Service Function Chaining is a mechanism for overriding the basic destination based forwarding that is typical of IP networks. It is conceptually related to Policy Based Routing in physical networks but it is typically thought of as a Software Defined Networking technology. It is often used in conjunction with security functions although it may be used for a broader range of features. Fundamentally SFC is the ability to cause network packet flows to route through a network via a path other than the one that would be chosen by routing table lookups on the packet’s destination IP address. It is most commonly used in conjunction with Network Function Virtualization when recreating in a virtual environment a series of network functions that would have traditionally been implemented as a collection of physical network devices connected in series by cables.

A very simple example of a service chain would be one that forces all traffic from point A to point B to go through a firewall even though the firewall is not literally between point A and B from a routing table perspective. A more complex example is an ordered series of functions, each implemented in multiple VMs, such that traffic must flow through one VM at each hop in the chain but the network uses a hashing algorithm to distribute different flows across multiple VMs at each hop.