Two-Way Active Measurement Protocol (TWAMP) YANG Data Model
RFC 8913

Document Type RFC - Proposed Standard (November 2021; Errata)
Authors Ruth Civil  , Al Morton  , Reshad Rahman  , Mahesh Jethanandani  , Kostas Pentikousis 
Last updated 2021-11-25
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Internet Engineering Task Force (IETF)                          R. Civil
Request for Comments: 8913                             Ciena Corporation
Category: Standards Track                                      A. Morton
ISSN: 2070-1721                                                AT&T Labs
                                                               R. Rahman
                                                                        
                                                         M. Jethanandani
                                                     Xoriant Corporation
                                                     K. Pentikousis, Ed.
                                                                 Detecon
                                                           November 2021

      Two-Way Active Measurement Protocol (TWAMP) YANG Data Model

Abstract

   This document specifies a data model for client and server
   implementations of the Two-Way Active Measurement Protocol (TWAMP).
   This document defines the TWAMP data model through Unified Modeling
   Language (UML) class diagrams and formally specifies it using the
   YANG data modeling language (RFC 7950).  The data model is compliant
   with the Network Management Datastore Architecture (NMDA).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8913.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Motivation
     1.2.  Terminology
     1.3.  Document Organization
   2.  Scope, Model, and Applicability
   3.  Data Model Overview
     3.1.  Control-Client
     3.2.  Server
     3.3.  Session-Sender
     3.4.  Session-Reflector
   4.  Data Model Parameters
     4.1.  Control-Client
     4.2.  Server
     4.3.  Session-Sender
     4.4.  Session-Reflector
   5.  Data Model
     5.1.  YANG Tree Diagram
     5.2.  YANG Module
   6.  Data Model Examples
     6.1.  Control-Client
     6.2.  Server
     6.3.  Session-Sender
     6.4.  Session-Reflector
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Detailed Data Model Examples
     A.1.  Control-Client
     A.2.  Server
     A.3.  Session-Sender
     A.4.  Session-Reflector
   Appendix B.  TWAMP Operational Commands
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   The Two-Way Active Measurement Protocol (TWAMP) [RFC5357] is used to
   measure network performance parameters such as latency, bandwidth,
   and packet loss by sending probe packets and measuring their
   experience in the network.  To date, TWAMP implementations do not
   come with a standard management framework, and, as such, implementers
   have no choice except to provide a proprietary mechanism.  This
   document addresses this gap by defining the model using Unified
   Modeling Language (UML) class diagrams [UML] and formally specifying
   a TWAMP data model that is compliant with the Network Management
   Datastore Architecture (NMDA) [RFC8342], using YANG 1.1 [RFC7950].

1.1.  Motivation

   In current TWAMP deployments, the lack of a standardized data model
   limits the flexibility to dynamically instantiate TWAMP-based
   measurements across equipment from different vendors.  In large,
   virtualized, and dynamically instantiated infrastructures where
   network functions are placed according to orchestration algorithms,
   proprietary mechanisms for managing TWAMP measurements pose severe
   limitations with respect to programmability.

   Two major trends call for standardizing TWAMP management aspects.
   First, it is expected that in the coming years large-scale and multi-
   vendor TWAMP deployments will become the norm.  From an operations
   perspective, using several vendor-specific TWAMP configuration
   mechanisms when one standard mechanism could provide an alternative
   is expensive and inefficient.  Second, the increasingly software-
   defined and virtualized nature of network infrastructures, based on
   dynamic service chains [NSC] and programmable control and management
   planes [RFC7426], requires a well-defined data model for TWAMP
   implementations.  This document defines such a TWAMP data model and
   specifies it formally using the YANG 1.1 data modeling language
   [RFC7950].

1.2.  Terminology

   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.  Document Organization

   The rest of this document is organized as follows.  Section 2
   presents the scope and applicability of this document.  Section 3
   provides a high-level overview of the TWAMP data model.  Section 4
   details the configuration parameters of the data model, and Section 5
   specifies in YANG the TWAMP data model.  Section 6 lists illustrative
   examples that conform to the YANG data model specified in this
   document.  Appendix A elaborates these examples further.

2.  Scope, Model, and Applicability

   The purpose of this document is the specification of a vendor-
   independent data model for TWAMP implementations.

   Figure 1 illustrates a redrawn version of the TWAMP logical model
   found in Section 1.2 of TWAMP [RFC5357].  The figure is annotated
   with pointers to the UML diagrams [UML] provided in this document and
   associated with the data model of the four logical entities in a
   TWAMP deployment, namely the TWAMP Control-Client, Server, Session-
   Sender, and Session-Reflector.  A UML Notation Guide is available in
   Section 5 of UML [UML].

   As per TWAMP [RFC5357], unlabeled links in Figure 1 are left
   unspecified and may be proprietary protocols.

            (Figure 3)                              (Figure 4)
        +----------------+                          +--------+
        | Control-Client |  <-- TWAMP-Control -->   | Server |
        +----------------+                          +--------+
                ^                                        ^
                |                                        |
                V                                        V
        +----------------+                     +-------------------+
        | Session-Sender |  <-- TWAMP-Test --> | Session-Reflector |
        +----------------+                     +-------------------+
            (Figure 5)                               (Figure 6)

                  Figure 1: Annotated TWAMP Logical Model

   As per TWAMP [RFC5357], a TWAMP implementation may follow a
   simplified logical model, in which the same node acts as both
   Control-Client and Session-Sender, while another node acts at the
   same time as both TWAMP Server and Session-Reflector.  Figure 2
   illustrates this simplified logical model and indicates the
   interaction between the TWAMP configuration client and server using,
   for instance, NETCONF [RFC6241] or RESTCONF [RFC8040].

     o-------------------o                       o-------------------o
     |   Config client   |                       |   Config client   |
     o-------------------o                       o-------------------o
              ||                                          ||
      NETCONF || RESTCONF                         NETCONF || RESTCONF
              ||                                          ||
     o-------------------o                       o-------------------o
     |   Config server   |                       |   Config server   |
     | (Figures 3 and 5) |                       | (Figures 4 and 6) |
     +-------------------+                       +-------------------+
     |   Control-Client  | <-- TWAMP-Control --> |      Server       |
     |                   |                       |                   |
     |   Session-Sender  |  <-- TWAMP-Test -->   | Session-Reflector |
     +-------------------+                       +-------------------+

               Figure 2: Simplified TWAMP Model and Protocols

   The data model defined in this document is orthogonal to the specific
   protocol used between the Config client and Config server to
   communicate the TWAMP configuration parameters.

   Operational actions such as how TWAMP-Test sessions are started and
   stopped, how performance measurement results are retrieved, or how
   stored results are cleared, and so on, are not addressed by the
   configuration model defined in this document.  As noted above, such
   operational actions are not part of the TWAMP specification [RFC5357]
   and hence are out of scope for this document.  See also Appendix B.
   In addition, for operational state, the information provided in the
   Performance Metrics Registry [RFC8911] and [PERF-METRICS] can be used
   to develop an independent model for the Performance Metrics that need
   to be captured and retrieved.

3.  Data Model Overview

   The TWAMP data model includes four categories of configuration items.

   First, global configuration items relate to parameters that are set
   on a per-device level.  For example, the administrative status of the
   device with respect to whether it allows TWAMP sessions and, if so,
   in what capacity (e.g., Control-Client, Server, or both) is a typical
   instance of a global configuration item.

   A second category includes attributes that can be configured on a
   per-TWAMP-Control-connection basis, such as the Server IP address.

   A third category includes attributes related to per-TWAMP-Test-
   session attributes -- for instance, setting different values in the
   Differentiated Services Code Point (DSCP) field.

   Finally, the data model includes attributes that relate to the
   operational state of the TWAMP implementation.

   As the TWAMP data model is described in the remaining sections of
   this document, readers should keep in mind the functional entity
   grouping illustrated in Figure 1.

3.1.  Control-Client

   A TWAMP Control-Client has an administrative status field set at the
   device level that indicates whether the node is enabled to function
   as such.

   Each TWAMP Control-Client is associated with zero or more
   TWAMP-Control connections.  The main configuration parameters of each
   control connection are:

   *  A name that can be used to uniquely identify at the Control-Client
      a particular control connection.  This name is necessary for
      programmability reasons because at the time of creation of a
      TWAMP-Control connection not all IP and TCP port number
      information needed to uniquely identify the connection is
      available.

   *  The IP address of the interface the Control-Client will use for
      connections.

   *  The IP address of the remote TWAMP Server.

   *  Authentication and encryption attributes such as KeyID, Token, and
      the Control-Client Initialization Vector (Client-IV); see also
      Section 3.1 of "A One-way Active Measurement Protocol (OWAMP)"
      [RFC4656] and "Randomness Requirements for Security" [RFC4086].

   Each TWAMP-Control connection, in turn, is associated with zero or
   more TWAMP-Test sessions.  For each test session, the following
   configuration items should be noted:

   *  The test session name, which uniquely identifies a particular test
      session at the Control-Client and Session-Sender.  Similar to the
      control connections mentioned above, this unique test session name
      is needed because at the time of creation of a TWAMP-Test session,
      for example, the source UDP port number is not known to uniquely
      identify the test session.

   *  The IP address and UDP port number of the Session-Sender on the
      path under test by TWAMP.

   *  The IP address and UDP port number of the Session-Reflector on
      said path.

   *  Information pertaining to the test packet stream, such as the test
      starting time; which Performance Metric is to be used, as defined
      in "Registry for Performance Metrics" [RFC8911]; or whether the
      test should be repeated.

3.2.  Server

   Each TWAMP Server has an administrative status field set at the
   device level to indicate whether the node is enabled to function as a
   TWAMP Server.

   Each Server is associated with zero or more TWAMP-Control
   connections.  Each control connection is uniquely identified by the
   4-tuple {Control-Client IP address, Control-Client TCP port number,
   Server IP address, Server TCP port}.  Control connection
   configuration items on a TWAMP Server are read-only.

3.3.  Session-Sender

   A TWAMP Session-Sender has an administrative status field set at the
   device level that indicates whether the node is enabled to function
   as such.

   There is one Session-Sender instance for each TWAMP-Test session that
   is initiated from the sending device.  Primary configuration fields
   include:

   *  The test session name, which MUST be identical to the
      corresponding test session name on the TWAMP Control-Client
      (Section 3.1).

   *  The control connection name, which, along with the test session
      name, uniquely identifies the TWAMP Session-Sender instance.

   *  Information pertaining to the test packet stream, such as the
      number of test packets and the packet distribution to be employed;
      see also "Network performance measurement with periodic streams"
      [RFC3432].

3.4.  Session-Reflector

   Each TWAMP Session-Reflector has an administrative status field set
   at the device level to indicate whether the node is enabled to
   function as such.

   Each Session-Reflector is associated with zero or more TWAMP-Test
   sessions.  For each test session, the REFWAIT timeout parameter,
   which determines whether to discontinue the session if no packets
   have been received (TWAMP [RFC5357], Section 4.2), can be configured.

   Read-only access to other data model parameters, such as the Sender
   IP address, is foreseen.  Each test session can be uniquely
   identified by the 4-tuple mentioned in Section 3.2.

4.  Data Model Parameters

   This section defines the TWAMP data model using UML [UML] and
   introduces selected parameters associated with the four TWAMP logical
   entities.  The complete TWAMP data model specification is provided in
   the YANG module presented in Section 5.2.

4.1.  Control-Client

   The client container (see Figure 3) holds items that are related to
   the configuration of the TWAMP Control-Client logical entity (recall
   Figure 1).

   The client container includes an administrative configuration
   parameter (client/admin-state) that indicates whether the device is
   allowed to initiate TWAMP-Control connections.

     +-------------+
     | client      |
     +-------------+                   1..* +-----------------------+
     | admin-state |<>----------------------| mode-preference-chain |
     |             |                        +-----------------------+
     |             |  1..* +------------+   | priority              |
     |             |<>-----| key-chain  |   | mode                  |
     +-------------+       +------------+   +-----------------------+
            ^              | key-id     |
            V              | secret-key |
            |              +------------+
            | 0..*
     +------------------------+
     | ctrl-connection        |
     +------------------------+
     | name                   |
     | client-ip              |
     | server-ip              |
     | server-tcp-port        |    0..* +----------------------+
     | control-packet-dscp    |<>-------| test-session-request |
     | key-id                 |         +----------------------+
     | max-count              |         | name                 |
     | client-tcp-port   {ro} |         | sender-ip            |
     | server-start-time {ro} |         | sender-udp-port      |
     | state             {ro} |         | reflector-ip         |
     | selected-mode     {ro} |         | reflector-udp-port   |
     | token             {ro} |         | timeout              |
     | client-iv         {ro} |         | padding-length       |
     +------------------------+         | test-packet-dscp     |
                                        | start-time           |
                 +-------------+ 1      | repeat               |
                 | pm-reg-list |------<>| repeat-interval      |
                 +-------------+        | state           {ro} |
                 | pm-index    |        | sid             {ro} |
                 +-------------+        +----------------------+

              Figure 3: TWAMP Control-Client UML Class Diagram

   The client container holds a list (mode-preference-chain) that
   specifies the mode values according to their preferred order of use
   by the operator of this Control-Client, including the authentication
   and encryption modes.  Specifically, mode-preference-chain lists the
   mode and its corresponding priority, expressed as a 16-bit unsigned
   integer.  Values for the priority start with zero, the highest
   priority, and decreasing priority value is indicated by every
   increase in value by one.

   Depending on the modes available in the Server Greeting, the Control-
   Client MUST choose the highest-priority mode from the configured
   mode-preference-chain list.

   Note that the list of preferred modes may set multiple bit positions
   independently, such as when referring to the extended TWAMP features
   in "Mixed Security Mode for the Two-Way Active Measurement Protocol
   (TWAMP)" [RFC5618], "Individual Session Control Feature for the
   Two-Way Active Measurement Protocol (TWAMP)" [RFC5938], "Two-Way
   Active Measurement Protocol (TWAMP) Reflect Octets and Symmetrical
   Size Features" [RFC6038], and "IKEv2-Derived Shared Secret Key for
   the One-Way Active Measurement Protocol (OWAMP) and Two-Way Active
   Measurement Protocol (TWAMP)" [RFC7717].  If the Control-Client
   cannot determine an acceptable mode, or when the bit combinations do
   not make sense, e.g., authenticated and unauthenticated bits are both
   set, it MUST respond with zero Mode bits set in the Set-Up-Response
   message, indicating that it will not continue with the control
   connection.

   In addition, the client container holds a list named "key-chain",
   which relates key-id with the respective secret-key.  Both the Server
   and the Control-Client use the same mappings from key-id to
   secret-key (in Figure 3); in order for this to work properly, key-id
   must be unique across all systems in the administrative domain.  The
   Server, being prepared to conduct sessions with more than one
   Control-Client, uses key-id to choose the appropriate secret-key; a
   Control-Client would typically have different secret keys for
   different Servers.  The secret-key is the shared secret, of type
   "binary", and the length SHOULD contain at least 128 bits of entropy.
   The key-id and secret-key encoding SHOULD follow Section 9.8 of YANG
   [RFC7950].  The derived key length (dkLen as defined in "PKCS #5:
   Password-Based Cryptography Specification Version 2.1" [RFC8018])
   MUST be 16 octets for the AES Session-key used for encryption and
   32 octets for the HMAC-SHA1 Session-key used for authentication; see
   also Section 6.10 of OWAMP [RFC4656].

   Each client container also holds a list of control connections, where
   each item in the list describes a TWAMP-Control connection initiated
   by this Control-Client.  There SHALL be one ctrl-connection per
   TWAMP-Control (TCP) connection that is to be initiated from this
   device.

   In turn, each ctrl-connection holds a test-session-request list.
   Each test-session-request holds information associated with the
   Control-Client for this test session.  This includes information
   associated with the Request-TW-Session/Accept-Session message
   exchange (see Section 3.5 of TWAMP [RFC5357]).

   There SHALL be one instance of test-session-request for each
   TWAMP-Test session that is to be negotiated by this TWAMP-Control
   connection via a Request-TW-Session/Accept-Session exchange.

   The Control-Client is also responsible for scheduling TWAMP-Test
   sessions; therefore, test-session-request holds information related
   to these actions (e.g., pm-index, repeat-interval).

4.2.  Server

   The server container (see Figure 4) holds items that are related to
   the configuration of the TWAMP Server logical entity (recall
   Figure 1).

   The server container includes an administrative configuration
   parameter (server/admin-state) that indicates whether the device is
   allowed to receive TWAMP-Control connections.

   A device operating in the Server Role cannot configure attributes on
   a per-TWAMP-Control-connection basis, as it has no foreknowledge of
   the incoming TWAMP-Control connections to be received.  Consequently,
   any parameter that the Server might want to apply to an incoming
   control connection must be configured at the overall Server level and
   applied to all incoming TWAMP-Control connections.

       +---------------------+
       | server              |
       +---------------------+
       | admin-state         |   1..* +------------+
       | server-tcp-port     |<>------| key-chain  |
       | servwait            |        +------------+
       | control-packet-dscp |        | key-id     |
       | count               |        | secret-key |
       | max-count           |        +------------+
       | modes               |
       |                     |   0..* +--------------------------+
       |                     |<>------| ctrl-connection          |
       +---------------------+        +--------------------------+
                                      | client-ip           {ro} |
                                      | client-tcp-port     {ro} |
                                      | server-ip           {ro} |
                                      | server-tcp-port     {ro} |
                                      | state               {ro} |
                                      | control-packet-dscp {ro} |
                                      | selected-mode       {ro} |
                                      | key-id              {ro} |
                                      | count               {ro} |
                                      | max-count           {ro} |
                                      | salt                {ro} |
                                      | server-iv           {ro} |
                                      | challenge           {ro} |
                                      +--------------------------+

                  Figure 4: TWAMP Server UML Class Diagram

   Each server container holds a list named "key-chain", which relates
   key-id with the respective secret-key.  As mentioned in Section 4.1,
   both the Server and the Control-Client use the same mapping from
   key-id to the shared secret-key; in order for this to work properly,
   key-id must be unique across all the systems in the administrative
   domain.  The Server, being prepared to conduct sessions with more
   than one Control-Client, uses key-id to choose the appropriate
   secret-key; a Control-Client would typically have different secret
   keys for different Servers.  key-id tells the Server which shared
   secret-key the Control-Client wishes to use for authentication or
   encryption.

   Each incoming control connection active on the Server is represented
   by a ctrl-connection.  There SHALL be one ctrl-connection per
   incoming TWAMP-Control (TCP) connection that is received and active
   on the Server.  Each ctrl-connection can be uniquely identified by
   the 4-tuple {client-ip, client-tcp-port, server-ip, server-tcp-
   port}.  All items in the ctrl-connection list are read-only.

4.3.  Session-Sender

   The session-sender container, illustrated in Figure 5, holds items
   that are related to the configuration of the TWAMP Session-Sender
   logical entity.

   The session-sender container includes an administrative parameter
   (session-sender/admin-state) that controls whether the device is
   allowed to initiate TWAMP-Test sessions.

         +----------------+
         | session-sender |
         +----------------+  0..* +---------------------------+
         | admin-state    |<>-----| test-session              |
         +----------------+       +---------------------------+
                                  | name                      |
                                  | ctrl-connection-name {ro} |
                                  | fill-mode                 |
                                  | number-of-packets         |
                                  | state                {ro} |
                                  | sent-packets         {ro} |
                                  | rcv-packets          {ro} |
                                  | last-sent-seq        {ro} |
                                  | last-rcv-seq         {ro} |
                                  +---------------------------+
                                               ^
                                               V
                                               | 1
                                   +---------------------+
                                   | packet-distribution |
                                   +---------------------+
                                   | periodic / poisson  |
                                   +---------------------+
                                       |           |
                            +-------------------+  |
                            | periodic-interval |  |
                            +-------------------+  |
                                                   |
                                           +--------------+
                                           | lambda       |
                                           | max-interval |
                                           +--------------+

              Figure 5: TWAMP Session-Sender UML Class Diagram

   Each TWAMP-Test session initiated by the Session-Sender will be
   represented by an instance of a test-session object.  There SHALL be
   one instance of test-session for each TWAMP-Test session for which
   packets are being sent.

4.4.  Session-Reflector

   The session-reflector container, illustrated in Figure 6, holds items
   that are related to the configuration of the TWAMP Session-Reflector
   logical entity.

   The session-reflector container includes an administrative parameter
   (session-reflector/admin-state) that controls whether the device is
   allowed to respond to incoming TWAMP-Test sessions.

   A device operating in the Session-Reflector Role cannot configure
   attributes on a per-session basis, as it has no foreknowledge of what
   incoming sessions it will receive.  As such, any parameter that the
   Session-Reflector might want to apply to an incoming TWAMP-Test
   session must be configured at the overall Session-Reflector level and
   applied to all incoming sessions.

                +-------------------+
                | session-reflector |
                +-------------------+
                | admin-state       |
                | refwait           |
                +-------------------+
                         ^
                         V
                         |
                         | 0..*
                +----------------------------------------+
                | test-session                           |
                +----------------------------------------+
                | sid                               {ro} |
                | sender-ip                         {ro} |
                | sender-udp-port                   {ro} |
                | reflector-ip                      {ro} |
                | reflector-udp-port                {ro} |
                | parent-connection-client-ip       {ro} |
                | parent-connection-client-tcp-port {ro} |
                | parent-connection-server-ip       {ro} |
                | parent-connection-server-tcp-port {ro} |
                | test-packet-dscp                  {ro} |
                | sent-packets                      {ro} |
                | rcv-packets                       {ro} |
                | last-sent-seq                     {ro} |
                | last-rcv-seq                      {ro} |
                +----------------------------------------+

            Figure 6: TWAMP Session-Reflector UML Class Diagram

   Each incoming TWAMP-Test session that is active on the Session-
   Reflector SHALL be represented by an instance of a test-session
   object.  All items in the test-session object are read-only.

   Instances of test-session are indexed by a Session Identifier (SID)
   (the sid parameter).  This SID value is auto-allocated by the TWAMP
   Server as test session requests are received and is communicated back
   to the Control-Client in the SID field of the Accept-Session message;
   see Section 4.3 of "Two-Way Active Measurement Protocol (TWAMP)
   Reflect Octets and Symmetrical Size Features" [RFC6038].

   When attempting to retrieve operational data for active test sessions
   from a Session-Reflector device, the user will not know what sessions
   are currently active on that device or what SIDs have been
   auto-allocated for these test sessions.  If the user has network
   access to the Control-Client device, then it is possible to read the
   data for this session under client/ctrl-connection/test-session-
   request/sid and obtain the SID (see Figure 3).  The user may then use
   this SID value as an index to retrieve an individual session-
   reflector/test-session instance on the Session-Reflector device.

   If the user has no network access to the Control-Client device, then
   the only option is to retrieve all test-session instances from the
   Session-Reflector device and then pick out specific test-session
   instances of interest to the user.  This could be problematic if a
   large number of test sessions are currently active on that device.

   Each Session-Reflector TWAMP-Test session contains the following
   4-tuple: {parent-connection-client-ip, parent-connection-client-tcp-
   port, parent-connection-server-ip, parent-connection-server-tcp-
   port}.  This 4-tuple MUST correspond to the equivalent 4-tuple
   {client-ip, client-tcp-port, server-ip, server-tcp-port} in
   server/ctrl-connection.  This 4-tuple allows the user to trace back
   from the TWAMP-Test session to the (parent) TWAMP-Control connection
   that negotiated this test session.

5.  Data Model

   This section formally specifies the TWAMP data model using YANG.

5.1.  YANG Tree Diagram

   This section presents a simplified graphical representation of the
   TWAMP data model using a YANG tree diagram.  Readers should keep in
   mind that the limit of 72 characters per line forces us to introduce
   artificial line breaks in some tree diagram nodes.  Tree diagrams
   used in this document follow the notation defined in "YANG Tree
   Diagrams" [RFC8340].

   Please note that the backslash ('\') character near the end of the
   diagram is used for formatting purposes only (i.e.,
   "reflector-udp-port]" should be treated as part of the same line as
   "[sender-ip sender-udp-port reflector-ip").

   module: ietf-twamp
     +--rw twamp
        +--rw client {control-client}?
        |  +--rw admin-state?             boolean
        |  +--rw mode-preference-chain* [priority]
        |  |  +--rw priority    uint16
        |  |  +--rw mode?       twamp-modes
        |  +--rw key-chain* [key-id]
        |  |  +--rw key-id        string
        |  |  +--rw secret-key?   binary
        |  +--rw ctrl-connection* [name]
        |     +--rw name                    string
        |     +--rw client-ip?              inet:ip-address
        |     +--rw server-ip               inet:ip-address
        |     +--rw server-tcp-port?        inet:port-number
        |     +--rw control-packet-dscp?    inet:dscp
        |     +--rw key-id?                 string
        |     +--rw max-count-exponent?     uint8
        |     +--ro client-tcp-port?        inet:port-number
        |     +--ro server-start-time?      uint64
        |     +--ro repeat-count?           uint64
        |     +--ro state?
        |     |       control-client-connection-state
        |     +--ro selected-mode?          twamp-modes
        |     +--ro token?                  binary
        |     +--ro client-iv?              binary
        |     +--rw test-session-request* [name]
        |        +--rw name                  string
        |        +--rw sender-ip?            inet:ip-address
        |        +--rw sender-udp-port?      union
        |        +--rw reflector-ip          inet:ip-address
        |        +--rw reflector-udp-port?   inet:port-number
        |        +--rw timeout?              uint64
        |        +--rw padding-length?       uint32
        |        +--rw test-packet-dscp?     inet:dscp
        |        +--rw start-time?           uint64
        |        +--rw repeat?               uint32
        |        +--rw repeat-interval?      uint32
        |        +--rw pm-reg-list* [pm-index]
        |        |  +--rw pm-index    uint16
        |        +--ro state?                test-session-state
        |        +--ro sid?                  string
        +--rw server {server}?
        |  +--rw admin-state?           boolean
        |  +--rw server-tcp-port?       inet:port-number
        |  +--rw servwait?              uint32
        |  +--rw control-packet-dscp?   inet:dscp
        |  +--rw count?                 uint8
        |  +--rw max-count-exponent?    uint8
        |  +--rw modes?                 twamp-modes
        |  +--rw key-chain* [key-id]
        |  |  +--rw key-id        string
        |  |  +--rw secret-key?   binary
        |  +--ro ctrl-connection*
        |          [client-ip client-tcp-port server-ip server-tcp-port]
        |     +--ro client-ip              inet:ip-address
        |     +--ro client-tcp-port        inet:port-number
        |     +--ro server-ip              inet:ip-address
        |     +--ro server-tcp-port        inet:port-number
        |     +--ro state?                 server-ctrl-connection-state
        |     +--ro control-packet-dscp?   inet:dscp
        |     +--ro selected-mode?         twamp-modes
        |     +--ro key-id?                string
        |     +--ro count?                 uint8
        |     +--ro max-count-exponent?    uint8
        |     +--ro salt?                  binary
        |     +--ro server-iv?             binary
        |     +--ro challenge?             binary
        +--rw session-sender {session-sender}?
        |  +--rw admin-state?    boolean
        |  +--rw test-session* [name]
        |     +--rw name                    string
        |     +--ro ctrl-connection-name?   string
        |     +--rw fill-mode?              padding-fill-mode
        |     +--rw number-of-packets       uint32
        |     +--rw (packet-distribution)?
        |     |  +--:(periodic)
        |     |  |  +--rw periodic-interval       decimal64
        |     |  +--:(poisson)
        |     |     +--rw lambda                  decimal64
        |     |     +--rw max-interval?           decimal64
        |     +--ro state?                  sender-session-state
        |     +--ro sent-packets?           uint32
        |     +--ro rcv-packets?            uint32
        |     +--ro last-sent-seq?          uint32
        |     +--ro last-rcv-seq?           uint32
        +--rw session-reflector {session-reflector}?
           +--rw admin-state?    boolean
           +--rw refwait?        uint32
           +--ro test-session*
                   [sender-ip sender-udp-port reflector-ip \
                    reflector-udp-port]
              +--ro sid?                                string
              +--ro sender-ip                           inet:ip-address
              +--ro sender-udp-port
              |       dynamic-port-number
              +--ro reflector-ip                        inet:ip-address
              +--ro reflector-udp-port                  inet:port-number
              +--ro parent-connection-client-ip?        inet:ip-address
              +--ro parent-connection-client-tcp-port?  inet:port-number
              +--ro parent-connection-server-ip?        inet:ip-address
              +--ro parent-connection-server-tcp-port?  inet:port-number
              +--ro test-packet-dscp?                   inet:dscp
              +--ro sent-packets?                       uint32
              +--ro rcv-packets?                        uint32
              +--ro last-sent-seq?                      uint32
              +--ro last-rcv-seq?                       uint32

                        Figure 7: YANG Tree Diagram

5.2.  YANG Module

   This section presents the YANG module for the TWAMP data model
   defined in this document.  The module imports definitions from
   "Common YANG Data Types" [RFC6991] and references "Framework for IP
   Performance Metrics" [RFC2330], "Network performance measurement with
   periodic streams" [RFC3432], "A One-way Active Measurement Protocol
   (OWAMP)" [RFC4656], "A Two-Way Active Measurement Protocol (TWAMP)"
   [RFC5357], "Mixed Security Mode for the Two-Way Active Measurement
   Protocol (TWAMP)" [RFC5618], "Network Time Protocol Version 4:
   Protocol and Algorithms Specification" [RFC5905], "Individual Session
   Control Feature for the Two-Way Active Measurement Protocol (TWAMP)"
   [RFC5938], "Two-Way Active Measurement Protocol (TWAMP) Reflect
   Octets and Symmetrical Size Features" [RFC6038], "Advanced Stream and
   Sampling Framework for IP Performance Metrics (IPPM)" [RFC7312],
   "IKEv2-Derived Shared Secret Key for the One-Way Active Measurement
   Protocol (OWAMP) and Two-Way Active Measurement Protocol (TWAMP)"
   [RFC7717], "Well-Known Port Assignments for the One-Way Active
   Measurement Protocol (OWAMP) and the Two-Way Active Measurement
   Protocol (TWAMP)" [RFC8545], and "Registry for Performance Metrics"
   [RFC8911].

   <CODE BEGINS> file "ietf-twamp@2021-11-17.yang"
   module ietf-twamp {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-twamp";
     prefix ietf-twamp;

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

     organization
       "IETF IPPM (IP Performance Metrics) Working Group";
     contact
       "WG Web: <https://datatracker.ietf.org/wg/ippm/documents/>
        WG List: <mailto:ippm@ietf.org>

        Editor: Ruth Civil
                <mailto:ruthcivil@gmail.com>

        Editor: Al Morton
                <mailto:acmorton@att.com>

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

        Editor: Mahesh Jethanandani
                <mailto:mjethanandani@gmail.com>

        Editor: Kostas Pentikousis
                <mailto:kostas.pentikousis@detecon.com>";
     description
       "This YANG module specifies a vendor-independent data
        model for the Two-Way Active Measurement Protocol (TWAMP).

        The data model defines four TWAMP logical entities, namely
        Control-Client, Server, Session-Sender, and Session-Reflector,
        as illustrated in the annotated TWAMP logical model (Figure 1
        of RFC 8913).

        This YANG module uses features to indicate which of the four
        logical entities are supported by a TWAMP implementation.

        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) 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 8913; see the
        RFC itself for full legal notices.";

     revision 2021-11-17 {
       description
         "Initial revision.

          References RFC 5357, RFC 5618, RFC 5938, RFC 6038, RFC 7717,
          and RFC 8911.";
       reference
         "RFC 8913: Two-Way Active Measurement Protocol (TWAMP) YANG
          Data Model";
     }

     /*
      * Typedefs
      */

     typedef twamp-modes {
       type bits {
         bit unauthenticated {
           position 0;
           description
             "Unauthenticated mode, in which no encryption or
              authentication is applied in TWAMP-Control and
              TWAMP-Test.  KeyID, Token, and Client-IV are not used in
              the Set-Up-Response message.  See Section 3.1 of
              RFC 4656.";
           reference
             "RFC 4656: A One-way Active Measurement Protocol (OWAMP),
                        Section 3.1";
         }
         bit authenticated {
           position 1;
           description
             "Authenticated mode, in which the Control-Client and
              Server possess a shared secret, thus prohibiting
              'theft of service'.  As per Section 6 of RFC 4656,
              in 'authenticated mode, the timestamp is in the clear
              and is not protected cryptographically in any way,
              while the rest of the message has the same protection
              as in encrypted mode.  This mode allows one to trade off
              cryptographic protection against accuracy of
              timestamps.'";
           reference
             "RFC 4656: A One-way Active Measurement Protocol (OWAMP),
                        Section 6";
         }
         bit encrypted {
           position 2;
           description
             "Encrypted mode 'makes it impossible to alter
              timestamps undetectably' (Section 1 of RFC 4656).
              See also Section 4 of RFC 7717.";
           reference
             "RFC 4656: A One-way Active Measurement Protocol (OWAMP),
                        Section 6
              RFC 7717: IKEv2-Derived Shared Secret Key for the One-Way
              Active Measurement Protocol (OWAMP) and Two-Way Active
              Measurement Protocol (TWAMP), Section 4";
         }
         bit unauth-test-encrypt-control {
           position 3;
           description
             "When using the mixed security mode, the TWAMP-Test
              protocol operates in unauthenticated mode and the
              TWAMP-Control protocol operates in encrypted mode.";
           reference
             "RFC 5618: Mixed Security Mode for the Two-Way Active
              Measurement Protocol (TWAMP)";
         }
         bit individual-session-control {
           position 4;
           description
             "This mode enables individual test sessions using
              Session Identifiers.";
           reference
             "RFC 5938: Individual Session Control Feature
              for the Two-Way Active Measurement Protocol (TWAMP)";
         }
         bit reflect-octets {
           position 5;
           description
             "This mode indicates the reflect octets capability.";
           reference
             "RFC 6038: Two-Way Active Measurement Protocol (TWAMP)
              Reflect Octets and Symmetrical Size Features";
         }
         bit symmetrical-size {
           position 6;
           description
             "This mode indicates support for the symmetrical size
              sender test packet format.";
           reference
             "RFC 6038: Two-Way Active Measurement Protocol (TWAMP)
              Reflect Octets and Symmetrical Size Features";
         }
         bit IKEv2Derived {
           position 7;
           description
             "In this mode, the shared key is derived
              from an Internet Key Exchange Protocol Version 2 (IKEv2)
              security association (SA).";
           reference
             "RFC 7717: IKEv2-Derived Shared Secret Key for
              the One-Way Active Measurement Protocol (OWAMP)
              and Two-Way Active Measurement Protocol (TWAMP)";
         }
       }
       description
         "Specifies the configurable TWAMP-Modes supported during a
          TWAMP-Control connection setup between a Control-Client
          and a Server.  Section 7 of RFC 7717 summarizes the
          'TWAMP-Modes' Registry and points to their
          formal specification.";
     }

     typedef control-client-connection-state {
       type enumeration {
         enum active {
           description
             "Indicates an active TWAMP-Control connection to the
              Server.";
         }
         enum idle {
           description
             "Indicates an idle TWAMP-Control connection to the
              Server.";
         }
       }
       description
         "Indicates the Control-Client TWAMP-Control connection
          state.";
     }

     typedef test-session-state {
       type enumeration {
         enum accepted {
           value 0;
           description
             "Indicates an accepted TWAMP-Test session request.";
         }
         enum failed {
           value 1;
           description
             "Indicates a TWAMP-Test session failure due to
              some unspecified reason (catch-all).";
         }
         enum internal-error {
           value 2;
           description
             "Indicates a TWAMP-Test session failure due to
              an internal error.";
         }
         enum not-supported {
           value 3;
           description
             "Indicates a TWAMP-Test session failure because
              some aspect of the TWAMP-Test session request
              is not supported.";
         }
         enum permanent-resource-limit {
           value 4;
           description
             "Indicates a TWAMP-Test session failure due to
              permanent resource limitations.";
         }
         enum temp-resource-limit {
           value 5;
           description
             "Indicates a TWAMP-Test session failure due to
              temporary resource limitations.";
         }
       }
       description
         "Indicates the Control-Client TWAMP-Test session state.";
     }

     typedef server-ctrl-connection-state {
       type enumeration {
         enum active {
           description
             "Indicates an active TWAMP-Control connection
              to the Control-Client.";
         }
         enum servwait {
           description
             "Indicates that the TWAMP-Control connection to the
              Control-Client is in SERVWAIT as per the definition in
              Section 3.1 of RFC 5357.";
           reference
             "RFC 5357: A Two-Way Active Measurement Protocol (TWAMP),
                        Section 3.1";
         }
       }
       description
         "Indicates the Server TWAMP-Control connection state.";
     }

     typedef sender-session-state {
       type enumeration {
         enum active {
           description
             "Indicates that the TWAMP-Test session is active.";
         }
         enum failure {
           description
             "Indicates that the TWAMP-Test session has failed.";
         }
       }
       description
         "Indicates the Session-Sender TWAMP-Test session state.";
     }

     typedef padding-fill-mode {
       type enumeration {
         enum zero {
           description
             "TWAMP-Test packets are padded with all zeros.";
         }
         enum random {
           description
             "TWAMP-Test packets are padded with pseudorandom
              numbers.";
         }
       }
       description
         "Indicates what type of packet padding is used in the
          TWAMP-Test packets.";
     }

     typedef dynamic-port-number {
       type inet:port-number {
         range "49152..65535";
       }
       description
         "Dynamic range for port numbers.";
     }

     /*
      * Features
      */

     feature control-client {
       description
         "Indicates that the device supports configuration of the
          TWAMP Control-Client logical entity.";
     }

     feature server {
       description
         "Indicates that the device supports configuration of the
          TWAMP Server logical entity.";
     }

     feature session-sender {
       description
         "Indicates that the device supports configuration of the
          TWAMP Session-Sender logical entity.";
     }

     feature session-reflector {
       description
         "Indicates that the device supports configuration of the
          TWAMP Session-Reflector logical entity.";
     }

     /*
      * Reusable node groups
      */

     grouping key-management {
       list key-chain {
         key "key-id";
         leaf key-id {
           type string {
             length "1..80";
           }
           description
             "KeyID used for a TWAMP-Control connection.  As per
              Section 3.1 of RFC 4656, KeyID is 'a UTF-8 string, up to
              80 octets in length' and is used to select which 'shared
              secret the client' (Control-Client) 'wishes to use to
              authenticate or encrypt'.";
         }
         leaf secret-key {
           type binary;
           description
             "The secret key corresponding to the KeyID for this
              TWAMP-Control connection.";
         }
         description
           "Relates KeyIDs with their respective secret keys
            in a TWAMP-Control connection.";
       }
       description
         "Used by the Control-Client and Server for TWAMP-Control
          key management.";
     }

     grouping maintenance-statistics {
       leaf sent-packets {
         type uint32;
         config false;
         description
           "Indicates the number of packets sent.";
       }
       leaf rcv-packets {
         type uint32;
         config false;
         description
           "Indicates the number of packets received.";
       }
       leaf last-sent-seq {
         type uint32;
         config false;
         description
           "Indicates the last sent sequence number.";
       }
       leaf last-rcv-seq {
         type uint32;
         config false;
         description
           "Indicates the last received sequence number.";
       }
       description
         "Used for TWAMP-Test maintenance statistics.";
     }

     grouping count {
       leaf count {
         type uint8 {
           range "10..31";
         }
         default "15";
         description
           "Parameter communicated to the Control-Client as part of
            the Server Greeting message and used for deriving a key
            from a shared secret as per Section 3.1 of RFC 4656:
            MUST be a power of 2 and at least 1024.  It is configured
            by providing said power.  For example, configuring 20 here
            means count 2^20 = 1048576.  The default is 15,
            meaning 2^15 = 32768.";
       }
       description
         "Reusable data structure for count, which is used in both the
          Server and the Control-Client.";
     }

     grouping max-count-exponent {
       leaf max-count-exponent {
         type uint8 {
           range "10..31";
         }
         default "20";
         description
           "This parameter limits the maximum Count value, which MUST
            be a power of 2 and at least 1024 as per RFC 5357.  It is
            configured by providing said power.  For example,
            configuring 10 here means max count 2^10 = 1024.
            The default is 20, meaning 2^20 = 1048576.

            A TWAMP Server uses this configured value in the
            Server Greeting message sent to the Control-Client.

            A TWAMP Control-Client uses this configured value to
            prevent denial-of-service (DoS) attacks by closing the
            control connection to the Server if it 'receives a
            Server-Greeting message with Count greater that [sic] its
            maximum configured value', as per Section 6 of RFC 5357.

            Further, note that according to Section 6 of RFC 5357:

            'If an attacking system set the maximum value in Count
            (2**32), then the system under attack would stall for a
            significant period of time while it attempts to generate
            keys.  Therefore, TWAMP-compliant systems SHOULD have a
            configuration control to limit the maximum Count value.
            The default maximum Count value SHOULD be 32768.'

            In the case of this document, the default max-count-exponent
            value SHOULD be 15, which corresponds to a maximum value of
            2**15 or 32768.

            RFC 5357 does not qualify 'significant period' in terms of
            time, but it is clear that this depends on the processing
            capacity available, and operators need to pay attention to
            this security consideration.";
       }
       description
         "Reusable data structure for max-count that is used in both
          the client (Control-Client) container and the server
          container.";
     }

     /*
      * Configuration data nodes
      */

     container twamp {
       description
         "TWAMP logical entity configuration grouping of four models
          that correspond to the four TWAMP logical entities
          Control-Client, Server, Session-Sender, and Session-Reflector
          as illustrated in Figure 1 of RFC 8913.";
       container client {
         if-feature "control-client";
         description
           "Configuration of the TWAMP Control-Client logical entity.";
         leaf admin-state {
           type boolean;
           default "true";
           description
             "Indicates whether the device is allowed to operate as a
              TWAMP Control-Client.";
         }
         list mode-preference-chain {
           key "priority";
           unique "mode";
           leaf priority {
             type uint16;
             description
               "Indicates the Control-Client mode preference priority,
                expressed as a 16-bit unsigned integer.  Values for the
                priority start with zero, the highest priority, and
                decreasing priority value is indicated by every increase
                in value by one.";
           }
           leaf mode {
             type twamp-modes;
             description
               "The supported TWAMP-Modes matching the corresponding
                priority.";
           }
           description
             "Indicates the Control-Client preferred order of use of
              the supported TWAMP-Modes.

              Depending on the modes available in the TWAMP Server
              Greeting message (see Figure 2 of RFC 7717), the
              Control-Client MUST choose the highest-priority
              mode from the configured mode-preference-chain list.";
         }
         uses key-management;
         list ctrl-connection {
           key "name";
           description
             "List of TWAMP Control-Client control connections.
              Each item in the list describes a control connection
              that will be initiated by this Control-Client.";
           leaf name {
             type string;
             description
               "A unique name used as a key to identify this
                individual TWAMP-Control connection on the
                Control-Client device.";
           }
           leaf client-ip {
             type inet:ip-address;
             description
               "The IP address of the local Control-Client device,
                to be placed in the source IP address field of the
                IP header in TWAMP-Control (TCP) packets belonging
                to this control connection.  If not configured, the
                device SHALL choose its own source IP address.";
           }
           leaf server-ip {
             type inet:ip-address;
             mandatory true;
             description
               "The IP address of the remote Server device to which
                the TWAMP-Control connection will be initiated.";
           }
           leaf server-tcp-port {
             type inet:port-number;
             default "862";
             description
               "This parameter defines the TCP port number that is
                to be used by this outgoing TWAMP-Control connection.
                Typically, this is the well-known TWAMP-Control
                port number (862) as per RFC 5357.  However, there are
                known realizations of TWAMP in the field that were
                implemented before this well-known port number was
                allocated.  These early implementations allowed the
                port number to be configured.  This parameter is
                therefore provided for backward-compatibility
                reasons.";
           }
           leaf control-packet-dscp {
             type inet:dscp;
             default "0";
             description
               "The Differentiated Services Code Point (DSCP) value
                to be placed in the IP header of TWAMP-Control (TCP)
                packets generated by this Control-Client.";
           }
           leaf key-id {
             type string {
               length "1..80";
             }
             description
               "Indicates the KeyID value selected for this
                TWAMP-Control connection.";
           }
           uses max-count-exponent;
           leaf client-tcp-port {
             type inet:port-number;
             config false;
             description
               "Indicates the source TCP port number used in the
                TWAMP-Control packets belonging to this control
                connection.";
           }
           leaf server-start-time {
             type uint64;
             config false;
             description
               "Indicates the Start-Time advertised by the Server in
                the Server-Start message (RFC 4656, Section 3.1),
                representing the time when the current
                instantiation of the Server started operating.
                The timestamp format follows RFC 5905, according to
                Section 4.1.2 of RFC 4656.";
             reference
               "RFC 4656: A One-way Active Measurement Protocol (OWAMP),
                Sections 3.1 and 4.1.2
                RFC 5905: Network Time Protocol Version 4: Protocol and
                Algorithms Specification";
           }
           leaf repeat-count {
             type uint64;
             config false;
             description
               "Indicates how many times the test session has been
                repeated.  When a test is running, this value will be
                greater than 0.  If the repeat parameter is non-zero,
                this value is smaller than or equal to the repeat
                parameter.";
           }
           leaf state {
             type control-client-connection-state;
             config false;
             description
               "Indicates the current TWAMP-Control connection state.";
           }
           leaf selected-mode {
             type twamp-modes;
             config false;
             description
               "The TWAMP-Modes that the Control-Client has chosen for
                this control connection as set in the Mode field of
                the Set-Up-Response message.";
             reference
               "RFC 4656: A One-way Active Measurement Protocol (OWAMP),
                          Section 3.1";
           }
           leaf token {
             type binary {
               length "64";
             }
             config false;
             description
               "This parameter holds the 64 octets containing the
                concatenation of a 16-octet Challenge, a 16-octet AES
                Session-key used for encryption, and a 32-octet
                HMAC-SHA1 Session-key used for authentication; see
                also the last paragraph of Section 6.10 of RFC 4656.

                If the mode defined in RFC 7717 is selected
                (selected-mode), Token is limited to 16 octets.";
             reference
               "RFC 4656: A One-way Active Measurement Protocol (OWAMP),
                          Section 6.10
                RFC 7717: IKEv2-Derived Shared Secret Key for the
                One-Way Active Measurement Protocol (OWAMP) and
                Two-Way Active Measurement Protocol (TWAMP)";
           }
           leaf client-iv {
             type binary {
               length "16";
             }
             config false;
             description
               "Indicates the Control-Client Initialization Vector
                (Client-IV), which is generated randomly by the
                Control-Client.  As per RFC 4656:

                'Client-IV merely needs to be unique (i.e., it MUST
                never be repeated for different sessions using the
                same secret key; a simple way to achieve that without
                the use of cumbersome state is to generate the
                Client-IV values using a cryptographically secure
                pseudo-random number source.'

                If the mode defined in RFC 7717 is selected
                (selected-mode), Client-IV is limited to 12 octets.";
             reference
               "RFC 4656: A One-way Active Measurement Protocol (OWAMP)
                RFC 7717: IKEv2-Derived Shared Secret Key for the
                One-Way Active Measurement Protocol (OWAMP) and
                Two-Way Active Measurement Protocol (TWAMP)";
           }
           list test-session-request {
             key "name";
             description
               "Information associated with the Control-Client
                for this test session.";
             leaf name {
               type string;
               description
                 "A unique name to be used for identification of
                  this TWAMP-Test session on the Control-Client.";
             }
             leaf sender-ip {
               type inet:ip-address;
               description
                 "The IP address of the Session-Sender device,
                  which is to be placed in the source IP address
                  field of the IP header in TWAMP-Test (UDP) packets
                  belonging to this test session.  This value will be
                  used to populate the Sender Address field of the
                  Request-TW-Session message.

                  If not configured, the device SHALL choose its own
                  source IP address.";
             }
             leaf sender-udp-port {
               type union {
                 type dynamic-port-number;
                 type enumeration {
                   enum autoallocate {
                     description
                       "Indicates that the Control-Client will
                        auto-allocate the TWAMP-Test (UDP) port number
                        from the dynamic port range.";
                   }
                 }
               }
               default "autoallocate";
               description
                 "The UDP port number that is to be used by
                  the Session-Sender for this TWAMP-Test session.
                  The number is restricted to the dynamic port range.

                  By default, the Control-Client SHALL auto-allocate a
                  UDP port number for this TWAMP-Test session.

                  The configured (or auto-allocated) value is
                  advertised in the Sender Port field of the
                  Request-TW-Session message (see Section 3.5 of
                  RFC 5357).  Note that in the scenario where a device
                  auto-allocates a UDP port number for a session and
                  the repeat parameter for that session indicates that
                  it should be repeated, the device is free to
                  auto-allocate a different UDP port number when it
                  negotiates the next (repeated) iteration of this
                  session.";
             }
             leaf reflector-ip {
               type inet:ip-address;
               mandatory true;
               description
                 "The IP address belonging to the remote
                  Session-Reflector device to which the TWAMP-Test
                  session will be initiated.  This value will be
                  used to populate the Receiver Address field of
                  the Request-TW-Session message.";
             }
             leaf reflector-udp-port {
               type inet:port-number {
                 range "862 | 49152..65535";
               }
               description
                 "This parameter defines the UDP port number that
                  will be used by the Session-Reflector for
                  this TWAMP-Test session.  The default number is
                  within the dynamic port range and is to be placed
                  in the Receiver Port field of the Request-TW-Session
                  message.  The well-known port (862) MAY be used.";
               reference
                 "RFC 8545: Well-Known Port Assignments for the One-Way
                  Active Measurement Protocol (OWAMP) and the Two-Way
                  Active Measurement Protocol (TWAMP)";
             }
             leaf timeout {
               type uint64;
               units "seconds";
               default "2";
               description
                 "The length of time (in seconds) that the
                  Session-Reflector should continue to respond to
                  packets belonging to this TWAMP-Test session after
                  a Stop-Sessions TWAMP-Control message has been
                  received.

                  This value will be placed in the Timeout field of
                  the Request-TW-Session message.";
               reference
                 "RFC 5357: A Two-Way Active Measurement Protocol
                  (TWAMP), Section 3.5";
             }
             leaf padding-length {
               type uint32 {
                 range "64..4096";
               }
               description
                 "The number of padding bytes to be added to the
                  TWAMP-Test (UDP) packets generated by the
                  Session-Sender.

                  This value will be placed in the Padding Length
                  field of the Request-TW-Session message.";
               reference
                 "RFC 4656: A One-way Active Measurement Protocol
                  (OWAMP), Section 3.5";
             }
             leaf test-packet-dscp {
               type inet:dscp;
               default "0";
               description
                 "The DSCP value to be placed in the IP header
                  of TWAMP-Test packets generated by the
                  Session-Sender and in the UDP header of the
                  TWAMP-Test response packets generated by the
                  Session-Reflector for this test session.

                  This value will be placed in the Type-P Descriptor
                  field of the Request-TW-Session message.";
               reference
                 "RFC 5357: A Two-Way Active Measurement Protocol
                  (TWAMP)";
             }
             leaf start-time {
               type uint64;
               default "0";
               description
                 "Time when the session is to be started
                  (but not before the TWAMP Start-Sessions command
                  is issued; see Section 3.4 of RFC 5357).

                  The start-time value is placed in the Start Time
                  field of the Request-TW-Session message.

                  The timestamp format follows RFC 5905 as per
                  Section 3.5 of RFC 4656.

                  The default value of 0 indicates that the session
                  will be started as soon as the Start-Sessions
                  message is received.";
             }
             leaf repeat {
               type uint32 {
                 range "0..4294967295";
               }
               default "0";
               description
                 "This value determines if the TWAMP-Test session must
                  be repeated.  When a test session has completed, the
                  repeat parameter is checked.

                  The default value of 0 indicates that the session
                  MUST NOT be repeated.

                  If the repeat value is 1 through 4,294,967,294,
                  then the test session SHALL be repeated using the
                  information in the repeat-interval parameter, and the
                  parent TWAMP-Control connection for this test
                  session is restarted to negotiate a new instance
                  of this TWAMP-Test session.

                  A value of 4,294,967,295 indicates that the test
                  session SHALL be repeated *forever* using the
                  information in the repeat-interval parameter and
                  SHALL NOT decrement the value.";
             }
             leaf repeat-interval {
               when "../repeat!='0'" {
                 description
                   "This parameter determines the timing of repeated
                    TWAMP-Test sessions when repeat is more than 0.

                    When the value of repeat-interval is 0, the
                    negotiation of a new test session SHALL begin
                    immediately after the previous test session
                    completes.  Otherwise, the Control-Client will
                    wait for the number of seconds specified in the
                    repeat-interval parameter before negotiating the
                    new instance of this TWAMP-Test session.";
               }
               type uint32;
               units "seconds";
               default "0";
               description
                 "Repeat interval (in seconds).";
             }
             list pm-reg-list {
               key "pm-index";
               leaf pm-index {
                 type uint16;
                 description
                   "Numerical index value of a Registered Metric in
                    the Performance Metrics Registry (see RFC 8911).
                    Output statistics are specified in the
                    corresponding Registry Entry.";
               }
               description
                 "A list of one or more Performance Metrics Registry
                  Index values, which communicate packet stream
                  characteristics along with one or more metrics
                  to be measured.

                  All members of the pm-reg-list MUST have the same
                  stream characteristics, such that they combine
                  to specify all metrics that shall be measured on
                  a single stream.";
               reference
                 "RFC 8911: Registry for Performance Metrics";
             }
             leaf state {
               type test-session-state;
               config false;
               description
                 "Indicates the TWAMP-Test session state -- an accepted
                  request or an indication of an error.";
               reference
                 "RFC 5357: A Two-Way Active Measurement Protocol
                  (TWAMP), Section 3.5";
             }
             leaf sid {
               type string;
               config false;
               description
                 "The Session Identifier (SID) allocated by the Server
                  for this TWAMP-Test session and communicated back to
                  the Control-Client in the SID field of the
                  Accept-Session message.";
               reference
                 "RFC 6038: Two-Way Active Measurement Protocol (TWAMP)
                  Reflect Octets and Symmetrical Size
                  Features, Section 4.3";
             }
           }
         }
       }
       container server {
         if-feature "server";
         description
           "Configuration of the TWAMP Server logical entity.";
         leaf admin-state {
           type boolean;
           default "true";
           description
             "Indicates whether the device is allowed to operate
              as a TWAMP Server.";
         }
         leaf server-tcp-port {
           type inet:port-number;
           default "862";
           description
             "This parameter defines the well-known TCP port number
              that is used by TWAMP-Control.  The Server will listen
              on this port number for incoming TWAMP-Control
              connections.  Although this is defined as a fixed value
              (862) in RFC 5357, there are several realizations of
              TWAMP in the field that were implemented before this
              well-known port number was allocated.  These early
              implementations allowed the port number to be
              configured.  This parameter is therefore provided for
              backward-compatibility reasons.";
         }
         leaf servwait {
           type uint32 {
             range "1..604800";
           }
           units "seconds";
           default "900";
           description
             "TWAMP-Control (TCP) session timeout, in seconds.
              According to Section 3.1 of RFC 5357:

              'The Server MAY discontinue any established control
              connection when no packet associated with that
              connection has been received within SERVWAIT seconds.'";
         }
         leaf control-packet-dscp {
           type inet:dscp;
           description
             "The DSCP value to be placed in the IP header of
              TWAMP-Control (TCP) packets generated by the Server.

              Section 3.1 of RFC 5357 specifies that the Server
              SHOULD use the DSCP value from the Control-Client's
              TCP SYN.  However, for practical purposes, TWAMP will
              typically be implemented using a general-purpose TCP
              stack provided by the underlying operating system,
              and such a stack may not provide this information to the
              user.  Consequently, it is not always possible to
              implement the behavior described in RFC 5357 in an
              OS-portable version of TWAMP.

              The default behavior if this item is not set is to use
              the DSCP value from the Control-Client's TCP SYN.";
           reference
             "RFC 5357: A Two-Way Active Measurement Protocol (TWAMP),
              Section 3.1";
         }
         uses count;
         uses max-count-exponent;
         leaf modes {
           type twamp-modes;
           description
             "The bit mask of TWAMP-Modes this Server instance is
              willing to support; see the IANA 'TWAMP-Modes' Registry.";
         }
         uses key-management;
         list ctrl-connection {
           key "client-ip client-tcp-port server-ip server-tcp-port";
           config false;
           description
             "List of all incoming TWAMP-Control (TCP) connections.";
           leaf client-ip {
             type inet:ip-address;
             description
               "The IP address on the remote Control-Client device,
                which is the source IP address used in the
                TWAMP-Control (TCP) packets belonging to this control
                connection.";
           }
           leaf client-tcp-port {
             type inet:port-number;
             description
               "The source TCP port number used in the TWAMP-Control
                (TCP) packets belonging to this control connection.";
           }
           leaf server-ip {
             type inet:ip-address;
             description
               "The IP address of the local Server device, which is
                the destination IP address used in the
                TWAMP-Control (TCP) packets belonging to this control
                connection.";
           }
           leaf server-tcp-port {
             type inet:port-number;
             description
               "The destination TCP port number used in the
                TWAMP-Control (TCP) packets belonging to this
                control connection.  This will usually be the
                same value as the server-tcp-port configured
                under twamp/server.  However, in the event that
                the user reconfigured server/server-tcp-port
                after this control connection was initiated, this
                value will indicate the server-tcp-port that is
                actually in use for this control connection.";
           }
           leaf state {
             type server-ctrl-connection-state;
             description
               "Indicates the Server TWAMP-Control connection state.";
           }
           leaf control-packet-dscp {
             type inet:dscp;
             description
               "The DSCP value used in the IP header of the
                TWAMP-Control (TCP) packets sent by the Server
                for this control connection.  This will usually
                be the same value as is configured in the
                control-packet-dscp parameter under the twamp/server
                container.  However, in the event that the user
                reconfigures server/dscp after this control
                connection is already in progress, this read-only
                value will show the actual DSCP value in use by this
                TWAMP-Control connection.";
           }
           leaf selected-mode {
             type twamp-modes;
             description
               "The mode that was chosen for this TWAMP-Control
                connection as set in the Mode field of the
                Set-Up-Response message.";
           }
           leaf key-id {
             type string {
               length "1..80";
             }
             description
               "The KeyID value that is in use by this TWAMP-Control
                connection as selected by the Control-Client.";
           }
           uses count {
             description
               "The Count value that is in use by this TWAMP-Control
                connection.  This will usually be the same value
                as is configured under twamp/server.  However, in the
                event that the user reconfigures server/count
                after this control connection is already in progress,
                this read-only value will show the actual count that
                is in use for this TWAMP-Control connection.";
           }
           uses max-count-exponent {
             description
               "This read-only value indicates the actual max-count in
                use for this control connection.  Usually, this would be
                the same value as is configured under twamp/server.";
           }
           leaf salt {
             type binary {
               length "16";
             }
             description
               "A parameter used in deriving a key from a
                shared secret, as described in Section 3.1 of RFC 4656.
                It is communicated to the Control-Client as part of
                the Server Greeting message.";
           }
           leaf server-iv {
             type binary {
               length "16";
             }
             description
               "The Server Initialization Vector (Server-IV)
                generated randomly by the Server.";
           }
           leaf challenge {
             type binary {
               length "16";
             }
             description
               "A random sequence of octets generated by the Server.
                As described in client/token, a Challenge is used
                by the Control-Client to prove possession of a
                shared secret.";
           }
         }
       }
       container session-sender {
         if-feature "session-sender";
         description
           "Configuration of the TWAMP Session-Sender logical entity.";
         leaf admin-state {
           type boolean;
           default "true";
           description
             "Indicates whether the device is allowed to operate
              as a TWAMP Session-Sender.";
         }
         list test-session {
           key "name";
           description
             "List of TWAMP Session-Sender test sessions.";
           leaf name {
             type string;
             description
               "A unique name for this TWAMP-Test session to be used
                for identifying this test session by the
                Session-Sender logical entity.";
           }
           leaf ctrl-connection-name {
             type string;
             config false;
             description
               "The name of the parent TWAMP-Control connection that
                is responsible for negotiating this TWAMP-Test
                session.";
           }
           leaf fill-mode {
             type padding-fill-mode;
             default "zero";
             description
               "Indicates whether the padding added to the
                TWAMP-Test (UDP) packets (1) will contain pseudorandom
                numbers or (2) should consist of all zeros, as per
                Section 4.2.1 of RFC 5357.";
           }
           leaf number-of-packets {
             type uint32;
             mandatory true;
             description
               "The overall number of TWAMP-Test (UDP) packets to be
                transmitted by the Session-Sender for this test
                session.";
           }
           choice packet-distribution {
             description
               "Indicates the distribution to be used for transmitting
                the TWAMP-Test (UDP) packets.";
             case periodic {
               leaf periodic-interval {
                 type decimal64 {
                   fraction-digits 5;
                 }
                 units "seconds";
                 mandatory true;
                 description
                   "Indicates the time to wait (in seconds) between
                    the first bits of TWAMP-Test (UDP) packet
                    transmissions for this test session.";
                 reference
                   "RFC 3432: Network performance measurement with
                    periodic streams";
               }
             }
             case poisson {
               leaf lambda {
                 type decimal64 {
                   fraction-digits 5;
                 }
                 units "seconds";
                 mandatory true;
                 description
                   "Indicates the average time interval (in seconds)
                    between packets in the Poisson distribution.
                    The packet is calculated using the reciprocal of
                    lambda and the TWAMP-Test packet size (which
                    depends on the selected mode and the packet
                    padding).";
                 reference
                   "RFC 2330: Framework for IP Performance Metrics";
               }
               leaf max-interval {
                 type decimal64 {
                   fraction-digits 5;
                 }
                 units "seconds";
                 description
                   "Indicates the maximum time (in seconds)
                    between packet transmissions.";
                 reference
                   "RFC 7312: Advanced Stream and Sampling Framework
                    for IP Performance Metrics (IPPM)";
               }
             }
           }
           leaf state {
             type sender-session-state;
             config false;
             description
               "Indicates the Session-Sender test session state.";
           }
           uses maintenance-statistics;
         }
       }
       container session-reflector {
         if-feature "session-reflector";
         description
           "Configuration of the TWAMP Session-Reflector logical
            entity.";
         leaf admin-state {
           type boolean;
           default "true";
           description
             "Indicates whether the device is allowed to operate
              as a TWAMP Session-Reflector.";
         }
         leaf refwait {
           type uint32 {
             range "1..604800";
           }
           units "seconds";
           default "900";
           description
             "The Session-Reflector MAY discontinue any session that
              has been started when no packet associated with that
              session has been received for REFWAIT seconds.  As per
              Section 3.1 of RFC 5357, this timeout allows a
              Session-Reflector to free up resources in case of
              failure.";
         }
         list test-session {
           key "sender-ip sender-udp-port
                reflector-ip reflector-udp-port";
           config false;
           description
             "TWAMP Session-Reflector test sessions.";
           leaf sid {
             type string;
             description
               "An auto-allocated identifier for this TWAMP-Test
                session that is unique within the context of this
                Server/Session-Reflector device only.  This value
                is communicated to the Control-Client that
                requested the test session in the SID field of the
                Accept-Session message.";
           }
           leaf sender-ip {
             type inet:ip-address;
             description
               "The IP address on the remote device, which is the
                source IP address used in the TWAMP-Test (UDP) packets
                belonging to this test session.";
           }
           leaf sender-udp-port {
             type dynamic-port-number;
             description
               "The source UDP port used in the TWAMP-Test packets
                belonging to this test session.";
           }
           leaf reflector-ip {
             type inet:ip-address;
             description
               "The IP address of the local Session-Reflector
                device, which is the destination IP address used
                in the TWAMP-Test (UDP) packets belonging to this test
                session.";
           }
           leaf reflector-udp-port {
             type inet:port-number {
               range "862 | 49152..65535";
             }
             description
               "The destination UDP port number used in the
                TWAMP-Test (UDP) test packets belonging to this
                test session.";
           }
           leaf parent-connection-client-ip {
             type inet:ip-address;
             description
               "The IP address on the Control-Client device, which
                is the source IP address used in the TWAMP-Control
                (TCP) packets belonging to the parent control
                connection that negotiated this test session.";
           }
           leaf parent-connection-client-tcp-port {
             type inet:port-number;
             description
               "The source TCP port number used in the TWAMP-Control
                (TCP) packets belonging to the parent control
                connection that negotiated this test session.";
           }
           leaf parent-connection-server-ip {
             type inet:ip-address;
             description
               "The IP address of the Server device, which is the
                destination IP address used in the TWAMP-Control
                (TCP) packets belonging to the parent control
                connection that negotiated this test session.";
           }
           leaf parent-connection-server-tcp-port {
             type inet:port-number;
             description
               "The destination TCP port number used in the
                TWAMP-Control (TCP) packets belonging to the parent
                control connection that negotiated this test
                session.";
           }
           leaf test-packet-dscp {
             type inet:dscp;
             description
               "The DSCP value present in the IP header of
                TWAMP-Test (UDP) packets belonging to this session.";
           }
           uses maintenance-statistics;
         }
       }
     }
   }
   <CODE ENDS>

6.  Data Model Examples

   This section presents simple but complete examples of configuring all
   four entities in Figure 1, based on the YANG module specified in
   Section 5.  The examples are illustrative in nature but aim to be
   self-contained, i.e., were they to be executed in a real TWAMP
   implementation, they would lead to correctly configured test
   sessions.  For completeness, examples are provided for both IPv4 and
   IPv6.  The examples are shown using XML [W3C.REC-xml-20081126].

   More elaborate examples, which also include authentication
   parameters, are provided in Appendix A.

6.1.  Control-Client

   Figure 8 shows a configuration example for a Control-Client with
   client/admin-state enabled.  In a real implementation following
   Figure 2, this would permit the initiation of TWAMP-Control
   connections and TWAMP-Test sessions.

   <?xml version="1.0" encoding="utf-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <client>
         <admin-state>true</admin-state>
       </client>
     </twamp>
   </config>

          Figure 8: XML Instance Enabling Control-Client Operation

   The following example shows a Control-Client with two instances of
   client/ctrl-connection -- one called "RouterA" and another called
   "RouterB".  Each TWAMP-Control connection is to a different Server.
   The control connection named "RouterA" has two test session requests.
   The TWAMP-Control connection named "RouterB" has no TWAMP-Test
   session requests.

   <?xml version="1.0" encoding="utf-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <client>
         <admin-state>true</admin-state>
         <ctrl-connection>
           <name>RouterA</name>
           <client-ip>203.0.113.1</client-ip>
           <server-ip>203.0.113.2</server-ip>
           <test-session-request>
             <name>Test1</name>
             <sender-ip>203.0.113.3</sender-ip>
             <sender-udp-port>54001</sender-udp-port>
             <reflector-ip>203.0.113.4</reflector-ip>
             <reflector-udp-port>50001</reflector-udp-port>
             <start-time>0</start-time>
           </test-session-request>
           <test-session-request>
             <name>Test2</name>
             <sender-ip>203.0.113.1</sender-ip>
             <sender-udp-port>54001</sender-udp-port>
             <reflector-ip>203.0.113.2</reflector-ip>
             <reflector-udp-port>50001</reflector-udp-port>
             <start-time>0</start-time>
           </test-session-request>
         </ctrl-connection>
         <ctrl-connection>
           <name>RouterB</name>
           <client-ip>203.0.113.1</client-ip>
           <server-ip>203.0.113.3</server-ip>
         </ctrl-connection>
       </client>
     </twamp>
   </config>

   <?xml version="1.0" encoding="utf-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <client>
         <admin-state>true</admin-state>
         <ctrl-connection>
           <name>RouterA</name>
           <client-ip>2001:db8:203:0:113::1</client-ip>
           <server-ip>2001:db8:203:0:113::2</server-ip>
           <test-session-request>
             <name>Test1</name>
             <sender-ip>2001:db8:203:1:113::3</sender-ip>
             <sender-udp-port>54000</sender-udp-port>
             <reflector-ip>2001:db8:203:1:113::4</reflector-ip>
             <reflector-udp-port>55000</reflector-udp-port>
             <start-time>0</start-time>
           </test-session-request>
           <test-session-request>
             <name>Test2</name>
             <sender-ip>2001:db8:203:0:113::1</sender-ip>
             <sender-udp-port>54001</sender-udp-port>
             <reflector-ip>2001:db8:203:0:113::2</reflector-ip>
             <reflector-udp-port>55001</reflector-udp-port>
             <start-time>0</start-time>
           </test-session-request>
         </ctrl-connection>
         <ctrl-connection>
           <name>RouterB</name>
           <client-ip>2001:db8:203:0:113::1</client-ip>
           <server-ip>2001:db8:203:0:113::3</server-ip>
         </ctrl-connection>
       </client>
     </twamp>
   </config>

6.2.  Server

   Figure 9 shows a configuration example for a Server with
   server/admin-state enabled, which permits a device following Figure 2
   to respond to TWAMP-Control connections and TWAMP-Test sessions.

   <?xml version="1.0" encoding="utf-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <server>
         <admin-state>true</admin-state>
       </server>
     </twamp>
   </config>

              Figure 9: XML Instance Enabling Server Operation

   The following example presents a Server with the TWAMP-Control
   connection corresponding to the control connection name
   (client/ctrl-connection/name) "RouterA" presented in Section 6.1.

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <server>
         <admin-state>true</admin-state>
         <ctrl-connection>
           <client-ip>203.0.113.1</client-ip>
           <client-tcp-port>16341</client-tcp-port>
           <server-ip>203.0.113.2</server-ip>
           <server-tcp-port>862</server-tcp-port>
           <state>active</state>
         </ctrl-connection>
       </server>
     </twamp>
   </data>

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <server>
         <admin-state>true</admin-state>
         <ctrl-connection>
           <client-ip>2001:db8:203:0:113::1</client-ip>
           <client-tcp-port>16341</client-tcp-port>
           <server-ip>2001:db8:203:0:113::2</server-ip>
           <server-tcp-port>862</server-tcp-port>
           <state>active</state>
         </ctrl-connection>
       </server>
     </twamp>
   </data>

6.3.  Session-Sender

   Figure 10 shows a configuration example for a Session-Sender with
   session-sender/admin-state enabled, which permits a device following
   Figure 2 to initiate TWAMP-Test sessions.

   <?xml version="1.0" encoding="utf-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-sender>
         <admin-state>true</admin-state>
       </session-sender>
     </twamp>
   </config>

         Figure 10: XML Instance Enabling Session-Sender Operation

   The following configuration example shows a Session-Sender with the
   two TWAMP-Test sessions presented in Section 6.1.

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-sender>
         <admin-state>true</admin-state>
         <test-session>
           <name>Test1</name>
           <ctrl-connection-name>RouterA</ctrl-connection-name>
           <number-of-packets>900</number-of-packets>
           <periodic-interval>1</periodic-interval>
         </test-session>
         <test-session>
           <name>Test2</name>
           <ctrl-connection-name>RouterA</ctrl-connection-name>
           <number-of-packets>900</number-of-packets>
           <lambda>1</lambda>
           <max-interval>2</max-interval>
         </test-session>
       </session-sender>
     </twamp>
   </data>

6.4.  Session-Reflector

   This configuration example shows a Session-Reflector with
   session-reflector/admin-state enabled, which permits a device
   following Figure 2 to respond to TWAMP-Test sessions.

   <?xml version="1.0" encoding="utf-8"?>
   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-reflector>
         <admin-state>true</admin-state>
       </session-reflector>
     </twamp>
   </config>

        Figure 11: XML Instance Enabling Session-Reflector Operation

   The following example shows the two Session-Reflector TWAMP-Test
   sessions corresponding to the test sessions presented in Section 6.3.

      |  Note: '\' line wrapping is for formatting only.

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-reflector>
         <admin-state>true</admin-state>
         <test-session>
           <sender-ip>203.0.113.3</sender-ip>
           <sender-udp-port>54000</sender-udp-port>
           <reflector-ip>203.0.113.4</reflector-ip>
           <reflector-udp-port>50001</reflector-udp-port>
           <sid>1232</sid>
           <parent-connection-client-ip>203.0.113.1</parent-connection-\
   client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>203.0.113.2</parent-connection-\
   server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <sent-packets>2</sent-packets>
           <rcv-packets>2</rcv-packets>
           <last-sent-seq>1</last-sent-seq>
           <last-rcv-seq>1</last-rcv-seq>
         </test-session>
         <test-session>
           <sender-ip>203.0.113.1</sender-ip>
           <sender-udp-port>54001</sender-udp-port>
           <reflector-ip>192.0.2.2</reflector-ip>
           <reflector-udp-port>50001</reflector-udp-port>
           <sid>178943</sid>
           <parent-connection-client-ip>203.0.113.1</parent-connection-\
   client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>203.0.113.2</parent-connection-\
   server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <sent-packets>21</sent-packets>
           <rcv-packets>21</rcv-packets>
           <last-sent-seq>20</last-sent-seq>
           <last-rcv-seq>20</last-rcv-seq>
         </test-session>
       </session-reflector>
     </twamp>
   </data>

      |  Note: '\' line wrapping is for formatting only.

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-reflector>
         <admin-state>true</admin-state>
         <test-session>
           <sender-ip>203.0.113.3</sender-ip>
           <sender-udp-port>54000</sender-udp-port>
           <reflector-ip>203.0.113.4</reflector-ip>
           <reflector-udp-port>54001</reflector-udp-port>
           <sid>1232</sid>
           <parent-connection-client-ip>203.0.113.1</parent-connection-\
   client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>203.0.113.2</parent-connection-\
   server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <sent-packets>2</sent-packets>
           <rcv-packets>2</rcv-packets>
           <last-sent-seq>1</last-sent-seq>
           <last-rcv-seq>1</last-rcv-seq>
         </test-session>
         <test-session>
           <sender-ip>203.0.113.1</sender-ip>
           <sender-udp-port>54001</sender-udp-port>
           <reflector-ip>192.0.2.2</reflector-ip>
           <reflector-udp-port>55001</reflector-udp-port>
           <sid>178943</sid>
           <parent-connection-client-ip>203.0.113.1</parent-connection-\
   client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>203.0.113.2</parent-connection-\
   server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <sent-packets>21</sent-packets>
           <rcv-packets>21</rcv-packets>
           <last-sent-seq>20</last-sent-seq>
           <last-rcv-seq>20</last-rcv-seq>
         </test-session>
       </session-reflector>
     </twamp>
   </data>

7.  Security Considerations

   Virtually all existing measurement systems using TWAMP [RFC5357] are
   administered by the same network operator.  For example, attacks on
   the measurement infrastructure could be launched by third parties to
   commandeer the packet generation capability, corrupt the
   measurements, or perform other nefarious acts.

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

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

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

   *  If written, the 'admin-state' node can cause unintended test
      sessions to be created.

   *  If the node 'number-of-packets', which dictates how many packets
      are sent in any particular test session, is written with a large
      value, it can cause a test session to run longer than expected.

   *  Nodes that are particularly vulnerable include several timeout
      values put in the protocol to protect against sessions that are
      not active but are consuming resources.  These are the REFWAIT
      timeout parameter, which determines whether to discontinue the
      session if no packets are received; and the nodes 'count' and
      'max-count-exponent', which can cause a long time to be spent on
      Password-Based Key Derivation Function 2 (PBKDF2) iterations.

   *  In addition, a 'dscp' node marked with different DSCP markings can
      cause the test traffic on the network to be skewed and the result
      manipulated.

   *  Finally, nodes within 'mode-preference-chain', which specifies the
      'mode' and 'priority' values and indicates the preferred order of
      use by an operator, can be manipulated to send unauthenticated or
      non-encrypted traffic, enabling an on-path attack.

   *  Limiting access to these nodes will limit the ability to launch an
      attack in network environments.

   Some of the readable data nodes in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to these data nodes.  This is the subtree and data node
   and its sensitivity/vulnerability:

   *  The 'token' node defined in the model, containing a concatenation
      of a Challenge, an AES Session-key used for encryption, and an
      HMAC-SHA1 Session-key used for authentication, is sensitive from a
      privacy perspective and can be used to disrupt a test session.
      The ability to read the field should be limited to the
      administrator of the test network.

   The TWAMP YANG data model does not define RPC operations, as detailed
   in Appendix B, and defers the definition of NETCONF RPC operations to
   each implementation.  These RPC operations, when defined, may be
   considered sensitive or vulnerable in some network environments.  It
   is thus important to control access to these operations.

8.  IANA Considerations

   IANA has registered the following URI in the "IETF XML Registry"
   [RFC3688].

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

   IANA has registered the following YANG module in the "YANG Module
   Names" registry [RFC6020].

   Name:  ietf-twamp
   Namespace:  urn:ietf:params:xml:ns:yang:ietf-twamp
   Prefix:  twamp
   Reference:  RFC 8913

9.  References

9.1.  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>.

   [RFC3432]  Raisanen, V., Grotefeld, G., and A. Morton, "Network
              performance measurement with periodic streams", RFC 3432,
              DOI 10.17487/RFC3432, November 2002,
              <https://www.rfc-editor.org/info/rfc3432>.

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

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
              <https://www.rfc-editor.org/info/rfc4656>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [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>.

   [RFC6038]  Morton, A. and L. Ciavattone, "Two-Way Active Measurement
              Protocol (TWAMP) Reflect Octets and Symmetrical Size
              Features", RFC 6038, DOI 10.17487/RFC6038, October 2010,
              <https://www.rfc-editor.org/info/rfc6038>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

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

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

   [RFC7717]  Pentikousis, K., Ed., Zhang, E., and Y. Cui,
              "IKEv2-Derived Shared Secret Key for the One-Way Active
              Measurement Protocol (OWAMP) and Two-Way Active
              Measurement Protocol (TWAMP)", RFC 7717,
              DOI 10.17487/RFC7717, December 2015,
              <https://www.rfc-editor.org/info/rfc7717>.

   [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>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

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

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8545]  Morton, A., Ed. and G. Mirsky, Ed., "Well-Known Port
              Assignments for the One-Way Active Measurement Protocol
              (OWAMP) and the Two-Way Active Measurement Protocol
              (TWAMP)", RFC 8545, DOI 10.17487/RFC8545, March 2019,
              <https://www.rfc-editor.org/info/rfc8545>.

   [RFC8911]  Bagnulo, M., Claise, B., Eardley, P., Morton, A., and A.
              Akhter, "Registry for Performance Metrics", RFC 8911,
              DOI 10.17487/RFC8911, November 2021,
              <https://www.rfc-editor.org/info/rfc8911>.

   [UML]      ISO/IEC, "Information technology - Open Distributed
              Processing - Unified Modeling Language (UML) Version
              1.4.2", ISO/IEC 19501:2005, OMG-UML VER 1.3, April 2005.

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

9.2.  Informative References

   [NSC]      John, W., Pentikousis, K., Agapiou, G., Jacob, E., Kind,
              M., Manzalini, A., Risso, F., Staessens, D., Steinert, R.,
              and C. Meirosu, "Research directions in network service
              chaining", 2013 IEEE SDN for Future Networks and Services
              (SDN4FNS), Trento, Italy,
              DOI 10.1109/SDN4FNS.2013.6702549, November 2013,
              <https://doi.org/10.1109/SDN4FNS.2013.6702549>.

   [PERF-METRICS]
              IANA, "Performance Metrics",
              <https://www.iana.org/assignments/performance-metrics>.

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
              "Framework for IP Performance Metrics", RFC 2330,
              DOI 10.17487/RFC2330, May 1998,
              <https://www.rfc-editor.org/info/rfc2330>.

   [RFC5618]  Morton, A. and K. Hedayat, "Mixed Security Mode for the
              Two-Way Active Measurement Protocol (TWAMP)", RFC 5618,
              DOI 10.17487/RFC5618, August 2009,
              <https://www.rfc-editor.org/info/rfc5618>.

   [RFC5938]  Morton, A. and M. Chiba, "Individual Session Control
              Feature for the Two-Way Active Measurement Protocol
              (TWAMP)", RFC 5938, DOI 10.17487/RFC5938, August 2010,
              <https://www.rfc-editor.org/info/rfc5938>.

   [RFC7312]  Fabini, J. and A. Morton, "Advanced Stream and Sampling
              Framework for IP Performance Metrics (IPPM)", RFC 7312,
              DOI 10.17487/RFC7312, August 2014,
              <https://www.rfc-editor.org/info/rfc7312>.

   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <https://www.rfc-editor.org/info/rfc7426>.

   [RFC8018]  Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
              Password-Based Cryptography Specification Version 2.1",
              RFC 8018, DOI 10.17487/RFC8018, January 2017,
              <https://www.rfc-editor.org/info/rfc8018>.

   [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>.

Appendix A.  Detailed Data Model Examples

   This appendix extends the examples presented in Section 6 by
   configuring more fields, such as authentication parameters, DSCP
   values, and so on.

A.1.  Control-Client

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <client>
         <admin-state>true</admin-state>
         <mode-preference-chain>
           <priority>0</priority>
           <mode>authenticated</mode>
         </mode-preference-chain>
         <mode-preference-chain>
           <priority>1</priority>
           <mode>unauthenticated</mode>
         </mode-preference-chain>
         <key-chain>
           <key-id>KeyClient1ToRouterA</key-id>
           <secret-key>c2VjcmV0MQ==</secret-key>
         </key-chain>
         <key-chain>
           <key-id>KeyForRouterB</key-id>
           <secret-key>c2VjcmV0Mg0K</secret-key>
         </key-chain>
         <ctrl-connection>
           <name>RouterA</name>
           <client-ip>203.0.113.1</client-ip>
           <server-ip>203.0.113.2</server-ip>
           <control-packet-dscp>32</control-packet-dscp>
           <key-id>KeyClient1ToRouterA</key-id>
           <test-session-request>
             <name>Test1</name>
             <sender-ip>203.0.113.3</sender-ip>
             <sender-udp-port>54000</sender-udp-port>
             <reflector-ip>203.0.113.4</reflector-ip>
             <reflector-udp-port>55000</reflector-udp-port>
             <padding-length>64</padding-length>
             <start-time>0</start-time>
           </test-session-request>
           <test-session-request>
             <name>Test2</name>
             <sender-ip>203.0.113.1</sender-ip>
             <sender-udp-port>54001</sender-udp-port>
             <reflector-ip>203.0.113.2</reflector-ip>
             <reflector-udp-port>55001</reflector-udp-port>
             <padding-length>128</padding-length>
             <start-time>0</start-time>
           </test-session-request>
         </ctrl-connection>
       </client>
     </twamp>
   </data>

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <client>
         <admin-state>true</admin-state>
         <mode-preference-chain>
           <priority>0</priority>
           <mode>authenticated</mode>
         </mode-preference-chain>
         <mode-preference-chain>
           <priority>1</priority>
           <mode>unauthenticated</mode>
         </mode-preference-chain>
         <key-chain>
           <key-id>KeyClient1ToRouterA</key-id>
           <secret-key>c2VjcmV0MQ==</secret-key>
         </key-chain>
         <key-chain>
           <key-id>KeyForRouterB</key-id>
           <secret-key>c2VjcmV0Mg0K</secret-key>
         </key-chain>
         <ctrl-connection>
           <name>RouterA</name>
           <client-ip>2001:db8:203:0:113::1</client-ip>
           <server-ip>2001:db8:203:0:113::2</server-ip>
           <control-packet-dscp>32</control-packet-dscp>
           <key-id>KeyClient1ToRouterA</key-id>
           <test-session-request>
             <name>Test1</name>
             <sender-ip>2001:db8:10:1:1::1</sender-ip>
             <sender-udp-port>54000</sender-udp-port>
             <reflector-ip>2001:db8:10:1:1::2</reflector-ip>
             <reflector-udp-port>55000</reflector-udp-port>
             <padding-length>64</padding-length>
             <start-time>0</start-time>
           </test-session-request>
           <test-session-request>
             <name>Test2</name>
             <sender-ip>2001:db8:203:0:113::1</sender-ip>
             <sender-udp-port>54001</sender-udp-port>
             <reflector-ip>2001:db8:203:0:113::2</reflector-ip>
             <reflector-udp-port>55001</reflector-udp-port>
             <padding-length>128</padding-length>
             <start-time>0</start-time>
           </test-session-request>
         </ctrl-connection>
       </client>
     </twamp>
   </data>

A.2.  Server

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <server>
         <admin-state>true</admin-state>
         <servwait>1800</servwait>
         <control-packet-dscp>32</control-packet-dscp>
         <modes>authenticated unauthenticated</modes>
         <count>15</count>
         <key-chain>
           <key-id>KeyClient1ToRouterA</key-id>
           <secret-key>c2VjcmV0MQ==</secret-key>
         </key-chain>
         <key-chain>
           <key-id>KeyClient10ToRouterA</key-id>
           <secret-key>c2VjcmV0MTANCg==</secret-key>
         </key-chain>
         <ctrl-connection>
           <client-ip>203.0.113.1</client-ip>
           <client-tcp-port>16341</client-tcp-port>
           <server-ip>203.0.113.2</server-ip>
           <server-tcp-port>862</server-tcp-port>
           <control-packet-dscp>32</control-packet-dscp>
           <selected-mode>unauthenticated</selected-mode>
           <key-id>KeyClient1ToRouterA</key-id>
           <count>15</count>
         </ctrl-connection>
       </server>
     </twamp>
   </data>

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <server>
         <admin-state>true</admin-state>
         <servwait>1800</servwait>
         <control-packet-dscp>32</control-packet-dscp>
         <modes>authenticated unauthenticated</modes>
         <count>15</count>
         <key-chain>
           <key-id>KeyClient1ToRouterA</key-id>
           <secret-key>c2VjcmV0MQ==</secret-key>
         </key-chain>
         <key-chain>
           <key-id>KeyClient10ToRouterA</key-id>
           <secret-key>c2VjcmV0MTANCg==</secret-key>
         </key-chain>
         <ctrl-connection>
           <client-ip>2001:db8:203:0:113::1</client-ip>
           <client-tcp-port>16341</client-tcp-port>
           <server-ip>2001:db8:203:0:113::2</server-ip>
           <server-tcp-port>862</server-tcp-port>
           <control-packet-dscp>32</control-packet-dscp>
           <selected-mode>unauthenticated</selected-mode>
           <key-id>KeyClient1ToRouterA</key-id>
           <count>15</count>
         </ctrl-connection>
       </server>
     </twamp>
   </data>

A.3.  Session-Sender

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-sender>
         <admin-state>true</admin-state>
         <test-session>
           <name>Test1</name>
           <ctrl-connection-name>RouterA</ctrl-connection-name>
           <fill-mode>zero</fill-mode>
           <number-of-packets>900</number-of-packets>
           <periodic-interval>1</periodic-interval>
           <sent-packets>2</sent-packets>
           <rcv-packets>2</rcv-packets>
           <last-sent-seq>1</last-sent-seq>
           <last-rcv-seq>1</last-rcv-seq>
         </test-session>
         <test-session>
           <name>Test2</name>
           <ctrl-connection-name>RouterA</ctrl-connection-name>
           <fill-mode>random</fill-mode>
           <number-of-packets>900</number-of-packets>
           <lambda>1</lambda>
           <max-interval>2</max-interval>
           <sent-packets>21</sent-packets>
           <rcv-packets>21</rcv-packets>
           <last-sent-seq>20</last-sent-seq>
           <last-rcv-seq>20</last-rcv-seq>
         </test-session>
       </session-sender>
     </twamp>
   </data>

A.4.  Session-Reflector

      |  Note: '\' line wrapping is for formatting only.

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-reflector>
         <admin-state>true</admin-state>
         <test-session>
           <sender-ip>203.0.113.3</sender-ip>
           <sender-udp-port>54000</sender-udp-port>
           <reflector-ip>203.0.113.4</reflector-ip>
           <reflector-udp-port>55000</reflector-udp-port>
           <sid>1232</sid>
           <parent-connection-client-ip>203.0.113.1</parent-connection-\
   client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>203.0.113.2</parent-connection-\
   server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <test-packet-dscp>32</test-packet-dscp>
           <sent-packets>2</sent-packets>
           <rcv-packets>2</rcv-packets>
           <last-sent-seq>1</last-sent-seq>
           <last-rcv-seq>1</last-rcv-seq>
         </test-session>
         <test-session>
           <sender-ip>203.0.113.1</sender-ip>
           <sender-udp-port>54001</sender-udp-port>
           <reflector-ip>192.0.2.2</reflector-ip>
           <reflector-udp-port>55001</reflector-udp-port>
           <sid>178943</sid>
           <parent-connection-client-ip>203.0.113.1</parent-connection-\
   client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>203.0.113.2</parent-connection-\
   server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <test-packet-dscp>32</test-packet-dscp>
           <sent-packets>21</sent-packets>
           <rcv-packets>21</rcv-packets>
           <last-sent-seq>20</last-sent-seq>
           <last-rcv-seq>20</last-rcv-seq>
         </test-session>
       </session-reflector>
     </twamp>
   </data>

      |  Note: '\' line wrapping is for formatting only.

   <?xml version="1.0" encoding="utf-8"?>
   <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <twamp xmlns="urn:ietf:params:xml:ns:yang:ietf-twamp">
       <session-reflector>
         <admin-state>true</admin-state>
         <test-session>
           <sender-ip>2001:db8:10:1:1::1</sender-ip>
           <sender-udp-port>54000</sender-udp-port>
           <reflector-ip>2001:db8:10:1:1::2</reflector-ip>
           <reflector-udp-port>55000</reflector-udp-port>
           <sid>1232</sid>
           <parent-connection-client-ip>2001:db8:203:0:113::1</parent-c\
   onnection-client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>2001:db8:203:0:113::2</parent-c\
   onnection-server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <test-packet-dscp>32</test-packet-dscp>
           <sent-packets>2</sent-packets>
           <rcv-packets>2</rcv-packets>
           <last-sent-seq>1</last-sent-seq>
           <last-rcv-seq>1</last-rcv-seq>
         </test-session>
         <test-session>
           <sender-ip>2001:db8:203:0:113::1</sender-ip>
           <sender-udp-port>54001</sender-udp-port>
           <reflector-ip>2001:db8:192:68::2</reflector-ip>
           <reflector-udp-port>55001</reflector-udp-port>
           <sid>178943</sid>
           <parent-connection-client-ip>2001:db8:203:0:113::1</parent-c\
   onnection-client-ip>
           <parent-connection-client-tcp-port>16341</parent-connection-\
   client-tcp-port>
           <parent-connection-server-ip>2001:db8:203:0:113::2</parent-c\
   onnection-server-ip>
           <parent-connection-server-tcp-port>862</parent-connection-se\
   rver-tcp-port>
           <test-packet-dscp>32</test-packet-dscp>
           <sent-packets>21</sent-packets>
           <rcv-packets>21</rcv-packets>
           <last-sent-seq>20</last-sent-seq>
           <last-rcv-seq>20</last-rcv-seq>
         </test-session>
       </session-reflector>
     </twamp>
   </data>

Appendix B.  TWAMP Operational Commands

   TWAMP operational commands could be performed programmatically or
   manually, e.g., using a command-line interface (CLI).

   With respect to programmability, YANG can be used to define NETCONF
   Remote Procedure Calls (RPCs); therefore, it would be, in principle,
   possible to define TWAMP RPC operations for actions such as starting
   or stopping control connections, test sessions, or groups of
   sessions; retrieving results; clearing stored results; and so on.

   However, TWAMP [RFC5357] does not attempt to describe such
   operational actions.  Refer also to Section 2 and the unlabeled links
   in Figure 1.  In actual deployments, different TWAMP implementations
   may support different sets of operational commands, with different
   restrictions.  Therefore, this document considers it the
   responsibility of the individual implementation to define its
   corresponding data model for TWAMP operational commands.

Acknowledgments

   We thank Fred Baker, Kevin D'Souza, Gregory Mirsky, Brian Trammell,
   Robert Sherman, and Marius Georgescu for their thorough and
   constructive reviews, comments, and text suggestions.

   Haoxing Shen contributed to the definition of the YANG module in
   Section 5.

   Jan Lindblad and Ladislav Lhotka did thorough reviews of the YANG
   module and the examples in Appendix A.

   Kostas Pentikousis was partially supported by FP7 UNIFY, a research
   project partially funded by the European Community under the Seventh
   Framework Program (grant agreement no. 619609).  The views expressed
   here are those of the authors only.  The European Commission is not
   liable for any use that may be made of the information in this
   document.

Contributors

   Lianshu Zheng

Authors' Addresses

   Ruth Civil
   Ciena Corporation
   307 Legget Drive
   Kanata ON K2K 3C8
   Canada

   Email: ruthcivil@gmail.com
   URI:   www.ciena.com

   Al Morton
   AT&T Labs
   200 Laurel Avenue South
   Middletown, NJ 07748
   United States of America

   Phone: +1 732 420 1571
   Email: acmorton@att.com

   Reshad Rahman
   Canada

   Email: reshad@yahoo.com

   Mahesh Jethanandani
   Xoriant Corporation
   1248 Reamwood Avenue
   Sunnyvale, CA 94089
   United States of America

   Email: mjethanandani@gmail.com

   Kostas Pentikousis (editor)
   Detecon
   Winterfeldtstrasse 21
   10781 Berlin
   Germany

   Email: kostas.pentikousis@detecon.com