ServerIron ADX Global Server Load Balancing Guide 9
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Global Server Load Balancing overview 1
Round-trip time between the remote ServerIron ADX and the client
The Round-trip time (RTT) is the amount of time that passes between when the remote site
receives a TCP connection (TCP SYN) from the client and when the remote site receives the client’s
acknowledgment of the connection request (TCP ACK). The GSLB ServerIron ADX learns the RTT
information from the site ServerIron ADXs through the GSLB protocol and uses the information as a
metric when comparing site IP addresses.
The GSLB ServerIron ADX maintains a database of cache entries, which contains the information
about past DNS queries. The information is aggregated on a network-address prefix basis. When
the GSLB ServerIron ADX receives a DNS query, it creates or updates a cache entry. RTT
measurements reported by remote ServerIron ADXs are then sorted into the cache. The GSLB
ServerIron ADX uses this information for decisions on subsequent DNS queries. If a cache entry is
not refreshed for a while (there are no subsequent queries from the same address prefix), the
ServerIron ADX clears the entry from the RTT database.
When the GSLB ServerIron ADX compares two site IP addresses based on RTT, the GSLB ServerIron
ADX favors one site over the other only if the difference between the RTT values is greater than the
specified percentage. This percentage is the RTT tolerance. You can set the RTT tolerance to a
value from 0-100. The default is 10%.
Site ServerIron ADXs send RTT information only for the sessions that clients open with them. To
prevent the GSLB ServerIron ADX from biasing its selection toward the firs t site ServerI ron ADX that
sent RTT information, the GSLB ServerIron ADX intentionally ignores the RTT metric for a specified
percentage of the requests from a given client network. You can specify an RTT explore percentage
from 0-100. The default is 5. By default, the GSLB ServerIron ADX ignores the RTT for 5% of the
client requests from a given network.
To configure RTT parameters, refer to “Modifying round-trip time values” on page 53.
Geographic location of the server
For each client query, the GSLB ServerIron ADX can determine the geographic location from which
the client query came based on its IP address. The GSLB can determine whether the query came
from North America, Asia, Europe, South America, or Africa.
If multiple sites compare equally based on the metrics above, the GSLB ServerIron ADX prefers
sites within the same geographic region as the client query.
NOTE
The GSLB ServerIron ADX deduces the geographic region of the client’s local DNS server from the
destination IP address in the DNS reply, which is the address of the client’s local DNS server.
The GSLB ServerIron ADX determines the geographic region of a server IP address in its DNS
database in the following ways:
•For real IP addresses (as opposed to VIPs, which are logical IP addresses configured on the
site ServerIron ADXs), the geographic region is based on the IP address itself.
•For VIPs, the geographic region is based on the management IP address of the site ServerIron
ADX on which the VIP is configured.
•You can explicitly specify the region if the management IP address of the remote ServerIron
ADX is not indicative of the geographic location. For example, if the management IP address is
in a private subnet, the address does not indicate the ServerIron ADX’s geographic location. If
you specify the region, the ServerIron ADX uses the region you specify instead of the region of
the ServerIron ADX’s management IP address.