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ietf-nat@2017-11-16



  module ietf-nat {

    yang-version 1.1;

    namespace
      "urn:ietf:params:xml:ns:yang:ietf-nat";

    prefix nat;

    import ietf-inet-types {
      prefix inet;
    }
    import ietf-yang-types {
      prefix yang;
    }
    import ietf-interfaces {
      prefix if;
    }

    organization
      "IETF OPSAWG (Operations and Management Area Working Group)";

    contact
      "WG Web:   <https://datatracker.ietf.org/wg/opsawg/>
     WG List:  <mailto:opsawg@ietf.org>

     Editor:  Mohamed Boucadair
              <mailto:mohamed.boucadair@orange.com>

     Editor:  Senthil Sivakumar
              <mailto:ssenthil@cisco.com>

     Editor:  Christian Jacquenet
              <mailto:christian.jacquenet@orange.com>

     Editor:  Suresh Vinapamula
              <mailto:sureshk@juniper.net>

     Editor:  Qin Wu
              <mailto:bill.wu@huawei.com>";

    description
      "This module is a YANG module for NAT implementations.

     NAT44, Network Address and Protocol Translation from IPv6
     Clients to IPv4 Servers (NAT64), Customer-side transLATor (CLAT),
     Stateless IP/ICMP Translation (SIIT), Explicit Address Mappings
     for Stateless IP/ICMP Translation (SIIT EAM), and IPv6 Network
     Prefix Translation (NPTv6) are covered.

     Copyright (c) 2017 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).

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

    revision "2017-11-16" {
      description "Initial revision.";
      reference
        "RFC XXXX: A YANG Data Model for Network Address Translation
        	  (NAT) and Network Prefix Translation (NPT)";

    }


    typedef percent {
      type uint8 {
        range "0 .. 100";
      }
      description "Percentage";
    }

    feature basic-nat44 {
      description
        "Basic NAT44 translation is limited to IP addresses alone.";
      reference
        "RFC 3022: Traditional IP Network Address Translator
        	  (Traditional NAT)";

    }

    feature napt44 {
      description
        "Network Address/Port Translator (NAPT): translation is
      extended to include IP addresses and transport identifiers
      (such as a TCP/UDP port or ICMP query ID).

      If the internal IP address is not sufficient to uniquely
      disambiguate NAPT44 mappings, an additional attribute is
      required. For example, that additional attribute may
      be an IPv6 address (a.k.a., DS-Lite (RFC 6333)) or
      a Layer 2 identifier (a.k.a., Per-Interface NAT
      (RFC 6619))";
      reference
        "RFC 3022: Traditional IP Network Address Translator
        	  (Traditional NAT)";

    }

    feature dst-nat {
      description
        "Destination NAT is a translation that acts on the destination
       IP address and/or destination port number. This flavor is
       usually deployed in load balancers or at devices
       in front of public servers.";
    }

    feature nat64 {
      description
        "NAT64 translation allows IPv6-only clients to contact IPv4
       servers using unicast UDP, TCP, or   ICMP.  One or more
       public IPv4 addresses assigned to a NAT64 translator are
       shared among several IPv6-only clients.";
      reference
        "RFC 6146: Stateful NAT64: Network Address and Protocol
        	  Translation from IPv6 Clients to IPv4 Servers";

    }

    feature siit {
      description
        "The Stateless IP/ICMP Translation Algorithm (SIIT), which
       translates between IPv4 and IPv6 packet headers (including
       ICMP headers).

       In the stateless mode, an IP/ICMP translator converts IPv4
       addresses to IPv6 and vice versa solely based on the
       configuration of the stateless IP/ICMP translator and
       information contained within the packet being translated.

       The translator must support the stateless address mapping
       algorithm defined in RFC6052, which is the default behavior.";
      reference
        "RFC 7915: IP/ICMP Translation Algorithm";

    }

    feature clat {
      description
        "CLAT is customer-side translator that algorithmically
       translates 1:1 private IPv4 addresses to global IPv6 addresses,
       and vice versa.

       When a dedicated /64 prefix is not available for translation
       from DHCPv6-PD, the CLAT may perform NAT44 for all IPv4 LAN
       packets so that all the LAN-originated IPv4 packets appear
       from a single IPv4 address and are then statelessly translated
       to one interface IPv6 address that is claimed by the CLAT via
       the Neighbor Discovery Protocol (NDP) and defended with
       Duplicate Address Detection.";
      reference
        "RFC 6877: 464XLAT: Combination of Stateful and Stateless
        	  Translation";

    }

    feature eam {
      description
        "Explicit Address Mapping (EAM) is a bidirectional coupling
       between an IPv4 Prefix and an IPv6 Prefix.";
      reference
        "RFC 7757: Explicit Address Mappings for Stateless IP/ICMP
        	  Translation";

    }

    feature nptv6 {
      description
        "NPTv6 is a stateless transport-agnostic IPv6-to-IPv6
       prefix translation.";
      reference
        "RFC 6296: IPv6-to-IPv6 Network Prefix Translation";

    }

    identity nat-type {
      base 
      description
        "Base identity for nat type.";
    }

    identity basic-nat44 {
      base nat:nat-type;
      description
        "Identity for Basic NAT support.";
      reference
        "RFC 3022: Traditional IP Network Address Translator
        	  (Traditional NAT)";

    }

    identity napt44 {
      base nat:nat-type;
      description
        "Identity for NAPT support.";
      reference
        "RFC 3022: Traditional IP Network Address Translator
        	  (Traditional NAT)";

    }

    identity dst-nat {
      base nat:nat-type;
      description
        "Identity for Destination NAT support.";
    }

    identity nat64 {
      base nat:nat-type;
      description
        "Identity for NAT64 support.";
      reference
        "RFC 6146: Stateful NAT64: Network Address and Protocol
        	  Translation from IPv6 Clients to IPv4 Servers";

    }

    identity siit {
      base nat:nat-type;
      description
        "Identity for SIIT support.";
      reference
        "RFC 7915: IP/ICMP Translation Algorithm";

    }

    identity clat {
      base nat:nat-type;
      description
        "Identity for CLAT support.";
      reference
        "RFC 6877: 464XLAT: Combination of Stateful and Stateless
        	  Translation";

    }

    identity eam {
      base nat:nat-type;
      description
        "Identity for EAM support.";
      reference
        "RFC 7757: Explicit Address Mappings for Stateless IP/ICMP
        	  Translation";

    }

    identity nptv6 {
      base nat:nat-type;
      description
        "Identity for NPTv6 support.";
      reference
        "RFC 6296: IPv6-to-IPv6 Network Prefix Translation";

    }

    grouping port-number {
      description
        "Individual port or a range of ports.
       When only start-port-number is present,
       it represents a single port.";
      leaf start-port-number {
        type inet:port-number;
        description
          "Begining of the port range.";
        reference
          "Section 3.2.9 of RFC 8045.";

      }

      leaf end-port-number {
        type inet:port-number;
        must ". >= ../start-port-number" {
          error-message
            "The end-port-number must be greater than or
             equal to start-port-number.";
        }
        description "End of the port range.";
        reference
          "Section 3.2.10 of RFC 8045.";

      }
    }  // grouping port-number

    grouping port-set {
      description
        "Indicates a set of ports.

       It may be a simple port range, or use the Port Set ID (PSID)
       algorithm to represent a range of transport layer
       ports which will be used by a NAPT.";
      choice port-type {
        default 'port-range';
        description
          "Port type: port-range or port-set-algo.";
        reference
          "Section 7597: Mapping of Address and Port with
          	      Encapsulation (MAP-E)";

        case port-range {
          uses port-number;
        }  // case port-range

        case port-set-algo {
          leaf psid-offset {
            type uint8 {
              range "0..15";
            }
            description
              "The number of offset bits (a.k.a., 'a' bits).
             Specifies the numeric value for the excluded port
             range/offset bits.

             Allowed values are between 0 and 15 ";
            reference
              "Section 5.1 of RFC 7597";

          }

          leaf psid-len {
            type uint8 {
              range "0..15";
            }
            mandatory true;
            description
              "The length of PSID, representing the sharing
            ratio for an IPv4 address.

            (also known as 'k').

            The address-sharing ratio would be 2^k.";
            reference
              "Section 5.1 of RFC 7597";

          }

          leaf psid {
            type uint16;
            mandatory true;
            description
              "Port Set Identifier (PSID) value, which
             identifies a set of ports algorithmically.";
            reference
              "Section 5.1 of RFC 7597";

          }
        }  // case port-set-algo
      }  // choice port-type
    }  // grouping port-set

    grouping mapping-entry {
      description
        "NAT mapping entry.

       If an attribute is not stored in the mapping/session table,
       this means the corresponding fields of a packet that
       matches this entry is not rewritten by the NAT or this
       information is not required for NAT filtering purposes.";
      leaf index {
        type uint32;
        description
          "A unique identifier of a mapping entry. This identifier can be
         automatically assigned by the NAT instance or be explicitly
         configured.";
      }

      leaf type {
        type enumeration {
          enum "static" {
            value 0;
            description
              "The mapping entry is explicitly configured
             (e.g., via command-line interface).";
          }
          enum "dynamic-implicit" {
            value 1;
            description
              "This mapping is created implicitly as a side effect
             of processing a packet that requires a new mapping.";
          }
          enum "dynamic-explicit" {
            value 2;
            description
              "This mapping is created as a result of an explicit
             request, e.g., a PCP message.";
          }
        }
        description
          "Indicates the type of a mapping entry. E.g.,
           a mapping can be: static, implicit dynamic
           or explicit dynamic.";
      }

      leaf transport-protocol {
        type uint8;
        description
          "Upper-layer protocol associated with this mapping.
         Values are taken from the IANA protocol registry.
         For example, this field contains 6 (TCP) for a TCP
         mapping or 17 (UDP) for a UDP mapping.

         If this leaf is not instantiated, then the mapping
         applies to any protocol.";
      }

      leaf internal-src-address {
        type inet:ip-prefix;
        description
          "Corresponds to the source IPv4/IPv6 address/prefix
         of the packet received on an internal
         interface.";
      }

      container internal-src-port {
        description
          "Corresponds to the source port of the packet received
         on an internal interface.

         It is used also to indicate the internal source ICMP
         identifier.

         As a reminder, all the ICMP Query messages contain
         an 'Identifier' field, which is referred to in this
         document as the 'ICMP Identifier'.";
        uses port-number;
      }  // container internal-src-port

      leaf external-src-address {
        type inet:ip-prefix;
        description
          "Source IP address/prefix of the packet sent on an
         external interface of the NAT.";
      }

      container external-src-port {
        description
          "Source port of the packet sent on an external
         interface of the NAT.

         It is used also to indicate the external source ICMP
         identifier.";
        uses port-number;
      }  // container external-src-port

      leaf internal-dst-address {
        type inet:ip-prefix;
        description
          "Corresponds to the destination IP address/prefix
         of the packet received on an internal interface
         of the NAT.

         For example, some NAT implementations support
         the translation of both source and destination
         addresses and ports, sometimes referred to
         as 'Twice NAT'.";
      }

      container internal-dst-port {
        description
          "Corresponds to the destination port of the
         IP packet received on the internal interface.

         It is used also to include the internal
         destination ICMP identifier.";
        uses port-number;
      }  // container internal-dst-port

      leaf external-dst-address {
        type inet:ip-prefix;
        description
          "Corresponds to the destination IP address/prefix
         of the packet sent on an external interface
         of the NAT.";
      }

      container external-dst-port {
        description
          "Corresponds to the destination port number of
         the packet sent on the external interface
         of the NAT.

         It is used also to include the external
         destination ICMP identifier.";
        uses port-number;
      }  // container external-dst-port

      leaf lifetime {
        type uint32;
        units "seconds";
        description
          "When specified, it is used to track the connection that is
         fully-formed (e.g., once the three-way handshake
         TCP is completed) or the duration for maintaining
         an explicit mapping alive. The mapping entry will be
         removed by the NAT instance once this lifetime is expired.

         When reported in a get operation, the lifetime indicates
         the remaining validity lifetime.

         Static mappings may not be associated with a
         lifetime. If no lifetime is associated with a
         static mapping, an explicit action is required to
         remove that mapping.";
      }
    }  // grouping mapping-entry

    container nat {
      description "NAT module";
      container instances {
        description "NAT instances";
        list instance {
          key "id";
          description
            "A NAT instance. This identifier can be automatically assigned
           or explicitly configured.";
          leaf id {
            type uint32;
            must ". >= 1";
            description
              "NAT instance identifier.

             The identifier must be greater than zero as per RFC 7659.";
            reference
              "RFC 7659: Definitions of Managed Objects for Network
              	Address Translators (NATs)";

          }

          leaf name {
            type string;
            description
              "A name associated with the NAT instance.";
            reference
              "RFC 7659: Definitions of Managed Objects for Network
              	Address Translators (NATs)";

          }

          leaf enable {
            type boolean;
            description
              "Status of the  NAT instance.";
          }

          container capabilities {
            config false;
            description "NAT capabilities";
            leaf-list nat-flavor {
              type identityref {
                base nat-type;
              }
              description
                "Supported translation type(s).";
            }

            leaf-list per-interface-binding {
              type enumeration {
                enum "unsupported" {
                  value 0;
                  description
                    "No capability to associate a NAT binding with
                   an extra identifier.";
                }
                enum "layer-2" {
                  value 1;
                  description
                    "The NAT instance is able to associate a mapping with
                 a layer-2 identifier.";
                }
                enum "dslite" {
                  value 2;
                  description
                    "The NAT instance is able to associate a mapping with
                 an IPv6 address (a.k.a., DS-Lite).";
                }
              }
              description
                "Indicates the capability of a NAT to associate a particular
             NAT session not only with the five tuples used for the
             transport connection on both sides of the NAT but also with
             the internal interface on which the user device is
             connected to the NAT.";
              reference
                "Section 4 of RFC 6619";

            }

            list transport-protocols {
              key "protocol-id";
              description
                "List of supported protocols.";
              leaf protocol-id {
                type uint8;
                mandatory true;
                description
                  "Upper-layer protocol associated with this mapping.

               Values are taken from the IANA protocol registry:
               https://www.iana.org/assignments/protocol-numbers/
               protocol-numbers.xhtml

               For example, this field contains 6 (TCP) for a TCP
               mapping or 17 (UDP) for a UDP mapping.";
              }

              leaf protocol-name {
                type string;
                description
                  "The name of the Upper-layer protocol associated
               with this mapping.

               Values are taken from the IANA protocol registry:
               https://www.iana.org/assignments/protocol-numbers/
               protocol-numbers.xhtml

               For example, TCP, UDP, DCCP, and SCTP.";
              }
            }  // list transport-protocols

            leaf restricted-port-support {
              type boolean;
              description
                "Indicates source port NAT restriction support.";
              reference
                "RFC 7596: Lightweight 4over6: An Extension to
                	  the Dual-Stack Lite Architecture.";

            }

            leaf static-mapping-support {
              type boolean;
              description
                "Indicates whether static mappings are supported.";
            }

            leaf port-randomization-support {
              type boolean;
              description
                "Indicates whether port randomization is supported.";
              reference
                "Section 4.2.1 of RFC 4787.";

            }

            leaf port-range-allocation-support {
              type boolean;
              description
                "Indicates whether port range allocation is supported.";
              reference
                "Section 1.1 of RFC 7753.";

            }

            leaf port-preservation-suport {
              type boolean;
              description
                "Indicates whether port preservation is supported.";
              reference
                "Section 4.2.1 of RFC 4787.";

            }

            leaf port-parity-preservation-support {
              type boolean;
              description
                "Indicates whether port parity preservation is
             supported.";
              reference
                "Section 8 of RFC 7857.";

            }

            leaf address-roundrobin-support {
              type boolean;
              description
                "Indicates whether address allocation round robin is
            supported.";
            }

            leaf paired-address-pooling-support {
              type boolean;
              description
                "Indicates whether paired-address-pooling is
             supported";
              reference
                "REQ-2 of RFC 4787.";

            }

            leaf endpoint-independent-mapping-support {
              type boolean;
              description
                "Indicates whether endpoint-independent-
             mapping in Section 4 of RFC 4787 is
             supported.";
              reference
                "Section 4 of RFC 4787.";

            }

            leaf address-dependent-mapping-support {
              type boolean;
              description
                "Indicates whether address-dependent-mapping is
             supported.";
              reference
                "Section 4 of RFC 4787.";

            }

            leaf address-and-port-dependent-mapping-support {
              type boolean;
              description
                "Indicates whether address-and-port-dependent-mapping is
             supported.";
              reference
                "Section 4 of RFC 4787.";

            }

            leaf endpoint-independent-filtering-support {
              type boolean;
              description
                "Indicates whether endpoint-independent-filtering is
             supported.";
              reference
                "Section 5 of RFC 4787.";

            }

            leaf address-dependent-filtering {
              type boolean;
              description
                "Indicates whether address-dependent-filtering is
             supported.";
              reference
                "Section 5 of RFC 4787.";

            }

            leaf address-and-port-dependent-filtering {
              type boolean;
              description
                "Indicates whether address-and-port-dependent is
             supported.";
              reference
                "Section 5 of RFC 4787.";

            }

            leaf fragment-behavior {
              type enumeration {
                enum "unsupported" {
                  value 0;
                  description
                    "No capability to translate incoming fragments.
                 All received fragments are dropped.";
                }
                enum "in-order" {
                  value 1;
                  description
                    "The NAT instance is able to translate fragments only if
                 they are received in order. That is, in particular the
                 header is in the first packet. Fragments received
                 out of order are dropped. ";
                }
                enum "out-of-order" {
                  value 2;
                  description
                    "The NAT instance is able to translate a fragment even
                 if it is received out of order.

                 This behavior is the one recommended in RFC4787.";
                  reference
                    "REQ-14 of RFC 4787";

                }
              }
              description
                "The fragment behavior is the NAT instance's capability to
             translate fragments received on the external interface of
             the NAT.";
            }
          }  // container capabilities

          leaf type {
            type identityref {
              base nat-type;
            }
            description
              "Specify the translation type. Particularly useful when
           multiple  translation flavors are supported.

           If one type is supported by a NAT, this parameter is by
           default set to that type.";
          }

          leaf per-interface-binding {
            type enumeration {
              enum "disabled" {
                value 0;
                description
                  "Disable the capability to associate an extra identifier
               with NAT mappings.";
              }
              enum "layer-2" {
                value 1;
                description
                  "The NAT instance is able to associate a mapping with
               a layer-2 identifier.";
              }
              enum "dslite" {
                value 2;
                description
                  "The NAT instance is able to associate a mapping with
               an IPv6 address (a.k.a., DS-Lite).";
              }
            }
            description
              "A NAT that associates a particular NAT session not only with
           the five tuples used for the transport connection on both
           sides of the NAT but also with the internal interface on
           which the user device is connected to the NAT.

           If supported, this mode of operation should be configurable,
           and it should be disabled by default in general-purpose NAT
           devices.

           If one single per-interface binding behavior is supported by
           a NAT, this parameter is by default set to that behavior.";
            reference
              "Section 4 of RFC 6619";

          }

          list nat-pass-through {
            if-feature basic-nat44 or napt44 or dst-nat;
            key "id";
            description
              "IP prefix NAT pass through.";
            leaf id {
              type uint32;
              description
                "An identifier of the IP prefix pass through.";
            }

            leaf prefix {
              type inet:ip-prefix;
              mandatory true;
              description
                "The IP addresses that match should not be translated.

             According to REQ#6 of RFC6888, it must be possible to
             administratively turn off translation for specific
             destination addresses and/or ports.";
              reference
                "REQ#6 of RFC6888.";

            }

            leaf port {
              type inet:port-number;
              description
                "According to REQ#6 of RFC6888, it must be possible to
             administratively turn off translation for specific
             destination addresses and/or ports.

             If no prefix is defined, the NAT pass through bound
             to a given port applies for any destination address.";
              reference
                "REQ#6 of RFC6888.";

            }
          }  // list nat-pass-through

          list policy {
            key "id";
            description
              "NAT parameters for a given instance";
            leaf id {
              type uint32;
              description
                "An identifier of the NAT policy. It must be unique
            within the NAT instance.";
            }

            container clat-parameters {
              if-feature clat;
              description "CLAT parameters.";
              list clat-ipv6-prefixes {
                key "ipv6-prefix";
                description
                  "464XLAT double translation treatment is stateless when a
              dedicated /64 is available for translation on the CLAT.
              Otherwise, the CLAT will have both stateful and stateless
              since it requires NAT44 from the LAN to a single IPv4
              address and then stateless translation to a single
              IPv6 address.";
                reference
                  "RFC 6877: 464XLAT: Combination of Stateful and Stateless
                  	  Translation";

                leaf ipv6-prefix {
                  type inet:ipv6-prefix;
                  description
                    "An IPv6 prefix used for CLAT.";
                }
              }  // list clat-ipv6-prefixes

              list ipv4-prefixes {
                key "ipv4-prefix";
                description
                  "Pool of IPv4 addresses used for CLAT.
               192.0.0.0/29 is the IPv4 service continuity prefix.";
                reference
                  "RFC 7335: IPv4 Service Continuity Prefix";

                leaf ipv4-prefix {
                  type inet:ipv4-prefix;
                  description
                    "464XLAT double translation treatment is
                 stateless when a dedicated /64 is available
                 for translation on the CLAT.  Otherwise, the
                 CLAT will have both stateful and stateless
                 since it requires NAT44 from the LAN to
                 a single IPv4 address and then stateless
                 translation to a single IPv6 address.
                 The CLAT performs NAT44 for all IPv4 LAN
                 packets so that all the LAN-originated IPv4
                 packets appear from a single IPv4 address
                 and are then statelessly translated to one
                 interface IPv6 address that is claimed by
                 the CLAT.

                 An IPv4 address from this pool is also
                 provided to an application that makes
                 use of literals.";
                  reference
                    "RFC 6877: 464XLAT: Combination of Stateful and Stateless
                    	  Translation";

                }
              }  // list ipv4-prefixes
            }  // container clat-parameters

            list nptv6-prefixes {
              if-feature nptv6;
              key "internal-ipv6-prefix";
              description
                "Provides one or a list of (internal IPv6 prefix,
            external IPv6 prefix) required for NPTv6.

            In its simplest form, NPTv6 interconnects two network
            links, one of which is an 'internal' network link
            attached to a leaf network within a single
            administrative domain and the other of which is an
            'external' network with connectivity to the global
            Internet.";
              reference
                "RFC 6296: IPv6-to-IPv6 Network Prefix Translation";

              leaf internal-ipv6-prefix {
                type inet:ipv6-prefix;
                mandatory true;
                description
                  "An IPv6 prefix used by an internal interface of NPTv6.";
                reference
                  "RFC 6296: IPv6-to-IPv6 Network Prefix Translation";

              }

              leaf external-ipv6-prefix {
                type inet:ipv6-prefix;
                mandatory true;
                description
                  "An IPv6 prefix used by the external interface of NPTv6.";
                reference
                  "RFC 6296: IPv6-to-IPv6 Network Prefix Translation";

              }
            }  // list nptv6-prefixes

            list eam {
              if-feature eam;
              key "ipv4-prefix";
              description
                "The Explicit Address Mapping Table, a conceptual
            table in which each row represents an EAM.

            Each EAM describes a mapping between IPv4 and IPv6
            prefixes/addresses.";
              reference
                "Section 3.1 of RFC 7757.";

              leaf ipv4-prefix {
                type inet:ipv4-prefix;
                mandatory true;
                description
                  "The IPv4 prefix of an EAM.";
                reference
                  "Section 3.2 of RFC 7757.";

              }

              leaf ipv6-prefix {
                type inet:ipv6-prefix;
                mandatory true;
                description
                  "The IPv6 prefix of an EAM.";
                reference
                  "Section 3.2 of RFC 7757.";

              }
            }  // list eam

            list nat64-prefixes {
              if-feature siit or nat64 or clat;
              key "nat64-prefix";
              description
                "Provides one or a list of NAT64 prefixes
            with or without a list of destination IPv4 prefixes.

            Destination-based Pref64::/n is discussed in
            Section 5.1 of [RFC7050]). For example:
            192.0.2.0/24 is mapped to 2001:db8:122:300::/56.
            198.51.100.0/24 is mapped to 2001:db8:122::/48.";
              reference
                "Section 5.1 of RFC7050.";

              leaf nat64-prefix {
                type inet:ipv6-prefix;
                mandatory true;
                description
                  "A NAT64 prefix. Can be Network-Specific Prefix (NSP) or
              Well-Known Prefix (WKP).

              Organizations deploying stateless IPv4/IPv6 translation
              should assign a Network-Specific Prefix to their
              IPv4/IPv6 translation service.

              For stateless NAT64, IPv4-translatable IPv6 addresses
              must use the selected Network-Specific Prefix.

              Both IPv4-translatable IPv6 addresses and IPv4-converted
              IPv6 addresses should use the same prefix.";
                reference
                  "Sections 3.3 and 3.4 of RFC 6052.";

              }

              list destination-ipv4-prefix {
                key "ipv4-prefix";
                description
                  "An IPv4 prefix/address.";
                leaf ipv4-prefix {
                  type inet:ipv4-prefix;
                  description
                    "An IPv4 address/prefix.";
                }
              }  // list destination-ipv4-prefix

              leaf stateless-enable {
                type boolean;
                default 'false';
                description
                  "Enable explicitly stateless NAT64.";
              }
            }  // list nat64-prefixes

            list external-ip-address-pool {
              if-feature basic-nat44 or napt44 or nat64;
              key "pool-id";
              description
                "Pool of external IP addresses used to service internal
            hosts.

            A pool is a set of IP prefixes.";
              leaf pool-id {
                type uint32;
                must ". >= 1";
                description
                  "An identifier that uniquely identifies the address pool
              within a NAT instance.

              The identifier must be greater than zero as per
              RFC 7659.";
                reference
                  "RFC 7659: Definitions of Managed Objects for
                  	  Network Address Translators (NATs)";

              }

              leaf external-ip-pool {
                type inet:ipv4-prefix;
                mandatory true;
                description
                  "An IPv4 prefix used for NAT purposes.";
              }
            }  // list external-ip-address-pool

            container port-set-restrict {
              if-feature napt44 or nat64;
              description
                "Configures contiguous and non-contiguous port ranges.

            The port set is used to restrict the external source
            port numbers used by the translator.";
              uses port-set;
            }  // container port-set-restrict

            leaf dst-nat-enable {
              if-feature basic-nat44 or napt44;
              type boolean;
              default 'false';
              description
                "Enable/Disable destination NAT.

            A NAT44 may be configured to enable Destination
            NAT, too.";
            }

            list dst-ip-address-pool {
              if-feature dst-nat;
              key "pool-id";
              description
                "Pool of IP addresses used for destination NAT.";
              leaf pool-id {
                type uint32;
                description
                  "An identifier of the address pool.";
              }

              leaf dst-in-ip-pool {
                type inet:ip-prefix;
                description
                  "Is used to identify an internal IP prefix/address
               to be translated.";
              }

              leaf dst-out-ip-pool {
                type inet:ip-prefix;
                mandatory true;
                description
                  "IP address/prefix used for destination NAT.";
              }
            }  // list dst-ip-address-pool

            list transport-protocols {
              if-feature napt44 or nat64 or dst-nat;
              key "protocol-id";
              description
                "Configure the transport protocols to be handled by
             the translator.

             TCP and UDP are supported by default.";
              leaf protocol-id {
                type uint8;
                mandatory true;
                description
                  "Upper-layer protocol associated with this mapping.

               Values are taken from the IANA protocol registry:
               https://www.iana.org/assignments/protocol-numbers/
               protocol-numbers.xhtml

               For example, this field contains 6 (TCP) for a TCP
               mapping or 17 (UDP) for a UDP mapping.";
              }

              leaf protocol-name {
                type string;
                description
                  "The name of the Upper-layer protocol associated
               with this mapping.

               Values are taken from the IANA protocol registry:
               https://www.iana.org/assignments/protocol-numbers/
               protocol-numbers.xhtml

               For example, TCP, UDP, DCCP, and SCTP.";
              }
            }  // list transport-protocols

            leaf subscriber-mask-v6 {
              type uint8 {
                range "0 .. 128";
              }
              description
                "The subscriber mask is an integer that indicates
             the length of significant bits to be applied on
             the source IPv6 address (internal side) to
             unambiguously identify a user device (e.g., CPE).

             Subscriber mask is a system-wide configuration
             parameter that is used to enforce generic
             per-subscriber policies (e.g., port-quota).

             The enforcement of these generic policies does not
             require the configuration of every subscriber's
             prefix.

             Example: suppose the 2001:db8:100:100::/56 prefix
             is assigned to a NAT64 serviced CPE. Suppose also
             that 2001:db8:100:100::1 is the IPv6 address used
             by the client that resides in that CPE. When the
             NAT64 receives a packet from this client,
             it applies the subscriber-mask-v6 (e.g., 56) on
             the source IPv6 address to compute the associated
             prefix for this client (2001:db8:100:100::/56).
             Then, the NAT64 enforces policies based on that
             prefix (2001:db8:100:100::/56), not on the exact
             source IPv6 address.";
            }

            list subscriber-match {
              if-feature basic-nat44 or napt44 or dst-nat;
              key "match-id";
              description
                "IP prefix match.
             A subscriber is identified by a subnet.";
              leaf match-id {
                type uint32;
                description
                  "An identifier of the subscriber match.";
              }

              leaf subnet {
                type inet:ip-prefix;
                mandatory true;
                description
                  "The IP address subnets that match
               should be translated. E.g., all addresses
               that belong to the 192.0.2.0/24 prefix must
               be processed by the NAT.";
              }
            }  // list subscriber-match

            leaf address-allocation-type {
              type enumeration {
                enum "arbitrary" {
                  value 0;
                  description
                    "Arbitrary pooling behavior means that the NAT
                 instance may create the new port mapping using any
                 address in the pool that has a free port for the
                 protocol concerned.";
                }
                enum "roundrobin" {
                  value 1;
                  description
                    "Round robin allocation.";
                }
                enum "paired" {
                  value 2;
                  description
                    "Paired address pooling informs the NAT
                 that all the flows from an internal IP
                 address must be assigned the same external
                 address. This is the recommended behavior for
                 NAPT/NAT64.";
                  reference
                    "RFC 4787: Network Address Translation (NAT)
                    	  Behavioral Requirements for Unicast UDP";

                }
              }
              description
                "Specifies how external IP addresses are allocated.";
            }

            leaf port-allocation-type {
              if-feature napt44 or nat64;
              type enumeration {
                enum "random" {
                  value 0;
                  description
                    "Port randomization is enabled. A NAT port allocation
                 scheme should make it hard for attackers to guess
                 port numbers";
                  reference
                    "REQ-15 of RFC 6888";

                }
                enum "port-preservation" {
                  value 1;
                  description
                    "Indicates whether the NAT should preserve the internal
                 port number.";
                }
                enum
                  "port-parity-preservation" {
                  value 2;
                  description
                    "Indicates whether the NAT should preserve the port
                 parity of the internal port number.";
                }
                enum "port-range-allocation" {
                  value 3;
                  description
                    "Indicates whether the NAT assigns a range of ports
                 for an internal host. This scheme allows to minimize
                 log volume.";
                  reference
                    "REQ-14 of RFC 6888";

                }
              }
              description
                "Indicates the type of port allocation.";
            }

            leaf mapping-type {
              if-feature napt44 or nat64;
              type enumeration {
                enum "eim" {
                  value 0;
                  description
                    "endpoint-independent-mapping.";
                  reference
                    "Section 4 of RFC 4787.";

                }
                enum "adm" {
                  value 1;
                  description
                    "address-dependent-mapping.";
                  reference
                    "Section 4 of RFC 4787.";

                }
                enum "edm" {
                  value 2;
                  description
                    "address-and-port-dependent-mapping.";
                  reference
                    "Section 4 of RFC 4787.";

                }
              }
              description
                "Indicates the type of a NAT mapping.";
            }

            leaf filtering-type {
              if-feature napt44 or nat64;
              type enumeration {
                enum "eif" {
                  value 0;
                  description
                    "endpoint-independent-filtering.";
                  reference
                    "Section 5 of RFC 4787.";

                }
                enum "adf" {
                  value 1;
                  description
                    "address-dependent-filtering.";
                  reference
                    "Section 5 of RFC 4787.";

                }
                enum "edf" {
                  value 2;
                  description
                    "address-and-port-dependent-filtering";
                  reference
                    "Section 5 of RFC 4787.";

                }
              }
              description
                "Indicates the type of a NAT filtering.";
            }

            leaf fragment-behavior {
              if-feature napt44 or nat64;
              type enumeration {
                enum "drop-all" {
                  value 0;
                  description
                    "All received fragments are dropped.";
                }
                enum "in-order" {
                  value 1;
                  description
                    "Translate fragments only if they are received
                 in order.";
                }
                enum "out-of-order" {
                  value 2;
                  description
                    "Translate a fragment even if it is received out
                 of order.

                 This behavior is the recommended behavior.";
                  reference
                    "REQ-14 of RFC 4787";

                }
              }
              description
                "The fragment behavior instructs the NAT about the
              behavior to follow to translate fragments received
              on the external interface of the NAT.";
            }

            list port-quota {
              if-feature napt44 or nat64;
              key "quota-type";
              description
                "Configures a port quota to be assigned per subscriber.
             It corresponds to the maximum number of ports to be
             used by a subscriber.";
              leaf port-limit {
                type uint16;
                description
                  "Configures a port quota to be assigned per subscriber.
               It corresponds to the maximum number of ports to be
               used by a subscriber.";
                reference
                  "REQ-4 of RFC 6888.";

              }

              leaf quota-type {
                type uint8;
                description
                  "Indicates whether the port quota applies to
                 all protocols (0) or to a specific protocol.";
              }
            }  // list port-quota

            container port-set {
              when
                "../port-allocation-type = 'port-range-allocation'";
              if-feature napt44 or nat64;
              description
                "Manages port-set assignments.";
              leaf port-set-size {
                type uint16;
                mandatory true;
                description
                  "Indicates the size of assigned port sets.";
              }

              leaf port-set-timeout {
                type uint32;
                units "seconds";
                description
                  "inactivity timeout for port sets.";
              }
            }  // container port-set

            container timers {
              if-feature napt44 or nat64;
              description
                "Configure values of various timeouts.";
              leaf udp-timeout {
                type uint32;
                units "seconds";
                default '300';
                description
                  "UDP inactivity timeout. That is the time a mapping
                 will stay active without packets traversing the NAT.";
                reference
                  "RFC 4787: Network Address Translation (NAT)
                  	  Behavioral Requirements for Unicast UDP";

              }

              leaf tcp-idle-timeout {
                type uint32;
                units "seconds";
                default '7440';
                description
                  "TCP Idle timeout should be 2 hours and 4 minutes.";
                reference
                  "RFC 5382: NAT Behavioral Requirements for TCP";

              }

              leaf tcp-trans-open-timeout {
                type uint32;
                units "seconds";
                default '240';
                description
                  "The value of the transitory open connection
                 idle-timeout.

                 Section 2.1 of [RFC7857] clarifies that a NAT
                 should provide different configurable
                 parameters for configuring the open and
                 closing idle timeouts.

                 To accommodate deployments that consider
                 a partially open timeout of 4 minutes as being
                 excessive from a security standpoint, a NAT may
                 allow the configured timeout to be less than
                 4 minutes.

                 However, a minimum default transitory connection
                 idle-timeout of 4 minutes is recommended.";
                reference
                  "Section 2.1 of RFC 7857.";

              }

              leaf tcp-trans-close-timeout {
                type uint32;
                units "seconds";
                default '240';
                description
                  "The value of the transitory close connection
                idle-timeout.
                Section 2.1 of [RFC7857] clarifies that a NAT
                should provide different configurable
                parameters for configuring the open and
                closing idle timeouts.";
                reference
                  "Section 2.1 of RFC 7857.";

              }

              leaf tcp-in-syn-timeout {
                type uint32;
                units "seconds";
                default '6';
                description
                  "A NAT must not respond to an unsolicited
                inbound SYN packet for at least 6 seconds
                after the packet is received.  If during
                this interval the NAT receives and translates
                an outbound SYN for the connection the NAT
                must silently drop the original unsolicited
                inbound SYN packet.";
                reference
                  "RFC 5382 NAT Behavioral Requirements for TCP";

              }

              leaf fragment-min-timeout {
                when
                  "../../fragment-behavior='out-of-order'";
                type uint32;
                units "seconds";
                default '2';
                description
                  "As long as the NAT has available resources,
                the NAT allows the fragments to arrive
                over fragment-min-timeout interval.
                The default value is inspired from RFC6146.";
              }

              leaf icmp-timeout {
                type uint32;
                units "seconds";
                default '60';
                description
                  "An ICMP Query session timer must not expire
                 in less than 60 seconds. It is recommended
                 that the ICMP Query session timer be made
                 configurable";
                reference
                  "RFC 5508: NAT Behavioral Requirements for ICMP";

              }

              list per-port-timeout {
                key "port-number";
                description
                  "Some NATs are configurable with short timeouts
                 for some ports, e.g., as 10 seconds on
                 port 53 (DNS) and 123 (NTP) and longer timeouts
                 on other ports.";
                leaf port-number {
                  type inet:port-number;
                  description
                    "A port number.";
                }

                leaf timeout {
                  type uint32;
                  units "seconds";
                  mandatory true;
                  description
                    "Timeout for this port number";
                }
              }  // list per-port-timeout

              leaf hold-down-timeout {
                type uint32;
                units "seconds";
                default '120';
                description
                  "Hold down timer.

                 Ports in the hold down pool are not reassigned until
                 hold-down-timeout expires.

                 The length of time and the maximum number of ports in
                 this state must be configurable by the administrator.

                 This is necessary in order to prevent collisions
                 between old and new mappings and sessions. It ensures
                 that all established sessions are broken instead of
                 redirected to a different peer.";
                reference
                  "REQ#8 of RFC 6888.";

              }

              leaf hold-down-max {
                type uint32;
                description
                  "Maximum ports in the Hold down timer pool.
                 Ports in the hold down pool are not reassigned
                 until hold-down-timeout expires.

                 The length of time and the maximum
                 number of ports in this state must be
                 configurable by the administrator.
                 This is necessary in order
                 to prevent collisions between old
                 and new mappings and sessions. It ensures
                 that all established sessions are broken
                 instead of redirected to a different peer.";
                reference
                  "REQ#8 of RFC 6888.";

              }
            }  // container timers

            leaf fragments-limit {
              when
                "../fragment-behavior='out-of-order'";
              type uint32;
              description
                "Limits the number of out of order fragments that can
               be handled.";
              reference
                "Section 11 of RFC 4787.";

            }

            list algs {
              key "name";
              description
                "ALG-related features.";
              leaf name {
                type string;
                description
                  "The name of the ALG.";
              }

              leaf transport-protocol {
                type uint32;
                description
                  "The transport protocol used by the ALG
                 (e.g., TCP, UDP).";
              }

              container dst-transport-port {
                description
                  "The destination port number(s) used by the ALG.
                 For example,
                   - 21 for the FTP ALG
                   - 53 for the DNS ALG.";
                uses port-number;
              }  // container dst-transport-port

              container src-transport-port {
                description
                  "The source port number(s) used by the ALG.";
                uses port-number;
              }  // container src-transport-port

              leaf status {
                type boolean;
                description
                  "Enable/disable the ALG.";
              }
            }  // list algs

            leaf all-algs-enable {
              type boolean;
              description
                "Enable/disable all ALGs.

              When specified, this parameter overrides the one
              that may be indicated, eventually, by the 'status'
              of an individual ALG.";
            }

            container notify-pool-usage {
              if-feature basic-nat44 or napt44 or nat64;
              description
                "Notification of pool usage when certain criteria
               are met.";
              leaf pool-id {
                type uint32;
                description
                  "Pool-ID for which the notification criteria
                 is defined";
              }

              leaf high-threshold {
                type percent;
                description
                  "Notification must be generated when the defined high
                 threshold is reached.

                 For example, if a notification is required when the
                 pool utilization reaches 90%, this configuration
                 parameter must be set to 90.

                 0% indicates that no high threshold is enabled.";
              }

              leaf low-threshold {
                type percent;
                must
                  ". >= ../high-threshold" {
                  error-message
                    "The upper port number must be greater than or
                   equal to lower port number.";
                }
                description
                  "Notification must be generated when the defined low
                 threshold is reached.

                 For example, if a notification is required when the
                 pool utilization reaches below 10%, this
                 configuration parameter must be set to 10";
              }

              leaf notify-interval {
                type uint32 {
                  range "1 .. 3600";
                }
                units "seconds";
                default '20';
                description
                  "Minimum number of seconds between successive
                 notifications for this pool.";
                reference
                  "RFC 7659: Definitions of Managed Objects for
                  	  Network Address Translators (NATs)";

              }
            }  // container notify-pool-usage

            container external-realm {
              description
                "Identifies the external realm of the NAT instance.";
              choice realm-type {
                description
                  "Can be an interface, VRF instance, etc.";
                case interface {
                  description
                    "External interface.";
                  leaf external-interface {
                    type if:interface-ref;
                    description
                      "Name of the external interface.";
                  }
                }  // case interface
              }  // choice realm-type
            }  // container external-realm
          }  // list policy

          container mapping-limits {
            if-feature napt44 or nat64;
            description
              "Information about the configuration parameters that
             limits the mappings based upon various criteria.";
            leaf limit-subscribers {
              type uint32;
              description
                "Maximum number of subscribers that can be serviced
               by a NAT instance.

               A subscriber is identified by a given prefix.";
              reference
                "RFC 7659: Definitions of Managed Objects for
                	  Network Address Translators (NATs)";

            }

            leaf limit-address-mapings {
              type uint32;
              description
                "Maximum number of address mappings that can be
               handled by a NAT instance.

               When this limit is reached, packets that would
               normally trigger translation, will be dropped.";
              reference
                "RFC 7659: Definitions of Managed Objects
                	  for Network Address Translators
                	  (NATs)";

            }

            leaf limit-port-mappings {
              type uint32;
              description
                "Maximum number of port mappings that can be handled
               by a NAT instance.

               When this limit is reached, packets that would
               normally trigger translation, will be dropped.";
              reference
                "RFC 7659: Definitions of Managed Objects for
                	  Network Address Translators (NATs)";

            }

            list limit-per-protocol {
              if-feature napt44 or nat64 or dst-nat;
              key "protocol-id";
              description
                "Configure limits per transport protocol";
              leaf protocol-id {
                type uint8;
                mandatory true;
                description
                  "Upper-layer protocol associated with this mapping.

                 Values are taken from the IANA protocol registry:
                 https://www.iana.org/assignments/protocol-numbers/
                 protocol-numbers.xhtml

                 For example, this field contains 6 (TCP) for a TCP
                 mapping or 17 (UDP) for a UDP mapping.";
              }

              leaf limit {
                type uint32;
                description
                  "Maximum number of protocol-specific NAT mappings
                 per instance.";
              }
            }  // list limit-per-protocol
          }  // container mapping-limits

          container connection-limits {
            if-feature basic-nat44 or napt44 or nat64;
            description
              "Information about the configuration parameters that
             rate limit the translation based upon various criteria.";
            leaf limit-per-subscriber {
              type uint32;
              units "bits/second";
              description
                "Rate-limit the number of new mappings and sessions
               per subscriber.";
            }

            leaf limit-per-instance {
              type uint32;
              units "bits/second";
              mandatory true;
              description
                "Rate-limit the number of new mappings and sessions
               per instance.";
            }

            list limit-per-protocol {
              if-feature napt44 or nat64;
              key "protocol-id";
              description
                "Configure limits per transport protocol";
              leaf protocol-id {
                type uint8;
                mandatory true;
                description
                  "Upper-layer protocol associated with this mapping.

                 Values are taken from the IANA protocol registry:
                 https://www.iana.org/assignments/protocol-numbers/
                 protocol-numbers.xhtml

                 For example, this field contains 6 (TCP) for a TCP
                 mapping or 17 (UDP) for a UDP mapping.";
              }

              leaf limit {
                type uint32;
                description
                  "Rate-limit the number of protocol-specific mappings
                 and sessions per instance.";
              }
            }  // list limit-per-protocol
          }  // container connection-limits

          container notification-limits {
            description
              "Sets notification limits.";
            leaf notify-interval {
              if-feature basic-nat44 or napt44 or nat64;
              type uint32 {
                range "1 .. 3600";
              }
              units "seconds";
              default '10';
              description
                "Minimum number of seconds between successive
                 notifications for this NAT instance.";
              reference
                "RFC 7659: Definitions of Managed Objects
                	  for Network Address Translators (NATs)";

            }

            leaf notify-addresses-usage {
              if-feature basic-nat44 or napt44 or nat64;
              type percent;
              description
                "Notification of address mappings usage over
               the whole NAT instance.

               Notification must be generated when the defined
               threshold is reached.

               For example, if a notification is required when
               the address mappings utilization reaches 90%,
               this configuration parameter must be set
               to 90.";
            }

            leaf notify-ports-usage {
              if-feature napt44 or nat64;
              type percent;
              description
                "Notification of port mappings usage over the
               whole NAT instance.

               Notification must be generated when the defined
               threshold is reached.

               For example, if a notification is required when
               the port mappings utilization reaches 90%, this
               configuration parameter must be set to 90.";
            }

            leaf notify-subscribers-limit {
              if-feature basic-nat44 or napt44 or nat64;
              type uint32;
              description
                "Notification of active subscribers per NAT
               instance.

               Notification must be generated when the defined
               threshold is reached.";
            }
          }  // container notification-limits

          leaf logging-enable {
            if-feature basic-nat44 or napt44 or nat64;
            type boolean;
            description
              "Enable logging features.";
            reference
              "Section 2.3 of RFC 6908 and REQ-12 of RFC6888.";

          }

          container mapping-table {
            if-feature basic-nat44 or napt44 or nat64 or clat or dst-nat;
            description
              "NAT mapping table. Applicable for functions which maintain
             static and/or dynamic mappings, such as NAT44, Destination
             NAT, NAT64, or CLAT.";
            list mapping-entry {
              key "index";
              description
                "NAT mapping entry.";
              uses mapping-entry;
            }  // list mapping-entry
          }  // container mapping-table

          container statistics {
            config false;
            description
              "Statistics related to the NAT instance.";
            leaf discontinuity-time {
              type yang:date-and-time;
              mandatory true;
              description
                "The time on the most recent occasion at which the NAT
               instance suffered a discontinuity.  This must be
               initialized when the NAT instance is configured
               or rebooted.";
            }

            container traffic-statistics {
              description
                "Generic traffic statistics.";
              leaf sent-packets {
                type yang:zero-based-counter64;
                description
                  "Number of packets sent.";
              }

              leaf sent-bytes {
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter for sent traffic in bytes.";
              }

              leaf rcvd-packets {
                type yang:zero-based-counter64;
                description
                  "Number of received packets.";
              }

              leaf rcvd-bytes {
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter for received traffic in bytes.";
              }

              leaf dropped-packets {
                type yang:zero-based-counter64;
                description
                  "Number of dropped packets.";
              }

              leaf dropped-bytes {
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter for dropped traffic in bytes.";
              }

              leaf dropped-fragments {
                if-feature napt44 or nat64;
                type yang:zero-based-counter64;
                description
                  "Number of dropped fragments on the external realm.";
              }

              leaf dropped-address-limit-packets {
                if-feature basic-nat44 or napt44 or nat64;
                type yang:zero-based-counter64;
                description
                  "Number of dropped packets because an address limit
                  is reached.";
              }

              leaf dropped-address-limit-bytes {
                if-feature basic-nat44 or napt44 or nat64;
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter of dropped packets because an address limit
                  is reached, in bytes.";
              }

              leaf dropped-address-packets {
                if-feature basic-nat44 or napt44 or nat64;
                type yang:zero-based-counter64;
                description
                  "Number of dropped packets because no address is
                 available for allocation.";
              }

              leaf dropped-address-bytes {
                if-feature basic-nat44 or napt44 or nat64;
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter of dropped packets because no address is
                 available for allocation, in bytes.";
              }

              leaf dropped-port-limit-packets {
                if-feature napt44 or nat64;
                type yang:zero-based-counter64;
                description
                  "Number of dropped packets because a port limit
                 is reached.";
              }

              leaf dropped-port-limit-bytes {
                if-feature napt44 or nat64;
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter of dropped packets because a port limit
                 is reached, in bytes.";
              }

              leaf dropped-port-packets {
                if-feature napt44 or nat64;
                type yang:zero-based-counter64;
                description
                  "Number of dropped packets because no port is
                 available for allocation.";
              }

              leaf dropped-port-bytes {
                if-feature napt44 or nat64;
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter of dropped packets because no port is
                 available for allocation, in bytes.";
              }

              leaf dropped-subscriber-limit-packets {
                if-feature basic-nat44 or napt44 or nat64;
                type yang:zero-based-counter64;
                description
                  "Number of dropped packets because the subscriber
                 limit per instance is reached.";
              }

              leaf dropped-subscriber-limit-bytes {
                if-feature basic-nat44 or napt44 or nat64;
                type yang:zero-based-counter64;
                units "bytes";
                description
                  "Counter of dropped packets because the subscriber
                  limit per instance is reached, in bytes.";
              }
            }  // container traffic-statistics

            container mappings-statistics {
              description
                "Mappings statistics.";
              leaf total-active-subscribers {
                if-feature basic-nat44 or napt44 or nat64;
                type yang:gauge32;
                description
                  "Total number of active subscribers (that is, subscribers
                 for which the NAT maintains active mappings.

                 A subscriber is identified by a subnet, subscriber-mask,
                 etc.";
              }

              leaf total-address-mappings {
                if-feature basic-nat44 or napt44 or nat64 or clat or dst-nat;
                type yang:gauge32;
                description
                  "Total number of address mappings present at a given
                 time. It includes both static and dynamic mappings.";
                reference
                  "Section 3.3.8 of RFC 7659";

              }

              leaf total-port-mappings {
                if-feature napt44 or nat64;
                type yang:gauge32;
                description
                  "Total number of NAT port mappings present at
                 a given time. It includes both static and dynamic
                 mappings.";
                reference
                  "Section 3.3.9 of RFC 7659";

              }

              list total-per-protocol {
                if-feature napt44 or nat64;
                key "protocol-id";
                description
                  "Total mappings for each enabled/supported protocol.";
                leaf protocol-id {
                  type uint8;
                  mandatory true;
                  description
                    "Upper-layer protocol associated with this mapping.
                   For example, this field contains 6 (TCP) for a TCP
                   mapping or 17 (UDP) for a UDP mapping.";
                }

                leaf total {
                  type yang:gauge32;
                  description
                    "Total number of a protocol-specific mappings present
                   at a given time. The protocol is identified by
                   protocol-id.";
                }
              }  // list total-per-protocol
            }  // container mappings-statistics

            container pools-stats {
              if-feature basic-nat44 or napt44 or nat64;
              description
                "Statistics related to address/prefix pools
               usage";
              leaf addresses-allocated {
                type yang:gauge32;
                description
                  "Number of all allocated addresses.";
              }

              leaf addresses-free {
                type yang:gauge32;
                description
                  "Number of unallocated addresses of all pools at
                 a given time. The sum of unallocated and allocated
                 addresses is the total number of addresses of
                 the pools.";
              }

              container ports-stats {
                if-feature napt44 or nat64;
                description
                  "Statistics related to port numbers usage.";
                leaf ports-allocated {
                  type yang:gauge32;
                  description
                    "Number of allocated ports from all pools.";
                }

                leaf ports-free {
                  type yang:gauge32;
                  description
                    "Number of unallocated addresses from all pools.";
                }
              }  // container ports-stats

              list per-pool-stats {
                if-feature basic-nat44 or napt44 or nat64;
                key "pool-id";
                description
                  "Statistics related to address/prefix pool usage";
                leaf pool-id {
                  type uint32;
                  description
                    "Unique Identifier that represents a pool of
                   addresses/prefixes.";
                }

                leaf discontinuity-time {
                  type yang:date-and-time;
                  mandatory true;
                  description
                    "The time on the most recent occasion at which this
                   pool counters suffered a discontinuity.  This must
                   be initialized when the address pool is
                   configured.";
                }

                container pool-stats {
                  description
                    "Statistics related to address/prefix pool usage";
                  leaf addresses-allocated {
                    type yang:gauge32;
                    description
                      "Number of allocated addresses from this pool.";
                  }

                  leaf addresses-free {
                    type yang:gauge32;
                    description
                      "Number of unallocated addresses in this pool.";
                  }
                }  // container pool-stats

                container port-stats {
                  if-feature napt44 or nat64;
                  description
                    "Statistics related to port numbers usage.";
                  leaf ports-allocated {
                    type yang:gauge32;
                    description
                      "Number of allocated ports from this pool.";
                  }

                  leaf ports-free {
                    type yang:gauge32;
                    description
                      "Number of unallocated addresses from this pool.";
                  }
                }  // container port-stats
              }  // list per-pool-stats
            }  // container pools-stats
          }  // container statistics
        }  // list instance
      }  // container instances
    }  // container nat

    notification nat-pool-event {
      if-feature basic-nat44 or napt44 or nat64;
      description
        "Notifications must be generated when the defined high/low
       threshold is reached. Related configuration parameters
       must be provided to trigger the notifications.";
      leaf id {
        type leafref {
          path "/nat/instances/instance/id";
        }
        mandatory true;
        description
          "NAT instance Identifier.";
      }

      leaf policy-id {
        type leafref {
          path "/nat/instances/instance/policy/id";
        }
        description "Policy Identifier.";
      }

      leaf pool-id {
        type leafref {
          path "/nat/instances/instance/policy/external-ip-address-pool/pool-id";
        }
        mandatory true;
        description "Pool Identifier.";
      }

      leaf notify-pool-threshold {
        type percent;
        mandatory true;
        description
          "A threshold (high-threshold or low-threshold) has
         been fired.";
      }
    }  // notification nat-pool-event

    notification nat-instance-event {
      if-feature basic-nat44 or napt44 or nat64;
      description
        "Notifications must be generated when notify-addresses-usage
       and/or notify-ports-usage threshold are reached.";
      leaf id {
        type leafref {
          path "/nat/instances/instance/id";
        }
        mandatory true;
        description
          "NAT instance Identifier.";
      }

      leaf notify-subscribers-threshold {
        type uint32;
        description
          "The notify-subscribers-limit threshold has been fired.";
      }

      leaf notify-addresses-threshold {
        type percent;
        description
          "The notify-addresses-usage threshold has been fired.";
      }

      leaf notify-ports-threshold {
        type percent;
        description
          "The notify-ports-usage threshold has been fired.";
      }
    }  // notification nat-instance-event
  }  // module ietf-nat