What Is NetFlow? A Deep Dive into Its Functionality and Benefits

Understanding how data moves through your network is essential for ensuring optimal performance, robust security, and efficient resource utilization. This is where NetFlow steps in—a versatile protocol originally developed by Cisco that offers deep insights into network traffic patterns. By collecting and analyzing flow data, NetFlow enables administrators to pinpoint bottlenecks, uncover anomalies, and enhance overall network performance. In this blog post, we’ll dive into what NetFlow is, how it functions, and why it’s a vital component of modern network management. Let’s get started!

What is Netflow?

NetFlow is a protocol designed to collect, aggregate, and record traffic flow data within a network. It offers a more detailed and granular view of bandwidth usage and network traffic compared to other monitoring solutions like SNMP.

Originally developed by Cisco, NetFlow is integrated into the company’s IOS software and has been supported on nearly all Cisco routers and switches since the 11.1 release of Cisco IOS. Additionally, many other hardware manufacturers either support NetFlow or utilize similar flow technologies, such as jFlow or sFlow.

NetFlow Version

NetFlow has ten different versions, but not all were widely implemented or released beyond specific hardware or internal use.

• NetFlow Version 1: The original version is now considered obsolete and rarely used.
• Versions 2 to 4: These were internal releases and never made publicly available.
• Version 5: Still commonly in use today due to its large install base on older Cisco routers and switches. It introduced Border Gateway Protocol (BGP) information and flow sequence numbers but is limited to IPv4 flows.
• Version 6: This version saw limited release and is no longer supported.
• Version 7: Added support for Cisco Catalyst switches operating in hybrid or native mode.
• Version 8: Provided functionality for router-based NetFlow aggregation.
• Version 9: The current and most versatile version, it is template-based, allowing for greater flexibility without requiring changes to the flow record format. It supports both IPv4 and IPv6 flows and is the preferred version for IETF IP Flow Information Export (IPFIX) and Packet Sampling (PSAMP) working groups.

Although IPFIX is often called “NetFlow v10” because it is based on NetFlow v9, it is a distinct protocol and not an official version of NetFlow.

How does NetFlow works?

NetFlow is a powerful protocol that operates by creating, recording, and exporting flows to provide detailed insights into network traffic. Here’s a breakdown of its core components and processes:

Creating a Flow

A flow is essentially a way of grouping a unidirectional stream of packets with similar attributes into a specific set. These attributes typically include:

• Source IP address
• Destination IP address
• Source port
• Destination port
• Class of service
• Layer 3 protocol type
• Interface

When a packet passes through the network device, these attributes are inspected. The first packet creates a flow, and subsequent packets with matching attributes (e.g., source and destination IP and ports, class of service) are grouped into the same flow. Any change in any of these attributes generates a new flow.

For high-traffic devices, sampled NetFlow is often used. Instead of inspecting every packet, the device examines only a subset of packets (e.g., one out of every 1,000). This approach reduces the performance impact of flow monitoring while still providing valuable insights.

NetFlow Cache

To manage the vast amount of data generated, network devices use a NetFlow cache to store active flow records. The cache condenses data into a manageable format and periodically exports it to a NetFlow collector. Data is automatically expired from the cache based on flow timers, ensuring efficient operation. By default, the cache is checked every second.

NetFlow Export

Flow records are grouped into NetFlow Export datagrams, each containing up to 30 flows. These datagrams are transmitted to the NetFlow collector using User Datagram Protocol (UDP). On average, NetFlow exports use about 1.5% of the total analyzed traffic, making it an efficient monitoring solution.

The export process involves defining the IP address of the collector and the destination port on the network device. Popular ports include 2055, but any port can be used as long as it is correctly configured on the collector. Unlike SNMP, NetFlow data is “pushed” to the collector, so no polling is required, but it lacks auto-discovery capabilities.

NetFlow Record

NetFlow records in Version 9 are template-based, allowing flexibility and extensibility without changing the record format. Each record includes a packet header, template FlowSets, and data FlowSets:

• The template FlowSet describes the data format in the subsequent data FlowSets.
• The data FlowSet contains the actual flow data.

This modular design enables seamless integration of future enhancements without disrupting existing implementations.

NetFlow Collector

NetFlow data is sent to a collector, a dedicated server or system equipped with software to receive, filter, and analyze flows. The collector must support the same NetFlow version as the exporting device. For example, a NetFlow V5 Sensor is required to monitor devices using NetFlow v5, while a NetFlow V9 Sensor is needed for devices using v9.

NetFlow MIB

In addition to direct exports, some NetFlow data can be accessed via SNMP using the NetFlow MIB. While not a full replacement for NetFlow exports, the MIB provides useful metrics like the number of flows, packets per flow, and top talkers (high-traffic sources).

Types of Data Monitored

NetFlow can categorize and monitor a wide range of traffic types, enabling granular analysis of network usage. Common categories include:

• Chat protocols
• Citrix
• FTP/P2P
• Infrastructure traffic (DHCP, DNS, ICMP, SNMP)
• Mail services
• NetBIOS
• Remote control protocols
• Web traffic (WWW)
• Total traffic

NetFlow Use Cases

Monitoring Networks, Users, and Applications

NetFlow’s most prominent use case is network monitoring, offering detailed insights into bandwidth usage that can be segmented in various ways—by user, device, application, or time. The data collected by a NetFlow collector is nearly real-time, enabling administrators to perform granular monitoring while also aggregating data to view broader network trends as they unfold.

By analyzing traffic, user behavior, and application usage patterns, NetFlow helps administrators identify potential issues before they escalate. For example, a single device or service consuming excessive bandwidth can degrade performance for other users. With a comprehensive dashboard or user interface, administrators can proactively detect these anomalies, or set up alerts to notify them of unusual activity, ensuring timely intervention and smoother network operations.

Network Planning

Being able to detect and respond to changing network conditions is invaluable, but even more powerful is the ability to anticipate future needs and address potential issues proactively.

By capturing and analyzing NetFlow data over extended periods, administrators can identify trends and patterns that provide insight into the network’s future requirements. For instance, if certain applications generate increased traffic at the end of the month, adjustments can be made to schedule other high-bandwidth activities at less busy times, avoiding potential bottlenecks.

NetFlow data also helps assess when traffic growth is approaching the limits of current hardware capacity. This foresight allows organizations to plan ahead, ensuring ample time to procure, install, and configure upgraded routers or switches before performance issues arise, keeping the network running smoothly.

Usage-Based Billing and Reporting

NetFlow’s ability to pinpoint specific traffic streams—identifying their origins and the applications behind them—makes it a valuable tool for usage-based billing. Whether for charging clients, allocating internal costs, or simply understanding how much network bandwidth is being used by specific users, groups, or applications, NetFlow provides the insights you need.

With detailed traffic data at your fingertips, it becomes easy to adjust billing rates based on factors like time of day, application usage, or total bandwidth consumed, offering both transparency and flexibility in managing network resources.

Application Reporting and Profiling

NetFlow provides valuable insights into application behavior, showing not only how much traffic an application generates but also when it occurs and who is using it. For instance, it can reveal if an application optimized for the accounting team is unexpectedly generating significant traffic for another department. This level of visibility helps administrators understand application usage patterns and identify opportunities for optimization.

Security Analysis

NetFlow is an invaluable tool for bolstering network security. For instance, if a user suddenly generates an unusually high volume of traffic unrelated to their normal activities, it may signal a compromised account. NetFlow data enables swift detection of anomalies, such as worms spreading across the network, malware connecting to command-and-control servers, or even an insider transferring sensitive company data. By delivering detailed traffic insights, NetFlow empowers security professionals to quickly identify and address potential security threats.

Conclusion

NetFlow is a powerful protocol that provides valuable insights into network traffic, enhancing monitoring, planning, and security. By analyzing flow data, it helps administrators identify usage patterns, optimize bandwidth, and address bottlenecks proactively. With applications in billing, application profiling, and security analysis, NetFlow is essential for managing modern networks. Its real-time troubleshooting and long-term data collection capabilities enable organizations to plan for growth and maintain efficient, secure networks.