Loss-freedom, Order-preservation and No-buffering: Pick Any Two During Flow Migration in Network Functions

Network Functions (NFs) provide security and optimization services to networks by examining and modifying packets and by collecting information. When NFs need to be scaled out to manage higher load or scaled in to conserve energy, flows need to be migrated from one instance of an NF, called the source instance, to another, called the destination instance, or from one chain of instances to another chain of instances. Before flows are migrated, the state information associated with the source instance needs to be migrated to the destination instance. Packets that arrive at the destination instance meanwhile need to be either buffered or dropped until the state information is migrated, for correct functioning of some stateful NFs, while for some others, the destination NF may continue to function. We define the properties of Loss-freedom, where the flow migration system does not drop packets, No-buffering, where it does not buffer packets, and Order-preservation, where it processes packets in the same manner as the source NF, if there was no flow migration. We formalize these properties, for the first time, and prove that it is impossible for a flow migration algorithm in stateful NFs to guarantee satisfying all three of the properties of Loss-freedom (L), Order-preservation (O) and No-buffering (N) during flow migration, even if messages or packets are not lost. We demonstrate how existing algorithms operate with regard to these properties and prove that these properties are compositional.