Lancaster University Telefonica JHU/APL Huawei Time-Variant Routing (TVR) refers to calculating a forwarding path through a network where the time of message transmission (or receipt) is part of the overall route computation. This means that, all things being equal, a TVR computation might produce different results depending on the time that the computation is performed without other detectable changes to the network topology or other cost functions associated with the route. This document introduces requirements for the design and implementation of systems which perform TVR computations. It also explains different aspects of a TVR system which need to be considered during its design.