Brocade Communications Systems ServerIron ADX 12.4.00 - Manual (Page 145)

ServerIron ADX Global Server Load Balancing Guide 133
53-1002437-01
Smoothing mechanism for RTT measurements
1
The above output shows that the geographic location is Asia. The geographic location for
10.10.10.200 is still North America (N-AM) because it does not match the user-configured
geographic prefix.
Smoothing mechanism for RTT measurements
A GSLB ServerIron ADX learns the Round Trip Time (RTT) information from the Site ServerIron ADXs
through the GSLB protocol and uses the information as a metric when comparing IP addresses. For
each RTT value reported by the Site ServerIron ADX, the GSLB ServerIron ADX calculates the
effective RTT value by adding 90% of the existing RTT value in the cache entry to 10% of the new
RTT sample to obtain the effective RTT value for that cache entry.
For example, assume Site ServerIron ADX 1.1.1.101 is periodically reporting the RTT for a client IP
1.1.1.42 to the GSLB. The first value that the Site ServerIron ADX reports to the GSLB ServerIron
ADX is 20ms. The GSLB ServerIron ADX stores this RTT in its cache (1.1.1.101, rtt = 20ms). When
the Site ServerIron ADX again reports RTT for 1.1.1.42, the GSLB ServerIron ADX uses the following
formula to calculate the new RTT value.
effective RTT = 90 % of old RTT value + 10% of new RTT value
If the Site ServerIron ADX reports an RTT value of 40ms for 1.1.1.101, then effective RTT would be
90% of 20ms + 10% of 40ms = 22ms
This smoothing mechanism may not be effective in dealing with large variances in RTT
measurements. For example, if the Site ServerIron ADX reports just one very high value, for
example 1 second, then the RTT will be.
90% of 22ms + 10% of 1 sec = 119ms
After this calculation, even if the Site ServerIron ADX continues to report 20ms, it will take some
time for the resulting RTT to come down to 20ms. This formula is not adaptive enough to deal with
transient spikes in RTT values.
ServerIronADX# show gslb dns detail


ZONE: gslb1.com

HOST: www:

(Global GSLB policy)

Flashback DNS resp.

delay selection

(x100us) counters

TCP APP Count (%)

* 1.1.1.22: dns real-ip ACTIVE ASIA 5 16 ---

* 10.10.10.200: dns real-ip DOWN N-AM -- -- ---

* 1.1.1.76: dns v-ip DOWN ASIA -- -- ---

site: local, weight: 0, ServerIronADX: 1.1.1.102

session util: 0%, avail. sessions: 5999976

preference: 128

* 1.1.1.101: dns v-ip ACTIVE ASIA 0 0 ---

Active Bindings: 1

site: local, weight: 0, ServerIronADX: 1.1.1.102

session util: 0%, avail. sessions: 5999976

preference: 128