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The Basics of Flow Analysis

When it comes to enterprise network monitoring, flow-based solutions are by far the most popular, with 30-40 major flow-based network monitoring solutions on the market today. With that many solutions, how do they differentiate from one another, and which one will be best for your network? To determine this, let’s start at the beginning, with the basics. How does a flow-based solution work?

The Data Source

Switches and routers are the primary sources of flow data. Since every packet is traversing the device, it is relatively easy for the device to extract key data from the packets, of course requiring extra processing. Depending on the protocol being used to analyze the packets, and the current load on the router or switch, sampling may be employed, and this could lower the accuracy of the data being reported. All flow-based reporting protocols categorize packets into a flow based on the following seven characteristics: source IP address, destination IP address, source port, destination port, layer 3 protocol type, TOS byte, and input logical interface. The device keeps track of all the flows, storing the information in available RAM, and once every configured interval packages up the data into a stream of UDP packets following a predefined format (like NetFlow or sFlow) and transmits these packets to a user-configured IP address, known as the Collector.

The Data Collector

Once the UDP data stream with the flow-based information leaves the switch or router it is purged and forgotten. It is now the responsibility of the Collector to receive, process, and store the flow-based information. Keep in mind that the original delivery to the Collector is over UDP, which is not a reliable transport, so dropped packets from the switch to the Collector can be a problem (a protocol analyzer like the OmniPeek Network Analyzer can help to identify if this is an issue on your network). Also, the packet stream from the switch is adding to the traffic load on your network, so this should be taken into consideration. Each packet typically contains information on five to ten flows, so a busy network segment can generate a significant number of packets. The frequency of data pushed from the switch to the Collector is something that is configured on the switch, and is typically set to one minute, though you may find a different interval works best in your specific environment.

The Collector becomes the central repository for all data from that switch or router, and from many others, because a single Collector is designed to support multiple data sources. A Collector employs either a proprietary data structure or database to store the large volume of data that accumulates from the flow-based sources, and retains the data for long periods of times (months, at least) for reporting. A flow-based monitoring solution is a combination of a Collector, or set of Collectors, and a central server which processes user requests, communicates with Collectors, and returns the desired results to the user.

What is the difference between Flow-Based Solutions?

Differences between network monitoring solutions based on flow data come in two forms. The first is the type of flow data. Different network device vendors support different flow-based protocols. The most common protocols are NetFlow (Cisco), sFlow (Foundry), JFlow (Juniper), and IPFIX – a proposed industry standard. Each protocol deals with the generation of flow records just a bit differently, with the major difference centered on whether or not sampling is used and how aggressively it is used. The other difference in flow-based network monitoring solutions is in how the vendor presents (displays) the data, and any unique ways each vendor finds to process the data to provide unique results. Unique data processing and presentation is really the only way for vendors to differentiate themselves since the source and format of the data is essentially the same regardless of the underlying flow-based protocol.

What solution would you find most helpful for your company and why? We always suggest that enterprises have something greater than just a flow-based solution, as flow-based solutions tend to lack all the details required for root-cause analysis on your network. If you are interested in learning more about these issues, check out our blog post, “Is A Flow-Based Solution, A Whole-Based Solution?”.

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