hop limit for IPv6.";
     }

     /*
      * Groupings
      */

     grouping auth-parms {
       description
         "Grouping for BFD authentication parameters
          (see Section 6.7 of RFC 5880).";
       container authentication {
         if-feature "authentication";
         presence "Enables BFD authentication (see Section 6.7
                   of RFC 5880).";
         description
           "Parameters for BFD authentication.";
         reference
           "RFC 5880: Bidirectional Forwarding Detection (BFD),
            Section 6.7";
         leaf key-chain {
           type key-chain:key-chain-ref;
           description
             "Name of the 'key-chain' as per RFC 8177.";
         }
         leaf meticulous {
           type boolean;
           description
             "Enables a meticulous mode as per Section 6.7 of
              RFC 5880.";
         }
       }
     }

     grouping base-cfg-parms {
       description
         "BFD grouping for base configuration parameters.";
       leaf local-multiplier {
         type multiplier;
         default "3";
         description
           "Multiplier transmitted by the local system.";
       }
       choice interval-config-type {
         default "tx-rx-intervals";
         description
           "Two interval values or one value used for both transmit and
            receive.";
         case tx-rx-intervals {
           leaf desired-min-tx-interval {
             type uint32;
             units "microseconds";
             default "1000000";
             description
               "Desired minimum transmit interval of control packets.";
           }
           leaf required-min-rx-interval {
             type uint32;
             units "microseconds";
             default "1000000";
             description
               "Required minimum receive interval of control packets.";
           }
         }
         case single-interval {
           if-feature "single-minimum-interval";
           leaf min-interval {
             type uint32;
             units "microseconds";
             default "1000000";
             description
               "Desired minimum transmit interval and required
                minimum receive interval of control packets.";
           }
         }
       }
     }

     grouping client-cfg-parms {
       description
         "BFD grouping for configuration parameters
          used by BFD clients, e.g., IGP or MPLS.";
       leaf enabled {
         type boolean;
         default "false";
         description
           "Indicates whether BFD is enabled.";
       }
       uses base-cfg-parms;
     }

     grouping common-cfg-parms {
       description
         "BFD grouping for common configuration parameters.";
       uses base-cfg-parms;
       leaf demand-enabled {
         if-feature "demand-mode";
         type boolean;
         default "false";
         description
           "To enable Demand mode.";
       }
       leaf admin-down {
         type boolean;
         default "false";
         description
           "Indicates whether the BFD session is administratively
            down.";
       }
       uses auth-parms;
     }

     grouping all-session {
       description
         "BFD session operational information.";
       leaf path-type {
         type identityref {
           base path-type;
         }
         config false;
         description
           "BFD path type.  This indicates the path type that BFD is
            running on.";
       }
       leaf ip-encapsulation {
         type boolean;
         config false;
         description
           "Indicates whether BFD encapsulation uses IP.";
       }
       leaf local-discriminator {
         type discriminator;
         config false;
         description
           "Local discriminator.";
       }
       leaf remote-discriminator {
         type discriminator;
         config false;
         description
           "Remote discriminator.";
       }
       leaf remote-multiplier {
         type multiplier;
         config false;
         description
           "Remote multiplier.";
       }
       leaf demand-capability {
         if-feature "demand-mode";
         type boolean;
         config false;
         description
           "Local Demand mode capability.";
       }
       leaf source-port {
         when "../ip-encapsulation = 'true'" {
           description
             "Source port valid only when IP encapsulation is used.";
         }
         type inet:port-number;
         config false;
         description
           "Source UDP port.";
       }
       leaf dest-port {
         when "../ip-encapsulation = 'true'" {
           description
             "Destination port valid only when IP encapsulation
              is used.";
         }
         type inet:port-number;
         config false;
         description
           "Destination UDP port.";
       }
       container session-running {
         config false;
         description
           "BFD 'session-running' information.";
         leaf session-index {
           type uint32;
           description
             "An index used to uniquely identify BFD sessions.";
         }
         leaf local-state {
           type state;
           description
             "Local state.";
         }
         leaf remote-state {
           type state;
           description
             "Remote state.";
         }
         leaf local-diagnostic {
           type iana-bfd-types:diagnostic;
           description
             "Local diagnostic.";
         }
         leaf remote-diagnostic {
           type iana-bfd-types:diagnostic;
           description
             "Remote diagnostic.";
         }
         leaf remote-authenticated {
           type boolean;
           description
             "Indicates whether incoming BFD control packets are
              authenticated.";
         }
         leaf remote-authentication-type {
           when "../remote-authenticated = 'true'" {
             description
               "Only valid when incoming BFD control packets are
                authenticated.";
           }
           if-feature "authentication";
           type iana-bfd-types:auth-type;
           description
             "Authentication type of incoming BFD control packets.";
         }
         leaf detection-mode {
           type enumeration {
             enum async-with-echo {
               value 1;
               description
                 "Async with echo.";
             }
             enum async-without-echo {
               value 2;
               description
                 "Async without echo.";
             }
             enum demand-with-echo {
               value 3;
               description
                 "Demand with echo.";
             }
             enum demand-without-echo {
               value 4;
               description
                 "Demand without echo.";
             }
           }
           description
             "Detection mode.";
         }
         leaf negotiated-tx-interval {
           type uint32;
           units "microseconds";
           description
             "Negotiated transmit interval.";
         }
         leaf negotiated-rx-interval {
           type uint32;
           units "microseconds";
           description
             "Negotiated receive interval.";
         }
         leaf detection-time {
           type uint32;
           units "microseconds";
           description
             "Detection time.";
         }
         leaf echo-tx-interval-in-use {
           when "../../path-type = 'bfd-types:path-ip-sh'" {
             description
               "Echo is supported for IP single-hop only.";
           }
           if-feature "echo-mode";
           type uint32;
           units "microseconds";
           description
             "Echo transmit interval in use.";
         }
       }
       container session-statistics {
         config false;
         description
           "BFD per-session statistics.";
         leaf create-time {
           type yang:date-and-time;
           description
             "Time and date when this session was created.";
         }
         leaf last-down-time {
           type yang:date-and-time;
           description
             "Time and date of the last time this session went down.";
         }
         leaf last-up-time {
           type yang:date-and-time;
           description
             "Time and date of the last time this session went up.";
         }
         leaf down-count {
           type yang:counter32;
           description
             "The number of times this session has transitioned to the
              'down' state.";
         }
         leaf admin-down-count {
           type yang:counter32;
           description
             "The number of times this session has transitioned to the
              'admin-down' state.";
         }
         leaf receive-packet-count {
           type yang:counter64;
           description
             "Count of received packets in this session.  This includes
              valid and invalid received packets.";
         }
         leaf send-packet-count {
           type yang:counter64;
           description
             "Count of sent packets in this session.";
         }
         leaf receive-invalid-packet-count {
           type yang:counter64;
           description
             "Count of invalid received packets in this session.";
         }
         leaf send-failed-packet-count {
           type yang:counter64;
           description
             "Count of packets that failed to be sent in this session.";
         }
       }
     }

     grouping session-statistics-summary {
       description
         "Grouping for session statistics summary.";
       container summary {
         config false;
         description
           "BFD session statistics summary.";
         leaf number-of-sessions {
           type yang:gauge32;
           description
             "Number of BFD sessions.";
         }
         leaf number-of-sessions-up {
           type yang:gauge32;
           description
             "Number of BFD sessions currently in the 'Up' state
              (as defined in RFC 5880).";
         }
         leaf number-of-sessions-down {
           type yang:gauge32;
           description
             "Number of BFD sessions currently in the 'Down' or 'Init'
              state but not 'adminDown' (as defined in RFC 5880).";
         }
         leaf number-of-sessions-admin-down {
           type yang:gauge32;
           description
             "Number of BFD sessions currently in the 'adminDown' state
              (as defined in RFC 5880).";
         }
       }
     }

     grouping notification-parms {
       description
         "This group describes common parameters that will be sent
          as part of BFD notifications.";
       leaf local-discr {
         type discriminator;
         description
           "BFD local discriminator.";
       }
       leaf remote-discr {
         type discriminator;
         description
           "BFD remote discriminator.";
       }
       leaf new-state {
         type state;
         description
           "Current BFD state.";
       }
       leaf state-change-reason {
         type iana-bfd-types:diagnostic;
         description
           "Reason for the BFD state change.";
       }
       leaf time-of-last-state-change {
         type yang:date-and-time;
         description
           "Calendar time of the most recent previous state change.";
       }
       leaf dest-addr {
         type inet:ip-address;
         description
           "BFD peer address.";
       }
       leaf source-addr {
         type inet:ip-address;
         description
           "BFD local address.";
       }
       leaf session-index {
         type uint32;
         description
           "An index used to uniquely identify BFD sessions.";
       }
       leaf path-type {
         type identityref {
           base path-type;
         }
         description
           "BFD path type.";
       }
     }
   }
   <CODE ENDS>

2.13.  BFD Top-Level YANG Module

   This YANG module imports and augments "/routing/control-plane-
   protocols/control-plane-protocol" from [RFC8349].  It also references
   [RFC5880].

   <CODE BEGINS> file "ietf-bfd@2021-10-21.yang"
   module ietf-bfd {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd";
     prefix bfd;

     import ietf-bfd-types {
       prefix bfd-types;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-routing {
       prefix rt;
       reference
         "RFC 8349: A YANG Data Model for Routing Management
          (NMDA Version)";
     }

     organization
       "IETF BFD Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
        WG List:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains the YANG definition for BFD parameters as
        per RFC 5880.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC 5880: Bidirectional Forwarding Detection (BFD)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2021-10-21 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol" {
       when "derived-from-or-self(rt:type, 'bfd-types:bfdv1')" {
         description
           "This augmentation is only valid for a control-plane protocol
            instance of BFD (type 'bfdv1').";
       }
       description
         "BFD augmentation.";
       container bfd {
         description
           "BFD top-level container.";
         uses bfd-types:session-statistics-summary;
       }
     }
   }
   <CODE ENDS>

2.14.  BFD IP Single-Hop YANG Module

   This YANG module imports "interface-ref" from [RFC8343] and typedefs
   from [RFC6991].  It also imports and augments "/routing/control-
   plane-protocols/control-plane-protocol" from [RFC8349], and it
   references [RFC5881].

   <CODE BEGINS> file "ietf-bfd-ip-sh@2021-10-21.yang"
   module ietf-bfd-ip-sh {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh";
     prefix bfd-ip-sh;

     import ietf-bfd-types {
       prefix bfd-types;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-bfd {
       prefix bfd;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-interfaces {
       prefix if;
       reference
         "RFC 8343: A YANG Data Model for Interface Management";
     }
     import ietf-inet-types {
       prefix inet;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-routing {
       prefix rt;
       reference
         "RFC 8349: A YANG Data Model for Routing Management
          (NMDA Version)";
     }

     organization
       "IETF BFD Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
        WG List:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains the YANG definition for BFD IP single-hop
        as per RFC 5881.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC 5881: Bidirectional Forwarding Detection (BFD)
        for IPv4 and IPv6 (Single Hop)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2021-10-21 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Augments
      */

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol/bfd:bfd" {
       description
         "BFD augmentation for IP single-hop.";
       container ip-sh {
         description
           "BFD IP single-hop top-level container.";
         uses bfd-types:session-statistics-summary;
         container sessions {
           description
             "BFD IP single-hop sessions.";
           list session {
             key "interface dest-addr";
             description
               "List of IP single-hop sessions.";
             leaf interface {
               type if:interface-ref;
               description
                 "Interface on which the BFD session is running.";
             }
             leaf dest-addr {
               type inet:ip-address;
               description
                 "IP address of the peer.";
             }
             leaf source-addr {
               type inet:ip-address;
               description
                 "Local IP address.";
             }
             uses bfd-types:common-cfg-parms;
             uses bfd-types:all-session;
           }
         }
         list interfaces {
           key "interface";
           description
             "List of interfaces.";
           leaf interface {
             type if:interface-ref;
             description
               "BFD information for this interface.";
           }
           uses bfd-types:auth-parms;
         }
       }
     }

     /*
      * Notifications
      */

     notification singlehop-notification {
       description
         "Notification for BFD single-hop session state change.  An
          implementation may rate-limit notifications, e.g., when a
          session is continuously changing state.";
       uses bfd-types:notification-parms;
       leaf interface {
         type if:interface-ref;
         description
           "Interface to which this BFD session belongs.";
       }
       leaf echo-enabled {
         type boolean;
         description
           "Indicates whether Echo was enabled for BFD.";
       }
     }
   }
   <CODE ENDS>

2.15.  BFD IP Multihop YANG Module

   This YANG module imports typedefs from [RFC6991].  It also imports
   and augments "/routing/control-plane-protocols/control-plane-
   protocol" from [RFC8349], and it references [RFC5883].

   <CODE BEGINS> file "ietf-bfd-ip-mh@2021-10-21.yang"
   module ietf-bfd-ip-mh {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh";
     prefix bfd-ip-mh;

     import ietf-bfd-types {
       prefix bfd-types;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-bfd {
       prefix bfd;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-inet-types {
       prefix inet;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-routing {
       prefix rt;
       reference
         "RFC 8349: A YANG Data Model for Routing Management
          (NMDA Version)";
     }

     organization
       "IETF BFD Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
        WG List:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains the YANG definition for BFD IP multihop
        as per RFC 5883.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC 5883: Bidirectional Forwarding Detection (BFD) for
        Multihop Paths
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2021-10-21 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Augments
      */

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol/bfd:bfd" {
       description
         "BFD augmentation for IP multihop.";
       container ip-mh {
         description
           "BFD IP multihop top-level container.";
         uses bfd-types:session-statistics-summary;
         container session-groups {
           description
             "BFD IP multihop session groups.";
           list session-group {
             key "source-addr dest-addr";
             description
               "Group of BFD IP multihop sessions (for ECMP).  A
                group of sessions is between one source and one
                destination.  Each session has a different field
                in the UDP/IP header for ECMP.";
             leaf source-addr {
               type inet:ip-address;
               description
                 "Local IP address.";
             }
             leaf dest-addr {
               type inet:ip-address;
               description
                 "IP address of the peer.";
             }
             uses bfd-types:common-cfg-parms;
             leaf tx-ttl {
               type bfd-types:hops;
               default "255";
               description
                 "Hop count of outgoing BFD control packets.";
             }
             leaf rx-ttl {
               type bfd-types:hops;
               mandatory true;
               description
                 "Minimum allowed hop count value for incoming BFD
                  control packets.  Control packets whose hop count is
                  lower than this value are dropped.";
             }
             list sessions {
               config false;
               description
                 "The multiple BFD sessions between a source and a
                  destination.";
               uses bfd-types:all-session;
             }
           }
         }
       }
     }

     /*
      * Notifications
      */

     notification multihop-notification {
       description
         "Notification for BFD multihop session state change.  An
          implementation may rate-limit notifications, e.g., when a
          session is continuously changing state.";
       uses bfd-types:notification-parms;
     }
   }
   <CODE ENDS>

2.16.  BFD-over-LAG YANG Module

   This YANG module imports "interface-ref" from [RFC8343] and typedefs
   from [RFC6991].  It also imports and augments "/routing/control-
   plane-protocols/control-plane-protocol" from [RFC8349].
   Additionally, it references [RFC7130].

   <CODE BEGINS> file "ietf-bfd-lag@2021-10-21.yang"
   module ietf-bfd-lag {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-lag";
     prefix bfd-lag;

     import ietf-bfd-types {
       prefix bfd-types;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-bfd {
       prefix bfd;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-interfaces {
       prefix if;
       reference
         "RFC 8343: A YANG Data Model for Interface Management";
     }
     import ietf-inet-types {
       prefix inet;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-routing {
       prefix rt;
       reference
         "RFC 8349: A YANG Data Model for Routing Management
          (NMDA Version)";
     }

     organization
       "IETF BFD Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
        WG List:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains the YANG definition for BFD-over-LAG
        interfaces as per RFC 7130.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC 7130: Bidirectional Forwarding Detection (BFD) on
        Link Aggregation Group (LAG) Interfaces
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2021-10-21 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Augments
      */

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol/bfd:bfd" {
       description
         "BFD augmentation for a LAG.";
       container lag {
         description
           "BFD-over-LAG top-level container.";
         container micro-bfd-ipv4-session-statistics {
           description
             "Micro-BFD IPv4 session counters.";
           uses bfd-types:session-statistics-summary;
         }
         container micro-bfd-ipv6-session-statistics {
           description
             "Micro-BFD IPv6 session counters.";
           uses bfd-types:session-statistics-summary;
         }
         container sessions {
           description
             "BFD-over-LAG sessions.";
           list session {
             key "lag-name";
             description
               "List of BFD-over-LAG sessions.";
             leaf lag-name {
               type if:interface-ref;
               description
                 "Name of the LAG.";
             }
             leaf ipv4-dest-addr {
               type inet:ipv4-address;
               description
                 "IPv4 address of the peer, for IPv4 micro-BFD.";
             }
             leaf ipv6-dest-addr {
               type inet:ipv6-address;
               description
                 "IPv6 address of the peer, for IPv6 micro-BFD.";
             }
             uses bfd-types:common-cfg-parms;
             leaf use-ipv4 {
               type boolean;
               description
                 "Using IPv4 micro-BFD.";
             }
             leaf use-ipv6 {
               type boolean;
               description
                 "Using IPv6 micro-BFD.";
             }
             list member-links {
               key "member-link";
               config false;
               description
                 "Micro-BFD over a LAG.  This represents one
                  member link.";
               leaf member-link {
                 type if:interface-ref;
                 description
                   "Member link on which micro-BFD is running.";
               }
               container micro-bfd-ipv4 {
                 when "../../use-ipv4 = 'true'" {
                   description
                     "Needed only if IPv4 is used.";
                 }
                 description
                   "Micro-BFD IPv4 session state on a member link.";
                 uses bfd-types:all-session;
               }
               container micro-bfd-ipv6 {
                 when "../../use-ipv6 = 'true'" {
                   description
                     "Needed only if IPv6 is used.";
                 }
                 description
                   "Micro-BFD IPv6 session state on a member link.";
                 uses bfd-types:all-session;
               }
             }
           }
         }
       }
     }

     /*
      * Notifications
      */

     notification lag-notification {
       description
         "Notification for BFD-over-LAG session state change.
          An implementation may rate-limit notifications, e.g., when a
          session is continuously changing state.";
       uses bfd-types:notification-parms;
       leaf lag-name {
         type if:interface-ref;
         description
           "LAG interface name.";
       }
       leaf member-link {
         type if:interface-ref;
         description
           "Member link on which BFD is running.";
       }
     }
   }
   <CODE ENDS>

2.17.  BFD-over-MPLS YANG Module

   This YANG module imports typedefs from [RFC6991].  It also imports
   and augments "/routing/control-plane-protocols/control-plane-
   protocol" from [RFC8349].  Additionally, it references [RFC5586] and
   [RFC5884].

   <CODE BEGINS> file "ietf-bfd-mpls@2021-10-21.yang"
   module ietf-bfd-mpls {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-mpls";
     prefix bfd-mpls;

     import ietf-bfd-types {
       prefix bfd-types;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-bfd {
       prefix bfd;
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }
     import ietf-inet-types {
       prefix inet;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-routing {
       prefix rt;
       reference
         "RFC 8349: A YANG Data Model for Routing Management
          (NMDA Version)";
     }

     organization
       "IETF BFD Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
        WG List:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains the YANG definition for BFD parameters for
        MPLS LSPs as per RFC 5884.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC 5884: Bidirectional Forwarding Detection (BFD)
        for MPLS Label Switched Paths (LSPs)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2021-10-21 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Identity definitions
      */

     identity encap-gach {
       base bfd-types:encap-type;
       description
         "BFD with G-ACh encapsulation as per RFC 5586.";
       reference
         "RFC 5586: MPLS Generic Associated Channel";
     }

     identity encap-ip-gach {
       base bfd-types:encap-type;
       description
         "BFD with IP and G-ACh encapsulation as per RFC 5586.";
     }

     /*
      * Groupings
      */

     grouping encap-cfg {
       description
         "Configuration for BFD encapsulation.";
       leaf encap {
         type identityref {
           base bfd-types:encap-type;
         }
         default "bfd-types:encap-ip";
         description
           "BFD encapsulation.";
       }
     }

     grouping mpls-dest-address {
       description
         "Destination address as per RFC 5884.";
       reference
         "RFC 5884: Bidirectional Forwarding Detection (BFD)
          for MPLS Label Switched Paths (LSPs)";
       leaf mpls-dest-address {
         type inet:ip-address;
         config false;
         description
           "Destination address as per RFC 5884.
            Needed if IP encapsulation is used.";
       }
     }

     /*
      * Augments
      */

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol/bfd:bfd" {
       description
         "BFD augmentation for MPLS.";
       container mpls {
         description
           "BFD MPLS top-level container.";
         uses bfd-types:session-statistics-summary;
         container egress {
           description
             "Egress configuration.";
           uses bfd-types:client-cfg-parms;
           uses bfd-types:auth-parms;
         }
         container session-groups {
           description
             "BFD-over-MPLS session groups.";
           list session-group {
             key "mpls-fec";
             description
               "Group of BFD MPLS sessions (for ECMP).  A group of
                sessions is for one FEC.  Each session has a different
                field in the UDP/IP header for ECMP.";
             leaf mpls-fec {
               type inet:ip-prefix;
               description
                 "MPLS FEC.";
             }
             uses bfd-types:common-cfg-parms;
             list sessions {
               config false;
               description
                 "The BFD sessions for an MPLS FEC.  The local
                  discriminator is unique for each session in the
                  group.";
               uses bfd-types:all-session;
               uses bfd-mpls:mpls-dest-address;
             }
           }
         }
       }
     }

     /*
      * Notifications
      */

     notification mpls-notification {
       description
         "Notification for BFD-over-MPLS FEC session state change.
          An implementation may rate-limit notifications, e.g., when a
          session is continuously changing state.";
       uses bfd-types:notification-parms;
       leaf mpls-dest-address {
         type inet:ip-address;
         description
           "Destination address as per RFC 5884.
            Needed if IP encapsulation is used.";
       }
     }
   }
   <CODE ENDS>

3.  Data Model Examples

   This section presents some simple and illustrative examples of how to
   configure BFD.

   The examples are represented in XML [W3C.REC-xml-20081126].

3.1.  IP Single-Hop

   The following is an example configuration for a BFD IP single-hop
   session.  The desired transmit interval and the required receive
   interval are both set to 10 ms.

   <?xml version="1.0" encoding="UTF-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
       <interface>
         <name>eth0</name>
         <type xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type">
           ianaift:ethernetCsmacd
         </type>
       </interface>
     </interfaces>
     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
       <control-plane-protocols>
         <control-plane-protocol>
           <type xmlns:bfd-types=
               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
             bfd-types:bfdv1
           </type>
           <name>name:BFD</name>
           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
             <ip-sh xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh">
               <sessions>
                 <session>
                   <interface>eth0</interface>
                   <dest-addr>2001:db8:0:113::101</dest-addr>
                   <desired-min-tx-interval>
                     10000
                   </desired-min-tx-interval>
                   <required-min-rx-interval>
                     10000
                   </required-min-rx-interval>
                 </session>
               </sessions>
             </ip-sh>
           </bfd>
         </control-plane-protocol>
       </control-plane-protocols>
     </routing>
   </config>

3.2.  IP Multihop

   The following is an example configuration for a BFD IP multihop
   session group.  The desired transmit interval and the required
   receive interval are both set to 150 ms.

   <?xml version="1.0" encoding="UTF-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
       <control-plane-protocols>
         <control-plane-protocol>
           <type xmlns:bfd-types=
               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
             bfd-types:bfdv1
           </type>
           <name>name:BFD</name>
           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
             <ip-mh xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh">
               <session-groups>
                 <session-group>
                   <source-addr>2001:db8:0:113::103</source-addr>
                   <dest-addr>2001:db8:0:114::100</dest-addr>
                   <desired-min-tx-interval>
                     150000
                   </desired-min-tx-interval>
                   <required-min-rx-interval>
                     150000
                   </required-min-rx-interval>
                   <rx-ttl>240</rx-ttl>
                 </session-group>
               </session-groups>
             </ip-mh>
           </bfd>
         </control-plane-protocol>
       </control-plane-protocols>
     </routing>
   </config>

3.3.  LAG

   The following is an example of BFD configuration for a LAG session.
   In this case, an interface named "Bundle-Ether1" of interface type
   "ieee8023adLag" has a desired transmit interval and required receive
   interval set to 10 ms.

   <?xml version="1.0" encoding="UTF-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
       <interface>
         <name>Bundle-Ether1</name>
         <type xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type">
           ianaift:ieee8023adLag
         </type>
       </interface>
     </interfaces>
     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
       <control-plane-protocols>
         <control-plane-protocol>
           <type xmlns:bfd-types=
               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
             bfd-types:bfdv1
           </type>
           <name>name:BFD</name>
           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
             <lag xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-lag">
               <sessions>
                 <session>
                   <lag-name>Bundle-Ether1</lag-name>
                   <ipv6-dest-addr>2001:db8:112::16</ipv6-dest-addr>
                   <desired-min-tx-interval>
                     100000
                   </desired-min-tx-interval>
                   <required-min-rx-interval>
                     100000
                   </required-min-rx-interval>
                   <use-ipv6>true</use-ipv6>
                 </session>
               </sessions>
             </lag>
           </bfd>
         </control-plane-protocol>
       </control-plane-protocols>
     </routing>
   </config>

3.4.  MPLS

   The following is an example of BFD configured for an MPLS LSP.  In
   this case, the desired transmit interval and required receive
   interval are both set to 250 ms.

   <?xml version="1.0" encoding="UTF-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <routing xmlns="urn:ietf:params:xml:ns:yang:ietf-routing">
       <control-plane-protocols>
         <control-plane-protocol>
           <type xmlns:bfd-types=
               "urn:ietf:params:xml:ns:yang:ietf-bfd-types">
             bfd-types:bfdv1
           </type>
           <name>name:BFD</name>
           <bfd xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd">
             <mpls xmlns="urn:ietf:params:xml:ns:yang:ietf-bfd-mpls">
               <session-groups>
                 <session-group>
                   <mpls-fec>2001:db8:114::/116</mpls-fec>
                   <desired-min-tx-interval>
                     250000
                   </desired-min-tx-interval>
                   <required-min-rx-interval>
                     250000
                   </required-min-rx-interval>
                 </session-group>
               </session-groups>
             </mpls>
           </bfd>
         </control-plane-protocol>
       </control-plane-protocols>
     </routing>
   </config>

4.  Security Considerations

   The YANG modules specified in this document define a schema for data
   that is designed to be accessed via network management protocols such
   as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF layer
   is the secure transport layer, and the mandatory-to-implement secure
   transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
   is HTTPS, and the mandatory-to-implement secure transport is TLS
   [RFC8446].

   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 are a number of data nodes defined in these YANG modules that
   are writable/creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability from a write access perspective:

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-sh/
   sessions:
      This list specifies the IP single-hop BFD sessions.

      Data nodes "local-multiplier", "desired-min-tx-interval",
      "required-min-rx-interval", and "min-interval" all impact the BFD
      IP single-hop session.  The "source-addr" and "dest-addr" data
      nodes can be used to send BFD packets to unwitting recipients.
      [RFC5880] describes how BFD mitigates such threats.
      Authentication data nodes "key-chain" and "meticulous" impact the
      security of the BFD IP single-hop session.

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-mh/
   session-group:
      This list specifies the IP multihop BFD session groups.

      Data nodes "local-multiplier", "desired-min-tx-interval",
      "required-min-rx-interval", and "min-interval" all impact the BFD
      IP multihop session.  The "source-addr" and "dest-addr" data nodes
      can be used to send BFD packets to unwitting recipients.
      [RFC5880] describes how BFD mitigates such threats.
      Authentication data nodes "key-chain" and "meticulous" impact the
      security of the BFD IP multihop session.

   /routing/control-plane-protocols/control-plane-protocol/bfd/lag/
   sessions:
      This list specifies the BFD sessions over a LAG.

      Data nodes "local-multiplier", "desired-min-tx-interval",
      "required-min-rx-interval", and "min-interval" all impact the BFD-
      over-LAG session.  The "ipv4-dest-addr" and "ipv6-dest-addr" data
      nodes can be used to send BFD packets to unwitting recipients.
      [RFC5880] describes how BFD mitigates such threats.
      Authentication data nodes "key-chain" and "meticulous" impact the
      security of the BFD-over-LAG session.

   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
   session-group:
      This list specifies the session groups for BFD over MPLS.

      Data nodes "local-multiplier", "desired-min-tx-interval",
      "required-min-rx-interval", and "min-interval" all impact the BFD-
      over-MPLS-LSPs session.  Authentication data nodes "key-chain" and
      "meticulous" impact the security of the BFD-over-MPLS-LSPs
      session.

   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
   egress:
      Data nodes "local-multiplier", "desired-min-tx-interval",
      "required-min-rx-interval", and "min-interval" all impact the BFD-
      over-MPLS-LSPs sessions for which this device is an MPLS LSP
      egress node.  Authentication data nodes "key-chain" and
      "meticulous" impact the security of the BFD-over-MPLS-LSPs
      sessions for which this device is an MPLS LSP egress node.

   The YANG modules have writable data nodes that can be used for the
   creation of BFD sessions and the modification of BFD session
   parameters.  The system should "police" the creation of BFD sessions
   to prevent new sessions from causing existing BFD sessions to fail.
   In the case of BFD session modification, the BFD protocol has
   mechanisms in place that allow for in-service modification.

   When BFD clients are used to modify BFD configuration (as described
   in Section 2.1), the BFD clients need to be included in an analysis
   of the security properties of the system that uses BFD (e.g., when
   considering the authentication and authorization of control actions).
   In many cases, BFD is not the most vulnerable portion of such a
   composite system, since BFD is limited to generating well-defined
   traffic at a fixed rate on a given path; in the case of an IGP acting
   as a BFD client, attacking the IGP could cause more broad-scale
   disruption than would (de)configuring a BFD session.

   Some of the readable data nodes in these YANG modules may be
   considered sensitive or vulnerable in some network environments.  It
   is thus important to control read access (e.g., via get, get-config,
   or notification) to these data nodes.  These are the subtrees and
   data nodes and their sensitivity/vulnerability from a read access
   perspective:

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-sh/
   summary:
      Access to this information discloses the number of BFD IP single-
      hop sessions that are in the "up", "down", or "admin-down" state.
      The counters include BFD sessions for which the user does not have
      read access.

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-
   sh/sessions/session/:
      Access to data nodes "local-discriminator" and "remote-
      discriminator" (combined with the data nodes in the authentication
      container) provides the ability to spoof BFD IP single-hop
      packets.

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-mh/
   summary:
      Access to this information discloses the number of BFD IP multihop
      sessions that are in the "up", "down", or "admin-down" state.  The
      counters include BFD sessions for which the user does not have
      read access.

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-mh/
   session-groups/session-group/sessions:
      Access to data nodes "local-discriminator" and "remote-
      discriminator" (combined with the data nodes in the session
      group's authentication container) provides the ability to spoof
      BFD IP multihop packets.

   /routing/control-plane-protocols/control-plane-protocol/bfd/lag/
   micro-bfd-ipv4-session-statistics/summary:
      Access to this information discloses the number of micro-BFD IPv4
      LAG sessions that are in the "up", "down", or "admin-down" state.
      The counters include BFD sessions for which the user does not have
      read access.

   /routing/control-plane-protocols/control-plane-
   protocol/bfd/lag/sessions/session/member-links/member-link/micro-
   bfd-ipv4:
      Access to data nodes "local-discriminator" and "remote-
      discriminator" (combined with the data nodes in the session's
      authentication container) provides the ability to spoof BFD IPv4
      LAG packets.

   /routing/control-plane-protocols/control-plane-protocol/bfd/lag/
   micro-bfd-ipv6-session-statistics/summary:
      Access to this information discloses the number of micro-BFD IPv6
      LAG sessions that are in the "up", "down", or "admin-down" state.
      The counters include BFD sessions for which the user does not have
      read access.

   /routing/control-plane-protocols/control-plane-
   protocol/bfd/lag/sessions/session/member-links/member-link/micro-
   bfd-ipv6:
      Access to data nodes "local-discriminator" and "remote-
      discriminator" (combined with the data nodes in the session's
      authentication container) provides the ability to spoof BFD IPv6
      LAG packets.

   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
   summary:
      Access to this information discloses the number of BFD sessions
      over MPLS LSPs that are in the "up", "down", or "admin-down"
      state.  The counters include BFD sessions for which the user does
      not have read access.

   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
   session-groups/session-group/sessions:
      Access to data nodes "local-discriminator" and "remote-
      discriminator" (combined with the data nodes in the session
      group's authentication container) provides the ability to spoof
      BFD-over-MPLS-LSPs packets.

   This document does not define any RPC operations.

5.  IANA Considerations

   IANA has registered the following namespace URIs in the "IETF XML
   Registry" [RFC3688]:

   URI:  urn:ietf:params:xml:ns:yang:iana-bfd-types
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-types
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-lag
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   URI:  urn:ietf:params:xml:ns:yang:ietf-bfd-mpls
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   IANA has registered the following YANG modules in the "YANG Module
   Names" registry [RFC6020]:

   Name:  iana-bfd-types
   Namespace:  urn:ietf:params:xml:ns:yang:iana-bfd-types
   Prefix:  iana-bfd-types
   Reference:  RFC 9127

   Name:  ietf-bfd-types
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-types
   Prefix:  bfd-types
   Reference:  RFC 9127

   Name:  ietf-bfd
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd
   Prefix:  bfd
   Reference:  RFC 9127

   Name:  ietf-bfd-ip-sh
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-sh
   Prefix:  bfd-ip-sh
   Reference:  RFC 9127

   Name:  ietf-bfd-ip-mh
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-ip-mh
   Prefix:  bfd-ip-mh
   Reference:  RFC 9127

   Name:  ietf-bfd-lag
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-lag
   Prefix:  bfd-lag
   Reference:  RFC 9127

   Name:  ietf-bfd-mpls
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-bfd-mpls
   Prefix:  bfd-mpls
   Reference:  RFC 9127

5.1.  IANA-Maintained "iana-bfd-types" Module

   This document defines the initial version of the IANA-maintained
   "iana-bfd-types" YANG module.

   The "iana-bfd-types" YANG module mirrors the "BFD Diagnostic Codes"
   and "BFD Authentication Types" registries at
   <https://www.iana.org/assignments/bfd-parameters/>.  Whenever these
   registries change, IANA must update the "iana-bfd-types" YANG module.

6.  References

6.1.  Normative References

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

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/info/rfc5586>.

   [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>.

   [RFC5882]  Katz, D. 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>.

   [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>.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
              June 2010, <https://www.rfc-editor.org/info/rfc5884>.

   [RFC5885]  Nadeau, T., Ed. and C. Pignataro, Ed., "Bidirectional
              Forwarding Detection (BFD) for the Pseudowire Virtual
              Circuit Connectivity Verification (VCCV)", RFC 5885,
              DOI 10.17487/RFC5885, June 2010,
              <https://www.rfc-editor.org/info/rfc5885>.

   [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>.

   [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>.

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.

   [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>.

   [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>.

   [RFC8177]  Lindem, A., Ed., Qu, Y., Yeung, D., Chen, I., and J.
              Zhang, "YANG Data Model for Key Chains", RFC 8177,
              DOI 10.17487/RFC8177, June 2017,
              <https://www.rfc-editor.org/info/rfc8177>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [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>.

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
              <https://www.rfc-editor.org/info/rfc8343>.

   [RFC8344]  Bjorklund, M., "A YANG Data Model for IP Management",
              RFC 8344, DOI 10.17487/RFC8344, March 2018,
              <https://www.rfc-editor.org/info/rfc8344>.

   [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>.

   [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>.

   [RFC8960]  Saad, T., Raza, K., Gandhi, R., Liu, X., and V. Beeram, "A
              YANG Data Model for MPLS Base", RFC 8960,
              DOI 10.17487/RFC8960, December 2020,
              <https://www.rfc-editor.org/info/rfc8960>.

6.2.  Informative References

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC6428]  Allan, D., Ed., Swallow, G., Ed., and J. Drake, Ed.,
              "Proactive Connectivity Verification, Continuity Check,
              and Remote Defect Indication for the MPLS Transport
              Profile", RFC 6428, DOI 10.17487/RFC6428, November 2011,
              <https://www.rfc-editor.org/info/rfc6428>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

   [RFC8529]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Data Model for Network Instances", RFC 8529,
              DOI 10.17487/RFC8529, March 2019,
              <https://www.rfc-editor.org/info/rfc8529>.

   [RFC8530]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Model for Logical Network Elements", RFC 8530,
              DOI 10.17487/RFC8530, March 2019,
              <https://www.rfc-editor.org/info/rfc8530>.

   [RFC8532]  Kumar, D., Wang, Z., Wu, Q., Ed., Rahman, R., and S.
              Raghavan, "Generic YANG Data Model for the Management of
              Operations, Administration, and Maintenance (OAM)
              Protocols That Use Connectionless Communications",
              RFC 8532, DOI 10.17487/RFC8532, April 2019,
              <https://www.rfc-editor.org/info/rfc8532>.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, November 2008,
              <https://www.w3.org/TR/2008/REC-xml-20081126>.

Appendix A.  Echo Function Configuration Example

   As mentioned in Section 2.1.2, the mechanism to start and stop the
   Echo function, as defined in [RFC5880] and discussed in [RFC5881], is
   implementation specific.  In this appendix, we provide an example of
   how the Echo function can be implemented via configuration.

   module: example-bfd-echo
     augment /rt:routing/rt:control-plane-protocols
               /rt:control-plane-protocol/bfd:bfd/bfd-ip-sh:ip-sh
               /bfd-ip-sh:sessions:
       +--rw echo {bfd-types:echo-mode}?
          +--rw desired-min-echo-tx-interval?    uint32
          +--rw required-min-echo-rx-interval?   uint32

A.1.  Example YANG Module for BFD Echo Function Configuration

   This appendix provides an example YANG module for configuration of
   the BFD Echo function.  It imports and augments "/routing/control-
   plane-protocols/control-plane-protocol" from [RFC8349], and it
   references [RFC5880].

   module example-bfd-echo {
     namespace "tag:example.com,2021:example-bfd-echo";
     prefix example-bfd-echo;

     import ietf-bfd-types {
       prefix bfd-types;
     }
     import ietf-bfd {
       prefix bfd;
     }
     import ietf-bfd-ip-sh {
       prefix bfd-ip-sh;
     }
     import ietf-routing {
       prefix rt;
     }

     organization
       "IETF BFD Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd/>
        WG List:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains an example YANG augmentation for
        configuration of the BFD Echo function.

        Copyright (c) 2021 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 Simplified 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 9127; see the
        RFC itself for full legal notices.";

     revision 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC 9127: YANG Data Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Groupings
      */

     grouping echo-cfg-parms {
       description
         "BFD grouping for Echo configuration parameters.";
       leaf desired-min-echo-tx-interval {
         type uint32;
         units "microseconds";
         default "0";
         description
           "This is the minimum interval that the local system would
            like to use when transmitting BFD Echo packets.  If 0,
            the Echo function as defined in BFD (RFC 5880) is
            disabled.";
       }
       leaf required-min-echo-rx-interval {
         type uint32;
         units "microseconds";
         default "0";
         description
           "This is the Required Min Echo RX Interval as defined in BFD
            (RFC 5880).";
       }
     }

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol/bfd:bfd/bfd-ip-sh:ip-sh/"
           + "bfd-ip-sh:sessions" {
       description
         "Augmentation for the BFD Echo function.";
       container echo {
         if-feature "bfd-types:echo-mode";
         description
           "BFD Echo function container.";
         uses echo-cfg-parms;
       }
     }
   }

Acknowledgments

   We would like to thank Nobo Akiya and Jeff Haas for their
   encouragement on this work.  We would also like to thank Tom Petch
   for his comments on the document.  We would also like to thank Acee
   Lindem for his guidance.  Thanks also to Jürgen Schönwälder, who was
   instrumental in improving the YANG modules.

Authors' Addresses

   Reshad Rahman (editor)
   Canada

   Email: reshad@yahoo.com


   Lianshu Zheng (editor)
   Huawei Technologies
   China

   Email: veronique_cheng@hotmail.com


   Mahesh Jethanandani (editor)
   Xoriant Corporation
   1248 Reamwood Ave
   Sunnyvale, California 94089
   United States of America

   Email: mjethanandani@gmail.com


   Santosh Pallagatti
   VMware
   India

   Email: santosh.pallagatti@gmail.com


   Greg Mirsky
   Ericsson

   Email: gregimirsky@gmail.com
