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Manual Chapter: Configuration Guide for BIG-IP® Global Traffic Management: 12 - Collecting Metrics
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12

Collecting Metrics


Introducing metrics collection

In Chapter 10, Configuring Monitors , we described how the Global Traffic Manager uses specialized software components, called monitors, to capture data regarding the availability of a resource, such as a virtual server. Monitors represent one half of the statistical gathering capabilities of the Global Traffic Manager. The second half, metrics collection, captures data on how well network traffic flows between the Global Traffic Manager and the external Local Domain Name Systems (LDNS) servers and internal resources with which it communicates.

The resources you make available to your users over the Internet are often critical to your organization; consequently, it is vital that these resources are not only available, but highly responsive to your users. Typically, two main criteria determine the responsiveness of a resource: hops and paths. A hop is one point-to-point transmission between a host and a client server in a network. A network path that includes a stop at a network router would have two hops: the first from the client to the router, and the second from the router to the host server. A path is a logical network route between a data center server and a local DNS server.

It is important to remember that hops and paths can differ from each other widely on a per-connection basis. For example, an LDNS might take a long path to reach a specific resource, but require only a few hops to get there. On the other hand, that same LDNS might select a short path, yet have to move between a larger number of routers, increasing the number of hops it takes to reach the resource. It is up to you to determine what thresholds for hops and paths are acceptable for your network, as the needs of each network, and even each application within the same network, can vary widely.

Through the metrics collection capabilities of the Global Traffic Manager, you can accomplish several tasks related to improving the availability and responsiveness of your network applications and resources. You can:

  • Define the types of metrics that the Global Traffic Manager collects, and how long the system keeps those metrics before acquiring fresh data.
  • Assign probes to LDNS servers that attempt to acquire the metrics information.
  • Configure Time-to-Live (TTL) values for your metrics data.
  • Exclude specific LDNS servers from Global Traffic Manager probes.
  • Implement the Quality of Service load balancing mode, which uses metrics to determine the best resource for a particular name resolution request.

Defining metrics

When you decide to use the Global Traffic Manager to collect metrics on the LDNS servers that attempt to access your network resources, you can define the following characteristics:

  • The types of metrics collected (either hops, paths, both, or disabled)
  • The time-to-live (TTL) values for each metric
  • The frequency at which the system updates the data
  • The size of a packet sent (relevant for hop metrics only)
  • The length of time that can pass before the system times out the collection attempt
  • The number of packets sent for each collection attempt

While each of these settings is important, the ones that perhaps require the most planning beforehand are the TTL values. In general, the lower the TTL value, the more often the Global Traffic Manager probes an LDNS. This improves the accuracy of the data, but increases bandwidth usage. Conversely, increasing the TTL value for a metric lowers the bandwidth your network uses, but increases the chance that the Global Traffic Manager is basing its load balancing operations off of stale data

An additional consideration is the number of LDNS servers that the Global Traffic Manager queries. The more LDNS servers that the system queries, the more bandwidth is required to ensure those queries are successful. As you can see, setting the TTL values for metrics collection can require incremental fine-tuning. We recommend that you periodically check the TTL values that you set, and verify that they are appropriate for your network.

To define metrics

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The General properties screen opens.
  2. From the Global Traffic menu, choose Metrics Collection.
    The metrics collection screen opens.
  3. In the Configuration area, assign values to the different metrics-related settings.
    For detailed information on these settings, please see the online help.
  4. Click the Update button.

Assigning probes to local domain name servers

To capture accurate metrics data from the local domain name servers (LDNS servers) that send name resolution request to the Global Traffic Manager, you assign probes to each LDNS. A probe is a software component that employs a specific methodology to learn more about an LDNS.

You can assign one or more of the following probes to query LDNS servers:

  • DNS_REV
    The DNS_REV probe sends a DNS message to the probe target LDNS querying for a resource record of class IN, type PTR. Most versions of DNS answer with a record containing their fully-qualified domain name. The system makes these requests only to measure network latency and packet loss; it does not use the information contained in the responses.
  • DNS_DOT
    The DNS.DOT probe sends a DNS message to the probe target LDNS querying for a dot (.). If the LDNS is not blocking queries from unknown addresses, it answers with a list of root name servers. The system makes these requests only to measure network latency and packet loss; it does not use the information contained in the responses.
  • UDP
    The UDP probe uses the user datagram protocol (UDP) to query the responsiveness of an LDNS. The UDP protocol provides simple but unreliable datagram services. The UDP protocol adds a checksum and additional process-to-process addressing information. UDP is a connectionless protocol which, like TCP, is layered on top of IP. UDP neither guarantees delivery nor requires a connection. As a result, it is lightweight and efficient, but the application program must take care of all error processing and retransmission.
  • TCP
    The TCP probe uses the transmission control protocol (TCP) to query the responsiveness of an LDNS. The TCP protocol is the most common transport layer protocol used on Ethernet and Internet. The TCP protocol adds reliable communication, flow-control, multiplexing, and connection-oriented communication. It provides full-duplex, process-to-process connections. TCP is connection-oriented and stream-oriented.
  • ICMP
    The ICMP probe uses the Internet control message protocol (ICMP) to query the responsiveness of an LDNS. The ICMP protocol is an extension to the Internet Protocol (IP). The ICMP protocol generates error messages, test packets, and informational messages related to IP.

With these probes, it does not matter if the Global Traffic Manager receives a valid response, such as the name of the LDNS, as queried by the DNS_REV probe, or a request refused statement. The relevant information is the metrics generated between the probe request and the response. For example, the Global Traffic Manager uses the DNS_REV probe to query two LDNS servers. The first LDNS responds to the probe with its name, as per the request. The second LDNS, however, responds with a request refused statement, because it is configured to not allow such requests. In both cases, the probe was successful, because the Global Traffic Manager was able to acquire data on how long it took for both LDNS servers to respond to the probe.

You can configure the Global Traffic Manager to use a select number of probes, or you can assign all five. The more probes that the Global Traffic Manager uses, the more bandwidth is required.

To assign a probe

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The General screen opens.
  2. From the Global Traffic menu, choose Metrics Collection.
    The metrics collection screen opens.
  3. In the Local DNS (LDNS) area, use the options provided in the Metrics Collection Protocol option to assign the relevant probes.
  4. In the Metrics Caching box, define the number of seconds for which the Global Traffic Manager keeps the collected metrics data.
    This value determines how often the system probes a given LDNS. The default value is 3600 seconds, or one hour.
  5. In the Inactive Local DNS TTL box, define the number of seconds for which an LDNS can be inactive before the Global Traffic Manager considers it inactive.
    The Global Traffic Manager stops probing LDNS servers that are considered inactive. The default value is 2419200, or 28 days.
  6. Click the Update button to save your changes.

Configuring TTL and timer values

Each resource in the Global Traffic Manager has an associated time-to-live (TTL) value. A TTL is the amount of time (measured in seconds) for which the system considers metrics valid. The timer values determine how often the Global Traffic Manager refreshes the information.

Table 12.1 describes each TTL value, as well as its default setting.

Table 12.1 TTL values and default settings
Parameter
Description
Default
Hops TTL
Specifies the number of seconds that the Global Traffic Manager considers traceroute data to be valid.
604800 (seven days)
Paths TTL
Specifies the number of seconds that the Global Traffic Manager uses path information for name resolution and load balancing.
2400
Inactive Path TTL
Specifies the number of seconds that a path remains in the cache after its last access.
604800 (seven days)
Inactive Local DNS TTL
Specifies the number of seconds that a local DNS remains in the cache after its last access.
2419200
(28 days)

Each resource also has a timer value. A timer value defines the frequency (measured in seconds) at which the Global Traffic Manager refreshes the metrics information it collects. In most cases, the default values for the TTL and timer parameters are adequate. However, if you make changes to any TTL or timer value, keep in mind that an object's TTL value must be greater than its timer value.

Table 12.2 describes each timer value, as well as its default setting.

Table 12.2 Time values and default settings
Parameter
Description
Default
Hops data refresh
Specifies the frequency (in seconds) at which the Global Traffic Manager retrieves traceroute data (traceroutes between each data center and each local DNS).
60
Paths refresh
Specifies the frequency (in seconds) at which the Global Traffic Manager refreshes path information (for example, round trip time or ping packet completion rate).
120
Sync Time Tolerance
Specifies the number of seconds that one system's time setting is allowed to be out of sync with another system's time setting.
Note: If you are using NTP to synchronize the time of the Global Traffic Manager with a time server, leave the time tolerance at the default value of 10. In the event that NTP fails, the Global Traffic Manager uses the time_tolerance variable to maintain synchronization.
This setting is available in the General screen of the Global Traffic Manager's general properties section.
10
Timer Sync State
Specifies the interval (in seconds) at which the Global Traffic Manager checks to see if it should change states (from Principal to Receiver or from Receiver to Principal).
This setting is available in the General screen of the Global Traffic Manager's general properties section.
30
Metrics Caching
Specifies the interval (in seconds) at which the Global Traffic Manager archives the paths and metrics data.
This setting is available in the General screen of the Global Traffic Manager's general properties section.
3600

To configure global TTL and timer values

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The General screen opens.
  2. From the Global Traffic menu, choose Metrics Collection.
    The metrics collection screen opens.
  3. Add the TTL and timer values settings.
    For help on configuring the TTL and timer values settings, see the online help for this screen.
  4. Click the Update button to save your changes.

Excluding LDNS servers from probes

When the Global Traffic Manager attempts to probe a local domain name system (LDNS), it is actively attempting to acquire data from that LDNS. Certain Internet Service Providers and other organizations might request that you do not probe their LDNS servers, while other LDNS servers might be known to act as proxies, which do not provide accurate metrics data. In these situations, you can configure the Global Traffic Manager to exclude LDNS servers from probes. When you exclude an LDNS, the Global Traffic Manager does not probe that system; however, the Global Traffic Manager is also unable to use the Quality of Service load balancing mode to load balance name resolution request from that LDNS.

To exclude an LDNS from probes

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The General screen opens.
  2. From the Global Traffic menu, choose Metrics Collection.
    The metrics collection screen opens.
  3. In the Address Exclusions area, in the IP Subnet box, type the IP address and subnet that contains the LDNS servers you want to exclude.
  4. Click the Add button to add the LDNS or network segment to the address exclusion list.
  5. Click the Update button to save your changes.

Removing LDNS servers from the address exclusion list

You can remove an LDNS from the address exclusion list at any time. Situations in which you want to remove the LDNS include the LDNS becoming inactive, or the IP address of the LDNS changing to a different network subnet.

To remove an LDNS from the address exclusion list

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The General screen opens.
  2. From the Global Traffic menu, choose Metrics Collection.
    The metrics collection screen opens.
  3. In the Address Exclusion area, select the LDNS that you would like to remove and click the Remove button.
  4. Click the Update button to save your changes.



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