Dynamic round trip time method 编辑

Dynamic round trip time (RTT) is a measure of time or delay in the network between the client’s local DNS server and a data resource. To measure dynamic RTT, the Citrix ADC appliance probes the client’s local DNS server and gathers RTT metric information. The appliance then uses this metric to make its load balancing decision. Global server load balancing monitors the real-time status of the network and dynamically directs the client request to the data center with the lowest RTT value.

When a client’s DNS request for a domain comes to the Citrix ADC appliance configured as the authoritative DNS for that domain, the appliance uses the RTT value to select the IP address of the best performing site to send it as a response to the DNS request.

The Citrix ADC appliance uses different mechanisms, such as ICMP echo request or reply (PING), UDP, and TCP to gather the RTT metrics for connections between the local DNS server and participating sites. The appliance first sends a ping probe to determine the RTT. If the ping probe fails, a DNS UDP probe is used. If that probe also fails, the appliance uses a DNS TCP probe.

These mechanisms are represented on the Citrix ADC appliance as Load Balancing Monitors and are easily identified due to their use of the “ldns” prefix. The three monitors, in their default order, are:

• ldns-ping
• ldns-dns
• ldns-tcp

These monitors are built into the appliance and are set to safe defaults. But they are customizable like any other monitor on the appliance.

You can change the default order by setting it explicitly as a GSLB parameter. For example, to set the order to be the DNS UDP query followed by the PING and then TCP, type the following command:

set gslb parameter -ldnsprobeOrder DNS PING TCP
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