Implementation

The Two-Way Active Measurement Protocol (TWAMP) defines a method for measuring round-trip IP network performance between two TWAMP-capable devices. Figure 19-89 Figure 19-89 shows how TWAMP is implemented.

1. The performance management system instructs the control-client to establish a test session with a specific TWAMP server.
2. The control-client establishes and completes the test session.
3. The performance management system collects and saves statistics during the test. The statistics of on-demand measurement are reported through SNMP (MIBs), and the statistics of proactive measurement are reported through the PM module.

   TWAMP supports only proactive measurement.

Figure 19-89 TWAMP implementation

Intercommunication Model

TWAMP uses the client/server mode and defines four logical roles, as shown in Figure 19-90.

• Control-client: establishes, starts, and stops a test session and collects statistics.
• Session-sender: proactively sends probes for performance statistics after being notified by the control-client.
• Server: responds to the control-client's request for establishing, starting, or stopping a test session.
• Session-reflector: replies to the probes sent by the session-sender with response probes after being notified by the server.

Figure 19-90 Typical TWAMP logical architecture

To facilitate implementation, TWAMP unifies the four logical entities, as shown in Figure 19-91. Control signals are exchanged between the control-client and server through a TCP connection; probes are exchanged between the session-sender and session-reflector through a UDP connection. The control-client and server establish and start a test session. Once a test session starts, the control-client and server notify the session-sender and session-reflector respectively of the session information and allow the session-sender to send probes and the session-reflector to respond to the probes.

Figure 19-91 Typical TWAMP implementation architecture

On a live network, if a network element (NE) functions as a server and session-reflector alone, the NE participates in TWAMP session establishment and probe exchanges but does not compile statistics. If a device or tester functions as the control-client and session-sender, the device or tester proactively establishes a TWAMP session for statistics collection. Users manage the control-client alone to rapidly obtain statistics about the performance of the entire IP network.

Measurement Indicators

Establishment of a Control Session

A control session provides a basis for the establishment of a test session. Figure 19-92 shows how a control session is established.

1. The server specifies a TCP port number (the default port number is 862), and the control-client initiates a TCP connection.
2. The server replies with a Server-Greeting message to notify the control-client of the server configurations.
3. After receiving the Server-Greeting message, the control-client sends a Set-Up-Response message to the server to establish a control session.
4. The server verifies the Set-Up-Response message and replies with a Server-Start message.

A control session is established between the control-client and the server.

Figure 19-92 Establishment of a control session

Establishment of a Test Session

After a control session is established, users can specify an IP address and a UDP port number on the control-client to establish a test session. Figure 19-93 shows how a test session is established.

1. The control-client sends a Request-TW-Session message carrying an IP address and a UDP port number to the server through the TCP connection.
2. After receiving the Request-TW-Session message, the server establishes a test session based on the IP address and UDP port number in the Request-TW-Session message and replies to the control-client with an Accept-Session message.

A test session is established between the control-client and the server.

Figure 19-93 Establishment of a test session

Start of a Test Session

A test session is started based on a control session. After a test session is established, the control-client can send a Start-Session message to the server. After receiving the Start-Session message, the server instructs the control session to start all test sessions that are established based on the control session. Figure 19-94 shows how a test session is started.

1. The control-client sends a Start-Session message to the server.
2. After receiving the Start-Session message, the server notifies the session-reflector of the test session information to enable the session-reflector to respond to probes.
3. The session-reflector replies to the control-client with a Start-ACK message to start the test sessions.
4. After receiving the Start-ACK message, the control-client notifies the session-sender of the test session information to enable the session-sender to send probes.

Test sessions are started, and the session-reflector starts to respond to probes.

Figure 19-94 Start of a test session

Stop of a TWAMP Session

After statistics are collected, users can stop a TWAMP session. Figure 19-95 shows how a TWAMP session is stopped.

1. The control-client sends a Stop-Session message to instruct the server to stop collecting statistics.
2. After receiving the Stop-Session message, the server disables the session-reflector from responding to probes in a TWAMP session.

The TWAMP session is stopped.

Figure 19-95 Stop of a TWAMP session