Network Working Group

Internet Engineering Task Force (IETF)                    R. Rahman, Ed.
Internet-Draft                                             Cisco Systems
Intended status:
Request for Comments: 9127
Category: Standards Track                                  L. Zheng, Ed.
Expires: February 2, 2019
ISSN: 2070-1721                                      Huawei Technologies
                                                    M. Jethanandani, Ed.
                                                     Xoriant Corporation
                                                           S. Pallagatti
                                                                 Rtbrick
                                                                  VMware
                                                               G. Mirsky
                                                         ZTE Corporation
                                                          August 1, 2018
                                                                Ericsson
                                                            October 2021

      YANG Data Model for Bidirectional Forwarding Detection (BFD)
                         draft-ietf-bfd-yang-17

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). (NMDA) (RFC 8342).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for a maximum publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of six months RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 2, 2019.
   https://www.rfc-editor.org/info/rfc9127.

Copyright Notice

   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Tree Diagrams . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Design of the Data Model  . . . . . . . . . . . . . . . . . .   4
     2.1.  Design of the Configuration Model . . . . . . . . . . . . . .   5
       2.1.1.  Common BFD configuration parameters . . . . . . . . .   6 Configuration Parameters
       2.1.2.  Single-hop  Single-Hop IP . . . . . . . . . . . . . . . . . . . .   7
       2.1.3.  Multihop IP . . . . . . . . . . . . . . . . . . . . .   7
       2.1.4.  MPLS Traffic Engineering Tunnels  . . . . . . . . . .   8
       2.1.5.  MPLS Label Switched Paths . . . . . . . . . . . . . .   9
       2.1.6.
       2.1.5.  Link Aggregation Groups . . . . . . . . . . . . . . .   9
     2.2.  Design of the Operational State Model . . . . . . . . . . . .   9
     2.3.  Notifications . . . . . . . . . . . . . . . . . . . . . .  10
     2.4.  RPC Operations  . . . . . . . . . . . . . . . . . . . . .  10
     2.5.  BFD top level hierarchy . . . . . . . . . . . . . . . . .  10 Top-Level Hierarchy
     2.6.  BFD IP single-hop hierarchy . . . . . . . . . . . . . . .  10 Single-Hop Hierarchy
     2.7.  BFD IP multihop hierarchy . . . . . . . . . . . . . . . .  12 Multihop Hierarchy
     2.8.  BFD over LAG hierarchy  . . . . . . . . . . . . . . . . .  14  BFD-over-LAG Hierarchy
     2.9.  BFD over MPLS LSPs hierarchy  . . . . . . . . . . . . . .  18  BFD-over-MPLS-LSPs Hierarchy
     2.10. BFD over MPLS-TE hierarchy  . . . . . . . . . . . . . . .  20
     2.11. Interaction with other Other YANG modules . . . . . . . . . . .  22
       2.11.1. Modules
       2.10.1.  "ietf-interfaces" Module ietf-interfaces . . . . . . . . . . . . . . .  22
       2.11.2.
       2.10.2.  "ietf-ip" Module ietf-ip . . . . . . . . . . . . . . . . . . .  22
       2.11.3.
       2.10.3.  "ietf-mpls" Module ietf-mpls . . . . . . . . . . . . . . . . . .  23
       2.11.4.
     2.11. IANA BFD YANG Module ietf-te . . . . . . . . . . . . . . . . . . .  23
     2.12. IANA BFD Types YANG Module  . . . . . . . . . . . . . . . . . .  23
     2.13. BFD types Top-Level YANG Module . . . . . . . . . . . . . . . . . .  26
     2.14. BFD top-level IP Single-Hop YANG Module . . . . . . . . . . . . . . . .  39
     2.15. BFD IP single-hop Multihop YANG Module . . . . . . . . . . . . . .  41
     2.16. BFD IP multihop BFD-over-LAG YANG Module . . . . . . . . . . . . . . .  44
     2.17. BFD over LAG YANG Module  . . . . . . . . . . . . . . . .  47
     2.18. BFD over MPLS YANG Module . . . . . . . . . . . . . . . .  51
     2.19. BFD over MPLS-TE BFD-over-MPLS YANG Module  . . . . . . . . . . . . . .  55
   3.  Data Model examples . . . . . . . . . . . . . . . . . . . . .  58 Examples
     3.1.  IP single-hop . . . . . . . . . . . . . . . . . . . . . .  58 Single-Hop
     3.2.  IP multihop . . . . . . . . . . . . . . . . . . . . . . .  59 Multihop
     3.3.  LAG . . . . . . . . . . . . . . . . . . . . . . . . . . .  60
     3.4.  MPLS  . . . . . . . . . . . . . . . . . . . . . . . . . .  61
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  62
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  66
     5.1.  IANA-Maintained iana-bfd-types module . . . . . . . . . .  70 "iana-bfd-types" Module
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  70
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  70
     7.1.
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  70
     7.2.
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  73
   Appendix A.  Echo function configuration example  . . . . . . . .  73 Function Configuration Example
     A.1.  Example YANG module Module for BFD echo function configuration .  74
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .  76
     B.1.  Changes between versions -16 and -17  . . . . . . . . . .  76
     B.2.  Changes between versions -15 and -16  . . . . . . . . . .  76
     B.3.  Changes between versions -14 and -15  . . . . . . . . . .  76
     B.4.  Changes between versions -13 and -14  . . . . . . . . . .  76
     B.5.  Changes between versions -12 and -13  . . . . . . . . . .  76
     B.6.  Changes between versions -11 and -12  . . . . . . . . . .  76
     B.7.  Changes between versions -10 and -11  . . . . . . . . . .  76
     B.8.  Changes between versions -09 and -10  . . . . . . . . . .  77
     B.9.  Changes between versions -08 and -09  . . . . . . . . . .  77
     B.10. Changes between versions -07 and -08  . . . . . . . . . .  77
     B.11. Changes between versions -06 and -07  . . . . . . . . . .  77
     B.12. Changes between versions -05 and -06  . . . . . . . . . .  77
     B.13. Changes between versions -04 and -05  . . . . . . . . . .  78
     B.14. Changes between versions -03 and -04  . . . . . . . . . .  78
     B.15. Changes between versions -02 and -03  . . . . . . . . . .  78
     B.16. Changes between versions -01 and -02  . . . . . . . . . .  78
     B.17. Changes between versions -00 and -01  . . . . . . . . . .  78 Echo Function Configuration
   Acknowledgments
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  78

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 which that is used for liveness detection of
   arbitrary paths between systems.  Some examples of different types of
   paths over which we have BFD:

   1) 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) [RFC5881].

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

   3) Multihop Paths"
       [RFC5883].

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

   4)
       Label Switched Paths (LSPs)" [RFC5884].

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

   5.  Two systems connected via pseudowires (PWs), this (PWs).  This is known as
       Virtual Circuit Connectivity Verification (VCCV) (VCCV), as described in BFD
       "Bidirectional Forwarding Detection (BFD) for PW VCCV 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 operation, as described in Generic "Generic Application of
   BFD
   Bidirectional Forwarding Detection (BFD)" [RFC5882].  The obvious
   candidates which that use BFD are those which that do not have "hellos" to detect
   failures, e.g. e.g., static routes, and routing protocols whose "hellos"
   do not support sub-second failure detection, e.g. 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.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown
   here.

1.2.  Tree Diagrams

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

2.  Design of the Data Model

   Since BFD is used for liveliness liveness detection of various forwarding paths,
   there is no uniform key to identify a BFD session, and so the BFD
   data model is split in 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 module, and BFD for MPLS-TE MPLS is in another YANG
   module.  The main difference between these modules is how a BFD
   session is uniquely identified, i.e 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 which that are used by
   all the modules.

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

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

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

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

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

   BFD can operate in the following contexts:

   1.  At the network device level level.

   2.  In Logical Network Elements logical network elements (LNEs) as described in YANG "YANG Model
       for Logical Network
       Element [I-D.ietf-rtgwg-lne-model] Elements" [RFC8530].

   3.  In Network Instances network instances as described in YANG Logical "YANG Data Model for Network Element
       [I-D.ietf-rtgwg-ni-model]
       Instances" [RFC8529].

   When used at the network device level, the BFD YANG data model is
   used
   "as-is". "as is".  When the BFD YANG data model is used in a Logical Network
   Element an LNE or in a Network Instance, then
   network instance, the BFD YANG data model augments the mounted
   routing model for the Logical Network Element LNE or the
   Network Instance. network instance.

2.1.  Design of the Configuration Model

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

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

   The main BFD parameters of interest to a BFD client are mainly those related
   to the multiplier and interval(s) interval(s), since those parameters impact the
   convergence time of the BFD clients when a failure occurs.  Other
   parameters
   parameters, such as BFD authentication authentication, are not specific to the
   requirements of the BFD client.  Ideally  Configuration of BFD for all configuration clients
   should be
   centralized under BFD. centralized.  However, this is a problem for clients of BFD
   which clients
   that auto-discover their peers.  For example, IGPs do not have the
   peer address configured, instead configured; instead, the IGP is enabled on an interface interface,
   and the IGP peers are auto-discovered.  So  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 "client-cfg-parms" in Section 2.13 2.12 for BFD clients to
   configure BFD: this allows BFD clients clients, such as the IGPs 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 and similarly 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 typically this
   would typically be done by use of using an API implemented by the BFD module
   on the system.  For  In the case of BFD clients which that create BFD sessions
   via their own configuration, authentication parameters (if required)
   are still specified in BFD.

2.1.1.  Common BFD configuration parameters Configuration Parameters

   The basic BFD configuration parameters are: 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 which that is used for both transmit and receive intervals 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 a the
   "base-cfg-parms" grouping (Section 2.12), which is used by the YANG
   modules for the various forwarding paths.

   For BFD authentication authentication, we have: have the following:

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

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

2.1.2.  Single-hop  Single-Hop IP

   For single-hop IP, there is an augment of the "bfd" data node 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.  For  We use the configuration
   parameters we
   use what is defined in Section 2.1.1.  The "ip-sh" node also contains
   a list of interfaces, this interfaces and is used to specify authentication parameters
   for BFD sessions which that are created by BFD clients, see clients.  See Section 2.1.

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

      1)

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

      2)

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

2.1.3.  Multihop IP

   For multihop IP, there is an augment of the "bfd" data node 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:

   source of the following:

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

   destination Multihop Paths" [RFC5883].

   Destination address
      Address belonging to the remote system as per BFD for
           Multiple Hops [RFC5883]

   For [RFC5883].

   We use the configuration parameters we use what is defined in Section 2.1.1

   Here are some extra 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.

2.1.5.

2.1.4.  MPLS Label Switched Paths

   Here

   Here, we address MPLS LSPs whose FEC Forwarding Equivalence Class (FEC)
   [RFC3031] is an IP address.  The "bfd" node
   in Section 2 (Section 2) is augmented
   with "mpls" "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 uni-directional unidirectional, there is no LSP configuration on
   the egress node.

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

2.1.6.

2.1.5.  Link Aggregation Groups

   Per BFD "Bidirectional Forwarding Detection (BFD) on LAG Interfaces 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
   However, there is no LAG YANG data model which that we can augment.  So  So, a
   "lag" data node is added to the "bfd" node in node; see Section 2, the 2.  The
   configuration is per-LAG: per LAG: we have a list of LAGs.  The destination IP
   address of the micro-BFD sessions is configured per-LAG per LAG and per address-family
   address family (IPv4 and IPv6) IPv6).

2.2.  Design of the Operational State Model

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

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

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

   1.  The first category includes fundamental information of regarding a
       BFD session session, such as the local discriminator, the remote discriminator
       discriminator, and the capability of
   supporting demand detect mode are shown in the first category. ability to support Demand mode.

   2.  The second category includes a BFD session running "session-running" information, e.g.
       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
   configured, is shown in this category. interval.  Similar examples are include the
       actual
   received receive interval between the control packets and the
       actual transmit interval between the echo Echo packets.

   3.  The third category contains the detailed statistics of for the
       session, e.g. 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 1 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 end users of
   important events detected during the protocol operation.  Pair of  The local
   discriminator identifies the corresponding BFD session on the local
   system, and the remote discriminator identifies a the BFD session on local
   the remote system.  Notifications also give more important details
   about BFD
   sessions; e.g. sessions, e.g., new state, time in previous state, network-instance 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 Top-Level Hierarchy

   At the "bfd" node under control-plane-protocol, "control-plane-protocol", there is no
   configuration data, data -- only operational state data.  The operational
   state data consist consists of overall BFD session statistics, i.e. 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 Single-Hop Hierarchy

   An "ip-sh" node is added under the "bfd" node in control-plane-protocol. "control-plane-
   protocol".  The configuration data and operational state data for
   each BFD IP single-
   hop single-hop session is 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?    kc:key-chain-ref    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?    kc:key-chain-ref    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 Multihop Hierarchy

   An "ip-mh" node is added under the "bfd" node in cntrol-plane-
   protocol. "control-plane-
   protocol".  The configuration data and operational state data for
   each BFD IP multihop session is are under this "ip-mh" node.  In the
   operational state model model, we support multiple BFD multihop sessions
   per remote address (ECMP), (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?    kc:key-chain-ref    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  BFD-over-LAG Hierarchy

   A "lag" node is added under the "bfd" node in control-plane-protocol. "control-plane-
   protocol".  The configuration data and operational state data for
   each BFD LAG session
   is 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?    kc:key-chain-ref    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  BFD-over-MPLS-LSPs Hierarchy

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

2.11.

2.10.  Interaction with other Other YANG modules

   Generic Modules

   "Generic YANG Data Model for the Management of Operations,
   Administration, and Maintenance (OAM) Protocols That Use
   Connectionless OAM protocols
   [I-D.ietf-lime-yang-connectionless-oam] Communications" [RFC8532] describes how the LIME 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.

2.11.1.

2.10.1.  "ietf-interfaces" Module ietf-interfaces

   The following boolean configuration is defined in A "A YANG Data Model
   for Interface Management 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.

2.11.2.

2.10.2.  "ietf-ip" Module ietf-ip

   The following boolean configuration is defined in A "A YANG Data Model
   for IP Management 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.

2.11.3.

2.10.3.  "ietf-mpls" Module ietf-mpls

   The following boolean configuration is defined in A "A YANG Data Model
   for MPLS Base [I-D.ietf-mpls-base-yang]:

   /rt:routing/mpls:mpls/mpls:interface/mpls:config/mpls:enabled 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.

2.12.

2.11.  IANA BFD YANG Module

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

   <CODE BEGINS> file "iana-bfd-types@2018-08-01.yang" "iana-bfd-types@2021-09-03.yang"
   module iana-bfd-types {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:iana-bfd-types";
     prefix "iana-bfd-types"; iana-bfd-types;

     organization
       "IANA";
     contact
      "        Internet
       "Internet Assigned Numbers Authority

        Postal: ICANN
                12025 Waterfront Drive, Suite 300
                Los Angeles, CA 90094-2536
                United States of America
        Tel:    +1 310 823 9358 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) 2018 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
       (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";

    // RFC Ed.: replace XXXX with actual RFC number and remove
    // this note
     reference
       "RFC XXXX"; 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: IANA BFD 9127: YANG Data Types."; Model for Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Type Definitions definitions
      */

     typedef diagnostic {
       type enumeration {
         enum none {
           value 0;
           description "None";
             "No Diagnostic.";
         }
         enum control-expiry {
           value 1;
           description
             "Control timer expiry"; Detection Time Expired.";
         }
         enum echo-failed {
           value 2;
           description
             "Echo failure"; Function Failed.";
         }
         enum neighbor-down {
           value 3;
           description
             "Neighbor down"; Signaled Session Down.";
         }
         enum forwarding-reset {
           value 4;
           description
             "Forwarding reset"; Plane Reset.";
         }
         enum path-down {
           value 5;
           description
             "Path down"; Down.";
         }
         enum concatenated-path-down {
           value 6;
           description
             "Concatenated path down"; Path Down.";
         }
         enum admin-down {
           value 7;
           description "Admin down";
             "Administratively Down.";
         }
         enum reverse-concatenated-path-down {
           value 8;
           description
             "Reverse concatenated path down"; Concatenated Path Down.";
         }
         enum mis-connectivity-defect {
           value 9;
           description
             "Mis-connectivity defect as specified in RFC6428"; 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 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";
             "Reserved.";
         }
         enum simple-password {
           value 1;
           description
             "Simple password"; Password.";
         }
         enum keyed-md5 {
           value 2;
           description
             "Keyed MD5"; MD5.";
         }
         enum meticulous-keyed-md5 {
           value 3;
           description
             "Meticulous keyed MD5"; Keyed MD5.";
         }
         enum keyed-sha1 {
           value 4;
           description
             "Keyed SHA1"; SHA1.";
         }
         enum meticulous-keyed-sha1 {
           value 5;
           description
             "Meticulous keyed SHA1"; Keyed SHA1.";
         }
       }
       description
         "BFD authentication type as defined in RFC 5880, values 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>

2.13.

2.12.  BFD types Types YANG Module

   This YANG module imports typedefs from [RFC6991], [RFC8177] [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@2018-08-01.yang" "ietf-bfd-types@2021-09-03.yang"
   module ietf-bfd-types {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-types";
     prefix "bfd-types";

  // RFC Ed.: replace occurences of XXXX with actual RFC number and
  // remove this note bfd-types;

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

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

     Editors:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman (rrahman@cisco.com),
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng (vero.zheng@huawei.com),
                  <mailto:veronique_cheng@hotmail.com>

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

        Copyright (c) 2018 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
     (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC XXXX"; 5880: Bidirectional Forwarding Detection (BFD)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: 9127: YANG Data Model for BFD"; Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Feature definitions
      */

     feature single-minimum-interval {
       description
         "This feature indicates that the server supports configuration
          of one minimum interval value which 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.";
          Section 6.7";
     }

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

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

     /*
      * Identity definitions
      */

     identity bfdv1 {
       base "rt:control-plane-protocol"; rt:control-plane-protocol;
       description
         "BFD protocol version 1.";
       reference
         "RFC 5880: Bidirectional Forwarding Detection (BFD)."; (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."; single-hop.";
       reference
         "RFC 5881: Bidirectional Forwarding Detection (BFD)
          for IPv4 and IPv6 (Single Hop)."; Hop)";
     }

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

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

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

     /*
      * Type Definitions definitions
      */

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

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

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

     typedef hops {
       type uint8 {
         range 1..255; "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 Section 6.7 of RFC 5880).";
       container authentication {
         if-feature authentication; "authentication";
         presence "Enables BFD authentication (see section 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 kc:key-chain-ref; key-chain:key-chain-ref;
           description
             "Name of the key-chain 'key-chain' as per RFC 8177.";
         }
         leaf meticulous {
           type boolean;
           description
             "Enables a meticulous mode as described in section per Section 6.7 " +
          "of of
              RFC 5880.";
         }
       }
     }

     grouping base-cfg-parms {
       description
         "BFD grouping for base config configuration parameters.";
       leaf local-multiplier {
         type multiplier;
         default 3; "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; "microseconds";
             default 1000000; "1000000";
             description
               "Desired minimum transmit interval of control packets.";
           }
           leaf required-min-rx-interval {
             type uint32;
             units microseconds; "microseconds";
             default 1000000; "1000000";
             description
               "Required minimum receive interval of control packets.";
           }
         }
         case single-interval {
           if-feature single-minimum-interval; "single-minimum-interval";
           leaf min-interval {
             type uint32;
             units microseconds; "microseconds";
             default 1000000; "1000000";
             description
               "Desired minimum transmit interval and required " +
            "minimum
                minimum receive interval of control packets.";
           }
         }
       }
     }

     grouping client-cfg-parms {
       description
         "BFD grouping for configuration parameters
          used by clients of BFD, e.g. BFD clients, e.g., IGP or MPLS.";
       leaf enable enabled {
         type boolean;
         default false; "false";
         description
           "Indicates whether the 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; "demand-mode";
         type boolean;
         default false; "false";
         description
           "To enable demand Demand mode.";
       }
       leaf admin-down {
         type boolean;
         default false; "false";
         description
        "Is
           "Indicates whether the BFD session is administratively
            down.";
       }
       uses auth-parms;
     }

     grouping all-session {
       description
         "BFD session operational information"; information.";
       leaf path-type {
         type identityref {
           base path-type;
         }
         config "false"; false;
         description
           "BFD path type, this type.  This indicates the path type that BFD is
            running on.";
       }
       leaf ip-encapsulation {
         type boolean;
         config "false"; false;
         description "Whether
           "Indicates whether BFD encapsulation uses IP.";
       }
       leaf local-discriminator {
         type discriminator;
         config "false"; false;
         description
           "Local discriminator.";
       }
       leaf remote-discriminator {
         type discriminator;
         config "false"; false;
         description
           "Remote discriminator.";
       }
       leaf remote-multiplier {
         type multiplier;
         config "false"; false;
         description
           "Remote multiplier.";
       }
       leaf demand-capability {
         if-feature demand-mode; "demand-mode";
         type boolean;
         config "false"; false;
         description
           "Local demand 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"; false;
         description
           "Source UDP port"; port.";
       }
       leaf dest-port {
         when "../ip-encapsulation = 'true'" {
           description
             "Destination port valid only when IP encapsulation
              is used.";
         }
         type inet:port-number;
         config "false"; false;
         description
           "Destination UDP port.";
       }
       container session-running {
         config "false"; false;
         description
           "BFD session running '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; "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"; 1;
               description
                 "Async with echo.";
             }
             enum async-without-echo {
               value "2"; 2;
               description
                 "Async without echo.";
             }
             enum demand-with-echo {
               value "3"; 3;
               description
                 "Demand with echo.";
             }
             enum demand-without-echo {
               value "4"; 4;
               description
                 "Demand without echo.";
             }
           }
           description
             "Detection mode.";
         }
         leaf negotiated-tx-interval {
           type uint32;
           units microseconds; "microseconds";
           description
             "Negotiated transmit interval.";
         }
         leaf negotiated-rx-interval {
           type uint32;
           units microseconds; "microseconds";
           description
             "Negotiated receive interval.";
         }
         leaf detection-time {
           type uint32;
           units microseconds; "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; "echo-mode";
           type uint32;
           units microseconds; "microseconds";
           description
             "Echo transmit interval in use.";
         }
       }
       container session-statistics {
         config "false"; 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 in to the
           down
              'down' state.";
         }
         leaf admin-down-count {
           type yang:counter32;
           description
             "The number of times this session has transitioned in to the
           admin-down
              '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 which 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 up the 'Up' state
              (as defined in RFC 5880).";
         }
         leaf number-of-sessions-down {
           type yang:gauge32;
           description
             "Number of BFD sessions currently in down the 'Down' or init 'Init'
              state but not admin-down 'adminDown' (as defined in RFC 5880).";
         }
         leaf number-of-sessions-admin-down {
           type yang:gauge32;
           description
             "Number of BFD sessions currently in admin-down the 'adminDown' state
              (as defined in RFC 5880).";
         }
       }
     }

     grouping notification-parms {
       description
         "This group describes common parameters that will be sent " +
      "as
          as part of BFD notification."; 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 "BFD
           "Reason for the BFD state change reason."; 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>

2.14.

2.13.  BFD top-level 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@2018-08-01.yang" "ietf-bfd@2021-09-03.yang"
   module ietf-bfd {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd";
     prefix "bfd";

     // RFC Ed.: replace occurences of XXXX with actual RFC number and
     // remove this note bfd;

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

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

        Editors:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman (rrahman@cisco.com),
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng (vero.zheng@huawei.com),
                  <mailto:veronique_cheng@hotmail.com>

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

        Copyright (c) 2018 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
        (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC XXXX"; 5880: Bidirectional Forwarding Detection (BFD)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: 9127: YANG Data Model for BFD"; 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 top-level container.";
         uses bfd-types:session-statistics-summary;
       }
     }
   }
   <CODE ENDS>

2.15.

2.14.  BFD IP single-hop Single-Hop YANG Module

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

   <CODE BEGINS> file "ietf-bfd-ip-sh@2018-08-01.yang" "ietf-bfd-ip-sh@2021-09-03.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";

     // RFC Ed.: replace occurences of XXXX with actual RFC number and
     // remove this note bfd-ip-sh;

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

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

        Editors:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman (rrahman@cisco.com),
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng (vero.zheng@huawei.com),
                  <mailto:veronique_cheng@hotmail.com>

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

        Copyright (c) 2018 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
        (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC XXXX"; 5881: Bidirectional Forwarding Detection (BFD)
        for IPv4 and IPv6 (Single Hop)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: A 9127: YANG data model Data Model for BFD IP single-hop"; 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"; single-hop.";
       container ip-sh {
         description
           "BFD IP single-hop top level container"; 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
          implementation may rate-limit notifications, e.g. e.g., when a " +
         "session
          session is continuously changing state.";
       uses bfd-types:notification-parms;
       leaf interface {
         type if:interface-ref;
         description
           "Interface to which this BFD session belongs to."; belongs.";
       }
       leaf echo-enabled {
         type boolean;
         description "Was echo
           "Indicates whether Echo was enabled for BFD.";
       }
     }
   }
   <CODE ENDS>

2.16.

2.15.  BFD IP multihop Multihop YANG Module

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

   <CODE BEGINS> file "ietf-bfd-ip-mh@2018-08-01.yang" "ietf-bfd-ip-mh@2021-09-03.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";

   // RFC Ed.: replace occurences of XXXX with actual RFC number and
   // remove this note bfd-ip-mh;

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

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

      Editors:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman (rrahman@cisco.com),
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng (vero.zheng@huawei.com),
                  <mailto:veronique_cheng@hotmail.com>

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

        Copyright (c) 2018 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
      (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC XXXX"; 5883: Bidirectional Forwarding Detection (BFD) for
        Multihop Paths
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: A 9127: YANG data model Data Model for BFD IP multihop."; 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 top-level container.";
         uses bfd-types:session-statistics-summary;
         container session-groups {
           description
             "BFD IP multi-hop multihop session groups.";
           list session-group {
             key "source-addr dest-addr";
             description
               "Group of BFD IP multi-hop multihop sessions (for ECMP).  A " +
             "group
                group of sessions is between 1 one source and 1  " +
             "destination, each one
                destination.  Each session has a different field " +
             "in
                in the UDP/IP hdr 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; "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.";
                  destination.";
               uses bfd-types:all-session;
             }
           }
         }
       }
     }

     /*
      * Notifications
      */

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

2.17.  BFD over LAG

2.16.  BFD-over-LAG YANG Module

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

   <CODE BEGINS> file "ietf-bfd-lag@2018-08-01.yang" "ietf-bfd-lag@2021-09-03.yang"
   module ietf-bfd-lag {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-lag";
     prefix "bfd-lag";

    // RFC Ed.: replace occurences of XXXX with actual RFC number and
    // remove this note bfd-lag;

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

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

       Editors:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman (rrahman@cisco.com),
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng vero.zheng@huawei.com),
                  <mailto:veronique_cheng@hotmail.com>

        Editor:   Mahesh Jethanandani (mjethanandani@gmail.com)";
                  <mailto:mjethanandani@gmail.com>";
     description
       "This module contains the YANG definition for BFD over LAG BFD-over-LAG
        interfaces as per RFC7130. RFC 7130.

        Copyright (c) 2018 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
       (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC XXXX"; 7130: Bidirectional Forwarding Detection (BFD) on
        Link Aggregation Group (LAG) Interfaces
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: A 9127: YANG data model Data Model for BFD over LAG"; Bidirectional Forwarding
          Detection (BFD)";
     }

     /*
      * Augments
      */

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol/bfd:bfd" {
       description
         "BFD augmentation for LAG"; a LAG.";
       container lag {
         description "BFD over LAG top level container";
           "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";
             "BFD-over-LAG sessions.";
           list session {
             key "lag-name";
             description
               "List of BFD over LAG BFD-over-LAG sessions.";
             leaf lag-name {
               type if:interface-ref ; if:interface-ref;
               description
                 "Name of the LAG"; 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 BFD-over-LAG session state change. " +
        "An
          An implementation may rate-limit notifications, e.g. e.g., when a " +
        "session
          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>

2.18.  BFD over MPLS

2.17.  BFD-over-MPLS YANG Module

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

   <CODE BEGINS> file "ietf-bfd-mpls@2018-08-01.yang" "ietf-bfd-mpls@2021-09-03.yang"
   module ietf-bfd-mpls {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-mpls";
     prefix "bfd-mpls";

    // RFC Ed.: replace occurences of XXXX with actual RFC number and
    // remove this note bfd-mpls;

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

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

       Editors:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman (rrahman@cisco.com),
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng (vero.zheng@huawei.com),
                  <mailto:veronique_cheng@hotmail.com>

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

        Copyright (c) 2018 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
       (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";
     reference
       "RFC XXXX"; 5884: Bidirectional Forwarding Detection (BFD)
        for MPLS Label Switched Paths (LSPs)
        RFC 9127: YANG Data Model for Bidirectional Forwarding
        Detection (BFD)";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: A 9127: YANG data model Data Model for BFD over MPLS LSPs"; 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"; encapsulation.";
       leaf encap {
         type identityref {
           base bfd-types:encap-type;
         }
         default bfd-types:encap-ip; "bfd-types:encap-ip";
         description
           "BFD encapsulation"; 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"; 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 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
             "BFD-over-MPLS session groups.";
           list session-group {
             key "mpls-fec";
             description
               "Group of BFD MPLS sessions (for ECMP).  A group of " +
              "sessions
                sessions is for 1 FEC, each one FEC.  Each session has a different " +
              "field
                field in the UDP/IP hdr 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. Local " +
                "discriminator  The local
                  discriminator is unique for each session in the " +
                "group.";
                  group.";
               uses bfd-types:all-session;
               uses bfd-mpls:mpls-dest-address;
             }
           }
         }
       }
     }

     /*
      * Notifications
      */

     notification mpls-notification {
       description
         "Notification for BFD over MPLS BFD-over-MPLS FEC session state change. " +
        "An
          An implementation may rate-limit notifications, e.g. e.g., when a " +
        "session
          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 Examples

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

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

3.1.  IP single-hop 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 10ms. 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>
                   <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 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 150ms. 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
   "ieee802eadLag"
   "ieee8023adLag" has a desired transmit interval and required receive
   interval set to 10ms. 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
   250ms. 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 module modules specified in this document defines 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
   [RFC5246].
   [RFC8446].

   The NETCONF access control model [RFC6536] 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 this these YANG module 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: sensitivity/vulnerability from a write access perspective:

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

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-sh/
   sessions: data

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

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

   /routing/control-plane-protocols/control-plane-protocol/bfd/ip-mh/
   session-group: data

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

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

   /routing/control-plane-protocols/control-plane-protocol/bfd/lag/
   sessions: data

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

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

   /routing/control-plane-protocols/control-plane-protocol/bfd/mpls/
   session-group: data

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

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

   The YANG module has writeable modules have writable data nodes which 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.  For
   In the case of BFD session modification, the BFD protocol has
   mechanisms in place which that allow for in service 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 BFD-using 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 could cause. session.

   Some of the readable data nodes in this these YANG module 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: sensitivity/vulnerability from a read access
   perspective:

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

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

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

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

   /routing/control-plane-protocols/control-plane-
   protocol/bfd/lag/sessions/session/member-links/member-link/micro-bfd-
   ipv4: access
   protocol/bfd/lag/sessions/session/member-links/member-link/micro-
   bfd-ipv4:
      Access to data nodes local-discriminator "local-discriminator" and remote-
   discriminator "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
      Access to this information discloses the number of micro BFD micro-BFD IPv6
      LAG sessions which that are up,
   down and admin-down. in the "up", "down", or "admin-down" state.
      The counters include BFD sessions for which the user does not have read-access.
      read access.

   /routing/control-plane-protocols/control-plane-
   protocol/bfd/lag/sessions/session/member-links/member-link/micro-bfd-
   ipv6: access
   protocol/bfd/lag/sessions/session/member-links/member-link/micro-
   bfd-ipv6:
      Access to data nodes local-discriminator "local-discriminator" and remote-
   discriminator "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
      Access to this information discloses the number of BFD sessions
      over MPLS LSPs which that are up, down and admin-down. in the "up", "down", or "admin-down"
      state.  The counters include BFD sessions for which the user does
      not have read- read access.

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

   This document does not define any RPC operations.

5.  IANA Considerations

   This document registers

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

   --------------------------------------------------------------------

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

   --------------------------------------------------------------------

   --------------------------------------------------------------------

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

   --------------------------------------------------------------------

   --------------------------------------------------------------------

   This document registers

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

   RFC Editor: Replace RFC XXXX with actual RFC number and remove this
   note.

   --------------------------------------------------------------------

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

   --------------------------------------------------------------------

   -------------------------------------------------------------------- 9127

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

   --------------------------------------------------------------------

   -------------------------------------------------------------------- 9127

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

   --------------------------------------------------------------------

   -------------------------------------------------------------------- 9127

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

   --------------------------------------------------------------------

   -------------------------------------------------------------------- 9127

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

   --------------------------------------------------------------------

   -------------------------------------------------------------------- 9127

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

   --------------------------------------------------------------------

   -------------------------------------------------------------------- 9127

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

   --------------------------------------------------------------------

   -------------------------------------------------------------------- 9127

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

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

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

7.

6.  References

7.1.

6.1.  Normative References

   [I-D.ietf-mpls-base-yang]
              Saad, T., Raza, K., Gandhi, R., Liu, X., and V. Beeram, "A
              YANG Data Model for MPLS Base", draft-ietf-mpls-base-
              yang-06 (work in progress), February 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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>.

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536,
              DOI 10.17487/RFC6536, March 2012,
              <https://www.rfc-editor.org/info/rfc6536>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

7.2.  Informative References

   [I-D.ietf-lime-yang-connectionless-oam]
              Kumar, D., Wang, Z., Wu, Q., Rahman,

   [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 S. Raghavan,
              "Generic V. Beeram, "A
              YANG Data Model for the Management of Operations,
              Administration, 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 Maintenance (OAM) Protocols that use
              Connectionless Communications", draft-ietf-lime-yang-
              connectionless-oam-18 (work in progress), 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 2017.

   [I-D.ietf-rtgwg-lne-model] 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 Logical Network Elements", draft-
              ietf-rtgwg-lne-model-10 (work in progress), Instances", RFC 8529,
              DOI 10.17487/RFC8529, March 2018.

   [I-D.ietf-rtgwg-ni-model] 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 Instances", draft-ietf-rtgwg-
              ni-model-12 (work in progress), Elements", RFC 8530,
              DOI 10.17487/RFC8530, March 2018.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K., 2019,
              <https://www.rfc-editor.org/info/rfc8530>.

   [RFC8532]  Kumar, D., Wang, Z., Wu, Q., Ed., Rahman, R., and R. Wilton, "Network S.
              Raghavan, "Generic YANG Data Model for the Management Datastore Architecture
              (NMDA)", of
              Operations, Administration, and Maintenance (OAM)
              Protocols That Use Connectionless Communications",
              RFC 8342, 8532, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>. 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 Function Configuration Example

   As mentioned in Section 2.1.2, the mechanism to start and stop the
   echo
   Echo function, as defined in [RFC5880] and discussed in [RFC5881], is
   implementation specific.  In this section appendix, we provide an example of
   how the echo 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 Module for BFD echo function 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,2018:example-bfd-echo"; "tag:example.com,2021:example-bfd-echo";
     prefix "example-bfd-echo"; example-bfd-echo;

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

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

     Editors:  <mailto:rtg-bfd@ietf.org>

        Editor:   Reshad Rahman (rrahman@cisco.com),
                  <mailto:reshad@yahoo.com>

        Editor:   Lianshu Zheng (vero.zheng@huawei.com),
                  <mailto:veronique_cheng@hotmail.com>

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

        Copyright (c) 2018 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
     (http://trustee.ietf.org/license-info).
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; 9127; see the
        RFC itself for full legal notices.";

     revision 2018-08-01 2021-09-03 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: A 9127: YANG data model example augmentation Data Model for BFD echo
       function"; Bidirectional Forwarding
          Detection (BFD)";
     }

  // RFC Ed.: replace XXXX with actual RFC number and remove this
  // note

     /*
      * Groupings
      */

     grouping echo-cfg-parms {
       description
         "BFD grouping for echo config parameters"; Echo configuration parameters.";
       leaf desired-min-echo-tx-interval {
         type uint32;
         units microseconds; "microseconds";
         default 0; "0";
         description
           "This is the minimum interval that the local system would
            like to use when transmitting BFD echo Echo packets.  If 0,
            the echo Echo function as defined in BFD [RFC5880] (RFC 5880) is
            disabled.";
       }
       leaf required-min-echo-rx-interval {
         type uint32;
         units microseconds; "microseconds";
         default 0; "0";
         description
           "This is the Required Min Echo RX Interval as defined in BFD
        [RFC5880].";
            (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 Echo function.";
       container echo {
         if-feature bfd-types:echo-mode; "bfd-types:echo-mode";
         description
           "BFD echo Echo function container"; container.";
         uses echo-cfg-parms;
       }
     }
   }

6.  Acknowledgements

Acknowledgments

   We would also like to thank Nobo Akiya and Jeff Haas for their
   encouragement on this work.  We would also like to thank Rakesh
   Gandhi and Tarek Saad Tom Petch
   for their help his comments on the MPLS-TE model. 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)
   Cisco Systems
   Canada

   Email: rrahman@cisco.com reshad@yahoo.com

   Lianshu Zheng (editor)
   Huawei Technologies
   China

   Email: vero.zheng@huawei.com veronique_cheng@hotmail.com

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

   Email: mjethanandani@gmail.com

   Santosh Pallagatti
   Rtbrick
   VMware
   India

   Email: santosh.pallagatti@gmail.com

   Greg Mirsky
   ZTE Corporation
   Ericsson

   Email: gregimirsky@gmail.com