The Round-trip time (RTT) is the amount of time that passes between when the remote site receives a TCP connection (sends a TCP SYN) from the client and when the remote site receives the client’s acknowledgment of the connection request (sends a TCP ACK). A site ServerIron ADX sends RTT data to the GSLB ServerIron ADX every five seconds.
You can modify RTT parameters to change processing of the RTT information reported by the GSLB and remote site ServerIron ADXs. You can change the following parameters, on an individual basis:
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RTT cache interval – The site ServerIron ADXs use the Brocade GSLB protocol to send RTT information to the GSLB ServerIron ADX. The GSLB ServerIron ADX stores this information in a cache. The GSLB ServerIron ADX uses the entries in the cache when using the RTT metric to evaluate IP addresses in a DNS reply. Entries in the cache age out if they remain unused. The default aging interval for RTT cache entries is 120 seconds. You can change the interval to a value from 10 – 1,000,000 seconds (about 11-1/2 days).•
RTT cache prefix – The entries in the RTT cache include IP address information for the clients. To avoid overflowing the cache, cache entries are aggregated based on the IP information. For example, if the GSLB ServerIron ADX receives RTT information for clients at 10.21.4.69 and 10.21.4.18, and the cache prefix is 31 bits, both addresses go in as separate entries. However, if the prefix is 16 bits, the GSLB ServerIron ADX aggregates the addresses. In this case, only one entry, 10.21.x.x goes in the cache. The default number of bits in the prefix is 20. You can specify a value from 1 – 31.•
RTT tolerance – 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 from0 – 100. The default is 10%.
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RTT explore percentage – Site ServerIron ADXs send RTT information only for the sessions that clients open with them. These are clients referred to the site ServerIron ADX by the GSLB ServerIron ADX. If the metrics that come before this one (based on the GSLB policy order) do not select a “best” site, the ServerIron ADX selects a site based on RTT.Since the only RTT information received by the GSLB ServerIron ADX comes from the site ServerIron ADXs to which the GSLB ServerIron ADX has referred clients, it is possible for the GSLB ServerIron ADX to continually bias its selection toward the first site ServerIron ADX that sent RTT information. To prevent this from occurring, 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.
You also can add static RTT prefix cache entries. Changing the RTT cache interval
You can change the round trip time (RTT) cache interval, which specifies how often the site ServerIron ADXs use the Brocade GSLB protocol to send RTT information to the GSLB ServerIron ADX. The GSLB ServerIron ADX stores this information in a cache. The GSLB ServerIron ADX uses the entries in the cache when using the RTT metric to evaluate IP addresses in a DNS reply. To change the RTT cache interval from 10 seconds to 30 seconds, enter commands such as the following.
ServerIronADX(config)#gslb policy
Syntax: [no] round-trip-time cache-interval <num>
The <num> parameter specifies the aging interval and can be from 10 – 1,000,000 seconds (about 11-1/2 days). The default is 120 seconds.
Changing the RTT cache prefix
You can change the RTT cache prefix, which specifies the level of aggregation that occurs in the GSLB ServerIron ADX’s RTT cache.
The entries in the RTT cache include IP address information for the clients. To avoid overflowing the cache, cache entries are aggregated based on the IP information. For example, if the GSLB ServerIron ADX receives RTT information for clients at 10.21.4.69 and 10.21.4.18, and the cache prefix is 31 bits, both addresses go in as separate entries. However, if the prefix is 16 bits, the GSLB ServerIron ADX aggregates the addresses. In this case, only one entry, 10.21.x.x goes in the cache.
To change the RTT cache prefix from 20 bits to 16 bits, enter commands such as the following.
ServerIronADX(config)#gslb policy
ServerIronADX(config-gslb-policy)#round-trip-time cache-prefix 16
Syntax: [no] round-trip-time cache-prefix <num>
The <num> parameter specifies the number of significant bits in the prefix and can be from 1 – 32. The default is 20.
Changing the RTT tolerance
To change the RTT tolerance from 10% to 70%, enter commands such as the following.
ServerIronADX(config)#gslb policy
ServerIronADX(config-gslb-policy)#round-trip-time tolerance 70
Syntax: [no] round-trip-time tolerance <num>
The <num> parameter specifies the percentage above which the RTTs of two sites must differ for the GSLB ServerIron ADX to favor one site over the other based on the RTT. You can specify a value from 0 – 100. The default is 10%.
Change the RTT explore percentage
You can change the RTT explore percentage, which prevents the GSLB ServerIron ADX from unfairly biasing selection of the best site based on previous RTT responses.
Site ServerIron ADXs send RTT information only for the sessions that clients open with them. These are clients referred to the site ServerIron ADX by the GSLB ServerIron ADX. If the metrics that come before this one (based on the GSLB policy order) do not select a “best” site, the ServerIron ADX selects a site based on RTT.
Since the only RTT information received by the GSLB ServerIron ADX comes from the site ServerIron ADXs to which the GSLB ServerIron ADX has referred clients, it is possible for the GSLB ServerIron ADX to continually bias its selection toward the first site ServerIron ADX that sent RTT information. To prevent this from occurring, 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.
ServerIronADX(config)#gslb policy
ServerIronADX(config-gslb-policy)#round-trip-time explore-percentage 10
The command in this example changes the RTT explore percentage from 5% to 10%. Syntax: [no] round-trip-time explore-percentage <num>
The <num> parameter specifies the explore percentage and can be from 0 – 100. The default is 5. Adding static prefix cache entries
The GSLB ServerIron ADX maintains a cache of round-trip time (RTT) information received from the site ServerIron ADXs through the GSLB protocol. The RTT is the amount of time that passes between when a remote site initiates a TCP connection from the client and when the remote site receives the client’s acknowledgment of the connection request. Each site ServerIron ADX sends RTT information to the GSLB ServerIron ADX at one-second intervals.
The GSLB ServerIron ADX uses the RTT information in the prefix cache when evaluating a site using the GSLB policy. Thus, the cache entry provides the RTT information used for the RTT metric during evaluation of the GSLB policy.
When the GSLB ServerIron ADX receives RTT information from a site ServerIron ADX, the IP address of the client is compared to the prefixes in the cache. If the address fits within a network in one of the prefixes, the GSLB ServerIron ADX stores the RTT information for that site under the prefix entry. If the client address is within more than one prefix entry, the GSLB ServerIron ADX selects the entry with the longer prefix (the more exact match).
The GSLB ServerIron ADX makes a dynamic entry in the prefix cache of the length specified by the cache prefix the first time the ServerIron ADX processes a DNS query or response from that prefix. After that, each time the GSLB ServerIron ADX receives a subsequent DNS query from within that prefix, the ServerIron ADX resets the aging timer for the cache prefix entry. If a dynamic entry is not refreshed by subsequent queries, the entry ages out.
You can manually add static prefix information to the cache. For example, you can add static cache entries with longer prefix information than the dynamic cache entries to ensure that RTT
information is stored under the static entries instead of dynamic cache entries with shorter prefixes. This is useful when you want to ensure that certain prefixes are always present in the cache regardless of how often the GSLB ServerIron ADX receives RTT data for them. Static prefixes do not age out.
NOTE
The GSLB ServerIron ADX uses the most exact match when more than one prefix entry can apply to the same site address. To ensure that the GSLB ServerIron ADX uses a static entry instead of certain dynamic entries for a given address, make sure prefix of the static entry is longer than the prefix for dynamic entries.
NOTE
Since RTT information is stored under individual domain names that are queried, the RTT
information reported from remote ServerIron ADXs are not recorded under the static records until the GSLB ServerIron ADX receives the first DNS query or response.
To add a static prefix cache entry, enter commands such as the following.
ServerIronADX(config)#gslb policy
ServerIronADX(config-gslb-policy)#static-prefix 10.1.1.1/20
The <ip-addr> specifies the address of the cache entry. This is not necessarily the address of a remote site. The address you specify here is combined with the prefix length to result in a network prefix (network portion of an IP address). The prefix length can be from 1 – 31.
NOTE
The prefix length 0 is not applicable to this feature and is ignored by the software.
You can enter more than one prefix on the same command line. Separate each prefix with a space. You can configure up to 250 static prefixes on a ServerIron ADX.
The command in this example configures an entry for address 10.1.1.1 with a prefix of 20 bits. (Due to the prefix length, the value actually stored in the cache is 10.1.0.0.20.) When the GSLB ServerIron ADX receives RTT information for an address within the specified prefix, the GSLB ServerIron ADX stores the information in the static prefix entry configured above, instead of creating a dynamic entry.