YANG Groupings for TCP Clients and TCP Servers

Internet-Draft	Groupings for TCP Clients and Servers	December 2021
Watsen & Scharf	Expires 17 June 2022	[Page]

Abstract

This document defines three YANG 1.1 modules to support the configuration of TCP clients and TCP servers. The modules include basic parameters of a TCP connection relevant for client or server applications, as well as client configuration required for traversing proxies. The modules can be used either standalone or in conjunction with configuration of other stack protocol layers.¶

1. Introduction

This document defines three YANG 1.1 [RFC7950] modules to support the configuration of TCP clients and TCP servers (TCP is defined in [RFC0793]), either as standalone or in conjunction with configuration of other stack protocol layers.¶

The modules focus on three different types of base TCP parameters that matter for TCP-based applications: First, the modules cover fundamental configuration of a TCP client or TCP server application, such as addresses and port numbers. Second, a reusable grouping enables modification of application-specific parameters for a TCP connections, such as use of TCP keep-alives. And third, client configuration for traversing proxies is included as well. In each case, the modules have a very narrow scope and focus on a minimum set of required parameters.¶

1.1. Relation to other RFCs

This document presents one or more YANG modules [RFC7950] that are part of a collection of RFCs that work together to, ultimately, enable the configuration of the clients and servers of both the NETCONF [RFC6241] and RESTCONF [RFC8040] protocols.¶

The modules have been defined in a modular fashion to enable their use by other efforts, some of which are known to be in progress at the time of this writing, with many more expected to be defined in time.¶

The normative dependency relationship between the various RFCs in the collection is presented in the below diagram. The labels in the diagram represent the primary purpose provided by each RFC. Hyperlinks to each RFC are provided below the diagram.¶

                               crypto-types
                                 ^      ^
                                /        \
                               /          \
                      truststore         keystore
                       ^     ^             ^  ^
                       |     +---------+   |  |
                       |               |   |  |
                       |      +------------+  |
tcp-client-server      |     /         |      |
   ^    ^        ssh-client-server     |      |
   |    |           ^            tls-client-server
   |    |           |              ^     ^        http-client-server
   |    |           |              |     |                 ^
   |    |           |        +-----+     +---------+       |
   |    |           |        |                     |       |
   |    +-----------|--------|--------------+      |       |
   |                |        |              |      |       |
   +-----------+    |        |              |      |       |
               |    |        |              |      |       |
               |    |        |              |      |       |
            netconf-client-server       restconf-client-server

Table 1: Label to RFC Mapping
Label in Diagram	Originating RFC
crypto-types	[I-D.ietf-netconf-crypto-types]
truststore	[I-D.ietf-netconf-trust-anchors]
keystore	[I-D.ietf-netconf-keystore]
tcp-client-server	[I-D.ietf-netconf-tcp-client-server]
ssh-client-server	[I-D.ietf-netconf-ssh-client-server]
tls-client-server	[I-D.ietf-netconf-tls-client-server]
http-client-server	[I-D.ietf-netconf-http-client-server]
netconf-client-server	[I-D.ietf-netconf-netconf-client-server]
restconf-client-server	[I-D.ietf-netconf-restconf-client-server]

1.2. Specification Language

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

1.3. Adherence to the NMDA

This document is compliant with the Network Management Datastore Architecture (NMDA) [RFC8342]. It does not define any protocol accessible nodes that are "config false".¶

1.4. Conventions

Various examples used in this document use a placeholder value for binary data that has been base64 encoded (e.g., "BASE64VALUE="). This placeholder value is used as real base64 encoded structures are often many lines long and hence distracting to the example being presented.¶

2. The "ietf-tcp-common" Module

This section defines a YANG 1.1 module called "ietf-tcp-common". A high-level overview of the module is provided in Section 2.1. Examples illustrating the module's use are provided in Examples (Section 2.2). The YANG module itself is defined in Section 2.3.¶

2.1. Data Model Overview

This section provides an overview of the "ietf-tcp-common" module in terms of its features and groupings.¶

2.1.1. Model Scope

This document defines a common "grouping" statement for basic TCP connection parameters that matter to applications. In some TCP stacks, such parameters can also directly be set by an application using system calls, such as the sockets API. The base YANG model in this document focuses on modeling TCP keep-alives. This base model can be extended as needed.¶

2.1.2. Features

The following diagram lists all the "feature" statements defined in the "ietf-tcp-common" module:¶

Features:
  +-- keepalives-supported

2.1.3. Groupings

The "ietf-tcp-common" module defines the following "grouping" statement:¶

tcp-common-grouping¶

This grouping is presented in the following subsection.¶

2.1.3.1. The "tcp-common-grouping" Grouping

The following tree diagram [RFC8340] illustrates the "tcp-common-grouping" grouping:¶

  grouping tcp-common-grouping
    +-- keepalives! {keepalives-supported}?
       +-- idle-time         uint16
       +-- max-probes        uint16
       +-- probe-interval    uint16

Comments:¶

The "keepalives" node is a "presence" node so that the mandatory descendant nodes do not imply that keepalives must be configured.¶
The "idle-time", "max-probes", and "probe-interval" nodes have the common meanings. Please see the YANG module in Section 2.3 for details.¶

2.1.4. Protocol-accessible Nodes

The "ietf-tcp-common" module defines only "grouping" statements that are used by other modules to instantiate protocol-accessible nodes.¶

2.1.5. Guidelines for Configuring TCP Keep-Alives

Network stacks may include "keep-alives" in their TCP implementations, although this practice is not universally accepted. If keep-alives are included, [RFC1122] mandates that the application MUST be able to turn them on or off for each TCP connection, and that they MUST default to off.¶

Keep-alive mechanisms exist in many protocols. Depending on the protocol stack, TCP keep-alives may only be one out of several alternatives. Which mechanism(s) to use depends on the use case and application requirements. If keep-alives are needed by an application, it is RECOMMENDED that the aliveness check happens only at the protocol layers that are meaningful to the application.¶

A TCP keep-alive mechanism SHOULD only be invoked in server applications that might otherwise hang indefinitely and consume resources unnecessarily if a client crashes or aborts a connection during a network failure [RFC1122]. TCP keep-alives may consume significant resources both in the network and in endpoints (e.g., battery power). In addition, frequent keep-alives risk network congestion. The higher the frequency of keep-alives, the higher the overhead.¶

Given the cost of keep-alives, parameters have to be configured carefully:¶

The default idle interval (leaf "idle-time") MUST default to no less than two hours, i.e., 7200 seconds [RFC1122]. A lower value MAY be configured, but keep-alive messages SHOULD NOT be transmitted more frequently than once every 15 seconds. Longer intervals SHOULD be used when possible.¶
The maximum number of sequential keep-alive probes that can fail (leaf "max-probes") trades off responsiveness and robustness against packet loss. ACK segments that contain no data are not reliably transmitted by TCP. Consequently, if a keep-alive mechanism is implemented it MUST NOT interpret failure to respond to any specific probe as a dead connection [RFC1122]. Typically, a single-digit number should suffice.¶
TCP implementations may include a parameter for the number of seconds between TCP keep-alive probes (leaf "probe-interval"). In order to avoid congestion, the time interval between probes MUST NOT be smaller than one second. Significantly longer intervals SHOULD be used. It is important to note that keep-alive probes (or replies) can get dropped due to network congestion. Sending further probe messages into a congested path after a short interval, without backing off timers, could cause harm and result in a congestion collapse. Therefore it is essential to pick a large, conservative value for this interval.¶

2.2. Example Usage

This section presents an example showing the "tcp-common-grouping" populated with some data.¶

<!-- The outermost element below doesn't exist in the data model. -->
<!--  It simulates if the "grouping" were a "container" instead.  -->

<tcp-common xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-common">
  <keepalives>
    <idle-time>15</idle-time>
    <max-probes>3</max-probes>
    <probe-interval>30</probe-interval>
  </keepalives>
</tcp-common>

2.3. YANG Module

The ietf-tcp-common YANG module references [RFC6991].¶

<CODE BEGINS> file "ietf-tcp-common@2021-12-14.yang"¶

module ietf-tcp-common {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-tcp-common";
  prefix tcpcmn;

  organization
    "IETF NETCONF (Network Configuration) Working Group and the
     IETF TCP Maintenance and Minor Extensions (TCPM) Working Group";

  contact
    "WG Web:   <http://datatracker.ietf.org/wg/netconf/>
               <http://datatracker.ietf.org/wg/tcpm/>
     WG List:  <mailto:netconf@ietf.org>
               <mailto:tcpm@ietf.org>
     Authors:  Kent Watsen <mailto:kent+ietf@watsen.net>
               Michael Scharf
               <mailto:michael.scharf@hs-esslingen.de>";

  description
    "This module defines reusable groupings for TCP commons that
     can be used as a basis for specific TCP common instances.

     Copyright (c) 2021 IETF Trust and the persons identified
     as authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with
     or without modification, is permitted pursuant to, and
     subject to the license terms contained in, the Simplified
     BSD License set forth in Section 4.c of the IETF Trust's
     Legal Provisions Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC DDDD
     (https://www.rfc-editor.org/info/rfcDDDD); see the RFC
     itself for full legal notices.

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
     'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
     'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
     are to be interpreted as described in BCP 14 (RFC 2119)
     (RFC 8174) when, and only when, they appear in all
     capitals, as shown here.";

  revision 2021-12-14 {
    description
      "Initial version";
    reference
      "RFC DDDD: YANG Groupings for TCP Clients and TCP Servers";
  }

  // Features

  feature keepalives-supported {
    description
      "Indicates that keepalives are supported.";
  }

  // Groupings

  grouping tcp-common-grouping {
    description
      "A reusable grouping for configuring TCP parameters common
       to TCP connections as well as the operating system as a
       whole.";
    container keepalives {
      if-feature "keepalives-supported";
      presence
        "Indicates that keepalives are enabled.  This statement is
         present so the mandatory descendant nodes do not imply that
         this node must be configured.";
      description
        "Configures the keep-alive policy, to proactively test the
         aliveness of the TCP peer.  An unresponsive TCP peer is
         dropped after approximately (idle-time + max-probes
         * probe-interval) seconds.";
      leaf idle-time {
        type uint16 {
          range "1..max";
        }
        units "seconds";
        mandatory true;
        description
          "Sets the amount of time after which if no data has been
           received from the TCP peer, a TCP-level probe message
           will be sent to test the aliveness of the TCP peer.
           Two hours (7200 seconds) is safe value, per RFC 1122.";
        reference
          "RFC 1122:
            Requirements for Internet Hosts -- Communication Layers";
      }
      leaf max-probes {
        type uint16 {
          range "1..max";
        }
        mandatory true;
        description
          "Sets the maximum number of sequential keep-alive probes
           that can fail to obtain a response from the TCP peer
           before assuming the TCP peer is no longer alive.";
      }
      leaf probe-interval {
        type uint16 {
          range "1..max";
        }
        units "seconds";
        mandatory true;
        description
          "Sets the time interval between failed probes. The interval
           SHOULD be significantly longer than one second in order to
           avoid harm on a congested link.";
      }
    } // container keepalives
  } // grouping tcp-common-grouping

}

<CODE ENDS>¶

3. The "ietf-tcp-client" Module

This section defines a YANG 1.1 module called "ietf-tcp-client". A high-level overview of the module is provided in Section 3.1. Examples illustrating the module's use are provided in Examples (Section 3.2). The YANG module itself is defined in Section 3.3.¶

3.1. Data Model Overview

This section provides an overview of the "ietf-tcp-client" module in terms of its features and groupings.¶

3.1.1. Features

The following diagram lists all the "feature" statements defined in the "ietf-tcp-client" module:¶

Features:
  +-- local-binding-supported
  +-- tcp-client-keepalives
  +-- proxy-connect
  +-- socks5-gss-api
  +-- socks5-username-password

3.1.2. Groupings

The "ietf-tcp-client" module defines the following "grouping" statement:¶

tcp-client-grouping¶

This grouping is presented in the following subsection.¶

3.1.2.1. The "tcp-client-grouping" Grouping

The following tree diagram [RFC8340] illustrates the "tcp-client-grouping" grouping:¶

  grouping tcp-client-grouping
    +-- remote-address                inet:host
    +-- remote-port?                  inet:port-number
    +-- local-address?                inet:ip-address
    |       {local-binding-supported}?
    +-- local-port?                   inet:port-number
    |       {local-binding-supported}?
    +-- proxy-server! {proxy-connect}?
    |  +-- (proxy-type)
    |     +--:(socks4)
    |     |  +-- socks4-parameters
    |     |     +-- remote-address    inet:ip-address
    |     |     +-- remote-port?      inet:port-number
    |     +--:(socks4a)
    |     |  +-- socks4a-parameters
    |     |     +-- remote-address    inet:host
    |     |     +-- remote-port?      inet:port-number
    |     +--:(socks5)
    |        +-- socks5-parameters
    |           +-- remote-address               inet:host
    |           +-- remote-port?                 inet:port-number
    |           +-- authentication-parameters!
    |              +-- (auth-type)
    |                 +--:(gss-api) {socks5-gss-api}?
    |                 |  +-- gss-api
    |                 +--:(username-password)
    |                          {socks5-username-password}?
    |                    +-- username-password
    |                       +-- username                string
    |                       +---u ct:password-grouping
    +---u tcpcmn:tcp-common-grouping

Comments:¶

The "remote-address" node, which is mandatory, may be configured as an IPv4 address, an IPv6 address, a hostname.¶
The "remote-port" node is not mandatory, but its default value is the invalid value '0', thus forcing the consuming data model to refine it in order to provide it an appropriate default value.¶
The "local-address" node, which is enabled by the "local-binding-supported" feature (Section 2.1.2), may be configured as an IPv4 address, an IPv6 address, or a wildcard value.¶
The "local-port" node, which is enabled by the "local-binding-supported" feature (Section 2.1.2), is not mandatory. Its default value is '0', indicating that the operating system can pick an arbitrary port number.¶
The "proxy-server" node is enabled by a "feature" statement and, for servers that enable it, is a "presence" container so that the descendant "mandatory true" choice node does not imply that the proxy-server node must be configured.¶
This grouping uses the "tcp-common-grouping" grouping discussed in Section 2.1.3.1.¶

3.1.3. Protocol-accessible Nodes

The "ietf-tcp-client" module defines only "grouping" statements that are used by other modules to instantiate protocol-accessible nodes.¶

3.2. Example Usage

This section presents two examples showing the "tcp-client-grouping" populated with some data. This example shows a TCP-client configured to not connect via a proxy:¶

<!-- The outermost element below doesn't exist in the data model. -->
<!--  It simulates if the "grouping" were a "container" instead.  -->

<tcp-client xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-client">
  <remote-address>www.example.com</remote-address>
  <remote-port>443</remote-port>
  <local-address>0.0.0.0</local-address>
  <local-port>0</local-port>
  <keepalives>
    <idle-time>15</idle-time>
    <max-probes>3</max-probes>
    <probe-interval>30</probe-interval>
  </keepalives>
</tcp-client>

This example shows a TCP-client configured to connect via a proxy:¶

<!-- The outermost element below doesn't exist in the data model. -->
<!--  It simulates if the "grouping" were a "container" instead.  -->

<tcp-client xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-client">
  <remote-address>www.example.com</remote-address>
  <remote-port>443</remote-port>
  <local-address>0.0.0.0</local-address>
  <local-port>0</local-port>
  <proxy-server>
    <socks5-parameters>
      <remote-address>proxy.my-domain.com</remote-address>
      <remote-port>1080</remote-port>
      <authentication-parameters>
        <username-password>
          <username>foobar</username>
          <cleartext-password>secret</cleartext-password>
        </username-password>
      </authentication-parameters>
    </socks5-parameters>
  </proxy-server>
  <keepalives>
    <idle-time>15</idle-time>
    <max-probes>3</max-probes>
    <probe-interval>30</probe-interval>
  </keepalives>
</tcp-client>

3.3. YANG Module

The ietf-tcp-client YANG module references [RFC6991].¶

<CODE BEGINS> file "ietf-tcp-client@2021-12-14.yang"¶

module ietf-tcp-client {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-tcp-client";
  prefix tcpc;

  import ietf-inet-types {
    prefix inet;
    reference
      "RFC 6991: Common YANG Data Types";
  }

  import ietf-crypto-types {
    prefix ct;
    reference
      "RFC AAAA: YANG Data Types and Groupings for Cryptography";
  }

  import ietf-tcp-common {
    prefix tcpcmn;
    reference
      "RFC DDDD: YANG Groupings for TCP Clients and TCP Servers";
  }

  organization
    "IETF NETCONF (Network Configuration) Working Group and the
     IETF TCP Maintenance and Minor Extensions (TCPM) Working Group";

  contact
    "WG Web:   <http://datatracker.ietf.org/wg/netconf/>
               <http://datatracker.ietf.org/wg/tcpm/>
     WG List:  <mailto:netconf@ietf.org>
               <mailto:tcpm@ietf.org>
     Authors:  Kent Watsen <mailto:kent+ietf@watsen.net>
               Michael Scharf
               <mailto:michael.scharf@hs-esslingen.de>";

  description
    "This module defines reusable groupings for TCP clients that
     can be used as a basis for specific TCP client instances.

     Copyright (c) 2021 IETF Trust and the persons identified
     as authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with
     or without modification, is permitted pursuant to, and
     subject to the license terms contained in, the Simplified
     BSD License set forth in Section 4.c of the IETF Trust's
     Legal Provisions Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC DDDD
     (https://www.rfc-editor.org/info/rfcDDDD); see the RFC
     itself for full legal notices.

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
     'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
     'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
     are to be interpreted as described in BCP 14 (RFC 2119)
     (RFC 8174) when, and only when, they appear in all
     capitals, as shown here.";

  revision 2021-12-14 {
    description
      "Initial version";
    reference
      "RFC DDDD: YANG Groupings for TCP Clients and TCP Servers";
  }

  // Features

  feature local-binding-supported {
    description
      "Indicates that the server supports configuring local
       bindings (i.e., the local address and local port) for
       TCP clients.";
  }

  feature tcp-client-keepalives {
    description
      "Per socket TCP keepalive parameters are configurable for
       TCP clients on the server implementing this feature.";
  }

  feature proxy-connect {
    description
      "Proxy connection configuration is configurable for
       TCP clients on the server implementing this feature.";
  }

  feature socks5-gss-api {
    description
      "Indicates that the server supports authenticating
       using GSSAPI when initiating TCP connections via
       and SOCKS Version 5 proxy server.";
    reference
      "RFC 1928: SOCKS Protocol Version 5";
  }

  feature socks5-username-password {
    description
      "Indicates that the server supports authenticating using
       username/password when initiating TCP connections via
       and SOCKS Version 5 proxy server.";
    reference
      "RFC 1928: SOCKS Protocol Version 5";
  }

  // Groupings

  grouping tcp-client-grouping {
    description
      "A reusable grouping for configuring a TCP client.

      Note that this grouping uses fairly typical descendant
       node names such that a stack of 'uses' statements will
       have name conflicts.  It is intended that the consuming
       data model will resolve the issue (e.g., by wrapping
       the 'uses' statement in a container called
       'tcp-client-parameters').  This model purposely does
       not do this itself so as to provide maximum flexibility
       to consuming models.";

    leaf remote-address {
      type inet:host;
      mandatory true;
      description
        "The IP address or hostname of the remote peer to
         establish a connection with.  If a domain name is
         configured, then the DNS resolution should happen on
         each connection attempt.  If the DNS resolution
         results in multiple IP addresses, the IP addresses
         are tried according to local preference order until
         a connection has been established or until all IP
         addresses have failed.";
    }
    leaf remote-port {
      type inet:port-number;
      default "0";
      description
        "The IP port number for the remote peer to establish a
         connection with.  An invalid default value (0) is used
         (instead of 'mandatory true') so that as application
         level data model may 'refine' it with an application
         specific default port number value.";
    }
    leaf local-address {
      if-feature "local-binding-supported";
      type inet:ip-address;
      description
        "The local IP address/interface (VRF?) to bind to for when
         connecting to the remote peer.  INADDR_ANY ('0.0.0.0') or
         INADDR6_ANY ('0:0:0:0:0:0:0:0' a.k.a. '::') MAY be used to
         explicitly indicate the implicit default, that the server
         can bind to any IPv4 or IPv6 addresses, respectively.";
    }
    leaf local-port {
      if-feature "local-binding-supported";
      type inet:port-number;
      default "0";
      description
        "The local IP port number to bind to for when connecting
         to the remote peer.  The port number '0', which is the
         default value, indicates that any available local port
         number may be used.";
    }
    container proxy-server {
      if-feature "proxy-connect";
      presence
        "Indicates that a proxy connection has been configured.
         Present so that the mandatory descendant nodes do not
         imply that this node must be configured.";
      choice proxy-type {
        mandatory true;
        description
          "Selects a proxy connection protocol.";
        case socks4 {
          container socks4-parameters {
            leaf remote-address {
              type inet:ip-address;
              mandatory true;
              description
                "The IP address of the proxy server.";
            }
            leaf remote-port {
              type inet:port-number;
              default "1080";
              description
                "The IP port number for the proxy server.";
            }
            description
              "Parameters for connecting to a TCP-based proxy
               server using the SOCKS4 protocol.";
            reference
              "SOCKS, Proceedings: 1992 Usenix Security Symposium.";
          }
        }
        case socks4a {
          container socks4a-parameters {
            leaf remote-address {
              type inet:host;
              mandatory true;
              description
                "The IP address or hostname of the proxy server.";
            }
            leaf remote-port {
              type inet:port-number;
              default "1080";
              description
                "The IP port number for the proxy server.";
            }
            description
              "Parameters for connecting to a TCP-based proxy
               server using the SOCKS4a protocol.";
            reference
              "SOCKS Proceedings:
                 1992 Usenix Security Symposium.
               OpenSSH message:
                 SOCKS 4A: A Simple Extension to SOCKS 4 Protocol
                 https://www.openssh.com/txt/socks4a.protocol";
          }
        }
        case socks5 {
          container socks5-parameters {
            leaf remote-address {
              type inet:host;
              mandatory true;
              description
                "The IP address or hostname of the proxy server.";
            }
            leaf remote-port {
              type inet:port-number;
              default "1080";
              description
                "The IP port number for the proxy server.";
            }
            container authentication-parameters {
              presence
                "Indicates that an authentication mechanism
                 has been configured.  Present so that the
                 mandatory descendant nodes do not imply that
                 this node must be configured.";
              description
                "A container for SOCKS Version 5 authentication
                 mechanisms.

                 A complete list of methods is defined at:
                 https://www.iana.org/assignments/socks-methods
                 /socks-methods.xhtml.";
              reference
                "RFC 1928: SOCKS Protocol Version 5";
              choice auth-type {
                mandatory true;
                description
                  "A choice amongst supported SOCKS Version 5
                   authentication mechanisms.";
                case gss-api {
                  if-feature "socks5-gss-api";
                  container gss-api {
                    description
                      "Contains GSS-API configuration.  Defines
                       as an empty container to enable specific
                       GSS-API configuration to be augmented in
                       by future modules.";
                    reference
                      "RFC 1928: SOCKS Protocol Version 5
                       RFC 2743: Generic Security Service
                                 Application Program Interface
                                 Version 2, Update 1";
                  }
                }
                case username-password {
                  if-feature "socks5-username-password";
                  container username-password {
                    leaf username {
                      type string;
                      mandatory true;
                      description
                        "The 'username' value to use for client
                         identification.";
                    }
                    uses ct:password-grouping {
                      description
                        "The password to be used for client
                         authentication.";
                    }
                    description
                      "Contains Username/Password configuration.";
                    reference
                      "RFC 1929: Username/Password Authentication
                                 for SOCKS V5";
                  }
                }
              }
            }
            description
              "Parameters for connecting to a TCP-based proxy server
               using the SOCKS5 protocol.";
            reference
              "RFC 1928: SOCKS Protocol Version 5";
          }
        }
      }
      description
        "Proxy server settings.";
    }

    uses tcpcmn:tcp-common-grouping {
      augment "keepalives" {
        if-feature "tcp-client-keepalives";
        description
          "Add an if-feature statement so that implementations
           can choose to support TCP client keepalives.";
      }
    }
  }
}

<CODE ENDS>¶

4. The "ietf-tcp-server" Module

This section defines a YANG 1.1 module called "ietf-tcp-server". A high-level overview of the module is provided in Section 4.1. Examples illustrating the module's use are provided in Examples (Section 4.2). The YANG module itself is defined in Section 4.3.¶

4.1. Data Model Overview

This section provides an overview of the "ietf-tcp-server" module in terms of its features and groupings.¶

4.1.1. Features

The following diagram lists all the "feature" statements defined in the "ietf-tcp-server" module:¶

Features:
  +-- tcp-server-keepalives

4.1.2. Groupings

The "ietf-tcp-server" module defines the following "grouping" statement:¶

tcp-server-grouping¶

This grouping is presented in the following subsection.¶

4.1.2.1. The "tcp-server-grouping" Grouping

The following tree diagram [RFC8340] illustrates the "tcp-server-grouping" grouping:¶

  grouping tcp-server-grouping
    +-- local-address                 inet:ip-address
    +-- local-port?                   inet:port-number
    +---u tcpcmn:tcp-common-grouping

Comments:¶

The "local-address" node, which is mandatory, may be configured as an IPv4 address, an IPv6 address, or a wildcard value.¶
The "local-port" node is not mandatory, but its default value is the invalid value '0', thus forcing the consuming data model to refine it in order to provide it an appropriate default value.¶
This grouping uses the "tcp-common-grouping" grouping discussed in Section 2.1.3.1.¶

4.1.3. Protocol-accessible Nodes

The "ietf-tcp-server" module defines only "grouping" statements that are used by other modules to instantiate protocol-accessible nodes.¶

4.2. Example Usage

This section presents an example showing the "tcp-server-grouping" populated with some data.¶

<!-- The outermost element below doesn't exist in the data model. -->
<!--  It simulates if the "grouping" were a "container" instead.  -->

<tcp-server xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-server">
  <local-address>10.20.30.40</local-address>
  <local-port>7777</local-port>
  <keepalives>
    <idle-time>15</idle-time>
    <max-probes>3</max-probes>
    <probe-interval>30</probe-interval>
  </keepalives>
</tcp-server>

4.3. YANG Module

The ietf-tcp-server YANG module references [RFC6991].¶

<CODE BEGINS> file "ietf-tcp-server@2021-12-14.yang"¶

module ietf-tcp-server {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-tcp-server";
  prefix tcps;

  import ietf-inet-types {
    prefix inet;
    reference
      "RFC 6991: Common YANG Data Types";
  }

  import ietf-tcp-common {
    prefix tcpcmn;
    reference
      "RFC DDDD: YANG Groupings for TCP Clients and TCP Servers";
  }

  organization
    "IETF NETCONF (Network Configuration) Working Group and the
     IETF TCP Maintenance and Minor Extensions (TCPM) Working Group";

  contact
    "WG Web:   <http://datatracker.ietf.org/wg/netconf/>
               <http://datatracker.ietf.org/wg/tcpm/>
     WG List:  <mailto:netconf@ietf.org>
               <mailto:tcpm@ietf.org>
     Authors:  Kent Watsen <mailto:kent+ietf@watsen.net>
               Michael Scharf
               <mailto:michael.scharf@hs-esslingen.de>";

  description
    "This module defines reusable groupings for TCP servers that
     can be used as a basis for specific TCP server instances.

     Copyright (c) 2021 IETF Trust and the persons identified
     as authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with
     or without modification, is permitted pursuant to, and
     subject to the license terms contained in, the Simplified
     BSD License set forth in Section 4.c of the IETF Trust's
     Legal Provisions Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC DDDD
     (https://www.rfc-editor.org/info/rfcDDDD); see the RFC
     itself for full legal notices.

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
     'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
     'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
     are to be interpreted as described in BCP 14 (RFC 2119)
     (RFC 8174) when, and only when, they appear in all
     capitals, as shown here.";

  revision 2021-12-14 {
    description
      "Initial version";
    reference
      "RFC DDDD: YANG Groupings for TCP Clients and TCP Servers";
  }

  // Features

  feature tcp-server-keepalives {
    description
      "Per socket TCP keepalive parameters are configurable for
       TCP servers on the server implementing this feature.";
  }

  // Groupings

  grouping tcp-server-grouping {
    description
      "A reusable grouping for configuring a TCP server.

       Note that this grouping uses fairly typical descendant
       node names such that a stack of 'uses' statements will
       have name conflicts.  It is intended that the consuming
       data model will resolve the issue (e.g., by wrapping
       the 'uses' statement in a container called
       'tcp-server-parameters').  This model purposely does
       not do this itself so as to provide maximum flexibility
       to consuming models.";
    leaf local-address {
      type inet:ip-address;
      mandatory true;
      description
        "The local IP address to listen on for incoming
         TCP client connections.  INADDR_ANY (0.0.0.0) or
         INADDR6_ANY (0:0:0:0:0:0:0:0 a.k.a. ::) MUST be
         used when the server is to listen on all IPv4 or
         IPv6 addresses, respectively.";
    }
    leaf local-port {
      type inet:port-number;
      default "0";
      description
        "The local port number to listen on for incoming TCP
         client connections.  An invalid default value (0)
         is used (instead of 'mandatory true') so that an
         application level data model may 'refine' it with
         an application specific default port number value.";
    }
    uses tcpcmn:tcp-common-grouping {
      augment "keepalives" {
        if-feature "tcp-server-keepalives";
        description
          "Add an if-feature statement so that implementations
           can choose to support TCP server keepalives.";
      }
    }
  }
}

<CODE ENDS>¶

5. Security Considerations

5.1. The "ietf-tcp-common" YANG Module

The "ietf-tcp-common" YANG module defines "grouping" statements that are designed to be accessed via YANG based management protocols, such as NETCONF [RFC6241] and RESTCONF [RFC8040]. Both of these protocols have mandatory-to-implement secure transport layers (e.g., SSH, TLS) with mutual authentication.¶

The NETCONF access control model (NACM) [RFC8341] provides the means to restrict access for particular users to a pre-configured subset of all available protocol operations and content.¶

Since the module in this document only define groupings, these considerations are primarily for the designers of other modules that use these groupings.¶

None of the readable data nodes defined in this YANG module are considered sensitive or vulnerable in network environments. The NACM "default-deny-all" extension has not been set for any data nodes defined in this module.¶

None of the writable data nodes defined in this YANG module are considered sensitive or vulnerable in network environments. The NACM "default-deny-write" extension has not been set for any data nodes defined in this module.¶

This module does not define any RPCs, actions, or notifications, and thus the security consideration for such is not provided here.¶

5.2. The "ietf-tcp-client" YANG Module

The "ietf-tcp-client" YANG module defines "grouping" statements that are designed to be accessed via YANG based management protocols, such as NETCONF [RFC6241] and RESTCONF [RFC8040]. Both of these protocols have mandatory-to-implement secure transport layers (e.g., SSH, TLS) with mutual authentication.¶

The NETCONF access control model (NACM) [RFC8341] provides the means to restrict access for particular users to a pre-configured subset of all available protocol operations and content.¶

Since the module in this document only define groupings, these considerations are primarily for the designers of other modules that use these groupings.¶

One readable data node defined in this YANG module may be considered sensitive or vulnerable in some network environments. This node is as follows:¶

The "proxy-server/socks5-parameters/authentication-parameters/username-password/password" node:¶
- The cleartext "password" node defined in the "tcp-client-grouping" grouping is additionally sensitive to read operations such that, in normal use cases, it should never be returned to a client. For this reason, the NACM extension "default-deny-all" has been applied to it.¶

This module does not define any RPCs, actions, or notifications, and thus the security consideration for such is not provided here.¶

Implementations are RECOMMENDED to implement the "local-binding-supported" feature for cryptographically-secure protocols, so as to enable more granular ingress/egress firewall rulebases. It is NOT RECOMMENDED to implement this feature for unsecure protocols, as per [RFC6056].¶

5.3. The "ietf-tcp-server" YANG Module

The "ietf-tcp-server" YANG module defines "grouping" statements that are designed to be accessed via YANG based management protocols, such as NETCONF [RFC6241] and RESTCONF [RFC8040]. Both of these protocols have mandatory-to-implement secure transport layers (e.g., SSH, TLS) with mutual authentication.¶

The NETCONF access control model (NACM) [RFC8341] provides the means to restrict access for particular users to a pre-configured subset of all available protocol operations and content.¶

Since the module in this document only define groupings, these considerations are primarily for the designers of other modules that use these groupings.¶

This module does not define any RPCs, actions, or notifications, and thus the security consideration for such is not provided here.¶

7. References

7.1. Normative References

[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>.
[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>.
[RFC6991]: Schoenwaelder, J., Ed., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, July 2013, <https://www.rfc-editor.org/info/rfc6991>.
[RFC7950]: Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016, <https://www.rfc-editor.org/info/rfc7950>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8341]: Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, March 2018, <https://www.rfc-editor.org/info/rfc8341>.

7.2. Informative References

[I-D.ietf-netconf-crypto-types]: Watsen, K., "YANG Data Types and Groupings for Cryptography", Work in Progress, Internet-Draft, draft-ietf-netconf-crypto-types-21, 14 September 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-crypto-types-21>.
[I-D.ietf-netconf-http-client-server]: Watsen, K., "YANG Groupings for HTTP Clients and HTTP Servers", Work in Progress, Internet-Draft, draft-ietf-netconf-http-client-server-07, 18 May 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-http-client-server-07>.
[I-D.ietf-netconf-keystore]: Watsen, K., "A YANG Data Model for a Keystore", Work in Progress, Internet-Draft, draft-ietf-netconf-keystore-22, 18 May 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-keystore-22>.
[I-D.ietf-netconf-netconf-client-server]: Watsen, K., "NETCONF Client and Server Models", Work in Progress, Internet-Draft, draft-ietf-netconf-netconf-client-server-23, 18 May 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-netconf-client-server-23>.
[I-D.ietf-netconf-restconf-client-server]: Watsen, K., "RESTCONF Client and Server Models", Work in Progress, Internet-Draft, draft-ietf-netconf-restconf-client-server-23, 18 May 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-restconf-client-server-23>.
[I-D.ietf-netconf-ssh-client-server]: Watsen, K., "YANG Groupings for SSH Clients and SSH Servers", Work in Progress, Internet-Draft, draft-ietf-netconf-ssh-client-server-25, 18 June 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-ssh-client-server-25>.
[I-D.ietf-netconf-tcp-client-server]: Watsen, K. and M. Scharf, "YANG Groupings for TCP Clients and TCP Servers", Work in Progress, Internet-Draft, draft-ietf-netconf-tcp-client-server-10, 18 May 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-tcp-client-server-10>.
[I-D.ietf-netconf-tls-client-server]: Watsen, K., "YANG Groupings for TLS Clients and TLS Servers", Work in Progress, Internet-Draft, draft-ietf-netconf-tls-client-server-25, 18 June 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-tls-client-server-25>.
[I-D.ietf-netconf-trust-anchors]: Watsen, K., "A YANG Data Model for a Truststore", Work in Progress, Internet-Draft, draft-ietf-netconf-trust-anchors-15, 18 May 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-trust-anchors-15>.
[RFC0793]: Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <https://www.rfc-editor.org/info/rfc793>.
[RFC1122]: Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, <https://www.rfc-editor.org/info/rfc1122>.
[RFC3688]: Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004, <https://www.rfc-editor.org/info/rfc3688>.
[RFC6056]: Larsen, M. and F. Gont, "Recommendations for Transport-Protocol Port Randomization", BCP 156, RFC 6056, DOI 10.17487/RFC6056, January 2011, <https://www.rfc-editor.org/info/rfc6056>.
[RFC6241]: Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011, <https://www.rfc-editor.org/info/rfc6241>.
[RFC8040]: Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017, <https://www.rfc-editor.org/info/rfc8040>.
[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>.
[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>.