LRTM method
Note: LRTM stands for Least response time method using monitors (LRTM).
When a load balancing virtual server is configured to use the LRTM method, it uses the existing monitoring infrastructure to get the fastest response time. The load balancing virtual server then selects the service with the smallest number of active transactions and lowest response time. Before you use the LRTM method, you must bind application-specific monitors to each service and enable LRTM mode on these monitors. The Citrix ADC appliance then makes load balancing decisions based on the response times it calculates from monitoring probes.
You can use the LRTM method to load balance non-HTTP and non-HTTPS services also. You can also use this method when several monitors are bound to a service. Each monitor determines the response time by using the protocol that it measures for the service that it is bound to. The virtual server then calculates an average response time for that service by averaging the results.
The following table summarizes how response times are calculated for the various monitors.
| Monitor | Response time calculation |
|---|---|
| PING | Time difference between the ICMP ECHO request and the ICMP ECHO response. |
| TCP | Time difference between the SYN request and the SYN+ACK response. |
| HTTP | Time difference between the HTTP request (after the TCP connection is established) and the HTTP response. |
| TCP-ECV | Time difference between the time the data send string is sent and the data receive string is returned. A TCP-ECV monitor without the send and receive strings is considered to have an incorrect configuration. |
| HTTP-ECV | Time difference between the HTTP request and the HTTP response. |
| UDP-ECV | Time difference between the UDP’s send string and the receive string. A UDP-ECV monitor without the receive string is considered to have an incorrect configuration. |
| DNS | Time difference between a DNS query and the DNS response. |
| TCPS | Time difference between a SYN request and the SSL handshake completion. |
| FTP | Time difference between the sending of the user name and the completion of user authentication. |
| HTTPS (monitors HTTPS requests) | Time difference is the same as for the HTTP monitor. |
| HTTPS-ECV (monitors HTTPS requests) | Time difference is the same as for the HTTP-ECV monitor |
| USER | Time difference between the time when a request is sent to the dispatcher and the time when the dispatcher response is received. |
下面的例子显示了如何Citrix ADC应用liance selects a service for load balancing by using the LRTM method. Consider the following three services:
- Service-HTTP-1 is handling 3 active transactions and the response time is five seconds.
- Service-HTTP-2 is handling 7 active transactions and the response time is one second.
- Service-HTTP-3 is not handling any active transactions and the response time is two seconds.
The following diagram illustrates the process that the Citrix ADC appliance follows when it forwards requests.
Figure 1. How the LRTM Method Works
The virtual server selects a service by using the value (N) in the following expression:
N = (Number of active transactions * Response time that is determined by the monitor)
The virtual server delivers requests as follows:
- Service-HTTP-3 receives the first request, because this service is not handling any active transaction.
- Service-HTTP-3 receives the second, third, and fourth requests, because this service has the lowest N value.
- Service-HTTP-2 receives the fifth request, because this service has the lowest N value.
- 因为Service-HTTP-2和Service-HTTP-3曲线ntly have the same N value, the Citrix ADC appliance switches to the round robin method. Therefore, Service-HTTP-3 receives the sixth request.
- Service-HTTP-2接收第七和第八点播uests, because this service has the lowest N value.
balancin Service-HTTP-1不考虑负载g, because it is more heavily loaded (has the highest N value) when compared to the other two services. However, if Service-HTTP-1 completes its active transactions, the Citrix ADC appliance again considers that service for load balancing.
The following table summarizes how N is calculated for the services.
| Request Received | Service Selected | Current N Value (Number of Active Transactions * TTFB) | Remarks |
|---|---|---|---|
| Request-1 | Service-HTTP-3;(N = 0) | N = 2 | Service-HTTP-3 has the lowest N value. |
| Request-2 | Service-HTTP-3; (N = 2) | N = 4 | Service-HTTP-3 has the lowest N value. |
| Request-3 | Service-HTTP-3; (N = 4) | N = 6 | Service-HTTP-3 has the lowest N value. |
| Request-4 | Service-HTTP-3; (N = 6) | N = 8 | Service-HTTP-3 has the lowest N value. |
| Request-5 | Service-HTTP-2; (N = 7) | N = 8 | Service-HTTP-2 has the lowest N value. |
| Request-6 | Service-HTTP-3; (N = 8) | N = 10 | Service-HTTP-2 and Service-HTTP-3 have the same N values. Citrix ADC appliance switches to the round robin method and selects Service-HTTP-3 |
| Request-7 | Service-HTTP-2; (N = 8) | N = 9 | Service-HTTP-2 has the lowest N value. |
| Request-8 | Service-HTTP-2; (N = 9) | N = 10 | Service-HTTP-2 has the lowest N value. |
Service-HTTP-1 is again selected for load balancing when it completes its active transactions or when its N value is less than the other services (Service-HTTP-2 and Service-HTTP-3).
Selection of services when weights are assigned
The Citrix ADC appliance also performs load balancing by using the number of active transactions, response time, and weights if different weights are assigned to services. The Citrix ADC appliance selects the service by using the value (Nw) in the following expression:
Nw = (N) * (10000 / weight)
Where N = (Number of active transactions * Response time that is determined by the monitor)
The following diagram illustrates how the virtual server uses the LRTM method when weights are assigned.
Figure 2. How the Least Response Time Load Balancing Method Works When Weights Are Assigned
In this example, suppose Service-HTTP-1 is assigned a weight of 2, Service-HTTP-2 is assigned a weight of 3, and Service-HTTP-3 is assigned a weight of 4.
foll Citrix ADC设备发送请求ows:
- Service-HTTP-3 receives the first request, because it is not handling any active transactions.
- Service-HTTP-3 receives the second, third, fourth, and fifth requests, because this service has the lowest Nw value.
- Service-HTTP-2 receives the sixth request, because this service has the lowest Nw value.
- Service-HTTP-3 receives the seventh request, because this service has the lowest Nw value.
- Service-HTTP-2 receives the eighth requests, because this service has the lowest Nw value.
Service-HTTP-1 has the lowest weight and the highest Nw value, so the Citrix ADC appliance does not select it for load balancing.
The following table summarizes how Nw is calculated for various monitors.
| Request Received | Service Selected | Current Nw Value (N) * (10000 / Weight) | Remarks |
|---|---|---|---|
| Request-1 | Service-HTTP-3; (Nw = 0) | Nw = 5000 | Service-HTTP-3 has the lowest Nw value. |
| Request-2 | Service-HTTP-3; (Nw = 5000 | Nw = 10000 | Service-HTTP-3 has the lowest Nw value. |
| Request-3 | Service-HTTP-3; (Nw = 10000) | Nw = 15000 | Service-HTTP-3 has the lowest Nw value. |
| Request-4 | Service-HTTP-3; (Nw = 15000) | Nw = 20000 | Service-HTTP-3 has the lowest Nw value. |
| Request-5 | Service-HTTP-3; (Nw = 20000) | Nw = 25000 | Service-HTTP-3 has the lowest Nw value. |
| Request-6 | Service-HTTP-2; (Nw = 23333.34) | Nw = 26666.67 | Service-HTTP-2 has the lowest Nw value. |
| Request-7 | Service-HTTP-3; (Nw = 25000) | Nw= 30000 | Service-HTTP-3 has the lowest Nw value. |
| Request-8 | Service-HTTP-2; (Nw = 26666.67) | Nw = 30000 | Service-HTTP-2 has the lowest Nw value. |
Service-HTTP-1 is selected for load balancing when it completes its active transactions or when its Nw value is less than other services (Service-HTTP-2 and Service-HTTP-3).
To configure the LRTM load balancing method by using the CLI
At the command prompt type;
set lb vserver [-lbMethod ] Example:
set lb vserver Vserver-LB-1 -lbMethod LRTM To configure the LRTM load balancing method by using the GUI
Navigate toTraffic Management>Load Balancing>Virtual Servers, and open a virtual server.
In Advanced Settings, selectLRTM.
To enable the LRTM option in monitors by using the CLI
At the command prompt type;
set lb monitor [-LRTM ( ENABLED | DISABLED )] Example:
set lb monitor monitor-HTTP-1 HTTP -LRTM ENABLED To enable the LRTM option in monitors by using the GUI
- Navigate toTraffic Management>Load Balancing>Monitors, and open a monitor.
- In Advanced Parameters, selectLRTM (Least Response Time using Monitoring).
For more information about configuring monitors, seeConfiguring Monitors in a Load Balancing Setup.