NetFlow


NetFlow is a feature that was introduced on Cisco routers around 1996 that provides the ability to collect IP network traffic as it enters or exits an interface. By analyzing the data provided by NetFlow, a network administrator can determine things such as the source and destination of traffic, class of service, and the causes of congestion. A typical flow monitoring setup consists of three main components:
s and switches that support NetFlow can collect IP traffic statistics on all interfaces where NetFlow is enabled, and later export those statistics as NetFlow records toward at least one NetFlow collector—typically a server that does the actual traffic analysis.

Network flows

Cisco standard NetFlow version 5 defines a flow as a unidirectional sequence of packets that all share seven values:
  1. Ingress interface
  2. Source IP address
  3. Destination IP address
  4. IP protocol
  5. Source port for UDP or TCP, 0 for other protocols
  6. Destination port for UDP or TCP, type and code for ICMP, or 0 for other protocols
  7. IP Type of Service
Note that the Egress interface, IP Nexthop or BGP Nexthops are not part of, and may not be accurate if the route changes before the expiration of the flow, or if load-balancing is done per-packet.
This definition of flows is also used for IPv6, and a similar definition is used for MPLS and Ethernet flows.
Advanced NetFlow or IPFIX implementations like Cisco Flexible NetFlow allow user-defined flow keys.
A typical output of a NetFlow command line tool when printing the stored flows may look as follows:
Date flow start Duration Proto Src IP Addr:Port Dst IP Addr:Port Packets Bytes Flows
2010-09-01 00:00:00.459 0.000 UDP 127.0.0.1:24920 -> 192.168.0.1:22126 1 46 1
2010-09-01 00:00:00.363 0.000 UDP 192.168.0.1:22126 -> 127.0.0.1:24920 1 80 1

Export of records

The router will output a flow record when it determines that the flow is finished. It does this by flow aging: when the router sees new traffic for an existing flow it resets the aging counter. Also, TCP session termination in a TCP flow causes the router to expire the flow. Routers can also be configured to output a flow record at a fixed interval even if the flow is still ongoing.

Packet transport protocol

NetFlow records are traditionally exported using User Datagram Protocol and collected using a NetFlow collector.
The IP address of the NetFlow collector and the destination UDP port must be configured on the sending router. A common value is UDP port 2055, but other values like 9555 or 9995, 9025, 9026 etc. can also be used.
For efficiency reasons, the router traditionally does not keep track of flow records already exported, so if a NetFlow packet is dropped due to network congestion or packet corruption, all contained records are lost forever. The UDP protocol does not inform the router of the loss so it can send the packets again.
This can be a real problem, especially with NetFlow v8 or v9 that can aggregate a lot of packets or flows into a single record. A single UDP packet loss can cause a huge impact on the statistics of some flows.
That is why some modern implementations of NetFlow use the Stream Control Transmission Protocol to export packets so as to provide some protection against packet loss, and make sure that NetFlow v9 templates are received before any related record is exported. Note that TCP would not be suitable for NetFlow because a strict ordering of packets would cause excessive buffering and delays.
The problem with SCTP is that it requires interaction between each NetFlow collector and each routers exporting NetFlow. There may be performance limitations if a router has to deal with many NetFlow collectors, and a NetFlow collector has to deal with lots of routers, especially when some of them are unavailable due to failure or maintenance.
SCTP may not be efficient if NetFlow must be exported toward several independent collectors, some of which may be test servers that can go down at any moment.
UDP allows simple replication of NetFlow packets using Network taps or L2 or L3 Mirroring. Simple stateless equipment can also filter or change the destination address of NetFlow UDP packets if necessary. Since NetFlow export almost only use network backbone links, packet loss will often be negligible. If it happens, it will mostly be on the link between the network and the NetFlow collectors.

Packet headers

All NetFlow packets begin with version-dependent header, that contains at least these fields:
A NetFlow record can contain a wide variety of information about the traffic in a given flow.
NetFlow version 5 contains the following:
For ICMP flows, the Source Port is zero, and the Destination Port number field codes ICMP message Type and Code.
The source and destination Autonomous System number fields can report the destination AS or the immediate neighbor AS depending on the router configuration. But the AS number will be zero if the feature is not supported, the route is unknown or not announced by BGP, or the AS is the local AS. There is no explicit way to distinguish between these cases.
NetFlow version 9 can include all of these fields and can optionally include additional information such as Multiprotocol Label Switching labels and IPv6 addresses and ports,
By analyzing flow data, a picture of traffic flow and traffic volume in a network can be built. The NetFlow record format has evolved over time, hence the inclusion of version numbers. Cisco maintains details of the different version numbers and the layout of the packets for each version.

Interfaces

NetFlow is usually enabled on a per-interface basis to limit load on the router components involved in NetFlow, or to limit the amount of NetFlow records exported.
NetFlow usually captures all packets received by an ingress IP interface, but some NetFlow implementations use IP filters to decide if a packet can be observed by NetFlow.
Some NetFlow implementations also allow the observation of packets on the egress IP interface, but this must be used with care: all flows from any ingress interface with NetFlow enabled to any interface with NetFlow enabled could be counted twice.

Sampled NetFlow

Standard NetFlow was designed to process all IP packets on an interface. But in some environments, e.g. on Internet backbones, that was too costly, due to the extra processing required for each packet, and large number of simultaneous flows.
So Cisco introduced sampled NetFlow on Cisco 12000, and that is now used in all high-end routers that implement NetFlow.
Only one packet out of n is processed, where n, the sampling rate, is determined by the router configuration.
The exact selection process depends on the implementation:
Some implementations have more complex methods to sample packets, like per-flow sampling on Cisco Martinez Catalysts.
The sampling rate is often the same for all interfaces, but can be adjusted per interface for some routers.
When Sampled NetFlow is used, the NetFlow records must be adjusted for the effect of sampling - traffic volumes, in particular, are now an estimate rather than the actual measured flow volume.
The sampling rate is indicated in a header field of NetFlow version 5 or in option records of NetFlow version 9

Versions

VersionComment
v1First implementation, now obsolete, and restricted to IPv4.
v2Cisco internal version, never released.
v3Cisco internal version, never released.
v4Cisco internal version, never released.
v5Most common version, available on many routers from different brands, but restricted to IPv4 flows.
v6No longer supported by Cisco. Encapsulation information.
v7Like version 5 with a source router field. Used on Cisco Catalyst switches.
v8Several aggregation form, but only for information that is already present in version 5 records
v9Template Based, available on some recent routers. Mostly used to report flows like IPv6, MPLS, or even plain IPv4 with BGP nexthop.
v10Used for identifying IPFIX. Although IPFIX is heavily based on NetFlow, v10 does not have anything to do with NetFlow.

NetFlow and IPFIX

NetFlow was initially implemented by Cisco, and described in an "informational" document that was not on the standards track: RFC 3954 - Cisco Systems NetFlow Services Export Version 9. The NetFlow protocol itself has been superseded by Internet Protocol Flow Information eXport. Based on the NetFlow Version 9 implementation, IPFIX is on the IETF standards track with RFC 5101, RFC 5102, etc. which were published in 2008.

Equivalents

Many vendors other than Cisco provide similar network flow monitoring technology. NetFlow may be a prevalent name in the area of flow monitoring, because of Cisco dominant market share in the networking industry. NetFlow is thought to be a Cisco trademark :
Also flow-tools collection of software allows to process and manage NetFlow exports from Cisco and Juniper routers.

Support

Variants

Cisco's NetFlow Security Event Logging

Introduced with the launch of the Cisco ASA 5580 products, utilizes NetFlow v9 fields and templates in order to efficiently deliver security telemetry in high performance environments. NetFlow Security Event Logging scales better than syslog while offering the same level of detail and granularity in logged events.

Monitoring based on standalone probes

NetFlow collection using standalone NetFlow probes is an alternative to flow collection from routers and switches. This approach can overcome some limitations of router-based NetFlow monitoring. The probes are transparently connected to the monitored link as a passive appliance using the TAP or SPAN port of the appliance.
Historically, NetFlow monitoring is easier to implement in a dedicated probe than in a router. However, this approach also has some drawbacks:
The easiest way to address the above drawbacks is to use a packet capture appliance inline in front of the router and capture all of the NetFlow output from the router. This method allows for storage of large amount of NetFlow data and does not require reconfiguration of the network.
NetFlow collection from dedicated probes is well suited for observation of critical links, whereas NetFlow on routers provides a Network-wide view of the traffic that can be used for capacity planning, accounting, performance monitoring, and security.

History

NetFlow was originally a Cisco packet switching technology for Cisco routers, implemented in IOS 11.x around 1996.
It was originally a software implementation for the Cisco 7000, 7200 and 7500, where it was thought as an improvement over the then current Cisco Fast Switching. It carries U.S. patent # 6,243,667.
The idea was that the first packet of a flow would create a NetFlow switching record. This record would then be used for all later packets of the same flow, until the expiration of the flow. Only the first packet of a flow would require an investigation of the route table to find the most specific matching route. This is an expensive operation in software implementations, especially the old ones without Forwarding information base. The NetFlow switching record was actually some kind of route cache record, and old versions of IOS still refer to the NetFlow cache as ip route-cache.
This technology was advantageous for local networks. This was especially true if some of the traffic had to be filtered by an ACL as only the first packet of a flow had to be evaluated by the ACL.
NetFlow switching soon turned out to be unsuitable for big routers, especially Internet backbone routers, where the number of simultaneous flows was much more important than those on local networks, and where some traffic causes lots of short-lived flows, like Domain Name System requests.
As a switching technology, NetFlow was replaced around 1995 by Cisco Express Forwarding. This first appeared on Cisco 12000 routers, and later replaced NetFlow switching on advanced IOS for the Cisco 7200 and Cisco 7500.
As of 2012, technologies similar to NetFlow switching are still in use in most firewalls and software-based IP routers. For instance the conntrack feature of the Netfilter framework used by Linux.