It has been recently observed that Software Defined Networks (SDN) can change the
paths of different connections in the network at a relatively frequent pace to improve the overall
network performance, including delay and packet loss, or to respond to other needs such as security.
These changes mean that a network that SDN controls will seldom operate in steady state; rather,
the network may often be in transient mode, especially when the network is heavily loaded and
path changes are critically important. Hence, we propose a transient analysis of such networks to
better understand how frequent changes in paths and the switches’ workloads may affect multi-hop
networks’ performance. Since conventional queueing models are difficult to solve for transient
behaviour and simulations take excessive computation time due to the need for statistical accuracy,
we use a diffusion approximation to study a multi-hop network controlled by SDN. The results show
that network optimization should consider the transient effects of SDN and that transients need to be
included in the design of algorithms for SDN controllers that optimize network performance.