Correctness of flow migration across Network Function instances

Abstract

Network Functions (NFs) improve the safety and efficiency of networks. Flows traversing NFs may need to be migrated from a source NF instance (sNF) to a destination NF instance (dNF) to balance load, conserve energy, etc. When NFs are stateful, the information stored on an sNF per flow must be migrated to the corresponding dNF before the flow is migrated, to avoid problems of consistency. Our main contribution is to examine what it means to correctly migrate flows from a stateful NF instance. We define the property of Weak-O, where only the state information required for packets to be correctly forwarded from an sNF is migrated first to the corresponding dNF, while the remaining states are eventually migrated. Weak-O can be preserved without buffering or dropping packets, unlike existing algorithms. We propose an algorithm that preserves Weak-O and prove its correctness. Even though this may cause packet re-ordering, we experimentally demonstrate that the goodputs with and without migration are comparable when the old and new paths have the same delays and bandwidths. This is also true when the new path has larger bandwidth or at most 5 times longer delays. Thus flow migration without buffering is practical, contrary to what was thought before. We also prove that no criterion stronger than Weak-O can be preserved in a flow migration system that requires no buffering or dropping of packets and eventually synchronizes its states.