Flow Migration for Service Function Chains: Preserving External-Ordering

Network Functions (NFs) are chained to form Service Function Chains (SFCs). We consider the issue of migrating a flow from a set of NFs in an SFC instance to another. The NFs in the original chain are called the sNFs, and those in the new chain are called dNFs. Most work in the literature consider a correct migration to be one where the Flow Migration System (FMS) consisting of sNFs and dNFs, processes packets in the same order as the sNFs would have processed them in the absence of migration (property O). We argue that it is also useful to preserve the order in which packets leave the FMS in the face of migration, thereby preserving the property of External-Ordering (E). There is no existing work that preserves E. We propose an algorithm for flow migration of an SFC that preserves both O and E. It uses an NF to buffer packets instead of existing nodes, for scalability. Our experiments demonstrate that our solution causes no packet re-ordering while a solution that does not preserve E causes, in the worst case, 0.72% packets of a flow to be reordered on an average, for SFCs of length 8.