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Server-based measurement is one of two methods that the CNA system measures the performance of your network’s traffic flow.
Since the objective of both methods (server-based measurement with the single pixel GIF is the other) is to gather as many measurements as possible for prefixes of interest, you can use either of the two, or both simultaneously.
An agent-based measurement is made through a TCP session, an ICMP ping, or execution of a traceroute type utility initiated by the CNA system at regular intervals.
The CNA system measures the time required to complete a TCP handshake or for the ICMP echo request to be answered, or, in the case of a traceroute probe, measures the time it takes for the traceroute packet to return from the last hop it was able to reach.
Single pixel GIF measurement is a passive test that is conducted as a consequence of a web client downloading a page on your web site.
The principal advantages of the single pixel GIF are its passive nature and its broad coverage of the Internet. Web surfers--your website clients--initiate the test whenever they download a page that contains a test-related URL. Data is collected for the prefix in which the web surfer’s IP address is located. Over time, the CNA measurement database will grow to cover virtually the entire spectrum of prefixes representing people who have an interest in your web site.
The principal advantages of the active probing mechanism are that it is predictable and it is configured to meet your needs. You choose in advance which prefixes will be measured, and CNA will send probes on a regular, predetermined schedule. You are guaranteed measurements for each designated prefix.
In addition, you can begin collecting measurements as soon as your CNA configuration is in place, without having to first make changes to your web pages.
If you are using an CNA system to manage a VPN exclusively, then agent-based measurements are your sole source of measurements.
If you are using your CNA system to optimize traffic between your web site and end users somewhere in the Internet cloud, or between your local network users and external web sites, then you should have an understanding of both the server-based measurement and agent-based measurement processes.
See Chapter 5: Configuring Agent-Based Measurement.
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