sFlow, short for "sampled flow", is an industry standard for packet export at Layer 2 of the OSI model. It provides a means for exporting truncated packets, together with interface counters. Maintenance of the protocol is performed by the sFlow.org consortium, the authoritative source of the sFlow protocol specifications. The current version of sFlow is v5.
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Operation
sFlow uses sampling to achieve scalability and is, for this reason, applicable to high speed networks (gigabit per second speeds and higher). sFlow is supported by multiple network device manufacturers and network management software vendors.
An sFlow system consists of multiple devices performing two types of sampling: random sampling of packets or application layer operations, and time-based sampling of counters. The sampled packet/operation and counter information, referred to as flow samples and counter samples respectively, are sent as sFlow datagrams to a central server running software that analyzes and reports on network traffic; the sFlow collector.
Flow samples
Based on a defined sampling rate, an average of 1 out of n packets/operations is randomly sampled. This type of sampling does not provide a 100% accurate result, but it does provide a result with quantifiable accuracy.
Counter samples
A polling interval defines how often the network device sends interface counters. sFlow counter sampling is more efficient than SNMP polling when monitoring a large number of interfaces.
sFlow datagrams
The sampled data is sent as a UDP packet to the specified host and port. The official port number for sFlow is port 6343. The lack of reliability in the UDP transport mechanism does not significantly affect the accuracy of the measurements obtained from an sFlow agent. If counter samples are lost then new values will be sent when the next polling interval has passed. The loss of packet flow samples results in a slight reduction of the effective sampling rate.
The UDP payload contains the sFlow datagram. Each datagram provides information about the sFlow version, the originating device’s IP address, a sequence number, the number of samples it contains and one or more flow and/or counter samples.
Related technologies
There are several other technologies that appear to be similar to sFlow, usually due to the word "flow" in their name, such as NetFlow and OpenFlow. These technologies are however fundamentally different from sFlow and several key differences can be identified: