NETCONF Working Group

Internet Engineering Task Force (IETF)                         K. Watsen
Internet-Draft
Request for Comments: 9643                               Watsen Networks
Intended status:
Category: Standards Track                                      M. Scharf
Expires: 7 October 2024
ISSN: 2070-1721                                     Hochschule Esslingen
                                                            5 April
                                                            October 2024

             YANG Groupings for TCP Clients and TCP Servers
                draft-ietf-netconf-tcp-client-server-26

Abstract

   This document presents 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 data models defined by these modules can may be used either standalone
   directly (e.g., to define a specific TCP-client or TCP-server) or in
   conjunction with configuration of other stack protocol layers.

Editorial Note (To be removed by RFC Editor)

   This draft contains placeholder values that need to be replaced with
   finalized values at the time of publication.  This note summarizes
   all of the substitutions that are needed.  No other RFC Editor
   instructions are specified elsewhere in this document.

   Artwork in this document contains shorthand references to drafts in
   progress.  Please apply the following replacements:

   *  AAAA --> the assigned RFC value for draft-ietf-netconf-crypto-
      types

   *  DDDD --> the assigned RFC value for this draft

   Artwork in this document contains placeholder values configuration defined for the date of
   publication of this draft.  Please apply the following replacement:

   *  2024-04-04 --> the publication date higher level protocols
   that depend on TCP (e.g., SSH, TLS, etc.).  Examples of this draft

   The "Relation higher level
   protocol configuration designed to other RFCs" section Section 1.1 contains the text
   "one or more YANG modules" and, later, "modules".  This text is
   sourced from a file in a context where it is unknown how many modules
   a draft defines.  The text is not wrong as is, but it may be improved
   by stating more directly how many modules are defined.

   The "Relation to other RFCs" section Section 1.1 contains a self-
   reference to this draft, along with a corresponding reference in the
   Appendix.  Please replace the self-reference used in this section conjunction with
   "This RFC" (or similar) and remove the self-reference in the
   "Normative/Informative References" section, whichever it is in.

   Tree-diagrams in this draft may use the '\' line-folding mode defined
   configuration are in RFC 8792.  However, nicer-to-the-eye is when the '\\' line-folding
   mode is used.  The AD suggested suggested putting a request here for
   the RFC Editor to help convert "ugly" '\' folded examples to use the
   '\\' folding mode.  "Help convert" may be interpreted as, identify
   what looks ugly RFCs 9644 and ask the authors to make the adjustment.

   The following Appendix section is to be removed prior to publication:

   *  Appendix A.  Change Log 9645.

Status of This Memo

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

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

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

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

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

   This Internet-Draft will expire on 7 October 2024.
   https://www.rfc-editor.org/info/rfc9643.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info)
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Relation to other Other RFCs  . . . . . . . . . . . . . . . . .   4
     1.2.  Specification Language  . . . . . . . . . . . . . . . . .   6
     1.3.  Adherence to the NMDA . . . . . . . . . . . . . . . . . .   6
     1.4.  Conventions
   2.  The "ietf-tcp-common" Module  . . . . . . . . . . . . . . . .   6
     2.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .   7
     2.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .   9
     2.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   9
   3.  The "ietf-tcp-client" Module  . . . . . . . . . . . . . . . .  12
     3.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  12
     3.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  15
     3.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  The "ietf-tcp-server" Module  . . . . . . . . . . . . . . . .  24
     4.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  24
     4.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  25
     4.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  25
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     5.1.  Considerations for the "ietf-tcp-common" YANG Module  . .  28
     5.2.  Considerations for the "ietf-tcp-client" YANG Module  . .  29
     5.3.  Considerations for the "ietf-tcp-server" YANG Module  . .  30
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
     6.1.  The "IETF XML" IETF XML Registry . . . . . . . . . . . . . . . . .  31
     6.2.  The "YANG YANG Module Names" Names Registry  . . . . . . . . . . . .  32
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  32
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  33
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  35
     A.1.  00 to 01  . . . . . . . . . . . . . . . . . . . . . . . .  35
     A.2.  01 to 02  . . . . . . . . . . . . . . . . . . . . . . . .  35
     A.3.  02 to 03  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.4.  03 to 04  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.5.  04 to 05  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.6.  05 to 06  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.7.  06 to 07  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.8.  07 to 08  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.9.  08 to 09  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.10. 09 to 10  . . . . . . . . . . . . . . . . . . . . . . . .  37
     A.11. 10 to 11  . . . . . . . . . . . . . . . . . . . . . . . .  37
     A.12. 11 to 12  . . . . . . . . . . . . . . . . . . . . . . . .  37
     A.13. 12 to 13  . . . . . . . . . . . . . . . . . . . . . . . .  37
     A.14. 13 to 14  . . . . . . . . . . . . . . . . . . . . . . . .  37
     A.15. 14 to 15  . . . . . . . . . . . . . . . . . . . . . . . .  37
     A.16. 15 to 16  . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.17. 16 to 17  . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.18. 18 to 19  . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.19. 18 to 19  . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.20. 19 to 20  . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.21. 20 to 22  . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.22. 22 to 23  . . . . . . . . . . . . . . . . . . . . . . . .  39
     A.23. 23 to 24  . . . . . . . . . . . . . . . . . . . . . . . .  39
     A.24. 24 to 25  . . . . . . . . . . . . . . . . . . . . . . . .  39
     A.25. 25 to 26  . . . . . . . . . . . . . . . . . . . . . . . .  39
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

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
   [RFC9293]), either as standalone
   [RFC9293]).  The data models defined by these modules may be used
   directly (e.g., to define a specific TCP-client or TCP-server) or in
   conjunction with the configuration defined for higher level protocols
   that depend on TCP (e.g., SSH, TLS, etc.).  Examples of other stack higher level
   protocol layers. configuration designed to be used in conjunction with this
   configuration are in [RFC9644] and [RFC9645].

   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. keepalives.  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.

   Please be advised that while this document presents support for some
   TCP proxy techniques, there are other TCP proxy techniques that are
   not part of this document, document but could be added by augmenting the YANG
   module.

1.1.  Relation to other Other RFCs

   This document presents one or more three YANG modules [RFC7950] that are part of
   a collection of RFCs that work together to, ultimately, to ultimately support the
   configuration of both the clients and servers of both the
   NETCONF Network
   Configuration Protocol (NETCONF) [RFC6241] and RESTCONF [RFC8040] protocols. [RFC8040].

   The dependency relationship between the primary YANG groupings
   defined in the various RFCs is presented in the below diagram.  In
   some cases, a draft document may define secondary groupings that introduce
   dependencies not illustrated in the diagram.  The labels in the
   diagram are a shorthand name names for the defining RFC. RFCs.  The citation
   reference
   references for shorthand name is names are provided below the diagram.

   Please note that the arrows in the diagram point from referencer to
   referenced.  For example, the "crypto-types" RFC does not have any
   dependencies, whilst the "keystore" RFC depends on the "crypto-types"
   RFC.

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

   +========================+==========================+
   | Label in Diagram       | Originating RFC Reference                |
   +======================+===========================================+
   |crypto-types
   +========================+==========================+
   | [I-D.ietf-netconf-crypto-types] crypto-types           |
   +----------------------+-------------------------------------------+
   |truststore [RFC9640]                | [I-D.ietf-netconf-trust-anchors]
   +------------------------+--------------------------+
   |
   +----------------------+-------------------------------------------+
   |keystore truststore             | [I-D.ietf-netconf-keystore] [RFC9641]                |
   +----------------------+-------------------------------------------+
   |tcp-client-server
   +------------------------+--------------------------+
   | [I-D.ietf-netconf-tcp-client-server] keystore               |
   +----------------------+-------------------------------------------+
   |ssh-client-server [RFC9642]                | [I-D.ietf-netconf-ssh-client-server]
   +------------------------+--------------------------+
   |
   +----------------------+-------------------------------------------+
   |tls-client-server tcp-client-server      | [I-D.ietf-netconf-tls-client-server] RFC 9643                 |
   +----------------------+-------------------------------------------+
   |http-client-server
   +------------------------+--------------------------+
   | [I-D.ietf-netconf-http-client-server] ssh-client-server      | [RFC9644]                |
   +----------------------+-------------------------------------------+
   |netconf-client-server
   +------------------------+--------------------------+
   | [I-D.ietf-netconf-netconf-client-server] tls-client-server      |
   +----------------------+-------------------------------------------+
   |restconf-client-server| [I-D.ietf-netconf-restconf-client-server] [RFC9645]                |
   +----------------------+-------------------------------------------+
   +------------------------+--------------------------+
   | http-client-server     | [HTTP-CLIENT-SERVER]     |
   +------------------------+--------------------------+
   | netconf-client-server  | [NETCONF-CLIENT-SERVER]  |
   +------------------------+--------------------------+
   | restconf-client-server | [RESTCONF-CLIENT-SERVER] |
   +------------------------+--------------------------+

          Table 1: Label in Diagram to RFC Mapping

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 in this document use the XML [W3C.REC-xml-20081126]
   encoding.  Other encodings, such as JSON [RFC8259], could
   alternatively be used.

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). Section 2.2
   ("Example Usage").  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 presents a common "grouping" statement for basic TCP
   connection parameters that matter to applications.  It is "common" in
   that this grouping is used by both the "ietf-tcp-client" and "ietf-
   tcp-server" modules.  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 data model in this document focuses on
   modeling TCP keep-alives. keepalives.  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

   The diagram above uses syntax that is similar to but not defined in
   [RFC8340].

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" container 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  Protocol-Accessible Nodes

   The "ietf-tcp-common" module defines only "grouping" statements that
   are used by other modules to instantiate protocol-accessible nodes.
   Thus
   Thus, this module, when implemented, does not itself define any
   protocol-accessible nodes.

2.1.5.  Guidelines for Configuring TCP Keep-Alives Keepalives

   Network stacks may include "keep-alives" keepalives in their TCP implementations,
   although this practice is not universally implemented.  If keep-alives keepalives
   are included, [RFC9293] mandates that the application MUST be able to
   turn them on or off for each TCP
   connection, connection and that they MUST
   default to off.

   Keep-alive

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

   A TCP keep-alive keepalive 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 [RFC9293].  TCP keep-alives keepalives may consume
   significant resources both in the network and in endpoints (e.g.,
   battery power).  In addition, frequent keep-alives keepalives risk network
   congestion.  The higher the frequency of keep-alives, keepalives, the higher the
   overhead.

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

   *  The default idle interval (leaf "idle-time") is two hours, i.e.,
      7200 seconds [RFC9293].  A lower value MAY be configured, but idle
      intervals SHOULD NOT be smaller than 15 seconds.  Longer idle
      intervals SHOULD be used when possible.

   *  The maximum number of sequential keep-alive keepalive 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 keepalive
      mechanism is implemented implemented, it MUST NOT interpret failure to respond
      to any specific probe as a dead connection [RFC9293].  Typically,
      a single-digit number should suffice.

   *  TCP implementations may include a parameter for the number of
      seconds between TCP keep-alive keepalive 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 keepalive 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  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"
   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>7200</idle-time>
       <max-probes>9</max-probes>
       <probe-interval>75</probe-interval>
     </keepalives>
   </tcp-common>

2.3.  YANG Module

   The ietf-tcp-common "ietf-tcp-common" YANG module references [RFC6991] and [RFC9293].

   <CODE BEGINS> file "ietf-tcp-common@2024-04-04.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:   https://datatracker.ietf.org/wg/netconf
                  https://datatracker.ietf.org/wg/tcpm
        WG List:  NETCONF WG list <mailto:netconf@ietf.org>
                  TCPM WG list <mailto:tcpm@ietf.org>
        Authors:  Kent Watsen <mailto:kent+ietf@watsen.net>
                  Michael Scharf
                  <mailto:michael.scharf@hs-esslingen.de>";

     description
       "This module define a reusable 'grouping' that is common
        to both TCP-clients TCP clients and TCP-servers. TCP servers.  This grouping statement
        is used by both the 'ietf-tcp-client' and 'ietf-tcp-server'
        modules.

        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.

        Copyright (c) 2023 2024 IETF Trust and the persons identified
        as authors of the code.  All rights reserved.

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

        This version of this YANG module is part of RFC DDDD
        (https://www.rfc-editor.org/info/rfcDDDD); 9643
        (https://www.rfc-editor.org/info/rfc9643); 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."; notices.";

     revision 2024-04-04 {
       description
         "Initial version"; version.";
       reference
         "RFC DDDD: 9643: 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, aligning to
                   the requirement in Section 3.8.4 of RFC 9293 that
                   states keepalives are off by default.";
         description
           "Configures the keep-alive policy, keepalive 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.  Further guidance can be found
            in Section 2.1.5 of RFC DDDD."; 9643.";
         reference
           "RFC 9293: Transmission Control Protocol (TCP)";
         leaf idle-time {
           type uint16 {
             range "1..max";
           }
           units "seconds";
           default 7200; "7200";
           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 if no data has been received from the TCP peer.
              Two hours (7200 seconds) is a safe value, per RFC 9293 Section 3.8.4.";
              3.8.4 of RFC 9293.";
           reference
             "RFC 9293: Transmission Control Protocol (TCP)";
         }
         leaf max-probes {
           type uint16 {
             range "1..max";
           }
           default 9; "9";
           description
             "Sets the maximum number of sequential keep-alive keepalive 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";
           default 75; "75";
           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). Section 3.2
   ("Example Usage").  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
         +-- socks4-supported {proxy-connect}?
         +-- socks4a-supported {proxy-connect}?
         +-- socks5-supported {proxy-connect}?
             +-- socks5-gss-api {socks5-supported}?
             +-- socks5-username-password {socks5-supported}?

   Comments:

   *  The "local-binding-supported" feature indicates that the server
      supports configuring local bindings (i.e., the local address and
      local port) for TCP clients." clients.

   *  The "tcp-client-keepalives" feature indicates that per socket TCP keepalive
      parameters are configurable for TCP clients on the server
      implementing this feature.

   *  The "proxy-connect" feature indicates the TCP-client TCP client supports
      connecting through TCP proxies.

   *  The "socks4-supported" feature indicates the TCP-client TCP client supports
      Socks4-based proxies.

   *  The "socks4a-supported" feature indicates the TCP-client TCP client supports
      Socks4a-based proxies.  The difference between Socks4 and Socks4a
      is that Socks4a enables the "remote-address" to be specified using
      a hostname, in addition to an IP address.

   *  The "socks5-supported" feature indicates the TCP-client TCP client supports
      Socks5-based proxies.

   *  The "socks5-gss-api" feature indicates that the server, when
      acting as a TCP-client, TCP client, supports authenticating to a SOCKS Version
      5 proxy server using GSSAPI Generic Security Service Application Program
      Interface (GSS-API) credentials.

   *  The "socks5-username-password" feature indicates that the server,
      when acting as a TCP-client, TCP client, supports authenticating to a SOCKS
      Version 5 proxy server using 'username' "username" and 'password' "password"
      credentials."

   The diagram above uses syntax that is similar to but not defined in
   [RFC8340].

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-supported}?
       |     |  +-- socks4-parameters
       |     |     +-- remote-address    inet:ip-address
       |     |     +-- remote-port?      inet:port-number
       |     +--:(socks4a) {socks4a-supported}?
       |     |  +-- socks4a-parameters
       |     |     +-- remote-address    inet:host
       |     |     +-- remote-port?      inet:port-number
       |     +--:(socks5) {socks5-supported}?
       |        +-- 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, or a hostname.

   *  The "remote-port" node is not mandatory, but its default value leaf is defined with neither a "default" nor a
      "mandatory" statement.  YANG modules using this grouping SHOULD
      refine the invalid value '0', thus forcing grouping with a "default" statement, when the consuming data model port
      number is well-known (e.g., a port number allocated by IANA), or
      with a "mandatory" statement, if a port number needs to
      refine it in order always be
      configured.  The SHOULD can be ignored when the port number is
      neither well-known nor mandatory to provide it an appropriate default value. configure, such as might be
      the case when this grouping is used by another grouping.

   *  The "local-address" node, which is enabled by the "local-binding-
      supported" feature (Section 2.1.2), 3.1.1), 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), 3.1.1), is not mandatory.  Its default
      value is '0', "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.  The proxy-server node uses
      a "choice" statement to allow one of several types of proxies to
      be configured.  The choices presented in this document include
      Socks4, Socks4a, and Socks5, each enabled by a YANG feature (see
      Section 3.1.1).  Other proxy types may be added by future work.

   *  This grouping uses the "password-grouping" grouping discussed in
      [I-D.ietf-netconf-crypto-types].
      [RFC9640].

   *  This grouping uses the "tcp-common-grouping" grouping discussed in
      Section 2.1.3.1.

3.1.3.  Protocol-accessible  Protocol-Accessible Nodes

   The "ietf-tcp-client" module defines only "grouping" statements that
   are used by other modules to instantiate protocol-accessible nodes.
   Thus
   Thus, this module, when implemented, does not itself define any
   protocol-accessible nodes.

3.2.  Example Usage

   This section presents two examples showing the "tcp-client-grouping"
   grouping populated with some data.  This example shows a TCP-client 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>8443</remote-port>
     <local-address>192.0.2.2</local-address>
     <local-port>12345</local-port>
     <keepalives>
       <idle-time>7200</idle-time>
       <max-probes>9</max-probes>
       <probe-interval>75</probe-interval>
     </keepalives>
   </tcp-client>

   This example shows a TCP-client TCP client configured to connect via a proxy: 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>8443</remote-port>
     <local-address>192.0.2.2</local-address>
     <local-port>12345</local-port>
     <proxy-server>
       <socks5-parameters>
         <remote-address>proxy.example.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>7200</idle-time>
       <max-probes>9</max-probes>
       <probe-interval>75</probe-interval>
     </keepalives>
   </tcp-client>

3.3.  YANG Module

   The ietf-tcp-client "ietf-tcp-client" YANG module references [SOCKS], [SOCKS_4A],
   [RFC1928], [RFC1929], [RFC2743], [RFC6991], [RFC9293], and
   [I-D.ietf-netconf-crypto-types]. [RFC9640].

   <CODE BEGINS> file "ietf-tcp-client@2024-04-04.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: 9640: YANG Data Types and Groupings for Cryptography";
     }

     import ietf-tcp-common {
       prefix tcpcmn;
       reference
         "RFC DDDD: 9643: 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:   https://datatracker.ietf.org/wg/netconf
                  https://datatracker.ietf.org/wg/tcpm
        WG List:  NETCONF WG list <mailto:netconf@ietf.org>
                  TCPM WG list <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) 2024 IETF Trust and the persons identified
        as authors of the code.  All rights reserved.

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

        This version of this YANG module is part of RFC DDDD
        (https://www.rfc-editor.org/info/rfcDDDD); 9643
        (https://www.rfc-editor.org/info/rfc9643); 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."; notices.";

     revision 2024-04-04 {
       description
         "Initial version"; version.";
       reference
         "RFC DDDD: 9643: 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
         "TCP keepalive parameters are configurable for
          TCP clients on the server implementing this feature.";
       reference
         "RFC 9293: Transmission Control Protocol (TCP)";
     }

     feature proxy-connect {
       description
         "Indicates the TCP-client TCP client supports connecting through
          TCP proxies.";
     }

     feature socks4-supported {
       if-feature proxy-connect; "proxy-connect";
       description
         "Indicates the TCP-client TCP client supports Socks4-based proxies.";
       reference
         "SOCKS Proceedings: 1992 Usenix Security Symposium."; Symposium";
     }

     feature socks4a-supported {
       if-feature proxy-connect; "proxy-connect";
       description
         "Indicates the TCP-client TCP client supports Socks4a-based proxies.";
       reference
         "OpenSSH message:
            SOCKS 4A: A Simple Extension to SOCKS 4 Protocol
            https://www.openssh.com/txt/socks4a.protocol.";
            <https://www.openssh.com/txt/socks4a.protocol>";
     }

     feature socks5-supported {
       if-feature proxy-connect; "proxy-connect";
       description
         "Indicates the TCP-client TCP client supports Socks5-based proxies.";
       reference
         "RFC 1928: SOCKS Protocol Version 5";
     }

     feature socks5-gss-api {
       if-feature socks5-supported; "socks5-supported";
       description
         "Indicates that the server, when acting as a TCP-client, TCP client,
          supports authenticating to a SOCKS Version 5 proxy server
          using GSSAPI GSS-API credentials.";
       reference
         "RFC 1928: SOCKS Protocol Version 5";
     }

     feature socks5-username-password {
       if-feature socks5-supported; "socks5-supported";
       description
         "Indicates that the server, when acting as a TCP-client, TCP client,
          supports authenticating to a SOCKS Version 5 proxy server
          using 'username' and 'password' credentials.";
       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 of the remote peer to establish a
            connection with.  An invalid default value is used
            so that importing modules may 'refine' it with the
            appropriate default port number value."; TCP server.";
       }
       leaf local-address {
         if-feature "local-binding-supported";
         type inet:ip-address;
         description
           "The local IP address/interface 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 which the server
            can bind to any IPv4 or IPv6 address.";
       }
       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 {
             if-feature socks4-supported; "socks4-supported";
             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,
                 "SOCKS Proceedings: 1992 Usenix Security Symposium."; Symposium";
             }
           }
           case socks4a {
             if-feature socks4a-supported; "socks4a-supported";
             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. Symposium
                  OpenSSH message:
                    SOCKS 4A: A Simple Extension to SOCKS 4 Protocol
                    https://www.openssh.com/txt/socks4a.protocol";
                    <https://www.openssh.com/txt/socks4a.protocol>";
             }
           }
           case socks5 {
             if-feature socks5-supported; "socks5-supported";
             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.";
                    <https://www.iana.org/assignments/socks-methods>.";
                 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 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 {
         refine "keepalives" {
           if-feature "tcp-client-keepalives";
           description
             "An if-feature '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). Section 4.2
   ("Example Usage").  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

   The diagram above uses syntax that is similar to but not defined in
   [RFC8340].

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-bind* [local-address]
       |  +-- local-address? 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 leaf is defined with neither a "default" nor a
      "mandatory" statement.  YANG modules using this grouping SHOULD
      refine the invalid value '0', thus forcing grouping with a "default" statement, when the consuming data model port
      number is well-known (e.g., a port number allocated by IANA), or
      with a "mandatory" statement, if a port number needs to
      refine it in order always be
      configured.  The SHOULD can be ignored when the port number is
      neither well-known nor mandatory to provide it an appropriate default value. configure, such as might be
      the case when this grouping is used by another grouping.

   *  This grouping uses the "tcp-common-grouping" grouping discussed in
      Section 2.1.3.1.

4.1.3.  Protocol-accessible  Protocol-Accessible Nodes

   The "ietf-tcp-server" module defines only "grouping" statements that
   are used by other modules to instantiate protocol-accessible nodes.
   Thus
   Thus, this module, when implemented, does not itself define any
   protocol-accessible nodes.

4.2.  Example Usage

   This section presents an example showing the "tcp-server-grouping"
   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-bind>
       <local-address>192.0.2.2</local-address>
       <local-port>49152</local-port>
     </local-bind>
     <keepalives>
       <idle-time>7200</idle-time>
       <max-probes>9</max-probes>
       <probe-interval>75</probe-interval>
     </keepalives>
   </tcp-server>

4.3.  YANG Module

   The ietf-tcp-server "ietf-tcp-server" YANG module references [RFC6991] and [RFC9293].

   <CODE BEGINS> file "ietf-tcp-server@2024-04-04.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: 9643: 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:   https://datatracker.ietf.org/wg/netconf
                  https://datatracker.ietf.org/wg/tcpm
        WG List:  NETCONF WG list <mailto:netconf@ietf.org>
                  TCPM WG list <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) 2024 IETF Trust and the persons identified
        as authors of the code.  All rights reserved.

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

        This version of this YANG module is part of RFC DDDD
        (https://www.rfc-editor.org/info/rfcDDDD); 9643
        (https://www.rfc-editor.org/info/rfc9643); 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."; notices.";

     revision 2024-04-04 {
       description
         "Initial version"; version.";
       reference
         "RFC DDDD: 9643: YANG Groupings for TCP Clients and TCP Servers";
     }

     // Features

     feature tcp-server-keepalives {
       description
         "Per socket TCP
         "TCP keepalive parameters are configurable for
          TCP servers on the server implementing this feature.";
       reference
         "RFC 9293: Transmission Control Protocol (TCP)";
     }

     // 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.";
       list local-bind {
         key local-address; "local-address";
         min-elements 1;
         description
           "A list of bind (listen) points for this server
            instance.  A server instance may have multiple
            bind points to support, e.g., the same port in
            different address families or different ports
            in the same address family.";
         leaf local-address {
           type inet:ip-address;
           description
             "The local IP address to listen on for incoming
              TCP client connections.  To configure listening
              on all IPv4 addresses addresses, the value must be '0.0.0.0'
              (INADDR_ANY).  To configure listening on all IPv6
              addresses
              addresses, the value must be '::' (INADDR6_ANY).";
         }
         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."; connections.”;
         }
       }
       uses tcpcmn:tcp-common-grouping {
         refine "keepalives" {
           if-feature "tcp-server-keepalives";
           description
             "An if-feature 'if-feature' statement so that implementations
              can choose to support TCP server keepalives.";
         }
       }
     }
   }
   <CODE ENDS>

5.  Security Considerations

   The three YANG modules in this document define groupings and will not
   be deployed as standalone modules.  Their security implications may
   be context dependent context-dependent based on their use in other modules.  The
   designers of modules which that import these grouping groupings must conduct their
   own analysis of the security considerations.

5.1.  Considerations for the "ietf-tcp-common" YANG Module

   This section follows is modeled after the template defined in Section 3.7.1
   of [RFC8407].

   The "ietf-tcp-common" YANG module defines "grouping" statements that
   are designed to be accessed via YANG based 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 Secure
   Shell (SSH) [RFC4252], TLS [RFC8446], and QUIC [RFC9000]) and
   mandatory-to-implement mutual authentication.

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular users to a pre-configured
   preconfigured subset of all available protocol operations and
   content.

   Please be aware that this YANG module uses groupings from other YANG
   modules that define nodes that may be considered sensitive or
   vulnerable in network environments.  Please review the Security
   Considerations security
   considerations for dependent YANG modules for information as to which
   nodes may be considered sensitive or vulnerable in network
   environments.

   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
   thus, the security consideration considerations for such is are not provided here.

5.2.  Considerations for the "ietf-tcp-client" YANG Module

   This section follows is modeled after the template defined in Section 3.7.1
   of [RFC8407].

   The "ietf-tcp-client" YANG module defines "grouping" statements that
   are designed to be accessed via YANG based 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 Secure
   Shell (SSH) [RFC4252], TLS [RFC8446], and QUIC [RFC9000]) and
   mandatory-to-implement mutual authentication.

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular users to a pre-configured
   preconfigured subset of all available protocol operations and
   content.

   Please be aware that this YANG module uses groupings from other YANG
   modules that define nodes that may be considered sensitive or
   vulnerable in network environments.  Please review the Security
   Considerations security
   considerations for dependent YANG modules for information as to which
   nodes may be considered sensitive or vulnerable in network
   environments.

   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 "password" node defined in the "tcp-client-grouping"
         grouping is defined using the "password-grouping" grouping
         presented in [I-D.ietf-netconf-crypto-types]. [RFC9640].  This grouping enables both cleartext
         and encrypted passwords to be configured.  As the referenced
         document states, configuration of cleartext passwords is NOT
         RECOMMENDED.  However, in the case cleartext values are
         configured, this node 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 "default-deny-
         all" extension "default-deny-all" has been applied to it.

   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
   thus, the security consideration considerations for such is are not provided here.

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

5.3.  Considerations for the "ietf-tcp-server" YANG Module

   This section follows is modeled after the template defined in Section 3.7.1
   of [RFC8407].

   The "ietf-tcp-server" YANG module defines "grouping" statements that
   are designed to be accessed via YANG based 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 Secure
   Shell (SSH) [RFC4252], TLS [RFC8446], and QUIC [RFC9000]) and
   mandatory-to-implement mutual authentication.

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular users to a pre-configured
   preconfigured subset of all available protocol operations and
   content.

   Please be aware that this YANG module uses groupings from other YANG
   modules that define nodes that may be considered sensitive or
   vulnerable in network environments.  Please review the Security
   Considerations security
   considerations for dependent YANG modules for information as to which
   nodes may be considered sensitive or vulnerable in network
   environments.

   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
   thus, the security consideration considerations for such is are not provided here.

6.  IANA Considerations

6.1.  The "IETF XML" IETF XML Registry

   This document registers three URIs

   IANA has registered the following URI in the "ns" subregistry registry of the
   IETF
   "IETF XML Registry Registry" [RFC3688].  Following the format in [RFC3688], the
   following registrations are requested:

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

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

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

6.2.  The "YANG YANG Module Names" Names Registry

   This document registers

   IANA has registered the following three YANG modules in the YANG "YANG
   Module Names Names" registry [RFC6020].  Following the format in [RFC6020], the following
   registrations are requested:

      name:

   Name:  ietf-tcp-common
      namespace:
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-tcp-common
      prefix:
   Prefix:  tcpcmn
      reference:
   Reference:  RFC DDDD

      name: 9643

   Name:  ietf-tcp-client
      namespace:
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-tcp-client
      prefix:
   Prefix:  tcpc
      reference:
   Reference:  RFC DDDD

      name: 9643

   Name:  ietf-tcp-server
      namespace:
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-tcp-server
      prefix:
   Prefix:  tcps
      reference:
   Reference:  RFC DDDD 9643

7.  References

7.1.  Normative 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-34, 16 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              crypto-types-34>.

   [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>.

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [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>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/info/rfc9293>.

   [RFC9640]  Watsen, K., "YANG Data Types and Groupings for
              Cryptography", RFC 9640, DOI 10.17487/RFC9640, September
              2024, <https://www.rfc-editor.org/info/rfc9640>.

7.2.  Informative References

   [I-D.ietf-netconf-http-client-server]

   [HTTP-CLIENT-SERVER]
              Watsen, K., "YANG Groupings for HTTP Clients and HTTP
              Servers", Work in Progress, Internet-Draft, draft-ietf-
              netconf-http-client-server-20, 16 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              http-client-server-20>.

   [I-D.ietf-netconf-keystore]
              Watsen, K., "A YANG Data Model for a Keystore and Keystore
              Operations", Work in Progress, Internet-Draft, draft-ietf-
              netconf-keystore-35, 16 March
              netconf-http-client-server-23, 15 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              keystore-35>.

   [I-D.ietf-netconf-netconf-client-server]
              http-client-server-23>.

   [NETCONF-CLIENT-SERVER]
              Watsen, K., "NETCONF Client and Server Models", Work in
              Progress, Internet-Draft, draft-ietf-netconf-netconf-
              client-server-36, 16 March
              client-server-37, 14 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              netconf-client-server-36>.

   [I-D.ietf-netconf-restconf-client-server]
              netconf-client-server-37>.

   [RESTCONF-CLIENT-SERVER]
              Watsen, K., "RESTCONF Client and Server Models", Work in
              Progress, Internet-Draft, draft-ietf-netconf-restconf-
              client-server-36, 16 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              restconf-client-server-36>.

   [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-40, 16 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              ssh-client-server-40>.

   [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-26, 4 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              tcp-client-server-26>.

   [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-41, 16 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              tls-client-server-41>.

   [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-28, 16 March
              client-server-38, 14 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              trust-anchors-28>.
              restconf-client-server-38>.

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              DOI 10.17487/RFC1928, March 1996,
              <https://www.rfc-editor.org/info/rfc1928>.

   [RFC1929]  Leech, M., "Username/Password Authentication for SOCKS
              V5", RFC 1929, DOI 10.17487/RFC1929, March 1996,
              <https://www.rfc-editor.org/info/rfc1929>.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743,
              DOI 10.17487/RFC2743, January 2000,
              <https://www.rfc-editor.org/info/rfc2743>.

   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [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>.

   [RFC8407]  Bierman, A., "Guidelines for Authors and Reviewers of
              Documents Containing YANG Data Models", BCP 216, RFC 8407,
              DOI 10.17487/RFC8407, October 2018,
              <https://www.rfc-editor.org/info/rfc8407>.

   [SOCKS_4A] Project, T. O., "SOCKS 4A: A Simple Extension to SOCKS 4
              Protocol", <https://www.openssh.com/txt/socks4a.protocol>.

Appendix A.  Change Log

A.1.  00 to 01

   *  Added 'local-binding-supported' feature to TCP-client model.

   *  Added 'keepalives-supported' feature to TCP-common model.

   *  Added 'external-endpoint-values' container and 'external-
      endpoints' feature to TCP-server model.

A.2.  01 to 02

   *  Removed the 'external-endpoint-values' container and 'external-
      endpoints' feature from the TCP-server model.

A.3.  02 to 03

   *  Moved the common model section to be before the client and server
      specific sections.

   *  Added sections "Model Scope" and "Usage Guidelines for Configuring
      TCP Keep-Alives" to the common model section.

A.4.  03 to 04

   *  Fixed a few typos.

A.5.  04 to 05

   *  Removed commented out "grouping tcp-system-grouping" statement
      kept for reviewers.

   *  Added a "Note to Reviewers" note to first page.

A.6.  05 to 06

   *  Added support for TCP proxies.

A.7.  06 to 07

   *  Expanded "Data Model Overview section(s) [remove "wall" of tree
      diagrams].

   *  Updated the

   [RFC8446]  Rescorla, E., "The Transport Layer Security Considerations section.

A.8.  07 to 08

   *  Added missing IANA registration for "ietf-tcp-common"

   *  Added "mandatory true" for the "username" and "password" leafs

   *  Added an example of a TCP-client configured to connect via a proxy

   *  Fixed issues found by the SecDir review of the "keystore" draft.

   *  Updated the "ietf-tcp-client" module to use the new "password-
      grouping" grouping from the "crypto-types" module.

A.9.  08 to 09

   *  Addressed comments raised by (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9641]  Watsen, K., "A YANG Doctor in the ct/ts/ks drafts.

A.10.  09 to 10

   *  Updated Abstract and Intro to address comments by Tom Petch.

   *  Removed the "tcp-connection-grouping" grouping (now models use the
      "tcp-common-grouping" directly).

   *  Added XML-comment above examples explaining the reason for the
      unusual top-most element's presence.

   *  Added Securty Considerations section Data Model for the "local-binding-
      supported" feature.

   *  Replaced some hardcoded refs to <xref> elements.

   *  Fixed nits found by YANG Doctor reviews.

   *  Aligned modules with `pyang -f` formatting.

   *  Added an "Acknowledgements" secetion.

A.11.  10 to 11

   *  Replaced "base64encodedvalue==" with "BASE64VALUE=" in examples.

   *  Minor editorial nits

A.12.  11 to 12

   *  Fixed up the 'WG Web' and 'WG List' lines in YANG module(s)

   *  Fixed up copyright (i.e., s/Simplified/Revised/) in a Truststore",
              RFC 9641, DOI 10.17487/RFC9641, September 2024,
              <https://www.rfc-editor.org/info/rfc9641>.

   [RFC9642]  Watsen, K., "A YANG module(s)

A.13.  12 to 13

   *  NO UPDATE.

A.14.  13 to 14

   *  Updated per Shepherd reviews impacting the suite of drafts.

A.15.  14 to 15

   *  Updated per Shepherd reviews impacting the suite of drafts.

A.16.  15 to 16

   *  Updated per Tom Petch review.

   *  Added refs to RFC9293 and SOCKS 4A.

   *  Fixed examples to use IETF-sanctioned values Data Model for examples.

A.17.  16 to 17

   *  Addresses AD review comments.

   *  Added note to Editor to fix line foldings.

   *  Added Security Considerations text to also look a SC-section from
      imported modules.

   *  Fixed bug: s/augment "keepalives"/refine "keepalives"/

   *  Set defaults Keystore", RFC 9642,
              DOI 10.17487/RFC9642, September 2024,
              <https://www.rfc-editor.org/info/rfc9642>.

   [RFC9644]  Watsen, K., "YANG Groupings for idle-time, max-probes, SSH Clients and probe-interval
      (removed "mandatory true").

   *  Updated examples to use IETF recommended values for examples.

A.18.  18 to 19

   *  Addresses AD review by Rob Wilton.

A.19.  18 to 19

   *  Replace RFC 1122 with SSH
              Servers", RFC 9293.

A.20.  19 to 20

   *  Addresses 1st-round of IESG reviews.

A.21.  20 to 22

   *  Addresses issues found in OpsDir review of the ssh-client-server
      draft.

   *  Updated to address Mallory Knodel's Gen-ART review.

   *  Renamed Security Considerations section s/Template for/
      Considerations for/

   *  s/defines/presents/ in a few places.

   *  Add refs to where the 'operational' 9644, DOI 10.17487/RFC9644, September 2024,
              <https://www.rfc-editor.org/info/rfc9644>.

   [RFC9645]  Watsen, K., "YANG Groupings for TLS Clients and 'system' datastores are
      defined.

   *  Added more 'feature' statements TLS
              Servers", RFC 9645, DOI 10.17487/RFC9645, September 2024,
              <https://www.rfc-editor.org/info/rfc9645>.

   [SOCKS]    Koblas, D. and descriptions for them

   *  Added M. Koblas, "SOCKS", USENIX UNIX Security Considaration for the "password" node

A.22.  22 to 23

   *  Address IESG review comments.

A.23.  23 to 24

   *  Nothing changed.  Bumped only for automation.

A.24.  24 to 25

   *  Updated to support dual-stacks

A.25.  25 to 26

   *  Updated
              Symposium III, September 1992, <https://www.usenix.org/leg
              acy/publications/library/proceedings/sec92/full_papers/
              koblas.pdf>.

   [SOCKS_4A] Lee, Y., "SOCKS 4A: A Simple Extension to ensure at least one local-bind is specified. SOCKS 4
              Protocol", <https://www.openssh.com/txt/socks4a.protocol>.

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

Acknowledgements

   The authors would like to thank the following for lively discussions
   on list and in the halls (ordered by first name): Éric Vyncke, Joe
   Clarke, Jürgen Schönwälder, Ladislav Lhotka, Mallory Knodel, Martin
   Duke, Michael Tüxen, Mohamed Boucadair, Nancy Cam-Winget, Nick
   Hancock, Per Andersson, Rob Wilton, Roman Danyliw, Tom Petch, and Wim
   Henderickx.

Authors' Addresses

   Kent Watsen
   Watsen Networks
   Email: kent+ietf@watsen.net

   Michael Scharf
   Hochschule Esslingen -
   University of Applied Sciences
   Kanalstr. 33
   73728 Esslingen am Neckar
   Germany
   Email: michael.scharf@hs-esslingen.de