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Transport IPPM WG This document specifies a data model for client and server implementations of the Two-Way Active Measurement Protocol (TWAMP). We define the TWAMP data model through Unified Modeling Language (UML) class diagrams and formally specify it using YANG.

The Two-Way Active Measurement Protocol (TWAMP) 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, configuration depends on proprietary mechanisms developed by the corresponding TWAMP vendor. This document addresses this gap by formally specifying the TWAMP data model using YANG.

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 as discussed in , proprietary mechanisms for managing TWAMP measurements pose severe limitations with respect to programmability. Two major trends call for standardizing TWAMP management aspects. First, we expect that in the coming years large-scale and multi-vendor TWAMP deployments will become the norm. From an operations perspective, dealing with several vendor-specific TWAMP configuration mechanisms is simply unsustainable in this context. Second, the increasingly software-defined and virtualized nature of network infrastructures, based on dynamic service chains and programmable control and management planes requires a well-defined data model for TWAMP implementations. This document defines such a TWAMP data model and specifies it formally using the YANG data modeling language .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

The rest of this document is organized as follows. presents the scope and applicability of this document. provides a high-level overview of the TWAMP data model. details the configuration parameters of the data model and specifies in YANG the TWAMP data model. lists illustrative examples which conform to the YANG data model specified in this document. elaborates these examples further.

The purpose of this document is the specification of a vendor-independent data model for TWAMP implementations. illustrates a redrawn version of the TWAMP logical model found in Section 1.2 of . The figure is annotated with pointers to the UML diagrams 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. As per , unlabeled links in are left unspecified and may be proprietary protocols.

| Server | +----------------+ +--------+ ^ ^ | | V V +----------------+ +-------------------+ | Session-Sender | <-- TWAMP-Test --> | Session-Reflector | +----------------+ +-------------------+ [Fig. 5] [Fig. 6] ]]>

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

| Server | | | | | | Session-Sender | <-- TWAMP-Test --> | Session-Reflector | +-------------------+ +-------------------+ ]]>

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 and hence are out of scope of this document. See also .

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 we describe the TWAMP data model in the remaining sections of this document, readers should keep in mind the functional entity grouping illustrated in .

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 which 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 Client Initialization Vector (Client-IV); see also the last paragraph of Section 6 in and . Each TWAMP-Control connection, in turn, is associated with zero or more TWAMP-Test sessions. For each test session we note the following configuration items: The test session name that uniquely identifies a particular test session at the Control-Client and Session-Sender. Similarly to the control connections 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 , or whether the test should be repeated.

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.

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 that MUST be identical with the corresponding test session name on the TWAMP Control-Client (). The control connection name, which along with the test session name uniquely identify the TWAMP Session-Sender instance. Information pertaining to the test packet stream, such as, for example, the number of test packets and the packet distribution to be employed; see also .

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 (, 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 .

This section defines the TWAMP data model using 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 .

The client container (see ) holds items that are related to the configuration of the TWAMP Control-Client logical entity (recall ). The client container includes an administrative configuration parameter (client/admin-state) that indicates whether the device is allowed to initiate TWAMP-Control connections.

----------------------| 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} | +-------------+ +----------------------+ ]]>

The client container holds a list (mode-preference-chain) which 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, where zero is the highest priority and subsequent values are monotonically increasing. 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 bit position combinations when necessary, such as when referring to the extended TWAMP features in , , , and . If the Control-Client cannot determine an acceptable Mode, it MUST respond with zero Mode bits set in the Set-up Response message, indicating it will not continue with the control connection. In addition, the client container holds a list named key-chain which relates KeyIDs with the respective secret keys. Both the Server and the Control-Client use the same mappings from KeyIDs to shared secrets (key-id and secret-key in , respectively). The Server, being prepared to conduct sessions with more than one Control-Client, uses KeyIDs 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, an octet string of arbitrary length whose interpretation as a text string is unspecified. The key-id and secret-key encoding SHOULD follow Section 9.4 of . The derived key length (dkLen in ) 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 . 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 list of test-session-request. 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 ). 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 so test-session-request holds information related to these actions (e.g. pm-index, repeat-interval).

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

------| 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} | +--------------------------+ ]]>

Each server container holds a list named key-chain which relates KeyIDs with the respective secret keys. As mentioned in , both the Server and the Control-Client use the same mappings from KeyIDs to shared secrets. The Server, being prepared to conduct sessions with more than one Control-Client, uses KeyIDs 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 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.

The session-sender container, illustrated in , 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.

-----| 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 | +--------------+ ]]>

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.

The session-reflector container, illustrated in , 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 are applied to all incoming sessions.

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). This value is auto-allocated by the TWAMP Server as test session requests are received, and communicated back to the Control-Client in the SID field of the Accept-Session message; see Section 4.3 of . 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 ). 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. 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.

This section formally specifies the TWAMP data model using YANG.

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.

This section presents the YANG module for the TWAMP data model defined in this document.

file "ietf-twamp@2017-10-16.yang" module ietf-twamp { namespace urn:ietf:params:xml:ns:yang:ietf-twamp; prefix ietf-twamp; import ietf-inet-types { prefix inet; } organization "IETF IPPM (IP Performance Metrics) Working Group"; contact draft-ietf-ippm-twamp-yang@tools.ietf.org; description "This YANG module specifies a vendor-independent data model for the Two-Way Active Measurement Protocol (TWAMP). The data model covers four TWAMP logical entities, namely, Control-Client, Server, Session-Sender, and Session-Reflector, as illustrated in the annotated TWAMP logical model (Fig. 1 of draft-ietf-ippm-twamp-yang). This YANG module uses features to indicate which of the four logical entities are supported by a TWAMP implementation."; revision 2017-10-16 { description "Revision appearing in draft-ietf-ippm-twamp-yang-05. Covers RFC 5357, RFC 5618, RFC 5938, RFC 6038, RFC 7717, and draft-ietf-ippm-metric-registry"; reference draft-ietf-ippm-twamp-yang; } /* * 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)"; } 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)"; } bit encrypted { position 2; description "Encrypted mode 'makes it impossible to alter timestamps undetectably.' See also Section 4 of RFC 7717 and Section 6 of RFC 4656."; reference "RFC 4656: A One-way Active Measurement Protocol (OWAMP)"; } bit unauth-test-encrpyt-control { position 3; description "When using the Mixed Security Mode, the TWAMP-Test protocol follows the Unauthenticated mode and the TWAMP-Control protocol the 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 the shared key is derived from an 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 Server."; } enum idle { description "Indicates an idle TWAMP-Control connection to Server."; } } description "Indicates the Control-Client TWAMP-Control connection state."; } typedef test-session-state { type enumeration { enum accepted { value 0; description "Indicates that 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 of Section 3.1 of RFC 5357."; } } 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 pseudo-random 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 shared secret the [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 10; 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 15 here means count 2^15 = 32768. The default is 10, meaning 2^10 = 1024."; } description "Reusable data structure for count which is used both in the Server container."; } grouping max-count { leaf max-count { type uint8 { range "10..31"; } default 15; 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 15, meaning 2^15 = 32768. 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 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 sets 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. TWAMP-compliant systems SHOULD have a configuration control to limit the maximum count value. The default max-count value SHOULD be 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 which is used both at the Control-Client and the Server containers."; } /* * Configuration data nodes */ container twamp { description "TWAMP logical entity configuration grouping of four models which correspond to the four TWAMP logical entities Control-Client, Server, Session-Sender, and Session-Reflector as illustrated in Fig. 1 of draft-ietf-ippm-twamp-yang."; container client { if-feature control-client; presence "Enables TWAMP Control-Client functionality."; description "Configuration of the TWAMP Control-Client logical entity."; leaf admin-state { type boolean; mandatory 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, where zero is the highest priority and subsequent values monotonically increasing."; } leaf mode { type twamp-modes; description "The supported TWAMP Mode 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 Fig. 2 of RFC 7717), the this 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, which the TWAMP-Control connection will be initiated to."; } 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 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; 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 advertized 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 1305 according to Section 4.1.2 of RFC 4656."; } leaf state { type control-client-connection-state; config false; description "Indicates the current state of the TWAMP-Control connection state."; } leaf selected-mode { type twamp-modes; config false; description "The TWAMP Mode that the Control-Client has chosen for this control connection as set in the Mode field of the Set-Up-Response message (RFC 4656, 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 in RFC 4656. If the Mode defined in RFC 7717 is selected (selected-mode), Token is limited to 16 octets."; reference "RFC 4086: Randomness Requirements for Security 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), that 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 Contol-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 advertized 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 dynamic-port-number; description "This parameter defines the UDP port number that will be used by the Session-Reflector for this TWAMP-Test session. The number is restricted to the dynamic port range and is to be placed in the Receiver Port field of the Request-TW-Session message."; } 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 (RFC 5357, Section 3.8). This value will be placed in the Timeout field of the Request-TW-Session message."; } 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 (RFC 4656, 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 (RFC 5357)."; } 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 1305 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 union { type uint32 { range 0..4294967294; } type enumeration { enum forever { description "Indicates that the test session SHALL be repeated *forever* using the information in repeat-interval parameter, and SHALL NOT decrement the value."; } } } 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 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. The implementation MUST decrement the value of repeat after determining a repeated session is expected."; } 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 Metric Registry (see ietf-ippm-metric-registry). Output statistics are specified in the corresponding Registry entry."; } description "A list of one or more Performance Metric 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 "ietf-ippm-metric-registry: Registry for Performance Metrics"; } 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 test-session-state; config false; description "Indicates the TWAMP-Test session state (accepted or indication of an error); see Section 3.5 of RFC 5357."; } leaf sid { type string; config false; description "The 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; see Section 4.3 of RFC 6038."; } } } } container server { if-feature server; presence "Enables TWAMP Server functionality."; description "Configuration of the TWAMP Server logical entity."; leaf admin-state { type boolean; mandatory 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, 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, as per Section 3.1 of RFC 5357."; } uses count; uses max-count; leaf modes { type twamp-modes; description "The bit mask of TWAMP Modes this Server instance is willing to support; see 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 re-configured 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 re-configures 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 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 re-configured 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 { description "This read-only value indicates the actual max-count in use for this control connection. Usually this would be the same value as 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 (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, Challenge is used by the Control-Client to prove possession of a shared secret."; } } } container session-sender { if-feature session-sender; presence "Enables TWAMP Session-Sender functionality."; description "Configuration of the TWAMP Session-Sender logical entity"; leaf admin-state { type boolean; mandatory 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 will contain pseudo-random numbers, or whether it should consist of all zeroes, 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; presence "Enables TWAMP Session-Reflector functionality."; description "Configuration of the TWAMP Session-Reflector logical entity"; leaf admin-state { type boolean; mandatory 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-Reflectortest 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 dynamic-port-number; 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; } } } } ]]>

This section presents a simple but complete example of configuring all four entities in , based on the YANG module specified in . The example is illustrative in nature, but aims to be self-contained, i.e. were it to be executed in a real TWAMP implementation it would lead to a correctly configured test session. For completeness, examples are provided for both IPv4 and IPv6. A more elaborated example, which also includes authentication parameters, is provided in .

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

true ]]>

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.

true RouterA 203.0.113.1 203.0.113.2 Test1 203.0.113.3 54001 203.0.113.4 50001 0 Test2 203.0.113.1 54001 203.0.113.2 50001 0 RouterB 203.0.113.1 203.0.113.3 true RouterA 2001:DB8:203:0:113::1 2001:DB8:203:0:113::2 Test1 2001:DB8:203:1:113::3 54000 2001:DB8:203:1:113::4 55000 0 Test2 2001:DB8:203:0:113::1 54001 2001:DB8:203:0:113::2 55001 0 RouterB 2001:DB8:203:0:113::1 2001:DB8:203:0:113::3 ]]>

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

true ]]>

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

true 203.0.113.1 16341 203.0.113.2 862 active true 2001:DB8:203:0:113::1 16341 2001:DB8:203:0:113::2 862 active ]]>

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

true ]]>

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

true Test1 RouterA 900 1 setup Test2 RouterA 900 1 2 setup ]]>

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

true ]]>

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

true 203.0.113.3 54000 203.0.113.4 50001 1232 203.0.113.1 16341 203.0.113.2 862 2 2 1 1 203.0.113.1 54001 192.68.0.2 50001 178943 203.0.113.1 16341 203.0.113.2 862 21 21 20 20 true 203.0.113.3 54000 203.0.113.4 54001 1232 203.0.113.1 16341 203.0.113.2 862 2 2 1 1 203.0.113.1 54001 192.68.0.2 55001 178943 203.0.113.1 16341 203.0.113.2 862 21 21 20 20 ]]>

The YANG module defined in is designed to be accessed, among other protocols, via NETCONF. Protocols like NETCONF use a secure transport layer like SSH that is mandatory to implement. The NETCONF Access Control Module (NACM) provides the means to restrict access for particular users to a pre-configured set of NETCONF protocol operations and attributes. There are a number of nodes defined in this YANG module which are writeable. These data nodes may be considered sensitive and vulnerable to attacks in some network environments. Ability to write into these nodes without proper protection can have a negative effect on the devices that support this feature. Examples of 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.

This document registers a URI in the IETF XML registry . Following the format in , the following registration is requested to be made. URI: urn:ietf:params:xml:ns:yang:ietf-twamp Registrant Contact: The IPPM WG of the IETF. XML: N/A, the requested URI is an XML namespace. This document registers a YANG module in the YANG Module Names registry . name: ietf-twamp namespace: urn:ietf:params:xml:ns:yang:ietf-twamp prefix: twamp reference: RFC XXXX

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 . Jan Lindblad and Ladislav Lhokta did thorough reviews of the YANG module and the examples in . Kostas Pentikousis was partially supported by FP7 UNIFY (http://fp7-unify.eu), 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.

Lianshu Zheng, Huawei Technologies, China Email: vero.zheng@huawei.com

John, W., Pentikousis, K., et al.

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

true 0 authenticated 1 unauthenticated KeyClient1ToRouterA secret1 KeyForRouterB secret2 RouterA 203.0.113.1 203.0.113.2 32 KeyClient1ToRouterA Test1 203.0.113.3 54000 203.0.113.4 55000 64 0 ok 1232 Test2 203.0.113.1 54001 203.0.113.2 55001 128 0 ok 178943 true 0 authenticated 1 unauthenticated KeyClient1ToRouterA secret1 KeyForRouterB secret2 RouterA 2001:DB8:203:0:113::1 2001:DB8:203:0:113::2 32 KeyClient1ToRouterA Test1 2001:DB8:10:1:1::1 54000 2001:DB8:10:1:1::2 55000 64 0 ok 1232 Test2 2001:DB8:203:0:113::1 54001 2001:DB8:203:0:113::2 55001 128 0 ok 178943 ]]>

true 1800 32 authenticated unauthenticated 1024 KeyClient1ToRouterA secret1 KeyClient10ToRouterA secret10 203.0.113.1 16341 203.0.113.2 862 active 32 unauthenticated KeyClient1ToRouterA 1024 true 1800 32 authenticated unauthenticated 1024 KeyClient1ToRouterA secret1 KeyClient10ToRouterA secret10 2001:DB8:203:0:113::1 16341 2001:DB8:203:0:113::2 862 active 32 unauthenticated KeyClient1ToRouterA 1024 ]]>

true Test1 RouterA zero 900 1 setup 2 2 1 1 Test2 RouterA random 900 1 2 setup 21 21 20 20 ]]>

true 203.0.113.3 54000 203.0.113.4 55000 1232 203.0.113.1 16341 203.0.113.2 862 32 2 2 1 1 203.0.113.1 54001 192.68.0.2 55001 178943 203.0.113.1 16341 203.0.113.2 862 32 21 21 20 20 true 2001:DB8:10:1:1::1 54000 2001:DB8:10:1:1::2 55000 1232 2001:DB8:203:0:113::1 16341 2001:DB8:203:0:113::2 862 32 2 2 1 1 2001:DB8:203:0:113::1 54001 2001:DB8:192:68::2 55001 178943 2001:DB8:203:0:113::1 16341 2001:DB8:203:0:113::2 862 32 21 21 20 20 ]]>

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 (RPC), therefore it would be, in principle, possible to define TWAMP RPC operations for actions such as starting or stopping control connections or test sessions or groups of sessions; retrieving results; clearing stored results, and so on. However, does not attempt to describe such operational actions. Refer also to and the unlabeled links in . 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 TWAMP operational commands data model.